Patent ID: 9667178
Date: 2017-05-30
CPC Classifications: H02P

Claim:
1. A variable frequency speed control system having a Low Voltage Ride Through (LVRT) function which comprises a variable frequency drive (VFD) having a rectifier, a DC bus and an inverter, wherein the rectifier converts a grid voltage to a DC bus voltage, the inverter converts the DC bus voltage to a three phase AC signal by the DC bus, the variable frequency speed control system further comprising: a voltage drop detecting module electrically coupled to a grid, for sampling the grid voltage, generating a voltage drop coefficient according to the sampled grid voltage and a rated grid voltage, and generating a corresponding operation mode switching signal according to the voltage drop coefficient; a bus voltage detecting module electrically coupled to the DC bus, for sampling the DC bus voltage; a current detecting module electrically coupled to the inverter, for sampling a three phase current signal of the three phase AC signal outputted from the inverter; an active current decomposing module coupling with the current detecting module, for retrieving a load active current component of the inverter from the three phase current signal sampled by the current detecting module to generate a load active current component; a power reduction control module coupling with the voltage drop detecting module, for receiving the voltage drop coefficient from the voltage drop detecting module and the load active current component from the active current decomposing module to generate a first frequency decreasing amount; renewable power control module coupling with the bus voltage detecting module and the active current decomposing module, for receiving the DC bus voltage from the bus voltage detecting module, a maintaining command voltage and the load active current component from the active current decomposing module to generate a second frequency decreasing amount; a frequency control module for generating target frequency signal according to a given frequency control signal; an operation mode selecting module coupling with the power reduction control module, the renewable power control module and the frequency control module, for receiving the target frequency signal, the first frequency decreasing amount and the second frequency decreasing amount to generate an output frequency signal according to the operation mode switching signal; and an inverter control module for generating a three phase modulating signal according to the output frequency signal, which is used to control the operation of inverter.