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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