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Power electronic control in electrical systems 259

Fig. 6.94 Four-wire four-pole VSC based shunt connected active filter.

Fig. 6.95 Four-wire three-phase three bridge VSC shunt connected active filter.

6.9 Combined active and passive filters

In high power applications with DC and AC motor drives, DC power supplies or
HVDC conventional thyristor-based systems 12-pulse rectifier loads are widely used.
Such configurations are typically used because they generate significantly lower
harmonic currents when compared to a six-pulse rectifier circuit.
In order to meet harmonic requirements (i.e. IEEE 519) although the 12-pulse sys-
tem offers improved harmonics it still requires filtering. Only an 18-pulse rectifier circuit
can meet IEEE 519 requirements without any additional filtering. In conventional