Insulation Coordination

[timm33333]

I have a question regarding insulation coordination. The conventional insulation coordination study involves hand calculations of protective margins (PRL1, PRL2, PRS) of surge arrester.

However there is also frequency scans based insulation coordination study done by software, which involves switching transients like TOV, TRV, SFO, FFO, VFTOV, and Ferroresonance. In his method surge arrester performance graphs (KV vs time) are created, and also surge arrester energy dissipation graphs (kJ/kV) are created.

My question is that what is the difference between these two methods of insulation coordination? Under what conditions the conventional protective margins method is used, and under what conditions the frequency scans based method is used? Thanks.

 

[cuky2000]

There are two approaches for insulation coordination studies
a) The conventional deterministic approach bases on empirical formulas to calculate conservative safety margin.
b) The statistical or stochastic method using smaller time frames and frequency scan during the transient phenomena.
The statistical method is inherently more accurate than the traditional deterministic approach, allows also to discriminate among probability of occurrence of flashovers on one, two, or on all phases. The statistical method is recommended for high and ultra light voltage using Electromagnetic Transient Programs such as ATP, PSCAD, EMTP, etc. requiring correct equipment modeling.

 

[crshears]

"ultra light voltage"

I've never heard of that . . . ?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Kiribanda]

I donot know what you meant by frequency scan in insulation coordination study. Normally it is for harmonics.
You can do the equivalent calculations of protective margins (PRL1, PRL2, PRS) for the surge arrester using a EMTP software.
After modeling the substation at least for 5 spans, you allow the the transmission line to hit with a certain value of surge current
(based on the ground flash density of the area) and then calculate the,
(1) Voltage applied at the transformer/ CB terminals and compare it with the BIL level of the equipment
(2) Energy dissipated at the terminals of the surge arrester and compare it with the energy handling capability of the surge arrester given by the manufacturer.
It should be noted here that per IEEE C62 we use either station class for substations and distribution class for distribution lines.
Therefore the energy handling capability is already defined.
But in the IEC world there is no station or dis class, but the line discharge class.
In that case the energy handling capability has to be compared.

 

[timm33333]

Thanks for the information. Also I need some clarification that what are the acceptable values of transient voltages (in kV) of different types for switching transients.

Switching transients are of several types including TOV (temporary overvoltage), TRV (transient recovery voltage), SFO (slow front overvoltage), FFO (fast front overvoltage), VFTO (very fast transient overvoltage), and Ferroresonance.

In Statistical/Stochastic method of insulation coordination: we plot graphs of transient voltages (kV) against time (sec) at different chopping currents for each type of switching transients. If the graphs show that the values of transient voltages (kV) are within acceptable limits, then no action is needed. Otherwise we will have to change the ratings of the surge arresters, or we will have to add more surge arresters.

For 230 kV: line to ground voltage = 230/1.732 = 132.8kV
Peak line to ground voltage = 132.8 * SQRT (2) = 132.8 * 1.414 = 187.8 kV
BIL for 230kV system = 1050 kV

So the acceptable values of different switching transients would be as below:

Acceptable TOV for 230kV system = 187.8 * 1.4 p.u. = 187.8 * 1.4 = 263 kV
Acceptable TRV for 230 kV system = 187.8 * 2.5 p.u. = 187.8 * 2.5 = 469.5 kV
Acceptable SFO for 230kV system = 80% of 1050 = 840kV
Acceptable FFO for 230kV system = 80% of 1050 = 840kV
Acceptable VFTO for 230kV system = 80% of 1050 = 840kV

Are these acceptable values correct? Thanks

 

[cuky2000]

Crshears, there is a typo. Instead light should be high. Following ANSI C84. 1-2020 defines high voltage as 115 kV to 230 kV, extra-high voltage as 345 kV to 765 kV, and ultra-high voltage as 1,100 kV.

 

[cuky2000]

Kiribanda, Temporary Overvoltage (TOV) is a typical phenomenon accounted for in insulation coordination studies and is impacted by harmonic resonance.

Modern modeling guidelines recommend a frequency scans form an initial screening process to identify and select relevant/onerous system conditions that require further investigation in the time domain. The EMTP-RV software has good capability to perform this assessment.

 

[timm33333]

Also does ferro-resonance only create temporary overvoltage (TOV), or does it also create other types of switching transients?