H.J. van Breen

Standardization of on-line VHF PD measurements on turbo generators

Based on field experiences on a number of different turbo-generators in the Netherlands, several fundamental and applied aspects of on-line very high frequency (VHF) partial discharge (PD) diagnostics are discussed. In particular the systematic results presented in this paper contribute to: 1. the characteristics of different on-line detection using suitable VHF sensors, 2. the methods to suppress disturbances and to distinguish between different types of discharges and 3. the interpretation of measured results to obtain knowledge about the insulation condition.

The importance of generator operating conditions on PD data evaluation

This paper summarizes the results of a preliminary study on the influence of generator operating conditions on its partial discharge (PD) activity. This study covers results from the field measurements as well as from laboratory investigations. The turbo generator discussed in this paper showed an extra PD source at high temperature. This extra PD source was activated at high temperatures and vanished at lower temperatures. The activation and the de-activation process had a clear hysteresis character. Both the measurements on the turbo generator and in the laboratory showed that there is certain dependence between the internal PD and the actual temperature of the stator coil. Due to the fact that in practice the moment at which a periodic measurement is performed does not always represent the PD activity of a generator under all its operating conditions, it is of practical interest to investigate the dependences between the power, temperature and the actual PD activity. A full investigation into the relation between generator operating conditions and its PD activity is justified.

Partial discharge detection in generator stator insulation using oscillating voltage waves

In this paper, preliminary results of testing generator stator insulation using PD detection at oscillating voltage waves are presented. Based on laboratory investigation, on a full scale stator insulation system, a comparison of PD behavior: the PD inception conditions and PD levels at 50 Hz AC stresses and at oscillating voltages has been performed. The usefulness of PD detection at oscillating voltage waves has been discussed.

Localizing the source of partial discharges in large generators

This paper presents the preliminary results of a study to localize partial discharges in the stator of a generator. This is achieved by measuring the voltage between the stators core and frame on several locations. Based on measurements done on a 23 MVA stator two algorithms were tested: the delay time technique and the top-top technique. The delay time technique assumes it takes time for the signal to travel through the stator. The top-top technique assumes the amplitude of the partial discharge is larger near its origin. The test results reported in this paper show that the delay time technique works better than the top-top technique.

Recovery voltage measurements on XLPE cables

The dielectric response provides information on the physical and chemical structure of a material. For aging processes that affect physically and/or chemically insulating materials, the change in polarization phenomena can be exploited to monitor insulation degradation. One economic way for estimation of the dielectric response are recovery voltage measurements (RVM). Originally, recovery voltage measurements have been used to assess the depolymerization of the paper insulation in transformers. In this paper, it is discussed to what extent RVM can be applied to evaluate the condition of XLPE cables. An adoption of the common RVM test circuit is presented in order to ensure reproducible and meaningful results. Two models have been developed to explain the RVM spectra obtained from unaged cables and a cable containing water trees.

Partial discharge diagnosis of stator insulation using damped AC voltages

In searching process for new, advanced technologies which could be applied for on-site diagnostics of machine insulation this paper discuses the application of damped AC voltages to detect and to analyze the PD processes. Based on laboratory investigation on a full scale stator insulation system, as well as during field inspections on large motors, a comparison of PD behavior: the PD inception conditions and PD levels at 50 Hz AC stresses and damped AC stresses has been performed. The applicability of PD detection at damped AC voltage has been discussed

Several aspects of stator insulation condition based maintenance

The objective of power plant maintenance is to reduce the probability that an unforeseen forced outage takes place. Balancing maintenance with reliability is a key issue for the economical performance of a power plant. The paper shows that the turbo generator is the third cause for forced outages hours in a power plant. It will zoom into the generator failure statistics and show that the HV insulation system is the root cause for generator failure outage hours. Also the paper provides insight into the benefit of condition based maintenance (CBM) on the stator insulation system of generators. These benefits include: steering of insulation reliability, lengthening economical lifetime of the machine and decrease maintenance cost. A third topic of the paper is decision process from condition assessment to maintenance. This process is currently in use in some form or other at most power plants. Measurements are performed, interpreted and maintenance decisions are taken. However, this decision process is often not applied to its maximum efficiency. The main reason for this decreased efficiency is the problem of providing reliability numbers on the insulation system. The challenge of tomorrow would be to generate failure probability numbers based on measurements performed on machine insulation.

Detection of water trees in medium voltage cables by RVM, without reference measurements

Measurement of the polarization spectrum by Recovery Voltage Measurement (RVM) is one among several methods, which are proposed in the literature to characterize the state of deterioration of water-treed cable insulation. It has previously been shown that it is possible to use RVM for detection of water trees in PE and XLPE cables. In this paper the authors present a methodology that allows the detection of water trees at low DC test voltage without the need for a reference measurement. This methodology makes use of a so called division spectrum, which is obtained from the results of two separate recordings of the polarization spectrum.

Detection of water trees in medium voltage XLPE cables by return voltage measurements

Several methods are proposed in literature to characterize the state of deterioration of water-treed cable insulation. One of these methods is the return voltage measurement (RVM) method, which was initially introduced as a diagnostic method for oil-paper insulating systems. It is mostly used to determine the condition of the paper insulation of transformer windings. It has been shown that it is possible to use RVM for detection of water trees in XLPE cables. In this paper, a methodology is presented that allows the detection of water trees in PE and XLPE insulated cables, without the need for a reference measurement. Also, the influence of temperature on RVM measurements was examined, for cables with and without water trees.

Experience with on-line PD measurements on generators in frequency and time domain

This paper presents a method to perform online time domain partial discharge measurements in a high noise and disturbance environment. The method combines the sensitivity of an online VHF PD (very high frequency partial discharge) measurement with the power of a time domain measurement.. Two measurements on running turbogenerators are presented: one on an 150 MVA generator without disturbances and. one on an 650 MVA turbo generator in a high disturbance environment. The paper shows that it is possible to detect and record partial discharges in an environment with disturbance pulses over 1000 times their size. Using frequency analysis a signal to noise ratio can be obtained, which can be used to determine the optimum measuring frequencies for online VHF PD measurements. Using typical time and frequency wave shape parameter extraction it is possible to separate the recorded pulses into PDs and disturbances.