Since the 21st century, the energy crisis and environmental pollution are gradually becoming two major problems plaguing countries around the world. For the sustainable development of modern society, the development and use of renewable energy is being paid more and more attention to, among them, solar energy as a widely distributed clean renewable energy with good prospects for application. UPS as a continuous, stable provision of high-quality, highly reliable power supply of important equipment, is now widely used in: home, industry, communications, national defense, hospitals, radio and television, computer Business terminals, network servers, network equipment, data storage equipment and other fields. The MPPT control technology based off-grid solar UPS introduced in this paper not only enables the use of new energy sources, but also transforms them into highly reliable AC power supplies for various industries and equipment.
1. System structure of the UPS
The system structure of solar UPS is mainly divided into solar charging panel based on MPPT control technology, battery bank, utility charging module, inverter module and power factor correction module, the functions of each part are described as follows:
MPPT control technology based solar charging panel
MPPT (Maximum Power Point Tracking), i.e. Maximum Power Point Tracking, can detect the power generation voltage of the solar panel in real time and track the highest output voltage and current value (VI) to make the system output at maximum power, using the TI28034 chip, a highly efficient MPPT computing scheme The output voltage is 24V/48V/72V and the output current is 50A to serve as the energy input of the inverter module.
The most popular one is lithium iron phosphate battery, whose role is to store the output energy of MPPT controller and utility charging module, to be supplied to the inverter module when utility power is lost and solar energy is insufficient.
Utility charging module
provide energy for the battery bank when there is no solar energy.
Convert the 24V/48V/72V DC voltage output from MPPT controller or the voltage of the battery bank into high quality 220V AC voltage for customer's load.
Power factor correction module
The so-called power factor correction is to improve the current waveform to make it consistent with the voltage waveform as much as possible, so that the load will show the characteristics of resistive load, and the power factor will be close to 1. Therefore, after rectification, a DC/DC power factor corrector (PFC) is added to correct the input current to a sinusoidal waveform similar to the voltage waveform by the action of the active switch, in order to improve the power factor. The purpose is to improve the power factor.
2. Working principle of solar UPS system
2.1 This section first briefly introduces the working principle of several modules, and then the working principle of the system
1) MPPT controller: MPPT is essentially an optimization process. Its hardware circuit is a BUCK circuit, through the perturbation method, the solar panel from the open circuit voltage V slowly tuned to slightly higher than the battery V1, and record the output power of each voltage point in the process, to determine the voltage of the peak output power point. After the search using the hill climbing method, the voltage is controlled to move towards the peak power point, and eventually the system will work near the maximum power point.
2) Among the above modules, power factor correction module, battery bank, utility charging module, and inverter module constitute the UPS.
Its overall block diagram and working principle are as follows:
When the utility power is normal, the input utility power is output to the load through the input filter, AC/DC booster converter, inverter and output filter on the one hand; on the other hand, the battery is charged through the input filter and charger; when the utility power is abnormal, the battery voltage is output to the load through the DC/DC booster converter, inverter and output filter; when the UPS is abnormal, the input utility power is directly output to the load through the bypass.
2.2 The above describes the working principle of the 2 main parts of the solar UPS system. The working principle of the solar UPS system is now described as follows in conjunction with the above 2 points and the following schematic diagram of the whole machine:
The following parameters are defined: utility input power Pin, load power Pout, maximum power provided by solar energy Pmax, and battery discharge power Pbat.
2.2.1 When the utility power is normal, the battery is normal, the solar power is normal, and the solar UPS works in utility mode
1) If the load power Pout is less than the maximum power Pmax provided by the solar energy, then the energy provided by the solar energy is controlled by MPPT --- DC-DC boost ----DC-AC inverter and finally supplied to the load, and the other part of the energy is used to charge the battery through diode D1. If the battery has already reached the floating charge voltage at this time, then the MPPT controller will control the maximum power Pmax from the solar energy to make it equal to the load power Pout.
2) If the load power Pout is greater than the maximum power Pmax provided by the solar energy, then the solar energy provides the maximum power Pmax, energy through MPPT control --- DC/DC boost ----DC/AC inverter, the other part of the load required energy is supplied by the utility through AC/DC --- DC/AC.
2.2.2 When the utility power is abnormal, the battery is normal, the solar energy is normal, and the solar UPS works in battery mode
1) If the load power Pout is less than the maximum power Pmax provided by the solar energy, then the energy provided by the solar energy is controlled by MPPT --- DC-DC boost ----DC-AC inverter, and finally supplied to the load, and another part of the energy is used to charge the battery through diode D1. If the battery has already reached the floating charge voltage at this time, then the MPPT controller will control the maximum power Pmax from the solar energy to make it equal to the load power Pout.
2) If the load power Pout is greater than the maximum power Pmax provided by the solar energy, then the solar energy provides the maximum power Pmax, and the energy is supplied to the load through MPPT control --- DC/DC boost ----DC/AC inverter, and the other part of the energy required by the load is supplied to the load by BAT through --- Q1 --- DC/DC boost ----DC/AC inverter.
2.2.3 When the utility is normal, the battery is normal, and the solar is abnormal, the solar UPS works in utility mode, when all the energy required by the load is supplied by the utility through AC/DC--DC/AC.
2.2.4 When the utility is abnormal, the battery is normal and the solar is abnormal, the solar UPS works in battery mode, when all the energy required by the load is supplied to the load by the BAT via --- Q1 --- DC/DC boost ----DC/AC inverter.
3. Solar UPS vs. Solar Inverter
At present, the common solar power equipment on the market are solar off-grid inverters and solar off-grid inverters, compared with them, solar UPS has its own unique advantages and shortcomings, which are compared as follows:
3.1. Compared with off-grid inverter (working principle: when utility is normal, it runs in grid-connected mode and the inverter feeds power to the grid. When utility power is abnormal, disconnect utility power and feed the load through battery inverter):
(1) PV efficiency is slightly lower, solar UPS to go through the PV charger, DC-DC boost, DC-AC inverter link, while off-grid inverter only DC-DC boost, DC-AC inverter link.
(2) PV utilization rate will be lower when the load is insufficient and the battery is fully charged, at this time, the MPPT controller will be out of MPPT control, into the output constant voltage control.
(1) Not affected by policies and regulations, no impact on the grid and no influence from the grid.
(2) Wide utility range, off-grid inverter supports 184-264V, solar UPS supports 110V-285V, and 110V input, solar UPS can work in utility mode if PV power is more than half of the load.
(3) The output voltage is stable with only 1% error, off-grid inverter follows the utility voltage.
(4) Utility mode battery mode conversion does not have a conversion time.
(5) Good input performance when working in utility mode, with power factor close to 1 and current harmonics less than 8%. While off-grid inverter depends on the load, the power factor is usually 0.7 and the current harmonics are about 30%.
3.2. Compared with solar off-grid inverter, the disadvantages are the same as solar off-grid inverter, and the advantages are compared as follows:
(1) The output voltage is stable, with an error of only 1%. Off-grid inverter output voltage follows the utility voltage.
(2) Good input performance when working in utility mode, with power factor close to 1 and current harmonics less than 8%. While off-grid inverter depends on the load, the power factor is usually 0.7 and the current harmonics are about 30%.
(3) The battery will not be charged and discharged frequently, which can effectively increase the battery life.
(4) There is no conversion time for utility mode battery mode conversion.
Compared to the traditional UPS with only battery and utility energy supply, the solar UPS has a new way of injecting a new energy source, solar energy, and it works preferentially with the MPPT control technology to maximize the utilization of solar energy. Compared to the solar inverter, the solar UPS has an additional level of power factor correction in utility mode, and its high-quality pure sine wave output is unattainable by general inverters, which is especially important for some equipment with high input voltage accuracy requirements. In addition, it is equipped with active operation function, after sunrise in the morning, the sun's radiation intensity gradually strengthened, the solar panel output also increased, when the required output power to reach the UPS task, the UPS is actively on and running, therefore, it eliminates the tedious manual operation. The above features will determine the solar UPS is a market prospect products, I believe that with the development and use of new energy is becoming increasingly important to mankind, solar UPS will play an important role in a variety of fields.
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