1. CASE : ENHANCE INTER-REGION POWER TRANSFER IN EXTRA HIGH VOLTAGE
POWER GRID
OBJECTIVE :
1. ENERGY OPTIMIZATION BY INCREASING THE POWER TRANSFER CAPABILITY OF THE
EXISTING INFRASTRUCTURE. MINIMIZE THE NEED OF COSTLY INFRASTRUCTURE.
2. REDUCE TRANSMISSION LOSSES
3. IMPROVE SYSTEM STABILITY
4. IMPROVE VOLTAGE PROFILE

2. Objective of Series Compensation
Reduce load dependent voltage drops
Influence load flow in parallel transmission lines
Increase transmission capacity
Increase system stability

3. Problems of Shunt Compensation
The reactive power output of capacitors is proportional to the square of voltage. The voltage across the Shunt Capacitor is proportional to the line voltage. As a result, the reactive power output is significantly lower at low line voltages when maximum compensation is required. It has seldom usage in Power Transmission.

4. Usefulness of Series Capacitor
The usefulness of Series Capacitor is demonstrated by well-known expressions relating to active power transfer and voltage:
P= V1 V2 sin δ / X …………………… (1)
V= f (P, Q) …………………………….. (2)
From eqn. (1) it is evident that the flow of active power can be increased by decreasing the effective reactance of the line. Otherwise, the angular separation can be reduced by decreasing effective reactance keeping the same power flow in the line.

5. Usefulness of Series Capacitor (contd…)
By varying Transmission Line Reactance of two parallel lines by
use of Series Capacitance, it is possible to influence Load Sharing
between two parallel lines.
By reactive compensation, it is possible to reduce apparent
power flow for a given active power flow. So, transmission line
losses are reduced for a given amount of active power flow by
incorporating series capacitance.
From eqn. (2), it is seen that line voltage is a function of both
active and reactive power. However, it is seen from analysis of
voltage phasors with variation of reactive power that voltage
increase significantly with leading reactive power and decrease
significantly with lagging reactive power. By varying series capacitance
in transmission line the transmission voltage can be controlled.

6. Improvement of Voltage and Phase Angle Stability with Series Capacitance

7. Controllable Series Compensation using TCSC
The Current through the inductor is controlled by using an anti-parallel connection arrangement of thyristors and adjusting the firing angle α of the gate pulse applied to the thyristors.
When the thyristors are in full conduction mode, the current through the inductor is maximum. When the thyristors are in full blocking mode, , the current through the inductor is zero.

8. Controllable Series Compensation using TCSC (contd…)
Inductor current lags the voltage by 90° and the firing pulse
for full conduction of the thyristor is to be applied at the peak of
the voltage.
When gate pulse delay is α, the current in the inductor
can be expressed with an applied voltage of as,
Or,

9. Controllable Series Compensation using TCSC (contd…)
The term in the eqn. is simply a constant offset depending
upon the value of α , which decreases the instantaneous value of
inductor current in each positive half cycle and increases the same
in each negative half cycle
inductor current reaches its zero value before ωt = π when there is
some delay (α ) applied to the firing pulses and the thyristor will stop
conduction. With a delay of α=0, the term in eqn. vanishes and the
thyristors are in full conduction mode of operation.
With a delay of α=π/2, the offset value reaches its maximum value
which will decrease the instantaneous value of the inductor current
(also reaches its peak value at the same instant ωt= π/2 ) to zero
and both the thyristors are in blocking state.

10. Thyristor Controlled Inductor Current

11. Controllable Series Compensation using TCSC (contd…)
Applying Fourier Series expansion, the average value of the fundamental current can be expressed as
Or,

12. Controllable Series Compensation using TCSC (contd…)
To find the maximum value of xL(α), differentiating with respect to and make the value equals to zero, we have,
Or,
The solution of this eqn. is α=π/2. For this value of α, xL(α) = ∞
Thus, as the firing angle varies between 0 ≤ α ≤ π/2, xL(α) can be varied within the range xL ≤ xL(α) ≤ ∞.

13. TCSC Operating Regions

14. Voltage and Current Waveform in Capacitive Mode
The expression for capacitor current can be written as,

15. Voltage and Current Waveform in Capacitive Mode (Contd…)
To stop conduction of the thyristor associated with the inductor, the value of delay should be π/2 measured from the instant where inductor current changes its polarity from negative to positive. Therefore, for capacitive operation of TCSC, the net value of delay angle should be α = π + π/2 = 3π/2 measured from the zero crossing of the line current (changes its value from negative to positive).
If delay of the firing pulse α is slightly decreased from 3π/2, this will allow a small current through the inductor. The inductor current will stop conduction at an angle 2π – α. In this region (π+α to 2π–α) of operation, capacitor current will increase in positive half cycle and decrease in negative half cycle.

16. Voltage and Current Waveform in Capacitive Mode (contd…)
Small increase in inductor current will also increase the capacitor current . This sudden increment of capacitor current will change the polarity of capacitor voltage sharply from its negative to positive and finally sets at higher value than previous condition (zero inductor current).
For TCSC circuit, the resonant charge reversal will create a positive dc offset in positive half cycle and negative dc offset in negative half cycle of the capacitor voltage.

17. Voltage and Current Waveform in Inducitive Mode
Similarly, in case of inductive mode of control, if the delay of the firing pulse α is increased from 00 slightly, inductor current will decrease :

18. Case Study taken in Purnea – Muzaffarpur 400 KV Double
Case Study – Enhanced Power Flow between ER – NR using Fixed Series Capacitor (FSC) & Thyristor Controlled Series Compensation (TCSC)
Case Study taken in Purnea – Muzaffarpur 400 KV Double
Circuit line in 400/220 KV New Purnea Substation.
New Purnea substation was commissioned in to create
a power corridor for evacuation of power from TALA hydroelectric
project, Bhutan to Northern Region.
For Purnea – Muzaffarpur 400 KV Line I & II, 40% Fixed Series
compensation and 5 – 15 % Thyristor Controlled Series
compensation have been commissioned in

19. Scheme 3 – Flexibility in Power System Operation using Thyristor
Controlled Series Compensation (TCSC)

20. FSC AND TCSC EQUIPMENTS WITH ASSOCIATED BAYS AT NEW PURNEA:
NAME OF SUBSTATION
TYPE OF SUBSTATION
CONVENTIONAL/GIS
VOLTAGE LEVEL KV
NO. OF T/Fs/REACTORS/SVC etc
(WITH CAPACITY)
NO. OF BAYS
DATE OF COMMERCIAL OPERATION
400/220KV New Purnea Substation
Conventional
400KV
FSC & TCSC connected to 400KV D/C New Purnea-Muzaffarpur T.L.
FSC: MVAR per circuit
TCSC: MVAR per circuit.
Total 6 platforms of TCSC & FSC with associated equipments.
26.08.
2006
NOMENCLATURE
DESCRIPTION
LOCATION/INSTALLED QUANTITY
TOTAL QUANTITY
FSC & TCSC
ASSOCIATED
WITH 400KV
PURNEA-MUZFR-I
PURNEA-MUZFR-II
Platform equipments
DS1 with ES1 & DS2 with ES2
MBS
BBR1 & BBR2
FSC & TCSC bank
HCB Isolators with 1 E/S
HCB Isolators without E/S
SF6 CB with associated equipments.
1 set
2 set
4 set

21. FSC and TCSC Purnea system specifications
Degree of compensation
40%
5 % - 15%
FSC capacitor bank impedance
24.2 ohm
TCSC capacitor bank impedance
3.03 ohm
FSC capacitance
131.5 micro farad
TCSC capacitance
micro farad
Rated current
3200 A
Rated capacitor current (50Hz)
3841 A
Rated voltage
77.4 KV
Rated bank voltage
11.7 KV
Rated power
743.4 MVAR
Nominal power
143.1 MVAR
Unit rating
469 KVAR, V
616 KVAR, 5954 V
Series connection 6 2
Parallel connection 88 38

22. Single line diagram of FSC & TCSC of New Purnea Substation

23. FSC & TCSC of New Purnea Substation
If Main Bypass Switch is open and Disconnect Isolators are closed and
Bypass breakers across FSC and TCSC are open then both FSC and
TCSC are in service.
Metal Oxide Varistors (MOV) are used for protection of Capacitor banks.
Damping reactors are used to minimize the effect of switching surges
produced due to operation of bypass breaker.
Triggered spark gap will operate under the condition of bypass breaker
failure.
Different CTs are installed to measure line current, capacitor current,
Thyristor current, capacitor unbalance current, MOV current. Temperature
of MOVs are monitored to maintain smooth operation.

24. Thyristor Valve Components

25. Thyristor Valve Circuit

26. Thyristor Circuit
Thyristor Voltage Monitoring (TVM) card monitor voltage across each valve
pair and give feed back to protection.
Snubber circuit is used for protection of individual Thyristor valve. Grading
resistors are used to maintain even voltage distribution across the valves.
As the thyristors used are of Light Triggered type so an opto-coupler (MSC)
is installed in the platform to interface between Valve Base Electronics
(VBE) of protection panel and the thyristor valves.
A column of optical fiber is installed connecting VBE with MSC of the
platform. Another Column containing cold circulating purified water is also
installed for the purpose of Thyristor cooling.

27. Thyristor Valves Gating and Monitoring System

28. Thyristor Circuit
Thyristor Voltage Monitoring (TVM) card monitor voltage across each valve
pair and give feed back to protection.
Snubber circuit is used for protection of individual Thyristor valve. Grading
resistors are used to maintain even voltage distribution across the valves.
As the thyristors used are of Light Triggered type so an opto-coupler (MSC)
is installed in the platform to interface between Valve Base Electronics
(VBE) of protection panel and the thyristor valves.
A column of optical fiber is installed connecting VBE with MSC of the
platform. Another Column containing cold circulating purified water is also
installed for the purpose of Thyristor cooling.

29. Data taken Before and After Insertion of FSC & TCSC
Date : Time : 22:15 hr. MW
MVAR KV
Before I
-135
418 II
After I
-121
420 II
The above data is for example only. As the total compensation capacity of
The FSC & TCSC combination is 55%, the maximum possible enhancement
Of power flow is 55% in case of high value of reactive power in the line.

30. THANK YOU