Volker Hinrichsen

Diagnostics and monitoring of metal-oxide surge arresters in high-voltage networks-comparison of existing and newly developed procedures

Various diagnostic methods-either on-line or off-line-for monitoring of metal-oxide surge arresters in power systems are known. In most cases they are based on the measurement of the leakage current. This paper describes the most important ones in detail, such as measuring of the V-I-characteristics or analysis of the third harmonic content of the leakage current. Their advantages and weak points, respectively, are discussed, based on the most recent investigation results on the change of the V-I-characteristics due to electrical and nonelectrical stresses. It turns out that almost none of these commonly applied methods really work satisfactorily under either technical or economical aspects. As an alternative the temperature of the MO-insert is proposed to be used for diagnostics purposes. This is made possible by newly developed passive temperature sensors, which can be remotely accessed without any hardware connection to the environment. It is shown how the temperature can be used for monitoring of all kinds of degradation, energy absorption, thermal stability or even pollution performance.

Experimental investigations on water droplet oscillation and partial discharge inception voltage on polymeric insulating surfaces under the influence of AC electric field stress

Water droplets on hydrophobic surfaces of polymeric insulators oscillate under applied high voltage electric field stress. Deformation of the droplets increases local electric field stress in the triple zones, causing lower partial discharge inception voltage and consequently affecting the ageing performance of the insulator. For this contribution dominant modes of single water droplet oscillation on hydrophobic surfaces were investigated in a wide range of droplet volume and frequency of applied electric field using a high speed camera. 3D frame analysis showed for all volumes an increasing trend of mode number with frequency in the range of 20 to 100 Hz. Experimentally found resonance frequencies for water droplets on a hydrophobic surface approximately correlate with published frequencies of free water droplets. Conductivity showed no effect on the pattern of water droplet oscillation. Partial discharge inception voltages on two different hydrophobic surfaces (silicone rubber and epoxy resin) with conductive and non-conductive (distilled) water droplets were measured. These measurements showed that inception voltage makes a step whenever the oscillation mode of water droplet changes. Conductivity of the water droplets was found to have no effect on the inception voltage on silicone rubber surfaces having a high degree of hydrophobicity, whereas conductive droplets on epoxy resin with lower hydrophobicity lead to lower inception voltages.

Recent Experimental Findings on the Single and Multi-Impulse Energy Handling Capability of Metal–Oxide Varistors for Use in High-Voltage Surge Arresters

This contribution discusses different aspects of impulse energy handling capability of metal-oxide (MO) varistors used in high-voltage surge arresters. The investigations were performed on more than 3000 MO varistors of different manufacturers worldwide. It can be confirmed that the single-impulse energy handling capability increases with increasing current density of the energy impact, a finding that was also published earlier but based on a very limited number of test samples. In addition, the impact of the MO varistor diameter is discussed. Thereafter, the differentiation of single and multi-impulse energy handling capability is addressed. Finally, investigations with a repeated energy injection (up to 100 prestresses per MO varistor at time intervals that allow the MO varistors to cool down to ambient temperature) are reported. At none of the investigated repeated energy stresses, any change of the V-I characteristic in the leakage current region was observed that would affect the arresters overall thermal stability.

The impact of capacitor bank inrush current on field emission current in vacuum

Field emission current measurements during the recovery voltage are investigated to understand the origin of restrikes in vacuum interrupters in case of the interruption of capacitive loads. Measurement and analysis of very small field emission currents (0.01 – 1 mA) from the current zero crossing until the restrike are performed both in an experimental circuit as well as in a full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of some kA and kHz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.

Experimental investigations on electrohydrodynamic phenomena at single droplets on insulating surfaces

Polymeric insulating material can lose its hydrophobic proper-ties when is stressed by various environmental factors under service conditions. Humidity is one very important factor for ageing phenomena. This paper reports investigations on single droplets which are electrically stressed on hydrophobic polymer surfaces. With the help of two digital optical diagnostic systems deformations and movements of the droplets can be recorded and analyzed in an electrical ac and dc field. The experiments show several electrohydrodynamic phenomena which have an effect on the process of the very early stage of ageing of polymer insulating surfaces. When a critical field stress is exceeded different phenomena occur, such as transient structures on the surface, creation of filaments, formation of water paths between droplets, collapsing of several droplets, separation of droplets and ejection of very small droplets from bigger ones. These effects lead to a deformation and rearrangement of the original droplets. As a result the droplets cover a wider area of the insulating surface and minimize the dry zones of the solid surface. Thus, bridging by a water path between the electrodes becomes more probable.

Field emission currents in vacuum breakers after capacitive switching

Field electron emission current measurements during the recovery phase of vacuum interrupters are carried out to understand the origin of restrikes in case of interrupting capacitive loads. A special shunt is designed for this measurement. To compensate the capacitive current component due to the stray capacitance of the interrupter, hardware (electronic, on-line) as well as software (off-line) processing are developed for different applications. Measurement and analysis of field emission currents (0.01-5 mA) from current zero until the moment of restrike are performed both in an experimental circuit as well as in full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). In this context, the influence of capacitive breaking currents on the dielectric recovery of the vacuum gap and the contacts surface condition is studied. Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of several kilo-amperes and kilo-hertz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.

Measurement of field emission current during switching of capacitive current in vacuum

To check the dielectric behavior of vacuum interrupters after capacitive current interruption and find the reasons for possible restrikes, measurements of field emission current during the recovery voltage is performed. An experimental set-up is built up for current interruption test and dielectric measurement. This set-up allows the measurement of very small field emission current in the range of some 10 µA to some mA. A special measurement current shunt is developed for this measurement. To measure the field emission current compensation of the capacitive component of the current which flows due to the stray capacitance of the vacuum interrupter is necessary. In some cases the capacitive component of the current is even much higher than the field emission current. To compensate the capacitive component hardware (electronic, on-line) compensation as well as software compensation (off-line) is developed. Furthermore, with the help of recovery voltage and field emission current measurements for every half cycle of the recovery voltage the Fowler-Nordheim-Equation (FNE) is used to evaluate the field enhancement factor and the emitting areas, which are indicators for the surface condition of the contacts and possibly for the dielectric behavior of the interrupter.

Investigation of composite insulators with microvaristor filled silicone rubber components

This paper reports possible applications of microvaristor filled silicone rubber at composite insulators. Different insulator variations fitted with microvaristor filled sheds or shanks have been investigated. The insulators are dimensioned for Um = 170 kV and Um = 420 kV. Different variations of the 420 kV type were tested for their a.c. behaviour under artificial rain. In these tests the insulators with microvaristor filled silicone rubber components showed interesting properties. The suitability of microvaristor filled silicone rubber for application both in the sheds and in the sheath are discussed based on these positive results. The applicability for outdoor applications is investigated by an “inclined plane test” (material test) as well as by a “weather ageing test” under salt fog (material and design test).

Application of varistors for overvoltage protection of machine windings in inverter-fed drives

This contribution deals with overvoltage protection in inverter-fed drives by varistors. Several kinds of overvoltage in combination with very low rise times impose extreme dielectric stress on the insulation of electrical machines. Here a novel approach of overvoltage protection in inverter-fed drives is presented. After a discussion of the origins of the overvoltages and of overvoltage limitation by varistors in general, the dimensioning for the particular application, which is quite unusual for varistors, will be presented. Two special cases of actual overvoltages are addressed. Oscillograms are analyzed, and comparisons of measurements and calculations are made as well. Finally, possible applications for certain cases are discussed.

Electroquasistatic-Thermal Modeling and Simulation of Station Class Surge Arresters

A coupled electroquasistatic-thermal method for the simulation of surge arresters is developed. In order to cope with the extremely short-time scales associated with the strongly nonlinear electrical characteristic of the metal-oxide resistor material, a multi-rate time integration technique is adopted. Besides, a model for the heat transfer in the arrester air gap is developed, which considers radiation and natural convection by means of an equivalent material model. 2-D finite-element simulations for a station class arrester in continuous operation are carried out and validated against measurements. Furthermore, the method is used to investigate thermal stability under pulsed overvoltages as specified by the IEC operating duty test.