P.C.J.M. van der Wielen

Accurate estimation of the time-of-arrival of partial discharge pulses in cable systems in service

Accurate location of the origins of partial discharges in power cable systems, based on arrival times, is imperative for the identification and assessment of defects. This paper evaluates different time-of-arrival algorithms in order to determine which method yields most accurate location under different circumstances. These methods are based on trigger level, Akaike Information Criterion, energy criterion, Gabors signal centroid and phase in frequency domain. Several criteria are defined by which the algorithms are evaluated. These criteria include the sensitivity to noise, pulse shape and effect of load impedance. The sensitivity of the methods upon varying these quantities is evaluated analytically and by means of simulations. Further, the methods are tested on a medium-voltage cable system by injecting PD pulses in a cable with one joint. Each algorithm is applied to the measured pulses and the resulting location is compared with the known joint location. From the results the energy criterion method and the phase method show the best performance.

Fundamental aspects of excitation and propagation of on-line partial discharge signals in three-phase medium voltage cable systems

On-line partial discharge (PD) detection of three-phase belted medium voltage cable connections results in a number of interpretation differences as compared to off-line measurements where only one phase is energized. The induced currents and charges in the phase conductors and earth screen upon a PD not only depend on the discharge site, but also become phase angle dependent. Furthermore, simulations show that the PD distribution itself varies with the amount of eccentricity of the rotating electric field and may differ from the off-line distributions obtained with a linear field. Finally, the PD propagation in a multi-conductor cable also alters the signals measured at the cable terminals. In this paper, induced charges and PD distributions are studied by means of computer simulation. The cable propagation characteristics are verified by measurements.

Sensors for on-line PD detection in MV power cables and their locations in substations

For developing a system capable of on-line detecting and localizing PDs in MV power cables, the choice of the sensor type is crucial. Sensors for detecting PDs can be divided into two main groups: capacitive and inductive sensors. In this paper, the advantages and disadvantages of these two types of sensors are discussed for on-line application. In multi-conductor cables, e.g. a three-phase belted cable, it is essential to distinguish the different propagation channels. When the cable is under normal operating conditions, all three the phases are energized simultaneously, implying that the PD pulses are propagating through two distinctive propagation channels: the Phase-to-Phase (PP) channel and the Shield-to-Phase (SP) channel. Measuring sensors can therefore also be subdivided with respect to the detected channel. If sensors can detect signals in all three conductors separately, both SP and PP channels are obtained. In this paper different positions in a substation for placing sensors are evaluated, with respect to the measured propagation channels, signal-and interference sensitivity, safety and practical applicability.

Influence of Ring Main Units and Substations on Online Partial-Discharge Detection and Location in Medium-Voltage Cable Networks

Online partial-discharge (PD) monitoring of medium-voltage power cables is traditionally performed on a cable between two consecutive ring main units (RMUs). PD location is achieved using a sensor at each cable end. Monitoring multiple consecutive cables, with RMUs in between, using a single monitoring system is more efficient. In addition, substations and RMUs without possibilities for sensor installation can be circumvented by installing the sensor at the next RMU. This paper studies how RMUs and substations along the cable under test affect online monitoring, including their influence on detection sensitivity, location accuracy, and charge estimate accuracy. Single-sided PD measurement, including PD location, with an RMU or substation at the far end is also considered. Models for RMUs and substations are proposed and verified by measurements. The performance of online PD monitoring is studied for a number of network configurations.

Partial discharge parameters to evaluate the insulation condition of on-line located defects in medium voltage cable networks

Partial discharge analysis has a long record as reliable diagnostic tool to assess the integrity and the quality of electrical insulation of power systems. On-line partial discharge monitoring systems for live medium voltage cable connections have recently been introduced in Dutch grids and also a few worldwide. To enhance the applicability of on-line cable diagnostics there is a need to develop a PD interpretation approach suitable for on-line monitoring. This paper discusses various PD parameters, including PD charge magnitude, PD occurrence rate and PD charge density and their related statistical values such as mean and maximum, used to interpret the PD activity. Trend watching of these parameters is employed to study the degradation state of cable connections and their components. The approach is applied to field data obtained over a year through monitoring several live circuits. In this paper, three examples are presented. Weak spots were detected and failures were prevented by early warning.

Synchronization of on-line PD detection and localization setups using pulse injection

The development of an on-line PD detection system has its main challenges in the area of sensors and signal processing techniques. If localization is included by means of simultaneous signal detection at both cable ends, the problem arises of synchronizing the measuring systems at both sites accurately. One method, already successfully applied in off-line PD measurements, is the use of GPS. Besides its high costs, this system has one main disadvantage: it requires an external antenna in the line of sight of several satellites. This requirement may be a problem for on-line measurements because of the long measuring time. Another option to perform the synchronization is the injection of high-frequency pulses into one cable end and detecting them at the other end. Taking the propagation time along the cable length into account, this results in accurate synchronization of the measuring sides, without being dependent on any other system. This paper discusses several aspects of the pulse injection technique. The obtained resolution realized during field experiments with the method of pulse injection is within the resolution obtainable by means of GPS (100 ns).

Approximation of transmission line parameters of single-core and three-core XLPE cables

A transmission line model of a power cable is required for the analysis of the behavior of high-frequency phenomena, such as partial discharges, lightning impulses and switching transients, in cables. A transmission line is characterized by its characteristic impedance, attenuation coefficient and propagation velocity. The semiconducting layers in an XLPE cable have a significant influence on these parameters. Unfortunately, the dielectric properties of these layers are usually unknown and can differ between similar types of cables. In this paper it is shown that nevertheless the characteristic impedance and propagation velocity of single-core and three-core XLPE cables can be estimated using available information from the cable specifications. The estimated values are validated using pulse response measurements on cable samples.

Effect of cable load impedance on coupling schemes for MV power line communication

Coupling of carrier wave frequencies up to 95 kHz (within the European CENELEC A-band) for online diagnostic data transfer in medium voltage cables is studied. Inductive and capacitive signal coupling is considered not only on basis of technical performance, but also on basis of practical aspects. The effectiveness of coupling schemes depends on the impedances of substation equipment at the cable terminals. The frequency response of a 10-kV, 400-kVA three-phase cast resin distribution transformer is investigated. In the frequency range of interest, the behavior is well described by a capacitance of typically 1 nF. The signal transfer over a 4-km paper cable, terminated by various load impedances to mimic real equipment is studied. From the results it is concluded that for inductive coupling performance within the CENELEC A-band may be sufficient, except for substations at the end of a grid. Transferring signals containing frequencies up to several megahertz, which is already required for synchronization of partial discharge detection and location equipment, is feasible under all conditions. Measurements on life substations indicate that up to these frequencies substation components can still be accurately modeled as lumped circuit impedances.In this work coupling of communication frequencies within the CENELEC A-band to medium voltage cables is studied. Inductive and capacitive signal coupling is considered not only on basis of technical performance, but also on basis of practical aspects as installation. The effectiveness of coupling schemes depends on the impedances of installed equipment at the cable terminals. The frequency response of a 10 kV, 400 kVA cast resin distribution transformer is investigated. In the frequency range of interest, the behaviour is best described as a capacitance of typically 1 nF. The signal transfer over a 4 km paper cable terminated by various load impedances to mimic real equipment is studied. From the results it is concluded that for inductive coupling performance can be low within the CENELEC A-band, but can become competitive if frequencies up to 1 MHz are allowed. Further, the location of signal coupling has a strong effect on the noise spectrum.

On-line PD monitoring system for MV cable connections with weak spot location

A unique measuring system is presented for the online monitoring of partial discharges (PDs) in medium-voltage power cables. The system is able to locate the origin(s) of PDs by using two inductive sensors, each at one cable end. The measuring system is called PD-OL, which stands for PD detection on-line with location. A pulse injection system is used for the time synchronization of the data intake at both cable ends and for the on-line calibration. The control center at KEMA collects data from all the PD-OL measuring systems for final presentation and interpretation, which can be made visible on a secured Web-site for the network owners. This paper discusses the basics of PD-OL and some first measurement results.

Algorithms for Arrival Time Estimation of Partial Discharge Pulses in Cable Systems

Partial discharge (PD) detection and location in cable systems is a valuable tool for the estimation of the condition of the system. Accurate localization of the PD origin, based on arrival times, is required for the identification and assessment of the defect. This paper evaluates different time-of-arrival algorithms to determine which method yields most accurate location under different circumstances. These methods are based on trigger level, Akaike Information Criterion (AIC), energy criterion, Gabors signal centroid and phase in frequency domain. Several criteria are defined by which the algorithms are evaluated. These criteria include the sensitivity to the noise level, the sensitivity to the pulse shape and others. The methods are tested on a medium-voltage cable system by injecting PD pulses in a cable with a joint at a known location. Each algorithm is applied to the measured pulses and the resulting location is compared with the known location. From the result the methods using the energy criterion and the phase are preferred.