1. Field of the Invention
The present invention is related to automatic control, electronics, DC-DC conversion, DC-AC conversion and energy technology, specifically, solar photovoltaic (PV) energy conversion system being used in power grid.
2. Description of the Prior Art
Advance in science and technology has brought many conveniences in daily lives, and at the same time various problems, such as, reduction in the mount of fossil fuel, causing higher price and energy crisis; new energy source development becomes increasingly important. New energy source has less impact on environment and less pollution to the air and water. More importantly, these kinds of energy are reusable and inexhaustible. Renewable energy includes solar energy, wind, thermal energy, ocean and water energy etc [1, 2]. Solar power becomes heated research area due to its cleanness and readily available and many configurations have been developed. Solar energy conversion system parallel connected to utility-power grid uses solar panels to convert solar energy to DC current, then uses power conditioner to convert DC to AC, which is fed into utility-power grid [3,4]. The system usually includes DC input source, power conditioner, distribution box, transformer and rechargeable batteries. The power conditioner is consist of DC/DC converter, DC/AC inverter and system controller, and may vary depending on different applications. Because output voltage of a solar panel is relatively low, conventional method uses series connection to form required output voltage on DC bus bar. However, the voltage can be easily affected by varying load, making it difficult to design subsequent stage inverter and degrading power quality. Furthermore, if one module in the series does not work in optimum condition, the whole system's efficiency will be suffered. Therefore, two-stage conversion method is usually used to achieve AC power output, first using DC/DC converter to boosting input voltage and then using DC/AC inverter to output AC voltage.
Conventional DC/DC step-up circuit utilizes a boost converter with a single inductor; power semiconductor switches in the circuit have to bear high voltage, large current and reverse recovery current spike in output diode; this decreases conversion efficiency and limits boost ratio to seven times ratio. Also, by using transformer to boost voltage, winding ratio will limit boosting range; if leakage induction is not effectively dealt with, conversion efficiency will suffer. To solve the problem, the present invention utilizes a conversion circuit using bi-directional induction energy transfer proposed in reference [7]; it has high boost ratio and fair conversion efficiency, up to thirty times DC/DC power conversion.
To stabilize power output from solar energy system, the present invention controls the inverter by a microprocessor. To solve control problem, due to parameters changing and various undeterminable conditions, variety control theories have been proposed, such as Proportional-Integral-Derivative (PID)[8], Computed Torque Control, Sliding-Mode Control)[9, 10]. PID is widely used in the industry due to its simple in structure, easy to design and low cost; however, for a system with uncertain dynamic state, PID cannot provide effective solution. Computed Torque Control obtains linear equation by eliminating part or all non-linear factors in the non-linear equation, and then a linear feedback controller is designed to have a character of closed-loop control. Since the Computed Torque Control is based on idealization in eliminating non-linear dynamic state, it does not fully understand system's uncertain variables in the time domain, including changes in system parameters and external disturbances. Hence, larger boost gain is chosen in order to achieve desired strength and to guarantee stability.
The variable Structure Control or Sliding-Mode Control is one of effective non-linear control method [9-18], since under the Sliding-Mode Control controlled system states are not affected by the system uncertain variables and disturbances. The design of a Sliding-Mode Control system can be accomplished in two steps. First, the sliding plain in state variable space is chosen based on a required close-loop control; then the control rules are designed to make system state moving to and keeping on the sliding plain. At the beginning, the condition of system state locus touching the sliding plain is called the Reaching Phase; once the system state locus reaches the sliding plain, the system state will be kept on the plain and moving to a target point; this condition is call a Sliding Phase. When the system state is in the Reaching Phase, it is still affected by the changes of system parameters and external disturbances. Many design methods for Reaching Phase or Total Sliding-Mode Control are proposed to reduce the impact from system uncertain variables [12-15]. Study on special reaching rules is done by Gao and Hung [12] to explain the system state at a reaching phase; however, under this condition, system uncertain variables still affect system control capability. A Total Sliding-Mode Control [13-15] does not have reaching phase in control process and all the states are on the sliding plain; all control process is not affected by the system uncertain variables, but it still may cause vibration in control power and induce unstable state in the system. Few years ago, the Boundary Layer concept is used to eliminate the vibration phenomena in control power; however, if an improper width of boundary layer is chosen, the system can become unstable. Therefore, adaptive calculation [18], which can estimate uncertain variables, is used to reduce vibration phenomena and the present invention also adapts the method in all-bridge inverter.
Output from a solar panel will vary greatly depending on the strength of Sunlight. Fixed panel cannot absorb maximum solar energy throughout a day and the effectiveness of a solar energy conversion system will be limited. Recently, great emphasize has been made on Sun tracking system. A conventional Sun tracking system uses photo sensors mounted on both ends of the solar panel, and when the radiation measured by the sensors are equal, the solar panel is directed straight to the Sun; however, the sensors are not easy to be tuned and have limited life span. Therefore, the present invention proposes an active Sun tracking system, utilizing that open voltage from the solar panel is proportional to the strength of radiation, to track the Sun and to overcome the drawback of the conventional tracking system.
Relationships between the output voltage, the current and the power of a solar panel are non-linear. Radiation strengths, temperatures, deterioration of components and sensing materials will all influence output power of the solar panel. For different conditions, each has an unique working curve; each curve has a maxim power point and this point is the optimal position for the solar panel. To maximize utilization of a solar panel, a proper design for controlling is required in order to absorb maximum solar energy under various working environments. The method is so-called maximum power point tracking method [22-25], such as, the Power Feedback Method, the Incremental Conductance Method, the Linear Approximation Method, the Practical Measurement Method and the Perturbation and Observation Method. The Perturbation and Observation Method is the most popular method due to its simplicity, but it can cause energy loss due to difficulty in balancing the response time and the energy loss. Therefore, the present invention proposes an adaptive step disturbance method to overcame the drawback of the Perturbation and Observation Method, and to speed up maximum power point tracking and reduce energy loss.
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