- Classification of inverters
There are many types of modern inverter technology, and the main classification methods are as follows.
(1) According to the destination of the inverter output energy, the active inverter and the passive inverter are classified. For solar photovoltaic power generation systems, active inverters are required in grid-connected photovoltaic power generation systems, while passive inverters are required in off-grid photovoltaic systems (ie, stand-alone photovoltaic systems).
(2) Classification according to the number of inverter phases Single-phase inverter, three-phase inverter, multi-phase inverter.
(3) Classification according to the frequency of the inverter output AC power: industrial frequency inverter (50~60Hz), intermediate frequency inverter (hundreds of Hz~10kHz), high frequency inverter (10kHz~1MHz).
(4) According to the form of the main circuit of the inverter, it is classified into single-ended type (including forward excitation and flyback type), push-pull type, half-bridge type and full-bridge type.
(5) According to the type of inverter main switching device, thyristor (also known as thyristor SCR) inverter, high-power transistor (GTR) inverter, high-power thyristor (GTO), field effect transistor inverter (VMOSFET), Insulated Gate Bipolar Transistor (IGBT), MOS Controlled Thyristor (MCT), etc.
(6) According to the stable output parameters of the inverter, it is classified into voltage type inverter and current type inverter.
(7) According to the waveform of the inverter output AC power, sine wave inverter, non-sine wave inverter (square wave, step wave, quasi-square wave, etc.).
(8) Classification by control mode Frequency Modulation (PFM) Inverter and Pulse Width Modulation (PWM) Inverter.
(9) Classification according to the working mode of the inverter switching circuit Resonant inverter, fixed-frequency hard-switching inverter and fixed-frequency soft-switching inverter.
- Test method of inverter
The grid-connected control inverter is a relatively complex electronic device. It consists of DC input circuit, power switch circuit, pulse width modulation and maximum power point tracking control circuit, harmonic smoothing, high frequency suppression circuit, island protection, and overload protection. Therefore, the test of the inverter is a very complicated process, which is mainly carried out before production and products leave the factory.
After the inverter is connected to the photovoltaic system, the inverter manufacturer should be responsible for testing before the system is activated. In the regular testing after the system is put into use, generally only its insulation resistance and insulation withstand voltage performance are tested. The experimental circuit of the inverter insulation resistance is shown in Figure 1.
According to the different rated voltage of the inverter, choose different insulation resistance meters. The test items include the insulation resistance test of the input circuit and the insulation resistance test of the output circuit. When inputting the circuit, first separate the switchboard from the power grid, short-circuit the input circuit and output circuit of the inverter, and then measure the insulation resistance between the input circuit and the ground. When measuring the insulation resistance of the output circuit, also short-circuit the input circuit and output circuit of the inverter, and then measure the insulation resistance between the output circuit and the ground. The input and output insulation resistance values of the inverter are generally above 0.1MΩ.
For the inverter in the photovoltaic system, the dielectric withstand voltage test is sometimes required to measure the dielectric withstand voltage value of the inverter circuit. The measurement conditions are generally the same as the insulation resistance test. During the test, take the open-circuit voltage of the standard inverter as the maximum operating voltage, and add 1.5 times the DC voltage or 1 times the AC voltage under the maximum operating voltage to the system. The test time is about 10 minutes to check whether there is insulation damage. During the test, the lightning arrester is generally removed, and then the test is carried out.
Read more: Solar Grid-Connected PV System Terminology