Distributed Power Supply Stabilization via Third Harmonic Injection
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Solution Overview
Problem
Distributed power supply systems, such as photovoltaic and wind power systems, face instability when experiencing momentary voltage fluctuations, leading to phase differences and excessive currents, causing disconnection from the power system.
Innovation Solution
Incorporating a three-phase voltage command signal generation unit with a three-phase fundamental wave command signal generation unit and a third harmonic command signal generation unit to generate voltage command signals that match the system voltage, along with a filter circuit and phase adjustment units to maintain phase synchrony and prevent overcurrent.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional PLL computation unit with PI regulation is used for frequency and phase control, then the system can maintain stable operation under normal conditions, but the system becomes unstable during momentary voltage fluctuations due to integration delays causing phase differences and excessive currents
Solution Approach 1:
The system performs preliminary detection of momentary voltage fluctuations before they cause significant phase deviations. By detecting the fluctuation early and preemptively adjusting the output voltage phase and frequency, the system avoids the integration delay problem of conventional PI regulators, preventing excessive currents and maintaining stability throughout the fluctuation event
Solution Approach 2:
The system continuously monitors system voltage and compares it against reference values to detect momentary fluctuations. This feedback mechanism triggers immediate corrective actions by adjusting the output voltage characteristics, creating a closed-loop control that responds faster than conventional PI-based PLL systems and eliminates phase synchronization issues during transients
2Duration of action of moving object
If the inverter circuit operates during momentary voltage fluctuations, then power supply continuity is maintained, but phase differences occur between system voltage and inverter output voltage causing excessive currents
Solution Approach 1:
The system dynamically changes the output voltage parameters (phase angle and frequency) in response to detected momentary fluctuations. By adjusting these parameters to match the distorted system voltage characteristics, the inverter maintains synchronization without generating excessive currents, enabling continuous operation during transient events that would normally cause disconnection
3Reliability
If multiple distributed power supply systems disconnect simultaneously during system errors, then individual system protection is achieved, but system-wide instability occurs due to collective disconnection
Solution Approach 1:
The system detects momentary fluctuations that precede full system failures and takes preliminary protective actions by adjusting its output characteristics. This preliminary anti-action prevents the system from reaching a state that would trigger complete disconnection, allowing it to remain online and contribute to grid support during transient events, thereby preventing collective disconnection scenarios
Data Source
AI summary
A distributed power supply system in which no simultaneous disconnection from the system occurs when a system voltage momentarily drops is provided. It includes an inverter circuit that converts a direct current power generated by a direct current power supply and that supplies the alternating current power to an alternating current power supply power system, and an inverter control circuit for carrying out PWM control of the inverter circuit, wherein the inverter control circuit includes a three-phase voltage command signal generation unit, that is configured of a three-phase fundamental wave signal generation unit that generates three-phase fundamental wave signals from two phase components of voltage detected by a voltage detector, and a third harmonic signal generation unit that adds together third harmonic components of respective phases, having a predetermined amplitude, generated based on the three-phase fundamental wave signals.