3. An industrial plant is to be supplied by three identical transformers. A deci

Question

3. An industrial plant is to be supplied by three identical transformers. A decision must be made whether to purchase a fourth transformer to build a sparing transformer system as shown in Figure 1.16, or to configure the source as a secondary selective system as shown in Figure 1.14. If the secondary selective option is chosen, a. how much larger must each transformer in the secondary selective option be compared to each transformer in the sparing transformer option if each transformer is to be able to supply the full plant load in the event that one transformer fails? tie capability for the secondary selective option compared to the sparing transformer option?

Explanation / Answer

When an electrical load is connected to the secondary winding of a transformer and the transformer loading is therefore greater than zero, a current flows in the secondary winding and out to the load. This secondary current is due to the induced secondary voltage, set up by the magnetic flux created in the core from the primary current.

The secondary current, IS which is determined by the characteristics of the load, creates a self-induced secondary magnetic field, S in the transformer core which flows in the exact opposite direction to the main primary field, P. These two magnetic fields oppose each other resulting in a combined magnetic field of less magnetic strength than the single field produced by the primary winding alone when the secondary circuit was open circuited.

This combined magnetic field reduces the back EMF of the primary winding causing the primary current, IP to increase slightly. The primary current continues to increase until the cores magnetic field is back at its original strength, and for a transformer to operate correctly, a balanced condition must always exist between the primary and secondary magnetic fields. This results in the power to be balanced and the same on both the primary and secondary sides.

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2)

Metal-clad circuit breakers are used as the feeder protective devices. Fused metal-enclosed-feeder switches could be utilized for this, but caution must be used if this is considered since the feeder fuses would have to be able to serve both transformers and the feeder and transformer fuses would have to coordinate for maximum selectivity.

This is designed to be operated with the loop open, i.e., one of the four loop switches shown would be normally-open. If closed-loop operation were required, metal-clad circuit breakers should be used instead to provide maximum selectivity .

Momentary paralleling to allow maintenance of one section of the loop without causing an outage to one part of the facility can be accomplished with metal-enclosed loop switches, however, if caution is used in the system design and maintenance.

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