Current control based on hysteresis algorithms are widely used in different applications, such as motion control, active filtering or active/reactive power delivery control in distributed generation systems. The hysteresis current control provides to the system a fast and robust dynamic response, and requires a simple implementation in standard digital signal platforms. On the other hand, the main drawback of classical hysteresis current control lies in the fact that the switching frequency is variable, as the hysteresis band is fixed. In this paper a variable band hysteresis control algorithm will be presented. As it will be shown, this variable band permits overcoming the aforementioned problem giving rise to an almost constant switching frequency. The performance of this algorithm, together with classical hysteresis controls and proportional resonant (PR) controllers, has been evaluated in three different single-phase PV inverter topologies, by means of simulations performed with PSIM. In addition, the behavior of the thermal losses when using each control structure in such converters has been studied as well.
Fig. 1. Basic Current Control Scheme in a single phase inverter.
EXPECTED SIMULATION RESULTS:
Fig. 2. Behavior of the current and the voltage at the output of the converter when using the H5 topology.
Fig. 3. Behavior of the current and the voltage at the output of the converter when using the HERIC topology.
Fig. 4. Behavior of the current and the voltage at the output of the converter when using the or HB-ZVR topology.
A hysteresis current control algorithm based on an adaptive hysteresis band for single phase PV converter topologies has been presented in this paper. As it has been shown analytically and by means of simulations this algorithm permits obtaining a fixed switching frequency in all the tested topologies. The main drawback of the conventional fixed hysteresis band current control is that generates excessive current ripple because modulation frequency varies within a band. This modulation frequency variation makes complicated the output filter design. Adaptive hysteresis band current control keeps the good performance of the fixed band hysteresis current control and additionally permits an easier output filter design due that the switching frequency is almost constant. On the other hand, switching losses can be reduced by using this adaptive hysteresis band current control. The analyzed topologies are the more widely used in transformerless single-phase PV systems (H5 and HERIC). Based in the previously comparative simulations results it can be concluded that in the case of H5 topology losses are concentrated in S5. In case of HERIC topology losses are located among S1, S2, S3 and S4. Finally in HB-ZVR single phase topology, losses are located in S5. These results mean that in each case, the losses distribution is not the same and a different thermal design should be done.
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