Virtual Synchronous Inverter Control for Stable Grid Relay Connection
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Solution Overview
Problem
In the system interconnection inverter device, it is challenging to stably maintain the voltage of the commercial AC power system after the system interconnection relay is turned on, without an inrush current prevention circuit.
Innovation Solution
A power convertor that includes an inverter, a relay, and a controller, which converts DC voltage to AC voltage and selectively switches the connection state between the inverter and the AC power system. The controller uses measurement values of voltage and current to control the inverter and relay, employing a virtual synchronous generator controller to simulate the operation of a synchronous generator and stabilize the AC power system voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a current control method is performed after the system interconnection relay is turned on, then the inverter can operate in system interconnection mode, but it becomes difficult to stably maintain the voltage of the commercial system
Solution Approach 1:
The patent implements dynamic control mode switching capability, allowing the inverter to transition between self-sustained operation mode and system interconnection operation mode. The controller dynamically adjusts control parameters based on the operational state, enabling stable voltage maintenance during system interconnection by switching to appropriate control strategies that accommodate grid connection requirements while preserving voltage stability capabilities.
Solution Approach 2:
The patent changes control parameters dynamically based on operational mode. During self-sustained operation, the inverter uses voltage control parameters to maintain stable output. When transitioning to system interconnection mode, the controller adjusts parameters to match grid requirements while maintaining voltage stability through coordinated control of amplitude, phase, and frequency parameters.
2Device complexity
If the system interconnection relay is turned on without an inrush current prevention circuit, then the device structure is simplified, but a large inrush current flows to the capacitor constituting the LC filter
Solution Approach 1:
The patent applies preliminary action by performing self-sustained operation before system interconnection. During this preliminary phase, the inverter builds up voltage and synchronizes its output characteristics (amplitude, phase, frequency) with the commercial system. This preparatory operation ensures that when the relay closes, voltage differences are minimized, preventing large inrush currents to the LC filter capacitor while avoiding the need for additional inrush current prevention circuits.
Solution Approach 2:
The patent implements preliminary anti-action by pre-synchronizing the inverter output voltage with the commercial system voltage before connection. The controller adjusts the inverter's amplitude, phase, and frequency to match grid parameters during self-sustained operation, creating opposing conditions that prevent harmful inrush current when the relay closes. This preliminary synchronization counteracts the potential for current surges without requiring extra protection circuits.
3Stability of the object's composition
If voltage control is performed to maintain stable voltage, then the voltage stability is improved, but the inrush current suppression capability may be compromised
Solution Approach 1:
The patent performs voltage control and synchronization as a preliminary action before system interconnection. The controller maintains stable voltage during self-sustained operation and simultaneously adjusts amplitude, phase, and frequency to match commercial system parameters. This preliminary voltage stabilization and synchronization ensures that when the relay closes, both voltage stability is maintained and inrush current is suppressed, as the voltage difference at connection moment is minimized.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The power convertor effectively stabilizes the voltage of the AC power system while suppressing inrush current when interconnected with the AC power system, ensuring stable operation and preventing voltage instability.
Implementation Method 1
an inverter to convert DC power into AC power
Data Source
AI summary
A virtual synchronous generator controller included in a controller of a power convertor generates a first command value of a frequency of a second voltage and a second command value of a phase of the second voltage by making an active power value and an active power target value correspond respectively to electrical energy and mechanical energy of a motion equation of a rotor of a synchronous generator to simulate an operation of the synchronous generator. When a turn-on condition is satisfied, a determination unit switches a connection state from a non-conduction state to a conduction state by controlling a relay that causes the connection state between an inverter and an AC power system to be selectively switched between the conduction state and the non-conduction state.