Our power system is made of of different
electrical components, big and small, and super expensice at that. From power
transformers, to insulators and conductors.
One of the most important facets in the
power system is the protection side of it. And one of the most important factor
involved is the protective relaying.
Objectives Of Protective Relaying
The objectives of electrical system
protection and coordination are to
a. Limit the extent and duration of service
interruption whenever equipment failure, human error, or adverse natural events
occur on any portion of the system
b. Minimize damage to the system components
involved in the failure
The circumstances causing system
malfunction are usually unpredictable; however, sound design and preventive
maintenance can reduce the likelihood of system problems. The electrical
system, therefore, should be designed and maintained to protect itself
automatically.
Protective Relaying's Role In Ensuring Safety
Prevention of human injury is the most
important objective when designing electrical systems. Interrupting devices
should have adequate interrupting capability. Energized parts should be
sufficiently enclosed or isolated to avoid exposing personnel to explosion,
fire, arcing, or shock. Safety should always take priority over service
continuity, equipment damage, or economics.
These fundamental principles of safety have
always been adhered to by responsible engineers engaged in the design and
operation of electrical systems. The National Electrical Code ® (NEC®) (NFPA 70
1999), National Electrical Safety Code® (NESC®) (Accredited Standards Committee
C2-2002), NFPA 70E-2000, and state and local codes all have prescribed
practices intended to enhance the safety of electrical systems.
In recent years, an increased concern about
safety has led to many studies resulting in detailed recommendations [from the
National Institute of Occupational Safety and Health (NIOSH)] and regulations
relating to electrical systems. Prominent among these are the regulations of
the Occupational Safety and Health Administration (OSHA) of the U.S. Department
of Labor.
Engineers and other personnel engaged in
the design and operation of electrical system protection should be familiar
with the most recent OSHA regulations, NIOSH Safely Alerts, and all other
applicable codes and regulations relating to human safety.
The essential electrical infrastructure in
industrial and commercial establishments employs protective devices, many of
which are addressed in this recommended practice, that function to de-energize
the electrical system in the event of a malfunction. However, electric shock
can result in serious injury far more quickly than the available technology of
interruption can perform its task.
Therefore, the limitation of shock hazard,
such as the step and touch potentials in electrical substations, depends upon
the speed of interrupting the fault. Rapid fault isolation is important (see
IEEE Std 80-2000). Therefore, the most efficient form of electrical shock
protection is to avoid shock altogether, and such avoidance is best
accomplished through proper system design and operation, and through effective
maintenance.
Equipment damage versus service
continuity
Whether minimizing the risk of equipment
damage or preserving service continuity is the more important objective depends
upon the operating philosophy of the particular industrial plant or commercial
business. Some operations can afford limited service interruptions to minimize
the possibility of equipment repair or replacement costs, while others would
regard such an expense as small compared with even a brief interruption of
service.
In most cases, electrical protection should
be designed for the best compromise between equipment damage and service
continuity. One of the prime objectives of system protection is to obtain
selectivity to minimize the extent of equipment shutdown in case of a fault.
Therefore, many protection engineers would
prefer that faulted equipment be de-energized as soon as the fault is detected.
However, for certain continuous process industry plants, high-resistance
grounding systems that allow the first ground fault to be alarmed instead of
automatically cleared are employed.
Economic and reliability considerations
The cost of system protection determines
the degree of protection that can be feasibly designed into a system. Many
features may be added that improve system performance, reliability, and
flexibility, but incur an increased initial cost.
However, failure to design into a system at
least the minimum safety and reliability requirements inevitably results in
unsatisfactory performance, with a higher probability of expensive downtime.
Modifying a system that proves inadequate is more expensive and, in most cases,
less satisfactory than initially designing these features into a system.
The system should always be designed to
isolate faults with a minimum disturbance to the system and should have
features to give the maximum dependability consistent with the plant
requirements and justifiable costs. Evaluation of costs should also include
equipment maintenance requirements.
In many instances, plant requirements make
planned system outages for routine maintenance difficult to schedule. Such
factors should weigh into the economic versus reliability decision process.
When costs of downtime and equipment maintenance are factored into the
protection system cost evaluation, decisions can then be based upon total cost
over the useful life of the equipment rather than simply the first cost of the
system. In-depth coverage of reliability versus economic decisions can be found
in IEEE Std 493-1997.
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