13-11-2012, 05:13 PM
Reactive Power Compensation of Transmission Lines
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General Introduction
During the past two decades, the increase in electrical energy demand has presented higher requirements from the power industry. More power plants, substations, and transmission lines need to be constructed. However, the most commonly used devices in present power grid are the mechanically-controlled circuit breakers. The long switching periods and discrete operation make them difficult to handle the frequently changed loads smoothly and damp out the transient oscillations quickly. In order to compensate these drawbacks, large operational margins and redundancies are maintained to protect the system from dynamic variation and recover from faults. This not only increases the cost and lowers the efficiency, but also increases the complexity of the system and augments the difficulty of operation and control. Severe black-outs happened recently in power grids worldwide and these have revealed that conventional transmission systems are unable to manage the control requirements of the complicated interconnections and variable power flow.
Shunt compensation
Shunt compensation, especially shunt reactive compensation has been widely used in transmission system to regulate the voltage magnitude, improve the voltage quality, and enhance the system stability [5]. Shunt-connected reactors are used to reduce the line over-voltages by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage levels by compensating the reactive power to transmission line.
A simplified model of a transmission system with shunt compensation is shown in Figure 1.2(a). The voltage magnitudes of the two buses are assumed equal as V, and the phase angle between them is δ. The transmission line is assumed lossless and represented by the reactance XL. At the midpoint of the transmission line, a controlled capacitor C is shunt-connected. The voltage magnitude at the connection point is maintained as V.
Series compensation
Series compensation aims to directly control the overall series line impedance of the transmission line. Tracking back to Equations (1-1) through (1-5), the AC power transmission is primarily limited by the series reactive impedance of the transmission line. A series-connected can add a voltage in opposition to the transmission line voltage drop, therefore reducing the series line impedance.
A simplified model of a transmission system with series compensation is shown in Figure 1.3(a). The voltage magnitudes of the two buses are assumed equal as V, and the phase angle between them is δ. The transmission line is assumed lossless and represented by the reactance XL. A controlled capacitor is series-connected in the transmission line with voltage addition Vinj. The phase diagram is shown in Figure 1.3(b)
Flexible AC Transmission System (FACTS)
The history of FACTS controllers can be traced back to 1970s when Hingorani presented the idea of power electronic applications in power system compensation. From then on, various researches were conducted on the application of high power semiconductors in transmission systems. The shunt-connected Static VAR compensator (SVC) using solid-state switches and the series-connected controllers were proposed in AC transmission system application. In 1988, Hingorani defined the FACTS concept and described the wide prospects of the application in [6]. Nowadays, FACTS technology has shown strong potential. Many examples of FACTS devices and controllers are in operation
As presented in [7].
Converter-based Compensator
Static Synchronous Compensator (STATCOM) is one of the key Converter-based Compensators which are usually based on the voltage source inverter (VSI) or current source inverter (CSI), as shown in Figure 1.5(a). Unlike SVC, STATCOM controls the output current independently of the AC system voltage, while the DC side voltage is automatically maintained to serve as a voltage source. Mostly, STATCOM is designed based on the VSI.