The anti-interference design of the secondary circuits in the distribution cabinet is a crucial aspect of ensuring the stable operation of the power system. Its core lies in using technical means to suppress the impact of external interference on relay protection, measuring instruments, and other equipment, preventing malfunctions that could lead to power outages or equipment damage. As the "nervous system" of the distribution cabinet, the secondary circuits are responsible for transmitting control signals and monitoring electrical parameters. However, their signal amplitude is small, and their anti-interference capability is weak, making them susceptible to electromagnetic interference, electrostatic coupling, and ground potential differences. Therefore, an anti-interference system needs to be constructed from multiple dimensions, including shielding, grounding, wiring, and component selection.
Shielding design is the primary measure to suppress electromagnetic interference. Steel-tape armored or copper-braided shielded cables should be preferred for secondary circuits. The shielding layer must be reliably grounded at both ends to create a Faraday cage effect, blocking the intrusion of external electromagnetic fields. For high-frequency interference, the shielding layer of non-magnetic materials (such as aluminum) can utilize the eddy current effect to cancel the interfering magnetic field; while for low-frequency interference, high-permeability materials (such as silicon steel) are needed to guide the magnetic flux around the circuit. In addition, the distribution cabinet should be made of special steel plate, and a conductive rubber strip should be installed between the cabinet door and the cabinet body to ensure shielding continuity and prevent electromagnetic wave leakage from gaps.
A reasonable grounding system is the foundation of interference immunity. Secondary circuit grounding must follow the "single-point grounding" principle, meaning only one grounding point is allowed in the same electrical connection to prevent circulating current interference caused by ground potential differences. The secondary windings of current transformers and voltage transformers must be grounded separately, and the grounding points should be connected to the substation grounding grid to avoid ground grid current entering the circuit due to multiple grounding points. For long-distance cables, additional grounding points can be added in the middle, but it must be ensured that the potential of each grounding point is consistent. At the same time, copper busbars or copper cables with sufficient cross-sectional area should be used for grounding to reduce grounding impedance and common-mode interference voltage.
Optimized cable routing can significantly reduce interference coupling. Secondary circuit cables should be laid perpendicular to the primary high-voltage busbar to reduce the length of parallel sections; if parallel arrangement is necessary, the distance between them should be increased to reduce the mutual inductance coefficient. Cables for equipment with different sensitivities should be laid in groups. Low-level signal lines (such as protection device signal lines) and high-level power lines (such as circuit breaker operating circuits) must be arranged separately to avoid crossing. Furthermore, AC and DC circuits, as well as high-voltage and low-voltage circuits, must not share the same cable to prevent AC/DC interference or high-voltage induced voltage from entering the low-voltage system.
Component selection and circuit design must consider anti-interference requirements. Relay protection devices should be microprocessor-based protection devices with high anti-interference capabilities, and anti-interference capacitors should be installed at their power inputs to suppress high-frequency noise. For output relays susceptible to interference, diodes or RC snubber circuits can be connected in parallel to prevent maloperation caused by operational overvoltage. Simultaneously, circuit design should avoid the influence of distributed capacitance in long cables. For example, shortening cable length, selecting cables with low distributed capacitance, or adding intermediate relays to segment signal transmission can reduce induced voltage accumulation.
Suppression of electrostatic coupling interference needs to be controlled at the source. Electrostatic coupling between primary and secondary equipment is mainly achieved through distributed capacitance; interference voltage can be reduced by increasing the coupling impedance. Specific measures include: rationally arranging the relative positions of primary busbars and secondary cables to reduce parallel laying lengths; adding anti-interference capacitors at appropriate locations in the secondary circuit (such as the power inlet of protection devices and the secondary side of instrument transformers) to reduce the coupling impedance between interference sources and the circuit. Furthermore, connecting natural shielding elements within the substation (such as reinforcing bars and metal components in cable trenches) to the grounding grid can further enhance electrostatic shielding.
Protection against operational overvoltages requires targeted design. Circuit breaker opening and closing, contactor contact disconnection, and other operations generate broadband interference, with high-frequency components easily entering the secondary circuit through capacitive coupling. To suppress this interference, diodes, varistors, or RC absorption circuits can be connected in parallel across the relay coil to dissipate overvoltage energy to the grounding grid. Simultaneously, avoid sharing the same cable between relay circuits without anti-interference measures and circuits connected to electronic equipment to prevent interference from being conducted through the common circuit.
Dynamic monitoring and maintenance are long-term guarantees for interference immunity. During distribution cabinet operation, the insulation performance, grounding reliability, and shielding integrity of the secondary circuits must be checked regularly to promptly identify and address issues such as loose grounding wires and damaged cables. For equipment already in operation, anti-interference performance can be optimized by adding filters and adjusting circuit parameters. For example, adding an RC filter module to a circuit with severe induced voltage can effectively eliminate residual voltage on the opening and closing coils and prevent contactor malfunction.
