This document provides an introduction to power system transients. It discusses the sources of transients, both internal like capacitor switching and external like lightning. It classifies transients into three categories based on speed: ultrafast surges, medium-fast short-circuit phenomena, and slow transient stability issues. The effects of transients are outlined, such as damage to insulation, semiconductors, and contacts. The importance of studying transients for insulation design is emphasized to prevent breakdown under overvoltage conditions.

1. Power System Transient
Chapter – 1
Introduction
Prepared by Viren Pandya

2. Contents
 Source of transients
 Various types of power systems transients
 Effect of transients on power systems
 Importance of study of transients for insulation
design

3. Source of transients
The sudden, short time and large magnitude surges
in energy system are termed as transients.
If the energy system is operated in static
equilibrium, system models are normally nonlinear
but algebraic equations e.g. power flow analysis,
optimal power flow etc.

4. Source of transients
Theory of small-scale dynamic perturbations
around the static equilibrium point requires system
models to be linear differential equations e.g.
ALFC and AVR study under steady state with
normal load variations.
Large scale disturbances -transients resulting into
dynamic state fluctuations in the energy system
need models to be nonlinear differential equations.

5. Source of transients
Internal Sources (Inside facilities)
Capacitor switching
Current interruption (motors, etc.)
Power electronics operation (SCRs, etc.)
Electrostatic discharge
(Arc) Welding
Copy machines
Faulty wiring or circuit breaker operation
Contact and relay closure
Load startup or disconnect

6. Source of transients
Internal sources do not produce surges of large
magnitude.
Experience shows that transients due to internal
sources hardly increase the system voltage to
twice the normal value.

7. Source of transients
External Sources (Outside facilities)
Lightning
Capacitor switching
Line/cable switching
Transformer switching
Current limiting fuse operation

8. Types of power systems transients
Depending upon the speed of the transients, they
can be classified into three categories.
Class A: Ultrafast transients – Surge phenomena
Class B: Medium-fast transients – Short-circuit
phenomena
Class C: Slow transients – Transient stability

9. Types of power systems transients
Class A: Ultrafast transients – Surge phenomena
Due to lightning or switching operation of large
component in the energy system
Entirely electric in nature and normally involves
transmission system only
Electromagnetic transient - travelling wave travels
almost the speed of the light along the lines, giving
rise to reflected wave at the line termination

10. Types of power systems transients
A few milliseconds after initiation – then get attenuated
due to system R and die out
High inductance of transformers in most instances
effectively provides protection to generator against
such transients.
Lightning arrester provide protection against lighting
strokes striking on the lines.
These transients’ study basically provides a basis for
selection of insulation level of the line equipment.

11. Types of power systems transients
It is to be noted that if insulation of the system gets
destroyed due to such overvoltage transient, this type
of transient can lead to a short circuit, which is an
abrupt and abnormal structural change of the network
which will give rise to a new – slower type of transient
that belongs to Class-B.

12. Types of power systems transients
Class B: Medium-fast transients – Short-circuit
phenomena
These are caused by normally abrupt and abnormal
structural changes – short circuits in the system.
These could be caused due to Class A transients, salt
spray on insulators, birds sitting, other mechanical
causes etc.
LLL/LLLG, LG, LL, LLG, 1CO, 2CsO
Important to study for selection of CBs, protection
system design, transient stability analysis.

13. Types of power systems transients
Class C: Slow transients – Transient stability
Short circuit will always be accompanied by an
instantaneous collapse (partial or full) of bus voltages
throughout the system.
Now the generator voltages drop suddenly with sudden
reduction in generator output power under short circuit.
Since the mechanical/turbine input power to generator at
the time of fault stays constant for first instants, each
generator will be subjected to a surplus accelerating torque.

14. Types of power systems transients
If it is sustained, it can develop into the most
dangerous type of transient situation to which a power
system can be subjected - mechanical oscillations of
the synchronous machine rotors.
These electromechanical transients may under
unfortunate circumstances pull some or all the of the
machines out of synchronism.
Such an occurrence constitutes a partial or complete
breakdown of the energy system.

15. Types of power systems transients
Here the system is said to have reached its transient
stability limit.
It may take hours to resynchronize a fully blacked-out
system.
These rotor swings, being mechanical in nature, are
relatively slow.
Thus such transients' study becomes necessary to
assess transient stability.

17. Effect of transients on power systems
Effects on insulation
Effects on semiconductors
Effects on electromechanical contacts
Noise generation
Downtime or running outages
Efficiency of components gets affected.

18. Effect of transients on power systems
Under severity, black out of power system will be
produced.
Lightning transient produces steep fronted wave on
transmission line.
Travelling wave produced due to transient will
shutter the insulations and week poles.
Cause damage to windings of transformer and
generators.

19. Importance of study of transients for insulation design
Failures in insulation cause safety risks for human,
fires and system outages.
Transient voltage peak value is very much
important for insulating material breakdown in the
power system.
Design of the insulation needs to be done in such a
way that it does not breakdown under lighting
impulses, switching surges or any other overvoltage
transients.

20. Importance of study of transients for insulation design
The EMTP has became popular for the calculation
of power system transients especially a switching
overvoltage from the viewpoint of insulation design
and coordination of a transmission line and a
substation.