High-Voltage Test and Measuring Techniques

High-voltage (HV) test and measuring techniques are considered in most general HV text books (e.g. Kuechler 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, 2016). 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: “High-Voltage and High-Current 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, partial discharges, 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) originating in weak spots in the insulation of HV apparatus and their components. As most HV equipment used to generate, transmit and distribute electric power are energized by high voltage alternating current (HVAC), this chapter focuses primarily on PD measurements under alternating voltage. However, specific problems which arise for PD tests under DC and impulse voltage will also briefly be considered. Sensitive PD measurements are often interfered by electromagnetic noises in the measuring surroundings. Therefore advanced features developed in the past to cancel disturbing noises will also be highlighted in this chapter as well as in the relevant sub-sections of Sects. 10.​3 and 10.​4. For a better understanding of the principles, procedures and instrumentation required for the measurement of electrical PD quantities, first some fundamentals of the PD occurrence will be presented, where also the PD quantities recommended in IEC 60270:2000 for the insulation condition assessment are considered. In the following section, the existing PD models proposed to estimate the PD charge transfer will critically be reviewed. Thereafter specific aspects of pulse charge measurements, as specified in IEC 60270:2000, are considered, which include the major components required to design PD measuring systems used to acquire and display the captured PD data and even the procedures employed to calibrate PD measuring systems. The following sections are dealing with the localization of PD faults, the visualization of PD events, and features developed to reduce or cancel electromagnetic noises interfering sensitive PD measurements. Finally, so-called non-conventional PD detection methods will briefly be highlighted, such as the measurement of electromagnetic PD transients up to the ultra-high frequency range as well as the detection of ultrasonic signals emitted from PD sources.

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, an ultimate breakdown might occur, which causes not only an unexpected outage of HV equipment but also physical, environmental and financial damages. To ensure a reliable operation of 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, while the measurement of integral dielectric properties, such as capacitance and loss factor measurements, became of interest for insulation condition assessment of HV equipment already since the beginning of the last century when the first HV transmission systems above 100 kV were erected. In this context, it should be noted that the dielectric properties are often determined at test frequencies different from the service frequency (50/60 Hz). So valuable information on the insulation condition may also be gathered by measuring the dielectric response under DC voltage after this is switched on and even off, as will also 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 HV tests as well as PD and dielectric measurements under direct voltage are becoming of increasing importance. Design and testing of HVDC insulation has to be performed under a difficult understanding of the acting electric fields. The reason are space and surface charges which are generated even below the PD inception voltages and strongly influenced by thermal effects. There are many publications on these phenomena, e.g. (Hering et al. 2017; Ghorbani et al. 2017; Christen 2014). 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 even 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, LI and VFT 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. It should be mentioned that these voltages are sometimes called “hybrid” or “superimposed” voltages, also the summarizing term “mixed” voltages is in use. This book follows the terminology of the Standard IEC 60060-1 (2010).

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.