CSPs do not need a primary-side cutout with a fuse. • The internal primary fuse protects against an internal failure in the transformer. The weak link • has less fault-clearing capability than a fuse in a cutout, so they need external current-limiting fuses where fault currents are high. • Secondary breakers provide protection against overloads and secondary faults. • The breaker responds to current and oil temperature. • Tripping is controlled by deflection of bimetallic elements in series.
Three Phase Distribution Transformers ECE5530 Power Distribution: Lecture 2 42 • Three-phase overhead transformer services are normally constructed from three single-phase units. • Three-phase transformers for underground service are normally single units, usually on a three- or five-legged core. • The kVA rating for a three-phase bank is the total of all three phases. • The full-load current in amps in each phase of a three-phase unit or bank is
Distribution Transformers Theory Three-phase transformer with a three-limb core and two windings per phase ECE5530 Power Distribution: Lecture 2 43
Distribution Transformers Loading Calculations ECE5530 Power Distribution: Lecture 2 44 • Loading calculations are needed to determine the apparent power load to be expected so that the proper kVA rating for a transformer can be specified. • For single-phase three wire services, transformer loading can be determined by summing connected loads. • For three-phase, four wire loads (i.e. grounded wye), transformer loading is easily calculated assuming that the loads are balanced. • Both the primary and secondary windings may be connected in different ways: delta, floating wye, or grounded wye. • This notation describes the connection of the transformer windings, not the configuration of the supply system.
Example 2. Loading Calculations ECE5530 Power Distribution: Lecture 2 45 The single phase load is 30 kW at 0.95 lagging power factor and the three-phase load is 150 kW at 0.8 lagging power factor. Assume the single phase load is distributed evenly on each of the 3 phases. We know P and we need to determine S and Q for each of the loads. • For the single-phase load:
Example 2. Loading Calculations ECE5530 Power Distribution: Lecture 2 46 • Similarly, for the 150 kW three-phase load we find: • The loading on each transformer is thus • Since 75 kVA is a standard single-phase transformer size, three 75 kVA transformers are required.
Example 3. Loading Calculations ECE5530 Power Distribution: Lecture 2 47 Three-phase load 300 kW @ 0.7 lagging power factor: P = 300 kW, Q = 306.1 kVA. Single-phase load 30 kW @ 0.95 lagging power factor: P = 30 kW, Q = 9.9 kVA. Solution: Therefore, we need three 167 kVA transformers.
Distribution Transformers Single Phase Transformer Paralleling ECE5530 Power Distribution: Lecture 2 48 • Two single-phase transformers of the same or different kVA ratings can be connected in parallel, with either additive or subtractive polarity, if the following is true: 1. All transformers have the same turns ratio.
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