assignment 1

i

Tshwane University of Technology

Department of Electrical Engineering

Student name: Ayub Machiri Wanjala

Student no.: 212 492 884

Power Systems V

Assignment 1

ii

Table of Contents

1. INTRODUCTION............................................................................................................ 1

2. SYSTEM MODELLING USING POWERWORLD SIMULATOR AND PSAT ..... 2

2.1. About PowerWorld simulator and PSAT .................................................................... 2

2.1.1. PowerWorld ..................................................................................................... 2

2.1.2. PSAT ................................................................................................................ 2

2.2. Line Modelling ............................................................................................................ 2

2.3. Load Modelling ........................................................................................................... 3

2.4. Generator Modelling ................................................................................................... 4

3. SYSTEM MODELLING WITHOUT COMPENSATION .......................................... 6

3.1. Analysis of System ...................................................................................................... 6

3.2. Observation and Results .............................................................................................. 7

4. SYSTEM MODELLING WITH COMPENSATION ................................................ 10

4.1. Choice of Compensation ........................................................................................... 10

4.1.1. Shunt Compensator ............................................................................................ 10

4.1.2. Synchronous Condenser..................................................................................... 11

4.1.3. Static Var Compensators.................................................................................... 12

4.2. Analysis of system .................................................................................................... 14

4.2.1. Point of Installation ........................................................................................ 14

4.3. Observation and Results ............................................................................................ 16

5. CONCLUSION .............................................................................................................. 18

6. References ....................................................................................................................... 19

1

1. INTRODUCTION

Power flow studies are performed to determine the voltages, active and reactive power etc. at

various points in the network for different operating conditions subject to the constraints on

generator capacities and specified net interchange between operating systems and several

other restraints.

Power flow or load flow solution is essential for the continuous evaluation of the

performance of the power systems so that suitable control measures can be taken in case of

necessity. In practice it will be required to carry out numerous power flow solutions under a

variety of conditions.

With the advent of the modern digital computers possessing large storage and high speed the

mode of power flow studies have changed from analog to digital simulation. A large number

of algorithms are developed for digital power flow solutions. These methods basically

distinguish between themselves in the rate of convergence, storage requirement and time of

computation. Loads are generally represented by constant power [P, Q].

Network equations can be solved in a variety of ways in a systematic manner. The most

popular method is node voltage method. When nodal or bus admittances are used complex

linear algebraic simultaneous equations will be obtained in terms of nodal or bus currents.

However, as in a power system since the nodal currents are not known, but powers are known

at almost all the buses, the resulting mathematical equations become non-linear and are

required to be solved by iterative methods. The bus admittance matrix is invariably utilized in

power flow solutions

This report describes a 10 bus system which is composed of 10 buses and 17 lines

implemented in the power system simulation tool PowerWorld (Version 16) and PSAT

Matlab toolbox. The system consists of 3 generator buses and 7 load buses.

The total real and reactive power demand of the system are 1350 MW and 670 MVAR,

respectively. The main objective of this study is to carry out the load flow study on the

system and compensate it accordingly to ensure that the required voltage levels are

maintained within the permissible level of for all the buses

within the given network. This is done using shunt compensation devices since the capability

of using FACT devices on PowerWorld is not available. I have explained briefly on the usage

of FACT devices as well as their respective advantages in this report.

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2. SYSTEM MODELLING USING POWERWORLD SIMULATOR

AND PSAT

2.1. About PowerWorld simulator and PSAT

2.1.1. PowerWorld

PowerWorld is simulation software designed by PowerWorld Corporation in USA, which is

used to carry out the analysis of a network using a visual interface. It is used to show the

various states of the network when carrying out load flow studies on the network. The

software works by using the Single Line Diagram to define the parameters of the given

network and the state of the network at a given instance. The main figures used in this report

are generated using this software.

2.1.2. PSAT

PSAT is a Matlab toolbox developed by Federico Milano and uses the computational power

of the Matlab software. It is used to carry out various network studies such as load flow

analysis. In this case, I used PSAT to be able to generate a complete outlook of the network

parameters which I was unable to do using PowerWorld. I also used it to carry out the

verification of the parameters I got using PowerWorld software. The main reports used in this

report are generated using this toolbox.

2.2. Line Modelling

To be able to carry out the load flow studies, it is paramount that the given data is

standardised to suit the given software interface.

This data usually consists of the base value of the voltages as well as the base value for the

power. In this case the values specified for the base are as follows:

Table 1: Basic formulae’s used

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From the data given, we can specify the line data as follows:

Where:

is a complex of resistive [R] and reactive [Q] components as shown below:

Line 1-2

Given that

From To R X R X

Bus Bus [p.u.] [p.u.] [ ] [ ]

1 2 0.00477 0.05103 7.632 81.648

1 4 0.00569 0.06008 9.104 96.128

1 5 0.00272 0.02872 4.352 45.952

2 10 0.00676 0.09429 10.816 150.864

3 8 0.00297 0.03706 4.752 59.296

3 9 0.00145 0.01802 2.32 28.832

5 8 0.00388 0.4834 6.208 773.44

6 7 0.0004 0.004 0.64 6.4

7 5 0.0043 0.0477 6.88 76.32

7 4 0.00589 0.05995 9.424 95.92

7 9 0.00289 0.03603 4.624 57.648

10 6 0.00546 0.06794 8.736 108.704 Table 2: Line parameters

2.3. Load Modelling

From the data given, we can specify the load data as follows:

Where:

is a complex of real [P] and reactive [Q] power as shown below

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Load 9

Given that

Load P Q P Q

Bus [ ] [ ]

9 3 2 300 200

7 2.5 1 250 100

6 1.8 0.5 180 50

10 0.5 0.2 50 20

4 1 0.6 100 60

8 3.2 2 320 200

5 1.5 0.4 150 40

Total 13.5 6.7 13500 670 Table 3: load parameters

2.4. Generator Modelling

From the data given, we can specify the generator data as follows:

Where:

is a complex of real [P] and reactive [Q] power as shown below

Generator 1

Given that ⁄

⁄ ⁄

PV bus

Generator

Bus [ ]

1

2 Table 4: PV parameters

5

Slack bus

Generator

Bus [ ]

3

Table 5: Slack generator parameters

Using the data obtained in the above tables (1 – 5), it is now possible to enter the data into the

PowerWorld software’s to perform the load flow analysis as shown in the following sections.

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3. SYSTEM MODELLING WITHOUT COMPENSATION

Consider the given 10 bus system with 2 PV buses, 7 PQ buses and 1 slack bus. The system is

modelled using PowerWorld simulator to obtain the initial state of the respective voltage nodes

without any compensation coupled to the network. The figure below shows the Single Line Diagram

(SLD) obtained:

Fig 1: single line diagram for uncompensated 10 bus network using PowerWorld

3.1. Analysis of System

The given system shows a typical traditional power system. The main generators (bus 1 and

2) are centralised and coupled by a tie line (line 1-2). These provide the main power supply

for the loads (bus 4 - 10) through a transmission grid comprising of 12 lines. The slack

generator (bus 3) is used as the reference for modelling the load flow analysis.

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The network configuration is such that the loads can be characterised by the distance from the

main generators. It can be seen that the 1st tier loads are the ones closest to the generators,

which are buses 10, 4 and 5 with respect to generator 1 and 2; buses 8 and 9 are the ones

closest to the slack generator 3. The 2nd

tier loads are further away from the generating source

and these are buses 6 and 7.

It can be noted that the major loads are found on the 2nd

tier level from the main generators (1

and 2) compared to the 1st tier level. This means that the electrical distance from the

generator to the loads is high and as a result the system will incur transmission losses due to

the inductive reactance of the transmission lines. It is assumed that the lined do not have any

line charging and as a result it is mainly an inductive (lagging) transmission network.

3.2. Observation and Results

From the simulation of the load flow using PowerWorld, it is noted that the voltage levels at

the respective buses fall far below the recommended limit; 0.95 p.u.

A complete report is generated using PSAT to show the respective voltage levels [p.u.] and

the total line flows as well as the losses in the lines.

POWER FLOW REPORT

P S A T 2.1.6

Author: Federico Milano, (c) 2002-2010

e-mail: [email protected]

website: http://www.uclm.es/area/gsee/Web/Federico

File: C:\MATLABR61\psat\model.mdl

Date: 20-Oct-2012 19:17:36

NETWORK STATISTICS

Buses: 10

Lines: 12

Generators: 3

Loads: 7

SOLUTION STATISTICS

Number of Iterations: 4

Maximum P mismatch [p.u.] 0

Maximum Q mismatch [p.u.] 0

Power rate [MVA] 100

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POWER FLOW RESULTS Bus V phase P gen Q gen P load Q load

[p.u.] [rad] [p.u.] [p.u.] [p.u.] [p.u.] Bus1 1 0.03483 4.848 2.4335 0 0 Bus10 0.93961 -0.07789 0 0 0.48912 0.19565 Bus2 1 0.03445 1.154 0.62069 0 0

Bus3 1 0 6.6575 4.891 0 0 Bus4 0.94894 -0.05704 0 0 0.49888 0.19955 Bus5 0.9476 -0.06112 0 0 1.4925 0.39799

Bus6 0.91808 -0.12829 0 0 1.6811 0.46696

Bus7 0.9196 -0.12356 0 0 2.3426 0.93702

Bus8 0.92605 -0.09275 0 0 3.0407 1.9004

Bus9 0.94281 -0.07557 0 0 2.9548 1.9698

Minimum voltage limit violation at bus <Bus10> [V_min = 0.95]







LINE FLOWS

From Bus To Bus Line P Flow Q Flow P Loss Q Loss

[p.u.] [p.u.] [p.u.] [p.u.] Bus10 Bus2 1 -1.1498 -0.45654 0.01172 0.16345

Bus10 Bus6 2 0.66066 0.26089 0.00312 0.03883

Bus8 Bus3 3 -2.4381 -1.545 0.02885 0.36004

Bus9 Bus7 4 1.1974 0.53899 0.00561 0.06989

Bus2 Bus1 5 -0.0075 0.0007 0 0

Bus1 Bus4 6 1.5221 0.77232 0.01658 0.17503

Bus4 Bus7 7 1.0067 0.39773 0.00766 0.078

Bus3 Bus9 8 4.1905 2.9859 0.03839 0.47711

Bus7 Bus6 9 1.024 0.25015 0.00053 0.00526

Bus5 Bus1 10 -3.2809 -1.2663 0.03746 0.39557

Bus5 Bus7 11 1.1837 0.48527 0.00784 0.08693

Bus5 Bus8 12 0.60483 0.38302 0.00221 0.02759

GLOBAL SUMMARY REPORT

TOTAL GENERATION

REAL POWER [p.u.] 12.6595

REACTIVE POWER [p.u.] 7.9451

TOTAL LOAD



9

TOTAL LOSSES



LIMIT VIOLATION STATISTICS

No of voltage limit violations: 7

All reactive power within limits.

All current flows within limits.

All real power flows within limits.

All apparent power flows within limits.

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4. SYSTEM MODELLING WITH COMPENSATION

4.1. Choice of Compensation

The choice of compensation to be used depends mainly on the instantaneous characteristic of

the network at any given point. This is usually done by analysing the voltage levels at the

given nodes (buses) which give us a general idea as to how the condition of the network is.

When the voltages at a given node are higher than the recommended value, it indicates that

the line capacitance is great and therefore injecting reactive power into the network.

Similarly, when the voltage at the node is lower than the recommended value, it indicates that

the line reactance is high and as a result the reactive power is absorbed by the line which

lowers the voltages.

It is therefore important to know that the compensation devices are usually inductive or

capacitive in nature. A brief look at the types of compensators and there use is discussed

below.

4.1.1. Shunt Compensator

The shunt compensator is the connection of a reactive power component in a power network.

These reactive power components are either an inductor or capacitor. The types of shunt

compensation used at a given voltage node is directly dependant on the voltage profile at the

point.

4.1.1.1. Shunt capacitor bank

The shunt capacitor bank is used to inject reactive power into the network at the given node.

It consists of a bank of capacitors connected to the network and operated individually at a

time. The amount of reactance to be injected into the given node is determined by the voltage

at the node itself.

The capacitors are used in an inductive (leading) network. In such a transmission network the

voltage profile at the node is usually below the recommended level of 0.9p.u, and this is to be

improved.

4.1.1.2. Shunt reactor

The shunt reactor is used to absorb excess reactive power in the network at a given node. It is

an inductive reactor made of air or iron core. Traditionally, the shunt reactor has a fixed

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rating and is either connected to the power line all the time or switched in and out depending

on the load. Recently Variable Shunt Reactors (VSR) have been developed and introduced on

the market. The rating of a VSR can be changed in steps ranging from 100-200MVars. The

variability brings several benefits compared to the traditional fixed shunt reactors.

The VSR can continuously compensate reactive power as the load varies and thereby

securing voltage stability.

Variable Shunt Reactors are used in high voltage energy transmission systems to stabilize the

voltage during load variations. They are used in capacitive (lagging) networks which usually

experience voltages above the recommended 1.1p.u voltage level.

4.1.2. Synchronous Condenser

In the initial power networks, the control of reactive power flow in the system to ensure unity

power factor was done by the use of synchronous condensers.

A synchronous condenser is a device which is identical to the synchronous motor, whose

shaft is not connected to a load, but is let to spin freely. The main purpose for this

synchronous condenser is to adjust the conditions in a power transmission grid. The field is

controlled by a voltage regulate to either generate or absorb reactive power in the network as

needed to improve the power factor.

Principle of operation:

When the devices field excitation is increased it resulted in the injection of reactive power

into the system operating as a capacitor bank. When the excitation on the field reduced, the

motor absorbs the reactive power from the network effectively working as an inductor. It is

through this principle that it was implemented to help in stabilizing the power system during

short circuits or rapidly fluctuating loads.

The installation and operation of this synchronous condenser is largely identical to large

electrical motors.

When compared to the capacitor banks, the value of reactive power from a synchronous

condenser can be continuously adjusted depending on the network conditions which was

difficult to do when suing the capacitor banks. In addition, reactive power from a capacitor

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bank decreases with voltage decrease, while a synchronous condenser can increase current as

voltage decreases.

Even though it does offer some advantages compared to the static capacitor banks, it does

have higher losses, which makes them to be uneconomical. Most synchronous condensers

connected to electrical grids are rated between 20 Mvar and 200 Mvar and many are

hydrogen cooled.

4.1.3. Static Var Compensators

SVCs are part of the Flexible AC transmission system device family which is used for

regulating voltage and stabilizing the power system. It is an electrical device used to provide

fast acting reactive power on high voltage transmission networks. It encompasses both the

capacitive and inductive elements which are controlled by the switching action of the

thyristors and the voltage at the given point of installation in the network.

The SVCs have no significant moving parts. Prior to the invention of the SVC, power factor

compensation and voltage regulation was done using synchronous condensers or switched

capacitor banks as explained in the above section.

Principle of operation:

The SVC is designed to automatically inject or absorb the required reactive power at a given

bus depending on the instantaneous load present in a system. It is designed to bring the

system closer to a unity power factor.

In the transmission network, it is used to regulate the transmission voltage levels. The SVC

may also be used near large industrial loads to improve the power quality.

The components of the SVC comprise of the following elements:-

Thyristor switched capacitor (TSC)

Considering a power network, which is under inductive (lagging) conditions, the SVC uses

Thyristor switched capacitors (TSC) which are automatically switched on hence injecting

reactive power into the network. This results in an increase in the voltage profile at the given

node.

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Thyristor controlled reactor (TCR),

When the network is under capacitive (leading) conditions, The SVC will use Thyristor

controlled reactors (TCR) which are turned on hence absorb reactive power in the system

which results in the decrease of the voltage profile at the given.

Harmonic filter(s)

This device is used to filter out the harmonic frequencies which are generated by the

switching action of the Thyristor.

Mechanically switched capacitors or reactors (switched by a circuit breaker)

In case of an emergency or failure of the thyristors, a mechanical switch may be provided to

control the reactive power injected or absorbed by use of a circuit breaker which is

mechanically operated.

Advantages

1. The main advantage of SVCs over simple mechanically-switched compensation schemes

is their near-instantaneous response to changes in the system voltage. For this reason they

are often operated at close to their zero-point in order to maximize the reactive power

correction they can rapidly provide when required.

2. They are, in general, cheaper, higher-capacity, faster and more reliable than dynamic

compensation schemes such as synchronous condensers. However, static var

compensators are more expensive than mechanically switched capacitors, so many system

operators use a combination of the two technologies (sometimes in the same installation),

using the static var compensator to provide support for fast changes and the mechanically

switched capacitors to provide steady-state Vars.

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Fig 2: Single Line Diagram representing the SVCs

4.2. Analysis of system

Considering the 10 bus network given above, we are able to characterise the network as a

lagging network due to the low voltage levels at the respective nodes. The network has to be

compensated by using shunt capacitor banks or SVC’s so as to inject reactive power into the

network. This is done to improve the voltage profile to the recommended levels.

4.2.1. Point of Installation

Now that we know we are supposed to inject reactive power into the network, we are

supposed to establish the nodes in the network which will offer the optimal performance for

the network.

On observing the network, we can note that the network is radial in nature. The system can be

identified by using three main radial configurations. These radial configurations are

1. Bus < 2 – 10 – 6 – 7 – 4 – 1 – 2 >

2. Bus < 2 – 10 – 6 – 7 – 5 – 1 – 2 >

3. Bus < 3 – 9 – 7 – 5 – 8 – 3 >

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From the initial results in the uncompensated system, it was noted that the voltage deviation

from the recommended value occurred in buses 4, 5, 6, 7, 8, 9 & 10 with the maximum

deviation occurring on bus 6.

According to the radial networks assumed observed above, radial 1 has bus 6 with the

maximum voltage deviation at 0.91808 and radial 2 also has bus 6 with the maximum voltage

deviation and in radial 3, the bus with the most voltage deviation is bus 7 with 0.9196.

Radial network 1 & 2 share a total of 4 lines. This basically means that the placement of the

capacitor bank is able to inject reactive power into both networks if placed correctly.

It can be noted that Bus 6 is part of the 1st and 2

nd radial networks. It is also noted that the

distance from the generating source is high as it forms part of the 2nd

tier loads. From this

observation, we can say that the best place to install the shunt capacitor banks is at bus 6.

When we consider the 3rd

radial network, the main load is installed at bus no 8, which also

has a relatively low voltage profile and the distance from the generator 1 and 2 are great.

Since the bus is independent of both the 1st and 2

nd radial network, it was noted that for the

regulation of voltage at this node, it was important to add a shunt capacitor bank at this node

too.

The system was compensated as shown in the SLD below by injecting reactive energy at bus

6 and bus 8 respectively.

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Fig 3: single line diagram for compensated 10 bus network using PowerWorld

4.3. Observation and Results

POWER FLOW REPORT

P S A T 2.1.6

Author: Federico Milano, (c) 2002-2010

e-mail: [email protected]

website: http://www.uclm.es/area/gsee/Web/Federico

File: C:\MATLABR61\psat\model.mdl

Date: 20-Oct-2012 19:14:23

NETWORK STATISTICS

Buses: 10

Lines: 12

Generators: 3

Loads: 7

SOLUTION STATISTICS

Number of Iterations: 4

Maximum P mismatch [p.u.] 0

Maximum Q mismatch [p.u.] 0

Power rate [MVA] 100

17

POWER FLOW RESULTS Bus V phase P gen Q gen P load Q load

[p.u.] [rad] [p.u.] [p.u.] [p.u.] [p.u.] Bus1 1 0.02092 4.848 0.34605 0 0 Bus10 0.99091 -0.09301 0 0 0.5 0.2 Bus2 1 0.01937 1.154 0.08065 0 0 Bus3 1 0 7.1331 1.4636 0 0 Bus4 0.98757 -0.0716 0 0 0.5 0.2 Bus5 0.98929 -0.07405 0 0 1.5 0.4 Bus6 1.0058 -0.14041 0 0 1.8 -2.8151 Bus7 0.99623 -0.1349 0 0 2.5 1 Bus8 1.0012 -0.10054 0 0 3.2 -0.67833 Bus9 0.96829 -0.08018 0 0 3 2

LINE FLOWS

From Bus To Bus Line P Flow Q Flow P Loss Q Loss

[p.u.] [p.u.] [p.u.] [p.u.] Bus10 Bus2 1 -1.1746 0.05496 0.00952 0.13278 Bus10 Bus6 2 0.67458 -0.25496 0.00289 0.03598 Bus8 Bus3 3 -2.6809 0.38325 0.02173 0.27117 Bus9 Bus7 4 1.3984 -0.82281 0.00811 0.10116 Bus2 Bus1 5 -0.0301 0.00284 0 5e-005 Bus1 Bus4 6 1.531 0.13217 0.01344 0.14188 Bus4 Bus7 7 1.0176 -0.20971 0.00652 0.06635 Bus3 Bus9 8 4.4305 1.5756 0.03206 0.39845 Bus7 Bus6 9 1.1313 -2.494 0.00302 0.03023 Bus5 Bus1 10 -3.2574 0.09495 0.02951 0.31162 Bus5 Bus7 11 1.2369 -0.21706 0.00693 0.07686 Bus5 Bus8 12 0.52045 -0.27789 0.00138 0.01719

GLOBAL SUMMARY REPORT

TOTAL GENERATION



TOTAL LOAD

REAL POWER [p.u.] 13


TOTAL LOSSES



LIMIT VIOLATION STATISTICS

All voltages within limits.

All reactive power within limits.

All current flows within limits.

All real power flows within limits.

All apparent power flows within limits.

18

5. CONCLUSION

In this report the 10 bus test system was initially analysed and compensated for voltage

instability. It was observed that generators have the capability of providing reactive power

but are limited to a certain extent.

The reactive power produced by the generators cannot be effectively utilized since the

demand for the reactive power is far from its location. A Shunt capacitor bank was used for

compensation to provide local reactive power support at the buses 6 and 8.

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6. References

1. P.S.R Murty-2007, “Power system analysis”. BS Publications

2. Prabha Kundur-1994“ Power System Stability and Control”. McGraw Hill inc.

assignment 1

Documents

limit violation

single line

apparent power

2nd tier loads

injecting

matlabr61psatmodel

shunt capacitor

inject reactive