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The islanding phenomenon occurs when the power supply from the grid is interrupted due to failures or maintenance work; the distributed generation systems connected to individual users’ systems (such as photovoltaic, wind, fuel cell generators, etc.) fail to detect this power outage immediately and therefore do not disconnect from the main power grid. As a result, an isolated system composed of these distributed generation systems and the loads connected to them is formed, capable of operating independently. When an islanding situation occurs, the unknown status of the system’s power supply can lead to various negative consequences; for example, it may put the lives of grid line maintenance personnel and users at risk ； Or interfere with the normal closing of the power grid ； Or the power grid is unable to control the voltage and frequency in isolated islands, thereby damaging distribution equipment and user equipment. Therefore, if the power control circuit breaker of a photovoltaic grid-connected inverter trips, an oscilloscope is needed to measure the device’s islanding prevention power-off time in order to verify whether it meets the standards. For an oscilloscope, this process is essentially measuring the changes in instantaneous values. So today we will discuss how to use an oscilloscope to measure the changes in the instantaneous values of waveforms. This process is actually divided into 2 parts; the first part is capturing the waveform at the moment of power loss. The second part is to calculate the power-off time of the captured waveform. Let’s first talk about how to use an oscilloscope to capture the waveform of the power-off moment. Signals can be divided into alternating current and direct current, while power-off methods can be divided into active power-off and passive power-off. Depending on various factors, the methods of capture also vary. For example, in case of ① passive power interruption of direct current (the device is running, and it’s not possible to know when the power will be cut off), an oscilloscope can use single-trigger mode to capture the waveform. We set the oscilloscope’s trigger mode to automatic, the trigger type to edge-trigger on descent, and moved the trigger level below the DC voltage and above the zero level. Then press single trigger. When a power outage occurs, the oscilloscope captures the process of voltage drop; then we pause the waveform, expand it to an appropriate position, and use a cursor to measure the time taken for the waveform to drop. As shown in the figure below, the power outage duration is around 500 μs. In the case of ② active disconnection of direct current (the device is running and the power is manually cut off), since the duration of the power outage can be controlled, the oscilloscope can not only use single-trigger mode to capture the signal but also employ a long time base to record the shutdown signal. Assuming that the entire power-off procedure will take about 10 seconds, we can set the time base to 1 second and then carry out the power-off operation; this way, one screen display cycle will last 14 seconds. After the operation is complete, by pressing the stop button on the oscilloscope, this time interval is just right for us to record the power-off signal. Then, just as before, expand the waveform to the appropriate position and use the cursor to measure the time it takes for the waveform to decline. It can be seen that around 580 μs is required for active power disconnection of the ③ AC current. We can also use the same long time base method, but it is necessary to set the storage depth to its maximum value in order to ensure that the waveform remains undistorted. So, if it’s ④ alternating current that is passively powered off, how can the waveform be captured? Since it is a passive power outage, and it’s not known when the power loss will occur, using a long time base is not suitable; therefore, a single trigger must be used, and an appropriate triggering method has to be found in order to capture the signal. Let’s give an example: suppose it’s a sine wave, and its positive pulse width is around 10 ms. After the power is cut off, its pulse width will inevitably decrease. Therefore, we use pulse width triggering, setting the condition to be that the positive polarity should be less than 9 ms. By pressing the single-trigger button, the oscilloscope will capture that moment when the power is cut off. Then, using the same method, we can expand the waveform and measure the time with the cursor.