IMPLEMENTATION OF A PV INVERTER

-PWM Modulator

Y=AB K

zero-p hase error compensator

zero-p hase error compensator EMI Filter

Fig. 4. Digital line current control of a grid-connected inverter for unit power factor.

The voltage-source current-regulated inverter for distributed power generation needs to feed sinusoidal current in phase with the grid voltage with low total harmonic distortion. Fig. 4 shows the block diagram of a grid-connected PV inverter using digital current control technique. A second-order T-type filter is used to smooth the output current. The output current and grid voltage are sensed for the current regulation in synchronous with the grid voltage. The dc-link voltage is regulated to maintain at a constant value for power balance control. Digital control techniques provide flexibility for the implementation of sophisticated PWM generation scheme for low-order harmonic control. Digital control techniques can used to improve the converter efficiency by a reduction of the switching frequency. These new control methods may lead to the development of a PV inverter to achieve a specified power factor with a lower switching frequency.

5. ISLANDING DETECTION TECHNIQUES

A fault occurring in the power distribution system is generally cleared by the protective relay that is located closest to the faulty spot. As a result, a distributed generation system (DG) tries to supply its power to part of the distribution system that has been separated from the utility's power system. In usually conditions, this DG assumes an overloaded condition, where its voltage and frequency are lowered and it is finally led to stoppage.

However, though this is a rare case, a DG (or a group of generators) connected to this islanded system may provide with a capacity that is large enough to feed power to all the loads accommodated in the islanded system. When the loads are fed power only from the DG even after the power supply is suspended from the power company, such a situation is called an "islanded operation" or "islanding" [14].

Fig. 5 shows the block diagram of an inverter side islanding prevention mechanism. Many methods for the islanding protection have been proposed [15]. These

techniques can be classified into two categories: passive and active methods. The passive schemes monitor variations of voltage, frequency, phase, reactive power, harmonic contents, etc. to decide an occurrence of the islanding operation. The active schemes monitor variations of characteristics of the grid by using a perturbation of the injected line current. The status of islanded operation can be detected even under the perfect equilibrium state. Active islanding detection scheme is the major approach in the design of an anti-islanding inverter.

The non-detection zone (NDZ) is the range of local loads for which the islanding prevention method under consideration can be made to fail to detect islanding. The (NDZ) can be used to evaluate the performance of an islanding detection method. Active islanding detection schemes with characteristics of minimum NDZ and easy to implement are major concerns in selection of an islanding scheme.

The Sandia’s active islanding algorithm consists of the Sandia frequency shift (SFS) and the Sandia voltage shift (SVS) schemes [16]. The principle of both the methods is an accelerated frequency and voltage drift created with positive feedback to unstabilize the inverter when loose of the utility.

In the presence of the utility, the frequency and voltage shifts are not effective in drifting the two parameters.

However, once the grid is disconnected, these methods force the frequency and/or voltage to shift outside the operating windows, causing the inverter to disconnect due to o/u voltage and frequency protection. This method may become a standard method in prevention islanding.

6. IMPLEMENTATION OF A PV INVERTER

The development of microelectronics has made low-cost high-performance single-chip digital signal processor (DSP) controller a feasible solution for complicated control functions required in modern motor drives and power electronics. Applying DSP control technique instead of using the conventional analog discrete PWM control ICs, one primary advantage is achieved by replacing hardware with flexible software.

vdc vo z_o Detectio n Algorithm

Si

Fig. 5. Inverter side islanding prevention mechanism.

ISES 2005 Solar World Congress, Orlando, USA, Aug. 6-12, 2005.

Utility Line 220V, 60Hz H-Bridge Inverter

S2

Load

Q3 Q5

Q4 Q6

Buck-Boost Converter

MPPT

PV module power–voltage curves.

Solar Output Power ( Unit P.U. )

Solar Terminal Voltage ( Unit P.U. ) 0.2

Line voltage and current.

Q1

Utility Line 220V, 60Hz H-Bridge Inverter

S2

Load

Q3 Q5

Q4 Q6

Buck-Boost Converter

MPPT

PV module power–voltage curves.

Solar Output Power ( Unit P.U. )

Solar Terminal Voltage ( Unit P.U. ) 0.2

Line voltage and current.

Fig. 6. Integrated control of a two-stage grid-connected PV inverter.

In general, the size of the inverter is dominated by its output LC filter and heatsinks. We can reduce the filter size by increasing the switching frequency; however, this increases overall losses and requires bigger heatsinks with more cooling. Sophisticated digital current and PWM control techniques using advanced fixed-point DSP enables possibility to reduce the size of a dc-ac inverter, increase efficiency, and improve the total harmonic distortion (THD).

The synchronous sampling of the inductor current with high current ripples can get a much more accurate and faster current measurement with lower sampling and switching frequency compared with conventional analog current control techniques.

Fig. 6 shows the proposed integrated control scheme for a two-stage grid-connected PV inverter. The front power stage is a buck-boost converter and takes charge in the MPPT control; the rear power stage is a full-bridge converter takes charge in line-fed current control. A balance controller is used for the integration of MPPT and current control. The dc-link voltage is controlled at a specified voltage level according the in-coming solar power.

Fig. 7 shows the block diagram of two-stage grid-connected PV inverter. A fuzzy MPPT control algorithm is developed for the maximum tracking of the solar energy conversion. The algorithm employs heuristic search for maximum power point based on current perturbation of the PV module output current.

In order to reduce the harmonic contents of the inverter output current cascaded output filter and EMI filter are usually used. However, these capacitors will also introduce low frequency dynamics which may instabilize the current control loop or add extra phase lag to its output current. A

digital zero-phase error compensator is included in the voltage control loop to compensate phase error resulted by the output power filter and EMI filter. Predictive deadbeat current control scheme with synchronous sampling technique is employed for the current loop control.

The practical implementation of the digital inverter controller requires careful considerations in signal sampling and scaling of the feedback signals. Being an inherently wide-band controller, with a very quick speed of response, the controller is very sensitive to feedback noise and disturbances. For this reason, particular care must be taken in the controller PCB layout for low-level signal sampling and ADC, DAC interface circuit design.

A prototype PV inverter has been implemented for the verification of the proposed control scheme using advanced fixed-point DSP controller. The 2kW anti-islanding inverter is designed for an optimal operation of 110V, 60 Hz power distribution system. The PV inverter can operate under a wide dc input voltage range from 40V to 200V. The inverter uses double-conversion when operating in low voltage mode and automatically switches to single-conversion when operating in high voltage mode.

Q3 Q1

Q2

Q4 S4 S2

S1 S3

Solar Cell Array Utility Lines

AC 110V (50/60Hz) High-frequency

Inverter Bridge DC filter AC filter

Input Power Stage Output Power Stage

Fuzzy MPPT Controller

Current Control Gate Drive Gate Drive

Multiplier

Vdc reference Ipv

Vpv Vdc bus

+ –

EMI Filter

Fig. 7. Block diagram of the designed two-stage grid-connected PV inverter.

Fig. 8. Photographs of the PV inverter under development.

ISES 2005 Solar World Congress, Orlando, USA, Aug. 6-12, 2005.

Input voltage and current

(a) Input vo ltage and current

Vo(V)

Input voltage and current

(a) Input vo ltage and current

(b) C urrent Harmo nic S pectrum

0 2k 4k 6k 8k 10k

(b) C urrent Harmo nic S pectrum

0 2k 4k 6k 8k 10k

Fig. 9. Experimental results of the inverter with a rated power of 2kW.

The designed PV inverter consists of two major parts:

one is a DSP-based inverter controller and the other is an IGBT-based power converter. Fig. 8 shows the photographs of the PV inverter under testing. Fig. 9 shows the experiment results of the implemented grid-connected PV inverter operating at rated output. The inverter achieves an efficiency of 94% at 2kW when operating in single-conversion mode. The output current harmonic spectrum is shown in Fig. 9(b) with a THD of 2.4%.

7. CONCLUSION

The status of technologies development for grid-connected PV inverters has been reviewed. Some practical design considerations are given for a DSP-controlled PV inverter. An integration of the-state-of-art PV inverter control techniques has been developed for a single-chip DSP-controlled grid-connected PV inverter with a rating of 2kW. Experimental results of the designed inverter show a high energy-efficiency of 94% and a low distortion on the line current with a current THD of 2.4% at rated power. It can be concluded that with the integration of advanced power electronics and digital control techniques, low-cost high-efficiency small-power anti-islanding inverters for distributed power generation will be popular home appliances within next ten years.

8. REFERENCES

[1] B. Yordi and W. B. Gillett, “Future trends in European PV power generation,” Progress in Photovoltaics: Research and Applications, vol. 5, no. 3, pp. 175-185, 31 Dec 1998.

[2] R. Bonn, “Developing of next generation PV inverter,” IEEE 29th Photovoltaic Specialists Conference Record, pp. 1352-1355, 19-24 May 2002.

[3] H. Haeberlin, B. Fachhochschule, etc., “Evolution of inverters for grid connected PV-systems from 1989 to 2000,”

17th Europeian Photovoltaic Solar Energy Conference, Munich, Germany, Oct. 22-26, 2001.

[4] R. Bonn ([email protected]), Inverter for the 21st Century, Sandia National Laboratories, 1999.

[5] M. Calais, V. G. Agelidis, and M. Mcinhardt, “Multilevel converters for single-phase grid connected photovoltaic systems: an overview,” Solar Energy, vol. 66, no. 5, pp. 325-336, August 1999.

[6] M. Calais, V. G. Agelidis, L. J. Borle, and M. S Dymond , “A transformerless five level cascaded inverter based single phase photovoltaic system,” IEEE PESC Conf. Rec., 2000.

[7] B. Verhoeven, et. al. Utility aspects of grid connected photovoltaic power systems, International Energy Agency PVPS task V, 1998.

[8] Yaosuo Xue, Liuchen Chang, Sren Baekhj Kjaer, J.

Bordonau, and T. Shimizu, “Topologies of single-phase inverters for small distributed power generators: an overview,” IEEE Transactions on Power Electronics, vol. 19, no. 5, pp. 1305-1314, Sept. 2004.

[9] G. Walker, “Evaluating MPPT converter topologies using a MATLAB PV model,” J. Elect. Electron. Eng., vol. 21, no. 1, pp. 49–56, 2001.

[10] Lyon van de Merwe and Gawie J. van der Merwe,

“Maximum power point tracking - implementation strategies,” IEEE ISIE Conf. Rec., pp. 214-217, 1998.

[11] Chihchiang Hua, Jongrong Lin, and Chihming Shen,

“Implementation of a DSP-controlled photovoltaic system with peak power tracking,” IEEE Transactions on Industrial Electronics, vol. 45, no. 1, pp. pp. 99-107, Feb. 1998.

[12] Eftichios Koutroulis and Nicholas C. Voulgaris,

“Development of a microcontroller-based, photovoltaic maximum power point tracking control system,” IEEE Trans.

on Power Electronics, vol. 16, no. 1, pp. 46-54, Jan. 2001.

[13] M. J. Ryan, W. E. Brumsickle, and R. D. Lorenz, “Control topology options for single-phase UPS inverters,” IEEE Trans. on Ind. Applications, vol. 33, no. 2, pp. 493-501, March/April 1997.

[14] G. Kern, R. Bonn, J. Ginn, S. Gonzalez, “Results of Sandia National Laboratories Grid-Tied Inverter Testing,”

Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, Austria, July 1998.

[15] T. Funabashi, K. Koyanagi, and R. Yokoyama, “A review of islanding detection methods for distributed resources,” IEEE Bologna Power Tech Conference Proceedings, pp. 608-613, June 23-26, 2003.

[16] J. Stevens, R. Bonn, J. Ginn, S. Gonzalez, and G. Kern, Development and testing of an approach to anti-islanding in utility interconnected photovoltaic systems, Technical Report, Sandia Nat. Labs, Albuquerque, NM, 2000.