“With the development of human society, the consumption of energy is increasing, and the world’s fossil energy will one day reach its limit. At the same time, due to the massive burning of fossil energy, the global ecological environment is deteriorating day by day, posing a great threat to the survival and development of human beings. In this context, as a huge amount of renewable energy, solar energy has attracted people’s attention, and governments around the world are gradually promoting the development of the solar photovoltaic power generation industry.
Author: Huang Hua
With the development of human society, the consumption of energy is increasing, and the world’s fossil energy will one day reach its limit. At the same time, due to the massive burning of fossil energy, the global ecological environment is deteriorating day by day, posing a great threat to the survival and development of human beings. In this context, as a huge amount of renewable energy, solar energy has attracted people’s attention, and governments around the world are gradually promoting the development of the solar photovoltaic power generation industry. In my country, the application of photovoltaic systems has just started, and the market situation is still unclear. It is believed that as one of the most rapidly developing high-tech technologies, solar photovoltaic power generation technology, especially photovoltaic grid-connected power generation technology, will bring new changes to the power industry and energy structure in the future.
As a new type of green renewable energy, solar energy is the most ideal renewable energy compared with other new energy sources. Especially in recent decades, with the continuous advancement of science and technology, solar energy and its related industries have become one of the fastest growing industries in the world. Because it has the following characteristics: abundant reserves, cleanliness and economy, and wide distribution.
These technologies are widely used in the northern and western regions of my country and have achieved remarkable results. Photovoltaic utilization with photovoltaic cell technology as the core has become the most important application field in solar energy development and utilization. Using photovoltaic power generation has obvious advantages:
(1) Simple structure, small size and light weight;
(2) It is easy to install and transport, and the construction period is short;
(3) Simple maintenance and convenient use;
(4) Clean, safe and noise-free;
(5) High reliability, long life and wide application range.
Solar photovoltaic power generation system
Classification of photovoltaic power generation systems
Solar photovoltaic power generation systems can be roughly divided into three categories: independent power generation systems, grid-connected power generation systems, and hybrid power generation systems.
5kw Microgrid Inverter System Design
5kw Microgrid Inverter Design
The system structure of the 5kw microgrid inverter is shown in Figure 1.
The microgrid inverter is a key part of the microgrid inverter system. The output of the micro-grid inverter is three-phase alternating current, and it has two working modes: grid-connected and independent operation. The main circuit of the micro-grid inverter adopts the intelligent power module for inversion, and generates three-phase alternating current, which is isolated and boosted through a three-phase transformer (Δ-γ), and becomes a three-phase four-wire output.
Static Switching and Energy Metering Design
Static switches are an important part of a microgrid inverter system. The static switch consists of three groups of triacs, two air switches and a circuit breaker, and the closing and opening driving signals are generated by DSP.
During normal operation, switches Switch1, Switch2, Switch3, and Switch4 are closed at the same time to provide electrical energy for local loads; when abnormal conditions such as grid phase loss and serious voltage drop occur, the DSP detects the abnormal situation, makes judgments, and controls The switch is turned on and 0 is turned off. At this time, turn on Switch1 and Switch2, and turn off Switch3 to ensure the power supply of important loads. When the inverter fails, Switch1 is immediately disconnected, the inverter exits, and Switch4 is disconnected at the same time, and the power grid supplies power to the important loads. When the inverter fault disappears, after synchronizing with the grid, open Switch1 and close Switch4 to restore the power supply to the local load. When the inverter needs to be repaired, first open the switch Switch2, and after the inspection is completed, close the Switch2 again.
Design of DC/DC Converter Scheme
The DC/DC converter adopts the Boost topology to realize the boost function of the DC voltage and the maximum power point tracking (MPPT) of the battery. The PWM drive signal is generated by the DSP. By collecting the output voltage and current of the solar panel, the instantaneous output power is calculated and compared with the output power at the previous moment to track the maximum output power of the solar panel.
Design of battery charging and discharging equipment
The hardware circuit of the battery charging and discharging equipment adopts Buck-Boost topology, and the driving signal is generated by PIC microcontroller. When charging, according to the current state of the battery, the equalizing mode or the floating charging mode is enabled to realize the intelligent charging of the battery. When the system needs to discharge the battery, the PIC microcontroller generates a PWM drive pulse to discharge the battery to the load.
Inverter requirements for photovoltaic grid-connected systems
The core of the grid-connected photovoltaic power generation system is the grid-connected inverter, and a dedicated inverter is required in this system to ensure that the output power meets the requirements of the grid power for electrical performance indicators such as voltage and frequency. Therefore, when connecting to the grid, higher requirements are placed on the inverter, mainly including:
(1) The inverter is required to output a sine wave current;
(2) The inverter is required to operate efficiently under the conditions of large changes in load and sunshine;
(3) The inverter is required to make the photovoltaic array work at the maximum power point;
(4) The inverter is required to have the characteristics of small size and high reliability;
(5) In the case of mains power failure, the inverter can supply power independently during sunshine.
Topological structure of photovoltaic grid-connected system
The topology of photovoltaic grid-connected systems can be divided into the following three categories:
(1) Single-stage grid-connected inverter topology
(2) Two-stage grid-connected inverter topology
(3) Multi-level grid-connected inverter topology
5kw Microgrid Inverter Circuit Design
The overall hardware diagram of the system is shown in Figure 2.
Figure 1 5kw microgrid inverter system structure diagram
Figure 2 System hardware diagram
Selection of main control chip
The functions to be realized by the control chip are: A/D conversion for the detection signal; generation of PWM waveform; completion of MPPT; calculation process of electric energy measurement and anti-islanding effect. The core device of the control circuit adopts TMS320F2812DSP (referred to as 2812) of TI Company in the United States.
The inverter is the core component of the photovoltaic grid-connected power generation system. Selecting a high-reliability inverter module is a necessary condition for the normal operation of the circuit. The following describes the inverter composed of IPM (Intelligent Power Module) and the inverter composed of separate components.
Introduction of IPM Inverter Module
IPM is an advanced power switching device, which has the characteristics of high current density, low saturation voltage and high voltage resistance of GTR (high power transistor), and has the advantages of high input impedance, high switching frequency and low driving power of MOSFET (field effect transistor). . IPM integrates logic, control, detection and protection circuits, which not only reduces the size and development time of the system, but also enhances the reliability of the system.
Design of Protection Circuit of IPM Inverter Module
The IPM fault output signal blocks the IPM control signal channel. The software protection does not need to add hardware, which is simple and easy to implement, but may be affected by the software design and computer failure; the hardware protection responds quickly and works reliably. The method of combining software and hardware in the application can better make up for the lack of IPM’s own protection and improve the reliability of the system.
Inverter module circuit design composed of IR2130
IR2130 is a high-voltage integrated drive device below 600V. It has six input signals and six output signals, and only one power supply can drive six power switching devices of a three-phase bridge inverter circuit. One piece of IR2130 can replace three pieces of IR2110. Make the whole drive circuit simpler and more reliable.
Microgrid Inverter Power Design
The microgrid inverter power system directly affects the three-phase AC power output by the inverter and the stability of the entire system, so a stable voltage system is another necessary condition for the stable operation of the inverter. Power systems that power batteries require high efficiency and low ripple. The following describes the power supply design of external 220V power supply and battery power supply. During the experiment, the external 220V power supply system can be used first; when the experiment is completed and the product becomes a product, in order to simplify the circuit, it is necessary to use only the internal battery for power supply.
Design of Signal Conditioning Circuit for Microgrid Inverter
Since the DSP cannot input negative voltage, the output line voltage and line current of the inverter, the line voltage and line current of the grid terminal, a total of 12 signals can only be sent to the DSP through signal conditioning.
Selection of voltage transformers
The output of this system is three-phase alternating current, and the output line voltage is 380V. Therefore, TV19E voltage transformer is selected. Its output load resistance can be connected to 0~500Ω, and the output AC voltage is 0~2.5V. This system uses 240Ω resistance, and the output voltage is -1.2V~1.2V. Meet the input requirements of DSP. The circuit is shown in Figure 3.
Selection of current transformers
The output current of this system is less than 1A, so choose a voltage-type current transformer TA1410 that can measure up to 1A, the load resistance is 200 ohms, and the output voltage is -1V~1V AC voltage. The circuit is shown in Figure 4.
Design of Level Boosting Circuit
Since the DSP input terminal cannot input negative level, it is necessary to increase the signal of the voltage transformer and the current transformer by +1.25V, so that the input signal is between 0 and 3.3V.
Microgrid inverter switch drive circuit design
In order to realize the connection between the micro-grid inverter, load, and power grid, when the circuit fails, it needs to be switched quickly, so static switches (thyristors), AC contactors, and air switches are used in the circuit.
Design of energy metering circuit for microgrid inverter
This system uses two ATT7022Bs to measure the energy on the inverter side and the grid side respectively. ATT7022B is a high-precision three-phase electric energy metering chip, which integrates 6-channel differential input second-order sigma-delta ADC, suitable for three-phase three-wire and three-phase four-wire applications, and is not within the input dynamic operating range (1000:1). Linear measurement error is less than 0.1%. Main functions include: energy metering, parameter measurement, digital interface and digital calibration.
Microgrid inverter DC-DC circuit design
In order to input to realize MPPT, the input DC-DC adopts BOOST circuit. SG3525 is used as the main control chip.
Design of battery charging and discharging circuit for microgrid inverter
The intelligent charger and discharger adopts a buck-boost topology structure, and uses a PIC microcontroller for intelligent control. The circuit includes both an intelligent charging circuit and an intelligent power generation circuit.
Microgrid Inverter Transformer Design
The three-phase AC line voltage output by the inverter of this system is 190V. As a result, the three-phase step-up transformer (transformation ratio of 1:2) is boosted to 380V, and the △-Y connection method is adopted, and the power is 5kw. This transformer plays the role of boosting and also plays the role of isolation.
Software Design and Test Results
According to the previous analysis and discussion, a set of 5kw photovoltaic grid-connected power generation device based on TI’s DSP chip TMS320LF2812 was developed. Due to the powerful control and data processing capabilities of DSP, the hardware structure of the whole machine is relatively simple. Except for the main circuit, sampling detection circuit and driving circuit, all operations and data processing are completed by DSP. Therefore, a reasonable and effective control strategy and a concise software framework are powerful guarantees for the reliable operation of the system. According to the previous analysis and the basic requirements of the photovoltaic grid-connected power generation system, the DSP should complete the maximum power point tracking control, independent power supply operation control, genlock and grid-connected control, island detection protection control and other corresponding protections. This chapter mainly gives the corresponding software architecture and main experimental results according to the above requirements.
The overall software architecture of the system
The purpose of the micro-grid inverter photovoltaic power generation system is to supply the electric energy generated by the photovoltaic devices to the local load first, and the excess electric energy is fed back to the power grid. The design of the software should not only accurately and reliably reflect the control idea, but also ensure that the system is stable and reliable to prevent interference. The effect of the signal on the system.
In order to realize grid connection of photovoltaic grid-connected power generation system, the amplitude, phase and frequency of inverter output and grid voltage must be consistent, otherwise the harmonics of grid will increase, power quality will decrease, and grid-connected circulating current will be generated, and even cause photovoltaic power generation. damage to the system. Therefore, in the process of grid connection, genlock control and output voltage amplitude control must be performed to meet the requirements of grid connection. According to IEEE Std 1547-2003, the maximum phase error is 20 degrees, the instantaneous voltage error cannot exceed 10% of the grid voltage, and the maximum frequency error cannot exceed 0.3Hz. Figure 5 shows the voltage waveform across a purely resistive load in stand-alone mode.
Island detection and protection
The islanding effect is an important issue that must be paid attention to in distributed energy including photovoltaic power generation. The so-called islanding effect means that during the grid-connected operation of the inverter of the distributed energy system, when the mains input is manually disconnected or the power supply is stopped due to a fault, the inverter continues to supply power to the local grid, thereby making the local load power supply. The power supply continues to work.
Figure 5 Voltage waveform across purely resistive load in stand-alone operation mode
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