Gerdau plant in Charlotte, US

An Hitachi Energy SVC Light^® rated at 13.2 kV, 0-64 Mvar (capacitive) has been installed at the Gerdau plant in Charlotte, N.C., USA operating an electric arc furnace (EAF) with continuous charging for scrap-based steel production. The EAF, rated at 30/33 MVA, as well as a ladle furnace (LF) rated at 18 MVA are taking their
power from a 100 kV supply grid. The fast control of SVC Light will improve the power quality and particularly reduce the flicker levels generated. As a valuable by-product, improved productivity and decreased specific operating costs of the EAF can be attained. SVC Light is extremely well tested, with close to 15 years of track record as a flicker compensator.

The case
For many years, Gerdau in Charlotte has been operating an EAF of continuous scrap charging type in their industrial process. In this type of furnace, the cold scrap is fed into the side of the furnace from a conveyor system where it is immersed into liquid steel, with continuous melting taking place (Fig. 1). However, during recent years the surroundings of the steel plant have been developed into both a business and a residential area. Due to this the power utility has set up more strict requirements on mitigation of flicker. For the EAF
operated with continuous scrap charging, the highest flicker peaks occur during the initial period when the EAF is started with cold scrap charged into a cold furnace, referred to as top charge. During that time, unless remedied, particularly annoying flicker can be expected.

Previously, the steel plant ran a conventional SVC, based on thyristor control. The main purpose of that compensator was to stabilize the AC bus voltage during EAF operation and keep up the steel production, but also to provide flicker mitigation. From a flicker point of view, however, the performance of a conventional SVC was no longer sufficient to meet the new requirements.

The EAF is fed from a double 100 kV sub-transmission circuit. Various scenarios for improved flicker mitigation were considered, such as re-conductoring the existing 100 kV lines, building a dedicated 3rd 100 kV line to the plant, or replacing the existing SVC with a STATCOM, a faster, more powerful flicker mitigating device than a conventional SVC. As a result, it was decided to dismantle the existing SVC and install a STATCOM instead, which was found to be the most attractive solution. Hitachi Energy´s trade name for STATCOM is SVC Light, based on a three-level voltage source converter (VSC) design, utilizing IGBT (insulated-gate bipolar transistors) as switching elements. Hitachi Energy was awarded the order and supplied an SVC Light to the steel plant, commissioned at the end of 2006.