Patent Number: 
Section: claims

1. A nuclear power plant, comprising:a nuclear steam supply system including a nuclear reactor generating heat to produce steam;a power generating turbine configured to be driven with the steam supplied by the nuclear steam supply system;a condenser configured to condense the steam after driving the power generating turbine;at least three feedwater pumps configured to supply water condensed and generated by the condenser to the nuclear steam supply system, each of the feedwater pumps is driven by a corresponding electric motor, and a flow rate provided by each of the feedwater pumps can be controlled by controlling a rotation speed of the corresponding electric motor, wherein during a normal operation of the reactor, one of the feedwater pumps is a backup feedwater pump standing by without operation and the other feedwater pumps are operating feedwater pumps;a water level controller configured to calculate a feedwater flow rate to the nuclear steam supply system and to output a feedwater flow rate command signal based on a detected value indicating a condition of the nuclear power plant and a setpoint of a water level of the nuclear steam supply system;at least three flow rate controllers equipped in correspondence with each electric motor, wherein each flow rate controller is configured to generate a rotation speed command signal of the corresponding electric motor based on the feedwater flow rate command signal, wherein each controller is configured to vary a speed of the corresponding electric motor such that a flow rate provided by each of the operating feedwater pumps can be varied;at least three electric power converters configured to convert electricity from a power supply of fixed voltage and fixed frequency to supply electricity of variable voltage and variable frequency to the electric motors of the operating feedwater pumps; anda trip compensation device configured to increase a rotation speed of the electric motor driving at least one of the operating feedwater pumps that has not tripped if one of the operating feedwater pumps trips;wherein the trip compensation device has a backup start up circuit configured to start up the electric motor that drives the backup feedwater pump by directly connecting the electric motor to a power supply of fixed voltage and fixed frequency by bypassing the electric power converter of the backup feedwater pump if one of the operating feedwater pumps trips. 2. The nuclear power plant of claim 1, wherein the trip compensation device has a trip detecting circuit configured to determine if a feedwater pump has tripped, wherein the trip detecting circuit determines a trip if a deviation of the rotation speed of the electric motor, as indicated by the rotation speed command signal, continues to be equal to or more than a predetermined deviation for a predetermined period. 3. The nuclear power plant of claim 2, wherein the trip compensation device has a control switching device configured to disconnect the electric motor driving the backup feedwater pump from the power supply and to connect to the electric power converter after a predetermined period passes from a start up of the backup feedwater pump. 4. The nuclear power plant of claim 3, wherein the trip compensation device has a flow rate adjustment device configured to change a rotation speed of the electric motor driving the backup feedwater pump so that a rotation speed of the backup feedwater pump decreases with progress of time, and the water level controller is configured to output a command to change a rotation speed of a feedwater pump so as to compensate a change of the feedwater flow rate accompanying a decrease of the rotation speed of the backup feedwater pump. 5. The nuclear power plant of claim 2, wherein all of the feedwater pumps are primary feedwater pumps configured to be usually operated, and the trip compensation device has a post-trip controller configured to transmit a post-trip control signal increasing the feedwater flow rate indicated by the feedwater flow rate command signal to the water level controller if one of the feedwater pumps trips. 6. The nuclear power plant of claim 5, further comprising: an on-restart flow rate adjustment device configured to transmit the feedwater flow rate command signal increasing at a predetermined value per unit time to the flow rate controller corresponding to a feedwater pump that has tripped and to transmit a correction signal decreasing the feedwater flow rate at the predetermined value per unit time to the water level controller when the feedwater pump that has tripped restarts. 7. The nuclear power plant of claim 1, wherein all of the feedwater pumps are primary feedwater pumps configured to be usually operated, and the trip compensation device has a post-trip controller configured to transmit a post-trip control signal increasing the feedwater flow rate indicated by the feedwater flow rate command signal to the water level controller if one of the feedwater pumps trips. 8. The nuclear power plant of claim 7, further comprising: an on-restart flow rate adjustment device configured to transmit the feedwater flow rate command signal increasing at a predetermined value per unit time to the flow rate controller corresponding to a feedwater pump that has tripped and to transmit a correction signal decreasing the feedwater flow rate at the predetermined value per unit time to the water level controller when the feedwater pump that has tripped restarts. 9. The nuclear power plant of claim 1, further comprising:a first connection directly between the power supply of fixed voltage and fixed frequency and the electric motor that drives the backup feedwater pump,a second connection between the power supply of fixed voltage and fixed frequency, an electric power converter, and the electric motor that drives the backup feedwater pump, anda control switching device configured to switch between the first connection and the second connection,wherein the control switching device is configured to use the first connection when the operating feedwater pump trip occurs and is configured to subsequently use the second connection thereafter. 10. The nuclear power plant of claim 9, wherein each of the feedwater pumps, including the backup feedwater pump, has a control switching device, a first connection directly between the power supply and a respective electric motor, and a second connection between the power supply, a respective electric power converter, and a respective electric motor,wherein each of the feedwater pumps is capable of being used as a primary or a backup feedwater pump. 11. The nuclear power plant of claim 9, wherein the first connection does not include an electric power converter between the power supply and the electric motor that drives the backup feedwater pump. 12. A feedwater controlling method of a nuclear power plant, the method comprising:supplying water to a nuclear steam supply system, including a nuclear reactor generating heat to produce steam, via an arrangement having at least three feedwater pumps, each of the feedwater pumps is driven by a corresponding electric motor connected to (1) a corresponding electric power converter or (2) directly to a power supply of fixed voltage and fixed frequency, and a flow rate provided by each of the feedwater pumps can be controlled by controlling a rotation speed of the corresponding electric motor, wherein during a normal operation of the reactor, one of the feedwater pumps is a backup feedwater pump standing by without operation and the other feedwater pumps are operating feedwater pumps;calculating a feedwater flow rate to the nuclear steam supply system and outputting a feedwater flow rate command signal based on a detected value indicating a condition of the nuclear power plant and a setpoint of a water level of the nuclear steam supply system;generating rotation speed command signals of all of the operating electric motors based on the feedwater flow rate command signal, wherein a speed of each electric motor is variable such that a flow rate provided by each of the feedwater pumps can be varied;converting electricity from a power supply of fixed voltage and fixed frequency with the corresponding electric power converters to supply electricity of variable voltage and variable frequency to the electric motors of the operating feedwater pumps; andusing a trip compensation device to increase a rotation speed of the electric motor driving at least one of the operating feedwater pumps that has not tripped if one of the operating feedwater pumps trips;wherein the trip compensation device has a backup start up circuit configured to start up the electric motor that drives the backup feedwater pump by directly connecting the electric motor to the power supply of fixed voltage and fixed frequency by bypassing the electric power converter of the backup feedwater pump if one of the operating feedwater pumps trips. 13. The feedwater controlling method of claim 12, further comprising the steps of:using a first connection between the power supply of fixed voltage and fixed frequency and the electric motor that drives the backup feedwater pump when the operating feedwater pump trip occurs, andsubsequently using thereafter a second connection between the power supply of fixed voltage and fixed frequency, an electric power converter, and the electric motor that drives the backup feedwater pump. 14. The feedwater controlling method of claim 13, wherein each of the feedwater pumps, including the backup feedwater pump, has a first connection directly between the power supply and a respective electric motor, and a second connection between the power supply, a respective electric power converter, and a respective electric motor,wherein each of the operating feedwater pumps and the backup feedwater pump is capable of being used as a primary or a backup feedwater pump by switching between the first connection and the second connection. 15. The feedwater controlling method of claim 13, wherein the first connection does not include an electric power converter between the power supply and the electric motor that drives the backup feedwater pump.