Power System Protection
EE454
Lecture ppt. # 1
• Note:
The materials in this presentation are only for the use of students enrolled in this course in the
specific campus; these materials are for purposes associated with this course and may not be
further disseminated or retained after expiry of the course.
Some of the Contents
Lect. File 1 – Chapter 1 of PS Relaying
1 Basic concepts – Relaying – Philosophy of PSP – Major
challenges – Disturbances in the system occur
2 Layers (Sub-systems) in the power system
3 Grounding of power system
4 Radial/Network system – Substation Bus configuration
5 Dependability – Zones - Primary and backup protection
6 Circuit Breakers
• Contents for this lecture from the course plan for ODL
CLOs and Books
The CLOs and Books may have changed after 2020 - Please refer to course outline file provided to you.
Disturbances in Power System
This gives you an idea of
power system disturbances
from a non-bookish source
https://newsghana.c
om.gh/2019-a-year-
of-power-system-
collapses-part-ii/
Accessed – 29 Sept.
2020
An example of a temporary fault
An example of a
multi-layered
system
You may see these in your
course of PSO&C – e.g. AVR
Concept of a smart power
system – showing layer of
communication (may be
referred to as control)
Problems with un-grounded system
Co provides path for flow of
a ground fault current even
if the system is
ungrounded. Relaying
problem becomes
detection of such low
magnitude of fault current.
Problems with un-grounded system
If phase 3 conductor
touches ground (due to a
fault), then ground is at
phase 3 voltage – hence LG
voltages of other phases
have now become LL
voltages.
Grounding
Solid Grounding –
Leads to high fault current
Reactance Grounding –
Reduces fault current level
Grounding
Peterson Coil (also known
as Ground Fault Neutralizer If 3Xn is made equal to Xco – the parallel
GFN) combination becomes open circuit and no
ground fault current flows.
GFN needs to be tuned to the system.
Substation Bus
Schemes
Very inflexible –
Maintenance work of
primary breaker will
shutdown the
substation
Substation Bus
Schemes
L1
B1
Two bus single breaker
scheme.
For maintaining a breaker – e.g. B1 Only one breaker
Close bus tie breaker can be maintained
And supply power to L1 through at a time
transfer bus.
The bus tie breaker becomes main
breaker for L1.
Substation Bus
Schemes
Two-bus two-breaker scheme
Allows any bus or breaker to
be maintained. Gives highest
flexibility however cost is high Breaker and a half scheme
too.
The basic philosophy of relaying/PSP is to disconnect the faulty/abnormal portion from the healthy portion. Thus
i. operation of the healthy part is maintained
ii. excessive damage in the faulty part is avoided
Major challenges of the protection engineer are:
i. Detection of fault
ii. Isolation of minimum system portion
Zone of protection – Region where a
relay should work; beyond this it
should not work.
Zone of bus protection –
Overlaps line protection -
Fault in overlapping region
causes both protections
(line and bus) to operate.
Operation of bus protection
(may be a differential relay)
trips all Breakers associated
with the bus.
Three important elements are:
Relay – Brain of the protection system
CB - The switch that makes/breaks
CT,PT - The transducers through which relay sees
Free standing CTs can further be live tank or dead tank –
and they can have multiple secondaries
Free standing CB will have separate CT - this one CT can have
two secondary windings - as shown above
