High-Voltage Test and Measuring Techniques

High-voltage (HV) test and measuring techniques are considered in most general HV text books (e.g. Kuchler 2009; Kuffel et al. 2007; Beyer et al. 1986; Mosch et al. 1988; Schufft et al. 2007; Arora and Mosch 2011). There are teaching books on HV test techniques for students (Marx 1952; Kind and Feser 1999) as well as few text books on special fields, e.g. on HV measuring technique (Schwab 1981; Schon 2010). It is the aim of this book to supply a comprehensive survey on the state of the art of both, HV test and measuring techniques, for engineers in practice, graduates and students of master courses. A certain guideline for this is the relevant worldwide series of standards of the Technical Committee 42 (TC42: “HV Test and Measuring Techniques”) of the International Electrotechnical Commission (IEC), largely identical with the corresponding standards of the Institute of Electrical and Electronic Engineers (IEEE). This introduction contains also the relation between HV test and measuring techniques and the requirements of power systems with respect to the increasing transmission voltages and the principles of insulation coordination. Furthermore, HV testing for quality assurance and condition assessment in the life cycle of power equipment is investigated.

High-voltage (HV) testing utilizes the phenomena in electrical insulations under the influence of the electric field for the definition of test procedures and acceptance criteria. The phenomena—e.g., breakdown, conductivity , polarization and dielectric losses —depend on the insulating material, on the electric field generated by the test voltages and shaped by the electrodes as well as on environmental influences. Considering the phenomena, this chapter describes the common basics of HV test techniques, independent on the kind of the stressing test voltage. All details related to the different test voltages are considered in the relevant Chaps.​ 3–8.

HVAC test voltages represent the stress of insulations by operational alternating voltages (50 or 60 Hz) and temporary over-voltages. For that reason they are the most important test voltages and applied for all kinds of withstand tests, lifetime tests and dielectric or partial discharges (PD) measurements. After the detailed description of HVAC voltage generation, the requirements of HVAC test voltages and the interaction between test system and test object are investigated. Measuring systems for HVAC test voltages are mainly based on capacitive voltage dividers and peak voltmeters, but for measurements during HVAC tests with an expected voltage drop, with harmonics or fast voltage changes, digital recorders become more and more necessary. The chapter is closed with a section on procedures for HVAC breakdown and withstand tests, dry, wet as well as pollution tests, long duration or lifetime testing. Examples for HVAC tests on cables, gas-insulated switchgear, power and instrument transformers are given.

This chapter is devoted to the measurement of partial discharges (PD) localized in weak spots of dielectrics if subjected to high test voltages. As standards for PD measurement s have been introduced only for quality assurance tests of HV apparatus energized by power frequency AC voltages (50/60 Hz), the following treatment refers mainly to this test voltage. Specific aspects to be taken into account for DC test voltages and also impulse voltages will only briefly be treated in the relevant chapters. Nowadays, PD measurements are performed not only in electromagnetically well-shielded laboratories but also under noisy on-site conditions, as discussed in this chapter.

The ageing of the insulation of HV apparatus is not only caused by the high electric field strength but also by thermal and mechanical stresses that evolve during normal operation condition. This may lead to chemical processes associated with a gradual deterioration of the integral insulation properties. Finally, weak spots and, in extreme case, a breakdown might occur, which causes not only an unexpected outage of HV equipment but also physical, environmental and financial damage. To ensure a reliable operation of the HV assets encourages high standards of quality assurance tests after manufacturing as well as advanced tools for preventive diagnostics in service. As treated already in Chap.​ 4, PD measurements have become an indispensable tool to trace local dielectric imperfections since the 1960s. In contrast to this, the measurement of the integral dielectric properties became already of interest for insulation condition assessment of HV equipment used for the first HV transmission systems at the beginning of the twentieth century. In this context, it seems noticeable that the dielectric properties are not only determined at test frequencies close to the service frequency, as in the case of loss factor measurements, but also at very low frequencies not representative for service conditions. Valuable results are also obtained if the dielectric response against DC exciting voltages is measured, as will be treated in the following.

HVDC test voltages represent the stress of insulations in HVDC transmission systems. Today, HVDC transmission systems are long point-to-point connections for the transmission of high power. These links are realized by HVDC overhead lines and HVDC cables, especially submarine cables. In the near future, it is expected that the application of the HVDC technology will increase and also HVDC networks will be established (Shu 2010). Therefore, also HVDC test voltage is becoming more important. This chapter starts with the different circuits for HVDC test voltage generation. Then, the requirements for HVDC test voltages according to IEC 60060-1: 2010 and the consequences for the components of test systems are considered. The interactions between test generator and test object are investigated for capacitive load—e.g. of submarine cables—and for resistive load in case of wet, pollution and corona tests. A short description of test procedures with HVDC test voltages follows. Finally, it is described how direct voltages can be measured by suitable measuring systems of resistive dividers and suited measuring instruments, and how measurements—e.g. PD measurements—at DC voltage are performed.

Lightning impulse (LI) over-voltages and switching impulse (SI) over-voltages are caused by direct or indirect lightning strokes or by switching operations in electric power systems, respectively. They cause transient stresses to the insulations, much higher than the stresses due to the operational voltages. Therefore, insulations must be designed to withstand LI and SI over-voltages, and the correct design has to be verified by withstand testing using LI test voltages , respectively, SI test voltages . This chapter deals with the generation of aperiodic and oscillating LI and SI impulse voltages and the requirements for their application in HV tests. Special attention is given to the interactions between the LI/SI generator and the test object. The deviations from the standardized impulse shape, e.g. by an over-shoot on the LI peak, are analysed, and the evaluation of recorded pulses according to IEC 60060-1:2010 and IEEE St. 4 (Draft 2013) is described. This is completed by the description of components and of the procedures for the correct measurement of LI/SI test voltages. Also the measurement of the test currents in LI voltage tests and the PD measurement at SI test voltages are included.

In power systems, the over-voltage stresses of insulations are often combinations of the operational voltage with over voltages. This can be neglected as long as the over-voltage value includes the contribution of the operational voltage. It cannot be neglected when the insulation between phases or of switching devices is considered. In that case, the resulting voltage is the combination of two voltage stresses on three-terminal test objects. In other cases, the stressing voltage is composed of two different voltage components, e.g. in certain HVDC insulations, as a composite voltage of AC and DC components. This chapter is related to the definition, generation and measurement of combined and composite test voltages on the basis of IEC 60060-1:2010 and IEEE Std. 4. Also some examples for tests with combined and composite voltages are given.

Efficient HV testing, research work or students training requires well-designed HV laboratories. This chapter is related to the planning of HV laboratories. The basis is a clear analysis of the requirements to the laboratory and corresponding selection of the HV test systems. This includes a general principle of the control and measuring systems, the internal data evaluation and communication structures. The planning of test buildings or test rooms depends strongly on the objective of the laboratory and of the available funds. The general principles for the grounding and shielding, for power supply, transportation and auxiliary equipment are explained. An important part of the planning is a safety system which guarantees both safety for the operators and reliable, quick testing. Some specialities to outdoor test laboratories and updating of existing test fields are submitted.

High-voltage (HV) tests and measurements are applied for two different reasons on site: When new equipment or systems are finally assembled on site, a quality acceptance test, usually a HV withstand test (more and more completed by PD or dielectric measurements), is required to demonstrate the necessary quality and reliability for commissioning. This test completes the quality tests in the factory and should follow their philosophy based on insulation coordination. The aim of the second group of tests on service-aged equipment, the so-called diagnostic tests, is related to the condition assessment of insulation for the estimation of the remaining lifetime. For diagnostic purposes, usually a set of tests and measurements is performed. HV tests after repair are in between the two, because the repaired part is new, but the other insulation of the equipment or system is service-aged. After a consideration of the general requirements to HV test systems used on site, the applied test voltages according to IEC 60060-3: 2006 are introduced. The chapter is closed with examples for both, quality acceptance withstand tests and diagnostic tests on site, for compressed-gas-insulated equipment (GIS), cable systems, power transformers and rotating HV machines.