As it is known, the SCADA systems are able to measure and monitor different parameters and variables, such as: the level of liquids, the volume of the gases, the volumetric flow rate, the mass of liquids and gases, the pressure, the temperature, the humidity, the viscosity, the position and the moving, etc.
For this, some signals must be acquired based on the appropriate sensors and transducers from different electrical and electronic devices, before to be processed and analyzed by SCADA system. These data can be used for supervision of remote processes.
For this, the data are processed and analyzed to generate reports which will be transmitted to control the processes parameters within certain limits.
The SCADA applications have two basic levels:
- The customer level, which will be performed the human-machine interaction;
- The data server level which will control the entire process.
Thus, the data server sends these data to the customer level via remote equipment such as PLCs (connected to the database server either directly or via a communications network, using various communication protocols).
The SCADA systems may be relatively simple or very complex, depending of the dimension and complexity of the process monitored. These systems operate in real-time using a database system named Real Time Data Base (RTDB), which can be found on one of servers.
The servers are responsible for the acquisition and management of data for a set of parameters. Is it possible to have dedicated servers for certain tasks (for example, if the objective is to develop the process servers), servers dealing with the handling of alarms, or file servers.
The OPC and RTU work as a data concentrator. The data is structured using a man-machine interface (HMI) in a convenient format for the operator in order to be able to take any decisions that would optimize the communication between the equipment.
RTU performs the following:
- The connection with supervised equipment;
- Reading of equipment status (such as open/closed position of the valve or relay);
- Acquisition of measured signals, such as the pressure, flow, voltage or current;
- The control of equipment by sending command signals, such as the closing of a valve or relay or setting the speed of a pump;
- Reading the digital or analog signals, and sending the commands using both
digital or analog signals.
The PLC contains a microprocessor used for the automatization of processes. A PLC is provided with input/output sensors and relays. PLC is less expensive alter to old systems which used tens or hundreds of relays and timers. The PLC is programmed using the structured programming languages and elementary logic operations.
The term “Supervisory Station” refers to the servers and software responsible for the communication with the equipment (RTUs, PLCs, etc.) and HMI software which run on master-station in the control room or elsewhere.
The master station may be composed of one PC in small SCADA system. In large SCADA systems, the master station may include multiple servers, distributed software applications, and disaster recovery strategies.
To increase the integrity of the system, the multiple servers will be often configured in a dual-redundant or hot-standby topology, providing control and monitoring even in the event of a server failure.
The SCADA applications are implemented based on the software architecture (Figure 2 below). SCADA software architecture has at least two components:
- The SCADA server application;
- The SCADA client application;
SCADA server and SCADA client
The SCADA server application is multi-tasking being responsible for storing the SCADA client applications in a database. The SCADA client application receives data via serial port using RS232 and MODBUS RTU protocols.
In many cases the SCADA applications shall be addressed to and remove the users who have not an Internet connection. In this case it is necessary a new software component on a Web server. A Web server is used for generating dynamic Web pages, which are supplied to customers decrease.
But, the Web SCADA clients do not receive the same level of graphical user interfaces and have the same HMI features as the regular SCADA clients. The regular SCADA clients are accustomed to running specific applications as opposed to The Web SCADA clients who view the Web pages provided by Web SCADA server.
In order to reduce as much difference between the regular SCADA clients and Web SCADA clients some drives, services and virtual instruments are implemented in HMI. Visualization software used for monitoring and control of industrial process is called “software” HMI or “SCADA software”.
HMI/SCADA appeared as a terminal dedicated for users from the system implemented with PLC units. A PLC is programmed to automatically control a process. Because the PLC units are distributed in a large system, the data from PLC are stored and then transmitted. HMI/SCADA has the role to gather and combine the data from the PLC using a specific protocol.
HMI software can be connected to a database in order:to fulfill the charts and diagrams in real time:
- To analyze the PLC data;
- To treat the planned maintenance;
- To drill the schemes for a particular sensor or machine;
- To apply the troubleshooting methods of the system.
The basic SCADA protocol is MODBUS, which is designed to send data to the master station even when the master interrogates the RTU station. MODBUS is based on a master-slave architecture known also as client-server architecture.
The protocol is designed to be used by Modicon in own PLCs. Now, it became a standard of communication for the industry and is currently the most widely used to connect all the industrial devices. The protocols contain extensions to operate over Transmission Control Protocol (TCP/IP).
In practice, it is suggested to avoid the security of Internet connection in order to reduce risks of cyber-attacks.
This protocol has the following features:
- It is an open protocol with available documentation;
- It can be quickly implemented (in few days, not months);
- It works with bits and bytes, so no particular requirements on implementation.
MODBUS allows the management of the network devices, which gives new features to the SCADA system related to flexibility, reliability and safety in operation at high performance.
Reference // SCADA Applications for Electric Power System by Naser Mahdavi Tabatabaei, Ali Jafari Aghbolaghi