Connected Converter System Research Articles

Large-Signal Stability Analysis of DC Distribution Systems With Cascading Converter Structure

DC-DC converters are major components of the DC distribution systems. The converters interface with external power inputs, internal DC buses and loadings of subsystems. The interfacing DC-DC converters should be stable locally and globally under mutual interactions through a DC bus within the system. Current research efforts have focused on the analysis of the stability of the DC distribution system subject to small-signal disturbance. However, in practice, the system routinely operates under large-signal disturbances, such as when an additional subsystem is turned on after being connected to the DC bus. In this scenario, the small-signal model may fail to fully describe the dynamics of the system. In this paper, we identify and analyze the bifurcation process when the system undergoes abrupt load changes. According to the nonlinear operation of the interconnected system, a large-signal stability criterion is derived. This criterion is simple, and can be easily extended to multiple connected converter systems. The criterion is also consistent with the result from bifurcation analysis. Finally, the validity of the proposed criterion is verified by the full-circuit simulations and the experimental works.

Small-Signal Modeling and Analysis of Converter Interactivity in 48 V DC Grid

Due to the flexibility of realization in decentralized form, environmentally genial, and the easy integration with renewable energy sources, storage units, and loads, the direct current (DC) microgrid has shaped as a subject matter worldwide. In a DC microgrid structure, the concurrent operation among multiple DC-DC converters, connected in parallel fashion across the common DC bus, realizes substantially with the average current information-based droop control technique. However, such control logic requires two minimal controllers with an inter-reliant design process to implement the overall control action. Considering the coupling effect during converter operation, this paper analyses the implementation of a mixed-signal peak current information-based droop control logic for the parallel connected converter system. A thorough small signal and switch-based modeling process for the interconnected systems are carried out with consideration of resistive and constant power loads. Adaptability to modeling with a first-order approximation-based approach makes the methodology easy for the design of the controller and stability analysis process in a parallel connected converter system. The veracity of the designed model is verified using simulation studies. A hardware prototype with two boost converters operating in parallel and feeding power to a resistive load and a buck converter (behaving as a constant power load) is fabricated to study the performance of the designed controller, along with the effect of different transient events on overall system performance.

Gradient Descent Optimization Based Parameter Identification for FCS-MPC Control of LCL-Type Grid Connected Converter

Aging and temperature changes in the passive components of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filter grid connected converter system (GCCs) may lead to parameter uncertainties, which can in turn influence its modeling accuracy for finite-control-set model predictive control (FCS-MPC). The presence of model errors will change the resonance point and deteriorate the power quality of the grid current, in turn degrading the active damping performance. In this situation, there is a serious possibility that the GCCs may malfunction and automatically disconnect from the grid, causing great challenges to the system stability. To solve this problem, first, prediction error analysis in FCS-MPC due to the model parameter errors is presented. Second, to achieve high accuracy and fast filter parameter estimation in utility, an adaptive online parameter identification method based on gradient descent optimization (GDO) has been proposed. Finally, to further reduce the searching time needed by the optimal iteration step, a variable iteration step searching method based on the root-mean-square-prop (RMSprop) GDO method is proposed. Experimental studies of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -GCCs prototype in the laboratory have been conducted to validate the effectiveness of the proposed method.

Integrated Current Balancing Transformer Based Input-Parallel Output-Parallel LLC Resonant Converter Modules

The input-parallel output-parallel (IPOP) connected converter system allows the use of a low-power converter module for high-power applications. However, mismatches in the IPOP circuit parameters cause uneven current sharing between the constituent modules, which makes the IPOP system unreliable for practical use. Different techniques have been proposed so far; however, they cause an increase in magnetic volume, cost, component count, and complexity of the IPOP system. In this article, a reliable and efficient magnetic integration technique is proposed for the current sharing requirements of the IPOP system based on LLC resonant converter modules. The proposed integrated magnetic current balancing transformer (ICBT) based IPOP LLC resonant converter modules ensures both the input current sharing and output current sharing between constituent LLC modules, under the open-loop condition quite effectively. The proposed ICBT is the resonant inductors of LLC resonant converter modules, coupled to perform the function of current balancing as well as generates enough leakage inductances for resonance operation, simultaneously. Thus, it can reduce the overall magnetic volume and improves power density of the IPOP LLC modules. Moreover, the current sharing performances of the proposed ICBT in both the steady-state and dynamic-state are analyzed based on the magnetic and electrical model. To validate the performance of the proposed ICBT based IPOP two LLC modules, a 2.6-kW hardware prototype was designed, fabricated, and tested successfully.

Bifurcation-Based Stability Analysis of Photovoltaic-Battery Hybrid Power System

Photovoltaic-battery hybrid power system (PBHPS) is a good candidate for rural electrification in remote areas. In this system, multiple energy sources are combined together to power the user loads under various system structures. We identify and analyze the bifurcation process when the system enters unstable operation states. Then, a small signal stability criterion is derived for the system based on reasonable simplification of the bifurcation analysis. Furthermore, according to the nonlinear dynamics of the interconnected system with closed-loop control, a large-signal stability criterion is derived with mixed potential theory. The criterion that is straightforward with the dual-loop control can be considered and easily extended to multiple connected converter systems. The criterion is also coincident with the result of bifurcation analysis. Finally, the validity of proposed criteria is verified by simulation and experimental works in a practical PBHPS of 1 kW.

Automatic Current Sharing of an Input-Parallel Output-Parallel (IPOP)-Connected DC–DC Converter System With Chain-Connected Rectifiers

Input-parallel output-parallel (IPOP)-connected converter systems allow the use of low-power converter modules for high-power applications. An IPOP converter topology with half-wave, daisy chain-connected rectifiers is presented which consists of multiple half-bridge (HB) dc–dc converter modules. By utilizing a common-duty-ratio control scheme, without a dedicated current-sharing controller, automatic sharing of input current and load current in the IPOP converter is achieved even in the presence of differences of more than 10% in various module parameters. The steady-state and dynamic-state current-sharing performance of the proposed IPOP converter is analyzed by using a steady-state dc model and a small-signal model of the system, respectively. It is concluded that steady-state current sharing among modules can be realized by applying a common-duty-ratio control scheme and by reducing the difference in transformer turn ratios, while dynamic-state current sharing is only slightly affected by substantial module parameter mismatches. The stability and current-sharing performance are verified by Saber simulation and an 800-W prototype consisting of two HB modules. The IPOP converter topology under the common-duty-ratio scheme can be extended to any system of three or more converter modules, including full-bridge dc–dc converters.

Common-Duty-Ratio Control of Input-Parallel Output-Parallel (IPOP) Connected DC–DC Converter Modules With Automatic Sharing of Currents

Input-parallel output-parallel (IPOP) connected converter systems allow the use of low-power converter modules for high-power applications, with interleaving control scheme resulting in smaller filter size, better dynamic performance, and higher power density. In this paper, a new IPOP converter system is proposed, which consists of multiple dual-active half-bridge (DAHB) dc-dc converter modules. Moreover, by applying a common-duty-ratio control scheme, without a dedicated current-sharing controller, the automatic sharing of input currents or load currents is achieved in the IPOP converter even in the presence of substantial differences of 10% in various module parameters. The current-sharing performance of the proposed control method is analyzed using both a small-signal model and a steady-state dc model of the IPOP system. It is concluded that the equal sharing of currents among modules can be achieved by reducing the mismatches in various module parameters, which is achievable in practice. The current-sharing performance of the IPOP converter is also verified by Saber simulation and a 400-W experimental prototype consisting of two DAHB modules. The common-duty-ratio control method can be extended to any IPOP system that consists of three or more converter modules, including traditional dual-active bridge dc-dc converters, which have a characteristic of current source.

General Control Considerations for Input-Series Connected DC/DC Converters

This paper discusses the general control problems of dc/dc converters connected in series at the input. As the input voltage is shared by a number of dc/dc converters, the resulting converter relieves the voltage stresses of individual devices and hence is suitable for high input-voltage applications. At the output side, parallel connection provides current sharing and is suitable for high output-current applications. Moreover, series connection at the output side is also possible, resulting in output voltage sharing. Theoretically, from a power balance consideration, one can show that fulfillment of input-voltage sharing implies fulfillment of output-current or of output-voltage sharing, and vice versa. However, the presence of right-half-plane poles can cause instability when the sharing is implemented at the output side. As a consequence, control should be directed to input-voltage sharing in order to ensure a stable sharing of the input voltage and of the output current (parallel connection at output) or output voltage (series connection at output). In this paper, general problems in input-series connected converter systems are addressed. Minimal control structures are then derived and some practical design considerations are discussed in detail. Illustrative examples are given for addressing these general control considerations. Finally, experimental prototypes are built to validate these considerations.

Maximum power tracking in parallel connected converters

A control scheme for parallel connected converter systems that will transfer the maximum available power from a nonideal voltage source is presented. Monitoring the rates of change in both the average input current and average input power from the source in each switching period, the proposed control method can dynamically regulate the DC-DC converter system to track the peak power of the source. The amplitude and frequency of the oscillation due to a limit-cycle around the system peak power point is analyzed. In addition, the central-limit current distribution control for the system is incorporated in this control scheme to improve the system efficiency and reliability by uniformly distributing the total peak power among the converters connected in parallel.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>