Environmental Engineering Reference
In-Depth Information
Appendix 1
FACTS technology
Flexible alternating current transmission system (FACTS) devices are expensive, and
it is necessary to ensure that their functionality is required before specifying them.
They perform four basic functions, which can be combined in different devices:
1.
Power transfer between electrically separated systems
2.
Active power management
3.
Reactive power management
4.
Waveform quality management.
Active power management devices (for example phase shifters, static syn-
chronous series compensators and unified power flow controllers) that are less
relevant to wind technology, either internally or in connection terms, will not be
discussed here. Current source converters for high-voltage direct current (HVDC)
are dealt with for completeness, although it is likely that most large wind farms
would be connected by voltage source converter technology, and doubly fed
induction generators (DFIGs) apply this technology in their rotor circuits.
Synchronous connection by lines and cables makes two electrical systems
behave as one. This clearly is inappropriate if the frequencies are different or one
system has a stability or fault level problem that would be exacerbated by con-
nection with another source. In these circumstances the systems may be main-
tained as separate entities through a converter/inverter DC path. In each of the
other applications, traditional technology exists but performs its function slowly
or within a narrow range. It is the need for rapid action in controlling active and
reactive power and the need to manage a wide range of waveform distortion
problems that requires the application of FACTS devices.
Two fundamental converter technologies are used in separating electrical
systems:
The current source converter
. This is usually a line-commutated device, that is, the
thyristors are switched on by a gate control but remain on until the current reaches
zero. Self-commutated current source converters have been constructed using gate
turn-off (GTO) or insulated-gate bipolar transistor (IGBT) technology. In line-
commutated devices the current must lag the voltage, whereas in self-commutated
devices either lead or lag is possible, giving power factor control. The converters
are normally arranged in a 6- or 12-pulse bridge. A 6-pulse bridge is shown in
Figure A.1.
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