Control Strategy Study For Three-level Three-phase Grid-connected Photovoltaic Inverter Under Grid Fault Condition

Nowadays, with the development of renewable energy sources, the majority of thephotovoltaic power systems are connected to the grid. Many problems of grid connectedcontrol scheme occur when voltage dips appear in the grid due to grid faults such as shortcircuits. Grid-connected PV power systems should be satisfied with several criterions ofgrid-connected techniques such as a fast and accurate synchronization of interconnectedinverter, the control of unbalanced current, reactive power support, an improved powerfactor, and the grid fault ride through capability. In particular, non-linear loads at the pointof common coupling or the most usual grid faults such as two-phase and single-phasefaults will lead to imbalance and non-sinusoidal distortion in the grid voltage.Correspondingly, negative sequence components of grid voltages appear and variablessuch as the grid voltage phase locked by converters, instantaneous active power andinstantaneous reactive power, dc-link voltage, the positive and negative sequencecomponents of the grid-connected current and voltage, start oscillating at twice thefundamental frequency, affecting the performance of the inverter.This dissertation presents many effective control skills of a three-level neutral pointclamped (NPC) topology inverter under grid fault to solve the problems of grid-connectedvariables oscillations and the grid fault ride through capability. Firstly, dc-link middlepoint voltage is controlled by taking into account the switching state of the inverter basedon conservation of charge. Secondly, a proposed positive sequence voltage phase detectorbased on a new decoupled double synchronous reference frame phase-locked loop(DDSRF–PLL) can completely eliminate the phase detection errors of conventionalsynchronous reference frame PLL and accurately separate the positive and negativesequence components of the grid voltage. In addition, a new scheme for controlling thepositive and negative sequence components of grid-injected currents using PI controllersis presented, based on a cross decoupling DDSRF currents loop, could cancel out theoscillations at twice the fundamental frequency caused by the presence of both sequences.Meanwhile, during the grid faults, the PV system should operate with increasing grid-injected currents to maintain the injection power generated by photovoltaic cell. Thisphenomenon may result in PV system disconnection due to overcurrents. This paper willpropose a flexible power control algorithm with currents-limited based on positive andnegative sequence control (PNSC) and a designated power control scheme forphotovoltaic cell to deal with the overcurrents. Finally, a series of methods are proposed tocalculate the oscillations exists in instantaneous active power and instantaneous reactivepower, then a certain method can be selected according to the physical conditions of thegrid faults to eliminate the power oscillations.In this dissertation, based on the DSP and the FPGA control systems, part oftheoretical and experimental studies has been tested on a three-level NPC topologyinverter with an LCL fiter connected to the grid and results confirm the validity of themethod.