How to choose an inverter for a PV off-grid system?
In the photovoltaic off-grid system, the power of the off-grid inverter is generally confirmed according to the user's load type and power.
Loads are classified according to their impedance properties: resistive loads, inductive loads, and capacitive loads.
(1) Resistive load: The load with no phase difference between current and voltage is a resistive load, such as rice cooker, light bulb, electric stove, electric soldering iron, etc.
(2) Inductive load: The load when the current lags the voltage with a phase difference is an inductive load, such as washing machines, air conditioners, refrigerators, water pumps, range hoods and other loads with motors, transformers, relays, compressors, etc.
(3) Capacitive load: The load with a negative phase difference in current leading voltage is a capacitive load, such as compensation capacitors, computers, TVs, etc., which are used in switching power supplies at home.
The starting power of inductive loads such as motors is 5-7 times the rated power. When calculating the power of the inverter, the starting power of these loads should be taken into account.
If the load is an elevator or the like, it cannot be directly connected to the output terminal of the inverter, because when the elevator descends, the motor reverses, which will generate a counter electromotive force, which will damage the inverter when it enters the inverter. If an off-grid system must be used, it is recommended to add a frequency converter between the inverter and the elevator motor.
领英推荐
If the photovoltaic system is only used for pumping water, and a water tower can be built, it is recommended to use a photovoltaic water pumping inverter, which can save costs. For example, if a 2P water pump is used, an off-grid inverter of more than 5KW is required to use an ordinary inverter. To drive, a battery of more than 200AH is required, and the total cost is more than 10,000 yuan. However, if a water-lifting inverter is used, it only needs 2KW, and the price is less than 3,000 yuan.
The waveform of the inverter is mainly divided into two categories, one is the sine wave inverter, and the other is the modified wave inverter. The modified sine wave inverter uses PWM pulse width modulation to generate modified wave output. Due to the harmonic distortion of about 20%, it cannot be used with inductive loads such as air conditioners, but it can be used with resistive loads such as lamps. The modified sine wave inverter adopts a non-isolated coupling circuit with simple components and high efficiency. The pure sine wave inverter adopts the isolation coupling circuit design, the circuit is more complicated and the cost is higher, and it can be connected to any common electrical equipment (including TVs, LCD monitors, etc., especially inductive loads such as refrigerators) without interference.
Inverters are divided into high-frequency machines and power-frequency machines according to isolation. There is a 50Hz power-frequency isolation transformer behind the inverter, called power-frequency inverter; using high-frequency switching technology, high-frequency switching elements or high-frequency isolation The transformer replaces the power frequency transformer in the inverter, which is called a high frequency inverter. The advantages of power frequency inverters are stable and reliable power components, strong overload capacity and impact resistance, but the disadvantages are low efficiency, heavy weight and high price. There are two types of high-frequency inverters: high-frequency isolation and non-isolation. The advantages are small size, high efficiency, and low price. The disadvantage is that the impact resistance is slightly lower. If the load is an inductive load such as a motor with a large impact, but it does not move frequently, it is recommended to choose a power frequency inverter. If the load is a resistive load with a small impact and needs to be moved frequently, it is recommended to choose a high frequency inverter.
The output power of the inverter must be greater than the power of the load. For monitoring stations, communication stations and other occasions with strict requirements, the output power is the sum of all load powers. But for ordinary poor families, considering that all loads cannot be turned on at the same time, in order to save costs, the sum of load power can be multiplied by a factor of 0.7-0.9.