Feeder Protection Solution with Microprocessor-Based Relays for Modern Smart Distribution Networks

1. Introduction and Core Challenges​
   The increasing integration of distributed energy resources (DERs) (such as PV and wind power) into distribution networks, coupled with rising user demands for power supply reliability and safety, poses severe challenges to traditional feeder protection schemes. This solution is designed to address the following three core challenges:

• ​Arc Flash Hazards:​​ Internal short circuits in equipment like switchgear can trigger highly destructive arc flashes, threatening equipment and personnel safety, which demands an extremely fast response from the protection system.
• ​High-Impedance Ground Faults:​​ Particularly single-phase ground faults occurring in rural areas or regions with high soil resistivity, characterized by low fault current, are difficult to detect reliably by traditional zero-sequence overcurrent protection, posing a risk of protection failure to operate.
• ​Impact of Distributed Energy Resources (DERs) Integration:​​ The integration of DERs alters the power flow direction and short-circuit current characteristics of distribution networks, potentially causing protection maloperation (false tripping) or failure to operate, and introducing the risk of unintentional islanding.

This solution, based on advanced microprocessor-based protective relays and integrating multiple innovative algorithms, provides comprehensive, rapid, and reliable feeder protection for modern distribution networks.

​2. Solution Details​
Our feeder protection relay adopts a modular design, integrating the following core protection functions to address the aforementioned challenges.

​2.1 Multi-Band Arc Flash Protection (AFP) Module​

• ​Technical Principle:​​ Employs a proprietary multi-band detection technology, simultaneously monitoring light intensity (via dedicated arc light sensors) and the rate of change of current (di/dt). A fault is confirmed as an arc flash only when both conditions – "intense arc light signal" AND "high-speed overcurrent characteristic (>10 kA/ms)" – are met (logical AND operation). This dual criterion effectively prevents maloperation caused by external light sources or switching overcurrents.
• ​Performance Advantage:​​ Features ultra-fast operating speeds, designed to minimize arc flash energy.
• ​Application Case:​​ After deployment in the medium-voltage distribution system of a large data center, this module achieved a total fault clearance time of less than 4 milliseconds, representing a speed increase of over three times compared to traditional current-only protection schemes, significantly reducing equipment damage risk.

​2.2 High-Sensitivity Low-Current Ground Fault Protection Module​

• ​Technical Principle:​​ Utilizes the zero-sequence admittance method. This method involves real-time, precise measurement of the system's zero-sequence voltage (3U₀) and zero-sequence current (3I₀), calculating the corresponding admittance value. This algorithm is relatively insensitive to variations in the system's capacitive ground fault current, effectively distinguishing between normal capacitive current and fault-induced resistive current, thereby accurately identifying high-impedance ground faults with resistance values up to 1 kΩ or higher.
• ​Performance Advantage:​​ Solves the issue of insufficient sensitivity in traditional protection schemes during faults through high transition resistance, greatly reducing risks of electric shock and fire.
• ​Application Case:​​ In a pilot project within a rural network (characterized by high capacitive ground fault current and uneven line insulation levels), the application of this technology increased the overall ground fault detection rate from 65% with traditional schemes to 92%, significantly enhancing power supply safety.

​2.3 Adaptive Anti-Islanding Protection Module​

• ​Technical Principle:​​ To address the islanding risk introduced by DER integration, this module combines passive and active detection methods.
• ​Passive Monitoring:​​ Continuously monitors abnormal parameters at the Point of Common Coupling (PCC), such as voltage frequency deviation (Δf > 0.5 Hz) and phase angle jump (Δφ > 10°).
• ​Active Determination:​​ When passive monitoring indicators exceed set thresholds, it incorporates active methods like Active Frequency Drift to rapidly confirm an islanding condition.
• ​Performance Advantage:​​ Ensures rapid disconnection of DERs within a very short time frame (< 200 ms, compliant with grid code requirements) after islanding occurs, preventing hazards to grid equipment and maintenance personnel from unintended islanded operation.
• ​Application Case:​​ Validated in a microgrid project containing multiple PV arrays, this anti-islanding module achieved an accuracy rate of 99.7%. It effectively prevents islanding while minimizing unnecessary trips caused by normal grid disturbances, thereby improving the utilization rate of distributed energy resources.

​3. Core Value Summary​
This microprocessor-based protection solution, by integrating multiple intelligent algorithms, achieves:

• ​Enhanced Safety:​​ Maximizes the protection of personnel and equipment through millisecond-level arc flash protection and ultra-high-sensitivity ground fault protection.
• ​High Reliability:​​ Effectively addresses the complexities introduced by DER integration, accurately identifying islanding conditions and high-impedance faults, eliminating protection "blind spots".
• ​Rapid Restoration:​​ Enables fast fault clearance, facilitating rapid network self-healing, reducing outage duration, and improving power supply reliability.