Grid-Connected Inverter Angle Mirroring for EMC Harmonic Limits
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Solution Overview
Problem
Grid-connected inverters often fail to meet EMC standards for Total Harmonic Content (THC) and Partial Weighted Harmonic Content (PWHC) in generative operation, particularly when synchronized with line voltage, due to current patterns that do not align with regulatory limits.
Innovation Solution
A method for operating grid-connected inverters that involves switching circuit breakers at defined ignition and extinction angles based on mirrored motor operation angles, adjusted by constant angular values, to optimize current profiles in generative operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the inverter switches are synchronized with line voltage in generative operation, then the switching pattern is stable and simple to control, but the current has a block form that exceeds THC and PWHC limits
Solution Approach 1:
The patent applies inversion by mirroring the switching angles from motor operation to generative operation. Instead of directly synchronizing with line voltage (which causes block-form current), the extinction angle in generative operation is set to 180° minus the ignition angle from motor operation, and the ignition angle is set to 180° minus the extinction angle from motor operation. This inverted approach transforms the harmful block-form current into a sinusoidal shape that meets EMC standards.
Solution Approach 2:
The patent changes the switching angle parameters in generative operation based on measurements from motor operation. By adjusting the ignition and extinction angles using the relationships αig_gen = 180° - αex_mot and αex_gen = 180° - αig_mot, the current waveform parameters are transformed from block form to sinusoidal form, reducing harmonic content while maintaining stable control.
2Stability of the object's composition
If the inverter uses a 2-level rectifier with fundamental frequency switching and large direct current link, then the voltage can be kept constant, but the current draw rate causes resonances in the grid
Solution Approach 1:
The patent applies preliminary action by pre-determining the switching angles based on motor operation characteristics before applying them to generative operation. The ignition and extinction angles are calculated in advance using the mirrored relationships, which prepares the switching pattern to avoid high current draw rates and subsequent grid resonances while maintaining voltage constancy through the large DC link.
3Object-generated harmful factors
If the circuit breaker switches on before commutation of line voltages to decrease current draw rate, then resonances are reduced, but the switching timing becomes complex to optimize
Solution Approach 1:
The patent uses copying by replicating the switching angle pattern from motor operation in generative operation. Instead of developing a complex new switching strategy, the extinction and ignition angles in generative mode are copied from motor mode using the simple mirrored relationships (180° minus the respective motor operation angles). This copying approach simplifies the optimization process while effectively reducing current draw rates and grid resonances.
Data Source
AI summary
A grid-connected inverter is connected to an electric machine. At least one circuit breaker of the inverter is switched on at a firing angle αig for which αig=180°αa b+αc_ab applies when the inverter operates as a generator, where αab is the angle at which the grid current drops to zero for the last time within half a period when the inverter operates as a motor, and where αc_ab is a constant angle. The at least one circuit breaker of the inverter is switched off at an extinction angle αex for which αex=180°−αan+αc_an applies, where αan is the angle at which the grid current when the inverter operates as a motor rises from zero for the last time before dropping to zero at the angle αab, and where αc_an is a constant angle.