TRAFO PROTECTION SCHEME

[PUNGLU]

We are installing a 7.5 MVA TX under a tight schedule and REF scheme is included in the design. The following are the details:
An LV CT- 400/1A class Px, 15 VA, Vk >= 250V, Imag<= 30mA at Vk/2. Another neutral CT has ratio of 400/1A and is of clas Px, 15VA.
Other available CTs are:
LV line CTs: 450/1A class 5P20, 15 VA for O/C and Diff.
HVline CTs: 150/1A class 5P20, 15 VA for low impendance Diff.

How can you incoprporate an REF scheme? Since the LV outgoing cable is long (type XLPE. Does differential scheme and O/C suffice to protect the cable and TX?

 

[slavag]

Hi Punglu.
It's not so correct Q's.
You need check CT performence according to your relay requerments.
What is a long cable? 200m or 2 km?
You need check secondary burden of LV side CT.
Please give more information.
Regards.
Slava

 

[PUNGLU]

Sorry, the exact details are as follows:

An LV neutral CT- 400-200/1A class 5P10, 15 VA for E/F,Another LV neutral CT has ratio of 400/1A and is of clas Px, 15VA for high impendance REF.
Other available CTs are:
LV line CTs: 450/1A class 5P20, 15 VA for O/C and Diff.
HVline CTs: 150/1A class 5P20, 15 VA for low impendance Diff.

The trafo is 7.5 MVA 33/11 kV: Ratings: 131 Amps on HV and 393.5 Amps on LV
NOTE: All are bushing CTs and the outgoing 11 kV XLPE cable type is 150m.

Relays available are: SIPROTECT 7UT61, SPAJ 140C AND SPAJ 115C and would like to include

I hope this is clearer.

 

[slavag]

Hi Punglu.
Several additional Q's.
1. For curios only. Why you use for this size of xfr. relays from several companies. Possible use same series:
or SPACOM relay or SIPROTEC relays?
2. I'm not sure, if you need REF (HI version) for this size of xfr. Maybe enough LI version of REF, additional wiring and varistor and resistor is additional cost only.
3. SPAJ115, as I remember, used for two function REF (HI)and BEF.
4. I checked your previos posts and see, that you used only solidly grounded systems.
5. I would like recommend you some simple and cheaper system
( for this I prefer used SPACOM relay of ABB, I have all documentation on the my laptop).
6. SPAD 346 relay with all 3 modules.
provide 87T protection ( it's not long cable and your CT with 1A secondary, but I strongly recommend use 6mm^2 current wiring,that mean you have good performrnce of CT's) second module provide REF for the HV side and REF for the LV side, and last module 3-stages of OC (50,51) protection, one as back-up of diff protection and one IDMT covered OC protection of LV side.
7. SPAJ140 you used as LV OC (51,50) side.
To I0 input of the SPAJ140 you can connect or residual current of LV CT or connect additional CT on the LV neutral.
Please see attached example ( w/o SPAJ140).
I used this connection as my standard and SPAD346 for me is standard relay for the "small" xfr's.
You can use same scheme with SIPROTEC relays, but, as I remember you need 3 relays ( I'm not sure if SIPROTEC diff relay have enough current inputs).
Regards and Good Luck.
Slava
and additional tip.
Electromechanical trips from xfr. ( bucholtz, OLTC relay, etc,) you can connect directly to SPAD346 ( 5 Digital inputs),of course SIPROTEC also)

 

[PUNGLU]

Thanks very much. The project is kind of tight and I have been provided only with the relays I mentioned

 

[slavag]

OK, also no problem.
You haven't set of LV CT for HI REF, you must set of 3xCT's class X, 400/1A, same as neutral CT for HI REF
Are you have varistor and resistor for SPAj115 ( HI REF)?
You haven't any option for the HI REF

1. LV neutral CT 400/1 class X you connect to SPAJ115 terminals 25,27.
2. HV side of SIPROTEC you connect to HV CT 150/1A 5P20.
3. LV CT 400/1A you must connect in series with SPAJ140 and LV side of SIPROTEC.

I don't remember if Siprotec also include LI REF, I assume, have.
LV neutral CT 5P10 you will connect to SIPROTEC.
Regards.
Slava

 

[slavag]

Hey.
REF in 7UT61 it's optional function. Check if you have it.
Punglu, between us, 7UT61 + SPAJ140 for this size of xfr.
are good protection. HI REF is nice to have.
Regards.
Slava

 

[PUNGLU]

Thanks so much! Infact I have decoded to exclude HI REF. Will connect SPAJ 140 in series to the Siprotec on LV side then use stand by E/F. I also think this suffices to protect the trafo

 

[PUNGLU]

One more thing, I desperately need to protect the cable to the incomer? I suppose a unit protection to it is important knowing evry well the seriuosness of cable faults!

 

[slavag]

Morning Punglu.
Protection of cable ( I assume you mean, 150m cable of LV side) covered by diff 87T protection and by two stages of O/C ( 50,51)of HI side of 7U61x relay.
Regards.
Slava

 

[PUNGLU]

I suppose the locaton of the CTs (Tx bushing)doesn't affect sensitivity for cable faults

Thanks

 

[slavag]

OOPS!!!
My bad!!!
I thinked about LV CT installed on the LV swg.
If you have only xfr bushing CT and connect to them SPAJ140,
then SPAJ140 will coverd cable O/C ( 50,51, 50N, 51N) protection.
Good practic it's covered with 87T also cables (HV and LV).
Check this option. What do you have at LV swg infeed bay?
Could you send us some drw. of your connections.
regards.
slava

 

[PUNGLU]

All the CTs are in the Trafo bushings. The breaker after the 11 kV cable (Client's and not the utility's) has CTs on the Red and Blue phases only for O/C and E/F protection

 

[slavag]

Hi.
Your scheme:
1.7UT61x will covered 87T of trafo only and include 50,51 (will connected to HV CT)
DT will covered 87T back-up protection, IDMT will back-up of LV cable. Additional current input ( 7) will connected to first neutral CT and will provide 50N, 51N protection.
2. SPAJ140 covered 50,51 of cable and provide back-up of 11kV swg protective relay. Second neutral CT will be connected to I0 input of SPAJ140 and will provide 50N,51N with same setting as 7UT61x.
SPAJ140 CT will wiried in series with LV CT of 7UT61x
3. You don't need SPAJ115.
4. You need also coordinate protection 50,51,50N,51N of SPAJ140 with protection relay are installed at 11kV swg.
Regards.
Slava

 

[PUNGLU]

Thanks alot, I am already working on this and you've been of great help!!!

 

[RRaghunath]

I think you can have low impedance REF protection for the 11kV windings. This feature is available in 7UT61 and one of the neutral CTs can be used in addition to the 11kV phase bushing CTs (already available for the relay for 87T).
11kV cable protection can be taken care by the Standby earth fault protection connected to the second neutral CT that is available in the transformer.