Multi-converter DC Research Articles

New Mesh Configurations With Decentralized Droop Control Method for DC Microgrids

This paper proposes new, practical, and scalable mesh configurations for DC microgrids. The new mesh configurations are inspired by the concepts of graphs in graph theory. A decentralized secondary droop control method without involving communication complexity is also presented in this paper. The proposed control method eliminates the limitations of the conventional droop control method in the scenarios where the cable resistances in a DC microgrid system cannot be neglected. The new mesh configurations with the proposed control method achieve accurate current sharing among all the converters. The effectiveness and performance of the proposed control method are validated using circuit simulations and hardware-based experiments on existing and proposed configurations of multi-converter DC microgrid systems.

An adaptive voltage control compensator for converters in DC microgrids under fault conditions

This paper proposes a voltage compensator for converters in multi-converter DC microgrids (MG), which enhances the DC-MG behavior under fault conditions. Among the four existing categories in the classification of the faults, this paper focuses on those faults that affect voltage generation of converter-based distributed generation units (DGUs). Such faults conceptually occur from the terminal voltage reference to actual voltage generated by DGUs. To compensate the adverse effects of the faults, an adaptive scheme is proposed, which considers the most general case of the faults by using the time-varying multiplicative and additive fault models. The proposed scheme benefits from an integrated structure, which does not require separate fault detection, isolation, and identification blocks. As an advantage of the proposed approach, it is designed independent of the primary and secondary controllers of DGUs, and hence its performance is not affected by using different primary and secondary controllers. The effectiveness of the proposed scheme is verified under various fault conditions and uncertainty in the system parameters. Using the proposed scheme, the results show that the performance of system under the fault condition is similar to that of the normal operation.

AC/DC fault handling and expanded DC power flow expression in hybrid multi-converter DC grids

Hybrid multi-converter dc grids are extended dc grids formed by combinations of different converter topologies such as the line-commutated converter (LCC), modular multilevel converter (MMC), alternate arm converter (AAC) and other various topologies. The stable and secure operation of such systems in steady-stage and transient conditions is critical. This paper investigates the ac and dc fault-ride-through (FRT) capability and the expanded expression of dc power flow in a hybrid multi-converter dc grid consisting of LCCs, MMCs and AACs. Fault handling schemes of single converters are combined, extended and coordinated to provide satisfactory transient response of the hybrid dc grid under both ac and dc faults. This paper proposes an expanded expression of dc power flow considering the initial power flow determination and the power flow after converter outages under mixed P/V and I/V droop control. The expanded dc power flow expression can be used in LCC and modular VSC-based hybrid multi-converter dc grids to derive the initial dc power flow and assess the system static security after converter outages. Simulation results for steady-state and transient operation based on a detailed equivalent model verify the accuracy of the proposed dc power flow expression, and validate the fault handling capability of the hybrid dc grid.

Interoperability of modular VSC topologies in Multi-converter multiterminal DC systems

The ongoing transformation of power systems driven by the need for more interconnected networks will eventually lead multiterminal dc (MTDC) grids to become more commonplace in future electricity networks. Addressing interoperability challenges associated with multiple converter topologies for future MTDC implementations is a paramount requirement. This paper introduces a multi-converter MTDC (MC-MTDC) system combining both state-of-the-art and emerging modular voltage source converter (VSC) topologies. The developed MC-MTDC system, (i) demonstrates operation of the modular multilevel converter (MMC) and the alternate arm converter (AAC) in different MTDC configurations, and (ii) sets the basis for further development of MTDC configurations with diverse modular VSC topologies. The developed MC-MTDC system will also facilitate accurate testing and validation of multi-converter interoperability using real-time electromagnetic transient (EMT) models. The design parameters of the AAC-based terminals are determined ensuring compliance with existing MMC-based dc power systems. Detailed analysis and a complete set of results based on real-time implementation for different operating scenarios including steady-state, transient, and contingencies, verify multi-converter interoperability and associated control functions in different MTDC configurations.

Modeling of DC Distribution System Based on High Frequency Transient Components

The fault characteristics of multi-converter DC system are complex and variable. How to use fault transient information for fault characteristic analysis and protection setting is one of the challenges faced by the development of DC power distribution system. In this article, the high-frequency characteristics of faults in DC systems are analyzed. Then based on the transient fault circuit analysis method, the frequency domain impedance model of typical converters is obtained. The model established in the higher frequency band is not affected by the control strategy and can provide a stable impedance value, which lays a theoretical foundation for the protection principle. At the same time, the characteristics of the high frequency impedance superposition of multi-converter system are also given, which provides a basis for system-level protection setting and coordination. Finally, the DC power distribution system model is established in PSCAD/EMTDC for simulation, and the results prove the accuracy and validity of the DC system model.

Multiconverter Medium Voltage DC Power Systems on Ships: Constant-Power Loads Instability Solution Using Linearization via State Feedback Control

Bus voltage stability is a key issue in future medium-voltage DC (MVDC) power systems on ships. The presence of high-bandwidth controlled load converters (Constant Power Load, CPL) may induce voltage instabilities. A control design procedure is presented which starts at the modeling level and comes to control implementation. A control method based on a Linearization via State Feedback (LSF), is proposed to face the CPL destabilizing effect and to ensure the MVDC bus voltage stability. A multiconverter shipboard DC grid is analyzed by means of a new comprehensive model, which is able to capture the overall behavior in a second-order nonlinear differential equation. Exploiting DC-DC converters that interface power sources to the bus, LSF technique is able to compensate for system nonlinearities, obtaining a linear system. Then, traditional linear control techniques can be applied to obtain a desired pole placement. With reference to system parameters mismatch, LSF control design is verified by means of a sensitivity analysis, evaluating the possibility of an over-linearization strategy. Time-domain numerical simulations are used to validate the proposed control, in presence of relevant perturbations by means of a two-way comparison (average value model and detailed switching model).

Electronic load interface for improving PV dual‐converter system operational margin

An electronic load interface (ELI) for improving the operational margin of a photovoltaic (PV) dual-converter system under dynamic conditions is presented. The ELI – based on a modified buck–boost converter – interfaces the output of the converters and the load system. It improves the operational margin of the PV dual-converter system by extending the conditions under which the dual-converter system operates at the maximum power point. The ELI is activated as and when needed, so as minimise system losses. By employing the ELI, utilisation and efficiency of a PV dual-converter system increases. In general, the concept of the ELI can be applied to multi-converter PV systems – such as multi-converter inverters, and multi-converter DC–DC converter systems – for performance and efficiency improvement.

Realization of Parasitics in Stability of DC–DC Converters Loaded by Constant Power Loads in Advanced Multiconverter Automotive Systems

Distributed DC power systems are becoming increasingly common in advanced automotive systems. Constant power load (CPL) behavior of tightly regulated DC-DC converters in multiconverter power systems is equivalent to the dynamic negative impedance, destabilizing the DC bus and, consequently, the system. Various techniques have been developed to control ideal/lossless converters loaded by CPLs. Loss components significantly enhance the stability of the cascaded converters. In this paper, the effects of parasitics in the behavior of DC-DC converters loaded by CPLs are investigated. Furthermore, the stability of power converters loaded by CPLs in parallel to conventional constant voltage loads in the presence of loss components is analyzed. Design criteria are presented for converter operation in continuous and discontinuous conduction modes, which gives recommendations on the design of multiconverter DC power systems to avoid negative impedance instability. The proposed stable operation criteria are validated with hardware prototypes and simulation analyses.

Suitability of the pulse adjustment technique to control single DC/DC choppers feeding vehicular constant power loads in parallel with conventional loads

The increasing use of power electronic and motor drives in land vehicles, marine vessels and aircraft, for lots of applications such as propulsion, braking, steering, and actuation is creating particular problems when the power distribution system is in the form of multi-converter dc electrical systems. This is because of many power electronic loads and associated controllers which have a 'constant power' characteristic resulting in negative incremental input impedance. The effect of such loads in a dc system is to cause instability. The purpose of this paper is first to identify the most appropriate theoretical techniques for analysis of this nonlinear instability, and then to devise a novel technique for damping the instabilities and ensuring stable operation of system. We call this new control technique, Pulse Adjustment. In spite of its simplicity it offers a very fast and smooth response. Simulation and experimental results are presented to verify the proposed technique.

Modeling and Analysis of Multiconverter DC Power Electronic Systems Using the Generalized State-Space Averaging Method

This paper presents a modular approach for the modeling and simulation of multiconverter DC power electronic systems based on the generalized state-space averaging method. These systems may consist of many individual converters connected together to form large and complex systems. In addition to simplifying the analysis procedure, by using the proposed method, the time step for analysis of the system can be increased. Therefore, the required computation time and computer memory for complex systems can be reduced considerably. In this paper, after introducing the proposed approach, results of applying the method to a representative system are presented.