The rapid increase in the proportion of renewable energy in the East China Power Grid System and the continuous decline in the rotational inertia and frequency modulation resources in the system will pose a threat to the system's frequency immunity and stability. The existing research focuses on the development and application of renewable energy system inertia and primary frequency modulation technology，while there is less research on the medium and long-term development priorities of multiple renewable energy frequency response technologies such as wind power，photovoltaic，and energy storage. Therefore，a research on the evolution path of collaborative frequency modulation technology in the East China Power Grid System was proposed. Firstly，based on the disturbance events of the East China Power Grid over the years，the system's multi-source collaborative frequency response system was analyzed，and the influencing factors of various frequency modulation entities in the disturbance events were quantitatively analyzed by decision-making trial and evaluation laboratory（DEMATEL）model. Secondly，the development priority and optimal evolution path of collaborative frequency modulation technology in East China Power Grid were prospectively analyzed based on the weight distribution of influencing factors，providing a theoretical basis for the orderly development of renewable energy frequency modulation technology in East China Power Grid. This has consolidated the frequency security of the East China Power Grid under the high proportion of new energy feeds.

Aiming at the poor robustness and system vibration problem of asynchronous motors under the traditional sliding mode control，a variability coefficient power exponent reaching law （VCPERL） was proposed to dynamically change the control coefficients according to the position of the sliding mode surface，and the sliding mode controller was designed based on this reaching law. Firstly，according to the advantages of quick-power reaching law （QPRL） and the double-power reaching law （DPRL），which have different reaching rates at different positions of the sliding mode surface，meanwhile，the tanh（x）was used to replace the sign（x），and a variable coefficient was introduced into the reaching law. Secondly，the stability of the new reaching law was proved based on theoretical analysis and simulation. Finally，the VCPERL was applied to design the sliding mode controller according to the theory of vector control oriented by rotor magnetic chain and the dynamic mathematical model of asynchronous motor. The simulation results show that compared with the traditional PI control，QPRL and DPRL，VCPERL has strong immunity to load disturbances and fast recovery capability，which can effectively improve the dynamic response performance of the system.

When the switching frequency of the four-quadrant rectifiers in different multiple units deviates，the current will experience low-frequency oscillation，which is not conducive to the operation of the traction power grid. Therefore，a matrix small-signal modeling method for SPWM comparators was proposed to analyze this issue，which achieves a more accurate description of the switching frequency characteristics of the single-phase PWM rectifier. Then，the matrix small-signal modeling of parallel four-quadrant rectifiers was further established，and then the model was used to analyze the causes and rules of differential frequency oscillation during mixed running of different multiple units. Finally，the correctness of the established model and analysis conclusions were verified through experiments.

The problem of AC train passing neutral section can be solved by ground's flexible passing neutral section method. However，for multisystem electric locomotives，the switching of power supply system in neutral zone is much more complicated，but there is little research at present. Based on the approach of vehicle switching combined with vehicle kinetic energy，a method to stabilize the DC-link voltage of multisystem electric locomotive in the process of power supply system switching was proposed，and selected the typical multisystem electric locomotive of certain model as the research object. Firstly，the traction drive system topology of the multisystem electric locomotive was introduced. Then，a power supply system switching method was designed for it. Finally，real-time simulation of the proposed power supply switching method was conducted on a hardware-in-the-loop experimental platform，and the good DC-link voltage stabilization effect of the proposed method was verified.

In order to make full use of the frequency modulation capability of the wind turbines，considering that a conventional wind-power coordinated frequency control strategy not only results in some fluctuations in the frequency of the system，but also requires high energy storage capacity and poor economic efficiency，therefore，based on the traditional wind storage coordinated frequency control strategy，an active support control strategy for wind storage coordinated regulation was proposed，which is based on a virtual synchronous generator （VSG） third-order model，and the related wind storage collaborative regulation model was built，and the control strategies for wind turbines and energy storage were designed. Based on this，the inertial response and the primary frequency response of the wind turbine and the energy storage plant to control the frequency were investigated under different control parameters. Through simulation analysis，it can be concluded that this strategy not only approximates the energy storage inverter as a synchronous voltage source，meets the inertia and damping characteristics required by the new energy grid，suppresses system frequency fluctuations，reduces energy storage capacity configuration，but also makes full use of the potential of wind farms to participate in frequency regulation，increasing the frequency stability of the electricity system.

Thermal and magnetic protection circuit breakers provided short circuit protection by instantaneous tripping devices. In the magnetic field formed by the coil with current，the moving armature and stationary core overcomed the spring force and engage under the influence of electromagnetic force，thereby pushing the circuit breaker mechanism to operate and causing the product tripping. During the entire process，only when the electromagnetic force reaches a certain value could be the moving armature and stationary core reliably engage at the preset current value. In actual engineering design，in order to simplify the design and calculation process，issues such as magnetic leakage may be ignored. Therefore，the theoretical calculated magnetic attraction force，i.e. spring reaction force，will be smaller than the preset target. By analogy calculation method，it is possible to directly eliminate the influence of magnetic leakage and part manufacturing accuracy on the test results during the manufacturing process，and complete the design of the instantaneous release device once. Finally，practical application cases were used to illustrate how to accurately and quickly design an instantaneous release devices through a combination of theoretical calculations and experiments.

High-frequency transformers are an essential component in series-resonant DC/DC converters，and their stable operation has a decisive impact on the reliability，efficiency，and power density of power supply systems. Nanocrystalline magnetic cores are widely used due to their advantages of high saturation magnetic flux，high magnetic permeability，low coercivity，and low high-frequency losses. In order to improve the power density of DC/DC converters，the leakage inductance of high-frequency transformers is commonly used as a resonant inductor to achieve the soft-switching characteristics of the switching devices. However，nanocrystalline alloys possess a high electrical conductivity，and when high-frequency leakage magnetic flux passes perpendicularly through the surface strip，it induces strong eddy currents，which could result in potential thermal issues，thereby compromising the stable and safe operation of the transformer and the system. An improved finite element simulation method for high-frequency transformers was proposed for determining the distribution characteristics of additional eddy current losses induced by high frequency leakage flux at large leakage inductance by building a three-dimensional model of nanocrystalline strips at the edge of the independent core，and its validity was verified by temperature rise experiments.

In view of the problem that the transfer branch of the current high-voltage DC circuit breaker mostly adopts uncontrollable full-bridge transformer coupling energy supply，which is not conducive to controllable and stable power supply，and low energy utilization，a combined DC circuit breaker energy supply method was proposed. A bidirectional DC-DC converter topology with energy storage device （battery or super capacitor） was used to control the voltage control and energy recovery and reuse of the precharged capacitor in the resonant transfer branch of the combined DC circuit breaker. A control strategy for DC-DC energy supply using dual-active-bridge （DAB） was developed as an example. The improved energy mode of supply can not only ensure the fast and stable supply of transferred branch energy during the rapid opening of the combined high-voltage DC circuit breaker in the event of DC power grid failure，but also use the recovered energy for the reclosing process after fault recovery. The simulation and experimental results show that the proposed bidirectional DC-DC converter replaces the uncontrollable full-bridge combined high-voltage DC circuit breaker energy supply method，and the output voltage is more stable，and the energy utilization rate of the system is improved.

In view of the need to test the operation of power electronic equipment under various working conditions in the current new power system，a control strategy using linear auto-disturbance rejection technology was proposed to study the power grid simulator with line impedance simulation function. On this basis，in order to separate and control the harmonics，the improved filter bank was used to separate and filter the grid voltage to achieve the effect of independent impedance simulation and harmonic output. Simulation and experiments show that the proposed control strategy has good output effect.

A microgrid optimization scheduling model considering dynamic network reconstruction was proposed for the parallel and off grid scheduling problem of microgrids. The proposed model was based on Benders decomposition and decomposes the optimal scheduling problem of microgrids into a grid-connected operation master problem and an islanded operation subproblem. When the grid-connected operation scheduling plan cannot guarantee the feasibility of island operation，the network topology structure，unit output，and scheduling plan of energy storage and adjustable loads in the main problem can be dynamically adjusted to meet the relevant constraints of the subproblem. A new linearized power flow model for distribution networks that considers network reconstruction was proposed to support the solution of the proposed optimized scheduling model. A random framework based on unscented transform（UT）was introduced to make the proposed model more in line with the actual power grid situation. The effectiveness and superior performance of the proposed model were verified through simulation analysis of the IEEE33 node test network.