Hybrid Voltage-Current Control of Grid-Forming and Grid-Following Inverters

Abstract

Grid-connected inverters are required to operate stably under a wide range of grid conditions. However, conventional grid-following (GFL) control may suffer from instability under weak-grid conditions, while grid-forming (GFM) control may exhibit unstable oscillations under strong-grid conditions. To address these issues, a hybrid voltage-current control method is proposed in this article. A voltage control is introduced on the d-axis, while a current control is adopted on the q-axis, enabling the inverter to exhibit voltage-source characteristics on the d-axis and current-source characteristics on the q-axis. In this way, the proposed control integrates the characteristics of both conventional GFL and GFM control. A full-order model is established to analyze the port characteristics and small-signal stability of the systems. Finally, the effectiveness of the proposed control strategy is validated through simulations and experiments on a 1.5 kW inverter experimental platform. The results show that the proposed control maintains stable operation under different grid conditions with varying short-circuit ratios (SCRs).

Figures (11)

• Figure 1: Control structure of hybrid voltage-current inverter.
• Figure 2: Port admittance comparison among GFM, GFL, hybrid voltage-current control in the complex vector dq frame.
• Figure 3: Modeling the hybrid voltage-current control of inverter.
• Figure 4: The diagram of four control methods. (a) GFM. (b) GFL. (c) Hybrid magnitude control with $P - \omega$ droop synchronous control. (d) Hybrid magnitude control with PLL synchronous control.
• Figure 5: Pole maps of the two control methods when $Z_g$ varies from 0.2 p.u. to 0.9 p.u..(a) GFL. (b)Hybrid voltage-current control with PLL synchronous control.