Earth Fault And Earth Leakage

electricalexpert65:

Thank you for your reply, after installation, the earth fault protection relay is tripped. The calculation showed me that the unblance current protection is set at 30%. Does it mean that the power system has accidental connection with earth? Or the relay tripped because of unbalance currents only?

It could be either. If you don't want your earth fault relay to trip for unbalance currents, provide a CT in the neutral and connecet it residually to the phase CTs. Then the earth fault relay would trip only for genuine earth faults.

electricalexpert65:

Still not so clear with what you mean. I have 4 CTs installed to 3 phase and 1 neutral already.

What you mean by "provide a CT in the neutral and connecet it residually to the phase CTs".

The relay manual says that the percent of ILn "earth fault current" shall be from 120% to 200%. What does it mean by that? Does it mean the unbalance current? If not, why it provides us such a big limit of 200%?

Regards,

What is the relay (make & model) you are using? And, what is the earth fault element setting?

If you already have provided 4 CTs and still the e/f element is tripping, then it could indicate a genuine earth fault.

But, wait, you had said that this element trips on switching ON. Then, probably you must be using a Numerical Relay, where the earth fault current is sensed by sensing the zero sequence component of the phase currents. And, during switching ON of reactive loads, there could be great unbalance in the phase currents, due to inrush currents, and, the zero sequence current will be more than the setting made.

The solution is to introduce a time delay of about 100 milli seconds in the erth fault element, by which time the inrush would have subsided. Or you can de-senstise the e/f element by providing a stabilising resistor.

With more details about the relay and settings, I can comment further.

electricalexpert65:

It is a Mikro relay and a combination of overcurrent (OC) and earth fault (EF) protection. We have set it as 30% of the maximum current on the total loads.

Yeah, it is correct, as I have stated. But, several days later after this question was posted on here. We got the other EF tripping of the breaker. We thought that the the current was too low and we added up with the fire pump capacity (capacity up to 95kW). So, now, the setting current is 30% of the total load plus the fire pump. It was around 150Amp. The total current is around 570Amp.

However, the breaker tripped again and we one more time increased the setting up to 50% of the total load. Will update if there is anything new.

By the time setting, we have set is at 5 seconds to increase the endurance.

Special note: I have recorded the three phases and seen that the worst unbalance case was just around 10%. The average unbalance during 4 hours or recording was 8.1%. This cannot explain if we say the breaker was tripped by unbalance current because the setting is at 30%.

What you mean by adding up with stabling resistor?

I am wondering about your question. Fist of all please note that the difference between those relay: 1. Unbalance Protection (66 as per ansi relay code) 2. Earth fail protection (51N, 50N..) 3. Earth leakage protection If you are talking about Unbalance protection (nomaly for motor protection) then 30% setting for unbalance current is ok, and your relay tripping is not cause by unbalance current. and i agree with electricalexpert the reason maybe due to the inrush current. if so farther study need to be considered.

nguyenkhoadung98:

If it was not tripped by unbalance current, we have inscreased the total current as large as 578A then set the EF current at 30% (which means 173A).

The breaker still tripped by EF.

Base on those facts, is there any possbility that our power system has contacted to earth in some area? Can we eliminate the possibility of unbalance current?

We are clear that your relay donot trip by unbalance current, so no need to care about unbalance current. The matter cause by other function (earth fail for exam.). we need many information to check your earth fail setting is correct or not. 1. Short circuit study report 2. System information like: location of your relay, transformer vector group,.... ...... with those information you gave, i have no idear for relay setting, excluding unbalance setting.

The location of the relay is after the transformer.

But I have the other question right here since you mentioned about the Vector group. Is there any affection to the EF relay function if we change the phase sequence.

Lets say from transformer outgoing: (L1,L2,L3) and to the beaker incoming: (L2,L3,L1)...

If your CT (current transformer) is correctly installation (phase sequence as per transformer outgoing)=> no problem. If your CT install according to CB outgoing => no problem for 50, 51, 50N, 51N, and other overcurrent relay, but for differential relay (87, 64R...) very big problem

electricalexpert65:

Still dont understand what you want to imply from this:

"And, during switching ON of reactive loads, there could be great unbalance in the phase currents, due to inrush currents, and, the zero sequence current will be more than the setting made".

Are you really electrical expert?

I am very much tempted to reciprocate your "mud-slinging"; but, this forum and my maturity has taught me to be calm. I don't need to prove my expertise to anybody.

Now, to answer your question, please refer to the Areva NPAG. It is mentioned in this guide book - in the chapter on Motor Earth fault protction - that what I had said can very much happen. If you are still not staisfied, then please attend to our 4-days training programme on protective relaying. You can, then understand, first-hand, whether I am an electrical expert or not.

By the by, politeness is a virtue.

I appologize if my words hurted you. I hope there would not any misunderstanding between us. I repect you and the other gurus by what you have been contributing to the forum.

You know, not only knowledge and experience. What I have learnt from this forum is the positive attitude like what you did in the previous post. To me, it is great. That is the first virtue for ones who want to become a professional engineer.

From this, we see that CR4 is a truthful place of engineers for discussion.

Thank you,

This protection relay gimme NUT !

I think that the problem is much clearer at the moment. All I need to do is to find if it is really a case of earth fault where the phase is connecting to earth.

This is an improvement project and sometimes the people in this factory uses the earth wire as a neutral wire. This is I have just known.