The National Metrology Institute (NMI) of The Netherlands (VSL) developed a new 1200-kV reference measuring system for on-site calibration of lightning impulse (LI) voltage measuring systems. Improvement of the low-voltage grounding loop and triaxial signal transmission reduced oscillation ripples and enhanced the shielding of environmental high-frequency interference. Calibration and measurement capabilities are realized in the test voltage (U<inf>t</inf>), front time (T<inf>1</inf>), and time to half-value (T<inf>2</inf>) with uncertainties of 0.7%, 3.0%, and 1.5%, respectively, up to 600 kV and 1.2%, 5.0%, and 3.0%, respectively, up to 1200 kV for direct comparison calibration. With additional linearity experiments, VSL is now able to provide LI voltage calibrations on-site up to 3000 kV. The VSL system was used as the reference for calibration and tuning of a 4000-kV LI divider to improve its accuracy performance. A measurement campaign was held to compare different LI measuring systems and research the linearity of the measuring system up to 3000 kV. The comparison results confirmed the good performance of the VSL 1200-kV reference measuring system and also indicated that the 4000-kV measuring system was improved after calibration and tuning with the VSL reference system

The increasing integration of power electronics and renewable energy sources in modern electrical grids has introduced new challenges in medium voltage transformer calibration, particularly in the frequency range from 9 kHz to 150 kHz. Accurate and traceable calibration methods are essential to ensure reliable voltage measurement in electrical grids, where power quality and compliance with IEC 61869-1:2024 standards are critical. This study presents advancements in the development of traceable calibration techniques for medium voltage transformers operating within this frequency range. The approach includes the design of generators capable of simulating real world grid disturbances and the establishment of reference measurement systems at National Metrology Institutes. The study further explores generator configurations, such as parallel and series source arrangements, to extend calibration capabilities beyond existing limits. The proposed calibration methodology incorporates precision voltage dividers with high resolution, filtering and amplification techniques, and advanced data acquisition methods to achieve measurement uncertainties below 0.01% for fundamental components and between 0.2 % and 1 % for harmonic components. A key challenge is the ability to measure a fundamental voltage as high as 50 kV while superimposing harmonics with amplitudes as low as 5 V in the 9 kHz to 150 kHz range. These developments aim to enhance power system reliability, improve voltage measurement accuracy, and support the transition to more efficient and resilient electrical grids. The presented results lay the foundation for future calibration services. © The Authors, published by EDP Sciences.

The lower power quality and shrinking inertia of renewable generation puts new demands on monitoring, controlling and balancing energy flows in the electricity grid. This calls for new high-voltage (HV) metrology solutions to support the HV manufacturing industry. The recently started EU project ’ENSURE’ is addressing this need by developing new HV measurement technology in four areas: wideband live connectable current or voltage sensors to monitor and calibrate substation instrumentation; new capability for determining the losses in power transformers caused by grid harmonics; new methods for loss determination in HVAC cables; and new partial discharge monitoring techniques for fault detection and localization in HVDC cables. 

High voltage testing and metering beyond voltage levels covered by presently available metrology infrastructures are needed to secure availability and quality of supply. Calibration has been available up 1000 kV and there is a need to extend the calibration capabilities for testing and with voltage instrument transformers for sub-station metering up to 1200 kV. This paper presents the results of calibration of two precision modular UHVDC dividers, providing traceability to 1200 kV with an unprecedented measurement uncertainty. The scale factors and measurements of the two dividers were calibrated with LV resistance measurement, using a HV step-up procedure, and in two comparisons. The agreement of LV scale factors traceable to RISE and PTB is within 2 μV/V, and the combined expanded uncertainty for a measurement system is better than 15 μV/V at 1200 kV giving new CMC claims of 20 μV/V at 1200 kV. 

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. © 2023 by the authors.

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. © 2023 by the authors.

High-frequency current transformers are popular noninvasive inductive wideband sensors. Despite simplicity in design and operational principle, implementation of such sensors for partial discharge applications requires careful consideration, particularly in the higher frequency range where traveling wave attenuation and distortion is relevant. First, the role of design variables, including core materials, winding design, and shielding practices on sensor sensitivity and frequency characteristics (transfer impedance) are presented. Next, the suitability of the constructed sensors for partial discharge applications is assessed. The designed wideband sensors are suitable for laboratory applications with standard measurement circuits and controlled conditions. The low-level magnitude and frequency spectrum of the discharge pulses hinders signal integrity in relation to the placement of the sensors within the measurement circuit, signal amplification, and pulse repetition rate (pulse resolution). To enable most stringent detection levels under 1 pC, efforts are needed in distortionless amplifier design and interference mitigation. 

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.

As part of the production of equipment for high voltage electricity grids, high voltage tests are performed to verify that the equipment can withstand the operational voltage stresses. Such 'withstand tests' are performed using combined and composite waves, where lightning impulse (LI) or switching impulse (SI) waves are superimposed on AC or DC. Both the technical understanding of the generation and measurement of such high voltage wave forms and the regulations in international standards are not sufficient for the current requirements. Therefore, a European metrology project with industrial partners, universities and seven European metrology institutes (NMI) is started in 2020. In three technical work packages the metrological and standardisation need will be issued. First, the relationship between impulse voltages with AC or DC measurement, and related detrimental effects due to combining wave shape tests will be reliably determined. New measurement instruments, low voltage generators and measurement evaluation software will be created. Traceable high voltage measurement systems and calibration services for composite and combined wave shapes, with a target amplitude uncertainty of less than 2 % will be developed at the NMIs. The capabilities of these NMIs will be confirmed with aid of a high voltage comparison campaign at PTB. The uncertainty of existing voltage dividers and measuring systems used in tests with composite and combined wave shapes will be accurately determined. Finally, the research results will be forwarded to the ongoing revision of the IEC 60060 series.

The stochastic nature of partial discharge (PD) and the inability to directly detect charge displacement makes representative measurement of such events very challenging. High frequency current transformers (HFCTs) have been favored among sensors due to their non-invasiveness and wide bandwidth. Although simple in design, HFCTs for PD measurements need careful consideration to ensure suitability for their intended environment and task. Traditional phase resolved partial discharge analysis (PRPDA) does not impose strenuous requirements for measurements systems as pertinent analysis can be extracted from peak magnitude and power frequency phase information. However, detailed assessment examining waveform parameters from individual discharge pulses (such as DC PD) requires higher bandwidth, sensitivity, and noise suppression, as well as a clear understanding of the measurements system's influence on observed phenomena.