Micom P721, Technical Manual

Page: 150 / 334

P72x/EN AP/C21 Application Guide
Page 14/52 MiCOM P721/P723
With a sinusoidal voltage applied across the Metrosil, the rms current is as follows:
4
RMS
RMS
C
2 V
52 . 0 I
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
Where:
VRMS = rms value of the sinusoidal voltage applied across the Metrosil
This is due to the fact that the current waveform through the Metrosil is not sinusoidal but
appreciably distorted.
For satisfactory application of a non-linear resistor (Metrosil), the current through the
nonlinear resistor at the relay voltage setting should be as low as possible but no
greater than approximately 30 mA rms for 1 A CTs and approximately 100 mA rms for
5 A CTs.
2.6.1.1 Metrosil units for relays with 1 A CT
The Metrosil units with 1 A CTs have been designed to comply with the following restrictions:
•
At the relay voltage setting, the Metrosil current should be less than 30 mA rms.
•
At the maximum secondary internal fault current, the Metrosil unit should limit the
voltage to 1500 V rms if possible.
The Metrosil units normally recommended to be used with 1 A CTs are shown in the
following table:
Nominal Characteristics Recommended Metrosil Type
Relay Voltage
setting
C
β
Single pole Relay Triple pole relay
Up to 125 V rms
125 to 300 V rms 450
900 0.25
0.25 600A/S1/S256
600A/S1/S1088 600A/S3/1/S802
600A/S3/1/S1195
NOTE: Single pole Relay Metrosil Units are normally supplied without
mounting brackets unless otherwise specified by the customer.
2.6.1.2 Metrosil units for relays with 5 A CT
These Metrosil units have been designed to comply with the following requirements:
•
At the relay voltage setting, the Metrosil current should be less than 100 mA rms. The
actual maximum currents passed by the units are shown in the following table, below
their type description.
•
At the maximum secondary internal fault current the Metrosil unit should limit the
voltage to 1500 V rms for 0.25 seconds. At the higher relay settings, it is not possible
to limit the fault voltage to 1500 V rms, hence higher voltages have to be tolerated
(indicated by *, **, ***).