Site power- Delta Star Isolation Transformer

[Sathyajith]

Hello,
I'm in a situation where a Delta Star Isolation Transformer is used to get 400V power from a HV Substation(Temporary construction works). Secondary star side is earthed (MEN/Neutral link). Attached a sketch on the arrangement.

The Supply side Circuit breaker (CB feeding primary side) trips when energised? No secondary side loads are connected. Any thing fundamentally wrong here/ or any thoughts what is the reason ?
P.S Need to earth neutral point as a regulatory requirement (MEN connection)

 

[xnuke]

Some ratings would be nice...

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[beyond86]

Did you check protection settings at supply side relay?
The resistor at neutral point doesn't affect the primary side if you don't have a short circuit on the secondary.
I think CB trip from inrush current and incorrect protection settings.

 

[dpc]

If it is tripping immediately upon energization, it is most likely due to the transformer inrush. If the primary breaker has adjustable instantaneous function, increase it to maximum setting. It's possible that you may have to increase the breaker size. Assume inrush of 10 time full load current for 0.1 seconds as a starting point.

 

[waross]

Under the Canadian code, a transformer with a 415 Volt primary supply must have a breaker rated at not more than 150% of the transformer rated primary current. If the current rating does not correspond to a standard breaker size, the next larger size may be used.
If the feeder is protected by a breaker set at not more than 300% of the transformer rated current then the breaker at the transformer may be ommitted, (or a larger breaker used as a local disconnect).
If the 415 V is a typo and the primary voltage is over 750 Volts, the breaker may be rated or set at 300% of the transformer rated current.
The instantaneous trips may be set to 10X.

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

 

[Sathyajith]

Hi All
Thanks heaps for the answers.
Inrush current could be a good reason, We tried without the star side earth and tried to energise, then the CB does not trip ?

Few more info:
Supply MCCB is 80 A Rated, 3 pole breaker ( no realy..etc is connected, and no setting for the CB)
Transformer Rating 50 KVA, 415/415 V, Dyn11 Dry type

The Substation has a auxiliary + Earthing TF - So need to use a Isolation transformer to get external power.
The transformer has same voltage on same side, only used for isolation

Need to check using a higher breaker size.

 

[waross]

70 Amps FLC with an 80 Amp breaker?
Under the Canadian code you could use the next size larger breaker than 104 Amps.
or
A 200 Amp breaker (300% of 70 Amps) ahead of the feeder. Then there is no limit to the size of the local breaker.

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

 

[FreddyNurk]

Is this a non-adjustable MCCB and it's effectively a C curve breaker?
Similar experience with step-down transformers of similar sizes resulted in us using D curve breakers as the inrush would trip the incoming breaker half the time. We got away with MCBs rather than MCCBs though as the fault level was just about non-existent, and we ended up 'scoping the inrush currents before we changed the breakers, to ensure that there wasn't an actual fault.

EDMS Australia

 

[dpc]

Tripping on inrush can seem a bit random since it depends on voltage angle on closing and remnant flux - it often does not occur 100% of the time. But that breaker seems too small to handle the inrush reliably. Not sure on your local codes, but as waross, I think you will need to go up at least one size. This could impact your conductor size on the primary side unfortunately.

 

[Imdsahay]

Inrush current looks most likely reason but if you share the ratings, then it will be easier to pinpoint it. Hope you must have completed the transformer dry out, in case of oil-filled transformer.

 

[Kiribanda]

Sathyajith,
You have mentioned that,
1) You are using this 415/415V transformer for isolation from the existing earthing transformer of the system.
2) When the earthing of the star side of the isolation transformer was removed and the star point is floating, there was no tripping.
Could you please upload your SLD showing how the above two transformers are connected in your system?

 

[waross]

We tried without the star side earth and tried to energise, then the CB does not trip ?

Tripping on inrush can seem a bit random since it depends on voltage angle on closing and remnant flux - it often does not occur 100% of the time.

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