Over-voltage pressure testing
High Potential (HiPot) cable system testing can be performed using DC, AC or Very Low Frequency AC (also known as VLF) test voltage. Such testing can be conducted as an acceptance test for new installations or as a maintenance test for existing installations. HiPot testing involves applying an over-voltage to the cable system for a short duration to verify the dielectric integrity of the system (cable, splices, and terminations). In most cases, HiPot testing is applied as a pass/fail or go/no-go test. If the cable system does not fail during the test, it is considered to have passed the test and can be placed back into service.
DC Hipot Testing
A DC HiPot unit is relatively easy to control, provides accurate leakage current data and is relatively small/lightweight. Current standards allow the use of DC HiPot testing for acceptance testing of cables with extruded (e.g. cross-linked polyethylene [XLP] or ethylene propylene rubber [EPR] insulation or laminated construction (such as paper insulated lead covered [PILC]).
But most industry standards, especially the IEEE 400 standard, no longer recommend DC HiPot testing for maintenance testing of field aged XLP or EPR cables. The standard states that DC HiPot testing of field aged XLP and EPR cables may not provide meaningful information, and in fact may cause damage.
Cable fault location and diagnostics
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 occur 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.
Cable sheath fault location and repairs
A common cause of cable faults is damage to the plastic casing (referred to as the cable sheath). Such damage permits water ingress to the cable, creating “Water Trees” and other corrosion-based damage within the cable. Water trees are one of the primary reasons for cable faults, and the ability to identify these faults is a vital step in the cable fault testing process.
Locating cable sheath faults is one of the last steps in cable testing. This only occurs after you have identified the existence of a cable fault via the identification procedure based on your specific site variables and the type of system you are working on.
Following cable fault identification, a pre-location procedure estimates the location of the fault. Cable sheath fault location is the final stage in this process, pinpointing the flaw on site. Faults can be identified by ground fault locators, which pass a test current along the cable and measuring the voltage gradient at the point it exits to ground.
There are inherent dangers to a cable sheath fault, which are elevated in a high voltage scenario. If the current leaks to ground, it is possible to complete a circuit with a secondary cable nearby causing collateral damage.
Very low frequency (VLF) tan delta testing is a precise and non-destructive mehod to provide information on the extent of ageing in cable insulation. The test applies an AC sinusoidal waveform at 0.1 Hz frequency and measures the degree of real power dissipation (or losses) in a dielectric material. Knowledge and understanding of the cable condition can help to reduce unnecessary outages and to assist in condition-based and cost-optimized maintenance programs.
Partial discharge testing and location
Partial discharge (PD) phenomena are local dielectric breakdowns of a small portion of a solid or liquid electrical insulation that is subjected to high voltage stress. PD often precedes insulation breakdown in power cables and cable accessories, such as joints and terminations, which can result in cost-intensive repairs and possibly prolonged outages.
PD can be the result of internal weak spots in power cables, such as voids, cracks or particles. It is also caused by damage to the outer semi-conductive layer of cables or to joints and terminations during installation.
Partial discharge measurement is a critical criterion for inspecting the quality of power cables and cable accessories during on-site cable commissioning.
Cable tracing and identification
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.