Fault (power engineering)

In an electric power system, a fault is any abnormalelectric current. For example, a short circuit is a fault inwhich current bypasses the normal load. An open-circuitfault occurs if a circuit is interrupted by some failure.In three-phase systems, a fault may involve one or morephases and ground, or may occur only between phases.In a “ground fault” or “earth fault”, charge flows into theearth. The prospective short circuit current of a fault canbe calculated for power systems. In power systems, pro-tective devices detect fault conditions and operate circuitbreakers and other devices to limit the loss of service dueto a failure.In a polyphase system, a fault may affect all phasesequally which is a “symmetrical fault”. If only somephases are affected, the resulting “asymmetrical fault”becomes more complicated to analyze due to the sim-plifying assumption of equal current magnitude in allphases being no longer applicable. The analysis of thistype of fault is often simplified by using methods such assymmetrical components.Design of systems to detect and interrupt power systemfaults is the main objective of power system protection.

1 Transient fault

A transient fault is a fault that is no longer present ifpower is disconnected for a short time and then restored.Many faults in overhead power lines are transient in na-ture. When a fault occurs, equipment used for power sys-tem protection operate to isolate the area of the fault. Atransient fault will then clear and the power-line can bereturned to service. Typical examples of transient faultsinclude:

• momentary tree contact• bird or other animal contact• lightning strike• conductor clashing

Transmission and distribution systems use an automaticre-close function which is commonly used on overheadlines to attempt to restore power in the event of a tran-sient fault. This functionality is not as common on under-ground systems as faults there are typically of a persistentnature. Transient faults may still cause damage both atthe site of the original fault or elsewhere in the networkas fault current is generated.

2 Persistent fault

A persistent fault does not disappear when power is dis-connected. Faults in underground power cables are mostoften persistent due to mechanical damage to the cable,but are sometimes transient in nature due to lightning.[1]

3 Symmetric fault

A symmetric or balanced fault affects each of the threephases equally. In transmission line faults, roughly 5%are symmetric.[2] This is in contrast to an asymmetricalfault, where the three phases are not affected equally.

4 Asymmetric fault

An asymmetric or unbalanced fault does not affecteach of the three phases equally. Common types of asym-metric faults, and their causes:

• line-to-line - a short circuit between lines, caused byionization of air, or when lines come into physicalcontact, for example due to a broken insulator.

• line-to-ground - a short circuit between one line andground, very often caused by physical contact, forexample due to lightning or other storm damage

• double line-to-ground - two lines come into contactwith the ground (and each other), also commonlydue to storm damage.

5 Bolted fault

One extreme is where the fault has zero impedance, giv-ing the maximum prospective short-circuit current. No-tionally, all the conductors are considered connected toground as if by a metallic conductor; this is called a“bolted fault”. It would be unusual in a well-designedpower system to have a metallic short circuit to groundbut such faults can occur by mischance. In one type oftransmission line protection, a “bolted fault” is delibratelyintroduced to speed up operation of protective devices.

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2 8 ANALYSIS

6 Realistic faults

Realistically, the resistance in a fault can be from close tozero to fairly high. A large amount of power may be con-sumed in the fault, compared with the zero-impedancecase where the power is zero. Also, arcs are highly non-linear, so a simple resistance is not a good model. Allpossible cases need to be considered for a good analysis.

7 Arcing fault

Where the system voltage is high enough, an electric arcmay form between power system conductors and ground.Such an arc can have a relatively high impedance (com-pared to the normal operating levels of the system) andcan be difficult to detect by simple overcurrent protection.For example, an arc of several hundred amperes on a cir-cuit normally carrying a thousand amperes may not tripovercurrent circuit breakers but can do enormous dam-age to bus bars or cables before it becomes a completeshort circuit. Utility, industrial, and commercial powersystems have additional protection devices to detect rela-tively small but undesired currents escaping to ground. Inresidential wiring, electrical regulations may now requireArc-fault circuit interrupters on building wiring circuits,to detect small arcs before they cause damage or a fire.

8 Analysis

Symmetric faults can be analyzed via the samemethods asany other phenomena in power systems, and in fact manysoftware tools exist to accomplish this type of analysisautomatically (see power flow study). However, there isanother method which is as accurate and is usually moreinstructive.First, some simplifying assumptions are made. It is as-sumed that all electrical generators in the system are inphase, and operating at the nominal voltage of the sys-tem. Electric motors can also be considered to be gen-erators, because when a fault occurs, they usually supplyrather than draw power. The voltages and currents arethen calculated for this base case.Next, the location of the fault is considered to be sup-plied with a negative voltage source, equal to the voltageat that location in the base case, while all other sourcesare set to zero. This method makes use of the principleof superposition.To obtain amore accurate result, these calculations shouldbe performed separately for three separate time ranges:

• subtransient is first, and is associated with the largestcurrents

• transient comes between subtransient and steady-state

• steady-state occurs after all the transients have hadtime to settle

An asymmetric fault breaks the underlying assumptionsused in three-phase power, namely that the load is bal-anced on all three phases. Consequently, it is impos-sible to directly use tools such as the one-line diagram,where only one phase is considered. However, due tothe linearity of power systems, it is usual to considerthe resulting voltages and currents as a superposition ofsymmetrical components, to which three-phase analysiscan be applied.In the method of symmetric components, the power sys-tem is seen as a superposition of three components:

• a positive-sequence component, in which the phasesare in the same order as the original system, i.e., a-b-c

• a negative-sequence component, in which the phasesare in the opposite order as the original system, i.e.,a-c-b

• a zero-sequence component, which is not truly athree-phase system, but instead all three phases arein phase with each other.

To determine the currents resulting from an asymmetricalfault, onemust first know the per-unit zero-, positive-, andnegative-sequence impedances of the transmission lines,generators, and transformers involved. Three separatecircuits are then constructed using these impedances. Theindividual circuits are then connected together in a par-ticular arrangement that depends upon the type of faultbeing studied (this can be found in most power systemstextbooks). Once the sequence circuits are properly con-nected, the network can then be analyzed using classicalcircuit analysis techniques. The solution results in volt-ages and currents that exist as symmetrical components;these must be transformed back into phase values by us-ing the A matrix.Analysis of the prospective short-circuit current is re-quired for selection of protective devices such as fusesand circuit breakers. If a circuit is to be properly pro-tected, the fault current must be high enough to operatethe protective device within as short a time as possible;also the protective device must be able to withstand thefault current and extinguish any resulting arcs without it-self being destroyed or sustaining the arc for any signifi-cant length of time.The magnitude of fault currents differ widely dependingon the type of earthing system used, the installation’s sup-ply type and earthing system, and its proximity to the sup-ply. For example, for a domestic UK 230 V, 60 A TN-Sor USA 120 V/240 V supply, fault currents may be a few

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thousand amperes. Large low-voltage networks withmul-tiple sources may have fault levels of 300,000 amperes.A high-resistance-grounded system may restrict line toground fault current to only 5 amperes. Prior to select-ing protective devices, prospective fault current must bemeasured reliably at the origin of the installation and atthe furthest point of each circuit, and this information ap-plied properly to the application of the circuits.

9 Detecting and locating faults

Overhead power lines are easiest to diagnose since theproblem is usually obvious, e.g., a tree has fallen acrossthe line, or a utility pole is broken and the conductors arelying on the ground.Locating faults in a cable system can be done either withthe circuit de-energized, or in some cases, with the circuitunder power. Fault location techniques can be broadly di-vided into terminal methods, which use voltages and cur-rents measured at the ends of the cable, and tracer meth-ods, which require inspection along the length of the ca-ble. Terminal methods can be used to locate the generalarea of the fault, to expedite tracing on a long or buriedcable.[3]

In very simple wiring systems, the fault location is oftenfound through inspection of the wires. In complex wiringsystems (for example, aircraft wiring) where the wiresmay be hidden, wiring faults are located with a Time-domain reflectometer.[4] The time domain reflectometersends a pulse down the wire and then analyzes the return-ing reflected pulse to identify faults within the electricalwire.In historic submarine telegraph cables, sensitivegalvanometers were used to measure fault currents; bytesting at both ends of a faulted cable, the fault locationcould be isolated to within a few miles, which allowedthe cable to be grappled up and repaired. The Murrayloop and the Varley loop were two types of connectionsfor locating faults in cablesSometimes an insulation fault in a power cable will notshow up at lower voltages. A “thumper” test set applies ahigh-energy, high-voltage pulse to the cable. Fault loca-tion is done by listening for the sound of the discharge atthe fault. While this test contributes to damage at the ca-ble site, it is practical because the faulted location wouldhave to be re-insulated when found in any case.[5]

In a high resistance grounded distribution system, afeeder may develop a fault to ground but the system con-tinues in operation. The faulted, but energized, feedercan be found with a ring-type current transformer col-lecting all the phase wires of the circuit; only the circuitcontaining a fault to ground will show a net unbalancedcurrent. To make the ground fault current easier to de-tect, the grounding resistor of the systemmay be switchedbetween two values so that the fault current pulses.

10 Batteries

The prospective fault current of larger batteries, such asdeep-cycle batteries used in stand-alone power systems,is often given by the manufacturer.In Australia, when this information is not given, theprospective fault current in amperes “should be consid-ered to be 6 times the nominal battery capacity at theC120

A·h rate,” according to AS 4086 part 2 (Appendix H).

11 See also• Fault (technology)

12 References[1] http://www.lightning.ece.ufl.edu/PDF/01516222.pdf

[2] Grainger, John J. (2003). Power System Analysis. TataMcGraw-Hill. p. 380. ISBN 978-0-07-058515-7.

[3] Murari Mohan Saha, Jan Izykowski, EugeniuszRosolowski Fault Location on Power Networks Springer,2009 ISBN 1-84882-885-3, page 339

[4] Smth,Paul, Furse, Cynthia and Gunther, Jacob. “Analy-sis of Spread Spectrum Time Domain Reflectometry forWire Fault Location.” IEEE Sensors Journal. December,2005.

[5] Edward J. Tyler, 2005 National Electrical Estimator ,Craftsman Book Company, 2004 ISBN 1-57218-143-5page 90

General

• Sarma, M.S. (2002). Power System Analysis andDesign. Brooks/Cole. ISBN 0-534-95367-0.

• Burton, G.C. Power Analysis.

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