In the field of substation automation, with the development of intelligent electrical equipment, especially the emergence of intelligent switches and optoelectronic instrument transformers as mechatronic devices, substation automation technology has entered a new digital stage. In high-voltage and extra-high-voltage substations, the I/O units of protection devices, measurement and control devices, fault recorders, and other automatic devices, such as A/D conversion, optical isolation components, and control operation circuits, will be separated and become part of intelligent primary equipment. Conversely, the digital sensors and digital control circuits of intelligent primary equipment replace the I/O parts of conventional relay protection and measurement devices. In medium and low voltage substations, protection and monitoring devices are miniaturized and compactly installed on switchgear cabinets, achieving mechatronic design of the substation.
The structure of a digital substation automation system can be physically divided into two categories: intelligent primary equipment and networked secondary equipment. Logically, it can be divided into three levels. According to the IEC 61850 communication protocol draft, these three levels are called the "process level", "bay level", and "station level".
1. Process level
The process level is the interface between primary and secondary equipment, or it refers to the intelligent part of intelligent electrical equipment. The main functions of the process level are divided into three categories: (1) real-time electrical quantity detection for power operation; (2) status parameter detection of operating equipment; (3) execution and driving of operation control.
(1) Real-time electrical quantity detection for power operation.
Similar to traditional functions, it mainly involves the detection of current, voltage, phase, and harmonic components. The transformer comprehensive test bench is mainly used for no-load and load characteristic tests on power transformers and distribution transformers, and can also perform frequency-doubled induction and power-frequency withstand voltage tests on the tested transformer. Other electrical quantities such as active power, reactive power, and electric energy can be calculated by the bay level equipment. Compared with conventional methods, the difference is that traditional electromagnetic current transformers and voltage transformers are replaced by photoelectric current transformers and photoelectric voltage transformers; collecting traditional analog quantities is replaced by directly collecting digital quantities. The advantages are strong anti-interference performance, good insulation and anti-saturation characteristics, and miniaturization and compactness of switching devices.
(2) Online detection and statistics of operating equipment status parameters.
The equipment requiring status parameter detection in substations mainly includes transformers, circuit breakers, disconnectors, busbars, capacitors, reactors, and DC power systems. The content of online detection mainly includes data such as temperature, pressure, density, insulation, mechanical characteristics, and working status.
(3) Execution and driving of operation control.
The execution and driving of operation control include transformer tap changer adjustment control, capacitor and reactor switching control, circuit breaker and disconnector closing and opening control, and DC power supply charging and discharging control. The control execution and driving at the process level are mostly passive, that is, they act according to upper-level control commands, such as receiving trip commands from bay level protection devices, switching commands for voltage and reactive power control, and remote opening and closing commands for circuit breakers. When executing control commands, it has intelligence, can judge the authenticity and rationality of commands, and can control the accuracy of upcoming actions. It can enable circuit breakers to close with phase synchronization and open with phase selection, achieving closing and opening of circuit breakers at selected phase angles, requiring operation time to be limited within specified parameters. For example, synchronous operation of vacuum switches requires that the switch contacts close at zero voltage and open at zero current.
2. Bay level
The main functions of bay level equipment are: (1) summarizing real-time data information from the process level of this bay; (2) implementing protection and control functions for primary equipment; (3) implementing operation interlocking functions for this bay; (4) implementing synchronization and other control functions for operations; (5) having priority control over data collection, statistical calculation, and issuance of control commands; (6) having communication functions that connect upper and lower levels, that is, completing network communication with both the process level and station level at high speed. When necessary, the upper and lower network interfaces should have dual-port full-duplex mode to improve the redundancy of information channels and ensure the reliability of network communication.
3. Station level
The main tasks of the station level are: (1) summarizing real-time data information of the entire station through two-level high-speed networks, continuously refreshing the real-time database, and logging into the historical database on time; (2) sending relevant data information to the dispatch or control center according to established protocols; (3) receiving control commands from the dispatch or control center and transferring them to the bay level and process level for execution; (4) having online programmable station-wide operation interlocking control functions; (5) having (or being equipped with) local monitoring and human-machine interface functions in the station, such as display, operation, printing, alarm, and even multimedia functions like images and sound; (6) having functions for online maintenance, online configuration, and online parameter modification of bay level and process level equipment; (7) having (or being equipped with) functions for automatic fault analysis and operation training of the substation.
