[Brett Welsh]

Earth fault tripping on starting of Large 3 phase motor. Can current imbalance and CT connection scheme explain this. Or other reason?
Earth fault tripping on starting of Large 3 phase motor. Can current imbalance and CT connection scheme explain this. Or other reason?

With 2 different motors we changed the protection settings as recommended (by others) protection study. Earth fault protection on both motors was sensitised to grade with power main switchboard bus bar protection. Now on starting the motors they trip on earth fault.

At starting there is a 4-5% phase current imbalance. Can this explain the earth fault detection due to using a residual connection of the three phase CTs? (Holmgreen connection below) Or is other cause likely?

Motor 1 – 11kV, 3P3W, FLA 243A. Protection relay – GE multilin 269+
Sample log of trip

I1= 206A
I2= 202A
I3= 220A
I0= 23.9A

new setting (trips on start) – 0.1xCT Def Time 0.6sec
old setting (okay) – 0.25xCT Def Time 2 sec

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Topic

[Hamid]

In starting situation we front to unbalance and zero sequence secondary currents due to unequal three ct characteristics (CT magnetizing current and ratio error), possible unequal CT core saturation and unequal CTs secondary circuit impedance. Generally there are some recommendations in this regards as following: To provide earth-fault protection, it is recommended the core-balance CT solution whatever type of network earthing is used. This method allows for the detection of small resistive values of earth fault current. Furthermore, an earth fault unit supplied from the residual connection of three line CTs as mentioned above carries the risk of mal-operation during starting. In order to obtain stability the pick-up must not be set below 0.15 to 0.2 In of the CTs, which is often too high compared to the maximum earth-fault current. Most LV systems fall into solidly earthed category, for reasons of personnel safety. Two types of earth fault protection are commonly found

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