Negative Sequence Overcurrent Protection Logic; Earth Fault Protection; Earth Fault Protection Elements - GE P50 Agile P253 Technical Manual

P50 Agile P253
• DT (Definite Time)
This is achieved using the cells
• I2>(n) Function for the overcurrent operate characteristic
• I2>(n) Reset Char for the overcurrent reset characteristic
where (n) is the number of the stage.
The IDMT-capable stages, (1, 2 and 3) also provide a Timer Hold facility. This is configured using the
cells I2>(n) tRESET, where (n) is the number of the stage. Timer Hold facility is applicable for both
IEC and IEEE curves. IEEE & US curves have IDMT reset curve option in addition to DT.
2.6.3

Negative Sequence Overcurrent Protection Logic

For Negative Phase Sequence Overcurrent Protection, the energising quantity I2> is compared with
the threshold current I2>(n) Current Set. If the value exceeds this setting a Start signal (Strt I2>(n)) is
generated, provided there are no blocks. 5% hysteresis is built into the comparator such that the drop-
off value is 0.95 x of the current set threshold.
The Strt I2>(n) signal is fed into a timer to produce the Trip I2>(n) signal.
The Negative Sequence overcurrent trip signal can be blocked by:
• The Second Harmonic blocking function which is for all three phases. The blocking is activated
by setting the I2>(n) 2H Blocking cell to 'Enabled', where (n) is the number of the stage.
• User defined blocking logic using 'IO Mask' functionality.
2.7

Earth Fault Protection

One of the most common faults on a motor is a stator winding fault. This is usually the result of
prolonged or cyclic overheating, which causes the insulation to deteriorate. Since the windings are
surrounded by an earthed metal case, stator faults usually manifest themselves as earth faults. The
type of earth fault protection and CT arrangement adopted depends on the amount of earth fault
current available during a fault. The magnitude of current is dependent on the system earthing
arrangements.
Earth faults are simply overcurrent faults where the fault current flows to earth (as opposed to between
phases). They are the most common type of fault. There are a few different kinds of earth fault, but the
most common is the single phase-to-earth fault. Consequently this is the first and foremost type of
fault that protection devices must cover.
Typical settings for earth fault IEDs are around 30-40% of the full load current. If greater sensitivity is
required, then Sensitive Earth Fault should be used.
Earth faults can be measured directly from the system by means of:
• A separate CT located in a power system earth connection
• A separate Core Balance CT (CBCT)
• A residual connection of the three line CTs, whereby the Earth faults can be derived
mathematically by summing the three measured phase currents.
Depending on the device model, it will provide one or more of the above means for Earth fault
protection.
2.7.1

Earth Fault Protection Elements

Earth fault protection is implemented in the columns EARTH FAULT 1 and EARTH FAULT 2 of the
relevant settings group.
P253/EN M/C
6 Protection Functions
6-19