Study Of The Low Voltage Ride Through Control For Grid-connected Photovoltaic Inverter

As the proportion of grid-connected photovoltaic system in energy structure becoming heavier, if they take off from the grid frequently when grid voltage sags, the safety and reliability of grid would be impacted. Therefore the latest grid connection standards demand that the PV inverter must have low voltage ride through(LVRT) capability. By taking three-phase grid-connected inverter as a target, a further study of LVRT on the basis of the control of grid converters under grid faults is discussed in this paper.First, power control and current control structure under unbalanced grid conditions are discussed. SOGI is used to achieve separation of positive and negative sequence. On basis of Mathematical model of the three-phase inverter model, decoupled double synchronous reference frame current controllers and resonant controllers is introduced in order to control negative-sequence and positive-sequence current exactly. According to instantaneous power theory, five different strategies for determining the reference current vectors are discussed. It is shown that these strategies intend to provide different features, such as the cancellation of active power oscillations, the injection of balanced sinusoidal currents.Secondly, flexible positive and negative sequence compensation control and positive and negative compensation control for eliminating active oscillation are discussed. Flexible positive and negative sequence compensation can calculate reactive power and adjusted factor depend on requirements and the voltage condition. Then it can inject positive and negative sequence reactive current flexibly to support grid voltage. At the same time, reactive power support capacity and power oscillation are also analyzed. Since the presence of output power oscillations in previous methods,Positive and negative compensation control for eliminating active oscillation method is put forward. Positive-Negative-Sequence Compensation(PNSC) method is used as the active current reference to ensure a constant active power and sinusoidal current. As these LVRT requirements also suggest that the PV grid-connected inverter should inject reactive power to support grid voltage, a reactive current injection that can cancel the oscillations in active power is derived. When the voltage dips, the grid current is unbalanced. In order to reduce the risk of overcurrent, a reactive power constraint is also derived. The results of simulation verified the feasibility and effectiveness of both mentioned LVRT methods.Finally, a experiment platform of three-phase grid-connected inverter which is controlled by DSP is completed. And positive and negative compensation control for eliminating active oscillation in this paper has been carried on the experimental verification using the platform. Compared with flexible positive and negative sequence compensation control, positive and negative compensation control for eliminating active oscillation can eliminate active oscillation and reduce the risk of overcurrent.