A Family of Neutral-Point-Clamped Circuits of Single-Phase PV Inverters: Generalized Principle and Implementation
Abstract
The common-mode leakage current should be carefully considered when designing a transformer-less photovoltaic (PV) inverter since the leakage current can cause the output current distortion and increase the operational risk. The unipolar SPWM of traditional H-bridge inverter can produce the superior output performance but will cause a high-frequency fluctuated common-mode voltage and consequently the non-negligible leakage current. To attenuate the fluctuation phenomena of common-mode voltage, few neutral-point-clamped circuits have been designed to clamp the neutral point voltage and maintain the common mode voltage constant. This paper analyzes the equivalent common mode circuit of single-phase inverters and proposes a generalized design principle of multi-terminal neutral-point-clamped circuits, whose unidirectional and bidirectional variations are fully analyzed respectively. Subsequently, two types of single-phase PV inverters with the neutral-point-clamped circuits are proposed, respectively. Also, the operational losses and component counts are compared between the proposed topologies and the traditional NPC inverters. The experimental results verified the theoretical findings.
EXISTING SYSTEM:
The main problem of transformerless PV inverter is that it has no galvanic isolation and then the common-mode leakage current can flow through the parasitic capacitor between ground and PV array, which will threaten people safety and cause other severe problems. To attenuate the common mode leakage current in transformerless inverter, the most effective solution is to keep the common mode voltage constant . So far, several single-phase topologies have been proposed to maintain the common-mode voltage constant as half of DC bus voltage, for example, H5, HERIC and H6 topologies . However, the common-mode voltage is indeed variable during the freewheeling stage because of the potential variation induced by charging and discharging the switch junction capacitance. Thus, the leakage current still exists in the common-mode circuit. Besides, few neutral-point-clamped circuits have been proposed to effectively clamp the common mode voltage to be half of dc bus voltage during the whole fundamental period . These NPC topologies are mainly applicable in the transformerless PV generation systems with the required low level leakage current. But the general design principles and the implementation procedures of the neutral-point-clamped circuits were not included in the already published literatures
PROPOSED SYSTEM:
this paper presents the general design method of clamping circuits and proposes the corresponding unidirectional and bidirectional neutral-point-clamped circuits and analyzes their operational theory. And then two types of neutral-point-clamped single-phase PV inverters are proposed by using the unidirectional and bidirectional clamping circuits, respectively. The full comparison between the generalized NPC PV inverters and the traditional NPC PV topologies in terms of their operational efficiency and component counts is also presented. The experimental results verified the performance of the proposed circuits.
CONCLUSION
This paper generalizes two kinds of neutral-point-clamped circuits with the unidirectional or bidirectional current flow capability. Then this paper implements the specific NPC circuits in single-phase photovoltaic inverters, where two types of single-phase NPC inverters are fully analyzed. Doing so, the common mode voltage of single-phase inverter can be effectively maintained constant as half of dc-link voltage. This paper also presents the comparison between the generalized inverters and the traditional NPC topologies. The experimental results verified the performance of the NPC inverters
REFERENCE
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