Rainer Haller

Overvoltages in photovoltaic systems induced by lightning strikes

The high number of photovoltaic systems raises questions regarding their operational security even under extremely weather conditions like thunderstorms. Therefore a simplified model of a photovoltaic system, consisting of typical arrangement of solar-panels, was investigated. The influence of lightning strike was simulated by different lightning current shapes and striking points into the solar-panels. It was found, that besides the construction size and shape of the lightning current the striking point has a main influence on the value of induced overvoltages, occurring on the connection point of the solar-panels. The simulation results were confirmed by measurement with an impulse current. The results demonstrate the dangerousness of lightning currents with very fast rise time e.g. subsequent impulse as well as the reducing of induced overvoltages by an appropriate earthing connection. This has to be considered at the design of the lightning protection system.

PD behaviour of basic test arrangements under different measurement and evaluation conditions

The PD behaviour of so called basic test arrangements as for corona discharge or other discharge sources seems to be well known from the physical side of view. On the other hand the measurement results and, in the same manner, their identification and interpretation can be different, because this is depending on the measurement or evaluation conditions. Therefore some typical corona basic arrangements representing corona needle-plane, needle-plane with insulating material and needle-sphere are investigated. The measurements are carried out under different measurement conditions especially the selected frequency range of the measuring system. For some effects the reproducibly, the voltage dependence and the changes over time of the PD behaviour are investigated. These results were evaluated using the applied and the updated version of IEC 60270. Furthermore the IEC 60270 and additional publications provide a lot of possible of the post evaluation of the measured PD signals. In this paper the most and most recognized values are compared at the well defined laboratory set ups as defined. Additional these results are compared with the results of the Pulse-Sequence-Analyzing method. The presented results show surprisingly characteristic shapes of the PD pattern, which allow the better classification of the types of PD sources and their identification respectively.

PD behaviour of basic test arrangements under different measurement conditions

The PD behaviour of so-called basic test arrangements as for corona discharge or other discharge sources seems to be well known from the physical point of view. On the other hand the measurement results and, in the same manner, their identification and interpretation can be different, because this is depending on the measurement or evaluation conditions. Therefore some typical basic arrangements representing corona discharges as needle-plane, needle-plane with insulating sheet and needle-sphere as well as artificial cavity into insulating material were investigated. In a previous paper the pd behaviour of the corona set-ups in air were already discussed. Additional to that the pd behaviour of typical internal pd source under different measurement condition is described. In this paper the most recognized values are compared at the well defined laboratory set ups as defined. During these investigations the standard evaluation methods according to IEC 60270, PRPD pattern and pulse sequence analysis is done. By comparing these three evaluation types the most significant method for the classification of failure types is discussed. For closer evaluation of image based evaluation methods like PRPD pattern and pulse sequence analysis different methods of classification are discussed. The presented results show surprisingly characteristic shapes of the PD pattern, which allow the better classification of the types of PD sources and their identification respectively.

Aspects of PD measurement in hybrid gaseous insulation at DC voltage

The increased application of HVDC components in electrical energy systems has led to new challenges in diagnostic testing and measurement of electrical insulation. In addition to other diagnostic methods Partial Discharge (PD) diagnosis at DC voltage is currently subject to much discussion worldwide. This is because the established and approved procedures, methods and evaluation techniques at AC voltages cannot be so easily applied under DC conditions. This paper deals with some problems associated with PD measurement, which can be carried out in hybrid gaseous insulation using DC testing. In principle, such insulation types are mostly characterized by the presence of insulating (solid) materials. These interfaces can become electrically charged, should an electric field be present within the insulation, e.g. by the application of an external electric field during PD testing. For the study of such phenomena, a simple needle-plane electrode set with an insulating sheet was investigated at DC voltage with different polarities and electrical stress durations. It could be shown, that the charging process on the insulating sheet already takes place at test voltages below the measured PD inception voltage level. The charge intensity on interface, and as a consequence, its impact on the electrical field conditions e.g. the inception field strength, depends both on polarity and applied voltage duration. It is assumed, that any charging process below the (self-sustaining) ionisation level, characterized by the electrical inception field strength, should be caused by natural ionisation processes which take place in the test object. This assumption was supported by different results obtained if the test set was under open- and closed- volume condition. In the same manner, the results obtained were confirmed by electrical field calculations for different charging conditions on the insulating sheet and applied voltage polarity. The results described in this paper lead to the main conclusion that the charging ability of interfaces in hybrid gaseous insulation must be considered if PD measurement at DC voltage should be performed. For a deeper understanding of these processes further investigation, e.g. in a more compact insulation type appears to be necessary.

Partial discharges evaluation at DC voltage

There are discussed the adequate problems to estimate the partial discharge (PD) behavior of typical PD arrangements under AC and DC conditions n the paper. The investigations were performed using typical PD test arrangements, corona, gliding and internal PD. For estimation of the PD behavior the standard phase resolved characteristics and PSA characteristics were used.

Aspects of statistical treatment of electrical breakdown behaviour in gas-insulated apparatus

The knowledge of stochastic breakdown behaviour of environmental friendly gas mixtures become more and more important with respect to the required replacement of SF6 - insulated apparatus. To obtain the necessary withstand values of breakdown voltage at the variety of desired parameters a statistical treatment of the measured data is required. The paper deals with the estimation of these values typical for such type of gas- insulated systems e.g. the approximation by a theoretical distribution function and necessary conditions. Problems at estimation will be discussed as non-conventional statistical treatment including some recommendations for their overcoming.

Partial discharge measurement at high voltage test circuit having static frequency converters

For testing of high voltage (h.v.) components under so-called unconventional frequency test conditions as h.v.-test source very often static frequency converters (SFC) were applied. A typical example for that is the testing of power transformers at higher frequency. In the past this test was provided by standardized motor-generator sets, but more and more alternative power sources having SFC-units are used. In that case the performing of sensitive diagnostic test procedure like partial discharge (pd) measurement seems to be impossible caused by vast electromagnetic disturbances which are generated by the switching part of the SFC. On the other hand the performing of such sensitive diagnostic measurement is obligatory. For overcoming of these problems two possibilities will be described in the paper. At first the EMI pulses which are coupled into the test circuit must be suppressed. Unfortunately, because of the mostly unknown “current-way” of these disturbances, the optimal solution of suppression must be found by a “half-empirical” way, taking into account the well-known EMC requirements. In addition to that the right choice of the commonly used filter located after the SFC-component is very important and should be designed following the performed EMC measures. For an exemplary h.v. test circuit the appropriate measures to minimize the EMI disturbances are described, at which a pd-sensitivity of ~ 30 pC could be obtained at the full range operation of SFC. For further increasing of pd-sensitivity additional measuring methods were investigated. For a desirable measuring sensitivity of ~ 10 pC the suppression of EMI signals, reached by additional measures should have at least a value of ~ 10 (20 dB), what could be obtained either by digital or by analogous methods. Using all the described measures for h.v. test circuits having SFC a pd measuring sensitivity in the range of few pC seems to be reachable.

Influence of transmission line wave properties on surge voltage wave propagation

The paper deals with an analysis of transmission line wave properties and their influence on the voltage wave propagation in the system EHV/UHV consisting of a transmission line and a grid transformer where a gas-insulated switchgear (GIS) are set up. Switching on/off operations in the GIS generate pulses with very short rise time it results in travelling wave phenomena which are depending on the type of the line-connection and wave properties of both the line and the transformer. The analysis of the system transmission line-grid transformer was carried-out numerically in time domain and an algorithm based on FDTD has been developed. It provides the time and space distribution of voltage and current waves along the line and the transformer windings.

Dielectric behaviour of gaseous insulation with synthetic air at AC stress

The paper deals with basic investigation of AC breakdown behaviour of gaseous insulation with synthetic air at different level of field homogeneity and gas pressure. The measurement was performed at different gas pressure ranging from 0.1 to 0.25 MPa and gap distances from 1 to 9 cm, using an electrode test-set “tip-bowl”. The test parameters were chosen in such a way that different field types could be realized. At small distances the electrical field is weakly inhomogeneous and the breakdown values correspond to the discharge inception level. The measured values show a reasonable agreement to those values based on the calculation of the streamer criterion by using a relevant approximation formula for the effective ionisation coefficient. For larger distances the electrical field becomes more inhomogeneous and the calculated streamer inception voltage corresponds to the partial discharge (PD) inception voltage only. The calculated inception voltage values fit the measured values over the applied gap distance and gas pressure quite sufficiently. The best fit to the measured results was reached by slightly changing the parameters of the approximation formula depending on the gas pressure value. Additional PD measurement shows different PD activity dependent on gas pressure and gap distance.

Corona pulse sequence analysis at DC voltage

The high voltage diagnostics must follow the actual state of the industry devices and the problems related to diagnostics methods. One of the diagnostic methods necessary to be solved at this time is the partial discharge detection, diagnostics and analyses in HVDC systems. There is no standardized method for detection of partial discharges in HVDC systems supplied with DC voltage. The presented study shows the application of pulse sequence analysis to analyse the partial discharges at DC voltage. There has been prepared the experiment with typical corona arrangement, needle-plane, to study and analyse the partial discharges at DC voltage. The measured data at DC voltage are compared to data measured at the same arrangement at AC voltage. The pulse sequence analysis has been applied for both AC and DC voltage and the data are plotted and compared. The aim of the study is to present the connection of pulse sequence data of apparent charge and pulse time differences between AC and DC.