FTEEE-1693 Highly Reliable Transformer less Photovoltaic Inverters With Leakage Current and Pulsating Power Elimination – IEEE EEE Project 2016-2017
A transformer less inverter topology, which is capable of simultaneously solving leakage current and pulsating power issues in grid-connected photovoltaic (PV) systems. Without adding any additional components to the system, the leakage current caused by the PV-to-ground parasitic capacitance can be bypassed by introducing a common mode (CM) conducting path to the inverter. The resulting ground leakage current is therefore well controlled to be below the regulation limit. Furthermore, the proposed inverter can also eliminate the well-known double line frequency pulsating power that is inherent in single-phase PV systems. By properly injecting CM voltages to the output filter capacitors, the pulsating power can be decoupled from the dc-link. Therefore, it is possible to use long lifetime film capacitors instead of electrolytic capacitors to improve the reliability of the PV system. The mechanism of leakage current suppression and the closed-loop control of pulsating power decoupling are discussed in the paper in details. A 500 W prototype was also built and tested in the laboratory, and both simulation and experimental results are finally presented to show the excellent performance of the proposed PV inverter.
Transformer less grid-connected photovoltaic (PV) inverters are widely accepted in the PV market mainly because of their high efficiency, low cost, small volume, and light weight. These features may not be possessed by their transformer galvanic ally isolated counterparts. However, since the PV panels in such systems have direct electrical connection with the power grid, the PV-to-ground parasitic capacitance, the PV inverter, and the utility grid may then form a conduction loop. High frequency common mode (CM) voltage-induced leakage current can flow through this loop if the unipolar modulation strategy is adopted for full-bridge inverters. The leakage current is definitely adverse to the system performance, and it may potentially lead to a series of problems, e.g. harmonic current, increased power losses, safety issues, and electromagnetic interference (EMI) issues.
The proposed transformer less PV inverter is essentially derived from a conventional full-bridge inverter with an output LC filter. The LC filter is split into two identical parts, having Lf1 = Lf2 = Lf and Cf1 = Cf2 = Cf. They are distributed into the two switching legs as shown in Fig. 1. More advanced LCL or LLCL filters can also be adopted, but they may increase the complexity of the system. The midpoint of the two capacitors is then connected to the negative dc bus in order to provide a conducting path for the CM current. Because of the symmetrical circuit configuration, its differential mode (DM) operation, i.e. active power injection and reactive power support will not be affected.
- Circuit configuration and leakage current elimination
- Design of the closed-loop power decoupling control
- Review of the pulsating power elimination 5. Simulation Setup
- Report Generation
Circuit configuration and leakage current elimination:
The dc current injection is mainly caused by the measurement errors from current sensors, and it may affect the normal operation of distribution transformers. To cope with this issue, an additional dc suppression loop can be introduced to control the dc current injection as discussed. Regarding the safety issue, the PV module frames should be grounded in transformer less PV systems in order prevent electric shocks and fire hazards. Moreover, a residual current device (RCD) should also be installed to monitor the leakage current. The RCD will disconnect the inverter if a 30 mA jump in the leakage current is detected, or the leakage current exceeds 300 mA.
Design of the closed-loop power decoupling control:
It can be seen, vcomp may consist of multiple even order harmonics, and this explains why a single second order term cannot completely eliminate the pulsating power. This also implies that several resonant controllers can be paralleled in the control loop in order to minimize the fluctuating power in the dc-link. In fact, the grid voltage may contain background harmonics, which may interact with the sinusoidal grid current and give rise to higher order pulsating power. In this case, vcomp will correspondingly be distorted by higher even order harmonics, and therefore more resonant controllers should be implemented for pulsating power elimination.
Review of the pulsating power elimination:
The inner current control loop of the CM controller is a simple proportional gain kip CM, and its main purpose is to make the plant model seen by the voltage controller to be a first order system. In this case, higher control bandwidth and more accurate power decoupling can be realized by using. The overall block diagram of the implemented control system is presented where the measured variables used for the closed-loop control are red highlighted. All the designed controllers are further discretized by the Tustin method and implemented in a digital control platform. The controller gains should be designed with the considerations of the system delays caused by the digital computation of duty cycles and the pulse width modulation (PWM) of the power converter.
Simulations were conducted with PLECS, based on the system. The main objectives were to show the low ground leakage current, and the effectiveness of the proposed power decoupling control. The system parameters can be found, and the resulting resonance frequency of the output LCL-filter is at 1062 Hz, which is less than 1/6 of the sampling frequency. In this case, the DM current control will be inherently stable, because converter side current feedback is adopted here. It should be noted that, only a 30 μF film capacitor was used in the dc-link, and this can better show the performance improvement achieved by the proposed power decoupling control.
A single-phase transformer less inverter topology has been presented for grid-connected PV applications. By introducing a CM conducting loop into the inverter system, the ground leakage current issue can be solved without adding additional active components to the circuit, and the system cost and complexity can be reduced. The inherent pulsating power in single-phase PV systems can also be eliminated by injecting proper CM signals to the modulation of the inverter. Therefore, long lifetime film capacitors can be used in the dc-link to replace those less reliable electrolytic capacitors. A dual mode closed-loop controller is also proposed to completely decouple the pulsating power even in the presence of system uncertainties and disturbances. Comprehensive experimental results have been presented to show the excellent performance of the proposed inverter, and the results can well match with those obtained from PLECS simulations.
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 Automatic Disconnection Device between a Generator and the Public Low-Voltage Grid, Germany Standard DIN VDE 0126, 2006, 2010.