The basic characteristics of the inverter and its evaluation

The basic characteristics of the inverter and its evaluation

Various inverters have different basic characteristics and technical specification requirements. The basic characteristics and evaluation of inverters matched with solar photovoltaic power generation are as follows.

(1) Output waveform
Common waveforms are square waves, staircase waves (sometimes called quasi-sine waves), and sine waves, as shown in Figure 1.

Figure 1 - Common Inverter Waveforms
Figure 1 – Common Inverter Waveforms

①Square wave inverter
The line is simple and the cost is low. The disadvantages are high harmonics, small voltage adjustment range, low efficiency with inductive loads such as motors, and large electromagnetic interference;
②Step wave inverter
The use of pulse width modulation PWM circuit makes up for the deficiency of square wave inverter to a certain extent. Its waveform is similar to sine wave, so it can carry various loads including inductive load. Although its DC/AC conversion efficiency is high, this waveform cannot be applied to the grid.
③Sine wave inverter
It is an ideal inverter. When used in an independent photovoltaic system, the waveform distortion requirement should not exceed 5%.

(2) Output frequency
Common power frequency 50Hz inverters and high frequency (20~200kHz) inverters and very high frequency (>200kHz) inverters. Due to the emergence of high-performance semiconductor switching tubes, the frequency of the inverter is greatly increased, thus greatly reducing the volume of the transformer, reducing copper loss and iron loss, and improving the efficiency of the inverter. In addition, the high-frequency inverter control The speed is fast, the precision is high, and the response to the protection signal is relatively fast, which increases the reliability of the system.

(3) DC/AC conversion efficiency
For solar photovoltaic power generation systems, the DC/AC conversion efficiency of the inverter is very important. Usually the efficiency of the inverter is 85%~90%, and the high-quality inverter can reach 90%~96%. It should be noted that the efficiency of the inverter tends to vary with the load rate. Often when the load factor is lower than 20% and higher than 80%, the DC/AC conversion efficiency is lower. Some inverters are not efficient at low load, but after the load rate exceeds 30%, the DC/AC efficiency has been maintained at a high level, as shown in Figure 2.

Figure 2 - The relationship between inverter DC/AC conversion efficiency and load rate
Figure 2 – The relationship between inverter DC/AC conversion efficiency and load rate

It is particularly worth noting that when measuring the efficiency of non-sine wave and non-50Hz inverters, it cannot simply be measured by a general-purpose instrument measuring 50Hz sine wave, but must be measured by a special method and other calibrated special instruments.

(4) Working temperature
The working temperature of the inverter power device directly affects many important characteristics of the inverter, such as output voltage, waveform, frequency, phase, etc., and the working temperature is related to the ambient temperature, the altitude, humidity and working state of the working location. When the inverter is to be used in extremely hot and cold areas, its operating temperature should be pre-designed.

(5) 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 or local energy, which affects the work. In areas with high humidity, it is easy to condense and cause local short circuit. Therefore, for each inverter, its applicable working environment must be specified.

(6) Electromagnetic interference and noise
The switching circuit in the inverter is very easy to generate electromagnetic interference, and it is easy to generate noise due to vibration on the inferior iron core transformer. Therefore, the indicators of electromagnetic interference and noise must be controlled in both design and manufacture to meet the requirements of relevant standards and users.

(7) Overload capability
When some loads such as TV sets and motors are started, their instantaneous power can be 3 to 6 times that of normal operation. Therefore, the inverter is required to have instantaneous overload capability, also known as the crest factor. In addition, under special circumstances, there will be some additional load increases, which requires the inverter to have a certain rated overload capacity, and there should be room when designing the photovoltaic system.

(8) Others
Other indicators of the inverter such as input and output rated voltage, current range and accuracy requirements, power factor, rated output power, continuous fault-free time, whether it can be operated in parallel with several inverters at the same time.

Read more: Marketization of PV systems.