2013 International Conference on Electrical Machines and Systems, Oct. 26-29, 2013, Busan, Korea
Multiple-Input Multiple-Output Medium Frequency-Link Based Medium Voltage Inverter for Direct Grid Connection of Photovoltaic Arrays Md. Rabiul Islam, Youguang Guo, and Jianguo Zhu Centre for Electrical Machines and Power Electronics University of Technology Sydney, 15 Broadway, Ultimo, New South Wales 2007, Australia E-mail:
[email protected] and
[email protected]
EMC filter
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I.
frequency transformers are heavy and large, in a lot of cases, inefficient. For example, the volume and weight of a 0.4/36 kVA, 1 MVA vacuum cast coil transformer are about 4.3 m3 and 3,250 kg, and the no-load and full-load losses are 3.1 kW and 11.5 kW, respectively [3]. Moreover, the dry type transformers can be sensitive to water, micro-cracks, temperature variations, and pollution which can block cooling ducts.
Solar panels
Abstract — Recent advances in magnetic materials and power semiconductor devices have led to the development of compact and lightweight medium voltage inverter with medium frequency-link, which would be a possible solution to reducing the weight and size of grid-connected photovoltaic (PV) inverter systems. This paper presents the design and analysis of a multiple-input multiple-output medium frequency-link based medium voltage inverters for step-up transformer-less direct grid connection of PV arrays. The multiple-output medium frequency-links generates multiple isolated and balanced dc supplies for all of the H-bridge inverter cells of modular multilevel cascaded (MMC) inverter. To verify the feasibility of the proposed system, a scaled down 1 kV laboratory prototype test platform with 5-level MMC inverter is developed. The design and implementation of the prototyping, test platform, and the experimental results are analyzed and discussed.
INTRODUCTION
Solar photovoltaic (PV) generates electricity in well over 100 countries and continues to be the fastest growing renewable source in the world [1]. Since 2007 medium and large scale PV power plants have attracted great attention. More than 200 PV power plants have already been installed in the world; each of them generating an output of more than 10 MW. More than 250 PV power plants will be installed in the next few years. These multi megawatt PV power plants require large area of land, and thus they are usually installed in remote areas, far from cities. For power transmission, medium voltage grids are commonly used. For grid integration, a power-frequency (i.e. 50 or 60 Hz) transformer is usually used in the PV inverter systems to step-up the voltage to the grid voltage levels, which results in high capital and installation costs because of its heavy weight and large size. A. Classical Inverters for PV Power Plants ASEA brown boveri (ABB) central inverters are especially designed for medium scale PV power plants. The PVS800 version is a 3-phase inverter with a power capacity in the range of 100–500 kW. The PVS800 inverter topology allows a parallel connection directly on the ac side, for grid connection through a step-up power frequency transformer [2]. The transformer step-up the inverter output voltage from 300 V ac to grid voltage level (i.e. 6–36 kV). The central inverter design and grid connection is depicted in Fig. 1. ABB has been delivering worldwide vacuum cast coil dry-type transformers for PV applications. However, the power
Fig. 1. ABB central inverter.
As well as its low-voltage system, Siemens also developed SINVERT PVS inverter based medium voltage system for medium scale PV plants. The ac output voltage and power capacity of PVS version inverters are in the range of 288–370 V and 500–630 kW, respectively. The 1 MW to 2.52 MW central inverters were designed by paralleling 2 to 4 PVS inverters through a transformer and switchgear at the grid side. The design and grid connection of the 2 inverters based system is illustrated in Fig. 2 [4]. Siemens developed GEAFOL cast-resin transformers for grid connection of PV arrays. However, no-load and full-load losses of a 0.4/30 kV, 1 MVA GEAFOL transformer are about 3.1 kW and 10 kW, and the volume and weight are about 3.5 m3 and 2,990 kg, respectively [5]. These heavy and large size power frequency step-up transformers significantly increase the weight and size, and thus the capital and installation costs of the inverter system as well as the running and maintenance costs of the plants. These penalties are critical for remote area applications, where the
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arrays makes the inverter operation complex and limits the range of maximum power point tracking (MPPT) operation.
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costs of installation and regular maintenance are extremely high. In order to eliminate the step-up transformer, different medium voltage inverter topologies have been proposed in the last few years as presented in next Section.
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B. Research and Development for Medium Voltage PV Inverters In 2012, by combination of a quasi-Z source inverter into a MMC as shown in Fig. 3, a medium voltage PV inverter was proposed in [6]. The proposed inverter does not have isolation between PV array and medium voltage grid. Multiphase isolated dc/dc converter based MMC inverter topology as shown in Fig. 4 were proposed in [7, 8]. In the proposed configuration, the voltage balancing is the challenging issue, since each H-bridge cell is connected to a PV array through a dc/dc converter.
Fig. 4. Multiphase isolated converter based MV PV inverter (one phase).
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Fig. 2. Siemens SINVERT PVS inverter.
Fig. 5. Single dc-link based MV PV inverter (one phase).
II. PROPOSED MEDIUM VOLTAGE INVERTER
Fig. 3. Quasi-Z source converter based PV inverter.
Common dc-link may be one of the possible solutions to minimize voltage unbalance problem. Single dc-link based inverter in Fig. 5 has been presented in [9, 10]. Although this design may reduce the voltage balancing problem in grid side, the generation of common dc-link voltage from different PV
Medium voltage grid
As an alternative approach to minimize the voltage balancing problem with wide range MPPT operation, a common medium frequency-link (common magnetic-link) based medium voltage inverter is proposed as shown in Fig. 6. In the proposed inverter, the PV array dc power is converted to medium frequency ac through a medium frequency fullbridge inverter. The medium frequency inverter also ensures constant output voltage.
Fig. 6. Proposed 3-phase PV inverter system for direct medium voltage grid integration.
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The inverter is connected to a primary winding of a multi winding medium frequency-link. Each secondary winding works as an isolated source and is connected to H-bridge cell through a bridge rectifier. The number of primary windings depends on the number of PV arrays and the number of secondary windings depends on number of levels of the inverter. The proposed 3-phase 5-level PV inverter system is given in detail as shown in Fig. 7. In large PV power plants, several PV arrays are operated in parallel, where each PV array is connected to a primary winding through a booster and medium frequency inverter. The common medium frequencylink provides electrical isolation between PV array and grid, thus inherently overcomes common mode and voltage imbalance problems.
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possible candidate for medium voltage applications. Several power semiconductor vendors like Semikron, ABB, IXYS, and Mitsubishi Electric produce switching devices in a module form, all the devices of an H-bridge inverter in a single pack with gate drive circuit, which reduces the complexity and overall size of the inverter, and save the development time and semiconductor costs significantly. The modularity also gives the ability to change or rectify the inverter circuit easily without changing other section of the inverter. The MMC inverter does not required any auxiliary devices where as neutral point clamped (NPC) and flying capacitor (FC) inverters requires a huge number of auxiliary diodes and capacitors, respectively. Although, at around 3-level the number of auxiliary devices of FC inverter are comparable with that of NPC inverter, at higher number of levels which is a few times higher. For example, a 3-phase 3-level FC and NPC inverters requires 3 and 6 auxiliary devices, and a 3phase 17-level FC and NPC inverters requires 360 and 90 auxiliary devices, respectively. Fig. 8 shows a comparison of auxiliary device requirement in FC and NPC inverters. A comparative study among these three multilevel inverter topologies has been carried out in [11, 12]. Based on distortion of the output voltage, control complexity, and semiconductor cost, the MMC inverter may be the natural choice for medium voltage applications. Fig. 9 shows the comparative results for an 11 kV system. It can be seen that the 11-level MMC inverter has gained lowest index value (harmonic distortion, complexity, and cost) among all inverters.
High-frequency-link Rectifier
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Fig. 7. Medium/high-frequency-link MMC inverter based direct grid connection of PV arrays
Fig. 8. Number of auxiliary devices in NPC and FC inverters.
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A. Medium Voltage MMC Converter In comparison with conventional two level inverters, multilevel inverters present lower switching losses, lower voltage stress on switching devices and better harmonic performance. These remarkable features enable the connection of renewable energy systems directly to the grid without using large, heavy and costly power transformers and also minimize the input and output filter requirements. Although several multilevel inverter topologies have been used in low voltage applications, most of the topologies are not suitable in medium voltage applications. Because of some special features: the number of components scales linearly with the number of levels, and individual modules are identical and completely modular in constriction and hence enable high-level number attainability, MMC inverter topology can be considered as a
5-level NPC
III. DESIGN AND DEVELOPMENT
Fig. 9. Overall comparison of different multilevel inverters.
Fig. 10 plots the line voltage waveform of an 11 kV 11level MMC inverter. There are 20 steps in peak to peak line voltage waveform and each step contributes 1,618 V to the line voltage of 32,360 V peak to peak. The 11 kV 11-level MMC inverters give 7.70% total harmonic distortions (THDs).
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The frequency spectrum of line voltage of an 11-level MMC inverter is depicted in Fig. 11. However, the MMC inverter requires multiple-isolated dc sources that must be balanced, therefore its application is not straightforward, especially in renewable power generation systems.
frequency transformer-link design is depicted in Fig. 12. The volume and weight of the transformer needs to be optimized by selecting proper parameters. The winding dimensions depend on the diameter and number of the conductors, and winding structure. Single layer winding provides low ac/dc resistance ratios, which increases the winding and core dimension significantly. For simplicity of the winding process a toroidal structure core is considered. Different factors are considered during the selection of core dimensions, such as the winding dimensions, hole reserve for natural cooling, maximum temperature limits, maximum power loss, availability of core material stripe dimensions, leakage inductance, and possibility to induce equal voltage in multiple secondary windings. Therefore, the design process involves multi-physics problems with some critical decision making tasks.
Fig. 10. Line voltage of 11 kV 11-level MMC inverter with no filter circuit.
Fig. 11. Frequency spectrum of 11 kV 11-level MMC inverter.
B. Multiple-Output Medium Frequency-Link In transformer design, because the winding electromotive force (emf) is proportional to the number of turns, frequency, and magnetic flux linking the winding, for a given power capacity, as the operating frequency increases, the required cross sectional area of magnetic core and the number of turns of the primary and secondary windings can be dramatically reduced. To couple the renewable energy source to the MMC inverter, a medium-frequency transformer-link with multiple secondary windings was developed and electromagnetic performances were reported in [13]. The medium-frequency transformer-link was used to generate the isolated balanced multiple dc supplies for all of the H-bridge inverter cells of MMC inverter from a single low voltage power source. Compared with the conventional transformers operated at the power frequency (50 or 60 Hz), the medium frequency (in the range of a few kHz to MHz) transformers have much smaller and lighter magnetic cores and windings, and thus much lower costs. At the beginning, according to power inverter rating the transformer-link specifications, such as the rated power, frequency, excitation current waveform, and voltage, are calculated. Considering the availability, system requirements, and cost the core material is selected. From the specifications of transformer-link and data sheets of core materials, transformer-link initial parameters are calculated with some assumptions. These parameters are used as initial values of the optimization process. The flow chart of the proposed methodology for multiple secondary windings medium-
Fig. 12. Medium-frequency transformer-link design technique flow chart.
Taking into account the flux density, specific core loss, cost, and availability, we chose Metglas 2605SA1 stripe of 20 μm thickness and 25 mm width as the core material. The other parameters are mass density of 7.18 g/cm3, saturation flux density of 1.56 T, and specific core loss of 180 W/kg at 10 kHz sinusoidal excitation of 1 T. In order to develop a test core the core material Metglas 2605SA1 Alloy sheet was collected from Metglas Inc. The coefficients (k, m and n) of Steinmetz equation (1) from the datasheet for this material are experimental results under sinusoidal voltage excitation, where f is the frequency in kHz and B the magnitude of flux density in T. Usually the medium frequency transformer are operated with non-sinusoidal voltage excitation. Therefore, for this design new coefficients are calculated by measurements under squire voltage
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excitation. The newly derived coefficients and datasheet coefficients are summarized in Table I.
Pcore = kf m B n
(1)
TABLE I COEFFICIENTS OF SPECIFIC CORE LOSS Coefficients Excitation voltage type k m Sinusoidal (from datasheet) 6.50 1.51 Squire (from curve-fitting) 6.2567 1.5800
techniques require special softwares such as HDL coder, System Generator, PSIM and ModelSim, which increase the development time and cost. In this paper, the most common software such as Matlab/Simulink and Xilinx ISE based an alternative design technique is proposed which may save the development time and cost of the switching controller. The Simulink and Xilinx ISE 13.2 Design Suite software based design technique is illustrated in Fig. 14.
n 1.74 1.6195
Using these two sets of coefficients, the plotted specific core losses curves as shown in Fig. 13 have been compared. About 20–30% extra loss is observed due to non-sinusoidal excitation waveform.
15 kHz
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0.32 0.5 0.8 Flux density (T) Fig. 13. Core loss in terms of flux density and frequency: diamond marker indicates sinusoidal excitation and pentagram indicates squire wave excitation. 0.2
C. FPGA Based Switching Controller The PWM generator requirements of multilevel inverters scale quadratically with the number of levels. A 3-phase voltage source N-level MMC inverter needs a 3×(N-1) pair PWM generator. The available microcontrollers at present can only provide about 6 pairs of PWM channels, which are clearly insufficient for multilevel inverter systems. On the other hand, field programmable gate array (FPGA) may provide multiple PWM generators according to the converter requirements. Unlike digital signal processor (DSP) which runs a sequential program in its microprocessor, an FPGA may run all the operations in parallel with the clock signal. The capability of parallel processing of FPGA gives the opportunity to switching controller to updates all gate signals simultaneously. In most of cases, the processing time is independent of the number of inverter levels. Therefore, FPGA technology is a natural choice for the control of multilevel inverters. Various design techniques and software environments are available for the modeling of switching control schemes with FPGA technology. The most commonly used design techniques are: (i) modeling the switching circuit and target system in Matlab/Simulink environment and generation of programming file with HDL coder, and (ii) modeling the switching circuit and target system in Matlab/Simulink environment and generation of programming file with System Generator. In order to verify the performance of generated very high speed integrated hardware description language (VHDL) code in simulation environment the ModelSim and PSIM with ModCoupler are of the possible options. These
Fig. 14. Simulink and Xilinx ISE based design technique.
In this technique the switching control scheme with target system is modeled in the Matlab/Simulink environment first. After getting satisfactory performance from Simulink, the updated model is used for behavioral modeling of the switching controller in the Xilinx ISE environment. The behavioral simulation results are observed, and they were found highly consistent with the Simulink results. After getting satisfactory simulation results, the design is implemented and verified with timing simulation. Before connecting with the target system, the gate signals were measured and they were found highly consistent with the Simulink results and the theoretical values. IV.
RESULTS AND DISCUSSIONS
A scaled down 1 kV 1.26 kVA 3-phase 5-level MMC inverter was developed by using Semikron SK 30 GH 123 IGBTs. Tektronix DPO 2024 Digital Phosphor Oscilloscope with P5200 high voltage differential probe and Tektronix TCPA300 current probe are used to observe waveforms. The total loss (core loss plus copper loss) was measured by a Voltech PW3000A universal power analyzer. Fig. 15 shows the measured primary and secondary side voltage waveforms
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of multiple-output medium frequency-link. The secondary winding current waveform is shown in Fig. 16.
development have brought these technologies to a certain level, more researches are still required for further developments as the new magnetic materials and new high power semiconductor devices are being introduced in the market. The new technology will enable step-up transformerless compact and lightweight inverter design, and has a great potential to be implemented in the future PV power plants and smart grid applications. The compact and lightweight grid integration system will save large installation, running and maintenance costs of PV power plants, especially in remote areas.
Fig. 15. Primary and secondary side voltage waveforms.
Fig. 18. Measured line voltages of the prototype inverter before filter circuit Fig. 16. Secondary winding current waveform.
The output of each secondary winding is connected to a fast recovery diode based rectifier with a low pass RC filter circuit. The dc-link voltages were found approximately equal at about 370 V, which can serve satisfactorily as the isolated and balanced dc sources for the proposed MMC inverter. Without using any special control algorithm regarding capacitor voltage unbalancing the prototype inverter generates satisfactory output voltage waveform as the transformer can provides balanced sources for all of the H-bridge inverter cells of the MMC inverter. Fig. 17 depicts the output phase voltage and phase current of the prototype inverter. As measured, before the filter circuit, the 3-phase 5-level output voltage waveform contains about 20% THD and after the filter it is reduced to less than 5%. Measured output line voltages before filter circuit are illustrated in Fig. 18.
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[2] [3] [4] [5] [6]
[7]
[8]
[9]
[10]
Fig. 17. Measured phase voltage and line current of the prototype inverter (phase voltage before LC filter circuit). V.
CONCLUSION
In this paper, the existing inverter topologies, and research and development trends to design compact and lightweight inverters for recently introduced medium and large scale PV power plants have been reviewed and presented chronologically. The installation of large scale PV power plants has been pushing the inverter technology to more competitive research areas. Although the current research and
[11]
[12]
[13]
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