Protective Relay Harmonic Unlocking for Faster Fault Response
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Solution Overview
Problem
Conventional protective relays malfunction due to inrush currents, leading to delayed operations during system faults, as they rely on second harmonic locking methods that are transient and cause false locking.
Innovation Solution
A protective relay device that calculates the fundamental and second harmonic components of differential currents, locks the protection signal when a specific ratio condition is met, and cancels the lock based on the rate of change in the second harmonic component within a defined time period.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If second harmonic locking method is used to prevent inrush current malfunction, then reliability is improved, but operation speed deteriorates due to transient false locking during system faults
Solution Approach 1:
The patent applies dynamics by making the locking state changeable based on real-time detection of second harmonic component trends. The locking is not static but dynamically adjusted according to whether the second harmonic content is increasing or decreasing, allowing the system to adapt its protection state based on the evolving fault condition rather than maintaining a fixed locking state throughout the inrush current duration
Solution Approach 2:
The patent changes the parameter being monitored from simply the presence of second harmonic components to the rate of change of second harmonic effective values. By detecting whether the second harmonic content is increasing or decreasing over time, the system can distinguish between inrush current (where harmonics persist or increase) and system faults (where harmonics transiently increase then decrease), enabling timely unlocking during faults while maintaining reliability during inrush current
2Speed
If operation matching is started during false locking to shorten operation time, then operation speed is improved, but reliability deteriorates because matching operates during incorrect conditions
Solution Approach 1:
The patent applies preliminary action by detecting the trend of second harmonic components (whether increasing or decreasing) before initiating operation matching. This preliminary detection of the harmonic trend allows the system to determine the appropriate timing for starting and canceling operation matching, ensuring that matching only operates under correct conditions while still achieving fast response during genuine system faults
Data Source
AI summary
A protective relay device includes a processor configured to: calculate a first effective value of a fundamental wave component of a differential current; calculate a second effective value of a second harmonic component of the differential current; determine whether a first condition is satisfied, the first condition being a condition that a ratio of the second effective value to the first effective value is equal to or more than a threshold value; perform a locking process for locking output of a protection signal, when the first condition is satisfied; and cancel the locking process based on a predetermined condition. When, within a first time period since a determination that the first condition is satisfied, a state in which the rate of change is less than a reference value continues for a second time period or longer, the processor outputs an unlock signal for canceling the locking process.