In a Biomass power plant the generator Transformer with following rating got failed.

HV - 110 KV ,Star connection and solidly earthed.
LV -11KV , Delta connected
Rating - 20 MVA,
Vector Group - YNd1

The following power transformer only now available.
11 KV - Star connected
110 KV - Delta connected
Rating - 20 MVA
Vector group - Dy11

To avoid generation loss whether this transformer can be used?. If possible whether the 11 KV star should be earthed or not. What are the other precautions to be taken ?

 

Responding to Pmmeeran's 15-Mar-10 (16:10)... in answer to your questions...

Q1) Add an 11kV Earthing System?
A1) Yes! I certainly recommend an earthing system be added to the 11kV system! This reduces "iron-core" damage to the 20MVA transformer, generator(s) and other xfmrs, if connected, as well as lowering sheath currents, if metallic, associated with the 11kV feeders! On the other hand, the 'n' connection is not required if the generator(s) have their neutral(s) earthed!

Q2) Replacing the YNd1 with a Dyn11 xfmr?
A1) While it could provide an earthing source for the 11-kV system, that of the 110kV system is lost, thus, presenting other problems!

Regards,
Phil Corso (cepsison [at] aol [dot] com)

 

Pmmeeran, further to my earlier reply...

Q3) Any other precautions?
A3) Yes! Metering and Protection circuits must me modified to account for polarity change

Phil Corso

 

But wouldn't there be a problem of circulating currents between grid and Xmer due to phase difference arising out of different vector group connections? The utility Xmers connected to grid (external to the plant) might be of different vector group/s! (although system voltages are identical). Pls. elaborate.

Regards,
Shahvir

 

Aha, the plot thickens. How about forwarding a Single-Line-Diagram!

Regards,
Phil Corso

 

We have delta/star connection 110/35kv and 50MVA power transformer.But our energy system mostly used Star/Delta connection type of 110 and 220kv voltage level.

Could you please, advice me , what do make need to connect grid our transformer.

Do know any manufacturer of the grounding transformer(Zig zag) for 110kv ? What company have can do it ?

Regards,
P.Jamiyandorj

Senior Electrical Engineer
Newcom group

 

Mr. Jamiyandorj... your questions encompass a rather large topic. But if maximum reliability is the goal, then here are a few suggestions for Grid-connection via a GSU xfmr:

1) The generator-voltage winding (call it the primary) should be Delta-connected.

2) The high-voltage winding (call it the secondary) should be Wye-Connected! Its neutral should be solidly-earthed! Do not use a high-voltage Zig-Zag or cross-connected Delta-Wye xfmr for this purpose!

3) A third LV winding (call it the tertiary) should be included. Its normal function is to ameliorate effects of transient phenomena, but it can fulfill an even greater function.

4) The tertiary-winding, Delta-connected, can supply power as one of two 3-phase sources for the site’s LV Auxiliary-Services bus!

5) The second 3-phase source for the Auxiliary-Services bus can be supplied by an Auxiliary-Services xfmr connected between the generator and the GSU!

6) A relatively small Zig-Zag xfmr can be installed to provide earth-fault protection for the connection between the generator and GSU, when the Grid is energized, but the Generator is out of service!

My intent concentrates on Reliability. Of course other factors must be considered. One example is Energy metering... a second, operability, a third, maintainability. A fourth, but extremely critical factor, is availability of a GSU replacement should it fail!

Regards, Phil Corso ([email protected])