Research On Fault Ride Through Strategic Techniques Of Grid-connected Photovoltaic System

Grid-connected photovoltaic(GCPV) generation system using green and renewable energy to generate electric power It can deal with global problems such as environmental pollution, energy shortages and climatic deterioration. Generally, the research area of GCPV generation belongs to the part of “Global Gnergy Interconnection”. There are still many technical problems required to stimulate the application of large-scale GCPV generation. From the dynamic view, GCPV generation should manage to be capable of resuming to the state of equilibrium quickly and accurately, that is the ability to ride through the fault period when the grid suffers unbalanced fluctuations or even breakdown faults. Several key technologies of solving fault ride-through(FRT)problems in GCPV generation should be addressed, such as flexible grid current instruction to meet the need of different occasions in grid side, the quick and accurate separation of positive and negative sequences of grid voltage, the fast and correct phase-locked loop tracking capability, the protection device to reduce short-circuit current on serious grid fault when control strategies cannot meet the requirement of nonstop operation.Focus on the fault ride-through techniques on the grid side, this paper mainly analyses the active and reactive power control strategies by calculating current coordinate instructions; the principle of proportional and first-order resonant controller to separate positive and negative sequence components; the phase locked loop tracking ability using first-order resonant controller and the design of the frequency locked loop synchronously monitoring method; a design of cheap, strong operable short-circuit current suppression method from the view of energy dissipation.This paper firstly establishes a unbanlanced transient mathematical model of GCPV generation. The instantaneous power control principle of PV inverter based on stationary coordinate frame is analized. Calculation algorithms of several current reference instructions are also discussed. Meanwhile, a flexible active and reactive power vector control strategy is put forward. By using them, a method of a current reference instruction to flexiblely switch to varieties of control objectives by using the adjustable coefficient k1 and k2 is achieved.In order to solve the problem of time delay and filter error during the separation of positive and negative sequence components, this paper proposes a method of reduced-order resonant(ROR) regulator. The proposed controller not only remains the advantage of infinite resonance gain, but supplement the function of separating positive and negative frequency polarity. To meet the error of parameters from engineering hardware and software, a cut-off frequency to improve the frequency bandwidth is introduced, the Bode diagram tool can d select the best cut-off frequency parameters. By using the complex variable function theory to realize the complex factor j.The discretization can be achieved by Tustin transform method.In order to solve the problem of phase locked loop unable to track grid fundamental signal in real-time because of the interference of negative sequence component, a synchronization technique of frequency-locked loop based on ROR controller(ROR-FLL) is put forward.. The FLL replaces of the classical PLL to solve the PLL overshoot. An integral feedback loop is introducd to eliminate interference of DC component of grid. FLL negative gain coefficient is linearized to get first-oder frequency response characteristics by the method of Small Signal Model. The adaptive capacity in the frequency-changed occasions is improved.When control strategies are difficult to meet the ride-through requirements of the serious grid fault, the reactive power compensation device should be firstly priority. In order to suppress the impact of large fault current to photovoltaic inverter or other equipment. For low cost and reliable operation, this paper proposes a multi-steps dissipative resistance switching method on grid side to inhibitite short-circuit current. By setting value of dissipative resistance, debugging switching operation time, designing logic switching dually in response to voltage and frequency with stable domain and designing the multi-steps switching methods for dissipating energy smoothly. Performance of dissipative resistance switching is optimized with great extent during fault ride-through, incluing supporting the sag voltage to recovery fastly, suppress rising voltage quickly and inhabiting of hopping frequency effectively.