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This paper proposes and compares transient currents limiting techniques for a variable speed drive short term power interruption ride-through improvement module. The module comprises of an additional capacitor that is switched onto the dc link during a short-term power interruption. The switching on of the additional capacitor to the dc-link results in high inrush currents which if not mitigated could damage the variable speed drive components and cause nuisance tripping of incoming supply breakers. Proposed inrush current limiting techniques are investigated which include the use of a fixed resistor, inductor, variable resistor and the equipotential voltage switching. Experimental results indicate that the variable resistor technique results in the least torque pulsations of 12% and 9% at the start and end of dc-link compensation. The inductor technique produced the greatest disturbance at the start of compensation with a torque pulsation of 15%. The equipotential voltage switching technique produced torque pulsations of 6.8% and 9.8% at start and end of compensation respectively. It also produced the least drops in torque and speed of 9.9% and 3.2% respectively.

This paper investigates improvement of under voltage ride-through capability of a variable speed drive to short-term power interruptions by use of switched capacitors. Variable speed drives are sensitive to voltage dips. Voltage dip related trips can lead to loss of key processes. To support the grid in South Africa, the transmission operator requires that power station equipment withstand certain voltage dips. Simulation and experimental results demonstrate feasibility of the proposed solution. Simulation results show a torque ripple of 16% and 25% during voltage dip initiation and clearance respectively. Minor speed disturbances are observed at both instances. Inrush charging currents are observed at both ride-through module activation and dip clearance as the variable speed drive capacitor charges up. Minimization of capacitor charging inrush currents to avoid damage to VSD components through the use of resistive elements is explored. The proposed solution can be extended to super/ultra-capacitors and batteries with little or no hardware modifications.