Power Electronic Converters for Grid-connected Photovoltaic Systems Aravinda Perera Ezekiel Muyembe Jacobus Brink Muhammad Shahbaz October 29, 2010
Contents 1 Introduction 1 1.1 Motivation................................. 1 1.2 Overview of a Grid Connected Photovoltaic System.......... 1 1.3 Inverters in Grid Connected Photovoltaic System............ 1 1.4 Project Rationale, Objectives and Scope................. 2 2 PV inverter topologies 3 2.1 Transformer-based Versus Transformerless Inverter Topologies..... 3 2.2 Country Policies Regarding Grid Connected PVs............ 3 2.3 Transformerless PV inverter topologies.................. 3 2.3.1 H4 (H-Bridge) topology with bipolar modulation....... 3 2.3.2 H4 (H-Bridge) topology with unipolar switching........ 4 2.3.3 H5 Topology (H-Bridge with DC bypass)............ 4 3 Ground leakage current in transformerless topologies 7 4 Simulations 9 4.1 Simulation settings............................. 9 4.2 Limitations................................. 9 4.3 Results.................................... 10 4.3.1 H4 topology with unipolar switching and 100nF stray capacitance to ground........................... 10 4.3.2 H4 topology with unipolar switching and 1µF stray capacitance to ground.............................. 12 4.3.3 H4 topology with bipolar switching and a split inductor output filter................................. 14 4.3.4 H4 topology with bipolar switching and a single inductor output filter................................. 16 5 Conclusion 19 A PSCAD circuits 21 i
List of Figures 2.1 The H4 topology.............................. 4 2.2 The H5 topology from SMA........................ 4 2.3 The H5 topology s freewheeling path during the positive half..... 5 2.4 The H5 topology s switching signals, at a much lower frequency for display purposes.............................. 5 3.1 Ground leakage current path in a grid-connected transformerless inverter system................................ 8 4.1 Simulated PV array voltage fluctuation for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground)................... 10 4.2 Simulated output voltage before and after the LC filter for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground)............ 11 4.3 Simulated harmonics of output voltage for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground)................... 11 4.4 Simulated ground leakage current for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground)....................... 12 4.5 Simulated PV array voltage fluctuation for the full bridge topology using a unipolar switching scheme with a split inductor output filter (1µF stray capacitance to ground).................... 12 4.6 Simulated output voltage before and after the LC filter for the full bridge topology using a unipolar switching scheme with a split inductor output filter (1µF stray capacitance to ground)............. 13 4.7 Simulated harmonics of output voltage for the full bridge topology using a unipolar switching scheme with a split inductor output filter (1µF stray capacitance to ground).................... 13 4.8 Simulated ground leakage current for the full bridge topology using a unipolar switching scheme with a split inductor output filter (µf stray capacitance to ground)........................... 14 4.9 Simulated PV array voltage fluctuation for the full bridge topology using a bipolar switching scheme with a split inductor output filter.. 14 4.10 Simulated output voltage before and after the LC filter for the full bridge topology using a bipolar switching scheme with a split inductor output filter................................. 15 4.11 Simulated harmonics of output voltage for the full bridge topology using a bipolar switching scheme with a split inductor output filter.. 15 4.12 Simulated ground leakage current for the full bridge topology using a bipolar switching scheme with a split inductor output filter...... 16 4.13 Simulated PV array voltage fluctuation for the full bridge topology using a bipolar switching scheme with a single inductor output filter. 16 4.14 Simulated output voltage before and after the LC filter for the full bridge topology using a bipolar switching scheme with a single inductor output filter................................. 17 4.15 Simulated harmonics of output voltage for the full bridge topology using a bipolar switching scheme with a single inductor output filter. 17 4.16 Simulated ground leakage current for the full bridge topology using a bipolar switching scheme with a single inductor output filter...... 18 ii
A.1 H4 unipolar................................. 21 A.2 H4 bipolar.................................. 22 A.3 H5...................................... 22 iii
Abstract Renewable energy source are increasingly becoming popular for power generation. Amongst the available renewable energy sources, solar power particularly has impressed the power specialists due to its inherent advantages discussed in the introduction. In the Photovoltaic (PV) System, substantial percentage of energy loss will be saved if the transformer, interfacing the PV panel and the grid, is replaced by special switching topologies. The pros and cons of the transformerless topologies and its features are discussed in the next chapter. Ground leakage current and efficiency are the challenges to be addressed in the transformerless inverter topologies. These issues are discussed with simulation results in the subsequent chapters. Safety of the transformerless topologies is even regulated in certain countries due to the safety issues resulting from the absence of galvanic isolation. In conclusion, the results of our analysis are summarized in the final chapter.
Chapter 1 Introduction The global energy demand proliferates every year with increasing technology and fast industrial growth rates of high dense regions like China, India and Brazil. However, the fossil fuels are in rapid extinction. On the other hand, supplying electric power with quality and reliability has been a challenge due to the overloads in the grid transmission systems. Renewable energy sources provide the ideal answer to the above questions. Sources as such as sun, wind, hydro etc are free and will remain many a years to come. Also power generation from these sources can be done in the close proximity to the consumers hence the transmission losses and the stress on the grid is greatly avoided. Out of the renewable sources, solar energy is the most promising hence popular due to following inherent advantages; 1. Available in abundance 2. Environmentally friendly power generation. 3. Since there are no moving parts, the hardware is very robust, and has a long lifetime and low maintenance requirements [5]. Today, many giant companies around the globe have pumped billions of dollars to make the Photovoltaic (PV) systems more efficient and reliable. Spain, Germany, Japan and USA are some of the leading countries that already use grid-connected PV Systems. 1.1 Motivation Grid-Connected Solar energy technology is the fastest growing energy technology in the world today [11]. Sun casts an amount of energy on the earth surface daily which would be sufficient for the whole world population for their energy consumption for more than 27 years. However, the efficiency of the solar systems is yet to reach its climax. Many researches around the globe are carried out to improve the efficiency of the power electronics of the system. Our motivation is to analyse the effect of removing the transformer which is used for galvanic isolation and analyse the main two transformerless topologies, H4 and H5. 1.2 Overview of a Grid Connected Photovoltaic System PV grid-connected system is one of the most commonly found distributed generations. Such a system typically comprises a photovoltaic generator (PVG), a DC-bus, a pulse width modulation (PWM) inverter, a grid filter and a grid utility. [6] 1.3 Inverters in Grid Connected Photovoltaic System A power electronic interface is developed to convert the available direct current generate by the PV panels and feed it into the utility grid. This power electronic interface consists mainly of an inverter. The inverter has to fulfil three main functions in order to feed energy from a PV array into the utility grid: 1. To shape the current into a sinusoidal waveform; 2. To invert the current into an AC current, and 3. If the PV array voltage is lower than the grid voltage, the PV array voltage has to be boosted with a further element. The way these three functions 1
are sequenced within an inverter design determines the choice of semiconductor and passive components and consequently their losses, sizes and prices. [6] 1.4 Project Rationale, Objectives and Scope PV inverter systems can be improved in terms of efficiency using transformerless topologies, but new problems related to leakage current need to be dealt with. The work presented in this report deals with analyzing and modelling of transformerless PV inverter systems regarding the leakage current phenomenon that can damage solar panels and pose safety problems. The major task of this research was the investigation and verification of transformerless topologies and control strategies to minimize the leakage current of PV inverter topologies in order to comply with the standard requirements and make them safe for human interaction. 2
Chapter 2 PV inverter topologies Depending on the electrical isolation between the PV panels and utility grid, the inverter can be isolated or non-isolated. This galvanic isolation is usually realized by the means of a transformer, and thus transformer-based inverters can be distinguished from their transformerless counterparts. Below is a review of some salient differences between these two inverter categories used in case of grid connected PV systems. 2.1 Transformer-based Versus Transformerless Inverter Topologies Galvanic isolation can be on the DC side, in the form of a high frequency DC-DC transformer or on the grid side in the form of a big-bulky AC transformer. Both of these solutions offer the safety and advantage of galvanic isolation, but the use of transformer introduces losses in the system and also needs more space and leads to noisy operation. Introduction of a boost converter between the solar system and the inverter eliminates the use of a transformer, thereby reducing the losses. This way, the efficiency of the whole PV system can be increased with an extra 1-2One disadvantage of transformerless systems is that the missing line-frequency transformer can lead to DC currents in the injected AC current by the inverter, which can saturate the core of the magnetic components in the distribution transformer, leading to overheating and possible failure [9]. 2.2 Country Policies Regarding Grid Connected PVs The presence of the galvanic isolation in a grid connected PV system depends on the local country regulations [4]. In some countries, like the UK and Italy, galvanic isolation is a requirement and is done either by a low-frequency step-up transformer on the grid side or by a high-frequency transformer on the DC side of the converter. On the other hand, there are countries like Germany and Spain, where the galvanic isolation can be left out, in case another technological solution is used to separate the PV array from the electrical grid [3]. 2.3 Transformerless PV inverter topologies We discuss only the most common single-phase transformerless topologies. 2.3.1 H4 (H-Bridge) topology with bipolar modulation This is the most basic topology which is formed up of two half bridges. To control the four switches of this topology, several PWM techniques can be implemented. The simplest one is the bipolar PWM [8], which modulates switches T1-T4 (Figure 4) complementary to T2-T3, resulting in a two level output voltage (+VDC and -VDC). The conversion efficiency is reduced due to the fact that during the free-wheeling period the grid current finds a path and flows back to the DC-link capacitor. 3
Figure 2.1: The H4 topology Figure 2.2: The H5 topology from SMA 2.3.2 H4 (H-Bridge) topology with unipolar switching H4 with unipolar switching is preferable to bipolar switching because three voltage levels are output, and has a higher efficiency. However, it introduces a serious problem for transformerless systems: high ground leakage currents. This makes H4 with unipolar switching unsuitable for use in highly efficient systems. One way of adressing this issue, while still using a unipolar switching type, is implemented in the H5 topology. 2.3.3 H5 Topology (H-Bridge with DC bypass) The H5 topology [7] uses a modified version of the H-Bridge with a unipolar type of switching which allows disconnection of the PV array from the grid during the zero voltage vector, increasing the efficiency of the inverter due to the fact that the freewheeling current will not go back to the DC-link capacitor. T1 and T3, in Fig. 2.3.3, are switched with the grid frequency; T1 is continuously ON during the positive half, while T3 is continuously ON during the negative half of the reference voltage. To make the positive voltage vector, T5 and T4 are switched simultaneously with high frequency, while T1 is ON and the current will flow through T5-T1 returning through T4. During the zero voltage vector, T5 and T4 are turned OFF and the freewheeling current finds its path through T1-T3, as detailed in Fig. 2.3.3. The negative voltage vector is done by switching T5 and T2 simultaneously with high frequency, while T3 is ON, during the corresponding half period of the reference voltage and the current will flow through T5-T3 returning through T2. 4
Figure 2.3: The H5 topology s freewheeling path during the positive half Figure 2.4: The H5 topology s switching signals, at a much lower frequency for display purposes 5
A circuit for H5 was constructed in PSCAD, but correct output waveforms were not obtained. The circuit and switching generation for H5 and H4 bi/unipolar is shown in Appendix A. 6
Chapter 3 Ground leakage current in transformerless topologies A transformerless topology lacks the galvanic isolation between the PV array and grid. This way the PV panels are directly connected to the grid, which means that there is a direct path for the leakage ground currents caused by the fluctuations of the potential between the PV array and the grid. These voltage fluctuations charge and discharge the parasitic capacitance formed between the surface of the PV and grounded frame, shown as C G P V in Fig. 3. The effect of different values of C G P V and of a split or single inductor in the output filter was simulated, and can be seen in the following chapter. The parasitic capacitance together with the DC line that connects the PV array to the inverter, form a resonant circuit and the resonance frequency of this circuit depends on the size of the PV array and the length of the DC cables [2]. A study, presented in [10] discusses the electrical hazards when a person touches the surface of the PV array. Based on the inverter topology, PV panel structure and modulation strategy, when touching the surface of the panels, a ground current could flow through the human body and if the current is above a certain levels it could lead to a shock or resulting in personal injury, as also discussed in [1]. The path of the ground current (IG-PV) flowing through the parasitic capacitance of the PV array is shown with an intermittent line in Fig. 3 In [10] several recommendations are given, which lead to the minimization of the before mentioned leakage current, by: - grounding the frame of the PV array, which reduces the capacitance, thereby minimizing the ground leakage current. - carefully choosing the topology and the modulation strategy, thereby reducing the voltage fluctuations between the PV array and ground. - disconnecting the inverter under service maintenance. 7
Figure 3.1: Ground leakage current path in a grid-connected transformerless inverter system 8
Chapter 4 Simulations All simulations were done on PSCAD, a time-step based simulator which solves differential equations. 4.1 Simulation settings Simulation step size 1µs Switching frequency 20100Hz DC Voltage 400V The bipolar switching frequency was chosen to be a multiple of 50 (the output frequency) and 3 (to minimize harmonics), and to be above 20kHz to disable audible effects. Because the unipolar output frequency is double the input switching frequency, the unipolar switching frequency was chosen as 10050Hz. In the simulation figures the ground leakage current is labelled i c 4.2 Limitations The majority of PV inverters on the market include a boost stage in order to raise the low voltage of the PV array to the needed DC-link voltage of around 400V (singlephase system in Europe) or 700V (three-phase system in Europe). During this research only single stage DC to AC topologies for single-phase grid connection have been studied with a power rating of up to 3kW/phase for the low power utility grid. The PV array has been simplified by using a DC power source in simulations. All the active and passive components within the modelled electrical circuit were taken to be ideal. 9
4.3 Results 4.3.1 H4 topology with unipolar switching and 100nF stray capacitance to ground Figure 4.1: Simulated PV array voltage fluctuation for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground) 10
Figure 4.2: Simulated output voltage before and after the LC filter for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground) Figure 4.3: Simulated harmonics of output voltage for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground) 11
Figure 4.4: Simulated ground leakage current for the full bridge topology using a unipolar switching scheme with a split inductor output filter (100nF stray capacitance to ground) 4.3.2 H4 topology with unipolar switching and 1µF stray capacitance to ground Figure 4.5: Simulated PV array voltage fluctuation for the full bridge topology using a unipolar switching scheme with a split inductor output filter (1µF stray capacitance to ground) 12
Figure 4.6: Simulated output voltage before and after the LC filter for the full bridge topology using a unipolar switching scheme with a split inductor output filter (1µF stray capacitance to ground) Figure 4.7: Simulated harmonics of output voltage for the full bridge topology using a unipolar switching scheme with a split inductor output filter (1µF stray capacitance to ground) 13
Figure 4.8: Simulated ground leakage current for the full bridge topology using a unipolar switching scheme with a split inductor output filter (µf stray capacitance to ground) 4.3.3 H4 topology with bipolar switching and a split inductor output filter Figure 4.9: Simulated PV array voltage fluctuation for the full bridge topology using a bipolar switching scheme with a split inductor output filter 14
Figure 4.10: Simulated output voltage before and after the LC filter for the full bridge topology using a bipolar switching scheme with a split inductor output filter Figure 4.11: Simulated harmonics of output voltage for the full bridge topology using a bipolar switching scheme with a split inductor output filter 15
Figure 4.12: Simulated ground leakage current for the full bridge topology using a bipolar switching scheme with a split inductor output filter 4.3.4 H4 topology with bipolar switching and a single inductor output filter Figure 4.13: Simulated PV array voltage fluctuation for the full bridge topology using a bipolar switching scheme with a single inductor output filter 16
Figure 4.14: Simulated output voltage before and after the LC filter for the full bridge topology using a bipolar switching scheme with a single inductor output filter Figure 4.15: Simulated harmonics of output voltage for the full bridge topology using a bipolar switching scheme with a single inductor output filter 17
Figure 4.16: Simulated ground leakage current for the full bridge topology using a bipolar switching scheme with a single inductor output filter 18
Chapter 5 Conclusion The H4 topology serves as a useful base for more complex topologies which result in greater efficiencies. These topologies overcome issues of safety and efficiency by isolating the DC source from the grid during the zero current vector and greatly minimizes the ground current leakage caused by the PV frame s stray capacitance. We would advise the introduction of the simulation package earlier in the course. The interface, and simulation in general, takes time to learn. That takes time away from grasping the learning opportunities offered during the project. 19
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Appendix A PSCAD circuits Figure A.1: H4 unipolar 21
Figure A.2: H4 bipolar Figure A.3: H5 22