A Photovoltaic Power Conversion System with Flat Efficiency Curve Over a Wide Load Range

A Photovoltaic Power Conversion System with Flat Efficiency Curve over a Wide Load Range

Yantao Song Bingsen Wang Department of Electrical and Computer Engineering Michigan State University 2120 Engineering Building East Lansing, MI 48824, USA [email protected]; [email protected]

Maximum power point tracking (MPPT) techniques based on Abstract-This paper proposes a modular power conditioning power electronic technology enable maximum power unit (PCU) that is intended to improve light load efficiency of extracted from solar panels. Many MPPT schemes, such as photovoltaic (PV) generation systems. The proposed PCU hill climbing, perturb-and-observe and incremental topology consists of many modules in parallel with lower power conductance methods, are proposed to further improve rating, rather than a single module with power rating matching with PV panels. The new structure features high efficiency over efficient utilization of solar cells [7-10]. a wide load range. Equalizing of utilization rate among PCU In PV generation systems, the power conditioning unit modules and maximum power point tracking control are (PCU), which converts electric power from solar panels to the presented. A case study of a 5 kW PV system demonstrates the form in parity with grid, typically contributes to energy loss superior efficiency performance of the proposed PCU structure and high energy cost of the systems. Therefore, improving of and verifies the control strategy. the overall conversion efficiency from the PV panel to loads or grids becomes important as evidenced by the research I. INTRODUCTION effort from both academic and industrial fields. Significant The continuously rising demand for electricity, in research attention has been devoted to the investigation of conjunction with the increasing cost of traditional fossil fuels novel topologies and control strategies aimed to improve the and environmental-safety concerns associated with energy peak efficiency of power electronic converters for consumptions, has favored the development of alternative photovoltaic application [11-14]. Nowadays the peak sources such as solar energy in recent years [1-2]. In efficiency of commercial power electronic systems for comparison with various traditional sources such as coal, oil, residential utility-interface application has already reached gas and hydro, the high energy production cost is the main 97%. In contrast, relatively less attention has been paid to the barrier that hinders the large-scale application of photovoltaic overall efficiency, in particular light-load efficiency. It is well (PV) generation systems. Besides reducing initial investment understood that converter efficiency normally reaches its cost, maximizing energy harvest in the service life of PV peak value around full power rating and drops quickly under systems as much as possible is an effective solution to partially loaded condition. It is a fact that PV panels more decrease their life-cycle energy cost since the abundant solar than often operate with output power being lower or much energy is free. In order to compensate for the high cost of lower than their rated peak power when power converters solar systems, many efforts are made to improve solar energy have lower-than-peak efficiency. This leads to deteriorated harvest of PV generation systems. utilization of solar generation systems and accordingly high PV modules are responsible for converting solar energy energy production cost. Improving efficiency of power into electricity power. Their low-conversion efficiency has converters under light load holds strong potential to increase contributed to high cost of PV-based solar energy. Many new energy harvest of photovoltaic generation systems. materials and technologies have been developed to improve This paper presents a new modular PCU architecture and the efficiency of solar cells [3-4]. However, single-junction corresponding control scheme that will enable a flat crystalline Si and GaAs solar cells are approaching their efficiency curve over a very wide load range. Thus the life- upper limits in terms of thermodynamic maximum efficiency. cycle energy production cost of PV generation systems can be Thin-film and other solar cells have disadvantages in large- reduced. The rest of the paper is organized as the following. scale applications. The conversion efficiency of solar cells Section II introduces the status of PCU efficiency and PV also depends on operating conditions such as temperature, module characteristics. The proposed PCU architecture and solar irradiation density and load conditions. Direct tracking control strategy are described in Section III. A case study is control techniques are developed to maintain sun radiation given in Section IV, followed by some discussions and perpendicular to surfaces of solar panels and maximize concluding remarks in Section V. performance by regulating angles of solar panels [5-6].

1. PCU Efficiency The PCU in a PV generation system generally consists of DC/DC converters and DC/AC inverters. The main function of PCU is to convert electricity generated by PV panels into electric power that is compatible with utility grid. The power conversion process will result in energy loss, which exerts an important influence on the energy harvest of PV systems. Power losses of converters can be divided into constant losses and variable losses that are closely related to operating current or actual power of converters. The former mainly consist of driving losses of power semiconductors, controller losses and core losses of magnetic components. The latter (a) include conduction losses of semiconductors and copper losses of magnetic components, which follow a linear relation to the square of load current. In addition, the variable losses are associated with switching losses of semiconductor devices that vary in proportion with load current. Since these loss components can be described by different functions of load power, it is possible to shape the efficiency curve of converters with different design options. The resultant system may reach peak efficiency at full power, middle power or other loading conditions. Although the PCU design with efficiency optimized at middle power performs well according to CEC weighted average efficiency measurement because efficiency under three fourths of full rated power has (b) the highest weight, it renders thermal management a Fig. 2. Illustration of output characteristics of PV panels affected by (a) solar challenging task. insolation and (b) cell temperature. A typically designed power electronic converter reaches its peak efficiency at or slightly lower than full power. When The P-V curves and I-V curves are obtained from ideal model load power drops, constant losses maintain unchanged. As a of PV cells. The reverse saturation current is calculated based result, the converter has much lower efficiency than full load. on nominal parameters from the module datasheet such as Fig. 1 shows a typical efficiency curve versus changes of load open-circuit voltage and short-circuit current at 25 °C cell 2 power. Efficiency of this PCU is optimized under heavy load temperature and 1000 W/m insolation density [15]. The to mitigate pressure of thermal management. series and parallel resistors are neglected. As a result, the calculated peak powers deviate slightly from the measured 2. Output Characteristics of PV Modules values. At constant temperature, as insolation density drops The output characteristic of PV panels is strongly short-circuit current ISC drops proportionally. Open-circuit influenced by the solar irradiation density and the cell voltage VOC decreases modestly because it follows a temperature. Fig. 2 illustrates the calculated output logarithmic relationship with short-circuit current [16]. For characteristics of the Sanyo PV module HIT-Power-215A example, the peak output power of the PV module also under various insolation conditions and cell temperatures. decreases to a half if solar insolation density drops by one Efficiency Versus Load half as shown in Fig. 2(a). Likewise, in reference to Fig. 2(b), 0.97 cell temperature affects peak output power of the PV module. 0.96 0.95 For the temperature rise of 1 °C, ISC of this PV module 0.94 increases slightly by 0.035% while VOC decreases 0.93 0.92 approximately by 0.27%. It is apparent that the peak power 0.91 drops as the cell temperature rises. 0.9 0.89 The insolation density and temperature vary dramatically in 0.88 a very wide range during a day. Consequently the peak output 0.87 0.86 power of an installed PV module will fluctuate during the 0.85 daytime. Fig. 3(a) shows average solar insolation density of 10% 20% 40% 60% 80% 100% twelve days (twenty-first day of each month) in a year in a

Fig. 1. Efficiency versus load for a 5 kW PV PCU. single-axis tracking system which is located in a place of the

1145 1. PCU Structure Insolation Density Versus Time 1100 Analysis of power losses of a power conditioning unit 1000 suggests that lower efficiency at light load is mainly caused 900 by constant portion of power losses. The reduction of 800 constant losses under partial load conditions will lead to the 700 improved PCU efficiency. This paper proposes a modular 600 PCU structure and control strategy to attain optimized 500 efficiency over a wide power range. The block diagram of the 400 proposed system is shown in Fig. 4. Rather than using a 300 single PCU that matches the full power rating of PV panels, 7 8 9 1011121314151617 the proposed structure employs N interleaved PCU modules with lower rated power to interface PV panels and grid. Each (a) PCU module has the same power rating

Normalized Output Power Versus Time Pac 1.1 P  (1) moduel N 1 where Pac is the total power rating of the PCUs, which 0.9 matches the total power rating of the PV panels. Each module 0.8 has identical components and design. The operating principle 0.7 is explained in detail in the following subsection. 0.6 2. Operating Principle 0.5 The basic principle is that the number n of PCU modules 0.4 that are in operating state depends on the actual output power 0.3 of PV panels, which is mathematically described by 7 8 9 10 11 12 13 14 15 16 17 n  Quotient(Ppv ,Pmodule ) 1 (2)

(b) where Quotient ( X 1 , X 2 ) is the function that returns integer Fig. 3. Peak output power of PV module HIT-215A during a day: (a) part of X1 divided by X2; Ppv is instantaneous DC power average insolation density; (b) normalized output power. generated by PV panels; and 1  n  N . In the morning of a

normal day, solar irradiance is low and the peak power of latitude 20 °N [16]. It is obvious that the solar irradiation density varies across a wide range during a day. With the solar panels is low. A few PCU modules may suffice to assumption that the PV module HIT-215A is installed at this handle the total DC power. The proposed control strategy will location and the cell temperature is 25 °C, the normalized only engage some PCU modules in converting DC power to peak output power is shown in Fig. 3(b), where the base value AC electricity required by grid while other modules are is the maximum output power. Fig. 3(b) clearly illustrates that turned off. The inactive modules will not incur constant within four hours when there is solar irradiation during a day, losses, such as driving losses, controller losses and core the output power of the PV module is less than 90% of its losses of magnetic components, especially high frequency maximum power. transformers. As a result, each of the operating modules can Power rating of grid-connected inverters of PV generation work with high load power and thus high efficiency. As solar systems is designed to match the peak power of PV panels. insolation density increases and the output power of the PV Peak efficiency of converters is obtained usually at approximate rated power to reduce power losses under full load conditions and to ease thermal management. According to the analysis aforementioned, the efficiency of power conditioning unit decreases following reduction of load power. Therefore grid-connected PV inverters operate with deteriorated efficiency performance during most of their operating time, which is adverse to maximize energy harvest and to reduce energy cost of PV generation systems. It is strongly favored for the PCU efficiency to maintain close to peak efficiency over a wide power range. III. INTERLEAVED PCU STRUCTURE WITH HIGHER LIGHT-LOAD EFFICIENCY AND ITS CONTROL STRATEGY

Fig. 4. The proposed PCU architecture with interleaved modules.

1146 panels also becomes higher, more PCU modules are one by one following the same order. For example, in the activated. At noon, solar insolation density hits its peak value, morning the 1st PCU module starts operating, and then the and the PV panels also generate maximum DC power. In such 2nd, 3rd,.., nth module is put into service one by one with the case all PCU modules become active. At afternoon, with increased PV output power. At noon, all modules participate decrease in solar irradiance and PV output power, some PCU in energy conversion. When the PV power starts decreasing modules stop working one by one to maintain remaining PCU in the afternoon, the 1st PCU module is turned off first, then modules operating under heavy load conditions. The the 2nd, 3rd,.., nth module shuts off gradually. If on certain day objective is to minimize constant part of power losses in the the solar irradiance is weak, then only n (nSOLAR INVERTER algorithm is independent of the number of active PCU A 5 kW grid-connected photovoltaic generation system modules. with the proposed power conditioning unit is studied to For DC/DC part, interleaved control can reduce high- demonstrate the improvement of efficiency in comparison frequency current ripple in both input and output filters. Thus with a traditional single-module topology. The specification power losses and filter sizes can be reduced. In addition, of the system is shown in Table I. dynamic response of PV panels can be improved. For DC/AC In order to meet safety requirements and to optimize inverters, interleaved control mitigates current ripple in efficiency of the power conditioning unit, a system of three output filter and common-mode voltage. Interleaved control stages is selected as shown in Fig. 6. The first stage is a of PCU modules is implemented by regulating phase-shifting DC/DC boost converter, which is mainly responsible for angle among modules. The phase-shifting angle  between two consecutive modules is determined by 2   (3) n where n is the number of active PCU modules. 4. Equalizing Utilization Rate of PCU Modules The proposed control strategy may lead to unequal utilization rate among all PCU modules, which means some modules may work under heavy-load conditions for a long duration while others may experience relatively shorter operating time. The differences in operating conditions among these modules may cause them to have different lifetimes or failure rates. Modules with the highest utilization rate will be worn out earlier while those experiencing less operating time may still be in sound status. This undesired scenario will increase maintenance cost of PV systems. A simple circulating pipeline control strategy is designed to actively balance the utilization of PCU modules. The order of activating and deactivating of PCU modules is based on the pipeline principle. The first PCU module that starts to work will stop firstly. When the PV output power increases, the PCU modules are engaged in operation by a preset order. When the PV power decreases from the peak value gradually, the PCU modules that are already operating will be turned off Fig. 5. Circulating pipeline control of PCU modules

1147 TABLE I TABLE II SPECIFICATION OF A 5 KW PV GENERATION SYSTEM COMPONENTS LIST PV panel output voltage range 200-600 V Components 5000W module 1300W module Boost Two SPW47N65C3 in SPP15N65C3 MPPT range 200-450 V switch parallel, Infineon Infineon Single phase grid voltage 240 V, 60 Hz Boost Three C3D10065D in C3D06065D, Output peak power 5000 W diode parallel, Cree Cree Boost Two inductors in series, 3mH, high flux core inductor each: 380uH, two 58907, stacked high flux cores Magnetics 58866, Magnetics LLC SPW47N65C3 SPP15N65C3 switch Infineon Infineon LLC Rectifier IDP18E120, Infineon IDP18E120, Infineon LLC Ferrite core U101/25/R, Ferrite core UR64/R, transformer 36:32:32, 87:77:77 Magnetics Magnetics Fig. 6. Power circuit of a 5 kW grid-connected PV generation system. LLC Ferrite core U57/R, Ferrite core resonant inductor 28uH U26/2/7/R, 28uH regulating PV panels output voltage to a constant value and Magnetics Magnetics DC-link capacitor Six 450HXC00470M Six 450HXC00470M implementing MPPT of PV panels. An open-loop LLC Rubycon Rubycon resonant converter works as a DC/DC transformer, which Inverter FGY75N60SMD IRG4BC20U, provides a galvanic isolation between PV panels and grid. top switch Fairchild IR The ZVS-on characteristic of primary-side MOSFETs Inverter IRG4PC50S, IRG4BC20S, bottom switch IR IR reduces their switching losses. The ZCS-off characteristic of Inverter C3D10065D, C3D04065D, secondary rectifiers removes reverse recovery losses of freewheeling diode Cree Cree diodes. Silicon Carbide diodes are not required in this Inverter Four inductors in series, 5.6mH ,two stacked inductor each: 280uH, high flux cores topology. Instead, less expensive ultrafast diodes can be two stacked high flux 58099, selected. The last stage, a full-bridge inverter that works as cores 58098, Magnetics Magnetics dual buck converters, fulfills utility-interface function and controls power quality of the PV generation system. of proposed structure is much higher than the traditional PCU The traditional solution is a single PCU module with 5 kW structure under light-load conditions although they approach power rating. The proposed structure consists of four (4) the same under heavy-load conditions. The efficiency of the interleaved modules. The rated power of each module is 1300 proposed PCU is higher by ten percentage points at five W. The DC/DC part includes the boost converter and LLC percent of full load and by two percentage points at twenty- resonant converter, and DC/AC part is the full bridge inverter. five percent of full load. This will improve energy harvest of The power components for two structures are listed in Table the PV generation system under operation conditions below II. The maximum permissible current ripple determines the the half of the full load, and therefore reduce the life-cycle inductances of the inductors. Herein, it is assumed that the energy cost of the systems. steady-state peak-to-peak value of the current ripple should 2. Verification of Circulating Pipeline Control not exceed 20% of the average or root-mean-square value of A simulation model based on Matlab is built to verify the the inductor current. The principle of component selection circulating pipeline control strategy. A hysteresis band of 50- and design is to optimize efficiency under full load.

1. Efficiency Comparison of Two Structures Efficiency Versus Load For efficiency calculation, power losses of magnetic 0.97 components are obtained from analytical derivation, while 0.95 losses of semiconductors and electrolytic capacitors are 0.93 obtained from simulation based on SIMetrix and Pspice 0.91 models provided by component manufacturers. Both the Existing Architecture analytical and numerical models are not detailed in this paper 0.89 Proposed Architecture due to space limitation. The boost converter operates with 0.87 switching frequency 40 kHz. The switching frequency of the 0.85 LLC resonant converter and the inverter is 20 kHz. The boost 0.83 input voltage is 300 V. The fixed losses of the converters are assumed to be 0.6% of their rated power. The efficiency 0.81 5% 10% 20% 25% 40% 50% 60% 75% 80% 100% curve of the proposed topology is plotted in comparison with Percent of Full Load a conventional one as shown in Fig. 7. It is evident that the Fig. 7. Efficiency comparison of the proposed and existing PCU proposed structure of power conditioning unit features a flat architectures. efficiency curve over a very wide load range. The efficiency

1148 improvement in efficiency is verified through a case-study of a 5 kW grid-connected solar generation system. Although this structure with many modules in parallel means comparably higher cost of PCU, the PCU cost just occupies a small portion of the total system cost, and increased cost can be offset by more energy harvest. The control strategy provides a viable option for control and operation management of large solar plants in which many grid-connected solar inverters works in parallel. As to the reliability of the system, the PCU topology reduces the unscheduled downtime and improves the mean time to failure of the overall system since other modules can maintain continuous operation with the failed PCU module being isolated from the system.

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