Station Service Transformer 138 kV-208Y/120 V 2

[redtrumpet]

We are designing a new 600 MVA 240 kV/138 kV substation. The AC station service is 208Y/120 V, 3 phase, 4 wire. The primary source is a 75 kVA padmount transformer fed from a local 25 kV distribution line. The client wants a full-capacity alternate source for the station service. The estimated 1 hour demand is 39 kVA; a 45 kVA or larger transformer capacity is recommended.

We are proposing to use the 138 kV substation bus as the alternate source. Standard metering VT's are limited to approximately 6000 VA capacity, or 18 kVA for our 3 phase service. This is too small. We have looked at Kuhlman SSVT transformers; the 138 kV class models have minimum 50 kVA power secondaries for a total capacity of 150 kVA. This is larger than we need and the cost is substantial (approximately $50k CDN each plus installation).

Can anyone suggest any alternatives for transformers? I perused the Ritz and Trench websites but all I found were metering transformers with too-small secondaries. Do any manufacturers offer something similar to the Kuhlman SSVT, but with a smaller secondary?

 

[busbar]

Try ABB's PVT series, {Can't find it at their website, but I think they have dual 120V secondaries and 50kVA rating} Saw some in a local 230kV yard.

Possibly Alstom/Balteau in Medford, OR.

FWIW, Kuhlman SSVT literature states "available with isolated metering winding option."

 

[jbartos]

Suggestion: Visit

http://www.thomasregister.com

and type Transformers: Potential under Product or Service that will return 58 companies to inquire about your PT needs. Some of them appear to offer custom produced PTs.

 

[scottf]

Ritz does have a PVT at 230 kV only (Ritz type OTET245). It is 25 kVA/phase, but at 230 kV, but it probably costs more than CAD$50k. if you can feed off of the 230 kV side, you may want to give them a call.

ABB does not make their PVT design any longer, since they closes their plant in Bloomington, IN. Alstom does not have a real design either.

 

[busbar]

Thanks for the update, scottf.

 

[jghrist]

Do you need three-phase for station service? Would one Kuhlman SSVT do? Three-phase service is not usually required for substations.

What is the 230-138 kV transformer? If it has a delta tertiary, this may be usable for station service.

 

[redtrumpet]

Thanks all for the responses.

jghrist - transformers have no tertiary winding. We are moving existing transformers from another sub that have 3 phase 208 V fans (14 per transformer) and tapchangers.

scottf - thanks for the tip on the Ritz 230 kV transformer. I will check out pricing.

busbar - the separate metering winding on the Kuhlman SSVT is a definite option to reduce cost by combining the station service with the metering for one side of the 138 kV bus (breaker-and-a-half scheme).

 

[scottf]

A word of caution on the metering winding on the Kuhlman SSVT. You need to confirm with them that the metering winding accuracy rating is valid when the power winding is in use. This is not always the case!!

The metering windings are typically included so they can be used in lieu of a normal VT, since these units are often used for back-up station power, i.e. the power rating is only used when the local distribution line trips off. Since this is treated as an occational and temperary condition, the thought is that the metering winding accuracy can be sacrificed during that period.

 

[redtrumpet]

Thanks for the caution, Scott. You have described our scenario exactly - the 138 kV source for station supply would be used only if we lost the 25 kV distribution line.

 

[busbar]

redt — Forgive me, but in an extreme case using only two phase-to-ground-powered transformers, there is an odd, almost bizarre transformer interconnection that might possibly be suitable for the application at hand. From LEWIS, H. S. Two Phases Can Supply 4-Wire 120/208 V. Electrical World, p 95, Nov 18, 1957; Discussion, Electrical World, p 124, Dec 16, 1957. Referenced in §9 of ANSI/IEEE C57.105-1978 …Transformer Connections in Three-Phase Distribution Systems. {Used it once for 3ø radiator fans, and a second as an open-delta variation for a remote electric-gate motor. }

To use this arrangement, there must be dual 120-volt secondaries on each of the two transformers {as in the obsolete ABB PVTs} and voltage imbalance must be considered for this non-symmetrical arrangement—possibly including fan-motor horsepower derating.

Caption to “Figure 17 — Open-Y Primary to 4-Wire Secondary A—Quasi-Phasor Diagram B—Connection Diagram

Open-Y primary to 4-Wire 208 V Service — If a 4-wire 3-phase 208 V service must be installed and only an open-Y primary is available, and the expense of adding the third-phase conductor is considerable, resort may be had to a connection of two single-phase transformers with
dual-voltage secondaries. At, least, one transformer must have four secondary bushings, that, is, the secondary voltage rating must be 120/240 V. The other transformer may be a 3-bushing transformer rated 240/120 V. The connection is made as shown in Fig 17. Each single-phase transformer must be sized for 2/3 of the balanced 3-phase load.”

 

[peebee]

Wow, busbar, that is wacky, but I like it. Red star for you.

 

[busbar]

Thank you very much peebee, but I suppose I have had an odd penchant for off-center transformer configurations. If it will reduce $50k components by even one, well...

The arrangement might conceivably eliminate one of three 138kV single-pole circuit switchers, interrupters or fuses.

 

[busbar]

Oops. Forgot a critical bureaucracy aspect. The image above is copyright IEEE, 1994.

 

[jghrist]

<<The arrangement might conceivably eliminate one of three 138kV single-pole circuit switchers, interrupters or fuses.>>

Is it necessary to provide high side overcurrent protection for the VT's? The bus is likely to have instantaneous protection that will not coordinate with any circuit switcher, interrupter, or fuse anyway. What would the protection accomplish in the unlikely event of a fault?

 

[busbar]

jghrist — that is a good point. Other than possibly providing manual isolation, protection of the primaries for station power would likely be just short of &quot;cosmetic&quot;. [At the risk of beating a dead horse, there have been 1-ampere, 138kV expulsion links ‘out there’.]

 

[redtrumpet]

Novel connection, busbar. I doubt if we will use it for this application, but I am tucking it in my back pocket for future use if required.

FWIW, the bus has instantaneous 87 protection and the VT primaries are not fused.

 

[peterb]

Have you thought of a UPS + standby generator + transfer switch for this duty? Providing you alarm the auto-start of the genset, should allow time for restoration of the 25 kV source. Should also be lower cost, even factoring in scheduled maintenance.

 

[redtrumpet]

The generator is one option under consideration, albeit without a UPS. Our only critical AC load is the tapchanger mechanism. A loss of power to the tapchanger for a few seconds while the generator starts and transfers isn't critical.

 

[scottf]

How much power does the tapchanger draw?

 

[redtrumpet]

I think I know where you are going, Scott. Our original recommendation to the client was to have both critical and non-critical load AC busses. The normal AC source would energize both busses. If the normal source failed, we would energize the critical load bus only with the alternate source. We proposed to use three instrument VTs with a total capacity of 18 kVA as the alternate source.

However, the client wanted a fully redundant alternate source with capacity equal to or greater than the normal source, which leads us to where we are today.

If we use a critical load bus (ie. reduced capacity alternate source), I would add the DC system battery charger and telecom UPS to the critical load bus in addition to the tapchanger. However, the tapchanger is the only critical load that would have a power interruption if we used a generator as the alternate source.

Personally, I believe the cost of a fully redundant alternate source will be too high for the client and our original recommendation will be adopted.

I appreciate everybody's suggestions but I believe we have considered all available options. I was really only looking for availability of VTs with larger capacity secondaries than standard.