Pumps & Systems , October 2007
With highly reliable electrical systems, protective relays may be called upon to operate very infrequently. However, the effects of faults and abnormal conditions can be severe and protective relay systems must be designed carefully to protect against the worst possible fault conditions.
This article briefly describes the basic goals and philosophies behind relay system design and the types of protection that are applied in water and wastewater treatment facilities. As motors for pumping applications are particularly critical to water and wastewater facility operations, the major faults and abnormal conditions that affect motors are also covered. The relay schemes discussed here are typically applied to systems with operating voltages greater than 1000-V.
Purposes of Protective Relaying
An electrical fault is the establishment of an unintentional conducting path. On a three-phase system, the unintentional path may be between two- or three-phase conductors, or between one or more phases and a metal enclosure or the earth. A fault can be established through:
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Insulation failure due to age
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Overheating
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Exposure to the elements
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A lightning strike
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Mechanical failure of equipment
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Misapplication of equipment
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Accidental forced contact between conductors, such as from a maintenance error, vehicle accidents, or animal contact
Abnormal conditions may exist with or without an actual failure, but may lead to a failure if not corrected. Abnormal conditions include:
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Overloading
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Low voltage
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High voltage
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Incorrect frequency
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Unbalanced current
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Unbalanced voltage
A complete protective relaying system consists of all the components necessary to detect faults or abnormal conditions and operate the appropriate switching devices, such as circuit breakers or automatic switches. Proper operation requires integrating a variety of electrical and electronic technologies at both high and low power levels. Major components are as follows:
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Current transformers (CTs) and voltage transformers (VTs) to reduce the voltage and currents of the electrical power system to levels suitable for relay inputs. Secondary wiring and disconnects are associated with the current and voltage transformers.
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The protective relays and associated auxiliary relays, selector switches, control circuit disconnects, indicating lamps, and control wiring to the circuit breakers. The system may also include separate relays for primary and backup protection to avoid losing protection if a relay fails.
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The circuit breakers or automatic switches that will perform the switching at power circuit voltage and current levels. These devices consist of switching mechanisms, interrupter assemblies, trip and close coils and control circuits.
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A reliable source of control power for the relays and circuit breakers. Control power must be available even if the power system is faulted or otherwise unavailable. While various control power arrangements have been used, the most reliable is a substation battery with a charger.
Basic Design Goals
Regardless of the complexity, any protective relay system design is governed by a few basic goals and philosophies. These goals include speed, selectivity and reliability.
Speed
High-speed operation of a protective relay system is necessary to limit the effects of a fault, which can include equipment damage, process upset, and hazards to personnel.
A fault typically causes an "overcurrent" condition, with the current exceeding the rating of the line conductors, switches and transformers that must carry the current. This may cause a violent arc at the fault location. The fault may also cause abnormally high or low voltage.
By definition of a fault, the equipment is damaged or destroyed, but the overcurrent, arcing and abnormal voltage may also damage equipment at other locations on the system. The risk of more widespread damage increases if the fault is allowed to persist.
Even if damage does not occur, the abnormal conditions may result in tripping other devices and upsetting the processes supplied by the system. High-speed operation of the protective relay system is essential in limiting the equipment damage and risk of a wider system disturbance due to a fault.
Beyond risk of equipment damage and process upset, the failed insulation and arcing associated with the fault presents hazards to personnel who might be in the vicinity. Electric shock hazards are well-known. Becoming more formally recognized are the hazards associated with an arc flash, including flash burns, hearing loss from the blast, vision loss from the flash, and injury due to the impact of particles expelled by the blast.
The National Electrical Code (NEC) requires equipment to be marked with an arc flash warning if it's likely to require servicing while energized. OSHA enforces requirements for arc flash hazard analysis and personal protective equipment found in NFPA 70E -2004, Standard for Electrical Safety in the Workplace . The energy received by a person exposed to an arc flash depends on the current magnitude and the time it persists. High-speed protective relaying can dramatically reduce arc flash energy and hazards to personnel.
Selectivity
Selectivity is the ability of a protective relay system to isolate the smallest portion of a system necessary to isolate a fault or abnormal condition.
Tripping a switchgear main breaker for a fault on a motor circuit supplied from a feeder breaker and downstream motor control center (MCC) is obviously a major process upset. While limiting damage and system upset demands high speed, selectivity often demands some delay to allow time for protective relays closest to the disturbance to operate.
Coordination studies attempt to determine settings for time-overcurrent devices that balance the opposing demands of speed and selectivity. Time-overcurrent coordination generally requires longer time delays for devices closer to the supply point of a system. As discussed later, differential relaying can be used to avoid excessive time delays at certain locations.
Reliability
Reliability of a protective relay system is defined in terms of two components: dependability and security. Dependability is the ability of the relaying system to always operate correctly for a fault or abnormal condition. Security is the ability of the relaying system not to operate when there is no abnormal condition present, or if the condition is temporary or should be isolated by relays in another part of the system.
As with speed and selectivity, dependability and security are opposing demands. Improving
