## 2 Background and Related Work
### Renewable Energy Generation Integrated in the Grid  
As renewable energy power stations are being constructed all over the world, the problems resulting from a grid penetrated with renewables have emerged. The photovoltaic power generation will be greatly affected by the change of solar radiation, while the wind generation will be affected by the wind speed variation. This makes renewable power generation intermittent and unstable. To improve the utilization efficiency of interconnected devices, improve the power quality of the renewable energy interconnection grid system, maintain the normal operation of the grid and get good economic profit, [PERSON] et al. [46] proposed a hybrid power generation system that connected the photovoltaic power station and batteries in parallel, which absorbed and supplied unbalanced power from the battery and flattened grid-connected power. [PERSON] et al. and [PERSON] et al. [47; 48] established a hybrid energy storage system composed of batteries and supercapacitors and optimized the power distribution of the storage equipment through different filtering algorithm controls, which improved the economy of the mixed energy storage equipment. [PERSON] et al. [49] proposed the IP-IQ current detection method of two kinds of control schemes of the photovoltaic grid inverter based on the instantaneous reactive power theory in order to realize the dynamic reactive power compensation and flexible combination of an active filter, while at the same time improving the power quality. In Reference [50], a control strategy of the unified power quality conditioner was proposed to stabilize the power system and increase the utilization ratio of the equipment. Although battery storage systems can effectively increase grid stability after integrating renewable energy, electricity storage is not the optimum solution to integrate the large inflows of fluctuating renewable energy since more efficient and cheaper options can be found by integrating the electricity sector with other parts of the energy system and by creating a smart energy system. [PERSON] et al. in Reference [51] investigated the most efficient and lowest cost storage options as a part of a smart energy systems approach, as defined in Reference [52]. By using this approach, it was explained why the best storage solutions could be found by integrating the individual sub-sectors of the energy system. One of the main reasons why a cross-sector approach could identify more economically viable solutions was that the cheaper and more efficient storage technologies that existed in the thermal and transport sectors, compared to the electricity sector. [PERSON] et al. [53] also made a state-of-the-art description of different single-sector approaches for the transformation towards future sustainable energy solutions within the electricity, gas, building and industrial sectors. They discussed the smart energy systems concept with regard to the issues of the definition of the term, identification of renewable systems design, the integration of holistic storage solutions and the modelling of national energy systems. In addition, some researchers have also studied various kinds of controllable loads. [PERSON] et al. [54] put forward the dynamically controllable load-refrigerators. The refrigerator could change its temperature automatically as the grid frequency changed, which meant changing the power of the load. In the case of an increase (or decrease) in power generation, the load power can be increased (or decreased) by effectively controlling the controllable dynamic load. However, a controllable load changing with the frequency of power grid is still relatively rare at present, so it will take some time for large-scale application. [PERSON] et al. [55] considered that each customer's load and each generator's active and reactive power should be controlled in order to stabilize and optimize the grid, comparing independent and cooperative control techniques as applied to load regulation using electric water heaters.