27-06-2013, 01:10 PM
Power System Protection
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INTRODUCTION
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.
For this reason, the technology and philosophies utilized in protection
schemes can often be old and well-established because they must be
very reliable.
Overview of Power Smicsystem Dyna
System protection requires study of system dynamics and control.
Power system dynamics
1. Can be described in terms of a set of differential and algebraic
equations.
2. Analysis is done by EMTP.
3. Post fault system stability is determined by transient stability
studies.
Components
Protection systems usually comprise five components:
Curren t 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; Following relays are used to detect system
disturbances
and for enhancing system stability.
Under frequency and over frequency relays.
Rate of change of frequency relays.
Under voltage relays.
Reverse power flow relays.
Vector shift relays.
For example, consider a voltage distribution system with
local generation which is synchronized with the grid. During
grid disturbances, the local plant generators have to be
isolated from the grid.
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.
Types of protection
Generator sets – In a power plant, the protective relays are
intended to prevent damage to alternators or to the transformers in
case of abnormal conditions of operation, due to internal failures,
as well as insulating failures or regulation malfunctions. Such
failures are unusual, so the protective relays have to operate very
rarely. If a protective relay fails to detect a fault, the resulting
damage to the alternator or to the transformer might require costly
equipment repairs or replacement, as well as income loss from the
inability to produce and sell energy.
Performance measures
Protection engineers define dependability as the tendency of the
protection system to operate correctly for in-zone faults. They
define security as the tendency not to operate for out-of-zone
faults. Both dependability and security are reliability issues. Fault
tree analysis is one tool with which a protection engineer can
compare the relative reliability of proposed protection schemes.
Quantifying protection reliability is important for making the best
decisions on improving a protection system, managing
dependability versus security tradeoffs, and getting the best
results for the least money. A quantitative understanding is
essential in the competitive utility industry. [