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Power-system protection is a branch of electrical power engineering that deals
with the protection of electrical power systems from faults through the isolation
of faulted parts from the rest of the electrical network. The objective of a
protection scheme is to keep the power system stable by isolating only the
components that are under fault, whilst leaving as much of the network as
possible still in operation. Thus, protection schemes must apply a very
pragmatic and pessimistic approach to clearing system faults.
Components
Protection systems usually comprise five components:
Current and voltage transformers to step down the high voltages and
currents of the electrical power system to convenient levels for the relays
to deal with
Protective relays to sense the fault and initiate a trip, or disconnection,
order;
Circuit breakers to open/close the system based on relay and autorecloser
commands;
Batteries to provide power in case of power disconnection in the system.
Communication channels to allow analysis of current and voltage at
remote terminals of a line and to allow remote tripping of equipment.
For parts of a distribution system, fuses are capable of both sensing and
disconnecting faults.
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Failures may occur in each part, such as insulation failure, fallen or broken
transmission lines, incorrect operation of circuit breakers, short circuits and
open circuits. Protection devices are installed with the aims of protection of
assets, and ensure continued supply of energy.
Switchgear is a combination of electrical disconnect switches, fuses or circuit
breakers used to control, protect and isolate electrical equipment. Switches are
safe to open under normal load current, while protective devices are safe to
open under fault current.
Protective device
Protective relays control the tripping of the circuit breakers surrounding the
faulted part of the network
Automatic operation, such as auto-re-closing or system restart
Monitoring equipment which collects data on the system for post event analysis
While the operating quality of these devices, and especially of protective relays,
is always critical, different strategies are considered for protecting the different
parts of the system. Very important equipment may have completely redundant
and independent protective systems, while a minor branch distribution line may
have very simple low-cost protection.
There are three parts of protective devices:
Instrument transformer: current or potential (CT or VT)