Demand for transmission system voltage support is increasing amid greater renewable penetration and retirement of fossil and nuclear power plants. Synchronous compensators such as STATCOM provide reactive power; yet its operational characteristics differ from a Static VAR Compensator.
GE said STATCOM undervoltage performance would be “superior”, while SVCs “masters overvoltages.”
Exhibiting at Cigré2016 in Paris this week, GE presented its utility-grade static synchronous compensator (STATCOM) which is based modular multilevel converter architecture with HVDC MaxSine full-bridge power submodules.
Shunt compensation
Development of insulated gate bipolar transistor technology enables high power shunt compensation systems with voltage source. “The shunt compensation market is turning to STATCOMs thanks to their harmonic and undervoltage performance. Moreover, stringent harmonic performance requirements are more easily met, even in weak networks,” explained Jussi Pöyhönen, Grid Solutions senior lead design engineer.
Each phase of the VSC valve consists of a string of series-connected full-bridge valve submodules, each with its own DC-link capacitor. The current is controlled by switching valve submodules to its positive or negative DC-link voltage or zero voltage for voltage difference over the coupling impedance of a step-down transformer or a series reactor.
AC voltage is controlled with capacitive reactive power, when the converter voltage is greater than the system voltage. If the converter voltage is less than the system voltage, the STATCOM produces inductive reactive power. VSC valve reactive current capability is symmetrical for inductive and capacitive operation.
New switching algorithms
A modulator is in charge of valve submodule switching, applying novel algorithms including DC-link balancing and valve submodule sorting. GE's says its STATCOM VSC valve has “inherent transient performance thanks to the strongest DC-capacitor rating on the market.”
Extensive self-diagnostic capabilities help to pinpoint performance degradation and component faults, e.g. through a dual-lane redundancy control system configuration. A built-in event logger with automatic time stamping of 1 millisecond resolution and a synchronized transient fault recorder with up to 10 microsecond sampling allow for detailed analysis post event.
Remote access to the control system allows remote diagnostics. According to Pöyhönen, the STATCOM solution “provides grid operators with reactive power to support grid stability in difficult network conditions and weak grids in a more compact package compared to traditional SVC solutions.”