van der L Lou Sluis

Experimental and Theoretical Analysis of Vacuum Circuit Breaker Prestrike Effect on a Transformer

The work presented in this paper deals with the investigation of circuit breaker prestrike effect that occurs during energizing a distribution transformer. An experimental test setup that consists of a supply transformer, a vacuum circuit breaker (VCB), a cable and a test transformer is built, and the prestrikes in the VCB are recorded. The test transformer is a prototype distribution transformer, with installed measuring points along transformer windings in each phase. Voltage oscillations are measured along the windings and transformer terminals. The transformer is modeled by lumped parameters extracted from telegraphers equations in discrete form. Voltage oscillations during switching-in operations are recorded and calculated with and without a cable installed between the VCB and the transformer. Computed voltages show good agreement with the measured voltages. Described method can be used by transformer manufacturers to estimate voltage wave forms during switching or lightning, to provide useful information for insulation coordination studies, and to investigate resonance effects in transformer windings.

Vacuum circuit breaker current-zero phenomena

Post-arc current phenomena that occur when interrupting high currents with vacuum circuit breakers have been investigated. High resolution measuring equipment has been used to measure both the post-arc current and the arc voltage in the current-zero region. Three examples of frequently observed phenomena are described. The first describes the phenomenon that in the event of a current-chopping, the current is zero for a short period of time just before the natural alternating current zero, but continues to flow afterwards, in the form of a post-arc current. The second and third example deal with the post-arc phenomena after currents that are much higher than the test breakers rated short-circuit current. These examples show a low-voltage period after current-zero. Apparently, during this post-arc period, the residual plasma between the breakers contacts conduct well. In addition to the voltage-zero period, the voltage trace in the third example also shows evidence of current-chopping. This means that the plasma conducts poorly just before current-zero, but conducts well immediately afterwards. The post-arc current model of Andrews and Varey is verified with measurements.

Digital testing of high-voltage circuit breakers

The functionality of high-voltage circuit breakers is tested in high-power laboratories. Due to the necessary power and the physical size of the equipment, testing is rather expensive and time consuming. The steps followed so far by the authors in order to enable the digital testing of HV circuit breakers are described in this paper. At the end of the article, examples of digital testing are also presented.

Vacuum Circuit Breaker Postarc Current Modelling Based on the Theory of Langmuir Probes

High-resolution measurements on the postarc current in vacuum circuit breakers (VCBs) reveal a period, immediately following current-zero, in which the voltage remains practically zero. The most widely used model for simulating the interaction between the postarc current with the electrical circuit lacks a proper explanation for this event, and hence, it needs to be complemented. We demonstrate that the breakers electrical behavior during this zero-voltage period can be explained by using the theory of a Langmuir probe. Such probes are used to investigate plasma properties such as the ion density and the electron temperature, and we extrapolate its theory to the VCB. After the voltage-zero period, when the transient recovery voltage starts to rise, the breakers electrical behavior is mainly determined by the expansion of an ionic space-charge sheath in front of the cathode. In addition to the current from the Langmuir probe model, the time change of the electric field inside the sheath gives a displacement current. Instead of solving the complicated plasma equations to find the displacement current, we use an approximation by simulating it with the aid of a voltage-dependent sheath capacitance. We programmed the model as a function block in Matlabs SimPowerSystems to facilitate its application in different electrical circuits.

Protection, transient stability and fault ride-through issues in distribution networks with dispersed generation

This paper describes the transient stability analysis of a 10 kV distribution network with wind generators, microturbines and CHP plants. The network being modeled in Matlab/Simulink takes into account detailed dynamic models of the generators. Fault simulations at various locations are investigated. For the studied cases, the critical clearing times are calculated. Some network parameters which influence transient stability are also observed. The compliance of DG protection to stability criteria requirements is further studied. The ride-through capability of the DG units is also investigated through transient stability analysis and the equivalent CCT-voltage dip curves are derived. It is concluded that during fault DG unit availability can be increased through modification of undervoltage protection requirements.

An ATP-EMTP-based model for analysis of shielding properties of ferromagnetic cable sheaths

Ferromagnetic materials, such as steel, are used for shielding of power and telecommunication cables. These shields often provide path for currents during faults or lightning and are important for the integrity and safety of electrical systems and devices. Since such currents might be with low- or high-frequency content, and of low or high intensity, ferromagnetic shield behavior might be quite different due to hysteresis. This paper describes a method for analyzing the dynamic behavior of ferromagnetic cable shields based on the Jiles model of hysteresis. The solution is implemented in ATP-EMTP, which allows one to analyze the complex behavior of ferromagnetic cable shields in conjunction with that of the other elements of the power system. Due to limitations of the model of hysteresis, application of the proposed model is limited to frequencies up to about 10 kHz. The model is illustrated for a practical 100-m-long cable.