Grid connection - Analysis of low voltage ride through and island coexistence

Independent analysis of island protection and low voltage ride through

When the inverter is connected to the grid , Will continue to pass through the frequency offset 、 Active islanding protection methods such as reactive power disturbance and passive anti islanding protection methods such as detecting whether the grid voltage frequency and grid voltage amplitude exceed the specified value to judge whether it is in an island state . Once it is judged as an island , The inverter stops immediately , And send out fault alarm , At the same time, wait for the power grid to return to normal . continued 5min after , The inverter is re connected to the grid .
The specific judgment logic is as follows ：
When the inverter starts grid connected operation , First, carry out normal grid connection voltage outer loop and current inner loop control , Conduct active disturbance （ Such as reactive power disturbance and frequency disturbance ）, Then check four items ： First, check whether the voltage value of the three-phase power grid is 80%~115% Between , if , Then return to the initial normal grid connection voltage outer loop and current inner loop control , If it is not , Then it is less than or equal to 2s Under voltage protection and detect whether the grid voltage value returns to the normal value =（80%-115%）, Maintain after returning to normal value 5min Return to the normal grid connection voltage outer loop and current inner loop control ; The second is to detect whether the three-phase voltage imbalance of the power grid is lower than 4%, if , Then return to the initial normal grid connection voltage outer loop and current inner loop control , If it is not , Then it is less than or equal to 2s Protect and detect whether the voltage imbalance of the three-phase power grid returns to the normal value （ lower than 4%）, Maintain after returning to normal value 5min Return to the normal grid connection voltage outer loop and current inner loop control ; Third, check whether the frequency value of the power grid is 47.5-51.5Hz Between , If it is not , Less than or equal to 0.2s Over and under frequency protection and detect whether the grid frequency returns to the normal value , After returning to the normal value, return to the normal grid connection voltage outer loop and current inner loop control , if , Then check whether the voltage phases of the three-phase power grid are different from each other 20° And the error is 10° within , According to the result, it is decided to return to the normal grid connection voltage outer loop and current inner loop control or carry out less than or equal to 0.2s Protect and detect whether the phase difference of the power grid returns to the normal value ; Fourth, check whether the phase deviation between the phase voltage and phase current of the power grid is 25° within , if , Then return to the normal grid connection voltage outer loop and current inner loop control , If not, conduct less than or equal to 0.2s Protect ,5min Then restart .

The action conditions of low voltage ride through and the working state of the inverter after action are as follows ：
When the inverter is connected to the grid , First of all, it will detect whether the grid voltage drops in real time . Based on the detected results , If the voltage after drop is lower than the rated value 20%, Voltage protection is required ; If it falls to 20% To 90% Between time , It is necessary for the inverter to send reactive power to the grid to support the supplementary grid voltage , That is, low voltage ride through control . meanwhile , Pay attention to whether the grid voltage recovers and whether the duration of low voltage exceeds the specified value , To decide whether to carry out undervoltage protection . Until the grid voltage recovers, and finally the inverter is connected to the grid normally .

Feasibility of coexistence of low voltage ride through and anti islanding protection

To realize the coexistence of low voltage ride through and anti islanding protection of inverter , We need the maximum opening time of power grid undervoltage protection to meet the time requirements of low voltage ride through . In addition, the instant when the grid voltage drops or recovers , Grid frequency is also prone to major changes , At this time, the passive islanding protection mode may also think that the photovoltaic grid connected system is in an island state because of this frequency change , So as to carry out anti island protection .
thus , We need to avoid passive island protection in the transient of voltage drop or recovery , Prevent the anti islanding protection mode from misjudging whether the photovoltaic grid connected system is in the islanding state . Or try to cut off the island protection function at the moment of voltage drop or recovery , Wait until the grid voltage returns to stability before resuming . in other words , In the low voltage steady state, the active anti islanding method is used to judge whether the photovoltaic grid connected system is in the islanding state , And carry out low voltage ride through . The logical relationship is as follows ：
During the grid connected operation of inverter , At the same time, active and passive anti islanding protection methods are used to judge whether the grid connected system is in an island state , if , Then start the anti island protection mode , Inverter shutdown , If it is not , Then continue to judge whether the grid voltage drops , If it is not , Then there is no problem with the grid connected system , Normal operation is enough , If it is judged that the grid voltage has fallen , Immediately check whether the voltage of the power grid is lower than the rated voltage 20%, if , Then the undervoltage protection is turned on and the inverter is shut down , otherwise , It means that the grid voltage is 20%~90% Between , Carry out low voltage ride through control , Reactive power compensation to the power grid , So we can judge whether it is in an island state through active and passive anti island protection , If so, start the anti island protection mode , Inverter shutdown , Otherwise, check whether the grid voltage is restored to the rated voltage 90%, if , Then normal grid connection , Otherwise, whether the duration of low voltage exceeds the standard value , if , It means that undervoltage protection needs to be started , Start the inverter and stop , Otherwise, re judge whether the grid voltage is 20%-90% Between , Carry out low voltage ride through control , Reactive power compensation to the power grid , And repeat the above process , Until the grid voltage recovers to the rated voltage 90%, It indicates that the grid can be connected normally .