GE P642, Technical Manual, Page: 213 / 592

Secondary Internal Fault Current Recommended Metrosil types for various voltage settings
50A
600A/S1/S1213
C = 540/640
35 mA RMS 600A/S1/S1214
C = 670/800
40 mA RMS 600A/S1/S1214
C =670/800
50 mA RMS 600A/S1/S1223
C = 740/870
50 mA RMS
100A
600A/S2/P/
S1217
C = 470/540
70 mA RMS
600A/S2/P/S1215
C = 570/670
75 mA RMS 600A/S2/P/S1215
C =570/670
100 mA RMS 600A/S2/P/S1196
C =620/740
100 mA RMS
150A
600A/S3/P/
S1219
C = 430/500
100 mA RMS
600A/S3/P/S1220
C = 520/620
100 mA RMS 600A/S3/P/S1221
C = 570/670
100 mA RMS 600A/S3/P/S1222
C =620/740
100 mA RMS
In some situations single disc assemblies may be acceptable, contact Alstom Grid for detailed applications.
Note:
The Metrosils recommended for use with 5 Amp CTs can also be used with triple pole devices and consist of three single pole
units mounted on the same central stud but electrically insulated from each other. To order these units please specify "Triple
pole Metrosil type", followed by the single pole type reference. Metrosil for higher voltage settings and fault currents are
available if required.
5.3.4 CT REQUIREMENTS - HIGH IMPEDANCE REF
In a high impedance REF scheme, the required stability voltage requirement is described in terms of an external
fault (I
F
), burden (2R
L
+ R
CT
) and a stability factor (K), as follows:
V
s
=> KI
F
(2R
L
+ R
CT
)
where:
● I
F
= maximum external fault level
● R
CT
= resistance of CT secondary winding
● R
L
= resistance of a single lead from device to current transformer
The assumption that one CT is completely saturated for an external fault does not describe what actually happens
when asymmetric CT saturation occurs. The CT that saturates will only saturate during parts of each current
waveform cycle. This means that the spill current waveform seen by the differential element will be highly non-
sinusoidal. The sensitivity to non-sinusoidal spill waveforms for through-faults will be a function of the REF
frequency response, the REF operating time, the REF current setting and the wave shapes.
The frequency response and the operating speed are factors that are inherent to the design. Spill current wave
shapes will be related to the ratio of the CT kneepoint voltage (V
K
) to the circuit impedance. The stability voltage is
determined by the current setting and the stabilising resistor. The stability of the High Impedance REF function
during through faults is determined by the ratio V
K
/V
S
. Where V
K
is the CT knee point voltage and V
S
is the stability
voltage.
The relationship between the V
K
/V
S
ratio and the required stability factor K has been found to be of a general form
for various designs that have undergone conjunctive testing. It is the absolute values of V
K
/V
S
and K that vary in
the relationship for different device designs.
Once stability has been considered, the next performance factor to take into account is the operating time for
internal faults. The CT kneepoint voltage as a multiple of the protection stability voltage setting (V
K
/V
S
) will govern
the operating time of a differential relay element for heavy internal faults with transiently offset fault current
waveforms. With the aid of the operating time curves derived for the device, it is possible to identify the ratio V
K
/V
S
that is required to achieve a desired average operating speed for internal faults.
P64x Chapter 8 - Restricted Earth Fault Protection
P64x-TM-EN-1.3 189