Page 378 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 359

Another gate trigger signal can be obtained, delayed with respect to the zero crossing
of the AC source voltage. The control scheme diagram is shown in Figure 8.79.
The implementation of the test power system in PSCAD/EMTDC is shown in
Figures 8.80±8.84.
The results obtained through the transient simulation are presented for the time
interval of 0.06±0.18 s. At the beginning of the simulation the shunt AF is discon-
nected from the AC system, i.e. Brk is in the open state. At t ˆ 0:1s Brk is closed and
the active filter is connected to the AC system and starts operating. Figure 8.85 shows
the source current I S , the load current I L and the VSC current I VSC for both operating
conditions. It can be seen in Figure 8.85(a) that when the shunt AF starts operating
at t ˆ 0:1 s the source current recovers its sinusoidal waveform as the shunt AF is
blocking properly the low order current harmonics generated by the load.
It is important to observe in Figure 8.85(b), that the line current drawn by the load
remains unchanged even when the filter operates. The effect of the filter is to block
the line current harmonics generated by the load from flowing back into the dis-
tribution network and disturb other network components in the vicinity.
Figure 8.86 shows the harmonic spectrum of the source current I S before and after
the shunt AF operates. Without filtering, I S has a current total harmonic distortion
ITHD ˆ 30:52% with high contents of low order harmonics such as the 5th, 7th, 11th
and 13th. With the shunt AF in operation, the total harmonic distortion of the source
current decreases to ITHD ˆ 8:73% as the content of the low order harmonics is
significantly reduced as shown in Figure 8.86(b). It must be observed that the
fundamental component of the source current is different before and after the filter
operation. The fundamental component value is greater when the filter is in oper-
ation. This can be explained bearing in mind that the VSC of the active filter has a
DC link capacitor whose voltage must be kept constant for the correct operation of
the filter. That is, the source current increases as the active filter is drawing active
power from the AC system to charge the capacitor and maintain constant DC link
voltage.
Figure 8.87 shows the harmonic current component of the load current I L , the
active filter VSC current I VSC and the source current I S . It can be clearly appreciated
in this figure how the harmonic current component generated by the active filter and
the harmonic current component due to the load have the same wave shape but
opposite direction. The response of the filter controller is fast and it only needs half a
cycle to start tracking the reference currents and drive the filter to generate the
appropriate harmonic currents to cancel those of the load current. Figures 8.88
and 8.89 show the harmonic current components of the load, filter and source
currents in the rotating dq0 frame.
Specifically, the harmonic current components in the rotating frame q-axis are
shown in Figure 8.88 and the harmonic current components in the d-axis are shown
in Figure 8.89. Once again, it can be seen that the harmonic current dq components
generated by the active filter and the harmonic current dq components due to the
load have the same wave shapes but opposite directions.
The voltage V DC through the DC link capacitor of the active filter is shown in
Figure 8.90(a) and the active and reactive powers absorbed by the shunt AF
are shown in Figures 8.90(b) and 8.90(c) respectively. After the transient period
when the filter is connected to the network, the shunt AF absorbs active power to