MULTI-LEVEL CONTROL OF A STATIC SYNCHRONOUS COMPENSATOR COMBINED WITH A SMES COIL FOR APPLICATIONS ON PRIMARY FREQUENCY CONTROL MARCELO G. MOLINA, PEDRO E. MERCADO Consejo Nacional de Investigaciones Científicas y Técnicas - CONICET, Instituto de Energía Eléctrica - IEE, Universidad Nacional de San Juan - UNSJ Av. Lib. Gral. San Martín Oeste 1109 - J5400ARL - San Juan - Argentina E-mails: mgmolina@iee.unsj.edu.ar, pmercado@iee.unsj.edu.ar Abstract At present, the advance of technology makes possible to include new energy storage devices in the electric power system. These energy storage systems can be incorporated into power electronic devices based on Flexible Alternating Current Transmission Systems (FACTS) using switching power converters. This combined system permit to independently exchange active and reactive power with the utility grid. In this way, a very effective primary frequency control can be carried out, which allows to reduce the generation reserve. This article proposes a multi-level control algorithm for a Static Synchronous Compensator (STATCOM) combined with Superconducting Magnetic Energy Storage (SMES) used for controlling the frequency of the power system. This multi-level control configuration allows dividing a complex control system into three comparative simpler subsystems, which are easier to design. A detailed model of the Static Synchronous Compensator with Superconducting Magnetic Energy Storage system is also presented. Dynamic systems simulation is used to study the performance of the proposed controls and models. Keywords Primary Frequency Control, FACTS, STATCOM, Energy Storage, SMES, Modeling, Multi-level Control. 1 Introduction One of the most important requirements during the operation of the power system is the operation security. This concept is related to the system capability of maintaining operational in case of an unexpected failure of some components, by providing totally or partially the customers load demanded. From this, the need of having available enough “short-term generation reserve” is derived in order to maintain acceptable security levels. This reserve must be appropriately activated by means of the primary frequency control (PFC) in order to keep during the transient the power system frequency above the admissible minimum level. Otherwise, serious problems could occur in the utility system that may lead to the system collapse. Nowadays, the new energy storage systems (ESS) are a feasible alternative to carry out a very effective PFC and to decrease the primary spinning reserve. By using appropriate energy storage devices, excess energy may be stored to substitute the generation reserve during the action of the primary frequency control. In this sense, research has been extended with the aim of incorporating power electronics devices into electric power systems. The goal pursued is to control the operation of the power system, a fact which clearly affects the operation security. In bulk power transmission systems, power electronics-based controllers are frequently called Flexible AC Transmission Systems (FACTS). Presently, these devices are a viable alternative as they allow controlling voltages and currents of appropriate magnitude for electric power systems at an increasingly low cost (Hingorani, 2000). While the FACTS/ESS combination has been proposed in theory (IEEE, 1996), the development of this FACTS/ESS combination has lagged far behind that of FACTS alone. Significant interest has been given to developing control strategies for a variety of FACTS devices in order to mitigate a wide range of power transmission problems (Song and Johns, 1999). However, a comparable field of knowledge on FACTS/ESS control is quite limited. Consequently, this work proposes a methodology to control the frequency of the power system, which uses FACTS controllers combined with ESS. This can be carried out by using power converter-based FACTS controllers. Among the different variants of FACTS devices, Static Synchronous Compensators (STATCOMs) are proposed as the most adequate for the present application (Hingorani, 2000). The DC inner bus of the STATCOM allows incorporating substantial amount of energy storage in order to enlarge the degrees of freedom of the STATCOM controller by exchanging active and reactive power with the utility grid. Based on a previous study of all energy storage technologies currently available (Molina and Mercado, 2003), the use of Superconducting Magnetic Energy Storage systems (SMES) has been suggested for the considered application. The current article proposes a three-level control scheme for applications of the STATCOM/SMES system on the primary frequency control of the electric utility system. In this way, a complex control system design is divided into three quite simpler control designs, which are easier to solve. Furthermore, a detailed model of the whole system is presented.