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MAGNETICALLY CONTROLLED SHUNT REACTOR APPLICATION FOR AC HVAND EHV TRANSMISSION LINES

A.BRYANTSEV V.DOROFEEV M.ZILBERMAN A.SMIRNOV

JSV “ELUR” FNG “UPS of Russia” Siberian IPG North-West IPG

S.SMOLOVIK*

St.-Petersburg State Polytechnical University

Russia

SUMMARY

Great extension of the Russian and other CIS countries power grid and variable loading schedules

produce possibility of considerable voltage rise above the rated values during minimal loading because

of excessive reactive power generated by line capacitance. High power losses are there due to that in

networks. The application of controllable shunt compensation by magnetically controlled shunt

reactors is considered for improving the characteristics (normal operation, small disturbance andtransient stability) of 500 kV long-distance transmission systems. First 500 kV, 180 MVA

magnetically controlled shunt reactors are currently considered to be installed at the 1000 km AC

transmission systems, such as line between Ural and Siberia regions (Russia) as well as Kazakhstan

North-South Interconnections. The proposed type of AC transmission system may be used as a reliable

interconnection between the distant power systems. It has proved to have very good properties in

damping low frequency inter-area oscillations when using appropriate control systems.

Voltage stabilizing by MCSR enlarges the static stability margins and does not deteriorate oscillatory

stability even in cases of considerable voltage deviation gains. It may be recommended to add

frequency control signals to improve MCSR impact on the static stability Such control is similar to

that used for generator excitation control known as “forced control” and based on the use as control

signals not the parameter deviation only but their derivatives as well.MCSR control provides the possibility of transmission line operation with acceptable stability margin

at both one-direction and reverse energy transfer. The MCSR may be considered as one of the simplest

and cheapest devices of FACTS technology.

At present there are already six MCSR of 110, 220, and 330 kV (4 u 25, 100 and 180 MVA

respectively) in operation in the ex-USSR power systems. Three 500 kV, 180 MVA MCSR are

considered to be installed on the 1000 km AC transmission line between Ural and Siberian energy

pools. Three others are to be installed on the two-circuit 500 kV transmission line from North

Kazakhstan to the Southern part of the country and further to the Central Asia (1500 km).

KEYWORDS: Transmission Line, Magnetically Controlled Shunt Reactor, Voltage Control, Power

System Stability.

*[email protected] 1

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1 INTRODUCTION

The conventional shunt reactors are one of the most important long-distance transmission system

elements without which its normal operation is a matter of considerable technical difficulties [1]. At

the same time, conventional reactors can make a negative impact on system operation through

increased active power losses. The main disadvantage of conventional reactors is possible operational

problems due to lack of switching ability. Preventing overvoltages results in demand of reactors

operation regardless of transmitted power amount which leads to reduce transfer capability. The

installation of controllable consumer of reactive power at the intermediate point of transmission

system gives the advantage of line sectioning and increase of transmission line capability (with proper

voltage control). The reactive power consumed by reactors in any transmission line mode can be

coordinated with the power flow through the line. Transfer capability in such case is only limited by

permissible current through the conductors [2]. It allows to eliminate using of some other complicated

and expensive devices such as synchronous condenser or TCSC [3, 4]. Magnetically controlled shunt

reactors (MCSR) are the perspective devices for reactive power shunt compensation in EHV long-

distance transmission lines. Using CSR allows

x to control maintenance of voltage or any other operation parameter without using circuit

breakers in automatic switching systems;x to decrease active power losses in networks and to improve their operational reliability by

reducing the number of switching in on-load tap-changing transformers;

x to enlarge small signal stability margin;

x to improve power system damping;

to minimize using of synchronous generators as a controlled sources of reactive power. The

application of controllable shunt compensation by example of magnetically controlled shunt reactors is

considered in this paper. It is well-known that 110, 220 and 330 kV MCSRs of this type are in

operation (25, 100 and 180 MVA, respectively) in ex-USSR power systems (PermEnergo, Siberia,

Belarus). Moreover, the first 500 kV, 180 MVA MCSRs are currently considered to be installed at the

1000 km AC transmission lines, one of which will connect Ural (G1) and Siberian (G2) energy pools

(Fig. 1) and another one is Kazakhstan North-South Interconnection. Because of strict technical

requirements from power system operators it has to be ensured the wide range of operating conditions – direct transmission from Siberian power system to Ural and North Kazakhstan ones as well as

reverse operation with different amount of energy transfer. In most cases, the problem of reactive

power shunt compensation is coupled with operator plans (in North-West part of Russia, Siberia,

Kazakhstan and some other ex-USSR regions) of parallel line construction for improving power

supply reliability.

Fig. 1. Circuit representation for stability study.

2. OPERATION PRINCIPLES

The magnetically controlled shunt reactor is a three-phase powerful extension of magnetic

amplifier with inverse-parallel connection of control windings (Fig. 2), which allows todecrease the power of these windings considerably (by the factor of 100÷1000). The MCSR

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has steel magnetic core with the main winding (UHV) and control one (UC). The latter is fed by

power electronics controlled rectifier providing variable DC superposed magnetization

current. In terms of small disturbance stability, it means the increment of MCSR equivalent

time constant Tp (up to 3-4 sec.). At the same time, it can be decreased in special cases by

application of magnetic field forcing for short period down to Tp = 0,1 s. When control voltage is

equal to zero reactor stays in one of the fixed operating modes, for instance idling (I), rated load (II),rated overload (III). Increasing or decreasing of the phase current is accomplished by corresponding

hange of the control voltage UC.c

Fig. 2. Operation principles of MCSR.

NS OF LONG-DISTANCE TRANSMISSION LINES WITH

ONTROLLED SHUNT REACTORS

r transferred

on at every intermediate point of transmission system (tripping this reactor can cause

balance at every substation (SS) will

3. THE OPERATING CONDITIO

C

Evaluation of shunt compensation amount required to be installed is based on calculations of reactive

power balance at intermediate (without generator emf) points of transmission system when

maintaining some predetermined voltage profile along the line and varying active powe

through the line from zero to transfer capability (or in the range of operating boundaries).If transmission system is only equipped by conventional reactors (SR), voltage profile along the line

depending on operating conditions can be obtained taking into consideration the number of reactors in

operation (it is supposed SR rated power to be equal to 180 Mvar). It is considered at least one reactor

to be in operati

overvoltages).

In another extreme case, if transmission system is only equipped by controlled reactors (MCSR),

voltage profile can be represented as almost straight line in a wide range of operating conditions

(actually, from zero to transfer capability) since reactive power

be equal to zero when applying MCSR terminal voltage control.

Installation of controlled shunt compensation devices can be based on the rate of reactive power

unbalance change at every intermediate point of transmission system depending on active power transferred through the line (Fig. 3, SS1 and SS2). If required amount of reactive power shunt

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compensation is weakly dependent on active power variation (Fig. 3., SS3 and SS4), installation of

180 Mvar conventional reactors will be sufficient condition to provide system normal operation (one

operation, the number of which should be equal to the integer number of ticks in Fig. 3. The value of

tick at Fig. 3 corresponds to rated power of one 180 Mvar reactor).

Regarding the real case of mixed installation (conventional and controlled devices), voltage

maintenance in wide range of active power transfer can be ensured by being reactors (SR) in

0.3 0.4 0.5 0.6 0.7 0.8 0.9-1

-0.8

-0.6

-0.4

-0.2

0

, p.u.

QCSR

P

, p.u.

L

SS 1

SS 2

SS 3

SS 4

Fi e

intermediate substations of transmission system

active power by regulating its

account MCSR control action and subsequent natural

ting

onditions without switching conventional reactors at least 10-20 percent (up to 1550-1600 MW).

. LONG-DISTANCE TRANSMISSION SYSTEM SMALL DISTURBANCE STABILITY

ment. Generally, CSR control law for small signal stability investigations

can be expressed as follows

g. 3. Evaluation of reactive power balance at som

reactive power consumed by CSR will match difference between reactive power unbalance shown in

Fig. 3 and power of 180 Mvar conventional reactors in operation. For that reason, the installation of

only one controlled shunt compensation device is enough to guarantee perfect voltage maintenance

despite switching large amounts of reactive power (180 Mvar conventional reactor). After switching

conventional reactor, the controlled one will compensate unbalance of re

total conductivity on the basis of MCSR terminal voltage control mode.

Moreover, evaluation of transmission system normal operation without switching of conventional

reactors should be done (Fig. 4) taking into

voltage drop when MCSR is out of operation.

In off-peak conditions (Fig. 4, P L = 400-700 MW), when additional reactive power consumption isrequired, SS1 terminal voltage does not exceed permissible value (1,05 p.u. or 525 kV). But if both

CSRs at substations SS1 and SS2 are out of operation (Fig. 4, P L = 1100-1350 MW), terminal voltage

drop is increased considerably. Minimum permissible voltage level (0,95 p.u. or 475 kV) corresponds

to maximum permissible active power transferred through the line (1350 MW). From this point,

switching of conventional reactors is needed to provide normal voltage profile along the line. It means

that switching of conventional reactors can be completely eliminated from all range of transmission

system operating conditions due to applying controlled shunt compensation devices. It can be noted

that installation of second controlled device at substation 1 (SS1) can increase the range of opera

c

4

The reactive shunt compensation is one of the most effective way of energy pool operating condition

and stability control improve

,1 1

00 p

u

u p p p pT ¸

¹¨©

where b

1 1u U p K

K bb pT '¸ ·

¨§

(1)

time constant. Moreover, some additional

p, b p0 are actual and initial (at predetermined operation) MCSR conductivity; K 0u, K 1u are

terminal voltage deviation U p and derivative control gains; T p – equivalent time constant of MCSR

control system; T 1u

– voltage derivative control loop

improvements of control law (1) will be discussed below.

The quality indices of 500 kV long-distance transmission system with conventional shunt reactors for

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reactive power compensation are exclusively determined by equivalent generator excitation control

(AVR+PSS). It is clear that very low damping of some electromechanical modes (Table 1) results

from extremely high capacity of interconnected power systems as well as large distances of

ansmission systems themselves (1000 km and more).tr

ional voltage control (a) by controlled shuFig. 4. Operat nt reactor (b)without switching conventional reactors.

can be noted that oscillatory stability indices are weakly dependent

on operating conditions (Table 1).

TRANSMI WITH CONVENTIONAL REACTORS

Practice of calculations shows that transmission capacity limits of long-distance lines with

conventional reactors correspond to aperiodic instability (real positive eigenvalue). The impact of

reactors in normal operations consists in maintaining voltage at intermediate substations in the range

of 475-525 kV by discontinuous switching of large amounts of reactive power shunt compensation

(±180 Mvar). At the same time, it

TABLE 1.

STEADY-STATE STABILITY INDICES OF

ISION SYSTEM

P L = 0,7 p.u. P L = 0,85 p.u.

-0.56562+1.7376i

-0.45146+9.4234i

-0.15

-0.0345

-0. 2i

-0.027384

617+6.2555i

-0.50756+1.562i

-0.44632+9.3596i

14762+6.16

The following key functions of MCSR voltage deviation control system as well as generator excitation

1. in voltage at CSR terminals in the range of 0,995-1,005 p.u. (if control accuracy is equal

olled shunt compensation devices with considerable

regulators (AVR+PSS) can be identified:

to mainta

to 1%);

2. to decrease the impact of transmitted power value on real eigenvalues nearest to the stability limit.

Application of terminal voltage deviation control in MCSR as it is in generator AVR without power

system stabilizer can cause system poor damping or even oscillatory instability. The results of

calculation shown in Table 2 demonstrate that long-distance transmission system oscillatory stability

does not deteriorate when embedding contr

voltage deviation gains (down to K 0u = -100).

The eigenvalues nearest to oscillatory stability limit (e.g.,

-0.15664+6.2519i, Table 2, second column) have practically the same values as it was when usingconventional reactors (Table 1). But aperiodic stability index of the system with conventional reactors

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is changed by more than 20 percent (from -0.0345 to -0.027384) when transmitted power growth is

equal to about 15 percent (from 0,7 p.u. to 0,85 p.u.). On the other hand, stability margin of the

systems with MCSR is located on the same level (-0.033698 in comparison with -0.03262) even when

transmitted power is increased 65 percent (from 0,3 p.u. to 0,85 p.u.).

TRANSMIISION SYSTEM WITH CONTROLL

P K 0u = -100

P ;

K 0u = -100

P .;

TABLE 2.STEADY-STATE STABILITY INDICES OF

ED REACTORS

L

L = 0,3 p.u.; L = 0,85 p.u.

= 0,85 p.u

K 0u = -10

with additional

frequency control

-0.4741+9.626i

-0.36416+2.0459i

-0.1 5i

-0.033698

-0.1 19i

-0.03262

-0.3 6i

-0.034946

6935+6.389

-0.46418+9.5615i

-0.41166+1.9022i

5664+6.25

-0.44704+9.5725i

-0.35337+2.0384i

1665+6.472

The equal damping curves shown in Fig. 5 can be referred to the first principle described above. It is

to be noted that maximum damping variation is equal to 0,016 whereas equivalent time constant T p is

varied from 0,01 to 20 sec. Obviously, from small disturbancel stability point of view, there is no

reason to reduce this constant by special expensive countermeasures.

a) b)

2 4 6 8 10 12 14 16 18 20

2

4

6

8

10

12

14

16

18

20

Tp R1, sec.

Tp R2, sec.

- 0. 1 4 0

3

- 0 .1 4 1

- 0 .141

-0.1 41-0.141

- 0

. 1 4 3

- 0 . 1

4 3

- 0 .143 -0.143 -0.143

- 0 . 1 4 3

- 0 . 1 4 3

- 0 .14 3

- 0

. 1 4 6

- 0

. 1 4

6

- 0 .146 -0.146 -0.146

- 0 . 1 4 6

- 0 .1 4 6 -

0 .

1 5

- 0 . 1 5

-0.15 -0.15 -0.15

- 0

. 1

5 3

- 0

. 1 5 3

- 0 . 1 5 3

-0.153 -0.153

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Tp R1, sec.

Tp R2, sec.

- 0 .15 6

- 0 .1 5 6

- 0 .1 5 6

- 0 .1 5 5

- 0 .1 5 5

- 0 .15 5

- 0 .1 5 4

- 0 .1 5 4

- 0 .1 5 4

- 0 .1 5 3

- 0 .1 5 3

- 0 .1 5 2

ig. 5. Impact of CSR control system equivalent time constanton oscillatory stF ability of

t on small signal stability with voltage deviation and derivative control concept (1)

The line current control mode can be expressed as follo

transmission system. a) T p in a range of 0 – 20 s, b) T p in a range of 0 – 1 s.

In some cases [5, 6], the application of CSR line current control is considered. It is interesting to

compare its impac

described above.

ws

1 0 IL p p p K bb pT L I ' , (2)

ge deviation control gain K 0u and line current one K IL

where K IL is line current deviation ', L control gain.

It can be followed that the sign of line current control gain K IL must be opposite to the sign of voltage

deviation control gain K 0u. When short circuit occurs, terminal voltage decreases, while line current

grows. MCSR control action have to be identical in both cases. Reactor conductivity is needed to

diminish down to zero. For that reason, volta

have to be negative and positive, respectively.

Fig. 6, b illustrates the aperiodic stability region (dotted line, the system has stable equilibrium

inside fourth quadrant and the part of second one) and the curves of equal damping (oscillatory

stability) on the plane of line current control gains( K 0U

and K 1U

for Fig.6,a). It turned out, that higher

quality indices can be obtained with line current control (in comparison with case of using voltage

deviation and derivative control mode as it is shown in Fig. 6, a), but control gains have to be negative

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(e.g., K IL R1 = -1 and K IL R2 = -1, Fig. 6, c), which is not matched with system requirements for MCSR

behavior during large disturbances considered afterwards. On the other hand, taking small positive

gains from the aperiodic and oscillatory stability region ( K IL R1 = 0,1 and K IL R2 = 0,1) makes the reactor

s show that reactor conductivity can never be reduced to zero when applying so

all control gains.a)

to be practically uncontrollable.

Computer simulation

sm

-20 -15 -10 -5 0 5 10 15 20-10

-8

-6

-4

-2

0

2

4

6

8

10

K0u R1

K1u R1

-0.4

-0.4

- 0 . 4

-0.4

-0.4

- 0. 4

- 0

. 4

- 0 .4 - 0 . 4 2

- 0

. 4 2

- 0. 4 2

-0.42

-0.42

- 0 . 4 2

-0.42

-0.42

- 0 . 4 4

- 0

. 4 4

- 0 . 4 4

-0.44

-0.44

- 0

. 4 4

-0.44

-0.44

- 0. 4 6

- 0. 4 6

- 0 . 4

6

- 0

. 4 6

-0.46

-0.46

- 0 . 4 6

-0.46

-0.46

- 0 . 4 8

- 0 . 4 8

- 0. 4 8

-0.48

-0.48

-0.48

- 0 . 4 8

-0.48

- 0 .4 8

-0.5

-0.5

-0.5-0.5

- 0 .5

- 0. 5

- 0 .

5

-0.5

- 0. 5

- 0 .5 2

- 0. 5 2

-0.52

-0.52- 0 . 5 2

- 0. 5 2

- 0 . 5 2

- 0. 5 4

-0.54

-0.54

-0.54

- 0 . 5

4

- 0. 5 6

- 0 . 5 6

- 0 .5 6

- 0 . 5 6

- 0 .5 8

- 0 .5 8

- 0 .5 8

- 0

. 5 8

0

0

0

0

b) c)

-10 -8 -6 -4 -2 0 2 4 6 8 10

-10

-8

-6

-4

-2

0

2

4

6

8

10

KIL R1

KIL R2

0

0

0

0 0

0

0

0 00

0 0

- 0

. 5

- 0

.5

- 0

. 5

-0.5

- 0

. 5

- 0 . 5

-0.5

-0

. 5

- 0

. 5- 0 .5

- 0

. 6

- 0

. 6

-0.6

- 0

. 6

- 0

. 6

- 0

. 6

- 0 . 6

-0.6

- 0

. 7

- 0

. 7

-0.7

- 0 . 7

- 0 .7 -0.7

- 0 . 7

- 0

. 7

2

- 0 . 7

2

-0.72

- 0 . 7 2

- 0 .7 2

-0.72

- 0

. 7

4

- 0

. 7 4

-0.74

- 0 .7 4

- 0 . 7 4 - 0 .7 4

-0.74

- 0

. 7

6

- 0

. 7 6

-0.76

- 0 . 7 6

- 0 . 7 6

- 0 . 7 6

- 0. 8

- 0

. 8

- 0 .8

- 0 . 9

-0.9

- 1

-1

0

00 0

0

0

-1.5 -1 -0.5 0 0.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

KIL R1

KIL R2- 0 .6

- 0 .6

- 0 . 7

- 0 . 7

- 0

. 7

- 0 .7

- 0 .7

- 0 .7

- 0 . 7 2

- 0 . 7 2

- 0

. 7 2

- 0 .7 2

- 0 .7 2

- 0 .7 2

- 0 . 7 4

- 0 . 7 4

- 0 . 7 4

- 0 .7 4

- 0 .7 4

- 0 .7 4

- 0 .7 6

- 0 . 7 6

- 0 . 7 6

- 0 .7 6

- 0 .7 6

- 0 .7 6

- 0 . 7 8

- 0 .7 8

- 0 .7 8

- 0

. 7

8

- 0 . 7 8

- 0 .7 8

- 0 . 7 9

- 0 .7 9

- 0 . 7 9

- 0 .7 9 - 0 . 8

- 0 . 8

transmission systems with voltage deviation tive (a) and line current (b, c) control mode

most

es only for small

cuit tripping)

has shown good influence of MCSR operation on damping of the post-fault oscillations.

USIONS

n small-signal and transient stability of 500 kV long-distance transmission systems is considered.

Fig. 6. Curves of equal damping (solid lines) and aperiodic stability region (dotted lines) for

and deriva

MCSRs.

Commonly used inputs of synchronous generator power system stabilizer in Russia are the derivative

of terminal voltage, the deviation and derivative of terminal frequency and the derivative of field

current. It has shown [6] that terminal frequency (both deviation and derivative) modes are

applicable for a purpose of enhancing damping of power oscillations through excitation control.

The result of terminal frequency deviation control mode implementation in MCSR to enhance

transmission system damping is shown in the third column of Table 2. As it is realized in generator

excitation systems, it can be recommended to apply additional frequency control mod

perturbations with blocking them when terminal voltage is out of range 475÷525 kV.

Investigation of 500 kV long-distance transmission system transient stability for the most severe faults

(double phase-to-ground one with subsequent unsuccessful auto-reclosing and faulted cir

5. CONCL

Impact of various MCSR control systems o

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1. Installation and tuning of controlled shunt compensation devices in combination with using

equivalent synchronous generator excitation control allows to decrease the impact of transmitted

power value on real eigenvalues and, therefore, to enlarge aperiodic stability margin.

2. Oscillatory stability of long-distance transmission system does not deteriorate when embedding

controlled shunt compensation devices with considerable voltage deviation gains whereas MCSR

terminal voltage quality can be improved substantially.

3. Application of voltage frequency (both proportional and derivative) control of MCSR allows to

obtain the acceptable stability margin even without generator PSS optimization.

4. The control gains selected can be recommended for wide range of transmission system operating

conditions (both direct and reverse power transfer).

5. It has been proved on the basis of stability region construction that the MCSR equivalent time

constant T p in the range of 0,01-20 sec. does not influence on system properties considerably. So

there is no reason to reduce this constant by special expensive countermeasures.

6. The application of MCSR line current control system does not give the priority to MCSR voltage

control since its stability region is too narrow, which makes difficult to choose the appropriate

control gains in the wide range of operating conditions. This is the main reason why the using of

CSR line current control can not be recommended for transmission systems with reversing energy

transfer.7. The positive impact of MCSR voltage and frequency control on transient stability of transmission

system is confirmed. Taking into account the actual control signal constraints, MCSR conductivity

variation is sufficiently high to provide reliable damping.

BIBLIOGRAPHY

[1] Bernard, S.; Trudel, G.; Scott, G. A 735 kV shunt reactors automatic switching system for Hydro-

Quebec network (IEEE Trans. on Power Systems, Vol. 11 , No. 4 , Nov. 1996. pp. 2024–2030)

[2] Belyaev A.N., Smolovik S.V. An improvement of AC electrical energy transmission system with

series compensation by implementation of Controllable Shunt Reactors (Proceedings of IEEE

PES PowerTech 2003, Bologna, Italy)[3] Gama, C. Brazilian North-South Interconnection control-application and operating experience

with a TCSC (IEEE Power Engineering Society Summer Meeting, 18-22 July 1999, Vol. 2, pp.

1103–1108)

[4] Gerin-Lajoie, L.; Scott, G.; Breault, S.; Larsen, E.V.; Baker, D.H.; Imece, A.F. Hydro-Quebec

multiple SVC application control stability study (IEEE Trans. on Power Delivery, Vol. 5, No. 3,

July 1990, pp. 1543–1551)

[5] Evdokunin G.A., Ragozin A.A., Seleznev Yu.G. New technical solution to the problems of long-

distance AC power transmission lines (9th International Power System Conference, St.

Petersburg, 1994)

[6] Kashin I.V., Smolovik S.V. The AC long-length transmission lines operation stability with

controllable source of reactive power shunt compensation (Elekrtrichestvo, 2001, No. 2, pp.8-15

(in Russian))

[7] Single phase tripping and auto reclosing of transmission lines. IEEE Committee Report (IEEE

Trans. on Power Delivery, Vol. 7, No. 1, January 1992, pp. 182–192)

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