31-08-2012, 10:53 AM
Flexible AC Transmission Systems (FACTS) technology
Introduction
Overview
Modern electric power utilities are facing many challenges due to ever-increasing complexity in their operation and structure. In the recent past, one of the problems that got wide attention is the power system instabilities. With the lack of new generation, transmission facilities and over exploitation of the existing facilities geared by increase in load demand make these types of problems are more imminent in modern power systems.
In recent years, several major phenomena have been observed and reported in many Countries such as France, Belgium, Sweden, Germany, Japan, the United States, etc. These phenomena usually result in widespread blackouts. Information gathered and preliminary analysis, so far, from the most recent blackout incident in North America on 14th August 2003, are pointing the finger on voltage instability due to some unexpected contingency. Even though it is premature to make it as a conclusive remark, the voltage instability could have had a major role in the incident as has been the case in the past in major blackout incidents. In this incident, reports indicate that approximately 50 million people interrupted from continuous supply for more than 15 hours. Most of the incidents are believed to be related to heavily stressed system where large amounts of real and reactive power are transported over long transmission lines while appropriate real and
reactive power resources are not available to maintain normal system conditions. Many electric utilities have made lot of efforts in system study in order to relieve the system from stability problem.
Unified Power Flow Controller (UPFC)
Combining the static compensator and the synchronous series capacitor into a single device with a common control system represents the third generation of FACTS. The device is known as “Unified Power Flow Controller (UPFC)”. It has the unique ability to simultaneously control all three parameters of power flow (voltage, line impedance and phase angle). In this configuration, the series-capacitor component, connected in series with a line, injects an AC voltage with controllable magnitude and phase angle. The static compensator component, connected as a shunt, supplies or absorbs the real power demanded by the series capacitor through a common DC link, and provides var control. The first utility demonstration of a Unified Power Flow Controller is being constructed at the Inez substation of American Electric Power in 1998. Recently, 80 MVA UPFC were being constructed at Gangjin substation in South Korea. Table 2.1. below shows the list of UPFC.
The Principle of Operation
The UPFC is the most versatile FACTS controller developed so far, with all encompassing capabilities of voltage regulation, series compensation, and phase shifting. It can independently and very rapidly control both real- and reactive power flows in a transmission line. It is configured as shown in Fig. 2.2 and comprises two VSCs coupled through a common dc terminal. One VSC—converter 1—is connected in shunt with the line through a coupling transformer; the other VSC—converter 2—is inserted in series with the transmission line through an interface transformer. The dc voltage for both converters is provided by a common capacitor bank. The series converter is controlled to inject a voltage phasor, Vpq, in series with the line, which can be varied from 0 to Vpq max. Moreover, the phase angle of Vpq can be independently varied from 0 to 360. In this process, the series converter exchanges both real and reactive power with the transmission line. Although the reactive power is internally generated/ absorbed by the series converter, the real-power generation/ absorption is made feasible by the dc-energy–storage device—that is, the capacitor.
Introduction
Overview
Modern electric power utilities are facing many challenges due to ever-increasing complexity in their operation and structure. In the recent past, one of the problems that got wide attention is the power system instabilities. With the lack of new generation, transmission facilities and over exploitation of the existing facilities geared by increase in load demand make these types of problems are more imminent in modern power systems.
In recent years, several major phenomena have been observed and reported in many Countries such as France, Belgium, Sweden, Germany, Japan, the United States, etc. These phenomena usually result in widespread blackouts. Information gathered and preliminary analysis, so far, from the most recent blackout incident in North America on 14th August 2003, are pointing the finger on voltage instability due to some unexpected contingency. Even though it is premature to make it as a conclusive remark, the voltage instability could have had a major role in the incident as has been the case in the past in major blackout incidents. In this incident, reports indicate that approximately 50 million people interrupted from continuous supply for more than 15 hours. Most of the incidents are believed to be related to heavily stressed system where large amounts of real and reactive power are transported over long transmission lines while appropriate real and
reactive power resources are not available to maintain normal system conditions. Many electric utilities have made lot of efforts in system study in order to relieve the system from stability problem.
Unified Power Flow Controller (UPFC)
Combining the static compensator and the synchronous series capacitor into a single device with a common control system represents the third generation of FACTS. The device is known as “Unified Power Flow Controller (UPFC)”. It has the unique ability to simultaneously control all three parameters of power flow (voltage, line impedance and phase angle). In this configuration, the series-capacitor component, connected in series with a line, injects an AC voltage with controllable magnitude and phase angle. The static compensator component, connected as a shunt, supplies or absorbs the real power demanded by the series capacitor through a common DC link, and provides var control. The first utility demonstration of a Unified Power Flow Controller is being constructed at the Inez substation of American Electric Power in 1998. Recently, 80 MVA UPFC were being constructed at Gangjin substation in South Korea. Table 2.1. below shows the list of UPFC.
The Principle of Operation
The UPFC is the most versatile FACTS controller developed so far, with all encompassing capabilities of voltage regulation, series compensation, and phase shifting. It can independently and very rapidly control both real- and reactive power flows in a transmission line. It is configured as shown in Fig. 2.2 and comprises two VSCs coupled through a common dc terminal. One VSC—converter 1—is connected in shunt with the line through a coupling transformer; the other VSC—converter 2—is inserted in series with the transmission line through an interface transformer. The dc voltage for both converters is provided by a common capacitor bank. The series converter is controlled to inject a voltage phasor, Vpq, in series with the line, which can be varied from 0 to Vpq max. Moreover, the phase angle of Vpq can be independently varied from 0 to 360. In this process, the series converter exchanges both real and reactive power with the transmission line. Although the reactive power is internally generated/ absorbed by the series converter, the real-power generation/ absorption is made feasible by the dc-energy–storage device—that is, the capacitor.