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Hybrid circuit breaker (HCB) has shown good performance for interrupting dc fault current. However, a massive number of IGBTs in series is needed. These will lead to high costs and significant difficulty in balancing IGBT dynamic voltage. This paper proposes a bidirectional hybrid circuit breaker based on a cascaded half-bridge submodule structure to reduce the overall costs and implementation difficulty of HCB. The number of IGBTs is reduced by 50%, and the operating speed is fast enough. A model of a four-terminal modular multilevel converter (MMC) system with a proposed hybrid circuit breaker in different stages is developed to describe the fault blocking process more clearly. The performance of the proposed HCB is validated and compared with other HCB structure through both theoretical derivation and time-domain simulation.

A new perspective of impedance matching is proposed to study the power flow operation of the three-phase four-wire (3P4W) unified power quality conditioner (UPQC) system. Specifically, the UPQC system is equivalent to an adjustable impedance network, when external conditions change, such as grid voltage sag/swell and unbalanced loads, the node impedances in the network will be dynamically matched. On the one hand, the three nodes of the series converter (SC) will be matched as positive or negative impedances with the change of grid voltages, which means that the SC operates in a rectifier or inverter state to absorb or transmit the energy. On the other hand, to compensate the unbalanced load currents, the three nodes of the parallel converter (PC) will be matched as positive and negative impedances at the same time, which means that the PC operates in both rectifier and inverter state to absorb and transmit the energy. After matching the node impedances, the system's output node impedances are equal to the load impedances, thus achieving the power balance of the whole system. Experimental results of the power flow and impedance matching from a laboratory-scale prototype hardware are presented to evaluate the correctness of the impedance analysis.

Energy storage systems (ESSs) are promising solutions for the mitigation of power fluctuations and the management of load demands in distribution networks (DNs). However, the uncertainty of load demands and wind generations (WGs) may have a significant impact on the capacity allocation of ESSs. To solve the problem, a novel optimal ESS capacity allocation scheme for ESSs is proposed to reduce the influence of uncertainty of both WG and load demands. First, an optimal capacity allocation model is established to minimize the ESS investment costs and the network power loss under constraints of DN and ESS operating points and power balance. Then, the proposed method reduces the uncertainty of load through a comprehensive demand response system based on time-of-use (TOU) and incentives. To predict the output of WGs, we combined particle swarm optimization (PSO) and backpropagation neural network to create a prediction model of the wind power. An improved simulated annealing PSO algorithm (ISAPSO) is used to solve the optimization problem. Numerical studies are carried out in a modified IEEE 33-node distribution system. Simulation results demonstrate that the proposed model can provide the optimal capacity allocation and investment cost of ESSs with minimal power losses.

Except for pumped storage, other existing electric energy storage technologies are difficult to achieve large-capacity energy storage and not easy to simultaneously meet the requirements in terms of site selection, cost, efficiency, and response. For this end, this paper combines the advantages of maglev technology and vacuum technology, proposes a new type of mechanical large-capacity energy storage technology which is vacuum pipeline maglev energy storage. Based on the idea of vacuum pipeline maglev energy storage technology, we have constructed a case system of vacuum pipeline magnetic maglev energy storage technology. The key technical parameters of the energy storage system, such as the maglev train's weight ratio and speed per hour, the mode of levitation and guidance, the car-track structure, the type and size of the vacuum pipeline, the type of the motor, the process of charging with acceleration, the process of no-load with uniform-speed and the process of generating electricity with deceleration were studied. On this basis, the energy storage capacity and cost of the vacuum pipeline maglev energy storage system were obtained through analysis. By comparing the energy storage capacity and cost of Fengning Pumped Storage Power Station in China, the advantages of vacuum pipeline maglev energy storage technology in economy and technology were verified. Finally, based on the technical characteristics of the vacuum pipeline maglev energy storage system, we analyzed its broad applications in renewable energy power consumption, optimization and upgrade of distribution network structure, urban emergency power supply and pulse power supply.

With the rapid development of distributed power and new energy vehicles, the traditional distribution network is facing enormous challenges. The development of multi-station integration technology is an effective way to solve the above problems. In this paper, a new multi-station integration topology is proposed. The new multi-station integration topology uses soft open point (SOP) as the key equipment of the system, and adopts the hierarchical power supply mode for data center, electric vehicle charging pile and other function stations. Then, based on the new topology of multi-station integration and smooth switching of converter control mode, a coordinated control strategy is proposed. Through the coordinated control strategy to control the working mode of each function station, and then further optimize the system operation conditions. Based on Matlab/Simulink, the simulation model of multi-station integration is established, and two different application scenarios are analyzed. The simulation results verify that the new topology of multi-station integration and its coordinated control strategy effectively improve the voltage level of the distribution network, improves the reliability of sensitive load power supply, and ensure the safe and reliable operation of multi-station integration system.

The analysis and calculation of the short-circuit fault current in the DC grid is of great significance to the design and configuration of the converter station and DC circuit breaker parameters. The existing flexible DC system not only includes the modular multilevel converter (MMC) converter, but also needs power equipment such as the fault current limiter and DC circuit breaker. Therefore, the system modeling and short circuit calculation of the multi-terminal DC system after adding the DC circuit breaker are also of great significance for the design of DC power system parameters and the grid troubleshooting ability. In this paper, firstly, the parameters of the four-terminal DC system of the modular multilevel converter (MMC) are simplified, and the analytical solution of the short circuit fault current of the multi-terminal DC system is given. Then, the external characteristics of the cascaded hybrid DC circuit breaker are modeled. Based on the equivalent circuit of the fault current in different stages, the short circuit calculation method of four-terminal MMC system with DC circuit breaker is obtained. This method can effectively describe the overall trend of fault current and provide the basis for the configuration of DC line protection settings and DC circuit breaker related parameters.

In a 100% clean energy town, to meet the energy balance and reduce the impact of power fluctuations on the main grid, in this paper, a hierarchical optimal energy management strategy (EMS) for a hybrid energy storage system (HESS) is proposed. The EMS consists of three layers. To meet the requirement of 100% clean energy, in the upper layer, a HESS economic operation model based on the mixed integer non-linear programming (MINLP) is presented. Then, considering the uncertainty of renewable energy and load, a power prediction model is presented in the middle layer. In addition, to reduce the power disturbance on the main grid caused by the stochastic power, a stochastic model predictive control (SMPC) strategy is implemented to optimize the power allocation of a HESS. In the lower layer, to reduce the power fluctuations of the lithium-ion battery (LiB) when mitigating minute-scale power fluctuations, a HESS optimal power allocation strategy based on Pontryagin's minimum principle (PMP) is proposed. In every control period, each layer is optimized based on the results of the previous layer. Finally, a simulation study is provided to validate the effectiveness of the proposed EMS. The results show that the proposed strategy has good performance in typical scenarios.

Each arm of modular multilevel converter (MMC) consists of a large number of sub-module (SM) units. However, it also increases the probability of SM failure during the long-term system operation. Focusing on the fault-tolerant operation issue for the MMC under SM faults, the traditional zero-sequence voltage injection fault-tolerant control algorithm is analyzed detailed and its disadvantages are concluded. Based on this, a novel fault-tolerant control strategy based on phase disposition pulse-width modulation (PD-PWM) is proposed in this paper, which has three main benefits: (i) it has carrier and modulation wave dual correction mechanism, which control ability is more higher and flexible；(ii) it only needs to inject zero-sequence voltage in half a cycle of the modulation wave, which simplifies the complexity of traditional zero-sequence voltage injection control algorithms and much easier for implement; (iii) furthermore, the zero-sequence voltage can even be avoided injecting under the symmetrical fault conditions. Finally, the effectiveness of the proposed control strategy is verified with the simulation and experiment studies under different fault conditions.

DC line fault is one of the important factors restricting the development of flexible DC system. DC circuit breaker is an effective means to realize the short circuit fault isolation on DC side of flexible DC system. Therefore, it is necessary to add DCCB into the short-circuit current calculation and modeling of MMC-HVDC power grid for comprehensive analysis. Firstly, the external characteristics of the hybrid HVDC circuit breaker and the modular multilevel converter (MMC) are modelled. Then, the modeling of short-circuit fault current in different stages was analysed. The analysis method of short circuit fault current of four-terminal MMC system with DC circuit breaker is obtained. Finally, the simulation results and the calculation results are compared and results show the effectiveness of the short-circuit calculation method proposed by this paper.

Lithium-ion capacitors is a novel kind of electrochemical energy devices integrating advantages of lithium-ion batteries and supercapacitors, and its thermal behaviors have not yet attracted sufficient attentions so far. In this study, the thermal behaviors of lithium-ion capacitor upon galvanostatic charge–discharge have been systematically investigated. The capacitors have been measured in a specially designed climate chamber where the ambient temperature remains constant (maximum fluctuation: ±0.5 °C). The surface temperatures of lithium-ion capacitors have been monitored while their C-rates and the ambient temperature changed in different charging and discharging tests; moreover, their high and low temperature performances have been studied and their thermal charge–discharge behaviors have been discussed in comparison with those of lithium-ion batteries and electrical double layer capacitors. The results show that the lithium-ion capacitors drop in temperature while the charging voltage remains 2.0–4.0 V but are heated up while they discharge. Such observations are different from both electrical double layer capacitors and lithium-ion batteries. These results can help proper thermal design and management of large capacity lithium-ion capacitors and modules for fast-charging applications.

Lithium-ion capacitors (LICs) are asymmetric electrochemical supercapacitors combining the advantages of high power density and long cycle life of electrical double-layer capacitor (EDLC), and high energy density of lithium-ion battery. A three-electrode LIC cell has been assembled employing activated carbon (AC) cathode and soft carbon anode. Self-discharge behavior and leakage current of LIC cell have been investigated using this three-electrode cell. It has been demonstrated that, in a LIC cell, the constant-voltage charge process and the applied voltage have significant impacts on self-discharge, which mainly occurs on AC cathode. Meanwhile, self-discharge and leakage current of LIC is much superior to EDLC. An approach to diminish self-discharge and leakage current has been proposed accordingly. The results will be beneficial to understand self-discharge behavior of LICs.

As a new type of energy storage devices with both high energy density and power density, lithium-ion capacitors have exhibited applicable prospects in the field of renewable energy and transportation. In order to explore possible applications of lithium-ion capacitors, equivalent circuit models have been extensively studied. The models are capable of representing the terminal characteristics of a cell. This paper establishes and compares two equivalent circuit models for lithium-ion capacitors: a classical model and a two-branch model. The experimental and simulation results of the two equivalent circuit models are compared under various working conditions to validate the accuracy of the models. A new parameter identification method combining numerical fitting and circuit analytical is developed for the two-branch model. Moreover, the assumptions and preconditions of the two-branch model are deeply analyzed to obtain a general parameter identification method.

For the purpose of drastically reducing the computational time of alternate arm voltage source converters in electromagnetic transient simulations, an accelerated model of AAMC is proposed based on Thevenin's theorem. This real-time simulation model is fast and efficient, and it has strong universality. The simulation results validate the simulation accuracy and acceleration ratio of the new model.

The future custom power park will realize the hierarchical power supply of different voltage quality grade, meanwhile and present the grid-friendly characteristics including low pollution for grid, organic combination with distributed generation (DG) and energy storage (ES). In order to gradually achieve the goals, the function reuse and integration in the equipment level should be solved firstly. It means the fusion among power quality equipments, DGs and ES in the equipment level. However, the research on series and parallel combination operation mode among some same or different types of custom power equipments, which could provide the basis for a variety of combination way of realization, provide the convenient and flexibility to deploy the functions of different equipments. It needs to study of problem which the share series or parallel connected converter among power quality equipments, DGs and ES. To seek a smooth switching method between variety of functions and status, and a coordination control method to balance functions of the grid generation, short-term support and power quality regulation. In this paper, mathematical models about custom power equipments are proposed and analysis is provided to show the principal and effect of interactions. Meanwhile the configuration guidelines for the different equipments are given and a novel integration strategy is proposed. Finally, simulation results are provided to verify the analysis.

Custom power park is an industrial/commercial park that integrates state-of-the-art power quality devices. The power quality devices can be classified into two catalogues: voltage quality enhancement devices and current quality enhancement devices. In order to meet the requirement of large capacity, high reliability, high flexibility in both the voltage and current compensation, several operation modes of multiple DVRs and multiple DSTATCOMs are proposed. Moreover, aiming at the negative interaction which may occur when DVR and DSTATCOM are installed on the same feeder, the paper analyzes the mechanism of the interaction with the harmonic impedance method. The stability problem may occur due to impedance parameter mismatch of two devices. Finally, the analysis results are verified by time domain simulation.

The operation of micro-grid based renewable energy usually considers efficiency and economy in comprehensive. The control strategy based on master micro-source (MS) constant voltage constant frequency (CVCF) networking is introduced. The voltage and frequency of micro-grid is established by the master MS, and the slave MS operated on the maximum power point tracking mode, in order to achieved micro-grid efficient and economic operation. For improving the function of each MS in micro-grid, a multi-functional convertor operation strategy is proposed. According to this strategy, the master MS has the function of forming micro-grid and switching to the public grid seamlessly, while the slave MS has that of MPPT and energy exchange. The seamless transfer technology for micro-grid is introduced. The control strategy of master MS constant voltage constant frequency networking is used in the grid seamless transfer. The grid-connected and off-grid processes of the MS control topology are analyzed and experimentally studied. The results have showed the effectiveness of proposed control technology, which has the advantages of less impact on local sensitive load, easy hardware implementation and good system stability.

Aiming at the problem of nonlinear load which affect the output voltage quality of the micro-source inverter in Micro- grid, An integral repetitive control strategy is proposed in the paper. The principle of integral repetitive control strategy is derivation, and its good performance is verified by micro-source inverter. Experimental results show the efficiency and reliability of the proposed control strategy in harmonic control and system stability.

With the nonlinear and unbalanced loads and high penetration of distributed generations connected into the power distribution network, some deteriorative influences such as poor power quality and stability issues have been caused. Static synchronous compensator (STATCOM) and another custom power device, electric spring (ES) are competent for solving these problems. This paper provides a general comparative analysis of STATCOM and ES on fundamental principles, control strategies and power quality improvement functions including reactive power compensation capacities and voltage regulation capabilities. Necessary mathematical derivations supporting the theoretical framework of the principle and the final conclusion are detailed in the paper.

Nowadays, the design of the supercapacitor equalization circuits is a hot topic for researchers, but most literatures focus on the topology of the equalization circuits, and few pays attention to the characteristics analysis of voltage equalization circuit in supercapcitor modules. In this paper, quantitative analysis of the influence of the dispersion characteristics of supercapacitor are presented, and the design method of switching resistor circuit is proposed under two kinds of typical working conditions. Simulation and experimental results are presented to verify the analysis.

Traditional schemes of UPQC and OPEN UPQC are proposed in literature to solve both the voltage quality problem of the load and the current quality problem of the grid. However, the traditional UPQC is not flexible when configured to compensate an industrial park if not all loads are sensitive. The previously proposed OPEN UPQC scheme has the advantages that communication coordination are required. To solve the above problems, a new topology of OPEN UPQC is proposed in this paper. The advantages of the new topology are analyzed in detail and the control scheme and the corresponding power flows are given. Further application topology is also given to not only achieve the function of conventional UPQC, but also to improve the voltage quality within the supply region. Simulation results are presented to verify the effectiveness and feasibility of the proposed topology.

With the high penetration of distributed generations (DGs) have been connected into the local power distribution network, it will result in some deteriorative influences such as poor power quality and stability issues. And power distribution network itself with problems of power system nonlinear characteristic, power system faults and nonlinear loads makes this issue more severe. Meanwhile, in order to restraint the large output power fluctuation caused by great many DGs, enough power support, which can be implemented by energy storage technology, is needed. Thus, for solving the voltage problems in power distribution network with the high penetration of DGs, a novel Unified Power Quality Conditioner (UPQC) based on fast energy storage is introduced in this paper. Then the structure principle and the control strategy were explained at length. At last the practicality of the device was determined successfully by simulation and experiment research.

Custom power devices are a set of devices designed to protect customers' loads from power line disturbances. One class of custom power devices is the series-connected device (SC), such as dynamic voltage restorer (DVR) and series active power filter (SAPF). Recently, downstream fault limiting function of series custom power devices have been proposed and integrated in some literatures as an add-on feature, which can limit the flow of large line currents, and therefore restore the point of common coupling (PCC) voltage as well as protect the series custom power devices themselves. Though it sounds straightforward, there are many difficulties in specific implementation, such as it requires a 100% voltage compensation capacity and very quick dynamic control in some existing proposals. So the feasibility and performance of these methods should be carefully examined. But till now there is no literature available to clarify these problems. In this paper, advantages and disadvantages of existing measures are firstly presented and then new methods are proposed to further improve the feasibility and performance. Detailed analysis is presented and conclusions are given.

The combination of the grid and DG is considered as a main method which can save investment, reduce fossil energy consumption and improve the reliability and flexibility of the power system. However, the high penetration of DG will severely influence the distribution network. This paper studies a typical structure of distribution network, analyzes the principal theory and sets up the simulation model. The simulation analysis at different cases shows that in mid-voltage or low-voltage distribution network, the energy flow from DG is the essential reason incurring voltage quality problems. Moreover, the voltage quality problem mainly comes from the change of the real power due to the big impedance ratio R/X of the line. The voltage quality problem will be improved by injecting power to regulate voltage with the inverter-based DG. From the result of simulation analysis, this paper provides a method to improve voltage quality with the power electronic inverter of DG.

For some kinds of motor drive systems, such as the variable-pitch wind power system, the back-up power needs to output very high power in a short time to meet the emergency. And the work temperature range and cycle life is required more strictly. Supercapacitors are very suitable to be the energy storage component due to its advantages of high power density, high charging speed and extremely high cycling capability. However, the energy density of supercapacitor is relatively low, so the energy management method of supercapacitors based UPS is very important to increase the energy usage efficiency. In this paper the finite state machines theory is applied as management method for supercapacitors based uninterruptable power supply (UPS). Design procedure is discussed in detail. Finally, the simulation and experimental results are provided to verify the design.

In this paper three types of energy storage system for regenerative braking are presented. The advantages and disadvantages are discussed after the simulation and experimental results are presented. Problems are discussed and the design guidelines are suggested1.

As a new energy storage device, ultracapacitors are able to attain greater energy densities while still maintaining the characteristic high power density of conventional capacitors. As a result, ultracapacitor based power electronics system will become an attractive power solution to an increasing number of applications. But in this situation, the ultracapacitor usually has the shortest span, and then determines the system lifetime. Therefore, a deterioration diagnosis of the ultracapacitor is needed for preventive maintenance of the circuit. In this paper the deterioration characteristics of ultracapacitor is presented and an online deterioration diagnosis method is proposed. Experimenal results are given to verify the analysis.

Conventional photovoltaic grid-connected using PI current loop control, there are some shortcomings: low precision control system, on the system parameters perturbation robustness is not strong, rapid, no static error, and strong anti-disturbance load capacity is limited and so on. In order to improve the dynamic performance of the grid-connected PV inverter control system, reduce the hardware and software cost of inverter control design, in view of grid- connected inverter non-linear mathematical model, this paper proposes the design of grid-connected inverter control system using the Exponential Sliding Mode Control. Theory testify of the Exponential Sliding Mode Approach Law applied in the grid-connected inverter is completed, and system stability was proven using Lyapunov theorem. The unit continuous of control method is used to solve the buffeting. Compared with the conventional control system, the result of the proposed control strategy had proven accurate, and the system has a good control precision and followed characteristic. Finally, the THD Harmonic Analysis is completed, proved the validity of the theory.

Supercapacitor is an emerging technology in the field of energy storage systems that can offer higher power density than batteries and higher energy density over traditional capacitors. Supercapacitor will become an attractive power solution to an increasing number of applications, such as renewable energy power generation, transportation, power system and many others, because of its advantages which include high charge/discharge current capability, very high efficiency, wide temperature range, etc. In this paper, the advantages and disadvantages of supercapacitor are discussed and some critical technologies for designing supercapacitor energy storage system are presented in detail. Finally, the role of the supercapacitor in renewable energy power system is discussed and a supercapacitor based uninterrupted power system (UPS) for the wind turbine pitch systems is presented as a design example.

As a new device in energy storage systems, ultracapacitor has drawn more and more attention recently. Many equivalent circuit models have been proposed to analyze its characteristics. However, for the design of energy storage systems, more insights are needed to analyze the transient response for many design aspects, such as the design of the voltage balance circuit, the optimization of the energy management scheme, etc. In this paper, an improved ultracapacitor equivalent circuit model is proposed, and the parameter identification algorithm based on nonlinear least-squares fitting of time response data is designed. The experimental results are presented and some important parameters regarding the proposed equivalent circuit model are discussed in detail.

Now pitch systems are widely used in the wind turbine system. To enhance stability, the pitch system is equipped with a ride through system to immediately ensure the reliable functioning of the fast blade pitch system in the event of a total power failure. Supercapacitor, as a new energy storage component, offers excellent performance, high power density, wide temperature range, and long life, particularly suitable for this application. In this paper, compared with other forms of energy storage components, the advantages and disadvantages of supercapacitor used in the ride through system for wind turbine pitch systems are discussed, and the design of supercapacitor based ride through system is presented in detail. Finally, experimental results are reported to verify the design.

Now it is a trend that active rectifier is used to replace the traditional non-controlled diode bridge in AC-DC-AC industrial drive system. Though direct power control (DPC) of three-phase voltage type active rectifier has drawn more and more attention, conventional DPC results in variable switching frequency, and consequently the system may suffer undesirable harmonics and the input EMI filter may be difficult to design. To alleviate this problem, in this paper, we propose a novel control strategy that combines DPC, Space Vector Modulation (SVM) and predictive control. The stability of the proposed control scheme is examined through Lyapunov's direct method, and a special control law suitable for transient operation is derived. The experimental results are also presented to verify the proposed scheme.

As a new technology, supercapacitor can attain greater energy density than those of conventional capacitors and greater power density than those of batteries. As a result, supercapacitors may become an attractive power solution for an increasing number of applications. In this paper, after a brief introduction of supercapacitor components, a supercapacitor based ride-through system for industrial drive applications is presented. The design details are discussed and experimental results are given.