ICSET 2008

Abstract—In this paper we introduce a series connected photovoltaic inverter that delivers both real and reactive power to a consumer load. It operates in series with the grid supply and injects power to the load along with the grid. Since the installed PV capacity is less than the load, the grid real power supply drops by an equivalent amount thereby resulting in energy savings. The inverter can also provide reactive power compensation so that the load voltage is kept constant even when the grid voltage fluctuates by around 6% of its rated voltage. The inverter serves a dual purpose during nighttime, when the circuitry is configured as a reactive power compensator. This way the proposed inverter is used to provide voltage compensation to the load all the time throughout day and night.

I. INTRODUCTION

Previous strategies to interconnect photovoltaic power inverters to the grid consist of parallel connections [3]. For single PV panels, with typical power ratings below 140W and maximum terminal voltage below 50 V DC, it was necessary to include a step up stage to increase the voltage to around 400 V DC [12] - [13], with typical efficiency under 95%. Furthermore, under cloudy conditions, the output voltage could fall below the minimum voltage required for the inverter causing it to stop working. Alternatively, if there are faults in the grid, it may cause damage to the inverter. The inverter in such a parallel connection only works during daytime, and therefore is used for about 50% of the time.

The series connected photovoltaic power inverter presented in this paper can be used more efficiently throughout day and night as a real and reactive compensator. For example, during a voltage sag, the load voltage can still be maintained equal to the rated voltage as the sum of the grid voltage and PV output voltage. A similar principle applies during a voltage swell, where the load voltage is kept constant by reducing the voltage contribution from the inverter [7]-[9].

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Fig.1.Circuit diagram of the series connected photovoltaic power inverter

Fig.1 shows the schematic diagram of the series connected photovoltaic inverter being connected to a resistive load.

II. ENERGY SAVINGS WITH SERIES INVERTER

To demonstrate the effectiveness of the voltage compensation offered by the series connected inverter, an experiment using an ac power source has been carried out. The source provides 24 Vrms and is connected to the mains via an isolation transformer as shown in Fig 2.

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Fig 2 Circuit diagram of the test power savings

Table-1 shows the results of the experiment. We can see that the system is able to save 0.214 kWh of energy per day. That is 5.136 kWh in one month and 61.6 kWh per year. This experiment is tested at a power rating of 500 W, and the proposed inverter power rating is 1700 W. Therefore, if the inverter operates at full load, the energy saving is 209.44 kWh per year.

Total energy from grid without inverter 7.245 kWh Total energy to the load with inverter 7.256 kWh Energy from ac inverter 0.283 kWh Energy from grid with series inverter 6.973 kWh

Power saving 0.214 kWh One day

Table 1 Energy flow with/ without ac model.

III. SIMULATION OF SERIES CONNECTED INVERTER

Fig 3 shows the circuit diagram of the inverter. A diode is connected in series with the DC source and a capacitor in parallel in order to stop the current being fed back into the PV panel. The full bridge output is connected to an isolating transformer in order to isolate the grid from the PV panel and eliminate DC offsets. The transformer secondary output is connected in series with the grid, and the net voltage passes through the LC filter and appears across the load.

Series Connected Photovoltaic Power Inverter Fei Kong*1, Cuauhtemoc Rodriguez2

Gehan Amaratunga1, Sanjib Kumar Panda3

1 Electrical Engineering Division, Engineering Department, University of Cambridge UK, CB3 0FA 2 Electromechanical Group, Cambridge Consultants Limited, UK, CB4 0DW

3Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576 *Email: fk248@cam.ac.uk

595978-1-4244-1888-6/08/$25.00 c© 2008 IEEE

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Fig.4 shows simulation results of the proposed series connected inverter under a voltage sag condition of 20%. The red signal is the inverter output voltage, the blue signal is the grid voltage, and the green one is the load voltage. These three voltages are in phase, and the load voltage equals the inverter output voltage plus grid voltage and the load energy will be taken from both sources.

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Fig 4 Simulation output result AC voltage compensation During nighttime, the inverter will operate by taking input energy from the AC grid source and provide voltage compensation to the grid voltage to maintain the net load voltage constant at its rated value. In this mode the inverter is operated using an ac type buck converter in order to step down the voltage.

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1. AC mode buck converter

A series connected ac voltage regulator, normally uses an AC to DC rectifier and then followed by a DC to AC inverter [4, 5]. In this mode of operation of the converter, the power will be lost in both stages. The output voltage range would be limited and cannot vary from 0 to full grid voltage.

The ac mode Matlab circuit diagram with LCR load is shown in Fig. 5. In the proposed series connected photovoltaic power inverter, it uses an ac mode buck converter, and the efficiency of the single-stage converter is higher than that of the two-stage converter [10]–[11]. It also operates, by using only four switches. The buck converter can share the same H-bridge inverter, between DC operation and AC operation. The output voltage can vary from 0 to full grid voltage.

2. Operation of the ac mode buck converter Fig. 6 (a) shows the operation of the ac mode buck converter operation in the voltage sag case. Switches S1 and S4 operate at high frequency, and S3 and S2 operate at low frequency. In this case, S3 is always off, and S2 is always on. S1 and S4 are switched in complementary ON and OFF mode.

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Fig.6. AC mode buck converter operation diagram.

When S1 is on, the current goes through S1 into L and passes through the primary winding to S2, and goes back to the source, as given in Fig.6.a.(1).When S1 is off and S4 is on, the energy stored in the inductor and primary winding will be discharged through S4, Fig.6.a.(2) shows the equivalent circuit for this stage. In the negative cycle, the current will go through S2, primary winding, and the inductor, and go through S4 back to the source, as shown in Fig.6.a.(3). When S1 is off and S4 is on, the energy stored in primary winding and inductor will be discharged through S4 and S2, but the current direction is opposite. Fig.6.a.(4) shows the equivalent circuit for this stage.

The output voltage equals the buck converter output voltage plus grid ac source voltage. Because the voltage across the secondary winding is in the same direction as the ac source, the voltage across the load equals grid ac source voltage plus buck converter output voltage. Fig 6. (b) shows the operation of the ac model buck converter operates in voltage swell model. Switches S3 and S2 are operating at high frequency and are switched in complementary ON and OFF mode. S1 and S4 are switched at low frequency. The difference is that only S4 remains always ON and S1 is always OFF. The output voltage will be out of phase with the load voltage in order to compensate it. The load voltage equals to the grid ac voltage minus the buck converter output voltage because the voltage across the secondary winding is opposite to the ac source.

The output voltage uses the same equation as the conventional buck converter, Vout = Vin x D. Here D is the duty cycle of switch S1 in voltage sag model, and the duty cycle of switch S3 in swell model. The simulation result is shown in Fig 4. The green waveform is the load output voltage, which is 30 V higher than the blue grid voltage waveform.

Fig 7 shows the ac model in voltage swell mode, where the load voltage (green color waveform) is lower than the grid voltage (blue color waveform) by about 30 V. It can be seen that the inverter output voltage waveform in red color is out of phase with the grid voltage to compensate the voltage swell.

Fig 8 shows the waveform for an inductive load. The current and voltage have a phase shift. The red trace is the load voltage, the green trace is the inverter output voltage, and the blue trace is the current through the load.

Fig 9 (a) shows the Orcad Pspice simulation result of the voltage across inductor L, when switches S1 and S4 operate without dead time. Fig 9 (b) shows the simulation result of the voltage across inductor L, when switches S1 and S4 operate with the dead time of 10 ns. From Fig 9(a) and Fig 9 (b), the voltage across the inductor is higher when S1 and S4 operate with dead time. Because when both S1 and S4 are off during the dead time, the inductor L will immediately be open circuited. From the equation L=V*di/dt, the time is short, so di/dt , making the voltage across the inductor very large. Without dead time, S1 is off and S4 is on immediately. The experimental results are shown in section IV.

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Fig 9 (a) Voltage across the inductor without dead time

Fig 9 (b) Voltage across the inductor with dead time

3. PV operation

During daytime the inverter will operate in PV mode and the energy from the PV panel will be fed into the load. The energy delivered to the load has two parts, one from the grid and the other from the PV module.

A diode connected in series between inverter and PV panel is used to stop current being fed back. Maximum Power Point Tracking (MPPT) using sliding-mode observer method is reported in [1]. Inverter operation with a PV source limits real power injection to instances where the grid is below its rated voltage. Above rated voltage, e.g. during a voltage swell, operation is stopped because the energy cannot be fed back into the PV panel. The normal output peak voltage of the inverter is 30 V, and the PV panel typical output voltage at maximum power point is around 36 to 40 V. During adverse weather conditions the output voltage of the panel can drop to 32 V, and in this case the inverter can still work properly. In other words the inverter operation time is longer than in the parallel connected case. Voltage compensation using this scheme ranges from 0-10% of the grid voltage, so that a 140W inverter suits loads greater than 1400W.

4. Operation from DC source to AC source

During nighttime, or when the weather changes or ac grid voltage exceeds the maximum voltage, then the inverter in PV mode cannot compensate the ac voltage swell (ac voltage exceeds the maximum voltage), the inverter will switch from PV source to AC grid source and would work as an ac voltage regulator providing ac voltage compensation to the load. Fig 11 shows the voltage change when there is a change in the source from DC to AC.

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Fig 11 shows the load voltage (in green) during fluctuation on the grid supply (blue). This simulation illustrates the effectiveness of the inverter in maintaining a constant supply during sags and swells.

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Fig 11 Inverter output voltage changed when load voltage exceeds 264 Vrms

From Fig12(c) the angle between the grid ac source Vs and the load current IL is different than in case (a). The angle between IL and Vs is in voltage control method and in Fig12 (b), the angle between IL

and Vs is in phase control. From the phasor diagram > , so the voltage control method can improve the power factor.

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Fig12. (a) Phasor diagram of the proposed inverter operating in the undervoltage sag situation. (b) Phasor diagram of the proposed inverter operating in the overvoltage swell situation. (c) Phasor diagram of phase controlled inverter operating in the overvoltage situation. Here Vs is the grid ac source, VL is the load voltage, Vi is the buck converter output voltage, IL1 is the inductive load current, and IL2 is the capacitive load current.

The ac buck converter uses a voltage control method, because the inverter output voltage is always in phase or out of phase with the grid ac source. The ac mode buck converter can feed power backinto the converter, under voltage swell situation. This is different from the two step ac voltage regulator. The buck converter also works in real power mode. During daytime, when the input power is from the PV panel, the inverter works in phase with the grid ac source in order to feed more real power to the load.

IV. EXPERIMENTAL RESULTS

The concept above was experimentally implemented as a series connected photovoltaic power inverter with rating of 1 kW. Xilinx’s 2S300E FPGA controller is used as the main controller. It controls

instantaneous voltage feedback, and generates in-phase PWM signals for the inverter. The FPGA receives analogue signals from the ADC and passes signals to the gate drivers.

Fig 13 Grid voltage during the sag mode and load voltage

Fig14 Inverter reduces output when load voltage exceeds the maximum voltage

Fig15 Series connected Photovoltaic inverter prototype

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Fig 16 AC mode buck converter output voltage series connected with grid voltage

Fig.13 shows that the proposed inverter operating in PV mode when the grid voltage sags, the yellow waveform shows load voltage, blue waveform shows the waveform of the load current.

When the load voltage exceeds the desired limit 255 Vrms , inverter output voltage is reduced in order to keep the load voltage constant as shown in Fig 14. Fig.16 shows that the proposed inverter operating in AC model during the grid voltage sag, the yellow waveform shows the load voltage is bigger than the grid voltage blue waveform.

V. Conclusion

The series connected photovoltaic power inverter is proposed and validated by means of experimentally. The control system has been implemented using an FPGA platform, allowing easy reconfiguration of the system. The simulation results validate the theoretical predictions that this system is able to operate with both DC and AC sources. The inverter is specially suitable for developing countries, because during the summer and winter times the grid supply power demand is often greater than the installed capacity, resulting in the grid voltage dropping. The series connected inverter can provide voltage compensation to maintain the load voltage constant and reduce energy demand from the grid.

REFERENCES

[1] Il-Song Kim,Myung-Bok Kim Myung-Joong Youn “New maximum Power Point Tracker using-Mode Observer for Estimation of Solar Array Current in the Grid-Connected Photovoltaic System”. IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, pp 1027-1035, August 2006

[2] Ali O Al-Mathnani, Azah Mohamed Mohd Alauddin Mohd Ali “Photovoltaic Based Dynamic Voltage Restorer For Voltage Sag Mitigation” The 5th Student Conference on Research and Development–SCOReD 2007 11-12 December 2007, Malaysia

[3] Cuauhtemoc .Rodriguez, GA.J Amaratunga “Dynamic stability of grid-connected photovoltaic systems,” IEEE power Engineering Society General Meeting, Vol. 2,pp2193-2199, Denver, USA June 2004.

[4] Ming Tsung Tsai “Design of a Compact Series-connected AC Voltage Regulator With an Improved Control Algorithm” IEEE

Transactions on industrial electronics, VOL.51 No.4 pp 933-936 August 2004

[5] Cuauhtemoc .Rodriguez, GA.J Amaratunga “Long-Lifetime Power Inverter for Photovoltaic AC Modules,” IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 7, JULY 2008, pp. 2593-2601

[6]Konstantinos Angelopoulos, “Integration of Distributed Generation in Low Voltage Networks: Power Quality and Economics” M.Sc. thesis in Energy Systems and the Environment, University of Strathclyde, UK, 2004.

[7] S. J. Chiang, C. Y. Yen, and K. T. Chang, A multimodule parallelable series-connected PWM voltage regulator, IEEE Trans.Ind. Electron., vol. 48, pp. 506 516, June 2001.

[8] S. J. Jeon and G. H. Cho, A series-parallel compensated uninterruptible power supply with sinusoidal input current and sinusoidal output voltage, in Proc. IEEE PESC 97, 1997, pp. 297 303.

[9] X. Z. Zhang, Analysis and design of switched-mode ac/ac voltage regulator with series connected compensation, in Proc. IEE Power Electronics and Variable-Speed Drives Conf., Oct. 1994, pp. 181 187.

[10]P.D.Ziogas, D. Vicenti, and G. Joos, “Practical PWM ac-ac controller topology” in Proc, Record, IEEE IAS con, Houston, TX, 1992, pp.880-887.

[11]F. Z. Peng, L. Chen, and F. Zhang, “Simple topologies ac-ac converter” IEEE Pow. Electron Letters, vol.1 no. pp. 10-13, March 2003

[12] M. Calais, V.G. Agelidis, "Multilevel converters for single-phase grid connected photovoltaic systems - An overview" 0-7803-4756-0/98 IEEE 1998.

[13] S. B. Kjaer, J. K. Pedersen, F. Blaabjerg, "A review of single-phase grid-connected inverters for photovoltaic modules", Transactions on Industry Applications, Vol. 41, No 5, IEEE 2005.

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