990XCA65609 SCHNEIDER

MiCOM P125/P126 & P127 Page 31/96 

The general stability conditions can be obtained when: 

Vs> K.If (RCT + 2RL) 

Where K is the stability factor. 

This stability factor is influenced by the ratio Vk/Vs which in turns governs the stability of the 

REF protection element for through faults . 

Vk = The Knee point voltage of the CT. 

To obtain a high speed operation for internal faults, the Knee point voltage Vk of the CT must 

be significantly higher than the stability voltage Vs . A ratio of 4 or 5 would be appropriate. 

For MiCOM P121, P122 and P123, we found the following results: 

K= 1 for Vk/Vs less or equal to 16 and 

K= 1.2 for Vk/Vs > 16. 

NOTE: The maximum internal fault level for stage 3 of 0.002 to 1In board 

must not exceed 20In. 

STAB

A

B

C

E/F Input

P0343ENa

CT CONNECTION DIAGRAM FOR HIGH IMPEDANCE REF APPLICATION 

2.10.3 Setting guide 

The characteristics of the relay and the value of K influence the stability of the scheme as 

explained here above. 

The typical setting values shall be chosen to provide a primary operating current less than 

30 % of the minimum earth fault level for a resistance earthed system. For a solidly earthed 

system, the typical setting shall provide an operating current between 10 and 60 % of the 

rated current. 

The primary operating current, at the secondary, depends on the following factors: 

− Current Transformer ratio 

− Relay operating current IS

− Number of CT in parallel with the relay element (n) 

− The inrush current of each CT (Ie) at the stability voltage 

Iop= CTRatio .(Is + n.Ie) 

Current setting should be selected for a high impedance element so that the primary current 

reaches its nominal current with a given CT, according to the following equation: 

Is < {(Iop / CTRatio) - n.Ie}