Harmonic and electromagnetic interference (EMI) filters are two important output filters used to sup-press the harmonic distortion and EMI noise in grid-connected inverters. Harmonic and EMI filters are combined by planar magnetic integration to reduce the volume and weight. Through the selection of an appropriate magnetic core, the common mode and differential mode inductors are integrated into the same core by drawing PCB planar coil. To integrate the discrete capacitors and further realize the planar magnetic integration of EMI filter, the dielectric is inserted into the PCB and the layer connection mode is reasonably planned. A symmetric LCL filter is used to replace the traditional asymmetric structure of magnetic integration. Furthermore, by designing the air gap in the center pillar of the magnetic core and reasonably arranging the planar windings, the inductors of LCL harmonic filter are also integrated into the same magnetic core unit to form an LCL-EMI planar magnetic integrated filter. A gallium nitride single-phase inverter platform was built, and the LCL-EMI filter with planar magnetic integration was experimentally analyzed to verify the feasibility of the planar magnetic integration method.

The traditional fault diagnosis methods for power transformers cannot detect the power faults accurately or ensure their normal operation. Therefore, a fault diagnosis method for power transformers based on wavelet packet transform and support vector machine (SVM) is proposed. For the power signal collected from a power transformer, the improved minimum noise fraction (MNF) transform denoising is used to denoise, and the noise matrix is estimated by the weighted neighborhood mean method. After the estimation, the improved MNF transform is used to effectively realize image dimensionality reduction and denoising, extract the signal characteristics, and divide the signal into low- and high-frequency part by means of wavelet packet transform to obtain the wavelet packet energy feature vector. The obtained wavelet packet energy feature vector is input into an SVM classifier, and the output results from the SVM classifier are used to realize the state recognition and fault diagnosis of power transformer. Experimental results show that the proposed method can effectively diagnose the faults in the power transformer, such as iron core short-circuit, coil interlayer short-circuit, bushing-to-ground breakdown, coil insulation resistance drop and bushing-to-bushing discharge, and the fault diagnosis accuracy was higher than 98.5%.

Aimed at the 5th, 7th, 11th, 13th and other low-order harmonics which account for a large proportion in a voltage source converter based high-voltage direct current (VSC-HVDC) AC system, on the basis of proportional integral (PI) control, a selective harmonic current control strategy based on a vector proportional integral (VPI) regulator in dq coordinate system is proposed, in which PI is used to control the DC component of current error while VPI is used to suppress the frequency doubling fluctuation of current error. Different from the proportional integral resonance (PIR) regulator, VPI contains a second-order numerator, which can achieve an ideal 0° phase delay of the closed-loop transfer function of the control system at the resonance frequency point. As a result, its control accuracy of harmonic current is better than that of PIR. A two-terminal VSC-HVDC system is established by using the Simulink software, and the AC current on two sides of VSC in three control modes of traditional PI, PIR and PI parallel VPI is simulated. Through a comparison of harmonic content, the superior harmonic suppression performance of VPI is verified.

Aimed at the problem that the simulation of electromagnetic radiation interference of a switching power supply module is not combined with its actual working conditions, a method combined with near-field scanning was proposed. First, a simulation model of the switching power supply was built by software Cadence Allegro, and the software Cadence Sigrity was used to obtain the near-field radiation image and data by performing eletromagnetic radiation simulations of the switching power supply module. Then, a near-field scanning system of electromagnetic radiation was built to test the electromagnetic radiation interference of the switching power supply module, and its near-field electromagnetic radiation cloud map and electric field distribution data were obtained. Finally, by comparing the electromagnetic radiation distribution data obtained from near-field scanning and simulations, the simulation results were verified, and the MOSFET switch tube device and the surrounding circuit in the switching module were determined to be the source of electromagnetic radiation with the highest intensity.

In view of the flat hysteresis characteristics of open circuit voltage(OCV) and state of charge(SOC) of LiFePO₄ batteries, the OCV estimated by using the traditional equivalent circuit model has the problem of low accuracy under the charge-discharge switching conditions, so the hysteresis modeling of battery is proposed. To highlight the necessity of considering the hysteretic characteristics of LiFePO₄ batteries, three battery models are compared to comprehensively evaluate their complexity, accuracy and applicability. Results show that the first-order RC model is only suitable for pure charge or pure discharge conditions without considering the influence of hysteresis. The first-order RC hysteresis model adds a hysteresis on the basis of the first-order RC model. Although the influence of hysteresis characteristics is considered, the hysteresis is greatly affected by parameter identification and the OCV estimation fluctuates. The Preisach model has a good accuracy under the charge-discharge switching conditions, but the corresponding training data and time cost are high. Under the new European driving cycle(NEDC) charging and discharging conditions, SOC estimation is carried out for different models combined with algorithms, and the estimation errors are all within 5%, among which the Preisach error is within 3%.

Aimed at the problem that the particle swarm optimization (PSO) algorithm has a poor global search capability and is easy to fall into local optimum when it is applied to the selective harmonic elimination pulse width modulation (SHEPWM) of an inverter, an improved PSO algorithm is proposed. The vertical crossover operation of genetic algorithm is introduced to the search process, and the elite retention strategy is used to improve the global and local search capabilities of the algorithm and retain excellent individuals, thereby increasing the accuracy of switching angle and improving the performance of SHEPWM. This algorithm is used to solve the SHEPWM nonlinear transcendental equation of a novel asymmetric cascaded switched capacitor multi-level inverter, which overcomes problems such as the high dependence of traditional numerical methods on initial value and the low accuracy of the traditional PSO algorithm in solving the switching angle. Finally, the feasibility of the proposed topology and the effectiveness of the proposed algorithm applied to SHEPWM were verified by simulation and experimental results.

The traditional maximum power point tracking (MPPT) method is prone to falling into a local optimum under partial shading conditions and failing, while the common intelligent optimization algorithms often have disadvantages such as a low convergence accuracy, a slow convergence speed, and a low system stability. Aimed at these problems, a maximum power tracking strategy for photovoltaic (PV) system is proposed, which is based on the hybrid control of sailfish optimization (SFO) algorithm and perturbation and observation (P&O) method. The SFO algorithm uses two populations of sailfish (predator) and sardine (prey) at the same time to ensure the exploration of particles in the global space. The hybrid algorithm uses the SFO algorithm to quickly track the neighborhood of maximum power point at first, and then it uses the P&O method with a small step size to finely search for the maximum power point. In this way, it takes advantage of the piecewise step method to meet the requirements of MPPT search speed and search accuracy. Simulation results show that the hybrid control strategy effectively improves the response speed and tracking accuracy of the control system, as well as its stability.

In the actual operation of a battery, its temperature will vary with the ambient temperature, which undoubtedly increases the difficulty in estimating its state-of-charge (SOC). To address this problem, the relationship of temperature with the charge and discharge capacities, internal resistance and open circuit voltage of the battery is studied, and an equivalent circuit model considering temperature is established accordingly. The battery SOC is estimated based on this model by combining the extended Kalman filter algorithm, which can update the temperature-dependent variables in real time and adapt to the temperature change of the battery. In addition, this method is validated at variable temperatures. Results show that the proposed method can quickly and accurately estimate the battery SOC with an estimation error within 2%.

A novel nine-level inverter topology with double boost and reduced components is proposed by combining the voltage boost capacity of switched capacitors with the voltage halving characteristics of a coupling inductor. This inverter consists of 12 switches, 2 switched capacitors and 1 reverse polarity coupling inductor. The charging and discharging of each capacitor in the inverter can maintain a self-balance, so the additional balance circuit is not needed. Compared with the topologies of most of the existing nine-level inverters that are based on switched capacitors, the proposed topology has a smaller capacity of switched capacitor. The output voltage levels are greatly increased by the adoption of the reverse polarity coupling inductor, and the current stresses on some switches are reduced by half, which can further reduce the switching loss. The working modes, modulation strategy, design of switched capacitors and loss analysis of the proposed topology are discussed in detail. In addition, the proposed topology is compared with those of other nine-level inverters, and its advantages are introduced. Finally, simulation and experimental results verified the effectiveness of the proposed topology.

The cascaded H-bridge is considered as one of the most suitable topologies for photovoltaic (PV) power generation. Aimed at the problems of the traditional three-phase cascaded H-bridge PV inverter such as a large capacitor volume, a short service life, inter-phase power mismatch and a complex control communication system, a novel modular three-phase PV inverter and its distributed control strategy are proposed based on the principle of magnetic flux cancellation. First, the basic structure of the proposed modular topology is introduced. Then, the basic principle of magnetic flux cancellation power decoupling and the influencing factors of double-line frequency voltage ripple are analyzed in detail, and a distributed control strategy is proposed to suppress the double-line frequency voltage ripple and ensure the balance of three-phase output power. Finally, the correctness of theoretical analysis and the feasibility of the proposed control strategy were verified by simulation and experimental results.