Partial discharge (PD) is an electric breakdown in the weak region of an extended insulating system, see figure 9.17.
An electric breakdown of the insulating material between two electrodes means generally that the distance between them has been bridged by electric charges (high ohm resistance has changed to low ohm resistance). Electric breakdown occurs if the following conditions are fulfilled:
• local electric field E in kV/mm is greater than the breakdown field strength Eb in kV/mm of the specific insulating material (different values for different insulating materials)
• starting electrons are available
For a homogeneous electric field (main insulation between the windings; see figure 9.17) the breakdown field strength Eb is defined as:
where:
Eb = breakdown field strength in kV/mm Vb = breakdown voltage in kV
d = distance between two electrodes in mm Note:
There are many places in the transformer insulating system, where the electric field is non-homogeneous, see figure 9.17.
At these locations, the calculated homogeneous electric field must be multiplied by the electrode shape factor to estimate the maximum electric field. When designing a transformer, the maximum electric field strength of critical locations must be calculated using a field program.
The following physical mechanisms describe the electric break-down between two electrodes [111] and [220]:
• generation of primary electrons to start the electric breakdown
Figure 9.17: PD source in insulating system of the transformer
PR = press ring BI = barrier insulation LV = low voltage HV = high voltage RW = regulation winding
9. Partial Discharge Measurements 9. Partial Discharge Measurements
Generation of primary electrons
Generation of primary electrons depends on:
• maximum electric field Emax
• electrode material (conducting, non-conducting)
• electrode shape (homogeneous or non-homogeneous electric field)
• electrode surface condition (uncovered, or covered with insulating material)
• smoothness of the surface (micro-tip electrodes)
The best condition for generating starting electrons occurs for an uncovered metallic tip electrode as shown in figure 9.18.
Starting electrons are generated by field emission from the surface if the magnitude of the local electric field E exceeds the field emission values of the specific material. PD activity appears as soon as the local inception electric field is reached.
Figure 9.18: Charge distribution (upper graph) and electric field distribution (lower graph) for negative tip electrode (upper figure, a) and positive tip electrode (lower figure b).
The most difficult location for generating a starting electron is a weak region without contact to the metallic electrodes, as shown in figure 9.19. In this situation the starting electrons are generated by radioactive ionization (cosmic, X-ray, etc.). Due to the lack of starting electrons, there is a delay before the PD-activity starts (up to several minutes), even if the local electric field exceeds the inception value.
Note:
For PD sources in the transformer insulating system the electrode material can be either an interface between different insulating materials (for example, solid insulation with a gas bubble) or a conducting material (for example copper covered with paper insulation).
E = electrical field [kV/cm]
x = distance between the electrodes [cm]
Charge multiplication and transportation
Charge multiplication and transport processes, see figure 9.20, are based on an avalanche mechanism described by the equation:
n = n0eαd where:
n = number of electrons at distance d d = distance between electrodes
n0 = number of available starting electrons eαd= electron avalanche
An electron avalanche can only cause an electric breakdown (bridging of the distance between electrodes) if the following condition is fulfilled:
eαd≈ 107 where:
α = factor which is a function of the local electric field E d = distance between the electrodes
Both physical processes described above take time to develop (breakdown delay), see figure 9.21. During this time the applied voltage (i.e. the local electric field) must be constant.
The limiting parameters for PD activity in the weak region of the insulating system are:
• local electric field E exceeds the design rating (α-factor)
• size of the weak region (bubble) is sufficient (distance between electrodes in the weak region)
• duration of the applied voltage (local electric field) is long enough to develop the discharge processes
Note:
In order to detect dangerous PD sources in the insulating system of the transformer, the test voltage must be applied long enough to fulfill the requirement for the described breakdown process.
Figure 9.19: PD defect without metallic electrodes
Figure 9.20: Development of electric breakdown
→E = local electric field E0 = electric field
ε = dielectric permitivity of the material
d = distance between electrodes
9. Partial Discharge Measurements 9. Partial Discharge Measurements
Charge storage
Charge storage mechanisms are important for the weak regions with no contact to conducting electrodes (bubbles in insulating system), see figure 9.22. Charge storage and de-trapping mechanisms strongly influence successive electric breakdowns in the weak region. The repetition rate of the PD impulses and the type of PD-pattern (PD-type PD-pattern type 5 in Table 3) are permanently changing.
The physical phenomena caused by the charge storage mechanism can be observed by the recorded PD patterns during a long duration test (several hours at a constant value of the test voltage). An increasing repetition rate in the PD impulses indicates continuous damage to the insulating system in the vicinity of the PD source.
Note:
This short introduction into the physics of partial discharge has shown that electric breakdown in insulating materials is strongly influenced by the statistical behavior of the discharge mechanisms.
To interpret PD results, it is necessary not only to consider the am-plitude of the apparent charge, but also to analyze the statistical behavior of the PD source. Statistical analysis of PD activity is performed using an advanced PD-system (Phase Resolving Partial Discharge Analyser, see clause A 9.5).