A Three-Phase Transformerless T-Type- NPC-MLI for Grid Connected PV Systems with Common-Mode Leakage Current Mitigation

Abstract

DC to AC inverters are the well-known and improved in various kinds photovoltaic (PV) and gird tied systems. However, these inverters are require interfacing transformers to be synchronized with the grid-connected system. Therefore, the system is bulky and not economy. The transformerless inverter (TLI) topologies and its grid interface techniques are increasingly engrossed for the benefit of high efficiency, reliability, and low cost. The main concern in the TL inverters is common mode voltage (CMV), which causes the switching-frequency leakage current, grid interface concerns and exaggerates the EMI problems. The single-phase inverter two-level topologies are well developed with additional switches and components for eliminating the CMV. Multilevel inverters (MLIs) based grid connected transformerless inverter topology is being researched to avail additional benefits from MLI, even through that are trust topologies presented in the literature. With the above aim, this paper has proposed three -phase three-level T type NP-MLI (TNP-MLI) topology with transformerless PV grid connected proficiency. The CM leakage current should handle over mitigating CMV through removing unwanted switching events in the inverter pulse width modulation (PWM). This paper is proposes PV connected T type NP-MLI interface with three-phase grid connected system with the help of improved space vector modulation (SVM) technique to mitigate the CM leakage current to overcome the above said requests on the PV tied TL grid connected system. This proposed the SVM technique to mitigate the CM leakage current by selecting only mediums, and zero vectors with suitable current control method in order to maintain the inverter current and grid interface requirements. The proposed PV tied TNP-MLI offering higher efficiency, lower breakdown voltage on the devices, smaller THD of output voltage, good reliability, and long life span. The paper also investigated the CM leakage currents envisage and behavior for the three-phase MLI through the inverter switching function, which is not discussed before. The proposed SVM on TL-TNP-MLI offers the reliable PV grid interface with very low switching-frequency leakage current (200mA) for all the PV and inverter operation conditions. The feasibility and effectiveness of the TLI and its control strategy is confirmed through the MATLAB/Simulink simulation model directly as compared with 2kW roof top PV plant connected TL-TNP-MLI experimentation, showing good accordance with theoretical investigation. The simulation and experimental results are demonstrated and presented in the good stability of steady state and dynamics performances. The proposed inverter reduces the cost of grid interface transformer, harmonics filter, and CMV suppressions choke.

Analytical Modeling of Neutral Point Current in T-type Three-level PWM Converter

“…To implement the voltage balancing control, a simple PI controller was used based on the voltage error input between the upper and lower capacitor to change the modulation index in vertical axis; however, there was no analytical model about that. In [12], a three-phase transformerless T-type multilevel inverter for photovoltaic application was proposed, and a novel hysteresis space vector current control (HSVCC) scheme was used to mitigate the common-mode leakage current and assure capacitor balancing; however, there was no specific explanation how the HSVCC scheme affects the capacitor balancing. In [13], the simplified model predictive control for a T-type inverter was proposed, and the predicted neutral point voltage was used just to select the suitable small vectors without the weighting factor in the cost function for balancing capacitor voltages with reduced execution time.…”

A Simple Multilevel Space Vector Modulation Technique and MATLAB System Generator Built FPGA Implementation for Three-Level Neutral-Point Clamped Inverter

“…Among them, space vector modulation (SVM) offers better-quality voltage and current output with higher DC-link utilization. In addition, SVM provides a switching state selection opportunity to improve the performance of the MLI [4][5][6][7][8][9][10][11][12][13][14][15]. Particularly, the SVM contributions in NPC-MLI are widely researched and applied in the various fields of drives and renewable energy integration applications [5].…”

“…The switching selections in the multilevel SVM are mainly associated with the space vector diagram (SVD) synthesization and switching states on-time calculation. The multilevel SVM using two-level concepts are widely explored than the other MLI SVM methods [4,9,[12][13][14][15][16]. However, these methods include complex mathematics to calculate the target reference voltage vector and recognize inner sub-triangle etc.…”

“…In this method, the three-level SVD has fragmented to a six, equal, two-level SVD, and its location of the centre of six virtual hexagons originated through segregation of the SVD [17]. Similar to this method, in the literature, many papers have been reported in which shifting the origin to one of the six centres, and αβ-axes are rotated by 60 • to use two-level on-time calculations [12][13][14][15][16][18][19][20][21]. Even though these methods are calculating the individual switch on-times from segregated two-level SVM, while extending to higher levels that need complex mathematical functions to calculate the sub-triangle.…”

“…Due to the development of ASIC technology, the FPGA based implementations have become popular, since it has an ability to implement custom hardware solutions and reprogramming flexibility. The SVM implementations on FPGA are showing a higher interest in the current era [6,12,14,[32][33][34][35][36][37][38][39][40][41][42][43][44]. These implementations are mainly done through Altera and Xilinx Spartan family.…”