## 4 Case Study
### Discussion
#### 4.2.2 VPP Operating Result Analysis  
The optimization results of VPP are shown in Figure 6.  
As illustrated in Figure 6a, during the periods from 01:00 to 08:00 and from 16:00 to 24:00, VPP strategically opts to procure the majority of its electricity at lower prices from the grid and SES systems, rather than relying solely on self-generation. This approach effectively minimizes generation costs. Furthermore, it selectively curtails or shifts electric load to time slots with lower tariffs, thereby contributing to a reduction in overall electricity consumption expenses. At 9:00-15:00, VPP does not purchase power from the grid or SES. On the one hand, because the lower PV output is large, it offsets most or all of the electricity load. On the other hand, the renewable energy and generator output in VPP can meet the lower power supply. At this time, the cost of its own power generation is less than the cost of purchasing power from superior power or SES, which can also save power supply costs and improve its own benefits. Secondly, the output of VPP units and renewable energy  
Figure 5: Purchase and sale price of electricity between SES and VPP. output is greater than the energy demand of the lower layer, hence VPP reverse sells most of the surplus electricity to the grid to obtain benefits. As shown in Figure 6b, most of the heat load is met by the gas turbine, and a small part of the heat load demand is met by the gas boiler. Because the gas turbine will also emit heat while meeting the electrical energy demand of the lower PVP alliance. Therefore, for the sake of the economy of the VPP system, the gas turbine has always been the main part of providing energy. Therefore, the gas turbine provides a large part of the heat required by the lower PVP alliance, but when the gas turbine output is low and the heat load is high, the gas boiler will also serve as an auxiliary to supply the missing heat energy. Figure 6: VPP optimization results: **(a)** VPP electric load balance; **(b)** VPP heat load balance. Figure 8: Power purchase price between VPP and PVP for PVP1. Figure 9: Heat purchase price between VPP and PVP for PVP1. The electric and heat optimization outcomes of PVP1 are depicted in Figure 7. From 01:00 to 08:00 and 16:00 to 24:00, PVP1 fulfills the majority of its electricity demand by procuring electricity from the VPP during this timeframe. The reason is that the photovoltaic output is extremely low or nearly nil during this period, and thus fails to meet its electricity requirements. From 9:00 to 15:00, the vast majority of the power demand of PVP1 is fulfilled either by photovoltaic power output or through the purchase of power from other members of the PVP alliance. The cost of purchasing power from other members is lower than that from the VPP, making it more economical. As is evident from Figure 8, the electricity price set by the VPP for the PVP alliance is largely consistent with the power selling price trend of the power grid. However, during the period from 12:00 to 15:00, when the lower-level demand is low, the electricity price is set at its lowest purchase level to attract lower-level users to purchase power. From 19:00 to 22:00, when the PVP alliance has a high-power demand, the electricity price is set at a higher level, nearly the same as the power purchase price of the grid, in order to obtain a higher income. As can be observed from Figure 9, the heat purchase price has no distinct rule and varies with the change in VPP heat production. The electrical energy interaction power between PVP1 and other PVP members is in accordance with Figure 10. During the intervals of 01:00-7:00 and 17:00-24:00, PVP1 conducts electrical energy transactions with other members. Simultaneously, PVP1 also acquires electricity from the VPP to maintain its own electrical energy balance. The rationale for this is that PVP1 incurs a minor increase in its own operating costs while maximizing the overall benefits of the PVP alliance.