Cable Fault Location with Syscompact Series
- January 25, 2019
- Posted by: Dominique
- Category: Cable Fault Location
Service faults in cables can occur due to many reasons, such as simple ageing of the insulation over time, sudden over-voltages, thermal overload, corrosion of the outside jacket, and poor workmanship during transport, installation, or repair, among other reasons. For the utility or electric power delivery company, it is imperative to have equipment that can quickly and efficiently locate the faults to allow for repairs as soon as possible. The shorter the downtime during a fault, the less associated costs and frustration are accrued.
In general, two different types of fault location are distinguished: pre-location and pin-pointing. HV TECHNOLOGIES, Inc. offers a long line of instruments that can be used to locate faults with ease. This post will focus on pre-location methods that are possible with the Syscompact Series of equipment, which combine high voltage surge generators, or thumpers, with time domain reflectometry (TDR) measuring devices to form a compact and fully integrated cable fault location system. These systems allow pre-location of low-resistive (i.e., short-circuit), high-resistive (i.e., cable cut), and intermittent cable faults according to the following fault location methods:
- Time domain reflectometry (TDR)
- Fault Conditioning
- Secondary Impulse Method / Multiple Impulse Method (SIM/MIM)
- Impulse Current Method (ICM)
Time Domain Reflectometry (TDR)
TDR is the most established method used to determine the location of short-circuit faults in a cable. TDR can also be used for the determination of:
- the total length of a cable
- the location of low resistive cable faults
- the location of cable interruptions
- the location of joints along the cable
In a coaxial cable system with a parallel path of two conductors (i.e., conductor and metallic sheath/ground), a low voltage TDR pulse is sent along the cable which can detect changes in impedance. A positive reflection will be seen at open ends (cable end or cable cut), a negative reflection will be seen at short-circuits or low-resistive faults, and S-shaped reflections can be seen where joints are located. By measuring the time until the TDR pulse returns from a reflection, the distance can be calculated. A typical set-up for a TDR measurement is seen below.
The basic measurement principle of TDR is the basis for all other cable fault location methods utilized with the Syscompact Series of instruments. Even more information on TDR is given in another blog post The Basics of Time Domain Reflectometry (TDR).
Fault Conditioning
By using the DC surge generator in the Syscompact, continuous thumping can be conducted to supply sufficient voltage and current to a high-resistive or intermittent fault to produce charring of the insulation material or metal fusion, which converts the fault into a low-resistive fault to easily locate using a TDR. This type of fault location can also be achieved by using Burn Down Transformers that supply a very high current to dry and carbonize the fault converting it to a low-resistive fault. The below TDR trace shows the difference of the TDR signal before (blue trace) and after (red trace) cable conditioning.
In reality, fault conditioning is an outdated method that should only be used on PILC-type cables as extended thumping can cause space charge build-up or generate new faults in the cable. Faults in solid dielectric cables can also reinsulate or melt rather than carbonize, which negates the use of TDR afterwards to locate the fault. For this reason, the next measurement method, SIM/MIM, is an excellent substitute for fault conditioning.
Secondary Impulse Method / Multiple Impulse Method (SIM/MIM)
The Secondary Impulse Method is a measurement method patented by BAUR and is regarded as the most reliable and precise cable fault pre-location method available. The measurement principle is also known as surge arc reflection and allows for locating faults at the lowest possible voltage levels without potential risk of generating new faults in the cable. Every cable fault that is either a high resistive or intermittent fault cannot be indicated by means of the TDR method. The low voltage impulse sent out by the TDR is not reflected at the fault position, as the fault impedance compared to the insulation impedance of the healthy part of the cable is not significantly lower.
Based on this fact, the SIM is supported by a single high voltage impulse that is generated by the coupled surge generator. Like this it is possible to change the high resistive fault temporarily into a short circuit (flash over, temporary low resistive fault condition) and therefore can be detected by a second TDR impulse. The low voltage TDR impulse is coupled to the high voltage output of the surge generator via the coupling unit SA 32.
The first measurement shows a TDR trace with the positive reflection of the cable end. The second measurement shows the TDR trace with a negative reflection from the arcing fault.
The timing response of a fault can be very unpredictable, depending on the cable length and the condition of the fault (i.e., water in a joint or oil-reflow in oil-filled cables). This can make it difficult to trigger the breakdown at the right moment. A simple and reliable solution is the Multiple Impulse Method with the Syscompact 4000 in which many TDR traces (up to 20) are sent down the cable in order to increase the possibility of having one or more TDR pulses reflect at the arcing fault.
Water can cause ignition of the breakdown of the cable fault to be delayed and can also extinguish the flashover very quickly. By emitting a sequence of TDR pulses with different trigger delays, a larger timeframe can be monitored increasing the likelihood that one or more TDR pulses are reflected at the arc.
Impulse Current Method (ICM)
Cable fault location methods based on a TDR impulse are affected by damping of the signal in very long cables, by reflections arising at joints and other impedance changes along the cable, or by corrosion of the cable sheath. In extreme cases the TDR pulses may be damped to such a degree before returning to the time domain reflectometer that interpretation of the TDR traces is very difficult. The Impulse Current Method (ICM) is a great method to divert to in such circumstances.
An inductive coupling unit SK 1D is connected to the sheath of the surge voltage generator output cable. The surge generator (thumper) releases a HV impulse that causes the fault position to break down, which in turn creates a transient current wave traveling along the cable sheath between the surge generator and the cable fault. The initial impulse is emitted as a negative pulse and every reflection at the fault position and generator causes a reversal in polarity, as both positions are low resistive points of reflection.
The first impulse reflection is influenced by the ignition delay time of the fault breakdown. The distance to the fault can be taken as the distance between the 2nd and 3rd pulse or 3rd and 4th pulse or 4th and 5th pulse. Due to damping of the signals over time, further pulses should not be analyzed and would lead to inaccurate results. As the pulse width of the transient current pulse is very wide, the ICM method is only accurate for very long cables. In short cables, the transient pulses influence each other and cause difficult TDR interpretations.
HVT’s Range of Syscompact Cable Fault Locators
| Syscompact 400 Portable |  |
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| Syscompact 400 |  |
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| Syscompact 4000 |  |
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