HVDC Bus Research Articles

A Modular DC-DC Converter as a Hybrid Interlink Between Monopolar VSC and Bipolar LCC-Based HVDC Links and Fault Management

High voltage dc (HVdc) transmission has gradually gained its popularity in long distance transmission of power. The possibility of multi-terminal HVdc network has further projected HVDC as a potential mode of power trading across geographical boundaries. The emergence of modular multilevel converter (MMC)-based dc-dc converters has led to the interconnection of HVdc networks across voltage levels, converter technologies and line topologies. This paper puts forward a topological modification in an existing MMC-based dc-dc converter topology, acting as a hybrid interconnect between different converter technologies, for interface of an asymmetric monopolar voltage source converter (VSC) and a bipolar line commutated converter (LCC)-based HVdc links. The proposed operation of the topology is validated experimentally in a down-scaled laboratory prototype. The proposed topology is further investigated for short-circuit fault conditions in any of the poles of HVdc bus and appropriate fault mitigation strategies are adopted for short-cicuit fault in both LCC and VSC bus. The fault mitigation strategies are validated by simulation in Matlab/Simulink platform.

Active Disturbance Rejection Control of DC-Bus Voltages Within a High-Speed Aircraft Electric Starter/Generator System

Wide-speed range operation capabilities in both motoring and generation modes are essential for an aircraft electrical starter/generator (ESG) drive system for more-electric aircraft applications. When running as a generator, the ESG is supplying power to the main HVdc bus within the onboard electrical system. A fast and robust dc bus voltage would be beneficial to electrical loads connected to it. In this article, an active disturbance rejection control (ADRC) is proposed to realize a fast and robust regulation of onboard dc bus voltages. The ADRC is achieved with a second-order extended state observer (ESO) to generate torque reference for the ESG. All the key characteristics, such as the stability of ESO, dc voltage tracking performance, and disturbance rejection capability of the ADRC for aircraft ESG applications, are thoroughly analyzed. To enhance system stabilities, an innovative and simple design method for the ADRC and ESO gains is introduced based on the gain-margin and phase-margin criteria. Experimental results have shown that, compared with the conventional PI-based dc voltage control, the proposed method can realize a faster and more robust dc voltage control.

Analysis of an LLC–DAB integrated DC–DC converter topology with low loss and low cost

AbstractThis paper proposes an LLC‐DAB integrated high‐frequency isolated DC–DC converter topology structure to reduce the loss and cost of the DC power distribution system. Furthermore, the complexity and efficiency of the system also can be improved. The proposed converter consists of a DAB converter and two input‐parallel‐output‐parallel half‐bridge LLC resonant converters. It provides two DC voltage level buses, namely the HVDC bus and the LVDC bus. Compared with the typical converter topology of the DC power distribution system, the number of power semiconductors of the proposed converter topology is reduced by sharing the power semiconductors of the DAB converter and LLC resonant converter. As a result, with the number of power semiconductors decreasing, the cost and loss of the system are reduced. Moreover, the proposed converter can control the flow direction of power conversion and supply power to the loads. The single‐phase‐shift modulation method is adopted for the DAB converter, and two LLC resonant converters operate as the DC transformer under resonant frequency with open‐loop control. The operation principles and characteristics of the proposed converter are discussed. Finally, the simulation is carried out and an experimental prototype is developed to verify the effectiveness of the proposed topology.

Multi-Functional DC Collector for Future All-DC Offshore Wind Power System: Concept, Scheme, and Implement

DC collection and transmission is one of the main developing directions of the large-scale offshore wind power system. This article proposes a multifunctional dc collector (MDC) to construct a low-cost, high reliability, and high flexibility all-dc offshore wind power system. By introducing the MDC to achieve energy collection and cascade boost, not only the system cost, size, and intermediate links of power conversion are reduced but also the coupling among wind turbines (WTs) is greatly weakened. Accordingly, the maximum power point tracking operation can be achieved independently for each WT unit. Furthermore, a constant voltage ratio control with first harmonic elimination iterative algorithm is proposed for dc collector to link the HVdc bus voltage and discretized dc voltage for triggering the energy consumption operation and suppressing the inductor current ripple, and the detailed control parameter design is provided. Meanwhile, the various operating modes are considered for MDC, including practical engineering configuration, fault isolation, dynamic cut-in/out, and energy consumption operation, which provides a practical solution for the construction of a long-distance and large-capacity offshore system. Finally, the comprehensive experimental results are given to illustrate the effectiveness of MDC.

AC unbalanced and DC load management in multi-bus residential microgrid integrated with hybrid capacity resources

This paper presents a control scheme including resources and load management in the residential DC microgrid. The DC microgrid is supported by fuel-cell, solar-cell and battery. The DC, AC single-phase and AC three-phase loads with 50 Hz frequency are integrated. The DC microgrid is connected to the external 60 Hz AC three-phase network. An efficient multi-bus topology is proposed for the microgrid and it is formed by various AC/DC buses to supply the loads and managing the resources. The main bus of system is a 470 V DC bus and it is connected to the external 440 V/60 Hz AC grid. The main DC bus supplies three LV, MV and HV DC buses with 100, 220, and 110–380 V, respectively. The HV DC bus produces a variable output DC voltage between 110 and 380 V in order to regulate the load power (i.e., motor speed). The MV DC bus is connected to 220 V/50 Hz AC single-phase loads. The connections between DC microgrid with AC loads and AC external gird are made by single-phase or three-phase inverters. The interface inverters between DC bus and AC loads are operated to control power, torque, speed, frequency and voltage of loads. The unbalanced AC loads are appropriately balanced by proper control of interface inverters. The resources and inverters are efficiently controlled to enable operation of residential building under both off-grid or grid-tied conditions. The coordination of fuel-cell, solar-cell and battery can supply a fixed 8 kW power to external grid and supply the internal loads under all outages and off-grid conditions. The simulations demonstrate that the proposed control realizes all the objectives including AC/DC load management, unbalanced load amendment, frequency adaptation, and off-grid operation.

Power Quality Improvement Using an Active Power Sharing Scheme in More Electric Aircraft

This article proposed a harmonic suppression scheme that used a dc-dc converter as an active harmonic injector to cancel voltage harmonics on the HVdc bus within a hybrid power generation centre (HPGC). A permanent magnet synchronous generator and a battery are considered to supply power to a common HVdc bus through their dedicated ac-dc and dc-dc converters respectively within the HPGC. We proposed simplified mathematical models of harmonics from the ac-dc and dc-dc converters. Thereafter, an active power sharing scheme between the PMSG and the battery is developed to control the magnitudes of targeted harmonics to be the same. The targeted harmonics on the HVdc bus can thus be cancelled by properly tuning the carrier signal phase angles within ac-dc and dc-dc converters. A closed-loop control scheme has been developed and this scheme is with no extra hardware cost. To demonstrate the effectiveness of the proposed scheme, we selected the first-band harmonic ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_c</i> − 3 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f₀</i> ) as the targeted harmonic component. The harmonic cancellation scheme for this component has been developed and validated using experimental results. It has been demonstrated that the proposed method can achieve over 90% reduction of this specific harmonic component on the HVdc bus within this HPGC using one dc-dc converter.

An improved nonlinear robust control design for grid-side converter of VSC-HVDC connected to wind power generation system

This article proposes an improved nonlinear (IN) robust control strategy for the grid-side voltage-source converter (GSVSC) of a VSC-based high-voltage direct current (VSC-HVDC) transmission system connected to a large wind farm by the using Hamiltonian function method. With the help of variable transformation, the nonlinear model with parameter uncertainties and external disturbances of the GSVSC is changed into the port-controlled dissipative Hamiltonian (PCDH) system. Based on the PCDH system, an IN robust control law is established. In order to demonstrate the effectiveness and robustness of the IN robust control, the backstepping power control (BPC), which is developed by using the backstepping design procedure in the sense of Lyapunov stability theorem for the GSVSC to satisfy the control objectives of a stable HVDC bus and the grid connection with a unity power factor, is selected as a comparison object. Generally speaking, the system used by the backstepping method must have special strict feedback of the lower triangular structure, but the wind farm connected to the power grid with the VSC-HVDC is a multivariable structure and highly coupled nonlinear system. So, the BPC cannot effectively maintain and utilize the nonlinear characteristics of the VSC-HVDC system, while this nonlinear physical structure characteristics is very useful for the design of a nonlinear controller. The greatest advantage of the Hamilton function method is that it can effectively keep and utilize the nonlinear characteristics of the system. The simulation results indicate that the proposed IN control designed by using the Hamilton function method gains the advantage over the BPC under a variety of operating conditions.

Assessment and improvements of inorganic insulation for high temperature low voltage motors

The increase of internal temperature capability of electrical machines is a major challenge for building high power density actuators for many applications in transport systems and in aerospace industry. Today, the main limit is the thermal class of the organic Electrical Insulation System (EIS), but a breakthrough toward very high temperature is possible with inorganic insulation technologies. An inorganic EIS is proposed and tested for designing motors able to work permanently up to 500 °C in the heart of windings. After a review of several high temperature (HT°) insulation solutions, the paper focuses on currently available ceramic-coated wires insulated with a very thin inorganic layer, which is a favorable solution for high current densities in machines of small sizes. The electrical limits of this technology are investigated, the weak points of the HT° wire are circumvented using impregnating cements able to provide adequate mechanical and electrical properties up to 500°C for designing HT ° electrical machines fed by a standard PWM inverter connected to the HVDC bus of the more electric aircraft, for example.

Distributed Photovoltaic Architecture for HVDC-bus Feeding with a Simple Evaluation of Optimal Tracking

This contribution describes, compares, and analyses two structures and their operating modes dedicated to renewable energy production from photovoltaic (PV) sources. Between the two different technical approaches, photovoltaic sources placed in a distributed architecture supplying a high DC voltage HVDC bus points large advantages. Thus, after preliminary comparison of both solutions and concluding phases, this efficient solution finally constitutes the main original analysis presented in this contribution. The distributed PV structure is investigated, implemented and simulated in an original way under the OrCAD/Pspice software environment. The adaptation stage for maximum power transfer is modelled in detail. A method to calculate the optimal duty cycle for optimal use of PV panels power is proposed, tested and validated by the use of a marketed PV module datasheet.

Circular Switching Surface Technique: High-Performance Constant Power Load Stabilization for Electric Vehicle Systems

Electric vehicles make use of energy storage systems, such as batteries and/or ultracapacitors to power the electric power drive train, as well as auxiliary automotive system for control, safety, and comfort. This relatively complex power structure can be described as a distributed multiconverter system. The constant power behavior of tight-speed controllers in the vehicle's traction system and tightly regulated dc–dc converters connected to the HV-DC bus produces instability effects. This paper proposes a simple and practical geometric control, using circular switching surfaces, to address constant power load instability in electric vehicle's power systems. The proposed switching surfaces provide a solution in the geometrical domain to constant power loading conditions, while achieving outstanding dynamic response compared to state-of-the-art controllers. The controller is implemented in a bidirectional Buck + Boost cascade converter as a battery charge/discharge unit and ensures reliable system operation. The predictable and consistent behavior of the converter with constant power load is presented by analyzing the system curves in the normalized state plane with the switching surfaces employed. Simulation and experimental results on a scaled 1-kW Buck + Boost cascade converter validate the proposed switching surfaces and predictions regarding the converter's behavior under constant power loading conditions.

Optimization of Power Line Communication System Using a Resonant HVDC Bus in a Distributed Renewable Energy Generator

The development of smart grid architecture for renewable energy production system is intensively studied for the possibilities that this architecture offers to optimize the global production and to increase the efficiency of the generator by monitoring individually the various sources subjected to diverse working conditions. To assume monitoring and intelligence in the grid and to communicate between remote stations and a central master one, a communication system needs to be associated to the power grid of the global production equipment. In this contribution we analyzepower line communication solutions in terms of quality of transmitted signal, efficiency and electromagnetic compatibility. We analyze the working modesof communication interfaces based on power line communication using the amplitude-shift-keying (ASK) modulation and integrating a light protocol of Modbus type. We point out that basic solutions offer poor quality of signal and generate a lot of harmonics of the fundamental carrier inducing important electromagnetic perturbations. An original solutionis thus developed, based on a narrow spectrum carrier on resonant HVDC bus working at a low frequency allowing to minimize the generation of harmonics and thus to increase the quality of the transmitted information. A PLC interfaceis then implemented in a hardware circuit and test pointing out the advantages of the PLC communication based on a resonant HVDC bus with sinusoidal carrier for renewable energy generator.

Modeling of the Characteristics of Photovoltaic Sources Feeding a HVDC Bus

In this paper, focuses on how to create a special library of any marketed solar panels in the Orcad-Capture environment using the design features provided by the founder, we established and implanted in Orcad-Capture the electric scheme of the solar panels according to their architecture. The main idea is to obtain PV station within output voltage exceed one kilovolts in order to simulate PV networks using transmission in HVDC bus. This makes it possible to model the operation of each panel powering a resistive load or other charges. Also, according to temperature and solar irradiation changes, simulation is used to determine with high accuracy the electric characteristics of the panels and the characteristics of the single or multiple maximum power points.

Individual Step-up Converter with Active Recovery Stage for High Efficiency Conversion of Photovoltaic Energy

The incessant involving of smart grids associated with renewable energies increases the demand in high efficiency converters. So it is important to consider all possible improvements involving reliability and simple structures. In this study we demonstrate the improvement potentiality of a DC/DC step-up converter, connected to a HVDC bus, based on a MCB-RS model (Magnetically Coupled Boost with Recovery Stage). We suggest a new possibility of improvement of the converter by mastering its active switch allowing an easier control of the internal voltage inherent to the recovery stage. The suggested solution permits a saving of the internal energy and prevents the over heating of certain components, like recovery diode. In the MCB-RS converter, the losses are for a main part dissipated in the recovery diode, representing a non-negligible amount of power. Thus, we propose to save this energy by the insertion in the recovery circuit of an additional parallel switch. This solution offers an additional advantage by making possible the flow back of a certain amount of energy by keeping in the on state the parallel switch. Finally we compare the simulated results with a real system and validate this new performing structure.

Micro-controlled Pulse Width Modulator Inverter for Renewable Energy Generators

A new controller for pulse-width modulation (PWM) inverter based on a circuit integrating a microcontroller and a symmetric output stage composed of two switching transistors which operate as switches for PWM has been developed. A model of operation was developed followed by the simulation and the realization of a prototype. The inverter output is filtered and a pure sine wave signal was generated. The PWM inverter adopting this PWM technique shows a simple method allowing to obtain pure sinusoidal output signal. In the context and framework of renewable energy, when associated in a generator architecture based on an intermediate HVDC bus, it is appropriated for a wide-range of power systems and applications.

Design of backstepping power control for grid‐side converter of voltage source converter‐based high‐voltage dc wind power generation system

This study presents a backstepping power control (BPC) design for the grid-side voltage source converter (GSVSC) in a voltage source converter-based high-voltage dc (VSC-HVDC) wind power generation system. First, a dynamic model by taking parameter variations and external disturbances into account is derived on the basis of the space-vector theory to achieve the decoupling control characteristic of the GSVSC in the VSC-HVDC. Moreover, based on the backstepping design procedure, a BPC scheme is developed in the sense of Lyapunov stability theorem for the GSVSC to satisfy multiple objectives of a stable HVDC bus and the grid connection with a unity power factor. The salient feature of the proposed BPC is the introduction of additional error terms into the control laws to reduce the chattering phenomena in traditional backstepping control. In addition, the effectiveness of the proposed BPC scheme is demonstrated by numerical simulations on a doubly-fed induction generator wind farm with VSC-HVDC grid connection, and its advantage is indicated in comparison with a traditional proportional-integral control strategy under a wide range of operating conditions and the possible occurrence of uncertainties.

State Estimation of Electric Power Systems with FACTS and HVDC Devices

In this study, the state estimation of electric power systems which contain FACTS and HVDC devices is investigated. A detailed steady-state model for the Unified Power Flow Controller (UPFC), the most general power flow controller, is formulated and implemented. The model is specified by the steady-state model parameters, such as voltage magnitudes and phase angles for the parallel and the series branches, and reactances associated with the parallel and the series transformers. The DC side HVDC bus net injections are assumed to be measured and error-free for the state estimation of HVDC. In order to include the new devices in the system, formulations of the state estimation algorithm are modified. Simulations of the test system are presented to demonstrate the use of the developed program. Test results show that the developed solution algorithms and formulations for the FACTS and HVDC devices are valid and effective.

New architecture for high efficiency DC-DC converter dedicated to photovoltaic conversion

Abstract The latest researches on photovoltaic energy conversion clearly show that the next generation systems will be both smarter and efficient, with insurance of greater security and modularity. They are clearly oriented nowadays on the multi-point conversion, based on a parallel high voltage bus, with photovoltaic panels owner of their individual Maximum Power Point Tracker (MPPT) connected to an integrated DC-DC converter managing a global Maximum Power Point. This structure allows to avoid shadowing problems. We have studied in this paper the possibility to have a small universal DC-DC converter able to work in a large input voltage range (10 - 40 volts), and producing an output voltage up to around 300 volts with a global efficiency better than 96%. In this paper we describe an original Step-Up converter architecture tested on a standard inverter i.e. including its own MPPT. We also discuss the possibility to use high voltage parallel bus to improve the electro-magnetic compatibility and electro-magnetic pulse robustness by the low surface of wirer loops, which also represents an interesting way to decrease wiring prices. The global efficiency improvement of this architecture has been estimated for + 15 to 20% compared to classic ones. This structure allows the use of numerous technologies (amorphous, poly, mono.. . cells) working together on a single HVDC bus in an optimal efficiency. Finally, it is possible to migrate sequentially and progressively from an old standard technology towards this new one as described before.