Incoming current suppression device
A control unit of an electric power supply system causes, responsive to a closing command signal, a switch corresponding to R phase to be thrown at a timing when a stationary magnetic flux and a residual magnetic flux of R phase are coincident with each other and then subsequently causes other switches corresponding to S phase and T phase to be thrown at a timing which is delayed from a zero point of an alternating voltage of R phase by a predetermined phase angle of 0-30 degrees, such that the difference of the stationary magnetic flux and the residual magnetic flux of each of S phase and T phase is minimum. Therefore, an exciting incoming current which flows into a three-phase transformer at a time of throwing a three-phase breaker provided with interpole capacitors can be suppressed.
1. An incoming current suppression device for suppressing an exciting incoming current that flows from a three-phase AC power supply into a three-phase transformer through a three-phase breaker,
said three-phase breaker including three switches and three interpole capacitors,
one terminal of said three switches receiving alternating voltages of three phases from said three-phase AC power supply, respectively, and other terminals of said three switches being connected to three input terminals of a three-phase primary winding of said three-phase transformer, respectively, and said three interpole capacitors being connected in parallel to said three switches, respectively,
a stationary magnetic flux being generated at each phase of said three-phase transformer when said three-phase breaker is closed, and a residual magnetic flux being generated at each phase of said three-phase transformer when said three-phase breaker is opened, and
said incoming current suppression device comprising:
a voltage measuring unit for measuring a voltage of the other terminal of one of the three switches corresponding to a reference phase of three phases;
a magnetic flux operation unit for determining the residual magnetic flux of said reference phase of said three-phase transformer based on the measurement result from said voltage measuring unit at the time when said three-phase breaker is opened; and
a control unit, responsive to a closing command signal, for causing one of the three switches corresponding to said reference phase to be thrown at a timing when the stationary magnetic flux of said reference phase and the residual magnetic flux determined by said residual magnetic flux operation unit are coincident with each other, then subsequently causing the two remaining switches to be thrown at a timing which is delayed from a zero point of the alternating voltage of said reference phase by a predetermined phase angle of between 0-30 degrees such that a difference between the stationary magnetic flux and the residual magnetic flux of each phase other than said reference phase is minimum.
2. The incoming current suppression device according to claim 1 , wherein
said predetermined phase angle increases according to a capacity value of each of said three interpole capacitors.
3. The incoming current suppression device according to claim 2 , wherein
said predetermined phase angle is set to a value between 0-10 degrees when the capacity value of each of said three interpole capacitors is not more than 2000 pF,
said predetermined phase angle is set to a value between 10-20 degrees when the capacity value of each of said three interpole capacitors is between 2000-6000 pF,
said predetermined phase angle is set to a value between 20-30 degrees when the capacity value of each of said three interpole capacitors is not less than 6000 pF.
4. The incoming current suppression device according to claim 1 , wherein
said control unit causes the one of the three switches corresponding to said reference phase to be thrown at the timing when the stationary magnetic flux and the residual magnetic flux of said reference phase are coincident with each other in a period when an absolute value of the alternating voltage of said reference phase increases.