The present invention relates to an inrush-current suppressing device and an inrush-current suppressing method for suppressing an excessive excitation inrush current that possibly occurs in a three-phase transformer when three-phase power is input to the three-phase transformer.
Examples of a method of suppressing an excitation inrush current in a transformer include a phase-control input method in which a breaker is input in a specific phase of a three-phase power supply. Conventionally, the following technique is disclosed. That is, as for a first closing phase on which a closing operation is performed first among three phases, a closing phase in which an input magnetic-flux error that is the absolute value of a maximum error between a stationary magnetic flux and a residual magnetic flux at an input point becomes a minimum value is calculated and set as a target closing phase for the first closing phase based on the residual magnetic flux in the first closing phase and the already-obtained pre-arc characteristics and closing-time variation characteristics of the three-phase breaker while assuming an arbitrary reference phase of 0 degree as a reference point. As for the two remaining phases, a closing phase for which an input magnetic-flux error becomes a minimum value when a residual magnetic flux is zero is calculated and set as a target closing phase for the two remaining phases based on the already-obtained pre-arc characteristics and closing-time variation characteristics of the three-phase breaker while assuming the arbitrary reference phase of 0 degree as the reference point. The time obtained by adding up the time from the reference point to the target closing phase for the two remaining phases and the delay time corresponding to the integer multiple of the preset cycle of a three-phase power supply is set as a target closing time of the two remaining phases (for example, Patent Literature 1 mentioned below).
As described above, with the technique described in Patent Literature 1 mentioned above, the closing phase in which the input magnetic-flux error that is the absolute value of the maximum error between the stationary magnetic flux and the residual magnetic flux at the input point becomes the minimum value is calculated, and the calculated closing phase is input as the first phase. However, an actual input point does not necessarily match a target input point because of the closing-time variation characteristics of the first phase. If the actual input point deviates from the target input point, the input magnetic-flux error is not zero but the stationary magnetic flux applied after input is offset by as much as this input magnetic-flux error and has an asynchronous waveform with respect to a zero reference axis. At this time, when an offset amount is large, the magnetic flux of an iron core reaches a saturation range and an excitation inrush current occurs in this period. In the period in which the excitation inrush current occurs, gap voltages of the two remaining phases are high. Therefore, when the two remaining phases are input in this period, an input point becomes earlier than an assumed input point, and then there is a problem that an excitation inrush current higher than an assumed excitation inrush current occurs.
The present invention has been achieved in view of the above problems, and an object of the present invention is to provide an inrush-current suppressing device and an inrush-current suppressing method capable of suppressing a maximum value of an excitation inrush current and suppressing occurrence of an excessive excitation inrush current.
In order to solve the above problem and in order to attain the above object, in an inrush-current suppressing device applied to a configuration in which three-phase alternating-current power is supplied to and cut off from a three-phase transformer via a three-phase breaker, for suppressing an excitation inrush current that possibly occurs in the three-phase transformer during input of the three-phase breaker, the inrush-current suppressing device of the present invention, includes: a residual-magnetic-flux calculation unit that obtains a residual magnetic flux in each phase generated within the three-phase transformer based on a voltage of each phase generated in the three-phase transformer before and after closing the three-phase breaker; an input magnetic-flux-error calculation unit that obtains a closing-phase input magnetic-flux error for every phase based on the residual magnetic flux in each phase and in consideration of pre-arc characteristics and closing-time variation characteristics of the three-phase breaker; a closing-order determination unit that determines a closing order of phases of the three-phase breaker based on the residual magnetic flux in each phase; a target-closing-phase setting unit that calculates a phase in which the input magnetic-flux error in a first closing phase determined by the closing-order determination unit becomes a minimum value and sets the calculated phase as a target closing phase of the first closing phase, and that calculates a phase in which the input magnetic-flux error in the two remaining phases becomes a minimum value while assuming a predetermined phase in a predetermined reference phase as a reference point and sets the calculated phase as a target closing phase of a second closing phase; a target-closing-time setting unit that sets a time from the reference point to the target closing phase of the first closing phase as a target closing time of the first closing phase, and that sets a time obtained by adding up a time from the reference point to the target closing phase of the two remaining phases and a predetermined delay time as a target closing time of the second closing phase; and a closing control unit that generates a closing control signal and outputs the closing control signal to the three-phase breaker so as to close each phase at the target closing time of each phase set by the target-closing-time setting unit in response to a closing instruction to the three-phase breaker. Additionally, the predetermined delay time is set to exclude a period in which a magnetic flux in the first closing phase possibly saturates because of the input magnetic-flux error in the first closing phase.
The inrush-current suppressing device according to the present invention can suppress a maximum value of an excitation inrush current so as to suppress occurrence of an excessive excitation inrush current.
Exemplary embodiments of an inrush-current suppressing device and an inrush-current suppressing method according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments.
An operation performed by the inrush-current suppressing device 100 according to the first embodiment is explained next with reference to
First, in the present embodiment, any one of three phases (an A-phase, for example) of the three-phase power supply 1 is defined as a reference phase. As shown in
In
The closing-order determination unit 13 determines a closing order 14 of the phases of the three-phase breaker 2. In the present embodiment, the phase in which an absolute value of the residual magnetic flux 8 is the highest among those of the residual magnetic fluxes 8 calculated by the residual-magnetic-flux calculation unit 7 is explained as a first closing phase. However, the assumption of the first closing phase is not limited to that in the present embodiment. For example, the phase in which the magnitude of the residual magnetic flux is the second largest can be assumed as the first closing phase as long as the residual magnetic fluxes in the two out of the three phases do not excessively differ from each other.
The target closing-phase/time setting unit 15 calculates a target closing time 16 of each of the phases with reference to a reference phase of 0 degree of the power supply voltage 18. That is, as for the first closing phase determined by the closing-order determination unit 13, the closing phase in which the input magnetic-flux error that is the absolute value of a maximum error between a stationary magnetic flux and the residual magnetic flux at an input point becomes a minimum value is calculated and set as the target closing phase for the first closing phase based on the residual magnetic flux in the first closing phase and the already-obtained pre-arc characteristics 9 and the closing-time variation characteristics 10 of the three-phase breaker 2 while assuming a point at which the reference phase is 0 degree as the reference point. In addition, a time from the reference point to the target closing phase of the first closing phase is set as a target closing time of the first closing phase. As for the two remaining phases, a closing phase in which the input magnetic-flux error becomes a minimum value when the residual magnetic flux is zero is calculated and set as a target closing phase of the two remaining phases (that is, the target closing phase for a second closing phase) based on the pre-arc characteristics 9 and the closing-time variation characteristics 10 of the three-phase breaker 2 while assuming a predetermined reference phase (of 0 degree, for example) as a reference point. In addition, a time obtained by adding up a time from the reference point to the target closing phase of the two remaining phases and preset a predetermined delay time is set as a target closing time of the two remaining phases. The predetermined delay time mentioned here is explained later in detail.
When a closing instruction 19b is input, the closing control unit 17 outputs a closing control signal 20 to the three-phase breaker 2 so as to close each of the phases at the target closing time 16 of each phase set by the target closing-phase/time setting unit 15 while assuming the point at which the reference phase of the power supply voltage 18 measured by the voltage measuring device 4 is 0 degree as the reference point.
When three-phase power-supply voltages (an A-phase power-supply voltage 101, a B-phase power-supply voltage 102, and a C-phase power-supply voltage 103) as shown in
Referring back to
When the three-phase transformer 3 is cut off from the three-phase power supply 1, the three-phase breaker 2 is controlled to collectively open the three phases by simultaneously actuating main contacts of the three phases similarly to an ordinary three-phase breaker. When the phases are input to the three-phase transformer 3 from the three-phase power supply 1, the three phases are controlled to be closed based on the closing phase set for each phase. That is, the three-phase breaker 2 is a three-phase breaker with a phase control function that can control the input of respective phases independently.
The pre-arc characteristics and the closing-time variation characteristics when a transformer input operation is performed by the use of the three-phase breaker 2 as well as a magnetic flux error and an excitation inrush current generated due to these characteristics are explained next with reference to
First,
In this case, the power supply voltages in the two remaining phases (the B-phase and the C-phase) are a B-phase power-supply voltage 302 and a C-phase power-supply voltage 303 shown in
B-phase power-supply voltage=sin(θ−120°) (1)
C-phase power-supply voltage=sin(θ−240°) (2)
On the other hand, a B-phase transformer voltage and a C-phase transformer voltage after the input of the A-phase (not shown) are expressed as follows.
B-phase transformer voltage=sin(θ−180°)/2 (3)
C-phase transformer voltage=sin(θ−180°)/2 (4)
Therefore, a B-phase gap voltage and a C-phase gap voltage that are gap voltages of the two remaining phases are expressed as follows.
B-phase gap voltage=(B-phase power-supply voltage)−(B-phase transformer voltage)=−√3/2×sin(θ−90°) (5)
C-phase gap voltage=(C-phase power-supply voltage)−(C-phase transformer voltage)=+√3/2×sin(θ−90°) (6)
Furthermore,
However, as shown in
When the A-phase excitation inrush current 140 shown in
With reference to
The matters described above are those also described in “Technical Problem” Section, that is, the problem that “an input point becomes earlier than an assumed input point, and then an excitation inrush current higher than an assumed excitation inrush current occurs”.
To solve this problem, according to the first embodiment, a control is executed to set the target input points of the two remaining phases within the period in which the excitation inrush current as shown in
First, attention is paid to the fact that the excitation inrush current that occurs because of the first closing phase occurs repeatedly at 360° intervals, and that the target input point of the two remaining phases is present at 180° intervals. Therefore, the delay time of the two remaining phases (a delay time with respect to a conventional input time) is controlled to be either reduced by 0.5 cycle or extended by “0.5+n” cycle (where n is an integer). In other words, the control is executed to set the delay time of the two remaining phases to a time that corresponds to half a cycle of the three-phase alternating-current power or an odd multiple of the half cycle. This control makes it possible to input the B-phase and the C-phase in the period in which no excitation inrush current occurs, and to ensure the input time range within the scope of the assumption. An example shown in
In the present embodiment, an example of setting the delay time of the two remaining phases to the time that corresponds to half the cycle of the three-phase alternating-current power has been disclosed. However, the present invention is not limited thereto. What is important is that the delay time of the two remaining phases is set to the time that excludes a period in which the magnetic flux in the first closing phase possibly saturates because of the input magnetic-flux error in the first closing phase. It eventually suffices that the two remaining phases can be input in the period in which no excitation inrush current occurs, and this concept also forms the spirit of the present invention.
As described above, the inrush-current suppressing device according to the first embodiment calculates the period in which the excitation inrush current that exceeds the magnetic flux saturation threshold occurs due to the offset of the magnetic flux in the first phase from the input magnetic-flux error during the input of the first closing phase, and determines the target input point of the two remaining phases that are delayed phases in the period in which no excitation inrush current occurs. Therefore, it is possible to input the two remaining phases in the period in which the gap voltages in the two remaining phases are not unstable. Furthermore, this control makes it possible to input the two remaining phases within the assumed input time range, thereby suppressing an unintended excessive excitation inrush current.
When the transformer primary side is the Δ connection, an excitation current does not occur in the three-phase transformer during the input of the first phase but the excitation current occurs for the first time during the input of the second phase. That is, as it is assumed that the A-phase power-supply voltage is ypa, transformer voltages yta, ytb, and ytc during the input of the first phase (during the input of the A-phase, for example) are expressed as follows.
A-phase transformer voltage yta=A-phase power-supply voltage ypa (7)
B-phase transformer voltage ytb=A-phase power-supply voltage ypa (8)
C-phase transformer voltage ytc=A-phase power-supply voltage ypa (9)
Next, at the time of the input of the second phase (the input of the B-phase subsequent to the input of the A-phase), a rated voltage is applied only to both ends of an A-phase transformer winding and half a voltage opposite in phase to the rated voltage is applied to each of a B-phase transformer winding and a C-phase transformer winding. That is, as it is assumed that the B-phase power-supply voltage is ypa, a potential difference on both ends of each of the phase windings is expressed as follows.
Potential difference on both ends of A-phase winding=ypa−ypb=√(3)sin(θ+30) (10)
Potential difference on both ends of B-phase winding=−(ypa−ypb)/2=−√(3)/2 sin(θ+30) (11)
Potential difference on both ends of C-phase winding=−(ypa−ypb)/2=−√(3)2 sin(θ+30) (12)
Moreover, the C-phase transformer voltage and the C-phase gap voltage are expressed as follows.
C-phase transformer voltage=ypa−(ypa−ypb)/2=(ypa+ypb)/2 (13)
C-phase gap voltage=ypc−(ypa+ypb)/2 (14)
However, at the time of inputting the first closing phase and the second closing phase, an excitation inrush current often occurs because of the closing-time variation characteristics similarly to the first embodiment. The potential difference on the both ends of each of the phase windings in the period in which the excitation inrush current occurs are expressed as follows because a voltage decrement ΔV corresponding to (excitation inrush current)×(system impedance) is generated.
Potential difference on both ends of A-phase winding=ypa−ypb−ΔV=√(3)sin(θ+30)−ΔV (15)
Potential difference on both ends of B-phase winding=−(ypa−ypb)/2−ΔV=−√(3)/2 sin(θ+30)−ΔV (16)
Potential difference on both ends of C-phase winding=−(ypa−ypb)/2−ΔV=−√3/2 sin(θ+30)−ΔV (17)
Furthermore, the C-phase transformer voltage and the C-phase gap voltage are expressed as follows.
C-phase transformer voltage=ypa−((ypa−ypb)/2−ΔV)=(ypa+ypb)/2+ΔV (18)
C-phase gap voltage=ypc−(ypa+ypb)/2−ΔV (19)
As described above, when the C-phase that is the delayed phase is input in the period in which the excitation inrush current occurs, the C-phase gap voltage varies, and therefore it is impossible to input the C-phase in a desired phase.
In
However, as shown in
When the A-B inter-phase excitation inrush current 240 shown in
With reference to the example shown in
To solve this problem, according to the second embodiment, similarly to the first embodiment, a control is executed to set the target input point of the one remaining phase (a third closing phase) within a time that excludes the period in which the excitation inrush current occurs at the timing at which the two preceding phases are input. This control makes it possible to input the third closing phase in a desired phase without being influenced by the preceding input phases.
An example shown in
In the present embodiment, an example of setting a delay time of one remaining phase to a time that corresponds to half a cycle of a three-phase alternating-current power supply has been disclosed. However, the present invention is not limited thereto. What is important is that the delay time of the one remaining phase (the third closing phase) is set to the time that excludes the period in which the magnetic flux between the first and second closing phases possibly saturates because of the input magnetic-flux errors in the first and second closing phases. It eventually suffices that the one remaining phase can be input in the period in which no excitation inrush current occurs, and this concept also forms the spirit of the present invention.
As described above, the inrush-current suppressing device according to the second embodiment calculates the period in which the excitation inrush current that exceeds the magnetic flux saturation threshold occurs due to the offset of the magnetic flux in the second phase from the input magnetic-flux error during the input of the second closing phase, and determines the target input point of the one remaining phase that is the delayed phases in the period in which no excitation inrush current occurs. Therefore, it is possible to stabilize the gap voltage of the one remaining phase, to input the one remaining phase in an assumed and desired phase, thereby suppressing an unintended excessive excitation inrush current.
The configurations explained in the first and second embodiments described above are only an example of the configuration of the present invention. Therefore, it is needless to mention that these configurations can be combined with other commonly known techniques and can be modified within a range not departing from the scope of the present invention, such as omitting a part of the configurations.
As described above, the inrush-current suppressing device according to the present invention is useful as an invention that can suppress a maximum value of an excitation inrush current so as to suppress occurrence of an excessive excitation inrush current.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2010/056398 | 4/8/2010 | WO | 00 | 8/1/2012 |