summary:
Using a PLC to implement the RTU function for subway substation automation can well meet the requirements of "three remotes" (remote control, remote monitoring, and remote signal transmission). This system uses a Modicon Quantum series PLC to implement the RTU function for substation automation.
1 Introduction
The subway's power supply system provides electricity for subway operations. Both subway trains and auxiliary facilities rely on electrical energy. Subway power is generally drawn from the city's power grid, transmitted or transformed through the city's primary power system and the subway's power supply system, and then supplied to various subway equipment at an appropriate voltage level.
The subway system fully adopts substation automation design. Due to the large number of substations and their numerous devices, coupled with their comprehensive functions, the amount of information exchange is substantial, requiring fast and accurate information transmission. In the substation integrated automation system, the monitoring system is crucial, as it is the key to ensuring the reliable operation of the entire system.
Substation automation systems, after several generations of development, have entered the era of distributed control systems. Telemetry, remote signaling, remote command execution, and relay protection functions are all independently performed by field units, which then transmit this information to the back-end computer system via communication systems. The overall functions of substation automation are handled by the back-end computer system.
This system integrates microprocessor-based protection and monitoring devices in substations into a unified automation system using computer networks and modern communication technologies. It eliminates traditional control panels, meters, and other conventional equipment, thus saving on control cables and reducing the size of the control room.
2. Composition of Subway Substation Automation System
In the design of this subway substation automation system, a hierarchical distributed functional partitioning scheme is adopted. The system is vertically divided into three layers: the substation management layer, the network communication layer, and the bay equipment layer. This hierarchical design facilitates the division of system functions and provides a clear and concise structure. The system adopts a centralized management and decentralized deployment model, with each lower-level monitoring unit installed in its respective switchgear, and the upper-level monitoring unit monitoring and controlling it through the substation's internal communication network. The substation automation system needs to monitor and collect data from equipment such as the 35kV AC microprocessor protection and control device, the DC 1500kV traction system microprocessor protection and control device, the 380/220V monitoring device, the temperature control devices for transformers and rectifiers, and the DC/AC power supply panels.
Programmable logic controller (PLC) technology has matured considerably over the decades. With a complete range of models and diverse functions, it is widely used in various fields of industrial control. Using PLCs to implement the RTU function for subway substation automation can well meet the requirements of "three remote" (remote control, remote monitoring, and remote operation). This system uses Modicon Quantum series PLCs to implement the RTU function for substation automation. Quantum features a modular and scalable architecture for real-time control of industrial and manufacturing processes. PLCs of different capacities (large, medium, and small) can be selected according to the voltage level and number of control points in the substation.
With the increasing sophistication of local protection devices, remote control and remote signaling functions can be achieved through communication with these devices. In some special cases, DI, DO, and AI modules are used to implement remote control and remote signaling. The PLC's DI module is used for remote signaling, the PLC's DO module for remote control, the PLC's AI module for telemetry, and the PLC's communication function for communication with the microcomputer protection unit. The various modules of the PLC can easily realize the basic "three remote" functions (remote control, remote signaling, and remote communication).
3. Design of Subway Substation Automation System
3.1 System Structure
The main monitoring unit within the substation management unit utilizes a programmable logic controller (PLC). The CPU module employs an 80586 processor with a 66MHz clock speed and 2MB of memory, and is equipped with RAM for storing data, adjustable parameters, and software. The PLC automation system implements the RTU function design for substation automation using flash memory. It is capable of self-diagnosing the CPU and I/O.
The power supply module employs a redundant configuration. The system uses two DC power inputs to ensure uninterrupted and safe operation even if one power source fails, thus improving system reliability. The communication module uses Modbus+. The system structure is shown in Figure 1.
Figure 1 System Structure Diagram
The microcomputer protection devices in the bay layer are divided into several groups via RS-485 bus and connected to the Modbus communication port of the bridge. The bridge collects data and sends this data to the main monitoring unit PLC via the MB+ network.
The system's main monitoring unit can be programmed with different protocols via programmable bridges to meet the interface requirements of various intelligent devices. The MODBUS bridge NW-BM85C002 MB+ bridge/multiplexer has four communication ports for communication with intelligent devices in the interval layer. The bridge converts MODBUS protocol data and establishes network communication with the PLC via the MB+ network. Simultaneously, the central signal panel is equipped with a programmable bridge NW-BM85C485, which connects to the PLC via the MB+ network. Each programmable bridge has four RS-485 ports with programmable communication protocols. In this solution, two of these ports are programmed to communicate with the central control center via IEC-60870-7-101.
The system's network communication layer communicates upwards with the control center via the RS-422 interface of the programmable bridge, using the IEC60870-5-101 international standard protocol. Downwards, the network communication layer communicates with intelligent devices such as microcomputer-based integrated protection and control units within the main substation's switchgear or protection panels via the RS-422 interface of the bridge, using the MODBUS standard protocol. This meets the technical requirements for control, measurement, and protection in the substation's integrated automation system. Communication with intelligent devices and the control center via the bridge, with protocol conversion handled by the bridge, reduces the CPU module load of the PLC and improves system reliability.
Equipped with an LCD display, this substation serves as a human-machine interface for monitoring, software maintenance, equipment debugging, and station control layer operations. The LCD displays and controls substation data. It shows in real-time, in Chinese characters, all faults, warning signals, and the operating status of each microcomputer-based integrated protection and control unit within the substation. It also displays the content and time of event changes. When multiple fault signals occur simultaneously, the LCD alarm device displays various information sequentially within the substation's time resolution range, in the order of oldest and newest, and can store the information. Operators can select displays using buttons, and when necessary, can use these buttons for centralized substation control of switches.
A "local-remote" control switching device is installed in the central signal panel to facilitate system operation, enabling switching and interlocking between local and remote control modes. This facilitates hierarchical control and management in substation control.
The system employs a redundant power supply configuration, with two DC power inputs to ensure that the system can still operate safely and without disturbance even if one power source fails, thereby improving the system's reliability.
3.2 Open and scalable design
It can interconnect with other companies' related IEDs that meet the corresponding standard protocols (profibus, spabus, modbus, etc.) for information exchange. Taking into full account factors such as substation expansion and renovation, the bay-level equipment is based on a modular and standardized design, which can be configured arbitrarily according to requirements, making the substation-level equipment setup flexible.
The network communication layer design takes into account the interface design of fieldbuses such as industrial Ethernet, CAN, 422, and Modbus+, which can fully meet the real-time and reliability requirements of high-volume real-time data transmission.
3.3 Software Design
In terms of PLC software, the PLC executes programs using both cyclic scanning and interrupt methods. To complete all RTU functions, the PLC uses cyclic scanning to communicate with each bay-level protection unit. Through the Modbus bus, it reads telemetry and telesignaling information from each protection unit and simultaneously performs setting operations on each intelligent protection device via bus communication, realizing remote control of the switches. This system uses the FBD method in the Quantum series PLC's accompanying Concept programming software to configure the PLC and realize the three remote functions of substation automation.
Figure 2 shows the configuration of the remote control function. By using appropriate combinations of function blocks, you can achieve the desired function. These function blocks include standard function blocks provided by the FFB library in the Concept software, or you can define your own unique function blocks.
There are two ways to implement remote signaling. One is through communication. When a substation device changes position, the PLC communicates with the intelligent protection device to read the position change information, which is then sent to the control center. The other is through a DI module. By connecting to the device's position relay, the PLC's DI module can sense the device's position change information.
Figure 2. Configuration of remote control function
Telemetry can be implemented in two ways. One is through communication, where the PLC communicates with the intelligent protection device to acquire telemetry information collected by the device in real time, essentially allowing the protection device to perform field-level data acquisition. The other is through an AI module, where the PLC itself performs field telemetry acquisition and stores the acquired data in RAM. The bridge then uses the telemetry information in RAM as secondary data to communicate with the control center in real time.
The message receiving and analysis program in the bridge analyzes the messages sent from the control center. If the analysis determines that it is a remote control message, it parses the message, writes the obtained remote control object information into the PLC, and the PLC program communicates with the intelligent protection device to complete the remote control function.
3.4 System Functions and Features
Substation automation implements real-time control and data acquisition of various equipment in the substation, realizing microcomputer control, monitoring, logic interlocking, microcomputer measurement, and inter-station tripping functions for various equipment.
Characteristics of substation automation systems:
(1) Perfect self-testing function: In addition to monitoring each unit through communication, the protection and monitoring modules in each unit have strong self-testing functions. At the same time, the two monitor each other and will alarm in time if an abnormality occurs, thus improving the reliability of system operation.
(2) The status information of switches and disconnectors adopts normally open and normally closed dual-position contacts, and the validity is judged by software.
(3) The monitoring system uses PLC instead of traditional RTU, and the data collected by each intelligent module is uploaded to the communication controller through fieldbus.
(4) The conventional light signboard was cancelled and a computer-simulated light signboard was adopted, and it was displayed in a layered mode according to different voltage levels.
(5) Simplify the design of anti-misoperation interlocking. Hard wiring is used to realize the interlocking function between important equipment. The integrated automation software has software logic judgment function, but considering the existing operation and maintenance experience, interlocking is generally not performed in the background software.
(6) Transient displacement signals are processed by software and a self-holding method is adopted. The signals will not disappear without manual confirmation.
4. Conclusion
In actual operation, the dual-channel design of the bridge and control center greatly facilitates operation and maintenance. Because the switching is automatic via software, it overcomes the shortcomings of manual channel switching in imported systems. The channel status is determined by software, significantly improving the timeliness of problem detection. The probability of both channels failing simultaneously is not very high; in actual operation, there are instances where the backup channel runs for extended periods, thus providing maintenance personnel with ample time to investigate problems.
The PLC hardware, designed for industrial-grade reliability, is highly reliable in actual operation, rarely experiencing crashes. Its reliability far exceeds that of general-purpose computers using Windows operating systems, perfectly meeting the requirements of power supply monitoring. Since its delivery, the PLC has not experienced any hardware failures, highlighting its adaptability to the humid and high-temperature environment of the subway. The modular design also makes system maintenance and replacement more convenient.
One area that needs improvement is the communication aspect. Because the design does not employ fiber optic communication modules, the devices cannot effectively isolate high-voltage electricity that may leak in during insulation maintenance or cable damage, potentially causing high-voltage breakdowns and unnecessary losses. Future designs will focus on improving high-voltage isolation to effectively avoid this problem.
