What is Insulation Coordination?

Insulation Coordination is the process of determining the proper insulation levels of various components in a power system as well as their arrangements. It is the selection of an insulation structure that will withstand voltage stresses to which the system, or equipment will be subjected to, together with the proper surge arrester. The process is determined from the known characteristics of voltage surges and the characteristics of surge arresters.

Some common terms that must be known when performing an Insulation Coordination Study.

1. Basic Impulse Insulation Level (BIL)

This is the reference insulation level expressed as an impulse crest (or peak) voltage with a standard wave not longer than a 1.2 x 50 microsecond wave.

A 1.2 x 50 microsecond wave means that the impulse takes 1.2 microseconds to reach the peak and then decays to 50% of the peak in 50 microseconds. (Click here for a figure of the BIL waveform)
 

2. Withstand Voltage

This is the BIL level that can repeatedly be applied to an equipment without flashover, disruptive charge or other electrical failure under test conditions.
 

3. Chopped Wave Insulation Level

This is determined by using impulse waves that are of the same shape as that of the BIL waveform, with the exception that the wave is chopped after 3 microseconds. Generally, it is assumed that the Chopped Wave Level is 1.15 times the BIL level for oil filled equipment such as transformers. However, for dry type equipment, it is assumed that the the Chopped Wave Level is equal to the BIL level.
 

4. Critical Flashover Voltage

This is the peak voltage for a 50% probability of flashover or disruptive charge.
 

5. Impulses Ratio

This is normally used for Flashover or puncture of insulation. It is the ratio of the impulse peak voltage to the value of the 60 Hz voltage that causes flashover or puncture. Or, it is the ratio of breakdown voltage at surge frequency to breakdown voltage at normal system frequency (60 Hz).
 

Overvoltages that need to be considered when doing an Insulation Coordination Study.

There are three types of overvoltages that may occur on a plant:

• Internal Overvoltages
• Switching Surges
• External Overvoltages

These may usually be short power frequency overvoltages or weakly damped oscillatory voltages. The main causes of these overvoltages are:

• Phase to Earth Faults: Single line to Ground, Double line to Ground, 3 Phase to Ground.
• Load Rejection.
• Ferro Resonance.
• Ferranti Effect.

These surges are of short duration, irregular (or impulse form) and highly damped. The effects of such overvoltages are of great concern when the transmission voltage is greater than 300kV. However, below 300kV, some causes of these overvoltages are:

• Resonance effects when switching transformer feeders, or cables and overhead lines.
• Ferro resonance encountered on transformer feeder double circuits, when one circuit is switched out but the other parallel feeder remains energised.
• Line energisation may cause switching surges especially at the remote end of the line that is being energised.

Power systems that operate below 145kV (example the T&TEC system) overvoltages due to lightning are of greater concern than the previous two types of overvoltages. Lightning discharges are usually very short, unidirectional and have a shape similar to the BIL waveform.

The point of insulation flashover depends on

    (i) Geographical position of the lightning stroke
    (ii) Magnitude of the stroke
    (iii) Rise time of voltage wave
    (iv) System insulation levels
    (v) System Electrical characteristics
    (vi) Local atmospheric or ambient conditions
 

Overvoltage Surge Protection

There are two methods of overvoltage protection:

1. Rod or Spark Gaps

These devices are easy and cheap to install and are usually installed in parallel with insulators between the live equipment terminal and earth. Some disadvantages of these devices include:

• When they operate, they cause a short circuit fault, which may cause protection to operate and isolate the equipment.
• The sudden reduction in the voltage during operation causes high stresses on the Transformer interturn insulation.
• The breakdown of plant insulation varies with the duration of the overvoltage.
• At lower voltages, where short distance gaps are used, maloperation may occur due to debris being deposited on the terminals of the gaps.

Modern Surge arresters are of the gapless Zinc Oxide type. Previously, Silicon Carbide arresters were used, but their use has been superceeded by the ZnO arresters, which have a non-linear resistance characteristic. Thus, it is possible to eliminate the series gaps between the individual ZnO block making up the arrester.
 

Selection Procedure for Surge arresters

1. Determine the continuous arrester voltage. This is usually the system rated voltage.
2. Select a rated voltage for the arrester.
3. Determine the normal lightning discharge current. Below 36kV, 5kA rated arresters are chosen. Otherwise, a 10kA
    rated arrester is used.
4. Determine the required long duration discharge capability.
    For rated voltage < 36kV, light duty surge arrester may be specified.
    For rated voltage between 36kV and 245kV, heavy duty arresters may be specified.
    For rated voltage >245kV, long duration discharge capabilities may be specified.
5. Determine the maximum prospective fault current and protection tripping times at the location of the surge arrester
    and match with the surge arrester duty.
6. Select the surge arrester having porcelain creepage distance in accordance with the environmental conditions.
7. Determine the surge arrester protection level and match with standard IEC 99 recommendations.
 

Some Common ratings associated with surge arresters

1. Rated Voltage

The power frequency voltage across the arrester must never exceed its rated voltage, otherwise the arrester may not reseal and may catastrophically fail after absorbing the energy of the surge.

For effectively earthed system:
Maximum phase to earth voltage = 80% maximum line voltage

2. Rated Current

Arresters are tested with 8/20 microsecond discharge current waves of varying magnitudes.

3. Normal Voltage

Nominal continuous voltage that the arrester can with stand before failing or flashover.

4. BIL

Basic Impulse Insulation Level which is the maximum impulse for a 1.2 x 50 microsecond waveform.

5. Discharge voltage

When the overvoltage impulse reaches this value, the arrester begins to channel energy to earth.
 
 

For an example of an insulation coordination procedure, please click here.