Power-system protection | Wikipedia audio article


Power-system protection is a branch of electrical
power engineering that deals with the protection of electrical power systems from faults through
the disconnection 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. The devices that are used to protect the power
systems from faults are called protection devices.==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 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 ensuring 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 (some switches are not safe to operate under normal or abnormal conditions), while
protective devices are safe to open under fault current. Very important equipment may have completely
redundant and independent protective systems, while a minor branch distribution line may
have very simple low-cost protection.==Types of protection=====
High-voltage transmission network===Protection on the transmission and distribution
system serves two functions: protection of plant and protection of the public (including
employees). At a basic level, protection disconnects equipment
which experiences an overload or a short to earth. Some items in substations such as transformers
might require additional protection based on temperature or gas pressure, among others.===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.===Overload and back-up for distance (overcurrent)
===Overload protection requires a current transformer
which simply measures the current in a circuit. There are two types of overload protection:
instantaneous overcurrent (IOC) and time overcurrent (TOC). Instantaneous overcurrent requires that the
current exceeds a predetermined level for the circuit breaker to operate. Time overcurrent protection operates based
on a current vs time curve. Based on this curve, if the measured current
exceeds a given level for the preset amount of time, the circuit breaker or fuse will
operate. The function of both types is explained in
“Non-Directional Overcurrent Protection” on YouTube.===Earth fault/ground fault===
Earth fault protection also requires current transformers and senses an imbalance in a
three-phase circuit. Normally the three phase currents are in balance,
i.e. roughly equal in magnitude. If one or two phases become connected to earth
via a low impedance path, their magnitudes will increase dramatically, as will current
imbalance. If this imbalance exceeds a pre-determined
value, a circuit breaker should operate. Restricted earth fault protection is a type
of earth fault protection which looks for earth fault between two sets of current transformers
(hence restricted to that zone).===Distance (impedance relay)===
Distance protection detects both voltage and current. A fault on a circuit will generally create
a sag in the voltage level. If the ratio of voltage to current measured
at the relay terminals, which equates to an impedance, lands within a predetermined level
the circuit breaker will operate. This is useful for reasonably long lines,
lines longer than 10 miles, because their operating characteristics are based on the
line characteristics. This means that when a fault appears on the
line the impedance setting in the relay is compared to the apparent impedance of the
line from the relay terminals to the fault. If the relay setting is determined to be below
the apparent impedance it is determined that the fault is within the zone of protection. When the transmission line length is too short,
less than 10 miles, distance protection becomes more difficult to coordinate. In these instances the best choice of protection
is current differential protection.===Back-up===
The objective of protection is to remove only the affected portion of plant and nothing
else. A circuit breaker or protection relay may
fail to operate. In important systems, a failure of primary
protection will usually result in the operation of back-up protection. Remote back-up protection will generally remove
both the affected and unaffected items of plant to clear the fault. Local back-up protection will remove the affected
items of the plant to clear the fault.===Low-voltage networks===
The low-voltage network generally relies upon fuses or low-voltage circuit breakers to remove
both overload and earth faults.===Cybersecurity===
The bulk system which is a large interconnected electrical system including transmission and
control system is experiencing new cybersecurity threats every day. (“Electric Grid Cybersecurity,” 2019). Most of these attacks are aiming the control
systems in the grids. These control systems are connected to the
internet and makes it easier for hackers to attack them. These attacks can cause damage to equipment
and limit the utility professionals ability to control the system.==Coordination==
Protective device coordination is the process of determining the “best fit” timing of current
interruption when abnormal electrical conditions occur. The goal is to minimize an outage to the greatest
extent possible. Historically, protective device coordination
was done on translucent log–log paper. Modern methods normally include detailed computer
based analysis and reporting. Protection coordination is also handled through
dividing the power system into protective zones. If a fault were to occur in a given zone,
necessary actions will be executed to isolate that zone from the entire system. Zone definitions account for generators, buses,
transformers, transmission and distribution lines, and motors. Additionally, zones possess the following
features: zones overlap, overlap regions denote circuit breakers, and all circuit breakers
in a given zone with a fault will open in order to isolate the fault. Overlapped regions are created by two sets
of instrument transformers and relays for each circuit breaker. They are designed for redundancy to eliminate
unprotected areas; however, overlapped regions are devised to remain as small as possible
such that when a fault occurs in an overlap region and the two zones which encompass the
fault are isolated, the sector of the power system which is lost from service is still
small despite two zones being isolated.==Disturbance-monitoring equipment==
Disturbance-monitoring equipment (DME) monitors and records system data pertaining to a fault. DME accomplish three main purposes: model validation,
disturbance investigation, and assessment of system protection performance.DME
devices include: Sequence of event recorders, which record
equipment response to the event Fault recorders, which record actual waveform
data of the system primary voltages and currents Dynamic disturbance recorders (DDRs), which
record incidents that portray power system behavior during dynamic events such as low
frequency (0.1 Hz – 3 Hz) oscillations and abnormal frequency or voltage excursions==
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. Performance and design criteria for system-protection
devices include reliability, selectivity, speed, economy, and simplicity. Reliability: Devices must function consistently
when fault conditions occur, regardless of possibly being idle for months or years. Without this reliability, systems may cause
costly damages. Selectivity: Devices must avoid unwarranted,
false trips. Speed: Devices must function quickly to reduce
equipment damage and fault duration, with only very precise intentional time delays. Economy: Devices must provide maximum protection
at minimum cost. Simplicity: Devices must minimize protection
circuitry and equipment.==See also==
Fault current limiter Network analyzer (AC power)
Prospective short-circuit current==Notes

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