DC test voltage should be applied from phase to ground on each conductor with other conductors, shields, and metallic sheath connected to ground or other conductors guarded with shield and metallic sheath grounded.
The test voltage values for DC hi-pot tests are based upon final factory test voltage, which is determined by the type and thickness of insulation, the size of conductors, the construction of cable, and applicable industry standards. The DC test values corresponding to AC factory proof test voltages specified by the industry standards are usually expressed in terms of the ratio of DC to AC voltage for each insulation system. This ratio is designated as K, which when multiplied by the acceptance test factor of 80% and maintenance factor of 60% yields the conversion factors to obtain the DC test voltages for hi-pot tests.
The IEEE standard 400.1–2007 lists the voltage values for conducting hi-pot acceptance and maintenance tests in the field for laminated shielded power cables, which are shown in Table below.
About leakage current the current is relatively high as a voltage is applied owing to capacitance charging current and dielectric absorption currents. As time passes, these transient currents become minimum with the steady-state current remaining, which is the actual leakage current and a very small amount of absorption current. At each step of voltage, the leakage current reading is taken before proceeding to the next step. Usually, it is recommended that at least eight equal steps of voltage be used and at least 1–4 min be allowed between each step. The leakage current versus voltage are then plotted as a curve. As long as this plotted curve is linear for each step, the insulation system is in good condition. At some value of step voltage, if the leakage current begins to increase noticeably, an increase in the slope of the curve will be noticed, as shown in Figure below.
Maximum leakage current allowable for new cables acceptance can be determined from the ICEA formula for minimum allowable insulation resistance discussed earlier. The formula for leakage current then can be written as follows:
IL is the conduction or leakage current
E is the test voltage impressed
K is the specifi c insulation resistance megohms per 1000 ft at 60°F
D is the diameter over insulation
d is the diameter over conductor
Traditional DC hi-pot testing was not only found to be ineffective in trying to diagnose the failure mechanism before cable failure occurred, but the presence of these elevated DC test voltages was also found to be potentially damaging to PE and XLPE service-aged cables. Whereas many PILC cables were lasting well over 50 years before being replaced, some of the originally installed PE/XLPE cables were experiencing failures within 10–12 years of their service life.
The research investigating the premature failure of extruded dielectric insulated cables pointed to water tress and partial discharges (PDs) in the void cavity of the insulation as the main cause of these cable failures. Water trees are tree-like structures which, through a process of electrophoresis, grow and mature in extruded cables. Water trees do not occur in laminated insulated cables because these laminated cables do not have cavity voids as the extruded insulated cables.
The extruded solid dielectric cables are susceptible to voids during manufacturing of these cables. After these cables are installed in the ground (i.e., in duct banks or direct buried), the voids over time will fill-up with water or water vapor. Therefore, water filled voids in the extruded insulation are referred to as water trees because these voids when examined under a microscope resemble like a tree, i.e., each void has a trunk and branches.
Research has shown that water treeing is the most important form of degradation that may afflict older XLPE and high-molecular weight PE-extruded cables. As a result, the phenomenon of water treeing has been studied extensively, including means by which the degree of water tree-induced degradation can be assessed. Water treeing can be described as a self-propagating dendritic pattern of electrooxidation, which reduces the AC and impulse breakdown strengths of extruded insulation and is the primary mechanism of degradation of extruded medium-voltage power cables. Although studied extensively, the initiation and growth mechanisms of water treeing are not clearly understood; they are not a single mechanism but complex interactions of chemical, electrical, and mechanical phenomena that depend on the material and applied stresses. The visible manifestation of water treeing is strings of water-filled microcavities. The water-filled microcavities are connected by electrooxidized tracks, which are usually less than 0.1 μm in diameter, which is too small to see.
Water trees do not generate partial discharges (PD) by themselves. However water trees can lead to electrical trees as a result of a lightning impulse, or applied AC voltage, or during fault locating activities, or during DC high voltage (HV) testing. The likelihood of causing a preexisting water tree to lead to an electrical tree may increase during a cable testing with high test voltages and the test duration. In general, electrical trees are more difficult to initiate than to grow, so that an electrical tree, once initiated, tends to grow to failure by PDs. Thus one can conclude that growing water trees do not generate PD signals, unless they give rise to an electrical tree. Any PDs at a water tree imply the existence of one or more electrical trees at that water tree. In order for water trees to grow in extruded insulated cables, four factors need to be present in extruded cable insulation. These four factors are electrical field, time, water in void cavity, and entry point into the cable. Water trees slowly migrate across the insulation, ultimately bridging adjacent voids across the insulation of the cable. Literally thousands of these trees grow to form electrooxidized channels which are extremely small in diameter.
Intuitively, as these water tree channels start to bridge the insulation, the losses dissipated through the insulation increases and thus lead to cable failure over time.
This loss can be determined by measuring the dissipation factor (DF).
Although other methods are available to determine the degree of water treeing in cables, the most widely used method is the measurement of DF (or PF) of the cable insulation. A perfect cable can be electrically modeled by a single capacitor. The longer the cable, the larger the capacitance of this capacitor. As water trees start to bridge the once-perfect cable insulation, this capacitor now starts to have some resistive (water tree) paths in parallel with it. The result is that the resistive loss component (in-phase component) of the total current loss increases which is measurable by measuring the DF or the PF of the cable.
The DF readings (measurements) can be compared with previous test measurements and trended to assess the cable health.