The function of photovoltaic inverter is to convert the DC generated by photovoltaic modules into AC. In the process of conversion, there will be a part of loss, that is, the energy at DC side has not been completely converted into the energy at AC side. We call the ratio of the output energy at AC side of photovoltaic inverter to the input energy at DC side as the conversion efficiency of photovoltaic inverter.
The only measure to improve PV inverter efficiency is to reduce losses that mainly come from power switching tubes such as IGBT and MOSFET, and magnetic devices like transformers and inductors, as well as other devices like relay and electrolytic capacitors.
Energy losses caused by IGBT mainly include conducting loss and switching loss, among which the first one is related to the internal resistance of its components and the current that passes through it, while the switching loss is more relevant to the switching frequency of IGBT and the DC voltage/current that they bear.
There are basically two types of inductive losses: Cooper loss and iron loss. Copper loss refers to the loss caused by the resistance of the inductance copper wire. As we all know that P = I2 * R, reducing the resistance of the inductance copper wire can reduce the copper loss of the inductance; Rron loss is the loss caused by the change of magnetic field when the inductance works.
There are currently four technical routes to enhance inverter efficiency. First, silicon carbide materials are used to reduce the internal resistance of power devices and the reverse recovery current of diodes. Second, soft switching technology is adopted to reduce the switching power loss. Third, optimized designs of inductor are applied for that purpose. Fourth, circuit design is optimize to enhance inverter efficiency.
Silicon carbide is a new technology developed in recent years. Compared with conventional silicon devices, it's on resistance is much smaller, which can effectively reduce the reverse recovery current of diode, thus reducing the switching loss on the diode itself and the IGBT.
Soft Switching Topology
Soft switching technology makes use of resonance principle to make the current or voltage in switching devices change according to sinusoidal or quasi-sinusoidal law, turns off the devices when the current crosses zero naturally, and turns on the device when the voltage naturally crosses zero. The coincidence area of voltage and current is significantly limited, thus lowering the switching loss.
There are four power semiconductor devices on the one bridge arm of the three-level converter. It outputs multi-level step wave, which can make the waveform closer to the sine wave. Thanks to this advantage, under the same switching frequency, three-level inverters have smaller output filter inductance than two-level equipment. Thus, Under the same output harmonic content, three-level inverters have lower switching frequency than two-level inverters, so as to decrease switching loss and the loss of inductor.
In addition to the three methods mentioned above, optimizing the circuit design is also another effective way to improve inverter efficiency. For example, the loss of the relay can be reduced by making the relay work at the holding voltage rather than the rated voltage. By using more electrolytic capacitors in parallel to reduce the ripple current of the electrolytic capacitor, not only the power loss of the electrolytic capacitor is reduced, but also the life of the electrolytic capacitor is improved.
|Loss distribution||Loss components||Loss property||Efficiency improvement method|
|Power device||MOSFET, Diode and IGBT||Conducting loss and reverse recovery loss||SIC technology|
|Switching power loss||Soft switching technology|
|Magnetic components||Boost inductor and inverter inductor||Cooper loss and iron loss||Three-level technology|
|Other components||Electrolytic capacitor, relay, switching power supply, etc||Internal resistance of components||Optimize the circuit design, such as let the relay work at the holding voltage|
TSUN's TSOL-A5.0K-H energy storage unit uses the most advanced SIC, soft switching and three-level technologies, adopting optimized designs of circuit, drive and inductors, so that the efficiency of inverter breaks through the 98.3% bottleneck of traditional inverters, and the maximum efficiency reaches 98.3%. In consequence, the inverter performance is significantly improved while the radiator weight has been furtherly reduced.