## 1 Introduction  
With the increasing demand for electricity and the increasingly severe problems of energy shortage and environmental pollution in the world, the disadvantages of traditional energy generation are becoming more and more prominent [1]. As a result, countries around the world have introduced policies to encourage companies to optimize their energy structure, promote energy conservation and emission reduction, and achieve sustainable economic development. In this new situation, the distributed energy resources (DERs) have been vigorously developed. These DERs have the characteristics of environmental protection and low energy consumption, and small investment can also improve the flexibility of the power system [2,3]. However, single DER is difficult to directly participate in market transactions, so aggregation technology is widely used. To this end, virtual power plant (VPP), microgrids and load aggregators (LAs) have been developed one after another. The microgrid takes the distributed generation (DG) and user as the main control targets, but it is restricted by geographical areas [4]. The VPP can reduce the geographical restrictions of DG aggregation and aggregate various DERs. Either microgrids or VPPs are more widely used in the generation side, focusing more on the integration of DERs and optimal scheduling. The LA is a resource aggregator with external load characteristic and strong responsive capability [5]. Therefore, this paper mainly studies the VPP. In order to achieve effective management of DERs with large quantities and scattered geographical locations, and eliminate the intermittent and random effects of renewable energy power generation, the concept of VPP has been mentioned [6; 7; 8]. The VPP can aggregate DERs based on their respective characteristics, achieve efficient use of various types of resources, and reduce the generation of abandoned wind power (WP) and photovoltaic (PV). The VPP has developed rapidly in recent years, such as Vattenfall and Next Kraftwerke in Germany [9]. In China, a VPP demonstration project has come into use in Yunnan Province [10]. Moreover, the VPPs constructed in different regions have different characteristics, so the actual geographic information should be taken into account when constructing VPPs. From a microscopic perspective, the VPP can be considered as the integration and coordination optimization of DERs through advanced information and communication technologies and software systems. The VPP is a power coordination management system that participates in the power market and grid operation as a special power plant [11]. Therefore, how to select various DERs for the construction of VPP is a prerequisite for the efficient use of resources, and it has important research significance. In the reasonable selection of DERs for aggregation and construction of VPPs, it is necessary to research the characteristics and correlation of various DERs. Many scholars have achieved research results. In Reference [12], the VPP components are introduced, including the wind power plants (WPPs), PVs, conventional gas turbines (CGTs), energy storage systems (ESSs) and demand resource providers (DRPs). In Reference [13], the conditional value at risk (CVaR) and confidence degree theory are introduced to build models for VPP connections with the WPP, PV, CGT, ESSs and incentive-based demand response (IBDR). In Reference [14], the implementation costs of demand response (DR) has been firstly taken into account, and then WPPs, PV, electric vehicles and conventional power plants have also been researched. The VPP is selective to select DERs for aggregation and there are different methodological guidelines in the aggregation process. In Reference [12], a two-tier robust scheduling model is established. The upper layer uses the VPP operating revenue as the maximum objective function, and the lower layer uses the minimum system payload and operating cost as objective functions.