Voltage Stress Suppression Method For Photovoltaic 5L-ANPC Inverter

Multi-level inverters are widely used in the photovoltaic industry to improve efficiency and reduce raw materials due to their higher efficiency and higher power density.The five-level midpoint active clamp converter(5L-ANPC)has been applied to high-voltage and high-power inverters due to its advantages in topology and control performance.Due to the sensitivity of power devices to voltage stress,ensuring the safety of voltage stress of all power devices during operation is very important to the reliability of the 5L-ANPC inverter.This thesis mainly focuses on three types of problems that cause high voltage stress in power devices.In this paper the topology of the 5L-ANPC inverter is analyzed.The first type of voltage stress problem is the large turn-off voltage stress caused by the long commutation loop of the low-voltage tube.By introducing a snubber capacitor between the output points of the front-stage series half-bridge circuit of the inverter,the commutation loop is shortened and the turn-off voltage stress is suppressed.The second type of voltage stress problem occurs in the dead zone stage of the front-stage halfbridge switching process.By delaying the turn-off action of the high-voltage tube,the risky switch is kept on.Its terminal voltage is clamped to zero and the voltage stress is suppressed.The third type of voltage stress problem is caused by the traditional shutdown method of turning off all switches.The high-voltage switch always-on scheme is proposed.This scheme switches the high-voltage tube according to the polarity of the grid voltage.It can effectively suppress voltage stress and inductor current during inverter shutdown.Finally,an experimental platform of a 60 k W three-phase three-wire 5L-ANPC inverter was built to verify the correctness and effectiveness of the voltage stress suppression strategy mentioned in this paper.