Technology and Technic

Resistances Mag'

INTERVIEW

Mr Bernard Audouard, Sales Manager of MS RESISTANCES

"The Insulation Coordination Topic in Medium and High Voltage"

MB: Mr Audouard, would you please start by explaning us what the "Insulation Coordination" is?

BA: Of course. The "Insulation Coordination" is a discipline aiming at achieving the best possible technico-economic compromise, taking into consideration Network Voltage and possible Overvoltages whether caused by the network itself or by lightning. To tune the insulation coordination:

1/ We must know the level of the possible overvoltages occurring on the network;

2/ the correct overvoltage's withstand* level must be chosen from among the insulating voltages satisfying the particular constraints.
(*The withstand voltage varying in particular according to the type of overvoltage applied, i.e. voltage level, rising front, frequency, time...)

Moreover, you have to remember that, depending on the atmospheric conditions, such as wind, rain, altitude or coastal area, electrical networks are exposed to some external risks.
That is the reason why it is important to determine clearances and creepages, to select insulators.

MB: Well, we clearly understood the importance of taking care about the insulation coordination in MV and HV but what does "Insulation Coordination" concretely mean ?

BA: In our case, Insulation coordination is the ability to fixe each single potential inside our equipment in order to be able to reduce Insulation sizing internally without reducing overall Equipment withstand Level.

Then, the best insulation coordination definition would be the one provided by the International Electrotechnical Commission (IEC), i.e.: "...The selection of the dielectric strenght of equipment in relation to the voltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the avalaible protective devices..."

MB: So, what would you recommend in terms of protection level?

BA: Study of insulation coordination of an electrical installation is thus the definition, based on the possible voltage and overvoltage levels on this installation, of one or more overvoltage protection levels. Installation equipment and protective devices are thus chosen accordingly.
Protection level is determined by the following conditions:

1/ installation
2/ environment
3/ equipment use

Study of these «conditions» determines the overvoltage level to which the equipment could be subjected during use. Choice of the right insulation level will ensure that, at least as far as power frequency and switching impulses are concerned, this level will never be overshot.

MB: Now, may you please remind us the Insulation Coordination Standards and how do you apply them?

BA: Insulation coordination is dealt with in two main documents:

1/ IEC 60 664 for LV (but this one does not interest us as it deals with Low Voltage)
2/ IEC 60 071 for HV

One of the objectives of the IEC 60 071 standard is to explain and break down the various factors for achievement of withstand voltages. This approach encourages search for optimisation and even reduction in voltage withstand levels.

Tables of standardized rated withstand voltages exists for each "Standardized" Network Voltage Level. These tables have been drawn up according to various criterias, with a few reservations, by experience.

MB: After all these valuable informations, may you please explain us how you would solve the insulation coordination problem if a customer asks you to build a Resistor to be used on a 100 kV Network?

BA: In order to get the minimum requirements to design a resistor that will be able to sustain a specific voltage,
- we will firstly select the Lightning Impulse Voltage which is equivalent to the Network Insulation Voltage.
In this example, Network voltage is rated at 100 kV. Thus, Rated Lightning Impulse voltage will be selected as 450kV BIL .
- This will help selecting the minimum number of Cubicles we shall use in our design.
- The same logic will be applied to the number of resistor banks and resistor elements.

We are able to design Resistors with one or multiple paths in grounded Cubicles or in live ones.
Up to now, we designed up to 3 alive cubicles per one phase for Resistors up to 1050kV BIL

MB: As you know, I am not really a technical person. Would you mind being a bit more explicit and giving us some more details on how you would calculate your Resistor.

BA: being more explicit would force me to give you information which are part our our know-how. You can therefore easily understand that I cannot divulge such strategic information.
However, would a specific customer request some more details, we would be pleased to meet him and provide him with a valuable explanation.

MB: Mr Audouard, thank you very much for this explaination and for your kindliness.

Marion BOSSET
Marketing and Sales Assistant