P12x/EN T/Eb6 - Schneider - error

Application GuideP12x/EN AP/Eb6MiCOM P120/P121/P122/P123 Page 25/78The current threshold I> should be set to the rated rms value of the current that flows into thetransformer when the rectifier is delivering its rated load. The relay will give a start indicationwhen the current exceeds this setting but this is of no consequence because this function isnot used in this application. The rectifier curve should be an inverse time curve and shouldcut-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 tothe 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 toleratedload area.Typical settings for the TMS area:Light industrial service TMS = 0.025Medium duty service TMS = 0.1Heavy duty traction TMS = 0.8The high set is typically set at 8 times the rated current as this ensures HV AC protection willdiscriminate with faults covered by the LV protection. However, the high set could be set to 4or 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 currentcould enhance the protection. It is also common practice to provide restricted earth faultprotection for the transformer feeding the rectifier. Refer to the corresponding section dealingwith restricted earth fault protection.