Limited by the switching frequency, the frequency-controlled LLC resonant converter is difficult to achieve a wide output voltage range. To solve this problem, an expandable variable-mode interleaved parallel LLC resonant converter is studied. The secondary-side of this converter adopts a voltage doubling rectifier circuit, which can work in a parallel or series mode according to different switch combinations of two half-bridges on the primary-side, and it can adapt to the wide output voltage range of 1-3N times. A fixed-frequency PWM control method is proposed. In the middle region between the parallel and series modes, the fixed switching frequency is taken as the resonant frequency, and the duty cycle of one bridge arm is changed to realize voltage control. PSIM simulation results show that the wide output voltage range of 1-3N times can be realized by expanding 2N resonator cavities. The experimental results of a 100 W prototype demonstrate that the wide output voltage range of 1-3 times can be achieved with two half-bridges and two resonant cavities, and the effectiveness of the proposed converter and its control strategy was verified.

In an energy storage system, the current-fed dual-active-bridge converter has a large current stress and the corresponding soft-switching range is limited, which limits the converter’s efficiency and power density. To solve these problems, combined with the coupled inductor technology, a current-fed dual-active-bridge converter with a low current ripple on the energy storage side and a wide soft-switching range is proposed. Two current-fed full bridges are connected in parallel on the energy storage side, thus effectively reducing the current stress of switches therein. By adjusting the phase shift angle between the two parallel full bridges on the energy storage side, the current ripple is reduced. By reasonably designing the coupling filter inductance, the obtained mutual inductance current is large enough to satisfy the soft-switching conditions for switches. The working principle and steady-state analysis of the converter were given in detail, and a 400 W experimental prototype was designed to verify the superiority and feasibility of the proposed converter.

To address the issue that a dual-active-bridge DC-DC converter will produce large current stress when voltages mismatch and result in a large reduction of its efficiency, a combined dual-phase-shifting (CDPS) control strategy is proposed, which combines dual-internal-phase-shifting (DIPS) and interlaced-dual-phase-shifting (IDPS). First, the working principles of the two control strategies are analyzed, and the mathematical models of transmission power and current stress are established. Second, with the minimum current stress as the objective, the optimal phase-shifting ratios are solved by using the Lagrange multiplied method under the Karush-Kuhn-Tucker condition. Third, the optimization methods under the two control strategies are combined according to different voltage ratios and transmission power. The CDPS control is used to obtain the optimal solution of current stress, which is compared with those obtained under the existing single-phase-shifting and dual- phase-shifting control strategies. Results show that the proposed control strategy can further reduce the current stress and reactive power under the condition of high voltage ratios and improve the efficiency. Finally, an experimental prototype was built to verify the feasibility of the proposed control strategy.

The application of multi-phase interleaved parallel coupled inductors technology can effectively reduce the phase current ripple and improve the dynamic response speed. Aimed at different design objectives, the influencing factors for the steady-state and dynamic performances of direct- and indirect-coupled inductors are analyzed. Subsequently, based on the invariant equivalent dynamic inductance before and after coupling, the direct- and indirect-coupled inductors are designed to enhance the steady-state performance. Similarly, based on the invariant equivalent steady-state inductance before and after coupling, direct- and indirect-coupled inductors are designed to improve the dynamic performance. Finally, the cor-rectness and effectiveness of the theoretical analysis were verified by experimental results, demonstrating that the two different coupling methods can significantly enhance the steady-state and dynamic performances of the converter, respectively.

Aimed at the problem that the traditional control methods are difficult to achieve soft-switching in a wide load range due to the limitation of resonant inductor volume and duty cycle loss in phase-shifted full-bridge converters, a hybrid control method based on peak current and Burst mode is proposed. The output voltage is stabilized to a reference value by adjusting the Burst duty cycle, and the phase shift angle is changed to maintain the minimum primary current so as to realize the lagging bridge arm zero voltage switching. A simulation platform was built for the proposed control method, and a 250 W prototype was developed. The hybrid control of a phase-shifted full-bridge converter was realized through a digital signal processor, and the feasibility of the control method was verified by simulation and experimental results.

Aimed at the time-delay oscillation of a bidirectional H4 bridge converter in a single-phase energy storage inverter, a unified control method for the bidirectional H4 bridge converter is proposed. In this method, a voltage regulator is used to control the power flow of the converter, and a set of bidirectional feasible control parameters are derived based on the power balance theory. At the same time, in order to realize AC current tracking input voltage without static error and increase the stability, the current inner-loop adopts a quasi proportional resonance controller, and a second-order generalized integrator is used to design a phase-locked loop. PSIM simulation and experimental results show that the proposed method can realize seamless switching between the rectification and active inverter modes, and it also has a good effect in the startup and switching between different modes. Therefore, it can realize stable control of the bidirectional AC-DC bridge converter in a single-phase photovoltaic energy storage system and obtain a good dynamic performance.

The coupling between the boost control and neutral-point voltage balance control of a quasi-Z-source three-level inverter seriously limits its control performance. To solve this problem, a neutral-point voltage balance control strategy based on a virtual space-vector pulse width modulation method is proposed. The neutral-point voltage balance control is realized through a closed-loop control of the DC-bus capacitor voltage, and the low-frequency fluctuations in the neutral-point voltage are eliminated. Meanwhile, a constant shoot-through boost modulation strategy is employed, which avoids the adverse impact on the neutral-point voltage and guarantees an ample boosting capacity of the quasi-Z-source network. Finally, simulation and experimental results verified the validity of the proposed control strategy.

To reduce the cost and the number of components while ensuring the safe operation of the circuit, a single-phase isolated Δ-source AC-AC converter is proposed. This novel converter can provide a wider range of Buck-Boost output voltage, and the input voltage and output voltage can be in-phase or out-phase. Meanwhile, the surge and harmonic currents are suppressed, and the circuit reliability is improved. The working principle for the proposed circuit is analyzed, the voltage values of main components in each working process are deduced, and the relationship between input voltage and output voltage is formulated, which is further compared with those of other improved AC-AC converters. The theoretical analysis proves the performance of the novel AC-AC converter. A simulation model and an experimental model were built according to the designed parameters for verification, and simulation and experimental results verified the correctness and feasibility of the theoretical analysis.

Three-phase chain-link energy storage converters (TPCLESCs) are promising in enhancing the controllability of renewable energy in power grid, such as wind and solar power. Aimed at the problem of state-of-charge (SOC) imbalance of energy storage battery among phases of a TPCLESC, a phase-to-phase SOC balance method based on phase-to-phase circulating current power closed-loop control is proposed. Through the zero-sequence voltage injection into phases a, b and c, the active circulating current among phases is generated to realize the SOC balance in the three-phase energy storage battery groups. A mathematical model of the maximum phase-to-phase circulating current power of the chain-link energy storage converter and SOC deviation is established. On this basis, the phase-to-phase SOC balance in battery groups is realized at the maximum circulating current power through the phase-to-phase circulating current active power closed-loop control. As a result, the phase-to-phase SOC reaches its balance at the maximum speed, and the process of phase-to-phase SOC balance is accelerated. Finally, the correctness and feasibility of the proposed method were verified by a MATLAB simulation model and an experimental platform.

The flying capacitor clamped three-level converter has many advantages, e.g., it can reduce the voltage stress of a switch and the volume of a filter inductor. Under its operation, it is necessary to stabilize the flying capacitor voltage at half of the high-voltage side voltage, so a control strategy of adjusting the duty cycle is often used. However, this method has the problem of coupling control between flying capacitor voltage and output voltage, resulting significant fluctuations of inductance current in the process of flying capacitor voltage regulation. To solve this problem, the advantages of using the phase-shifting control strategy to realize the decoupling control of flying capacitor voltage and output volt-age are analyzed, and the corresponding control characteristics are also studied. Through the establishment of a harmonic model of flying capacitor voltage, the relationship between flying capacitor voltage and phase-shifting angle is given. A low-order harmonic function relationship is constructed, which indicates that the flying capacitor voltage is affected by the switch duty cycle D and phase-shifting angle ∆φ. The effective duty cycle interval of phase-shifting control and the duty cycle that optimizes the performance of phase-shifting control are delimited by combining with a time-domain model. A simulation model was established, and an experimental prototype with 3.6 kW was built. The control strategies of adjusting flying capacitor voltage based on phase-shifting angle and duty cycle are compared to verify the decoupling advantages and control characteristics of phase-shifting control.

When a modular multilevel converter (MMC) adopts the traditional carrier phase-shifted pulse width modulation strategy, the additional capacitor voltage balance strategy for submodules will cause the number of submodules in each phase circuit of the MMC to jump at a high frequency. Therefore, a large induced pulse voltage will appear on the inductance of the bridge arm, which will generate a high-frequency harmonic circulating current, thereby increasing the voltage and current stress of components. Based on the study of the traditional carrier phase-shifted modulation strategy and its application to the MMC, the implementation process is improved to ensure that the number of submodules in each phase circuit of the MMC is constant at any time, so as to avoid the above problems. The improved carrier phase-shifted modulation strategy is analyzed in detail, and the corresponding capacitor voltage balance control strategy is given. Simulation and experimental results show that the circulating current amplitude of the bridge arm is reduced after adopting the improved carrier phase-shifted modulation strategy.

To achieve the target of carbon neutrality, renewable energy power generation represented by photovoltaic (PV) power generation has become an important means. PV power generation systems usually require multiple PV modules to be connected in series to obtain a high output voltage. However, in series-connected PV modules, the mismatch of component characteristics due to partial shading or different degrees of aging of components will cause a serious loss of power generation. Therefore, many technical schemes have been proposed to solve this problem. The PV equalizer has become a promising solution, and it uses a power electronic converter to transfer the mismatched power and change the operating point of the component to obtain the maximum output power. First, the basic concepts and principles of PV equalizers are elaborated, then, the PV-bus, PV-PV and other special types of PV equalizers topologies are introduces in detail. In addition, a comparison and analysis of the paremeters and performance of the existing PV equalizer topologies is performed. Finally, the topological structures of PV equalizers are summarized and evaluated, providing a reference for engineers and practitioners in this field.

To improve the capability of microgrid in coping with new energy output and load uncertainty, an optimal control strategy for microgrid considering flexible resources is proposed. According to the source-storage-load characteristics of various flexible resources, a two-layer optimal scheduling model for microgrid is established. In the user layer, user-side flexibility resources are introduced, with an optimization goal of minimizing the difference between user costs and net load and decision variables of electric vehicles and output power of translatable loads. In the source-storage layer model, flexible resources are added to the energy storage and power generation sides, with an optimization goal of minimizing the microgrid operators’ cost and load loss rate and decision variables of gas turbines, main network tie-line and output power of energy storage unit. A case study based on scenario-reduced seasonal typical daily data is simulated, and the improved MOEA/D(multi-objective evolutionary algorithm based on decomposition) algorithm is used to solve the two-layer optimal scheduling model. Results show that the average annual user cost is reduced by 6.85%, the average annual total cost of operators is reduced by 14.68%, and the average annual load loss rate is reduced by 6.65%. The results verified the correctness and effectiveness of the proposed method.

An improved incremental conductance method with a variable step size is proposed to solve the problem that the traditional maximum power point tracking (MPPT) algorithm cannot strike a balance between the tracking speed and steady-state oscillation. The MPPT speed can be increased by the improved incremental conductance method using a zonal variable step size. Meanwhile, the steady-state oscillation problem is optimized by using the incomplete partial differential theory, thus improving the efficiency of photovoltaic power generation. The feasibility and effectiveness of the improved incremental conductance method is verified by comparing the traditional control algorithm with the improved incremental conductance method.

Since DC bias is one of the main reasons for increases in the vibration and noise of a large-scale transformer, it is essential to fully understand the vibration and noise characteristics of large-scale transformers under DC bias for the evaluation of the operating state of transformers and the reduction of noise and vibration. A 406 MVA EHV large-scale transformer is taken as the research objective, and its vibration and noise characteristics are studied. First, based on the field-circuit coupling finite element method, the no-load operation characteristics under different DC bias currents are simulated and analyzed, and the law of excitation current under different DC biases is analyzed. Second, a multi-physics coupling model of circuit-magnetic field-solid mechanics-pressure acoustics is established, and the effective value of vibration displacement and the time-frequency characteristics of noise signal at different measuring points of the transformer under DC bias are obtained considering the influence of magnetostriction. Third, the sound level is measured at different measuring points around the transformer, and the simulated value is compared with the actual measured value to verify the effectiveness of the proposed calculation method for transformer vibration and noise. Finally, the Hilbert-Huang transform method is used to extract the vibration and noise characteristic quantities of one large-scale transformer under DC bias, and a transformer vibration characteristic recognition method based on the energy ratio of the noise signal intrinsic mode function is proposed. This method can effectively recognize the severity of DC bias of the transformer and accurately grasp its operating state, providing a theoretical basis for timely taking measures to suppress the DC bias.

The control system for hybrid distributed energy storage virtual synchronous generator (VSG) composed of a battery and a supercapacitor is improved. A principle for the power allocation of hybrid energy storage is proposed, and a frequency division sliding filter method is designed according to different frequency bands. In view of the power fluctuations of different frequencies, the virtual moment of inertia is piecewise improved to achieve the coordination between the battery and the supercapacitor. Aimed that the problem that the traditional VSG cannot suppress the frequency oscillations, the virtual damping coefficient is modified, and adaptive virtual damping is used to improve the regulation capability of the control system when the frequency oscillations occur. MATLAB is used to verify the proposed control strategy through simulations. Results show that the hybrid energy storage VSG can be controlled in terms of frequency, i.e., when the load fluctuates irregularly, the supercapacitor responds to the high-frequency power fluctuation in time while the battery suppresses the low-frequency power, thus realizing the coordination between the battery and the supercapacitor. When faced with a sudden decrease or increase of 10 kW load, adaptive virtual damping can be used to deal with the frequency oscillations of active output caused by load power fluctuations of hybrid energy storage, and the frequency overshoot is controlled within 0.06 Hz. As a result, the proposed control strategy can adjust the small deviation of frequency regardless of the sudden increase or decrease of power.

Aimed at the problems such as voltage sag/surge resulting from strong fluctuations of high-permeability renewable energy, which cannot be dealt with by using the existing transformers, a novel hybrid distribution transformer (HDT) based on a three-bridge arm power converter is proposed. This method is realized by adding a series three-bridge arm power converter to the primary side of the existing distribution transformer. The proposed novel HDT has two advantages, i.e., it can reduce the rated power of the power converter, and it can improve the transformer’s degree of freedom by adding an additional current loop. In addition, to further improve the power quality of the transformer, the proposed method integrates the voltage vector, which can compensate the adverse effects of voltage sag/surge and grid voltage harmonics on the transformer. Therefore, it improves the power factor of power grid, as well as the transmission efficiency of the distribution network. Finally, the configuration and control strategy for the proposed HDT are discussed, and the effectiveness and superiority of the proposed method are verified by simulation analysis.

The converters, switching power supply and other power electronic equipment will inject a large quantity of supraharmonics into distribution network when they are connected to the grid on a large scale, resulting in the problem of power quality which becomes more and more serious. On this basis, the supraharmonics emission mechanism for an ordinary two-stage single-phase frequency converter is studied in depth. First, the Fourier series expression of harmonic current on the grid side is derived using the switching function method. Then, the ratio of supraharmonics is calculated, and its influencing factors are analyzed. Finally, the theoretical analysis is verified by simulation and measurement results. The research can provide a reference for the quantification, detection and monitoring of supraharmonics in distribution network.

Aimed at the problem of large harmonic content of grid-side current in a single-phase matrix converter based wireless power transfer (MC-WPT) system, a harmonic suppression modulation strategy is proposed to effectively reduce the low-order harmonic content and total harmonic distortion (THD) of grid-side current. The voltage and current characteristics of resonant tank are analyzed, the equivalent circuits at two fundamental frequencies are obtained based on the parameter normalization method, and the mathematical model of MC-WPT system is derived accordingly. On this basis, with an objective of eliminating the low-order harmonic content, the optimal modulation wave of the H-bridge on the receiving side is obtained by using the calculation method, so that the low-frequency component of grid-side current only contains the line frequency component, thereby reducing the THD of grid-side current. Finally, an experimental platform was built to verify the feasibility and effectiveness of the proposed harmonic suppression modulation strategy.

An anti-offset method for a wireless power transmission (WPT) system based on constant-voltage output interval tracking is proposed to deal with the problem of output voltage fluctuation when the coupling mechanism in the WPT system is shifted. First, a model of a CLC-S WPT system is established, and the relationship between the mutual inductance and output voltage gain in resonant and non-resonant states of the system is analyzed. Based on the analysis, it is found that the system has a stronger anti-offset capability in the constant-voltage output interval when it works in the non-resonant state. Second, an inductance compensation sequence is designed, and a constant-voltage output interval tracking control strategy is proposed to realize the constant output voltage control of the WPT system and improve its anti-offset capability. Finally, a simulation model and a test platform were built, and simulation and experimental results show that the constant-voltage output interval tracking control strategy can effectively reduce the output voltage fluctuation, thus verifying the robustness of the system under strong mutual inductance interference. Compared with the WPT system without constant-voltage output interval tracking, the CLC-S WPT system has a better dynamic regulation capability of output voltage.

Aimed at the problem of eddy current loss, a method for the parameter design and optimization of a wireless power transfer (WPT) system in seawater is proposed to optimize the power transmission efficiency of the system. First, based on the analysis of the electromagnetic field of coils under operation in seawater, the equivalent mutual inductance model of the WPT system in a marine environment is obtained by using the equivalent impedance of eddy current loss. Second, when the positions of the primary- and secondary-side coils are fixed, the corresponding relationship between the equivalent impedance of eddy current loss and the operating frequency of the system and the number of coil turns is established, and the feasibility of the calculation method for the equivalent impedance of eddy current loss is verified by using the coils on both sides of the WPT system. Finally, based on the energy model of an LCC/S-type WPT system in seawater, the particle swarm optimization algorithm is used to optimize the transmission efficiency. A test system was built with the optimized parameters, and results show that when it transmitted 1 kW of power in a simulated marine environment, its overall efficiency can reach 84%.

The double-sided LCC compensated inductive power transfer (IPT) system with constant-voltage (CV) output suffers from the problem of low efficiency under light load. To solve this problem, based on the idea of approximate optimal solution, a parameter design method for double-sided LCC compensation topology was proposed. The zero phase angle condition in CV output mode and the loss of a loosely coupled coil were analyzed, and a 6.6 kW prototype was built to verify the proposed method. Experimental results show that the system efficiency can be improved with the proposed compensation parameter design method, especially in the case of light load. The system efficiency can reach 95% under full load of 6.6 kW and 93% under light load of 1.32 kW.

A novel two-channel light emitting diode (LED) driver is proposed, and its operating principle and characteristics are analyzed in detail. This driver uses a novel Z-source resonant network and an active switch which has the same ground properties as the power supply, and the current balance is automatically realized by balancing capacitors, so its current control is simple. At the same time, owing to the use of Z-source resonant network to transfer energy, the proposed driver has advantages such as soft switching, high efficiency, small volume and low voltage stress. To verify the effectiveness of this driver, a 80 W prototype was built and tested.

Aimed at the problems of unbalanced capacitor neutral-point voltage and high common-mode voltage in a permanent magnet synchronous motor drive system powered by a T-type three-level inverter, a model predictive instantaneous torque control (MPITC) strategy based on finite voltage vector set optimization is proposed. First, in view of the influence of voltage vectors on the neutral-point voltage, only zero, small and large vectors are selected to participate in MPITC. Second, according to the relationship between the switching states and common-mode voltage, it is confirmed that 13 low common-mode voltage vectors participate in the control. To improve the operation performance of the motor, the long vector synthetic virtual voltage vector is used to replace the medium vector to participate in the model predictive control. Finally, according to the neutral-point potential and the motor current direction, the voltage vector which is favorable for maintaining the neutral-point voltage balance is selected from 19 voltage vectors as the preselected vector set. Experimental results show that the proposed control strategy can effectively reduce the electromagnetic torque, flux linkage pulsation and common-mode voltage amplitude, and the neutral-point voltage achieves balanced control.

To ensure the motor drive performance while saving cost in the field of electric industrial vehicles, low-precision encoders are often adopted. Although these encoders can measure the rotor’s accurate mechanical position, they introduce a long delay and large errors to the measured speed information. Therefore, the traditional load torque observers cannot obtain the accurate load torque information, and when the inaccurate information is used as feedforward of the current loop, the motor speed fluctuation cannot be effectively suppressed. To solve this problem, a second-order sliding-mode load torque observer based on a low-precision encoder is proposed, which can obtain the accurate load torque information based on the rotor position information and thus improve the anti-interference performance of the system. Finally, the correctness and effectiveness of the proposed method was verified by simulation and experimental results.

The development of industry and economy has caused a huge consumption of energy, which brings serious energy crisis and environmental pollution. Therefore, building a safe and clean energy interconnection network is a way to solve the relationship among social development, environment and energy at present. Nowadays, different countries have proposed their policies for the development of new energy electric vehicles (EVs). As the core component of EVs, lithium-ion batteries are directly related to the driving performance and safety of EVs. The state-of-charge (SOC) estimation is a core parameter of lithium-ion batteries used in various industries, and the estimation accuracy is directly related to the service life and efficiency of batteries. In this paper, the problem of battery SOC estimation accuracy in EV applications is studied, and an SOC estimation method based on the extended Kalman filter (EKF) optimized by the whale optimization algorithm (WOA) is proposed. On the basis of constructing the covariance matrix of system noise and observation noise, the improved and optimized WOA-EKF algorithm is used to optimize the noise covariance matrix under dynamic conditions, thus improving the SOC estimation accuracy. The model parameter identification and comparative simulation verification are carried out in MATLAB/ Simulink. Results show that the SOC estimation of lithium-ion batteries based on the WOA optimized EKF algorithm can control the SOC estimation error to be within 2% under different working conditions, which is of significance to the promotion of develop- ment of batteries in the new energy field.

Electric vehicle fast charging piles are prone to overheating of power devices under high-power operation, causing potential safety hazards. However, the existing cooling strategy adopts a rule-based forced air cooling method, and the cooling fan rotates at a high speed and generates large environmental noise. To protect the thermal safety of core components in the module while optimizing the cooling regulation strategy, an optimal thermal management method for electric vehicle fast charging module based on data-driven model predictive control (MPC) is proposed. This method adopts a data-driven method to construct a prediction model of module temperature distribution based on the long short-term memory neural network, and it combines MPC to control the fan speed, thus optimizing the thermal management strategy for the fast charging module and reducing the fan noise. Through experimental tests, it was verified that this method can effectively reduce the average fan speed by 1 293 rpm and reduce the average noise by 4.99 dB while ensuring that the key components are not overheated, which ensures the thermal safety of core components and the durability of the cooling fan.

A fast and accurate estimation of the state-of-charge (SOC) of lithium batteries is critical for the battery management system. Aimed at the problem that the Kalman filter algorithm lacks reasonable constraints on the resistance-capacitance (RC) parameters when estimating the SOC of lithium batteries, an optimization method of RC parameters filtering is proposed, and it is combined with unscented Kalman filter (UKF) to achieve the fast and accurate convergence of lithium battery SOC estimation. First, an equivalent circuit model of lithium battery is established by combing the polynomial equation. Then, forgetting factor recursive least squares is used to obtain the time-varying and time-invariant model RC parameters. The expression of RC parameters filtering relationship is established by setting the Kalman gain threshold, and an RC optimization UKF algorithm is proposed for lithium battery SOC estimation. Finally, hybrid pulse-power characteristic experiment, intermittent constant-current discharge experiment and dynamic stress test experiment were designed to verify the convergence and robustness of the proposed algorithm. The maximum estimation error of SOC was less than 1.0%, and the reference range of gain threshold was also given.

The accurate estimation of the state-of-charge (SOC) and state-of-health (SOH) of lithium-ion batteries is always a key scientific prob-lem that needs to be solved urgently. In this paper, based on a second-order fractional-order equivalent circuit model, the state space equation of a lithium-ion battery is established, and the discretization expressions of fractional-order differential and integral equations of battery parameters and SOC are derived. Then, a dual fractional-order extended Kalman filter method is studied to estimate the equivalent circuit parameters, SOC and battery capacity simultaneously. In addition, a time weighting sequence method based on estimated SOC and battery capacity is proposed, different discharge currents and cumulative time are monitored, and the available capacity of the battery is calculated online, thus achieving real-time estimation of the SOH of the battery at any discharge depth and any discharge rate. Finally, under the conditions of dynamic stress test, three lithium iron phosphate batteries of the same manufacturer, the same model and different aging degrees were used for experimental verification.

To enhance the accuracy of a lithium battery model and realize accurate state estimation of the lithium battery, a fractional-order electric model of the lithium battery was built, and the model parameters were identified using adaptive genetic algorithm. Based on the fractional-order electric model, the unscented Kalman filter was adopted to estimate the lithium battery’s state-of-charge (SOC) and state-of-health (SOH). Stimulation results show that, the established fractional- order electric model of the lithium battery can more accurately describe its dynamic characteristics during charging and discharging, and the accuracy of the proposed state estimation strategy was higher than that the conventional control strategy.

The switching speed and switching frequency of converters keep increasing, and higher requirements are also imposed on the DC-side decoupling capacitors. To solve the problems of overvoltage spikes and decoupling capacitor losses caused by voltage and current oscillations during the switch-off process, a transient circuit model considering the system’s stray parameters is established, and the evolution of overvoltage and decoupling capacitor current oscillation is analyzed. On this basis, a loss model considering the parasitic parameters of the main circuit and the equivalent series resistance of the decoupling capacitor is proposed. The limiting factors for the selection of decoupling capacitors in practical engineering are quantitatively analyzed, and an optimal selection method for the decoupling capacitance and loss limit condition is obtained. Finally, the proposed model and analysis method were verified by simulation and experimental results.

To improve the accuracy and efficiency of diagnosis when a converter fails, an active current limiting method is applied to realize the converter fault diagnosis in a power system. Active current limiting control is used to limit the output DC current from the converter to 1.2 times of rated current and limit the fault current, thus improving the stability of fault diagnosis. Based on a prediction model, the fault current distribution characteristics of the converter in the power system are diagnosed, and the bridge arm current is taken as the diagnosis parameter. The difference between the measured bridge arm current and predicted value is compared. If the difference is greater than the threshold value, it is judged that the converter has a fault. Experimental results show that the proposed method had a high accuracy and a high diagnosis efficiency in diagnosing the converter faults in an experimental power system, and the rates of false diagnosis and missed diagnosis were low, indicating that the converter diagnosis effect was satisfying.

In view of the recent situation in which power sources gradually reach their service terms in China, an evaluation method based on improved fuzzy analytical hierarchy process (FAHP) and entropy weight method (EWM) is proposed. First, an appropriate hierarchical structure is constructed based on the analytic hierarchy process to form a judgment matrix. Then, FAHP is used to process the judgment matrix between various layers, and the importance-oriented weight vector is obtained. At the same time, the Delphi survey method is used to form an evaluation matrix for the last sub criterion layer and the target layer. After normalization, EWM is used to obtain the value-oriented weight vector. The two weight vectors are synthesized to form a comprehensive weight vector. Finally, the final weight vector of the scheme layer to the target layer is formed, and the best scheme is given. The result of an example shows that the proposed evaluation method has strong flexibility and wide applicability. In addition, it also has a clear and reasonable process, as well as intuitive and accurate results.

To study the conducted common-mode (CM) electromagnetic interference (EMI) characteristics of a single-switch forward converter and reduce its CM noise, the analysis of the transmission mechanism of conducted CM noise in the single-switch forward converter is necessary. On this basis, a CM noise transmission path model is established, and a calculation method is proposed to determine the specific external capacitance to reduce the CM noise. In addition, aimed at the defects of the traditional calculation model of induced charge on the secondary side, an improved calculation model is put forward, and simulation results show that the accuracy of the improved model is higher under ideal conditions. Afterwards, the balanced winding method was used to reduce the CM noise flowing through the transformer, and a prototype of single-switch forward converter power supply was used for experimental verification. Results show that the method of calculating the external capacitance was effective, and the accuracy of charge calculated by the improved calculation model was higher when the windings were close or when the number of turns per unit length was relatively large.