DC power supply for relay protection

A reliable DC power supply for protection relays should provide stable voltage, handle typical relay loads (10–30 W), and include protective measures like flyback diodes or series diodes to safeguard against inductive and capacitive spikes.Power Supply RequirementsProtection relays can be self-powered or auxiliary powered. For auxiliary-powered relays, the DC supply must match the relay's voltage rating, typically 24 V, 48 V, 125 V, or 250 V DC, with power ratings ranging from ≤10 W for simple overcurrent relays to ~30 W for multifunction relays with communication features . The supply should comply with IEC61000-4-11 (AC) and IEC61000-4-29 (DC) standards for voltage dips and interruptions, ensuring reliable operation under transient conditions . Commercial options like the SEL-LPS Linear Power Supply provide multiple outputs (24 Vdc, 48 Vdc, 125 Vdc, 250 Vdc) and regulated low-voltage outputs (+5 Vdc, ±15 Vdc) suitable for laboratory or classroom setups . Reference designs, such as the 30-W Ultra-Wide Range Power Supply, can handle both AC and DC inputs and are optimized for numerical protection relays .Protective Measures for DC Relay CircuitsRelays with DC coils are inductive loads, which can generate back EMF when de-energized. To protect both the relay and the power supply:Flyback diode (freewheeling diode): Placed across the relay coil to safely dissipate the energy from the collapsing magnetic field .RC snubber: Can be used across the coil to reduce voltage spikes and suppress oscillations, especially for sensitive or high-speed relays .Series diode: Protects the power supply from reverse currents caused by capacitive loads or batteries connected to the relay circuit .NTC thermistor: Limits inrush current on the contact side when switching capacitive or resistive loads .MOVs or varistors: Can be added across contacts to suppress voltage surges in AC output circuits .Load ConsiderationsInductive and capacitive loads can feed energy back into the DC supply, potentially damaging it. Proper sizing of bulk capacitors on the DC bus is necessary for higher-power relays, and the supply should be rated to handle transient currents without oscillation . For laboratory or benchtop setups, compact linear supplies with multiple outputs provide flexibility and safe operation for various relay types .SummaryWhen designing or selecting a DC power supply for relay protection:Match the voltage and power rating to the relay type (10–30 W typical).Ensure compliance with voltage dip and interruption standards.Include protective components like flyback diodes, series diodes, RC snubbers, and MOVs to safeguard both the relay and the supply.Consider commercial or reference designs for reliable, tested solutions, especially in critical substation or industrial applications . This approach ensures stable operation, longevity of the relay, and protection of the DC power supply under normal and transient conditions.