Michael Kurrat

Distributed generation location and capacity effect on voltage stability of distribution networks

Analysis of the effect of distributed generation (DG) capacity and location on voltage stability enhancement of distribution networks is presented in this paper. The analysis is performed using a steady state voltage stability index which can be evaluated at each node of the distribution system. Different optimal capacities and locations are used to check this effect. The location of DG has the main effect on the system voltage stability. Voltage stability should be taken into account as one of the objectives of DG optimal allocation techniques. The analysis is conducted on a 69 node distribution system.

Switching behavior of different contact materials under capacitive switching conditions

For vacuum circuit-breakers capacitive current switching means a specific operating condition. Though it differs notably from the short-circuit current interruption situation, the circuit-breaker design needs to be reconsidered and adapted. Capacitive switching combines high inrush-currents at the connection of a capacitive load and considerable low breaking currents at its disconnection. A reliable dielectric performance of the breaker is required since the occurrence of a restrike under recovery voltage stress can cause voltage escalation. In terms of a reliable performance of the circuit-breaker the choice of contact design and material takes centre stage. The behavior of various contact materials and designs are studied during a series of tests, representing complete switching cycles for a 24 kilovolt (kV) system voltage. The test series is comprised of making operations and current interruptions followed by a subsequent capacitive recovery voltage. Under these conditions diverse variants of copper-chrome composite materials are analyzed and contrasted. Thereby the effect of additive materials is observed. The pre-arcing behavior and the occurrence of restrikes are observed as an indication of the alteration of the contact systems dielectric condition during the test series. In this regard the tested CuCr contact types show a distinctly differing behavior in their pre-arcing and restrike performance. Furthermore a serious erosion of the contact surfaces can be observed occasionally. This can affect the dielectric condition of the contact system significantly. In addition optical investigation of arcing periods by high-speed movies show the localization of an arc being established on the contacts and particles escaping the contact gap can be noticed.

Arcing Behavior on Different TMF Contacts at High-Current Interrupting Operations

Vacuum circuit breakers are widely used in the medium-voltage area. The majority of the installed vacuum tubes are equipped with electrodes using the transverse-magnetic-field design forcing the electric arc on a circular motion to avoid severe local overheating. A vacuum test switch was used to investigate the arc movement behavior between spiral- and cup-shaped electrodes at high-current interrupting operations. The switch was equipped with viewing ports allowing an observation from two rectangular views. Mounted sample contacts were made of CuCr 75/25 in different diameters. A digital 8-bit high-speed camera was used to record the arcing process with frame rates of 33 000 frames per second. Behavior patterns were investigated and compared with the arc voltage and the instantaneous current. Parameters such as the arc velocity and the current density on the contacts could be determined by means of the recordings. A static simulation model delivered Lorentz forces for a comparison between both designs. The experiments were conducted with short circuit currents from 20 to 60 kA (root mean square) with a frequency of 50 Hz.

Modeling the Arc Splitting Process in Low-Voltage Arc Chutes

Investigations on the arc splitting process in low- voltage arc chutes have shown that a threshold voltage has to be exceeded before new arc roots form. When an arc spot has been formed, the electrode fall is nearly constant and rather independent of the current. In simulations a thin layer of elements with a nonlinear current density-voltage characteristic surrounding the splitter plate represents the formation of new arc roots. To model this behavior a characteristic with a voltage hump at low current densities and a constant final voltage at higher current densities has been presented in former publications. In this paper, the influence of the magnitude of the voltage hump is studied. A lower voltage hump yields a smaller arc loop around the front edge of the splitter plate and an earlier subdivision of the arc. A too high voltage hump makes arc splitting impossible. A comparing simulation without any arc root model, but only with the properties of the metallic plates, shows that a realistic simulation of the arc splitting process needs special treatment of the arc roots. The results are compared and verified on the basis of experimental results. Hence, the arc splitting process is analyzed concerning arc voltage and current through the plate as well as location of the arc roots.

Thermal-Electric Simulations of Coated Conductors With a Variable Conductivity of the Buffer Layer

This work presents the results of thermal-electric simulations of coated conductors with systematic variations of the buffer layers electrical conductivity. As boundary conditions liquid nitrogen (LN2) cooling and an overcritical current density are used. For the simulations the commercially available 3D finite element program ANSYS, which was enhanced with a superconducting element, is used. The simulations are carried out with a virtual buffer layer, for the purpose of material selection, under the aspect of use for electrical power applications. The results help to clarify the influence of the conductivity of the buffer layer on the thermal-electric behavior of a high temperature superconductor. In addition the equations used to program the HTS element are described.

Effect of integration of Distributed Wind Generation into a real MV distribution network: A case study using measured wind data and simulated load profiles

The interest in the integration of distributed generation into distribution systems has been increased in different countries all over the world as economic and environmental factors drive new technologies to be more efficient and less polluting than their earlier counterparts. As the penetration level of DG increases, the distribution system performance has to be analyzed in details. This work investigates the effects of Distributed Wind Generation (DWG) on voltage ranges, voltage profiles, and system MWh losses of a real MV distribution network. The availability to integrate a new wind mill into the system that already contains three wind mills is examined with respect to the technical conditions for generation connection to the MV networks introduced by German Energy and Water Federation. In the simulation, we used a one year measured wind data and simulated load profiles of the households which are connected to the LV side at each MV substation.

Allocation of decentralized generators in distribution networks for enhancing normal operation loadability

This work presents a new approach for placement and dispersing of Decentralized Generation (DG) power in medium voltage distribution networks based on Continuation Power Flow (CPF) method. The main objectives of the proposed method are to maximize the Normal Operation Loadability (NOL) (i.e. the maximum loading which can be supplied by the power system while the voltages at all nodes are kept within the limits) and to obtain more benefits from the same amount of DG power. The results obtained with the proposed methodology for a 85 node distribution network demonstrate its applicability.

Optical Investigations of High-Current Vacuum Arc Behavior on Spiral-Shaped and Cup-Shaped RMF-Contacts

Vacuum circuit breakers are mostly equipped with radial magnetic field (RMF) contacts forcing the electric arc on a circular motion by a self-induced magnetic field. Using a high-speed-camera with a recording rate above 25,000 frames per second the appearance of vacuum arcs was investigated from two 90 degrees displaced views regarding three different contact designs with CuCr 75/25 as contact material. Spiral shaped contacts and cup-shaped contacts with diameter of 68 mm and 65 mm, respectively, and spiral shaped contacts with diameter of 90 mm were investigated with 45 Hz short-circuit currents of amplitudes up to 85 kA. From the high-speed films arc motion, shape and velocity as well as current density were determined in consideration of arc voltage and current strength. Results were compared to former publications.

Feasibility Study of Superconducting Current Limiter Application in a Cogeneration Plant

Fault current limiters (FCL) have been developed using high temperature superconductors. Possible applications of FCL are medium and high voltage energy supply systems. In this field conventional protection against high currents often cause high expenses. Using a superconducting FCL gives an economic advantage, if technical problems can be solved at lower costs or if the usage of a FCL makes savings in other equipment possible. This work deals with possible applications of a superconducting fault current limiter in a medium voltage auxiliary power network of a typical small cogeneration power plant. For possible installation locations in the existing energy supply system of Braunschweig, Germany, calculations of short circuit currents were made. It is investigated, whether the power plant can be expanded while staying or becoming short-circuit-proof. One result of the work is that the auxiliary power system of the power plant is an especially interesting installation location for a FCL. Advantages of a FCL compared to conventional strengthening of the installations are discussed.

Contact behavior in vacuum under capacitive switching duty

According to the relevant IEC standards vacuum circuit-breakers have to meet various needs, e.g. the interruption capability, making operations, and dielectric strength. Besides the interruption of short-circuit currents, switching of capacitive currents causes high stress of the circuit-breaker. Switching of capacitor banks, overhead lines, or cables leads to very small currents in comparison with short circuit currents. After current interruption the circuit-breaker must withstand twice the peak value of the system voltage. Furthermore, restrikes can lead to voltage multiplication. This conjunction of relatively small breaking currents with high voltage stress must be considered in detail. This work introduces a test arrangement for combined tests of making operation, current interruption, and dielectric stress of a vacuum gap under capacitive switching condition. A test vessel permits investigations of various contact materials and designs. It is connected to a synthetic test circuit which provides the appropriate test currents and capacitive voltage. During the test sequence the contacts are stressed by inrush-currents up to 4.5 kA peak, followed by a breaking operation at 500 A peak and a subsequent capacitive voltage up to 50 kV peak. Both the appearance of pre-ignitions at contact closing and the behavior under capacitive voltage stress after breaking are indications of the contact surface conditions.