Abstract
This paper proposes a novel control algorithm to enhance the fault ride-through (FRT) capability of a photovoltaic (PV) system. In this method, the overcurrent of the grid-tied inverter is suppressed to a preset value using model current predictive control (MCPC) algorithm, and its DC-link overvoltage is removed using a non-maximum power point tracker (non-MPPT) algorithm. Therefore, the inverter overcurrent and its DC-link overvoltage problems can be placed under decoupling control using a DC/AC converter controller and DC/DC converter controller, respectively. Using eight switching modes for the three-phase inverter, the MCPC of DC/AC converter controller establishes a value function between the current reference value and output current value. Then, by introducing the switching mode (corresponding to the minimum value function) into the DC/AC converter controller, if the current reference is set at the rated value, the inverter’s output current can be inhibited to the rated value under fault conditions. Moreover, a balanced three-phase rated current can always be obtained under either a symmetric or asymmetric fault. However, together with the MCPC, a DC-link overvoltage will appear. To remove this DC-link overvoltage quickly, based on the grid system voltage sag level, the non-MPPT algorithm is used to calculate the adjusted power for PV arrays. Then, based on the calculated power, the amended duty cycle can be obtained and immediately introduced into the DC/DC converter controller to tune the PV arrays’ output power, which significantly decreases the power imbalance between the inverter’s AC and DC sides, thus inhibiting the DC-link overvoltage. In addition, to inhibit DC-link voltage fluctuations further, DC-link voltage feedforward compensation is introduced into the DC/DC converter controller. In particular, under an asymmetric fault condition, with the help of the voltage feedforward compensation, the second harmonic frequency components of the DC-link voltage can also be removed. Finally, based on the theoretical derivation and simulation results, it can be proved that the major problems troubling a PV system’s FRT technologies can be resolved by the proposed method, especially the problem related to the harmonic components of the DC-link voltage.
Highlights
Because of its high-quality energy characteristics and economic support from governments, photovoltaic (PV) generation systems will be rapidly developed in the few decades [1], [2]
To improve a PV system’s fault ride-through (FRT) capability, previous studies have mainly focused on adopting additional circuitry [6]–[9] and active control [10]–[17] for a PV system
If the voltage values corresponding to the eight switching modes at time k are respectively substituted into (11), eight different predicted results for the output current, iLα−k+1 / iLβ−k+1, can be obtained
Summary
Because of its high-quality energy characteristics and economic support from governments, photovoltaic (PV) generation systems will be rapidly developed in the few decades [1], [2]. By introducing the fault voltage value of the power system and feedforward compensation of the inverter’s DC-link voltage, the modified non-MPPT algorithm can completely eliminate the DC-link overvoltage of a grid-connected inverter, as well as its harmonics. If the voltage values corresponding to the eight switching modes at time k are respectively substituted into (11), eight different predicted results for the output current, iLα−k+1 / iLβ−k+1, can be obtained. The strategy enters a loop where, for each possible grid-connected inverter switching state, the current predictions can be obtained from (6) and (11) considering voltage uinα/uinβ. Considering the inverter’s overcurrent problem, iLα_ref and iLβ_ref are amended as follows: IN2 − Iq2_set (15) In this case, the output current of the inverter can be restrained at its rated value
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