Method for compensating instantaneous power failure in medium voltage inverter and medium voltage inverter system using the same
According to an embodiment, a method for compensating instantaneous power failure includes determining whether an input voltage of a plurality of power cells is less than or equal to a first threshold voltage, decreasing, when the input voltage is less than or equal to the first threshold voltage, an output frequency of the inverter, determining whether a voltage of a direct current (DC) link is greater than or equal to a second threshold voltage, and increasing, when the voltage of the DC link is greater than or equal to the second threshold voltage, the output frequency of the inverter. Overvoltage trip may be prevented at the power restoration time and overcurrent trip caused by increase in the slip frequency may be prevented. Thereby, reliability of a medium inverter may be enhanced, and a continuous operation time increased compared to the conventional cases may be ensured.
1. A method for compensating instantaneous power failure in an inverter including a plurality of power cells, the method comprising:
determining whether an input voltage of the plurality of power cells is less than or equal to a first threshold voltage;
decreasing, when the input voltage is less than or equal to the first threshold voltage, an output frequency of the inverter;
determining whether a voltage of a direct current (DC) link is greater than or equal to a second threshold voltage; and
increasing, when the voltage of the DC link is greater than or equal to the second threshold voltage, the output frequency of the inverter,
wherein the increasing of the output frequency of the inverter comprises monitoring change of the voltage of the DC link resulting from regenerative energy supplied from a load to the plurality of power cells and a restored input voltage and increasing the output frequency by an increment of the voltage of the DC link at a time when the input voltage of the plurality of power cells greater than or equal to the first threshold voltage is restored,
the method further comprising: maintaining, when the increment of the voltage of the DC link resulting from the regenerative energy is eliminated, the output frequency of the inverter for a predetermined time,
wherein the maintaining is performed after the monitoring of the change and the increasing of the output frequency of the inverter.
2. The method according to claim 1 , further comprising:
decreasing, when an output current is greater than or equal to a threshold current, an output frequency of the plurality of power cells.
3. The method according to claim 1 , wherein a gradient of the output frequency of the inverter is predetermined according to a load amount at the time when the input voltage of the plurality of power cells greater than or equal to the second threshold voltage is restored.
4. The method according to claim 2 , wherein decreasing the output frequency of the plurality of the power cells comprises:
monitoring the output current according to increase in the voltage of the DC link and decreasing the output frequency of the plurality of power cells by a predetermined value.
5. The method according to claim 1 , further comprising:
increasing the output frequency of the inverter according to a predetermined acceleration gradient,
wherein the increasing of the output frequency of the inverter according to the predetermined acceleration gradient is performed after the maintaining of the output frequency for the predetermined time.
6. The method according to claim 5 , wherein the increasing of the output frequency of the inverter according to the predetermined acceleration gradient comprises:
increasing the output frequency of the inverter up to a frequency prior to the instantaneous power failure.