[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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Gade, S., Sound Intensity Instrumentation and Applications, Technical Review No. 4, Brüel & Kjaer, Nerum,<br />
Denmark, 1982.<br />
IEEE Audible Sound and Vibration Subcommittee Working Group, Guide for Sound Level Abatement<br />
and Determination for Liquid Immersed <strong>Power</strong> <strong>Transformer</strong>s and Shunt Reactors Rated to be<br />
500kVA, IEEE C57.156 (draft 9), Institute of <strong>Electric</strong>al and Electronics <strong>Engin</strong>eers, Piscataway, NJ.<br />
Replinger, E., Study of Noise Emitted by <strong>Power</strong> <strong>Transformer</strong>s Based on Today’s Viewpoint, CIGRE Paper<br />
12-08, Siemens AG, Transformatorenwerk, Nürnberg, Germany, 1988.<br />
Specht, T.R., Noise Levels of Indoor <strong>Transformer</strong>s, Westinghouse Corp. <strong>Transformer</strong> Division, 1955.<br />
Westinghouse <strong>Electric</strong> Corp., Bolt Beranek, and Newman Inc., <strong>Power</strong> <strong>Transformer</strong> Noise Abatement,<br />
ESERC Report EP 9-14, Empire State <strong>Electric</strong> Energy Research Corp., New York, 1981.<br />
3.10 Transient-Voltage Response<br />
Robert C. Degeneff<br />
3.10.1 Transient-Voltage Concerns<br />
3.10.1.1 Normal System Operation<br />
<strong>Transformer</strong>s are normally used in systems to change power from one voltage (or current) to another.<br />
This is often driven by a desire to optimize the overall system characteristics, e.g., economics, reliability,<br />
or performance. To achieve these system goals, a purchaser must specify — and a designer must configure<br />
— the transformer to meet a desired impedance, voltage rating, power rating, thermal characteristic,<br />
short-circuit strength, sound level, physical size, and voltage-withstand capability. Obviously, many of<br />
these goals will produce requirements that are in conflict, and prudent compromise will be required.<br />
Failure to achieve an acceptable characteristic for any of these goals will make the overall transformer<br />
design unacceptable. <strong>Transformer</strong> characteristics and the concomitant design process are outlined in the<br />
literature [1–4].<br />
Normally, a transformer operates under steady-state voltage excitation. Occasionally, a transformer<br />
(in fact all electrical equipment) experiences a dynamic or transient overvoltage. Often, it is these<br />
infrequent transient voltages that establish design constraints for the insulation system of the transformer.<br />
These constraints can have a far-reaching effect on the overall equipment design. The transformer must<br />
be configured to withstand any abnormal voltages covered in the design specification and realistically<br />
expected in service. Often, these constraints have great impact on other design issues and, as such, have<br />
significant effect on the overall transformer cost, performance, and configuration. In recent years, engineers<br />
have explored the adverse effect of transient voltages on the reliability of transformers [5–7] and<br />
found them to be a major cause of transformer failure.<br />
3.10.1.2 Sources and Types of Transient-Voltage Excitation<br />
The voltages to which a transformer’s terminals are subjected can be broadly classed as steady state and<br />
transient. The transient voltages a transformer experiences are commonly referred to as dynamic, transient,<br />
and very fast transients.<br />
The majority of the voltages a transformer experiences during its operational life are steady state, e.g.,<br />
the voltage is within 10% of nominal, and the frequency is within 1% of rated. As power-quality issues<br />
grow, the effect of harmonic voltages and currents on performance is becoming more of an issue. These<br />
harmonics are effectively reduced-magnitude steady-state voltages and currents at harmonic frequencies<br />
(say 2nd to the 50th). These are addressed in great detail in IEEE Std. 519, Recommended Practices and<br />
Requirements for Harmonic Control in <strong>Electric</strong>al <strong>Power</strong> Systems [8]. Strictly speaking, all other voltage<br />
excitations are transients, e.g., dynamic, transient, and very fast transient voltages.<br />
Dynamic voltages refer to relatively low-frequency (60 to 1500 Hz), damped oscillatory voltage.<br />
Magnitudes routinely observed are from one to three times the system’s peak nominal voltage. Transient<br />
voltage refers to the class of excitation caused by events like lightning surges, switching events, and line<br />
faults causing voltages of the chopped waveform [9]. Normally, these are aperiodic waves. Occasionally,<br />
the current chopping of a vacuum breaker will produce transient periodic excitation in the 10- to 200-kHz<br />
range [10]. The term very fast transient encompasses voltage excitation with rise times in the range of<br />
50 to 100 sec and frequencies from 0.5 to 30 MHz. These types of voltages are encountered in gasinsulated<br />
stations. The voltages produced within the transformer winding structure by the system is a<br />
part of the problem that must be addressed and understood if a successful insulation design is to be<br />
achieved [11]. Since transient voltages affect system reliability, in turn affecting system safety and economics,<br />
a full understanding of the transient characteristic of a transformer is warranted.<br />
3.10.1.3 Addressing Transient-Voltage Performance<br />
Addressing the issue of transient-voltage performance can be divided into three activities: recognition,<br />
prediction, and mitigation. By 1950 over 1000 papers had been written to address these issues [12–14].<br />
The first concern is to appreciate that transient-voltage excitation can produce equipment responses<br />
different than one would anticipate at first glance. For example, the addition of more insulation around<br />
a conductor may, in fact, make the transient-voltage distribution worse and the insulation integrity of<br />
the design weaker. Another example is the internal voltage amplification a transformer experiences when<br />
excited near its resonant frequency. The transient-voltage distribution is a function of the applied voltage<br />
excitation and the shape and material content of the winding being excited. The capability of the winding<br />
to withstand the transient voltage is a function of the specific winding shape, the material’s voltage-vs.-<br />
time characteristic, the past history of the structure, and the statistical nature of the voltage-withstand<br />
characteristic of the structure.<br />
The second activity is to assess or predict the transient voltage within the coil or winding. Today, this<br />
generally is accomplished using a lumped-parameter model of the winding structure and some form of<br />
computer solution method that facilitates calculation of the internal transient response of the winding.<br />
Once this voltage distribution is known, its effect on the insulation structure can be computed with a<br />
two- or three-dimensional finite element method (FEM). The resultant voltages, stresses, and creeps are<br />
examined in light of the known material and geometrical capability of the system, with consideration<br />
for desired performance margins.<br />
The third activity is to establish a transformer structure or configuration that — in light of the<br />
anticipated transient-voltage excitation and material capability, variability, and statistical nature — will<br />
provided acceptable performance margins. Occasionally, nonlinear resistors are used as part of the<br />
insulation system to achieve a cost-effective, reliable insulation structure. Additionally, means of limiting<br />
the transient excitation include the use of nonlinear resistors, capacitors, and snubbers.<br />
3.10.1.4 Complex Issue to Predict<br />
The accurate prediction of the transient-voltage response of coils and winding has been of interest for<br />
almost 100 years. The problem is complex for several reasons. The form of excitation varies greatly. Most<br />
large power transformer are unique designs, and as such each transformer’s transient-response characteristic<br />
is unique. Each has its own impedance-vs.-frequency characteristic. As such, the transientresponse<br />
characteristic of each transformer is different. Generally, the problem is addressed by building<br />
a large lumped-parameter model of inductances, capacitances, and resistances. Constructing the lumpedparameter<br />
model is challenging. The resultant mathematical model is ill-conditioned, e.g., the resultant<br />
differential equation is difficult to solve. The following sections outline how these challenges are currently<br />
addressed.<br />
It should be emphasized that the voltage distribution within the winding is only the first component<br />
of the insulation design process. The spatial distribution of the voltages within the winding must be<br />
determined, and finally the ability of the winding configuration in view of its voltage-vs.-time characteristic<br />
must be assessed.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC