assignment 1
DESCRIPTION
10 bus-power system analysisTRANSCRIPT
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Tshwane University of Technology
Department of Electrical Engineering
Student name: Ayub Machiri Wanjala
Student no.: 212 492 884
Power Systems V
Assignment 1
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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
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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]
Minimum voltage limit violation at bus <Bus4> [V_min = 0.95]
Minimum voltage limit violation at bus <Bus5> [V_min = 0.95]
Minimum voltage limit violation at bus <Bus6> [V_min = 0.95]
Minimum voltage limit violation at bus <Bus7> [V_min = 0.95]
Minimum voltage limit violation at bus <Bus8> [V_min = 0.95]
Minimum voltage limit violation at bus <Bus9> [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
REAL POWER [p.u.] 12.4995
REACTIVE POWER [p.u.] 6.0674
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TOTAL LOSSES
REAL POWER [p.u.] 0.15997
REACTIVE POWER [p.u.] 1.8777
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
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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 0.02092 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
REAL POWER [p.u.] 13.1351
REACTIVE POWER [p.u.] 1.8903
TOTAL LOAD
REAL POWER [p.u.] 13
REACTIVE POWER [p.u.] 0.30653
TOTAL LOSSES
REAL POWER [p.u.] 0.13512
REACTIVE POWER [p.u.] 1.5837
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.
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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.