Ct Requirements - High Impedance Ref - GE P642 Technical Manual

P64x
Secondary Internal Fault Current
50A
100A
150A
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 ), burden (2R + R
F L
V => KI (2R + R
s F L
where:
I = maximum external fault level
●
F
● R = resistance of CT secondary winding
CT
R = resistance of a single lead from device to current transformer
●
L
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
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
voltage.
The relationship between the V
for various designs that have undergone conjunctive testing. It is the absolute values of V
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
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
that is required to achieve a desired average operating speed for internal faults.
P64x-TM-EN-1.3
Recommended Metrosil types for various voltage settings
600A/S1/S1213
C = 540/640
35 mA RMS
600A/S2/P/
S1217
C = 470/540
70 mA RMS
600A/S3/P/
S1219
C = 430/500
100 mA RMS
) and a stability factor (K), as follows:
CT
)
CT
/V ratio and the required stability factor K has been found to be of a general form
K S
Chapter 8 - Restricted Earth Fault Protection
600A/S1/S1214
C = 670/800
40 mA RMS
600A/S2/P/S1215
C = 570/670
75 mA RMS
600A/S3/P/S1220
C = 520/620
100 mA RMS
) to the circuit impedance. The stability voltage is
K
/V . Where V is the CT knee point voltage and V
K S K
600A/S1/S1214 600A/S1/S1223
C =670/800 C = 740/870
50 mA RMS 50 mA RMS
600A/S2/P/S1215 600A/S2/P/S1196
C =570/670 C =620/740
100 mA RMS 100 mA RMS
600A/S3/P/S1221 600A/S3/P/S1222
C = 570/670 C =620/740
100 mA RMS 100 mA RMS
/V and K that vary in
K S
/V
K
is the stability
S
) will govern
S
/V
K S
189