SCADA system
1.1. The significance of SCADA systems
SCADA is an abbreviation for Supervisory Control and Data Acquisition System, which is a data acquisition, monitoring and control system for production systems with geographically dispersed production units.
Understanding production conditions is fundamental to implementing scientific production. If production processes are geographically close, local control methods can be used, including on-site wiring, monitoring, and control. For complex processes, DCS systems are commonly used, although PLCs or specialized controllers are also employed. However, for production processes where the various stages are geographically dispersed—for example, several kilometers, tens of kilometers, hundreds of kilometers, or even thousands of kilometers apart—such as substations, natural gas pipelines, oil fields, and water supply networks, remote data acquisition, monitoring, and control systems, known as SCADA systems, have gradually been developed with technological advancements.
The difference between SCADA systems and other systems is that:
Wide distribution area
The distance between the main station and the controlled object is far
The monitoring terminal operates under harsh conditions.
Communication systems are complex and ever-changing.
The communication system does not guarantee reliable transmission.
1.2. Composition of SCADA System
The SCADA system mainly consists of three parts: the master station, the communication system, and the remote terminal unit.
The main station typically uses advanced computers with excellent graphics support. Currently, PCs and Windows operating systems are most common, although historically many systems used UNIX and XWINDOWS graphical interfaces. The number of branch stations a single main station can range from dozens to hundreds or even thousands.
Communication systems are very complex. Wired systems include audio cables, carrier cables, optical fibers, and power line carriers, while wireless systems include radio stations, satellites, and microwaves.
There are many types of remote terminal units (RTUs or TeleControls). Large systems consist of many cabinets, while small systems may be just a small box.
1.3. SCADA Master Station System
In the past, many well-known SCADA systems were based on the UNIX operating system family and the XWINDOW graphical interface.
With the development of computer systems, especially PCs, PCs and the operating systems running on them are playing an increasingly important role.
A large main site system typically includes the following:
The communication front-end system is primarily responsible for parsing various protocols and processing communication interface data, including data forwarding. It includes a front-end computer, serial port pool or modem pool, rack, surge protection measures, and network interfaces.
A real-time database system mainly consists of servers that run real-time databases.
The engineer's workstation is responsible for system configuration, screen creation, and various system maintenance tasks.
The production scheduling workstation is the main user of the monitoring system, displaying screens, browsing screens, and implementing various alarms.
Various monitoring workstations are mainly used for particularly large systems where a few people can no longer monitor them. In such cases, various monitoring workstations are set up as needed, and each workstation has personnel working on it.
The historical database server is the server where the SCADA system stores historical data.
Web servers are a popular trend in SCADA master systems nowadays. As long as users have browser software installed and are authorized, they can access the data they care about.
Upper-level application workstations are primarily used for mining real-time and historical data. In power systems, this includes applications such as power flow analysis, load forecasting, fault replay, grid stability analysis, and energy management. In the water supply industry, it includes pipeline pressure loss analysis, pipeline economic analysis, and pipeline leakage analysis. In oilfield engineering, it includes dynamometer card display, dynamometer card analysis, pump condition analysis, and dynamometer card production calculation.
As a SCADA master station system, a large system may have dozens or even hundreds of workstations and multiple servers. To ensure system reliability, a dual front-end system, a multi-server system, and two networks are used. However, a simple SCADA master station system may only have one computer running one set of software.
While most SCADA systems for different applications are similar or identical, each industry has its own characteristics and focuses on different aspects. For example, the power industry is very concerned with the voltage, current, and power of each line, while the water supply industry is less concerned with these data and more focused on pipeline pressure and flow rate. This leads to the development of industry-specific systems. General-purpose human-machine interfaces (HMIs) exist, but these generally cannot meet the specific needs of each industry.
1.4. SCADA Communication System
SCADA communication systems are incredibly diverse, with many communication methods that are unfamiliar to most people. Generally, they can be broadly categorized into two types: wired and wireless. However, with the rapid development of communication technology, network-based communication methods have also evolved rapidly. These methods are difficult to classify into either wired or wireless categories and are therefore treated as a separate category.
There are many wired transmission methods: audio cables, overhead open lines, carrier cables, coaxial cables, fiber optics, power line carriers, etc. Wired transmission is broadly divided into baseband transmission and modulation transmission. Baseband transmission transmits digital signals over the medium, which may involve signal transformation. Modulation transmission involves analog-to-digital conversion. Many media can be used for both baseband and modulation transmission.
Wireless methods mainly include: radio, microwave, satellite, optical fiber, sound waves, etc.
Network-based communication systems are built on a computer network, such as Frame Relay, ATM, or IP networks. This type of communication can be wired or wireless, and may even cross between wired and wireless connections multiple times. Its performance is significantly different from wired and wireless systems. For example, it doesn't need to consider bit errors, message size, or system topology, but network latency may be relatively high. Transmitting SCADA system data via GPRS or CDMA networks is a typical example.
1.5. Remote Terminal Unit of SCADA
The remote terminal unit of a SCADA system has a specific term, RTU. It is now referred to as TeleControl.
An RTU typically consists of a communication processing unit, a digital input acquisition unit, a pulse input acquisition unit, an analog input acquisition unit, an analog output unit, a digital output unit, and a pulse output unit. There are also other interface types, such as tap changers for power transformers, Gray code interfaces for meteorology, and BCD code interfaces for hydrology.
Long-distance communication systems are usually installed together with the RTU for easy wiring.
Modern RTUs, in addition to performing their own data acquisition and protocol processing, also need to handle interface and protocol conversion with various IED devices. Furthermore, the communication processing capabilities of RTUs are becoming increasingly powerful, while their data acquisition functions are gradually diminishing, being replaced by various IED devices.
The most significant development of RTUs in China's power system was the direct calculation of line voltage, current, active power, reactive power, power factor, frequency, harmonics, etc., using AC sampling algorithms. This replaced traditional power transmitters, reducing costs, wiring complexity, error loops, and improving accuracy. Today, devices integrating protection, remote control, and metering have emerged, while traditional remote control systems have largely become obsolete.
The author's knowledge is limited in other industries, so he cannot write much about them.
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