Specifying the Insulation Strength
3.9 Chopped Wave Tests or Time-Lag Curves
In general, in addition to the tests to establish the BIL, apparatus are also given chopped wave lightning impulse tests. The test procedures is to apply a standard lightning impulse waveshape whose crest value exceeds the BIL. A gap in front of the apparatus is set to flashover at either 2 or 3 ps, depending on the applied crest voltage. The apparatus must "withstand" this test, i.e., no flashover or failure may occur. The test on the power transformers consists of an applied lightning impulse having a crest voltage of 1.10 times the BIL, which is chopped at 3 ps.
For distribution transformers, the crest voltage is a minimum of 1.15 times the BIL, and the time to chop varies from 1 to 3 ps. For a circuit breaker, two chopped wave tests are used: (1) 1.29 times the BIL chopped at 2 ps and (2) 1.15 times the BIL at 3 ps. Bushings must withstand a chopped wave equal to 1.15 times the BIL chopped at 3 ps.
These tests are only specified in ANSI standards, not in IEC standards.
Originally, the basis for the tests was that a chopped wave could impinge on the apparatus caused by a flashover of some other insulation in the station, e.g., a post insulator. Today, this scenario does not appear valid. However, the test is a severe test on the turn-to-turn insulation of a transformer, since the rapid chop to voltage zero tests this type of insulation, which is considered to be an excellent test for transformers used in GIs, since very fast front surges may be generated by discon- necting switches. In addition, these chopped wave tests provide an indication that the insulation strength to short duration impulses is higher than the BIL. The tests are also used in the evaluation of the CFO for impulses that do not have the lightning impulse standard waveshape. In addition, the chopped wave strength at 2 ps is used to evaluate the need for protection of the "opened breaker."
Chapter 1
CREST KV
Crest Voltage Time to Flashover
kV US
700 no flashover 780 no flashover
800 16
120 1000 4
1200 2
1000 1550 1
8 0 0 CFO 2050 0.5
6 0 0 ~ : I
.5 I 2 4 8 16 32
TIME
-
TO- FLASHOVERFigure 9 A sample time-lag curve.
To establish more fully the short-duration strength of insulation, a time-lag or volt-time curve can be obtained. These are universally obtained using the standard lightning impulse wave shape, and only self-restoring insulations are tested in this manner. The procedure is simply to apply higher and higher magnitudes of voltage and record the time to flashover. For example, test results may be as listed in Fig. 9.
These are normally plotted on semilog paper as illustrated in Fig. 9. Note that the time-lag curve tends to flatten out at about 16 ps. The asymptotic value is equal to the CFO. That is, for air insulations, the CFO occurs at about a time to flashover of 16 us. Times to flashover can exceed this time, but the crest voltage is approximately equal to that for the 16 ps point that is the CFO. (The data of Fig. 9 are not typical, in that more data scatter is normally present. Actual time-lag curves will be pre- sented in Chapter 2.)
4 NONSTANDARD ATMOSPHERIC CONDITIONS
BILs and BSLs are specified for standard atmospheric conditions. However, labora- tory atmospheric conditions are rarely standard. Thus correlation factors are needed to determine the crest impulse voltage that should be applied so that the BIL and BSL will be valid for standard conditions. To amplify, consider that in a laboratory nonstandard atmospheric conditions exist. Then to establish the BIL, the applied crest voltage, which would be equal to the BIL at standard conditions, must be increased or decreased so that at standard conditions, the crest voltage would be equal to the BIL. In an opposite manner, for insulation coordination, the BIL, BSL, or CFO for the nonstandard conditions where the line or station is to be constructed is known and a method is needed to obtain the required BIL, BSL, and CFO for standard conditions. In a recent paper [13], new and improved correction factors were suggested based on tests at sea level (Italy) as compared to tests at 1540 meters in South Africa and to tests at 1800 meters in Mexico. Denoting the voltage as measured under nonstandard conditions as VA and the voltage for standard condi- tions as Vs, the suggested equation, which was subsequently adopted in IEC 42, is
Specifying the Insulation Strength 17
where 5 is the relative air density, Hr is the humidity correction factor, and m and w are constants dependent on the factor Go which is defined as
where S is the strike distance or clearance in meters and CFOs is the CFO under standard conditions.
By definition, Eq. 5 could also be written in terms of the CFO or BIL or BSL.
That is,
The humidity correction factor, per Fig. 10, for impulses is given by the equation
where H is the absolute humidity in grams per m3. For wet or simulated rain conditions, Hc = 1.0. The values of m and w may be obtained from Fig. 11 or from Table 10.
0.85'-
Figure 10 Humidity correction factors. (Copyright IEEE 1989 [13].)
Chapter 1
Go
Figure I I Values of m and w. (Copyright 1989 IEEE [13].)
Lightning lmpulse
For lightning impulses, Go is between 1 .O and 1.2. Therefore
In design or selection of the insulation level, wet or rain conditions are assumed, and therefore Hc = 1.0. So for design
Switching Impulse
For switching impulses, Go is between 0.2 and 1, and therefore
Table I 0 Values of m and w
Specifying the Insulation Strength
For dry conditions
However, in testing equipment, the BSL is always defined for wet or simulated rain conditions. Also in design for switching overvoltages, wet or rain conditions are assumed. Therefore, Hc = 1 and so
The only remaining factor in the above correction equations is the relative air den- sity. This is defined as
where Po and To are the standard pressure and temperature with the temperature in degrees Kelvin, i.e., degrees Celsius plus 273, and P and T are the ambient pressure and temperature. The absolute humidity is obtained from the readings of the wet and dry bulb temperature; see IEEE Standard 4.
From Eq. 14, since the relative air density is a function of pressure and tem- perature, it is also a function of altitude. At any specific altitude, the air pressure and the temperature and thus the relative air density are not constant but vary with time.
A recent study 1141 used the hourly variations at 10 USA weather stations for a 12- to 16-year period to examine the distributions of weather statistics. Maximum altitude was at the Denver airport, 1610 meters (5282 feet). The statistics were segregated into three classes; thunderstorms, nonthunderstorms, and fair weather. The results of the study showed that the variation of the temperature, the absolute humidity, the humidity correction, and the relative air density could be approximated by a Gaussian distribution. Further, the variation of the multiplication of the humidity correction factor and the relative air density 6Hc can also be approximated by a Gaussian distribution.
The author of Ref. 14 regressed the mean value of the relative air density 6 and the mean value of SHc against the altitude. He selected a linear equation as an appropriate model and found the equations per Table 11. However, in retrospect, the linear equation is somewhat unsatisfactory, since it portrays that the relative air density could be negative-or more practically, the linear equation must be limited to a maximum altitude of about 2 km. A more satisfactory regression equation is of the exponential form, which approaches zero asymptotically. Reanalyzing the data, the exponential forms of the equations are also listed in Table 11.
These equations may be compared to the equation suggestion in IEC Standard 7 1.2, which is
20 Chapter 1
Table 11 Regression Equations, A in km
Linear equation Exponential equation Average standard
Statistic for mean value for mean value deviation
Relative air density, 6
Thunderstorms 0.9974.106A 1 .OOO e-A1x.59 0.019
Nonthunderstorms 1.0254.090A 1 .025 e-A19.82 0.028
Fair 1.0234.103A 1.030 e-A1x.65 0.037
6Hc
Thunderstorms 1.0354.147A 1.034 e-A16.32 0.025
Nonthunderstorms 1.0234.122A 1 .01 7 e-A1x.oo 0.031
Fair 1.025-0.132A 1 .O 13 eCAI7.O6 0.034
Either form of the equation of Table 11 can be used, although the linear form should be restricted to altitudes less than about 2 km. The exponential form is more satisfactory, since it appears to be a superior model.
Not only are the CFO, BIL, and BSL altered by altitude but the standard deviation of is also modified. Letting x equal 6Hc, the altered coefficient of variation (of/CFO)' is
2
CFO CFO
Considering that for switching overvoltages, the normal design is for wet con- ditions, Eq. 13 is applicable with the mean given by the first equation in Table 11, where the average standard deviation is 0.019. For a strike distance S of 2 to 6 meters, at an altitude of 0 to 4 km, the new modified coefficient of variation increases to 5.1 to 5.3Y0 assuming an original of/CFO of 5Yo. For fair weather, Eq. 12 applies, and the last equation of Table 11 is used along with the standard deviation of 0.034.
For the same conditions as used above, the new coefficient of variation ranges from 5.4 to 5.8Y0. Considering the above results, the accuracy of the measurement of the standard deviation, and that 5Y0 is a conservative value for tower insulation, the continued use of 5Y0 appears justified. That is, the coefficient of variation is essen- tially unchanged with altitude.
In summary, for insulation coordination purposes, the design is made for wet conditions. The following equations are suggested:
(7) For Lightning
Specifying the Insulation Strength
(2) For Switching Overvoltages
either Zi = 0.997 - 0.106A or Zi = e-(A'8.6)
(18)
where the subscript S refers to standard atmospheric conditions and the subscript A refers to the insulation strength at an altitude A in km. Some examples may clarify the procedure.
Example I. A disconnecting switch is to be tested for its BIL of 1300 kV and its BSL of 1050 kV. In the laboratory, the relative air density is 0.90 and the absolute humidity is 14 g/m3. Thus the humidity correction factor is 1.0437. As per standards, the test for the BIL is for dry conditions and the test for the BSL is for wet con- ditions. The CF~/CFO is 0.07. The test voltages applied for the BIL is
Thus to test for a BIL of 1300 kV, the crest of the impulse should be 1221 kV. For testing the BSL, let the strike distance, S, equal 3.5 m. Then
Thus to test for a BSL of 1050 kV, the crest of the impulse should be 1009 kV.
An interesting problem occurs if in this example a bushing is considered with a BIL of the porcelain and the internal insulation both equal to 1300 kV BIL and 1050 kV BSL. While the above test voltages would adequately test the external porcelain, they would not test the internal insulation. There exists no solution to this problem except to increase the BIL and BSL of the external porcelain insulation so that both insu- lations could be tested or perform the test in another laboratov that is close to sea level.
An opposite problem occurs if the bushing shell has a higher BILIBSL than the internal insulation and the laboratory is at sea level. In this case the bushing shell cannot be tested at its BILIBSL, since the internal insulation strength is lower. The
22 Chapter 1
solution in this case would be to test only the bushing shell, after which the internal insulation could be tested at its BILIBSL.
Example 2. The positive polarity switching impulse CFO at standard conditions is 1400 kV for a strike distance of 4.0 meters. Determine the CFO at an altitude of 2000 meters where 8 = 0.7925. Assume wet conditions, i.e., Hc = 1.
Example 3. Let the CFO for lightning impulse, positive polarity at standard atmo- spheric conditions, be equal 2240 kV for a strike distance of 4 meters. Assume wet conditions, i.e., Hc = 1. For a relative air density of 0.7925, the CFO is
Example 4. At an altitude of 2000 meters, 8 = 0.7925 and the switching impulse, positive polarity CFO for wet conditions is 1265 kV for a gap spacing of 4 meters.
Find the CFOs. This problem cannot be solved directly, since m is a function of Go and Go is a function of the standard CFO. Therefore the CFO for standard condi- tions must be obtained by iteration as in the table. Note that this is the exact opposite problem as Example 2 and therefore the answer of 1400 kV coordinates with it. This example represents the typical design problem. The required CFO is known for the line or station where it is to be built, i.e., at 2000 meters. The problem is to determine the CFO at standard conditions. Alternately, the required BILIBSL is known at the altitude of the station, and the BILIBSL to be ordered for the station must be determined at standard conditions.
Assumed
CFOs, kV GO m hm CFOs = 1265/hm
1300 0.650 0.3656 0.9185 1377
1377 0.689 0.4204 0.9069 1395
1395 0.698 0.4338 0.9040 1399
1399 0.700 0.4368 0.9034 1400
1400 0.700 0.4375 0.9033 1400
5 GENERATION OF VOLTAGES IN THE LABORATORY
Lightning impulses are generated by use of a Marx generator as shown schematically in Fig. 12. The same generator is used, except in the former USSR, to generate switching impulses. In the former USSR, the switching impulse is generated by discharge of a capacitor on the low-voltage side of a transformer.
Specifying the Insulation Strength
ONE STAGE
TRIGGER