FTEEE-1692 Transformelress Photovoltaic Inverter Based on Interleaving High-Frequency Legs Having Bidirectional Capability – IEEE EEE Project 2016-2017
A novel bidirectional transformelress Photovoltaic (PV) inverter based on the High Frequency Leg (HFL) technique is proposed which can work on Discontinuous Current Mode / Continuous Current Mode (DCM/CCM) having greatly enhanced reliability. With the high-frequency-leg, the smooth ac current is achieved as the higher equivalent switching frequency can reduce the inductor current ripple decreasing the passive components’ volume. There’s no dead time issue which can push the duty cycle to the theoretical limit and fully transfer the energy to grid through total Pulse Width Modulation (PWM). And the capacity of the PV (Photovoltaic) inverter ca34n be expanded easily by increasing the number of high-frequency legs. Additionally the proposed topology can work under the rectifier mode having the bidirectional power capability, which is attractive for PV (Photovoltaic) application. In the end the experimental results of 8-kW laboratory prototype have verified the feasibility and effectiveness of the proposed transformelress PV (Photovoltaic) inverter under standalone mode.
Grid-connected Photovoltaic (PV) systems, particularly low-power transformelress single-phase systems (The national standard GB/T 30427-2013 of the People’s Republic of China rules that the maximum output power of single-phase PV systems is 8kW), are becoming more important worldwide. There is a small PV power generation system, which consists of the PV panel, the battery, the inverter, AC grid and the users’ load. Among them, the inverter is the core of the whole system. In the daytime, PV panels produce the power to the grid or the users directly, and the redundant power is brought to the battery for charging at the same time. At night, PV panels stop working without sunlight, while the grid and users rely on the battery supplying power.
Transformelress PV grid-tied inverters have many advantages of higher efficiency, lower cost, smaller volume, and less complexity compared to that with transformer galvanic isolation. Taking into consideration these factors, highly efficient single-phase inverter topologies that will most likely reach a high level of efficiency at low cost are the ones constituted by a single-stage structure without transformer, called as transformer less PV inverters. One key issue for the transformer less PV inverter with high efficiency and reliability is that in order to achieve high efficiency over a wide load range it is necessary to utilize MOSFETs or some switches with better performance for all switching devices because of its low conduction and switching loss.
- Two Basic Switching Cells
- Traditional transformerless photovoltaic Inverters
- Ground loop leakage current analysis for the proposed transformerless inverter
- Report Generation
Two Basic Switching Cells:
All power electronics circuits are based on two simple switching cells and a combination of the basic switching cells defined as P-cell and N-cell. Each cell consists of one switching device (a MOSFET, IGBT or any other switching device) and one diode connected to three terminals: (+) which is connected to the positive of a voltage-source or capacitor, (-) which is connected to the negative of a voltage-source or capacitor, and a common terminal shown as (→) or (←). They can be used directly as the DC converter, and also applied commendably in the technique of inverter and rectifier. The advantage is that using P-cell and N-cell in the inverter topologies can avoid the problem of shoot-through. All inverters can be similarly constructed by the basic switching cells.
Traditional transformerless photovoltaic Inverters:
The HERIC topology is consisted of a normal full-bridge circuit with each group of diagonal switches being operated at high frequency during one half-wave of the output voltage. An additional branch placed in parallel with the filters and load has two switches in opposite directions, each one is active during one whole half-period of the grid waveform. The drawbacks here are the increased quantities of semiconductors and the reactive power incapability. Examples of commercial products with such topology are the NT-series of inverters from the manufacturer Sunway’s.
The two P-Cell legs are used for the selection of grid voltage directions. The common terminal of the P-Cell leg with Spf and Dpr choosing the positive direction is connected with the positive port of ac side and the negative N-Cell HFL with Ln1 and Ln2. The common terminal of the P-Cell leg with Snf and Dnr choosing the negative direction is connected with the negative port of ac side and the positive N-Cell HFL with Lp1 and Lp2. Spf and Snf would work at power frequency in switching cycle under inverter mode (PV mode), and the role of Dpr and Dnr is to make the choosing direction legs under rectifier mode.
Ground loop leakage current analysis for the proposed transformerless inverter:
A galvanic connection between the grounds of the grid and the PV array exists in transformer less grid-connected PV systems. Large ground leakage currents may appear due to the high stray capacitance between the PV array and the ground. So it is necessary to analyse the ground loop leakage current in the inverter structure. Leakage currents are avoided due to the absence of high frequency oscillations since the positive output of the PV array is directly connected to and to the phase positive output the phase negative output (the neutral), respectively, during the positive and negative half-waves. In the experiment, the waveforms of the common-mode voltage are monitored. The experiment monitor results confirmed the leakage currents is limited successfully in this circuit.
The discontinuous inductor current iLp1, the continuous total inductor iout and the output voltage vout under the 1.8kW load condition. The waveforms iLp1 are the inductor Lp1 current at the negative half-line cycle. The two inductor currents iLp1 and iLp2 operating at the positive half-line cycle, and the sum of them can be found. As shown, the iL is the sum of the two inductor current waveforms, and the total current ripple is reduced obviously compared with either one. It can be obviously seen that the circuit operates in the DCM at this time as the inductor currents are discontinuous shown in Fig.19. However the total current iL is continuous and it can be inferred that the proposed PV inverter topology can operate well in the DCM. When the circuit operates in the DCM, the switches can be turned on under ZCS condition, so that the losses of the switches are reduced and the efficiency of the system is improved.
A new bidirectional transformer less PV Inverter with DCM/CCM operation has been proposed in this paper using the interleaving high-frequency-leg Concept. With the high-frequency-leg, the smooth ac current is achieved as the high switch frequency can reduce the inductor current ripple based on DCM/CCM operation. With the application of the proposed topology the dead time is not needed at PWM switching commutation instants, so that the low ac output current distortion is achieved. And the capacity of the inverter can be expanded expediently by increasing the number of high-frequency-leg. Besides the topology proposed in this paper is a bidirectional inverter, can also work as a rectifier. With the advantages all above the proposed topology is very attractive for transformer less PV inverter applications.
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