reactive power management capabilities of swedish sub ...1272223/fulltext01.pdf · control methods...

IN DEGREE PROJECT ELECTRICAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2018

Reactive power management capabilities of Swedish sub-transmission and medium voltage level grid.

Öland's case.

MICHAL TOMASZEWSKI

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE

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Abstract

Rising penetration of renewable energy sources in electric power grids isboth a challenge and an opportunity to optimally utilize the potential of eitherwind or PV energy sources, to stabilize operation of future power systems.Bi-directional flows between distribution and transmission system operatorscause significant problems with keeping the voltages in the grid within ad-missible limits. This paper contains description of Öland’s island medium-and low-voltage electric power grid, ranging from 0.4 kV to 130 kV in thepurpose of quasi-static analysis of active and reactive power flows in the sys-tem. Goal of the analysis is to optimize reactive power exchange at the pointof connection with the mainland grid. In the analyzed grid system, thereis an enormous, 190 % penetration of wind sources. Capacity of the windparks connected to dedicated buses totals to 136.1 MW, that supply up to90.5 MW of load. With industry-wise reactive power capability limits, totalcontribution of wind parks reaches almost 66 MVAr, enabling to compen-sate deficits and extra surpluses of the reactive power in the grid. Presentedsystem is connected to the mainland’s grid through one point of connection,which is simulated as Thevenin equivalent circuit. Main objective of thethesis is to test and analyze viable solutions to minimize reactive power ex-change at the point of connection at Stävlö substation connecting Öland’sand Sweden’s electric grid keeping valid all necessary contingencies enforcedby current grid codes applied in Sweden as well as thermal limits of the linesand voltage limits of the system. Furthermore, state of the art of currentreactive power compensation methodologies and most promising techniquesto efficiently and effectively control reactive power flow are outlined. Droopcontrol methodologies, with focus on global and local objectives, and smartgrid solutions opportunities are being tested and modeled by the authors andare comprehensively presented in this paper. Moreover, economic costs ofcontrol methods are compared. Analysis of active power losses in the systemas well as cost of implementation of alternative solutions is presented, wheremost financially viable solutions are outlined, giving brief outlook into futureperspectives and challenges of electric power systems. It is shown that con-trollability of reactive power support by wind turbine generators can enhanceoperation of electric power grids, by keeping the reactive power flow mini-mized at the boundary between grids of distribution and transmission systemoperators. Furthermore, results indicate that extra reactive power supportby wind turbine generators can lead to diminishment of active power losses inthe system. Presented system is being modeled in the PSS/E software dedi-cated for power system engineers with use of Python programming languages.Analysis of data was done either in Python or R related environments. Thesiswas written with cooperation between KTH and E.On Energidistribution AB.

Keywords: electric power systems, reactive power, sub-transmission levelgrids, PSS/E, power flow analysis, wind farms, smart grids, droop control

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Abstract

Högre genomslagskraft av förnyelsebara energikällor i elnäteten är b̊adeen utmaning och möjlighet för att optimalt kunna utnyttja potentialen avvindkraft och PV källor, med avseende p̊a att stabilisera driften av framtidaelkraftsystem. Tv̊avägsflöden mellan distribution- och transmissionsoperatörerorsakar betydande problem att h̊alla spänningen i nätet inom till̊atna gränsvärden.Denna uppsats inneh̊aller en beskrivning av Ölands mellan- och l̊agspänningsnät,p̊a 0.4 kV till 130 kV i syftet att utföra en kvasistatisk analys av aktiva ochreaktiva effektflöden i systemet. Målet med analysen är att optimera detreaktiva effektutbytet i kopplingspunkten med fastlandets nät. I det analy-serade systemet, finns det en enorm potential p̊a 190% genomslagskraft avvindkraft. Kapaciteten p̊a vindkraftsparker kopplade till medtagna sam-lingsskenor i systemet uppg̊ar till 136,1 MW, som tillgodoser upp till 90.5MW last. Med industrimässigt begränsad reaktiv effektkapabilitet, uppg̊arvindkraftsparkernas bidrag till nästan 66 MVAr, vilken möjliggör kompensa-tion för underskott och överskott av reaktiv effekt i nätet. Det presenteradesystemet är kopplat till fastlandet genom en kopplingspunkt, där fastlandetär simulerat som en Thevenin ekvivalent. Huvudsakliga målet med dennauppsats är att testa och analysera g̊angbara lösningar för att minimera detreaktiva effektutbytet vid kopplingspunkten i Stävlö, som kopplar ihop Ölandmed resterande nät i Sverige, samtidigt som alla nödvändiga villkor enligt nu-varande nätkoder i Sverige bibeh̊alls, liksom termiska gränser för ledningarnaoch spänningsgränser för systemet. Ytterligare beskrivs den bästa tillgängligatekniken som finns idag för reaktiv effektkompensation, och de mest lovandeteknikerna för att effektivt och verkningsfullt kontrollera reaktiva effektflöden.Droop-kontroll-metodologier, med fokus p̊a globala och lokala tillämpningar,och smarta nät-möjligheter testas och modelleras av författarna och presen-terar djupg̊aende i detta arbete. Dessutom jämförs ekonomiska kostnader förolika kontrollmetoder. Analyser av aktiva effektförluster i systemet samt kost-nader för implementation av alternativa lösningar presenteras, där de flestag̊angbara lösningar behandlas, och ger en översk̊adlig bild av framtida per-spektiv och utmaningar i elkraftsystemet. Det visas att vindturbiners kontrollav reaktiv effekt, kan förbättra driften av elnäten, genom att minimera detreaktiva effektflödesutbytet i gränsen mellan distribution- och transmission-soperatörers nät. Ytterligare pekar resultat p̊a att extra understöd av reaktiveffekt fr̊an vindturbiner kan leda till förminskning av aktiva förluster i sys-temet. Det presenterade systemet modelleras i mjukvaruprogrammet PSS/Ededikerat för elkraftsingenjörer med hjälp av Python. Analys av data gjordesantingen i Python- eller R-relaterade miljöer. Detta arbete har gjorts tillsam-mans med KTH och E.ON Energidistribution AB.

Keywords: elektriska system, reaktiv effekt, mellan spänningsniv̊an, PSS/E,strömflödesanalys, vindkraftparker, smarta nät, droop control

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Acknowledgements

I would first like to thank my thesis advisor Stefan Stankovic of the Electric Powerand Energy Systems at KTH Royal Institute of Technology in Stockholm. I wouldlike to express my sincere gratitude for the continuous support of the research, forhis patience and motivation. He consistently allowed this paper to be my own work,but steered me in the right the direction whenever he thought I needed it.

I would also like to thank Ingmar Leisse, the expert who was involved in thisresearch project. His expertise allowed me to take a glimpse and understand var-ious challenges of electric power systems. I would like to sincerely thank for thecontinuous support during my stay at the E.On company.

I would also like to acknowledge professor Lennart Söder of the Electric Powerand Energy Systems at KTH Royal Institute of Technology in Stockholm as thesecond reader of this thesis, and I am gratefully indebted for his very valuablecomments on this thesis.

Micha l Tomaszewski,Stockholm, 25.09.2018

Table of contents

Table of contents 6

List of Figures 8

List of Tables 15

1 Introduction 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Research objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Thesis organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theoretical background and reactive power control techniques 52.1 Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Analytical expression of power flows . . . . . . . . . . . . . . 52.2 Reactive power control techniques . . . . . . . . . . . . . . . . . . . 9

2.2.1 Voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.2 Impedance control - Compensators . . . . . . . . . . . . . . . 102.2.3 Synchronous condensers . . . . . . . . . . . . . . . . . . . . . 112.2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 TSO-DSO Communication and literature review of reactive powercontrol methods 133.1 TSO-DSO communication . . . . . . . . . . . . . . . . . . . . . . . . 133.2 State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.2.2 Local objective control methods . . . . . . . . . . . . . . . . 163.2.3 Global objective control methods . . . . . . . . . . . . . . . . 193.2.4 Controllability of wind turbine generators . . . . . . . . . . . 213.2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4 System description 23

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TABLE OF CONTENTS 7

4.1 Öland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.1 General characteristics . . . . . . . . . . . . . . . . . . . . . . 234.1.2 Buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.1.3 Compensators . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1.4 Lines and cables . . . . . . . . . . . . . . . . . . . . . . . . . 284.1.5 Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2 Model and data validation . . . . . . . . . . . . . . . . . . . . . . . . 294.2.1 Location of measurement points . . . . . . . . . . . . . . . . 294.2.2 Initial model validation . . . . . . . . . . . . . . . . . . . . . 324.2.3 Final model validation . . . . . . . . . . . . . . . . . . . . . . 354.2.4 Model validation results . . . . . . . . . . . . . . . . . . . . . 384.2.5 Ölands electricity system characteristics . . . . . . . . . . . . 41

5 Simulation Results 515.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.2 Reactive power support capabilities of wind farms . . . . . . . . . . 535.3 Control Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.3.1 Local objective control schemes . . . . . . . . . . . . . . . . . 555.3.2 Global objective control schemes . . . . . . . . . . . . . . . . 625.3.3 Costs & Savings . . . . . . . . . . . . . . . . . . . . . . . . . 83

6 Conclusions and future work 856.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 856.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Bibliography 89

A Additional figures 93A.1 Öland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94A.2 PSS/E model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95A.3 Smart grid algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 96A.4 Costs and Savings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97A.5 Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

List of Figures

2.1 Representation of a line . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 Apparent power transfer through a line under constant receiving voltage

and sending voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1 Average monthly Swedish consumption profile in year 2016. . . . . . . . 244.2 Active power consumed and generated by loads and generators located

on the island. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.3 Reactive power consumed and generated by loads and generators located

on the island. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.4 Location of measurement point in real grid, case 1. . . . . . . . . . . . . 304.5 Location of measurement point in real grid, case 2. . . . . . . . . . . . . 304.6 Active Power difference between simulation and measurement without

correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.7 Reactive Power difference between simulation and measurement without

correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.8 Active Power difference between simulation and measurement after cor-

rection of line and transformers losses. . . . . . . . . . . . . . . . . . . . 344.9 Reactive Power difference between simulation and measurement after

correction of line and transformers losses. . . . . . . . . . . . . . . . . . 344.10 Relation in difference between measurement and simulation and active

power input to the wind farm at bus 37790. . . . . . . . . . . . . . . . . 364.11 Active Power difference between simulation and measurement after lin-

ear regression data correction. . . . . . . . . . . . . . . . . . . . . . . . . 374.12 Reactive Power difference between simulation and measurement after

linear regression data correction. . . . . . . . . . . . . . . . . . . . . . . 384.13 Logical process and steps taken to obtain accurate input data to the

PSS/E model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.14 WAE of active power. Comparison between different data correction steps. 404.15 WAE of reactive power. Comparison between different data correction

steps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.16 Active and reactive power consumption by loads in Öland in year 2017. 424.17 Active and reactive power generation by generators in Öland in year 2017. 43

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List of Figures 9

4.18 Maximum (left) and minimum (right) voltage profile in year 2017 presentin southern (top figure) and northern (bottom figure) part of Öland. . . 44

4.19 Maximum (left) and minimum (right) voltage profile in year 2018 presentin southern (top part of the figure) and northern (bottom part of thefigure) part of Öland. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.20 Active and Reactive power exchanged at the point of common connec-tion at Stavlö station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.21 Measured active against reactive power exchange at the point of con-nection at Stavlö station in year 2017. . . . . . . . . . . . . . . . . . . . 48

4.22 Simulated active against reactive power exchange at the point of con-nection at Stavlö station in year 2017. . . . . . . . . . . . . . . . . . . . 49



5.3 Exemplary droop curve used in one of the St. Istad wind farms. . . . . 56

5.4 Reactive power exchanged at swing bus in year 2017 after local objectivecontrol scheme have been implemented. Scenario without STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.5 Reactive power exchanged at swing bus in year 2017 after local objec-tive control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.6 Reactive power exchanged at swing bus in year 2018 after local objectivecontrol scheme have been implemented. Scenario without STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.7 Reactive power exchanged at swing bus in year 2018 after local objec-tive control scheme have been implemented. Scenario with STATCOMfunctionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.8 Exemplary of general droop curve created based on the reactive powerexchanged at swing bus and active power capacity used in wind farms. . 62

5.9 Influence of multiplication coefficients of droop curve on the absolutereactive power exchanged at swing bus. Scenario with STATCOM func-tionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.10 Influence of multiplication coefficients of droop curve on the absolutereactive power exchanged at swing bus. Scenario without STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.11 Reactive power exchanged at swing bus in year 2017 after global objec-tive control scheme have been implemented. Scenario without STAT-COM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5.12 Reactive power exchanged at swing bus in year 2017 after global objec-tive control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

10 List of Figures

5.13 Reactive power exchanged at swing bus in year 2018 after global objec-tive control scheme have been implemented. Scenario without STAT-COM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

5.14 Reactive power exchanged at swing bus in year 2018 after global objec-tive control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

5.15 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario without STAT-COM functionality and objective of 0 MVAr of reactive power exchangedat swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.16 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario with STATCOMfunctionality and objective of 0 MVAr of reactive power exchanged atswing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.17 Reactive power exchanged at swing bus in year 2018 after global smartgrid control scheme have been implemented. Scenario without STAT-COM functionality and objective of 0 MVAr of reactive power exchangedat swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.18 Reactive power exchanged at swing bus in year 2018 after global smartgrid control scheme have been implemented. Scenario with STATCOMfunctionality and objective of 0 MVAr of reactive power exchanged atswing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.19 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario without STAT-COM functionality and objective of ±5 MVAr of reactive power ex-changed at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.20 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario with STATCOMfunctionality and objective of ±5 MVAr of reactive power exchanged atswing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.21 Reactive power exchanged at swing bus in year 2018 after global smartgrid control scheme have been implemented. Scenario without STAT-COM functionality and objective of ±5 MVAr of reactive power ex-changed at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.22 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario with STATCOMfunctionality and objective of ±5 MVAr of reactive power exchanged atswing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.23 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario without STAT-COM functionality and objective of ±10% of active power exchanged atswing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

List of Figures 11

5.24 Reactive power exchanged at swing bus in year 2017 after global smartgrid control scheme have been implemented. Scenario with STATCOMfunctionality and objective of ±10% of active power exchanged at swingbus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.25 Reactive power exchanged at swing bus in year 2018 after global smartgrid control scheme have been implemented. Scenario without STAT-COM functionality and objective of ±10% of active power exchanged atswing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.26 Reactive power exchanged at swing bus in year 2018 after global smartgrid control scheme have been implemented. Scenario with STATCOMfunctionality and objective of ±10% of active power exchanged at swingbus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A.1 Öland’s map of wind parks of capacity exceeding 5 MW . . . . . . . . . 94

A.2 Öland’s PSS/E model representation. . . . . . . . . . . . . . . . . . . . 95

A.3 Smart grid control algorithm . . . . . . . . . . . . . . . . . . . . . . . . 96

A.4 Potential savings of global and local objective droop control schemesin year 2017. Assumed reactive power cost 26.3 SEK/MVArh and 42.2SEK/MVARh. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

A.5 Voltage profile in year 2017 present in the southern part of Öland afterlocal control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

A.6 Voltage profile in year 2017 present in the southern part of Öland afterlocal control scheme have been implemented. Scenario without STAT-COM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

A.7 Voltage profile in year 2017 present in the northern part of Öland afterlocal control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

A.8 Voltage profile in year 2017 present in the northern part of Öland afterlocal control scheme have been implemented. Scenario without STAT-COM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

A.9 Voltage profile in year 2018 present in the southern part of Öland afterlocal control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

A.10 Voltage profile in year 2018 present in the southern part of Öland afterlocal control scheme have been implemented. Scenario without STAT-COM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

A.11 Voltage profile in year 2018 present in the northern part of Öland afterlocal control scheme have been implemented. Scenario with STATCOMfunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

A.12 Voltage profile in year 2018 present in the northern part of Öland afterlocal control scheme have been implemented. Scenario without STAT-COM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

12 List of Figures

A.13 Voltage profile in year 2017 present in the southern part of Öland afterglobal droop control scheme have been implemented. Scenario withSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 102

A.14 Voltage profile in year 2017 present in the southern part of Öland afterglobal droop control scheme have been implemented. Scenario withoutSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 102

A.15 Voltage profile in year 2017 present in the northern part of Öland afterglobal droop control scheme have been implemented. Scenario withSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 103

A.16 Voltage profile in year 2017 present in the northern part of Öland afterglobal droop control scheme have been implemented. Scenario withoutSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 103

A.17 Voltage profile in year 2018 present in the southern part of Öland afterglobal droop control scheme have been implemented. Scenario withSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 104

A.18 Voltage profile in year 2018 present in the southern part of Öland afterglobal droop control scheme have been implemented. Scenario withoutSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 104

A.19 Voltage profile in year 2018 present in the northern part of Öland afterglobal droop control scheme have been implemented. Scenario withSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 105

A.20 Voltage profile in year 2018 present in the northern part of Öland afterglobal droop control scheme have been implemented. Scenario withoutSTATCOM functionality. . . . . . . . . . . . . . . . . . . . . . . . . . . 105

A.21 Voltage profile in year 2017 present in the southern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of 0 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.22 Voltage profile in year 2017 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of 0 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 106

A.23 Voltage profile in year 2017 present in the northern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of 0 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

A.24 Voltage profile in year 2017 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of 0 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 107

A.25 Voltage profile in year 2018 present in the southern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of 0 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

List of Figures 13

A.26 Voltage profile in year 2018 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of 0 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 108

A.27 Voltage profile in year 2018 present in the northern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of 0 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

A.28 Voltage profile in year 2018 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of 0 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 109

A.29 Voltage profile in year 2017 present in the southern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of ± 5 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

A.30 Voltage profile in year 2017 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 5 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 110

A.31 Voltage profile in year 2017 present in the northern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of ± 5 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

A.32 Voltage profile in year 2017 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 5 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 111

A.33 Voltage profile in year 2018 present in the southern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of ± 5 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

A.34 Voltage profile in year 2018 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 5 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 112

A.35 Voltage profile in year 2018 present in the northern part of Öland afterglobal smart grid control scheme have been implemented. Scenario withSTATCOM functionality and objective of ± 5 MVAr of reactive powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

A.36 Voltage profile in year 2018 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 5 MVAr of reactivepower exchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . 113

14 List of Figures

A.37 Voltage profile in year 2017 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowith STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

A.38 Voltage profile in year 2017 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

A.39 Voltage profile in year 2017 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowith STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

A.40 Voltage profile in year 2017 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

A.41 Voltage profile in year 2018 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowith STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

A.42 Voltage profile in year 2018 present in the southern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

A.43 Voltage profile in year 2018 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowith STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

A.44 Voltage profile in year 2018 present in the northern part of Öland af-ter global smart grid control scheme have been implemented. Scenariowithout STATCOM functionality and objective of ± 10 % active powerexchanged at swing bus. . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

List of Tables

3.1 Comparison of local and global control schemes. . . . . . . . . . . . . . 15

4.1 Bus summary of Öland’s model of electric grid . . . . . . . . . . . . . . 26

4.2 Bus summary of Öland’s electric grid system model . . . . . . . . . . . 27

4.3 Compensator summary of Öland’s electric grid system model . . . . . . 27

4.4 Lines summary of Öland’s electric grid system model . . . . . . . . . . . 28

4.5 Two-winding transformers summary of Öland’s electric grid system model 28

4.6 Three-winding transformers summary of Öland’s electric grid systemmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.7 Active and reactive power flow through a subsystem (MV line and twostep-up transformers) presented in figure 4.5. . . . . . . . . . . . . . . . 33

4.8 Active and Reactive power flow through a simple subsystem (MV lineand two step-up transformers) presented in figure 4.11 in step two ofdata correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.9 Comparison of statistical parameters before and after implementationof model optimization algorithms. . . . . . . . . . . . . . . . . . . . . . 39

4.10 Net and absolute active and reactive power exchange at swing bus andgeneration by wind farms in years 2017 and 2018. . . . . . . . . . . . . . 48

4.11 Number of hours lying outside of assumed limits of ±5 MVAR, ±10MVAR and 10% of absolute value of active power in years 2017 and 2018. 50

4.12 Absolute value of reactive power exceeding assumed limits of ±5 MVAR,±10MVAR, and 10 % of absolute value of active power in years 2017and 2018. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.1 Reactive power capabilities of WTG. Industry standard reference value- top, scaled reactive power support - bottom . . . . . . . . . . . . . . . 54

5.2 Reactive power capabilities of St. Istad wind farm generators. . . . . . . 57

5.3 Net and absolute active and reactive power exchange at swing bus andgeneration by wind farms in years 2017 and 2018 after local objectivecontrol scheme have been implemented. B - Base scenario, S - STATOMscenario, NS - No STATCOM scenario. . . . . . . . . . . . . . . . . . . . 58

15

16 List of Tables

5.4 Number of hours lying outside of assumed limits of ±5 MVAR, ±10MVAR and 10% of absolute value of active power in years 2017 and2018 after local objective control scheme have been implemented. B -Base scenario, S - STATOM scenario, NS - No STATCOM scenario. . . 58

5.5 Absolute value of reactive power exceeding assumed limits of ±5 MVAR,±10MVAR, and 10 % of absolute value of active power in years 2017and 2018 after local objective control scheme have been implemented.B - Base scenario, S - STATOM scenario, NS - No STATCOM scenario. 59

5.6 Comparison of different droop curves analyzed and their fit to the data. 63

5.7 Net and absolute active and reactive power exchange at swing bus andgeneration by wind farms in years 2017 and 2018 after global objectivecontrol scheme have been implemented. B - Base scenario, S - STATOMscenario, NS - No STATCOM scenario. . . . . . . . . . . . . . . . . . . . 66

5.8 Number of hours lying outside of assumed limits of ±5 MVAR, ±10MVAR and 10% of absolute value of active power in years 2017 and2018 after global objective control scheme have been implemented. B -Base scenario, S - STATOM scenario, NS - No STATCOM scenario. . . 66

5.9 Absolute value of reactive power exceeding assumed limits of ±5 MVAR,±10MVAR, and 10 % of absolute value of active power in years 2017and 2018 after global objective control scheme have been implemented.B - Base scenario, S - STATOM scenario, NS - No STATCOM scenario. 66

5.10 Net and absolute active and reactive power exchange at swing bus andgeneration by wind farms in years 2017 and 2018 after smart grid controlscheme with objective of 0 MVAr have been implemented. B - Basescenario, S - STATOM scenario, NS - No STATCOM scenario. . . . . . 71

5.11 Number of hours lying outside of assumed limits of ±5 MVAR, ±10MVAR and 10% of absolute value of active power in years 2017 and2018 after smart grid control scheme with objective of 0 MVAr havebeen implemented. B - Base scenario, S - STATOM scenario, NS - NoSTATCOM scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5.12 Absolute value of reactive power exceeding assumed limits of ±5 MVAR,±10MVAR, and 10 % of absolute value of active power in years 2017and 2018 after smart grid control scheme with objective of 0 MVAr havebeen implemented. B - Base scenario, S - STATOM scenario, NS - NoSTATCOM scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

5.13 Net and absolute active and reactive power exchange at swing bus andgeneration by wind farms in years 2017 and 2018 after smart grid controlscheme with objective of ±5 MVAr have been implemented. B - Basescenario, S - STATOM scenario, NS - No STATCOM scenario. . . . . . 75

5.14 Number of hours lying outside of assumed limits of ±5 MVAR and ±10MVAR in years 2017 and 2018 after smart grid control scheme withobjective of ±5 MVAr have been implemented. B - Base scenario, S -STATOM scenario, NS - No STATCOM scenario. . . . . . . . . . . . . . 76

List of Tables 17

5.15 Absolute value of reactive power exceeding assumed limits of ±5 MVAR,±10MVAR, and 10 % of absolute value of active power in years 2017and 2018 after smart grid control scheme with objective of ±5 MVArhave been implemented. B - Base scenario, S - STATOM scenario, NS- No STATCOM scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.16 Net and absolute active and reactive power exchange at swing bus andgeneration by wind farms in years 2017 and 2018 after smart grid con-trol scheme, with objective of 10% of active power exchanged, have beenimplemented. B - Base scenario, S - STATOM scenario, NS - No STAT-COM scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.17 Number of hours lying outside of assumed limits of ±5 MVAR, ±10MVAR and 10% of absolute value of active power in years 2017 and 2018after smart grid control scheme, with objective of 10% of active powerexchanged, have been implemented. B - Base scenario, S - STATOMscenario, NS - No STATCOM scenario. . . . . . . . . . . . . . . . . . . . 80

5.18 Absolute value of reactive power exceeding assumed limits of ±5 MVAR,±10MVAR, and 10 % of absolute value of active power in years 2017and 2018 after smart grid control scheme, with objective of 10% of ac-tive power exchanged, have been implemented. B - Base scenario, S -STATOM scenario, NS - No STATCOM scenario. . . . . . . . . . . . . . 80

5.19 Cost of electrical equipment needed to build bus station connected to130 kV grid compensating up to 40 MVAr of reactive power. . . . . . . 84

18 List of Tables

DSO Distribution System OperatorTSO Transmission System OperatorDG Distributed GeneratorDGU Distributed Generation UnitAC Alternating CurrentDC Direct CurrentEHV Extra High VoltageHV High VoltageMV Medium VoltageLV Low VoltageWTG Wind Turbine GeneratorPV PhotovoltaicMW MegawattGW GigawattMVAr Mega Voltage Ampere ReactiveGWh Gigawatt hourTWh Terawatt hourMWh Megawatt hourkV KilovoltENTSO-E European Network of Transmission System Operators for ElectricityFACTS Flexible Alternating Current Transmission SystemsPOC Point of ConnectionPCC Point of Common CouplingUS Sending VoltageUR Receiving VoltageIS Sending CurrentIR Receiving CurrentPS Sending Active PowerPR Receiving Active PowerQS Sending Reactive PowerQR Receiving Reactive PowerUn Reference VoltageX ReactanceR ResistanceZ ImpedancePST Phase Shifting TransformerSVC Static Var CompensatorTCR Thyristor Controlled ReactorTSR Thyristor Switched ReactorTSC Thyristor Switched CapacitorSTATCOM Static CompensatorVSC Voltage Source ConverterSSSC Static Synchronous Series CompensatorUPFC Unified Power Flow Controllerp.u. per unitWAE Weighted Average ErrorNC DCC Network Codes Demand Connection CodeVPRF Variation of Reactive PowerRPC Reactive Power CharacteristicsOPRD Optimal Reactive Power DispatchedBAU Business as UsualDFIG Doubly Fed Induction GeneratorGSC Grid Side Converter

Chapter 1

Introduction

1.1 Introduction

Throughout the time, electric power systems evolved and various energy carrierswere used to satisfy the demand. Since last several years, with development ofintermittent renewable energy sources like wind generators and photovoltaic pan-els, energy systems came into era of sustainable and local generation, but mostimportantly, generation independent of scarcity of natural resources like coal, gasor uranium.

1.1.1 Background

In 2017, total generation from all available energy sources was around 24817400000MWh. Giving on average 20985 megawatt hours consumed more every hour, dueto immense electrification lasting more than 135 years. Mostly in alternating form,electricity is being supplied to the consumers through transmission and distribu-tion lines. Some of them are several hundreds meters long with nominal voltageas low as 400 Volts. Others are extremely long, reaching up to 1050 kilometerswhile voltage, in alternating form, goes up to 1000 kV, [Liu, 2014]. Even highervoltages are obtained with direct current transmission systems. In most extremecases, electricity is transmitted to the length of 2385 km by a single DC link, whatis almost 6% of Earth’s perimeter. Accessible capacity of such link is equal to7.1 GW, what is being equivalent to constant supply of nearly 34 million house-holds in Brazil, every hour, [Mendonça et al., 2017]. Energy systems all aroundthe world are being a subject of many concerns of reliability and sustainability inthe times of extensive investments in renewable energy sources, especially in windand photovoltaic farms. Development of renewable energy sources imposed mean-ingful changes regarding topology of the electric grids and direction of electricityflow through low and medium voltage distribution grids as well as high voltageelectric transmission grids. One of the most problematic issues is reversed powerflow from low to higher voltage grid, leading to unwanted voltage increase under

1

2 CHAPTER 1. INTRODUCTION

low load conditions. Resulting in raise in the investment of compensating devicesand transformers.Unfortunately, renewable energy generation sources are usuallylocated in the lower, distribution voltage grids, where lines and equipment is notdesigned to withstand such high currents and voltages. Furthermore, alternatingcurrent flowing through the conductor, transforms the conductor to an inductor,making the current lag the voltage. In result, lines are absorbing reactive power,and dropping the voltage at the end of the line. On the other hand, in cables, thathave bigger capacitance values than overhead lines, due to Ferranti Effect, voltagelag is generated with respect to the current and lines supply the reactive power. Inresult there is increase of the voltage at the end of the line. In both cases compen-sating devices as shunt capacitors or shunt reactors are needed to supply or absorbextra reactive power and to keep the voltages and power flow within permissiblelimits, making grids safe and reliable [Chavan et al., 2016].

1.1.2 Motivation

”The price of failure is too high” [Energy, 2010]. Directive of European Parlia-ment was a breakthrough in terms of evolution of energy systems we knew before.Progress of energy policies was never so rapid, setting new thresholds and changingthe energy industry forever, giving boost to more local and sustainable sources ofenergy [Union, 2009]. As the legislation states, by 2020, Europe should be facinga goal to achieve 20 % of the overall share of energy coming from renewable en-ergy sources. From 2007, when first communication from European Commissionwas published, to 2018, two years before the deadline to fulfill the requirementsof European Parliament, wind power grew by more than 300% and surpassed, oil,nuclear, hydro and coal settling as the second largest generation source by capacityin Europe, reaching almost 170 GW. Only in 2017, wind assets in Europe increasedby 15.7 GW, with most of it in Germany and United Kingdom, [WindEurope,2018]. Further targets are coming into force from the policy makers, setting newobjectives for energy sector until 2030 and beyond. What will lead to subsequentincrease in renewable energy sources share in final energy generation, [Commision,2014]. On the the other hand, consumption and in result generation of electricityin Europe has been steady for the last couple of years, pointing that old generatingsources are being rather replaced by new capacities in wind and solar and are beingphased out from the system. This results in several, significant challenges that gridoperators and distribution companies are facing, [Coster et al., 2011].

Old type of generators were mostly of synchronous type that were able to ab-sorb or supply reactive power immediately in a controlled and synchronized way.This is also achievable by most of the modern wind turbine generators. However,many already installed assets more than 5 years old, are not capable of doing so. Itis possible to control reactive power output from Type III and Type IV wind tur-bine generators (WTGs), but some of the already existing wind farms, built severalyears ago, are composed of an old types of WTGs. Secondly, grid topologies and itsstructure was built to transport electricity from high voltage to low voltage, causing

1.1. INTRODUCTION 3

the protection and relay system to work in top-down coordination. Additional gen-erating units connected to lower voltage grid might cause unsynchronized reclosingor are in need for adjusted location of disconnectors and relays. Lastly, additionalgenerators located at the lower voltage grids might influence power quality undersudden drop of wind speed or cloudy periods over PV panels. This resulted indevelopment of new grid codes and technical requirements for connection of dis-tributed generators. ENTSO-E and other EU organizations force new legislationstating connection demands for generators, [Com, 2016]. Moreover, with new net-work codes, cooperation between DSO and TSO is at the stake of imminent need ofimprovement in European countries. ENTSO-E, organization connecting 43 elec-tricity transmission system operators from 36 countries across Europe, points outto improve interaction between transmission and distribution system operators forthe benefit of the consumer. Series of recommendations aim to serve as basis fordiscussion between all electricty market entities and participants to render success-ful story written by both. Introduction of vast amount of local and dispatchablegeneration sources like wind farms, PV panels or geothermal power plants, forcethe distribution system operators to extensively control active and reactive powerflows being transmitted to higher voltages and transmission grid.

In the future power grid, excess of energy generated in lower voltage parts ofthe system, will be pushed up, to transmission lines vastly rising the complexity ofthe system in terms of efficient and effective control. For example, back in the day,voltages were kept under the allowable limits by central synchronous generatorsconnected to high voltage substations, and transformers that varied the respectivetap ratios. Now, TSO instead of balancing the voltage and frequency dips by largegeneration units in the transmission grid, installs compensating equipment likeshunt reactors and capacitors or FACTS. The same occurs for DSO. Since 2011,E.On invested plenty of money into compensating devices. During the last 7 years,company installed more than 22 shunt capacitors at different voltage levels, startingas low as 6 kilovolts.

To fight this problem, it appeared that distributed generation plants could be aviable, both financially and technically, reactive power control and in result voltagecontrol units. Prospects of renewable energy sources reactive power compensationcapabilities is already successfully implemented in individual, smaller scale cases allaround the globe, making parts of the grid self-sufficient and working in islandedoperation, when control of frequency and voltage done by main power plants con-nected to extra high voltage grid. Local control systems utilizing potential of RES,after some adjustments, could be implemented in power grids as a whole. In thispaper, authors present viable substitutes to the existing practice. Transmission anddistribution system operators can no longer keep voltage within limits under highpenetration of generation in medium and low voltage grid causing reactive powerflow upwards, leading to overvoltages and violation of grid constraints.

4 CHAPTER 1. INTRODUCTION

1.2 Research objectives

Objective of the thesis is to:

1. Analyze viable solutions extending capabilities of distributed generation units,particularly, wind turbine generators, to help in addressing challenges of ex-cess reactive power exchange between sub-transmission and transmission levelgrids in Sweden,

2. Compare results of control strategies, both technically and financially, thatcould be implemented in existing topology of electric grid,

3. Contrast effects of potential grid codes limiting the exchange of reactive powerbetween sub-transmission and transmission level grids.

Control methods, extending capabilities of DGUs, are evaluated basing on po-tential grid codes stating limits of reactive power exchange at the point of con-nection (POC) in Sweden. Comparison of results puts main focus on contrastingavailable technical solutions and their advantages. As, for instance, Q(P ) droopcurves implemented locally in clustered wind generators with local objectives aswell as with global objectives in the excluded system. Furthermore, smart grid ap-proach is tested, evaluating potential of the wind farms to support voltage controlwithin Öland boundaries. In the last objective of the paper, limits stated by po-tential grid codes are divided into three different limits of reactive power exchangeat the POC.

1.3 Thesis organization

First chapter of this thesis includes introduction, where current state of develop-ment of electric power grid is described. Furthermore, explanation of causes ofchallenges energy systems are facing is written, highlighting the potential benefitsof use of renewable energy sources to solve them. Research objectives are enlisted,stating technical solutions extending capabilities of wind farms. In the secondchapter, theoretical background on physics behind active and reactive power flowin power lines is described as well as methods used to control reactive power flowwithin power grid. Third chapter contains literature review. Also, difficulties oflack of communication between transmission and distribution system operators arehighlighted. Most promising reactive power control methods are cited giving briefoutlook on academic research on the topic of reactive power controllability by dis-tributed generation sources. Fourth chapter includes analyzed system description.Furthermore, ways used to obtain real grid representation in PSS/E software arethoroughtly explained. Fifth chapter includes authors’ simulation results and con-clusions on reactive power support by wind farms in Öland. Last chapter presentsconclusions and recommendations for future work.

Chapter 2

Theoretical background andreactive power control techniques

2.1 Theoretical background

2.1.1 Analytical expression of power flows

Power flow through a line can be described with use of basic Kirchhoff’s Laws and aTwo-port scheme. Assuming that lines are, under normal conditions, transmittingelectricity in a perfect 3-phase balance, one could represent 3-phase line by a singlephase line equivalent (same procedure can be applied for cables). A simple systemshown in figure 2.1, with a line between two nodes, where receiving node is writtenwith subscript R and sending node written with subscript S and where Z is a line’simpedance and Y SR is a line’s shunt admittance, this representation is called πmodel of a line. Kirchhoff’s current and voltage law state that, respective sumof currents flowing into a node and from a node is equal to 0, whereas sum ofelectrical potential differences over a closed loop is equal to 0. Relating voltage andcurrent flowing through a line, which can be described by a simple matrix of size2x2, containing four constants A,B,C,D, multiplied by receiving end voltage andcurrent, giving sending end voltage and current.[

USIS

]=

[A BC D

]∗[URIR

]Sending and receiving end voltages and currents can be related as has been done

in equation 2.1 and equation 2.2.

UR + Z ∗ (IR + UR ∗ YSR) = US (2.1)

IR + US ∗ Y SR + UR ∗ Y SR = IS (2.2)Sending end voltage is equal to the sum of receiving end voltage and voltage drop(increase) over a line due to series impedance and shunt admittance. Sending end

5

6CHAPTER 2. THEORETICAL BACKGROUND AND REACTIVE POWER

CONTROL TECHNIQUES

Figure 2.1: Representation of a line

current is equal to sum of receiving end current, current due to shunt admittanceand current due to series impedance of a line. Expressing the left side of the equation(2.1) and equation (2.2) in form of a multiplication of receiving end voltage andreceiving end current, and rewriting, we find sending end voltage and current.

UR ∗ (1 + Z ∗ Y SR) + Z ∗ IR = US (2.3)

UR ∗ Y SR ∗ (2 + Z ∗ Y SR) + (Z ∗ Y SR + 1) ∗ IR = IS (2.4)

One can notice that equations look similar to the ones seen in the matrix form ofthe twoport scheme. Comparing left side of the equations to the right side of thetwoport matrix, we can find that constants A,B,C and D are equal to:

A = 1 + Z ∗ Y SR (2.5)

B = Z (2.6)

C = Y SR ∗ (2 + Z ∗ Y SR) (2.7)

D = Z ∗ Y SR + 1 (2.8)

Finding the equality A = D, which states that the analyzed line is symmetric.Moreover, depending on the voltage of the line, as well as its length, equations canbe simplified. If a line has low values of parallel and shunt admittances, which isvalid for most of overhad medium and high voltage lines. We can assume, thatY SR is equal to 0, imposing D = 1 and C = 0. Putting values of A,B,C and D intoequations (2.3) and (2.4), we come to:

UR +B ∗ IR = US (2.9)

IR = IS (2.10)

2.1. THEORETICAL BACKGROUND 7

Taken into account assumptions, which closely represent the reality, simplifiedour analyzed model to following statements:

Sending end voltage is equal to receiving end voltage plus voltage drop caused bythe line impedance.

Sending end current is equal to receiving end current.

It could be also presumed to take the receiving end voltage as a reference valueUR = |UR|∠0◦. Equation (2.3) can be rewritten, with respect to the receiving endcurrent IR, inserting A = |A|∠α, B = |B|∠β and US = |US |∠δ, gives a formulationfor a receiving end current:

IR =|US ||B|∗ ej(δ−β) − |A| ∗ |UR|

|B|∗ ej(α−β) (2.11)

Putting (2.11) into formula for apparent power at the receiving end, SR = UR ∗I∗R,

where I∗R is the complex conjugate of the receiving end current, we have:

SR =|US | ∗ |UR||B|

∗ ej(β−δ) − |A| ∗ |UR|2

|B|∗ ej(β−α) (2.12)

Knowing that SR = PR+ jQR, equation (2.12) can be split into active and reactivepower at the receiving end, giving:

PR =|US | ∗ |UR||B|

∗ cos(β − δ)− |A| ∗ |UR|2

|B|∗ cos(β − α) (2.13)

And

QR =|US | ∗ |UR||B|

∗ sin(β − δ)− |A| ∗ |UR|2

|B|∗ sin(β − α) (2.14)

For a line with a negligible parallel or shunt impedances (as stated before in theassumptions), the parameters are, thus, (A = 1 and B = Z, giving equation (2.15)and equation (2.21)).

PR =|US | ∗ |UR||Z|

∗ cos(β − δ)− |UR|2

|Z|∗ cos(β) (2.15)

QR =|US | ∗ |UR||Z|

∗ sin(β − δ)− |UR|2

|Z|∗ sin(β) (2.16)

This general equations describing active and reactive power flow, conclude that:

1. Maximum power that can be transmitted through the line is limited if receiv-ing end voltage UR and sending end voltage US are kept constant. If voltagesat the sending and receiving end are equal, there is no active power flow,

2. Active power flowing through a line is strongly linked to the angle δ, reachingmaximum when β − δ = 0,


CONTROL TECHNIQUES

3. Reactive power flow is strongly linked to the sending end voltage |US |.

Figure (2.2) shows relation between equations (2.13) and (2.14). Maximum reactivepower transfer occurs at specific point when β − δ = 45◦, which is equivalent totan βδ = 1. Further simplifications can be introduced, for purely inductive line

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

X

Y

β − αβ − δ

o

n

P

Q

S

φ

Figure 2.2: Apparent power transfer through a line under constant receivingvoltage and sending voltage.

(where Z ≈ X), giving |B|∠β ≈ X∠90◦, equations (2.13) and (2.14), result in:

PR =|US | ∗ |UR||X|

∗ sin(δ) (2.17)

QR =|US | ∗ |UR|

X∗ cos(δ)− |UR|

2

X(2.18)

For purely inductive line, what is a good approximation for high voltage lines (inour case sub-transmission level lines), control of the reactive power flow is possiblethrough variation of impedance (reactance in case of purely inductive line) of theline, sending voltage, receiving voltage or power angle between the nodes.

2.2. REACTIVE POWER CONTROL TECHNIQUES 9

Similar equations are derived in relation to the sending end node (taking refer-ence for the current from sender to receiver IR = IS):

PS =|US | ∗ |UR||X|

∗ sin(δ) (2.19)

QS =|US | ∗ |UR|

X∗ cos(δ)− |US |

2

X(2.20)

From the equations (2.17)(2.18)(2.19)(2.20) we see that controllability of active andreactive power can be influenced by several variables, namely, sending voltage US ,receiving end voltage UR, impedance X and power angle δ.

2.2 Reactive power control techniques

As stated in section 2.1.1, there are four ways to control reactive power flow andvoltage withing electric grid. This section highlights most used methods used inreal power systems.

2.2.1 Voltage control

Voltage at the sending node can be increased or decreased by varying the tap ratioof the transformer connecting the generator to the grid. Every, analyzed in thesystem generator, has been connected to the step-up transformer either individuallyor collectively. Voltage at the receiving end of the line, connecting remotely locatedwind farm, can be fluctuated to be as close to the desired one, as possible. Thisis done by changing the tap ratios between primary and secondary side of thetransformer. Furthermore, some of the generators could be connected to the samebuses as loads what imposes limits on maximum and minimum voltages at thatbus. Challenges occur when there are parallel lines connected to buses connectingother nodes with more transformers on the path, making the controllability ofsuch system extremely complex. Voltage regulation can be performed in discretesteps. For instance, by ±1.67% increase or decrease per step, up to several stepsin each direction. Output voltage of the generator could be also varied by, forexample, changing excitation levels of field winding of the generator. This method,might be more useful when analyzing low voltage distribution grids where smallwind generators or household PV systems are connected directly to the grid. Here,generators can contribute to voltage control in the feeder. In analyzed system, noneof the generators were directly connected to the grid, thus, this contorl method isnot further developed. Nonetheless, high penetration of the photovoltaic systemscould enable voltage control in the distribution systems as presented by [Phochaiet al., 2014]. More voltage control methods exist, like network reconfiguration ordemand side management of the loads.


CONTROL TECHNIQUES

2.2.2 Impedance control - Compensators

Formula 2.21 states that reactive power flow could be varied by impedance fluctu-ation.

QR =|US | ∗ |UR||Z|

∗ sin(β − δ)− |UR|2

|Z|∗ sin(β) (2.21)

Impedance of the line can be varied by connecting shunt or series compensatorsdirectly with the line or by putting compensator at the substation bridging twoends of a line. There are several types of compensators that could aid to stabilizeand optimize performance of the grid. Reactors and capacitors are used to, respec-tively, compensate for the effects of the line capacitance and line reactance to limitthe voltage rise or voltage drop. Shunt connected compensators are, as the nameindicates, placed between lines and the neutral point, what partially or completelyreduces shunt susceptance, minimizing the reactive power flow. Shunt reactors, forexample, limit energization overvoltages and fundamental frequency overvoltages.This is extremely common problem under no-load periods, when voltage at thereceiving end becomes too high. On the other hand, shunt capacitors increase thevoltage and are used to ensure that voltages at all points remain within the accept-able limits in a high load circumstances. Furthermore, when lines are inductive,shunt capacitors optimize power factor, by supplying reactive power at points whereit is consumed, in order to minimize the reactive power being transmitted from i.e.generators. In other words, reactive power, should be theoretically generated asclose to the reactive load as possible, so the lines are naturally loaded (reactivepower is consumed or generated by the line itself and covers reactive power needsof a line). Contrarily, series reactors and capacitors compensate series impedanceof the line. High impedance of the line causes high voltage drop along the linelowering the maximum capacity of the line. Series capacitors compensate inductivereactance, whereas series reactors are often used to reduce fault currents and matchimpedance of parallel feeders.

2.2.2.1 SVC

Other methods include SVC (static VAr compensators) which are devices as thyristor-switched capacitors or reactors. TCR (thyristor-controlled reactor) is a shunt con-nected inductor, which, by partially continuous conductance, due to thyristor valve,can regulate reactive power supply and absorption. Thyristor-switched reactor,contrarily to the TCR, is being controlled in steps, by delaying the angle betweencurrent and voltage in one half of a cycles, absorbing reactive power. Big advantageof TSR over TCR is that there is less harmonic distortion of current under switchedoperation. In thyristor-switched reactor, capacitor limits the effective reactance byvariable conductance through the thyristor activating consecutive capacitors in thebank. Component-wise TSR and TSC work on the same principle, but instead ofreactor, capacitor is installed. Due to the limitations of the TSC (can be switchedeither on or off due to characteristics of the capacitor) it is often linked with extra

2.2. REACTIVE POWER CONTROL TECHNIQUES 11

transistor-switched reactors to make the reactive power controllability more finelytuned by making many discrete steps.

2.2.2.2 STATCOMs

Static synchronous compensators (STATCOMs) objective is to provide precise,rapid and flexible adjustment of reactive power. In contrary to previously enlistedcompensators, STATCOMs are self-commutated devices. This compensation tech-nique base on modification of magnitude and polarity of the imaginary part of thecurrent flowing through the device. STATCOMs are often used as dynamic powerfactor compensators i.e. in industrial areas, with high peaks of reactive power, forexample when starting up and magnetizing the motors. Beside that, it could bealso used for voltage compensation at the receiving end of a line, where load isconnected.

2.2.2.3 SSSC

Static synchronous series compensators depend upon voltage source converter (VSC)and are connected in series with the transmission line through a transformer. Oper-ation principle of the SSSC is to generate voltage in quadrature with the line currentand display capacitive or inductive equivalent impedance by rising or declining thepower flow. One of the big drawbacks of SSSC is that it can only transmit reactivepower in one direction.

2.2.2.4 UPFC

Last methods used in practice are UPFC (Unified Power Flow Controller), whichcould be understood as an SSSC and STATCOM connected by a common dc link.Reactive power in the shunt or series converter can be controlled independentlyupon each other, giving great flexibility. Unified power flow controller can generateany voltage phasor in series with the transmission line, assuming the voltage phasoris within permissible limits. UPFC, apart from controlling the reactive power andtherefore voltage, can be used to magnify or decrease the active power flow througha line.

2.2.3 Synchronous condensers

Synchronous condensers are basically synchronous generators rotating without aload. In theory, any synchronous generator could participate in reactive powercontrol strategies. By changing the excitation of the field, freely rotating rotor ofsynchronous condenser can be slowed down or sped up in order to absorb or generateextra reactive power. Bottleneck of this type of compensation is that synchronouscondensers are machines of low efficiency and are highly affected by the losses,where energy as a form of motion and heat is being dissipated in the generatoritself. In most of the cases, significant amounts of heat need to be extracted out,


CONTROL TECHNIQUES

thus, synchronous condensers are, very often, hydrogen cooled machines. Thistype of compensation has been widely used before development of semi-conductingmaterials and introduction of power electronics in power systems. Nevertheless,in some cases, synchronous condensers are still used to adjust voltage under highwind fluctuations that could lead to rapid voltage drop. What is more, constantlyrotating synchronous condensers boost system with additional inertia. Which canbe used to, for example, recover voltage stability in the system.

2.2.4 Conclusions

This chapter provided a theoretical background on power flow formulations andhighlighted potential possibilities to compensate excess of shortage of reactive powerin real power grids. Power system engineering is a very complex and challengingbranch of electrical engineering, where plenty of compromises have to be made.Assuming that the receiving end voltage is kept constant, by injecting the reactivepower, sending end voltage is boosted. This is a huge concern when deliberatingabout future power grid because bidirectional flows are tough to control, due tofollowing reasons:

1. Excessive injections of reactive power from distribution grid to transmissiongrid vastly increases the voltage in higher voltage grids that can cause damagesto the equipment, thus, it has to be compensated and controlled by expensivemeans,

2. Some electrical machinery and equipment can compensate reactive power be-ing either inductive (capacitors) or capacitive (inductors). This imposes thatadvanced control techniques and vast communication has to be implementedin power systems,

3. Compensation of reactive power at POC, sometimes will come at the cost ofactive power, increasing the losses in the system, leading to higher operationalcost of the grid. This is, as shown later in the thesis, possible when extrareactive power have to be absorbed by wind turbine generators causing voltagedrop in the lines.

All enlisted methods have their drawbacks and advantages and it is up to indi-vidual cases, depending on the needs, time of usage and financial feasibility, whichapproach is preferred. Nonetheless, this thesis compromises potential advantagesof reactive power support by wind farms over enlisted mechanisms.

Chapter 3

TSO-DSO Communication andliterature review of reactive powercontrol methods

3.1 TSO-DSO communication

Data acquisition devices are mostly located at large loads, generators, points ofinterconnection between low, medium and high voltage parts of the grid. Readingsare seldom used for real-time computing to improve the way grid operates. The eraof smart grids could be a close or far future. Without a doubt smart grids will be es-sential to the electric power grid, automatically and intuitively controlling voltages,power flows, load shedding and frequency based on i.e. current electricity priceson the market, risk for the distribution companies and profit for the producers. Itbecomes even more promising under assumption that electric vehicles could be agrid balancing mean, remotely supplying or absorbing electricity while we sleep orwork. However, before communication network will be set down, grid should beequipped with advanced data acquisition apparatus. To dynamically control out-put of the wind farm in order to balance reactive power being exchanged at desiredpoint or within specific area, dynamic line rating systems should be of particularinterest for grid operators, [Fernandez et al., 2016].

In most cases, current transformer measuring power flow from a wind turbineis located at the very output of the step-up transformer or at the end of the lineconnecting remotely located wind generators with the rest of the system (depend-ing on the agreement between grid operator and producer). Next measuring point,could be located several substations later, leading to lack of proper accessibility tothe data needed for sufficient control of output of wind turbine. Furthermore, whenit comes to clusters of wind generators, there are cases, when current transformer ismeasuring the output of several WTGs. This causes several problems for controlla-bility of the wind generators, for example, measurements from the aggregated wind

13

14CHAPTER 3. TSO-DSO COMMUNICATION AND LITERATURE REVIEW

OF REACTIVE POWER CONTROL METHODS

farm model under very windy conditions, when there is nearly complete utilizationof wind generators capacity and with condition when one of the WTG is not op-erating due to maintenance. Simply, readings will be misleading even for the mostexcellent control algorithms. Especially when it comes to integration of wind parks,currently there is very limited or none one-way communication with WTG, which isbased on static or quasi-static models [Glinkowski et al., 2011]. More issues appeararound privacy, cyber security, computing power of modern processing units andhow data should be shared between different entities in the energy sector.

The last problem, communication between different entities, particularly in be-tween DSO and TSO is being highlighted in this section. Recent guidance onreactive power management at the T-D (Transmission-Distribution) interface re-garding national implementation for network codes on grid connection has beenpublished in 2014. In the document ENTSO-E requests by the means of NC DCC(Network Codes Demand Connection Code), states that TSO should not requireless than maximum range of 48 percent of the larger of the maximum import ca-pability. However, the way the information is processed between transmission anddistribution system operators is antique. In Europe, only in Germany, automaticactive control of exchange of reactive power between transmission and distributionsystems is being developed. Other countries are to follow, not having such a hugepenetration of distributed generators along the grid as Germany does.

However, TSO-DSO cooperation is set up by widely accepted rules that differfrom country to country. For instance in Spain, during peak periods, the powerexchanged between transmission and distribution systems has to have power factorof cos(φ) > 0.95 and during off-peak periods DSO is obligated to not exchangereactive power with TSO under penalty jeopardy. In most of the countries, thereis an obligation for the DSO to keep the ratio of real power to apparent power asclose to 1 as operator is able to. For example, the lowest possible power factoris kept in Poland and is equal to 0.928, [ENTSO-E, 2016], giving some flexibilitywhen it comes to reactive power compensation and improvement of grid operationat the stake of better communication between DSO and TSO. Vast investmentsin shunt and series compensators by most of the European distribution systemoperators, indicate that the regulations could not exactly be fulfilled and financialanalysis confirmed feasibility of the projects (it is more economically feasible fordistribution system operator to install shunt compensator than pay to TSO for theexcess of reactive power being exchanged).

Swedish legislation can be perceived as liberal, there are no specific rules statingmaximum and minimum exchange of reactive power at the intersection of trans-mission and sub-transmission level grids. Both parties benefited from the availableflexibility. Unfortunately, Swedish grid has been hit by storms in the second part of2000s, [Haanpää et al., 2006]. More than 30 000 km of power lines have been dam-aged cutting power from 730 000 people. Total costs of the disaster summed up to274 millions of Euro. Disaster lead to intensive investments in converting overheadlines into the cable systems. Cables, as explained, posses natural capacitance, anddue to Ferranti effect, extra reactive power is being generated within the cable. For

3.2. STATE OF THE ART 15

example, after cabling of the MV grid in France, local voltage increase and difficul-ties with reactive power flow management occured, as a consequence of saturationof the existing HV/MV transformers on load tap changers. When reactive powerstarts to flow from the MV to the HV network, the level of reactive power leadingto saturation depends upon the characteristics of HV/MV transformer. Neverthe-less, it has to be analyzed case by case, depending on the topology of the grid.However, the path had been paved by other countries of how important the properapproach to the challenges of cabling is. The higher the HV voltage, the loweris the limit of the reactive power flow. According to European network code ondemand connection, distribution networks are required to have the capacity to re-strain reactive power flows towards transmission systems especially at low activepower consumption. Currently in Sweden, exchange of active and reactive power isindividual matter in the contract settled for particular point of connections.

3.2 State of the art

3.2.1 Introduction

By the European standard EN50160 the end consumers in power grids is ensuredthat the voltage at points of coupling is kept within a bandwidth of plus minus 10%of the nominal voltage Un in 99%/95% of the time. During periods of low loads andhigh generation and high loads and low generation, voltage limits could be violated.Reactive power is needed to transport active power through a line, and surpluses ofit cause many problems with voltage stability in both distribution and transmissionsystems. This chapter, apart from ”typical” control methods i.e. on load tapchangers or installation of shunt or series compensators, discuss some pioneeringapproaches in reactive power and voltage controllability in medium voltage grids.Method of the control of reactive power is of huge importance, however also thescale of the control system to be implemented is decisive. Control systems can bedivided into two categories. In first category, there are techniques based on localcompensation with local objectives and local communication. Second categoryencloses techniques based on global compensation with global objectives and globalcommunication. Pros and cons of both approaches are presented in table 3.1.

Control Method Advantages Disadvantages

LocalTechnically simpler to implement Conflicts between controllersLittle need of communication equipment and topology knowledge Local objectives

GlobalControl of set of inputs and desired outputs Technically troublesomeGlobal and local objectives ensured without conflicts High investment costs

Table 3.1: Comparison of local and global control schemes.

To obtain optimized control function, maximizing the revenue or minimizingthe cost, global control method is preferred over local control method, naturallyonly if total cost of investment and pay-back period of the project is within desired



time frame. This cost and pay-back time will vary between different cases, location,voltage levels and complexity, therefore, proper balance between two solutions isneeded. Comparison of the two approaches are presented further in the thesisin section 5.3. Communication between distributed generators has been widelydiscussed by many authors on various levels, starting from type of communication(wireless or wired), voltage levels, going to frequency of the metering and delayof the control method. In this section, most focus will be put upon algorithmsand methodologies to effectively control the wind turbine generators as well as PVgenerators.

3.2.2 Local objective control methods

High penetration of DG capacity impacting weak parts of the grid causes signifi-cant problems for the optimal power flow control [Masters, 2002]. This problem isformulated by authors of [Ochoa et al., 2011] as the minimization of the reactivesupport for distributed generation, setting the objective for the distributed genera-tors to be self sufficient when it comes to reactive power consumption. Adaptationof passive enhanced power factor and substation settings as well as impact of smartgrid control scheme had been investigated. Tailored multi period AC optimal powerflow algorithm has been used, highlighting the volatility of demand and generation,not violating the N-1 contingency. Power factor equal to 1 is often not possibleor difficult to achieve, especially in rural areas, as a voltage rise is a serious issue.Excess absorption of reactive power by generators combined with weak volt-amperereactive backing capability of local transmission grid might result in poor voltageprofiles or depletion of reactive power reserves finally leading to instabilities, [Vit-tal et al., 2008], [Keane et al., 2009]. On the other hand, distributed generationusually operate under an unity power factor to benefit, in the best way, the ac-tive power production, which generates profit for producers. In the first method ofthe simulation presented in paper [Ochoa et al., 2011] enhanced passive operation(EPO), power factor of generator is adjusted, furthermore, on load tap changersare varied, in a manner to keep secondary voltage fluctuating, whose final value iskept within limits across all periods for all network topologies. Smart grid schemeincludes adaptive power factor (PF) control and coordinated voltage control, whereoptimization is conducted independently for each out of 198 time period (time peri-ods are characterized by different load and generation demand and were discretizedalong a year). Analysis included several assumptions, i.e. measurement and controlinfrastructure, to support the control schemes, was in place. Moreover, responsedelays by generators were negligible. Enhanced passive operation case, with onlypower factor control, enabled to save more than 70% of the reactive power beingimported into distribution grid. While, under power factor and voltage controlscheme, almost 96% of the reactive power being imported into distribution gridwas saved. What, respectively, gives a drop from 76.6 GVARh to 21.9 GVArh and3.2 GVArh of net imports per year, taking into account normal operation of thenetwork. Imposing the N-1 contingency, control schemes were able to drop reactive


power support by 55% of the initial value under only power factor control and by95.8% under power factor and voltage control. Smart grid solution with controllablepower factors, resulted in higher flexibility and better results, leading to reductionof 97% of reactive power exchange comparing to BAU scenario. Authors indicatethat EPO can be adopted for seasonal power factor setting requirement, as it isdone in Spain and is presented by [Espejo Maŕın and Garćıa Maŕın, 2010].

In distribution grids, which for most of the time have an inductive characterbecause of the loads, [Concordia and Ihara, 1982], a reduction of the total reactivepower consumption of distributed generation units, indeed, improves the reactivepower balance of the control area. Authors in Franz et al. [2015] test minimizationalgorithms on disperesed generation units. Analyzed droop control methods areimproved cosφ(P ) profile, which are implemented as a control scheme to reducevoltage rises. Algorithm, however, is independent of the current network charac-teristics and the current characteristics at the point of common coupling. Resultsof the two optimized cosφ(P ) curves for reactive power support are highlightedand analyzed. In standard cosφ(P ) control scheme, reactive power support startsat a generation of 50% of the rated power P, whereas, the highest inductive re-active power consumption occurs at maximum generation (cosφ = 0.95). In thefirst control method, point k( PPr ), at which unit starts to supply reactive power isbeing varied and is set in a range from 0.5 to 1. Point k is determined so that theDGU causes the same maximum voltage deviation 4V at the PCC independentlyof the feed in power when reactive power support is demanded. Second methodvaries cosφ by modifying the maximum feed in a range of 0.95 to 1. However,control method B is only applicable if there is a voltage margin left regarding thelimits of the technical guidelines. New optimized cosφ profile had been found foraggregated distributed generation units. Results indicated that first method saved3% of the active power losses throughout the system, while the second one lead tosavings of 3.3%. Cuts in a range of 8.1 and 9.4% of reactive power, were respec-tively achieved, for control method 1 and control method 2. Concluding, reductionof reactive power demand by distributed generators has direct financial benefits forthe DSO. Especially if there is a network charge in force for the overlaid TSO thatdepends on the power factor, [TenneT, 2018].

Authors in [Samadi et al., 2014] analyze droop-based voltage regulation in dis-tribution grids with high penetration of PV systems. Recent German grid codespropose a standard Q(P) characteristics for inverter-coupled distributed genera-tors. However, Q(P) function is not enough for explicit regulation of the voltage,therefore Q(V) characteristic is being addressed as a potential solution in pair withQ(P) control. Authors underline that constant power factor might cause unneces-sary line losses and reactive power consumption. Derived and presented algorithmoptimally adjusts the settings of individual droop based Q(V) characteristics of PVsystems such that the reactive power consumption profile and line losses profile areminimized. Results show that the proposed solution can successfully regulate thevoltage under the upper steady state voltage limit, with a drawback that algorithmmight lead to an uneven reactive power distribution along PV systems. Droop



control is a well known concept in conventional power systems used primarily forthe load sharing among multiple generation units. Frequency of each generator isallowed to droop in accordance with delivered active power in order to share thesystem load. In the proposed method voltage sensitivity matrix is employed tocoordinate the slope factor and the voltage threshold of each PV system along aradial feeder by considering the maximum critical voltage deviation at the last buson the feeder. Voltage sensitivity was used as a measure to quantify the sensitivityof voltage magnitudes and angles with respect to injected active and reactive pow-ers at the bus. Each characteristic is specified by two main parameters the voltagethreshold and the slope factor, which are determined based on the voltage sensi-tivity analysis and the multi objective approach in order to balance the individualreactive power distribution against the total reactive power consumption and linelosses. Comparable approach, with distinction that WTGs are analyzed instead ofPV generators, is performed in this paper.

In Tayab et al. [2017] authors analyze potential of implementation of droop con-trol techniques in microgrid applications. Assuming that inverter output impedance,in the conventional droop control, is purely inductive (neglecting resistance) due toits high inductive line impedance and large induction filter installed. In an inductivesystem the active and reactive power drawn from each inverter can be expressedas:

P =E ∗ VX

∗ sin(α) and Q = E ∗ VX

∗ cos(α)− V2

X(3.1)

Since the conventional method can not provide a balanced reactive power sharingamong parallel connected inverters under line impedance mismatch, imbalance inreactive power sharing is a serious problem. Similar problems occur for parallelWTGs when it comes to finding proper distribution of reactive power among everysingle generator. To fight this problem virtual impedance loop based droop controlhad been developed. In adaptive droop control the maximum reactive power takenfrom each unit is stored and compared with the reference value. If the maximumreactive power is less than the reference value then the voltage amplitude followsthe traditional Q/E equation, however, when it is bigger, the voltage amplitudefollows equation (3.2).

E = E∗ − nadd(Q−Qref ) (3.2)

When Q > Qref the voltage amplitude changes the slope of E(Q). On the otherhand, when Q once again becomes lower than Qref the voltage amplitude changethe line function again. Robust and adaptive methods were derived, but they willnot be further deliberated in this thesis.

Based on overall discussion on the control strategies for an AC microgrid case,problems of harmonic load sharing, stability, droop performance on RES and fre-quency against active power sharing have been tackled, where each of the methodshave their pros and cons. Robust droop control technique seem to be the mostpromising regarding stability, frequency and voltage regulation and reactive powersharing. Authors of [Cabadag et al., 2015], conducted particle swarm optimization


method to control wind farms located in 110 kV grids, ensuring desired reactivepower exchange value at the transmission system interface. Obtained results provedexistence of the problem of effective and efficient control of full reactive power ca-pabilities of wind farms which could benefit grid operators. Authors demonstratedthat, not only by hardware like shunt and series reactors or capacitors, effective com-pensation is possible, but also novel techniqu

reactive power management capabilities of swedish sub ...1272223/fulltext01.pdf · control methods...

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