Reactive Power Command Control for PWM Over-Modulation Limits
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Solution Overview
Problem
The increase in reactive power output by PV/ESS-PCSs can lead to a rise in grid voltage, causing over-modulation of PWM and an increase in harmonics, which may distort the voltage waveform and adversely affect connected equipment.
Innovation Solution
A reactive power control device that includes a voltage acquisition unit, modulation rate calculation unit, and reactive power command units to limit or compensate the reactive power command based on the modulation rate, preventing over-modulation and harmonic increase by controlling the reactive power command according to the PWM modulation rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the reactive power output of the PCS is increased, then the voltage of the impedances in the reactor components rises, but the grid voltage rises above a predetermined value causing over-modulation of the PWM
Solution Approach 1:
The control device monitors the grid voltage and PWM modulation rate in real-time, and when the modulation rate exceeds a predetermined threshold, the system automatically limits the reactive power command to prevent over-modulation. This closed-loop feedback mechanism ensures the PWM operates within reliable boundaries while maximizing reactive power output.
Solution Approach 2:
The reactive power command is dynamically adjusted based on the modulation rate. When the modulation rate is within acceptable limits, the full reactive power command is executed; when the modulation rate approaches dangerous levels, the reactive power command is automatically reduced. This dynamic adjustment allows the system to operate at maximum capacity when safe and prevents over-modulation when risks arise.
2Object-affected harmful factors
If the grid voltage continues to rise above a predetermined value, then a limiter operates to prevent the upper part of the sinusoidal waveform from rising above a certain level, but the waveform distorts into a trapezoidal shape increasing harmonics to the grid
Solution Approach 1:
The system proactively limits the reactive power command before the grid voltage rises to dangerous levels that would cause waveform distortion. By monitoring the modulation rate and preemptively adjusting the reactive power output, the system prevents the conditions that lead to trapezoidal waveform distortion and harmonic generation, rather than reacting after the distortion occurs.
Solution Approach 2:
The system applies partial limiting of the reactive power command only when necessary to maintain sinusoidal waveform integrity. Instead of continuously limiting reactive power output, the system applies just enough limitation to keep the modulation rate within safe boundaries, allowing maximum reactive power output while preventing the excessive action that would cause waveform distortion.
3Productivity
If the waveform of the voltage deforms and the PWM is over-modulated to increase harmonics, then the waveform of the current also deforms, but the PCS continues to operate as long as it is within the fault detection level
Solution Approach 1:
The control device continuously monitors the modulation rate and grid voltage, and when the modulation rate exceeds a predetermined threshold, the system automatically limits the reactive power command to prevent current waveform deformation. This real-time feedback control ensures the PCS operates continuously with clean waveforms, preventing harmful current deformation before it occurs.
Solution Approach 2:
The system proactively prevents current waveform deformation by limiting the reactive power command before over-modulation occurs. Instead of waiting for fault detection thresholds to be exceeded, the system takes preliminary action by monitoring modulation rate and adjusting reactive power output in advance, ensuring continuous operation with undistorted current waveforms.
Data Source
AI summary
A reactive power control device includes a voltage acquisition unit that detects a grid voltage and a DC voltage; a modulation rate calculation unit that calculates a modulation rate; a reactive power command acquisition unit that obtains a first reactive power command from a host device; a blind zone determination unit that determines whether the first reactive power command is at a blind zone level; a reactive power command calculation unit that limits or compensates the first reactive power command according to the modulation rate and calculates a second reactive power command when the blind zone determination unit determines that the first reactive power command is not at the blind zone level; and a reactive power command output unit that outputs the second reactive power command when the second reactive power command is calculated, and outputs the first reactive power command when the second reactive power command is not calculated.