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Odd differential protection behaviour

Odd differential protection behaviour

Odd differential protection behaviour

(OP)
Dear eng-tips colleagues, I was wondering if someone could give an alternative point of view about the following issue:
Months ago a transformer differential protection relay (7ut 612) was installed on a site.
Conditions:
The transformer which is protected by the relay stays permanently energized on the high voltage side (closed status on -52S1), but de-energized on its low voltage side (open status on -52S2). The low voltage side is closed when all the main transformers in the facility are de-energized.


Problem:
The busbar that feeds the transformer had a fail, which should be an external fail for the protection relay, thus no trip had to be carried out. Nonetheless a current flow was read by the relay and as a consequence -52S1gets tripped


The staff working on site are complaining about the settings in the relay, but in my opinion the settings are correct and the origin of the device trip is the operational conditions of the power system (closed on high and open on low voltage side) caused the unbalancein the current reading.

 

RE: Odd differential protection behaviour

What are the transformer windings?  How are the CTs connected? What kind of fault?  Details! Details! Details!  We aren't mind readers.

RE: Odd differential protection behaviour

In my humble opinion, if a trip was present then you should check your system (connections, site conditions,etc). Or you can try to change the conditions of your system, i mean isolate the transformer which is powered on the high voltage side and connect it when you really need it.  

RE: Odd differential protection behaviour

(OP)
Dear davidbeach:
- The power transformer characteristics are:
Capacity: 5000kVA
vectorial gruop: YD1
Primary voltage: 13200V
Secundary voltage: 2400V
ct's (primary side): 600/5
ct's (secondary side): 2000/5

- The fault was a three fase fault at the source side (for more information see the attached file).

If you need more information, just tell me.

Thank you davidbeach for the interest on the issue.

Ervin,
 

RE: Odd differential protection behaviour

(OP)
Dear batusai 18:
This power system is an 2400v Auxiliary Bus of two 75MW mitsubishi generators, and the mentioned transformer works as a back up of the main ones that feed the bus. Thats why when the main transformer fails or get out of services, the plant operator make the transfer and inmediatly close the low side breaker, to continuing energizing the auxiliary circuits of the plant.

Ervin.

RE: Odd differential protection behaviour

Hi,
I assume that vectot group is YND1 ( I assume RAT with resistive grounding) and was some ground fault on the 13.8lkV side.please verify and please attach settings of 87T function/power system data.
Low level of fault current, same current for diff, for all 3 phases are indicated about wrong setting.

RE: Odd differential protection behaviour

(OP)
Dear Slavag,
The vector gruop is YD1 without earthing the star point, please check the three line diagram attach file and see the connections. As you can see in the pdf the ct's connections are star-star in the both side, and are taken from the ct's circuit breaker. The HV ct's has a 150meters away from IED (7ut612) and LV ct's has a 25meters away from the IED.
Searching in the disturbance recorder, we can find a three similar currents in the HV side, and no currente in the LV side, that's why in the second attached file "indication fault" you see similar values in the Idiff current and Ibias or Istab current. and getting a trip.
Please check the following files:
- Disturbance recorder.
- Setting.

Thanks.
Ervin.

RE: Odd differential protection behaviour

Hi Ervin.
I put 99.9999%, this transformer is earthed on the Y side, according to disturbance recorder, its outside groun fault is closed via neutral of tranformer.
more, please see your setting.
gr 4.4
adrdress 0313 is isolated for Y side
address 0323 is solid grounded for!!! D connection.
I think this is mistake

smile Im also learned somthing in Spanish,.

RE: Odd differential protection behaviour

Sorry:  aislado=isolated, my mistake

will check again

RE: Odd differential protection behaviour

(OP)
Dear Slavag,
- address 0313, makes reference to star point of HV winding (wye),   wich is earthed,
(it has relation with address 0314 "winding 1 connection" -> Y(wye))
- address 0313, makes reference to star point of LV winding (delta), wich is isolated,
(it has relation with address 0324 "winding 2 connection" -> D(delta)).

Thank you slavag your interest on the issue.

Ervin.

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