IMM 8001/8002 – Digital Motor Protection Relay
ICE / CEE
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IMU307-1 GB Ind. B
62
5.9. PROTECTION AGAINST CURRENT UNBALANCES (PHASE LOSS) [46]
Function
Fuse-protected asynchronous motors are liable to operate with single-phase power supply following fuse
blowing. This single-phase operation is detrimental to the motor, since it is accompanied by a flow of
notable negative sequence currents with an increase in the total absorbed current and slippage. The
result of this is an increase of losses due to the Joule effect, especially at the rotor, to the prejudice of the
power factor and efficiency.
Motors with deep rotor level slots, or double squirrel-cage motors are, as far as they are concerned, quite
sensitive to the flow of negative sequence currents, since they generate an air gap field revolving at
twice the speed of the synchronism, and consequently heavy losses concentrated at the rotor surface due
to the skin effect.
During single-phase operation, the amplitude of negative sequence currents Ineg is :
- with a load equal to the nominal load : 0,7 In
≤
Ineg
≤
1,5 In
- off-load
: 0,15 In
≤
Ineg
≤
0,4 In
- on starting
: Ineg =
√
3 / 2 Irb = 0.866
Irb
with
Irb = stalled rotor current
When a resistive unbalance fault occurs in the motor, it is interesting to have a device which detects the
flow of negative sequence currents to eliminate it within a shortest period than that provided by the
fuses. However, the response time of the unit must remain compatible with the breaking power of the
switching device. In general, for modern contactors, this condition is satisfied if the response time is not
less than 0.5 seconds.
Some operating conditions may result in the flow of negative sequence currents for which the motor
must not cause :
- Close unbalance faults.
- Aperiodic component occurrence during start or faults.
- Voltage unbalances: these occur if the network load is unbalanced, or if the HV power supply
network may be subjected to slow single-phase reclosings.
To comply with the stability conditions required during external faults (Operating level adjustable from
15 % In, and suitable time-delay to let the other protections operate), and obtain a response
characteristic compatible with the thermal behaviour of the rotor and quick enough during internal
violent faults, the IMM 8000 relays are fitted with a phase negative sequence current unit with the
following characteristics :
