Intermittent fault in induction motor 2

[Aakash12845382]

I have one HV 3 phase star ungrounded induction motor. Sometimes there is a short duration of 5 milliseconds high magnitude currents recorded at its neutral side. The frequency of occurrence is random, sometimes even few days. How can a fault be of such type? Please suggest.

 

[Kiribanda]

What do you mean by "Its neutral side" because the star point of the motor is not grounded?
Do you mean that the 50/51N relay associated with the motor, is giving some current while the motor is running or at its start?

 

[Aakash12845382]

There are CTs mounted at its neutral side also and phase side also. Only neutral CTs are showing short pulse of high currents. Phase side there is a marginal increase.

 

[Kiribanda]

What is your voltage? Is it a 3-Wire or a 4-Wire system?
Could you please upload a copy of the SLD showing how 50/51N relay is connected specially showing how the NEUTRAL CT is connected?

 

[Aakash12845382]

The red plots show current spikes by only Y and B phase CTs (neutral side). Rest CTs show nearly same currents as previous.

 

[Kiribanda]

Looks like the Y & B phase CTs at the Neutral side have a little mismatch. Also per your dwg the two currents are out of phase by 180 degree.
Have you checked the wdg insulation for any possible faults?
Have you double checked the polarity connections of the CTs?
Is it a new motor commissioned recently or an old motor running without any trouble but giving this fault recently?

 

[waross]

Do you monitor the voltage?
There may be a reoccuring external event causing the Y-B voltage to go high momentarily.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Aakash12845382]

All tests done for motor and CT and found healthy. There is no drop in voltage during these current spikes.

 

[waross]

I would be looking for a spike in the supply voltage rather than a dip.
At this point I would consider a loose connection or a faulty relay.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

If the polarity is confirmed correct, then it looks like there is a slight CT mismatch.
But my concern is why 180deg out of phase.
Why don't you thinking replacing the CT set but it should be all six? Not that easy.

 

[waross]

I would be looking for a spike in the supply voltage rather than a dip.

A fault in the motor will cause a dip in the voltage.
A spike in the incoming voltage may cause a current spike.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Aakash12845382]

If the primary currents of a current transformer is fixed, how can it generate spikes on its secondary which are 3-4 times full load current. Won't it violate equations of transformer action?. Any chance of noise is also ruled out becuase spikes are being produced consistently in only two phases and 180 degree out of phase.
Also there are dual redundant relays mounted which recorded the same current nature.

 

[edison123]

Swap the R & Y CT's and see if the problem goes with CT or stays with the winding phase.

Muthu

http://www.edison.co.in

 

[waross]

Edison's suggestion is a great suggestion.
But, the spikes being 180 degrees out, that indicates to me that the current is flowing out from Y and returning through B.
A high voltage transient may saturate the windings and cause spikes like that but it would also show on the line side CTs.
Is your supply system grounded and is the ground in good condition?


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

N point is a cable connection or made by a contactor?