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
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
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
Overvoltages that need to be considered when doing an Insulation Coordination Study.
There are three types of overvoltages that may occur on a plant:
These may usually be short power frequency overvoltages or weakly damped oscillatory voltages. The main causes of these overvoltages are:
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:
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:
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.
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.