Page 360 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 343

1. voltage regulation and compensation of reactive power (STATCOM)
2. correction of power factor (PFC)
3. elimination of undesirable current harmonics (shunt active filter).
The design approach of the control system determines the priorities and functions
developed by the shunt-connected VSC in each case. In section 8.4 this topology was
used as a STATCOM to provide voltage regulation and compensation of reactive
power at the point of connection. In that case a simple control strategy based on
voltage measurements was implemented to control the operation of the shunt-con-
nected VSC. However, for the PFC model the implementation of the control strategy
is based on a current-controlled VSC scheme. The VSC is controlled as a current
source using PWM. Controlled in this way the VSC injects the proper amount of
reactive current required by the load so that the main supply delivers only the
required active current thus improving the power factor.
The principle of operation of the PFC can be explained using the schematic
diagram of Figure 8.61. Without compensation the load current I load is commonly
made up of the following terms:
I load ˆ I load 0 ‡ I load p ‡ I load q ‡ I load h (8:14)
where
I load 0 DC component
I load p in-phase line current
I load q reactive current
I load h harmonic currents.
Normally, the DC component is very small or it does not exist at all, then I load 0 is
excluded from Equation 8.14. For the specific case of power factor correction and
assuming a linear load, the term I load h can be considered zero and Equation 8.14
finally reduces to
I load ˆ I load p ‡ I load q (8:15)

In order to achieve unity power factor operation the mains supply must only deliver
I load p in Equation 8.15. This can be achieved if the PFC is controlled to generate and
inject into the system the reactive current I load q required by the load. From Equation
8.15 the reactive current I load q is calculated by subtracting the active current com-
ponent I load p from the measured current I load as
(8:16)
I load q ˆ I load I load p
In order to generate I load q the VSC must be provided with an appropriate current
control loop. At present there are several current control techniques available such as
hysteresis, predictive and indirect current control techniques which are implemented
using a stationary (abc) frame of reference. These techniques can be further explained
as follows:
1. Hysteresis current control keeps the error within a specified band. The advantages
of this technique are simplicity, good accuracy, and high robustness. The major
drawback is that the switching frequency varies within one load cycle; this results