Through applying a novel, patent pending algorithm, a lunchbox-sized instrument can now enable distribution network operators to confidently predict fault currents on their networks.
Hurricane Sandy and the documented explosion in a New York substation have reminded us all of the dangers associated with failures of the distribution network. Fortunately, on this occasion, there were no reported injuries as a result of the blast but it is well known that faults on the network, whether caused by man or natural phenomena, can result in the flow of very large currents until the supply can be interrupted by the safety mechanisms in place. Although a vital task, determining the fault currents that can flow on an electrical network in the event of a fault has always posed a challenge to utilities and industrial site managers. Accurate knowledge of the maximum current that could possibly flow, the Fault Level, is essential to maintaining safety on the network, to ensure optimal operation and to optimise investment in infrastructure.
The safety of the transmission and distribution networks depends on circuit breakers to interrupt and manage safely the electricity supply to affected sections of the grid in the case of a fault. These circuit breakers must be of a high enough capacity to make or break the flow of current even under the worst possible fault conditions. The network infrastructure must also be able to withstand the currents that flow during the period before the circuit breakers isolate the affected network to prevent destructive failure.
A circuit breaker is designed to interrupt or connect to a certain maximum current known as its fault level rating. If the actual fault level should be found to exceed this rating or more capacity needs to be added, the network operator must reinforce or reconfigure the network to ensure continued safe operation. However, upgrading of switchgear, cables or transformers, for example, are all costly and potentially disruptive activities. It is essential to know the fault level to maintain efficient, safe and predictable network operation.
This requirement to consider fault level is becoming increasingly relevant to today’s network operators. Since the amount of distributed generation on the network is on the rise, the network is being pushed closer to its existing Fault Level limits.
Existing fault level calculations
These limits are calculated by utilities in either of two ways, or a combination of both. Firstly by using “rules of thumb” – calculations based upon the known contribution from the networks major elements i.e. transformers – or secondly by creating a detailed, analytical model for the relevant section of the network. Although this latter method is used to provide an accurate value for the fault current, it is complex and time consuming to establish and so has been used mainly at higher network voltages (for example at 33kV and above in the UK).
Pressure is rising on the network operators to deliver a continuous, low carbon supply of clean, stable and value for money electricity. Unfortunately as this pressure mounts the job gets progressively more challenging as in many cases the infrastructure is ageing and becoming increasing unsuitable. Not only are nonlinear loads increasingly being connected to the electricity network, producing harmonics, as well as other undesirable network effects but the introduction and proliferation of Distributed Generation, for instance Combined Heat and Power plants, Solar Panels and Wind Turbines, have their own challenges. When DG is added to a network they increase uncertainty by not only taking power from the grid but also adding it back. Fault levels are continually changing. This means that rules of thumb go out of the window and models cannot with conviction take all this changing activity into account.
Search for a new solution
Following a discussion with the DNO Scottish Power Energy Networks (SPEN) on the merits of Outram’s Power Quality measuring products, SPEN optimistically asked whether Outram Research Ltd (ORL) could design a product that could measure fault level from natural disturbances. Outram Research Ltd rose to the challenge. For the past several years ORL has been working on a portable instrument which can predict fault current by measuring natural disturbances on a radial network during normal operation.
The Outram Fault Level Monitor (FLM) is a compact, portable device which takes measurements of natural voltage and current disturbances at a specific point on the network and predicts a fault level value for that point. This involved Outram Research developing a completely original set of algorithms and implementing them on a hardware platform, the 600V Category IV, Outram PM7000 Power Quality Analyser.
The Outram FLM works using a novel, patent-pending algorithm to determine worst case fault current, separating out peak ‘make’ current, RMS ‘break’ current and contributions from downstream motors. This is achieved by extracting the tiny characteristics of interest from voltage and current signals at the arbitrary times at which natural disturbances occur, e.g. tap changes and load switching. The instrument distinguishes the various components capable of delivering the fault level results, processing these mathematically to optimise signal to noise then filtering and computing the desired Fault Level values from this data. This innovative solution means that high quality results can be obtained from voltage disturbances of as little as 0.15%. No previous knowledge of the network is required and the FLM can be deployed at any voltage level assuming the necessary CT/VTs are in place.
A successful testing programme
Lab trials produced accurate and encouraging results. With partial funding from OFGEM’s Innovation Funding Incentive (IFI), field trials were carried out with ORL’s partners Scottish Power Energy Networks (SPEN) to compare results generated by the FLM with fault currents calculated by analytical modelling. The results compared very positively with Scottish Power’s detailed IPSA and DigSILENT models. They were consistently within 3-5% of the modelled values showing that the Outram FLM can accurately calculate fault current in real-world applications.
To produce these results, the Outram FLM must rely on voltage disturbances seen on the network during normal operation, so it stands to reason that if there are no disturbances then no fault level prediction can be given. The results generated for the user are therefore ascribed a confidence level dependent on the magnitude of the disturbance seen. Whilst voltage disturbances of <0.15% are given little significance the larger the disturbance the higher the weighting given for the instantaneous or composite results. The relative accuracy of the FLM result versus model result eventually depends on sensor accuracy and noise for the FLM, and fidelity of the network characterisation for the computer model. Clearly where network modelling is poor or non-existent and sizeable disturbances are plentiful, the FLM will produce superior results. Conversely where networks are interconnected and there are multiple current paths, or suitable disturbances are infrequent, a single FLM operating only on natural disturbances is likely to be inferior to a well characterised model. However while interconnected network complexity increases the difficulties for both model and FLM (the latter requiring multiple synchronised devices), the introduction of artificial disturbances on a known single current path is expected to cut through the uncertainty and embrace all current sources, enabling the FLM to produce correct results in the most complicated of interconnected arrangements.
Network disturbance on demand
Another UK DNO, Western Power Distribution (WPD), is planning to take the Outram FLM one step further. With funding from a Tier 1 Low Carbon Networks Fund (LCNF) project WPD has a vision for an active network management solution. With the benefit of further communications developments on the FLM, WPD aims to introduce a means of supplying it with a network disturbance on demand so that a Fault Level value with a high confidence rating can be obtained in real time. WPD have identified a real network environment in which they aim to trial active fault level-based control of a Distributed Generation connection.
Stage 1 of this project, 11kV lab testing, has already been achieved. WPD carried out trials with the Outram FLM and an IntelliRupter ® PulseCloser, a unique alternative to conventional automatic circuit reclosers manufactured by S&C Electric Company, at S&C’s Advanced Technology Center in Chicago, USA. (By comparison the IntelliRupter significantly reduces the amount of energy the network is exposed to when testing the circuit to see if a fault is still present). In the lab, non-customer affecting ¼ to ½ cycle current and voltage disturbances, generated during the IntelliRupter’s pulseclosing operations, were used to enable the FLM to predict the fault currents produced by separate, deliberately instigated and independently measured bolted faults produced under identical source conditions. Thus during the trials it was possible to compare the predicted fault level with the actual measured peak current seen in response to the bolted fault. These trials were outstandingly successful, the results showing that the FLM predictions were between 2.5 and 5% of the current measured by lab instrumentation – an error margin felt very hard to improve upon as noise on the network and sensor deficiencies will always produce some degree of uncertainty.
WPD’s ultimate aim is to further develop their ability to actively manage their distribution network by optimising the network arrangement using predicted Fault Level values.
Satisfying a growing requirement
Following the successful trials, two clear uses for the Outram FLM have emerged for both Distribution Network Operators (DNOs) and other network operators, large or small. The first is to validate and refine existing models by obtaining greater visibility of actual system fault levels and the contributions from customer equipment. The second is the identification of the fault level in areas of the network where models do not exist or it is difficult to build them, such as in sections of the 11kV or Low Voltage network.
The validation or prediction of unknown fault level values will deliver many benefits and therefore ensure a considerable return on any investment. Network operators will be able to reduce health and safety risks by identifying and subsequently managing fault level issues previously unknown due to inaccurate or non-existent models. Financial rewards may be achieved through optimal operation or interconnections on the network which previously could have been restricted due to perceived fault level issues. The DNOs should also be able to improve their regulatory performance through faster rectification of fault level issues following more accurate monitoring.
There are not just financial and operational but also environmental benefits to be realised. Measurement of the real day-to-day impacts on fault level could enable faster connection of renewable generation, important with today’s emphasis on low carbon generation, and can also delay the premature decommissioning of equipment through unnecessary fault level upgrades.
The Outram Fault Level Monitor is the only product of its kind available today: it is a solution that determines fault current by observing the behaviour of the electricity network, using a novel algorithm to do so. Furthermore, it is a practical solution – the equipment is about the same size as a lunchbox and can be quickly and easily deployed and relocated.
Fault Level is a parameter much needed by electrical network operators across the globe. Being able to predict it on any network will bring many benefits, not least enabling the presently assumed safety margins to be realistically assessed. John Outram said: “We are delighted with the success of the trials. The Outram FLM is a unique solution, well received by Distribution Network Operators, that meets an increasing need that, up until now, has remained unfulfilled. We worked very closely with SPEN on the requirements and have delivered a solution with all the functionality required by operators worldwide.”
Outram Research Ltd is currently in talks with other DNOs to get the Outram FLM deployed in a number of further sites. With the Outram FLM currently in a position to help give an indication of the fault level on a network, Outram are now accepting expressions of interest, hoping to reinforce their findings with more trials in expectation of shipping product to the wider market within six months.
Outram Research Ltd would like to thank the following for their help and support on the projects mentioned in this article: Scottish Power Energy Networks, Western Power Distribution, S&C Electric Europe Ltd and Parsons Brinckerhoff.
Outram Research Ltd is the 2012 winner of the British Engineering Excellence Award for Small Company of the Year.