1. Electrical Circuit Overview
The generation, transmission, distribution, and use of electric power require a large number of electrical devices of various types to form the main power system. To ensure the main system supplies sufficient, qualified electric energy to users safely, stably, continuously, and reliably, the system's operating modes must be adjusted frequently and its operating conditions monitored at all times.
When a device fails, the fault should be isolated selectively and as quickly as possible to protect electrical equipment and the power system. These functions are performed by electrical equipment outside the main power circuit. Accordingly, electrical equipment is classified into primary devices and secondary devices according to their roles in power production.
2. Secondary Circuit Overview
Importance of secondary circuits: Secondary circuits are an important part of substation electrical wiring and provide a reliable basis for safe and economical operation of the power system. The power system consists of primary wiring and secondary wiring, which together form an inseparable whole. If the power system is likened to a human body, the primary wiring corresponds to bones and muscles, while the secondary wiring corresponds to the nervous system. Both must be in good condition to ensure normal operation of the power system. In modern highly automated grids, the role of secondary circuits is even more prominent.
Scope of secondary circuits: Generally includes control circuits, relay protection circuits, measuring circuits, signal circuits, and automatic device circuits. By AC or DC classification, they can be further divided into AC voltage and current circuits and DC logic circuits.
Power supply systems for operation: Consist of power equipment and the supply network, usually including DC power systems and AC power systems. Their main function is to provide working power and operating power for control, protection, and signaling equipment.
Signal circuits: Comprised of signal senders, indicator lights, and signal relays. Their function is to reflect the operating status of primary and secondary devices.
Measuring circuits (AC circuits): Composed of various measuring instruments and their related circuits. Their function is to indicate or record operating parameters of primary devices and systems so operators can understand system operation. They are also the main basis for analyzing power quality, calculating economic indicators, and assessing the operating condition of major equipment.
Relay protection and automatic device circuits: Consist of measuring circuits, comparison elements, logic sections, and execution sections. According to the operating status of primary devices and the system, they determine faults or abnormalities and automatically issue trip commands to selectively isolate faults while signaling appropriately.
Control circuits: Composed of control switches and the transmission and execution mechanisms for controlled objects (such as circuit breakers and isolators). Their function is to execute "close" and "open" operations on primary devices.
