Capacitor Bank Harmonic Protection via Weighted RMS Calculation
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Solution Overview
Problem
Current methods for protecting power capacitor banks from harmonics fail to accurately measure harmonic components, leading to delayed disconnection and increased risk of damage or accidents, as they only measure fundamental waves and do not account for harmonic currents effectively.
Innovation Solution
A harmonic protection method that uses a CPU unit with current and voltage sampling, calculating the true RMS value of current through the capacitor bank by incorporating harmonic coefficients, allowing for real-time and accurate measurement and disconnection when the current exceeds a threshold.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If DFT algorithm is used to calculate harmonics, then harmonic components can be calculated, but the current RMS value measurement is inaccurate and protection cannot disconnect promptly
Solution Approach 1:
The patent changes the calculation parameters by introducing harmonic coefficients kn that account for the frequency-dependent resistance of capacitor banks. Instead of using a single RMS formula, the system calculates weighted RMS values for each harmonic component and combines them, transforming the measurement approach to achieve both accuracy and speed.
Solution Approach 2:
The patent segments the current measurement into fundamental wave components and harmonic components separately. Each component is measured and calculated independently using the DFT algorithm, then combined using the true RMS formula with harmonic coefficients, allowing accurate measurement without time delay.
2Reliability
If only fundamental waves are measured, then protection methods are simple, but harmonic components are not detected and protection is insufficient
Solution Approach 1:
The patent makes the protection device multi-functional by enabling it to measure both fundamental waves and harmonic components simultaneously. The same sampling and calculation system handles both types of measurements, allowing the device to provide comprehensive protection against different types of faults without requiring separate measurement systems.
Solution Approach 2:
The patent introduces harmonic coefficients kn as an intermediary parameter that bridges the gap between simple fundamental wave measurement and complex harmonic analysis. These coefficients serve as pre-calculated lookup values that simplify the real-time calculation process while enabling accurate harmonic component measurement.
3Measurement precision
If conventional protection methods are used, then device complexity is low, but measurement precision of harmonic currents is insufficient
Solution Approach 1:
The patent performs preliminary calculations of harmonic coefficients kn offline and stores them for real-time use. This preliminary action eliminates the need for complex real-time iterative calculations, allowing the system to achieve high measurement precision using simple lookup and multiplication operations during actual protection operations.
Data Source
AI summary
A harmonic protection method for a capacitor bank uses a harmonic protection apparatus, which includes a CPU unit, a current and voltage sampling unit, and a protection device. The CPU unit obtains effective value I1 of fundamental wave current of the capacitor bank, and effective value In of the n-th harmonic wave current of the capacitor bank, where n is harmonic order and ranges from 2 to ∞, and calculates the true effective value I of current of the capacitor bank byI=I12+∑n=2∞knIn2provided that kn=rn/r1, kn is the n-th harmonic coefficient, rn is power consumption equivalent resistance of the capacitor bank for the n-th harmonic, r1 is power consumption equivalent resistance of the capacitor bank at a fundamental frequency. When I≧1.3 IN, the CPU unit controls the protecting device to disconnect the capacitor bank, provided that IN is a threshold current of the capacitor bank.