Investigation into pulse sequence analysis of PD features due to electrical tree growth in epoxy resin

The effect of DC bias on electrical tree growth characteristics in epoxy resin samples is investigated, using three waveforms types: AC, AC with positive DC bias, and AC with negative DC bias. Point-to-plane samples were used. AC tests resulted in 62% and 48% longer average time to breakdown than positive and negative DC biased tests respectively. The negative DC bias test had 14% longer average time to breakdown than positive DC bias test. It is suggested that this is due to space charge injection modifying the field at the tree tip. 4 stages of distinct tree growth are identified in AC tests compared to 3 stages in DC bias tests. In particular, the phenomena of trees growing in the ‘reverse direction’ (from the planar to the point electrode) observed in the latter stages of AC tests, is not seen in the DC tests reported here. This is thought to be due to the peak field magnitudes involved in each case.

The effect of DC bias on electrical tree growth and partial discharge (PD) characteristics in epoxy resin (Araldite LY® 5052/Aradur® HY 5052 supplied by Huntsman) is investigated using three waveforms: AC, AC with positive DC bias, and AC with negative DC bias. Needle-plane samples are used. Tree growth is shown to be accelerated by the combined effect of DC and 50 Hz AC, beyond the AC growth rate. Positive and negative DC biased tests result in 62% and 54% reductions in average time to breakdown, respectively. Different tree structures and stages of development are associated with different partial discharge characteristics, with thick dark tree branches associated with high PD magnitudes, whereas fine tree channel growth is linked with PD magnitudes below 1 pC. AC tests showed five distinct stages of tree growth compared to four stages seen in DC biased tests. In particular, trees growing in the ‘reverse direction’ from the planar to the point electrode, which is observed in the latter stages of AC tests, is not seen in the DC biased tests. It is concluded that for composite voltages, the AC component is the essential driver of tree growth but the DC component can accelerate propagation. AC noise may therefore compromise the reliability of insulation in HVDC networks.

The growth characteristics of electrical trees under DC and AC voltages are quite different. In order to investigate the influences of AC component in HVDC cable system on the electrical tree properties, the growth and discharge characteristics of electrical trees under AC-DC composite voltages were studied in this paper. The results showed that the AC component greatly accelerated the developing process of the electrical trees. The influences of the positive and negative DC bias voltages on the electrical tree growth properties were quite different. The growth rate under negative DC bias voltage was similar to that under pure AC voltage and pine-branch type electrical trees were more likely to form. In contrast, the growth rate of the electrical tree increased with the increase of the positive DC bias voltage and it was much faster than the negative DC biased one. More branch-like electrical trees would form under positive DC bias voltage. When the AC component decreased, the developing process of the electrical tree was fairly tough and it was easy to form bush-like electrical trees, which were the typical conducive electrical trees with fairly small discharges. Under the positive DC bias voltage, there was a fast re-growth process of electrical tree after the bush tree formed, and the positive DC bias voltage could promote the coming of the re-growth process. However, the detected partial discharge during this process was quite small. The test results indicated that it may pose a great threat to the safety of the cable insulation if there is a large AC component in the HVDC cable system.

In high-voltage (HV) insulation, electrical trees are an important degradation phenomenon strongly linked to partial discharge (PD) activity. Their initiation and development have attracted the attention of the research community and better understanding and characterization of the phenomenon are needed. They are very damaging and develop through the insulation material forming a discharge conduction path. Therefore, it is important to adequately measure and characterize tree growth before it can lead to complete failure of the system. In this paper, the Gaussian mixture model (GMM) has been applied to cluster and classify the different growth stages of electrical trees in epoxy resin insulation. First, tree growth experiments were conducted, and PD data captured from the initial to breakdown stage of the tree growth in epoxy resin insulation. Second, the GMM was applied to categorize the different electrical tree stages into clusters. The results show that PD dynamics vary with different stress voltages and tree growth stages. The electrical tree patterns with shorter breakdown times had identical clusters throughout the degradation stages. The breakdown time can be a key factor in determining the degradation levels of PD patterns emanating from trees in epoxy resin. This is important in order to determine the severity of electrical treeing degradation, and, therefore, to perform efficient asset management. The novelty of the work presented in this paper is that for the first time the GMM has been applied for electrical tree growth classification and the optimal values for the hyperparameters, i.e., the number of clusters and the appropriate covariance structure, have been determined for the different electrical tree clusters.

Modulation efficiency and mechanisms of repetitively pulsed streamer discharge in humid air are ambiguous with dramatic variations in free electron availability, residual ion mobility, enhanced heat release, etc, caused by water molecules intentionally supplemented or existing in the surrounding environment. The inception and propagation patterns of repetitively pulsed streamer discharge modulated by superimposed DC bias are experimentally investigated in the needle-plane electrode configuration. The inception voltage decreases due to negative ion drift under positive DC bias. The secondary streamer with a bright glowing cloud prolongs towards the plane electrode and the diameter decreases under positive DC bias. The primary streamer tends to propagate along the off-axis direction under negative DC bias. The number of applied pulses before breakdown decreases with the increase in positive DC bias and illustrates an insignificant dependence on the negative DC bias. The effect of air humidity is more pronounced than the DC bias. The streamer inception, propagation, and morphological transition are explained by residual space charge distributions and drift velocity.

Dielectric charging/discharging in the real-life electrostatic microelectromechanical system device is proposed to be evaluated by the capacitance-voltage response for an analogous metal-insulator-semiconductor (MIS) structure. An analytical model based on this approach has been established. In the experiment, the relaxation behaviors of trapped charges in silicon nitride of MIS structure have been systematically investigated. For both positive and negative dc bias polarities, fast and slow discharge stages were clearly observed in the early and late charge relaxation processes, respectively. The discharge ratio (DR) was found to depend on both the bias polarity and magnitude. It was shown that negative dc bias would cause a high hole injection level but low DR, whereas positive dc bias would lead to a low electron injection level but high DR. Moreover, the DR will increase with the dc bias voltage. It was further found that the hole relaxation reaches the steady state faster than the electron relaxation. To explain the experimental results, we pointed out that the traps close to the interface play an important role in dielectric charging and discharging process.

Online observing the growth of electrical trees in XLPE cable section and monitoring its PD characteristics have been conducted. With different voltage levels, the electrical trees had different characteristics of growth. Secondary voltage applied promoted the growth of bush-like electrical trees. The PD data during the growth of electrical trees had been analyzed and found that the trends of the magnitude and times of PD in a frequency cycle with time are similar to the growth of bush-like electrical trees, which can be used to evaluate the degree of development of electrical trees in XLPE insulation, especially for HV and EHV cables. Keywords—XLPE cable; electrical trees; secondary voltage; partial discharge

Pulse Sequence Analysis (PSA) was carried on PD data from electrical trees grown in flexible epoxy resins. The samples used for the electrical tree experiments were conditioned in environments with different relative humidities in the range 15-100% prior to the electrical tests with the corresponding moisture content in the samples between 0.1 and 6.9%. The electrical treeing experiments were carried out at different temperatures in the range 20-70°C. The details of PD dynamics during the electrical tree growth have been found to change significantly with temperature and absorbed moisture. In this paper, it is shown that PSA could be successfully used to discriminate between PD data from electrical trees with different shapes and runaway tree growth.

Electrical tree growth through polymeric insulation is a key degradation process in HV cables. Partial discharges (PDs) are considered to be the energy conversion path for the tree growth. Several methods have been employed to study PDs in electrical tree channels, including the phase resolved PD (PRPD) pattern and pulse sequence analysis (PSA). Our recent work proposed a new method of estimating the PD inception and extinction voltages in non-conductive tree channel based on the PD signals. Here a typical non-conductive tree in polyethylene is considered. Both PRPD and PSA have been employed. It is found that after the PD inception, the PD inception and extinction voltages decrease with the tree growth, and become stable before breakdown. It is also found that the decrease of PD inception and extinction voltages causes an increase in the average number of PDs per cycle, and the shrinking of the dV-dV plot. In turn, the average number of PDs per cycle, and the dV-dV plot can be used to monitor the change of PD inception and extinction voltage during the tree growth.

Electrical tree growth in epoxy resin under square waves superimposed with DC voltages is studied in this paper. Using the needle-plane configuration, a ±15 kV DC voltage superimposed on a 15 kV pk 50 Hz square wave was applied to grow electrical trees. A standard 50 Hz sinusoidal source was also used for comparison. A CCD camera imaged the electrical tree growth during testing and associated partial discharges were recorded. It was observed that the tree shape under 15 kV square-wave voltages combined with positive or negative 15 kV DC voltages are branch-like, whereas bush-branch trees grow under 15 kV bipolar square-wave voltages. Positive unipolar square-wave voltages resulted in faster growth than negative unipolar square waves. The different tree structures were also associated with distinct characteristic PD activity.

The international thermonuclear experimental reactor (ITER) project has attracted much attention in the world and its major part, namely the superconducting magnet system, uses epoxy resin as the basic insulation material. Epoxy resin needs to address the challenge of the liquid nitrogen temperature and the pulse voltage with changing duration due to the special operating environment. This paper investigates the effect of pulse duration on the characteristics of electrical tree growth in epoxy resin under low temperature. The tested samples were stressed with different pulse durations in a needle-plate geometry electrode system. The pulse duration was set to 50, 110 and 220 µs and the experimental temperature was −30, −90 and −196 °C. Fractal dimension, accumulated damage and expansion coefficient (D/L) are employed to characterize the electrical tree. The experimental results indicate that the typical structures of electrical tree are obviously different with the variations of low temperature and pulse duration. It is revealed that the increase of pulse duration promotes the growth of electrical tree under the same low temperature. In addition, larger pulse duration may lead to a more complex tree structure, which indicates the larger value of fractal dimension and accumulated damage. Meanwhile, obtained results show that large pulse duration plays an important role in promoting the electrical tree's growth processes, including propagation and breakdown characteristics.

Electrical treeing is the main degradation mechanism in high voltage polymeric insulation, that leads to power system plant failure and the loss of electricity supply. Electrical trees grow under partial discharge (PD) activity, which can be measured and analyzed to understand and characterize electrical tree growth. In this work, PD measurements were analyzed for electrical trees grown in epoxy resin needle-plane samples under very low frequency (VLF, 0.1 Hz) voltage excitation. VLF is interesting as it is used for testing power cables and other high capacitance insulation loads. However, more experience and new methods are needed for PD interpretation. PDs were studied using two tools: pulse sequence analysis (PSA) and nonlinear time series analysis (NLTSA) from dynamic system theory. PSA was treated here as a particular case of NLTSA since their constructions are similar in their mathematical treatment. The experimental results showed that electrical trees grown at VLF had branch-type structure and times to breakdown about fifty times larger than samples aged at industrial frequency. PSA plots were compared with 2D projections of state-space trajectories that represent the dynamics of the nonlinear system (NLTSA approach). In terms of graphical representation, NLTSA 2D projections generated more clusters than the PSA plots, thus, it was interpreted that NLTSA revealed more details about the nonlinear dynamic system associated with electrical tree growth. On the other hand, using the NLTSA approach, the correlation dimension was estimated to characterize the electrical tree growth. The results showed a different evolution obtained for VLF excitation compared to the results reported for test samples aged at industrial frequency in other studies.

This experimental study sought to investigate the influence of interfaces and ventilated channels on electrical tree growth in epoxy resin. The electrical trees were developed in point-plane geometry samples and tested within the voltage range of 6 - 15 kV rms. Vented channels of diameters in the range 0.16 - 0.30 mm were employed. The distance to the grounded plane from both the epoxy resin interface and the ventilated channels varied between 0.5 - 1.5 mm. The time to breakdown is increased when the location of the interface and channels is centred near the insulation gap or is closer to the plane surface, but decreases when the interface is closer to the needle. A narrower breakdown path is observed when the interface layer is present.

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

Electric trees have been the main types of cable defects that lead to cable insulation failure and power transmission interruption. The electrical tree repair technology can reshape the cable insulation performance to a certain extent and avoid the characteristics of power transmission interruption, which makes it widely used in the future. Existing research mainly focuses on repairing electric tree in the way of pre-injection of insulation material in the early stage of the cable, which cannot achieve the suppression or repair of electric tree in the cable. It is of high research value to explore new and reliable methods for suppressing electrical trees in transport cables from the perspective of repair engineering technology. This paper first sets up a cable accelerated aging experiment platform, prepares two sets of dry electric tree and wet electric tree samples, and sets up a cable repair platform in the laboratory environment to inject nanocomposite repair fluid. Comparing the characteristic parameters of each electric tree sample before and after the injection of repair liquid, the influence law of nano-composite repair liquid injection on the growth of the electric trees in the cable was analyzed. The results show that: after injecting the repair liquid, the growth of the electric tree was suppressed and the maximum discharge was reduced, and the suppression effect of the wet electric tree was the most obvious; According to the measurement characteristics, it was found that the initial discharge voltage of the electric tree sample injected with the composite repair solution was too large, the maximum discharge amount was small, but neither could reach the insulation level of the new sample. That was to say, the water tree repair liquid has an inhibitory effect on the growth of the electric tree, and the nano-composite repair liquid has a more obvious inhibitory effect on the electric tree. Finally, the mechanism of the inhibition effect of nano-composite repair liquid on the growth of electrical trees in different states is summarized.