A Grid-tied PV Inverter with Sag-severity- independent Low-voltage Ride Through, Reactive Power Support, and Islanding Protection

Abstract

This paper proposes a grid-tied photovoltaic (PV) inverter capable of low-voltage ride through (LVRT), reactive power support, and islanding protection. Unlike other LVRT inverters, the proposed inverter is independent of sag severity while maintaining the maximum power-point tracking (MPPT) under normal and faulty conditions. The addition of an energy storage buffer stage mitigates the DC-link voltage surge during sags. At the same time, the inverter injects the reactive power during back-to-back sags of variable depths. The control system of the inverter generates the appropriate reference signals for normal, LVRT, and anti-islanding modes while the MPPT continues running. The salient features of the proposed inverter are: ① active power injection under normal grid conditions; ② sag-depth independent LVRT with reactive power support; ③ no DC-link fluctuations; ④ continuous MPPT mode; and ⑤ simultaneous LVRT and anti-islanding support during a grid outage. The inverter demonstrates an uninterrupted operation and seamless transition between various operating modes. Simulations and the experimental prototype have been implemented to validate the efficacy of the proposed PV inverter.

Multi-Functional PV Inverter With Low Voltage Ride-Through and Constant Power Output

“…However, the existing LVRT methods operate the boost stage at a predetermined fixed duty in both mild or severe sags. In a more severe sag, the fixed duty cycle operation cannot ensure the DC-link voltage stability and may still surge the DC-link voltage [13]. Therefore, the boost-stage duty cycle should be selected adaptively according to the sag intensity.…”

“…Therefore, the boost-stage duty cycle should be selected adaptively according to the sag intensity. However, for severe sag compensation a new duty cycle may be toolow which can make the boost converter falls from (CCM) to (DCM) [13,19]. The boost converter may not provide adequate gain in DCM, thus failing to achieve the required DC-link voltage, resulting in inverter shutdown.…”

“…𝑃 𝑀𝑃𝑃𝑇 = 0 + 𝑃 𝐷𝐶−𝐿𝑖𝑛𝑘 + 𝑃 𝑙𝑜𝑠𝑠𝑒𝑠 (12) Therefore, during severe sags, all the PV power PMPPT is available to be harvested into the battery buffer without causing any instabilities in inverter control, as shown in (13), (14).…”

“…Although (33) seems independent of grid voltage Vg. However, the studies [13,34] show that the grid voltage Vg implicitly affects the current loop, creating a hidden impedance path. So, when the inverter is controlled by a classical integrator-based controller (PI, Type-I, or Type-II), it fails to attain the zero steady-state error.…”

“…However, the fixed duty cycle approach for attaining the LVRT is only effective if the sag is also of fixed intensity. Most of the solutions for singlephase LVRT inverters presented in the literature can only deal with pre-determined fixed sag intensity [1,4,[6][7][8][9][10][11][12][13].…”