Context

Ultra high-voltage systems

In the production of equipment for high-voltage grids, dielectric testing is performed to verify that the equipment can withstand the operational environment, including high voltage and high current impulses. Methods and schemes for traceable calibration are defined in IEC 60060-2 for high voltage and in IEC 62475 for high current. However, system voltages are currently increasing to levels higher than those covered by this standard, and there is a need to extend the traceability of the test methods into the ultra-high voltage (UHV) range above 800 kV.

The expansion of UHV grids, now operating at 1100 kV system voltage for DC and 1200 kV for AC, requires testing with voltages up to 2000 kV and traceability of DC and AC signals are now established up to 1600 kV. For switching and lightning impulse measurements, the impact on measurements due to proximity effects, corona, front oscillations, divider topology and measuring cables has been studied and will be collected in a good practice guide.

The highest test voltages surpass 2500 kV for lightning impulse testing, 4000 kV for extreme cases, and the traceability is typically available up to 800 kV on site and up to 2000 kV at national metrology institutes (NMI) laboratories. New methods need to be developed to linearly extend traceability to the highest voltage in testing facilities. Large measurement systems are strongly affected by corona and proximity effects, and generally methods to handle wave shape distortions, like front oscillations and losses in measurement cables, need a revision. Providing traceability for these measurements is especially challenging in the case of impulse voltages above megavolt level. Traceability is also required for voltage dividers and measuring systems for composite and combined voltage tests. During these tests, a high impulse voltage is applied to the test object in addition to continuous high AC or DC voltage.

For verification of high-voltage DC systems, there is an increased need for traceable partial discharge (PD) measurements. Whilst PD measurements for AC grids is a well-developed area, the reliable measurement and categorisation of PD for high-voltage DC grids is a relatively new area that requires further metrological research and development of traceable reference instrumentation and measurement methods.

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High-voltage transformers and reactors

Loss measurements on large transformers and reactors are performed using complex measuring systems that rely on extremely precise voltage and current transducers connected to advanced power meters. For large power transformers it is necessary for manufacturers to measure the active power with an uncertainty of better than 3 % at a power factor that may be 0.01, which leads to an accuracy requirement of 0.03 % of the apparent power. Therefore, the calibration of such measuring systems at the manufacturer’s site should be done with a level of accuracy that is improved by at least a factor of 3. Individual component calibration is only partly suitable for the calibration of such systems and so new calibration facilities are required that provide a system calibration service for this purpose with sufficient accuracy (i.e., uncertainty smaller than 0.01 % in ratio or 100 Î¼rad in phase).

Transformer and reactor loss calibrations are presently performed under sinusoidal conditions, but the actual grid conditions suffer from an increasing number of harmonics . New metrology is needed to traceably quantify the impact of these harmonics on the losses of transformers, reactors and other grid components.

High-voltage components

High-voltage components such as cables, insulators, instrument transformers, capacitors, surge arresters or switchgears require thorough testing, which typically includes, in addition to measurement of withstand voltage, the measurement of loss factor, insulation resistance, partial discharge, and life-time assessment of power electronics. Non-destructive testing methods are also required for commissioning or preventive maintenance of large equipment used in installations such as power transformers, overhead lines and cables (e.g. new wave shapes for DC cable testing), high-voltage substations or high-voltage DC (HVDC) converter stations. This can involve superimposition of DC signals, high-frequency harmonics, switching patterns in power electronics, new wave shapes for DC cable testing. On-site testing brings about significant logistical and technical challenges with complex interconnection schemes. For instance, high-capacitive test objects such as power cables, generators or capacitors can be tested after their installation on-site using very low frequency technique (VLF) down to 0.1 Hz.

Measurement challenges

  • Traceability of very low frequency resonance testing methods including loss measurements
  • Loss measurements of power transformers, and high-voltage (HV) capacitors and reactors
  • On-site measurement of HV calibrations
  • General improvement of measurement accuracy in HV testing
  • Linear extension of lightning impulse voltage beyond 2500 kV level
  • Testing and diagnostic of specific High-voltage DC (HVDC) equipment and components
  • Traceability for ultra high-voltage (UHV) component testing up to 2000 kV
  • UHV testing diagnostic techniques
  • Composite voltage testing of cables
  • Combined voltage testing of AC and DC switches
  • Partial discharge testing of DC

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