## 3 The System Configuration and Operating Process
### Two-Level System Configuration and Its Operating Process  
In hierarchical systems, a number of regulators perform their control actions at different timescales. This approach can be useful when the overall process under control is characterized by slow and fast dynamic behavior [29]. In this paper, pumped hydro storage is a slow form of storage, and the fly wheel is a fast form of storage according to the response time. Therefore, hierarchical MPC is adopted to present the system. This two-level system is composed of a wind farm, a hybrid storage system, a grid, an MPC1 controller, and an MPC2 controller, as shown in Figure 3b.  
To understand the cooperation between the MPC1 controller and MPC2 controller, the above-mentioned Assumptions 1, 2 and 3 must be explained in detail. Assumption 3 is that the scheduling interval is 5 min; Assumption 2 is that a fly wheel can change its operating state at any scheduling time interval. Assumption 1 which is crucial, is that the pumped storage cannot change its operating state within a 30 min period until the next 30 min period starts. In other words, pumped hydro storage can change its operating state only at the beginning of the 30 min period and must then maintain its operating state until the 30 min period ends. Note that 30 min corresponds to six scheduling intervals.  
The level of MPC 1 is regarded as the inner level or short-term level, and the level of MPC 2 is viewed as the outer level or long-term level. Similar to the conventional hierarchical MPC, in this paper, the long-term MPC operates by first making decisions and then transmitting some decisions to the short-level MPC; that is, information is output from the long-term MPC and then input to the short-term MPC [29].  
The two-level MPC operating process is presented as follows:  
First, the long-term MPC (MPC 2) runs and makes decisions. Some decisions are then transmitted to the short-term MPC (MPC 1), and further decisions are made.  
Second, when the next scheduling interval arrives, the long-term MPC (MPC 2) stops working, and then the short-term MPC (MPC 1) starts working. MPC 1 makes decisions and then performs them.  
If a new 30 min period arrives, the first step is repeated, or else the second step is repeated.  
Figure 3: System configuration. (**a**) Single-level system configuration; (**b**) Two-level system configuration.  
There are three ways to communicate between the two controllers. The three ways are Ethernet communications, global system for mobile communications (GSM) and satellite communication technology, respectively. Ethernet communication solution is the cheapest solution among the three ways, and the satellite communication solution is the most expensive solution. We consider the three following implement cases:  
If internet is available in the pumped storage station and the fly wheel station then Ethernet communication is recommended.  
If the mobile signals exists in the pumped storage station and the fly wheel station, then GSM communication is adopted.  
Otherwise, the satellite communication has to be chosen.  
In this paper, the two controllers communicate once every five minutes, the speed of above three technologies is fast enough to ensure synchronization of their communication. Meanwhile, it also possible to adopt some protocols in the software to ensure synchronization--for example, the shake hand protocol is a common way to synchronize communication.