What are the causes of unbalance current during no load test of transformer?

[za1820]

What are the causes of unbalance current during no load test of transformer?

 

[plugsandsparks]

What are the causes of unbalance current during no load test of transformer?

During the no-load test of a transformer, the primary winding is connected to an AC source without any load connected to the secondary winding. In this condition, the transformer draws only magnetizing current which is required to magnetize the core and establish a magnetic field in the transformer. The unbalance current during the no-load test of a transformer may be caused by several factors including:

1. Magnetic imbalance: If the transformer core is not symmetrically designed, or if there is any damage or manufacturing defect, it may cause magnetic imbalance which will result in unbalanced current during no-load operation.
2. Open circuit: If there is an open circuit in the winding of the transformer, it may cause an unbalanced current during no-load operation.
3. Insulation failure: If there is an insulation failure in the transformer winding, it may cause a current imbalance during no-load operation.
4. Voltage imbalance: If there is any voltage imbalance in the power supply, it may cause current imbalance during no-load operation.
5. Unbalanced core magnetization: If the transformer core is not uniformly magnetized, it may cause an unbalanced current during no-load operation.

To determine the exact cause of the unbalanced current during no-load operation, a detailed analysis and testing of the transformer is required. It is important to identify and correct the cause of the unbalanced current to prevent any damage to the transformer and to ensure its proper operation.

 

[plugsandsparks]

 

[za1820]

Thanks for answering, Transformer was in service for approximately 20 years, when the problem arised we performed all the tests, including Insulation resistance, Winding resistance, TTR, Short cicuit test etc. all test results are satisfactory, yet Transformer takes unbalance current on no load.

 

[pc1966]

How big is the imbalance? Have you got the no-load current values? The incoming voltages?

Also when / how did you notice there was a problem?

If it really is recent then it is unlikely to be the core (unless someone applied a LOT of force, or an earthquake) and that would suggest insulation failure as most likely to me. But I don't know as much on this as the likes of @Julie. or @plugsandsparks

 

[za1820]

How big is the imbalance? Have you got the no-load current values? The incoming voltages?

Also when / how did you notice there was a problem?

If it really is recent then it is unlikely to be the core (unless someone applied a LOT of force, or an earthquake) and that would suggest insulation failure as most likely to me. But I don't know as much on this as the likes of @Julie.

How big is the imbalance? Have you got the no-load current values? The incoming voltages?

Also when / how did you notice there was a problem?

If it really is recent then it is unlikely to be the core (unless someone applied a LOT of force, or an earthquake) and that would suggest insulation failure as most likely to me. But I don't know as much on this as the likes of @Julie. or @plugsandsparks

How big is the imbalance? Have you got the no-load current values? The incoming voltages?

Also when / how did you notice there was a problem?

If it really is recent then it is unlikely to be the core (unless someone applied a LOT of force, or an earthquake) and that would suggest insulation failure as most likely to me. But I don't know as much on this as the likes of @Julie. or @plugsandsparks

TR rating: 5KVA, Step up: 0.470KV / 22KV, No Load current: 8Amp, 0.14Amp, 7.8Amp, Primary side applied Voltage: 0.4KV, Transformer service is as HV power supply unit including rectifier circuit.

 

[za1820]

TR rating: 5KVA, Step up: 0.470KV / 22KV, No Load current: 8Amp, 0.14Amp, 7.8Amp, Primary side applied Voltage: 0.4KV, Transformer service is as HV power supply unit including rectifier circuit.
Primay side winding resistance: 629m-Ohm, 643m-Ohm, 631m-Ohm

 

[Julie.]

TR rating: 5KVA, Step up: 0.470KV / 22KV, No Load current: 8Amp, 0.14Amp, 7.8Amp, Primary side applied Voltage: 0.4KV, Transformer service is as HV power supply unit including rectifier circuit.
Primay side winding resistance: 629m-Ohm, 643m-Ohm, 631m-Ohm

What is the winding group?

Yny Or Ynd1?

 

[plugsandsparks]

I can see why you are querying these numbers. When you say there is a rectifier on the output is this rectifier connected when you are seeing these numbers? - Has the rectifier been tested to ensure all stages are working ?

 

[za1820]

I can see why you are querying these numbers. When you say there is a rectifier on the output is this rectifier connected when you are seeing these numbers? - Has the rectifier been tested to ensure all stages are working ?

No, Rectifier is not connected, Primary is delta connected at 400V and Secondary is star connected at 22,800V.

 

[za1820]

What is the winding group?

Yny Or Ynd1?

dY11 and Primary is delta connected at 400V and Secondary is star connected at 22,800V.

 

[pc1966]

Have you got access to each set of windings separately at some connector board?

I.e. can you separate the delta and energise each of the 3 separately to test each on a single phase supply?

 

[Julie.]

dY11 and Primary is delta connected at 400V and Secondary is star connected at 22,800V.

OK

Since you see the current between two phases on the delta winding, this means the one winding is either at fault, or (more likely) the secondary has a fault - I would guess one high voltage winding has a fault somewhere along its winding to earth (the star point being earthed) or has an inter-turn winding fault.

This phase will be the one corresponding to the branch of the delta connecting both current taking phases (~8A).

Can you flash test the hv windings - ~27-35kV as it may be an insulation breakdown issue, so won't be shown via an insulation resistance test?

If its an inter-turn winding fault it is a little more difficult to detect with an assembled tx.

(The above assumes there is absolutely nothing connected on the hv side - obviously if there is a single phase load/fault in that part of the system it will reflect as a phase-phase load/fault on the primary )

 

[pc1966]

You give the primary resistance, have you also checked the secondary?

Edit: Beaten to it by @Julie.

 

[za1820]

Have you got access to each set of windings separately at some connector board?

I.e. can you separate the delta and energise each of the 3 separately to test each on a single phase supply?

As for now, we do not have access to each phase coil separately.

 

[za1820]

You give the primary resistance, have you also checked the secondary?

Edit: Beaten to it by @Julie.

Yes, Secondary winding resistance is: 2.83K-Ohm, 2.82K-Ohm, 2.83K-Ohm

 

[za1820]

OK

Since you see the current between two phases on the delta winding, this means the one winding is either at fault, or (more likely) the secondary has a fault - I would guess one high voltage winding has a fault somewhere along its winding to earth (the star point being earthed) or has an inter-turn winding fault.

This phase will be the one corresponding to the branch of the delta connecting both current taking phases (~8A).

Can you flash test the hv windings - ~27-35kV as it may be an insulation breakdown issue, so won't be shown via an insulation resistance test?

If its an inter-turn winding fault it is a little more difficult to detect with an assembled tx.

(The above assumes there is absolutely nothing connected on the hv side - obviously if there is a single phase load/fault in that part of the system it will reflect as a phase-phase load/fault on the primary )

The star point is not earthed, All the transformer test results are satisfactory, Test we conducted are following;
IR (Both Windings), TTR, Open Circuit Test, Winding resistance

 

[Julie.]

The star point is not earthed, All the transformer test results are satisfactory, Test we conducted are following;
IR (Both Windings), TTR, Open Circuit Test, Winding resistance

What voltage did you apply to the primary side?

 

[za1820]

What voltage did you apply to the primary side?

400V and no load current was; 7.79A, 0.14A, 8.15A

 

[Julie.]

400V and no load current was; 7.79A, 0.14A, 8.15A

Test voltages

As stated you either have a breakdown at high voltage, or you have inter-turn failure probably on the single hv winding.

Have you flash tested it or not?

If so what voltages hv & lv?