СТІЙКІСТЬ КОМБІНОВАНОЇ СИСТЕМИ НАКОПИЧЕННЯ ЕНЕРГІЇ НА ОСНОВІ СУПЕРКОНДЕНСАТОРА ТА АКУМУЛЯТОРНОЇ БАТАРЕЇ

The aim of the work is to analyze the stability of the battery-supercapacitor hybrid storage of power supply for resistance micro-welding equipment, considering the possible variation of the system parameters and taking into account parallel series resistance of the circuit components. Methodology. The sufficient accurate mathematical model of the hybrid energy storage system to stability analysis has been obtained by the state-space average method. According to the state-space averaging method, PWM switching converters are described by separate circuit topologies for each switching period. The system of differential equations for each time interval has been derived by use of the Kirchhoff rules. The small-signal model transfer function of the SEPIC converter has been obtained by applying the Laplace transform to linear state equations averaged over one switching cycle. Finally, the Nyquist stability criterion has been considered to evaluate the stability of the proposed energy storage system. Results. Bode diagrams of an open-loop system for different values of the duty cycle, average load current, and input voltage have been obtained by using MATLAB software. The gain margin ranges from 14.6 dB to 26.4 dB and the phase margin ranges from 45.4 degrees to 54.8 degrees. From these results, it is obvious that the proposed system meets the stability criteria regardless of the aforementioned parameter fluctuations. Originality. The high-efficiency energy storage system for micro resistance welding technology has been proposed. Developing of the energy storage system according to the battery semi-active hybrid topology enables to control the Li-ion battery discharge current within the maximum allowable value. SEPIC converter utilization ensures the high-efficient operation of the power supply despite the battery charge state. Moreover, this topology allows implementing series and parallel configuration of both batteries and supercapacitors to obtain the required value of voltage and current. Practical significance. The mathematical model of the SEPIC converter has been developed by applying the state-space averaging technique. The stability analysis for parameter variation, such as duty cycle and the average load current, the input voltage has been performed by using Nyquist criteria. В роботі розглянуто комбінований ємнісний накопичувач енергії на основі акумуляторної батареї (АБ) та суперконденсатора джерела живлення для установки контактного мікрозварювання. Для забезпечення рівномірного споживання струму від АБ обрано напівактивну топологію АБ та перетворювач SEPIC (Single-Ended Primary-Inductor Converter). Методом усереднення в просторі змінних стану аналітично отримано математичну модель системи. З метою проведення аналізу стійкості комбінованого накопичувача при різних значеннях коефіцієнта заповнення імпульсів, струму навантаження та напруги АБ отримано передавальну характеристику системи керування. Результати аналізу показали, що запропонована система є стійкою при зміні параметрів у встановлених межах.

Inverter-based voltage control of distribution networks : a three-level coordinated method and power hardware-in-the-loop validation

The reactive power of the photovoltaic (PV) inverters has great potential for voltage regulation of distribution networks. In this paper, a new three-level coordinated control method for PV inverters is proposed to address network voltage fluctuation and violation issues. In Level I, a ramp-rate control is designed to smooth the network voltage fluctuations, while in Level II, a droop control is designed to alleviate the network voltage deviations. If the local compensation provided by Level I and II is not enough to regulate the network voltages within the required limits, the Level III control based on dynamic average consensus can respond and share the reactive power requirement among other inverters in a distributed way. The proposed control method can smooth the voltage profiles, restrain the voltage rise/drop problem, and coordinate all PV inverters in real-time when there is no feasible local solution. The stability analysis of the proposed three-level coordinated control for network voltage regulation is provided. The power hardware-in-the-loop (PHIL) experiment has been conducted for validating the proposed control method under various scenarios

Time-delay stability analysis for hybrid energy storage system with hierarchical control in DC microgrids

Hybrid energy storage system (HESS) plays an important role in the operation of dc microgrids which have attracted significant research attention recently. The hierarchical control is widely adopted for the coordination of multiple energy storages in a HESS. As the hierarchical control comprises the centralized and the decentralized control levels, the time delays during signal transfer processes between two control levels may significantly affect HESS operation and may lead to instability. In this paper, considering the multiple delays in the hierarchical control processes, the maximum delayed time (MDT) is defined to assess the stability margin for a HESS. An accurate and effective method based on small signal stability model is then proposed to determine the MDT of a HESS to maintain its stability. The effectiveness and correctness of the proposed method are verified using a lab-scale dc microgrid