Operation and control of hybrid AC/DC Microgrids in unbalanced conditions

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Operation and control of hybrid AC/DC Microgrids in unbalanced conditions

ارائه دهنده: Provider: Saeed Heidari

اساتید راهنما: Supervisors: Alireza Hatami

اساتید مشاور: Advisory Professors: Mohsen Eskandari

اساتید ممتحن یا داور: Examining professors or referees: Saleh Razini, Hadi Delavari, Iraj Ahmadi

زمان و تاریخ ارائه: Time and date of presentation: 2022-09-20, 16:15

مکان ارائه: Place of presentation: Amphitheater of Faculty of Engineering

چکیده: Abstract: In recent years, the implementation and use of microgrids has increased due to economic and environmental reasons. Considering this issue, many studies have been done on microgrids and different types of microgrids have been designed and presented. Microgrids were initially divided into two general types, AC and DC. Next, in order to use the benefits of both types, AC-DC hybrid microgrids were introduced, which compared to AC and DC types, reduces power losses and also reduces the need for power conversion. In hybrid microgrids, AC and DC sub-grids are connected through one or more interface converters (ICs), which enable power exchange between two or more sub-grids. For this reason, the interface converter is one of the important parts in combined AC/DC microgrids, whose main task is to adjust the voltage and frequency in the AC section in island operation and to adjust the voltage of the DC section. However, this converter also has other capabilities such as compensating imbalanced and harmonic components in the AC microgrid and eliminating DC fluctuations in the DC microgrid. But using the interface converter for these applications occupies part of their power exchange capacity and can affect its main function. In addition, in unbalanced microgrids, the power exchange capacity of converters is reduced and the aforementioned challenge becomes more serious. According to the mentioned challenge, in this article, an intelligent strategy for managing the capacity of the interface converter is presented, which prioritizes the mentioned applications for the interface converter, in such a way that the main task of the converter is realized first, and then the empty capacity of the converter is used to provide services. In this strategy, initially, the capacity of the interface converter is used to exchange the reference value of active power between the AC and DC sections. This reference value is obtained based on droop control, which is a common method in microgrid control. In the second step, the proposed strategy uses the remaining capacity of the converter to compensate the harmonic components of the AC microgrid. One of the main advantages of this strategy compared to previous works is the possibility of managing the capacity of the converter in conditions of severe imbalance and harmonics of the microgrid voltage. In other words, the proposed strategy is designed in such a way that it is able to use the maximum capacity of the IC in all working conditions. Also, based on the power theory method, the control method of the interface converter is designed in such a way that in imbalance conditions, both in the grid-connected and island state, the active power fluctuation on the AC side and the DC link voltage fluctuation are completely eliminated. The interface converter is capable of compensating the imbalance as well as the harmonic components caused by the non-linear loads. Another important advantage of the proposed strategy is that the unbalance and harmonic components of the microgrid are estimated based on the empty capacity of the converter and using the IC terminal voltage. This feature means that there is no need to measure the current of loads and send them through telecommunications for compensation. This feature reduces the cost and also increases the reliability of the control method due to the elimination of telecommunications. Another important feature of the control strategy is the ability to fault ride through, which is reliably provided for the interface converter in any situation. In the proposed strategy for IC capacity management, the instantaneous capacity of the converter is calculated online based on the voltage and current of the AC side as well as the nominal current of the converter. Therefore, in any situation, the exchange power of the converter is smaller than its instantaneous capacity and the current limitation of the converter is always considered. The proposed strategy has been evaluated by implementing it on a combined AC/DC microgrid through MATLAB/Simulink and its effectiveness has been proven. The simulation results show that all the mentioned goals are achieved for the proposed strategy and this strategy is a big step towards maximum utilization of the capacity of converters in microgrids

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