Digital Control Strategy for Harmonic Compensation of Dynamic Voltage Restorer - Intelligent Computing for Sustainable Energy and Environment - page 11

voltage compensation effect, this paper presents a double closed loop digital PR
control strategy by using the fundamental PR to control voltage outer loop and
low order harmonics PR to control current inner loop electrical. At the reso-
nance frequency, the open loop gain of the PR controller is infinite, which can
achieve no static error control to particular frequency, and can provide enough
attenuation to prevent interference between adjacent frequencies in other fre-
quencies. Because the bandwidth of the voltage outer loop is narrow, the low
order harmonic PR control can't be applied in the outer loop. The paper can
use fundamental PR control in voltage outer loop and low order harmonic PR
control in current feedback inner loop to eliminate the steady state error and
compensate the low order harmonics. To avoid the influence produced by the
sampling, calculation delay and other discrete processes on the steady-state er-
ror and dynamic response characteristics, the paper use a digital control method.
The paper develops a 11KVA DVR prototype based on the theory and do some
corresponding test. The theory research and test result show that the proposed
control scheme is effective and feasible.
2 Inverter Unit Digital Proportion Resonance Control
Strategy
When DVR inverter unit uses a double closed loop PR control, since the band-
width of the voltage of the outer ring is small, and that of the voltage of the
inner ring is big, if the outer loop uses the harmonic PR control, though the
PR controller has a very big gain at the resonance frequency, the system will
be instable and it is still dicult to realize the satisfied compensation effect in
condition that the resonant frequency exceeds the bandwidth of the voltage of
the outer ring, and the reduction of the gain of the outer ring will greatly offset
the resonance gain. Thus, the fundamental wave PR controller is applied in the
outside loop and the harmonic PR controller in the inside current loop.
As the digital processor has a sampling and a calculation time delay, PWM
pulse update generally lags behind for a beat of sampling time. And for the
DVR emphasizes the dynamic characteristics, this influence can't be ignored.
The key to continuous domain discretization digital control is how to choose
an appropriate discretization method. And this paper uses the step response
invariant method of zero-order holder combined to discrete the consecutive time
model of the DVR inverter unit, which makes the system stable and the gain
unchanged. The high harmonics produced by the inverter unit switching device in
switch action can be filtered,while 3rd, 5th and 7th low order harmonics produced
by nonlinear factors such as the nonlinear load can only be compensated by
designing a reasonable controller. Therefore, as discussed and analyzed in Fig.1,
the paper presents a double loop PR control strategy, that the fundamental
wave PR controller is applied in the outer loop and the 3rd, 5th, 7th harmonic
low pass filters are applied in the inner loop. Among them, K p 1 and R 1 ( z )
respectively stand for the fundamental wave PR controller, K ph ,R 3 ( z ) ,R 5 ( z )
and R 7 ( z ) respectively stand for 3rd, 5th and 7th harmonic PR controllers.