Insulation coordination is "the selection of insulation strength in accordance with expected overvoltages to obtain an acceptable risk of failure" [I]. These two factors result in the minimum shock distance from the damper to the end of the tower.
Specifying the Insulation Strength
External Insulation
External insulation is the distances in open air or over the surfaces of solid insulation in contact with open air which are subjected to dielectric stress and to the effects of the atmosphere. Examples of external insulation are the porcelain shell of a bus, bus support insulators and disconnect switches.
Internal Insulation
Self-Restoring (SR) Insulation
Non-Self-Restoring (NSR) Insulation
BIL-Basic Lightning Impulse Insulation Level
BSL-Basic Switching Impulse Insulation Level
Standard Waveshapes
Although the tail of the switching pulse waveform is defined as the time to half value, the time is measured from the actual time zero and not the virtual time zero. The time to peak or front is measured from the actual time zero to the actual peak of the pulse.
Statistical vs. Conventional BILIBSL
Conventional BIL or BSL is more simply defined but has less significance in terms of insulation strength. One or more pulses with a standard waveform and peak value equal to BIL or BSL are used for isolation.
Tests to "Prove" the BIL and BSL
Ideally, the test passes if the actual probability of flashover is less than 0.10, and ideally the test fails if the probability is greater than 0.10. That is, even for an unacceptable device there is a chance that the test will pass.
Standard BILs and BSLs
In the ANSI insulation coordination standard, C92, no required values are given for alternating system voltages. The voltage is then increased and decreased to obtain further test points resulting in the data in the table.
Chopped Wave Tests or Time-Lag Curves
Furthermore, the variation of the multiplication of the humidity correction factor and the relative air density 6Hc can also be approximated by a Gaussian distribution. 14 regressed the mean value of the relative air density 6 and the mean value of SHc against the height.
- Apparatus Standards and Effects of Altitude
- CFO and nf/CFO
- Chopped Waves
- Atmospheric Correction Factors
Standard atmospheric conditions, ie sea level, relative air density 8 = 1. The BIL or BSL is equal to the peak value of the standard impulse. The average value of the relative air density 8 is related by the comparison with the height A in km.
Insulation Strength Characteristics
Wave Front
The effect of the wavefront or time to crest on the CFO is shown in Fig. First, note the U-shaped curves that show that a wavefront exists that produces a minimum insulation strength.
1 ---#/WET
Insulator Length
This is to be expected, because if the insulator length is less than the span, skip across the insulators will occur, and thus the insulator string limits the tower strength. In contrast, if the strike distance is less than the insulator length, overshoot will occur over the air strike distance, and the strike distance is limiting.
Strike Distance
For wet conditions, this saturation point increases to a level where the insulator length is 1.05 to 1.10 times the strike distance. Therefore, to obtain the maximum CFO within a tower "window" or for a fixed strike distance, the insulator length must be 5 to 10% greater than the strike distance.
Standard Deviation of Flashover
16 together with the graph of the above equation, illustrating the excellent fit of the equation to the data. PERCENTAGE DECREASE IN WET CFO OVER DRY CFO n PERCENTAGE DECREASE IN G WET CFO OVER DRY CFO a.
The Outside Phase
S is the minimum of the three distances (1) the span to the tower side, (2) the span to the top truss, and (3) the insulator strand length divided by 1.05. Because the gap factor and Go are both functions of the stroke distance, the stroke distance cannot be obtained directly.
The Conductor-Window Gap-Center Phase
Based on the previous test results, the proposed gap factor for the outer phase was 1.08 times the gap factor for the middle phase. Therefore, the suggestion remains valid, i.e. multiply the gap factor for the middle phase by 1.08 to obtain the gap factor for the outer phase.
The gap factor for the cross arm should be less than the 1.41 to 1.43 calculated here since the case of the cross arm adds an extra.
Rod-Rod Structure
Other Data
CFO of Insulators and Gaps-McAuley's Data
As an example, for 15 insulators these curves indicate a CFO of 1350 kV, whereas if the string were part of a V-string in the center phase of a tower, a CFO of 1225 kV would be expected . As will be shown, for all practical cases the CFO for positive polarity is less than that for negative polarity.
CFO of Insulators and Gaps-Present Day Data
The relationship between the gap and the CFO gradient for rod-plane gap is shown in Fig. While the CFO gradient for positive polarity is constant at 525 kV/m, the CFO gradient for negative polarity varies with the gap.
LI Strength of Towers
Therefore, the CFO gradient should be analyzed in terms of the CFO per meter of insulator length and is comparable to the values in Table 4. For these tower tests, the standard deviation for positive polarity was 1.0% of the CFO, which increased to 3.6 % of CFO due to negative polarity.
Suggested Values for Air Gaps and Insulators
Time-Lag (Volt-Time) Curves
Above this critical length of wood, the CFO is equal to the CFO of the wood alone, at a CFO gradient of about 300 kV/m (90 kV/ft). At this critical length the wood adds 100 kV/m wood to the CFO of the insulators.
Phase-Ground Switching Overvoltages, Transmission Lines
Stress and Strength Both Normal-For One Tower
Now let Z = (S - s) or more formally let the random variable Z equal the random variable S minus the random variable s. Since both distributions are normal, the Z distribution will be normal, and off (Z) is normal with parameters.
More Than One Tower
Voltage Profile
However, since the probability of at least one skip (on one tower) is desired, we must first calculate the probability of no skip (on any of the towers) and then subtract it from 1. The probability of no skip on tower 1 is ql, the probability of no skip on tower 2 is q2,.
Case Peaks To Be Used
For the normal low values of SSFORP, the SSFOR is simply three times the SSFORp.
Gaussian Distribution
The probability that the SOV is equal to or greater than E2 is 0.02, or in other words: 2% of the SOVs are equal to or greater than E2. Another disadvantage of the Gaussian distribution is that it is untruncated both to the left and to the right, that is, it is defined from - infinity to + infinity, while the SOVs are limited between 1.0 pu up to a maximum SOV of Em.
The Extreme Value Positive Skew Distribution
Therefore, for a linear SOV profile, the SOVs along the line can be simply described by the ratio y or Es/Ev and the SOV at the open end of the Ev line. To further clarify, the SOV distribution f,(V) always refers to the voltage at the open end of the line.
Strength-Stress ratio V 3 / &
The ratio described by y is used to find the voltages at other places along the line. Using E2 and V3, the force-strain ratio is expressed as VdE2 and SSFOR is given by the curves of Fig.
Number of Towers
19, as the number of towers increases, the strength characteristic becomes steeper or the standard deviation becomes smaller. To account for the SOV profile, the equivalent number of towers is calculated and then used in Eq.
Discussion of Estimating Method
Gaussian SOV Distribution Neglecting Em, the SSFOR is
As noted, the ratio V3/E2 is obtained directly from Kf, which is only a function of the strength, and KG, which is only a function of the voltage.
Extreme Value Positive Skew SOV Distribution For an extreme value positive skew SOV distribution, Eq. 67 becomes
Usually, the 100-hour wind speed can be obtained from nearby airports where weather statistics are recorded. Usually the 100-hour wind speed at the airport is greater than that at the line location.
SOV Distribution
Stress-Strength and Estimating the SSFOR
Note that the Weibull distribution is characterized to approximate the Gaussian distribution. The advantage of the method is that the CFO can be calculated directly without using a Gaussian probability table.
Estimating the Strike Distance
Use the estimation method described in this chapter and then check the answer with the appropriate computer program. Use the estimation method presented in this chapter and then check the answer with the appropriate computer program.
Phase-Phase Switching Overvoltages, Transmission Lines
The Alpha Method
4, plotting the CFOp or kgp as a function of ct results in an approximate linear relationship [I]. They found only small effects of conductor height and length (doubling the height increases the CFOp by 1.3Y0, doubling the length decreases the CFOp by 1.4Y0).
The V+-V- Method
However, for long conductor-to-conductor gaps, crFp is lower, from 2 to 3Y0 CFO. The limits of the data shown in [6.2] are indicated by two lines in the figure.
Correlating the Methods
By knowing two values of a and kgp, the values of KL and Kop can be determined. As noted, the values of opp/CFOo are assumed to be equal to values ofp/CFOp; more on this later.
Phase-Phase and Phase-Ground
Only for large air clearances does the separation of the phase-phase voltage into separate components become important, and for this case KL is less than one. That is, the programs easily obtain the maximum phase-to-ground voltage at T^ and the maximum phase-to-phase voltage at TI2.
However, this method is inherently conservative as it involves collecting the "worst" data at each moment. Today, the trend is to collect the V' data as it is necessary for the evaluation of the SSFOR for phase-ground isolations and also to collect the Vp data.
The Correlation Coefficients
Estimating the Value of EZp
Combined Phase-Phase and Phase-Ground SSFOR
Thus, within a few percent, the total SSFOR is equal to the phase-phase SSFOR plus the phase-ground SSFOR. In other words, the phase-phase SSFOR and the phase-ground SSFOR can be calculated separately.
Including the Phase-Ground, Negative Polarity
But first, to complete the analysis, consider the inclusion of the negative polarity strength and the use of "reversed parameters.".
Reversed Parameters
General
As for the phase-to-ground case, the data are usually approximated by a continuous distribution, and this distribution can be Gaussian (normal), extreme value, or some other distribution. In addition, the same simplifications and approximations used to calculate phase-to-earth SSFOR can be used to calculate the phase-to-phase SSFOR, e.g. Brown's method.
With SOVs Density as Normal or Gaussian
The same method can be used to estimate the phase-to-phase SSFOR or estimate the striking distance for a given SSFOR as used to calculate the phase-to-ground SSFOR or to calculate the phase-to-ground striking distance, except that the voltage profile for V z must be used. As far as phase-to-ground insulation is concerned, the equivalent number of towers or spans is no.
Reversed Parameters
This is shown more clearly in Table 4, which uses the same parameters as in the example above, except that yp = y+ = 1.0. Theoretically, the total SSFOR is the sum of the calculation performed in the example plus the SSFOR when the input voltage parameters are inverted.
Sensitivity
At this time, there was much confusion in the technical understanding of the process of phase-phase insulation coordination. Estimate the phase-phase and phase-ground strike distance for a phase-phase and phase-ground SSFOR of 1 skip per 100 breaker operations.
Switching Overvoltages, Substations
Coordination of Station and Line Insulation
The use of arresters on the track side of the circuit breaker essentially isolates the station from the track. In this case, the insulation of the station can be selected according to the characteristics of the arrester without taking into account the insulation strength of the lines.
Number of Insulations in Parallel
1, it is the end-of-line switching surges that affect the station isolation, or the station isolation is the end-of-line isolation. This means that the power of the switching pulse, i.e. the value of V3, for the station to be equal to or exceed the power of the switching pulse, V3, for the line.
Voltage Profile
Unequal Insulation Strengths
Estimating the SSFOR and Strike Distance
In anticipation of the effect of arresters, the values of KG and KE are given for small values of o0/E2 and PIE2. In anticipation of phase phase methods, the value of Ze for a Gaussian distribution is given in Table 2 (see Table 6 in Chapter 3).
Station Insulation Strengths Relationship of Strengths
However, it is never, or rarely, important, since, even though it has a low gap factor. For nonstandard atmospheric conditions, i.e., altitudes greater than sea level, correction procedures require the use of a gap factor.
Design Value of SSFOR
Equations-Review
Thus, BIL 1300 kV was chosen for the supporting bus insulators and disconnecting switches. For higher altitudes, the insulation strength of the bushing's outer insulation decreases, so the required BSL increases.
SOV Distribution
Estimating Method
As in the above, let the correlation coefficient between the phase-phase and the positive SOVs be equal to 1.0, and let the number of isolations be equal to one. For heights greater than zero, the impact distance is increased and can be calculated using the same methods used for phase-phase ground SOVs.
Arresters
So for the standard test, a positive voltage of 633.5 kV is applied to one phase and a negative voltage of 633.5 kV is applied to the other phase. Assuming the use of a 3 18 kV MCOV arrester with a switching impulse discharge voltage of 823 kV at 2 kA, the arrester, according to example 3, will change the SOVs on the positive phase, but not on the negative phase.
