Design requirements for photovoltaic inverters

Solar photovoltaic power generation systems are currently mainly used in remote areas without or lacking electricity, as independent power sources to power household appliances and lighting equipment. With the increasing severity of power shortages and environmental pollution, solar power generation systems connected to the public power grid have become increasingly competitive. The grid-connected operation of photovoltaic power generation will eliminate the energy storage link in the independent photovoltaic system - the battery, thereby greatly reducing the maintenance of the power station. Since the battery has a short life, after eliminating the battery, the life of the power generation system can be equivalent to the life of the solar cell. For family homes, equipping with photovoltaic power generation systems can ease the power shortage during the day, improve the power factor of the power grid and reduce line losses. The grid-connected power generation of photovoltaic power stations will eventually replace conventional energy generation. The grid-connected principle of photovoltaic power generation is shown in Figure 1. The solar cell array transmits power to the grid through a sinusoidal pulse width modulation inverter. The power fed to the grid by the inverter is determined by the array power and the local sunshine conditions at that time. In addition to the DC-AC conversion function, the inverter must also have the maximum power tracking function of the photovoltaic array and various protection functions. The inverter shown in Figure 1 is a voltage source inverter. At present, the technology of voltage source inverter has become increasingly mature, and the required hardware is also easy to purchase. This paper will further study the voltage source inverter.

1 Performance that a small photovoltaic grid-connected power station should have

The grid-connected operation of photovoltaic power stations places high demands on inverters. These requirements are as follows:

① The inverter is required to output a sinusoidal current. The power fed back to the public grid by the photovoltaic power station must meet the indicators specified by the grid, such as the output current of the inverter cannot contain a DC component, the high-order harmonics of the inverter output current must be minimized, and harmonic pollution to the grid cannot be caused.

② The inverter is required to operate efficiently under large changes in load and sunshine. The energy of photovoltaic power stations comes from solar energy, and the intensity of sunshine varies with the climate, which requires the inverter to operate efficiently under different sunshine conditions.

③ The inverter is required to enable the photovoltaic array to operate at the maximum power point. The output power of solar cells is related to changes in sunlight, temperature, and load, that is, its output characteristics have nonlinear characteristics [1]. This requires the inverter to have a maximum power tracking function, that is, no matter how the sunlight, temperature, etc. change, the inverter can automatically adjust to achieve the best operation of the array.

④ The inverter is required to be small in size and highly reliable. For home photovoltaic power stations, the inverter is usually installed indoors or hung on the wall, so there are restrictions on its size and weight. In addition, the reliability of the whole machine is also required to be high. Since the life of solar cells is more than 20 years, the life of its supporting equipment must also be comparable.

⑤ It is required that the inverter can provide power independently when there is sunshine when the mains power is off.

2 Implementation of Sine Wave Voltage Source Inverter

The circuit principle of photovoltaic power generation when connected to the grid is shown in Figure 2. Up is the inverter output voltage, Uu is the grid voltage, R is the line resistance, L is the series reactor, and Iz is the current fed back to the grid. In order to ensure that the feedback power factor is 1, the phase of the feedback current must be consistent with the phase of the grid voltage. Taking the grid voltage Uu as a reference, Iz is in phase with Uu, and its vector diagram is shown in Figure 3. The voltage UR across the internal resistance R is consistent with the grid voltage, while the phase of the voltage UL across the reactor lags behind UR by 90°. From this, the phase and amplitude of UP can be obtained:

Where ω is the angular frequency of the public power grid. In actual circuits, Uu

Abstract The design requirements of photovoltaic inverters for grid-connected power generation are proposed. Voltage source inverters are used to realize grid-connected operation of photovoltaic power generation, and 16-bit microprocessors and IGBT power devices are used to improve the output waveform of the inverter.

Solar photovoltaic power generation systems are currently mainly used in remote areas without or lacking electricity, as independent power sources to power household appliances and lighting equipment. With the increasing severity of power shortages and environmental pollution, solar power generation systems connected to the public power grid have become increasingly competitive. The grid-connected operation of photovoltaic power generation will eliminate the energy storage link in the independent photovoltaic system - the battery, thereby greatly reducing the maintenance of the power station. Since the battery has a short life, after eliminating the battery, the life of the power generation system can be equivalent to the life of the solar cell. For family homes, equipping with photovoltaic power generation systems can ease the power shortage during the day, improve the power factor of the power grid and reduce line losses. The grid-connected power generation of photovoltaic power stations will eventually replace conventional energy generation. The grid-connected principle of photovoltaic power generation is shown in Figure 1. The solar cell array transmits power to the grid through a sinusoidal pulse width modulation inverter. The power fed to the grid by the inverter is determined by the array power and the local sunshine conditions at that time. In addition to the DC-AC conversion function, the inverter must also have the maximum power tracking function of the photovoltaic array and various protection functions. The inverter shown in Figure 1 is a voltage source inverter. At present, the technology of voltage source inverter has become increasingly mature, and the required hardware is also
easy to purchase. This paper will further study the voltage source inverter.

1 Performance that a small photovoltaic grid-connected power station should have

The grid-connected operation of photovoltaic power stations places high demands on inverters. These requirements are as follows:

① The inverter is required to output a sinusoidal current. The power fed back to the public grid by the photovoltaic power station must meet the indicators specified by the grid, such as the output current of the inverter cannot contain a DC component, the high-order harmonics of the inverter output current must be minimized, and harmonic pollution to the grid cannot be caused.

② The inverter is required to operate efficiently under large changes in load and sunshine. The energy of photovoltaic power stations comes from solar energy, and the intensity of sunshine varies with the climate, which requires the inverter to operate efficiently under different sunshine conditions.

③ The inverter is required to enable the photovoltaic array to operate at the maximum power point. The output power of solar cells is related to changes in sunlight, temperature, and load, that is, its output characteristics have nonlinear characteristics [1]. This requires the inverter to have a maximum power tracking function, that is, no matter how the sunlight, temperature, etc. change, the inverter can automatically adjust to achieve the best operation of the array.

④ The inverter is required to be small in size and highly reliable. For home photovoltaic power stations, the inverter is usually installed indoors or hung on the wall, so there are restrictions on its size and weight. In addition, the reliability of the whole machine is also required to be high. Since the life of solar cells is more than 20 years, the life of its supporting equipment must also be comparable.

⑤ It is required that the inverter can provide power independently when there is sunshine when the mains power is off.

2 Implementation of Sine Wave Voltage Source Inverter

The circuit principle of photovoltaic power generation when connected to the grid is shown in Figure 2. Up is the inverter output voltage, Uu is the grid voltage, R is the line resistance, L is the series reactor, and Iz is the current fed back to the grid. In order to ensure that the feedback power factor is 1, the phase of the feedback current must be consistent with the phase of the grid voltage. Taking the grid voltage Uu as a reference, Iz is in phase with Uu, and its vector diagram is shown in Figure 3. The voltage UR across the internal resistance R is consistent with the grid voltage, while the phase of the voltage UL across the reactor lags behind UR by 90°. From this, the phase and amplitude of UP can be obtained:

Where ω is the angular frequency of the public grid. In the actual circuit, the position, period and amplitude of Uu are detected by the voltage sensor. Since R is difficult to obtain in the actual system, the phase of the feedback current Iz must be realized by current negative feedback, and the reference phase of the phase angle of the feedback current Iz is the phase of the public grid
. Use the current transformer to detect Iz at any time to ensure that Iz is consistent with the grid voltage phase to achieve feedback power generation with a power factor of 1.

The structure of a practical inverter for photovoltaic power generation grid-connected operation is shown in Figure 4. The inverter main circuit power tube uses IGBT with a capacity of 50A and 600V, and the model is 2MBI50N-060. The isolation drive circuit uses TLP250 produced by Toshiba. The control part of the inverter is completed by a microprocessor. The main control chip uses the latest 16-bit microprocessor 87C196MC dedicated to inverter or motor drive launched by INTEL. In addition to 16-bit operation instructions, this chip also has a dedicated pulse width
modulation (PWM) output port [2], including a 10-bit A/D converter, an event processing array, two 16-bit timers and a three-phase waveform generator. Each phase of the three-phase waveform generator can output two PWM signals with settable dead time.

This provides a lot of convenience for inverter applications. The microprocessor mainly completes the grid, phase real-time detection, current phase feedback control, photovoltaic array maximum power tracking and real-time sinusoidal pulse width modulation signal generation. Its working process is as follows: the voltage and phase of the public grid are sent to the microprocessor's A/D converter through the Hall voltage sensor. The microprocessor compares the phase of the feedback current with the voltage phase of the public grid. The error signal is sent to the PWM pulse width modulator after PID adjustment, which completes the power feedback process with a power factor of 1. Another major task completed by the microprocessor is to achieve the maximum power output of the photovoltaic array. The output voltage and current of the photovoltaic array are detected and multiplied by the voltage and current sensors respectively to obtain the array output power, and then the PWM output duty cycle is adjusted. The adjustment of this duty cycle is essentially to adjust
the feedback voltage size, so as to achieve maximum power optimization.

As can be seen from Figure 3, when the amplitude of Up changes, the phase angle φ between the feedback current and the grid voltage will also change to a certain extent. Since the current phase has achieved feedback control, the decoupling control of the phase and amplitude is naturally achieved, making the processing process of the microprocessor simpler. In addition, the grid-connected operation of photovoltaic power generation must also consider the working conditions when the public grid is out of power. Conventional photovoltaic power generation grid-connected systems stop the inverter when the public grid is out of power. If it is during the day, the photovoltaic array can still continue to generate electricity.

Its working principle is as follows: when the public power grid is powered off, the grid side is equivalent to a short-circuit state. At this time, the grid-connected inverter will automatically protect itself due to overload. When the microprocessor detects an overload, in addition to blocking the SPWM signal, it will also disconnect the relay RE. At this time, if the photovoltaic array has energy output, the inverter will operate in a single operation state. The control is relatively simple when operating alone, that is, the negative feedback state of the AC voltage. The microprocessor detects the inverter output voltage and compares it with the reference voltage (usually 220V), and then controls the PWM output duty cycle to achieve inversion and voltage stabilization operation. Of course, the premise of single operation is that the photovoltaic array can provide enough power at the time. If the load is too large or the sunshine conditions are poor, the inverter cannot output enough power, and the terminal voltage of the photovoltaic array will drop, thereby reducing the output AC voltage and entering a low-voltage protection state. When the power grid resumes power supply, it will automatically switch to the feedback state.

3 Conclusion

The 16-bit microprocessor and high-speed IGBT power module are used to realize the grid-connected power generation of medium and small-capacity photovoltaic power stations. The photovoltaic grid-connected inverter described in this article not only has high efficiency and less distorted output current waveform, but also can operate independently in the case of power outages in the grid, and has a certain prospect for promotion and application.