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UNIT-VII Static Series Compensators UNIT-VII Static Series Compensators

UNIT-VII Static Series Compensators - PowerPoint Presentation

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UNIT-VII Static Series Compensators - PPT Presentation

1Variable impedance type series compensators a GTO ThyristorControlled Series Capacitor GCSC b ThyristorSwitched Series Capacitor TSSC c ThyristorControlled Series Capacitor TCSC ID: 674074

capacitor voltage series line voltage capacitor line series current valve thyristor compensation transmission compensating sssc power tcsc controlled gto

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Slide1

UNIT-VII

Static Series CompensatorsSlide2

(1)Variable impedance type series compensators.

(a) GTO Thyristor-Controlled Series Capacitor (GCSC)

(b) Thyristor-Switched Series Capacitor (TSSC)

(c) Thyristor-Controlled Series Capacitor (TCSC)

(2)Switching converter type series compensators.

Static synchronous series compensator (SSSC)Slide3

GTO

Thyristor

-Controlled Series Capacitor (GCSC)

It consists of a fixed capacitor in parallel with a GTO

thyristor (or equivalent) valve (or switch) that has the capability to turn on and off upon command.

Fig. (a) Basic

GTO-Controlled Series

Capacitor, (b) principle

of turn-off

delay angle control,

and

(c) attainable

compensating voltage

waveformSlide4

The

objective

of

the GCSC scheme is to control the ac voltage vc across the capacitor at a given line

current i. Evidently, when the GTO valve, sw, is

closed

, the voltage across the

capacitor

is zero, and

when the valve

is open, it is maximum

. For

controlling the

capacitor

voltage

,

the closing and opening of

the

valve

is

carried out

in

each half-cycle in synchronism with the ac

system

frequency

.

The GTO valve

is stipulated to close

(

through

appropriate control

action

)

whenever

the

capacitor voltage crosses zero. (Recall

that the thyristor valve of the TCR opens whenever the current crosses zero.)Slide5

When the valve

sw

is opened at the crest of the (constant) line current (

γ = 0), the resultant capacitor voltage vc will be the same as that obtained in steady state with a permanently open switch. When the opening of the valve is delayed by the angle γ

with respect to the crest of the line current, the capacitor voltage can be expressed with a defined line current, i(t) = I

cos

ωt, as follows:

The amplitude of fundamental capacitor voltage can be expressed as a function of

γ

where

γ

is

the amplitude of the line current,

C

is the capacitance of the GTO

thyristor

controlled

capacitor, and

ω

is

the angular frequency of the ac

system.Slide6

Fundamental component of the series capacitor voltage vs. the turn-off delay angle

γ

.Slide7

This

impedance can

be

written as

In a practical application the GCSC can be operated either to control the compensating

voltage,

V

CF(γ

), or the compensating reactance,

XC(γ).

In the voltage compensation mode

, the GCSC is to maintain the rated compensating voltage in face of decreasing line current over a defined interval

I

min

<=

I

<=

I

max

as illustrated in Figure

(a1).

In this compensation mode the capacitive reactance

X

C

, is

selected so as to

produce the

rated compensating voltage with

I

=

I

min

, i.e

.,

V

Cmax

=

X

C

I

min

. As

current

I

min

is increased

toward

I

max

,

the turn-off delay angle

γ

is

increased to reduce the duration of the capacitor injection and thereby maintain the compensating voltage with increasing line current

.Slide8

In the impedance compensation mode, the GCSC

is to maintain the maximum rated

compensating reactance

at any line current up to the rated maximum. In this compensation mode the capacitive

impedance is chosen so as to provide the maximum series compensation at rated current,

X

C

= Vcmax/

Imax, that

the GCSC can vary in the 0 <= XC

) <=

X

C

range by controlling

the

effective capacitor voltage

V

CF

(

γ

), i.e

.,

X

C

(

γ

) =

V

CF

(

γ

)/

I

. Slide9

Thyristor-Switched Series Capacitor (TSSC)

The operating

principle: the degree of series compensation is controlled in a step-like manner by increasing or decreasing

the number of series capacitors inserted. A capacitor is inserted by turning off, and it is bypassed by turning on the corresponding thyristor valve. A thyristor valve commutates "naturally," that is, it turns off when the current crosses zero. Thus a capacitor can be inserted into the line by the thyristor

valve only at the zero crossings of the line current. Slide10

Since the insertion takes place at line current zero, a full half-cycle of the line current will charge the capacitor from zero to maximum and the successive, opposite polarity half-cycle of the line current will discharge it from this maximum to

zero.

As can be seen, the

capacitor insertion

at line current zero, necessitated by the switching limitation of the thyristor valve, results in a dc offset voltage which is equal to the amplitude of the ac

capacitor voltage

. In order to minimize the initial surge current in the valve, and the corresponding circuit transient, the

thyristor valve should be turned on for bypass only when the capacitor voltage is zero. With the prevailing dc offset, this requirement can cause

a delay of up to one full cycle, which would set the theoretical limit for the

attainable response time of the TSSC.Slide11

Thyristor-Controlled Series Capacitor (TCSC)

It

consists of the

series compensating capacitor shunted by a TCR. In

a practical TCSC implementation, several such basic compensators may be connected

in series to

obtain the desired voltage rating and operating

characteristics. This arrangement is similar in

structure to

the TSSC and, if the impedance of the reactor,

X1

,

is

sufficiently

smaller than that of

the capacitor

,

X

C

, it can

be

operated in an

on/off manner like

the TSSC

.

Basic TCSC SchemeSlide12

However, the basic idea behind the TCSC scheme is to provide a continuously variable capacitor by means of partially canceling the effective compensating capacitance by the TCR

.Slide13

Damping effects of TCSCSlide14

Applications of variable series compensation -TCSC

Power flow control

Enhancing

transient stability

Damping of power swings Sub-synchronous resonance damping Slide15

TCSC at a Substation Slide16

Static Synchronous Series Compensator

(SSSC)

The

SSSC is one of the most recent FACTS devices for power transmission series compensation. It can be considered as

a synchronous voltage source as it can inject an almost sinusoidal voltage of variable and controllable amplitude and phase

angle, in

series with a transmission line. The

injected voltage is almost in quadrature with the line current. A small part of

the injected voltage that is in phase with the line current provides the

losses in the inverter.

Most of the injected voltage, which is

in quadrature

with the line current, provides the effect of

inserting an

inductive or capacitive reactance

in series

with

the transmission

line. The variable reactance influences the

electric power

flow in the transmission line. The basic configuration of

a SSSC

is shown in Fig.Slide17

SSSC (a)WITH OUT STORAGE and (b)WITH STORAGESlide18

A static synchronous Series Compensator operated without an external energy source as Reactive Power with output voltage is in quadrature with and fully controllable independently of the transmission line current for the purpose of increasing or decreasing the overall reactive voltage drop across the transmission line and thereby controlling the electric power flow.

The SSSC FACTS device can provide either capacitive or inductive injected voltage compensation, if SSSC-AC injected voltage, (Vs), lags the line current IL by 90º, a capacitive series voltage compensation is obtained in the transmission line and if leads IL by 90º, an inductive series compensation is achieved.Slide19

Theory of the SSSC

Figure 1 shows a single line diagram of a simple Transmission line with an inductive transmission reactance, XL, connecting a sending

end voltage source, and a receiving end voltage source, respectively.Slide20
Slide21

The expression of power

flow is given bySlide22

Where

Xeff

is the effective total transmission line reactance between its sending and

Receiving power system ends, including the equivalent “variable reactance” inserted by the equivalent injected voltage (Vs) (Buck or Boost) by the SSSC-FACTS Compensator.