Impact of Unbalance Grid Voltage on the Performance of Doubly-fed Induction Machine at Motoring Mode

Hydro-turbines are usually optimized for an operating points defined by speed, head, and discharge. At fixed-speed operation, limited deviations from head and discharge are allowed. However, to improve operational efficiency of hydro turbines with partial loads and varying heads, variable-speed turbine and generator operation offers an attractive solution. Variable-speed operation is achieved using a doubly-fed induction machine with power electronics. During hybrid power generation, stable interconnected system is achieved with pumped storage schemes as renewable energy sources are highly erratic, especially solar and wind. Application of doubly-fed induction machine in pumped storage scheme is equivalently significant in both motoring and generating modes. In practice, the doubly-fed induction machine experiences power quality problems, depending upon the location of pumped storage scheme and the length of the distribution feeder. The objective of this study is to analyze the effects of voltage unbalance on a 2.2-kW doubly-fed induction machine at motoring mode through computer simulations and laboratory experiments. The test results and subsequent analysis confer hidden damage of negative-sequence voltage and effects of positive-sequence voltage magnitude on the performance of doubly-fed induction machine, particularly ripples in torque and speed and reduction in overall machine efficiency, which makes pumped storage scheme economically unviable.