The technical requirements and performance indicators of the inverter that people are most concerned about are as follows:
Inverter technical requirements
(1) High reliability
Due to the special location of the installation location and its all-weather operation, the photovoltaic power generation system cannot be maintained frequently and in time. This requires the road transformer to be safe and stable for long-term operation and should have high reliability.
(2) High inverter efficiency
At present, the power generation cost of photovoltaic power generation systems is relatively high, in order to maximize the rational use of the power generated by photovoltaic power generation. To improve the efficiency of the system, it is necessary to increase the inverter efficiency of the inverter as much as possible. Generally, the inverter efficiency requirement of the medium and small power inverters is 85% to 90% at full load, and the inverter efficiency requirement is 90% when the large speed converter is fully loaded. ~98%. In addition, the efficiency of the inverter under light load, that is, the weighting efficiency is required to be high.
(3) Wide DC input voltage range
Since the output voltage of the PV array will vary with load and irradiance, climatic conditions. The input voltage varies widely, so the inverter must have a wide DC input voltage range.
(4) Good power output quality
The quality of the photovoltaic system to provide electrical energy to or from the local AC load should meet the practical requirements and meet the standards. In the event of a deviation from the standard, the system should be able to detect these deviations and disconnect the PV system from the grid.
(5) Good value for money
In order to reduce the cost of photovoltaic power generation systems, in addition to the solar module components to reduce manufacturing costs, the system balance components must also have a good price/performance ratio. In general, inverters account for 8% to 10% of the system cost.
Inverter performance index
(1) Rated output voltage
The rated output voltage indicates the rated voltage that the inverter can output within the allowable fluctuation range of the specified input DC voltage.
For grid-connected inverters, according to GB/T19939-2005 (Photovoltaic system grid connection technical requirements), the allowable deviation of the three-phase voltage is 7% of the rated voltage, and the allowable deviation of the single-phase voltage is +7% of the rated voltage -10%. For off-grid inverters, the rated output voltage values are as follows:
1 During steady-state operation, the voltage fluctuation should have a certain range, and the range of agitation is not more than 3% or ±5% of the rated value;
2 Under the dynamic influence of sudden load (0-50%~100% of rated load) or other disturbance factors, the output voltage deviation should not exceed 8% or 10% of the rated value.
(2) Selection of rated output capacity and overload capability
In the selection of off-grid inverters, the first thing to consider is the sufficient rated output capacity to meet the power requirements of the equipment under the maximum load. The rated output capacity represents the ability of the inverter to supply power to the load. The rated output voltage gives the user more loads. However, when the load of the grader is not a purely resistive load, that is, when the output power factor is less than 1, the load capacity of the inverter is less than the rated output capacity value given.
For inverters that are loaded with a single device, the selection of the common rated capacity is relatively simple. When the electrical equipment is pure resistive load or the power factor is greater than 0.9, the rated output capacity of the inverter is 1.1 to 1.15 times of the capacity of the electrical equipment. When the inverter is loaded with multiple devices, the rated output capacity of the inverter should be considered considering the possibility that several consumers will work at the same time, that is, the "load simultaneous coefficient".
(3) Output voltage stability
The output voltage stability characterizes the regulation capability of the off-grid inverter output voltage. Most inverters give the percentage deviation of the inverter's output voltage within the allowable fluctuation range of the input DC voltage, commonly referred to as the voltage regulation. High performance is that the inverter also gives the percentage deviation of the output voltage of the inverter when the load changes from 0 to 100%, which is usually called load regulation. The voltage regulation rate of the inverter with good performance is within 3%, and the load regulation rate is within 6% of the soil.
(4) Waveform distortion of the output voltage
When the inverter output voltage is sinusoidal, the maximum allowable waveform distortion (or harmonic content) is specified. Usually expressed as the total waveform distortion of the output voltage, the value should not exceed 5% (10% for single-phase output specifications),
(5) Rated output frequency
The frequency of the output AC voltage of the inverter should be a relatively stable value, usually 50Hz. For grid-connected inverters, according to GB/T 19939 -2005 (Photovoltaic system grid connection technical requirements), the frequency deviation is soil 0.5. Hz. For off-grid inverters, the deviation should be within 1% of the soil under normal working conditions.
(6) Power factor
The power factor characterizes the ability of the inverter with inductive or capacitive loads. When the output of the grid-connected inverter is greater than 50% of its rated output, the average power factor should be no less than 0.9 (leading or lagging).
For off-grid inverters, under sine wave conditions, the load power factor is 0.7 to 0.9 (hysteresis) and the rated value is 0.9.
(7) Rated output current (rated output capacity)
This indicator indicates the rated output current of the inverter within the specified load power factor range. The inverter gives the rated output capacity and the rated capacity of the inverter is the product of the rated output voltage and the rated output current when the output power factor is 1 (i.e pure resistive load).