hvt 6 pedrow overvoltages

8/9/2019 Hvt 6 Pedrow Overvoltages

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Overvoltage Phenomena and Insulation Coordination

in Electric Power Systems-Part 1

The next frame is borrowed from our Introduction Section. It reminds us

again that there are three important types of overvoltages on electric power

systems: lightning overvoltages, switching overvoltages, and powerfrequency overvoltages.

In the next frame, you are asked to consider a thought experiment where a

voltage recorder captures a continuous record of the voltage at a particular

location on the power system (say that these voltages are measured from

phase conductor to earth for simplicity). Each voltage "event" is then

studied and the pertinent events are classified into our three types

(lightning, switching, and power frequency) and probability distributionfunctions are made for each category. In the hypothetical probability

distribution functions shown, it is assumed that only transients exceeding

the nominal system voltage are counted. This leads to a sharp cutoff at the

low voltage end of the distribution. It is also assumed that huge transients

will be "clipped" by system sparkovers. This leads to a sharp cutoff at the

high voltage end of the distribution. These plots are strictly hypothetical

and will be discussed later in this chapter.


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The next frame begins lecture notes on lightning.

Some supplementary references on lightning will be placed below:

1. W. Diesendorf, Insulation Co-ordination in High-voltage

Electric Power Systems, London: Butterworth, 1974.

2. J. G. Anderson, Lightning Performance of EHV-UHVLines, Chapter 12 in Transmission Line Reference Book 345 kV

and Above, Palo Alto: Electric Power Research Institute, 1975.

3. A. J. Eriksson, Lightning-Induced Overvoltages on Overhead

Distribution Lines, IEEE Transactions on Power Apparatus and

Systems, Vol. PAS-101, No. 4, April 1982, pp. 960-968.

4. R. Bernstein, R. Samm, K. Cummins, R. Pyle, and J. Tuel,

Lightning Detection Network Averts Damage and Speeds Restoration,

IEEE Computer Applications in Power, April 1996, pp. 12-17.

5. The Lightning Protection Design Workstation Version 4.0, Technical

Brief, Power Delivery Group, Electric Power Research Institute,

August 1996, P.O. Box 10412, Palo Alto, CA 94303, (415) 855-2000.

6. C. Zimmer, Heavens New Fires, Discovery, Vol. 18, No. 7,

July 1997, pp. 100-107. This describes phenomena recently observed

ABOVE the thunder clouds.

7. E. R. Williams, The Electrification of Thunderstorms, ScientificAmerican, November 1988, pp. 88-99.

8. M. A. Uman, Understanding Lightning, Carnegie, Pennsylvania: Bek

Technical Publications Inc., 1971.

9. K. Berger and R. B. Anderson, Parameters of Lightning Flashes,

Electra, International Conference on Large High Voltage Electric

Systems, CIGRE, No. 41, July 1975.

10. P. R. Elkin, Anaylysis of the Spatial, Temporal, and

Electrical Characteristics of Lightning in Portions of the Northwestern United States for 1985 and 1986, M. S. Thesis,

Washington State University, Pullman, Washington, December 1988.

11. C. F. Wagner, G. D. McCann, and J. M. Clayton, Chapter 16,

Lightning Phenomena, in Electrical Transmission and Distribution

Reference Book, East Pittsburgh, Pennsylvania: Westinghouse Electric

Corporation, 1964.


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The equivalent circuit shown at the bottom of the previous frame appearson page 48 of Reference #1 by Diesendorf.


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Much of the information given below on lightning detection networks was

taken from References # 4 and # 10 listed above.


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Note that the following frame has a self-contained reference #1. All otherreferences in this section are listed near the start of this section.


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These frames describe switching overvoltages including that due to circuit

breaker current chopping.


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The following frame mentions a text book by Greenwood. It covers the

techniques of superposition and current injection. The complete reference

and page numbers are: A. Greenwood, Electrical Transients in Power

Systems, NY: Wiley-Interscience, 1971, pp. 6-9, pp. 38-41, pp. 66-67, and

pp. 96-99.


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HSPICE was used to simulate current chopping at about 20 amps and at

about 100 amps. Results are shown below. Note that transient switching

overvoltages can easily exceed twice the peak power frequency voltage

(25 kV in these examples) in the presence of current chopping.


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End Part 2 of Overvoltage Phenomena and Insulation Coordination in

Electric Power Systems


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Overvoltage Phenomena and Insulation Coordination in Electric

Power Systems-Part 3

These frames describe power frequency overvoltages including that due to

the Ferranti rise and that due to ferro-resonance.

The following frame mentions text books by Gallagher & Pearmain and by

Greenwood. The complete references are:


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T. J. Gallagher and A. J. Pearmain, High Voltage Measurement, Testing

and Design, NY: Wiley, 1983, p. 27 and p. 225.

A. Greenwood, Electrical Transients in Power Systems, NY: Wiley-

Interscience, 1971, pp. 438-440.


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The following frame shows a scanned image of the MATLAB file used to

solve for the Ferranti rise. This file (ferranti.m) has also been placed in this

subdirectory for use by the student.


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The following MATLAB m-file has been placed in this subdirectory and is

available for student use.

The following HSPICE file has been placed in this subdirectory and is

available for student use.


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The following frame shows voltage plots from the HSPICE simulation.

Note that the magnetization branch of the transformer is nearly resonant

with C2 (as we planned for this example) and so these two circuit elements

develop large power frequency voltages. In this case the line to line

applied voltage is about 17 kV peak while the resonant voltage across the

transformer and across C2 exceeds 50 kV peak. This power frequency

overvoltage could easily damage the transformer and cable.


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We will now replace the linear magnetization element with a nonlinear

magnetization element and repeat the HSPICE simulation. The next few

frames will describe the nonlinear magnetization element for the

transformer.


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.

The following frame shows voltages when the nonlinear inductor was

present (compare with frame 8-56 above.) Note that the bottom capacitor

(C2 at node 6) and the transformer inductance both have large voltageswith significant harmonics. Comparing with frame 8-56, we see that the

nonlinear inductor has limited the overvoltage to a maximum during the

simulation of about 38kV while the linear inductor had a maximum

voltage during the simulation of about 52kV.

The next frame shows currents from the simulation with the nonlinear

inductor. Current from the sinusoidal voltage source contains strong

harmonics as does current through the nonlinear inductor and the capacitor

(C2) that is in resonance with the nonlinear inductor. Note that during this

simulation, current through the inductor did not exceed 10 A.




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These frames describe apparatus for overvoltage avoidance and

mitigation. A Classic reference for the older techniques is:1. Electrical Transmission and Distribution Reference Book, East

Pittsburgh, Pennsylvania: Westinghouse Electric Corporation, 1964, pp.

599-642.


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hvt 6 pedrow overvoltages

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