Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2000-02-25
2004-04-06
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C324S521000
Reexamination Certificate
active
06718271
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a fault detection apparatus and method of detecting faults in an electrical distribution network and more particularly to an apparatus and method for detecting faults in a network utilizing a neutral earthing system.
Three phase distribution networks carry energy along three separate conductors while maintaining a voltage difference between any two of the conductors (line voltages). Under normal operating conditions the three voltages are symmetrical about a neutral point. Voltage measured between a conductor and the neutral point is referred to as the phase voltage.
Where star (Y) connected transformer windings are used at a source station, the star point corresponds to the neutral position. For certain circuits this neutral point is brought out as a fourth conductor and can carry load current for loads connected between a phase and earth. This is the common arrangement for low voltage (LV) circuits and is used extensively in the United States of America.
Each conductor or phase also has a voltage with respect to its surrounding environment, mainly the local ground or earth. Load current normally flows out in one or more of the phases and returns through the other phase wire(s). In the event of a fault, for example a conductor coming in contact with the ground, some of the current flows into the ground. This fault current must find its way back to the source, i.e. into the electrical network. The network neutral point is generally connected to earth at the source to provide such a return path. Fault current for a network earth fault therefore generally flows from the phase conductor, through the earth and back through the neutral-earth connection.
There are many ways to connect the network neutral to earth, for example, it can be solidly connected. In this case large currents can flow in the event of a network earth fault. Such faults can be detected easily but must be cleared rapidly to minimise safety hazards and equipment damage. Solid earthing is predominantly used on cabled networks, where fault rates are lower and faults must be detected and cleared quickly to contain damage to cables.
Alternatively, the neutral point can be left isolated. In this case fault current flows back into the system through a weak capacitive connection between the earth and the remaining phase wires. Relatively little fault current can flow through this capacitive link in the event of a fault and faults are more difficult to detect. However by taking certain precautions, these faults can be tolerated on the network, that is it is not necessary to trip the circuit straight away and interrupt supply to customers. Isolated neutral earthing is predominantly used on rural overhead networks, where faults are much more frequent, damage is less severe and it is desirable to minimise frequent outages.
In another form of neutral earthing, an impedance such as a resistor is used in the neutral-earth link to limit the amount of current that can flow in a fault. Where low impedance devices are used, so that faults can be detected easily but must be cleared quickly, the arrangement is referred to as low impedance earthing. Where high impedance devices are used, fault currents are limited so that damage is limited and faults can be sustained on the network. This arrangement is referred to as a high impedance earthed network. Isolated neutral networks fall into the category of high impedance earthed networks, as the capacitive coupling referred to above is effectively a high impedance link to earth.
High impedance earthed networks offer better operational performance in rural overhead networks when earth faults can be detected reliably. Once a fault is detected the simplest corrective measure is to trip the circuit. However, where frequent faults events occur this can be very disruptive for customers. Where it is desired to maintain supply in the event of a fault, the fault current must be brought to a very low level at the fault site.
There are two method used for reducing the fault current at a fault site. One method is to install an arc-suppression coil in the neutral-earth link at the source station. The coil diverts or tunes out most of the fault current. The second method involves the use of an earthing switch to connect the faulted phase directly to earth in the source station. This switching shorts out the fault diverting the bulk of the fault current directly to the station. This method is referred to as faulted phase earthing (FPE).
The impedance of the fault at the fault site also determines the amount of current that flows in an earth fault. Low impedance faults facilitate current flow allowing the fault to be detected more readily, whereas high impedance faults restrict current flow making detection more difficult. High impedance faults are typically associated with fallen conductors or accidental contact with a live conductor and are extremely dangerous despite the restricted current flow. It is highly desirable to be able to detect such high impedance faults. The ability of a protection system for an electrical distribution network to be able to detect high impedance faults is referred to as its sensitivity.
There are also factors militating against sensitive protection. During the normal operation of a network there is always a certain amount of current flowing to earth through the inherent capacitive links between each phase conductor and earth. When one of the wires is switched, for example, during normal operations or load switching, these currents can be interrupted or become unbalanced. Unbalance currents flow in the earth and in any neutral-earth link, similar to the behaviour of current in an earth fault. As these are normal operational events, the protection system must either discriminate between such events and real faults or it must be de-sensitised to them.
These issues arise in relation to overhead medium voltage (MV) distribution networks which are much more extensive than underground networks. As the conductors are exposed to weathering they are considerably more fault prone. Additionally, bare conductors overground increases the risk of exposing the public to hazards in the event of an accident or plant failure.
In a low impedance earthed network, earth fault protection usually relies on detecting the fault currents associated with a fault and is referred to as over-current protection. A modified over-current protection system utilizes the power flow associated with an earth fault and is referred to as directional over-current protection. Neither protection system can discriminate between high impedance faults and operational events. The sensitivity of these systems is therefore limited by the need to avoid reacting to operational events.
In a high impedance earthed network, the phase voltages during an earth fault are disturbed. These disturbances can be used to detect earth faults in a protection system referred to as voltage displacement protection which is also affected by operational events and must operate with limited sensitivity. To limit spurious operation a threshold of 30% neutral displacement is often used. The sensitivity is thus limited to 30% of the maximum possible fault current, for example, in a 10 kV network with a 15 A maximum fault current, sensitivity would be 5 A. Any fault below 5 A would not be detected.
Wattmetric protection measures residual current on a feeder and forms a product with the neutral voltage displacement from earth. The product gives a directional indication of the source of the fault and can be used to identify faulty feeders. Wattmetric protection techniques however cannot discriminate between earth faults and imbalance effects.
The method of neutral earthing used has a significant impact on the performance of a MV distribution network, particularly when applied to an overhead network. Neutral earthing techniques determine the manner in which earth faults are detected and treated. They also determine the degree of stress, particularly voltage stress, on the network.
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Albrecht Ralph P.
Electricity Supply Board
Hoff Marc S.
Kim Paul
Venable LLP
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