Zero Sequence Impedance
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our electrical home
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
LABELS
Basic(21)
of transmission and distributionbasics (24)
devicesC.B (112)
cables(21)
BreakerCircuit (26)
GeneratorD.C. (42)
MotorD.C. (40)
and CircuitsDiode (41)
InstrumentsElectrical (23)
SystemGrounding (13)
Transformer and PowerInstrument
Management (P1)(20)
Library(18)
GenerationPower (28)
system protectionPower (68)
Transformer TheoryPower (29)
TheorySemiconductor (31)
Phase Induction MotorSingle (10)
Phase TransformersSingle (38)
MachinesSpecial (24)
TransformerSpecial (15)
generatorSynchronous (55)
MotorSynchronous (22)
Phase Induction MotorThree (57)
Line ParametersTransmission (36)
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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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