Pros/Cons of Two Power Transformers Connected

[Kiribanda]

Dear friends!
Two Power transformers each having 50 MVA/ 33/11 kV/ Dyn11/ OLTC, are going to be connected in Double Ended configuration ( i.e. each 11 kV secondary is connected to Bus A & Bus B having a NO Bus-Tie ) to a 11 kV switchgear. They are going to be fed from a remote 33 kV substation about 850 m away. Each of these 850 m long 33 kV cables are going to be directly terminated to the 33 kV side of each transformer without an additional 33 kV switchgear. The protective relay scheme for this Feeder & Transformer has got two different options.

Option #1: Feeder and Transformer as two Zones of protection.
First zone covers the 33 kV feeder with Overcurrent (50/51),Earth Fault (51N) & Sensitive Ground Fault (50GS) protection extending from the 33 kV SF6 breaker up to the primary Delta winding of the transformer, which is typical for a transformer feeder.
Second zone covers the 50 MVA transformer as a unit with Phase Differential (87T) and Restricted Earth Fault (87N) protection, also typical unit protection for a large power transformer.

Option #2: Feeder and Transformer as single Zone of protection.
This consists of Overcurrent (50/51), Earth fault (51N) & Sensitive Ground fault (50GS) for the feeder, Phase Differential (87) for the feeder plus transformer and Restricted Earth Fault (87N) protection, a typical “Translay Differential Scheme “ (similar to a conventional Phase Differential protection for a MV cable+motor )

As far as better coordination, cost, c/t locations, feeder length etc are concerned I always support the option 1. On the other hand some of our friends may already be using the option 2 ( if I am not mistaken it is the so called Translay scheme) in the field and may (or may not) support the same. Therefore I welcome your comments (pros & cons) on these two options.

Thanks in advance!

 

[jghrist]

Is option 2 a pilot wire differential scheme or a current differential scheme? If the latter, the CT lengths would be too large.

 

[Kiribanda]

Thanks for the reply.
Infact I donot have detail information about the option 2. I presume it to be a pilot wire scheme due to the following reasons.
1) It is not possible to install the star connected 33 kV c/t set at the substation which is 850 km away from the Dy11 transformer.(We cannot take the c/t output to such a long distance. The burden will be very high)
2) How about the charging current of the 33 kV feeder? There is no balancing current in the secondary side, isn't it?
Therefore I beleive that it is not be a usual phase differential scheme with two c/t sets at either ends of the feeder+transfomer unit and three 87R,87Y & 87B relays - which we normally install for a 33 kV feeder.Because this scheme has to look after the transformer in-rush current, 33 kV charging current etc.
That is why I called upon our friends to furnish some details of such a scheme(s) which is already in service. There may be some individuals who are doing maintenance of such systems.
Regards!

 

[alehman]

You can find some info on the Alstom MBCI "Translay" pilot wire differential relay here -

http://www.tde.alstom.com/servlet/C...b=Contacts and Documentation&id=1030012076605

I have no experience with this device and I'm not sure what it's length restrictions are. I would think you could compensate for charging current in some manner. It should be faily small for 850m of cable. Reading the brochure, it appears you would want to use the MCTH on the transformer end to provide blocking during energization to prevent false trips. They may have a version with harmonic restraint or you may be able to add that with a separate device.

Option 1 seems simpler to me. You loose differential protection of the cable though. I assume you are using 50SG because your source is not solidly grounded? If so, differential protection of the cable may have limited effectiveness anyway.

 

[jbartos]

Suggestion: The first option has an edge over the second option in a separate protection of the feeder and transformer. There will be another zone downstream that will protect the double ended switchgear buses. This configuration makes it qualitatively different from the cited "conventional Phase Differential protection for a MV cable+motor". It can be done, however, the downstream switchgear protection zone supposed to be overlapping the translay differential scheme. This may become too complex and difficult to test and troubleshoot.