**Distributed Control System (DCS)**
DCS, the abbreviation for Distributed Control System, is also known as a distributed control system in the domestic automation industry. Unlike centralized control systems, DCS represents a new generation of computer control systems that have evolved from them. It is a multi-level computer system composed of a process control level and a process monitoring level, integrated with computer, communication, display, and control technologies. The core idea of DCS is decentralized control, centralized operation, hierarchical management, flexible configuration, and easy setup.
The backbone of DCS is its system network, which serves as the foundation and core of the entire system. Since the network plays a crucial role in the real-time performance, reliability, and scalability of DCS, manufacturers have carefully designed it. A DCS network must meet real-time requirements, ensuring that information is transmitted within a specific time limit. This time limit is determined based on the real-time needs of the controlled process. Therefore, the performance of the system network is not measured by data transfer rate (in bits per second), but by how reliably and quickly it can transmit required information.
Reliability is essential for DCS networks. Communication must remain uninterrupted at all times. Most manufacturers use dual bus, ring, or double star topologies to ensure this. To support scalability, the number of nodes on the network should be several times more than actually used. This allows for future expansion without overloading the network, ensuring both real-time performance and reliability. During operation, internet access and offline states may occur, especially at operator stations. The network must support online reconfiguration without disrupting normal operations.
**Key Features of DCS**
1. **High Reliability**
By distributing control functions across multiple computers, DCS uses a fault-tolerant design. If one computer fails, the rest of the system continues to function. Each computer performs simple tasks, often using dedicated hardware and software, which enhances overall reliability.
2. **Openness**
DCS adopts open, standardized, modular, and serialized designs. Computers communicate via LAN, making it easy to expand or modify the system. New components can be added or removed without affecting other parts of the system.
3. **Flexibility**
DCS allows users to configure software and hardware according to specific process needs. It supports various control algorithms and graphic displays, enabling the creation of tailored control systems.
4. **Easy Maintenance**
With small, single-function computers, maintenance is straightforward. Faulty components can be replaced online without stopping the entire system, minimizing downtime.
5. **Coordination**
Workstations share data through the network, enabling seamless coordination and optimization of the entire system.
6. **Comprehensive Control Functions**
DCS supports continuous, sequential, and batch control. It includes advanced control methods like cascade, feedforward, and predictive control. It can integrate various devices such as management computers, operator stations, and field control units.
**Detailed Application Examples of DCS**
In a power plant, the DCS system is divided into seven functional areas: 1# boiler, 2# boiler, 3# boiler, 1# steam turbine, 2# steam turbine, utility system, and electrical. Each area is further divided into groups based on the process, allowing precise control of each device.
For example:
- Each boiler has five functional groups: steam-water, air supply, flue gas, fuel, and coal system.
- Each steam turbine has three groups: water, heat recovery, and oil system.
- The deaeration and decompression system includes groups for feedwater pumps, desuperheating, and industrial water.
**System Hardware Configuration and Network**
The DCS system consists of engineering stations, operator stations, and field control stations. Three boilers, two turbines, and the utility and electrical systems are controlled through different stations. The system uses a UPS for power supply with redundancy. The system includes 7 control units, 13 I/O cabinets, 6 relay cabinets, 1 power distribution cabinet, 1 engineering station, and 8 operator stations.
**Engineer Station**
The engineer station, AW7001, manages the entire system. It handles program development, configuration, and database editing. It also sets software protection passwords to prevent unauthorized changes. The basic configuration includes a host, monitor, and Windows XP operating system.
**Operator Station**
There are 8 operator stations, each responsible for displaying operational data and allowing operators to monitor and control unit conditions. These stations are equipped with standard hardware and software to ensure reliable performance.
**Field Control Station (I/O Station)**
Field control stations include main control units, intelligent I/O modules, power supplies, and field buses. They are distributed and scalable, supporting real-time control and communication between the engineering and operator stations.
**Communication Network**
DCS uses redundant communication networks, including a monitoring network and a control network. Both are redundantly designed to ensure reliability and real-time performance. Any failure in the network does not disrupt the system.
**Number of Field I/O Signals**
The total number of I/O signals across three 130t/h boilers, two 25MW steam turbines, and auxiliary systems is 4256 points. After adding a 15% margin, the actual configuration is 5120 points.
**Basic Functions and Control Methods**
- **Data Acquisition System (DAS):** Collects and processes key signals, providing real-time data to operators.
- **Analog Control System (MCS):** Automates loop control, maintaining pressure, temperature, and other critical parameters.
- **Sequential Control System (SCS):** Controls auxiliary equipment in a specified sequence.
- **Furnace Safety Monitoring System (SFS):** Prevents furnace explosions through automatic control and monitoring.
- **Turbine Trip Protection System (EST):** Safely stops the turbine under emergency conditions.
**Installation and Commissioning**
The DCS system was installed on July 5, 2005, and commissioned on July 20. Units were tested and connected to the grid in August and September 2005.
**Debugging Steps**
The commissioning process included wiring inspection, power-on testing, signal checks, actuator calibration, and static interlock tests. Several technical issues were addressed, including actuator oscillation, control logic errors, and sensor signal filtering.
**Application Effect Analysis**
The use of DCS in power plants has significantly improved automation, reduced downtime, and enhanced operational efficiency. It provides effective tools for accident investigation and analysis, improving safety and reliability.
**Software Programming**
In software programming, the PLC is configured based on allocated I/O points, divided into 12 networks to complete control tasks.
**System Debugging**
A notebook computer with Siemens software is used to monitor and debug the PLC program. Online monitoring ensures smooth system operation.
Overall, DCS has become an essential tool in modern industrial control, offering a robust, flexible, and reliable solution for complex processes.
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