Zhao Zhengming

A Single-Stage Three-Phase Grid-Connected Photovoltaic System With Modified MPPT Method and Reactive Power Compensation

Single-stage grid-connected photovoltaic (PV) systems have advantages such as simple topology, high efficiency, etc. However, since all the control objectives such as the maximum power point tracking (MPPT), synchronization with the utility voltage, and harmonics reduction for output current need to be considered simultaneously, the complexity of the control scheme is much increased. This paper presents the implementation of a single-stage three-phase grid-connected PV system. In addition to realize the aforementioned control objectives, the proposed control can also remarkably improve the stability of the MPPT method with a modified incremental conductance MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. Simulation and experimental results show the high stability and high efficiency of this single-stage three-phase grid-connected PV system.

Design and control of a three-phase grid-connected photovoltaic system with developed maximum power point tracking

In this paper an optimum design and control of three-phase grid-connected PV system has been proposed and analyzed. Also, a developed maximum power point tracking (MPPT) controller that uses the PV array differential power to voltage (dP/dV) as an index to provide the reference voltage of maximum power point (MPP) quickly and accurately is presented. The control scheme of a three phase current-controlled space vector pulse-width modulation (SVPWM) inverter in rotating synchronous coordinate d-q with the proposed MPPT algorithm and feed-forward compensation is discussed. The PV grid-connected system controller employs multi-loop control with the filter inductor current of the inverter in the inner loop to achieve fast dynamic response and the outer loop to control dc bus voltage for MPPT. The design method provided in the paper for the controller parameters and filter proved to be very practical and effective. Both the power quality into the grid and power factor correction comply with standards IEEE 929-2000. Good agreement is obtained between simulation and experimental results which illustrated the high performance of the proposed approach.

Comparison of three PWM strategies-SPWM, SVPWM & one-cycle control

Sinusoidal pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM) are the most popular modulation strategies applied to variable frequency and adjusting speed systems, and there are diverges in comparing the merits and demerits of their own. Also there is another modulation strategy called one-cycle control. This paper gathers the three strategies and does some comparisons through simulation based on a simulation tool-PSIM, and has drawn some profitable conclusions.

Performance evaluation of three control strategies for three-level neutral point clamped PWM rectifier

This paper presents the performance evaluation and comparisons of three typical control strategies for three-level neutral point clamped (NPC) rectifier with pulse width modulation (PWM). These are voltage oriented control (VOC), direct power control (DPC) and direct power control based on space vector modulation (DPC-SVM). The theoretical background is described, and the advantages and disadvantages of each scheme are studied through analyses and a series of experiments. A comprehensive comparison and evaluation are implemented for the three methods from several aspects such as algorithm complexity, steady state and dynamic performances, current THD with unbalanced power source voltage, and startup current which is important for the high power applications. The results show that all the three schemes can provide good abilities; and VOC and DPC-SVM possess better steady state performance while DPC is provided with best dynamic performance.

Speed sensorless control for three-level inverter-fed induction motors using an Extended Luenberger Observer

This paper proposes an extended Luenberger observer (ELO) for speed sensorless vector control of induction motor drive fed by a three-level neutral point clamped (NPC) inverter. Two simple methods for observer gain selection are proposed, which eliminates the speed dependent items and decreases the relevant errors caused by observed speed. Conventional speed adaptive observer observes only flux and speed, while ELO observes not only flux linkage and rotor speed, but also load torque, which makes use of mechanical model and more smooth and accurate speed estimation is obtained. Both simulation and experimental results are presented to validate the effectiveness of ELO proposed in this paper.

A High Voltage and High Power Adjustable Speed Drive System Using the Integrated

A filter device is indispensable in most ac adjustable speed drive systems to eliminate the high-frequency components in the output voltage of an inverter, which could protect the insulation of the ac motor and reduce electromagnetic interference. Additionally, due to the restraints of power electronic devices, especially in the high voltage and high power systems with neutral point clamped (NPC) three-level structure, the step-up transformer is usually applied to ascend the output voltage. Thus, the system would be bulky when both of the components, filter and transformer, are adopted simultaneously. This paper efficiently utilizes the leakage inductance of the transformer and effectively integrates the filter and step-up transformer to achieve boosting and filtering simultaneously, and economizes on cost and volume. Besides, some relevant dynamic processes, like dc pre-excitation at the starting process, the self-excitation in the pulse-blocking process, and the response to load variation are discussed, respectively. This integrated configuration is validated by corresponding simulations and experiments, and has also been successfully applied in the three-level NPC 6000V/1MVA system. The other relevant factors in this device are discussed in detail

LC

Single-stage grid-connected photovoltaic (PV) system has advantages such as simple topology, low cost, etc. However, since this kind of system has only one stage of power conversion, all the control objectives need to be considered simultaneously, such as maximum power point tracking (MPPT), synchronization with the utility voltage and harmonics reduction for output current, so that the complexity of control scheme is much increased. This paper presents a modified MPPT strategy, which can remarkably improve the stability of the single-stage grid-connected PV system during rapidly changing process of light intensity, besides realizing the control objectives above. Comparing it with traditional constant-step MPPT strategy, this paper illustrates the improvement of the novel strategy based on PSIM simulation platform. A 300 Wp grid-connected PV system is also established in laboratory to verify the stability of this modified MPPT strategy, in which DSP (TMS320F2407) is employed to implement the proposed MPPT controller. Simulation and experimental results show the high stability and high efficiency of this modified strategy applied in single-stage grid-connected PV system

and Step-Up Transforming Filter

This paper presents a novel control scheme for three-level neutral point clamped (NPC) back-to-back converter. The distribution and balance of instantaneous energy for the converter is analyzed, and the direct power control with space vector modulation (DPC_SVM) for front-end rectifier and rotor field oriented control (FOC) for back-end inverter-fed induction motor are adopted respectively. A power feed-forward control loop from inverter to rectifier side is introduced to obtain better dynamic performance for the DC-link. Experimental results show that with the proposed control scheme, the three-level NPC back-to-back converter exhibits excellent dynamic performance, and can minimize DC-bus voltage fluctuations with low size of DC-bus capacitors.

Modified MPPT strategy applied in single-stage grid-connected photovoltaic system

Various failures and destructions occur in the applications of the power electronic converter. The real practice shows that these failures are connected with the concentration of the transient power pulse. In allusion to the physical characteristics of power electronic converters, this paper proposed that the power pulse and its sequence are the basis for power electronics in the perspective of electromagnetic energy. The authors analyzed the transient processes in the power semiconductors, electric conduction loops and controller system and illustrated the power pulse phenomena in high voltage and high power inverters. This investigation on the power pulse sequence is very meaningful for the failure analysis and device protection and has become an important topic in power electronics.

A novel control scheme for three-level NPC back-to-back converter

The communication protocol is important for applying adjustable speed drive (ASD) systems to an industry network. This paper proposes a novel scheme of communication system for the ASD system based on Modbus protocol. The most remarkable advantage of the approach lies in the tradeoff between performance and economic cost for a power converter system. The overall system is built on master-slave structure and easy for interacting with other networks. An additional study on the cyclic redundancy check (CRC) in Modbus RTU is introduced to obtain the optimization design in the actual application and the optimization method in software design is also presented. The performance of the communication system obtained from experiment conducted on the 15 kW ASD system validates the effectiveness of the proposed design method.