Negative Sequence Overcurrent Protection; Loss Of Phase While Starting And Running; Negative Sequence Overcurrent Protection Implementation - GE P50 Agile P253 Technical Manual

6 Protection Functions
•
User defined blocking logic using 'IO Mask' functionality.
2.6

Negative Sequence Overcurrent Protection

When applying standard phase overcurrent protection the overcurrent elements must be set
significantly higher than the maximum load current, thereby limiting the element's sensitivity. Most
protection schemes also use an earth fault element operating from residual current, which improves
sensitivity for earth faults.
However, certain faults may arise which can remain undetected by such schemes. Negative
Sequence Overcurrent elements can be used in such cases.
Any unbalanced fault condition will produce a negative sequence current component. Therefore, a
negative phase sequence overcurrent element can be used for both phase-to-phase and phase-to-
earth faults. Negative phase sequence current is generated from an unbalanced current condition,
such as unbalanced loading, loss of one phase or single phase faults.
With positive phase sequence voltages applied to the motor, a rotating field is set up between the
stator and rotor. The resulting effect is that the direction of rotation of the rotor is equal to that of the
applied field. With negative phase sequence voltages, the field rotates in the opposite direction, cutting
a rotating rotor conductor at almost twice the system frequency.
The a.c. resistance of the rotor conductor to the induced negative sequence current is greater due to
the higher frequency, 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.
2.6.1

Loss of Phase while Starting and Running

If a motor is started with one phase open, it remains stationary and it can draw a current equalto 0.866
times the normal starting current. Under these circumstances, the negative phase sequence
component present in the current is equal to half the normal starting current value. This is an extreme
condition, as this amount of negative phase sequence current rapidly overheats the motor, and unless
corrective action is taken, the motor will be seriously damaged.
Loss of one phase of the supply to a motor during normal running conditions, results in the following
conditions:
• Heating increases considerably due to high rotor losses caused by the presence of negative
sequence current.
• Output of the motor is reduced and, depending on the load, an induction motor may stall.
• Motor current will increase
One common factor in the aforementioned conditions is the presence of negative phase sequence
current. The P253 relay therefore incorporates a negative phase sequence current element to detect
such extreme operating conditions.
2.6.2

Negative Sequence Overcurrent Protection Implementation

Negative Sequence Overcurrent Protection is implemented in the NEG SEQUENCE O/C column of
the relevant settings group.
The product provides three stages of negative sequence overcurrent protection with independent time
delay characteristics.
Stages 1, 2 and 3 provide a choice of operate and reset characteristics, where you can select
between:
• A range of standard IDMT (Inverse Definite Minimum Time) curves
6-18
P50 Agile P253
P253/EN M/C