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Figure 3.3. Active filter principle illustrated in time and frequency
3.3.
The ABB Active filter: the PQFA.
As we have just seen, the active filter is basically a compensating current
generator. The most important parts are then the current generator and the
control system.
The compensating current is in a first step created by a three-phase Insulated
Gate Bipolar Transistors (IGBT) inverter bridge that is able to generate any
given voltage waveform with PWM (Pulse Width Modulation) technology. The
IGBT bridge uses a DC voltage source realised in the form of a DC capacitor.
The inverter bridge is in fact the same technology than in AC drives.
The generated voltage is coupled to the network via reactors and a small filter
circuit. The desired current generator is thereby achieved.
The DC capacitors are loaded actively through the inverter bridge and there is
no need of external power source. Obviously, the DC voltage level must
always be higher than the peak value of the network voltage in order to be
able to inject currents to the network.
To control the active filter the choice stands between open loop and closed
loop current control. Under open loop current control, the harmonics currents
are measured on the load side of the active filter that computes the required
compensating current and injects it into the network.
Closed loop current control as performed by the PQFA is shown in Figure 3.4.
In this topology the resulting current to the network is measured and the
active filter operates by injecting a compensating current minimising this
resulting current. In this configuration, the filter directly controls its effect on
the filtration.
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Asea Brown Boveri Jumet S.A.
PQFA Instructions Manual
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Troubleshooting
