Design and Implementation of a 154-kV $\pm$ 50-Mvar Transmission STATCOM Based on 21-Level Cascaded Multilevel Converter

In this research work, the design and implementation of a 154-kV $\pm$ 50-Mvar transmission static synchronous compensator (T-STATCOM) have been carried out primarily for the purposes of reactive power compensation and terminal voltage regulation and secondarily for power system stability. The implemented T-STATCOM consists of five 10.5-kV $\pm$12-Mvar cascaded multilevel converter (CMC) modules operating in parallel. The power stage of each CMC is composed of five series-connected H-bridges (HBs) in each phase, thus resulting in 21-level line-to-line voltages. Due to modularity and flexibility of implemented HBs, each CMC module has reached a power density of 250 $\hbox{kvar/m}^{3}$, thus making the mobility of the system implementable. DC-link capacitor voltages of HBs are perfectly balanced by means of the modified selective swapping algorithm proposed. The field tests carried out at full load in the 154-kV transformer substation where T-STATCOM is installed have shown that the steady-state and transient responses of the system are quite satisfactory.

I. Introduction

Flexible ac transmission systems are being increasingly used in power systems, to enhance the system utilization and power transfer capacity as well as the stability, security, reliability, and power quality of ac system interconnections. The static synchronous compensator (STATCOM) converters used in transmission system applications should control a few tens or hundreds of megavars, operate at medium voltage (MV) levels, and are connected to extrahigh-voltage or high-voltage (HV) transmission system buses via specially designed coupling transformers. These are called transmission STATCOMs (T-STATCOMs) in the literature.