GE P24DM Technical Manual page 130

Chapter 6 - Current Protection Functions P24xM
Motor negative sequence impedance at a given slip s by the formula:
[(R1 + R'2/s)2 + (X1 + X'2)2]0.5
That means: [(R1 + R'2/2)2 + (X1 + X'2)2]0.5 when s << 1 at normal running speed.
Where:
PPS = positive phase sequence
NPS = negative phase sequence
R1 = PPS Stator Resistance
R'1 = PPS Rotor Resistance Referred to Stator
X1 = PPS Rotor Reactance
X'1 = PPS Rotor Reactance Referred to Stator
R2 = NPS Stator Resistance
R'2 = NPS Rotor Resistance Referred to Stator
X2 = NPS Rotor Reactance
X'2 = NPS Rotor Reactance Referred to Stator
s = Slip
The value of resistance is generally much less than the leakage reactance. Therefore, neglecting the resistance
term, the motor negative phase sequence impedance at normal running speed can be approximated to the
positive phase sequence impedance at standstill.
At normal running speed:
positive sequence impedance sta r r ting current
≅
negative sequence impedance normal load current
For example, if a motor has a starting current of six times the full load value, the negative sequence impedance
would be about 1/6 the positive sequence impedance.
Therefore, the presence of 5% negative sequence voltage in the supply would result in approximately 30% of
negative sequence current.
The a.c. resistance of the rotor conductor to the induced negative sequence current is greater due to the higher
frequency [(2-s)f], causing skin effect. The heating effect of negative sequence current is therefore greater and
increases the motor losses.
It is therefore essential to detect any negative sequence current present on the system and act accordingly before
dangerous temperatures occur in the motor.
The P24xM relay includes several methods for the detection of negative phase sequence currents, depending on
the cause of the unbalance. These methods are addressed in detail below.
6.5.1.1 THREE PHASE VOLTAGE CHECK (REVERSE PHASE DETECTION)
Principle
Incorrect phase rotation of the incoming supply to the motor results in the motor rotating in the opposite direction.
For certain directionally sensitive applications, such as lifts and conveyor belts, this is a potentially dangerous
condition and must be detected rapidly.
Although the above condition does not result in the flow of negative phase sequence current in the motor, 100%
negative phase sequence current are presented to the measuring circuitry of the relay.
106 P24xM-TM-EN-2.1