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Techniques for ultra-high voltage and very fast transients

Publishable Summary, December 2018

Overview

This project focuses on the pre-normative research required for the measurement and testing of Ultra High Voltage (UHV) equipment with an emphasis on direct current (d.c.) applications. UHV is the common denominator between the three areas being addressed in this project, which are the measurement of composite d.c. wave shapes, very fast transients, and fault detection. Future high voltage (HV) and ultra‑high voltage (UHV) grids need support for progress beyond the state of the art to be achieved in terms of measurement techniques for d.c. applications. This project will provide IEC TC38, TC42, TC62C and related standardisation groups with methods to develop standards for medical x-ray equipment, for the measurement of very fast transients (VFT) and for partial discharge (PD) measurements for d.c. All of these areas will create an impact on the electrical power industry.

The x-ray voltage measurement activities in this project will benefit from the divider design experience gained in EMRP ENG07 HVDC. This knowledge will be used to take a first step in the measurement of d.c. switching, which will prepare the way for the development of the new methods needed for the traceable measurement of UHV grid component testing. Switching in general has an unknown effect on the instrument transformers and sensors used for gas insulated system (GIS) applications, both in terms of very fast transient over‑voltage (VFTO) on the low voltage side in existing HV grids and to an even larger extent in planned UHV grids. Progress in providing new methods for the traceable measurement of fast transients in two time domains is addressed in the VFT section under “Progress beyond the state of art” and experience will be exchanged with the ongoing EMPIR project 14IND08 ElPow. New procedures for partial discharge (PD) measurements in d.c. grids and the traceability for ultra-sensitive PD calibrators will be addressed. Future UHV grids, with an emphasis on d.c. grids, will need methods for the detection of PD and for the determination of the source.

Need

This pre-normative research will support the metrology needed for the standardisation of UHV transmission measurement techniques. The work will further provide guidance for HV metrology in medical electrical equipment. The industry needs traceable methods for d.c. switching measurements, in this case applied to x‑ray acceleration voltage measurements. Instrument manufacturers are asking for improved standardisation in this field, and medical staff and patients need more accurate x-ray dosing. Thus, it is essential to provide a unified view between the manufacturers and the users in this case. The research will further provide input to measurements for VFT, which is essential for the measurement of transmitted over voltages, and which is also critical for the insulation coordination of GIS equipment. Manufacturers of instrument transformers and GIS need traceability and new methods for the measurement of VFT, which are not covered by the NMIs today. It will further provide input to PD measurement techniques for equipment under d.c. stress, to detect and prevent insulation failures, e.g. for d.c. transmission and distribution using cables. The power grid operators and manufacturers of equipment for d.c. grids need new methods for PD detection. Traceability for low level PD, for the early detection of faults in all power grids, is also needed for the development and testing of equipment.

Objectives

This project has the following scientific and technical objectives:

  • To provide a substantial contribution to TC62C work. This will contribute to the revision of IEC 61676 (Medical electrical equipment – Dosimetry instruments used for non‑invasive measurement of x‑ray tube voltage in diagnostic radiology). The project will deliver calibration procedures, including a statement of uncertainty, for invasive dividers for different x‑ray equipment applications.
  • To provide a substantial contribution to TC38 and TC42 work (via task-force D1.63). This will contribute to the development of measurement techniques relating to transmitted over voltages in terms of VFTO, thereby supporting the pre‑normative CIGRE working group D1.60 (Traceable measurement techniques for very fast transients), which in turn will report it to TC42.The project will deliver calibration procedures and CMC entries for VFTO to 100 kV, 10 ns, with a target uncertainty of 1 %.
  • To provide a substantial contribution to TC42 work (via task-force D1.63). This will be achieved by contributing to the revision of IEC 60060 series (High‑voltage test techniques), IEC TC42 MT17, and thereby supporting the pre‑normative CIGRE working group D1.63 (Partial discharge measurements). The project will deliver a calibration procedure and CMCs will be updated to include low‑level PD down to 0.1 pC with a target uncertainty of 0.01 pC.
  • To work closely with the European and International Standards Developing Organisations, and the users of the Standards they develop, to ensure that the outputs of the project are aligned with their needs, communicated quickly to those developing the standards, and in a form that can be incorporated into standards at the earliest opportunity.

Progress beyond the state of the art

15NRM02 will advance the state of the art in the following three areas:

X-ray. Users currently find that the methods of validation and calibration of x‑ray high voltage tubes, using invasive equipment, are vague and can be interpreted in a variety of ways. In addition, the practical peak voltage (PPV) was recently adopted by the IEC as a new quantity for the calibration of x‑ray high voltage tubes. The lack of traceability of this new quantity will lead to additional errors. This project will go beyond the state of the art by developing and validating appropriate measurement methods for the calibration of x‑ray units for every type of pulse (time duration from 200 µs to a few seconds). Traceability of the PPV will ensure better control of the dose received by the patient.

Very fast transients. In areas where very fast transients are measured, e.g. monitoring of GIS systems, this project will go beyond the state of the art by providing new methods and calibration capability, in response to end user’s needs, in a field where traceable calibration is not currently available. In GIS, wideband sensors working in the ultra-high frequency (UHF) range are adapted to GIS ducts by the manufacturers in order to measure internal VFTO in sulphur hexafluoride (SF6) insulated equipment without traceable calibration.

Partial discharge. In high voltage direct current (HVDC) transmission systems, the traceability of PD measurements will be improved, and metrological support will be given to the new measuring instruments that are used to analyse the insulation condition of a d.c. grid. Therefore, this project will go beyond the state of the art by developing reliable techniques for PD measurements and diagnosis. This will involve evaluating the correlation between apparent charge in pC and the voltage measured in a different frequency range for HVDC cable systems. The d.c. systems will be modelled and validated by means of measurements using reference PD measuring methods.

Results

To provide a substantial contribution to TC62C work. This will contribute to the revision of IEC 61676 (Medical electrical equipment – Dosimetry instruments used for non‑invasive measurement of x‑ray tube voltage in diagnostic radiology). The project will deliver calibration procedures, including a statement of uncertainty, for invasive dividers for different x‑ray equipment applications.

The work in this project on x‑ray charging voltage measurement will lead both to new services becoming available from European NMIs, and to input for the amendment of written standards. A very important set of x‑ray equipment performance characteristics will involve the determination of the measurement uncertainty and the reproducibility of the peak voltage and exposure time. The traceable calibration of x‑ray equipment will ensure a better control of the x‑ray dose.

Prior to selection of the voltage dividers for in-situ measurements of the acceleration voltage, a study of ionisation chambers was made. Two voltage dividers were selected from 4 candidate types, with 8 models of each. A method was selected to be used for the measurements, and two voltage dividers were purchased and have been characterized, determining the frequency response, DC and AC linearity, proximity effects, influence of output impedances and temperature effects. Both dividers have also been characterised at high voltage for different waveforms; AC, DC, LI, SI, square waveform with duration from 200 µs to two seconds.

A method of low current measurements under high voltage has been developed using a commercial clamp meter with an x-ray generator as source, applying mathematic post processing for the determination of correction factors for specific waveform parameters. With the calculated correction factors, a conversion of the standardised KVp quantity to a new quantity named Practical Peak Voltage (PPV) can now be validated by measurements in an X-ray generator. The conversion factor was found to be proportional to the ripples and the voltage level. In-situ measurements are now possible in this rather noisy environment.

High voltage waveforms for voltages as typically used by x-ray machines have been generated, for a time duration of between 200 μs and a few seconds in order to check the linearity of the selected invasive dividers. High voltage measurements have been performed at DC, AC and at switching impulse in order to validate that the voltage dividers are fast enough to measure possible transients at 200 μs. This gives input into calculating invasive divider correction factors for the waveforms.

Calibration procedures, including a statement of uncertainty, for invasive dividers for different x-ray equipment applications are now available.

A set-up composed of a fast X-ray generator, Ross dividers, and KVp meters has been built. The objective is to compare different measurement techniques (invasive and non-invasive methods) and to study the behaviours of non-invasive techniques by studying the influence quantities and to elaborate a guideline for the uncertainty calculations. Measurements have been conducted on a HV generator, with the developed reference measurement equipment, the internal kV meter and current sensor.

To provide a substantial contribution to TC38 and TC42 work (via task-force D1.60). This will contribute to the development of measurement techniques relating to transmitted over voltages in terms of VFTO, thereby supporting the pre‑normative CIGRE working group D1.60 (Traceable measurement techniques for very fast transients), which in turn will report it to TC42. The project will deliver calibration procedures and CMC entries for VFTO to 100 kV, 10 ns, with a target uncertainty of 1 %.

The work on VFTO will provide new calibration methods for fast transient sensors installed in GIS. VFTOs are critical for the insulation coordination of GIS equipment. Therefore, different techniques, most of them in the field, are used to reduce the risk of VFTO. For this reason, VFTO has to be measured in service conditions. In order to assure the reliability of corrective actions to reduce VFTO, wideband sensors have to be calibrated and traceability is being developed. A co-axial GIS setup has been designed for the calibration of very fast transient sensors.

System components for a VFTO reference measurement system, consisting of a newly developed transient recorder and a 600 kV voltage divider, have been selected, characterized and calibrated. The traceability of the transient recorder was published at the conference CPEM 2018 in July 2018.

FFII together with VTT and RISE have designed a GIS setup for the generation of very fast transient over-voltages. A co-axial GIS generator, combined in a calibration rig with a modified fast divider developed by VTT in the 14IND08 ElPow project, has been built for calibration of very fast transient sensors by FFII with support from RISE and VTT. Traceability for VFT will be established using two different transient recorders.

IEC TC42 is being regularly updated on the project’s development and contributions towards the work on technical brochure on VFT techniques within CIGRE D1.60.

For the traceability of transmitted over-voltages in instrument transformers, for the the standard IEC 61869-1 of IEC TC38, measurements have been performed at FFII, by RISE and FFII in May 2018. Traceability of the waveshape 0.5/50 ms for the over-voltages has been established. Further measurements of over-voltages on a second instrument transformer are scheduled for the coming period at RISE.

To provide a substantial contribution to TC42 work (via task-force D1.63). This will be achieved by contributing to the revision of IEC 60060 series (High‑voltage test techniques), IEC TC42, MT17 and MT23, and thereby supporting the pre‑normative CIGRE working group D1.63 (Partial discharge measurements). The project will deliver a calibration procedure and CMCs will be updated to include low‑level PD down to 0.1 pC with a target uncertainty of 0.01 pC.

This project’s work on PD measurement techniques will lead to new calibration services for the most sensitive PD measuring instruments. It will provide a significant understanding of PD phenomena in HVDC systems. Results will be directly inputted to the relevant standardisation committee (IEC TC42). Improvement of PD measurements under d.c. stress will ensure a better service continuity for d.c. transmission and distribution grids using cable systems. Moreover, a reduction of the risk of eventual explosion or blackouts due to a short‑circuit fault is expected, because incipient PD pulses caused by small defects will be detected, thus preventing insulation failures.

We expect that according to the project plan, the new calibration procedures for level PD down to 0.1 pC will be introduced by four partners, and respective CMCs submitted for EURAMET review. The target measurement uncertainty of 0.01 pC has already been reached. The experience will be shared within IEC TC42, and it will have influence onto the next revision of IEC 60270.

A paper titled “Traceability of partial discharge calibrations below 1 pC” has been submitted to IEEE Transaction on Instrumentation and Measurement. The setup of the low-level PD calibrator system is constructed by TUBITAK UME to generate precise PD charges from 2 pC to 500 pC and modified to also cover the 0.1 pC-2 pC level. A model function for the uncertainty calculations on the range of 2-500 pC was completed and reached the uncertainty level of less than 2 %.

An intercomparison of calibration of a low-level PD calibrator (0.1 pC) was agreed on at the meeting in Paris in July 2018. VTT will circulate a PD-calibrator in autumn 2018 between the participants FFII, RISE, TUBITAK and itself.

Representative test cells for corona PD, cavity PD, surface PD and floating PD have been designed and built. A database of reference PD pulses in High Voltage DC/AC for corona PD, cavity PD, surface PD and floating PD has been generated. A grid operator Red Eléctrica de España has provided recordings of representative noise in HV d.c. grids. A paper titled “Test Cells to Generate Reference Partial Discharge Series” has been presented at CPEM 2018. A new method for evaluation and qualification of measuring and diagnostic instruments for Partial Discharge measurements has been patented. The proposed method allows to determine the following performances: sensitivity of the PD pulses under electrical noise, location of the defects, PD location at border zone between two different equipments, the discrimination of different PD sources located in the same site.

To work closely with the European and International Standards Developing Organisations, and the users of the Standards they develop, to ensure that the outputs of the project are aligned with their needs, communicated quickly to those developing the standards, and in a form that can be incorporated into standards at the earliest opportunity.

This research will create impact by enhancing the metrology for high voltage and other related quantities through the development of new techniques for the application of precision measurements. Essential competencies will be developed in a multidisciplinary approach. Successful achievement of the objectives will enable a greater number of test laboratories to benefit from research projects in the field of high voltage metrology.

The work on X-ray charging voltage measurement, regarding traceable calibration of X-ray equipment to ensure a better control of the ray dose, has been presentented and discussed with IEC TC62C and 62B.

Plans for the measurement system for VFTO were presented and discussed at the meeting of CIGRE D1.60 in Paris during the CIGRE general meeting in 2016 and at the conference ISH2017. Input by this project and work on the technical brochure were discussed. IEC TC42 was informed about the ongoing activities at the annual meeting in Frankfurt in October 2016 and at a session in Toronto in 2017.

The experience on PD under d.c. voltage stress has been extensively discussed within the CIGRE working group D1.63, and is expected to contribute to the writing of a technical brochure. The results will further be presented to IEC TC42 for input to standardisation.

Impact

The uptake of calibration services to be provided by the project has been prepared by informing the standardisation bodies TC38, TC42 and TC62C during web meetings and at the 80th general IEC meeting in August 2016. The pre-normative bodies Cigré D1.60 (VFT) and D1.63 (PD under d.c. stress) are now preparing for uptake of the results. A publication of a new transient recorder for VFT measurements was presented at CPEM 2018.

Impact on industrial and other user communities

Improved calibration of X-ray systems is needed to minimize the X-ray dose on patients.

The project has triggered cooperation with the instrument manufacturer National Instruments, and a new transient recorder for VFT measurements, the PXIe-5164, has been characterized within the project and is now available on the market. This will have impact by improving diagnostics of discharges in power grids, especially in GIS systems. A good news story of the digitizer, titled “Innovative instrument developed for electrical networks”, has been published on the EURAMET and the project webpages.

The project will provide methods for identification of PD sources in d.c. grids, of great interest for grid operators. Grid operators are supplying data to the project in order to validate methods for the identification of PD sources. Correct assessment of source and position of PD in the grid will both have impact on reliability and failure prevention the power industry as well as on the user communities in grid operation. The power industry will be able to use the new methods for identification of PD under d.c. stress for monitoring of the systems.

Impact on the metrology and scientific communities

Cooperation with a manufacturer on the design of a transient recorder for VFT has provided the community with a technology that has a dramatic increase in dynamic range combined with a high bandwidth and excellent step response needed in transient digitisers. This technique has the potential to become the state of the art.

The development of techniques for traceable PD calibrators for the traceable low-level calibration of PD down to 0.1 pC will have an impact on metrology. The design and construction of standard source equipment for studies of various corona sources, providing standardised typical PD patterns for d.c. systems, will also have larger impact on the research in this field.

Impact on relevant standards

The calibration of X-ray systems and development of algorithms for improved calibration are needed for future standardisation by TC62C. With this new calibration method, X-ray exposure to patients and staff will be kept at a minimum. In the final stages of the project, this method will be presented to the IEC TC62C for possible uptake in the IEC 61676 standard.

Partners have taken part in CIGRE meetings in Paris 2016, where the measurement system for VFTO was presented and discussed in CIGRE WG D1.60. During the ISH2017 conference in Buenos Aires, the progress on generation, measurement and methods for calibration of systems for VFTO was discussed.

IEC TC42 was updated on the progress in the development of VFT and PD under d.c. stress. which will have an impact on standardisation. IEC TC38/WG50 is preparing documents that consider how transmitted over-voltages add to measurement uncertainties.

Longer-term economic, social and environmental impacts

Enhanced measurement capabilities for the calibration of x-ray equipment acceleration voltage will lead to more accurate dosing of e.g. cancer treatment. Target beneficiary groups will be x-ray equipment manufacturers and users; and finally the patients being treated.

A metrological infrastructure will be created for the calibration of PD at low levels of apparent charge and for the evaluation of the performance of PD measuring instruments under d.c. stress. Greater reliability of electricity continuity will avoid blackouts. An additional impact will be to reduce the risk of explosions and catastrophic fires due to short-circuits caused by insulation failures.

Contribution to CIGRE work on the development of measuring techniques for very fast transients will support the compatibility of testing between different test organisations and it will enable accurate measurements via the development of calibration services. Better quality control of the components for a high voltage transmission system will lead to more cost effective solutions.

This project also offers a unique opportunity for European NMIs to pool their collective strengths, and unique capabilities and facilities, to make a long-term impact on improving patient safety, through introducing improvements for x-ray control. The outputs related to charging voltage measurement will put in place means to control to x-rays during treatment on a scientific basis, with the aim of minimising exposure.

Reliable electrical delivery is one of the prime needs in modern society. HV transmission grids are the crucial backbone of the total electricity grid infrastructure and are often referred to as the most extensive and complex machine made by humankind. The developments in PD for d.c. and measurements of VFTO will have a wider positive impact on power delivery.

One of the European 20-20-20 targets is a 20 % reduction in CO2 emissions compared to 1990 levels. The improved PD measurement techniques and more reliable instrument transformers will contribute to the reliability of the power grid. Even the smallest contribution to this improvement will result in an equivalent reduction in CO2 emissions of many kilotons per year.

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