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CHAPTER 3

TRANSIENT STABILITY ENHANCEMENT IN A

REAL TIME SYSTEM USING STATCOM

3.1 INTRODUCTION

The modeling of the real time system with STATCOM using

MiPower simulation software is presented in this chapter. The system is

analyzed under severe disturbance to study the transient behavior by

simulating three phase to ground fault at various buses. To enhance the

transient stability of the system, STATCOM is inserted and tested to show the

effect of the same on the transient stability under severe disturbance. The

swing curve in degree, real power in MW, reactive power in MVAr and the

voltage in p.u. of 11 kV generator bus(11) and the voltage in p.u. for 11 kV

bus(14), 33 kV bus(6) and 110 kV grid bus(1) are taken for analysis. The

potential application of STATCOM on the improvement of voltage profile of

the various buses and reduction in rotor angle oscillation of the generator is

evaluated from the implementation results.

3.2 LITERATURE REVIEW

STATCOM includes the GTO and diode valves together with their

necessary snubber circuits that make a valuable tool in the power system

design process. The STATCOM is given this name because in a steady state

operating regime, it replicates the operating characteristics of a rotating

synchronous compensator (Gyugyi 1998 and Hingorani and Gyugyi 2000).

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Abido (2005) presented a singular value decomposition (SVD) based

approach to assess and measure the controllability of the poorly damped

electromechanical modes by the different control channels of STATCOM. It

is observed that the electromechanical modes are more controllable via phase

modulation channel. It is also concluded that the STATCOM-based damping

stabilizers extend the critical clearing time and enhance greatly the power

system transient stability. Haque (2004) demonstrated that by the use of

energy function, the STATCOM is capable to provide additional damping to

the low frequency oscillations. The damping characteristics of STATCOM

have also been analyzed and addressed, where different approaches to

STATCOM-based damping controller design have been adopted, such as

loop-shaping (Rahim et al 2002), pole placement (Lee and Sun 2002),

multivariable feedback linearization (Sahoo et al 2002, 2004), H control(Al-Baiyat 2005) and intelligent control (Morris et al 2003).

3.3 PROBLEM STATEMENT

Transient stabilization is an important factor in the power system

control. The key factor in transient stability prediction is the way in which the

transient swings either converges or diverges. It is important to prevent

generators from losing synchronism and damping the subsequent oscillations

quickly. The problem is formulated as the insertion of STATCOM in real

time system that is to be analyzed using MiPower simulation software for

enhancing the transient stability.

3.4 GENERAL REPRESENTATION OF STATCOM

The basic electronic block of a STATCOM is a VSC that converts a

DC voltage at its input terminal into a three-phase set of AC voltages at

fundamental frequency with controllable magnitude and phase angle. The

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VSC can be made up of three-phase, two-level, six pulse converters

connected by an appropriate magnetic circuit into a multi-pulse structure to

meet the practical harmonic, current and voltage rating requirements; or three-

phase, three-level, twelve-pulse converters in a multi-pulse structure; or

simply PWM controlled three phase, two-level converters. A STATCOM has

no inertia and can basically act in a fraction of a second, which is an

advantage over a synchronous compensator. Furthermore, STATCOM does

not significantly alter the existing system impedance, which gives it an

advantage over the SVC. In all STATCOM applications implemented in

transmission systems so far, only two of these methods have been used; PWM

are still considered uneconomical due to high switching losses of available

semiconductor switches with intrinsic turn-off capabilities. A STATCOM can

be used for voltage regulation in a power system, having as an ultimate goal

the increase in transmittable power, improvements of steady state

transmission characteristics and of the overall stability of the system. Under

light load conditions, the controller is used to minimize or completely

diminish line overvoltage; on the other hand, it can also be used to maintain

certain voltage levels under heavy loading conditions. In its simplest form, the

STATCOM is made up of a coupling transformer, a VSC, and a DC energy

storage device. The energy storage device is a relatively small DC capacitor,

and hence, the STATCOM is capable of only reactive power exchange with

the transmission system. If a DC storage battery or other DC voltage sources

are used to replace the DC capacitor, the controller can exchange real and

reactive power with the transmission system, extending its region of operation

from two to four quadrants. A functional model of a STATCOM is shown in

Figure 3.1. A STATCOM can support system voltage at extremely low

voltage conditions as long as the DC capacitor can retain enough energy to

supply losses. The STATCOM also has increased transient ratings in both

capacitive and inductive regions.

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Figure 3.1 Functional model of a STATCOM

The overload capability is about 20% for several cycles in both

regions. It is also worth noticing that the inductive transient current rating is

slightly larger due to the fact that the GTOs in the inductive region are

naturally commutated and hence, the amount and duration of this temporary

overload capability is limited by the maximum current of the free-wheel

diode. The capacitive transient rating is determined by the maximum current

turn-off capability of the GTO thyristors.

3.4.1 Modeling of STATCOM

STATCOM model shown in Figure 3.2, used to improve the

transient stability of the real time system (real time system is discussed in

section 2.6) considered, has been modeled in MiPower6.0. STATCOM is

modeled using FPB available in MiPower. This FPB is executed to verify the

correctness of the model. FPB is converted into FPD and this FPD is called as

a function for the simulation of transient stability.

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Figure 3.2 Modeling of STATCOM

K K

sT

K1Vref [1]

+ N[2]

K=2.23 K=0.9416

K=0.0584

T = 0.015

+

Max = -0.529

Min = 0.9959

STATCOM Q

Supply

N[14]N[9]N[6]

N[7]

N[5]

N[3]

N[16]

N[17]N[10]

N[13]

K = 1 K = 1

T=0.025 T = 0.01 Terminal Voltage [12]

N[4]+ N[18]

sT

K1sT

K1

-

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3.5 RESULTS WITH DISCUSSIONS

The effect of STATCOM on transient stability of power system is

analyzed by creating three phase to ground fault at 11 kV bus(14) and 33 kV

bus(6) using MiPower. The transient stability is considered with a fault

initiated at 1 s and cleared at 1.1 s. i.e., protection system cleared the fault

within 100 ms. Transient stability is executed upto 10s to view the response.

By inserting the STATCOM in the 33 kV bus(6) for the fault at 11 kV

bus(14) and in the 11 kV bus(14) for the fault at 33 kV bus(6), the responses

are presented. Swing curve and voltages are taken at different conditions like

normal, at fault and after the insertion of FACTS controllers.

During the steady state condition (without any disturbance) and

fault condition (with disturbance) for the fault at 11 kV bus(14), the various

responses of the system are discussed in sections 2.7.1 and 2.7.2.

3.5.1 Transient Stability of System with STATCOM at 33 kV Bus(6)

The STATCOM is placed at 33 kV bus(6) of the system to improve

the voltage and to enhance the transient stability. Three phase to ground fault

is simulated for a period of 100 ms in 11 kV bus(14). The swing curve and

voltage of 11 kV generator bus(11) and voltage of 110 kV grid bus(1) and 33

kV bus(6) are presented below.

STATCOM at 33 kV bus(6), for a three phase to ground fault of

100 ms duration at 11 kV bus(14), leads the generator to oscillate from 3.5 to

9 degrees (during disturbance its value ranges from 3 to 9.6 degrees) whereas

at the steady state condition, it is 6.3 degrees with respect to grid. This is

depicted in Figure 3.3.

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Figure 3.3 Swing curve of generator at fault condition (with STATCOM

at 33 kV bus(6))

Figure 3.4 110 kV grid bus(1) voltage in p.u. at fault condition

(with STATCOM at 33 kV bus(6))

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STATCOM at 33 kV bus(6), for three phase fault at 11 kV bus(14),

significantly contributes the reactive power and improves the voltage profile.

Since 110 kV grid bus(1) is far away from STATCOM at 33 kV bus(6), the

voltage profile improves from 0.96 p.u. to 0.983 p.u. This is depicted in

Figure 3.4.

Figure 3.5 33 kV bus(6) voltage in p.u. at fault condition (with

STATCOM at 33 kV bus(6))

STATCOM at 33 kV bus(6), for three phase fault at 11 kV bus(14),

significantly contributes the reactive power and improves the voltage profile.

Since the STATCOM is connected in the same bus, the impact on voltage

profile improvement is high. Even in the steady state, the STATCOM

increases the voltage profile to 0.995 p.u. and limits the voltage dip to 0.955

p.u. during fault. This is depicted in Figure 3.5.

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Figure 3.6 11 kV generator bus(11) voltage in p.u. at fault condition

(with STATCOM at 33 kV bus(6))

STATCOM at 33 kV bus(6), for three phase fault at 11 kV bus(14),

significantly contributes the reactive power and improves the voltage profile.

It limits the voltage drop from 0.98 p.u. to 0.99 p.u. This is depicted in

Figure 3.6.

Considering the STATCOM at 33 kV bus(6) for the fault at 11 kV

bus(14), it improves voltage stability to great extent by maintaining the

voltage profile during steady state and improves the voltage profile

significantly during fault. However, the impact of STATCOM on angular

stability is good.

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3.5.2 Transient Stability of System with STATCOM at 11 kV bus(14)

The transient stability is evaluated with the help of swing curve for

a three phase fault at 33 kV bus(6) for 100 ms duration. It is assumed that

protection system cleared the fault within 100 ms. Three phase fault at 33 kV

bus(6) is simulated from 1 s to 1.1 s and the response is plotted up to 10 s.

The swing curve, real power, reactive power and voltage of 11 kV generator

bus(11) and voltage of 33 kV bus(6) and 11 kV bus(14) are presented below.

Figure 3.7 Swing curve of generator at fault condition in 33 kV bus(6)

A 100 ms three phase to ground fault at 33 kV bus(6) leads the

generator to oscillate from -2 to 15 degrees whereas at the steady state

condition, it is 6.3 degrees with respect to grid. This is depicted in Figure 3.7.

The system is considered stable, for the three phase to ground fault at 33 kV

bus(6) for 100 ms duration, since the swing is well within the transient limit

of 180 degrees.

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Figure 3.8 Real power generation by generator in MW at fault

condition in 33 kV bus(6)

Whenever there is a three phase fault in the network, the real power

transfer and consumption will come down. However, mechanical input of the

generator remains constant. This will increase the accelerating torque in the

generator and in turn leads to unstable condition if not controlled properly.

Even after the clearance of fault, oscillation will not damp to zero

immediately. Since the droop characteristics of the generator are slow in

nature (high time constant), the oscillation takes longer time to damp.

Figure 3.8 indicates the oscillation of real power of the generator that varies

from 6.5 MW to 63 MW.

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Figure 3.9 Reactive power generation by generator in MVAr at fault

condition in 33 kV bus(6)

Whenever there is a three phase fault in the network, the voltage

will come down to zero at fault point and drastically reduce the voltage

profile in the vicinity of the fault. The reactive power of the generator

drastically increases from 14 MVAr to 75 MVAr during the fault without any

time delay because the exciter characteristics are fast in nature (very low time

constant). This is depicted in Figure 3.9. Also, after the fault clearance, the

voltage profile is improved to normal value.

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Figure 3.10 33 kV bus(6) voltage in p.u. at fault condition in 33 kV bus(6)

Since the fault is at this bus, the voltage is zero during fault and

recovers back to the normal after clearance of the fault. This is depicted in

Figure 3.10.

Figure 3.11 11 kV generator bus(11) voltage in p.u. at fault condition in

33 kV bus(6)

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Though the 11 kV generator bus(11) is far away from the faulty 33

kV bus(6) and two transformer impedances exist between the two buses, the

generator connected to the 11 kV generator bus(11) limits the voltage drop to

small value (from 1 p.u. to 0.97 p.u.) during fault and recovers back to normal

after clearance of the fault. This is depicted in Figure 3.11.

Figure 3.12 11 kV bus(14) voltage in p.u. at fault condition in 33 kV

bus(6)

Since the 11 kV bus(14) is far away from the faulty 33 kV bus(6)

and two transformer impedances exist between the two buses, the voltage

reduction is severe (from 1 p.u. to 0.95 p.u.) during fault and recovers back to

normal after clearance of the fault. This is depicted in Figure 3.12.

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The effect of STATCOM on transient stability of power system is

analyzed by simulating three phase to ground fault at 33 kV bus(6) using

MiPower, by inserting the STATCOM in the 11 kV bus(14). The swing curve

and voltage of 11 kV generator bus(11) and voltage of 11 kV bus(14) are

presented below.

Figure 3.13 Swing curve of generator at fault condition in 33 kV bus(6)

(with STATCOM at 11 kV bus(14))

STATCOM at 11 kV bus(14), for a three phase to ground fault of

100 ms duration at 33 kV bus(6), leads the generator to oscillate from -1 to 14

degrees (during disturbance its value ranges from -2 to 15 degrees) whereas at

the steady state condition it is 6.3 degrees with respect to grid. This is

depicted in Figure 3.13.

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Figure 3.14 11 kV generator bus(11) voltage in p.u. at fault condition in

33 kV bus(6) (with STATCOM at 11 kV bus(14))

STATCOM at 11 kV bus(14), for three phase fault at 33 kV bus(6),

significantly contributes the reactive power and improves the voltage profile.

It limits the voltage drop from 0.97 p.u. to 0.985 p.u. This is depicted in

Figure 3.14.

STATCOM at 11 kV bus(14), for three phase fault at 33 kV bus(6),

significantly contributes the reactive power and improves the voltage profile.

Since the STATCOM is connected in the same bus, the impact on voltage

profile improvement is higher and maintained at 1 p.u. This is depicted in

Figure 3.15.

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Figure 3.15 11 kV bus(14) voltage in p.u. at fault condition in 33 kV

bus(6) (with STATCOM at 11 kV bus(14))

Considering the STATCOM at 11 kV bus(14), it improves voltage

stability to great extent by maintaining the voltage profile during steady state

and improves the voltage profile significantly during fault. However, the

impact of STATCOM on angular stability is good.

The reactive power supplied by STATCOM at 11 kV bus(14) to

nearby buses 13 and 15 is depicted in the Figures 3.16 and 3.17 respectively.

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Figure 3.16 Reactive power Q supplied to bus13 (with STATCOM at

11 kV bus(14))

Figure 3.17 Reactive power Q supplied to bus15 (with STATCOM at

11 kV bus(14))

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The ability of the STATCOM to supply the reactive power during

voltage dip / drop improves the voltage profile of buses, which in turn

prevents the large motors connected to those buses to stall.

3.6 CONCLUSION

Modeling of real time system with the STATCOM using MiPower

simulation software is presented. STATCOM is modeled using MiPower for

enhancing the transient stability of the system. From the transient stability

analysis, it is observed that STATCOM improves the voltage profile of the

system for the following conditions:

i) STATCOM at 33 kV bus(6), three phase to ground fault at

11 kV bus(14)

ii) STATCOM at 11 kV bus(14), three phase to ground fault at

33 kV bus(6)

Also, the impact of STATCOM on damping the rotor angle

oscillation of generator is effective in the system considered.

Compared with the performance of SVC discussed in chapter 2, the

STATCOM provides much better control in damping the rotor angle

oscillation of the generator in addition to the improvement of voltage profile.

The damping of the rotor angle oscillation of the generator may

further be improved by increasing the inertia of the generator by means of

adding additional weight or flywheel.