Armando Rodrigo Mor

Partial discharge analysis of gas insulated systems at high voltage AC and DC

Partial discharge (PD) measurements are commonly used for monitoring and diagnostics of AC gas insulated systems (GIS). Because of the growing importance of DC GIS, a research project was started focused on the measurement and interpretation of PD signals obtained under DC stress. Results are presented using IEC60270 and unconventional methods through UHF antenna and high frequency current transformer (HFCT) on three different PD generating defects in a GIS chamber. The results of the PD analysis of the defects are compared for AC and DC stress. Phase resolved partial discharge patterns (PRPD) are used for the HVAC analysis, whereas time resolved partial discharge patterns (TRPD) are used for HVDC analysis. Finally, relevant characteristics of PD patterns in HVAC and HVDC are presented and related with the physical phenomena of the discharges.

Partial discharge behavior of mineral oil based nanofluids

A previous study showed that both mineral oil based nanofluids with 0.01% silica mass fraction and with 0.1% fullerene mass fraction have a higher AC breakdown strength than mineral oil. Breakdown occurs following discharge initiation and propagation in the oil. The breakdown strength value alone provides little information on the discharge process. Therefore, it is important to investigate the details of the discharge mechanisms in mineral oil and in nanofluids. Hence, this study focuses on the partial discharge (PD) behavior of mineral oil, silica and fullerene nanofluids. The total charge, voltage and pulse shape were recorded with the help of a high bandwidth PD measuring system. The discharge mechanism in mineral oil appeared to depend strongly on the polarity of the applied DC voltage. Under positive DC voltage, the silica and the fullerene nanofluids show increased inception voltage and a reduction of the total discharge magnitude compared to the reference mineral oil. Under negative polarity, inception voltage and discharge magnitude of the nanofluids and the reference mineral oil are virtually the same.

Refurbishing existing mvac distribution cables to operate under dc conditions

Refurbishing existing ac distribution cables to operate under dc conditions can offer several advantages in terms of capacity enhancement, efficiency and flexibility in power and voltage control, among others. In this paper, technical aspects such as insulation ageing, capacity and efficiency enhancement are explored. A novel idea of dynamic cable voltage rating based on the temperature dependent electric field is developed. The proposed algorithm can be incorporated in the dc link converters to obtain additional efficiency and capacity gains. Finally, challenges in the presented concept are highlighted.

Power transfer computations for medium voltage AC link by imposing rated current at sending end

This paper derives the mathematical expressions for computation of the load impedance magnitude, such that rated cable current is imposed at the sending end, with sending end voltage as the reference phasor. Thereby, the variation in maximum power transferred to the receiving end of a medium voltage cable link is described for varying link length, conductor cross-sectional area and load power factor. The percentage error in transmitted power computation due to simplification by neglecting the cable capacitance is quantified. The merit of the developed theory for future use is highlighted.

Partial discharge recognition of insulation defects in HVDC GIS and a calibration approach

Partial discharge (PD) monitoring is an established technique used to assess the condition of AC gas insulated switchgears (GIS). The growing application of High Voltage Direct Current (HVDC) transmission systems has triggered research projects focusing on the measurement and interpretation of PD signals in DC GIS. In this paper, several insulation defects are characterized using Phase Resolved Partial Discharge (PRPD) patterns in HVAC. The defects then are identified using Time Resolved Partial Discharge (TRPD) patterns in HVDC. Noticeable characteristics of PD patterns in HVDC are presented and related with the physical phenomena of the discharges. The potentiality of on-line defect recognition based on Radio Frequency (RF) PD detection measurement is discussed. In addition, the relationship between the apparent charge and the frequency spectrum of the PD signal is investigated.

Analysis of the arcing process in on-load tap changers by measuring the acoustic signature

Power transformers are fundamental parts of the power system. In order to keep the availability of power systems high, the reliability of a transformer, in all its parts, must be assured. This requires also paying attention to the reliability of the tap changer, as tap changer failures are major causes of power transformer breakdown. This paper focuses on the development of acoustic diagnostics for on-load tap changers (OLTC). A selector switch-type tap changer was used to detect and characterize different defects that may ultimately lead to a tap changer failure. Among other techniques, acoustic diagnostic was considered first because it is a method which can work online without affecting in any way the normal operation of a power transformer. Special emphasis was dedicated to recreate the arcing conditions in a tap changer. Arcing inside the tap changer chamber is unavoidable, but excessive arcing leads to a multitude of problems, so that efforts have been made to detect acoustically the signature of the arcing phenomenon. In order to have both flexibility and control over the experimental parameters, the tests were conducted on two sets of test objects, i.e. a model and a small-size tap changer. Focus was made to the type of defects that pose the most imminent threat to the tap changer operation, and the most appropriate methods of signal acquisition and processing were singled out. It is shown that these defects can be recognized based on their specific acoustic signature.

Partial discharge analysis and monitoring in HVDC gas insulated substations

Partial discharge (PD) detection is a widely used tool for monitoring and diagnostics of high voltage gas insulated substations (GIS). Despite the fast growing demand of high voltage direct current (HVDC) interconnections, little research has been dedicated to PD monitoring systems for HVDC GIS. In this paper, conventional and unconventional PD detection systems are employed for the detection of three different defects common in GIS, and relevant PD analysis methods are presented. Finally, the analysis methods and detection systems are compared to point out their potential and limitations in monitoring PD in HVDC GIS.

The effect of frequency on the dielectric strength of epoxy resin and epoxy resin based nanocomposites

In this study, the relative permittivity, dielectric loss and breakdown strength of epoxy resin and epoxy-hBN (hexagonal Boron Nitride) nanocomposites are studied under two different frequencies: 50 Hz and 1500 Hz. The effect of fill grade of the nanoparticles on the dielectric properties of the nanocomposites is also included. The following fill grades are investigated: 0.2 vol.%, 1 vol.% and 5 vol.%. The results indicate that the higher frequency (1500 Hz) affects the breakdown strength of the samples. The influence changes with the change of the filler concentration. The dielectric loss values of epoxy resin and epoxy-hBN nanocomposites show a significant increase with the increase of frequency. The decrease of the breakdown strength is in line with the increase of the tanô.

The effect of frequency on the dielectric breakdown of insulation materials in HV cable systems

In this study, the effect of frequency on the breakdown strength of polymeric insulating materials is investigated. Two types of polymeric insulating materials that are used in extruded high voltage (HV) cable systems have been investigated: cross-linked polyethylene (XLPE) and epoxy resin. Breakdown tests are performed under DC voltage and AC voltage with frequencies ranging from 50 Hz to 2500 Hz. Weibull statistical techniques are applied to analyse the breakdown results. The results show that, for both materials, the breakdown strength is significantly higher under DC voltage than AC voltage. For AC voltage, there is a statistically significant dependency between breakdown strength and frequency: the higher the frequency, the lower the breakdown strength.

Charge estimation methods in partial discharge cable tests

This paper presents different charge evaluation methods for partial discharge charge estimation. The paper describes methodologies based in time-domain calculations, frequency-domain calculations and impulse response of band-pass filters. Finally, the charge estimation accuracy of each methodology is compared for a given set of partial discharge pulses.