1. Technical requirements
The technical requirements of the inverter include the following points:
(1) Higher reliability. Due to the particularity of the installation location and all-weather operation of the photovoltaic power generation system, frequent and timely maintenance cannot be achieved. This requires the inverter to be able to operate safely and stably for a long time and have high reliability.
(2) Higher inverter efficiency. At present, the power generation cost of photovoltaic power generation systems is still relatively high. In order to maximize the reasonable use of the electrical energy generated by photovoltaic power generation and improve system efficiency, the inverter efficiency of the inverter must be increased as much as possible. Generally, the inverter efficiency of small and medium power inverters at full load is required to reach 85% to 90%, and the inverter efficiency of high-power inverters at full load is required to reach 90% to 95%.
(3) Wide DC input voltage range. Since the output voltage of the photovoltaic array will vary with the load, sunshine intensity, and climatic conditions, and its input voltage has a large variation range, the inverter must be required to have a wide DC input voltage range.
(4) Better power output quality. The quality of the electric energy provided by the photovoltaic power generation system to the local AC load or sent to the grid shall meet the practical requirements and meet the standards. Once there is an over-limit condition that deviates from the standard, the system should be able to detect these deviations and disconnect the photovoltaic power generation system from the grid.
2. Performance Index
The performance indicators of the inverter are as follows:
(1) Rated output voltage. The rated output voltage indicates the rated voltage value that the inverter can output within the specified allowable fluctuation range of the input DC voltage.
(2) The inverter should have sufficient rated output capacity and overload capacity. In the selection of off-grid inverters, the first thing to consider is sufficient rated capacity to meet the power requirements of the equipment under the maximum load. The rated output capacity indicates the ability of the inverter to supply power to the load. An inverter with a high rated output capacity can drive more loads. But when the load of the inverter is not a pure resistive load, that is, when the output power factor is less than 1, the load capacity of the inverter will be less than the rated output capacity value given.
For an inverter with a single device as a load, the selection of its rated capacity is relatively simple. When the electrical equipment is a pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter is selected to be 1.1 to 1.15 times that of the electrical equipment. When the inverter uses multiple devices as the load, the selection of the inverter capacity should consider the possibility of several electrical devices working at the same time, that is, the “load simultaneous coefficient”.
(3) Output voltage stability. The output voltage stability indicates the regulation ability of the output voltage of the off-grid inverter. Most inverters give the deviation percentage of the inverter’s output voltage within the allowable fluctuation range of the input DC voltage, which is usually called the voltage regulation rate. A high-performance inverter should also provide the deviation percentage of the inverter’s output voltage when the load changes from 0 to 100%, which is usually called the load regulation rate. The voltage regulation rate of the inverter with good performance is within ±3%, and the load regulation rate is within ±6%.
(4) Waveform distortion of the output voltage. When the inverter voltage is a sine wave, the maximum allowable waveform distortion (or wave content) is usually expressed by the total waveform distortion of the output voltage, and its value should not exceed 5% (single-phase output index allows 10%).
(5) Rated output frequency. The frequency of the AC voltage output by the inverter should be a relatively stable value, usually 50 Hz. For grid-connected inverters, according to GB/T 19939-2005 “Technical Requirements for Grid-connected Photovoltaic Systems”, the frequency deviation is ±0.5 Hz.
For off-grid inverters, the deviation should be within ±1% under normal working conditions.
(6) Power factor. The power factor represents the ability of the inverter to carry inductive loads. When the output of the grid-connected inverter is greater than 50% of its rated output, the average power factor should not be less than 0.9 (leading or lagging).
For off-grid inverters, under sine wave conditions, the load power factor is 0.7~0.9 (lagging), and the rated value is 0.9.
(7) Rated output current (rated output capacity). This index indicates the rated output current of the inverter within the specified load power factor range. Some inverters give the rated output capacity. The rated output capacity of the inverter is the product of the rated output voltage and the rated output current when the output power factor is 1 (that is, a pure resistive load).
(8) Rated output efficiency. Rated output efficiency refers to the ratio of output power to input power under specified working conditions. The high inverter efficiency of the whole machine is a significant feature that distinguishes the inverter used in the photovoltaic power generation system from the general inverter. The efficiency value of the inverter represents the size of its own power loss, usually expressed as a percentage. The rated output efficiency changes with the change of the load rate. The higher the load rate, the rated output efficiency increases. Inverters with larger capacity should also give full-load efficiency values and low-load efficiency values.
(9) DC component. When the photovoltaic power generation system is connected to the grid, the DC component fed by the inverter to the grid should not exceed 0.5% of its AC rating. For photovoltaic inverters that are directly connected to the grid without a transformer, it can be relaxed to 1% due to special factors such as inverter efficiency.
(10) Harmonic and waveform distortion. The output of the photovoltaic power generation system should have low current distortion to avoid adverse effects on other equipment connected to the grid. The total spectral wave current of the grid-connected inverter should be less than 5% of the rated output of the inverter.
(11) Voltage unbalance degree. When the photovoltaic power generation system is connected to the grid (only for three-phase output), the three-phase voltage unbalance at the grid interface should not exceed the value specified in GB/T 15543, the allowable value is 2%, and it should not exceed 4% in a short time.
(12) Protection function. During the normal operation of the photovoltaic power generation system, various faults may be caused due to load failure, misoperation of staff, and external interference. The inverter must have a reliable and complete protection function to ensure the stable and efficient output of electric energy. For grid-connected inverters, island protection is particularly important.
(13) Start-up characteristics. The inverter should be guaranteed to start reliably and stably under the rated load. High-performance inverters can be started repeatedly at full load without damaging the power devices. For their own safety, small inverters sometimes adopt soft start or current limit start.
(14) Noise. Components such as transformers, filter inductors, electromagnetic switches, and fans in power electronic equipment all generate noise. When the inverter is operating normally, its noise should not exceed 65 dB.