Power sharing of parallel operated DC-DC converters using current-limiting droop control

In this paper, a nonlinear current-limiting droop controller is proposed to achieve accurate power sharing among parallel operated DC-DC boost converters in a DC micro-grid application. In particular, the recently developed robust droop controller is adopted and implemented as a dynamic virtual resistance in series with the inductance of each DC-DC boost converter. Opposed to the traditional approaches that use small-signal modeling, the proposed control design takes into account the accurate nonlinear dynamic model of each converter and it is analytically proven that accurate power sharing can be accomplished with an inherent current limitation for each converter independently using input-to-state stability theory. When the load requests more power that exceeds the capacity of the converters, the current-limiting capability of the proposed control method protects the devices by limiting the inductor current of each converter below a given maximum value. Extensive simulation results of two paralleled DC-DC boost converters are presented to verify the power sharing and current-limiting properties of the proposed controller under several changes of the load

Estudo de estratégias de controle para conversores CC-CC conectados em paralelo com foco numa microrrede fotovoltaica operando em modo isolado

TCC (graduação) - Universidade Federal de Santa Catarina. Centro Tecnológico. Engenharia Elétrica.Neste trabalho de conclusão de curso de engenharia elétrica da Universidade Federal de Santa Catarina (UFSC) são estudadas técnicas de controle para conversores CC-CC conectados, em paralelo, a um barramento CC. Os conversores também estão conectados a um sistema fotovoltaico de geração de energia e foram projetados para processar até 8 kW de potência por meio de 40 módulos instalados no topo do prédio do Instituto de Eletrônica de Potência (INEP) da UFSC. O estudo foi constituído considerando que a microrrede (MR) proposta possa atuar em dois modos de operação: conectado e isolado. No primeiro modo, a estratégia deve garantir que os conversores sejam controlados individualmente para funcionarem como rastreadores do ponto de máxima potência ( do inglês Maximum Power Point Tracker - MPPT), pois podemos utilizar a rede elétrica como fonte de geração ou armazenamento infinito de energia. Isso faz com que não nos preocupemos num balanço de potências entre a carga e a fonte geradora e que pensemos na máxima extração possível de potência do arranjo fotovoltaico. No segundo modo, o controle tem o objetivo de garantir que os conversores mantenham, de forma compartilhada, a tensão do barramento CC regulada, pois com a ausência da rede elétrica, o balanço de potência é necessário para regular o barramento e garantir o funcionamento adequado do sistema. É importante ressaltar que este trabalho tem como foco o estudo da microrrede operando no segundo modo e, embora técnicas do primeiro sejam apresentadas, suas discussões serão resumidas ou omitidas. Além de uma introdução e motivação ao estudo do tema, o projeto e o dimensionamento de um conversor boost também são apresentados no decorrer do texto. Finalmente, resultados provenientes de simulação teórica computacional com o software PSIM® e de experimentação prática são apresentados e discutidos, considerando a microrrede operando no modo isolado.In this undergraduate thesis of electrical engineering of the Federal University of Santa Catarina (UFSC), control techniques are studied for DC-DC converters connected, in parallel, to a common DC bus. The converters are also connected to a photovoltaic power generation system and were designed to process up to 8 kW of power through 40 modules installed at the top of the building of the Institute of Power Electronics (INEP) of the UFSC. The study was constituted considering that the proposed microgrid can act in two modes of operation: connected and isolated. In the first mode, the strategy must ensure that the converters are individually controlled to function as Maximum Power Point Trackers (MPPT), because we can use the electrical network as a source of infinite power generation or storage. This means that we do not need to worry about a balance of power between the load and the generating source and we should think on the maximum possible extraction of power of the photovoltaic arrangement. In the second mode, the control has the objective that the converters maintain, in a shared form, the voltage of the DC bus regulated, because with the absence of the electric network, the power balance is necessary to regulate the bus and ensure the proper operation of the system. It is important to emphasize that this work focuses on the study of the microgrid operating in the second mode and, although techniques of the first one are presented, their discussions will be summarized or omitted. In addition to an introduction and motivation to study of the theme, the design of a boost converter is also presented throughout the text. Finally, results from theoretical computer simulation through PSIM® software and practical experimentation are presented and discussed, considering the microgrid operating in isolated mode

Development of an alternative droop strategy for controlling parallel converters in standalone DC microgrid

Most of parallel-connected DC-DC converters schemes are based on a high-bandwidth communication network to achieve minimum circulating current, proper load current sharing, and acceptable voltage regulation. However, in DC microgrids, the use of communication network can be costly and unsuitable considering the data reliability and cost investment because the load and renewable energy sources are connected to the point of common coupling. Therefore, the droop control as a decentralized method has gained more attention. However, the challenge for the conventional droop method is to overcome the issue of circulating current, poor load current sharing, and the drop in DC grid voltage due to the droop action. This thesis develops and tests an approach for minimizing the circulating current, as well as improving the voltage regulation and the load current sharing for the droop method. The developed approach is based on the concept of synchronized switching, which is implemented using an alternative droop strategy for controlling different sizes of parallel-connected DC-DC boost converters. In this thesis, synchronous switching, based on an optimized controller, is presented to eliminate the initiation of circulating current and minimize the ripple in the output current for parallel-connected boost converters. Furthermore, a modified droop method, including the cable resistance, is introduced. The modified droop method uses the measurements of the voltage and current at the point of common coupling to estimate the voltage set point for each converter locally. The communication network is eliminated by utilizing the modified droop method because, in the proposed method, there is no current and voltage measurement data transmitted from one converter to the other converter. Additional loop control is also applied for equal current sharing between parallel converters to overcome the issue of mismatch in parameters of the parallel converters. The additional loop control is added to improve the load current sharing in the modified droop control. The modified droop control method with additional loop control is verified using MATLAB/SIMULINK and validated with experimental results. However, the droop action of the modified droop and different cable resistances degrades the voltage regulation and load current sharing. Therefore, an improved droop method, which utilizes the virtual droop gain and voltage droop control gain, is proposed to overcome the problem of load current sharing and voltage regulation. The virtual droop gain compensates the differences in the cable resistances, and the voltage droop control gain regulates the voltage at the point of common coupling. This maintains the common DC bus at its rated value. The effectiveness of the improved droop method is demonstrated by MATLAB/Simulink and Laboratory prototype results. Finally, the proposed method is utilized in a standalone DC microgrid. An example of a DC microgrid of a residential building powered by a PV solar system illustrates the feasibility and the effectiveness of the proposed methods