Aimed at the transient instability of a voltage source converter (VSC) based on phase locked loop (PLL) under weak network conditions and considering the influences of power grid line impedance, VSC reactive power injection and PLL filtering, the transient instability boundary of VSC is comprehensively studied based on the critical voltage in a variety of fault scenarios such as grid voltage sag, frequency fluctuation and three-phase asymmetric fault. Through the analysis of the VSC grid-connected vector diagram in different operation scenarios, the mathematical models of relationships between the grid voltage of VSC grid-connected system and factors (e.g., line impedance and impedance angle, VSC operation power and its power factor, PLL phase-locked error, and grid frequency) in multiple scenarios are established. Then, the transient instability boundary of VSC is indicated based on the critical voltage. Results show that both the line resistance and reactive power injection can directly reduce the critical voltage and improve the stability of the system. The increase or decrease in grid-side frequency will directly lead to the phase lag or lead of PLL and an increase or decrease in line reactance, thus indirectly affecting the critical voltage and further affecting the transient stability of VSC. The PLL pre-filter may cause errors in the phase-locked result, and its phase lag or lead will reduce or increase the critical instability voltage of the system, respectively.

Aimed at the problems of DC bias and frequency variation in a weak grid, a modified inverse Park transform phase-locked loop (IPT-PLL) technology suitable for single-phase grid-connected inverters is proposed. First, the α component after Park transform is used as a reference voltage in the phase detector to solve the problem of DC bias in grid voltage, and an orthogonal component is constructed by the method of 1/4 fundamental periodic delay. Second, the fractional-order delay is approximated by Lagrange interpolation polynomial to reduce the calculation error of delay caused by frequency variation, and the design method for PI regulator is theoretically analyzed. Finally, experimental results show that the modified IPT-PLL proposed has a strong frequency adaptivity, and it can significantly suppress the interference of DC bias in grid voltage. In addition, its dynamic and static performances are satisfying.

With the large-scale integration of wind power and other renewable energy sources, the frequency regulation capacity and effect of traditional frequency regulation power sources are difficult to meet the requirements of power grid. To solve this problem, a comprehensive control strategy based on the frequency regulation signal optimization of a battery energy storage system which assists the thermal power unit to participate in secondary frequency regulation is proposed. First, a simulation model of energy storage that meets the power grid’s frequency regulation requirements is established. Based on this model, the allocation mode of area control error signal and area frequency regulation requirement signal is analyzed in the complex frequency domain, and the switching criterion for frequency regulation signal is determined by combining the advantages of the two control signals. Then, considering the economy and efficiency of energy storage frequency regulation, the allocation coefficient is optimized by a decomposed multi-objective evolutionary algorithm to reduce the frequency offset and optimize the cost of frequency regulation. Finally, the effectiveness of frequency regulation signal switching criterion and multi-objective evolutionary algorithm in optimizing the energy storage allocation coefficient is verified by step disturbance simulation. The comprehensive control strategy is verified by continuous disturbance simulation, and results show that it can not only reduce the system frequency offset effectively, but also lower the operating cost of energy storage.

In the impedance measurement process, since the inverter impedance varied widely, the magnitude of injection disturbance cannot be evaluated in advance. Therefore, it is necessary to adjust the disturbance energy adaptively. The impedance measurement device of disturbance voltage injected in series is taken as the research object, and an adaptive adjustment strategy of disturbance voltage based on disturbance current feedback is proposed. The magnitude of disturbance voltage is adjusted by detecting the responding disturbance current in real time, thus realizing the adaptive adjustment of disturbance energy. Both the disturbance voltage and responding disturbance current are controlled to be within 10% of the steady-state point of the system under test. The effectiveness of the proposed control strategy was verified by hardware-in-the-loop simulations in real time.

As the penetration rate of renewable energy resources in a new power system continues to rise while the proportion of traditional thermal power units continues to decline, the new power system faces severe frequency control problems. Distributed battery energy storage systems (BESSs) provide an effective way to solve these problems. On this basis, a robust load frequency control (LFC) method for distributed BESSs based on sparse communication network is proposed. To suppress the uncertainties related to system operation, a two-tier model predictive control (MPC) is designed to improve the response characteristics of BESSs, thus improving the performance of LFC. To minimize the area control error, the proposed method can satisfy various operating physical constraints of the system. In addition, the influence of communication delay on the performance of frequency modulation participated by BESSs is also considered, and a fuzzy coordination control device is designed to coordinate BESSs and the traditional generator, so that the mis-operation of the traditional generator under the condition of long delay can be avoided. Finally, simulation results show that the response capability and frequency modulation effect of distributed BESSs are better than the traditional methods under parameters such as different values of capacity, rated power, charge and discharge coefficient, state-of-charge and time constant.

With the increasing penetration rate of renewable energy, carbon emissions are reduced. However, the inherent intermittency and volatility of renewable energy also bring problems such as inertia, security and economy to the power system. The battery energy storage(BES) technology has become one of the important means to solve this problem. Under this background, an autonomous control method for BES system oriented to the active support of grid voltage is proposed based on full-state feedback. First, based on sagging K_v(V_g-v_g) and the virtual capacitor C inertia technology, static power support control and dynamic voltage support control modules are designed, so that the BES system can provide power(static) support and voltage(dynamic) support. Second, the voltage controller and current controller are combined by using the full-state feedback method, which makes the design of the proposed controller more systematic and flexible and reduces the voltage oscillations caused by single-phase ground fault. Third, in order to maintain the stability of state-of-charge(SOC) of BES, a BES SOC controller based on regulatory factors is also designed to further improve the autonomous operation ca-pability of the BES system. Finally, a case study of a 14-node DC system was carried out based on MATLAB and a semi-physical simulation platform, and simulation results verified the effectiveness of the proposed method in the cases of double-support of static power and dynamic voltage and single-line ground fault. With this method, the BES system can be connected to any key node in the grid, and the voltage at the point of common coupling in the grid can be actively supported through the local monitoring of disturbance, which is not affected by disturbance and can be operated and controlled independently. In addition, this method can also prevent the converter from overcurrent during transient low-voltage accidents, so that the autonomous operation capability of BES is realized.

Aimed at the problem that the accuracy of photovoltaic array fault diagnosis based on support vector machine (SVM) is not high and it is easily affected by the kernel function and penalty factor parameters, a photovoltaic array fault diagnosis method based on SVM optimized by the seagull optimization algorithm (SOA) is proposed. The SOA is introduced to optimize the parameters of the SVM model, and an SOA-SVM fault diagnosis model based on the optimal parameters is established. MATLAB software is used to build a photovoltaic array simulation model, and the characteristic parameters under different fault types are extracted and further inputted into the SOA-SVM model for fault diagnosis. Experimental results show that the fault diagnosis accuracy of the SVM model optimized by SOA is significantly improved. Compared with the ABC-SVM and PSO-SVM models, the SOA-SVM model converges faster in the optimization process and has a higher fault diagnosis accuracy.

With the scale expansion of a subsea observation network, the stability of its high-power power supply system has attracted attention. First, the impedance models of key parts in the subsea power supply system are established. Considering the characteristics of high power electronic penetration rate, multi-bus cascading and adjacent bus interactive coupling of the subsea DC power supply system, the stability and influencing factors of the system are explored by using the step-by-step analysis method. The analysis result shows that the integral parameter of the controller is the dominant parameter that leads to the instability of the Buck converter, and the proportional parameter of the controller is the dominant parameter that results in the instability of the junction box subsystem. Both an increase in the impedance parameter of the optoelectronic composite cable and a decrease in the inductance parameter are beneficial to improving the system stability. The simulation results based on the PLECS simulation software verify the stability analysis results.

With the large-scale network entry of electric vehicles (EVs), their disorder charging further increases the load peak-valley gap, which has a negative impact on the stable operation of power system. A two-stage optimization scheduling strategy which takes into account the EV load and the energy storage system of batteries is proposed. First, an orderly charging scheduling model for EV is established, which aims at minimizing the absolute peak-valley gap between user charging cost and load. The improved particle swarm optimization algorithm is used to solve this model to avoid peak charging. Second, an optimal scheduling model of peak-shaving and valley-filling for the energy storage system is established with an objective of minimizing the variance of load and the combined cost of energy storage life, which is solved by the improved Harris Hawks optimization algorithm to reduce the peak-valley gap of load. In addition, the optimization results are evaluated and analyzed based on the evaluation index of peak-shaving and valley-filling. Finally, a simulation experiment is carried out with the measured load power of one power network as an example. Results show that under the proposed two-stage optimization scheduling strategy, the peak load decreases by about 147 kW, the valley load increases by about 223 kW, and the peak-valley gap deceases by 46.73%, indicating that this strategy can effectively improve the load curve, alleviate the pressure on power supply during the peak load period and ensure the safe and stable operation of power grid.

To avoid the dynamic problems caused by dynamic and static loads in the operation of a power electronic power system, which affect the system stability, a modeling and analysis method for the voltage power angle dynamic stability is proposed. The continuation method is used to track the equilibrium solution manifold of the power system, and the small disturbance analysis method is used to calculate the voltage power angle state matrix to judge the dynamic stability of voltage power angle. By means of the power system stability mode discrimination method, the correlation between voltage and power angle state variables in the instability state is analyzed, the system instability category is determined, and the construction and analysis of voltage and power angle dynamic stability model is realized. Experimental results show that the load model is directly related to the category of dynamic instability of power system in an electronic environment. The static load model is easy to cause power angle instability, while the dynamic load model is easy to cause voltage instability. The time-domain simulation results are consistent with the discrimination results of state variable participation factor. Through the tests of power angle instability and voltage instability, the generator angle and node voltage are analyzed. It is proved that the proposed method has a good effect in the analysis of voltage and power angle dynamic stability, providing certain reference value for researches in the field of power system engineering.