[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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TABLE 3.6.3 <strong>Transformer</strong> Short-Circuit Test Categories<br />
Category Single-Phase, kVA Three-Phase, kVA<br />
I 5 to 500 15 to 500<br />
II 501 to 1,667 501 to 5,000<br />
III 1,668 to 10,000 5,001 to 30,000<br />
IV above 10,000 above 30,000<br />
1. Starting with an open circuit at the terminal to be faulted, energize the transformer and then apply<br />
the short circuit by closing a breaker across the terminal.<br />
2. Starting with a short circuit across the terminal to be faulted, close a breaker at the source terminal<br />
to energize the prefaulted transformer.<br />
The first fault-initiation method is the preferred method. Given in the order of preference as listed in<br />
C57.12.90 [2], the following fault types are permissible:<br />
• Three-phase source, three-phase short circuit.<br />
• Single-phase source, single phase-to-ground short circuit.<br />
• Single-phase source, simulated three-phase short circuit.<br />
• Single-phase source, single-phase short circuit, applied one phase at a time<br />
The simulated three-phase short circuit, using single-phase source, is conducted as follows, depending<br />
on whether the connection is delta or wye. For wye-connected windings, the fault or source is applied<br />
between one terminal and the other two terminals connected together. For delta-connected windings,<br />
the fault or source is connected between two line terminals, with no connection to the other terminal,<br />
which must be repeated for each of the three phases.<br />
3.6.7.1.4 Short-Circuit Current Duration, Asymmetry, and Number of Tests<br />
The required short-circuit current magnitude and degree of asymmetry of the first cycle of current is<br />
given in IEEE C57.1200 [1]. Each phase of the transformer is subjected to a total of six tests that satisfy<br />
the symmetrical current requirements. At least two of the six must also satisfy the asymmetrical current<br />
tests. Five of the six tests must have a fault duration of 0.25 sec. One of the tests, which is sometimes<br />
called the “long duration test,” must be of longer duration for Category I, II, and III transformers. The<br />
longer duration must be determined as follows:<br />
For Category I transformers,<br />
t = 1250/I 2<br />
where<br />
t = duration, sec<br />
I = fault current, per unit of load current<br />
For Category II transformers,<br />
t = 1.0 sec<br />
For Category III transformers,<br />
t = 0.5 sec<br />
Clause 7 of IEEE C57.12.00-1993 [1] and Clause 12 of IEEE C57.12.90-1993 [2] provide a detailed<br />
and excellent description of the requirements and procedures for short-circuit testing.<br />
3.6.7.1.5 Evaluation of Short-Circuit Test Results<br />
Clause 12.5 of IEEE C57.12.90-1993 [2], Proof of Satisfactory Performance, spells out the requirements<br />
for passing or failing the tests. These include:<br />
• Visual inspection<br />
• Dielectric tests following short-circuit tests<br />
• Observations of terminal voltage and current waveforms — no abrupt changes<br />
• Changes in leakage impedance are limited to values specified in the standards<br />
• Low-voltage impulse tests may be performed<br />
• Increases in excitation current are limited by the standards<br />
Clause 12.5 of the test code discusses these criteria in detail.<br />
3.6.7.2 Special Tests<br />
Many additional tests are available to obtain certain information about the transformer, usually to address<br />
a specific application issue. These are listed below.<br />
• Overload heat run<br />
• Gas-in-oil sampling and analysis (in conjunction with other tests)<br />
• Extended-duration no-load-loss tests<br />
• Zero-sequence impedance measurements<br />
• Tests of the load tap changer<br />
• Short-circuit-withstand tests<br />
• Fault-current capability of enclosures (overhead distribution transformers)<br />
• Telephone line voice frequency electrical noise (overhead distribution transformers)<br />
• Tests on controls<br />
These tests and others, of a specific nature, are beyond the scope of this general article. Interested<br />
readers will find more information about these and other tests in the national and international standards<br />
dealing with transformers.<br />
References<br />
1. IEEE, Standard General Requirements for Liquid-Immersed Distribution, <strong>Power</strong>, and Regulating<br />
<strong>Transformer</strong>s, IEEE Std. C57.12.00-1993, Institute of <strong>Electric</strong>al and Electronics <strong>Engin</strong>eers, Piscataway,<br />
NJ, 1993.<br />
2. IEEE, Standard Test Code for Liquid-Immersed Distribution, <strong>Power</strong>, and Regulating <strong>Transformer</strong>s;<br />
and Guide for Short-Circuit Testing of Distribution and <strong>Power</strong> <strong>Transformer</strong>s (ANSI), IEEE/ANSI<br />
Std. C57.12.90-1992, Institute of <strong>Electric</strong>al and Electronics <strong>Engin</strong>eers, Piscataway, NJ, 1992.<br />
3. IEEE, The New IEEE Standard Dictionary of <strong>Electric</strong>al and Electronics Terms, Std. 100-1992,<br />
Institute of <strong>Electric</strong>al and Electronics <strong>Engin</strong>eers, Piscataway, NJ, 1992.<br />
4. IEEE, Guide for <strong>Transformer</strong> Impulse Tests, IEEE Std. C57.98-1993, Institute of <strong>Electric</strong>al and<br />
Electronics <strong>Engin</strong>eers, Piscataway, NJ, 1993.<br />
5. IEEE, Guide for Partial Discharge Measurement in Liquid-Filled <strong>Power</strong> <strong>Transformer</strong>s and Shunt<br />
Reactors, IEEE Std. C57.113-1991, Institute of <strong>Electric</strong>al and Electronics <strong>Engin</strong>eers, Piscataway, NJ,<br />
1991.<br />
3.7 Load-Tap-Change Control and <strong>Transformer</strong> Paralleling<br />
<strong>James</strong> H. <strong>Harlow</strong><br />
3.7.1 Introduction<br />
Tap changing under load (TCUL), be it with load-tap-changing (LTC) power transformers or step-voltage<br />
regulators, is the primary means of dynamically regulating the voltage on utility power systems. Switched<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC