Introduction to Power System Transient

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.