ELR Tripped When Switching on the 3 Phase 60kVA Isolation Transformer

<...is it due to the inrush current that causes the leakage current to increase...>

No. It's due to the earth leak fault that is not related to the inrush current.

sorry for not being clear earlier. the tripping does not occur everytime but very frequent when switching on the transformer. once the ELR passes through this stage, the transformer runs perfectly. therefore, i think this leakage current that causes the ELR to trip is not real value but emplified...some how...?

It's all just speculation until the insulation tester advocated in reply #2 has been put to work. Why not look for the actual problem and then correct it?

Measure the insulation resistance (phase to earth) with a Megger tester (or similar) and report results.

it's 352Mohm

What is the insulation resistance of the wiring connected downstream of the ELR?

Is it anumerical type ELR?

You have not told us..

1. Transfo primary voltage.
2. Number and circuit positions of CT connected to relay.
   
3. Current transformer [CT] ratios.
4. Time delay setting.
5. Is this a new installation or one which used to work OK?
   

Without this, we have little idea of the significance of 3 amp setting.

How do the settings compare with those for 150 kVA transfo?

What are CT details for 150 kVA transfo?

Are primary voltage and relays same for both transfo? If relays are same, have you tried interchange (with change of settings, of course)? Or secondary injection test of relay?

Inrush (magnetizing) currents are never symmetrical between phases - this causes problems with the matching of the CTs [the CTs themselves have different saturation at switch-on] - and sets a lower limit to how low the earth fault setting current/time delay can be.

It does not seem like a real fault - it seems like a wrong setting or relay or CT problem..

1. the primary voltage of the transformer is 415V
2. there are 2 ELR incorporated into the system 1 before the transformer and the other one after the transformer.
3. ELR uses single zero sequence current transformer.
4. time delay setting is 3s
5. this is a new installation.
6. both transformers are set similarly.

It seems you are using a "core balance" CT. In this, all 3 phase cables pass through one CT. I guess same CT type may be used for both transfo, in that case, the smaller cables may be a loose fit.

This kind of CT avoids problems of CT balance, but perfect balance is only got if the cables are disposed symetrically about the CT centre and pass perpendicular to its plane - within CT and for some distance outside it.

Is it possible the cables are badly aligned? Have you checked against CT maker's requirements? Have you asked CT maker for help - they will have test experience of what CT "earth fault" current settings are practical for this CT and how much cable alignment affects operation.

I suggest

1. Open transfo input Isolator.
2. Check all transfo phases dead with voltmeter.
3. Short all cables together at transfo input.
4. Apply voltage to CT secondary with Variac, be sure transfo is not saturated (e.g. doubling test voltage only doubles CT current).
5. measure and record voltages between phases at isolator caused by CT field.

Ideally, voltages at 5/ are identical.

Do test on 150 kVA transfo also, for comparison.

Can you do a primary current injection into each CT primary cable, to check relay setting? A current of 3 amps is not much to get using Variac/step-down transfo and resistor or lamp ballast e.g. 240/12 V transfo with motor car headlamp bulb.

insrush current when making on a transformer may trip the ELR connected to it.

residual inrush current in 3 phases as seen by CBCT may be greater than 3A, thats why ELR trips.

This difference is due to inductance of transformer windings. Ensure proper useage of stabilizing resistors in case you use CBCT with ELR or Numerical relays with 3 CT's wiring inputs.