Design and Implementation of Distributed Control Architecture of an AC-Stacked PV Inverter Hamidreza Jafarian, Iman Mazhari, Babak Parkhideh
Saurabh Trivedi, Deepak Somayajula, Robert Cox, Shibashis Bhowmik
University of North Carolina, Charlotte Electrical and Computer Engineering Department Charlotte, North Carolina, USA Email:
[email protected]
SineWatts Incorporated 9319 Robert D. Snyder Road #304D Charlotte, North Carolina, 28223 USA Email:
[email protected]
Abstract— A control scheme for a distributed Inverter Molecule™ architecture is analyzed and implemented in this paper. An Inverter Molecule is a building block for a novel distributed inverter architecture suitable for PV/Battery applications which exceeds the performance metrics obtained by Central/String- inverters with or without DC/DC optimizers or by Micro-Inverters. The proposed transformerless architecture utilizes low-voltage semiconductor devices with much higher yields than their high-voltage counterparts. A group of Inverter Molecules operate autonomously and cooperatively to maintain the grid interconnection requirements while providing maximum power control on each molecule’s PV panel. This paper overviews the state-of the-art control methods implemented mostly on cascaded H-bridge derived topologies and further establishes a distributed control scheme for Inverter Molecule architecture that does not need any high bandwidth communications except a heartbeat signal at line frequency. Experimental results are presented for a lab-scaled prototype. Keywords— Single Phase inverter; Distributed Control; Photovoltaic System; Battery; Energy Storage System; Inverter Molecule; Micro Inverter; Cascaded H-bridge; AC-stacked Inverter
I. INTRODUCTION Recently, due to increased (green) electricity demand, there is unprecedented interest in electrical power generation from renewable energy. Moreover, renewable energy systems are playing an increasingly important role in power production, due to growing environmental concerns and energy independence. Wind energy and Photovoltaic are the major sources of renewable energy which are the 81.6% of renewable power capacity all over the world by the end of 2013 [1] and [2]. Due to advent of new technologies and cost reduction, Solar PV energy has received significant attention in recent years such that, in 2013 solar PV capacity in the world increased by 39%. However, still the role of power generated by PV is very small in when compared to other sources of This work and any subject invention is part of activities being performed under the US Department of Energy (DoE), SunShot Incubator Program, Incubator Round(s) 8 and 9. The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number(s) DEEE0006459 and DE-0006692.
978-1-4673-7151-3/15/$31.00 ©2015 IEEE
energy [2]. The main challenge involved in this, is the cost of power production by using solar energy. In recent years there has been a great effort from policy makers, researchers and industry to reduce the cost of PV energy production. One example is the ‘SunShot” initiative which was introduced by US Department of Energy in 2011. This program that has the goal to reduce the cost of utility scaled PV system to $1/W by 2020, was significantly successful in the first three years [3]. More than 80% of this cost reduction was due to the dramatic pricing pressures observed in the PV module marketplace during the last 3-4 years [4]. PV inverter prices have also reduced 10-15% during that time [2]; however, substantial performance and inverter smartness is now required for making the PV power plants suitable for high penetration of PV onto the electrical grid [5]. An innovative distributed inverter architecture, based on Inverter Molecule™ that promises to deliver utility-grade reliability and performance is being developed in this paper [6]. Different PV inverter configurations have been introduced to convert the solar energy to AC power that can be used in electrical network. These configurations are shown in Fig. 1. Central inverter architecture which is presented in Fig. 1.a, has the advantages of high efficiency power conversion and low first or hardware cost per watt. However, the system lacks reliability and flexibility in its configuration due to a central controller for regulating the system and is significantly expensive in its installation cost. String and multi-string inverters are more flexible and expandable configurations but the energy conversion efficiency is lower when compared to central inverters. Micro-inverters are module-level configurations which have the ability to perform the individual panel-level maximum power point tracking. The main disadvantage of this architecture is that it needs a boosting level which increases the cost and reduces the efficiency because of utilizing two or more stages of power conversion. Micro-inverter is becoming popular for residential and some commercial applications but because of its disadvantages it is typically not applicable for the large commercial and utility segments of the market.
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For all market segments PV inverter control scheme is a critical part of each inverter configuration and will become increasingly demanding as smart inverter functionalities are being mandated and inverters will be required to provide PV power plant intelligence. Since, proper control design for particular configuration can improve the performance and reliability of the configuration. In recent years, there has been a great deal of research work on central control of PV inverters [7], [8]. This paper investigates a new configuration based on an implementation of the Inverter Molecule™ architecture [1]. To the best of the authors’ knowledge, the proposed architecture has not been presented in literature and cannot be categorized in any standard commercially-termed architecture, i.e. Central-, String- or Micro inverter. In the proposed architecture, a few number of inverters operate cooperatively together to maintain the grid connection requirements while providing a maximum power control on each member. Since the building block is an inverter that goes into back of the panel or a battery pack and operates only based on local measurements but yet the architecture requires a few number of inverters, we call the architecture Inverter Molecule™. Moreover, a distributed control method is proposed in this paper that can provide an independent control of each module.
II. BACKGROUND ON MODULAR PV INVERTER CONTROL Since inverter molecule building blocks are single phase inverter, which are connected in series on AC side. In this section we review the state-of-the-art control methods for series connected inverter topologies such as cascaded H-bridge converters which are applied to PV system. In recent years, there has been significant contribution on control of single phase PV inverters [9], [10], [11] and [12]. Multi-loop control schemes have been used to control the DC input active power and output active and reactive power. Two examples of this control architecture are presented in Fig. 2 and Fig. 3. Block diagram presented in Fig. 2 shows one of the recent popular control schemes for regulating single phase PV inverters. In this controller, a proportional control has been used for compensating for dc voltage error and producing output modulation index. A resonant proportional controller controls the output current. Another approach for controlling single phase PV inverters is utilizing D-Q rotating frame principle and utilizing PI controller for regulating D and Q components of output AC current. The Block diagram of this controller is shown in Fig. 3 [11].
Fig. 2. Control block diagram of single stage single phase PV inverter in stationary plane
Fig. 3. Control block diagram of single stage single phase PV inverter in synchronous rotating plane
Fig. 1. Available PV inverter configurations. a. Central Inverter b. String inverter c. Multi-string inverter d. Micro-inverter
This paper is organized as follows: First, a background on modular PV inverter control is provided. In section III, Inverter Molecule configuration is presented. Proposed distributed control architecture is presented in section IV. In section V, the experimental result is provided to verify the proposed configuration. Finally, the last section concludes the work.
Controller methods presented in Fig. 2 and Fig. 3 are applied for regulating single inverters. For PV inverter architectures which require multiple units working cooperatively, significant handshaking and communications among modules are needed which have been proposed in literature [10], [12] and [13]. One of common approaches in literature is cascaded H-bridge.
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Fig. 4. State-of-the-art control schemes for PV-connected Cascaded H-bridge topology; a. control scheme proposed in [23], b. a modern control approach proposed in [13].
Cascaded H-bridge (CHB) topology belongs to family of multi-level converters that provides multiple connection points for isolated DC sources. Operating based on multi-level converter principles, the conversion units can be designed using medium voltage devices and relatively low switching frequency. Several central control architectures has been proposed in literature and two examples of these proposed schemes are presented in Fig. 4. Fig. 4.a presents a centralized control scheme in which the DC link voltage error are added up and a central voltage controller has been applied to it. The output after multiplication with normalized PLL gives the current reference. The string current error is calculated and fed to the current controller. The grid voltage was fed to the output of this controller to get the output voltage references. This signal has been normalized and added to an additional feedforward term from DC voltage reference. The outcome is the modulation index for individual inverters. This normalized value will be used based on predefined algorithm to determine the switching signal for each inverter. As can be seen, this control scheme is centralized.
Another central control scheme for PV string control proposed in [10] is presented in Fig. 4.b. Similar to single inverter control strategies an individual, module-level MPPT is performed and the output is used as reference to control the input DC voltage of inverter for n-1 inverters. The voltage controller outputs are multiplied by normalized sine signal synchronized by the grid information to build their switching functions. The remaining one inverter uses the information of reference DC voltages and measured DC voltages of all inverters to control the total string voltage. Since, it is a series configuration; the current flowing in all inverters is the same. Therefore, only one inverter is required to have a current controller. The current reference is generated by multiplying the voltage controller output with normalized sine signal. This controller output is the sum of all inverter switching functions and thus, switching functions of all other inverters should be subtracted from this to give the switching function for that individual inverter. The centralized PWM generator then generates gating signals for all the H-bridges. As can be seen, in this control scheme one individual inverter should have access to DC voltage and switching function information of all
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the other inverters. This requires high bandwidth communication infrastructure and also has impact on the reliability of the PV string in the case of losing that particular inverter or losing communication links between inverters. The Inverter Molecule configuration which is presented in the next section makes the distributed control of series connected PV inverter string feasible and not only increases the efficiency but also increases the reliability of the operation of PV inverter string. III. INVERTER MOLECULE Inverter Molecule™ is a building block for a novel distributed inverter architecture suitable for PV/Battery applications which exceeds the performance metrics obtained by Central/String- and Micro Inverter combined. A few number of Inverter Molecules operate cooperatively together to maintain the grid connection requirements while providing maximum power control on each member, i.e. PV panel or a low voltage battery pack.
From the control implementation point of view, the CHB is implemented with central controllers to generate switching signals and therefore has been proposed and demonstrated for systems with relatively low switching frequency of a few kHz. Comparing cascaded H-bridge configuration, Inverter Molecule has the benefit of using fully distributed control architecture. In other words, there is no need to transmit high frequency signals over wire or wirelessly and therefore, the speed of switching can be increased as much as semiconductor’s Safe Operating Area (SOA) allows. In the Inverter Molecule, all the molecules are synchronized with a heartbeat signal at line frequency for which we can implement it with a very low bandwidth communication medium. The power system analysis of this topology and islanding detection and protection is presented in a separate paper [14].
The Inverter Molecule™ in its core utilizes low voltage semiconductor devices. This configuration and its tangible advantages were introduced in [6]. Inverter Molecule can provide different topologies such as single-stage inverter and two-stage inverter by using Member Power Mismatch Resolver (MPMR) which is a dc-dc converter and it can be provided in accordance with an embodiment of the energy collection and conversion system. This architecture can also be implemented with Multi-Frequency Energy Coupler (MFEC) which is a dcdc converter connected in parallel with DC bus to supply 120 Hz component of the power to the grid and removes the 120 Hz DC bus ripple in the single phase inverters which reduces the loss in the inverter [6]. In this study, a single stage inverter topology without utilizing MFEC and MPMR is considered to represent the configuration of AC-stacked panel-level inverters. The proposed panel-level configuration has an advantage of utilizing low-voltage semiconductor devices like MOSFETs which can switch at much higher frequencies compared with high voltage switches such as IGBTs. As a result, there are significant opportunities in shrinking the size of passive components hence increasing the power density, implementing efficiency enhancement circuitries, and designing high performance converters with extremely enhanced control bandwidth. It is worth noting that having the Inverter Molecule with distributed control simplifies (if not enable) the implementation of high frequency converters. One of the differences between Inverter Molecule and CHB is related to the fact that in Inverter Molecule each H-bridge inverter output has been filtered by a LC low-pass filter network tuned at >50kHz and connection to the grid is through a very small interface inductor (a few uHs). The contradiction is due to difference in operation principle. In Inverter Molecule architecture after filtering in each inverter stage the output current and voltage are sinusoidal with minimum harmonic distortion content in the range of 0.1%. Applying advanced switching techniques further enhances the output power quality.
Fig. 5. Inverter Molecule string architecture circuitry scheme
IV. PROPOSED CONTROL SCHEME The control architecture and a schematic of Inverter Molecule string consists of two inverters is presented inFig. 5. Unlike, other presented control architectures, in this novel configuration, there is no need for communications between inverters. The control architecture is completely distributed and there are no links between inverters. Individual molecules are controlled locally by measuring individual voltage and current. The only required information is the synchronizing signal which is provided from grid voltage waveform through PLL function that synchs the inverter molecules. In this PV string configuration, one of the Inverter Molecule is assigned to regulate the string output current. This Inverter Molecule is called the Current Administrator Voltage Compensator (CAVC). The overall control block diagram of
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this molecule is presented in Fig. 6. In this controller, the DC voltage input of the inverter is controlled by a PI controller which provides reference for the output current amplitude. The phase of output current is set based on PF requirement of the network. This reference is multiplied to a Sine signal which is synchronized by the grid voltage and builds the current reference. This reference current is compared to string current measurement and after applying a PI or PR controller, the reference voltage is generated. This reference voltage is added up to a feedforward of the measured AC voltage and finally the modulation index for regulating the inverter is generated.
V. EXPERIMENTAL RESULTS In order to verify the proposed PV string configuration and control method, a hardware setup consisting two PV inverters, two PV emulators and a grid emulator is used. The string of inverters is connected to the grid through a relay. After receiving and verifying the heartbeat signal which is synchronization signal from grid, each inverter starts closed loop operation. The closed loop operation started with MPP of 30W and the reference voltage of 29.4 V which is the voltage in maximum power point.
Vdc Meas Vdc ref MPPT Algorithm
+
Ͳ
Vdc Controller Iac amplitude
Sin(wt) PLL Iac ref Ͳ
+
Current Controller
PWM
+
Iac meas
Vac ref
Grid Voltage
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AC Voltage Controller Vac masur
Fig. 6. Multi loop control scheme for CAVC
The remaining (n-1) inverters operate as Voltage Mode Molecules (VMM). These molecules are controlling their own DC voltage and generate the required output voltage to maintain MPP of their respective PV panel. In other words, the string current which is common for all the series connected inverters is controlled by CAVC and each VMM controls its own input voltage in order to deliver the maximum power of the respective PV panel. The simplified control scheme of the VMM inverters is presented in Fig. 7.
Fig. 7. Control architecture for VMM
Fig. 8. Experimental results of lab-scaled string of two Inverter Molecules with the proposed distributed control scheme. Test conditions: emulated grid voltage: 22Vrms, PV-side open-circuit voltage: 30V which is connected to an emulated grid with 22 Vrms, power reference: 30W for each molecule. a) 16 seconds of operation with MPP of 30W for each inverter; b) ramp-up transient operation for a 30W DC power command as MPP for each molecule; c) steady-state operation of the string at 60W (30W per molecule)
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REFERENCES Average mode current control has been used to control the hardware. Each molecule takes advantage of using a TI TMS320F28335 control card. Measured current and voltage signals are sent to the control card through 12 Bit ADC channels. ADC sampling rate is 40 kHz and this information has been used in control loops to compare with generated reference signals. The inverters switching frequency is also 40 kHz. The overview of grid-tied operation of a lab-scaled string consisting of two inverters (one CAVC and one VMM) for 16 seconds is illustrated in Fig 8.a. In this figure, green waveform represents grid voltage and red waveform is string output current. Blue and yellow are VMM and CAVC DC voltage respectively. Fig 8.b shows the system start-up. The system reaches to steady-state in 14 cycles as shown in Fig. 8.a and Fig. 8.c.
[1]
J. Rodríguez, J. I. Leon, S. Kouro, R. Portillo, and M. a M. Prats, “The Age of Multilevel Converters Arrives,” no. June, pp. 28–39, 2008.
[2]
M. Brower, D. Green, R. Hinrichs-rahlwes, S. Sawyer, M. Sander, R. Taylor, I. Giner-reichl, S. Teske, H. Lehmann, M. Alers, and D. Hales, “Renewables and global status report 2014,” 2014.
[3]
2014 SunShot Initiative Portfolioc US Department of Energy. 2014.
[4]
D. Feldman, G. Barbose, R. Margolis, N. Darghouth, and A. Goodrich, “Photovoltaic ( PV ) Pricing Trends : Historical , Recent , and NearTerm Projections Photovoltaic ( PV ) Pricing Trends : Historical , Recent , and Near-Term Projections,” 2012.
[5]
E. Malashenko, S. Appert, and W. Al-Mukdad, “Advanced Inverter Technologies Report Grid Pplanning and Reliability,” 2013.
[6]
S. Bhowmik, “Systems and Methods for Solar Photovolatic Energy Collection and Conversion,” U.S. Utility application number 13/546868, 2013.
[7]
L. Liu, H. Li, and Y. Xue, “Control strategies for utility-scale cascaded photovoltaic system,” in 2013 IEEE Energy Conversion Congress and Exposition, ECCE 2013, 2013, pp. 2391–2397.
[8]
B. Xiao, S. Member, L. Hang, J. Mei, C. Riley, S. Member, L. M. Tolbert, B. Ozpineci, and S. Member, “Modular Cascaded H-Bridge Multilevel PV Inverter With Distributed MPPT for Grid-Connected Applications,” vol. 51, no. 2, pp. 1722–1731, 2015.
[9]
M. Ciohotaru, R. Teodorescu, and F. Blaubjerg, “Control of single-stage single-phase PV inverter,” EPE J. (European Power Electron. Drives Journal), vol. 16, no. 3, pp. 20–26, 2006.
VI. CONCLUSION This paper introduced a novel distributed PV inverter architecture called Inverter Molecule™. The proposed architecture presents superior efficiency and reliability figures than what have been reported for conventional architectures including central-, string-, and micro-inverters. Inverter Molecule™ takes advantage of utilizing low-voltage MOSFETs for which it provides significant opportunities for increasing the efficiency and power density. To fully utilize the potentials of the proposed architecture, inverter members in the string should work cooperatively and autonomously and there must be no high bandwidth communications among the members. Therefore, we proposed a novel distributed control scheme for Inverter Molecule™ in which one string administers the string current and the rest of the inverters control their output voltages. The effectiveness of the proposed distributed control scheme was demonstrated on a lab-scaled prototype and some preliminary results were presented. DISCLAIMER The information, data, or work presented herein was funded in part by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
[10] E. Villanueva, P. Correa, J. Rodriguez, and M. Pacas, “Control of a Single-Phase Cascaded H-Bridge Multilevel Inverter for GridConnected Photovoltaic Systems,” IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 4399–4406, 2009. [11] S. Golestan, M. Ramezani, M. Monfared, and J. M. Guerrero, “A D-Q synchronous frame controller for single-phase inverter-based islanded distributed generation systems,” in International Review on Modelling and Simulations, 2011, vol. 4, no. 1, pp. 42–54. [12] A. Dell’Aquila, M. Liserre, V. G. Monopoli, and P. Rotondo, “Overview of PI-based solutions for the control of DC buses of a single-phase Hbridge multilevel active rectifier,” IEEE Trans. Ind. Appl., vol. 44, no. 3, pp. 857–866, 2008. [13] S. Kouro, B. Wu, A. Moya, E. Villanueva, P. Correa, and J. Rodriguez, “Control of a cascaded H-bridge multilevel converter for grid connection of photovoltaic systems,” in 2009 35th Annual Conference of IEEE Industrial Electronics, 2009, pp. 3976–3982. [14] I. Mazhari, H. Jafarian, B. Parkhideh, S. Trivedi, and S. Bhowmik, “Locking Frequency Band Detection Method for Grid-tied PV Inverter Islanding Protection” in IEEE Energy Conversion Congress & Exposition (ECCE), 2015, p. in press.
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