Micom P721, Technical Manual

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 3/52
1. INTRODUCTION
1.1 P72x High Impedance Differential Relay
The P72x relay, used with a stabilizing resistor, is designed for applications where sensitive
settings and stability on heavy through faults are required. It is recommended for balanced
and restricted earth fault schemes, bus-zone and certain forms of differential protection for
generators, auto-transformers, reactors and motors.
The sampling rate of the P72x is 32 samples per cycle. A Measurement Filter setting is
available in the P72x, so that the relay may operate in three different modes. The first one is
the Fourier mode, the relay is tuned to the supply frequency, and the harmonics and DC
component produced by current transformer saturation are filtered. The second option is
Sample mode. This mode is based on the peak to peak measurement of the differential
current. A fault is detected when
setting
2 2
I
peak peak
diff
>
×
−
. Finally, the relay may be set to
Fast sample mode, and a fault is detected when
setting 2 2 I
eous tan tan ins
diff
× × >
. In Fast
sample mode a trip is issued if four consecutive differential current samples are above
setting 2 2
× ×
in one half cycle.
1.2 Busbar Protection
Busbar protection must be completely reliable since the protection may only be called to
operate once or twice in the life of the installation. Failure to operate under fault conditions is
unacceptable because the damage that results from an uncleared fault may be extensive, for
example, the loss of the station by fire. In addition, the protection must be high speed in
operation to minimize damage and maintain system stability. It must also be absolutely
stable under all through fault conditions because failure to stabilize would cause
unnecessary widespread interruption of supply.
If the busbar is subdivided into sections, each of which is separately protected, a fault in one
section does not involve the tripping of the complete busbar. Therefore, the protection must
discriminate between sections of the busbars so that the minimum number of circuit breakers
is tripped to isolate the fault.
The following are commonly used busbar protections:
•
Frame to earth (leakage) protection
•
Differential protection
•
Directional comparison (blocking schemes) protection
The P72x relays operate on the high impedance principle. Therefore, the application of high
impedance busbar protection in different busbar arrangements is considered in the following
sections.
The high impedance differential scheme is simple and fast, but it requires good CT inputs
with well matched ratios and high CT kneepoints. At 5In the typical operating times when
the P72x is used in REF/BEF applications is 40 ms for Fourier, 34 ms for Fast and 52 ms for
Sample.
At 5In the typical operating times when the P72x is used in busbar applications is
18 ms for Fourier, 18 ms for Fast and 38 ms for Sample.
1.3 Reactor Protection
To calculate the stability voltage when protecting a reactor with a high impedance scheme, it
should be noted that the external fault current contribution from the reactor is much smaller
than the internal fault current. Therefore, the reactor inrush current should be used in
stability calculations instead of the external fault current.