Power Flow Control with Static Synchronous Series Compensator (SSSC) Abdul Haleem Chandra babu Nayudu Ravireddy M Project Manager College of engineering pune Project Developer Krest technologies, cbn.203@gmail.com ravireddy57@gmail.com haleemforyou@gmail.com

Abstract- The series compensation technique of long and medium transmission lines is

extensively employed in many countries including India as it offers considerable advantages and

better use of transmission lines. It can also be a technique in improving power system stability

and power flow through the intended transmission network. However, technical problems such

as reliability of capacitors and their protective equipments do exist; and more recently the

problem of sub synchronous resonance (SSR) has surfaced. To remove these drawbacks,

recently a series compensation technique for transmission line which uses a synchronous

voltage source (SVS). The static synchronous voltage source utilizes a power electronic voltage

source (VSC) converter employing GTO or IGBT depending upon power requirements. The VSC

may employ a two level or multilevel converter. In this paper a static synchronous series

compensator (SSSC) using a 6-pulse VSC employing sinusoidal pulse width modulation is

examined. The steady state performance and P-δ characteristics are obtained for a given

transmission network embedded with SSSC. A control circuit for the operation of SSSC is

developed and the performance of the control circuit is investigated in MATLAB-SIMLINK

Platform.

Keywords— 6-pulse VSC, SSSC, FACTS, Power Flow Control, Series compensation

1. Introduction Series capacitive compensation is

widely used in long transmission

lines to maintain the overall

impedance of the transmission

line. The capacitive series

compensation increases the power

transfer capacity as well as the

transient stability. The series

dielectric capacitors have been

installed all over the world as

efficient economical way of

providing capacitive series

compensation [1]. With the new

advances in the generation of the

power electronics devices based on

voltage source converter (VSC)

known as flexible ac transmission

system (FACTS), more flexible

operation and control of the

transmission networks are possible

[2]. FACTS controllers can be

classified as shunt, series, or phase

angle compensating devices or

devices which are a combination of i

the above three types such as

unified power flow controller

(UPFC) [3]. These FACTS devices

enable fast response using the

phase locked loop (PLL) with

minimum inherent time delay

during severe disturbances,

transient power swings, thus

allowing the transmission system

operating safely and close to the

theoretical stability limit. Two

FACTS devices can provide capacitive

series compensation, they are :(1) thyristor

controlled series capacitor (TCSC) [4] and

(2) static synchronous series compensator

[5,6].

There are several TCSCS are widely

installed [7]. The TCSC is used in practice

to significantly improve the small

disturbance and transient stability of the

power system [8,9]. Although the TCSC can

provide the capacitive series compensation,

it has several disadvantages [10]. It injects

low order harmonic components (typically

third, fifth, seventh and ninth) into the

power system because of phase control of

the thyristors [2]. Transient response of the

circuit is rather slow, because of controlling

thyristor firing pulse is available only once

in each half cycle. Deriving a closed-loop

model of TCSC is complicated [11].

Furthermore, it is susceptible to parallel

resonance due to the presence of inductors

and capacitors in parallel paths.

The SSSC is one of the most important

FACTS devices for power transmission line

series compensation. It is a power

electronic-based VSC that generates a

nearly sinusoidal three phase voltage which

is in quadrature with the line current

[3,12].The SSSC converter block is

connected in series with the transmission

line by a series coupling transformer. The

SSSC can provide either capacitive or

inductive series compensation independent

of the line current. Unlike other series

compensators, an ideal SSSC is essentially a

pure sinusoidal ac voltage source at the

system fundamental frequency. Its output

impedance at other frequencies is ideally

zero. Thus, SSSC does not resonate with the

inductive line impedance to initiate sub

synchronous resonance oscillations. This

paper deals with a 6 pulse (two levels) VSC

[13].

The objective of this paper is to analyze

and investigate the steady state performance

of the SSSC for providing dynamic series

compensation, voltage regulation. A control

circuit is proposed for the operation of the

SSSC. The proposed control scheme for the

SSSC is fully validated in both capacitive

and inductive modes of operation by

simulation.

2. Priciple of Operation of SSSC

ii

The SSSC is generally connected in

series with the transmission line with the

arrangement as shown in Fig.1. The SSSC

comprises a coupling transformer, a

magnetic interface, voltage source

converters (VSC) and a DC capacitor. The

coupling transformer is connected

in series with the transmission line

and it injects the quadrature

voltage into the transmission line.

The magnetic interface is used to

provide multi-pulse voltage

configuration to eliminate low

order harmonics.

Fig.1 static synchronous series compensator

The VSCs are either two-level

converter or three level converter.

One side of the VSC is connected

to the magnetic interface while the

other side is connected to the DC

bus. The VSC generates six-pulse

voltage waveform and it is

combined into multi-pulse (12

pulses) voltage waveform by Wye-

Delta connection of the magnetic

interface. More pulses (24 or 36

pulses) can be achieved if zigzag

transformers are used as the

magnetic interface. The DC

capacitor is used to maintain DC

voltage level on the DC bus. This

DC capacitor is selected to meet

harmonic and economic criteria of

the SSSC and the power system.

Figure.2 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 [3].

Fig.2 an Elementary Power Transmission System

The real and reactive power (P and Q) flow

at the receiving-end voltage source are

given by eq (1) and (2)

(1)

(2)

Where Vs and Vr voltage magnitudes and

are the phase angles of the

voltage sources. The voltage magnitudes are

chosen such that Vs = Vr =V and the

iii

difference between the phase angles is

.

An SSSC, limited by its voltage and current

ratings, is capable of emulating a

compensating reactance, Xq, (both inductive

and capacitive) the expression of power

flow given in equation (1) and equation (2)

becomes

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 (Vq) (Buck or Boost) by

the SSSC. The compensating reactance is

defined to be negative when the SSSC is

operated in inductive mode and positive

when SSSC operated in capacitive mode.

Fig.3 shows an example of a simple power

transmission system with an SSSC and the

related phasor diagrams.

Fig.3 Two machine system with SSSC

Fig.4 Phasor diagram

The SSSC injects the compensating

voltage in series with the line irrespective of

the line current. The transmitted power Pq

therefore becomes a parametric function of

the injected voltage and it can be expressed

as follows:

The normalized power Pq versus angle

plots are shown in Fig.4.6 as a function of

Vq These values are calculated for the

system whose specifications are given

earlier in A Programme in MATLAB has

been developed to obtain these

iv

characteristics for Vq= 0, 0.353, 0.707 and

these are shown in Fig.5

0 20 40 60 80 100 120 140 160 180-1

-0.5

0

0.5

1

1.5

2

TRANSMISSION ANGLE (DEGREES)

TR

AN

SM

ITT

ED

PO

WE

R (

p.u

)

Vq=0.707

Vq=0.353

Vq=0Vq=-0.353

Vq=-0.707

Fig.5 Transmitted power versus transmission angle

as a function of the degree of series compensating

voltage Vq by the SSSC

From the plots given Fig.5 we can say that

the SSSC increases the transmitted power

by a fixed fraction of the maximum power

transmittable by the uncompensated line,

independently of transmission angle and

SSSC not only increase the transmittable

power but also decreases it.

The transmittable active power, P, and the

reactive power, Q, supplied by the receiving

end bus can be expressed for the simple

two-machine system as functions of the

(actual or effective) reactive line impedance,

XL the line resistance, R, and transmission

angle, as follows:

P= [ sin -R (1-Cos )]

Q= [Rsin + (1-Cos )]

The normalized active power P and reactive

power Q versus angle transmission

characteristics described by equations and

are plotted as a parametric function of the

XL/R ratio for 7.4, 3.7, 1.85 in Fig.6 These

values are calculated for the system whose

specifications are given earlier. A

Programme in MATLAB has been

developed to obtain these characteristics for

XL/R = ∞, 3.7, 7.4, 1.85 and these are

shown in Fig.6 and the Programme is given

in Appendix-3

Fig.6 Transmitted real and reactive power versus

transmission angle as a function of ratio of

These plots clearly show that the maximum

transmittable active power decreases, and

the ratio of active to reactive power

increases, rapidly with decreasing XL/R

ratio.

Control circuit

Introduction An advanced control scheme is

introduced by Akagi [4] used for SSSC. The

development of this control scheme is

discussed in this chapter.

Development of Control circuit for SSSC

v

Fig.7 System Configuration of SSSC

The following assumptions are made in the analysis

1) The sending-end voltage is equal to

the receiving-end voltage

2) The SSSC device is assumed to be an ideal controllable voltage source. Output

voltage vector is equal to its reference

3) The three phase voltages at sending end

are balanced

Fig.7 shows a block diagram of the

control circuit [4]. The three- to two-phase

transformation obtains and from the

three-phase currents and. The d-q

transformation yields and from

and the phase information is generated

by a phase lock-loop (PLL)

Fig.8 Control circuit for SSSC

The injected voltage is independent of the

line current and controlled by using the

pulse width modulation switching

techniques. The voltage source converter

uses PWM switching techniques to ensure

fast response and to generate a sinusoidal

wave form. The output of The PLL is angle,

θ, which is used to transform the direct axis

and quadrature axis components of the ac

three phase voltages and current. The

measured quadrature voltage is compared

with the desired reference constant

quadrature voltage to the input of the AC

voltage regulator which is a PI controller.

Thus the voltage regulator provides the

quadrature component of the converter

voltage. Also the Measured direct axis

component voltage is compared with the

reference voltage; this driven error is an

input to the voltage regulator which is a PI

controller to compute the direct component

of the converter voltage. The injection

voltage is generated by transforming these

vi

direct axis and quadrature axis components

into three phase voltage and is applied to the

VSC to produce the preferred voltage, with

the help of pulse width modulation (PWM).

Simulation results

Simulation of the SSSC is performed in

MATLAB SIMULINK using the Akagi’s

control technique.

Steady state characteristics of SSSC [4].

Fig.9 Simple system taken for simulation

Fig.9 shows the simple system taken for

simulation. The main circuit of the SSSC

device consists of three phase voltage-fed

pulse width modulation (PWM) inverters. A

PWM control circuit compares reference

voltage with a triangle carrier signal in order

to generate gate signals. The ac terminals of

the PWM inverters are connected in series

through step-up transformers because

injecting voltage is very small compare to

transmission line voltage. A three-phase

diode rectifier is employed and reactor L

and resistor R representing the impedance of

the transmission line are inserted between

sending end and receiving end. DC

capacitor used for the charging and

discharging purpose. The function of the

control system is to keep the injecting

voltage in quadrature with the transmission

line current and only control the magnitude

of injected series reactance to meet the

desired reactance compensation level.

Fig.9 shows simulation model used for the

steady state performance of the SSSC

Fig.10 Static Synchronous Series Compensator Model in MATLAB

Fig.11 injecting voltage

The fig.11 shows that the injecting

voltage of the SSSC and this injected

voltage will be in quadrature with the line

current. The SSSC can provide either

capacitive or inductive series compensation

independent of the line current. By

controlling the magnitude of injected

voltage the amount of series compensation

can be adjusted.

When an SSSC injects an altemating voltage

lagging the line current as shown in the

Fig.12, it emulates a capacitive reactance in

vii

series with the transmission line causing the

power flow as well as the line current to

increase as the level of compensation

increases and then SSSC is operating in a

capacitive mode. The emulating capacitive

reactance of 0.22 ohms.

0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 0.7-100

-80

-60

-40

-20

0

20

40

60

80

100

Time in seconds

Capacitiv

e com

pensation

voltage (V)

current (A)

Fig.12 SSSC Operating in Capacitive Mode (Capacitive Compensation)

The emulating reactance value calculated by

using following relation is .where

Vq is the rms value of the injecting voltage

and I is the current flowing in the line (rms

value).

When an SSSC injects an alternating

voltage leading the line current as shown in

the fig.13, it emulates an inductive reactance

in series with the transmission line

causing the power flow as well as the

line current to decrease as the level of

compensation increases and the SSSC is

operating in an inductive mode. The

emulating inductive reactance of 2 ohms

2.04 2.05 2.06 2.07 2.08 2.09 2.1-40

-30

-20

-10

0

10

20

30

40

Time in seconds

Inductive

Com

pensation

voltage (V)

current (A)

Fig.13 SSSC Operating in Inductive Mode (Inductive Compensation)

0 0.5 1 1.5 2 2.5 3-1.5

-1

-0.5

0

0.5

1

1.5x 10

4

Time in Seconds

Inje

cte

d A

ctive P

ow

er

(Watt

)

Fig.14 Performance of a SSSC Operating in Capacitive Mode (Capacitive Compensation) and Inductive Mode (Inductive Compensation) in the case of injected Active Power

0 0.5 1 1.5 2 2.5 3-1500

-1000

-500

0

500

1000

1500

Time in Seconds

Inje

cte

d R

eactive P

ow

er

(VA

R)

Fig.15 Performance of a SSSC Operating in

Capacitive Mode (Capacitive Compensation) and

Inductive Mode (Inductive Compensation) in the

case of Injected Reactive Power

Fig 14 and 15 shows the simulation

results when an SSSC emulates a reactance

in series with the transmission line. At the

time 0 seconds, the SSSC injects no voltage.

At 0.2 seconds, capacitive reactance

compensation is requested. The injecting

voltage lags the line current, by almost 900.

Due to the capacitive reactance there is an

increase in the line current and the power

flow in the transmission line increases. At

0.8 seconds coming into the no injected

state. The time interval between 0.8 to 1.6

seconds SSSC does not inject any voltage.

At 1.6 seconds, the inductive reactance is

requested. The inverter voltage leads the

line current, by almost 900. Due to the

inductive reactance there is a decrease in the

line current and the power flow in the

transmission line. At 2.5 seconds it’s again viii

coming into the no injected state so it does

not emulates any reactance.

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

2.5x 10

4

Time in Seconds

Lin

e A

ctive P

ow

er

(Watt

)

]

Fig.16 Performance of a SSSC Operating in

Capacitive Mode (Capacitive Compensation) and

Inductive Mode (Inductive Compensation) in the

case of Line Active Power

0 0.05 0.1 0.15 0.2 0.25-100

-80

-60

-40

-20

0

20

40

60

80

100

Time in seconds

Voltage (

V)

Curr

ent

(A)

voltage

current

Fig.17 injected voltage and line current

0 0.5 1 1.5 2 2.5 30

500

1000

1500

2000

Time in Seconds

Lin

e R

eactive P

ow

er

(VA

R)

Fig.18 Performance of a SSSC Operating in

Capacitive Mode (Capacitive Compensation) and

Inductive Mode (Inductive Compensation) in the

case of Line Reactive Power

In the fig.16 at the time 0 seconds, the

SSSC did not emulate any reactance

compensation. At 0.2 seconds, capacitive

reactance compensation is requested. Due to

the capacitive reactance there is an increase

in the line current and the power flow in the

transmission line increases from 12 kW to

22 kW. At 0.8 seconds coming into the no

injected state. The time interval between 0.8

to 1.6 seconds SSSC does not injecting any

voltage. At 1.6 seconds, the inductive

reactance is requested. Due to the inductive

reactance there is a decrease in t the power

flow in the transmission line from 12 kW to

2 kW. At 2.5 seconds it’s again coming into

the no injected state so it does not emulates

any reactance.

Therefore, from the figures 16 and 18

when an SSSC emulates a reactance in

series with the transmission line, the power

flow in the transmission line always

decreases if the emulated reactance is

inductive. Also, the power flow always

increases if the emulated reactance is

capacitive.

The parameters of the test system

Controllable Power rating (P) =10 kW

Utility line to line Voltage=200V

Line inductance (L) = 1.0 mH

Line resistance (R) = 0.04 ohm

Frequency = 60 Hz

Phase difference=100

Rms voltage of Vc =12V

PI controller gains areKp =0.5Ki = 100 2 Level inverter employing IGBTCapacitor = 200 µF IGBT Snubber resistance = 1 x105 ohm

Snubber capacitance = ∞

On resistance of IGBT = 1 x10-4 ohm

Conclusion

The static synchronous series

compensator offers an alternative to

conventional series capacitive line

ix

compensation. Whereas the series capacitor

is an impedance that produces the required

compensating voltage as the line current

flows through it, the SSSC is a solid-state

voltage source that internally generates the

desired compensating voltage. However the

voltage is in quadrature to line current

(Leading or lagging as per requirement)

independent of the line current. The voltage

source nature of the SSSC provides the

basis for its superior operating and

performance characteristics not achievable

by series capacitor type compensators.

References

1. N.G Hingroni and L Gyugyi.

“Understanding FACTS: Concepts and

Technology of flexible AC Transmission

System”, IEEE Press, New York, 2000.

2. L.Gyugyi, C. D. Schauder, K. K. Sen.

“Static synchronous series compensator: a

solid-state approach to the series

compensation of transmission lines,” IEEE

Trans. on Power Delivery, vol. 12,no. 1,

1997,pp. 406-417.

3. K.K.Sen, “SSSC-static synchronous series

compensator: theory, modeling and

applications”, IEEE Trans. On Power

Delivery, v.13, no.1, 1998, pp.241-246.

4. Hideaki Fujita, Yasuhiro Watanabe

Hirofumi Akagi “Control and Analysis of a

Unified Power Flow Controller” IEEE

Trans.On Power Electronics vol.14, no.6,

November 1999, pp. 1021-1027.

5. Hideaki Fujita, Yasuhiro Watanabe

Hirofumi Akagi “Dynamic Performance of

a Unified Power Flow Controller for

Stabilizing AC Transmission Systems”IEEE

Trans.On Power Electronics

Vol.14,No.6,November 1999,pp.81-87.

6. B.Geetalakshmi, A.Saraswathi,

P.Dananjayan “Comparing and evaluating

the performance of SSSC with Fuzzy Logic

controller and PI controller for Transient

Stability Enhancement” Proceeding of India

International Conference on Power

Electronics 2006.

7. M.S. El-Moursi, A.M. Sharaf, “Novel

reactive power controllers for the

STATCOM and SSSC”, Electric Power

Systems Research 76 (2006) 228-241.

8. Mohammed El Mours A.M.Sharaf

KhalilEl-Arroud “Optimal control schemes

for SSSC for dynamic series compensation”

Electric Power Systems Research 78 (2008)

646–656.

9. Bruce S. Rigby and Ronald G. Harley ‘An

Improved Control Scheme for a Series-

Capacitive Reactance Compensator Based

on a Voltage-Source Inverter” IEEE

Transactions on industry

applications,vol.34,no.2, march/april1998.

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Student Member, “Development of a control

scheme for a Series-Connected Solid-State

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No. 2, April 1996.

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