ISLANDING

PREPARED BY :

(16MEEEPV009)Soni Divyangkumar R

INTRODUCTION

What is islanding ?Consider the power network as shown in fig.1• Now if we disconnect the line AB from

the infinite transmission grid there will be an isolated region . The D1, D2 are power sources (eg : inverter , solar power cells ). The power generated in this region is fed to the island only.

• We see that there no longer is any control over the island voltage at the bus X . Also there is no mechanism here for control of frequency.

• This state is referred to as islanding.

PAPERS

PAPER-1: Prevention of Islanding in Grid Connected PV System Using Twelve Pulse Line.

AUTHORS: 1. Dhanshree A. Diyewar 2. Jyoti M. Kumbhare

PAPER-2 : Micro-grid Islanding Detection Based on PQ Active method. AUTHORS: 1. Zhu Yipeng 2.Teng Yun

PAPER.1 PREVENTION OF ISLANDING IN GRID CONNECTED PV SYSTEM USING TWELVE PULSE LINE.

Outline :• Abstract • Introduction • 12-Pulse Line commutated converter (LCC)• Proposed Scheme• Photovoltaic Characteristics • Simulation results• Conclusion

ABSTRACT

One of the major drawback of connecting PV systems to the grid is unintentional islanding condition.

Islanding can be dangerous for utility workers and damage utility equipment so anti islanding is a crucial subject for grid connected PV systems.

For this reason inverter in the PV system must detect islanding and stop supplying power if the grid is down.

In this paper 12 pulse LCC is used in inversion mode for the grid connection of PV system.

A 12 pulse LCC converter needs commutating voltage of grid to operate. The LCC do not require maintaining synchronism between grid and converter and

having an ability to suppress all harmonics below 11 th order. The simulation results is carried out in Matlab/Simulink.

INTRODUCTION

PV generation have been come into prominence all over the world. Solar energy is the most developed energy sources and is receiving wide attention

now a days because the everlasting solar energy is the best alternative to conventional energy sources.

Grid connected PV systems is well recognized all over the world despite the fact that there have been some drawbacks about connecting them to the electrical grid.

One of the major drawback of connecting PV systems to the grid is unintentional islanding condition.

Islanding can be dangerous for utility workers and damage utility equipment so anti islanding is a important subject for grid connected PV systems.

Islanding detection methods 1. Active methods 2. Passive methods

Passive methods Active methods Based on measurement of the natural effects of islanding

The active methods use intentional transients or harmonic effects

passive methods fail due to the small natural effects of islanding. The passive methods have a non detection zone (NDZ).

The active methods can reduce the NDZ size. However, these methods reduce the grid power quality

Many islanding detection algorithm based on VSI inverter based PV- grid interfacing .

For high power conversion line commutated converter (LCC) is widely used.

In HVDC at receiving end LCC operate in an inversion mode. Commutation of thyristor takes place due to ac grid voltage LCC based grid connected PV system does not require extra synchronizing controller like VSI when connected to ac grid.

LINE COMMUTATED CONVERTER (LCC)

In this paper, twelve pulse LCC operates in an inversion mode for a firing angle greater than 90°.

Twelve pulse LCC is capable of suppressing all harmonics below 11 th order which improves power quality of the grid.

Twelve pulse LCC has come into anti-islanding feature since islanded operation will not occur as the converter needs commutating voltage of grid to operate.

The configuration of proposed scheme is shown in fig. 1

Fig. 1. Grid connected PV system using twelve pulse line commutated inverter for islanding prevention

In a twelve pulse LCC, two six pulse converter is connected in series.

There is a phase difference of 30°. It is obtained by lower bridge lags that

of upper by 30°. The upper bridge connected through

star-star transformer and lower bridge through star-delta transformer.

LINE COMMUTATED CONVERTER (LCC)

Fig. 2. Twelve pulse line commutated converter

PROPOSED SCHEME

Figure 1 shows, the schematic of proposed scheme for grid connected PV system using twelve pulse line commutated inverter.

PV system is connected to grid through twelve pulse line commutated inverter with transformer and breaker.

PV generates dc and using inverter it convert in to ac and fed to the grid. The dc source must be connected such that positive polarity is connected to the common anode of the bridge and negative polarity is connected to the common cathode so that power is transfer from the DC to AC side and converter operates in an inverter mode by making firing angle greater than 90°.

If islanding condition is occur then photovoltaic system continues to energies the load after the system has been disconnected from the grid. In islanding condition twelve pulse LCC in PV system will not operate and stop supplying power to the load.

PHOTOVOLTAIC CHARACTERISTICS

In the proposed scheme PV is connected to LCC.

For the operation of converter as inverter the DC voltage must be greater than the supply voltage then in that condition power is transfer from dc side to ac side and converter operates in inversion mode.

For this purpose PV voltage is 1000V and supply voltage is 400V. The simulation result of PV characteristic is shown in Fig.3.

Boost voltage of dc-dc converter is 1000V which is applied to LCC.

Fig. 3. Photovoltaic array voltage

SIMULATION RESULTS

Fig. 4. Waveform of twelve pulse LCC for 150 degree

Fig. 5. Grid side current at 33 kV

The nature of current wave is almost square wave contains harmonics from 11 th order and above which can be eliminated using filters.

SIMULATION RESULTS

In proposed scheme, simulation is done by considering islanding during 0.5 to 0.8 seconds. The islanding is shown in Fig. 6, from 0.5 to 0.8 seconds for which 33 kV grid voltage is zero.

Fig. 6. Voltage during islanding condition at 33 kV Grid side

SIMULATION RESULTS

Fig. 7. Voltage of transformer across 440 V side Fig. 8. Dc link voltage of twelve pulse LCC during islanding

CONCLUSION

In this paper, islanding is prevented using twelve pulse LCC. During islanding dc link voltage of twelve pulse LCC is zero thus converter will

not operate and will not supply to any local loads and workers working on converter side is prevented due to dangerous hazards.

One major advantage of LCC is, it does not require any synchronizing circuits and is robust topology for high power application

REFERENCES

I. John K. Pedersen and Soeren Baekhoej Kjaer, "A Review of SinglePhase Grid-Connected Inverters for Photovoltaic Modules" iEEE Transactions On industry Applications, vol. 41 ,no. 5, SEP/OCT 2005,

II. T. R. Sims, R. A. Jones and A. F. Imece, 'Investigation of potential islanding problems of a line-commutated static power converter in photovoltaic systems', iEEE Tr

III. S. Samerchur, S. Premrudeepreechacharn Y. Kumsuwun, and K. Higuchi , "Power Control of Single-PhaseVoltage Source Inverter for Grid-Connected Photovoltaic Systems", iEEE , pp. 01,2011

IV. C.Boonmee and Y. Kumsuwan, "Modified Maximum Power Point Tracking Based-on Ripple Correlation Control Application for SinglePhase VSI Grid-Connected PV Systems", IEEE. Department of Electrical Engineering, pp. 01, 2013. [ansactions on Energy Conversion" , pp. 429-435.1990

PAPER.2 MICRO-GRID ISLANDING DETECTION BASED ON PQ ACTIVE METHOD.

Outline :• Abstract • Introduction • Islanding Detection Method • Inverter modeling• Relationship of Frequency with Islanding Operation • Simulation results• Conclusion

ABSTRACT

Micro-grids are being developed as a building block for future smart grid system. Islanding detection is a great challenge in connecting distributed generation (DG)

in electrical power systems. Islanding detection methods are categorized into two groups, i.e., communication

based methods, active, and passive methods. Communication based methods are established upon communication between

utilities and DGs and have the inconvenience of being expensive and are limited in use.

Active methods perturb the system with a disturbance and the system response to the disturbance is analyzed.

This detection circuit model of the micro-grid islanding is simulated by the software of MATLAB/Simulink, and the simulation results show that the islanding detection method does achieve the effect of the micro-grid islanding detection.

INTRODUCTION

Renewable energy and the development of green power has become a hot topic in the field of electrical .

With the increasing penetration of renewable energy and the application of large area, the concentration of power production in the form of more decentralized development.

Therefore, the micro network as a part of the main network, provides a lot of auxiliary services to network.

It is a difficult task to guarantee the stability and reliability of micro grid, especially in the isolated island mode.

The micro-grid is aimed to provide electricity for small communities (buildings, schools, industry), which is a small scale grid.

The fossil fuels (diesel, gas turbine) and renewable energy (photovoltaic, wind turbine) is its main source of electricity.

The micro grid architecture is shown in Fig.1.The power is supplied by DG.

Distributed storage systems store energy when they produce more than they consume, and provide energy when consumption is greater than production.

In the micro-grid, the consumer of power is the load. Electrical connection between the micro grid and the main power grid through the

point of common coupling (PCC). Islanding is a major issue in penetration of DGs in the power system . Equivalent

islands Circuit is shown in Fig. 2.

Fig. 1 Architecture of a micro-grid Fig. 2 Equivalent Islands Circuit

the islanding condition happens when “a portion of the utility system that contains both load and distributed resources remains energized while it is isolated from the remainder of the utility system”.

Unintentional islanding may lead to adverse consequences such as uncoordinated protection, inadequate grounding, and safety aspects.

Current standards such as IEEE 929-1988 and IEEE 1547-2003 requires the disconnection of the DG immediately and with delay of 2 seconds, respectively.

If the DG is allowed to work autonomously , fast islanding detection is required to make appropriate decisions to control the DG in the autonomous mode.

Hence, detecting islanding correctly and as fast as possible is essential in connecting DGs to the utility system.

ISLANDING DETECTION METHODS

Islanding detection methods can be divided into two main groups.

(1) DG resident techniques (2) Communication based techniques Communication based methods : These

methods are based on communication between the DG and the utilities.

These methods have a zero Non Detection Zone (NDZ) but due to their high costs are rarely used .

Islands Circuit is shown in Fig. 3. DG resident methods: (a) Passive (b) Active (c) Hybrid methods

Fig. 3 Islands Circuit

(a). Passive methods In passive methods certain system parameters such as frequency, voltage, phase angle, and

total harmonic distortion are monitored continuously and islanding detection is performed from the variation of these parameters

The main drawback of these techniques is their large NDZ. Particularly, when the power which the DG generates is equal to the power absorbed by the

loads, no power is exchanged between the grid and the DG. The grid parameter changes are negligible and may not be detected by the islanding detection

technique. (b). Active methods Active techniques reduce the NDZ of passive methods. These techniques perturb the system with a periodic or transient disturbance and estimate the

systems response to detect islanding.

In one way, active methods could be classified in two subgroups. (i). Impedance measurement (ii). Detection One setback of these methods is that they enter a disturbance to the grid and may

degrade the power quality of the system. (c). Hybrid methods Hybrid islanding detection techniques combine the principles of active and passive

techniques. In a hybrid islanding detection method which uses total harmonic distortion and

continuous feedback and selection is presented. In a covariance index is used as the passive method, to activate adaptive reactive

power shift action.

In this paper a hybrid islanding detection method which is on the principles of VU and HF impedance is presented.

The imbalance of PCC voltage is measured, if it is above the selected threshold, a HF voltage is injected to the system and the impedance of the DG is estimated at the injected frequency.

The propose method can detect islanding in 32(ms) and discriminate islanding from other system disturbances.

Fig.4 Islanding test circuit

INVERTER MODELING

Inverters are the main interference sources of the micro-grid . In this paper, two control strategies are used to control the inverters installed in the micro-grid.

(1). PQ inverter control. (2). Voltage source inverter (VSI) control.

Fig .5 Basic structure of the PQ inverter control scheme

(1). PQ inverter controlFig.5 Pref in Fig. 2 represents the active power which is produced by the micro-source, which is connected to the Micro-grid by that inverter.Qref represents the amount of reactive power injected into or absorbed from the Micro-grid at the inverter’s bus. In this model, all PQ inverters operate at unity power factor ,which means there is no reactive power conversion between the PQ inverter and the Micro-grid system.In PQ inverters were worked at unity power factor to reduce the ratings of those inverters and the cost.

Fig. 6 Voltage source inverter control model.

This kind of inverter is used to feed the Micro-grid with predefined values of voltage and frequency subsequent to islanding occurrence.

VSI in this paper is used to interface the storage device (flywheel) to the Micro-grid and represents the reference bus (slack bus) for each Micro-grid during islanding mode.

The VSI emulates the behavior of a synchronous machine in conventional power systems.

The VSI is controlled through droops with the magnitude and frequency of the output voltage, as described by the following functional relation:

(2). Voltage source inverter (VSI) control

P and Q are the inverter active and reactive output powers respectively.PK and Q K are the F and V droop slopes respectively. f0 and V0 are the idle values of the frequency and voltagesA 3-ph model of a VSI implementing the droop concepts described by Equation no. (1)In this model, the amount of P and Q powers injected into or absorbed from the Micro-grid will control the V and frequency of the Micro-grid.

RELATIONSHIP OF FREQUENCY WITH ISLANDING OPERATION

Fig.4 shows the power flow chart of inverter ,which is based on DG (IBDG) and connected load in the presence of the utility.

Under the islanding operation, the power flow can be described as:

The load is replaced by equivalent RLC elements and is connected in parallel, PCC voltage across R and parallel LC are the same. The load power can be calculated as

From Eqs.(2) and (3), we can get

If IBDG output power Pinv +jQinv is equal or approximate to the rated load power Load Load Pload +jQload, the PCC voltage frequency will under the threshold at the moment, at the instant the utility is disconnected, the islanding detection fails.

SIMULATION

In this part, simulation results are shown to verify the effectiveness of the proposed islanding detection algorithm.

The difference between inverter output capacity and rated load capacity is described as:

In this case, the islanding frequency will go beyond the allowable range (49.5 Hz—50.5 Hz) immediately, and the islanding can be confirmedThe simulation parameters of the system are shown in Tab.I.

PARAMETERS VALUEVoltage ; Frequency 220V,50Hz

Pload0 5kW,9.68Ω

QL 10KVar, 15.4mH

QC 5KVar, 328.8µH

fmax=wmax/ 2pi ; fmin =wmin/ 2pi 50.3Hz;49.7Hz

Fig.8 Simulation result of Case(the islanding frequency exceeds the lower frequency limit for 3 times within 0.1 s. )

Fig.7. ∆P= -1%Pload0 and the utility is disconnected at t=0.1s

Fig.9 ∆P= 1%Pload0 , the islanding frequency should also shift continuously when the proposed algorithm is applied.

CONCLUSION

In this paper a hybrid islanding detection method using VU and HF impedance is presented. The voltage unbalance is measured at PCC and if it overreaches the threshold settings, the

situation is suspicious of islanding. Therefore, a HF voltage is injected in the DG control loop and the PCC voltages and current

are measured to estimate impedance at the injected frequency. This hybrid method merges the advantages of active and passive methods. Furthermore, the method does not lead to system instability since the active method does not

change any parameters of the system. The test system is simulated in PSCAD/EMTDC to investigate the credibility of the method.

The detection method can detect islanding in 32ms (3 cycles) which is supported by standard. In conclusion , compared to other existing algorithms, the non-detection region of this

algorithm is very small. at the same time, it also provides an excellent detection speed of the island. The effectiveness

of the algorithm is proved by the simulation results.

REFERENCES

I. V. Timbus, R. Teodorescu, F. Blaabjerg, and U. Borup, “ENS detection algorithm and its implementation for PV inverters,” Proc. Inst. Electr. Eng.—Electric Power Applications, vol. 153, no. 2, pp. 206--212, Mar. 2006.

II. L. A. C. Lopes and H. Sun, “Performance assessment of active frequency drifting islanding detection methods,” IEEE Trans. Energy Convers.,vol. 21, no. 1, pp. 171–180, Mar. 2006.

III. R. Lasseter, “Microgrids,” in IEEE PES Winter Meeting, New York, Jan. 27–31, 2002, vol. 1 and 2, pp. 305–308.

IV. IEEE Standard for Interconnecting Distributed Resources With Electric Power Systems, IEEE Std. 1547-2003, 2003.

V. F. Katirei, M. R. Iravan, and P. W. Lehn, “Micro-grid autonomous operation during and subsequent to islanding.