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.

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.

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.

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.

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.

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 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 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.

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.

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.