【正文】 sed. Index Terms —Dynamic Compensators, SVC, STATCOM, Short Circuit Capacity, Gain Sensitivity, Short Term Voltage Stability, Harmonics I. INTRODUCTION SHUNTconnected static var pensators (SVCs) are used extensively to control the AC voltage in transmission works. Power electronic equipment, such as the thyristor controlled reactor (TCR) and the thyristor switched capacitor (TSC) have gained a significant market, primarily because of wellproven robustness to supply dynamic reactive power with fast response time and with low maintenance. With the advent of high power gate turnoff thyristors and transistor devices (GTO, IGBT, …) a new generation of power electronic equipment, STATCOM, shows great promise for application in power systems [3,4]. This paper aims to explain the benefits of SVCs and STATCOMs for application in utility power systems. Installation of a large number of SVCs and experience gained from recent STATCOM projects throughout the world motivates us to clarify certain aspects of these devices. II. BASIC DESCRIPTION This section explains briefly the basic configuration of SVCs and STATCOMs: A. SVC The pensator normally includes a thyristorcontrolled reactor (TCR), thyristorswitched capacitors (TSCs) and harmonic filters. It might also include mechanically switched shunt capacitors (MSCs), and then the term static var system is used. The harmonic filters (for the 2 TCRproduced harmonics) are capacitive at fundamental frequency. The TCR is typically larger than the TSC blocks so that continuous control is realized. Other possibilities are fixed capacitors (FCs), and thyristor switched reactors (TSRs). Usually a dedicated transformer is used, with the pensator equipment at medium voltage. The transmission side voltage is controlled, and the Mvar ratings are referred to the transmission side. The rating of an SVC can be optimized to meet the required demand. The rating can be symmetric or asymmetric with respect to inductive and capacitive reactive power. As an example, the rating can be 200 Mvar inductive and 200 Mvar capacitive, or 100 Mvar inductive and 200 Mvar capacitive. B. STATCOM The voltagesourced converter (VSC) is the basic electronic part of a STATCOM, which converts the dc voltage into a threephase set of output voltages with desired amplitude, frequency, and phase. There are different methods to realize a voltagesourced converter for power utility application. Based on harmonics and loss considerations, pulse width modulation (PWM) or multiple converters are used. Inherently, STATCOMs have a symmetrical rating with respect to inductive and capacitive reactive power. For example, the rating can be 100 Mvar inductive and 100 Mvar capacitive. For asymmetric rating, STATCOMs need a plementary reactive power source. This can be realized for example with MSCs. VII. THE FUNCTIONAL RATING CONCEPT Traditionally, SVCs of a mon design have been used to handle different types of work problems. The trend today, however, is to tailor SVCs for their intended use. This is important in order to make SVCs cost efficient. For steady state voltage support, ., to follow the daily load pattern, bulk reactive power bined with stepless smooth voltage control is desired. Vernier voltage regulation can be provided by a TCR running in parallel with harmonic filters. The bulk reactive power is provided by mechanically switched capacitor banks (MSCs) or reactors (MSRs) governed by the SVC controller. Thus SVCs serve the purpose of continuously maintaining a smooth voltage, piloting the MSC switching. If the task is to support a system limited by post contingency voltage instability