In the photovoltaic system, the technical indicators and parameters of the photovoltaic inverter are mainly affected by the battery, load and grid connection requirements. The main technical parameters are as follows.
1. Rated output voltage
The photovoltaic inverter should be able to output the rated voltage value within the specified allowable input DC voltage range. Generally, when the rated output voltage is single-phase 220V and three-phase 380V, the voltage fluctuation deviation is specified as follows.
(1) When running in a steady state, the voltage deviation is generally required not to exceed ±5% of the rated value.
(2) When the load changes suddenly, the voltage deviation does not exceed ±10% of the rated value.
(3) Under normal working conditions, the unbalance of the three-phase voltage output by the inverter should not exceed 8%.
(4) The distortion of the voltage waveform (sine wave) of the three-phase output is generally required not to exceed 5%, and the single-phase output should not exceed 10%.
(5) Under normal working conditions, the frequency deviation of the inverter output AC voltage should be within 1%. The output voltage frequency specified in the national standard GB/T19064-2003 should be 49-51Hz.
2. Load power factor
The size of the load power factor indicates the ability of the inverter to carry inductive or capacitive loads. Under sine wave conditions, the load power factor is 0.7~0.9, and the rated value is 0.9. In the case of a certain load power, if the power factor of the inverter is low, the required capacity of the inverter will increase, resulting in an increase in cost. At the same time, the apparent power of the AC circuit of the photovoltaic system increases, the circuit current increases, and the loss will inevitably increase, resulting in a decrease in the system efficiency.
3. Rated output current and rated output capacity
The rated output current refers to the rated output current of the inverter within the specified power factor range, and the unit is A. The rated output capacity refers to the product of the rated output voltage and rated output current of the inverter when the output power factor is 1 (ie pure resistive load), and the unit is kVA or KW.
4. Rated output efficiency
Rated output efficiency refers to the ratio of output power to input power under specified working conditions, expressed as a percentage. In general, the nominal efficiency of a photovoltaic inverter refers to the efficiency under pure resistive load and 80% load. The efficiency of the inverter varies with the size of the load, and when the load factor is below 20% and above 80%, the efficiency is lower. The standard stipulates that when the output power of the inverter is greater than or equal to 75% of the rated power, the efficiency should be greater than or equal to 90%. At present, the nominal efficiency of mainstream inverters is 95%~99%, and the efficiency of low-power inverters is required to be no less than 85%. In the design of the photovoltaic power generation system, not only should high-efficiency inverters be selected, but also through reasonable system configuration, the photovoltaic system load should work as close as possible to the optimal efficiency point.
5. European efficiency and maximum efficiency
European efficiency is based on European lighting conditions, a PV inverter with a standard configuration array, given weights at different power points, used to estimate the overall efficiency of the inverter. Specifically, it refers to the probability weighted sum of the efficiency of the inverter under different load conditions. The specific formula is as follows:
It can be seen that the sum of the six coefficients is 1, and each coefficient reflects the probability of the inverter operating at its respective power point under European lighting conditions, and the overall efficiency of the inverter is reflected.
The maximum efficiency of the inverter refers to the maximum efficiency that the inverter can achieve.
6. Overload capacity
Overload capacity refers to the requirement that the inverter can continue to work for a certain period of time under a specific output power, and its standards are as follows.
(1) When the input voltage and output power are rated values, the inverter should be able to work continuously and reliably for more than 4 hours.
(2) When the input voltage and output power are 125% of the rated value, the inverter should be able to work continuously and reliably for more than 1 minute.
(3) When the input voltage and output power are 150% of the rated value, the inverter should be able to work continuously and reliably for more than 10s.
7. Rated DC input voltage
The rated DC input voltage refers to the rated DC voltage input to the inverter in the photovoltaic power generation system. The input voltage of low-power inverter is generally 12V, 24V and 48V, and the input voltage of medium and high-power inverter is 48V, 150V, 300V, 500V, etc.
8. Rated DC input current
The rated DC input current refers to the rated DC working current provided by the photovoltaic array to the inverter.
9. DC voltage input range
For off-grid PV inverters, the DC input voltage is allowed to vary within the range of 90% to 120% of the rated DC input voltage, and should not affect the output voltage variation. For grid-connected inverters, the DC voltage input range is generally wide, such as 160~800V, 200~1000V, etc., and there is also an MPPT operating voltage range that is generally 120~600V, 450~800V, etc.
10. Use environmental conditions
(1) Working temperature. The operating temperature of the inverter power device directly affects many important characteristics such as the output voltage, waveform, frequency, phase, etc. of the inverter, and the operating temperature is related to the ambient temperature, altitude, relative humidity and working state.
(2) Working environment. For high frequency and high voltage inverter, its working characteristics are related to the working environment and working state. In high-altitude areas, the air is thin, and it is prone to discharge between the circuit poles, which affects the work. In high humidity areas, condensation is easy to cause local short circuits. Therefore, the inverters all stipulate the applicable working range.
The normal operating conditions of the PV inverter are: ambient temperature -20~+50℃, altitude ≤5500m, relative humidity ≤93%, and no condensation. When the working environment and working temperature are outside the above range, consider reducing the capacity use or redesigning the customization.
11. Electromagnetic Interference and Noise
The switching circuit in the inverter is very easy to generate electromagnetic interference, and it is easy to generate noise on the iron core transformer. Therefore, electromagnetic interference and noise indicators must be controlled in design and manufacture to meet relevant standards and user requirements. The noise requirements are: when the input voltage is the rated value, measure the noise at 50% rated load and full load with a sound level meter at 1/2 of the equipment height and a frontal distance of 3m, and the noise value should be less than or equal to 65db.
12. Protection function
The photovoltaic power generation system should have high reliability and safety, and the inverter as an important part of the photovoltaic power generation system should have the following protection functions.
(1) Input under-voltage protection. When the input voltage is lower than the specified under-voltage disconnection (LVD) value, that is, lower than 85% of the rated voltage, the inverter should be able to automatically shut down for protection and display accordingly.
(2) Input overvoltage protection. When the input voltage is higher than the specified overvoltage disconnection (HVD) value, that is, higher than 130% of the rated voltage, the inverter should be able to automatically shut down for protection and display accordingly.
(3) Overcurrent protection. The over-current protection of the inverter should be able to ensure timely action when the load is short-circuited or the current exceeds the allowable value, so as to prevent it from being damaged by the surge current. When the working current exceeds 150% of the rated value, the inverter should be able to automatically protect. When the current returns to normal, the device can work normally again.
(4) Short circuit protection. When the inverter output is short-circuited, there should be short-circuit protection measures. The short-circuit protection action time of the inverter should not exceed 0.5s. After the short-circuit fault is eliminated, the equipment should be able to work normally.
(5) Reverse polarity protection. When the positive input terminal and negative input terminal of the inverter are reversely connected, the inverter should be able to automatically protect. After the polarity is positive, the device should work normally.
(6) Lightning protection. The inverter should have lightning protection function, and the technical indicators of its lightning protection device should be able to absorb the expected impact energy.
13. Safety performance requirements
The safety performance requirements of photovoltaic inverters have been described in the previous article.