New current controller for inverter fed medium voltage drives with LC filter

In high power medium voltage drives, the switching frequency of power semiconductor devices is restricted because of high switching losses. Such drives often include an LC output filter, introducing a resonant circuit that may be excited for instance by fast control transients. One way to avoid such oscillations is using damping control schemes, which in turn usually require switching frequencies well above the filter resonance. This paper proposes a new predictive stator current controller that avoids the excitation of the filter resonance, performs active damping and allows fast current control while maintaining low switching frequency. Simulation results of a 2.4 kV induction motor drive and experimental results obtained from a 55 kW prototype a.c. drive demonstrate feasibility and good dynamic performance of the proposed control approach.

I. Introduction

Medium voltage high power inverters usually operate at a switching frequency lower than 500 Hz [1]–[4] in order to minimize the switching losses of the power semiconductors. They are often applied to retrofit of existing fixed speed induction motors with medium voltage variable speed drives, i.e. to achieve energy savings. Because existing motors usually are not designed for inverter supply, the use of LC output filters becomes necessary in order to avoid isolation problems and bearing currents [5]. The filter creates sinusoidal output voltages and thereupon conditions for the machine similar to operation from the grid. The inverter switching frequency that is acceptable for thermal reasons lies only slightly above the resonance frequency of the LC filter. At the same time, a suitable motor current control is necessary that does not excite the filter resonance. Otherwise, sudden changes of the control reference would create considerable weakly damped transient currents [6].