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

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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?

 

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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.”