The selection of line protection
requires the consideration of several factors, some of which are
mutually exclusive. Knowledge of the most probable failures,
recommendations of equipment suppliers, and good practical judgment
can assist the protection engineer in determining which of the
factors deserve the most emphasis.
One of the most important design
considerations of relaying is reliability. Relaying reliability is
separated into two aspects: dependability and security. Dependability
is defined as “the degree of certainty that a relay or relay system
will operate correctly.
” Security is defined as “the
degree of certainty that a relay or relay system will not operate
incorrectly.” In other words, dependability is a measure of the
relay’s ability to operate when it is supposed to operate. Security
is a measure of the relay’s ability to avoid operation for all
other conditions for which tripping is not desired.
Dependability is relatively easy to
obtain in relay design or in the application of a number of relays.
Testing using operating conditions, fail-safe designs, and redundancy
are methods to ensure dependability. Security is harder to attain; an
almost infinite variety of tests would be needed to simulate all
possible conditions to which a relay may be exposed.
Various engineering practices can
enhance dependability. These include independence of design,
different operating principles, redundancy within the relay systems,
local backup methods, remote backup methods, and application of
relays and relay systems that cause undesirable trips upon failure.
Security can be enhanced by using
relays that fail into a “disarmed” mode, series connected
protection, improved monitoring and self-checking, and emphasis on
high-quality components. Another important design consideration is
protection selectivity or coordination.
Selectivity is the ability of relays
and relaying systems to cooperate with each other to minimize the
outaged area resulting from a fault. Coordination refers to the
process of applying relays to operate as fast as possible for
conditions with their primary zone, but to have delayed, or
coordinated operations for conditions within an extended backup zone.
Selectivity and coordination must be
achieved to ensure maximum service continuity. Fault clearing time is
an important consideration in the selection of line relaying.
Requirements for relaying speed must be carefully determined. If the
relaying is too slow, system instability, excessive equipment damage,
and adverse effects on customer service may result.
However, faster protection tends to
compromise relay system security and selectivity. There is a limit to
the speed with which a relay can correctly respond due to the
transients present in the power system itself.
Sensitivity of protection refers to the
minimum operating quantities that must be available for the relays to
detect an abnormal condition. While this factor is still important,
most modern relays are using solid state or microprocessor
technologies that are many times more sensitive than their
electromechanical predecessors.
Certain problems, such as
high-impedance ground faults, inherent system voltage unbalances, and
high source-to-line impedance ratios (SIRs) still challenge the
sensitivity of relays and should be considered in relay selection.
The line protection design may often
fail to recognize one of the more important design factors
simplicity. The multifunction and programmable capabilities of modern
relays have created an abundance of special solutions to possible
system problems.
The implementation of these solutions
challenges the application engineer, those responsible for setting
the relays, and operations and maintenance personnel. The problems
caused by incorrect or incomplete implementation of overly complex
protection may create more serious consequences than not providing
special solutions.
The protection engineer should
carefully weigh the consequences and probability of each problem to
determine if it justifies using complex special solutions. Economic
evaluations of protection options will continue to be necessary.
Protection engineers have long pointed to the relatively low cost and
high importance of relays compared with the equipment they protect.
However, it is fundamental to attempt
to achieve the required protection at the lowest cost. In recent
years, more importance has been placed on economic analysis that
considers more than just the lowest initial cost. Installation and
maintenance costs, as well as the cost of unreliable protection, are
sometimes considered.
In addition, modern protection usually
offers many features not previously available that may result in
improvements in operations, restoration of the system, and post-fault
analysis. The value of these improvements should be considered in a
complete economic evaluation of alternatives.
Certain transmission system
configurations and characteristics require protective relaying with
communication systems to provide high-speed clearing for all faults
within the zone of protection. Pilot relaying schemes are employed to
provide this high-speed fault clearing.
These pilot relaying systems require
the transmission of information during a fault between relays that
are located in different substations to determine if the fault is
internal or external to the zone of protection. This is accomplished
using several different methods, ranging from direct hardwire
communications to fiber optic communications systems.
Different protection schemes and
communication channels have different degrees of reliability.
Knowledge of these different communication options is necessary to
determine the reliability of the protection scheme being considered.
The network configuration and local system loading requirements may
also affect the type of pilot protection scheme chosen.
The protection scheme required will
often dictate the type of relaying communications that will be used.
On the other hand, the relaying communications may limit the types of
relay schemes that can be used. Obviously, the requirements of the
relaying scheme must be considered, along with the types of
communications available.
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