Abstract—In this study, a single stage, three-phase, three-level neutral point clamped inverter is designed for grid connected renewable energy systems. The proposed voltage source inverter is operated in current controlled mode and a PI current controller is used for the production of switching pattern. Also the proposed inverter is operated in parallel with public grid by using phase locked loop method. Maximum power point tracking process which is aimed to obtain maximum energy from the supply is performed via the proposed inverter without using any additional DC-DC converter. Maximum power point tracking algorithm generates the current reference for the current controller. Results of experimental studies show that inverter output current is in phase with line and unity power factor operation is obtained. Also inverter output current total harmonic distortion is measured as 3.95%. This value is in the limits of international standards (<5%).

Index Terms—Three-level inverter, MPPT, Grid interactive inverter, current control.

I. INTRODUCTION

Technological developments and increasing world population increase the world energy demand exponentially. Photovoltaic (PV) modules generate DC electrical energy from solar energy. This DC energy should be converted to AC energy and usually static inverters are used for this purpose. The inverters can be designed in voltage source or current source structure. Although, energy always flows from DC supply to AC system in current source converters, bidirectional energy flow can be obtained by using voltage source inverters. Current controllers or voltage controller, or combination of these controllers can be used for inverters. The use of current control for grid connected PWM inverters is becoming very popular in distributed generation, due to the need to control both the harmonic content and the power factor of the current [1].

On the other hand, voltage and current levels and switching frequencies of the static switches limit the power levels of the inverter. Also cost of the switches exponentially increases at high power levels. Several methods have been proposed to increase the power level of the inverters. Using multiple inverters is one of these methods. But this method is not economical. Using serial or parallel connected switches increase the voltage and/or current, thus inverter power level increases. But non-

This work was supported by Gazi University Academic Research Projects Unit under the grant number of 07/2011-52.

identical components, current paths, small differences between the control signals and differentiation of the switch characteristics with temperature may cause unbalanced voltage and/or current sharing between the serial and/or parallel connected switches. Additionally, unbalanced currents of parallel IGBT modules require a careful selection of IGBTs (e.g. according to switching times), the manual parameterization of gate units and a substantial current derating of parallel connected devices in state of the art Voltage Source Converters (VSCs). Obviously these are important disadvantages [2].

Recently, multilevel inverter structures are used to increase the inverter voltage and power level to achieve higher power levels. Although, number of switches is greater than two level inverters, higher voltage and power level can be achieved by using switches which are lower rated values in multilevel inverters. Also, use of these inverters becomes popular because of some advantageous such as decreasing size of the filters, lower dv/dt and lower electromagnetic interference (EMI) level, lower switching losses [3-6]. Furthermore, commercially available three level power modules decrease the cost and simplify the design of the multilevel inverters.

Different system structures are proposed to export the energy, produced from PV modules, to the grid. In these systems, the main scope is converting DC energy produced from PV modules to AC energy which is conditioned to variable grid conditions. Maximum power point (MPP) of the PV system should be tracked because the irradiation, temperature and the operation point of the system are variable. Maximum power point tracking (MPPT) process is usually implemented with DC-DC converters. In this case, this structure is called two-stage inverter because two power conversion stages (DC-DC and DC-AC) are used. In single stage inverter systems, both DC-AC conversion and MPPT process are carried out by inverter. Initially, this structure did not used because MPPT algorithm had negative effects on inverter output current quality. But today, this structure gains importance with the improving microcontroller and power switch technologies.

In past studies, generally cascaded multilevel, single stage inverters which require isolated power supplies are investigated. In these studies, indirect MPPT methods such as constant voltage method, constant current method, look-up table method and sample PV module method are used or output current quality defined in international standards cannot be achieved with direct

Single Stage Three-Level MPPT Inverter for Solar Supplied Systems

S. Ozdemir1, N. Altin2 and I. Sefa3 1Ataturk Vocational School, Gazi University, Ankara, Turkey, sabanozdemir@gazi.edu.tr

2Department of Electrical-Electronics Eng. Faculty of Technology, Gazi University, Ankara, Turkey, naltin@gazi.edu.tr 3Department of Electrical-Electronics Eng. Faculty of Technology, Gazi University, Ankara, Turkey, isefa@gazi.edu.tr

978-1-4673-1301-8/12/$31.00 ©2012 IEEE

2012International Symposium on Power Electronics,Electrical Drives, Automation and Motion

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MPPT methods. There are limited studies (generally simulation studies) about neutral point clamped (NPC) grid interactive inverters for PV applications. Removing of isolated power supply requirement and commercially available power switches module for a leg of the inverter are the some advantageous of the NPC inverters.

In this study, three phase, three-level grid interactive NPC inverter is proposed to export the energy generated from PV modules. The proposed inverter converts the DC energy to AC energy and exports to the grid and tracks the MPP of the PV system without using any additional converter. Perturb and Observe (P&O) method is used for tracking the MPP of the system and a PI current controller is used for shaping inverter output current. It is seen from the experimental results that inverter output current is in sinusoidal waveform and total harmonic distortion (THD) level of this current is in the limits of the international standards (3.95%<5%). Also unity power factor operation is achieved. Total efficiency of the MPPT algorithm and inverter is measured as 93.87%.

II. BASICS OF NPC INVERTER

First studies about multi-level inverters are carried out by Nabae [7]. Initially these inverters could not be used commonly because of control difficulties and higher number of components than used in conventional inverters. However, multi-level inverters become common especially in high power applications with progress in microcontroller technologies and decrease in power switching devices cost. The multi-level inverters have some advantages according to conventional two-level inverters such as:

• Lower output current/voltage harmonics, • High voltage and high power level can be achieved

by using low voltage switches, • Lower filter requirements for same frequency

levels, • Lower dv/dt ratio and lower electromagnetic

interference (EMI), Nevertheless, the multi-level inverters have

disadvantageous higher numbers of circuit components (power switches, capacitors, isolated gate drivers etc.) and complex circuit structures [3, 5, 6, 8].

There are different types of multi-level inverter topologies. These topologies are classified according to their specifications such as bidirectional power flow capability and number of phases [9]. Well known of them are the NPC inverter, the cascaded H-bridge (CHB) inverter and the flying capacitor (FC) inverter [3,8]. In NPC topology, DC bus voltage is divided by two capacitors and balancing of these two capacitors’ voltages is difficult. The FC topology requires bigger capacitor and this requirement increases the size of system. Also, CHB structure requires isolated DC voltage sources. Because the NPC inverter does not require isolated DC sources and power switch modules for a leg of the inverter are commercially available, design and implementation of the NPC inverter is simple. The NPC inverter scheme supplied from PV is given in Fig. 1.

Fig. 1. Structure of NPC Inverter

As seen from figure all power switches of the NPC inverter operate with only half of the DC bus voltage. This situation is valid P, O and N positions of the switches. Consequently, there is no problem about voltage sharing between the power switches [10]. Output voltage value according to switch position is given in Table 1.

Table 1. Output voltage value according to switch positions for a leg

of the inverter.

Switch Position S1 S2 S3 S4 Output voltage

P (Positive) 1 1 0 0 1/2V

O (Medium) 0 1 1 0 0

N (Negative) 0 0 1 1 -1/2V

III. MPPT ALGORITHMS FOR GRID INTERACTIVE

INVERTERS

Power level and efficiency of the PV modules vary directly with irradiation level and module temperature because the modules cannot store the energy. MPPT is a control method which is used to obtain maximum available power from PV modules. This control method is usually applied with power converter which is conditioning the PV generated power.

MPPT methods can be classified into two major groups such as direct (online, or true seeking) methods and indirect (offline or quasi seeking) methods [11]. The main scope of the classification is "True MPPT is implementing or not?". Indirect methods estimate the MPP of PV system by using some specifications of the system such as module open circuit voltage, module short circuit current, irradiation level, module temperature and etc. In indirect methods, real output power of the module is not calculated so only approximate MPP of system can be found. Constant Current (CC), Constant Voltage (CV), Plot Cell (PC) and Look Up Table (LUT) are the commonly used indirect MPPT methods.

In direct MPPT methods, voltage and/or current values of the PV modules are used to define the operation point and operation point is changed to detect the MPP. The real MPP can be found by using direct methods but an oscillation occurs around the MPP because of change of the operation point. The amount of oscillation depends on MPPT methods used in the system and circuit design.

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The most important advantageous of direct methods is irresponsive to variables such as PV module characteristics, temperature, irradiation, pollution/shading of module and etc. [11]. Perturb&Observe (P&O), Incremental Conductance (IC) are the most common direct MPPT methods.

In P&O methods, control signal is changed and output power is tracked. The next control signal is defined according to power variation (increase or decrease in power). In IC method, operation point of the system is found according to power variation versus module voltage (dP/dV). At the MPP dP/dV=0, in the left side of the MPP, dP/dV is negative, and in the right side of the MPP, dP/dV is positive. The next control signal is determined according to these data. It is determined that there is a constant ratio between the MPP, open circuit voltage and short circuit current of a PV module. The CC and the CV methods use these relations between the MPP and short circuit current and open circuit voltage respectively. In these methods, the PV system disconnected from load and voltage/current values of the PV system and sample PV are read periodically to define the control signal. In the LUT method, the control signal is determined by using PV module data (voltage, current, irradiation, temperature) and previously prepared tables [11]. In the PC method, parameters of an additional PV module which has the same characteristics with the PV system are used to determine the control signal.

In grid interactive inverters which are used to export energy generated form PV system to the grid one or more power conversion stages are used. Two-stage grid interactive inverter structure which is the most common structure and single stage inverter structure with different MPPT methods are seen in Fig 2(a) and (b) respectively. MPPT process is implemented with a converter and DC/AC conversion and other processes are implemented with the other converters in two or higher number state inverters. Besides, MPPT and grid interactive operation processes are carried out with same converter in single-stage grid interactive inverters. Increasing stage number of the system also increases the cost and decreases the efficiency, but makes control easier.

In single stage grid interactive inverter, the DC current drawn from PV is not pure DC current so some errors occur in instantaneous power calculations of the power based MPPT methods. A mean power value can be calculated by increasing the number of the sample to implement MPPT. Also filter capacitors and inductors can be added to linearization of the voltage and current waveform. In this case, filter values for the same ripple factor differs according to inverter structures. Using ultracapacitors or additional converter or converters may be solutions to linearize the voltage and current and sustain the stability of the system under the rapidly changing load or power conditions.

(a)

(b)

Fig 2 a) Double stage grid interactive inverter structure b) Single stage grid interactive inverter structure

The ultracapacitors can be connected at input of

inverter directly or via a DC/DC converter [12]. On the other hand, ultracapacitors and battery groups increase the size and cost of the system, and their life cycles are limited. Also different converter topologies and structures with additional switches are proposed [13].

Another difficulty with single stage MPPT grid interactive inverter is inverse power flow possibility. Instant power demands decrease the DC voltage and at that time power flows from grid to DC side and power calculation errors occur and operation around the MPP is difficult. So grid interactive system should be protected to the DC bus voltage collapses.

IV. PROPOSED THREE-LEVEL GRID INTERACTIVE

INVERTER

In this study, single stage, MPPT, three-phase, three-level NPC grid interactive inverter is designed and implemented. The proposed system consists of three-phase, three-level NPC inverter, MPPT algorithm, current controller, LCL output filter and line frequency transformer as seen in Fig. 3. System is controlled with TMS320F28335 eZdsp control board. This controller board has a 32 bit floating point processor with 150 MHZ clock frequency. This processor and control board are suitable for the control of the three phase grid interactive inverter. It has 12 bit ADC and 16 bit PWM resolutions. Also an USB2000 controller JTAG emulator board has been used to communicate with the computer and online monitoring the system variables.

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Fig. 3. Block diagram of the proposed system. P&O method, which is one of the direct MPPT

methods, decreases the oscillations around the MPP and can be easily implemented in digital systems. This method is also called “Hill-Climbing Method”. In this method, PV power is calculated and current reference of the inverter varied to increase the power. Measuring current and voltage of the PV is enough to find the MPP. The base relationships between the output power of the PV and reference current value is given in Table 2 [14].

Table 2 The rule base of the P&O method.

Change in Reference

Change in Power

Change in Next Reference

Positive Positive Positive Positive Negative Negative Negative Positive Negative Negative Negative Positive

V. EXPERIMENTAL RESULTS

Hardware of the proposed three-phase three-level NPC grid interactive inverter is seen in Fig. 4. The system consists of a three phase NPC three level inverter, a line frequency transformer and a LCL filter. A PI compensator is used for the current control. Inverter output voltage is boosted to the 230 V with a line frequency transformer. The line frequency transformer also prevents DC ripple injection in current waveform and provides galvanic isolation between the inverter and the utility line. In addition this transformer simplifies the grounding of the DC energy source.

The use of a LCL filter for the grid side and of a current controller which is able to perform harmonic compensation of the grid background distortion is almost mandatory. The LCL filter offers the possibility to reduce switching harmonics injection as well as to limit the value of the converter boost inductance decreasing encumbrance and mounting cost [1, 15]. LCL filter is obtained by using LC filter and transformer primary winding inductance. Static switch group is used to connect and disconnect the inverter to the grid.

In this study, MAGNA-POWER TDS III 600-8 Photovoltaic Power Profile Emulator (PPPE) is used as DC energy source. This power supply can operate as PV modules. PPPE is programmed as serial connected five KYOCERA KD245GX-LPB panels (P=245W, VOC=36.9V, ISC=8.91A) and emulated at 890W/m2 irradiation level. Semikron SKM 200 MLI 066 T IGBT modules are used for every leg of the NPC inverter.

Fig. 4. Hardware of the proposed three-phase grid interactive inverter.

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The general view of the proposed system is shown in Fig. 5. The output currents and transformer primary side voltages waveforms of the single stage, three-phase, and three-level NPC inverter are seen in Fig. 6. As seen from the figure, inverter output currents are in sinusoidal waveform and in phase with line voltage phase and the frequency.

Because the inverter is supplied from the PV modules, amount of the exported energy must be equal to the generated energy. The rated power of the proposed system is 1000 W. Inverter output current harmonics, output power, and power factor are depicted in Fig. 7(a) and Fig 7(b) for its rated power. As seen from these figures, inverter output current quality meets the international standards (3.95% < 5%).

Also power factor of the inverter is near unity (>0.99) and unity power factor operation is achieved. Proposed inverter is operated for different load levels (%30-%100) and inverter output current harmonics and power factor values are investigated. The proposed inverter current THD is in the limit of international standards (< 5%) and power factor is >0.98 for 40% of the rated power. Also power factor is >0.99 at 55% load level. The total efficiency of the inverter and MPPT method is measured as 93.87%.

Fig. 5. General view of the proposed system.

Fig. 6. Inverter output currents and line voltages.

(a)

(b)

Fig. 7. Output current, output power and power factor values of the proposed single stage, MPPT NPC inverter.

VI. CONCLUSIONS

In this study, current controlled NPC grid interactive inverter is designed and implemented. The current reference of the proposed inverter is generated by phase locked loop algorithm and MPPT method and single stage power conversion is achieved. P&O method is used for MPPT. Inverter output current is controlled by PI controller. It is seen from the experimental results that, unity power factor operation is achieved and current THD level is measured as 3.95%. Also, power quality indices are investigated at light load conditions and it is seen that inverter output current THD meets the international standards for >40% load conditions. PV supplied energy exported to the grid via the single stage inverter without using additional DC-DC converter for MPPT, thus efficiency of the system is improved. Also whole system is controlled via a TMS320f28335 without any additional hardware such as CPLD or FPGA.

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