ODES EMC-Enhanced Auxiliary Relay: Solution for Long-Cable Interference

TonyZhang
Sep 11
2 min read

Discovery of the Problem

At the start of our company’s journey, our first product was an optocoupler isolation terminal, designed to isolate and convert switch signals. For example, a 220 V DC input signal was isolated and converted to 24 V DC output for protection devices.

However, as domestic power plants expanded in scale, site environments grew increasingly complex. Even after isolation, signals were still vulnerable to interference. In 2003, at a major thermal power plant, unexplained nuisance trips in control boxes repeatedly disrupted operations.

As part of the expert task force, we investigated the issue. Using disturbance recorders, we eventually captured the key waveform that revealed the interference source—providing the breakthrough needed.

Root Cause Analysis

The investigation showed that nuisance operations occurred not during actual commands, but when adjacent bays were operated. Field inspection confirmed the cable run between the control box and control room was about 1.5 km—far beyond normal design limits.

This extended cable acted as a distributed capacitance. During switching operations, high-frequency components charged this capacitance. Even with open contacts, parasitic capacitance created induced voltages across relay circuits, leading to false pickups.

Solution Development

Filtering Low-Energy Interference

Voltage thresholds and dropout delays were introduced to suppress transient low-level disturbances.

Addressing High-Energy Interference

Waveform analysis showed large disturbances concentrated at the initial stage, with rapid decay. Following system requirements, relays must reliably operate at ≥80% Un. To balance disturbance immunity with compatibility, we set:

Pickup threshold upper limit: 75% Un
Pickup threshold lower limit: 55% Un

This design was validated in prototypes and accepted by the expert group for field trials.

Borrowing Protection Relay Concepts

Drawing on experience with electromagnetic differential relays, we applied a sub-threshold power criterion to reinforce disturbance immunity in the auxiliary relay design.

Verification and Improvement

The first prototypes were bulky and consumed ~25 W, but successfully eliminated nuisance trips. Over two years of operation, no further false trips occurred. The expert group endorsed the design parameters, which later influenced industry standards.

Subsequent optimization reduced power consumption and size while maintaining high EMC performance. Parameter settings were later validated against IEC models, confirming their scientific basis.

ODES EMC-enhanced Auxiliary Relay on Project Sites

Industry Application and Value

Today, the ODES EMC-enhanced auxiliary relay is widely deployed in high-voltage breaker control circuits, recognized as a proven solution to long-cable capacitive interference.

This journey reflects our values: