Zero Sequence Impedance

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Home Library C.B. Device Library Electrical Units

LABELS : POWER SYSTEM PROTECTION

Unbalanced Supply VoltageWEDNESDAY, SEPTEMBER 14, 2011

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

he 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

imes 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

o (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

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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

voltage.

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

hen 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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