0 Introduction
Microcomputer monitoring systems for substations are a hot topic in power construction. They enable real-time acquisition and display of the operating status of various switching equipment and the actual parameters of each circuit in the power distribution system. They also allow for remote control and automatic interlocking of equipment, parameter limit alarms, accident reporting, event logging, and remote equipment configuration. These features greatly improve the operation and management of power distribution equipment, providing conditions for unmanned operation. Furthermore, they significantly enhance the safety of the power distribution system, improve power quality, save energy consumption, and reduce the operating costs of power distribution equipment. Therefore, the integrated application of substation systems has become a trend.
Substation microcomputer monitoring systems generally adopt a decentralized, layered, and distributed structure. The entire system can be divided into a three-layer structure: the field layer (composed of intelligent communicable devices), the network management layer (composed of communication management units installed in various distribution centers or functional sections), and the system management layer (composed of the monitoring center computer system). This paper analyzes the fieldbus-type low-voltage switchgear used in the field layer and explores how to better utilize components and design the network system for low-voltage switchgear.
1. Research on Electrical Control Methods The components used in fieldbus-type low-voltage switchgear are generally intelligent electronic devices. These devices can independently perform the power distribution, control, and protection functions of traditional low-voltage equipment. Furthermore, because electronic devices can easily achieve accurate measurement, calculation, and complex control logic, they typically possess more precise and powerful functions. Most importantly, they can not only function as independent devices but also transmit collected signals to the outside and receive external commands to perform operations, thus providing the foundation for distributed monitoring systems. The fieldbus referred to here is a communication protocol used between industrial devices; it is an open communication protocol adopted and supported by numerous manufacturers. The promotion of fieldbus has made interconnection, interchangeability, and interoperability of equipment from different manufacturers possible, and the use of open fieldbuses has gained widespread support.
In power distribution monitoring systems, the field-level equipment has a decentralized, distributed structure, maintaining the original electrical system design's equipment bay divisions, and employing unitized and modular design, with each bay operating independently. Each device can independently complete its electrical functions without relying on the network, while centralized control equipment within the network enables coordination and interlocking control between devices, as well as system monitoring functions. Specifically, the following comparison can be made:
(1) Traditional electrical control methods. Traditional low-voltage switchgear mainly uses circuit breakers, contactors, thermal relays, fuses, various control relays, various master control devices, transformers, electrical instruments, etc. to realize power distribution, control, protection and monitoring functions. Such products are equipped with dozens or more analog pointer instruments, and a large number of relays bring great inconvenience to production, storage and maintenance. Moreover, they are mainly operated manually and cannot be managed by computers, making it difficult to realize more complex control logic.
(2) Electrical control method of fieldbus type low-voltage switchgear. Fieldbus type low-voltage switchgear completely abandons the management concept and model of traditional switchgear that uses analog instruments and relays as monitoring and control devices. Instead, it uses a large number of intelligent components (intelligent network instruments, network I/O, intelligent circuit breakers, intelligent distributed functional modules, etc.) and a very small number of ordinary control devices. It is a new type of network integrated fully distributed control equipment.
Bus-type low-voltage switchgear can monitor electrical parameters such as current, voltage, active power, reactive power, power factor, frequency, and electricity consumption; monitor and control the opening and closing status and fault information of circuit breakers; remotely set and adjust the operating parameters of circuit breakers; and provide comprehensive remote monitoring software, thereby realizing the "three remotes" or "four remotes" functions.
2. Research on Bus Network Technology 2.1 Characteristics and Current Status of Fieldbus Technology and Electrical Applications Fieldbus refers to a digital, serial, multi-point communication data bus between field devices installed in manufacturing or process areas and automatic control devices in the control room. Fieldbus technology is an industrial field-level network communication technology that realizes digital communication between field-level devices. Using fieldbus technology, automation can be achieved quickly, simply, and at low cost. The emergence of fieldbus technology has broken the original structure and model of control systems, making control systems more decentralized, networked, and intelligent. Its most obvious characteristics are: digital signal transmission, unified and fully open standards, and independent control units. In fieldbus, various analog signals are converted into standard digital signals on-site, using fully digital, bidirectional transmission. Transmission is less susceptible to interference from the surrounding environment and overcomes the attenuation problem of analog signals during transmission, thus ensuring good data integrity, reliability, and anti-interference capabilities, and extending signal transmission distance. The system is fully open from top to bottom, using open communication protocols. Devices from different manufacturers that adhere to the same communication standard can interconnect and exchange information. Users can flexibly select fieldbus products from different manufacturers to form actual control systems, achieving optimal system integration. Control functions are completely decentralized to the field, with highly intelligent field devices performing specific functions such as data acquisition and compensation, computation and control, equipment self-verification and self-diagnosis. This transforms the centralized and decentralized DCS system architecture into a new fully distributed architecture, simplifying the control system structure.
Currently, there are more than 40 types of fieldbuses used in industrial control systems. The main ones used in low-voltage electrical products are: the Profibus-DP protocol from Siemens (Germany); the DeviceNet protocol from Rockwell Automation (USA); the CC-Link protocol from Mitsubishi Electric (Japan); the Modbus protocol from Schneider Electric (France); and the ASI protocol (mainly for transmitting switching signals).
Each fieldbus has its advantages and disadvantages. Most experts believe that, given the current technological level, there is no absolutely advanced fieldbus. In terms of technical characteristics, Profibus and CC-Link are similar; both offer high communication speeds but have complex protocols, higher costs, and relatively simple master-slave communication methods that are less efficient. DeviceNet offers lower communication speeds than the above two, but its protocol is relatively simple, lower cost, and its communication methods are relatively "flexible." Modbus has a simpler and lower cost protocol, but its functionality is relatively weaker. ASI is mainly suitable for switching signal transmission and communication in low-voltage control appliances.
Based on the characteristics of low-voltage electrical products and the actual situation of my country's low-voltage electrical industry and market, the development of my country's communicable low-voltage electrical appliances mainly follows four bus types: DeviceNet, Profibus, Modbus, and Asi.
2.2 Fieldbus Networking Technology Analysis To achieve remote monitoring of intelligent field devices, an intelligent network must be formed, which typically consists of a monitoring computer or PLC and power distribution automation monitoring software.
Monitoring computers or PLCs can be categorized into front-end units and monitoring units based on their installation location. The front-end unit is optional depending on the system design; it can be directly installed on the bus cabinet or at the installation site, allowing users to directly view and modify various parameters on-site. The front-end unit connects to the communicable components within the bus cabinet. Data from these components is collected by the front-end unit and can connect to other monitoring computers via TCP/IP networks, serving as a data collection and relay mechanism. Considering the on-site working environment and reliability requirements, the front-end unit must be capable of operating in harsh environments. Front-end units typically use industrial control computers with high protection levels, capable of operating in the high-temperature environment of substations.
The monitoring unit is installed in the duty room or in various off-site remote monitoring scenarios. In cases where a front-end server is already installed, the monitoring unit does not need to be directly connected to the various communicable devices in the bus cabinet; the user only needs to connect the front-end server and the monitoring unit via a local area network. If needed, multiple monitoring units can be installed to monitor the bus cabinet simultaneously. While commonly used PCs can also be used as monitoring units, considering system reliability, the author believes that using a brand-name PC is preferable.
The power distribution automation monitoring software uses configuration software, which can easily form a human-machine interface according to the field configuration and various bus standards and user needs. Through intelligent devices connected to the bus cabinet, users can monitor the operation of the entire power distribution system in real time, anytime, anywhere. For example, the Riyear-PowerNet power distribution monitoring system software from Changshu Switchgear Manufacturing Co., Ltd., through high-efficiency data acquisition, can remotely display the field's working status, trend curves, historical records, operation and alarm records, etc. This system supports bus protocols such as Profibus-DP, Device Net, and Modbus, and has the following main functions:
(1) Configuration function. The human-machine interface of the system can be configured arbitrarily. The main wiring diagram of the power supply and distribution system, the circuit and layout diagram of the complete set of switch equipment can be drawn directly on the computer screen, the components on the main wiring diagram can be directly operated, and various operating statuses, reports and graphs can be viewed.
(2) Control functions. It can set, read, and control system equipment parameters. Such as circuit breaker opening and closing operations, protection characteristic settings, and remote measurement of current, voltage, power, and power factor.
(3) Alarm function. When an alarm occurs in the system, an alarm screen will automatically pop up, an alarm sound will be emitted, and the user can be notified by telephone or other means.
(4) Debugging and diagnostic functions. It can debug and diagnose the equipment, making it easy for users to quickly troubleshoot equipment faults.
(5) Report and graph functions. It can generate various daily reports, monthly reports, annual reports, and historical trend graphs according to user needs, which facilitates user analysis and processing, and allows for reasonable scheduling of the commissioning and operation of various production equipment.
(6) User management function. It can set different permissions for different users and has corresponding security checks.
(7) System expansion functions. The universal OPC interface can exchange data with other programs, generate HTML files, and be remotely monitored via the Internet or LAN.
2.3 Relationship between Fieldbus-type Low-voltage Switchgear and Components and Systems Fieldbus-type low-voltage switchgear uses standard, open fieldbus communication technology to connect low-voltage intelligent devices (intelligent circuit breakers, intelligent protection and control devices, intelligent measurement and control devices, and intelligent instruments, etc.) with communication functions via communication cables. Through a back-end main control system (PLC or PC system), it achieves functions such as data acquisition and remote monitoring of the entire power distribution system's operating status, adjustment of protection parameters, control ("four remote" functions), system data storage and analysis, and equipment operation management. As the field-level fieldbus-type low-voltage switchgear operates in harsh environments such as high temperatures and strong electromagnetic interference in the power distribution room, to ensure stable system operation, it is necessary to consider how to improve the reliability and anti-interference capabilities of components and communication systems. This is mainly reflected in the following aspects:
(1) The components integrate protection and measurement and control functions. Each protection and monitoring device can independently complete measurement, protection and control functions. The realization of its functions does not depend on the communication layer and the substation monitoring system layer. The selection of intelligent electrical components is mainly based on the overall design requirements of the project. Considering that the application is in unattended locations, its high reliability has become a more important indicator. At present, relatively excellent domestic components include various switchgear such as CM1, CM2, CM1Z, CW1, CW2 and CW3 from Changshu Switchgear Manufacturing Co., Ltd.
(2) The selection of electrical components must fully consider the fieldbus communication interface specifications (physical interface, protocol, transmission rate, data volume, etc.) while selecting equipment functions, quality stability, and appearance. For components with serialization requirements, products from the same manufacturer should be selected as much as possible. Since the power distribution system needs to accurately integrate various components, different products from the same manufacturer have a unified equipment layer description, which can be quickly configured and realize user goals. If network instruments, I/O modules, and intelligent electrical components, such as circuit breakers and motor protectors, are all produced by the same manufacturer, then the integration speed and efficiency will be the highest. Currently, Changshu Switchgear Manufacturing Co., Ltd. is the only company in China that can design a complete series of systems and components. It has not only developed host computer system software, but also CGZ1 bus-type low-voltage switchgear, and has electrical components such as CM1 and CW1, which are highly regarded in the industry.
(3) High anti-interference capability is required. Bus-type low-voltage switchgear used in a mixed application of high-voltage and low-voltage circuits must have electromagnetic compatibility capabilities that meet the IEC 61000 standard. Therefore, this factor should be considered in the design. In addition to selecting various electrical components with good electromagnetic compatibility performance, in order to improve the operational reliability and system maintainability of the communication network, a dedicated communication cable laying channel should be set up, away from the high-voltage environment. In the design, an independent communication circuit compartment can be set up, which is completely isolated from the main bus and high-current main circuit by metal partitions, thereby effectively suppressing the interference of the magnetic field generated by the change of main circuit current on the communication control system.
3 Conclusion Fieldbus-type low-voltage switchgear is an important foundation for realizing power distribution monitoring systems. It adopts standard and open fieldbus communication technology to closely integrate highly intelligent low-voltage devices with communication functions with computer networks, making the underlying devices of the monitoring system standardized, modularized, and simplified in terms of communication management. This makes the management of power distribution stations more convenient and practical, and is an ideal upgrade and replacement product for traditional power distribution switchgear.
