Harmonic Directional Overcurrent Relay for Microgrid Fault Detection
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Solution Overview
Problem
Microgrid protection schemes face challenges due to varying fault current magnitudes in grid-connected and islanded modes, low fault current contribution from inverter-based distributed generators (IBDGs), and the impracticality of existing directional relays, which complicates fault detection and coordination in microgrids with multiple IBDGs.
Innovation Solution
The system injects impedance-modulated harmonic currents from multiple IBDGs during fault conditions, measured by digital relays to create a selective and sensitive harmonic directional overcurrent relay (HDOCR) that decouples overcurrent functions for grid-connected and islanded modes, using inverse time-harmonic-current characteristics for fault detection and optimizing primary and backup relay settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing directional overcurrent relays are used for microgrid protection, then fault detection can be performed, but the relays fail to detect faults in islanded microgrids with IBDGs only due to low fault currents
Solution Approach 1:
The patent changes the parameter being measured from fundamental frequency current to harmonic frequency current. By detecting harmonic currents injected by IBDGs during faults, the protection scheme overcomes the limitation of low fundamental fault current magnitudes in islanded microgrids with IBDGs only.
Solution Approach 2:
The patent introduces harmonic current injection as an intermediary mechanism. IBDGs inject harmonic currents into the system during faults, and these harmonic currents serve as the mediator that enables fault detection without relying on high fundamental fault current magnitudes.
2Adaptability or versatility
If multiple IBDGs are used to provide distributed generation, then renewable energy integration is improved, but fault current contribution from each IBDG remains low and coordination becomes difficult
Solution Approach 1:
The patent segments the protection function by frequency. Different IBDGs inject harmonic currents at different harmonic frequencies, and the protection system detects faults by analyzing these segmented frequency components. This allows multiple IBDGs to operate independently while maintaining coordinated protection.
Solution Approach 2:
The patent introduces asymmetry in the harmonic injection strategy where each IBDG injects a unique harmonic frequency. This asymmetric approach enables the protection system to identify and coordinate between multiple IBDGs by detecting their distinct frequency signatures during fault conditions.
3Measurement precision
If communication-based protection approaches are used, then fault detection accuracy is improved, but the extensive communication network required is expensive and often unavailable in microgrids
Solution Approach 1:
The patent implements a self-service protection scheme where each IBDG autonomously injects harmonic currents and each protection device independently detects faults by analyzing locally measured harmonic currents. This eliminates the need for extensive communication networks while maintaining accurate fault detection through self-contained measurement and decision-making at each device.
Data Source
AI summary
A system and method for controlling microgrids composed of inverter-based distributed generation (IBDG) units. This includes a method using multiple IBDGs to inject impedance-modulated harmonic currents during fault conditions, with each IBDG injecting a unique, differentiable harmonic (i.e., non-fundamental) order from neighboring IBDGs. The method also involves using an inverse time-harmonic-current characteristic to detect faults by locally measuring the harmonic currents injected by IBDGs. A harmonic directional overcurrent relay is also used for fault detection.