Technical Field of the Invention

The present invention relates to a Fault Level monitoring and in particular to a method and apparatus for Fault Level monitoring for electricity network infrastructure components, such as substations.

Background to the Invention

In any electricity distribution network the term Fault Level is used to describe the potential energy which would be released in the event of a short circuit fault. The Fault Level can be expressed as an MVA (Megavolt-ampere) equivalent at the network voltage under consideration or in terms of the current which will flow into the short circuit. The total prospective fault current at a particular location is made up of the contributions from the network source infeed and from energy sources embedded within the network downstream.

It is necessary to know the Fault Level throughout an electricity distribution network to ensure that the fault current make and break duties of switchgear are not exceeded, to ensure that the through fault current withstand of network components are not exceeded and to allow the appropriate setting of protection equipment to detect and clear faults when they occur. Calculation of the Fault Level requires knowledge of the elements of the network which are in circuit, in the source infeed, details of any embedded generation downstream of the point of concern, as well as characteristics of the load such as number and size of motors. It is not practically possible to know all of these characteristics all of the time so generic factors are typically employed to estimate the effects of the load elements. The use of these generic elements can lead to errors in the assessment of the Fault Level. To avoid the danger of exceeding the Fault Current capability of the switchgear and other network elements artificial limits on these duties are typically imposed. This may lead to the full capability of the network not being utilised and in extreme situations to unnecessary work to replace and reinforce elements of the network.

It is of course possible to measure actual fault currents when they occur, however until a fault occurs this technique provides no knowledge of the total fault level on the network. Even when a fault occurs this may not be at a point which will provide information on the maximum fault current which may be expected either due to the location or the characteristics of the fault.

Detecting network disturbances, either naturally occurring or deliberately created, and recording the change in voltage and current conditions as result of the disturbance is a longstanding technique to determine the network Fault Level. Typically, a monitoring apparatus is provided operable to monitor the incoming feed to the substation. Disturbances in voltage and/or current are identified and categorised. Subsequently, the magnitude of the disturbances can be utilised to calculate a quantity indicative of the prospective fault level. Since calculation of the prospective fault level requires calculation of the source impedance at the fundamental line frequency, disturbances in harmonics of the line frequency are usually filtered out. Over time, information from multiple disturbances analysed using regression techniques can provide an improved assessment of the source impedance and hence the prospective fault current.

Algorithms for calculation of the prospective fault level based on network disturbances were developed by Shackshaft et al in the 1970s and have been implemented and developed since this time. To separate the source infeed and the downstream contributions it is typically necessary to distinguish between disturbances which occur downstream and upstream of the monitoring location. In a typical embodiment of this approach a monitor is located at the source infeed(s) to the substation being monitored. Disturbances which result from changes in the downstream load provides information which can be used to assess the source infeed contribution to the substation Fault Level, experience has suggested that around 100 events are required to provide the best results. To assess the contribution from embedded energy sources within the downstream network disturbances upstream of the monitoring point must be detected and analysed. Since upstream disturbances of sufficient magnitude are much less frequent than downstream disturbances and the magnitude of the contribution varies with the variation in load it can take a long time to gather sufficient numbers of disturbances to carry out this analysis.

It is therefore an object of the present invention to provide a method and apparatus for fault level monitoring that at least partially overcomes or alleviates the above problems.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of Fault Level monitoring for use on electricity network infrastructure components, the method comprising the steps of: identifying disturbances in voltage and/or current on each outgoing feed from said infrastructure component; calculating a value indicative of a potential fault infeed from each feeder based on the magnitude of each identified disturbance; and thereby determining the prospective fault infeed contribution of each outgoing feeder of the infrastructure component.

By concentrating on monitoring outgoing feeders as well as the incoming source feeders, a larger number of disturbances can be obtained for the assessment of downstream network fault infeed. This is achieved by increasing the number of monitoring points and placing these at the outgoing feeders, whereby the downstream disturbances on individual feeders become upstream disturbances with respect to every other feeder monitoring point. Typically there are larger numbers of comparable magnitude disturbances which occur on the downstream network as compared to the upstream network. Therefore applying additional monitoring, which has the effect of making the downstream disturbances on one feeder appear as upstream disturbances to every other feeder, substantially reduces the time taken to obtain sufficient numbers of measurements to improve the confidence levels of the result.

In addition the use of monitoring on each outgoing feeder allows the individual prospective fault current duties for each circuit breaker to be determined. Previous methods have tended to provide the total fault level which would apply to the next circuit breaker to be connected by aggregating the entire downstream contribution from all of the network embedded energy sources which overstates the actual duty presently experienced by circuit breakers already. By providing the individual duties for each circuit breaker it will be possible to identify particular sources of potential problems and carry out targeted remediation.

The invention may be applied to any suitable network infrastructure components including but not limited to substations. In the present invention, the term substation is used to encompass all electricity network infrastructure elements having one or more incoming feeders and a plurality of outgoing feeders. The method may involve categorising each disturbance before calculating a value indicative of a prospective fault infeed contribution from each feeder based on the magnitude of each identified disturbance.

The method may include collating information on the individual contributions of each outgoing feeder to provide an overall network infrastructure component prospective fault level. The method may involve the further step of monitoring disturbances on the incoming feeder so as to calculate a network infrastructure component's incoming source impedance and/or prospective fault infeed. Calculations of the incoming source impedance may be made according to any suitable known technique. In such embodiments, the method may include the further step of comparing the calculated incoming and outgoing fault level contributions or collating the calculated incoming and outgoing prospective fault infeed contributions. The method may include calculating the source impedance from; outgoing feeder monitoring; incoming feeder monitoring; or a combination of the two.

The method may include monitoring each of the outgoing feeders to determine a profile of time variation in prospective fault infeed. The determined profile of time variation in prospective fault infeed may be cyclically variable. In the event that the level of time variation is minimal, the profile may be treated as substantially time invariant. The determined profile of time variation in prospective fault infeed for each feeder may then be used in calculating the overall profile of time variation in prospective fault level for the network infrastructure component. Use of a measured profile of time variation in prospective fault infeed rather than a model of a profile based on a value linked to the load can enable greater accuracy.

In particular, the profile of time variation in prospective fault infeed may be determined against a daily, weekly or other episodic cycle. In this manner, variations in the profile of time variation in prospective fault infeed due to expected daily or weekly changes in the embedded energy sources can be demonstrated.

The monitoring of outgoing feeders may be achieved by an averaging methodology. Preferably, this averaging methodology will draw out trends in variation in prospective fault infeeds, against a daily, weekly or other episodic cycle. These trends may be due to circumstances that vary by time of day or day of the week. An advantage of knowing the profile of variation of prospective fault infeeds is the ability to assess the effect on Fault Level, at different times going forward, which would result from the connection of equipment which can increase the prospective fault infeed of the feeder it is connected to. In particular, the monitoring may include box car averaging of the measured profile of time variation in prospective fault infeed. In particular, voltage and current measurements may be averaged over an averaging period, the values averaged over the averaging period being aggregated from values averaged over successive sub- periods within the period. The sub-period averages may be aggregated from values averaged over successive sub-sub-periods say within the sub-period. In one suitable example, the periods may be of the order of 10 minutes, the sub-periods of the order of 3 seconds and the sub- sub-periods of the order of 10 cycles.

The method may include the step of monitoring voltage and/or current disturbances at the fundamental line frequency of the network and monitoring voltage and/or current disturbances at one or more harmonics of the fundamental frequency. By monitoring harmonics of the fundamental line frequency, additional information on the harmonic impedance of the network can be obtained.

According to a second aspect of the present invention there is provided a fault level monitoring apparatus for use with electricity network infrastructure components, the apparatus comprising: monitoring means operable to detect disturbances in voltage and/or current on each outgoing feeder from said infrastructure component; processing means operable to identify disturbances in voltage and/or current on each outgoing feeder from said infrastructure component; calculating a quantity indicative of a prospective fault level based on the magnitude of each identified disturbance for each outgoing feeder; and thereby determining the fault infeed contribution from each outgoing feeder of the infrastructure component; and an output interface operable to output information on the fault infeed contribution of each outgoing feeder.

The apparatus of the second aspect of the present invention may operate according to the method of the first aspect of the present invention and incorporate any features of the first aspect of the invention as are required or desired.

The monitoring means is preferably attached to the each outgoing feeder adjacent to the associated busbar. In cases where the incoming feeder is also monitored, further monitoring means is provided adjacent to the incoming feeder to the busbar. The monitoring means may comprise a single unit connected to and operable to monitor each feeder. Alternatively, the monitoring means may comprise a network of separate devices each dedicated to monitoring a single feeder. The processing means may be provided at the network infrastructure component. Alternatively, the processing means may be provided remotely and a suitable wired or wireless communications link may be provided between the monitoring means and the processing means. The processing means may further be provided with or connected to a data storage means operable to store historical data on performance. The storage means may further be operable to store the measured profiles of time variation in prospective fault infeeds.

The output interface may be provided locally to the processing means. Alternatively, the output interface may be provided remotely and a suitable wired or wireless communications link may be provided between the output interface and the processing means.

The output interface may comprise a display means operable to display data related to past or present disturbances and/or to display data relating to prospective fault levels. The output interface may be provided with one or more user actuable controls. The user actuable controls may be operable to vary the display output or may be operable to vary settings or operational modes for the monitoring means or the processing means.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic diagram illustrating the prior art method of fault level monitoring for electricity network infrastructure components; Figure 2 is a schematic diagram illustrating a first implementation of fault level monitoring for electricity network infrastructure components according to the present invention;

Figure 3 is a schematic diagram illustrating a second implementation of fault level monitoring for electricity network infrastructure components according to the present invention; and

Figure 4 is a schematic block diagram illustrating an example of a fault level monitoring apparatus according to the present invention.

Turning now to figure 1, in known fault level monitoring methods for electricity network infrastructure components, such as substation 10, a monitoring device 1 is provided for the incoming feeder 11. Power from the incoming feeder 11 is distributed via a busbar 12 to multiple outgoing feeders 13. Each outgoing feeder is attached to a load 14.

Disturbances in voltage and/or current are identified by monitoring means 21 are categorised and subsequently, the magnitude of the disturbances can be utilised to calculate a quantity indicative of the prospective fault level. The algorithms used for calculation of this quantity were originally developed by Shackshaft et al in the 1970s and have been refined since this time. In order to calculate the prospective fault level, these algorithms assume that outgoing feeder behaves according to a general load model. The above method can run into difficulties where loads on outgoing feeds do not comply to standard models. Additionally, since upstream disturbances are typically less frequent than downstream disturbances and a significant number of disturbances (-100) is typically required to allow regression analysis of the results to provide a meaningful and reliable indication of the potential downstream infeed this method can take some time to determine measurements to an acceptable confidence level.

Turning now to figure 2, a first implementation of fault level monitoring methods for electricity substations according to the present invention is illustrated. As before, an incoming feeder 11 provides power which is distributed via busbar 12 to multiple outgoing feeders 13. Each outgoing feeder is attached to a load 14. In this embodiment, instead of providing a monitoring device 21 on the incoming feeder 11, a monitoring device 21 is provided on each outgoing feeder.

In the present invention, disturbances in voltage and/or current on each outgoing feeder 13 from said substation are identified by the monitoring means 21 provided for each outgoing feeder 13. Subsequently, each disturbance can be used to calculate an individual quantity indicative of the prospective fault level for that feeder based on the magnitude of each identified disturbance for each outgoing feeder 13. Accordingly, the prospective fault level can be separately assessed for each individual feeder. Additionally, by collating information on the individual outgoing feeders, an overall substation prospective fault level can be determined and used to assess the potential effects of an additional outgoing feeder being added to the substation.

In an alternative embodiment of the present invention shown in figure 3, in addition to dedicated monitoring means 21 for each outgoing feeder 13, a monitoring means 21 can also be provided on the incoming feeder 11. In such cases, disturbances on the incoming feeder 11 can be used to calculate a substation incoming fault infeed as in the prior art. Subsequently, the calculated incoming and outgoing prospective fault levels may be compared or collated to provide overall prospective fault level information. Since the present invention provides for monitoring individual outgoing feeders 13, it is possible to use these measured disturbances to develop a profile of time variation in prospective fault infeeds rather than a theoretical profile of time variation in prospective fault infeeds based on expected or measured load profiles. This enables disturbances to be measured more accurately from measured performance rather than worst case scenarios based on theoretical profiles. It is also possible in the present invention to build up time variant or cyclically variable profiles for individual loads. This can allow for regular daily or weekly variations in a particular profile of time variation in prospective fault infeed to be taken into account when measuring disturbances.

Additionally, the present methodology makes it feasible to monitor voltage and/or current disturbances at harmonics of the fundamental line frequency of the outgoing feeder 13. By monitoring harmonics of the fundamental line frequency, additional information on the harmonic impedance of the network can be obtained. Turning now to figure 4, a schematic block diagram of a monitoring apparatus 20 for implementing the method of the present invention is shown. The apparatus 100 comprises a plurality of monitoring means 21, each connected to a processing means 22. The processing means 22 is operable to receive signals indicative of identified disturbances from the monitoring means 21 and thereby calculate prospective fault infeeds for the relevant feeds 13, 11. In calculating these factors, the processing means may be operable to retrieve load profiles and profiles of time variation in prospective fault infeeds from a data storage means 23. In the event that individual profiles of time variation in prospective fault infeeds are determined by measurement, the processing means may be operable to store averaged voltage and/or current data produced by the monitoring means 21 in the data store. The stored data can subsequently be retrieved by the processing means 21 and used to generate or update individual profiles of time variation in prospective fault infeeds.

The processing means is also connected to an output interface 24, which may have an associated display 25 and/or user input means 26. The output interface 24 allows an operator to review the operation of the apparatus 20 and gain information as to potential faults. The operator may also be able to vary the mode of operation of the apparatus where appropriate.

The processing means 22 and the output interface may be provided locally to the substation 10. Alternatively, they can be provided at a remote location, such as a network control centre. In such cases a suitable wired or wireless communications link may be provided. The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.