Monday, August 27, 2012

STATCOM

overview-
Electrical loads both generate and absorb reactive power. Since the transmitted load often varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations, a voltage depression, or even a voltage collapse.
Similarly to the SVC the STATCOM can provide instantaneous and continuously variable reactive power in response to grid voltage transients, enhancing the grid voltage stability. The STATCOM operates according to voltage source principles, which together with unique PWM (Pulsed Width Modulation) switching of IGBTs (Insulated Gate Bipolar Transistors) gives it unequalled performance in terms of effective rating and response speed. This performance can be dedicated to active harmonic filtering and voltage flicker mitigation, but it also allows for a STATCOM to be comparatively downsized, its footprint can be extremely small. ABB has branded this high performance STATCOM concept SVC Light®.

Installing a STATCOM at one or more suitable points in the network will increase the grid transfer capability through enhanced voltage stability, while maintaining a smooth voltage profile under different network conditions. The STATCOM provides additional versatility in terms of power quality improvement capabilities.
STATCOM/SVC Light Technology
SVC Light is based on a technology platform also used for HVDC applications (HVDC Light). The most important building block is the Voltage Source Converter (VSC) equipped with Insulated Gate Bipolar Transistors (IGBTs) that are controlled by Pulse Width Modulation (PWM). A VSC is capable of both generating and consuming reactive power. If required, air core reactors and high voltage AC capacitors can be used along with the VSC as additional reactive power elements to achieve any desired range.

STATCOM/SVC Light Principle

SVC Light can be seen as a voltage source behind a reactance. Physically it is builtlas a three-level inverter operating on a constant DC-voltage. It provides reactive power generation as well as absorption purely by means of electronic processing of voltage and current waveforms in a voltage source converter (the grid will see it as a synchronous machine without inertia). This means that capacitor banks and shunt reactors are not needed for generation and absorption of reactive power, facilitating a compact design, a small footprint. The high switching frequency of IGBT allows extremely fast control, which can be used in areas such as mitigation of voltage flicker caused by electric arc furnaces, voltage balancing, harmonic filtering and robust grid voltage recovery support. A DC capacitor bank is utilized to support and stabilize the controlled DC voltage needed for the converter operation. Voltage source converters connected in "back-to-back configuration" between two AC busbars have the capability to operate with active power allowing a Dual Purpose scheme to be feasible. Using such a back-to-back configuration enables active power transfer between two AC grids (synchronous or asynchronous or even with different frequencies) while, simultaneously, the converters provide reactive power support to the AC networks. 
Urbanized environmentally friendly grid voltage support, a case story.
Urbanisation seems to be an ever-present force, more or less worldwide. But another trend in the opposite direction involves the relocation of electricity production to places far away from the city centers. Our favourite working and living places shall become cleaner and the aging machines that brought people together are now retired, often along with the factories they initially supported. Computers and air-conditioning follow people downtown. The result is that the growth of city centers increases the stress on the power transmission system, consuming considerable reactive power in a destabilising manner.

Given this helicopter perspective, it becomes apparent that it is not straightforward simple to retire old downtown workhorses. The authorities in Austin, Texas were faced with this problem as a down-town gas/oil fired plant was doomed due to its environmental impact. The introduction of extremely compact STATCOM (in the form of SVC Light®) technology greatly facilitated for the municipal utility Austin Energy to take a fast-track approach for the closing of the power plant, while maintaining adequate voltage stability margins in the grid operation. Within the timeframe of two years, all the pre-project activities (studies, specification, supplier selection), design, delivery, installation and commissioning of the approximately +100Mvar/138kV STATCOM was completed. It went into commercial service in December 2004. The Austin example shows how elegantly STATCOM technology can replace a generator, or more correctly, the voltage support capability of the generator. It does so cost efficiently and with a minimum of environmental impact. 


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