Compensating Voltage Synchronization for Current Differential Protection
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Solution Overview
Problem
Current synchronization methods for current differential protection in power systems, such as the echo method and GPS, are unreliable due to channel asymmetry and GPS signal unreliability, and existing load current-based methods fail to handle light load or no load conditions, multi-terminal systems, and synchronization errors greater than half a cycle.
Innovation Solution
A synchronization method based on compensating voltages, which measures and calculates compensating voltages at selected points on the transmission line, detects synchronization errors by comparing these voltages, and distinguishes between errors caused by channel asymmetry and line parameter changes, allowing for accurate synchronization across multiple terminals and in varying load conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If echo method is used for synchronization, then data synchronization can be achieved, but reliability deteriorates due to channel asymmetry
Solution Approach 1:
The patent introduces compensating voltage as an intermediary physical quantity to establish synchronization relationship. Instead of directly comparing current measurements from different terminals (which requires symmetric channels), the system calculates compensating voltages at a selected point on the transmission line from current and voltage measurements at each terminal. The synchronization is then achieved by comparing these compensating voltages, which are less sensitive to channel asymmetry.
Solution Approach 2:
The patent changes the synchronization parameter from direct current measurement comparison to compensating voltage comparison. By transforming the synchronization basis from current phasors (which are directly affected by channel asymmetry) to compensating voltages (which incorporate line impedance and provide a more robust reference), the system achieves better reliability under asymmetric channel conditions.
2Reliability
If GPS synchronization is used, then absolute synchronized time can be provided, but reliability deteriorates due to GPS signal unreliability
Solution Approach 1:
The patent enables the protection system to synchronize itself using local measurements of current and voltage at each terminal. Instead of relying on external GPS signals, each terminal independently calculates its own compensating voltage and compares it with neighboring terminals to determine and correct synchronization errors. This self-synchronization capability eliminates dependence on unreliable external GPS signals.
3Reliability
If load current based synchronization is used, then external synchronization dependency is reduced, but measurement precision deteriorates in light load or no load conditions
Solution Approach 1:
The patent introduces compensating voltage as an intermediary that combines both voltage and current measurements. The compensating voltage calculation incorporates line impedance parameters and terminal measurements, creating a more robust synchronization reference than current alone. This intermediary quantity maintains measurement precision even when load current is small, because it leverages the voltage measurement which remains stable.
4Adaptability or versatility
If load current based synchronization is used, then two-terminal systems can be synchronized, but adaptability deteriorates for multi-terminal systems
Solution Approach 1:
The patent creates a universal synchronization method that works for transmission lines with any number of terminals. The compensating voltage calculation and comparison approach is generalizable - for multi-terminal systems, each terminal calculates its compensating voltage relative to a reference terminal, and the same basic principle applies regardless of whether there are 2, 3, or more terminals. This universal approach significantly improves adaptability while managing complexity through systematic extension of the two-terminal methodology.
Data Source
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AI summary
The present invention provides a synchronization method for current differential protection comprises the following steps: selecting a point on the transmission line protected by the current differential protection; measuring the current and the voltage of each of the terminals of said transmission line; calculating the compensating voltage at the selected point respectively according to the measured current and the voltage of the each terminal; detecting and calculating the synchronization error by comparing all the compensating voltages. The current and voltage of each of the terminals are measured before a fault or after a fault, and can be measured in vectors or sampling values. The current and voltage of each of the terminals are phase quantities or sequence quantities. The point can be selected from any points of transmission line; preferably the middle or the ends of the transmission line or the T connected point of the multi-terminal transmission lines. The synchronization method further comprises a step of distinguishing the calculated synchronization error caused by a serious line parameter change or an asymmetrical channel switching by calculating the changing speed of the phase angle difference or the wave shift of said compensating voltage.