This document describes a project to design a three-phase individual controlled fixed capacitor-thyristor controlled reactor (FC-TCR) static VAR compensator (SVC) to perform power factor correction and prevent negative sequence current. It includes an abstract discussing the issues with negative sequence current, an introduction to the FC-TCR SVC design, the design procedure and algorithm, results showing the SVC reduces negative sequence current both with and without power factor correction, the source code implementing the design, and a conclusion stating the SVC approach is effective and unique.

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COURSE: EE 5374
COURSE TITLE: POWER SYSTEMS PROTECTIVE RELAYING
INSTRUCTER: DR.W.J.LEE
TERM: SPRING‐2015
NAME: HARDIK PARIKH
STUDENT ID: 1001090431
SUBJECT: PROJECT-2
Project Title: DESIGN A THREE-PHASE INDIVIDUAL CONTROLLED FIXED
CAPACITOR-THYRISTOR CONTROLLED REACTOR (FC-TCR) STATIC
VAR COMPENSATOR (SVC) TO PERFORM POWER FACTOR
CORRECTION AND PREVENT NEGATIVE SEQUENCE CURRENT

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Sr.No
INDEX
Subject Page No
1 Abstract 3
2 Introduction 4
3 Design Procedure 5
4 Algorithm 6-7
5 Results 9
6 Source code 10-11
7 Conclusion 12
8 Learning Outcome 12
9 Reference 12

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Abstract:
In a transmission system, only the positive sequence is usually selected to analyze a
load flow because the power system is assumed to be balanced. In practice,
however, a completely balanced power system is almost impossible to be obtained,
so the zero and negative sequences should exist in the power system. Since a Δ-
Yground connected step-up transformer can block the zero sequence current on
the primary side, the current entering a generator just consists of positive and
negative sequence parts. Beside this negative sequence current produced by the
system heats the windings to possibly damage the generator, it may also cause a
mechanical resonant problem on the generator.
When a system is unbalanced, the frequency of negative sequence current will be
converted into a mechanical vibration frequency through the rotor shaft. This will
create small chronicle damage over the time and results in mechanical failure of
the turbine. As a result we are required adjust the setting of the I2 relay to limit the
negative sequence current going into the generator or to implement methods
which prevent I2 from entering back to generator.
The research has shown that SVC has been proved successful to prevent negative
sequence current more over it also has capabilities for Power factor correction.
• Negative-sequence current causes some problems in generator systems.
Though every generator is capable of withstanding a certain level of
negative-sequence current, excess and/or persistent amounts of negative
sequence current may cause rotor overheating and serious damage.
• Since its frequency quite matches the natural mechanical frequency of
turbine blades and the zero sequence current is blocked by delta connected
step-up transformer, the negative sequence current becomes the only
reason for the super synchronous resonance of a generator due to an
unbalanced system, especially in an isolated power system.
• SVC has the potential to overcome some adverse effects of the negative
sequence current to the turbine generator systems

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FC-TCR SVC can change the real and reactive power flow and force the output o f
the generator become balanced even though the load is unbalanced. Besides, a
correct and simplified mathematical model which is selected among several
compensating methods would be built up. This model would be easily implemented
in a control program and reduce negative sequence current to the expected value.
Introduction:
To analyze the effects to the whole system when using SVC to reduce the negative
sequence current entering a generator, a three-phase transmission load flow
program has been developed. The connection of an SVC, however, is delta to avoid
producing zero sequence problems; an appropriate delta connected load model for
a load flow program needs to be developed because the load o r shunt reactive
elements used to be in grounded wye. The function of an SVC to reduce the
negative sequence current entering a generator.
Before presenting the special application of a static var compensator system, the
principle of a fixed-capacitor Thyristor Controlled Reactor (FC-TCR) as shown in Fig.
1, is briefly discussed. The adding of the Fixed-Capacitor is to make this SVC also
have the capability of supplying reactive power. The basic elements of the thyristor
controlled reactor, as shown in the right half of Fig. 1, consist of a fixed reactor with
inductance L, and a bidirectional thyristor valve which conducts on alternate half-
cycles. The current flowing through the thyristors can be adjusted from zero to
maximum by controlling the delay angle a.
Fig 1. Basic Elements of a FC-TCR Circuit

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The fundamental of the negative sequence current reduction on the generator with
SVC is to lead the I
Design procedure:
2 current into the SVC instead of the generator. Beside
generating reactive power; the adjustment o f the firing delay angle o f the SVC can
obtain the unbalanced susceptance to balance the equivalent load impedance,
which connection is delta. The following will compare several different approaches
to reduce generator I2 current with an SVC.
Fig 2. A power system with compensator
Theoretically, a complete compensation can be obtained and the negative
sequence current can be compensated by an SVC.
The Equation can be divided into real and imaginary parts and will have three
variables and two equations. An additional constraint has to be added to obtain a
Unique solution. From the practical point of view, the following constraint is
selected.

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Case-I:
Algorithm:

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Case-II:

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Available Data:
1. i. The internal voltage of the generator is balanced.
2. ii. The zero sequence current is blocked by using the Δ‐ Y grounded step up
transformer.
3. iii. The magnitude of the phase A voltage is maintained at 1.0pu.
4. iv. The source impedance of the generator is j0.1 pu.
5. v. For simplicity, assume that the SVC is connected at the terminal of the
generator.
6. vi. To avoid the appearances of zero sequence current, the SVC is Δ
connected.
7. vii. The loads are Pa + jQa, Pb+jQb and Pc+jQc.
Procedure:
1. We are provided the phase powers Sa, Sb, Sc.
where Sa = Pa + jQa ; Sb = Pb + j Qb; Sc = Pc + jQc
2. The generator terminal voltage is set at 1.0 pu.
3. We calculate phase currents entering the generator using Ia = (Sa/Va)’
similarly for Ib and Ic and also consider the phase shifts.
4. From these phase currents we calculate the sequence currents I0, I1, I2.
5. Our aim is to reduce the I2 (negative sequence current) as close to zero as
possible.
6. The FC-TCR will generate reactive power Qab, Qbc, Qca and its susceptances
are calculated by dividing the reactive powers by V2
7. Once reactive power from FC-TCR is calculated the total power entering the
generator is calculated using formulas in method 3 of the reference
dissertation.
i.e. generator terminal
voltage.
8. Then the phase and sequence currents are calculated.

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Results:
Description
Case-I
Without PF Correction
B -1.4989e-006ab
B 1.9535e-007bc
B 1.3036e-006ca
I2 -8.7711e-007 -7.2809e-006isystem
I2 -8.7893e-007 -7.2808e-006isvc
I2 1.8287e-009 -6.1762e-011inet
Case-II
Description With PF Correction
PF = 0.8 PF = 1
B -1.4989e-006ab 0.0999
B 1.9535e-007bc 0.1002
B 1.3036e-006ca 0.0999
K 5.5511e-017 0.3000
I2 -8.7711e-007 -7.2809e-006isystem -8.7711e-007 -7.2809e-006i
I2 -8.7893e-007 -7.2808e-006isvc -9.8327e-004 -8.7025e-006i
I2 1.8287e-009 -6.1762e-011inet 9.8239e-004 +1.4217e-006i
PF New 0.8 1.0

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Source Code:
%% CASE:1- Without power factor correction
% Load data as provided
Pa= 0.4;
Qa= 0.3;
Sa= Pa+Qa*i;
Pb= 0.4;
Qb= 0.3;
Sb= Pb+Qb*i;
Pc= 0.4;
Qc= 0.3;
Sc= Pc+Qc*i;
% Terminal Voltage at Transformer primary
VA= 1;
VB= -0.5-0.866i;
VC= -0.5+0.866i;
% Terminal Voltage at Transformer secondary
Va= VA*(0.866025+0.5i);
Vb= VB*(0.866025+0.5i);
Vc= VC*(0.866025+0.5i);
a= -0.5+0.866i;
% Phase current, Transformer secondary side
Ia= conj(Sa/Va);
Ib= conj(Sb/Vb);
Ic= conj(Sc/Vc);
% Phase current, Transformer Primary side
IA = Ia*(0.866025-0.5i);
IB = Ib*(0.866025-0.5i);
IC = Ic*(0.866025-0.5i);
%Sequence current in system
Iseq = (1/3) * [1 a a^2; 1 a^2 a; 1 1 1]*[IA; IB; IC]; %
Iseq = [Iseq(1); Iseq(2); 0]; % As delta configuration will eliminate zero
serquence current.
I2= [real(Iseq(2)); imag(Iseq(2)); 0];
A= [1.5 -2.99 1.5; 2.598 -0.0001 -2.598; 1 1 1];
I2netbefore= Iseq(2)
B= inv(A)*I2;
Bab=B(1)
Bbc=B(2)
Bca=B(3)
A= [1 a^2 a];
V= [VA-VB 0 VA-VC; VB-VA VB-VC 0; 0 VC-VB VC-VA];
I2SVC= A*V*B
I2netafter = Iseq(2)-I2SVC

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%% CASE:2- With power factor correction
% Load data as provided
Pa= 0.4;
Qa= 0.3;
Sa= Pa+Qa*i;
Pb= 0.4;
Qb= 0.3;
Sb= Pb+Qb*i;
Pc= 0.4;
Qc= 0.3;
Sc= Pc+Qc*i;
% Terminal Voltage at Transformer primary
VA= 1;
VB= -0.5-0.866i;
VC= -0.5+0.866i;
% Terminal Voltage at Transformer secondary
Va= VA*(0.866025+0.5i);
Vb= VB*(0.866025+0.5i);
Vc= VC*(0.866025+0.5i);
a= -0.5+0.866i;
% Phase current, Transformer secondary side
Ia= conj(Sa/Va)
Ib= conj(Sb/Vb)
Ic= conj(Sc/Vc)
% Phase current, Transformer Primary side
IA = Ia*(0.866025-0.5i);
IB = Ib*(0.866025-0.5i);
IC = Ic*(0.866025-0.5i);
S1= VA*conj(IA);
P1= real(S1);
Q1= imag(S1);
% Phi1= atan(Q1/P1);
% PF1= cos(phi1);
P2=P1;
PF2= 0.8;
PF= 1; % As provided
Q2= P2*tan(acos(pf2))
Q3= Q1-Q2;
K= Q3/VA^2
% Q2new= Q3+Q2avg
pfnew= P2/(sqrt(P2^2+Q3^2))
%Sequence current in system
Iseq = (1/3) * [1 a a^2; 1 a^2 a; 1 1 1]*[IA; IB; IC]; %
Iseq = [Iseq(1); Iseq(2); 0] % As delta configuration will eliminate zero
serquence current.

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I2= [real(Iseq(2)); imag(Iseq(2)); K];
A= [1.5 -2.99 1.5; 2.598 -0.0001 -2.598; 1 1 1];
B= inv(A)*I2
Bab=B(1)
Bbc=B(2)
Bca=B(3)
A= [1 a^2 a];
V= [VA-VB 0 VA-VC; VB-VA VB-VC 0; 0 VC-VB VC-VA];
I2SVC= A*V*B
I2net = Iseq(2)-I2SVC
Conclusion:
In both the cases it is observed that the net negative sequence flowing in the
system is reduced with introduction of SVC. It is observed that the SVC based FCTCR
reduce the negative sequence current with variable switching of Thyristor. It is also
important that SVC also supplies reactive power in case of power factor
improvement.
In this project we have learnt about operation of FC-TCR based Static VAR
Compensator and how to implement SVC for negative sequence current reduction
and power factor improvement. The approach followed is quite unique and novel
method which can be widely implemented to overcome the consequences of
negative sequence current in modern power system.
Learning outcome:
1) Thesis: The prevention of super synchronous resonance problem on the
turbine system of generator with staticvar compensator
References:
By Jen-hung chen
2) Negative sequence current reduction for generator turbine protection
Wei-jen lee, Tze-yee ho, member, Jih-phong liu, Yuin-hong liu, IEEE member