# The Cascade Control of Natural Gas Pipeline Systems
## 1 Introduction  
Most natural gas is typically transported by means of pipelines from the production and storage facilities to market regions. Supervisory Control And Data Acquisition (SCADA) system is a standard configuration in modern pipeline systems that works as the control center of natural gas pipelines, but rather focuses on the supervisory level [1]. Unlike the advances of the hardware configuration in pipelines, the scheduling scheme is still an empirical decision process [2]. Current industrial practice for natural gas pipeline control involves regulatory control loops along with manual intervention by pipeline dispatchers. The transient process is regulated by the real-time change of compressor speed and valves openness [3]. It includes inlet pressure regulation, outlet pressure regulation and flow regulation. When more than two parameters need to be adjusted at the same time, the PID controller choose the actuator according to the principle of \"small value selection\". With the PID controller, the stability control of local parameters is determined by the signal of inlet pressure, outlet pressure or flow deviation. The input, output and reference signals of the system are all local variables in the station. Since the control process is not affected by the entire pipeline state, it is called the local PID control. During the scheduling process, the dispatchers adjust the parameters according to the state of the entire pipelines. The transient process is fast and steady under the operation of experienced dispatchers. Following the concept of unmanned vehicles, this paper attempts to develop a control algorithm replacing the experienced dispatchers, i.e., the SCADA system can schedule the piping systems independently without the manual intervention. The controller design method employed may not be the frontier theory. The way to solve the problem is inspiring with the rise of Artificial Intelligence (AI) technology. The hierarchical control structure of natural gas pipelines is shown in Figure 1. According to users' demand and pipeline capacity, the pipeline scheduling center carries out gas supply balance analysis,and formulates the medium and long term gas supply plan and pipeline scheduling scheme [4; 5]. The mid-long term and short term optimization process is calculated off-line and the result is the guideline of one month of operation. The short term optimal operation is to get the steady flow and pressure along the pipelines [6]. Since the gap between two steady states is large, it is impossible to achieve by just one set point command to the PID controllers along the pipelines [7]. The state of the piping systems has to move gradually by the set point command sequences based on the dispatchers' experience [8]. They make decisions usually following system dynamic characters with the state of the entire pipelines. On the other hand, the transient process control is more difficult mathematically than the steady state because of the fluctuations in gas supply and gas consumption [9]. The dynamic behaviors of natural gas pipelines are characterized by the fluid equations consisting of a set of nonlinear hyperbolic partial differential equations [10]. A control oriented mathematical model and the corresponding algorithm are necessary for the automatic scheduling process. Until recently, the state space model is used for the transient flow simulation in gas pipelines [11]. Since the state space model is the fundamental work of modern control theory, it is possible that the classical controller synthesis method can be used for the complicated gas pipeline controller design [12]. In this paper, a cascade control algorithm is deployed in the SCADA system as an additional control level between the scheduling process and the PID controllers. The virtual controller gives the continuous set point command sequences based on two successive steady hydraulic states in real time [13].