The energy transition towards decarbonization and modernization of the electric network requires the development of new technologies. Offshore windfarms, essential to this transition, rely on Gas-Insulated Systems (GIS) due to their compactness and extended lifespan. Despite the reliability of GIS, the high maintenance costs and severe consequences of power outages leave no room for failure risks. The primary source of failures in GIS is attributed to defects in electric insulation, producing Partial Discharges (PDs) that serve as a tool for defect detection. The current approach for standardized PD measurements proves impractical for implementation in GIS under operation. Consequently, unconventional methods have been employed with Ultra-High Frequency (UHF) sensors standing out. Despite the notable signal-to-noise ratio of the UHF system, it is still susceptible to interference and does not yield calibrated measurements suitable for comparison with other methods and for assessing insulation degradation. In response to these challenges, this study introduces a Very-High Frequency (VHF) system capable of providing calibrated and reliable measurements....@en

The correct identification of partial discharges (PDs) is instrumental for the maintenance plan in gas-insulated systems (GIS). However, onsite PD measurements are difficult, especially in HVDC systems, where partial discharges can be confused with interference. This paper proposes a method to discern PDs from interferences based on the GIS characteristic impedance. The characteristic impedance is measured using very-high frequency electric and magnetic sensors, and it is calculated using four approaches based on the PD charge magnitude, peak value, peak-to-peak value, and frequency spectrum. The method is first tested with a PD calibrator in a matched and open-circuited GIS testbench. Then, the identification of PDs and interference is tested in a full-scale GIS, where the measurements are subjected to pulse overlapping and noise. Five types of interferences and PDs are injected into the GIS in two positions and measured in multiple mounting holes. The results show that all four approaches can precisely calculate the characteristic impedance in a matched testbench. In the full-scale GIS, these approaches show more deviation, with the peak approach being the most accurate. A practical application of the method is demonstrated using a calibrator in the full-scale GIS. The proposed method contributes to a more reliable PD monitoring system for HVDC/AC GIS and allows better maintenance planning, reducing unnecessary costs, notably for offshore substations.@en

This paper introduces a directional coupler for partial discharge (PD) measurements in gas-insulated substations (GIS). The sensor comprises a combination of magnetic and electric couplers, effectively segregating forward and backward pulses to enhance PD charge estimation and defect location. The sensor's design was supported with finite element method simulations and measurements conducted in a transverse electromagnetic test bench. Comparative analyses were performed against independent magnetic and electric couplers. The charge estimation and the directional coupler's directivity were evaluated in both the test bench and a full-scale GIS with different PD defects. Initially, the combined magnetic and the electric couplers exhibited undesired interactions, prompting corrective measures. Subsequent adjustments included changes to the electric coupler material and modifications to the magnetic coupler construction. The resulting high-voltage directional coupler performed better than the separated couplers in a GIS with discontinuities. This partial discharge sensor emerges as a candidate for future SF₆-free alternative GIS.

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The insulation condition of HVDC grids consisting of cable systems, GIS, and converters should be monitored by partial discharge (PD) analysers using artificial intelligence (AI) tools for efficient insulation diagnosis. Although there are many experiences of PD monitoring solutions developed for the supervision of the insulation condition of HVAC grids using PD analysers, there are no standardised requirements for their qualification available yet. The international technical specification TS IEC 62478 provides general rules for PD measurements using electromagnetic methods but does not define performance requirements for qualification tests. HVDC and HVAC PD analysers must be tested by unambiguous test procedures. This paper compiles experiences of using PD analysers with HFCT sensors in HVAC grids (cable systems, GIS, and AIS) to define a qualification procedure for HVAC systems. This procedure is applicable to HVDC grids (cable systems, GIS, AIS, and converters) because the particularities related to the insulation behaviour under HVDC voltage are also considered. Representative PD sources are discussed in HVAC and HVDC positive and negative polarity. The PD pulse trend of representative insulation defects in HVDC cable systems is quite different from that of HVAC grids. Special attention should be paid to the acquisition of PD signals in HVDC grids since few pulses appear in solid insulations, mainly during voltage changes (polarity reversals or surges), but rarely in continuous operation with constant direct voltage. A synthetic PD simulator has been developed to reproduce trains of PD pulses or noise signals, similar to those that can appear in the power network. A set of three functionality tests has been developed for qualification of the diagnostic capabilities of PD analysers working up to 30 MHz addressed to HVDC or HVAC grids: (1) PD recognition test, (2) PD clustering test, and (3) PD location test. This qualification procedure has been validated by means of a round-robin test performed by five research institutes (RISE, FFII, TUDelft, TAU, and UPM) using commercial and in-development AI PD recognition and clustering tools to demonstrate its robustness and applicability. Applying this qualification procedure, two PD methods for electrical detection and prevention of insulation defects have been approved, one for HVAC and the other for HVDC grids.

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One of the main difficulties in measuring partial discharges (PD) in gas-insulated substations (GIS) is the overlapping of pulses at the sensor's location, which distorts the pulse resolution and the charge estimation. This research presents a new method called “synergy,” which identifies and suppresses reflections using magnetic and electric antennas in the very-high frequency range. By scaling the antennas’ outputs and adding them, it is possible to segregate forward and backward pulses. Additionally, by multiplying the electric and magnetic signals, the power flow of the pulses is obtained, which identifies the propagation direction and the location of discontinuities in the transmission path. The synergy method is evaluated in three scenarios: a fully matched test bench using a calibrated pulse, a full-scale GIS using a calibrated pulse, and a full-scale GIS using a PD defect. The results showed that the pulse reflections can be eliminated from the incident pulse, improving the charge calculation when the pulses overlap. The output of this research represents an improvement for PD monitoring in GIS, exhibiting a tool for better defect localization, pulse wave shape construction, charge estimation, and possible interference rejection.

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Driven by the voltage increase in high-voltage direct current (HVDC) gas-insulated substations (GISs), novel methods are needed for partial discharge (PD) detection and monitoring. This article shows a PD calibration method for very-high-frequency (VHF) magnetic and electric sensors in GIS. The calibration method uncertainty is tested in three laboratories using a low-voltage (LV) test bench and a high-voltage (HV) full-scale GIS. In the LV test, the calibration method's linearity, signal-to-noise ratio (SNR), and pulsewidth were compared against a reference charge, resulting in an error of around ±10%. The HV test consisted of different artificial defects introduced in a full-scale GIS, resulting in errors of around ±30%. The uncertainty is attributed mainly to random noise, which is critical in the charge estimation method. The electric and magnetic sensor combination showed better results, especially in the full-scale GIS, where reflections play an important role. This research has been performed in the framework of the project Future Energy 19ENG02 of EURAMET, resulting in a calibration method with the potential to measure PD pulses and discriminate impulse interferences, giving an advantage over conventional and ultrahigh-frequency (UHF) methods.

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On-site partial discharge (PD) measurements have turned out to be a very efficient technique for determining the insulation condition in high-voltage electrical grids (AIS, cable systems, GIS, HVDC converters, etc.); however, there is not any standardised procedure for determining the performances of PD measuring systems. In on-line and on-site PD measurements, high-frequency current transformers (HFCTs) are commonly used as sensors as they allow for monitoring over long distances in high-voltage installations. To ensure the required performances, a metrological qualification of the PD analysers by applying an evaluation procedure is necessary. A novel evaluation procedure was established to specify the quantities to be measured (electrical charge and PD repetition rate) and to describe the evaluation tests considering the measured influence parameters: noise, charge amplitude, pulse width and time interval between consecutive pulses. This procedure was applied to different types of PD analysers used for off-line measurements, sporadic on-line measurements and continuous PD monitoring. The procedure was validated in a round-robin test involving two metrological institutes (RISE from Sweden and FFII from Spain) and three universities (TUDelft from the Netherlands, TAU from Finland and UPM from Spain). With this round-robin test, the effectiveness of the proposed qualification procedure for discriminating between efficient and inappropriate PD analysers was demonstrated. Furthermore, it was shown that the PD charge quantity can be properly determined for on-line measurements and continuous monitoring by integrating the pulse signals acquired with HFCT sensors. In this case, these sensors must have a flat frequency spectrum in the range between several tens of kHz and at least two tens of MHz, where the frequency pulse content is more significant. The proposed qualification procedure can be useful for improving the future versions of the technical specification TS IEC 62478 and the standard IEC 60270.

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Partial discharge (PD) measurements in gasinsulated substations (GIS) are tested according to the standard IEC 60270. This “conventional” PD test method applies to electrically small devices. The equipment size increases the resonance, and attenuation, contributing to the total uncertainty. Additionally, when an ultra-high frequency (UHF) sensor is used as a coupling capacitor, the calibrator and PD pulse duration difference increase the measurement uncertainty. In this paper, the IEC method, using an external capacitor coupler and a UHF sensor, is simulated and tested in a full-scale GIS. The results show the uncertainty dependency with the IEC 60270 filter bandwidth. With proper measures, the UHF sensor correlates with the external coupling capacitor, resulting in a reasonable charge estimation for a 25-meter-long GIS. Knowing the calibration limits is critical to estimate the PD charge uncertainty.@en

Partial discharges (PD) measurements provide an estimation of the severity of the insulation degradation; additionally, it is used as a unit of standardization. Up to now, only the IEC 62478 briefly mentions partial discharge measurements using electromagnetic sensors in gas-insulated substations (GIS). The IEC60270 standard provides a method for PD charge measurement when the test object approximates to a lumped element; PD in SF 6 are in the range of nanosecond, and given the GIS length, it behaves as a transmission line. This work compares different sensors (commercially available and developed by the authors) used for measuring PD charge magnitude in GIS. The sensors’ sensitivity, time resolution, and charge estimation accuracy are tested in a full-scale 420 kV GIS. A nanosecond rise time pulse was connected to the GIS through a transition cone to provide a good PD representation. The pulse was measured by electric and magnetic sensors installed in mounting holes located in different sections of the GIS, and a directional coupler was used as a reference to the injected pulse. This work demonstrates that the charge magnitude can be extracted using different sensors, harmonizing the reading from different measuring systems. The results show the possibility of a standardized method for on-line PD measurements and routine and after-installation tests.@en

There are no accepted procedures that quantify the apparent charge of partial discharge (PD) in gas-insulated substations (GIS). This paper proposes a calibration method for PD charge estimation using unconventional electromagnetic sensors: a magnetic loop antenna (inductive coupler) and an electric antenna (capacitive coupler.) The calibration procedure is intended for the voltage double integral method, which is reviewed for magnetic antennas and extended for electric antennas. By injecting low-frequency sinusoidal signals, the calibration constants are determined for two different test setups: the first one being a testbench where the characteristic impedance is matched and the second one a full-scale 420 kV GIS. The calibration method is validated in three ways: with a calibrated pulse in the testbench, a calibrated pulse in a full-scale GIS, and PD defects in the full-scale GIS. The calibration procedure revealed a frequency limit range dependent on the GIS length and the sensor's signal-to-noise ratio. The three validation methods showed low charge estimation errors for the magnetic and electric antennas, demonstrating that the PD calibration method applies to any electric/magnetic detector with a low-frequency derivative response. This research paves the way for better GIS insulation monitoring and PD sensor harmonization.

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Recent research has found that Magnetic loop antennas can detect partial discharges (PD) in gas-insulated substations (GIS); however, unbalanced shielded loop (UBSL) antennas are affected by electric field coupling. This research proposes a balanced shielded loop (BSL) adapted for GIS PD sensing with different designs. The frequency response and PD charge estimation for BSL and UBSL antennas are analyzed and compared in a matched test bench using a very-fast pulse calibrator. The results confirmed a better common-mode noise rejection and thus a better charge estimation by using the balanced magnetic antenna. An alternative BSL design improved the antenna's gain by a factor of two, and by using a balun transformer, the gain was increased by a factor of 4. The results showed the criticality in the shielded gap placement and the influence of the antenna's parasitic elements on the balun's frequency response. This paper shows an improvement in the noise rejection using magnetic antennas, which leads to the possibility of better PD sensing in GIS.@en

Society's increasing demand for electrical energy, along with the increased integration of remote renewable generation has driven transmission levels to ever higher voltages in order to maintain (or improve) grid efficiency. Consequently, high voltage testing and monitoring beyond voltage levels covered by presently available metrology infrastructures are needed to secure availability and quality of supply. Calibration services for Ultra-High Voltage Direct Current (UHVDC) presently are only available up to 1000 kV. There is a need to extend the DC calibration capabilities for voltage instrument transformers up to 1200 kV and for factory component testing capabilities up to 2000 kV. Also, methods for linear extension of lightning impulse calibration, for dielectric testing of UHV grid equipment, urgently need revision. Recent research has raised questions regarding the validity of the current linearity extension methods for voltages beyond 2500 kV. Furthermore, new methods for calibration are needed for the 0.2 class HVAC voltage instrument transformers for system voltages up to 1200 kV. The current methods used for determination of the voltage dependence are very time consuming, raising the need for methods allowing faster assessment. Finally, with new HVDC transmission grids and associated components, novel methods are needed for detection, classification and localisation of partial discharge (PD) under DC stress. The industry needs methods for reliable monitoring of critical components such as cables, for both HVAC and HVDC, and gas insulated substations (GIS), and techniques for addressing new challenges introduced by HVDC technologies, such as the ability to distinguish PD signals from switching transients in converters and other sources of noise.

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Gas insulated substations (GIS) have increased in the electric power system in recent years and is expected to continue increasing in the following years. The GIS has remarkable advantages over air insulated substation in offshore windfarms where the installation space is limited. The outlying location of the GIS demands a robust monitoring system. It is well known from literature that partial discharge (PD) measurements are an accepted method for insulation diagnosis and in many cases a required part of the acceptance protocol for many high voltage (HV) assets. In a recent investigation, a new concept of PD measurements in GIS using a Very High Frequency (VHF) Sensor (so-called Magnetic Antenna) has been demonstrated. Unlike Ultra High Frequency (UHF) technology, the presented PD sensor has a sensitivity better than 5 pC and yield an estimation of the charge magnitude, thus providing information about the severity of the insulation degradation. In this paper, a test bench for the characterization of the magnetic antenna is proposed. The magnetic antenna and a traditional UHF antenna are tested in the frequency domain up to 1 GHz, and in the time domain using different wave-shapes. Additionally, using the test bench, it is demonstrated the homogeneity of the PD current propagating in the GIS in the transverse electromagnetic (TEM) mode. The charge is estimated using the double integral voltage method, showing low errors. The results from the research gives a preface for further investigation in the design of the magnetic antenna.@en

A recent investigation explored a new measuring concept used in partial discharges (PD) measurements in gas insulated substations (GIS), consisting of a magnetic loop antenna. The sensor's frequency response was characterized up to some tens of MHz. This paper proposes an improved version of the sensor with an extended bandwidth (BW) one order of magnitude higher: a resonance, attributed to a common mode current in the mounting hole, is identified and eliminated employing ferrite beads in the feeder cables. Moreover, this publication proposes an electric circuit model that fully covers the transverse electromagnetic mode (TEM) frequency range in GIS. The electric model is compared against experimental measurements using a 1 GHz bandwidth testbench, giving accurate results. Two contributions are achieved in this research: an improved magnetic loop antenna with extended bandwidth and an accurate electric circuit model. This publication paves the way for further research on time resolution and signal postprocessing techniques for magnetic loop antennas in GIS.

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