The development of renewable energy is a fundamental measure to resolve environmental pollution and energy crisis, and achieve the carbon peaking and carbon neutrality goals. However, the inherent volatility and fluctuation of renewable energy output bring unprecedented challenges to the planning and operation of the power grid. This paper studies the optimal ratio of renewable energy and energy storage, aiming to minimize power fluctuation. According to the complementary nature of wind and solar resources, the mode of optimal ratio of wind and solar power that leads to minimal power fluctuation is established and is further transformed into linear programming. The optimization problem of energy storage capacity aiming to smooth the renewable energy output is formulated as a multi-parameter linear program, where storage charging power and energy capacities are parameters. The power fluctuation index is expressed as an analytical function in storage parameters, which is convex and piecewise linear. On the basis of the fluctuation index function, the optimal storage capacities can be determined according to the costs. The proposed method provides an illustrative tool and more abundant information for policy and decision-making.

In order to study the ralationship between grid-forming （GFM） and grid-following （GFL） units that can support the stable operation of renewable systems, this paper firstly takes the single wind farm integrated system as an example to analyze the dynamic characteristics of the GFL and GFM units. Then, from the perspective of maintaining the oscillation stability of wind farms both in low frequency and subsynchronous frequency bands, the calculation principle of the GFM unit capacity to be deployed in wind farms is proposed. On this basis, based on the three-machine-nine-bus system, the feasibility of the stable operation of the 100% renewable system is explored, and the influence of the proportion and location of the GFM units on the dynamics of the system are studied. The results show that the subsynchronous oscillation （SSO） risk can be effectively improved by deploying a small number of GFM units in the wind farm, and the capacity of GFM units is mainly constrained by the low-frequency mode stability. Besides, in the 100% renewable system, the SSO due to inadequate support of GFM to the remote GFL units is the key constraint in determining the need for GFM units.

Large-scale renewable energy transmission from desert, Gobi and wilderness to load centers is critical measure to achieve the goals of "Carbon Peak and Carbon Neutrality". Two large-scale renewable energy ultra-long-distance transmission schemes are introduced, including line commutated converter based high voltage direct current (LCC-HVDC) and voltage source convert based high voltage direct current (VSC-HVDC). The two schemes are compared in terms of their applicable scenarios, overvoltage, voltage and reactive power adjustment capabilities, and economy, to highlight their respective advantages and disadvantages. VSC-HVDC is superior in terms of technology and economy. An integrated design scheme of ±800 kV/10 GW four-terminal VSC-HVDC system for 10 gigawatt renewable energy ultra-long-distance transmission is proposed, which includes the electrical main wiring, main circuit parameters, coordinated control strategy, and the configuration of AC/DC choppers. Key technologies such as bipolar single-valve and high-low valve scheme selection, AC voltage coordinated control of multi-valve at the sending end, and DC voltage balance control of high-low valve are studied, and the configuration scheme and switching strategy of the configuration of AC/DC choppers are proposed. The effectiveness of the HVDC transmission system scheme is verified through electromagnetic transient simulation.