EEE 462
Switch Gear and Protection
Lecture 2
Earthing Switch
• Objective:
• Long transmission line holds distributed capacitances to earth.
• So even the transmission line is isolated it contains trapped voltage for which
charging current discharges very slowly
• Now if maintenance is required to the transmission line the potential of the
line should be zero (ground potential).
• So earthing switch is necessary for maintenance on HV transmission line.
Earthing Switch
Earthing Switch vs Load Break swt.
• Load Break Switch (LB) Line to Load
• Earting Switch (ES) Line to Ground
EBS LBS
The sequence of operation during abnormal
condition
1. Fault occur
2. Relay sense the fault and close the trip circuit
3. Energize the trip coil unlatch the spring
4. Contacts start to apart and arc is drawn between the contact of CB
5. Arc is extinguished at the instant when fault ac currant becomes
zero
6. Fault interruption is completed so fault is cleared.
The Trip-coil Circuit
CB CT
Protected line
PT
Solenoid coil
Relay
Auxiliary
contact
Battery
Trip free feature of a CB
• Suppose a CB has been instructed to close by manual instruction (by
using the pushing of push button.
• So the operation mechanism will start (closing operation)
• Meanwhile a fault occur and relay give the instruction to open the
circuit
• Then the mechanism which permits the CB to be tripped even it is
under the process of closing
Bus-Bar Arrangements
When a number of generators or feeders operating at the
same voltage have to be directly connected
electrically, bus-bars are used as the common electrical
component.
(1) Single Bus-bar System.
The single busbar system has the simplest design and is
used for power stations. It is also used in small outdoor
stations having relatively few outgoing or incoming
feeders and lines.
Disadvantages. Single bus-bar system has the
following three principal disadvantages :
(i) The bus-bar cannot be cleaned, repaired
or tested without de-energising the whole system.
(ii) If a fault occurs on the bus-bar itself, there is complete
interruption of supply.
(iii) Any fault on the system is fed by all the generating
capacity, resulting in very large fault
currents.
(2) Single bus-bar system with Sectionalisation.
It is worthwhile to keep in mind that a
circuit breaker should be used as the
sectionalising Switch so that
uncoupling of the bus-bars
may be carried out safely during load
transfer. Moreover, thecircuit breaker
itself should be provided with isolators
on both sides so that its maintenance
can be done while the
bus-bars are alive.
(3) Duplicate bus-bar system.
In large stations, it is important that breakdowns and maintenance should interfere as little as possible with
continuity of supply. In order to achieve this objective, duplicate bus-bar system is used in important stations.
Such a system consists of two bus-bars, a “main bus-bar’’ and a “spare” bus-bar (see Fig). Each generator
and feeder may be connected to either bus-bar with the help of bus coupler which consists of a circuit
breaker and isolators.
Advantages
(i) If repair and maintenance it to be carried on the main
bus, the supply need not be interrupted
as the entire load can be transferred to the spare bus.
(ii) The testing of feeder circuit breakers can be done by
putting them on spare bus-bar, thus
keeping the main bus-bar undisturbed.
(iii) If a fault occurs on the bus-bar, the continuity of
supply to the circuit can be maintained by
transferring it to the other bus-bar.
