The internal primary fuse protects against an internal failure in the

The internal primary fuse protects against an

This preview shows page 41 - 49 out of 90 pages.

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.
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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
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Distribution Transformers Theory Three-phase transformer with a three-limb core and two windings per phase ECE5530 Power Distribution: Lecture 2 43
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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.
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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:
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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.
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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.
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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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