inrush current on overcurrent relay

[odlanor]

We use inverse time overcurrent relay to protect power transformer(low power). During setting we have to take care to avoid phase overcurrent relay to operate during magnetizing inrush current.Some people apply this criteria for neutral overcurrent relay too.
In my opinion its is not necessary because inrush current are very rich in second harmonic. These harmonics has no zero sequence.
Question is: Do we have to use this criteria for neutral overcurrent relay?

 

[jghrist]

If the transformer has a delta primary winding, then there will be no zero-sequence inrush current. If it has a grounded wye primary winding, I'm not so sure. Second harmonic currents are negative sequence if they are balanced. There may be a zero-sequence component in unbalanced second harmonic currents.

 

[xxjohnh]

Time setting of overcurrent relays are usually several seconds, unless it has instanteous element, inrush currents have long since died away.

 

[slavag]

Odlanor, is depend, what is a type of E/F input connection:
Holmogreen ( sum of phase currents) or dedicated toroidal CT.

In case of holmogreen connection and short time of trip about (0.4sec) you must take in acount inrush current ( see again jghrist's: There may be a zero-sequence component in unbalanced second harmonic currents).
, in second case -no need.

Best Regards.
Slava

 

[odlanor]

Guys,
Let me explain better a typical auxiliary system service for hydropower plant:
1) Radial system 13800-440-220V , each one three-phase balanced, solid grounded by source side.
2) Circuit breaker are 3-phase air-circuit breakers.
3) Protection is overcurrent relay (50/51-50N-51N) at all feeder
4) Panel and transformers are connected by insulated cable.
5) Transformer are dry type three-phase , Ynd or Dyn protected with ground overcurrent relay at neutral. Up to 1MVA has differential protection.

During transformer energization , assume inrush current of 8 or 12 times normal full-load current for 0.1seg.

Question 1-
If you suddenly energized a Dyn transformer will be inrush current in phase relay-51 and residual relay -51N of both sides of transformer and neutral overcurrent relay 51N ?

Question 2-
If you suddenly energized a YnD transformer will be inrush current in phase relay-51 and residual relay -51N of both sides of transformer and neutral overcurrent relay 51N ?

Question 3-
Because inrush current are very rich in balanced second harmonic , these harmonics has no zero sequence. During setting , do we have to take care mal-operation of ground or residual overcurrent relay?

 

[jghrist]

I assume you are energizing from the 13.8 kV side. There will be no inrush current in the 440 (220?) volt side if there is no source.

If the 13.8 kV side is delta, there will be no inrush current in the residual relay (except for a small amount due to CT error).

If the 13.8 kV side is wye, how is the neutral grounded? I think there might be zero-sequence inrush if it is solidly grounded, but I could be convinced otherwise. I've never seen oscillography of the inrush on a grounded-wye primary transformer.

On what basis do you assume a balanced second harmonic?

 

[odlanor]

jghrist
Yes. Energization begins at 13.8kV
1- there a transformer regulator 7.5MVA-13.8/13.8 kV
D(source)/yn(load).
2- There is a transformer 1MVA-13.8/0.46kV D(source)/yn(load).
4-13.8 kV side is wye, solid grounded.

I understand if a system is balanced ,harmonics of pair order are three-phase balanced with frequency in order pair from nominal. In this case will be 2*60Hz.
Only odd pair harmonic has the same angle phase for each phase (like zero sequence). It will be the case of 3*60Hz.
It should be understand that magnetizing current of transformer has 3rd harmonic. It is a normal condition of transformer with load.
Magnetizing inrush current has 2nd harmonics. It is only in energization.