Integration of online partial discharge monitoring and defect location in medium-voltage cable networks

Partial discharge (PD) diagnostics is a proven method to assess the condition of underground power cables. PDs are symptomatic for a defect (weak spot) that may evolve into a complete breakdown. A PD induces a small pulse in the conductor(s) and earth screen that propagates through the cable in both directions. In previous research an online PD detection and location system was developed for medium-voltage cables. This system is currently introduced by utilities on an increasingly large scale. This thesis deals with several new challenges that are related to the integration of online PD monitoring in different medium-voltage power cable network configurations. A transmission line model of the power cable is required to enable optimal PD detection. A three-core power cable with common earth screen has multiple coupled propagation modes. The propagation modes are decoupled into a modal solution. A practical method to measure and analyze the cable parameters is developed and validated by measurements on a cable sample. Detailed prediction of multiple reflections was achieved, including the mixing of propagation modes having distinct propagation velocities, validating both the model and the measurement method for three-core cables with common earth screen. The semiconducting layers in a cable with cross-linked polyethylene (XLPE) insulation have a significant influence on the transmission line parameters. Unfortunately, the dielectric properties of these layers are usually unknown and can differ between similar cable types. It is shown that nonetheless the characteristic impedance and propagation velocity of single-core and three-core XLPE cables can be calculated using information available from the cable specifications. The calculated values are validated using pulse response measurements on several cable samples. The accuracy of the calculated characteristic impedance and propagation velocity is 5–10%, which is sufficient for estimating PD pulse shape and amplitude in a cable circuit. The online PD monitoring system was initially developed to be installed on a single cable connection between two consecutive ring-main-units (RMUs). It is more efficient to monitor two or more consecutive cables, with one or more RMUs or substations in between, using a single monitoring system. Models for RMUs and substations are proposed and verified by measurements. The influence of RMUs and substations along the cable under test on the PD detection sensitivity and location accuracy is studied using these models and measurements. The influence of a compact RMU with two connected cables is neglectable if the total cable length is longer than approximately 1km. The longer the total cable length, the smaller the influence of an RMU along the cable under test. An RMU or substation along the cable under test with more than two connected cables introduces a significant signal loss, decreasing the detection sensitivity significantly. The higher the number of connected cables, the higher the signal loss. In the equipment found in some substations and RMUs it is not possible to install a PD measurement unit at a desired location. The models and measurements are used to study the feasibility of a single-sided PD measurement, including PD location, with the problematic RMU/substation at the far end. The study shows that a major part of an incoming pulse reflects on a large RMU/substation with many connected cables. This reflection enables a single-sided PD measurement from the other cable end. A single-sided measurement has the disadvantage that the maximum cable length that can be monitored is halved and that it is sensitive to reflections on joints in the cable under test and other connected cables. Accurate location of PDs in cables, based on arrival times, is imperative for the identification and assessment of defects. Five algorithms that determine the time-of-arrival of pulses are evaluated to investigate which method yields the most accurate location under different circumstances. The methods are evaluated analytically, by simulations, and by measurements on a cable system. From the results the energy criterion method and the phase method show the best performance. The energy criterion is a robust method that achieves location accuracy of 0.5% of the cable length or better. The phase method has a good performance only if the phase shift introduced at the transmission from the cable under test to the RMU is known accurately. For maximum detection sensitivity the PD monitoring system uses matched filters. These filters are constructed using predicted PD waveforms. These predictions are based on a series of online system identification measurements and a standard cable model. Due to signal distortions that are not taken in account in the model, e.g. an RMU along the cable under test, the predictions can be inaccurate, resulting in a sub-optimal PD detection. An automated procedure that creates new signal templates, based on measured PD signals, is proposed and tested on signals measured during online PD measurements on multiple cable systems. The algorithm generated new templates for PD signals and for disturbing pulses. New PD pulse templates for may be used to improve the cable system model. Disturbing pulse templates may be used to improve disturbance rejection by the measurement unit

Comparison of arrival time estimate methods for partial discharge pulses in power cables

Accurate location of partial discharges in power cable systems, based on arrival times, is critical 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 threshold, Akaikepsilas information criterion, energy criterion, Gaborpsilas signal epoch 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. From the results the energy criterion method and the phase method show the best performanc

Measurement of transmission line parameters of three-core power cables with common earth screen

In power cables fast transient signals arise because of partial discharges. These signals propagate to the cable ends where they can be detected for diagnostic purposes. To enable optimal detection sensitivity and to judge their severity the propagation parameters Z c (characteristic impedance) and ÿ¿ (propagation coefficient) need to be known. A three-core power cable with a single metallic earth screen around the assembly of the cores has multiple, coupled propagation modes with corresponding characteristic impedances and propagation coefficients. This paper presents a practical method to measure and analyse the cable parameters. The propagation modes are decoupled into a modal solution. The modal solution is interpreted in terms of convenient propagation modes: a shield-to-phase (SP) propagation mode between conductors and earth screen and two identical phase-to-phase (PP) modes between conductors. The measurement method, based on a pulse response measurement, to determine all transmission line parameters of the SP and PP modes is proposed and tested on a cable sample. The model is validated by predicting the time, shape and amplitude of multiple reflections in all modes resulting from an injected pulse

Detection limitation of high frequency signal travelling along underground power cable

The detection of the high frequency signal propagating along underground power cables is part of many monitoring techniques, e.g. partial discharge (PD) based diagnostics. On one hand, higher frequency corresponds to better spatial resolution, which means more accurate PD location. On the other hand, signal attenuation increases with frequency. Apart from the signal itself, noise level and detection equipment also play a role in the signal detection process. This paper focuses on the detection limitation of high frequency components in PD signals travelling along an underground power cable considering effects of signal attenuation, noise level and applied equipment. The attenuation coefficient is based on measurements from 10kV three-core XLPE cables. Though the attenuation coefficients for other types of cables differ, the measured value for this particular cable provides a practical parameter value, and it can be altered to match other cable types. The detected analog signal is digitized through an analog-to-digital converter (ADC) and may be averaged before being digitally stored. In addition, an amplifier and/or filter can be applied before the analog to digital (AD) conversion. The vertical resolution and the vertical sensitivity of the ADC are crucial for signal detection. Effect of noise is considered in this paper by analyzing Gaussian noise and typical noise characteristics obtained from field measured. Sinusoidal wave and Gaussian pulse shapes are applied as input signals for the cable. Firstly, the relationship between maximum cable length and detectable frequency components for a specific set of detection equipment conditions is analyzed without averaging. This is the limitation from ADC. Secondly, the merits and limits of averaging are studied. The required averaging time for different frequencies as a function of PD signal propagation length is studied. Finally, the effect of averaging and analog filtering is demonstrated with test measurements

Partial discharge propagation in three-core cables with common earth screen

Online partial discharge (PD) detection and location in cable systems is a valuable tool for the estimation of the condition of the system. Different types of cables are encountered in medium-voltage (MV) connections and appropriate models must be constructed to predict PD signal propagation. For three-core power cables with a single common earth screen a multiconductor transmission line (MTL) model is required. This paper presents an MTL model of a such a cable, including interpretation of the decoupled solution as convenient propagation modes. A complicating factor in the estimation of the transmission line parameters of these propagation modes is the presence of semiconducting layers. These layers have a significant influence on the transmission line parameters. Unfortunately, the dielectric properties of these layers are usually unknown for the frequency range of interest for PD diagnostics. It is shown that nonetheless the characteristic impedance and propagation velocity can be estimated using information available in most cable specifications. the estimated values are validated using pulse response measurements on a cable sample. ©2009 IEEE

The formation of atomic oxygen and hydrogen in atmospheric pressure plasmas containing humidity : picosecond two-photon absorption laser induced fluorescence and numerical simulations

Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications. We quantify ground-state densities of key species, atomic oxygen (O) and hydrogen (H), produced from admixtures of water vapour (up to 0.5%) to the helium feed gas in a radio-frequency-driven plasma at atmospheric pressure. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. These absolute O and H density measurements, at the nozzle of the plasma jet, are used to benchmark a plug-flow, 0D chemical kinetics model, for varying humidity content, to further investigate the main formation pathways of O and H. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio

Impedance Analysis of an RMU for On-line PD Measurement

Abstract A new system to monitor partial discharges (PD) online in medium-voltage power cables is being developed. On-line measurement implies that the equipment can be applied when the cable is in operation, i.e. without disrupting service. A PD gives a high frequency pulse, propagating through the MV power cable. At the termination cable, the PD pulses partially reflect and partially propagate into the connected Ring Main Unit (RMU) due to the impedance transition. For correct interpretation of PD signals from the cable under test, it is essential to know the involved impedances up to several megahertz. An analysis of the impedances of the RMU elements is performed. A theoretical analysis based on lumped impedances is verified using various measurements. The measurements have been carried out on a test-grid consisting of two RMUs and several cables and show good agreement with predictions. This method of analysis will be used in the on-line PD monitoring system