Off-Grid Inverter Control Using Virtual Generator Synchronization
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Solution Overview
Problem
In off-grid independent power supply systems, fluctuations in load cause frequency and voltage fluctuations in AC power generators, leading to potential disconnection of DC power supply devices like inverters.
Innovation Solution
A control device for the inverter that simulates a virtual power generator using rotor, governor, and AVR models to calculate and control active and reactive power, matching time constants with AC power generators, thereby stabilizing bus voltage frequency and preventing inverter disconnection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If the AC power generator performs rotation speed control by droop characteristics during off grid independent operation, then the system can operate autonomously, but the frequency of bus voltage tends to fluctuate due to load fluctuations
Solution Approach 1:
The inverter control device implements feedback control by monitoring the bus voltage frequency and adjusting the inverter's output frequency accordingly. The control device calculates the frequency deviation from the reference frequency and uses this information to generate a frequency command that stabilizes the bus voltage frequency, preventing inverter disconnection while maintaining autonomous operation.
Solution Approach 2:
The system dynamically changes the inverter's output frequency parameter based on the bus voltage frequency conditions. By adjusting the frequency command in response to load fluctuations, the inverter maintains synchronization with the AC power generator and prevents disconnection, resolving the contradiction between autonomous operation and frequency stability.
2Productivity
If the bus voltage frequency fluctuates due to load changes, then the AC power generator responds to load variations, but the inverter is likely to be disconnected from the linked state
Solution Approach 1:
The control device continuously monitors the bus voltage frequency and uses this feedback to adjust the inverter's output frequency. By comparing the reference frequency with the actual bus voltage frequency and generating appropriate frequency commands, the system maintains reliable inverter connection while responding to load changes.
Solution Approach 2:
The inverter proactively adjusts its output frequency in anticipation of frequency deviations by implementing frequency control based on bus voltage frequency monitoring. This preliminary action prevents disconnection by maintaining synchronization before complete frequency mismatch occurs.
Data Source
AI summary
A combined electric power generations' supply system includes an AC power generator that supplies power by off grid independent operation, a DC power supply device, an inverter that converts DC power output by the DC power supply device into AC power. The rotation calculation unit calculates a value relating to a rotation of a rotor when driving the virtual power generator according to an active power command based on a rotor model calculates a value relating to the rotation of the rotor of the virtual power generator and the active power command. The output determination unit determines values relating to an active power and a reactive power to be output to the inverter based on the calculated value relating to the rotation. The modulation control unit performs control of a pulse width modulation of the inverter based on the determined value relating to the active power and reactive power. A time constant relating to synchronization power of the rotor model matches a time constant relating to synchronization power with the AC power generator.