Setting of earth leakage protection relay
Setting of earth leakage protection relay
(OP)
What would be an acceptable setting for an earth leakage protection relay supervising a 1200KVA transformer on the 480V secondary (about 1440A line current). This relay used to be set at 2.5A and had not tripped for years. Recently, high power AC drives have been added to the feeder using most of the transformer's capacity. We had to raise the setting to 5A but still get some trips when the drives are running at full capacity. We have taken measurements and found no alarming causes for the tripping. It seems to be caused by random switching coincidence that sporadiquely increase the leakage level of the transformer. Before increasing the level by another 50%, we want to know what is the acceptable level. The transformer manufacturer does not have experience with this type of protection feeding large drives.
Thank you for sharing your experience.
Thank you for sharing your experience.





RE: Setting of earth leakage protection relay
You have run into a very common problem. VFDs cause a lot of "leakage current". But it is not a leakage current flowing through bad insulation resistance to ground. Instead, it is the high-frequency components in the PWM voltage that find their way through motor/ground capacitance and cable/ground capacitance. Shielded cables have much more capacitance than unshielded ones and if you also have filters added to the VFD outputs, you will have lots of current flowing to ground.
You may have to turn your relay up considerably to avoid nuisance tripping. So high, in fact, that it probably will not be of any use.
There is (have been along for quite a while, actually) a new breed of relays that work on entirely different principles and actually measure the insulation resistance. The are quite good and can be compensated for capacitance and other problems. They are often able to find 100 kohm faults in a plant with tens of VFDs. Google Bender and Isolation for more information.
RE: Setting of earth leakage protection relay
RE: Setting of earth leakage protection relay
"Venditori de oleum-vipera non vigere excordis populi"
RE: Setting of earth leakage protection relay