ET 601 – POWER SYSTEMET 601 – POWER SYSTEM
After completed this chapter, students should be
1. Understand the constitution of power system
2. Know the need of protection system
3. Understand basic requirement of protection
evaluation 4. Understand the type of fault and
5. Know the basic components of protection
6. Understand basic concept of protection system.
static load disconnect
Safeguard the entire system to maintain
continuity of supply
Minimize damage and repair costs where it
Ensure safety of personnel.
Why need protection system?
1. Normal operation means that there are no failures of
equipment and/or no mistakes committed by personnel.
This also means that there is no effect of any natural
calamity on the power system.
2. Prevention of electric failure means the provision
of features in the design that are aimed at preventing
failures. These features include adequate insulation,
insulation co-ordination in accordance with the
capabilities of lighting arresters, providing overhead
ground wires and low tower footing resistance, and
adoption of proper operation and maintenance
3. Mitigation of the effects of electric failure when
it occurs implies that certain ‘protection provisions
should be incorporated in the system. These provision
include limiting the magnitude of short circuit current,
provision for promptly disconnecting the faulted
section, and features that investigate and send out an
alert in the case of an incipient fault.
These requirements are necessary, firstly for early
detection and localization of faults, and
Secondly for prompt removal of faulty equipment
In order to carry out the above duties, protection
must have the following characteristics:
• minimum fault duration and
consequent equipment damage and
• assurance that the protection will
• maximum continuity of service with
minimum system disconnection.
• To detect even the smallest fault,
current or system abnormalities and
operate correctly at its setting
A power system fault may be defined as any
condition or abnormality of the system which
involves the electrical failure of primary
equipment such as generators, transformers,
busbars, overhead lines and cables and all other
items of plant which operate at power system
Electrical failure generally implies one or the
other (or both) of two types of failure, namely
insulation failure resulting in a short-circuit
condition or conducting path failure resulting in
an open-circuit condition, the former being by far
the more common type of failure.
FAULT IN POWER SYSTEM
Faults giving rise to equal currents in lines
equal phase angles i.e 120o
in three phase
Example: short circuit of all three phase
conductors of a cable at a single location
Faults in which not all the line currents are equal
and not all have the same phase.
Example (any one): single phase line to ground
fault (L-G), two phase to ground (LL-G) fault and
phase to phase (L-L) fault.
FAULT IN POWER SYSTEM
Figure shows basic connections of
circuit breaker control for the
opening operation. The protected
circuit X is shown by dashed line.
When a fault occurs in the
protected circuit the relay (2)
connected to CT and PT actuates
and closes its contacts (6).
Current flows from battery (5) in
the trip circuit (4). As the trip coil
of circuit breaker (3) is energized,
the circuit breaker operating
mechanism is actuated and it
operates for the opening operation.
Thus the fault is sensed and the
trip circuit is actuated by the relay
and the faulty part is isolated.
The power system is divided into protection zones
defined by the equipment and the available circuit
breakers. Six categories of protection zones are
possible in each power system:
1. Generators and generator-transformer units
3. Bus bars
4. Lines (transmission and distribution)
5. Utilization equipment (motors, static loads,
6. Capacitor or reactor banks (when separately
Although the fundamentals of protection are quite
similar, each of these six categories has protective
relays, specifically designed for primary protection,
that are based on the characteristics of the
equipment being protected. The protection of each
zone normally includes relays that can provide
backup for the relays protecting the adjacent
The protection in each zone should overlap that in
the adjacent zone; otherwise, a primary protection
void would occur between the protection zones. This
overlap is accomplished by the location of the CTs
the key sources of power system information for the
Main or primary protection
First in line of defense is main protection which ensures
quick action and selective clearing of faults within the
boundary of the circuit section or the element it protects.
Main protection is essentially provided as a rule.
It is the second line of defense in case in failure of primary
protection. It is designed to operate with sufficient time
delays so that primary relaying will be given enough time to
function if it is able to. Back up protection gives back up to
the main protection, when the main protection fails to
operate or is cut out for repairs etc.
PRIMARY & BACK UP PROTECTION
PRIMARY & BACK UP PROTECTION
WHICH ONE PRIMARY AND BACK UP???
- Able to detect and respond to fault occurring
only within its own zone protection.
- Have absolute discrimination. Its zone of
protection is well defined.
- It does not respond to the fault occurring beyond
its own zone protection.
- Also referred to as “main protection”
- Fast operating time
- Example: differential protection of alternators,
transformer or bus bars, frame leakage
protection, pilot wire and carrier current
NON UNIT PROTECTION
- Does not have absolute discrimination(selectivity).
- No defined area of coverage
- Also known as “back-up protection”
- Slower operating time
- Example: distance protection and time graded,
current graded or both time and current graded.