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Study of Control Strategy on Small Power Photovoltaic Grid-connected Generation System

LIU Sujuan ,JING Qiufeng Henan Zhengzhou Power Supply Company

State Grid Zhengzhou China liusj888@126.com

YANG Hai-zhu Department of Electrical Engineering and Automation

Henan Polytechnic Univ. Jiaozuo China

yanghaizhu@hpu.edu.cn

Abstract—Photovoltaic(PV) grid-connected system is the development trend of photovoltaic systems. According to the PV grid-connected system characteristics, analyzes the PV grid-connected inverter control and maximum power point tracking(MPPT) control strategy based on two-stage power conversion architecture, proposed a novel MPPT control algorithm, presents the design of a photovoltaic grid-connected inverse system based on the digital signal processor TMS320F2407. The experiment results verify that the system can track the maximum power point quickly, the grid-connected current wave achieves our need and the wave of grid-connected current has the same frequency and phase as the utility voltage.

Keywords- photovoltaic; grid-connected system; maximum power point tracking; control algorithm; inverter

I. INTRODUCTION Solar energy has a non-polluting, resource-rich, no

regional restrictions, etc., so as a renewable utilization of new energy sources has been widely used. Photovoltaic power generation is an important renewable energy use technical means, Since 1990s, foreign developed countries has set off the development of photovoltaic grid-connected system research and development climax. Grid-connected PV system has become the main form of photovoltaic power generation applications, its advantage is that no intermediate energy storage batteries, saving investment, but also make the system simplified, easy to maintain.

Grid-connected PV system is to change the quantity of heat that solar battery absorbs becoming DC, re-use of inverter to convert DC into AC line power requirements, and then access to the public power grid, as in [1-2]. Photovoltaic power generation system usually contents the PV array , the maximum power point tracking (MPPT) circuit, inverter, controllers. For this design low-power grid-connected PV systems, due to a lower output voltage of PV array, this article on the use of the two transformations, and network structure, the structure shown in Fig. 1.

Figure 1. grid-connected PV system schematic diagram

Grid-connected solar inverter control and maximum power point tracking control are two core issues of grid-connected PV systems. The system first go through the step-up DC-DC converters and achieve photovoltaic arrays maximum power point tracking, and through the DC-AC converter to achieve grid-inverter, the system uses DSP chip TMS320LF2407 as the control chip. The shortcomings of PV cells is low energy conversion rate, while its output power has a relationship with the external environment and load, so to enhance the utilization of solar cells, it is necessary to achieve PV array MPPT control. Grid-connected inverter output current requirements for a stable of high-quality sine wave current, at the same time demand grid-connected inverter output current and the net voltage waveforms remain strictly in phase with the same frequency, so that you can make the largest grid-connected system output the active power, but also to avoid pollution of the public power grid , as in [3].

II. GRID-CONNECTED INVERTER CURRENT CONTROL STRATEGIES

System main circuit topology structure, as shown in Fig. 2: The former level DC-DC boost converter, this paper adopts simple structure, easy control of Boost step-up circuit. After the class DC-AC converter, using a single-phase full-bridge inverter circuit. Intermediate bus capacitance DClink part, realizes the DC-DC converter to the DC-AC inverter power transmission.

Figure 2. Main circuit topology

Output voltage of the PV array is converted to DClink DC bus voltage Ud by Pre-class DC-DC converter, DClink is equivalent to the input voltage source inverters, post-level DC-AC link is to stabilize the value of Ud setting Uref and converts it into a sinusoidal AC 220V/50HZ, realize inverters transfer power to the grid. From Fig. 2 we can see that as a result of the buffer inductor LN existence at the AC-side, the fundamental component of the output current

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DOI 10.1109/ICICCI.2010.29

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to generate a voltage drop on it. so that the fundamental of inverter output voltage and grid voltage will produce a displacement between them, output side existence the relationship between a vector triangle shown in Fig. 3. By PWM control of inverter circuit switch duty cycle so that the inverter output voltage vector to satisfy the above relations, so in theory, can be achieved with the grid voltage output current in phase with the same frequency , as in [4].

Figure 3. Vector diagram inverter

In this paper, a fixed-frequency transient current control mode as shown in Fig. 4, the control introducing grid voltage uN feed-forward, that is, before the current error signal compare with the triangular carrier, added the AC-side voltage feedforward signal to offset the power grid disturbances ,and make the system approximate as a passive follow the system, thus simplifying the system control structure, improving the system's control effect. It can adjust the output current is given strictly follow the sinusoidal current, the elimination of all non-linear effects of factors on the output current. Of thought control as follows: DClink middle voltage Udcompared with the reference voltage Uref, the error regulatored by the PI obtain current instruction I ＊ , and then sync signal multiplication with the sine wave to be sinusoidal command iref, , and after instruction current iref compares with network current io, the value obtained by the difference △i multiplied the ratio of coefficient of the regulator (the physical meaning of the product equivalent the voltage of inductor), and the sum of grid voltage uN is equal to the inverter output voltage, and then compared with the triangle wave ,and rise to two-way wave PWM control inverter switch-off tube, after the final adoption of filter circuit ,it will get a better control effect.

Figure 4. Schematic diagram of fixed-frequency transient

current control

III. MAXIMUM POWER TRACKING CONTROL STRATEGY FOR POWER

Solar cell output characteristics of non-linear, but also by the impact of light intensity and temperature, as shown in Fig. 5, each working curve has a corresponding maximum power output point. In order to allow solar-powered system in the full capacity of its photoelectric

conversion, we need real-time control solar cell operating point to obtain the maximum power output .

Figure 5. Under different light intensity of P-V characteristic

curve of

Some commonly used MPPT control method is through the detection of the battery output voltage and current, continuous calculation of the battery output power, through the optimization of the method makes the battery output voltage approaches the voltage that the maximum power point corresponding to achieve the MPPT control. This paper is a small power grid power generation systems, so the system output voltage is fixed, the output current with output power P increases monotonically increasing, so the maximum output current means the maximum output power. According to power conservation, the system's output power is equal to the input power, therefore, the maximum output current tracking is equivalent to the maximum power point tracking ,as in [5-6].

This paper presents a MPPT algorithm which is based on single output parameter, instead of direct control of power, just concerned about the maximum output current, so ruled out a dual-parameter, and even multi-parameter testing, and do not require power calculations, reducing the control design more difficult. In this system, the first-level DC-DC converter duty cycle D can be controlled variables, the system based on sampled values of inverter output current through the current-sensing circuit input to the DSP's A / D converter module, after the values analysed and processed by the MPPT control procedures, obtained control conclusion and finally control the DC-DC converter switch duty cycle to adjust the system operating point. Relationship of power and duty cycle is shown as Fig.6.

Figure 6. P-D relationship

As shown in Fig.6, when the dP/dD=0, the output power reaches its maximum, this paper presents MPPT algorithm is through compare the current output power with the power of the previous one more time to determine the duty cycle increases and decreases. When the output power changes, the output current will also change, and the output current is also reached the maximum at maximum output power point. So, the control of D continuously

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variable , and to observe the output current changes. As long as the changes of D result in the output current increases, it should control D in the same direction of change, the contrary should control D in the opposite direction of change, and gradually approaching the maximum power point by this way. Figure 7 shows the system flow of MPPT control algorithm, the stability of the voltage reference value Uref of the middle DClink voltage Ud is set 400V, we can calculate initial duty cycle when system has just started its work from Boost circuit output voltage and input voltage as well as the relationship of duty cycle D.

Figure 7. MPPT control algorithm flow chart

Before the class through the MPPT method to control the duty cycle D of the DC-DC converter, and to control solar cell output power, the latter class is under the principle of power balance to carry out closed-loop control while achieving DClink regulator and the inverter output current keep the same frequency and phase with the net voltage ,as in [7-8].

System through the principle of power balance to achieve maximum power tracking principle is: When the solar cell output power is greater than the inverter output power, that is, the output power of solar cells at this time failed to timely fed to the grid up, then the DClink ,there will be some accumulation of residual energy make the DClink voltage rise, this time the system will detect the Ud is greater than 400V, the control direction is increased current to make the output current increases through the PI regulator, it means increase power that DC -AC inverter output to the grid to consume the remainder of the energy of Dclink,to make DClink voltage drop to the mid-point ,and keep voltage stability. On the contrary, it reduces the voltage-current instructions so that DClink increased, and so as to achieve a stable DClink voltage purposes.

IV. EXPERIMENTAL According to the previous control strategy analysis and

design, built in the laboratory and completed a rated power of 700W photovoltaic grid experimental apparatus, experimental circuit contents the main circuit and control panel TMS320LF2407A composition, software development environment is CCS3.1. Experimental system shown in Fig. 8, solar cell arrays using four 175W polysilicon solar arrays in series, rated input power of 700W, around the open circuit voltage of solar cell arrays is about 44.4V. Ammeter and voltmeter are used to measure solar cell voltmeter input voltage and current at input-side, power quality analyzer FLUKE is used to detect parameters and waveforms of output AC voltage and current of grid-connected inverter at the output side.

Figure 8. Test Sketch

In accordance with analysis and studying MPPT control problem of the grid-connected PV system which has two-stage transformation structure in front of, using new MPPT control algorithm on MPPT control experiments of the system, recorded a PV array operating point every 5 minutes one day , the specific solar cell output power, voltage and temperature curve of the relationship shown in Fig. 9.

Figure 9. MPPT control of solar array output power change

process

As shown in Fig. 9, the high voltage operating point to reach 140V before 10:05, while the output power is relatively low. As the light is growing, the ambient temperature and power is rising. It may be noted in the power curve, there are two obvious inflection point, one at 9:45, the second is about 10:05. The output power is small before the first inflection point, but growing slowly, following, the power is steady increase, the inflection point is the PV array as a whole has been generally shade moment after observation, and then the time to the second inflection point is the shadow on the PV array gradually

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disappear, this time, power output increased steadily. After the second inflection point there is no shadow effect, carefully observe , we can found the curve of the output voltage of solar energy works in a relatively constant voltage under the control of the MPPT, the voltage is also large when the larger the output power. From the MPPT Experimental waveforms, it can be seen that the photovoltaic power generation system has good stability at the external environment is relatively stable. If the external environment conditions changes suddenly , the system also can be fast and stable tracking maximum power point changes.

Figure 10. Output voltage and current waveforms

Figure 11. Output current harmonic analysis

Fig.10 and Fig.11 are experimental waveforms which output current to track grid voltage when a grid-connected inverter meet the passive load. It can be seen from the experimental waveforms that inverter output voltage is 219.6V, output current Valid values are 2.71A, output power of 570W, the output power factor of 0.95, close to 1. Grid current and grid voltage has remained keep the same frequency and phase with the net voltage, grid-connected current waveform well, deformity rate is low, and matched with simulation results, inverter has excellent steady state characteristics under the instantaneous voltage feedback control method. The output current fundamental component accounted for 99.7% of the total current. It can

be said that grid-connected inverter output power quality is satisfactory. From the experimental results we can see: the photovoltaic power generation system has good stability at the external environment is relatively stable. The control strategy based on improved fixed-frequency current-tracking can better complete the current control.

V. CONCLUSION This article is based on DSP chip TMS320F2407A

control of single-phase photovoltaic grid-connected inverter system, according to the system structure, it analyzes the two core grid control strategy in detail, propose a new maximum power point tracking algorithm, built laboratory prototype, the experimental results show that the correctness and feasibility of this design.

ACKNOWLEDGMENT The authors would like to express their gratitude to the

managers and teachers of Department of Electrical Engineering and Automation for kindly supporting this research. Thesis is supported by Henan Province key scientific and technological project (082102240008), Doctor Fund of Henan Polytechnic University (648193) and Young Backbone Teachers Fund of Henan Polytechnic University(649093), thank them for their support.

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