Operation Characteristics of Zone 3 Impedance Relays in Wind Power Systems with Fixed-Speed Induction Generators

The induction generators are basic to wind energy conversion. They produce the active power and consume reactive power, with the voltage characteristics fragile compared with that of the synchronous generators. In stressed system conditions, they may intensify var imbalance, causing undesirable operation of the zone 3 impedance relays (mho relays). In this paper, the static voltage characteristics of the induction generators is derived from theoretical and load flow analysis. A sensitivity model of the operation margin of the mho relays to wind power is proposed to quantify the relay performance against undesirable operation. The numerical results show that: (1) more wind power will yield lower stator voltage and impedance of the induction generator, and less operation margin; (2) the induction generator has negative resistance causing the apparent impedance approaching to the protection region from the left side of the R-X plane; (3) load surge near the wind farm may increase or decrease the operation margin.