| The development of human society is inseparable from energy, and the energy used by human is mainly the fossil energy which consists of coal, oil and natural gas. However, the fossil energy is non-renewable and its storage capacity declines sharply as the society development. The fossil energy will be completely exhausted in the near future. Moreover, pollution will be produced and environmental quality will be reduced in the utilization process of fossil fuels. In recent years, the countries all over the world are actively developing various types of renewable energy in order to respond to global energy and environmental crisis. Solar photovoltaic power generation has been widely used because of its large storage capacity, wide distribution, environment-friendly and easy maintenance. Large-scale grid-connected photovoltaic power station can utilize solar energy intensively and efficiently and help to expand the scale of solar photovoltaic power generation, so it has become the recent focus of solar photovoltaic power generation. As the scale of grid-connected photovoltaic power station is increasing remarkably, the strict gird codes are formulated by many countries in order to ensure the grid safety. In order to meet the gird codes of photovoltaic power station and ensure the safety operation of photovoltaic power station, fault ride through and key control technologies of large-scale grid-connected photovoltaic power station should be researched.Therefore, based on the analysis of large-scale grid-connected photovoltaic power station, this thesis studies the fault ride-through methods and the control methods by theoretical analysis and simulation and experiment verifications. This thesis proposes a fast dc component suppressing method for the phase locked loop which can filter out the DC component rapidly by the correlation calculation of adjacent sampling points, then the fundamental component information of point of common coupling (PCC) voltage of large-scale grid-connected photovoltaic power station can be detected rapidly and accurately; On the basis of accurate phase-locking, a voltage detection method for the low voltage ride through operation of photovoltaic power station is proposed, which can detect voltage drop depth when PCC voltages suffer three-phase unbalanced voltage sag, harmonic interference, frequency variation and phase-angle shift. What’s more, this method can calculate PCC voltage amplitude in sequence in order to provide the basis for accurate reactive power compensation and assist photovoltaic power station to achieve low voltage ride through; In order to achieve low voltage ride-through and islanding detection of photovoltaic power station simultaneously, a reactive power perturbation method for simultaneous low voltage ride-through and islanding detection is proposed. Compared to typical methods, the complexity of detection system is reduced significantly, detection result is more unified and detection time is shortened significantly. Specific contents are as follows:(1) Phase lock technology of large-scale grid-connected photovoltaic power station.Phase-locked loop is an important part of control system of three-phase inverter, which can ensure that output current of three-phase inverter and PCC voltage have the same frequency and phase angle. This paper analyzes the principle and classification of phase-locked loops and the common three-phase phase-locked loops. The synchronous reference frame phase-locked loop cannot achieve phase-locked task when PCC voltage suffers various types of failures, such as three-phase unbalanced voltage sag, frequency variation, phase-angle shift and harmonic interference. Therefore, this paper introduces several new phase-locked loops, including multiple reference frame phase-locked loop (MRF-PLL), decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL), second-order generalized integrator phase-locked loop (SOGI-PLL), double second-order generalized integrator phase-locked loop (DSOGI-PLL) and enhanced phase-locked loop (EPLL). Above phase-locked loops can detect the PCC voltage accurately when PCC voltage is distorted. The DC component which may exist in PCC voltage will also affect the normal operation of phase-locked loops. There are many factors introduce the DC component into the sampled grid voltage, such as 1) Zero drift of the feedback channel of control system; 2) Attenuated DC component in the grid voltage after fault occurs; 3) Different switching characteristics of semiconductors introducing the asymmetrical output sine wave. This paper analyzes several typical phase-locked loops with DC component elimination capability. These phase-locked loops calculate the DC component by using the integral branch and derive the fundamental voltage signal after sampling voltage signal subtracts DC component calculating value. Because the dynamic response speed of typical phase-locked loops with DC component elimination capability is slow, this paper proposes a new DC component suppressing method, which can eliminate the DC component rapidly. The DC component elimination module can meet most application requirements which need fast dynamic response. Moreover, cascaded delayed signal cancellation module based on cascaded delayed signal cancellation principle can be added into the three-phase locked loop to eliminate the effect of low order harmonics when harmonic content of PCC voltage is large. Further, the proposed improved phase-locked loop can detect PCC voltage information accurately when distortion (DC component and low order harmonics) in PCC voltage appear or disappear.(2) Voltage detection technology of low voltage ride through.This paper introduces the definition and classification of voltage sag and low voltage ride through requirement. Then this paper introduces voltage sag detection principle of four fundamental voltage sag detection methods (root mean square calculation method, peak value calculation method, discrete Fourier method and synchronous reference frame phase-locked loop) and analyzes the real-time performance and accuracy of PCC voltage amplitude detection of synchronous reference frame phase-locked loop under different PCC voltage condition by simulation. The analysis reveals that fundamental voltage sag detection methods cannot detect PCC voltage amplitude accurately when PCC voltage is distorted, then this paper analyzes some improved voltage sag detection methods whose performance are better than the performance of traditional methods. This paper indicates that the detection time of voltage drop depth should be shortened so that injection of reactive current will have plenty of time through analyzing the low voltage ride through requirement of Chinese photovoltaic power station (GB/T 19964-2012). The existing voltage sag detection methods, however, cannot meet the demand of Chinese low voltage ride through requirement. This paper proposes a new fast voltage sag detection method to assist the low voltage ride-through operation of photovoltaic power station. In particular, it consists of one synchronous reference frame phase-locked-loop and two voltage detection modules. The fast voltage detection module which consists of maximum point tracking block and zero point tracking block can calculate PCC voltage amplitude in 5ms when PCC voltage is normal by tracking maximum points or zero points of three-phase grid voltage waveforms. The accurate voltage detection module assumes an additional cascaded delayed cancellation block to filter out the low order voltage harmonics and then derive PCC voltage amplitude. This paper analyzes the performance of the proposed method when PCC voltage frequency varies and proposes control flow chart of the voltage sag detection method. Further, a progressive logic algorithm is designed to improve the rapidity and accuracy of reactive power compensation of the grid-connected inverter. This method can detect PCC voltage drop depth rapidly and accurately when PCC voltage suffers three-phase unbalanced voltage sag, harmonic interference, frequency variation and phase-angle shift. Simultaneously, this method can detect three-phase PCC voltage amplitude in sequence in order to provide the basis for accurate reactive power compensation. In summary, the proposed PCC voltage sag detection method can assist grid-connected photovoltaic power inverter to meet the low voltage ride-through requirement of the new standard.(3) Islanding detection technology of large-scale grid-connected photovoltaic power station.This paper introduces the definition, causes and hazards of islanding and principle of islanding detection methods. Three remote islanding detection methods (transmission circuit breaker trip signal method, power line carrier communication method and impedance insertion method), four passive islanding detection methods (under/over voltage islanding detection method, under/over frequency islanding detection method, harmonic islanding detection method and voltage phase jump islanding detection method) and five active islanding detection methods (impedance measurement method, active frequency drift method, active frequency drift with positive feedback method, slip frequency drift method and output power perturbation method) are introduced. Moreover, the cause of islanding detection failure is analyzed. The existing low voltage ride through and islanding detection methods cannot be combined simply and achieved simultaneously. This paper proposes a reactive power perturbation method, which consists of dual second order generalized integrator (DSOGI) phase-locked loop, power perturbation module and islanding detection module. This method detects the PCC voltage amplitude and frequency by DSOGI phase-locked loop and then controls the output reactive current of photovoltaic power station to form power perturbation. This method can meet the requirement of low voltage ride through and islanding detection in parallel. Compared to the existing methods, the complexity of detection system is reduced significantly, the detection result is more unified and the detection time is shortened significantly. This method can distinguish the islanding condition and low voltage condition of photovoltaic power station accurately and implement accurate operation for photovoltaic power station in order to ensure the safe operation of photovoltaic power station when photovoltaic power station is in rated and non-rated operating condition. Further, the effectiveness and practicality of the proposed reactive power perturbation method can still be guaranteed when photovoltaic power station is under special operating conditions. |
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