High Voltage Measurement Techniques

High direct, alternating and impulse voltages as well as the corresponding currents play an important role in the electrical energy supply, but also in many other areas of physics and technology. The high-voltage apparatus and other equipment used for this purpose are subjected to a series of tests prior to commissioning, which allow limited information on their reliability and expected lifetime. The decisive factor is the proper execution of high-voltage insulation, which is based inter alia on the specific knowledge of the solid, liquid and gaseous insulating materials used. All tests require accurate measurement techniques and the use of calibrated measuring systems. In this context are terms and content such as quality assurance, calibration, traceability of the measurements to the SI units, measurement uncertainty, internationally agreed test specifications, accredited testing and calibration laboratories. In the following chapters, the old but still valid fundamental basics and principles of high-voltage measurement techniques are combined with the more recent developments in all the above-mentioned fields under particular consideration of digital measurement technology and data transmission.

The transmission of electrical energy from the power plant to the consumer takes place predominantly with high alternating (AC) voltages, so that this voltage and thus also the alternating currents have special significance. Each apparatus for the electrical energy supply is therefore tested for reliability prior to commissioning. The test and measurement procedures as well as requirements for test voltages and test currents are specified in national and international test regulations. High AC voltages are also important because they are needed for the generation of high DC and impulse voltages as well as numerous applications in physics and engineering. Furthermore, high alternating voltages are used for tests on insulating material in terms of dielectric properties and partial discharges. The chapter introduces the standardized quantities and measurement methods, briefly describes the basic principles of AC voltage and current generators, and discusses the measuring systems and instruments in more detail. Although analog measurement methods and devices are still in use, including the standard sphere gap, to-day the main focus is on digital measuring systems with computer-aided data processing, allowing online and on-site tests.

High direct (DC) voltages and currents are becoming increasingly important because of the increasing number of high voltage direct current (HVDC) transmission systems worldwide. Numerous other application fields exist in physics and technology, and DC voltages are also needed to generate impulse voltages and currents. The basic requirements for test and measurement methods for electrical power equipment are laid down in national and international test specifications. The introduction of digital measurement techniques with computer aided data processing has greatly improved the scope, quality and accuracy of the measurements. This marks the imminent end of the analogue circuits and instruments used so far, though analog measurement methods and devices, including the standard rod-rod gap, are still used. This chapter first defines the standardized measurement quantities of DC voltages and currents, briefly describes a few selected generator systems and then discusses in detail the predominantly used digital measurement circuits and methods with computer-aided data processing, allowing online monitoring and on-site tests.

In power supply networks for the transmission and distribution of electrical energy at high voltage, transient voltages of more than 1 MV can occur due to lightning or switching operations. Because of their high magnitudes and short rise times, ranging from a fraction of a microsecond to milliseconds, the overvoltages cause enhanced stress on the insulation of the affected high-voltage apparatus, such as power transformers, switchgears, arrestors, insulators, power cables, etc. Therefore, before commissioning, each high-voltage apparatus must undergo acceptance tests with standardized impulse test voltages. The chapter introduces the standardized quantities and measurement methods and briefly describes the basic principles of impulse voltage generators. The various systems for measuring impulse voltages with resistive, capacitive or mixed voltage dividers and digital recorders are discussed in detail. The main focus is on digital measuring systems with computer-aided data processing that enable automated data acquisition, data filtering of impulses with overshoot, online monitoring and on-site tests. A well-founded measurement technique is essential and is achieved mainly by comparison measurement with a reference system. One important property of an impulse measuring system is its step response, which characterizes the transfer behavior. Short descriptions of analog measurement methods and devices that are still in use, including the standard sphere gap, are included. High impulse voltages are also used for applications in other fields of physics and engineering, such as plasma physics, electric spot-welding, electro-shock weapons, etc.

Tests with high impulse currents are performed to simulate the stress of power apparatus in the high-voltage grid caused by lightning and short circuits. Various impulse test currents and the corresponding measurement methods are standardized in IEC 62475, together with DC and AC currents. Impulse currents with peak values of up to several 100 kA are generated in the test laboratory, using capacitive energy storage units which are charged slowly and then discharged abruptly to the test object via an RC network. For the conventional measurement of impulse currents up to the highest current magnitudes, low-ohmic measuring resistors or magnetic coils with and without magnetic core are used together with digital recorders, enabling computer-assisted evaluation of the impulse parameters. A well-founded measurement technique is essential, which is achieved mainly by comparison measurement of the current measuring system with a reference system. One of the important properties of a current sensor is its step response, which characterizes the transfer behavior. Numerical convolution can be used for improving the dynamic behavior of a current measuring system.

The basics of electro-optic and magneto-optic effects, also named after their discoverers, have been known for more than a century. The Pockels effect and the Kerr effect characterize the optical properties of certain crystals, liquids and gases under the influence of an electric field, whereby the polarization of a light wave propagating in the optical axis of the medium is affected. According to the Faraday effect, a magnetic field also changes the polarization of a passing light wave. For all effects, a rotation of the polarization plane of the light occurs in the medium, which is indicated by a downstream analyzer and photodetector as the corresponding electric or magnetic field strength. The optical processes in the medium take place in the nanosecond range, so that bandwidths from zero to the GHz range can generally be achieved with electro-optic or magneto-optic sensors. The clause discusses the nature and characteristics of various recently developed sensors. In conjunction with fiber optics, there are good prospects for the use of these sensors in the high-voltage area. After calibration of the sensors on site, the voltages or currents that generate the fields can be displayed directly. The technical realization of sensors based on the Pockels or Faraday effect has progressed in the last two decades thanks to the solution of many individual problems, resulting in a variety of electro-optic and magneto-optic transducers in the high-voltage network.

The measurement of high direct, alternating and impulse voltages, as well as the corresponding currents, is carried out mainly with measuring systems, in which digital recorders or other digital instruments are used. Analog measuring devices, e.g. impulse oscilloscopes or impulse voltmeters, have practically no more meaning and are not treated here. An important component of digital measuring instruments is the A/D converter, which digitizes the analog measurement voltage and provides it as a digital data set for further evaluation with the PC. The requirements for A/D converters vary according to the voltage type. For example, the recording of impulse voltages and currents requires high sampling rates, which can only be realized by fast flash converters with limited amplitude resolution. The chapter describes a variety of test and calibration methods to determine the characteristics of digital devices. For the accurate calibration and verification of digital measuring instruments, calibrators are used which generate AC, DC and impulse voltages of several 100 V up to 2000 V. The analysis of the recorded data is carried out with evaluation software, which is also subjected to a comprehensive evaluation using the test data generator (TDG).

Single and continuous signals can be represented by their shape in the time domain or by their spectrum in the frequency domain. Both forms of representation are equivalent. Which form is preferred in a particular case depends on the measurement task and the specified target. Both the waveform and the spectrum can be used to derive requirements for the correct measurement of a signal. High-voltage impulses and high-current impulses are defined by their waveforms, which are characterized by the value of the test voltage – usually the peak value – and two time parameters. On the other hand, low-voltage measuring instruments, including analog oscilloscopes and digital recorders that are not specifically built to measure impulse voltages and currents, are more characterized by parameters in the frequency domain such as frequency response and bandwidth. In this chapter, the spectra as well as the time courses of some ideal impulse voltages and currents are calculated using the Laplace transform. This allows a statement as to whether the transfer behavior of the measuring instrument is suitable for the measurement task.

The transfer behavior of a linear system can be described by its input and output signals and expressed in the time domain or frequency domain. In order to analytically determine the transfer behavior of a system, the Laplace transform of a time function into the complex variable domain (spectral domain, frequency domain) and the subsequent inverse transform into the time domain is a very effective tool. The step responses of simple RC and RCL circuits, which represent the basic elements of voltage dividers, shunts and measuring coils, are calculated using the Laplace transform. With the convolution integral, the output signals of the RC and RCL circuits are calculated for some characteristic input signals. This opens up a variety of possibilities to thoroughly analyze and optimize impulse voltage and current measuring systems and their components without the need for extensive experimental investigations. For calculations with experimental step responses, numerical convolution is applicable due to the high computing power of the PC as well as the significantly improved properties of digital recorders.

For standardized tests of power apparatus used in the electrical power supply, approved measuring systems are used to measure the relevant test voltages and test currents. “Approved” in this case means that the measuring system meets the requirements of the test and calibration standards. The approval of a measuring system is achieved by a traceable calibration, i.e. a calibration that can be traced back to national or international measurement standards. After an introductory discussion on standardization, accreditation and traceability, the following sections deal with the mainly applied calibration methods, with emphasis on impulse voltage and current measuring systems. These are, for example, the determination of the scale factor by comparison with a reference system, the linearity test and the evaluation of the experimental step response. The principle of measurement methods and the requirements on the measuring systems are largely comparable for the various types of voltage and current, with few exceptions, which mainly concern on-site tests.

The optimum performance of high-voltage equipment and apparatus of the electrical energy transmission depends largely on the design and quality of the insulating materials and the error-free execution of the insulation. Solid, liquid or gaseous dielectrics, also in combination, are used as insulation material. Important characteristics of the dielectrics that are exposed to high AC or impulse voltages are the relative permittivity and the dissipation factor (dielectric loss factor). This chapter explains the basics of both measurement quantities and the various analog and digital measurement methods. The basic design of measuring devices such as the Schering bridge with and without Wagner’s auxiliary arm, the current comparator bridge and the digital measuring system with A/D converters are discussed. Examples of the calibration of the measuring instruments are given. The properties of compressed gas capacitors according to Schering and Vieweg, which serve as a virtually lossless reference in the measurement of capacitance and dissipation factor, are discussed in detail.

Partial discharge (PD) denotes a small, localized electrical discharge in the insulation between conductors, occurring when the local electric field strength exceeds a critical value. Prolonged exposure to partial discharges is known to degrade the dielectric behavior of the insulation and may even lead to complete breakdown. PD testing of high-voltage equipment and electrical apparatus is one of the most important and difficult tasks in high-voltage testing. This chapter describes the measuring instruments, the calibration methods and the test circuits for measuring the so-called apparent charge. For most apparatus, special PD measurement methods with computerized data processing have been developed, which are constantly being improved. Key words are on-site PD measurements after installation of the equipment, localization of single or multiple PD sources in spatially extended apparatus, synchronous multichannel PD measurement, VHF and UHF PD measurement techniques, permanent PD online monitoring and interference suppression methods. Partial discharges are associated with electromagnetic, acoustic, optical and chemical effects, which are also exploited for their detection and diagnosis, in particular in complex apparatus such as three-phase power transformers and gas-insulated switchgear (GIS). The PD measurement techniques at DC and impulse voltages are briefly discussed.

Each measurement is not perfect and therefore cannot give the "true" value of the quantity to be measured, but only a more or less accurate approximate value, called estimate (of the value). Even if the measurement is repeated under seemingly identical conditions, the measuring instrument with sufficiently high resolution will display values usually differing from one another. The imperfection, or, positively considered, the quality of a measurement is expressed quantitatively by a numerical value, the uncertainty of measurement. The result of a measurement is the more reliable, the smaller the uncertainty. If the specified measurement uncertainty limits are not met during tests and calibrations, the test object will not pass the acceptance test. This chapter describes the definitions and the concept of how the uncertainty of a measurement can be determined. Key words are model function of the measurement, Type A and Type B evaluation methods, standard uncertainties, expanded uncertainty, uncertainty budget and statement of uncertainties. Several examples are given in Appendix B.