DETECTION AND SUPPRESSION OF PARASITIC DC VOLTAGES IN 400 V AC GRIDS

Grid connected static power converters inject parasitic DC currents due to the offset in current sensing, control imperfections, assymetries in power switches and other secondary effects. Ever growing number of grid connected converters contributes to an increase of DC bias in AC grids, and this brings the cores of distribution transformers closer to saturation and increases their power losses. This paper provides sensitivity analysis of distribution transformers to the DC bias, and considers solutions for detecting and compensating the parasitic DC components in AC grids. Active compensation methods  can be advantageously used in suppressing the DC bias at grid connection point of the power converter. The sensing approach proposed in this paper makes use of saturable ferromagnetic cores and a low cost DSP for signal analysis and processing. Proposed algorithm uses distortion of the magnetizing current of a parallel connected saturable core due to the bias. Experimental results demonstrate the capability for detecting and compensating the bias voltages far below 1 mV in 0.4 kV grids. The paper describes the principles of DC bias detection and it provides the guidelines for the proper design of magnetic components. High precision of the proposed DC bias sensing is thoroughly verified on the experimental setup connected to a 0.4 kV grid

CONTROL OF SERIES IMPEDANCE OF POWER LINES USING POWER FLOW CONTROLLER

In this paper, the possibility of unified power flow controller (UPFC) to modulate both series resistance R and series reactance X of an overhead power line is discussed. The classical power flow control system of the UFPC is modified in the manner that standard input references signals (active and reactive powers) are replaced by reference signals of series resistance and reactance. Using the procedure described in this work, the reference signals for active and reactive powers are generated indirectly. The operation of UPFC in proposed operation mode is analyzed using computer simulation, based on a model of single machine infinite bus (SMIB) with constant impedance loads and two parallel lines. The goal is to show that UPFC is capable to control both series line parameters (R and X) directly and independently by means of a simple control system without additional decoupling controllers. An additional task is to show that power flows can be indirectly controlled this way. The step response of series line resistance and reactance is used to validate the operation of the proposed control system. The obtained results clearly show that all goals are fulfilled

Power electronics solution to dust emissions from thermal power plants

Thermal power stations emit significant amounts of fly ash and ultra fine particles into the atmosphere. Electrostatic precipitators (ESP) or electro filters remove flying ashes and fine particles from the flue gas before passing the gas into the chimney. Maximum allowable value of dust is 50 mg/m3 and it requires that the efficiency of the ESPs better than 99 %, which calls for an increase of active surface of the electrodes, hence increasing the filter volume and the weight of steel used for the filter. In previous decades, electrostatic precipitators in thermal power plants were fed by thyristor controlled, single phase fed devices having a high degree of reliability, but with a relatively low collection efficiency, hence requiring large effective surface of the collection plates and a large weight of steel construction in order to achieve the prescribed emission limits. Collection efficiency and energy efficiency of the electrostatic precipitator can be increased by applying high frequency high voltage power supply (HF HV). Electrical engineering faculty of the University of Belgrade (ETF) has developed technology and HF HV equipment for the ESP power supply. This solution was subjected to extensive experimental investigation at TE Morava from 2008 to 2010. High frequency power supply is proven to reduce emission two times in controlled conditions while increasing energy efficiency of the precipitator, compared to the conventional thyristor controlled 50Hz supply. Two high frequency high voltage unit AR70/1000 with parameters 70 kV and 1000 mA are installed at TE Morava and thoroughly testes. It was found that the HF HV power supply of the ESP at TE Morava increases collection efficiency so that emission of fine particles and flying ashes are halved, brought down to only 50 % of the emissions encountered with conventional 50 Hz thyristor driven power supplies. On the basis of this study, conclusion is drawn that the equipment comprising HF HV supplies are the best solution for new ESP installations, as well as for the reconstruction of existing facilities. The paper describes the topology of the HF HV power supply, power management and controls, and brings the most important details of the implementation. It is found that the HF HV solution achieves several significant improvements over the conventional thyristor system. It is possible to provide more precise control of the ESP parameters such as the output voltages and currents. It is also possible to make a rapid increase or decrease in voltage and to effectuate a very fast response to load changes. Due to this advantages it is possible to suppress the supply quickly in the case of sparking, reducing the spark energy and the quantity of ionized gasses produced by the electric arc. Reduction in the spark energy is up to 10 times compared to conventional thyristors solution. This means that the erosion of the electrode system is significantly reduced, and that the quality of the collection plates is preserved for much longer periods. At the same time, lower quantity of ionized gasses produced by the spark contribute to much shorter deionization intervals, required to quit sparking and evacuate charged particles in order to reinstate the voltage and proceed with the operation. In addition, HF HV power supply provides a significant reduction in size and weight of the complete ESP installation, hence reducing the tons of steel that has to be built in. Therefore, the HF HV power supply may be the key instrument to reducing the cost of the dedusting ecological equipment. Besides, size and weight reduction leads to cost savings of installation and maintenance. According to estimates, savings in steel may reach 30%, contributing to the overall cost savings of roughly 20%. Within this paper, in addition to describing the AR70/1000 unit topology and principles of operation, the paper presents the results and measurements obtained during extensive experimental investigations wherein performances of 50 Hz based thyristor units with T/R sets are compared to HF HV power supply

Unaprijeđeno upravljanje momentom visokobrzinskog pogona s asinkronim motorom bez mjerenja brzine

This paper presents improved torque control scheme for a high speed sensorless induction motor drive. The proposed high speed torque control scheme substitutes the flux oriented control by the voltage angle control in the flux weakening regime. This scheme uses maximum of available inverter voltage, alleviates well known problems of current control schemes in conditions with insufficient voltage margin and avoids the influence of estimated speed error to the achieved flux level. The algorithm uses similar slip control as flux oriented control algorithm, but is applied without an outer flux trajectory reference which is typical for the flux weakening, providing a fast and well damped torque response even if error in estimated speed is present. Experiments confirm the effectiveness of proposed torque control algorithm, smooth transition from the flux oriented control in the base speed region to the voltage angle control in the flux weakening, superior dynamic performance of the voltage angle torque control, and its robustness to an estimated rotor speed error.U radu je predstavljena unaprijeđena shema upravljanja za pogon visokobrzinskog asinkronog motora bez mjerenja brzine. Predloženi postupak zamjenjuje vektorsko upravljanje upravljačkom strukturom s upravljanjem kutom napona u slabljenju polja. Predložena shema koristi maksimalni raspoloživi napon invertora, eliminira dobro poznate probleme strujno reguliranih pogona u uvjetima s nedovoljnom rezervom napona i eliminira utjecaj greške u estimaciji brzine na dostignutu razinu toka. Algoritam koristi sličnu kontrolu klizanja kao i vektorsko upravljanje, ali bez tipičnog vanjskog zadavanja toka u slabljenju polja, pružajući brz i dobro prigušen odziv momenta čak i u slučaju greške u estimaciji brzine. Eksperimenti izvedeni na velikoj brzini vrtnje potvrđuju učinkovitost predložene regulacije momenta, gladak prijelaz iz baznog područja brzine u slabljenje polja, vrhunske dinamičke performanse upravljanja kutom napona i robusnost na pogrešku u estimiranoj brzini vrtnje

Designing Energy Conversion Systems for the Next Decade

Sustainable growth in energy consumption requires transition to clean and green energy sources and energy systems. Environment friendly and renewable energy systems deal with electrical energy and rely on efficient electrical power converters. High power electronics is the key technology to deal with the next generation of electrical energy systems. The door to future breakthroughs in high power electronics is opened by major improvement in semiconductor power devices and their packaging technologies.  New materials allow for much higher junction temperatures and higher operating voltages. Most importantly, advanced power semiconductor devices and novel converter topology open the possibility to increase the energy efficiency of power conversion and reduce the amount of heat. Although the waste heat created by high power converters can be put to use by adding on to heating systems, this option is not always available and the conversion losses are mostly wasted. At the same time, wasted heat is a form of pollution that threatens the environment. Another task for high power converters is efficient harvesting of renewable energy sources, such as the wind energy and the sun. Intermittent in nature, they pose a difficult task to power converter topology and controls. Eventually, high power converters are entering power distribution and transmission networks. With their quick reaction, with fast communication between the grid nodes and with advanced controllability of high power converters, a number of innovations can be introduced, facilitating the power system control and allowing for optimizations and loss reduction. Coined smart grid, this solution comprises two key elements, and these are intelligent controls and large static power converters. At virtually no cost, smart grids allow for a better utilization of available resources and it enlarges the stable operating range of the transmission systems. Therefore, it is of interest to review the future trends in designing high power converters

Analysis of the magnetomotive force in the multiphase machines within integrated battery chargers for the electric vehicles

In this paper we discuss magnetomotive force (MMF) in the multiphase machines during fast charging of batteries in electric vehicles (EV). Analytical considerations show that using an adequate topology of power converter and additional levels of freedom one can achieve a fast charging without creating any revolving magnetic field within the machine. They also show that this regime can be achieved by using only the elements of the traction system and without changing the configuration between charging and traction regimes. The simulation models have been developed for the relevant configurations of the stator asymmetrical winding in six-phase and nine- phase integrated electrical machines within the charging system of an EV. As the result, the time-space distribution of the magnetic field in charging mode is obtained. Based on the results, suggestions are given for the design of an integrated traction / charging system within electric vehicles

Finite Element Design of Rotor Permanent Magnet Flux Switching Machine with Arbitrary Slot, Pole and Phase Combinations

A two-dimensional finite element approach for designing RPMFS (Rotor Permanent Magnet Flux Switching) machines is presented in this paper. The proposed method enables fast, accurate and computationally efficient assessment of different RPMFS machine designs with an arbitrary number of rotor poles, stator slots and stator phases. The appropriate stator winding layout is assembled for any feasible slot, pole and phase combination by employing the winding distribution table, which contributes to automating the design process. Based on the proposed method, a program is developed using the Octave FEMM (Finite Element Method Magnetics) toolbox. The program is suited for the use in the design stage, where it is necessary to determine various machine parameters for given core dimensions, terminal voltage constraints and adopted value of current density in the conductors, while taking iron saturation effects into account. Verification was carried out by simulating torque and EMF waveforms for several RPMFS machine designs

Suppression of torsional oscillations based on IMPACT controlling structure

In this paper, an algorithm for suppression of the torsional oscillations, in the computer controlled servo systems, is purposed. A few modification of the IMPACT structure (Internal Model Principle and Control Together) in the digitally controlled electrical drives are proposed. The IMPACT structure suitable for suppression of the torsional oscillations in a servo system with the flexible coupling has been presented. The approach, proposed in this paper, gives better solutions for the problem of the mechanical resonance in the modern servo drive than the previous ones. The presented structure is simple with a small number of adjustable parameters that could be easily set to achieve the desired robust, filtering, and dynamic properties of the system

IN RESPECTFUL MEMORY OF PROFESSOR NINOSLAV STOJADINOVIĆ

Ninoslav Stojadinović, a friend of many, passed away on December 25, 2020, after one month of fight with Covid-19. Ninoslav, for friends Nino, was a chairman of many conferences, editor of many scientific journals, well-known and famous professor, a highly respected mentor to his students, and a role model for colleagues.He was Editor-in-Chief of the scientific journal Facta Universitatis, Series: Electronics and Energetics (University of Niš) in the period 2013-2020

A new signal injection‐based method for estimation of position in interior permanent magnet synchronous motors

International audienceSeveral heuristic procedures to estimate the rotor position of interior permanent magnet synchronous motors (IPMSM) via signal injection have been reported in the application literature, and are widely used in practice. These methods, based on the use of linear time-invariant (LTI) high-pass/low-pass filtering, are instrumental for the development of sensorless controllers. To the best of authors’ knowledge, no theoretical analysis of these methods has been carried out. The objectives of this note are (i) to invoke some recent work on the application of averaging techniques for injection-based observer design to develop a theoretical framework to analyse the LTI filtering used in sensorless methods and (ii) to propose a new method that on the one hand ensures improved accuracy and on the other hand can be related to the current filtering technique. An additional advantage of the new method is that it relies on the use of linear operators, implementable with simple computations. The effectiveness of the proposed scheme is assessed by experiments on an IPMSM platform driven by a 521 V DC bus with a 5-kHz pulse-width modulation