The concepts of Insulation coordination in sub stations are explained in this PPT

1. Insulation Coordination
K. Karthikeyan,
Associate Professor / EEE,
Ramco Institute of Technology

2. Definition
• Insulation coordination means the correlation of the insulation
of the various equipments in a power system to the insulation
of the protective devices used for the protection of those
equipments against over-voltages.
• IEC 60071-1
The selection of the dielectric strength of equipment in
relation to the operating voltages and over-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 available preventing and protective
devices.
• IEEE 1313.1
The selection of insulation strength consistent with expected
over voltages to obtain an acceptable risk of failure.

3. Basic Concepts
• In a power system various equipments like
transformers, circuit breakers, bus supports
etc. have different break-down voltages and
hence the volt-time characteristics.
• In order that all the equipments should be
properly protected it is desired that the
insulation of the various protective devices
must be properly coordinated.

4. • Electric systems insulation designers have two options
available to them:
o choose insulation levels for components that would withstand
all kinds of overvoltages,
o consider and devise protective devices that could be installed at
the sensitive points in the system that would limit overvoltages
there.
• The first alternative is unacceptable especially for e.h.v.
and u.h.v. operating levels because of the excessive
insulation required.
• Hence, there has been great incentive to develop and use
protective devices.
Basic Concepts

5. Basic Concepts
• Curve A is the volt-time curve of
the protective device and B the
volt-time curve of the equipment
to be protected.
• Figure shows the desired
positions of the volt-time curves of
the protecting device and the
equipment to be protected.
• Thus, any insulation having a
withstand voltage strength in
excess of the insulation strength of
curve B is protected by the
protective device of curve A.

6. Volt-time Curve Construction
• The volt-time curve is a graph showing the relation between the
crest flashover voltages and the time to flashover for a series of
impulse applications of a given wave shape.
• Waves of the same shape but of different peak values are applied
to the insulation whose volt-time curve is required.
• If flashover occurs on the front of the wave, the flashover point
gives one point on the volt-time curve.
• The other possibility is that the flashover occurs just at the peak
value of the wave; this gives another point on the V-T curve.
• The third possibility is that the flashover occurs on the tail side of
the wave. In this case to find the point on the V-T curve, draw a
horizontal line from the peak value of this wave and also draw a
vertical line passing through the point where the flashover takes
place. The intersection of the horizontal and vertical lines gives the
point on the V-T curve.

7. Volt-time Curve Construction

8. Process of Insulation Coordination
• The problem of coordinating the insulation of the
protective equipment involves not only guarding the
equipment insulation but also it is desired that the
protecting equipment should not be damaged.
• To assist in the process of insulation coordination,
standard insulation levels have been recommended.
• Basic impulse insulation levels (BIL) are reference levels
expressed in impulse crest voltage with a standard
wave not longer than 1.2/50 µ sec wave.
• Apparatus insulation as demonstrated by suitable tests
shall be equal to or greater than the basic insulation
level.

9. Three Steps for insulation coordination
1. Selection of a suitable insulation which is a
function of reference class voltage (i.e., 1.05
× operating voltage of the system).
2. The design of the various equipments such
that the breakdown or flashover strength of
all insulation in the station equals or exceeds
the selected level as in (1).
3. Selection of protective devices that will give
the apparatus as good protection as can be
justified economically.

10. Insulation coordination for Sub-station
Figure gives the relative position of the volt-time
curves of the various equipments in a substation for
proper coordination.

11. Statistical approach to insulation coordination
• Present-day practices of insulation coordination rely
on a statistical approach which relates directly the
electrical stress and the electrical strength.
• This approach requires a knowledge of the
distribution of both the anticipated stresses and the
electrical strengths.
• For the purpose of coordinating the electrical stresses
with electrical strengths it is convenient to represent
the overvoltage distribution in the form of probability
density function (Gaussian distribution curve as
shown in Fig.) and the insulation breakdown
probability by the cumulative distribution function

12. Statistical approach to insulation coordination
• The knowledge of these distributions enables us to
determine the ‘risk of failure’.
• As an example, let us consider a case of a spark gap for
which the two characteristics in Figs apply and plot these
as shown in Fig.

13. Statistical approach to insulation coordination
• If Va is the average value of overvoltage, Vk is the k th
value of overvoltage, the probability of occurrence of
overvoltage is p0 (Vk)du, whereas the probability of
breakdown is Pb(Vk)or the probability that the gap will
break down at an overvoltage Vk is Pb (Vk )p0 (Vk ) du.
• For the total voltage range we obtain for the total
probability of failure or ‘risk of failure’
• The risk of failure will thus be given by the shaded area
under the curve R.

14. Statistical approach to insulation coordination
• In engineering practice it would become uneconomical to
use the complete distribution functions for the
occurrence of overvoltage and for the withstand of
insulation and a compromise solution is accepted as
shown in Figs (a)and (b) for guidance.

15. Statistical approach to insulation coordination
• Curve (a) represents probability of occurrence of
overvoltages of such amplitude (Vs) that only 2
per cent (shaded area) has a chance to cause
breakdown. VS is known as the ‘statistical
overvoltage’.
• In Fig. (b) the voltage Vw is so low that in 90 per
cent of applied impulses, breakdown does not
occur and such voltage is known as the ‘statistical
withstand voltage’ Vw.

16. Statistical approach to insulation coordination
• In addition to the parameters statistical
overvoltage ‘VS’ and the statistical withstand
voltage ‘VW’ we may introduce the concept of
statistical safety factor.
• This parameter becomes readily understood by
inspecting Figs (a) to (c) in which the functions Pb
(V) and p0 (Vk) are plotted for three different cases
of insulation strength but keeping the distribution
of overvoltage occurrence the same.

17. Statistical approach to insulation coordination

18. Statistical approach to insulation coordination
• The density function p0Vk is the same in (a) to (c)
and the cumulative function giving the yet
undetermined withstand voltage is gradually
shifted along the V-axis towards high values of V.
• This corresponds to increasing the insulation
strength by either using thicker insulation or
material of higher insulation strength.
• As a result of the relative shift of the two curves
[Pb (V) and p0 (Vk)] the ratio of the values Vw/Vs
will vary.
• This ratio is known as the statistical safety factor
or

19. Statistical approach to insulation coordination
• In the same figure (d) is plotted the relation of this
parameter to the ‘risk of failure’.
• It is clear that increasing the statistical safety factor
(γ) will reduce the risk of failure (R), but at the same
time will cause an increase in insulation costs.
• The above treatment applies to self-restoring
insulations.
• In the case of non-self-restoring insulations the
electrical withstand is expressed in terms of actual
breakdown values.
• The statistical approach to insulation, presented here,
leads to withstand voltages (i.e. probability of
breakdown is very small), thus giving us a method for
establishing the ‘insulation level’.