Sparing Transformer System

The sparing transformer system concept came into use as an alternative to the capital cost intensive double ended secondary unit substation distribution system (see Two-Source Primary— Secondary Selective System). It essentially replaces double-ended substations with single-ended substations and one or more “sparing” transformer substations all interconnected on a common secondary bus—see Figure 1.1-38. 

Generally no more than three to five single-ended substations are on a sparing loop. 

The essence of this design philosophy is that conservatively designed and loaded transformers are highly reliable electrical devices and rarely fail. Therefore, this design provides a single common backup transformer for a group of transformers in lieu of a backup transformer for each and every transformer. This system design still maintains a high degree of continuity of service. 

Referring to Figure 1.1-38, it is apparent that the sparing concept backs up primary switch and primary cable failure as well. Restoration of lost or failed utility power is accomplished similarly to primary selective scheme previously discussed. It is therefore important to use an automatic throw-over system in a two source lineup of primary switchgear to restore utility power as discussed in the Two-Source “Primary” scheme—see Figure 1.1-37. 

A major advantage of the sparing transformer system is the typically lower total base kVA of transformation. In a double-ended substation design, each transformer must be rated to carry the sum of the loads of two buses and usually requires the addition of cooling fans to accomplish this rating. In the “sparing” concept, each transformer carries only its own load, which is typically not a fan-cooled rating. In addition to first cost savings, there is a side benefit of reduced equipment space.

The sparing transformer system operates as follows:

■■ All main breakers, including the sparing main breaker, are normally closed; the tie breakers are normally open

■■ Once a transformer (or primary cable or primary switch/fuse) fails, the associated secondary main breaker is opened. The associated tie breaker is then closed, which restores power to the single-ended substation bus

■■ Schemes that require the main to be opened before the tie is closed (“open transition”), and that allow any tie to be closed before the substation main is opened, (“closed transition”) are possible .

With a closed transition scheme, it is common to add a timer function that opens the tie breaker unless either main breaker is opened within a time interval. 

This closed transition allows power to be transferred to the sparing transformer without interruption, such as for routine maintenance, and then back to the substation. This closed transition transfer has an advantage in some facilities; however, appropriate interrupting capacities and bus bracing must be specified suitable for the momentary parallel operation. 

In facilities without qualified electrical power operators, an open transition with key interlocking is often a prudent design.

Note: Each pair of “main breaker/tie breaker” key cylinders should be uniquely keyed to prevent any paralleled source operations. 

Careful sizing of these transformers as well as careful specification of the transformers is required for reliability. Low temperature rise specified with continuous overload capacity or upgraded types of transformers should be considered. 

One disadvantage to this system is the external secondary tie system, see Figure 1.1-38. As shown, all single-ended substations are tied together on the secondary with a tie busway or cable system. Location of substations is therefore limited because of voltage drop and cost considerations. 

Routing of busway, if used, must be carefully layed out. It should also be noted, that a tie busway or cable fault will essentially prevent the use of the sparing transformer until it repaired. Commonly, the single-ended substations and the sparing transformer must be clustered. This can also be an advantage, as more kVA can be supported from a more compact space layout.

Figure 1.1-38. Sparing Transformer System