Sound Coming from NER/NGR with Minor Current Flow

What's a NER/NGR? A/c = airconditioner?

Let's see:

The amount of current flowing in the Neutral Grounding/Earthing Resistor is an indication of the amount of load/current imbalance in the generator. The vector sum of the three phase currents is flowing through the resistor is indicated by your ammeter, under perfectly balanced conditions it would be zero, otherwise current flows, and don't forget that any harmonic currents may also register.

Thanks for adding some sanity.

What is the frequency of the hum? 50, or 60 Hz?

With generators, there will be third harmonic currents circulating in the system even without any load unbalance or a phase-to-ground fault, i.e. even during normal 'system healthy' conditions. When generators are part of the system and the neutral earthing is designed to allow over 100A current during earth faults, it is advise to ensure that there is only one system earth. That is to say, in case there is another generator or a grid transformer, the subject equipment neutral shall remain disconnected from earth. This is to ensure that there is no closed loop for the third harmonic currents to circulate. You may like to verify.

With regard to the noise, check for any loose connections in the NER. I don't think the noise should be there if the NER is healthy.

Further, the NERs are generally short time rated. They do have a continuous rating but quite a small value. You may like to verify this as well.

It would be good if you could post the feedback.

I agree with Raghun. Do some additional checking with a tong type ammeter you may have current flowing that is well below the relay setting. This may be acceptable to you but really it should be zero. Make sure your CT & relay are also connected correctly - you would not like to be missing the fault.

The 3rd Harmonics can show as a hum also but if they are high then your main transformer will most likely be running hot - have a look at the operating temperatures as well.

Feedback would be good.

Hey thanx :)

We have four (04) generators each of 6.6kV, 50Hz on single bus; three (03) operate at a time. The NER or to clear any confusion the ''Neutral Earth Resistor'' is rated at 10 ohms, 350 Amp for 10 seconds.

Do you mean to say that only one generator's neutral be earthed and the rest be disconnected, just to stop the circulating harmonic current? wouldn't this lead to other adverse affects?

Would def check again for any loose connections and Trafo operating temperatures.

Btw how is the frequency of hum measured? its just sound isn't it?

Thanx again.

rizwan is right, dont disconnect any neutral-earth bonding in the generators. generators typically come with an internal neutral-earth bond that should not be disconnected.

That's right Rizwan. Here, NER value is such as to allow 350A current during earth faults. When there is an earth fault in generator stator winding we are talking of ~1000A flowing (3-generator contributing 350A each). As I said earlier you need to keep only one earth connection, disconnecting all other earths. You can check with the manufacturer for his recommendation.

Having said that, the caution is not to leave the system unearthed at any given time. This means an alarm in case of trip of the generator that is providing the system earth and operating procedures to select one of the other generators for star point earthing without loss of time.

imho, it would make more sense to earth the system at the Main distribution board.

or, leave all generators earthed, but connect together all the ground rods into a grid.