Voltage Stability Improvement using Static Var Compensator in Power Systems

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VOLTAGE STABILITY IMPROVEMENT USING STATIC VAR COMPENSATOR IN POWER SYSTEMS

 

CHAPTER ONE

INTRODUCTION

 

The focus of this research is on the application of Static Var Compensator to solve voltage regulation and system dynamic performance deficiencies. SVC is a mature thyristor- based controller that provides rapid voltage control to support electric power transmission voltages during and immediately after major system disturbances.

Since the advent of deregulation and the separation of generation and transmission systems in the electric power industry, voltage stability and reactive power-related system restrictions have become an increasingly growing concern for electric utilities. With  deregulation came an “open access” rule to accommodate competition that requires utilities to accept generation and load sources at any location in the existing transmission system. This “open access” structure has challenged transmission owners to continually maintain system security, while at the same time trying to minimize costly power flow congestion in transmission corridors. When voltage security or congestion problems are observed during the planning study process, cost effective solutions must be considered for such problems. Traditional solutions to congestion and voltage security problems were to install new costly transmission lines that are often faced with public resistance, or mechanically-switched capacitor banks that have limited benefits for dynamic performance due to switching time andfrequency.

One approach to solving this problem is the application of “Flexible AC transmission System” (FACTS) technologies, such as the Static Var Compensator (SVC). FACTS technologies are founded on the rapid control response of thyristor-based reactive power controls.

Over the last several years, there were numerous installations of FACTS in the United States and around the world [1] [2]. FACTS have proven to be environmental friendly and cost- effective solutions to a wide range of the power system needs. FACTS have given utilities the option to delay new transmission line construction by increasing capacity on existing lines and/or

providing dynamic control and compensation of the system voltages [3] [4] [5]. FACTS controllers are available in different forms such as static VAR compensators (SVCs), thyristor controlled series capacitors (TCSCs), static reactive compensators (STATCOMs), and unified power flow controllers (UPFCs).

Some of the technical/economic “attractiveness” of SVC are highlighted in the Table 1-1.

 

 

Table 1-l. Comparison of Var Compensation Methods Incorporating Both Technical and Economic Merits (extracted from [11])

The Static Var Compensator is the first generation of FACTS devices that has been in use in transmission systems worldwide since the 1970’s and in North America since the late 1970’s. Figure 1-1 presents the approximate number of transmission SVC installed in both North America and worldwide.

From Figure 1-1, a hyporesearch can be drawn that from 1998 to 2003 the North American SVC market was significantly stagnant most likely due to the uncertainties of the impact deregulation in the power industry. Then in 2004 deregulation in the electric utility industry became better implemented, and SVC offered a cost-effective and environmentally friendly solution to system problems that could in some cases delay the need for new transmission line.

The installation of transmission lines typically requires clearing of vegetation and trees from the area under/near the transmission line towers or poles.

North American SVC Installations

 

25

 

 

 

20

 

 

 

15

 

 

 

10

 

 

 

5

 

 

 

0

 

Installation Year

 

Worldwide SVC Installations

 

25

 

 

 

20

 

 

 

15

 

 

 

10

 

 

 

5

 

 

 

0

 

Installation Year

 

Figure 1-1.Approximate number of Transmission SVC installations from 1970 to 2006.

(based on 2004 List compiled by IEEE Working Group I4 on SVC and other manufacturers data)

The SVC provides an excellent source of rapidly controllable reactive shunt compensation for dynamic voltage control through its utilization of high-speed thyristor switching/controlled reactivedevices.

An SVC is typically made up of the following major components:

  • Couplingtransformer
  • Thyristorvalves
  • Reactors
  • Capacitors (often tuned for harmonicfiltering)

In general, the two thyristor valve controlled/switched concepts used with SVCs are the thyristor-controlled reactor (TCR) and the thyristor-switched capacitor (TSC). The TSC  provides a “stepped” response and the TCR provides a “smooth” or continuously variable susceptance.

CB
CB
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CB

Two “common” main SVC circuit arrangements shown in Figure 1-2 are: “FC/TCR”–fixed capacitor(filter)/thyristor(phase angle)-controlled reactor (Config A) “TSC/TCR”–thyristor-switched capacitor/thyristor-controlled reactor (Config B)

 

 

Figure 1-2. Common SVC main configurations.

An SVC is a controlled shunt susceptance (B) as defined by the SVC control settings that injects reactive power (Q) into the system based on the square of its terminal voltage. Figure 1-3 illustrates a TCR/FC SVC, including the operational concept. The control objective of the SVC is to maintain a desired voltage at the high-voltage bus. In the steady-state, the SVC will provide some steady-state control of the voltage to maintain it the high-voltage bus at a pre-definedlevel.

If the high-voltage bus begins to fall below its setpoint range, the SVC will inject reactive power (Qnet) into the system (within its controlled limits), thereby increasing the bus voltage back to its desired voltage level. If bus voltage increases, the SVC will inject less (or TCR will absorb more) reactive power (within its controlled limits), and the result will be to achieve the desired bus voltage. From Figure 1-3, +Qcap is a fixed capacitance value, therefore the magnitude of reactive power injected into the system, Qnet, is controlled by the magnitude of -Qind reactive power absorbed by theTCR.

SVC Controller / Automatic Voltage Regulator (AVR)
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High Voltage Bus

Figure 1-3. SVC with control concept briefly illustrated.

Pulse Generator

The fundamental operation of the thyristor valve that controls the TCR is described here. The thyristor is self commutates at every current zero, therefore the current through the reactor is achieved by gating (or firing) the thyristor at a desired conduction angle (or firing angle) with respect to the voltage waveform. Figure 1-4 describes the relationship between the fundamental frequency TCR current and firing angle.

     
     
     
     
     

 

1

0.8

0.6

0.4

0.2

0

90                                      120                                    150                                    180

Firing  Angle (deg.) (a)

TCR Current versus Firing Angle Charac t eristic s

Figure 1-4. Illustration of the relationship between TCR current and firing angle.

Figure 1-5 further illustrates the thyristor valve operating characteristics of a thyristor- controlled reactor. The firing pulses are on the order of 10 µs. So it is concluded that as the  firing angle increases above 90 degrees, the current in the TCR is reduced. Referring back to Figure 1-3, the “Pulse Generator” block after the AVR block utilizes the concepts discussed here and illustrated in Figures 1-4 and 1-5 to determine the firing angle for the thyristor valve controlling thereactor.

Figure 1-5. Illustration of the relationship between TCR current and firing angle (or conduction angle).

This research gives details of an 87 Mvar, 115 kV SVC installed in a transmission system. The SVC will effectively solve the voltage regulation problem in the study area and delay the costly construction of a new 40 mile, 230 kV transmission line. Budgetary cost of an 87 Mvar SVC is around $8 million, where a 40 mile 230 kV transmission line is approximately $19million.

 

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