Setting of earth leakage protection relay 1

[registigger]

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.

 

[Skogsgurra]

Hi marcosheila,

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.

 

[registigger]

Thank you skogsgurra. I will look into it.

 

[jraef]

We had a similar problem on a series of installations each with 3 900HP MV Soft Starters and 2 1200HP MV VFDs. The Multilin 469 relays on each individual motor controller were fine, set to trip at 2.5A, but the protective relay (I beleive it was ABB) on the main station transformer would nuisance trip several times, taking down the entire pump station at each site. They turned the GF settings on the transformer GF relay up to 10A to get it to stay in, but they were relatively confident that since 99% of the load was the soft starters and drives which each had their own 469 MPR, they still had adequate protection and left the transformer relays that way.

"Venditori de oleum-vipera non vigere excordis populi"

 

[registigger]

Thank you jraef. I have read the adjustment instructions from various manufacturers of protections relays and they advise to set the relay above the tripping threshold under normal operation. I have advised the customer to raise the setting by 50%. This transformer is dedicated to the drives that have their own protections. The risk is minimal.