Design considerations for six pulse and twelve pulse diode rectifier systems operating under voltage unbalance and pre-existing voltage distortion with some corrective measures

In this paper design considerations for six pulse and twelve pulse diode rectifier systems operating under utility voltage unbalance and pre-existing harmonic voltage distortion is discussed. For a six pulse diode rectifier system it is shown that voltage unbalance and pre-existing voltage distortion in the utility generates uncharacteristic harmonics in the line currents. The amplitude of these uncharacteristic harmonics are shown to be amplified if the diode rectifier is operating in discontinuous conduction mode. For a twelve pulse diode rectifier system connected in parallel to feed a common DC link via interphase reactor it is shown that a small amount of impedance mismatch, utility voltage unbalance or pre-existing voltage distortion drastically affects the current sharing capability of the rectifier bridges. This in turn generates additional uncharacteristic and characteristic harmonics thereby increasing the THD. In order to mitigate these effects and ensure proper operation of diode rectifiers, specially designed line reactors termed as harmonic blocking reactors (HBRs) are introduced The analysis and design procedure for the HBRs are discussed. Simulation results illustrate improved performance. Experimental results from a laboratory prototype system show close agreement with theory.