This paper proposes an improved maximum power point tracking (MPPT) method for wind power systems. The proposed method combines hysteresis control with tip speed ratio (TSR) control using a power coefficient curve. It has fast dynamic characteristics with the TSR control using data obtained from an anemometer. Moreover it can track the maximum power point (MPP) with hysteresis control even when there is incomplete data. Since the proposed method selects the operating mode according to the operation state of the generator of three control modes, the controllers do not interfere with each other and they provide excellent performance. The effectiveness of the proposed algorithm is verified by simulation and experiments based on a 3 kW wind turbine system.
This study proposes a space vector modulation (SVM) strategy to balance the neutral-point voltage of three-level inverter systems. The proposed method is implemented by combining conventional symmetric SVM with nearest three-vector (NTV) modulation. The conventional SVM is converted to NTV modulation by properly adding or subtracting a minimum gate-on time. In addition, using this method, the switching frequency is reduced and a decrease of switching loss would be yielded. The neutral-point voltage is balanced by the proposed SVM strategy without additional hardware or complex calculations. Simulation and experimental results are shown to verify the validity and feasibility of the proposed SVM strategy.
This paper presents a method to improve the quality of input-output currents in a three-level neutral-point clamped (NPC) inverter with small direct current-link (DC-link) capacitor systems. The inverter systems with the small DC-link capacitors have several advantages in terms of cost, volume, life-time, and reliability when compared to inverters that use large DC-link capacitors. However, there are problems with respect to the deterioration of the input current quality and a severe ripple of neutral-point voltage (NPV), which can cause an aggravated output current. To mitigate these issues, an additional circuit is applied for the input current shaping and a compensation algorithm is applied to reduce the ripple voltage of NPV. The effectiveness of the proposed design and control method is verified with various simulation and experimental results.
This paper proposes a variable DC-link reference voltage algorithm for wide range of maximum power point tracking (MPPT) for two-string photovoltaic (PV) systems. A multi-string system, which is a kind of PV system, is widely used due to its many merits (such as the ability to use low rating devices, high MPPT efficiency, and so forth). PV systems can choose their input voltages on the basis of their PV cell connection structure. The PV cell connection structure can be restricted because the input voltage and current affect the PV system design. This reduces the MPPT range under some weather conditions. In the restricted PV connection structure, this algorithm enlarges the MPPT range and minimizes the increment of the total harmonic distortion (THD) by selecting the appropriate DC-link voltage reference which is changed by comparing the sorted input voltage. To verify the proposed algorithm, simulation and experiments are conducted to show the results of the performance for the proposed algorithm.
Conventional single-phase Vienna rectifiers employ proportional-integral (PI) controllers which are appropriate for controlling DC components, to regulate their line currents. However, in the regions close to the line current's zero-crossing point, the dynamics of PI controllers are too slow to respond to the reference current, which has an AC component. Hence, the power factor (PF) of the device is degenerated, and total harmonic distortion (THD) increases. A controller with a fast dynamic response is thus required to solve this problem. In this paper, we investigate the use of a model-based predictive controller (MPC), which has a faster dynamic response than a PI controller, to improve the line current quality of a single-phase Vienna rectifier. With this method, the average current in both the continuous current mode (CCM) and the discontinuous current mode (DCM) of operation are controlled using a mode detection method. Moreover, we calculate the optimized duty cycle for the single-phase Vienna rectifier, by predicting the next current state. We verify the operation of the proposed algorithm using a PSIM simulation, and a practical experiment conducted with a 1-kW-rated single-phase Vienna rectifier prototype. With the proposed method, the quality of the line current near the zero-crossing point is improved, and the PF is controlled to unity.
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