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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.

Voltage stability has always been a hot topic in power system research. Traditional On-Load Tap-Charger (OLTC) transformer is considered to play a very important role in the system voltage stability. However, in the heavy load of distribution network, the tap adjustment of OLTC transformer will lead to the shift of critical stable operating point, which bring the “negative voltage regulating effect” of voltage adjustment, and even cause the instability of system voltage. This paper presents a Flexible On-Load Voltage Regulation (OLVR) transformer based on power electronic technology. The Flexible On-Load Voltage Regulation (OLVR) transformer is a combination of Power Electronic Converter (PEC) and OLTC transformer, which can realize voltage step-less regulation and reactive power regulation. Meanwhile, the paper presents the equivalent models of distribution network with Flexible OLVR transformer and analyzes the critical operating point. Through the step-less voltage regulation control of the Flexible OLVR transformer, the negative voltage regulation effect of the transformer in on-load voltage regulation is avoided, and the voltage stability of the distribution network is improved.

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.

The conventional half-bridge submodule (HBSM)-based multilevel modular converter (MMC) cannot block DC faults. To solve this problem, a novel flexible overhead-line high-voltage direct current (HVDC) transmission converter station topology is proposed in this study, which provides DC fault blocking capability. By adding blocking submodules (SMs) onto the positive and negative DC buses of a conventional HBSM converter station, a rapid fault current blocking can be achieved in the case of a DC bus short circuit. The DC fault blocking principle of the proposed topology and the rapid fault blocking capability of the blocking SM are analyzed. The quantity of required major components is also calculated. In addition, the IGBT, the system loss and the control complexity quantities are comprehensively compared with those of existing topologies. The validity of the proposed topology is demonstrated based on simulation and experimental studies.

Since each branch of the multiterminal DC circuit system relies on the DC circuit breaker for breaking and fault isolation, the prohibitive cost and huge volume of the Hybrid DC Circuit Breaker (HCB) limit its development and broad application in multiterminal flexible DC systems. Multiport hybrid DC circuit breaker (MP-HDCCB) based on device and branch sharing reduces the configuration cost of the circuit breaker to a certain extent. In order to further reduce the cost of MP-HDCCB, a novel MP-HDCCB topology based on hybrid switching devices is proposed, adopting full controlled switching devices to achieve rapidity of breaking fault current, and using semi-controlled switching devices in series to withstand the transient interruption voltage (TIV), so as to reduce the construction cost and technical difficulty. In this paper, the working principle and fault breaking strategy of the topology are introduced in detail, then the parameters of the major circuit are analyzed theoretically, and the parameter design of each branch is given. In the end, the rationality and validity of the proposed topology is tested and verified by simulations and experimental tests.

The application of on-load tap-charger (OLTC) transformer technology has become the most direct and effective way to solve the voltage fluctuation of power grid. With the development of active distribution technology, the research focus of on-load voltage regulation technology has gradually turned to the development direction of arc free, fast and intelligent. This paper presents a flexible on-load voltage regulation topology based on power electronic technology. The flexible on-load voltage regulation (flexible OLVR) transformer is a combination of power electronic technology and traditional on-load tap-charger transformer, which can realize fast arcless switching, voltage step-less regulation, power regulation and other functions. In this paper, a new type of flexible on-load voltage regulation transformer is proposed. The OLTC switches of the device adopts the power electronic switch of anti-parallel thyristors, which can realize step, fast and arcless voltage regulation; the power electronic converter (PEC) module is connected to the primary side of the main transformer. At the same time, it proposed a new reactive power compensation control strategy, which could realize the functions of step-less voltage regulation and reactive power compensation. In the end, the rationality and validity of the proposed topology is tested and verified by simulations and experimental tests.

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.

The widely used flywheel energy storage (FES) system has such advantages as high power density, no environment pollution, a long service life, a wide operating temperature range, and unlimited charging&ndash, discharging times. The flywheel array energy storage system (FAESS), which includes the multiple standardized flywheel energy storage unit (FESU), is an effective solution for obtaining large capacity and high-power energy storage. In this paper, the strategy for coordinating and controlling the charging&ndash, discharging of the FAESS is studied in depth. Firstly, a deep analysis is conducted on the loss generated during the charging&ndash, discharging process of the FESU. The results indicate that the loss is related to the charging&ndash, discharging of power. To solve the problems of over-charging, over-discharging, and overcurrent caused by traditional charging&ndash, discharging control strategies, this paper proposes a charging&ndash, discharging coordination control strategy based on the equal incremental principle (EIP). This strategy aims to minimize the total loss and establish a mathematical model of optimal coordination control with the constraints of total charging&ndash, discharging power, rated power limit, over-charging, over-discharging, and overcurrent. Based on the EIP, the optimal distribution scheme of power charging&ndash, discharging is determined. Secondly, this paper gives the specific implementation scheme of the optimal coordinated control strategy. Lastly, the charging&ndash, discharging coordinated control strategy is verified by examples. The results show that the coordinated control strategy can effectively reduce the loss during the charging&ndash, discharging process and can prevent over-charging, over-discharging, and overcurrent of the system. Overall, it has a better control effect than the existing charging&ndash, discharging control strategies.

CHB is a suitable type of topology for synchronous rectifier application which has two or more independent DC loads. Focusing on the individual DC-link capacitor voltage balancing problems caused by asymmetric parameters of independent DC loads, this paper developed a new dual modulation signals based phase disposition pulse width modulation (PD-PWM) strategy using dynamic carrier bias allocation method. Its main idea is to allocate the bias of the carrier wave dynamically according to the balance situation of the system. Compared with the traditional modulation strategies, this method is more easily to be realized and has much stronger dynamic regulation ability. This modulation strategy is widely applicable to various types of CHB structures. A detailed analysis of the DC-link capacitor charge-discharge profile is performed, and an operation condition based dynamic bias allocation method is described using the cascaded H-bridge rectifier (CHBR) structure of five sub-modules as an example. A simulation model and an experiment platform are developed, and the feasibility and effectiveness of the modulation strategy are verified by simulation and experiment results.

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.

Lithium-ion capacitor is a hybrid electrochemical energy storage device which combines the merits of lithium-ion battery and electric double-layer capacitor. It is of great importance to monitor the real capacity to evaluate failures of lithium-ion capacitors. Remaining Useful Life (RUL), which is referred to remaining cycle number before reaching its End of Life (EOL) threshold, is a key part in the prognostics and health management and an important indicator of the depletion capacity of lithium-ion capacitor. In this paper, we propose a hybrid neural network which combine with the convolutional neural network and Bidirectional Long Short-Term Memory Network (Bi-LSTM), the data will be used to train this model. Finally, the verifications among different prediction horizons and other methods are discussed. According to the experimental and analysis results, the proposed approach has high reliability and prediction accuracy, which can be applied to battery monitoring and prognostics, as well as generalized to other prognostic applications.

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 capacitor is a hybrid electrochemical energy storage device which combines the merits of lithium-ion battery and electric double-layer capacitor. The precise state of charge estimation is essential to utilize lithium-ion capacitor efficiently. In this paper, the electrical characteristics of lithium-ion capacitor have been studied and the corresponding equivalent circuit model is established. Then, an adaptive square root cubature Kalman filter with variable forgetting factor recursive least square has been proposed to achieve state of charge estimation at each sampling step. The proposed method identifies the model parameters and noise statistics in real time, leading to obtain accurate SOC estimation even if the method is initialized with inaccurate parameters. Estimations under four working conditions are quantitatively studied, the experimental results show that the proposed method has better performance and the corresponding maximum root mean square error is controlled within 1%. Furthermore, the verification of robustness illustrates that the proposed method remains stability and acceptable accuracy when external interference and inaccurate initial SOC are included.

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.