Environmental Engineering Reference
In-Depth Information
3-phase AC
DC
Figure A.3
Voltage source converter
transferred power. Clearly the commutation gives rise to ripple on the DC side,
which is smoothed through an inductor, and to imperfect sine waves on the AC-
side. These imperfections result in DC harmonics at m n , where m is an integer
and n is the number of pulses in the converter, so 12, 24, 36, 48 and so forth for a
12-pulse bridge. The AC side experiences harmonics of m n l, so 11, 13, 23, 25,
35, 37 and so forth.
The voltage source converter. This converter can produce an AC voltage that is
controllable in magnitude and phase, similar to a synchronous generator or syn-
chronous compensator. The device commutates independently of the AC-side
voltage and therefore the voltage source converter can be used on a load-only
system, that is, a system with zero fault level. This makes it useful for DFIG rotor
connection, wind farm connection, connection of oil platforms and so forth. The
device configuration is shown in Figure A.3.
At present, economics suggest that it is viable for transfers up to 200-300 MW,
above which it becomes too expensive. The efficiency of voltage source converters
is lower than current source devices which can achieve 98 per cent or greater.
Voltage source converters operate by switching the devices at frequencies higher
than line frequency using pulse width modulation (PWM). By varying the pattern
from the requirement for a standard sinewave, harmonics can be negated at source.
Thus the converter allows control over both the amplitude of the voltage and the
waveform quality. A penalty for higher frequency switching is increased losses. It
must therefore be part of the objective of any selective harmonic elimination (SHE)
scheme employed in PWM control to minimise switching frequency. Magnetic
circuits can be applied with coils suitably wired to cancel some harmonics and
minimise others, hence reducing the SHE duty. If PWM is not used, as when SHE
is applied to a fixed and restricted pattern of harmonics, then the output voltage
control is achieved by varying the DC bus capacitor voltage. This is achieved by
charging or discharging the DC bus capacitor through variation of the firing angle.
Clearly this process takes time and does not give the fast response of PWM. Current
loop control can be linked with the voltage control achieved in PWM, resulting in a
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