Off-grid power supply system 380V for relay protection

A reliable off-grid 380V power supply for relay protection can be implemented using inverter-based renewable energy sources with auxiliary DC/AC supply, designed to meet relay load and voltage dip standards.

System Overview

For off-grid applications, especially in relay protection systems, the power supply must ensure continuous operation even during faults or grid disturbances. Modern off-grid systems often rely on renewable energy sources (RES) such as photovoltaic (PV) panels and Battery Energy Storage Systems (BESS), connected through inverters to provide a stable 380V AC output for protection relays . Diesel generators can also be used as backup to maintain voltage stability.

Relay Load and Power Requirements

Protection relays typically have low power consumption, ranging from ≤10 W for single-function relays to ~30 W for multifunction relays with communication capabilities . The auxiliary power supply must be sized to handle the total relay load, including any surge currents during operation. For a 380V system, the DC bus or AC output must be designed to maintain voltage within IEC 61000-4-11 and IEC 61000-4-29 standards, ensuring immunity to voltage dips and interruptions .

Short-Circuit and Fault Considerations

In inverter-dominated off-grid systems, short-circuit currents are lower than in conventional synchronous generator systems, which can reduce relay sensitivity . To address this:

• Use real-time simulation to evaluate relay response under low short-circuit conditions.
• Implement adaptive or digital relays capable of detecting faults with reduced current magnitudes.
• Ensure the system can isolate faulty lines without affecting the rest of the network .

System Design Recommendations

1. Inverter Selection: Choose inverters capable of supplying stable 380V AC with low harmonic distortion and fast dynamic response.
2. Battery Sizing: Ensure BESS can supply the relay load during PV downtime or transient faults.
3. Auxiliary Supply: For relays without self-powering, provide an auxiliary AC or DC supply with sufficient capacity and voltage regulation.
4. Protection Coordination: Use directional, overcurrent, and distance relays coordinated for the off-grid network to ensure selective fault clearing .
5. Compliance and Testing: Verify the system against IEC standards for voltage dips, interruptions, and electromagnetic compatibility to ensure reliable relay operation .

Advanced Considerations

• Digital and AI-based relays can improve fault detection in low-inertia, inverter-dominated systems .
• Monitoring and metering can be integrated to track relay performance and system stability in real time .
• Redundancy: Consider dual inverters or parallel BESS to enhance reliability for critical relay protection. By carefully sizing the inverter, battery, and auxiliary supply, and by selecting relays compatible with low short-circuit currents, a robust off-grid 380V system can reliably support relay protection in renewable energy-based or isolated networks.