Enhanced voltage quality in grids with SVC Plus

ERLANGEN, GERMANY: Siemens Energy is expanding its product spectrum of Flexible AC Transmission Systems (FACTS) with its new reactive power compensation system SVC Plus (Static Var Compensator). This system operates on the basis of innovative Voltage-Sourced Converter technology (VSC), with continuously variable control through high-power transistors (IGBT).

The result is enhanced voltage quality and stability in the power system which allows grid access of fluctuating energy sources, such as offshore wind farms. Siemens is supplying the SVC-Plus system in space-saving, standardized and modular units. These pre-tested units of ±25, ±35 or ±50 MVAr can be operated individually or in parallel.

The core of SVC Plus, an advanced Statcom (static synchronous compensator), is made up of a modular multilevel converter technology. In contrast to other self-commutated converter topologies, the voltage waveshape generated by SVC Plus is nearly sinusoidal, thanks to the multilevel technology used. This renders the low-frequency harmonic filters, used in solutions up to now, superfluous and it substantially reduces the space requirements for the overall unit.

The SVC Plus converter consists of a number of capacitive voltage sources connected in series, thus giving the unit its name -– multilevel voltage-sourced converter. These series-connected voltage sources act as a voltage divider, which builds up the sinus-wave voltage to the grid. Each of these voltage sources consists of a capacitor and a transfer switch with standard industrial IGBT components. The electronic control system switches the power transistors such that the capacitor can be bypassed or activated as needed, allowing the SVC voltage that is tapped at the center of the voltage divider to be set to nearly any required level.

Using this multilevel technology, the SVC Plus system is not only excellently predestined for grid access of energy sources with fluctuating in-feed, such as offshore wind farms, but also fits extremely well in transmission systems for dynamic voltage support during and after system faults. A further field of application for this system is in industrial systems for reactive power and flicker compensation in the power range of up to approximately ±100 MVAr.

The first SVC Plus system will be put into operation this summer as the core component for grid access of the Thanet offshore wind farm that is being constructed around eleven kilometers off the coast of Kent in the North Sea. Siemens is currently fitting the substation, which constitutes the grid in-feed point on the coast, with an SVC Plus unit. This system will ensure that the power quality requirements (grid code) stipulated by the British power grid operator are fulfilled. Energy-efficient grid access solutions for offshore wind farms are part of the Siemens environmental portfolio, which earned the company revenues of nearly EUR 19 billion in fiscal 2008, roughly a quarter of Siemens total revenues.