Faults in cables can arise due to any defect, inconsistency, weakness, or non-homogeneity that affects the performance of the cable. Typically, faults are classified as:
- Low Resistive (short circuit): Damaged insulation leads to a low resistive connection, or short circuit, of two or more conductors at the fault location.
- Ground fault (short circuit to ground): Similar to a short circuit fault, a low resistive connection is caused to ground.
- Cable breaks: Mechanical damage or ground movements while digging can lead to the breaking of individual or multiple conductors leading to a high resistive fault.
- Intermittent faults: Sometimes faults are not constant and only occasionally happen depending on the load on the cable. An example could be the drying out of areas in laminated (oil insulated) cables with a low load or the presence of partial discharges in extruded cables.
- Sheath faults: Damage to the outer sheath of cables does not always lead to faults directly, but can cause a cable fault in the long-term, as the result of moisture penetration and insulation damage.
Depending on the type of cable fault, the voltage level at which the fault occurs, the design of the cable system, the surrounding area of the faulted cable (direct buried, conduit, overhead, etc.), and other factors, a variety of measurement methods can be employed. This blog post will briefly examine the most commonly used methods and the equipment that HV TECHNOLOGIES, Inc. supplies from BAUR to most efficiently and effectively locate the problematic cable and the type of fault causing the adverse performance or outage. The methods will be divided into fault analysis, pre-location, and pin-pointing.
Before starting a fault location procedure, one must first ascertain the fault characteristics in order to determine the best method(s) and procedure(s), and also the voltages to apply.
Insulation Resistance Measurement:
The resistance of the fault must be determined in order to know whether the fault is low resistive or high resistive. When conducting this all cable phases should be measured to neutral and to phase to determine all resistance values (6 measurement results in a 3-phase cable). As a rule of thumb, low resistive faults are considered to have a resistance value below 200 Ohm and high resistive or intermittent faults are above 200 Ohm.
If the fault is considered to be high resistive or intermittent, a DC voltage can be applied to the cable to determine at which voltage the fault breaks down. This breakdown voltage should be noted, as it serves as the minimum voltage value for the fault location procedure when a surge generator or thumper is applied for pre-location of the fault. The breakdown voltage of sheath faults can also be determined by applying a DC voltage to the cable sheath.
Following analysis of the fault type, pre-location is conducted to determine the fault position as precisely as possible so that the subsequent fault pin-pointing can be conducted as briefly and efficiently as possible. Generally, pre-location methods will locate the cable fault with an accuracy of approx. 1% of the cable length, letting the user know in which general vicinity the fault is located.
Time Domain Reflectometry (TDR):
The TDR method is the most established and widely used measuring method for establishing the total length of a cable and the distance of low resistive faults, cable interruptions, and the location of joints along a cable. When a low voltage pulse is sent into a cable that has a parallel path of two conductors, reflections will be seen at points along the cable that exhibit different impedances.
Fault Conditioning / Fault Burning
High resistive cable faults can be treated using powerful high-voltage burner units that supply very high current to dry and carbonize the fault and subsequently convert it to a low resistive fault. Afterwards it can easily be measured using the TDR method. Repetitive “thumping” of the cable can also condition the fault to become low resistive, as during this time a series of high voltage surges are causing many breakdowns of the fault.
Secondary Impulse Method / Multiple Impulse Method (SIM/MIM):
SIM/MIM is also known as surge arc reflection and is based upon a surge generator or thumper being coupled together with a TDR. A high voltage impulse is sent down the cable causing the fault to break down and temporarily transforms a high resistive fault into a low resistive fault, which can be detected by a TDR signal to measure the fault distance. Fault distance evaluation is conducted fully automatically.
Impulse Current Method (ICM):
ICM is the conventional location method for high resistive cable faults in very long cables. A surge generator/thumper is coupled to a TDR via an inductive coupler. The breakdown in the fault generates a current impulse, which travels along the cable sheath between the surge generator/thumper and the cable fault causing reflections that are detected by the TDR.
In certain cables the breakdown voltage of the fault may be higher than the rated output of the surge generator/thumper (> 32 kV for Syscompact 2000 and Syscompact 4000). In this case a VLF or DC source with higher voltage output needs to be used as the high voltage source.
The decay method is based on voltage decoupling by a capacitive voltage divider. The faulted cable is charged by applying high voltage VLF or DC up to the breakdown voltage. The cable acts as a capacitor, stores the energy, and once breakdown is achieved, a transient wave is created that travels between the cable fault and the high voltage source. The transient wave is recorded by a capacitively coupled TDR and the recorded period of oscillation is equal to the fault distance.
Compared to the ICM method above, the decay method is based on a transient voltage wave that is continuously recorded by the capacitive coupler.
The sequence below describes the steps during the decay method process. The cable is charged with negative voltage and the flash over at the cable fault creates a positive discharge transient wave that travels towards the near end of the cable. At the high voltage source, the pulse is reflected without a polarity change. Once the pulse travels back to the cable fault, the pulse is reflected and the polarity is changed. This process is repeated until the pulse dampens and loses energy.
Differential Impulse Current Method / Differential Decay Method:
The differential ICM or decay methods can be used for cable faults that are very difficult to locate, such as in very long cables, in a T-branch network, or in overhead transmission lines. For this method, two cables are required for the pre-location process – the faulted cable and a healthy auxiliary cable. The connection is similar to what is described above for ICM (< 32 kV breakdown voltage) and Decay methods (> 32 kV breakdown voltage), except that a 3-phase surge coil SK 3D is used as a coupler.
In a first step, the HV impulse is released into the healthy cable and faulted cable simultaneously, giving a first differential picture. Second, a linking bride is connected at the far end of the two cables. This then extends the effective length of the healthy cable from the far end to the cable fault. As this reflection characteristic is different compared to the open-end measurement in the first step, the impulse is reflected differently, whereas the reflection in the cable fault remains the same.
When laying both graphs on top of each other, the deviation point caused by the influence of the extension of the healthy cable, shows the fault distance from the end of the cable.
Cable Sheath Fault Location (Bridge Measurement):
When a fault occurs between two defined cores or parallel wires the theory of time domain reflectometry (TDR) can be applied, as is the case for all the pre-location methods described above. However, certain cable structures can allow faults to happen from a core to the outside ground or soil, such as in unshielded cables or when there is damage to the outer sheath of a cable. Cable sheath faults will not immediately cause cable failures, but over time the performance of the cable can be compromised because water can penetrate into the cable causing corrosion and the growth of possible water trees. For these types of faults measuring bridge techniques must be employed.
The measuring bridge techniques of Murray and Glaser are based upon the principle of the Wheatstone bridge, in which resistances of different cable sections are being balanced to find the location of the fault. The measuring circuit according to Murray requires an additional healthy core with the same diameter and conductor material as the faulted core. A linking bridge is connected at the far end of the cable and the bridge is then balanced to determine the distance of the fault.
The measuring circuit according to Glaser requires two auxiliary cores of equal cross section and material, but may be different than the faulted core. Two linking bridges are connected at the far end of the cable to the faulted core. The advantage of this set up is that the forward path defined via the two auxiliary lines is compensated.
Fault pre-location gives the user the general vicinity of the cable fault, in some cases even up to 1% of the total cable length. However, it is impossible to detect or recognize all deviations within a cable route in the ground, and for this reason, it is highly important to pin-point the cable fault in order to determine its exact location.
A successful cable fault location relies on knowing the position of the cable and other lines laid in the soil. If the exact route of the underground cable is not known magnetic frequency procedures can be used to establish the position and depth of the cable using the minimum or maximum method.
An audio frequency generator can be connected to a healthy phase of the faulted cable via galvanic connection, inductive connection with a clip-on CT clamp, or inductive connection with a frame antenna, as shown in the below image. Galvanic connection is considered the best method because the best signal values can be obtained. However, an inductive connection may be necessary in areas or circumstances in which a galvanic connection is not possible, such as when route tracing on live cables would like to be conducted. For galvanic coupling a lower frequency is advisable to minimize inductive coupling into other cables. A higher coupling effect is required when inductively coupling a signal, and therefore a higher frequency should be chosen.
When tracing the cable using an audio frequency receiver and detecting rod, the electromagnetic signal transmitted via the audio frequency generator can be measured. Depending on the direction of the coil in the detecting rod, the signal can be coupled differently. With the Minimum Method the detecting coil is vertical to the path of the cable and minimum signal is obtained when directly over the cable. For the Maximum Method the detecting coil is horizontal to the path of the cable and maximum signal is obtained when directly over the cable.
The Minimum Method can also be used to determine the depth of the cable. The distance between the minimum signal at 0° (directly over cable) and the minimum signal obtained when flipping the detecting coil to 45° is equal to the depth of the cable.
Acoustic Fault Pin-Pointing:
The acoustic fault location method is used for pin-pointing of high resistive or intermittent faults in buried cables in which the cable is “thumped”, i.e., a series of high voltage surge pulses are sent down the cable causing the fault is break down. During a flashover an audible acoustic signal is generated that can be detected on the ground surface by using a ground microphone, receiver, and headphone. The closer the distance to the fault, the higher the amplitude of the flashover sound.
The flashover at the fault location also produces an electromagnetic signal that can be detected with the ground microphone. In some instances, such as when the cable is situated in a conduit, the strongest acoustic signal may not be above the fault. By measuring the electromagnetic signal, a distance calculation can be conducted and the receiver will guide a user to the fault location. The propagation time of the electromagnetic signal is not affected by where the fault is located. This is similar to the differences in light and sound propagation in a thunder storm, as you will always see the lightning first before you hear it.
Step Voltage Fault Pin-Pointing:
Cable sheath faults or short circuit faults to ground do not allow for a flashover when thumping a cable, and therefore, acoustic fault pin-pointing cannot be applied. In this case, a sequence of voltage impulses (step voltages) is sent into the faulted cable, which produces a voltage drop to ground. The voltage drop results in a voltage gradient, which can be measured with the use of two earth probes above ground. When walking towards the fault, an increasing voltage should be detected and immediately over the fault a detectable polarity change will be measured and the resulting voltage will be zero when the earth probes are placed symmetrically above the fault.
Twist Field Location
The Twist Field Location Method patented by BAUR is successfully employed in signal and multi-core cable installations that contain a low resistive fault. A high frequency audio signal with sufficient current (> 8 A) is sent into the faulted cable and returns after reaching the fault position. Despite reverse current directions, a magnetic field is produced and using a detecting rod the maximum and minimum of the signal can be detected due to the twist or steady change of geometrical position of the cores in the cable. Since the audio signal returns at the fault position, the position at which no signal can be detected can be determined as the cable fault. The twist field method can also be used to detect cable joints, as the twisted field is interrupted according to the length of the joint. Since the twist field is always in the direction of the fault, this method is a major advantage when pin-pointing low resistive faults in T-branched networks. All healthy cable branches would give a continuous low signal.
Once the cable fault has been pin-pointed and uncovered, subsequent repair work has to be conducted in order to place the cable back into service. If a violent cable fault has occurred and is visible, then it is fairly easy to distinguish which cable needs to be repaired. However, in other circumstances, especially when multiple cables are bundled together, the correct cable has to be identified first to reduce the chances of cutting a section of healthy cable that does not require any repair.
Cable identification is conducted by connecting a transmitter onto the suspected faulted cable, either galvanically or inductively. The transmitter contains a capacitor that is charged and then discharged into the cable. A flexible coupler (Rogowski coil) is then used to measure the current pulse in the target cable. The computer-supported APT (Amplitude – Phase – Time) procedure developed by BAUR is the most reliable method of distinguishing one core from the other, which analyzes the direction, amplitude, and phase synchronization of the induced pulse.
The signal transmitted into the cable is considered to have 100% amplitude. In a 3-phase cable system, or when other cables are in the vicinity, a percentage of the signal will return through the shield of the cable the signal is being injected into and another percentage of the signal amplitude will return through the core and cable shield in each of the other phases, as the signal will flow through ground to the other cables. This results in the highest percentage positive signal on the cable the signal is being injected into and lower percentage negative signals in the other cables. Furthermore, the receiver is synchronized with the time interval of released signals into the cable from the transmitter. As a result, identification of the correct cable is simply the detection of the largest amplitude and the correct (positive) phase.