Line Protection Relay Microcomputer-Based Line Protection Solution

1. Abstract and Background​
   With the increasing complexity of power grid structures—particularly the development of ultra-high-voltage direct current (UHVDC) transmission, large-scale integration of renewable energy, and multiple parallel transmission lines—the performance requirements for transmission line protection have reached unprecedented levels. The core challenge lies in balancing two critical demands: ensuring extremely high-speed operation of protection devices during faults to maintain system stability, while also guaranteeing strong selectivity to prevent unnecessary tripping and fault escalation. This contradiction is especially pronounced in complex grid configurations such as parallel double-circuit lines, where traditional single-ended protection principles face significant limitations.

This solution leverages advanced microcomputer-based protection technology, integrating three core modules: power frequency variation distance protection, double-terminal traveling wave fault location, and adaptive auto-reclosing strategies. It aims to comprehensively enhance the reliability, speed, and intelligence of line protection, providing critical support for building a robust and smart grid.

​2. Core Challenge Analysis​

• ​Conflict between speed and selectivity: Traditional protection schemes often require delayed operation to ensure selectivity, conflicting with the need for rapid fault clearance to maintain system stability.
• ​Accurate fault location in parallel double-circuit lines: Mutual inductance between double-circuit lines complicates fault characteristics, significantly reducing the accuracy of traditional fault location methods and hindering fault identification and power restoration.
• ​Uncertainty introduced by renewable energy integration: The integration of wind and solar power plants alters short-circuit current levels and characteristics, potentially causing protection maloperation or failure. Additionally, their output fluctuations challenge the success rate of auto-reclosing strategies.

​3. Core Technologies of the Solution​

​3.1 Power Frequency Variation Distance Protection (ΔZ Protection)​​

• ​Technical Principle: This technology is unaffected by load current during normal system operation. It calculates fault impedance using only the power frequency variations in voltage and current generated at the instant of a fault. With high starting thresholds, it is inherently directional, highly selective, and insensitive to system oscillations and transition resistance.
• ​Performance Advantages:
• Ultra-high-speed operation: Extremely fast response, with typical operation times of less than 10ms.
• High reliability: Effectively avoids misoperation due to load current influences.
• ​Application Case: In an ±800kV UHVDC transmission line, this technology reduced the total fault clearance time (protection operation + circuit breaker tripping) for near-end faults to within 80ms, significantly enhancing the transient stability of the UHVDC system.

​3.2 Double-Terminal Traveling Wave Fault Location​

• ​Technical Principle: A fault generates traveling waves that propagate toward both ends of the line. Using high-precision GPS/BDS synchronized clocks, protection devices at both ends precisely record the arrival times of the initial current traveling waves (t1 and t2). The fault location is accurately calculated using the formula L = (v * Δt) / 2, where v is the wave velocity and Δt = |t1 - t2|.
• ​Performance Advantages:
• Ultra-high accuracy: Fault location is largely unaffected by line mutual inductance, system operation mode, transition resistance, or current transformer (CT) saturation.
• Parameter-independent: Does not rely on line impedance parameters, eliminating errors caused by inaccurate parameters in traditional impedance-based methods.
• ​Application Case: Deployment on a 500kV double-circuit line on the same tower reduced the fault location error to less than 200 meters, improving accuracy by over 80% compared to traditional single-ended impedance-based methods. This greatly facilitates rapid fault identification and maintenance.

​3.3 Adaptive Auto-Reclosing Strategy​

• ​Technical Principle: The microcomputer-based protection device intelligently distinguishes fault types (transient or permanent):
1. ​Transient faults: After tripping, the line dielectric strength self-restores. The device detects insulation recovery and promptly issues a reclosing command.
2. ​Permanent faults: The device detects the persistent fault and blocks reclosing to prevent secondary circuit breaker tripping, ensuring equipment safety.
   Additionally, the strategy dynamically adjusts the dead time of auto-reclosing based on real-time system conditions (e.g., renewable energy output share) to match system recovery characteristics.
• ​Performance Advantages:

• Increased success rate: Avoids reclosing on permanent faults, significantly improving the success rate of auto-reclosing and power supply reliability.
• Reduced impact: Prevents unnecessary secondary shocks to the system, safeguarding equipment.

• ​Application Case: Implementation on a critical wind farm outgoing line increased the auto-reclosing success rate from 72% to 93%, effectively reducing wind turbine disconnections caused by transient line faults.

​4. Solution Value Summary​
This integrated microcomputer-based protection solution delivers core value to customers through the synergistic application of its three key technologies:

1. ​Enhanced system stability: Ultra-high-speed protection isolates faults rapidly, securing critical time to maintain grid stability.
2. ​Improved power supply reliability: Intelligent adaptive auto-reclosing maximizes power restoration, reducing outage duration and losses.
3. ​Increased operational efficiency: High-precision fault location transforms maintenance from "line patrolling" to "point inspection," significantly reducing排查 costs and time.
4. ​Adaptability to new power systems: Its exceptional performance makes it highly suitable for complex modern grid scenarios, including UHVDC, renewable energy integration, and multi-circuit lines.