Method for DC islanding detection based on positive feedback of bus voltage at specific frequency

A method for direct current (DC) islanding detection based on positive feedback of a bus voltage at a specific frequency, comprising following steps:

• extraction of a component of the specific frequency of the bus voltage:
• collecting information of the bus voltage in real time and sending the information of the bus voltage to a bandpass filter by a power management unit in a DC system to obtain the component of the specific frequency of the bus voltage, wherein the bandpass filter is a unity gain resonance amplifier, and the component of the specific frequency is a resonance frequency of the unity gain resonance amplifier;
• injection of a disturbance component of the specific frequency:
• sending the component of the specific frequency of the bus voltage to a proportional amplifier to obtain the disturbance component, and superimposing the disturbance component on a converter control inner loop instruction value or superimposing the disturbance component on a converter control outer loop instruction value, and then forming the positive feedback of the component of the specific frequency of the bus voltage on a control loop of converter,
• wherein selecting the resonance frequency of the unity gain resonance amplifier comprises:
• a magnitude gain of a transfer function from the disturbance component to the bus voltage being represented by k2(f) in an islanding mode, the magnitude gain of the transfer function from the disturbance component to the bus voltage being represented by ki(f) in a grid-connected mode, a difference therebetween being defined as G=k2(f)−ki(f), and a frequency fo corresponding to a maximum value of G being selected as the resonance frequency of the unity gain resonance amplifier;
• and
• determination of DC islanding:
• determining whether DC islanding occurs according to a variation trend of the component of the specific frequency of the bus voltage, determining that the system is operating in the islanding mode if the component of the specific frequency of the bus voltage continuous to increase and the number of sustained oscillation cycles exceeds a detection set value, or determining that the system is operating in the grid-connected mode if the component of the specific frequency in the bus voltage doesn't exist or the component of the specific frequency continuous to decrease, wherein the detection set value of the number of the sustained oscillation cycles is three.