Wednesday, September 14, 2011

Unbalanced Supply Voltage

Induction Motor Protection (Part4)
Abnormal Operating Conditions from Supply Side (Part1)
       Large induction motors are very sensitive to unbalances in supply voltage. The negative sequence component, which comes into picture because of the unbalance in the supply, is particularly troublesome. This is because the motor offers very small impedance to the negative sequence currents. In fact, the negative sequence impedance is less than the positive sequence standstill-impedance as shown in Fig 1. Thus, a 4% negative sequence supply voltage causes more than 24% negative sequence current to be drawn by the motor, if the starting current of the motor is six times the full-load current. This causes increased heating of the stator.
Fig.1  Equivalent circuit of induction motor
       Further, the magnetic field due to negative sequence rotates at synchronous speed Ns in a direction opposite to that of the rotor which is rotating at a speed equal to (sNs), which is slightly less than the synchronous speed, where s is the slip of the motor. This causes currents of [f(2 - s)] frequency, i.e. almost double the supply frequency, to be induced in the rotor circuit. Because of skin effect, the rotor offers five to ten times its normal resistance to these double frequency currents, thus, causing excessive heating of the rotor.
       Therefore, for large motors any unbalance in the supply voltage needs to be quickly detected and corrective action taken. A scheme for detecting unbalanced supply voltage is shown in Figure 2.
Fig. 2 Negative sequence voltage relay for protection against unbalance in supply
       There are certain situations where the negative sequence relay does not operate correctly. For example, if there is an open circuit fault between the supply and the relay then the relay measures the negative sequence voltage across the motor, which is substantial and, therefore, the relay operates correctly. This is shown in Fig.3.
Fig. 3  Negative sequence voltage relay correctly detects an open circuit between supply
and relay.
       However, if the open circuit is between the relay and the motor then the negative sequence relay ends up measuring the negative sequence component of the supply voltage, which is very small and does not cause the relay to pick up as shown in Figure 4,
Fig. 4  Negative sequence voltage relay fails to detect an open circuit between relay ~ and motor
       For positive detection of such faults, we need to detect the negative sequence component in the line currents.

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