Congshan Li

With the deepening of the construction of the new power system, the penetration rate of new energy in the power system continues to increase significantly. However, the traditional assessment of the ability of new energy (wind, solar, and storage) to participate in the primary frequency response (PFR) of the system exhibits limitations due to the diffi-culty in fully considering their differentiated characteristics. Therefore, this study develops a comprehensive evaluation index system for the PFR performance of wind-solar-storage new energy systems based on the entropy weight method. This system fully incorporates the characteristics of wind power generation, photovoltaic power generation, and energy storage systems in terms of frequency response and operational constraints, establishing overall PFR capability evaluation indicators of the system, as well as specialized and re-fined evaluation indicators for each technology. Simulation results demonstrated the meth-odology's effectiveness, demonstrating its capability to accurately assess frequency regula-tion capacities while considering the distinct operational characteristics of wind, solar, and storage technologies. These findings provide theoretical guidance for optimizing frequency regulation resource allocation and operational strategies in renewable-dominated power systems.

Droop control is widely used in multi-terminal flexible DC (VSC-MTDC) transmission systems by virtue of the advantage of multi-station cooperative unbalanced power dissipation, however, the essence of the droop control strategy is to change the DC current to realize the unbalanced power dissipation, and the resulting DC voltage deviation will affect the normal operation of the system. Firstly, this paper theoretically analyses the working characteristics of the conventional droop control and proposes a control method to realize the quasi-differential-free regulation of DC voltage by translating the droop curve. Second, according to the power margin of the converter station, the feedforward compensation amount of each converter station is reasonably set to avoid the power impact on the converter station. Finally, for the problem that the actual value of DC voltage still deviates from the rated value, a control strategy containing secondary regulation of DC voltage is proposed to further restore the DC voltage to the initial value on the basis of ensuring the effect of power regulation, which improves the stability of the operation of the VSC-MTDC system. The final simulation results verify the effectiveness of the proposed method.

In response to the inability of the flexible DC transmission system connected to the AC grid under conventional control strategies to provide inertia to the system as well as to participate in frequency regulation, a virtual synchronous generator (VSG) control strategy is proposed for a voltage source converter (VSC)-based multi-terminal high-voltage direct current (VSC-MTDC) interconnection system. First, the virtual controller module is designed by coupling AC frequency and active power through virtual inertia control, so that the VSC-MTDC system can provide inertia response for AC grid frequency. Second, by introducing the power margin of the converter station into the droop coefficient, the unbalanced power on the DC side is reasonably allocated to reduce the overshoot of the DC voltage in the regulation process. Finally, the power regulation capability of the normal AC system is used to provide power support to the fault end system, reducing frequency deviations and enabling inter-regional resource complementation. The simulation model of the three-terminal flexible DC grid is built in PSCAD/EMTDC, and the effectiveness of the proposed control strategy is verified by comparing the conventional control strategy and the additional frequency control strategy.

Since the additional frequency control of multi-terminal flexible DC transmission system is easy to property DC voltage abnormality, this paper investigates the mathematical relationship from AC system frequency and DC voltage, and proposes an improved adaptive frequency regulation strategy. This strategy connects VSC-MTDC systems containing wind energy to make sure that the DC voltage is stable during system frequency tuning. Finally, A three-terminal flexible DC transmission system containing wind power is constructed at PSCAD/EMTDC, and the results show that the proposed method is able to reduce voltage swings while achieving mutual power support between AC systems.

DC voltage stability and power balance are the key conditions for stable operation of DC transmission systems. The droop control does not depend on inter-station communication and has the advantage of multi-station cooperative unbalanced power dissip...

In order to solve the problem that the voltage source converter based multi-terminal direct current (VSC-MTDC) system cannot provide inertia and participate in frequency modulation after connecting to the AC power grid under the traditional control strategy, an adaptive control strategy based on virtual inertia is proposed. First, the relationship between AC frequency and DC voltage was established by a virtual inertia control, allowing the VSC-MTDC system to provide inertia to the AC side. Second, to address the limited inertia coefficient selection due to DC voltage deviation, an adaptive control was adopted. When the DC voltage deviation is small, the inertia coefficient is increased to obtain a better inertial response; on the contrary, the inertia coefficient is reduced to prevent the DC voltage from exceeding the limit. Finally, to solve the problem of insufficient flexibility of the fixed droop coefficient, this paper introduces the power margin of a VSC-station into the droop coefficient to dynamically adjust the distribution ratio of unbalanced power and reduce the DC voltage deviation. The three-terminal VSC-MTDC system was modelled on the PSCAD/EMTDC simulation platform, and the superiority of the control strategy was highlighted in this paper by comparing it with conventional droop control and a fixed virtual inertia coefficient.

Background: With the continuous penetration of renewable energy in modern power systems, the problem of excessive frequency deviation of power systems, which is caused by the integration of wind power into electricity grids, needs to be solved urgently. Objective: Make up for frequency fluctuations when AC systems fail, improve the frequency characteristics of main system networks, and reasonably distribute unbalanced power. Methods: According to the principle of active power distribution, a frequency division coordinated control strategy that uses low-pass filter control to divide power fluctuations into highfrequency fluctuations and low-frequency fluctuations is proposed. High-frequency fluctuations are attached to the wind farm side, and low-frequency fluctuations are attached to the DC system to realize frequency division control. Result: The simulation results showed that the coordination of the frequency division control of the wind farm and flexible DC system could effectively reduce the main grid’s frequency fluctuations, which are caused by the AC system failure, and improve the stable operation ability of the AC and DC systems. Conclusion: The frequency division control can overcome the unbalanced power distribution caused by traditional droop control, reduce the frequency deviation of the AC system, realize frequency autonomy, and enhance system robustness.

Voltage source converter-based multi-terminal high-voltage direct current (VSCMTDC) transmission system can realize a multi-point power supply, multi-drop power receiving, and mutual coordination between the converter stations to ensure the reliability of the transmission. Based on the PSCAD/EMTDC platform, a five-terminal DC transmission system model is established. According to the fast power regulation capability and overload capacity of theVSC-MTDC power transmission system, an analysis of additional emergency power support for a transmission system under large disturbance conditions was carried out. A new control strategy for emergency power support that introduces its basic principle is proposed in this paper. It uses the short-term overload capability of the DC system. By changing the power reserve of the converter station and the electrical distance between the converter stations, the influence of the power reserve and the electrical distance on the emergency power supply guarantee is analyzed the stability of the system is improved, thereby improving the sudden change of power caused by voltage fluctuations, and the feasibility of the control module is verified by PSCAD simulation. The simulation results show that when the system power supply suddenly changes, the converter stations at a short distance and large power reserve has a better effect on emergency power supply protection. A comparative study of the active power support of a single converter station and multiple converter stations is carried out. The research results show that the use of emergency power support in the DC transmission system has a good effect on maintaining the stability of the inter-connection system and the reliability of the power supply.

A hybrid multi-infeed HVDC (HMIDC) system is composed of line-commutated converter-based high-voltage direct current (LCC-HVDC) and voltage-source converterbased high-voltage direct current (VSC-HVDC), whose receiving ends have electrical coupling. To solve the problem of low-frequency oscillation (LFO) caused by insufficient damping in the HMIDC system, according to the multi-objective mixed H2/H∞ output feedback control theory with regional pole assignment, an additional robust damping controller is designed in this paper, which not only has good robustness, but also has strong adaptability to the change of system operation mode. In the paper, the related oscillation modes and transfer function of the controlled plant are obtained, which are identified by the total least squares estimation of signal parameters via rotary invariance technology (TLS-ESPRIT). In addition, the control-sensitive point (CSP) for suppressing LFO based on the small disturbance test method is determined, which is suitable for determining the installation position of the controller. The results show that the control sensitivity factor of VSC-HVDC is greater than that of LCC-HVDC so that adding an additional damping controller to VSC-HVDC is better than LCC-HVDC in suppressing the LFO of HMIDC. Finally, a hybrid double infeed DC transmission system with three receiving terminals is built on PSCAD/EMTDC where the time-domain simulations are performed to verify the correctness of the CSP selection and the effectiveness of the controller.