Predictive Current Controller for Inverter Fed Medium Voltage Drives with LC Filter

The switching frequency of power semiconductor devices in high power medium voltage drives is limited because of high switching losses. These drives often comprise an LC output filter. The filter introduce a resonant circuit that may be excited for instance by fast control transients. One option to avoid such oscillations is applying damping control methods, which on the other hand usually require switching frequencies well above the filter resonance. This paper presents a new predictive stator current controller that avoids the excitation of the filter resonance, realizes active damping and enables fast current control while maintaining low switching frequency. Feasibility and good dynamic performance of the proposed control method is demonstrated by simulation results of a 2.4 kV induction motor drive and by experimental results obtained from a 55 kW prototype a.c. drive.

I. Introduction

In order to minimizethe switching losses of the power semiconductors high power medium voltage inverters typically operate at a switching frequency lower than 500 Hz [1]–[5]. They are frequently applied to the 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 designedfor inverter supply, the use of LC output filters becomes necessary in order to avoid isolation problems and bearing currents [6]. 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 abovethe resonance frequency of the LC filter. At the same time, a suitable motor current control is necessary that doesnot excite the filter resonance. Otherwise, sudden changes of the control reference would create considerable weakly damped transient currents [7].