ABB PQFA Instruction Manual page 10

PQFA Instructions Manual
AF
Target Output
Control
Measurement Feedback
Figure 3.4. Closed loop control
In addition to being more precise, the closed loop control system also allows
for a direct control of the degree of filtering. Furthermore, the closed loop
control system ensures that measurement errors do not result in a higher
distortion.
To fully exploit the potential of an active filter we need a digital measurement
and control system that is fast enough to operate in true real time. We need to
be able to track the individual harmonics and control the compensating
current according to the requirements of the plant and this with full control at
every instant in time. To achieve this, we need advanced Digital Signal
Processors, DSP' s.
Among the physical signals needed by the PQFA, the three line currents have
obviously to be measured. Standard CTs with 5A secondary are usually
sufficient. Those analogue signals must first be acquired, levelled and
antialias-filtered before digitalisation. Fast and high precision analogue-to-
digital converters are used to create a digital representation of the analogue
signals. The digitised signals are then sent to the powerful DSP that controls
all measurements and calculations in real time, and builds the PWM
references for the inverter. It is another processor, a microcontroller, which
handles all digital input/output (including the command of the PWM inverter).
More dedicated to control than to calculations, this microcontroller ensures for
instance the closing of the relays and contactors.
One control is needed per PQFA system and can handle more than one
power module simultaneously.
Asea Brown Boveri Jumet S.A.
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