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.