## 2 Multiple Resources Coordination Control Framework for EFC  
The number of DER devices is huge, and the regulation capability of individual devices is limited, so they cannot be directly controlled by the grid dispatching center. Fortunately, the form of VPP can aggregate a cluster of limited-capacity and dispersed DERs to participate in bulk grid stability control. There may be several auxiliary services that VPP can provide, in order to realize the decoupling of different control objectives; this paper proposes a hierarchical control architecture to manage DERs, as shown in Figure 1. There are four layers, which are DERs devices, a VPP control center, a VPP cloud control platform, and a grid dispatching center from bottom to top.  
When participating in EFC, a VPP is required to fully respond to the allocated capacity within 1 s after receiving the control instruction, which is the same response speed as fast frequency response. For EFC, the communication between the VPP control center and the DERs adopts a low-delay communication technology, such as optical fibers or 5G. To achieve a rapid power decrease, EV can immediately stop charging or reverse discharge, and AC can immediately shut down. To reduce the computational burden of VEPR capacity evaluation, the cloud-side collaboration technique is adopted in this paper. The intelligent terminal unit (ITU) of the DER evaluates the EPR capacity of individual devices based on the current status of the DER device, device constraints, and user comfort settings. The ITU sends the evaluation result to the VPP control center, and then the VPP control center can refresh its EPR capability periodically. The VEPR capacity of all VPPs in the receiving power grid will be aggregated through the VPPs cloud control platform to the grid dispatching center for online EFC decision-making.  
The hierarchical management architecture of DERs decouples the VPP regulation capacity aggregation and control instruction allocation between different layers. For the grid dispatching center, the VPPs connected to the transmission bus can be regarded as controllable loads.  
The resources for EFC considered in this work include HVDC emergency power support, ELS and VEPR. This work proposes a bi-level optimized EFC caused by HVDC blocking. Figure 2 shows the EFC framework.  
Figure 1: Hierarchical control architecture of DERs.  
Figure 2: Schematic diagram of emergency control including VPP.  
Through the aggregation and disaggregation roles of VPP, when massive controllable DERs participate in EFC, the decision complexity can be reduced by hierarchical optimization. The upper-level optimization model is a multiple resources coordination scheme with minimal control cost solved by the grid dispatching center. The participation of VEPR can reduce the impact of ELS on residents' lives. In the upper-level model, the grid dispatching center needs to know the regulation capacity of all resources and their response process characteristics. The power grid dispatching center updates its decision table according to the latest operation status of the power grid and the support capability of all resources under the stable operation condition. Usually, the update period of the decision table can be 10 min.  
The lower-level optimization model is the instruction allocation of each control resource. Since HVDC emergency power support and ELS are mature technologies in the existing stability and control system, this work only presents the instruction allocation of VPP. After receiving the control instruction, the VPP control center quickly determines the DER device that needs to participate according to the DERs dynamic response priority sequence. Under the stable operation condition, the VPP control center updates the dynamic response priority sequence according to the regulation capability and margin information of each DER device.