Grid Conductance Estimation for Inverter Control
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Solution Overview
Problem
Existing methods for estimating grid conductance and susceptance in low-voltage grids are inefficient due to high computational requirements and temporary degradation of power quality, as they often rely on injecting test signals or assume constant grid impedance, which is not accurate for low-voltage grids.
Innovation Solution
A method for operating a voltage source inverter (VSI) connected to a power grid that estimates grid conductance and susceptance based on active and reactive power exchange without injecting test signals, using a negative feedback loop to correct voltage magnitude and phase, thereby decoupling active and reactive power control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If test signals are injected into the grid to estimate grid impedance, then measurement precision is improved, but power quality is degraded temporarily
Solution Approach 1:
The system uses the inverter's own normal operating signals (voltage and current measurements during regular operation) to estimate grid impedance, eliminating the need for external test signals. The control unit calculates conductance and susceptance values by processing measurements already taken for power control, thereby achieving impedance estimation without injecting additional disturbances into the grid.
Solution Approach 2:
The method extracts grid impedance information from the inverter's normal operating measurements rather than requiring separate test signal injections. By utilizing the existing voltage and current data already captured during regular inverter operation, the system separates the impedance estimation function from the power control function, achieving both goals without compromising power quality.
2Measurement precision
If discrete Fourier transformation and optimal control are used to adapt power rotation angle, then measurement precision is improved, but computational power requirement increases significantly
Solution Approach 1:
The system uses simple algebraic calculations based on instantaneous voltage and power measurements rather than computationally intensive Fourier transformations. The control unit computes conductance G and susceptance B using direct mathematical relationships from measured values, providing accurate impedance estimation with minimal computational resources and fast response time.
Solution Approach 2:
The patent replaces complex signal processing mechanisms (Fourier transformation, optimal control algorithms) with simpler algebraic calculations. By using direct mathematical relationships between measured voltage, power, and the unknown impedance parameters, the system achieves the same measurement precision with significantly reduced computational complexity and faster execution.
3Measurement precision
If high frequency tone is superimposed on AC current for impedance monitoring, then measurement precision is improved, but power quality is degraded and response is delayed
Solution Approach 1:
The system utilizes the inverter's own operating current and voltage measurements to detect grid impedance changes, rather than superimposing external test tones. The control unit continuously monitors the relationship between the inverter's output current and the resulting voltage at the point of common coupling, extracting impedance information from normal operation without adding harmful frequency components to the grid.
4Device complexity
If grid impedance is assumed constant for detection methods, then device complexity is reduced, but measurement precision deteriorates in low-voltage grids
Solution Approach 1:
The system dynamically adapts to varying grid impedance conditions by continuously estimating conductance and susceptance values during operation. Rather than assuming constant impedance, the control unit recalculates the grid parameters based on current operating conditions, allowing accurate control in low-voltage grids where impedance varies with location and loading conditions.
Data Source
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AI summary
A method for operating a voltage source inverter (VSI) connected to a power grid having a grid voltage. The method comprises controlling a magnitude of a VSI voltage and a phase between the VSI voltage and the grid voltage, determining active and reactive powers exchanged between the VSI and the grid, estimating one or more values indicative of the ratio of a grid conductance and of a grid susceptance based on the active and reactive powers. The method further comprises determining an active power deviation between the determined active power and an active power setpoint as well as a reactive power deviation between the determined reactive power and a reactive power setpoint, mapping the active and reactive power deviations to a phase deviation and to a magnitude deviation, the mapping being dependent on the estimated one or more values indicative of the ratio of the grid conductance and the grid susceptance,andcorrecting the magnitude of the VSI voltage and the phase between the VSI voltage and the grid voltage, based on the phase and magnitude deviations.