In recent years, solar energy (photovoltaic) power generation using natural energy is rapidly expanding from the viewpoints of global environmental protection and depletion of fossil energy. In addition, in many countries, policy support such as the system of subsidy for the introduction of equipment and the power purchase compensation system has been implemented, and the popularity of solar power has been continuously promoted. The world's photovoltaic market has reached 5.95 GW in 2008 and is expected to steadily increase and expand in the future.
A solar cell made of a single crystal of silicon is called a cell, and a member made of a majority of solar cells is called a solar module. A plurality of modules are connected in series and connected to form a solar cell module group. Large devices made up of modular groups are called solar arrays.
The solar power system is shown in Fig. 1. It is composed of a solar battery module group and an inverter. The role of the inverter is to convert the emitted direct current into alternating current. This power electronic converter is also called a power conditioner. ).
The power regulator has the following functions:
(1) Output solar power as large as possible [Maximum Power Point Tracking (MPPT) Control Function]
(2) Efficient DC power conversion to AC power (inverter function)
(3) Convey the transmitted power to the power system (to the system grid connection function)
(4) Stop power generation after detecting an abnormality in the power system (system connection protection function)
This article introduces the function and structure of the power conditioner and the high efficiency technology of the power conditioner.
2 power regulator function and structure
As described above, the power conditioner has an MPPT function and a function of converting DC to AC. This section describes the implementation of these functions and the structure of this converter.
The energy of solar radiation, when the temperature changes, the solar cell module voltage and current, voltage and power generation relationship, as shown in Figure 2. As can be seen from the figure, there is a voltage condition for the maximum output power of the solar cell. The intensity of sunlight irradiation energy, that is, the temperature of the solar cell, changes its characteristics. Power conditioners are devices that constantly control the voltage at which a solar cell obtains maximum power under its operating conditions.
2.2 Power Regulator Structure
The power regulator circuit structure shown in Figure 3. The DC power of the solar battery is input to the chopper circuit. When the solar battery power (maximum power point tracking) is taken out efficiently, it should be converted into the necessary DC voltage before the inverter; then the input DC power is converted into AC. Power, and control its output current to reach the desired alternating current. The output of the inverter is a pulsed voltage waveform. Therefore, the output filter composed of coils and capacitors is further converted into a smooth sine wave voltage. The output filter and the system voltage must correspond to each other. Connect and cut off the connection between the system and the inverter. The switch of the grid-connected (incorporated into the system) should be assembled, and each part of the circuit should be controlled through the control loop.
The chopper circuit of the power conditioner generates power loss in the inverter. The semiconductor switching elements, coils, capacitors, and wiring that constitute each circuit generate a normal fixed loss (conduction loss) due to the circulation of current, and a transition loss (switching loss) occurs when the semiconductor switching element is switched on and off. The important factor of loss. In order to achieve high efficiency of power regulators, the key is how to reduce the conduction losses and switching losses. After the conduction loss is reduced, the portion depending on the performance of the circuit element used is increased. To reduce the switching loss, efforts are made to reduce the switching voltage and reduce the number of switching operations. 3 High Efficiency Inverter (Ladder Control Inverter)
It is known that a stepped inverter is a way to reduce the loss of the inverter. Fig. 4 shows an example of the structure of the inverter and the waveform of the output voltage. A stepped inverter is a method in which a plurality of inverters having different voltages are connected in series to combine the outputs of the inverters to obtain an analog sine wave voltage. The voltage of each inverter forms an equal-difference series relationship with a difference of 2 or 3 steps. The output waveform of the ladder-controlled inverter has a stepped shape, and its respective level is called a tone level. For example, in the case of a three-digit (Vo2Vo4Vo) step-controlled inverter with a step of 2, two steps of 15 steps can be obtained, and for a three-digit step (Vo3Vo6Vo) with a step of three, 27 steps can be obtained. The output of the ladder.
The characteristics of the ladder modulator inverter are as follows: (1) Since the switching frequency can be greatly reduced, it is a low-loss, low-noise device; (2) The output filter can be designed to be smaller due to the small voltage amplitude. It can realize miniaturization; (3) Because the output voltage is obtained by the sum of the voltages generated by each inverter, an AC voltage higher than the input voltage can be generated.
In addition, according to the feature (3), the DC bus voltage can be set lower than that of the original system, and the chopper loop loss in the front stage of the inverter can also be reduced.
The ladder-controlled inverter is composed of a main inverter that inputs a DC voltage from outside and a sub-inverter connected in series with it. The DC voltage of the main inverter is input, and the DC voltage is supplied to the sub-inverter through the DC/DC converter. If the power conversion loss of the DC/DC converter is added, high efficiency cannot be achieved. In the step-controlled inverter, a technique of supplying DC power to the sub-inverter without adding a DC/DC converter is adopted.
Input the main inverter DC power, according to the instructions listed in Table 1, respectively sent to the sub-inverter to output the required voltage. In the table, the three equal-difference progressions with a difference of 2 are listed. At this time, eight levels of voltage (0Vo to 7Vo) can be output in unipolarity. For example, when Vo is output, three methods (0, 0, Vo), (0, 2Vo, -Vo), (4Vo, -2Vo, -Vo) can be selected. From the case of (4Vo, -2Vo, -Vo), it can be seen that the main inverter supplies sub-inverter power. In this way, not only the required voltage is output but also the power supply to the sub-inverter is realized.
4 Application of ladder regulator type inverter power regulator
The power regulator using step-controlled inverter technology is described. The circuit structure and operating principle are described below. The power conditioner is a method for supplying power to the sub-inverter; a method for supplying power to the sub-inverter, and a method for reducing leakage current through leakage current flowing through the parasitic capacitance of the solar cell. The situation is as follows.
4.1 Power Regulator with Step Control-type Inverter Technology
The ladder-type power regulator circuit structure shown in Figure 5. A newly specially developed power conditioner is shown in Fig. 6. A chopper circuit that regulates the output DC power of a solar cell is composed of the same coil as the original one, a semiconductor switching element, and a diode. The inverter part is composed of three inverters composed of three full bridge circuits, including the DC voltage of the switching input of the main inverter (1B), and the auxiliary inverter connected in series with it (2B, 3B). In general, the DC power generated in the solar cell generates power loss when the power conditioner section is converted into AC power. The original inverter uses an inverter to generate a high-voltage, high-frequency square wave through a filter loop and trim it into a sine wave.
As described above, a stepped inverter is a combination of three inverters with different voltages and generates a stepped analog sine wave. Since the amplitude of the voltage is small, the filter circuit can be miniaturized, so that the loss during power conversion is greatly reduced. Figure 7 shows the waveforms of the output voltage of the main inverter and the sub inverter, the output voltage waveforms of all the inverters, and the output voltage waveform after passing through the output filter. In the step-controlled inverter of the power conditioner, since there are few two sub-inverters, the sub-inverter performs PWM operation that compensates for the difference voltage between the sinusoidal voltage and the main-inverter rectangular wave voltage, and will output the The voltage waveform is trimmed to a sinusoidal waveform.
4.2 Method of Supplying Power to Sub Inverter
The step-controlled power regulator is connected in series by three inverters. The power obtained from the solar battery is sent to the main inverter having the highest input voltage, and the power is supplied to the other sub-inverters 2B-1NV and 3B-1NV, and it is necessary to pass the power to other sub-inverters 2B-1NV and 3B-1NV. The following describes the method of supplying power to the sub-inverter. The output voltage waveform of 1B-1NV is shown in Fig. 8. A 1-pulse waveform is output once in a half period of a sine wave. On the other hand, the output voltages of 2B-1NV and 3B-1NV are equal, and output is performed by PWM control that compensates for the target output voltage and the 1B-1NV output voltage difference. Let the power handled by each of the inverters 1B-1NV, 2B-1NV, and 3B-1NB be P1B, P2B, and P3B, and the output effective power is P. Let the input voltage of each inverter be VC1, VC2, VC3, the peak value of the output sine wave current is Im, the voltage peak value is Vm, the frequency of AC voltage is fs, each power is expressed as follows:
For the load connected to the power conditioner, it is assumed that a current with a sine wave and power factor = 1 flows. When Po=P1B, the integrated power output from 2B-1NV and 3B-1NB is zero. Therefore, by increasing or decreasing the output pulse width of 2B-1NV and 3B-1NV, the amount of power flowing to 2B-1NV and 3B-1NV can be controlled. In the region A shown in FIG. 8, the energy is discharged from 2B and 3B-1NV, and in the region B, 2B and 3B-1NV are charged.
4.3 How to reduce the leakage current flowing through the parasitic capacitance of the solar cell
As shown in Fig. 9, the main inverter is equipped with zero switching circuits consisting of two MOSFETs (metal oxide semiconductor field effect transistors) QZ1 and QZ2. The zero switching is in the low voltage output range where 1B-1NV does not operate and does not pass. The inverter circuit formed by Q1~Q4 directly connects 2B-1NV and 3B-1NV and outputs zero voltage. In an inverter circuit using a zero-switching circuit, once the zero voltage is output, the high-side or low-side switch of the inverter circuit is switched on. When this zero voltage output or the operation from this zero voltage output is alternately performed, the leakage current is rapidly generated by the parasitic capacitance of the solar cell module. To prevent this leakage current, 1B-1NV outputting 1 pulse is equipped with a zero-switching circuit, and zero-voltage is output by zero-switching operation.
5 Application of Step Regulator-Type Inverter Power Regulator Features
This section describes the features of the step-controlled power regulator PV-PN40G. This power regulator reduces power loss by 44% compared to the original, reaching 97.5% of the industry's highest power conversion efficiency (Mitsubishi's survey results so far in June 2010). In addition, as shown in FIG. 10, in a wide range of output power regions, conversion efficiency higher than the rated efficiency (97.5% or more) is achieved.
Heat generated by power loss during conversion is greatly reduced. Therefore, there is no need for air inlets for heat dissipation, and the degree of sealing is improved. As a result, moisture resistance has also been greatly improved. Facilities such as undressing rooms and washrooms that could not be installed can now be configured. At the same time, due to the reduction of the voltage amplitude, the noise generated by the coils of the filter circuit can be suppressed, and in the actual operating conditions, the industry's top 30dB low noise is achieved.
In addition, the input voltage range
From the original DC115-380V extended to DC50-380V. As a result, the original multi-matrix converter (container with built-in boost function) must have 3 to 6 modules in series. Now, there is no standard connection box to achieve the boost function. Not only is the configuration easy to design, but it also reduces the cost of the system.
For the purpose of high efficiency, the ladder control inverter technology applicable to solar power generation power conditioners is described. In addition, a method of supplying power to the sub-inverter has been developed for the application of a step-controlled inverter. A zero-switching circuit for reducing the leakage current caused by the parasitic capacitance of the solar cell, and an inverter structure equipped with zero-switching are described. In addition, the characteristics of the power regulator PV-PN40G suitable for this technology are also shown. This product achieves the industry's top efficiency and excellent noise performance.
In recent years, photovoltaic power generation systems are rapidly expanding. Therefore, the impact of solar power generation on the power system cannot be ignored. For power conditioners, due to system accidents that cause instantaneous power and voltage drops, the power conditioner cannot be de-energized or stopped. It is required to have a continuous operation function, not only to satisfy this new function, but also to achieve high efficiency. It is necessary to develop such a power conditioner.
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