Short Circuit Withstand Capabilities
Distribution transformers are subjected to external short circuits on the secondary side. Such external faults can develop on the service line, in the house wiring or in connected loads due to numerous environmental reasons. These faults can be line-to-ground, double line-toground or line-to-line.
To meet these operating conditions, the American National Standard Institute (ANSI) has set standards concerning short circuit withstand capability. These standards require that distribution transformers shall be designed and constructed to withstand the mechanical and thermal stresses produced by these external short circuits.
The current standards relating to short circuit strength are ANSI C57.12.00 which sets the short circuit withstand requirements for distribution transformers and ANSI C57.12.90 which provides procedures for short circuit testing.
For distribution transformers, the magnitude of the short circuit current, the numbers of short-circuit tests and the duration of each short circuit test are defined by ANSI standards as follows.
A. Magnitude
| Category | Single Phase kVA | Three Phase kVA | Capability* |
|---|---|---|---|
| I | 5-25 | 15-75 | 40 |
| 37.5-100 | 112.5-300 | 35 | |
| 167-500 | 500 | 25 | |
| II | 750-2500 | 1/Z T** | |
| * Base current (Symmetrical) per unit for all distribution transformers with secondary rated 600 V and below. ** The short circuit current will be limited by the transformer impedance only. |
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B. Number of Tests
Each phase of the transformer shall be subjected to a total of six tests, four with symmetrical fault currents and two with asymmetrical fault currents.
C. Duration of Short Circuit Tests
When short circuit tests are performed the duration of each test shall be 0.25 s except that one test satisfying the symmetrical current requirement shall be made for a longer duration on distribution transformers. The duration of the long test in each case shall be as follows:
Category I:
T=1250/I2
Where T is the duration in seconds,
And I=Isc/IR=symmetrical short circuit current, in multiples of normal base current except I shall not exceed the maximum symmetrical current magnitudes listed in A.
Where Isc=IR/ZT=symmetrical short circuit current, in rms amperes
IR=rated current on the given tap connection, in rms amperes
ZT=transformer impedance on the given tap connection in per unit on the same apparent power base as IR
Category II:
T=1.0 second
Criteria of Satisfactory Performance
According to ANSI Standards a unit is considered to have passed the test if it passes a visual inspection and dielectric tests. Recommended additional checks include examination of wave shape of terminal voltage and current, leakage impedance measurement and excitation current test. (Refer to ANSI C57.12.90.)
The standard allows the following variations in the leakage impedance:
ZT (Per Units) Percentage Variation
0.0299 or less 22.5-500 (ZT)
0.0300 or more 7.5
ZT=per unit impedance of the transformer.
