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Transcript
Delta V - Fault Location Made Easy ‘Location of intermittent faults on low voltage electricity distribution cables’ In the developed world the electricity supply is so reliable that it is often taken for granted. When there is a single disruption, it is seen by most as an inconvenience; however when the disruptions to the supply become more frequent, users become less tolerant, and the matter becomes a serious problem for the distribution companies. Kelman, an engineering company based in the UK, which specialises in producing fault management and asset management tools for the global electricity utility market, has developed a revolutionary new tool to solve the problem of fast accurate fault location, even on intermittent faults. This new device is called Delta V. Industry Reaction “… I am confident the capital investment in this technology will quickly be recovered and complaints in respect of intermittent faults will be reduced.” Chris Watson R&R Zone Manager Yorkshire Electricity Distribution Ltd Most faults on low voltage cables are caused by failure of the outer metallic sheath allowing moisture to penetrate into the cable. Although such failures can occur as a result of poor repair work or through the deterioration of the cable over age, it is more likely that accidental contact during excavation work in the vicinity of the cable is the cause. Such damage normally goes unnoticed and the excavation is filled in. Over a short period of time moisture will penetrate the insulation and enter the cable resulting in transitory faults that may take some time to develop into permanent faults. Unless the fault level in the damaged region is high, it is unlikely that a developing transient fault will cause the circuit fuse to operate, and will probably go unnoticed. However as this cycle continues the integrity of the cable will worsen eventually leading to a fuse rupture at the sub-station, disconnecting the cable and all of the customers joined to it from the electricity supply. Engineers called out to investigate the outage will typically attempt a fuse replacement and if the fault is in a non-permanent state the supply will be restored to the consumers. After a period of time the moisture will start to ingress the cable and the cycle starts over again. Each time a fuse operation occurs consumers are disconnected for a period of several hours causing severe disruption. Locating such a transient fault is not easy, especially as the cables that supply electricity, unlike telecommunications cables, are in a branched network. A cable may have many joints and branches, any one of which could be the source of the fault. The Transgradient Principle and Permanent Faults – Complicated and time consuming Figure 1 Graph detailing the Transgradient principal in action An established method for determining the location of permanent faults on low voltage underground cables is the ‘Transgradient Principle.’ This technique uses the fact that the fault current flowing in the cable is large compared to normal load currents. This current causes a voltage drop along the cable in relation to its impedance. Voltage recorders that record the maximum voltage seen can be placed along the length of the cable, and down various branches. The faulted cable is then briefly energised and the voltage recorded. The voltage measured at the source will be the highest. Measurements made down the cable towards the fault will see less voltage, and once the fault is passed, there will be a consistent reading, indicating the voltage of the fault. In permanent faults, this voltage is often zero (see Fig 1). It is possible to plot the voltage at various points along the cable allowing a gradient to be drawn from the source towards the fault. The fault voltage can also be plotted from readings taken beyond the fault, a region in which no fault current flows. The fault lies where the intersection of the two lines occurs. In its basic form, this is a simple application of Ohms Law. However, this method can be very time consuming and requires highly skilled and experienced engineers in order to be successful. A more simplified method of permanent fault location is obviously required. The Transgradient Principle and Intermittent Faults – An even more complicated problem When dealing with transitory or intermittent type faults, it is not possible to use the simple voltage measurement devices employed in the transgradient permanent fault location described above. In fact a method of accurately locating intermittent faults on low voltage underground cables had until recently never been found, which has caused more than a few headaches for distribution companies. In the UK, the governmental regulatory body OFGEN, penalises disruptions in supply by imposing substantial fines on distribution companies based on the number of minutes that customers are without supply. Therefore intermittent fault occurrence and the inability to locate them can have significant financial consequences for distributors. Kelman has been the leader in this field for many years offering a range of products to aid in low voltage fault management. The new Delta V has been designed to locate permanent or intermittent faults and to work in conjunction with these existing tools. These include: Rezap - Designed to temporarily replace a fuse in a distribution panel to re-energise a faulted feeder after an overcurrent event. It essentially is a single phase auto-recloser with intelligent electronics providing overcurrent protection and programmable delay characteristics, thereby allowing intermittent faults to be cleared completely or conditioned to accelerate the onset of a permanent fault and subsequent repair. The Rezap is also available in a Sigma version equipped with a 2 channel transient recorder and a GSM modem that can be programmed to alert the engineer of Rezap operation by SMS. Once again it is particularly useful under intermittent fault conditions. Figure 2 – Rezap connected to Sub Retrace – Identifies the phase and feeder of any low Station voltage circuit from a given substation using a coded power line carrier signal. It can be invaluable in identifying locations for Delta V node installation that are supplied from a faulted phase, especially under intermittent fault conditions. Figure 3 – Retrace and accessories Delta V – Simple and accurate permanent and intermittent fault location The Delta V uses the Transgradient principle, but Kelman have employed transient voltage recorders in place of the maximum voltage sensors previously used, and developed unique software for analysing the waveforms captured by these devices. After Kelman Comment “Using the analysis techniques within Delta V it is often possible to find the fault before the next fuse interruption.” Bob Beasant Senior Engineer Kelman successful field trials, the first production units were delivered in the UK in April 2004 with excellent reports being received from the end users of these units. Delta V – The System To implement Transgradient Analysis on an intermittent system, Kelman has designed a unique ‘Node’ – a small transient recording device, about the size of a mobile phone charger. These come in two forms, a Domestic Node for placement directly into a socket at a premises and System Nodes, which can be connected directly to street lamps and link boxes in the network where fault activity is suspected. The system also includes a PDA type handheld device into which network topography and node location information is entered. This handheld can then be connected to each node individually in order to download the nodes fault activity logs which it will then analyse and use to provide the user with diagrammatic and synoptic report details of the fault location. The Delta V system is delivered in a robust carry case which also acts as a charging unit for the nodes and handheld. Figure 4 – The Delta V System Delta V – How it works In order to implement Delta V successfully it is necessary to first identify the phase on which the fault is occurring. In many cases this can prove difficult if not impossible for engineers to decipher from the existing documentation. However the Kelman Retrace phase identifier provides engineers with a definitive, simple and quick solution to this problem (see Fig 5). Figure 6 – Delta V Handset illustrating a typical network Figure 5 - Retrace Phase Identifier Ideally a Rezap Sigma should then be deployed on the faulted phase. This will not only ensure that customers will remain supplied even in the case of a fault, but will also notify engineers by SMS when a fault has occurred. This means that engineers know exactly when to visit the Delta V nodes and download their event logs. Next the topography of the cable is entered into the Delta V Handset using a simple graphic interface. It is not necessary for the exact cable configuration to be described, but it is important that major junctions in the backbone are identified. The Handset will then suggest the best locations for the Nodes to be placed (see Fig 8). Figure 7A – Delta V domestic node deployed in a standard UK socket Figure 7B – Delta V domestic and system nodes The first Node is always fitted at the substation, ideally with a Rezap to manage any fault activity. The remaining Remote Nodes are then installed in suitable premises on the faulted phase by simply plugging the Nodes into domestic sockets or fitting into UDBs (Underground Disconnecting Box). There is no need to connect to a phone service. The location of each Node can be entered on-screen to allow the operator to find them again after the fault has been repaired. In general, the greater the number of nodes - the more accurate the fault location, but the system can operate with as few as three Nodes with a simple cable topography. In the case of an intermittent fault it is likely that the Rezap will not trip immediately and normal supply will be maintained until fault current is detected. The Nodes then sense the voltage drop and record several cycles of data immediately prior to the Rezap operation. Each record is time tagged permitting information from several fault events to be gathered (see Fig 9). In many cases the fault can be located successfully even if the Rezap has not seen sufficient current to trip. The network can be revisited at any time and the data collected and used to calculate a fault location. Figure 8 – Typical Delta V network topography including suggested node location points Sn = System Node Dn = Domestic Node In the case of a permanent fault the Rezap will trip immediately, but the Nodes will be able to collect enough data to calculate a fault location. If necessary the process can be repeated to zero-in on a given location. After a fault of any kind each Node can be visited and the data extracted using the Handset. The Handset guides the user through a menu to give the cable parameters necessary for an accurate location. The Handset then calculates a fault position based on the voltage gradient and the topography of the network. Results are presented in both graphical and text form, giving the fault location in metres from the nearest major cable junction (see Fig 10). Figure 9 – Graph of Typical Transient Waveforms Delta V In Action Company: EDF South, UK Date: April 2004 Figure 10A – Example of a Delta V Fault Location Diagram Intermittent fault active since August 2003, which was resulting in, fuses blowing on a regular basis. EDF had been unable to locate the fault using traditional methods. It can be seen from the system configuration illustrated in Figure 11 that it would always prove difficult to identify any type of fault on this piece of network. Prior to connecting Delta V the network was scanned using a Retrace unit and points identified at which to place the nodes. Figure 10B – Example of a Delta V Fault Location Text Report To make things easier a more simplified system configuration was entered into the Delta V handset and a mixture of system nodes [S/n] & domestic nodes [D/n] were installed as illustrated in Figure 12. Figure 12 – Delta V Representation of EDF Network Configuration Figure 11 – Actual EDF Network Configuration When the next intermittent fault occurred 5 days later, the Delta V nodes were interrogated and a fault position calculated to be 11 meters +/- 4 meters from the tee joint of the service to where the domestic node was connected and the tee joint to Pelham St s/s. Rezap was closed onto the fault and the fault was identified approximately 1 meter further towards Pelham st s/s than the 11meter position had indicated but still within +/- 4 meter tolerance. From the network drawings this indicated the position of an old tee joint /pot end, and on excavation it was found that the main cable had faulted at the tee joint just outside the joint towards Pelham St s/s. Industry Reaction “Engineers are usually quite reserved in embracing new technology…. however, the Delta V unit is simple to install, very user friendly and on its first trial it has proved very reliable and accurate.” Mark Parris Senior Engineer EDF South. UK The fault was repaired and supplies returned to normal the same day. Industry Reaction “In the space of a month, we have successfully used the new Delta V unit on three occasions to aid in the location of intermittent faults. This has provided us with increased confidence in our ability to locate intermittent faults.” “We found the system very easy to use. It took less than 5 minutes in each property to confirm the phase and install a node. Our customers thought it was excellent and we have received excellent support from our friends at Kelman.” Chris Watson R&R Zone Manager Yorkshire Electricity Distribution Ltd. Company: YEDL, UK Date: June 2004 On this intermittent fault in Marshland Road, Moorends a Rezap Sigma was installed at the source substation. This fault was very active with many current spikes in between fuse failures and Rezap operations. YEDL carried out a test with the Retrace and made sure that the nodes were installed on the same phase as the Sigma and also at the premises of a business customer who had complained of flicker and supply failures. On this occasion, as the network was fairly simple, YEDL decided to install only 5 Delta V nodes. The Rezap Sigma was set up to send an SMS message for current values over 600A as well as trips. The day after the installation, the Rezap sent an SMS indicating fault activity. The data was duly collected and the circuit details requested by the software were entered. The Delta V told YEDL engineers that the fault was near or beyond the last node but was unable to give a more accurate position unless more nodes were installed. Although this would have been very simple to do, YEDL found that after examining the fault statistics and the mains records, there was a likely position for the fault beyond the last node. The cable had previously been cut and an insulated end made to disconnect an old 3-phase supply. Excavation and tests at this position revealed the fault to be there, and it was repaired immediately. No further faults have occurred on this network section since the repair. Why Delta V? • Accurately locates permanent and intermittent faults; • The user friendly interface means that the system an be used by less skilled engineers; • Automatic operation detects fault activity 24 hours/day; • Capable of finding intermittent faults before another outage occurs. For further information on Delta V or any of the other products discussed in this article please contact: Kelman Ltd Lissue Industrial Estate East Lissue Road Lisburn, BT28 2RB United Kingdom Telephone: +4428 9262 2915 or email: [email protected] or see our website on www.kelman.co.uk