Areva MiCom P120, Technical Manual, Page: 246 / 668

P12x/EN AP/A96 Application Guide
Page 24/80
MiCOM P120/P121/P122/P123
The current threshold I> should be set to the rated rms value of the current that flows into the
transformer when the rectifier is delivering its rated load. The relay will give a start indication
when the current exceeds this setting but this is of no consequence because this function is
not used in this application. The rectifier curve should be an inverse time curve and should
cut-off currents below 1.6 times allowing the rectifier to carry 150% overload for long periods.
If this is not acceptable, the I> setting can be adjusted to move the cut-off point relative to
the current scale. The operation time can be modified by adjusting the time multiplier setting
(TMS) so that the time lies between limiting characteristic of the rectifier and the tolerated
load area.
Typical settings for the TMS area:
Light industrial service TMS = 0.025
Medium duty service TMS = 0.1
Heavy duty traction TMS = 0.8
The high set is typically set at 8 times the rated current as this ensures HV AC protection will
discriminate with faults covered by the LV protection. However, the high set could be set to 4
or 5 times the rated current if the AC protection is not trustworthy.
Use of the thermal element to provide protection between 70% and 160% of rated current
could enhance the protection. It is also common practice to provide restricted earth fault
protection for the transformer feeding the rectifier. Refer to the corresponding section dealing
with restricted earth fault protection.