Using Bridge Measuring Techniques to Locate Difficult to Find Cable Faults
For faults that occur in medium or high voltage cables, traditionally, equipment is used that combines high voltage surge generators, or thumpers, with time domain reflectometry (TDR) measuring devices, such as with the Syscompact Series of equipment. More information on the different pre-location methods that can be used with the Syscompact equipment can be found under Cable Fault Location with Syscompact Series.
While fault locating with TDR and thumpers are great tools for locating faults in cables, they do pose some disadvantages, especially when signal propagation is an issue for TDR-based techniques. In a coaxial cable system, if the cable sheath is damaged at a point that covers the cable diameter, essentially causing a “cable cut” of the sheath, or if the sheath is heavily corroded, signal propagation becomes extremely difficult. A TDR signal requires a parallel path of two conductors, and if signal propagation is impeded, this will lead to misleading results and erroneous diagrams.
A great alternative solution is to use the Shirla Sheath Test and Fault Location Device, in which fault location is based on the measuring bridge principle according to Murray or Glaser. Most people view this instrument as purely a sheath tester and sheath fault locator; however, with its 10 kV high voltage power source, the measuring bridge is also able to locate low resistive or high resistive cable faults that occur between the core and sheath. The distance to the fault is determined by measuring and balancing the resistances of the cable. More information on the Murray and Glaser measuring bridges is given in Cable Sheath Fault Location Using Bridge Methods.
Case Study on Electrical Submersible Pump (ESP) Cables
ESP cables are subject to very harsh conditions in a wellbore environment, such as extremely high temperatures, fluid infiltration, rapid decompression, or impact damage during equipment run-in. When a cable does fail in service, this cable must be pulled and tested with cable fault location procedures to try and find the source of the failure as soon as possible to determine whether the cable can be reused following repair. With the amount of corrosion and the harsh conditions ESP cables experience, or when you have unshielded poly cables, then fault locating with a thumper/TDR can be extremely difficult. When there is a great amount of corrosion on the shield of the cable, signal propagation becomes increasingly difficult, giving rise to misleading results using traditional cable fault location techniques.
After days of unsuccessfully thumping a 5280 ft faulted ESP cable, HV TECHNOLOGIES, Inc. was called in to demonstrate the Shirla fault locator. According to an insulation resistance measurement, phase 1 (L1) was determined to have a cable fault present. For a core to sheath resistive fault, the Shirla allows for 2 different measurements to be conducted – either using the Murray or Glaser bridge methods. The measuring bridge method according to Murray is applied on arrangements where besides the faulted core, one healthy core is required with the same cross-section and conductor material. A linking bridge has to be created at the cable end between the faulted and healthy core. When using the measuring bridge method according to Glaser, 2 healthy cores of equal cross-section and conductor material are required. This method requires 2 linking bridges to be placed between each healthy core and the faulted core. The image below shows the general set up of the Shirla when using Murray or Glaser bridge methods.
Another advantage of the Shirla is when a cable is situated on a drum and both cable ends are accessible, the Glaser method can be used by direct connection at the cable end and eliminating the use of linking bridges. This method is particularly advantageous in a situation in which all phases would contain a fault at some point along the cable, meaning no healthy cores would be available to connect to, as shown above. The Glaser connection on a cable drum is shown below.
All 3 methods were used to determined the distance of the fault on phase 1 of the cable. The results for each method were 1245 ft, 1229 ft, and 1256 ft for using the Murray, Glaser, and Glaser on a cable drum method, respectively. At that point, the cable was spooled to 1250 ft and a thumper was connected. A distinct audible breakdown could be heard and this was pinpointed to be located at a distance of 1264 ft. Therefore, the Shirla successfully and quickly located the cable fault in which the operators were previously unable to locate using a traditional thumper/TDR method.
Locating Faults in T-branched Networks
Cable faults located in T-branched networks are extremely difficult to find, regardless of the cable fault location method used. One of the best methods to use is the Differential Impulse Current Method or Differential Decay Method, which is explained in more detail in a previous blog post titled Cable Fault Location Measuring Methods. This equipment requires special couplers and is fairly large, which is why it is typically installed in dedicated cable test vans, such as the Titron. Using traditional methods such as a thumper and TDR to locate faults in T-branched networks is very difficult as you will receive a distance to the fault, but cannot know exactly which branch this stems from. Thumping the cable multiple times trying to locate the fault is also detrimental to the cable, as further damage can be created to healthy parts of the cable over an extended period of time.
The bridge measurement technology within the Shirla can help to locate these difficult to find faults without having to apply very high voltages or thump unnecessarily. Below is an example in which there is a fault in phase 3 (L3) in a T-branched network. Connecting the Shirla using the Glaser method as shown above, in which a connection on healthy L1 and L2 from instrument is made and at the end of the main strand short-circuit bridges (supplied with Shirla) are made from L1 and L2 going to L3. The measurement conducted would then provide the position of the T where the leakage current leads away from the main strand.
In a second step you would then change the short circuit bridges to the cable coming off of the T containing the phase with a fault. Afterwards another measurement would be conducted and the length to the fault could be determined. At this point you could also enter different cross sections, length, and cable material of the different cables if the cable coming off the T is different from the main strand. This helps in compensating for differing resistances, improving the accuracy of the measurement.