Chapter 4 - Series Compensators

Series compensators

Chapter – 4

Prof. B. S. Sree Shailan

�Education is a progressive

discovery of our own ignorance.

- Will Durant

�Proverb:

• Little things are pretty.

• Good things come in small

packages.

Introduction�The series controller could be a variable

impedance or a variable source, both arepower electronics based.

� In principle, all series controllers injectvoltage in series with the line.

�Combined series-series controllers:

The combination could be separate seriescontrollers or unified series-seriescontroller--- Interline Power Flow Controller.

Basic Principle

�Series compensation control the overall

series impedance of transmission line.

�AC power transmission is primarily limited

by the series reactive impedance of

transmission line.

� It can add a voltage in opposition to the

transmission line voltage drop, thereby

reducing the series line impedance.

BUS 1 BUS 2

I

V δ 0VjXLC

VL

VC

0VV δ

Phasor Diagram

Simplified Model

Capacitance C is

defined as a portion

of the line reactance,

C LX kX=

�A simplified model of a transmission

system with series compensation is shown.

�The voltage magnitudes of the two buses

are assumed equal as V and the phase

angle between them is δ.

�The transmission line is assumed lossless

and represented by the reactance XL.

�A controlled capacitor is series-connected

in the transmission line with voltage

addition Vinj.

� Overall series inductance of the transmission line

is X = XL – XC = (1 – k)XL.

� Active power transmitted is

� Reactive power supplied by the capacitor is

calculated as:

� Thus, it can be seen that the transmitted

active power increases with k.

( )

2

sin1

L

VP

k Xδ=

−

( )( )

2

22 1 cos

1C

L

V kQ

X kδ= −

−

Relative importance of Controllers

�For a given MVA, series controller isseveral times more powerful than shuntcontroller in controlling the power flow.

�Drawing from or injecting current into theline, the shunt controller is a good way tocontrol voltage at and around the point ofconnection.

�Shunt controller serves bus independentlyof the individual lines connected to the bus.

�Series connected controllers have to bedesigned to ride through contingency anddynamic overloads, and ride through orbypass short circuit currents.

�A combination of series and shuntcontrollers can provide the best of effectivepower/current flow and line voltage.

�Principle of controllers are based on dc toac converters with bidirectional power flowcapability.

�Energy storage systems are needed whenactive power is involved in the power flow.

�Battery, capacitor, superconductingmagnet, or any other source of energy canbe added in parallel through an electronicinterface to replenish the converter’s dcstorage.

Thyristor Controlled Series Compensator

�Like SVC, TCSC connects a thyristor

controlled reactor (TCR) in parallel with a

fixed capacitor.

�By varying the firing angle of anti-parallel

thyristors that are connected in series with

a reactor in the TCR, the fundamental

frequency inductive reactance of the TCR

can be changed.

�This effects a change in the reactance of

TCSC and it can be controlled to produce

either inductive or capacitive reactance.

�The basic TCSC module comprises a series

capacitor C in parallel with a thyristor-

controlled reactor LS.

� In addition, practical TCSC module includes

a metal-oxide varistor (MOV) connected

across series capacitor to prevent over-

voltages.

�TCSC is assumed to be connectedbetween buses k and m in a transmissionline where it is simplified like continuouslycontrollable reactance (capacitive).

T2

T1

LS

Ci

iL(α)

iC(α)=i+iL(α)

VC(α)

( )( )

( )C L

T

L C

X XX

X X

αα

α=

−

�TCSC behaves as a tunable parallel LC-

circuit to the line current.

�As the impedance of XL is varied from its

maximum (infinity) toward its minimum ωL,

TCSC increases its capacitive impedance.

TCSC Characteristic

Impedance versus firing delay angle

Typical TCSC Waveforms

TCR Internal Control Scheme

�A “Thyristor controlled series compensator

TCSC“ is planned for better utilization in

India in 2000.

�The FACTS system was to be installed on

the 400kV, 395km long, Kanpur-

Ballabgarh line.

�Kanpur – Ballabgarh and Kanpur – Agra

400kV lines in Northern grid of UP carry

about 800MW power from Singrauli and

Rihand belt to western UP and Rajasthan.

Static Synchronous Series Compensator

�Alternatively, a SSSC can be used for

series compensation.

�An SSSC is an SVS with all GTO based

device which contains a VSC which is

driven by a dc capacitor. The output of VSC

is connected to a 3-phase transformer

whose other end is connected in series with

the transmission line.

�This device work the same way as the

STATCOM.

�Unlike TCSC which changes the impedance

of the line, an SSSC injects a voltage in the

line in quadrature with the line current.

�By making the SSSC voltage to lead or lag

the line current by 90°, the SSSC can

emulate the behavior of an inductance or

capacitance.

�A SSSC is able to exchange active and

reactive power with the transmission

system.

� But if our only aim is to balance the reactive

power , the energy source could be quite small.

� The injected voltage can be controlled in phase

and magnitude if we have an energy source that

is big enough for the purpose.

� With reactive power compensation only the

voltage is controllable, because the voltage

vector forms 90º with the line intensity.

� Here serial injected voltage can delay or advance

the line current. Thus, SSSC can be uniformly

controlled in any value, in the VSC working slot.

� The Static Synchronous Series Compensator

(SSSC) uses a VSC interfaced in series to a

transmission line, as shown below:

Basic configuration of SSSC

transformer to the transmission line.

A source of energy is required for providing and

maintaining the DC voltage across the DC

capacitor and compensation of SSSC losses.

SSSC is placed in the

group of series connected

FACTS devices.

SSSC consists of a VSI

connected in series

through a coupling

Equivalent circuit of SSSC

�SSSC model consists of a series

connected voltage source in series with an

impedance. This impedance represents

the impedance of coupling transformer.

� SSSC when operated with an appropriate DC

supply (an energy source) can inject a component

of voltage in anti-phase with the voltage

developed across the line resistance, to

counteract the effect of the resistive voltage drop

on the power transmission.

� Capability of SSSC to exchange both active and

reactive power makes it possible to compensate

for the reactive and resistive voltage drops,

maintaining a high effective X/R ratio

independently of degree of series compensation.

�Thus, SSSC can work like a controllable

serial condenser and a serial reactance.

�Main difference is that the voltage injected

through SSSC is not related to the line

intensity and can be controlled

independently.

�Thus, SSSC can be used with excellent

results with low loads as well as with high

loads.

Inductive and Capacitive modes of operation

�Figure shows an example of a simple power

transmission system with an SSSC

operating both in inductive and capacitive

modes, and related phasor diagrams

�Transmission line with inductive reactance

XL delivering power form the sending-end

voltage source to the receiving-end voltage

source, having no compensation of any

kind, is said to be in a steady-state.

�Voltage impressed by the effective

reactance is the same with the voltage drop

across uncompensated line because the

degree of series compensations is zero.

�Line reactance is constant and by adding

variable series (capacitive/inductive)

reactance, the amount of compensation

can be controlled.

�Degree of series compensation in this case

is defined as:

line inductive reactance and Xq is the

emulated series reactance.

% 100 q

L

L

XCompensation where X is th

Xe= ×

Thyristor-Switched Series Capacitor (TSSC)

� Equivalent capacitance is between 0 and C/m.

� A capacitor is inserted by turning-off and

bypassed by turning-on the corresponding

thyristor switch.

Series Static VAR Compensator(SSVC)

�Control strategy of SSVC is typically based

on achieving an objective line power flow in

addition to the capability of damping power

oscillations.

Advanced SSVC: series-connected STATCOM

Series-connectedSTATCOM is the dualcircuit of shunt-connected STATCOM.

This type of seriescompensation canprovide a continuousdegree of seriescompensation byvarying |VC|.

Also, it can reverse the phase of VC, therebyincreasing the overall line reactance; this can bedesirable to limit fault current or to dampen poweroscillations.

Phase-Angle Compensator (PAC)

The phase shifter controls the magnitude of Vq andthus the phase shift α to the sending-end voltage.

Transforming arrangement between the excitation & series transformers ensures that Vq is always at 900 to V (called quadraturebooster)

Series connected controllers

Combined shunt and series

connected controllers

Other controllers

UPFC (Unified Power Flow Controller)

�UPFC combines together the features of

two FACTS devices: Static Synchronous

Compensator (STATCOM) and the Static

Synchronous Series Compensator (SSSC).

�DC terminals of two underlying VSCs are

coupled and this creates a path for active

power exchange between the converters.

�Hence, active power supplied to the line by

the series converter can now be supplied by

the shunt converter, as shown in the Figure.

�This topology offers four degrees of

freedom (two associated with each VSC)

with one constraint (active powers of the

VSCs must match).

�Thus, a fundamentally different range of

control options is available compared to

STATCOM or SSSC.

�UPFC can be used to control the flow of

active and reactive power through the line

and to control the amount of reactive power

supplied to the line at the point of

installation.

�Representative of the last generation of

FACTS devices is UPFC.

�UPFC is a device which can control

simultaneously all three parameters of line

power flow (line impedance, voltage and

phase angle).

�Basic components of the UPFC are two

voltage source inverters (VSI's) sharing a

common dc storage capacitor, and

connected to the system through coupling

transformers.

�One VSI is connected in shunt to the

transmission system via a shunt

transformer, while the other is connected in

series through a series transformer.

�Shunt inverter is used for voltage regulation

at the point of connection, injecting an

opportune reactive power flow into the line

and to balance the real power flow

exchanged between the series inverter and

the transmission line.

�Series inverter can be used to control the

real and reactive line power flow inserting

an opportune voltage with controllable

magnitude and phase in series with the

transmission line.

�Thereby, the UPFC can fulfill the functions

of reactive shunt compensation, active and

reactive series compensation and phase

shifting.

�Further, the UPFC allows a secondary but

important function such as stability control

to suppress power system oscillations

improving the transient stability of power

system.

Assignment - 4

1) Explain the basic principle of series

compensation in power transmission

systems.

2) Explain the relative importance of

controllers in power transmission system.

3) Explain the operation of Thyristor

controlled series compensator (TCSC)

with the help of a typical characteristics

and waveforms.

4) Explain the working of Static Synchronous

Series Compensator (SSSC) with the help

of equivalent circuit.

5) Explain the inductive and capacitive

modes of operation of SSSC with the help

of phasor diagrams.

6) Explain the principle of operation of

Thyristor Switched Series Compensator

(TSSC).

7) Explain the principle of operation of

Series Static VAR compensator (SSVC).

8) Explain the operation of Unified Power

Flow Controller (UPFC) and describe the

various components of UPFC.

Last date for submission:

13th April ‘13

Chapter 4 - Series Compensators

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