Guorong Zhang

Control strategy of low voltage ride-through for grid-connected photovoltaic inverter

With increasing penetration of grid-connected photovoltaic (PV) power generation, the grid codes about low-voltage ride-through (LVRT) of PV plants and their interface inverters have been established. Under a grid voltage sag, the underlying reasons may cause overcurrent of the grid-connected inverter and failure in ride-through. In this paper, a positive-sequence current injection with feedforward of positive- and negative-sequence grid voltages is proposed to meet the LVRT requirement. The proposed method not only reduces the risk of overcurrent but also suppresses the negative sequence current during the LVRT operation. Whats more, a rapid detection of voltage sag is proposed to disable the DC link voltage control and the PV maximum power point tracking (MPPT) control during the grid sag, which is another way to reduce the risk of overcurrent. A test bench has been established, and testing results of a 500kVA prototype inverter are presented to validate the effectiveness of the proposed methods.

Resonant mechanism of multi grid-connected inverters in distribution power systems

With increasing penetration of grid-connected photovoltaic (PV) power generation in distribution network, the equivalent impedance of the distribution network and the distribution capacitance of the interconnection cables are changed. The changed impedance network may be excited by the harmonic current of inverters or the harmonics of the grid voltage, which will affect the power quality and stable operation of the distribution network. The frequency characteristic of the grid current is given firstly. Then a Norton equivalent model of grid-connected inverter with LCL filter is established. Basing on the inverter model, the Norton model of distribution network considering the cable distribution capacitance is proposed. By analyzing of the series and parallel resonant characteristics, the resonant mechanism is demonstrated. Simulative results verify the validity of theoretical analysis.

Circulating current suppressing in modular parallel photovoltaic grid-connected inverter

Modular parallel photovoltaic grid connected inverter (PV GCI) with common array bus can optimize the operation mode of the modules to improve the power generation and the reliability of the system. However, the shunt of DC side serves a channel for zero sequence current, that will cause zero sequence circulating current. In this paper, the model of zero sequence circulating current in abc coordinate and dq coordinate is established seperately. It is concluded that the rate of zero sequence circulating current is mainly depended on the difference between the zero sequence voltage of the two modules. The influence of carrier asynchronization and PWM modulation method on zero sequence circulating current is also analysised. Furthermore, this study proposes a carrier synchronization and module master-slave control method to suppress the circulating current without combining a zero sequence current controller. Test results on PV GCI with two 10kW modules are presented to validate the performances of the proposed method.

Quantitative analysis on economic impacts of installation at different sites on microgrids with multi-energy

Economic performances of a microgrid with multi-energy are effected by many factors such as the configuration of microgrid systems for the specific installation sites and cluster of loads, operation modes and dispatching schedules of the microgrid, the electricity market, energy policy and alike. And the configuration of the microgrid is the premise of all factors. This paper presents an optimization model to quantitatively evaluate the economic impacts of different installation sites on microgrids with multi-energy. In this model, optimal configuration schemes for microgrids installed at typical sites are designed by particle swarm optimization method. The economic performances of optimal configuration schemes are evaluated by the operation simulation with the proposed indexes. Those typical installation sites mentioned above belong to different regions which are divided into the five-level classifications in terms of the richness of wind and solar resource and the distributions of loads in China. The calculation results may be used for the decision-making references by the decision-makers.