## 5 Key Problems for the Development of the GVPP  
In this paper, the concept, fundamental design, and dispatch and control architecture of GVPPs are proposed. But this research on GVPPs is preliminary. Moreover, the operation  
Figure 10: Flowchart of the operation of the bi-level dispatch and control architecture. strategies of GVPPs are various and system specified, only one of which is discussed. There are key problems and research directions for the development of GVPPs. Therefore, the suggestions for further research in this field can be depicted in Figure 11 and summarized as follows. 1. The mechanism and quantification analysis of the interaction between GVPPs and power systems. The GVPP consists of diversified and heterogeneous DERs, which have distinguished operation characteristics and bring about different impacts on power systems. So the interaction between a single GVPP and power systems is complicated. In addition, the cooperation of GVPPs makes the interaction more complex. The impacts of GVPPs on power systems should be figured out before large-scale application. Thus, how to analyze the interaction between GVPPs and power systems will be the future requirement of large-scale GVPP integration. 2. The model establishment and parameter identification for GVPPs. The model and parameter are the basis of the analysis and optimization for GVPPs. Moreover, the composition of GVPPs makes the model complicated. However, the complexity of established model reduces the effectiveness on the on-line assessment and optimization. A compromise between the complexity and effectiveness should be made. Thus, how to establish an appropriate model and propose corresponding parameter identification approach is a problem to be researched urgently. The estimation of operation states for GVPPs. In the GVPP, the RESs, especially WTGs and PV panels, are deployed, of which the operation states are time variant. So, the operation sates of GVPPs are influenced by the RESs and vary with the changing of meteorological condition, such as wind speed and solar irradiation. Moreover, the operation states of GVPPs are significant for the dispatch and control. Thus, how to estimate the operation states for GVPPs is also a problem to be researched. 3. The optimal planning for GVPPs. The GVPP consists of numerous DERs, which are dispersed in space. The planning of a GVPP involves the sizing and siting of heterogeneous DERs. There are massive objectives, such as economic efficiency and RES accommodation, that should be taken into account and traded off. The optimization of GVPP planning is a high-dimensional, non-linear, and multi-objective mathematical problem. Moreover, the coordination planning of multiple GVPPs makes the mathematical problem more complicated. Thus, how to establish an optimization model for the planning of GVPPs and provide a high-efficiency solving approach involves extensive research. 4. The coordinated control strategy for GVPPs. Figure 11: Suggestions for further research on the GVPP. The control of GVPPs involves different time scales. The control objectives, constraint, and resources of GVPPs in normal and emergency conditions are different. Moreover, the coordination among GVPPs and the coordination between GVPPs and power systems should be considered. There are high-dimensional, non-linear, multi-objective, and multi-level control problems for a single GVPP, GVPPs, and power systems. Thus, how to control the GVPPs synergistically for the economic, secure and stable operation under different time scales and conditions remains unanswered. The design of communication infrastructure and demonstration project for GVPPs. As for the transmission of data and commands, the communication infrastructure is necessitated. Considering the integration of extensive DERs and GVPPs, there are high requirements on GVPPs with respect to the indices of latency, packet loss, retransmissions, bandwidth, and amount of traffic.