Paaf Peter Wouters

HV design of vacuum components

We discuss a number of practical implications from recent studies on HVDC design concepts for vacuum components. These studies dealt with microwave tube technology. The conclusions, however, are valid for a wide range of components. The goal of this work is to provide a scientific basis for the design of HVDC vacuum components. From a study of breakdown and emission mechanisms, and from the measured insulating performance of many different geometries, we have derived guidelines for the design of, for example, insulators and cables. It is further shown how conditioning procedures and operating conditions (operating pressure, insulator charging) should be reflected in the design. We discuss a number of practical implications regarding insulator design, conditioning, vacuum vs. air operation, HV cables in vacuum and potting. >

The effect of insulator charging on breakdown and conditioning

As part of a study on HV design concepts for microwave tubes, a number of different insulator designs have been studied. Analysis of the measured DC current, partial discharge activity and breakdown voltage shows that surface charging of insulators is a key mechanism in the breakdown process and in the conditioning process. Insulator parameters are not only the breakdown voltage, but also the conditioning speed and the sensitivity to gas exposure or charge leakage. In all these respects insulators with a field enhancement at the anode are superior. Field enhancements at the cathode are less harmful if stepped insulator shapes are chosen. Effective conditioning requires at least a limited number of breakdowns. With sufficient conditioning breakdowns, all insulator geometries tested reached a breakdown field exceeding 12 kV/mm. >

Remaining lifetime modeling of power transformers: individual assets and fleets

Uncertainty analysis is an essential part of models that estimate remaining lifetimes of power transformers. Uncertainty can be included in modeling the insulation aging process. If the uncertainty becomes too large, the insulation condition can, in principle, be determined by measuring the DP value, and more accurate projections can then be made. A drawback of the model presented here is the required input data, i.e., average values of dynamic load and ambient temperature, and maximum values, because the degradation rate is not a linear function of temperature. This information is not available for most transformers. However, even if the loading data are available only over times when the load approaches the rated value, a reliable estimate of the paper degradation can still be made. The observation that tap-changer failure is still the major transformer failure mode supports the simulated results presented above, which show that a transformer failure peak due to paper insulation failure is expected only after several decades of service. Although prediction of transformer performance may be considered speculative because of uncertainties in the model parameters and incomplete service data, it can serve as a tool to compare different maintenance and replacement strategies. Thus annual replacement strategies appear superior to equal loading because the latter provides no guidance on the order of replacement. It has also been shown that transformers should be replaced before the beginning of the failure wave and that variation in annual load growth between transformers leads to spreading of the replacement wave. A disadvantage of a distributed transformer loading is the reduction of the average remaining lifetime due to relatively fast degradation of the more heavily loaded transformers. Comparison of these strategies requires a technical reliability model, which is the foundation of risk-based asset management.

Forecasting Transformer Reliability

In this paper the concept of an integral transformer lifetime model is presented. The model provides the best possible prediction of future behaviour given the data available. It treats remaining life in terms of future failure probability, thereby giving better support to the decision taking process than a mere remaining life estimate. The core of the model is a generic description of ageing processes, coupled to a probabilistic approach. The approach presented utilizes various techniques to reduce the uncertainty that is inherent to modelling processes with incomplete knowledge of the operational history. One technique couples the process model to externally measurable quantities; another technique involves a sensitivity analysis, which shows what additional input data gives the most efficient way to improve accuracy. We will illustrate the approach by applying it to a well-known degradation process: thermal degradation of the transformer winding insulation.

Capacitive Current Interruption With Air-Break High Voltage Disconnectors

Capacitive current interruption with air-break disconnectors in a high-voltage network is an interactive event between the circuit and arc with a variety of interruptions and reignitions. In this contribution, first, a theoretical analysis related to this interaction is presented. The effect of capacitances at the source side (Cs ) and load side (Cl ) is investigated. Three distinct frequencies are identified as contributing to the voltage and current events in the circuit. Besides the power frequency quantities, a medium frequency transient arises related to the excursion of voltage across capacitances to the applied voltage, and a high-frequency transient arises due to charge redistribution between load- and source-side capacitance at reignition. Second, experimental results from an interruption measurement are studied in detail. Typical waveshapes of voltages across the capacitances, disconnector, and currents through the disconnector show that the transients during interrupted are in agreement with the theoretical analysis. Reignition voltage of the air gap and energy input to the arc on reignition are also studied. It is concluded that besides a higher interruption current and a higher power supply level, a lower Cs /Cl ratio leads to more severe interruption and longer arc duration. Finally, the actual status of IEC recommendations on testing, that has taken into account this arc-circuit interaction, is summarized.

Remaining lifetime modelling for replacement of power transformer populations

The age of the majority of power transformers applied in the western electricity network varies between 25 and 50 years. Depending on the load history and time of operation, replacement on short term is imminent. A technically sound policy concerning the replacement of these assets must be based on knowledge of (i) the life expectancy or reliability of individual components, (ii) how these failure probabilities cumulate to a replacement wave, and (iii) how to manage an expected replacement wave. The population reliability is obtained from individual transformer reliabilities using Arrhenius based modelling of paper insulation degradation. This modelling technique includes measures to cope with inherent uncertainties in available data. Population reliability figures are obtained using an adapted k-out-of-N failure model. The modelling method is applied to existing populations of power transformers in The Netherlands, to evaluate their expected replacement wave.

Propagation of PD pulses through ring-main-units and substations

Online partial discharge (PD) monitoring systems are traditionally installed at a single mediumvoltage (MV) cable connection between two ring-main-units (RMUs). It is more efficient to monitor two or more consecutive cables using a single monitoring system. Moreover, practical experience with the PD-OL system [1], shows that for substations, with many parallel MV cables, and RMUs installing the inductive sensor may be hampered or even impossible. In this paper the influence of RMUs and substations on the propagation of PDs is studied. An RMU or substation can be modeled as a combination of complex impedances representing switchgear, transformer and MV cables. A PD pulse from a cable encounters a load impedance that does not match the cables characteristic impedance, resulting in partial reflection and partial transmission transmission to other cables. Models for RMUs and substations are proposed and verified by measurements. Feasible options for online PD monitoring through RMUs or substations are determined.

Reliability estimation of paper insulated components

In view of the coming major asset replacement wave of power grid components, new lifetime prediction tools are developed to assist asset managers in making technically and economically sound decisions. This paper discusses the modeling technique based on the concept of quality parameters applied to paper insulated systems. The concept of quality parameters was recently introduced for power transformers to obtain a measurable quantity which relates the actual degradation process to an observable quantity. The present paper discusses a general approach for modeling other power components along the same lines. In case of a power transformer, oil samples can be taken and analyzed in order to estimate the DP (degree of polymerization) value, which is linked to the mechanical strength of the insulation material. This quantity then serves as quality parameter. Using a probabilistic approach based on the physical degradation mechanism a remaining lifetime model was developed. This resulted in a scheme, which is flexible in the sense that any new available data can be included to enhance the prediction reliability. The question addressed here is whether a similar approach can be applied to other power components as well. In particular we have investigated the possible use of quality parameters which are not directly correlated to one degradation mechanism. We will define different kinds of quality parameters and propose a component reliability model that incorporates all of these.

Influence of solar irradiation on power transformer thermal balance

In countries with a high ambient temperature and strong solar irradiation, transformer winding hot-spot temperature may increase over its maximum permissible limit. This can considerably reduce the insulation life of the transformer by enhanced degradation of the paper insulation. According to current loading guides, for each 6 K increase in working temperature, the ageing rate increases with approximately a factor two. Therefore, it is important to take into consideration the impact of the sun on the power transformer thermal behavior. In this paper, a modified hot-spot temperature model is presented to account for the effect of transformer winding temperature rise by solar irradiation. The effects of solar irradiation on transformer winding paper insulation are shown by comparing the degree of polymerisation (DP), the fault probability and the remaining life. Here, the fault probability is defined as the probability that the estimated DP-value at a certain moment in time is below a certain end-of-life criterion (threshold value). An additional winding hot-spot temperature rise of 9 K during the summer and a temperature rise of 6 K during the winter may occur in countries with strong solar irradiation. This may result in a reduction of the remaining lifetime by up to 40%.

Capacitive Current Interruption by HV Air-Break Disconnectors With High-Velocity Opening Auxiliary Contacts

Air-break disconnectors (DS) in a high-voltage network can interrupt small capacitive currents (e.g., occurring when disconnecting short unloaded transmission lines, current transformers, and so forth). The performance of a device with high-velocity opening auxiliary contacts attached to the DS enhancing the interrupting capability is presented. A series of laboratory experiments is carried out with the interrupted current range between 5 and 27 A. The experimental observations include voltage and current characteristics of the DS, and synchronous optical recording of the arc with a high-speed camera. Electrical and optical data are studied for fail and successful interruptions. The results show: 1) The short arcing time by the fast opening contacts limits the energy input into the arc, enhancing the probability of arc extinction; 2) the contribution to the arc energy mainly comes from the oscillations upon restrike. The energy input to the arc increases with higher breakdown voltage, larger air gap, and higher interrupted current; 3) the arc brightness is synchronous with arc current. With higher interrupted current, the overall arc is brighter and its remnants decay slower. Based on the results, potential successful approaches to increase the interruption capability are discussed.