APPLICATION OF POWER ELECTRONICS IN THE TRANSMISSION OF ELECTRICAL.pdf

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APPLICATION OF POWER ELECTRONICS IN THE TRANSMISSION O F ELECTRICAL

ENERGY

T

Adhikari

Transmission Projects Division

BHEL,

New Delhi

email

:

[email protected]

ABSTRACT

Application of Powe r Electronics in the field

of power distribution and transmission systems

is attracting wide attention becau se of its

anticipated large scale application

in

future.

Traditional industry struc ture and operating

principles have built arou nd tightly and

centrally controlled single entity in a typical

vertically integrated utility. The demands that

are made on the power system because of

unbundling of vertically integrated utilities to

allow open access to third parties for

transmission of power will force application of

advance d controllers. It

is

then relevant to take

.

a look at how and to what extent power

electronics can contribute to effective control

of power distribution on the network. In this

paper, trends and h tu re prospects in utility

applications of power electronics are

presented. Idea is to highlight some practical

applications particularly related with FAC TS

(Flexible AC Transmission Systems) which

cover a wide range of power electronics

equipment in power systems.

1.

INTRODUCTION

With the radical restructuring

of electrical

energy industry throu ghou t the world, bulk

transm ission system will not continue to be

controlled in the sam e manner

as

has been done

in

the past.

So

far, the pow er system has mostly

comprised of vertically integrated utilities.

This

is

slowly giving way to a multitude of

diversified corpo rate entities having diverse

interests. roles and equipment

in

the power

system. For example, there are independent

generating entities, transmission entities,

distribution entities and brokering entities.

The governments and regulating agencies all

over the world are considering restructuring

and privatisation of the industry. The aim

is

to

increase efficiency through better investment

decisions, better use of existing plants, better

management and better choice for customers.

Whatever be the driving force, the unbundling

of vertically integrated utilities in the open

access en vironme nt will fo rce application of

advanced controllers in the power system.

Operating principles and control of system in

the emerging scenario will be much different

from the conventional way of control.

Consequent to different ownership for

different physical components of the system,

the dec isio n, processes will be distributed

amo ng various entities and the data related

to

hnc tion al control will widely differ from one

block to another. This can become quite

complex for the power system conrol engineer

particularly

when switching. between various

control systems necessitated either

by

system

contingencies or

by

return on the investment.

The requirement of alternative routes, the

loading of lines to their thermal limits without

sacrificing security, the requirement of dynamic

stability make us think on voltage control,

angle control and enhanced damping. The

controllers described in the following sections

can perform many of these hnctions. All of

them _a re an attempt to improve the

performa nce of the transmission- system by

using various types of pow er electronic circuits.

Some of the applications which utilize these

circuits are High Voltage DC (HVDC), Static

VAR Compensators (SVC), Thyristor

Controlled series Capacitors (TCSC), Phase

Angle Regulators (PAR), Static Condensers

(STATCON), Active Filters, Unified Power

Flow Controller (UPFC) etc.

0-7803-4886-9/98/ 10.00 1998

EEE

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2. POWER ELECTRONICS DEVICES

In the past decade, there has been a

remarkable progress in the high-power semi-

conductor devices such

as

GTO (Gate Turn-

oil) thyristors and light-triggered thyristors

(LTT) ; both due to sophisticated semi-

conductor technology and also owing to

demand of higher reliability in the power

systenis. Newer circuits are now evolving

which utilize turn-off devices to achieve self-

commutated configurations. In all utility

applications the semiconductor devices which

are used as switches offer a whole new

dimension in controlling the high voltage

systems which wa s not feasible earlier. No t only

are these switches much faster, they a lso do not

wear o ut like the mechanical switches.

From the many variations of power electronic

devices it appears that three types will be

dominant for FACTS and HVDC applications

during the next

5

to

10 years, viz. the line

commutated thyristor, the gate turn-off

thyristor, and the insulated gate bipolar

transistor.

Line Conimirtniecf

T h y i s

ors:LTT and ETT

are available. The peak blocking voltage is

expected to be limited to the 10

kV 12

kV

range. With new or firther improved

manufacturing technologies, improvement in

current handling capability as well as in

conduction losses and dynamic parameters

appear possible (e.g. recovery charge, dildt,

dvldt).

ale

l uni-igf

hyristor GTO)

blocking voltage and current handling

capabilities are still increasing: 6kV, 6kA have

been announced and higher ratings are

expected. Progress in manufacturing

technologies should result

in

reduced switching

losses and snubber requirements. H oweve r gate

power requirements are not expected to reduce.

The

Inszilated Gate Bipolar

finmistor

IGBT : It

appears reasonable to expect that for future

modules, blocking voltages will increase above

5kV

and current handling capability for

modules to at least

2kA.

To filly make use

of

such parameters for FACTS applications,

it

would be necessary that matching fast

switching diodes ar e also available. Much work

seems to be necessary to substantially reduce

the high on-state voltage.

3 POWER ELECTRONICS SYSTEMS /

APPLICATIONS

H VDC:

The first application of power electronics in

power transmission is the technology of HVDC

which started by using mercury ionic valves,

then switched to thyristors and has been

instrumental in pushing thyristor technology to

ever increasing device ratings. Adoption of

LTT has made a great contribution to the

development of com pac t and reliable thyristor

valves. LTT of

8

kV, 3 . 5 kA with the

forward voltage drop of 2.7 V at 3.5 kA,

fabricated on a silicon wafer of 6 inches in

diameter has been developed in Japan for

realizing an HVD C transmission system

of

2 . 8 GW (+/- 500 kV, 2 8 kA) by beginning

of next century. The HV DC system would

control bi-directional

flow

of 2.8

GW

on 50.5

kin

long submarine transm ission cables and 50

km long overhead transmiss ion lines between

two electric power companies in Japan thus

resulting in a higher degree of stability in

power systems. Similar long distance 6-10

GW

HVDC transmission

is

planned in Brazil from

Amazon to Sao Paul0 and

Rio

de Janeiro. Such

a big project

is

feasible only by application of

high-power electronics technology.

Fkxible

A

C Trnnsnzission

Systems:

Recent advances in power electronics have

made FACTS a reality.

FACTS

controllers

can be used to increase the transmission

capacity upto the thermal limit

of

transmission

lines, aid in fast voltage con trol in event of

contingencies and avoid loop flows causing

undesirable loading of certain transmission

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facilities. FACTS thus enables increased

transmission over existing right

o f

way by more

effective use of network and provides

coordinated control for better total system

damping to enhance system stability and

security.

FACTS technology is not a single high power

controller but a collection of controllers, which

can be applied individually or collectively in a

staged manner

to

control inter-related

parameters. The various FACTS controllers

along with their attributes are listed in Table s 1

&

2. Some o f these controllers are described

below.

Stntic VAR Conzpensntor

The SVC

uses

the conventional thyristor to

achieve fast control of shunt connected

capacitors and reactors. The SVC provides a

rapid and fine control of voltage without

moving parts and is readily available in current

market. The TCR portion of SVC consists

of

antiparallel thyristors in series with shunt

reactors usually in delta configuration. These

thyristors may be sw itched at any point over the

half wave (90 to

180

electrical deg rees behind

the voltage wave) to provide a hlly adjustable

control from 100 to zero reactive power

absorption. Harmonic currents are generated at

any angle other than 90 (full conduction) and

180 (zero conduction). Thyristor switched

reactor (TSRj or thyristor switched capacitor

(TSC configurations are also used which have

only two states of operation zero or full

conduction.

Figure- 1 shows a typical operation

characteristic for SVC . At point A all TSC s and

the fixed capacitors (FC) are switched on,

providing rated reactive generation at the

specified voltage, typically

1

O pu reactive

power output at

0.95

pu voltage. At point B

the TCR (or TSR)

is

fully switched on, and all

TSCs or FCs off , to give rated reactive

absorption (not necessarily equal to rated

generation).

Between A and B the T CR output is off and the

characteristic follows the natural impedance line

of a capacitor to some minimum voltage point

A l , below which the TSC is switched off.

Above point

B,

similarly, the TSC

is

off

and the

TCR fblly on; the characteristic follows the

natural impedance of the shunt reactor. Point C

indicates some thermal limit of current, above

which thyristor junction temperatures might

exceed a safe level for blocking. The higher

point

D

represen ts a voltage limit of equipment.

Thyristor Controlled Series C apacitor

The thyristor controlled series capacitor can

vary the impedance continuously to levels

below and up t o th e line s natural impedance.

This helps in increasing the power flow over the

line in steady state and also it will respond

rapidly to control signals to change the line

impedance, thereby damping oscillations during

and after a disturbance. In the TCSC scheme,

part of the compensation could be fixed and

part could be made variable which can be

varied during transient conditions. n example

circuit is shown in figure-2.

Plztise

Ang le Regulator

:

Another way to control the power flow on the

transmission line is through phase angle

regulator shown in figure-3. The phase shift is

accomplished by adding or subtracting a

variable voltage component that is

perpendicular to the phase voltage of the line.

This perpendicular voltage component is

obtained from a transformer connected between

the other two phases. In the scheme shown, the

three secondary windings have voltages

proportional to

1 3 : 9

Thyristor switches, one

per winding, allow each winding to be included

or excluded in the positive or negative

direction. The choice of 1, 3,

9

along with the

plus or m inus polarity for each w inding - yields

a switchable voltage range

of

-13 to

+13,

thus

giving a variable high speed control of the

perpendicular voltage component. The voltage

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corresponding to each

unit

step will ofcourse

determine the total phase shift that results.

Static Synclzronous Generator

SSG)

All

evolving applications of self commutated

circuits to transmission systems include the

basic main circuit components illustrated in

figure-5, the combination of which is termed

Static Synchronous generator. The

SSG

consists of a self commutated converter

connected to an AC power system through a

magnetic interface. A capacitor which acts as

an internal voltage source is connected across

the DC terminals of the converter. Some

applications also require energy storage or

supply to the DC link.

The principle of operation of the SSG is shown

in figure-5. The voltage source converter can

be considered as an AC voltage generator,

whose output voltage,

frequency and phase

angle are controllable Accordingly, the

magnitude and the phase angle of AC currents

are adjustable by changing the magnitude and

phase angle of the AC output voltage El of the

GTO converter, because of its connection to

the AC power system through the reactance of

the magnetic interface

(X,)

When E1 is

in

phase

with VT, currents flowing from the converter t o

the AC system have reactive power component

only (i.e.

=

k90 due to impedance being

nearly pure reactance from converter AC

voltage to AC system). In this case, reactive

power is capacitive when El

>

VT, or is

inductive when

Er

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