Unified Power Flow Controllers Without Energy Storage: Designing Power Controllers for the Matrix Converter Solution

Abstract

In the last years the growing economic, environmental and social concerns have increased the difficulty to use fossil fuels, as well as to obtain new licenses to build either transmission lines (right-of-way) or high power facilities. This led to the continuous growth of decentralized electricity generation (using renewable energy resources) (Hingorani, 2000). This scenario has introduced new problems and technical challenges to power systems researchers and electricity markets participants. One of the main consequences of these changes has been the substantial increase of power transfer within transmission networks, approaching their rated capacity and requiring adequate control capability to supply the continuously growing demand of electric power. To solve these issues Flexible AC Transmission Systems (FACTS) became a well known power electronics based solution to control power flow in transmission lines. These systems are switching controlled converters that operate in real time increasing the transmission lines power flow capacity up to their thermal limits. Currently, Unified Power Flow Controllers (UPFC) are the most versatile and complex FACTS enabling accurate and reliable control of both active and reactive power flow over networks, through load sharing between alternative line paths (Song et al , 1999). The original UPFC concept was proposed by L. Gyugyi (Gyugyi, 1992), and consisted of the combination of a Static Synchronous Compensator (STATCOM) and a Static Synchronous Series Compensator (SSSC) connected by a common DC link, using large high-voltage DC storage capacitors. The AC converters sides of these compensators are connected to a transmission line, through coupling transformers, in shunt and series connection with the line. This arrangement operates as an ideal reversible AC-AC switching power converter allowing shunt and series compensation and bidirectional power flow, between the AC terminals of the two converters. The DC capacitor bank used in the UPFC topology to link the two back-to-back converters increases the UPFC weight, cost, occupied area and introduces additional losses. Replacing the double three-phase inverter by one three phase matrix converter the DC link capacitors are eliminated, reducing costs, size, maintenance, and increasing reliability and lifetime. The