FTEEE-1695 Efficient Single Phase Transformer less Inverter for Grid-Tied PVG System With Reactive Power Control – IEEE EEE Project 2016-2017
There has been an increasing interest in transformelress inverter for grid-tied photovoltaic (PV) system due to low cost, high efficiency, light weight, etc. Therefore, many transformelress topologies have been proposed and verified with real power injection only. Recently, almost every international regulation has imposed that a definite amount of reactive power should be handled by the grid-tied PV inverter. According to the standard VDEAR-N 4105, grid-tied PV inverter of power rating below 3.68KVA, should attain power factor (PF) from 0.95 leading to 0.95 lagging. In this paper, a new high efficiency transformer less topology is proposed for grid-tied PV system with reactive power control. The new topology structure and detail operation principle with reactive power flow is described. The high frequency common mode (CM) model and the control of the proposed topology are analyzed. The inherent circuit structure of the proposed topology does not lead itself to the reverse recovery issues even when inject reactive power which allow utilizing MOSFET switches to boost the overall efficiency. The CM voltage is kept constant at midpoint of dc input voltage, results low leakage current. Finally, to validate the proposed topology, a 1 kW laboratory prototype is built and tested. The experimental results show that the proposed topology can inject reactive power into the utility grid without any additional current distortion and leakage current. The maximum efficiency and European efficiency of the proposed topology are measured and found to be 98.54% and 98.29%, respectively
RECENTLY, the photovoltaic power generation system has been focused as one of the most significant energy sources due to the rising concern about global warming, and the increase of electrical power consumption. In addition, the PV module has no moving parts, which have made it very robust, long lifetime and low maintenance device. Though the PV module is still expensive, but due to the large-scale manufacturing it has become increasingly cheaper in the last few years. It has been reported that the milestone of 100GW installed PV power all over the world was achieved at the end of 2012 and increased to 140GW at the end of 2013, and the majority were grid connected. Therefore, a prediction has been made that the future grid tied PV system will play an important role in the regulation of the conventional power system.
The proposed transformer less inverter topologies consisting of six MOSFET switches (S1-S6) and six diodes (D1-D6). L1A, L1B, L2A, L2B, L1g, L2g and Co make up the LCL type filter connected to the grid. VPV and Cdc represent the input dc voltage and dc link capacitor. The proposed topology is derived from the topology presented to overcome the low reverse-recovery issues of MOSFETs body-diode when injects reactive power into the utility grid. Therefore, the proposed topology can be implemented with MOSFET switches without reliability and efficiency penalty. The proposed topology can also employ unipolar-SPWM with three-level output voltage.
- High frequency cm model of the proposed Topology for leakage current analysis
- Control of the proposed topology
- Verification with Real Power Injection
- Report Generation
High frequency cm model of the proposed Topology for leakage current analysis
The PV module generates an electrically chargeable surface area which faces a grounded frame. In case of such configuration, a capacitance is formed between the PV module and the ground. Since this capacitance occurs as an undesirable side effect, it is referred as parasitic capacitance. Due to the loss of galvanic separation between the PV module and the grid, a CM resonant circuit can be created. An alternating CM voltage that depends on the topology structure and control scheme, can electrify the resonant circuit and may lead to higher ground leakage current. In order to analyse the CM characteristics, an equivalent circuit of the proposed topology.
In order to illustrate the CM model at switching frequency, could be replaced for the bridge-leg. The grid is a low frequency (50–60 Hz) voltage source; thus the impact of grid on the leakage current can be neglected. The DM capacitor Co can also be removed since it has no effect on the leakage current.
Control of the proposed topology
In order to control the grid current, several existing control methods such as conventional PI controller, repetitive controller (RC), proportional resonant (PR) controller, and deadbeat (DB) controller can be adopted due to the capability of tracking reference signal without steady state error. Since the PR controller has better performance of tracking the reference signal if compared to the normal PI and RC controller, it is selected to control the output current of the proposed topology. The block diagram of the PR controller with harmonic current compensator, where Gc(s), Gh(s), and Gd(s) are the transfer function of fundamental current controller, harmonic compensator, and inverter respectively.
The dynamic response of the system when it is subject to 750W load to 1000W load step change. It can clearly be seen that fast and effective response under the changes of active power reference are achieved with the proposed topology. Therefore, it can be concluded that the proposed topology can inject real power into utility grid with low leakage current and low THD at output.
Verification with Real Power Injection
The proposed topology is verified with 1 kW power injection. The experimental gate drive signals for the proposed topology. It can be seen that the switching signals are fully matched with the proposed PWM scheme, and the gate drive voltages are kept constant at the desired level. The waveforms of CM characteristics. It is clear that the voltages V1N, V2N, V3N, and V4N are clamped at 200V during the freewheeling period of positive and negative half cycle. As a result, the CM voltage is kept constant at 200V for the whole grid cycle except a small fluctuation during grid zero crossing instant as witnessed. Consequently, the leakage current flows through the system are well reduced. During zero crossing instant, a small spike can be observed due to the fluctuation of CM voltage
A new high efficiency transformer less topology for grid-tied PV system is presented. The main advantages of the proposed topology can be summarized as: The inherent circuit configuration of the proposed topology does not lead itself to the reverse recovery issues which allow utilizing MOSFET switches even though when inject reactive power. Therefore, without compromising the overall efficiency, proposed topology can inject reactive power into the utility grid. The CM voltage is kept constant at the mid-point of dc bus voltage; as a result, low leakage current flows through the system which is lower than the H6-type topology. PWM dead time is not required for the proposed topology that reduces the THD at the output.
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