A hydropower generation system comprising: a frequency converter which is provided with two AC output terminals (108a, 108b); a water turbine generator which is interconnected to a power system via the frequency converter; a bypass breaker which includes both ends for short-circuiting the two AC output terminals (108a, 108b) in the frequency converter; a control device which controls the frequency converter and the bypass breaker; and a voltage detector (110a, 110b) which detects two AC voltages at both ends of the bypass breaker, the control device being configured to adjust an output voltage of the frequency converter to a direction in which two AC voltage amplitudes at both ends of the bypass breaker coincide with each other and two voltage phases at both ends of the bypass breaker coincide with each other when the bypass breaker is switched from opening to closing, and output a closing command for closing the bypass breaker when an amplitude difference and a phase difference of the two AC voltages at both ends of the bypass breaker are within respective certain values2 characterized in that: the control device includes a power command correction unit for correcting each power command value (P GFCref; Q SFCref; Q GFCref) of active power (P GFC) and reactive power (Q GFC) exchanged between the frequency converter and a generator, and is configured such that, when the bypass breaker is switched from opening to closing, the frequency converter outputs active power (P GFC) and reactive power (Q GFC) such that the amplitude difference and the phase difference between the two AC voltages are zero or within respective certain values. a frequency converter which is provided with two AC output terminals (108a, 108b); a water turbine generator which is interconnected to a power system via the frequency converter; a bypass breaker which includes both ends for short-circuiting the two AC output terminals (108a, 108b) in the frequency converter; a control device which controls the frequency converter and the bypass breaker; and a voltage detector (110a, 110b) which detects two AC voltages at both ends of the bypass breaker, the control device being configured to adjust an output voltage of the frequency converter to a direction in which two AC voltage amplitudes at both ends of the bypass breaker coincide with each other and two voltage phases at both ends of the bypass breaker coincide with each other when the bypass breaker is switched from opening to closing, and output a closing command for closing the bypass breaker when an amplitude difference and a phase difference of the two AC voltages at both ends of the bypass breaker are within respective certain values2 characterized in that: the control device includes a power command correction unit for correcting each power command value (P GFCref; Q SFCref; Q GFCref) of active power (P GFC) and reactive power (Q GFC) exchanged between the frequency converter and a generator, and is configured such that, when the bypass breaker is switched from opening to closing, the frequency converter outputs active power (P GFC) and reactive power (Q GFC) such that the amplitude difference and the phase difference between the two AC voltages are zero or within respective certain values. the control device includes a power command correction unit for correcting each power command value (P GFCref; Q SFCref; Q GFCref) of active power (P GFC) and reactive power (Q GFC) exchanged between the frequency converter and a generator, and is configured such that, when the bypass breaker is switched from opening to closing, the frequency converter outputs active power (P GFC) and reactive power (Q GFC) such that the amplitude difference and the phase difference between the two AC voltages are zero or within respective certain values.