Medium Voltage DC Bus Research Articles

DC Cascaded Energy Storage System Based on DC Collector With Gradient Descent Method

With the continuous development of distributed energy, the energy storage system (ESS) is indispensable in improving power quality. Aiming at the application of large-capacity storage battery access to medium voltage dc power grid, a dc cascaded energy storage system based on the dc collector is proposed, and the characteristic, topology and control are presented in detail. In this scheme, the low-voltage storage batteries are accessed to medium voltage dc bus directly with dc collectors, which not only has higher efficiency, but also improves power density. Especially, the diversity of state of charge (SOC) is only reflected in the duty cycle of submodules (SMs), rather than the voltage deviation. Further, a variable angle carrier phase-shifted modulation with gradient descent method is discussed to eliminate the dominant current harmonics. By calculating the harmonic amplitude and solving the Jacobian matrix, the gradient that reduces harmonic current is obtained, and the optimal combination of the carrier phases is deduced by iterative calculation. Meanwhile, the method can be extended in the dc collector with any number of SMs to eliminate the dominant harmonics. Finally, the experimental prototype is built and the benefit of proposed solution is verified.

Topology of Wind Power DC Collection Converter Based on Double Bus Structure and Its Power Balancing Control Strategy

Wind power collection technology is the key to grid-connected wind power generation. Compared with AC wind integration, DC wind power collection is more advantageous. Using a triple active bridge (TAB) converter as a submodule, this paper proposes a wind power DC converter topology and its control strategy for medium-voltage dc (MVDC) grid connection. The converter adopts a double bus structure, with the MVDC bus connected to the grid and the low-voltage dc (LVDC) bus handling the mismatched power. It has independent maximum power point tracking (MPPT) control, power balancing, and autonomous voltage balancing on the output side. The converter system is simulated and verified on the MATLAB/SIMULINK software platform. The simulation results show the effectiveness of the control strategy and the ability of the converter to operate stably under multiple conditions.

A Quasi-Square-Wave Modular Multilevel Resonant DC/DC Converter With ZCS and Current-Shaping Capacity for High Step-Ratio Application

High step-ratio dc/dc converter is one of the key equipments for dc grids to satisfy the requirement of interconnecting low voltage dc and medium voltage dc buses. This article proposes a high step-ratio modular multilevel resonant dc/dc converter, where two full bridges are connected in parallel at LV side, and two submodules (SMs) strings are connected in series at MV side. Due to the series connection of two SM strings, the number of SMs and the size of the passive filter at MV terminal can be significantly reduced. Moreover, with the quasi-square-wave (QSW) modulation, the proposed converter can achieve flexible current-shaping capability. On this basis, through an internal phase-shifted angle of QSW modulation, the output voltage can be regulated with a fixed switching frequency, simplifying the design procedure of magnetic components. And zero current switching turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> of full-bridge switches at LV side can be realized in forward and backward mode. In addition, based on the analysis of the operation principle with the time-domain method, the basic control strategy and parameter design procedure are introduced in detail. Finally, the feasibility of the proposed converter is validated by a 100 V/1 kV/4 kW prototype.

Research on Stability of MMC-Based Medium Voltage DC Bus on Ships Based on Lyapunov Method

Research on Stability of MMC-Based Medium Voltage DC Bus on Ships Based on Lyapunov Method

Multiobjective Optimization Based Framework for Early Stage Design of Modular Multilevel Converter for All-Electric Ship Application

The Medium Voltage DC (MVDC) architecture for All Electric Ships (AES) has the potential to provide superior features compared to traditional 60-HZ AC distribution systems in terms of power density, power quality, and system stability. The MVDC system introduces extensive use of power electronics equipment into the shipboard power system (SPS) design that brings complexity to the system design. These power electronics equipment connect the power sources and load centers to the MVDC bus and play a major role in handling system faults. This paper focuses on developing a framework to determine the volume and failure rate of a Modular Multilevel Converter (MMC) for early stage ship design. Two different methodologies (Taguchi method and a genetic algorithm) were used to determine the best design from a robust set of design options. Once the design parameters have been identified, the Taguchi method forms orthogonal array to explore and evaluate designs. At the end of the design cycle, it identifies the best parameters from a large set of design parameters to achieve lower volume and failure rate. These parameters are used as input to the optimization process. This helps to narrow out the number of inputs for optimization algorithm. The Nondominated Sorting Genetic Algorithm II (NSGA-II) has been integrated with converter design tool to minimize the volume and failure rate of MMC. The results show that the optimization algorithm coupled with Taguchi Method provides the lowest volume and failure rate for MMC. One of the goals of early-stage ship design is to develop preliminary design and evaluation of trade space to narrow it down. This paper is expected to aid early-stage ship design of power electronics converter design for MVDC systems in SPS.

Stability analysis and control for power electronic transformer with FRT capability

In this paper, a power electronic transformer (PET) topology with the fault ride-through (FRT) capability is presented. In the PET, modular multilevel converters (MMCs) are adopted to provide medium voltage DC (MVDC) ports. The positive and negative isolated modular multilevel DC converters (IMMDCs) are cascaded respectively with the positive and negative MVDC bus. Under pole-to-ground (PTG) fault condition, the healthy pole of MMC and the healthy IMMDC can operate as normal with the capability of half rated active power transmission. As a cascaded system, the stability of the PET may be affected by the impedance-mismatching of the cascaded converters. Therefore, based on impedance analysis method, the stability analysis of the proposed PET is developed. Then, the stability control method based on series virtual resistance is presented to improve the PET stability. Besides, to fast the dynamic respond speed, a coordinated control scheme based on low voltage DC port current feedforward is introduced. Finally, the presented PET topology and the control strategy is verified by simulations in MATLAB/Simulink.

Optimal Scheduling Strategy of AC/DC Hybrid Distribution Network Based on Power Electronic Transformer

The AC/DC hybrid distribution network is composed of a medium-voltage DC bus, a low-voltage DC bus, and a power electronic transformer, and has the characteristics of multi-voltage level, multi-DC bus, and multi-converter, so its operation mode and optimal scheduling strategy are more complex. Firstly, this paper constructs the AC/DC hybrid distribution network using an power electronic transformer. Then, a two-layer control structure including a scheduling management layer and a bus control layer is proposed, which simplifies the control structure and gives full play to the role of “energy routing” function of the power electronic transformer. Moreover, the minimum operation cost of the AC/DC hybrid distribution network in the whole scheduling cycle is taken as the optimization objective, considering the characteristics of various distributed generations, the structure of AC/DC hybrid distribution network, and the interaction of “source–load–storage”. Finally, the optimal scheduling model of the AC/DC hybrid distribution network based on power electronic transformer is established, and the feasibility of the optimal scheduling strategy is verified by the open-source solver, which can realize the complete absorption of renewable energy and the optimal coordinated control of “source–load–storage”.

Capacitor Commutation Method for MVDC Hybrid Circuit Breakers

The medium voltage DC (MVDC) type system can connect multiple terminals to a common MVDC bus, so it is possible to connect several renewable DC power sources to the common MVDC bus, but a DC circuit breaker is needed to isolate short circuit accidents that may occur in the MVDC bus. For this purpose, the concept of a hybrid DC circuit breaker that takes advantage of a low conduction loss contact type switch and an arcless-breaking semiconductor switch has been proposed. During break the hybrid switch, a dedicated current commutation device is required to temporarily bypass the load current flowing through the main switch into a semiconductor switch branch. Existing current commutation methods include a proactive method and a reverse current injection method by a LC (Inductor-capacitor) resonant circuit. This paper proposes a power circuit of a new MVDC hybrid circuit breaker using a low withstanding voltage capacitor branch for commutation and a sequence controller according to it, and verifies its operation through an experiment.

A Multi-port DC Solid-state Transformer for MVDC Integration Interface of Multiple Distributed Energy Sources and DC Loads in Distribution Network

In traditional DC distribution networks with both a low-voltage DC (LVDC) and a medium-voltage DC (MVDC) bus, DC units are connected to the LVDC bus through LVDC converters. In this approach no galvanic isolation is provided and two power conversion stages are needed between the DC units and the MVDC grid, hence leading to a high number of converters and higher costs. To address these shortcomings, this paper proposes a novel multi-port DC solid-state transformer (MDCSST) to interface DC units directly to the MVDC bus. Multiple modules are connected in series on the medium voltage side to the MVDC bus, whereas DC units are independently connected on the low-voltage side of the MDCSST. Compared to a traditional DC distribution network, the proposed scheme connects DC units to the MVDC bus without an extra converter or an LVDC bus, therefore saving cost and reducing the number of converters. In addition, DC units are galvanically isolated by high-frequency transformers. The main challenge of the MDCSST is to resolve the voltage-imbalance problem on the MVDC side which is caused by the DC units power differences. An LC-branch is used to balance the voltages among the modules and to transfer their differential powers.

Energy flow control and sizing of a hybrid battery/supercapacitor storage in MVDC shipboard power systems

The propulsion system of a medium-voltage dc (MVDC) ship is subject to large thrust/torque variations due to interactions of the ship and the propeller with sea waves. These variations induce steep power fluctuations on the MVDC bus, adversely impacting the stability, efficiency and power quality. Hybrid energy storage system (HESS) is a promising solution for mitigating these power fluctuations. Dictating the energy that the HESS components must deliver/absorb, the energy management strategy (EMS) impacts the size/capacity of the energy storage system (ESS). Based on this consideration, sizing and EMS of a battery/supercapacitor (SC) HESS are jointly optimised by using a deep reinforcement learning-based method. The proposed method splits the power between the HESS components such that while the operational constraints are satisfied, energy storage size and losses are minimised. Its features are adaptability to varying sea states, real-time implementation feasibility, while obviating the requirement for knowledge of the ship propulsion power profile. The efficacy of the joint sizing/EMS on reducing the ESS size is validated by comparing the sizes when battery-only, SC-only and HESS are employed in the MVDC shipboard power system. Real-time implementation feasibility and adaptability to various ship propulsion power profiles is confirmed through real-time simulations.

A New Control Strategy for Plug-in Electric Vehicle of DC Microgrid with PV and Wind Power Integration

This paper presents a plug-in electric vehicle (PEV) charging unit supplied by PV, wind and the battery in an autonomous mode of DC microgrid (MG) system. With the traditional control methods of PEV, the EV is charged without considering generation and the load limits. The control strategy proposed for PEV in this article is to increase the maximum rate of charging capacity of an EV only if generation exceeds the load demand using new current-loop control technique. The variation of photovoltaic irradiance, DC and AC loads, battery charging and discharging characteristics are considered which are directly coupled to a medium voltage DC bus. This tends to be an attractive option for increasing the efficiency and operation of electric vehicle charging station. The power reference based droop controller is designed for the battery to provide an efficient power management of a DC MG system by regulating the DC link voltage with less deviation. The effectiveness of the proposed MG system with Escape Ford EV vehicle has been validated by MATLAB/Simulink. The reported simulation results show that the approach presented is capable of governing the PEV charging unit and regulation of DC bus voltage.

Quasi-Square-Wave Modulation of Modular Multilevel High-Frequency DC Converter for Medium-Voltage DC Distribution Application

In a direct current (dc) distribution network, the modular multilevel high-frequency dc converter (MDCC) can achieve electrical isolation, voltage conversion and power transmission between the low- and medium-voltage dc buses. In this paper, quasi-square-wave (QSW) modulation, which avoids the $dv/ dt$ stress problem and maintains a high dc voltage utilization, is applied in MDCC, and the mathematical model of QSW modulation with a dual-phase-shift (DPS) scheme is established. Based on the model, the influences of staircase rising and falling process in QSW modulation on MDCC performance, including the power characteristic, current stress, switching characteristic, and efficiency characteristic, are analyzed. The influences of QSW modulation under DPS scheme on MDCC performance are also investigated. Moreover, to achieve a comprehensive optimization of MDCC performance, a multiobjective optimized control strategy based on QSW modulation under DPS scheme is proposed. Finally, a MDCC prototype is set up and the experimental results verify the correctness and effectiveness of the analysis and proposed strategy.

Power management and operational planning of multiport HPCS for residential applications

Renewable energy integration to the power grid has seen a tremendous rise in last few years. A novel hybrid AC and DC bus layout (i.e. medium-voltage DC bus of 380 V and low-voltage DC bus of 48 or 12 V DC) for the residential consumer premises has been proposed in this study. This would enable the consumer to directly connect DC loads to the system, without the need of an individual power conversion device. This study summarises the strategical operational modes of the proposed hybrid power conditioning system (HPCS) for multiple distributed energy resources (DER) integrated residential premises located within urban/rural area. Modified HPCS converter control strategy has been proposed, such that the system operation is efficient and self-adaptive in both, grid forming (or, islanded) and feeding (or, grid connected) modes. System is configured by using a single power conditioning unit, capable of performing wide range of operations, such as, multiple DER interface, battery power management, grid power control and accessibility to AC and DC household loads at the desired voltage levels. Performance of the designed system has been tested via simulation and experimental studies.

Approach to Scalable Model Development for Navy Shipboard Compatible Modular Multilevel Converters

This paper describes a methodology for developing a scalable metamodel for power conversion equipment that produces size/weight/efficiency and dimensions using a building block to achieve required voltage and power ratings. There are provisions for capturing requirement-based shipboard environmental constraints, such as voltage dielectric standoff, shock/vibration mitigation, accessibility, and thermal management without the use of conservativeness factors. The method is suitable to produce equipment data for the smart ship systems design environment. A multimodule converter that provides the interface between a medium voltage ac generator and the ship medium voltage dc bus(es) is modeled using a reference hardware system as a starting point. Allocations for spacing and support structure impacts which are populated using experiential rules of thumb with the goal of incorporating physics-based methods in the future. The reference system is converted to a shipboard design using wide bandgap 1.7- and 10-kV power semiconductor modules. A study of 6-, 10-, 20-, and 30-kVdc systems is performed. This approach will enable the study under various shipboard power system architectures.

Performance of Medium-Voltage DC-Bus PV System Architecture Utilizing High-Gain DC–DC Converter

The number of large-scale solar photovoltaic (PV) systems continues to increase, while the size of the largest systems has already reached several hundred megawatts. This trend will challenge existing PV system architectures and will require new PV system architectures with higher power ratings and higher voltage levels at the point of common coupling (PCC). This paper reports a medium-voltage (MV) dc-bus PV system architecture based on a high-gain soft-switched interleaved boost (SSIB) dc-dc converter. The interleaved characteristic increases the flexibility of the converter, allowing for either a higher voltage and/or current rating, thus increasing the power rating of the converter. The high-gain capability of the SSIB converter allows it to be connected directly to an MV dc bus. This will facilitate direct connection of a PV system to an MV ac grid (i.e., 20 kV) using only one step-up transformer. Simulation and experimental results are presented to verify the operation of the SSIB converter and to confirm the steady-state and dynamic performance of the proposed PV system architecture.

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).

Generation control system for improving design and stability of medium-voltage DC power systems on ships

Voltage stability in medium-voltage DC (MVDC) power systems on ships is a key design goal. MVDC bus voltage stability can be impaired because of the presence of power electronic converters that can induce negative incremental resistance instabilities. This study presents a control strategy that stabilises MVDC bus voltage in the presence of destabilising constant power load converters. A state-feedback adaptive control is designed by linearisation and implemented using controlled interface converters connecting the MVAC alternators to the MVDC bus. Large signal stability of the control system is assessed using Lyapunov techniques. The proposed control is verified against both average-simplified and detailed time-domain numerical simulations, thus providing a two-way comparison.