one, Inverter grid connection impact mechanism
At the moment a photovoltaic power generation system is connected to the grid, the grid-connected inverter often experiences a short-term inrush current. This phenomenon directly affects the stability of power equipment and the power quality of the grid , severely impacting the reliability of inverter products . The core mechanism of the grid-connection inrush current stems from the instantaneous deviation in amplitude, phase, or frequency between the output voltage and the grid voltage during the inverter's synchronization process with the grid , as well as the rapid charging and discharging characteristics of the DC-side filter capacitor . Furthermore, the high-frequency switching action of power electronic switching devices and the coupling effect with grid impedance parameters further exacerbate the pulse characteristics of the transient current .
The inherent limitations in shock resistance of existing devices have become a key bottleneck restricting system reliability —data shows that approximately 32% of inverter failures originate from device-level damage caused by grid connection shocks . Against this backdrop, breakthroughs in the shock resistance performance of RCMU devices are providing the industry with a better solution.
II. Impact Failure Phenomena and Model Analysis
2.1 Description of actual phenomena of grid connection impact
The waveform of the inrush current at the moment of grid connection of the inverter, which was actually captured at the client, is shown in the figure below.
Actual inrush current waveform
The CH4 waveform represents the grid-connected inrush current, exhibiting as a damped resonant current. This current peaks at 77A and resonant at 1.852MHz. This inrush current has an extremely high di/dt at the moment of oscillation initiation. According to Faraday's law of electromagnetic induction, this drastic electric field transformation will induce a strong magnetic field in the conductor, which will then be converted into an induced voltage, damaging the components in the product.
In the formula , i1 is the inrush current , V2 is the induced voltage in the device , and M is the equivalent mutual inductance between the grid connection line and the device's internal circuitry. Devices with magnetic components and those closer to the grid connection line have greater mutual inductance and are most susceptible to damage under grid connection surges, such as RCMUs and relays. User feedback indicates that this surge frequently damages relays and RCMUs on the inverter output side.
2.2 Analysis of Equivalent Circuit Model During Grid Connection Impact
Consider the circuit conditions on the inverter output side and the grid at the moment of grid connection. At the instant of grid connection, the control system's response speed is insufficient to adjust the inverter output voltage to be in phase with the grid; a phase difference exists between the two, equivalent to applying a step impulse to the line. The inverter's output filter and parasitic inductances in the line form an LC series structure, generating a resonant current under the impulse. The equivalent circuit at the moment of grid connection impulse consists of L, C, and an equivalent impulse voltage source.
Circuit analysis and equivalent model
If a voltage source generates a step signal with a value of V at time 0, then the inrush current is:
This formula represents a gradually decaying sinusoidal resonant signal with a resonant frequency of and a peak value of in the resonant circuit . The inrush current expression derived from this equivalent model matches the actual waveform. Using this expression to assess the worst-case scenario of the inrush current, the strongest impact occurs when the step voltage amplitude is at its maximum, corresponding to a complete phase misalignment between the inverter and the grid, approximately 440V. The resonant frequency is the same as the resonant frequency of the equivalent LC circuit, and the larger the C/L ratio, the larger the peak value of the inrush current.
Therefore, by applying a step impact to a specific LC series circuit, the inrush current at the moment of inverter grid connection can be simulated.
three, Impact Failure Simulation and Impact Resistance Testing
3.1 Circuit Simulation Evaluation
First, a circuit model was built in LTspice for simulation to attempt to model the inrush current. Taking the switch on the voltage source V1 side as S1 and the switch between the LC circuits as S2, the circuit operates in two modes. Initially, S1 is on and S2 is off, charging C1 to 400V; then, the moment S2 turns on, S1 turns off, and C1 discharges into the RLC circuit. To achieve a sufficiently high resonant frequency, the value of C1 needs to be sufficiently small, which causes its voltage to drop rapidly due to the discharge loop formed by its internal equivalent parallel circuit. Therefore, S1 and S2 must use power semiconductor devices with a switching speed an order of magnitude faster than their discharge time to ignore the effect of capacitor leakage.
Simulation circuit
The circuit simulation results using LTspice are shown in the figure below. Both the capacitor and the GaN HEMT are modeled using high-precision LTspice models of actual devices, taking into account the effects of parasitic parameters. The simulation waveforms show a peak resonant current of 92A and a resonant frequency of 1.607MHz, indicating that the circuit under these parameters basically meets the design specifications.
3.2 Analog Circuit Testing
To simulate the inverter grid connection impact as accurately as possible and to verify the reliable operation of Magtron's RCMU products under this condition, an actual circuit was built to simulate the situation, with RCMU101SN-4P16A-5S/RCMU101SN-2P12A-5N products connected in series in the circuit. Different voltages were controlled from the high-voltage power supply output to manage the peak inrush current.
Impact simulation circuit
During the actual test, the power supply was set to a maximum of 500V (because the switching transistor's withstand voltage is 600V), the simulated peak inrush current was 98.4A, and the resonant frequency was 1.33MHz, basically simulating the actual grid-connected inrush current. Three pieces of each of the two models were randomly selected for testing. Under this impact, neither of Magtron's two RCMU products failed, verifying that the product has good immunity performance and is sufficient to cope with the impact of inrush current in inverter scenarios.
Test procedures for RCMU101SN-4P16A-5S and RCMU101SN-2P12A-5N
Four, Technical recommendations for dealing with inverter impact grid connection
1. Components with superior shock resistance are selected. Tests have verified that the Magtron RCMU series products can maintain reliable and accurate operation under grid-connected surges, achieving over 50kA under 8/20us surge tests, meeting the requirements of grid-connected inverters in harsh real-world operating conditions.
2. Grid-connection pre-synchronization technology is employed. The root cause of grid-connection shock is the voltage phase difference between the inverter and the grid at the moment of grid connection. Some papers have pointed out that deploying voltage sensors on the grid side and performing phase synchronization before grid connection can effectively solve the problem of phase asynchrony at the moment of grid connection.
3. Adjust the output filter parameters appropriately. As shown in the equivalent circuit model of grid connection, the grid-connected inrush current is related to the parameters of the current equivalent LC circuit, which are mainly determined by the output filter. While meeting filtering and EMC requirements, modifying the filter's LC parameters can control the maximum amplitude and oscillation frequency of the inrush current, thus reducing its impact.
