DC power supply system relay protection

DC power supplies for relay protection provide reliable auxiliary power to protective relays, typically ranging from 12 V to 250 V DC, ensuring proper operation during faults and system disturbances.

Overview

Protective relays are critical for monitoring electrical systems and issuing trip commands to circuit breakers during faults. They require a stable DC power supply to operate reliably, whether in substations, industrial plants, or laboratory environments . DC supplies for relays can be self-powered or auxiliary-powered, depending on the relay design and system configuration .

Voltage and Power Requirements

• Typical trigger voltages for DC protection relays range from 12 V to 415 V DC, with common auxiliary supply voltages being 24 V, 48 V, 110 V, 125 V, and 250 V DC .
• The power consumption of relays varies: simple overcurrent relays may require ≤10 W, while multifunction relays with communication and multiple protection functions can require up to 30 W .
• Laboratory or classroom DC supplies, such as the SEL-LPS, provide multiple outputs (e.g., two 24 V DC and two 125 V DC) that can be combined to achieve higher voltages like 48 V or 250 V DC .

Design Considerations

• Voltage stability and ripple: Relays require low-ripple DC to ensure accurate sensing and tripping. Linear regulators or low-dropout regulators (LDOs) are often used for regulated outputs .
• Protection features: DC supplies should include input transient protection (TVS), overload protection (eFuse), reverse polarity protection (ideal diode controller), and voltage supervision to prevent relay malfunction .
• Compliance with standards: Supplies must meet IEC61000-4-11 (AC) and IEC61000-4-29 (DC) for voltage dips and interruptions, especially for high-power relays .
• Auxiliary vs. self-powered: In substations without batteries, AC mains can feed an auxiliary DC supply. In battery-backed systems, DC supplies ensure relay operation during outages .

Implementation Examples

• Reference designs: TI's TIDA-010055 and 30-W ultra-wide range power supply designs provide practical guidance for generating 5 V, 12 V, or 24 V DC for protection relays using DC/DC converters and integrated FETs .
• Commercial solutions: SEL-LPS linear power supplies offer compact, benchtop solutions with multiple DC outputs suitable for lab or classroom relay testing .
• Industrial relays: Devices like the ELR-4MD DC Earth Leakage Relay or JY1000-C2 are designed for DC systems under 1000 V and include integrated monitoring and protection functions .

Summary

A reliable DC power supply is essential for protective relays to function correctly. Key factors include appropriate voltage selection, sufficient power capacity, low ripple, protection features, and compliance with standards. Depending on the application, options range from reference design DC/DC converters to commercial linear power supplies capable of supporting multiple relay outputs. Proper design ensures relays operate accurately during faults, maintaining system safety and reliability.