## 2 Microgrid Structure and Dispatching Strategy
### Expression Construction  
#### 2.3.1 Objective Function  
In this paper, objective functions are established for the economy of the wind-photovoltaic-storage electric vehicle microgrid system under a grid-connected operation. Consider the operation, maintenance, and investment cost of each unit in a system, and the cost of grid sales and purchases. Meanwhile, taking into account the operating costs of EVs and the cost policies issued by the government for new energy, the optimization goal can be achieved through relevant scheduling. The expression is as follows:  
\[Minimize\ F(x)=F_{uv}+F_{x}-F_{G}-F_{EV}+F_{BT} \tag{3}\]  
where the operation and maintenance costs of wind and photovoltaic generation are represented as \(F_{uv}\). The power generation and investment costs of the battery are determined as \(F_{x}\). The power grid sales and purchase costs are simplified as \(F_{G}\). The operating costs of EVs and policy expenses related to new energy power generation are given as \(F_{EV}\) and \(F_{BT}\). The decision variable of the objective function is the output power of each unit, which is the real number. Moreover, the specific coupling relationships between decision variables and objective functions (3) are shown in subsections a-e.  
a. Wind and photovoltaic generation  
The equipment loss, manual repair, and operating reserve storage costs are included in \(F_{uv}\), which can be expressed as follows:  
\[F_{uv}=\sum_{t=1}^{T}C_{uv}P_{wf}(t)+\sum_{t=1}^{T}C_{v}P_{bf}(t)+F_{RE} \tag{4}\]  
where the operation and maintenance cost coefficients of wind and photovoltaic generation are defined as \(C_{uv}\) and \(C_{v}\), respectively. These coefficients are based on the ratio of the total operation and investment costs of the equipment to the total annual generated power within a year obtained in enterprise. The output power of wind and photovoltaic power  
Figure 3: The scheduling strategy of EV. generation is computed as \(P_{wf}\) and \(P_{bf}\), respectively. The operating reserve storage cost is simplified as \(F_{RE}\), which can be expressed as follows:  
\[F_{RE}=C_{RE}[P_{vf}(t)+P_{wf}(t)] \tag{5}\]  
where the operating reserve storage cost coefficients of wind and photovoltaic generation are given as \(C_{RE}\). b. Battery generation  
In Equation (6), \(F_{x}\) consists two parts, which are the acquisition and maintenance costs of batteries. The acquisition cost is converted into each work process, and the specific expression is calculated as follows:  
\[F_{x}=\frac{F_{cap}C}{P_{bess}T_{n}}|\sum_{t=1}^{T}P_{bess}(t)|+\frac{K_{o}P_{ bess,max}}{365}+K_{M}\frac{1}{\Delta t}P_{bess}(t)\Delta t \tag{6}\]  
where the acquisition cost and depreciation coefficient are defined as \(F_{cap}\) and \(C\), respectively. The annual operating cost coefficient and hours are represented as \(Ko\) and \(T_{n}\), respectively. \(P_{bess}\) and \(P_{bess,max}\) are battery-rated power and maximum charge and discharge power, respectively. Meanwhile, the maintenance cost coefficient is described as \(K_{M}\), and the duration from the beginning to the end of charging and discharging the battery is shown as \(T\). c. Interaction with main grid  
The microgrid system can interact with the main grid when the power is vacant or redundant. When the interactive power is positive, the power is purchased from the main grid; otherwise, the interaction cost \(F_{G}\) is the sum of the two. The equation is simplified as follows:  
\[F_{G}=\sum_{t=1}^{24}P_{grid}(t)C_{price,grid} \tag{7}\]  
where \(P_{grid}(t)\) refers to interactive power with the main grid, and the time-sharing sale and purchase price of the main grid is expressed as \(C_{price,grid}\). d. Policy of government  
Subsidies are paid for wind and photovoltaic generation, which encourage companies to adopt new sources of electricity. Meanwhile, fees are charged for energy waste to reduce the energy loss caused by wind and solar discarding.