David Topolanek

The method of the additional earthing of the affected phase during an earth fault and its influence on MV network safety

The paper deals with the description and analysis of the method of the additional earthing of the affected phase used for the elimination of the fault current during an earth fault in a compensated network. The efficiency of this method is strongly influenced by the fault location and resistance and, in some cases, its application can lead to deteriorating the situation at the point of the fault and thus to increasing the risk of dangerous touch voltage. The paper analyses the results of the simulations of selected fault cases where the negative influence of the additional earthing of the affected phase on the fault current magnitude (and therefore on the occurrence of the dangerous touch voltage at the point of the fault and in the neighboring LV network) can be expected. All the theoretical conclusions and assumptions were consequently verified by experiments in a real distribution network. Their results were then used in the final part as a basis for proposing the measures to apply the method in MV distribution networks.

Practical experience of using additional earthing of the faulty phase during a ground fault

The paper is focused on the operational experience with an automatic system for suppression of a fault current during a ground fault by the additional earthing of the faulted phase in a supply substation. Such system was experimentally installed in the Brno-Medlánky 110/22 kV substation. To verify the benefits of the system as well as to assess its possible failings, twenty-four experimental measurements based on a theoretical study were performed. During those experiments, low-resistance and arc ground faults were monitored in the urban and suburban part of the compensated network with a capacitive current of 300 A and 800 A. The results of the crucial experiments and the evaluation of the system functioning are summed up in the final part of the paper.

Voltage regulation optimization in low voltage network based on Voltage Quality Index

The paper deals with an optimization procedure in terms of a network voltage parameters improvement. A software based approach is used to select optimal type of voltage regulator, its power and ideal location for application in a Low-Voltage (LV) test network, where it is required to enhance Voltage Quality (VQ). The test network presents a case where application of on-load tap changer transformer instead of conventional distribution transformer MV/LV is not adequate to maintain voltage profile in the LV system in limits. Therefore other type of voltage regulator has to be employed to increase VQ in the entire LV network. Principal measure for VQ evaluation there is adopted a Voltage Quality Index expressing the VQ level in dependence on selected and specified input parameters, which may be voltage magnitude, unbalance, harmonic distortion, flicker level and frequency. Within optimization process, maximal current carrying capacity of lines, network losses and cost of solutions are also taken into account.

Experimental measuring of the touch voltages in large compensated networks

A connection between a phase conductor and the ground in MV networks is called earth fault. The fault current in this case does not depend on the point of the fault, but only on the total capacity (size) of a network. Distribution system operator (DSO) sometimes needs to operate the large network which means operating of the network with big capacitive current (in Czech Republic more than 800A). This operation puts the emphasis not only on the setting of the earth fault relays but on the safety of the point of fault too. The paper describes the tests in real compensated distribution network with capacitive currents 300A and 800A. There were tested low-ohmic and arc fault. The first results show that the operation of large network is possible with using of the automatics of additional earthing of affected phase. The touch and step voltages were reduced under level allowed by standard EN 50522.

Earthing system resistance calculation using analytical approach with mutual coefficients

The basic formulas for analytical determination of the earthing system resistance are given by the European standard EN 50522:2010 [1] in its annex J. As these formulas are rather vague about how to determine the earthing system resistance for more complicated shapes of earthing systems consisting of more basic earthing electrodes (e.g. strip, rod, ring), this formulas are cited in the Czech Republics specific document PNE 33 0000-4 [2] and they are supplemented with the mutual coefficients. These mutual coefficients should incorporate into the calculation the effect of mutual potentials caused by the potential of other electrodes and thus make it possible to use the basic formulas even for the complicated shapes of earthing systems. Thus the method introduced by the PNE document is introduced in this paper. To assess the correctness of the mutual coefficients two other calculation methods were taken into account. First, the numerical finite element method was used as the most precise method and the second method was chosen as the analytical method introduced by Dwight [6]. The results of both analytical methods were compared with the results of finite element method and it was pointed out to the possible usage limitation and possible error being made using the PNE document analytical approach.

Evaluation of different solutions of faulted phase earthing technique for an earth fault current limitation

The study is focused on evaluation of different types of prototypes of automatics for additional faulted phase earthing (FPE), which are used for earth fault (EF) current reduction in resonant earthed distribution network. Three prototypes of these automatics have been installed in Czech distribution network, the first one utilises direct connection of faulty phase–to-earthing system of supply substation, the second one utilises connection through resistor and the last one through reactor. The contribution is mainly focused on detail analysis of operational differences of these types of FPE systems based on case study of compensated distribution network. The main aim is to specify and describe benefits and disadvantages of individual FPE applications. The results could be used for evaluation of best solution of FPE application which could be chosen for an EF current reduction in compensated distribution network.

Ground faults simulation model in comparison with performed experiments

This paper deals with the description and of the mathematical model of compensated MV distribution network. The model was created in PS Cad in order to solve the ground faults localization using the method of the additional grounding of the healthy phase. This method was experimentally tested for different types of ground faults. The comparison of the measured values and the simulation results is performed in this paper. The aim of the network model is verification of the methodology for simultaneous ground faults calculation.

Assessment of short-circuit conditions in the interconnected local distribution system

This paper analyzes how to calculate the symmetrical and unsymmetrical short-circuit currents. The main reason of this thesis is to give, step by step, a complete method for the calculating of short-circuit currents and his mechanical or thermal stresses in local distribution network. The calculation is applying on medium-voltage AC power networks and takes into account the recommendations of the IEC 60909-0 standard. Thesis also deals with limiting of short-circuit current when the short-circuit withstand capability of equipment within power network is exceeded and evaluation of individual variants of this limitation.

Hesteresis and its influence on the creation of the ferroresonance

The effort to decrease the power dissipation in the energetic system stemming from increasing ecological and economical requirements is the reason of the growing frequency of occurrence of various incidental phenomena. It is important to consider the system as a whole, so it is necessary to take into account the fact that lowering the power dissipation will lead to absorption in the system. However, that results in increasing and lengthening the systems response to any change, and therefore also to the character of transient phenomena. In the electric power system these are usually a short-term increase or decrease in voltage. It can also happen that as a result of a change in the system, the voltage will increase permanently, which might have a destructive effect on most electrical appliances.