Direct current (DC) power systems are gaining interest in the last decade due to increased utilization of DC outputted power sources, DC based energy storage (ES) elements and DC inputted loads. Microgrids are also becoming widely researched as the main foundation of smart grid. It is therefore logical to try to utilize DC microgrid (DCMG) concepts in organization of power systems in wide range of applications. DC microgrids have several important advantages compared to alternating current (AC) microgrids. The control system is essential in order to keep DCMG operating properly, reliable and efficient. Their control structures, with special interest in hierarchical control are explored and compared in this paper in terms of architecture and techniques. This paper presents real world applications using DCMG concept. Future research propositions given in the final chapter can be used as a foundation for researchers exploring the area.

A building-integrated microgrid (BIM) has been a widely utilized concept in low-carbon smart cities. The key advantages of microgrids are using locally available renewable energy sources (RES) and reducing dependency on fossil fuels. Solar photovoltaic (PV) systems and battery storage systems play a crucial role in BIM to achieve desired goals. Due to legal/regulatory and technical restrictions, the distribution system operator (DSO) often imposes zero energy export (ZEE) for these microgrids. Therefore, the sizing of solar-battery systems in BIM, which will be technically feasible and economically optimized, is a challenge for designers, owners and DSO. The objective of this paper is to show the practical approach for design and sizing a microgrid for public buildings using the real data sets of a power consumption and solar energy production. As an example, the BIM for the Faculty of Electrical Engineering, University of Sarajevo is presented.

Abstract Simulation of unsteady flow of SF6 gas in a simplified high-voltage circuit breaker model describing the nozzle, contacts and their nearest surrounding is presented. SF6 is considered as viscous, compressible, real gas described by Redlich-Kwong model. Heat transfer is taken into account due to the gas compressibility. The heat source, triggered by the electric arc between the contacts, was out of the scope of the current research, thus it was not included in the simulations presented. Turbulence, caused by the gas viscosity, is described using realizable k-ε model. In the simulation model, one of the contact sides – electrodes, is considered as moving at prescribed velocity. The part of the space ‘swept’ by the moving electrode is considered as the gas with imposed artificially increased viscosity in order to imitate the rigid body behaviour. Thus, no moving parts of the computational mesh are used in the model. The conservation equations of mass, momentum and energy, given in integral form, are solved using a finite-volume method on unstructured computational grids.

Abstract This paper describes the advantages of using data acquisition systems and software modelling tools to support the assessment and therefore redesign of the existing medium voltage switchgear. A 38kV/630A load break linear puffer (LP) will be used as an example for this study. In house testing was conducted to capture important design parameters of the switch such as displacement, velocity of mechanical parts and gas pressure using various sensors and three different measurement setups. The first setup, which is primarily intended for no-load measurements, consists of a DAQ system equipped with different types of sensors - two rotational encoders, three laser-based distance sensors, six pressure sensors, contact separation measurement, and a high-speed camera integrated and synchronized with the measurement system. The second and third setups, which are suitable both for no-load and on-load measurements, are based on state-of-the-art DAQ systems, which use three piezo-electric based pressure sensors, two fibre-optic based pressure sensors, three laser-based distance sensors and a high speed camera synchronized with the measurement system. The data acquired by the measurement systems is used in combination with an in-house developed simulation software HV CB Simulation, which enables simulating and predicting various variables of switching devices. Moreover, high speed camera videos analysed with both commercial and in-house developed image processing software, visualize and reveal many otherwise inaccessible occurrences. In addition to a comprehensive analysis of the proposed data acquisition and simulation setups, three design improvements in the linear puffer design - increase of the opening speed, removal of the flexible conductors and the length increase of the puffer cylinder - are presented and discussed in this paper.

Abstract The breaking capacity of a medium voltage (MV) rotary SF6 load break switch (LBS) can be improved by incorporating permanent magnets into the stationary contacts. The magnetic field is intended to blow the switching arc root towards a recessed space at the stationary contacts thereby preventing reignition of the arc after current zero. Making and breaking tests of load current 630 A were performed comparing the switching performance of load break switches equipped without a permanent magnet, with a ferrite and with a neodymium magnet. The impact of different polarity arrangements of the magnets in the three phases is also considered and analysed. In order to understand the arc behaviour caused by the effect of permanent magnet, arcing times and arc voltage were measured and evaluated. The results show that the arc voltage depends on the direction of the electromagnetic force, which is determined by the phase current direction but also by the polarity of the magnets. When the force is directed towards the recessed space at the stationary contacts, the arc voltage is notably higher than in the case where the arc is blown in the opposite direction. The higher arc voltage is a reliable indication that the length of the arc is increased, which significantly reduces the risk of both thermal and dielectric breakdowns after the first current zero. The consequences are noticed first in the reduction of the number of missed current zeroes and second in shorter minimum arcing times. An adverse arrangement of the magnet polarity in the three phases increases the number of missed current zeroes.

Connecting a photovoltaic solar power plant (PVSP) to a radial low-voltage network can significantly influence the power quality. According to grid rules in our country, the PVSP with rated power of up to 150 kW can be connected to the low-voltage power distribution network. Common case is that the PVSP with rated power of approximately 100 kW is installed at the rooftop of a residential or commercial building and connected to one connection point. Usually, the owner tends to regulate PVSP to operate at unity power factor to maximize profit. However, when the power plant operates with unity power factor, voltages at the connection point and at the nearby buses are often increased. Also, a significant increase of power losses can occur. This paper describes the procedure for determining the maximum power of the PVSP operating with unity power factor while simultaneously meeting the following criteria: (1) the range of slow voltage variations must be within the limits defined by the EN 50160 standard; (2) after connecting the PVSP, the active power losses should be less or equal to active power losses calculated with the disconnected PVSP.

This paper describes the usage of voltage VAr control (VVC) in closed loop mode (CLVVC). The focus is the application in single feeder radial distribution systems, where the CLVVC is executed in combination with short term load forecast (STLF) and distribution system state estimation (DSSE). Although well-known benefits, this paper especially deals with pitfalls and drawbacks of implementing and using closed loop.

Nowadays, high voltage circuit breaker (CB) simulations are mostly based on Computational Fluid Dynamics (CFD) models. Such simulations require significant computer resources. An alternative approach is to use enthalpy flow models, which do not use space discretization of the interrupter unit chambers and valves. Gas flow is calculated based on state of gas in adjacent chambers and valve settings. However, the valve shape has significant influence on the effective flow cross section between chambers. Therefore, in order to ensure correct simulation results, it is necessary to determine the correct values of discharge coefficients for all valves in the interrupter unit of a circuit breaker. In this paper, the discharge coefficients were determined by combining a series of CFD and enthalpy flow simulations for each valve in the interrupter unit. After that, discharge coefficients are used as input for further simulations based on the enthalpy flow model. This way, benefits from both models are combined: more precise gas flow calculation and faster simulations. The proposed novel approach is validated in a high-power laboratory by pressure measurements on a 420 kV 63 kA self-blast circuit breaker.

Abstract This paper presents the design and development of a distributed measurement system for measuring pressure in high voltage circuit breakers (HV CB) and other switching apparatuses, during no-load operations. Instead of using traditional pressure transducers which require significant installation space, additional data acquisition cards and often demand for complex wiring, an in-house solution of pressure measurement is proposed. The system consists of miniature sensors, accompanied with a suitable amplifier, microcontroller unit and communication module, which may be distributed inside the interrupter unit in convenient locations. Due to the fact that the measurement values are transmitted digitally, measurement noise is significantly reduced while the wiring of the system is additionally simplified. The proposed measurement system is tested using two different interrupters (HV CB and a load break switch). The experimental results have demonstrated that the developed system is applicable, accurate, cost-effective, flexible and simple to use.

The self-blast type circuit breaker has been developed to reduce mechanical operation energy by building up the pressure of arc extinguishing gas flow from the heat of the arc itself. Unlike a puffer type, breaking performance for self-blast type are influenced and sensitive by various factors inside interrupter parts, such as the nozzle structure, chamber shape as well as amplitude of short circuit current. These days, particularly, it has been difficult to secure a low current breaking performance as the circuit breaker has been compacted. The currents for breaking test duties belong to from 10% to 30% of the rated breaking current in accordance with IEC standard. Although the arc energy for interruption is lower than the rated breaking current test duties, the breaking performance could be lower than the tests because the transient recovery voltage (TRV) after the current zero is relatively high. The capability of interruption is related to dielectric recovery after the arc quenching. Therefore, a complex analytical method is needed to secure the breaking performance for the current and to improve the performance by using the limited gas flow inside the interrupter parts. In this paper, it described the techniques to verify breaking performance such as hot gas flow analysis and dielectric analysis. And it has studied a method for improving the performance with various design parameters using computational fluid dynamics (CFD) programs and high power laboratory test. Finally, this paper shows us the improvement of dielectric recovery performance for the self-blast type circuit breaker.

The breakdown voltage during interruption of capacitive currents is defined by two physical quantities: the electric field and the gas density field, which are calculated in different calculation domains and using different mashes. In order to calculate the breakdown voltage, it is necessary to map these two mashes and calculate the ratio density/electric field in every calculation point. The straightforward solution is to pair each density cell with the nearest cell from the electric field mesh, based on their coordinates. Although this solution gives good results, it is very time consuming. Therefore, this paper presents a new approach for mapping of two meshes based on the algebra of fractal vector, so called Bosnian algebra. This approach does not search the meshes for the closest pair based on the coordinates of each point, but instead uses only the assigned cell indexes and simple fractal operations to determine the neighboring cells. This way, the search for the nearest pair is much more efficient and faster.

Superior characteristics of SF6 gas have enabled its successful use as the interruption media in high voltage circuit breakers. Unique combination of physical properties considers high dielectric strength, high thermal interruption capability and high heat transfer performance. However, having in mind its high global warming potential (GWP), long term use of SF6 gas in the future should be called into question. Considering that CO2 has much lower GWP than the SF6, it represents a promising alternative, but still with known inferior breaking performance. Therefore, careful investigation of the CO2 gas behavior should be obtained. In the present work, pressure rise in a high voltage circuit breaker during no-load operation has been simulated and measured for SF6 and CO2 gas, followed with the results comparison. For simulation of high voltage circuit breakers operation, computer program HV CB Simulation, which takes into consideration interaction between the interrupting unit and the driving mechanism is used. Thermodynamic properties of gases are calculated using highly accurate equations of state explicit in Helmholtz free energy.

Interruption of short circuit currents in high voltage circuit breakers requires high SF6 gas pressure, which can be generated by compression of SF6 gas and/or energy transfer from the arc. The pressure difference between circuit breaker chambers causes SF6 gas flow. The rise of pressure in the chambers is highly dependent on the design of the circuit breaker. This paper presents a universal approach to modeling of gas flow and pressure rise, which is based on a real gas model and implemented into computer software for high voltage circuit breaker interruption simulation. The approach allows simulation of SF6 and alternative media circuit breakers with any number of chambers and different types of valves and connections between chambers. The calculated pressure rise is validated by comparison with experimental measurement results from high power laboratory.

Special attention needs to be given to the processes that occur in alternating current arcs at the instant when the current naturally passes through a zero. The short interval of time around current zero, named the "interaction period" [1], is of crucial importance for arc-quenching, when it is decided whether the interruption will be definite or whether arc reignition will occur. As a matter of fact, regardless of the SF6 circuit breaker working principle, its rated voltage and history of the short circuit current interruption process, there is a critical value of arc extinction voltage peak which must be reached to make a successful interruption possible [2]. The arc extinction voltage peak can be increased by elongating the arc (in other words by increasing the distance between arcing contacts) and/or by increasing the pressure of SF6 gas inside the thermal chamber around current zero instant. This means that it is possible to find a critical value of distance between arcing contacts and a critical value of SF6 gas pressure inside the thermal chamber at current zero instant as criteria for successful short circuit current interruption. Based on the results of type tests in high power laboratory (including current zero measurement - CZM) and the calculation results of the software HV CB Simulation, these criteria for short circuit current interruption are successfully established for a real SF6 circuit breaker 245 kV 40/50 kA.