Patent Publication Number: US-8526155-B2

Title: Phase-control switchgear and phase-control method for switchgear

Description:
TECHNICAL FIELD 
     The present invention relates to a phase-control switchgear that breaks a current at a desired phase and a phase-control method for the switchgear, and in particular to a device and a method for suppressing a transient voltage generated by breaking a current flowing through a switchgear when step-out occurs between generators on both sides of the switchgear. 
     BACKGROUND ART 
     As a device for detecting step-out of an electric power system, for example, a device described in Japanese Patent Laying-Open No. 2007-60870 (Patent Document 1) has been known. In a plurality of electric power systems each including at least one generator and bus and coordinated with each other by connecting the buses via a link line, the device predicts step-out of the generators. In particular, the device predicts that step-out will occur if the generators continue operation, based on a voltage of a bus and a current flowing from the link line to the bus.
     Patent Document 1: Japanese Patent Laying-Open No. 2007-60870   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     When a step-out detection device as described above predicts step-out, the step-out detection device outputs a breaking instruction to a switchgear provided to a link line. In this case, a current is broken by the switchgear, independently of a phase difference between voltages on both sides of the switchgear. As a result, a transient voltage exceeding an upper limit value prescribed by step-out current switching test duty in alternating current (AC) circuit breaker standards (JEC-2300, IEC62271-100, IEEE C37.079) is generated, depending on timing of breaking the current by the switchgear. 
     The present invention has been made in consideration of the above problem, and one object of the present invention is to provide a phase-control switchgear capable of suppressing a transient voltage generated after a current is broken, and a method of controlling the switchgear. 
     Means for Solving the Problems 
     According to an aspect, the present invention is directed to a phase-control switchgear provided to a multi-phase AC power transmission line connecting between first and second buses, including a circuit breaker, a phase difference detection unit, a storage unit, and a control unit. Here, first and second multi-phase generators are connected to the first and second buses, respectively. The circuit breaker breaks a current flowing through the power transmission line. The phase difference detection unit detects a phase difference between a voltage of a specific phase of the first bus and a voltage of one of a plurality of phases of the second bus that is identical to the specific phase, at a plurality of time points. The storage unit stores the phase differences at the plurality of time points detected by the phase difference detection unit. When the control unit receives a breaking instruction for the circuit breaker, the control unit estimates a breaking time point at which the phase difference between the voltage of the specific phase of the first bus and the voltage of one of the plurality of phases of the second bus that is identical to the specific phase will be a predetermined phase difference, based on the phase differences at the plurality of time points stored in the storage unit, and opens the circuit breaker to break the current at the breaking time point. 
     According to another aspect, the present invention is directed to a phase-control method for a switchgear provided to a multi-phase AC power transmission line connecting between first and second buses. Here, first and second multi-phase generators are connected to the first and second buses, respectively. The method of controlling the switchgear according to the present invention includes: a step of detecting a phase difference between a voltage of a specific phase of the first bus and a voltage of one of a plurality of phases of the second bus that is identical to the specific phase, at a plurality of time points; a step of storing the detected phase differences at the plurality of time points; a step of estimating, when a breaking instruction for the switchgear is received, a breaking time point at which the phase difference between the voltage of the specific phase of the first bus and the voltage of one of the plurality of phases of the second bus that is identical to the specific phase will be a predetermined phase difference, based on the phase differences at the plurality of time points stored in the step of storing; and a step of opening the switchgear to break a current at the breaking time point. 
     Effects of the Invention 
     According to the present invention, since timing of opening the circuit breaker is determined to break the current when the phase difference is a predetermined phase difference, based on the phase differences at the plurality of time points stored in the storage unit, a transient voltage generated after the current is broken can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a phase-control switchgear  50  according to Embodiment 1 of the present invention. 
         FIG. 2  is a view showing the relationship between a phase difference between voltages of a U-phase of buses  11 ,  21  and a recovery voltage. 
         FIG. 3  is a view for explaining timing of activating an opening operation signal  46  to be output to a circuit breaker  30 . 
         FIG. 4  is a flowchart illustrating a procedure for controlling circuit breaker  30  by a computer  40  in  FIG. 1 . 
         FIG. 5  is a block diagram showing a configuration of a phase-control switchgear  50 A according to Embodiment 2 of the present invention. 
         FIG. 6  is a block diagram showing a configuration of a phase-control switchgear  50 B according to Embodiment 3 of the present invention. 
     
    
    
     DESCRIPTION OF THE REFERENCE SIGNS 
       10 ,  20 : three-phase generator,  11 ,  21 : bus,  12 ,  22 : instrument transformer,  25 : power transmission line,  30 ,  30 A: circuit breaker,  40 ,  40 A: computer,  41 : phase difference detection unit,  42 : storage unit,  43 : step-out determination unit,  44 : circuit breaker control unit,  45 : breaking signal,  46 : opening operation signal,  50 ,  50 A: phase-control switchgear,  70 : step-out determination device,  110 ,  120 : single-phase generator,  111 ,  121 : bus,  125 : power transmission line. 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It is to be noted that identical or corresponding parts will be designated by the same reference numerals, and the description thereof will not be repeated. 
     Embodiment 1 
       FIG. 1  is a block diagram showing a configuration of a phase-control switchgear  50  according to Embodiment 1 of the present invention. Referring to  FIG. 1 , phase-control switchgear  50  is provided to a three-phase AC power transmission line  25  connecting a first bus  11  and a second bus  21 . A first three-phase generator  10  is connected to bus  11 , and a second three-phase generator  20  is connected to bus  21 . Further, an instrument transformer  12  for measuring a voltage is provided to bus  11 , at a U-phase among U-, V-, and W-phases. Similarly, an instrument transformer  22  is provided to bus  21 , at the same U-phase. Although the U-phase is selected in  FIG. 1  as a specific phase to which instrument transformers  12 ,  22  are provided, any of the U-, V-, and W-phases may be selected. 
     Phase-control switchgear  50  includes a circuit breaker (CB)  30  that breaks a current flowing through power transmission line  25  in response to an opening operation signal  46 , and a computer  40  for controlling circuit breaker  30 . Computer  40  determines whether or not three-phase generators  10 ,  20  are out of synchronization based on the voltages of the U phase of buses  11 ,  21  detected by instrument transformers  12 ,  22 , respectively. 
     Here, loss of synchronization (also referred to as step-out) is caused by a generator continuing acceleration or deceleration when a balance between a mechanical input and an electrical output of the generator is lost. For example, if a short-circuit or a grounding fault occurs at power transmission line  25  in the vicinity of three-phase generator  10 , an electrical output of three-phase generator  10  is decreased, and thus three-phase generator  10  continues acceleration, resulting in step-out when the acceleration exceeds a limit. Generally, if a phase difference between the voltages of a specific phase (here, the U-phase) of buses  11 ,  21  exceeds 180°, such a state is determined as a step-out state. Since the generator continues acceleration or deceleration even after it is determined that step-out has occurred, a phase shift between the voltages of the specific phase of buses  11 ,  21  is further increased. 
     In the description below, the magnitude of the phase shift between the voltages of the specific phase of buses  11 ,  21  caused by step-out will be referred to as a step-out phase angle. Specifically, the step-out phase angle means a phase shift from a state where the voltages of buses  11 ,  21  are completely in synchronization. For example, a step-out phase angle of 360° means that there occurs a phase shift shifted from an original synchronized state by one cycle. In addition, a step-out phase angle of 720° means that there occurs a phase shift shifted from the original synchronized state by two cycles. 
     When computer  40  determines that step-out has occurred, computer  40  activates opening operation signal  46  to be output to circuit breaker  30 , at appropriate timing. The timing on this occasion is determined to minimize a transient voltage (referred to as a recovery voltage) generated between electrodes of circuit breaker  30  after the current is broken, based on the phase difference between the detected voltages of the U-phase of buses  11 ,  21 . The magnitude of the recovery voltage varies depending on the phase difference between the voltages of the U-phase of buses  11 ,  21  when circuit breaker  30  breaks the current. 
       FIG. 2  is a view showing the relationship between the phase difference between the voltages of the U-phase of buses  11 ,  21  and the recovery voltage. The axis of ordinates in  FIG. 2  represents the magnitude of the recovery voltage based on a phase voltage E of each of buses  11 ,  21 . The axis of abscissas in  FIG. 2  represents the phase difference between the voltages of the U-phase detected between buses  11 ,  21 . The axis of abscissas in  FIG. 2  also represents the step-out phase angle. The phase difference between the voltages of buses  11 ,  21  actually detected when the step-out phase angle is 360° and 720° is 0°. 
     The recovery voltage indicated by curves  61 ,  63  in  FIG. 2  is given as a value obtained by multiplying a maximum value of a difference between the voltages of buses  11 ,  21  by a first-phase breaking coefficient prescribed in the AC circuit breaker standards (JEC-2300, IEC62271-100, IEEE C37.079). The first-phase breaking coefficient is 1.3 in the case of an effectively-grounded system (curve  61  in the figure), and 1.5 in the case of a non-effectively grounded system (curve  63  in the figure). 
     As shown in  FIG. 2 , the recovery voltage has a maximum magnitude in a complete step-out state where the voltage of the U-phase of bus  11  and the voltage of the U-phase of bus  21  have opposite phases (i.e., a phase difference of 180 degrees). On this occasion, since the maximum value of the difference between the voltage of the U-phase of bus  11  and the voltage of the U-phase of bus  21  is 2.0E (E represents the phase voltage of each of buses  11 ,  21 ), the maximum value of the recovery voltage is 2.6E in the case of the effectively-grounded system (curve  61  in the figure), and 3.0E in the case of the non-effectively grounded system (curve  63  in the figure). 
     According to the provision of the step-out current switching test duty in the AC circuit breaker standards (JEC-2300, IEC62271-100, IEEE C37.079), the upper limit value of the recovery voltage is prescribed as 2.5E (a straight line  64  in the figure) for a circuit breaker for the non-effectively grounded system, and 2.0E (a straight line  62  in the figure) for a circuit breaker for the effectively-grounded system. 
     Specifically, in the case of  FIG. 2 , the phase difference between the voltages of the U-phase of buses  11 ,  21  when the magnitude of the recovery voltage is equal to the upper limit value of the standards is about 115 degrees and 245 degrees in the case of the non-effectively grounded system, and about 105 degrees and 255 degrees in the case of the effectively-grounded system. 
     Therefore, a phase difference θ between the voltages of the U-phase of buses  11 ,  21  accepted by the step-out current switching test duty in the case of the non-effectively grounded system is represented as:
 
−115°≦θ≦115°  (1).
 
The range of phase difference θ in the above formula (1) corresponds to the range of a step-out phase angle Θ represented for example as:
 
245°≦Θ≦475°,605°≦Θ≦835°  (2).
 
In addition, phase difference θ accepted in the case of the effectively-grounded system is represented as:
 
−105°≦θ≦105°  (3).
 
The range of phase difference θ in the above formula (3) corresponds to the range of step-out phase angle Θ represented for example as:
 
255°≦Θ≦465°,615°≦Θ≦825°  (4).
 
Accordingly, unless circuit breaker  30  breaks the current such that the phase difference is within this range of phase difference θ, a voltage exceeding the upper limit value of the standards is generated.
 
     Thus, computer  40  according to Embodiment 1 controls timing of opening circuit breaker  30  such that the current flowing through power transmission line  25  is broken when phase difference θ between the voltages of the U-phase of buses  11 ,  21  is in the range of:
 
−80°≦θ≦80°  (5),
 
considering variations in a breaking time period for the circuit breaker. The range of phase difference θ in the above formula (5) corresponds to the range of step-out phase angle Θ represented for example as:
 
280°≦Θ≦440°,640°≦Θ≦800°  (6).
 
The most preferable case is that phase difference θ is 0° (the step-out phase angle is 360°, 720°, and the like), because the magnitude of the recovery voltage is 0.
 
     Hereinafter, a method of controlling timing of opening circuit breaker  30  will be described in detail. Referring to  FIG. 1  again, when seen functionally, computer  40  includes a phase difference detection unit  41 , a storage unit  42 , a step-out determination unit  43 , and a circuit breaker control unit (CB control unit)  44 . Functions of these components are implemented by executing a program in a Central Processing Unit (CPU) of computer  40 . 
     Phase difference detection unit  41  successively detects the phase difference between the voltage of the U-phase of bus  11  measured by instrument transformer  12  and the voltage of the U-phase of bus  21  measured by instrument transformer  22 . On this occasion, outputs of instrument transformers  12 ,  22  are subjected to digital conversion by an Analog to Digital (A/D) converter (not shown) built in computer  40 , and input into phase difference detection unit  41 . Specifically, phase difference detection unit  41  detects the phase difference between the voltage of the U-phase of bus  11  and the voltage of the U-phase of bus  21  at each cycle of the voltage of the U-phase of bus  11 . 
     Storage unit  42  sequentially stores data of the phase difference detected by phase difference detection unit  41  at each cycle of the voltage of the U-phase of bus  11 . Storage unit  42  includes a storage device (not shown) built in computer  40 . 
     Step-out determination unit  43  determines whether or not step-out has occurred between three-phase generators  10  and  20 , and if it determines that step-out has occurred, it outputs an activated breaking signal  45  (breaking instruction) to circuit breaker control unit  44 . A specific criterion for determining occurrence of step-out is that the phase difference detected by phase difference detection unit  41  exceeds 180 degrees (i.e., a complete step-out state). 
     When breaking signal  45  is switched into an active state, circuit breaker control unit  44  determines an approximate curve of a temporal change in the phase difference based on data of the phase difference at a present time point received from phase difference detection unit  41  and data of a plurality of phase differences up to the present time point stored in storage unit  42 . As an approximation technique in this case, n-order (n is an integer) polynomial approximation may be used, or a known time-series prediction technique such as an Auto-Regressive (AR) model may be used. 
     Circuit breaker control unit  44  estimates a breaking time point at which the phase difference between the voltages of the U-phase of buses  11 ,  21  will be a preset appropriate phase difference, by extrapolating the determined approximate curve. The appropriate phase difference is set to be included in the range represented by the above formula (5). Preferably, the appropriate phase difference is set to be equal to 0 degrees. Thereafter, circuit breaker control unit  44  activates opening operation signal  46  to be output to circuit breaker  30  at timing such that the current will be broken at the estimated breaking time point, considering the breaking time period for circuit breaker  30 . 
       FIG. 3  is a view for explaining timing of activating opening operation signal  46  to be output to circuit breaker  30 .  FIG. 3  shows, from the top, a temporal change in the phase difference output from phase difference detection unit  41  in  FIG. 1  (represented by the step-out phase angle in  FIG. 3 ), a waveform of breaking signal  45  output from step-out determination unit  43  in  FIG. 1 , and a waveform of opening operation signal  46  output from circuit breaker control unit  44  in  FIG. 1 . 
     Referring to  FIGS. 1 and 3 , at a time point t 1  when the phase difference between the voltages of the U-phase of buses  11 ,  21  reaches 180 degrees, step-out determination unit  43  switches breaking signal  45  from an H level to an L level to activate breaking signal  45 . 
     Here, generally, a breaking time period Tbrk for circuit breaker  30  is given as the sum of an opening time period from when circuit breaker  30  receives opening operation signal  46  to when a main contact point is opened and an arc time period after the main contact point is opened. Breaking time period Tbrk for typical circuit breaker  30  is about 50 milliseconds. Therefore, if circuit breaker control unit  44  activates opening operation signal  46  immediately after time point t 1  at which breaking signal  45  is activated, the current is broken when the step-out phase angle is around 210°. In this case, a voltage exceeding the upper limit value of the recovery voltage prescribed by the step-out current switching test duty described above is generated. 
     Thus, circuit breaker control unit  44  estimates a breaking time point t 3  at which the phase difference between the voltages of the U-phase of buses  11 ,  21  will be an appropriate phase difference of 0° (corresponding to a step-out phase angle of 360°), based on the temporal change in the phase difference between the voltages of buses  11 ,  21  prior to time point t 1  at which breaking signal  45  is activated. Then, circuit breaker control unit  44  switches opening operation signal  46  to an L level to activate it at a time point t 2  obtained by subtracting breaking time period Tbrk for circuit breaker  30  from estimated breaking time point t 3 . A time period from time point t 1  to time point t 2  is a delay time period Td from when breaking signal  45  is activated to when opening operation signal  46  is activated. As a result, the current is broken when the phase difference between the voltages of the U-phase of buses  11 ,  21  is around 0° (the step-out phase angle is around 360°), and thus the voltage generated between the electrodes of circuit breaker  30  after the current is broken is substantially 0, satisfying the provision of the step-out current switching test duty described above. 
       FIG. 4  is a flowchart illustrating a procedure for controlling circuit breaker  30  by computer  40  in  FIG. 1 . Hereinafter, the procedure for controlling circuit breaker  30  will be described, summarizing the above description. 
     Referring to  FIGS. 1 and 4 , in step S 1 , phase difference detection unit  41  of computer  40  detects a phase difference between the voltages of the U-phase of buses  11 ,  21  at each cycle of the voltage of the U-phase of bus  11 . 
     In subsequent step S 2 , storage unit  42  of computer  40  stores the phase difference detected by phase difference detection unit  41 . 
     In subsequent step S 3 , step-out determination unit  43  of computer  40  determines whether or not the phase difference detected by phase difference detection unit  41  is in a step-out state exceeding 180°. If the phase difference is not in the step-out state (NO in step S 3 ), the procedure returns to step S 1 , and steps S 1  and S 2  are repeated again. In this case, the phase differences detected at a plurality of time points are sequentially stored in storage unit  42 . On the other hand, if step-out determination unit  43  determines that the phase difference is in the step-out state (YES in step S 3 ), the procedure proceeds to step S 4 . In this case, step-out determination unit  43  activates breaking signal  45 , and activated breaking signal  45  is received by circuit breaker control unit  44 . 
     In step S 4 , circuit breaker control unit  44  estimates a breaking time point at which the phase difference between the voltages of buses  11 ,  21  will be a preset appropriate phase difference, based on data of the phase difference at a present time point and data of the phase differences at the plurality of time points prior to the present time point stored in storage unit  42 . Here, the appropriate phase difference is set to satisfy the provision of the step-out current switching test duty in the AC circuit breaker standards, and is included in the range represented by the above formula (5), as described above. 
     In subsequent step S 5 , circuit breaker control unit  44  activates opening operation signal  46  at a time point obtained by subtracting the breaking time period for circuit breaker  30  from the breaking time point. As a result, the current is broken by circuit breaker  30  at substantially the breaking time point. 
     As described above, phase-control switchgear  50  according to Embodiment 1 controls the timing of activating opening operation signal  46  such that the current is broken when the phase difference between the voltages of the U-phase of buses  11 ,  21  on both sides of circuit breaker  30  is an appropriate phase difference, considering the breaking time period for circuit breaker  30 . The appropriate phase difference is set to be included in the range represented by the above formula (5). As a result, a transient voltage generated between the electrodes of circuit breaker  30  after the current is broken can be suppressed to be not more than the upper limit value of the recovery voltage prescribed by the step-out current switching test duty in the AC circuit breaker standards. 
     In Embodiment 1 described above, a case where circuit breaker  30  is provided to power transmission line  25  connecting two three-phase generators  10  and  20  has been described. More generally, in a case where multiple three-phase generators are connected to an electric power system, phase-control switchgear  50  controls timing of breaking a current by circuit breaker  30  by detecting a phase difference between voltages of a specific phase of buses on both sides of circuit breaker  30  to which nearby three-phase generators are connected. 
     Further, in phase-control switchgear  50  according to Embodiment 1, an appropriate value of the phase difference between the voltages of the U-phase of buses  11 ,  21  when the current is broken is set to be in the range represented by the above formula (5) to satisfy the provision of the step-out current switching test duty even if the breaking time period for circuit breaker  30  varies. It is needless to say that, if it is possible to suppress variations in the breaking time period for circuit breaker  30 , circuit breaker  30  only needs to be opened such that the current is broken when the phase difference between the voltages of the U-phase of buses  11 ,  21  is in the range represented by the above formula (1) in the case of the non-effectively grounded system, and in the range represented by the above formula (3) in the case of the effectively-grounded system. 
     Embodiment 2 
       FIG. 5  is a block diagram showing a configuration of a phase-control switchgear  50 A according to Embodiment 2 of the present invention. A computer  40 A in  FIG. 5  is different from computer  40  in  FIG. 1  in that computer  40 A does not include step-out determination unit  43 . In the case of Embodiment 2, phase-control switchgear  50 A breaks a current flowing through power transmission line  25  in response to breaking signal  45  received from an externally provided step-out determination device  70 . 
     Step-out determination device  70  in  FIG. 5  can be configured to determine whether step-out has occurred between three-phase generators  10  and  20  based on a phase difference between voltages of a specific phase of buses  11 ,  21 , as in the case of Embodiment 1. Alternatively, step-out determination device  70  can also be configured to determine whether step-out has occurred based on the voltage of bus  11  and the current flowing from power transmission line  25  to bus  11 , as in Japanese Patent Laying-Open No. 2007-60870 (Patent Document 1) described above. In any of these cases, step-out determination device  70  outputs activated breaking signal  45  to circuit breaker control unit  44  of phase-control switchgear  50 A when it determines that step-out has occurred. Since other components in  FIG. 5  are identical to those in  FIG. 1 , identical or corresponding parts will be designated by the same reference numerals, and the description will not be repeated. 
     Embodiment 3 
       FIG. 6  is a block diagram showing a configuration of a phase-control switchgear  50 B according to Embodiment 3 of the present invention. Referring to  FIG. 6 , phase-control switchgear  50 B is provided to a single-phase AC power transmission line  125  connecting a first bus  111  and a second bus  121 . A first single-phase generator  110  is connected to bus  111 , and a second single-phase generator  120  is connected to bus  121 . Further, instrument transformers  12 ,  22  for measuring a voltage is provided to buses  111 ,  121 , respectively. 
     Phase-control switchgear  50 B includes a circuit breaker  30 A that breaks a current flowing through power transmission line  125  in response to opening operation signal  46 , and computer  40  for controlling circuit breaker  30 A. Computer  40  determines whether or not single-phase generators  110 ,  120  are out of synchronization based on the voltages of buses  111 ,  121  detected by instrument transformers  12 ,  22 , respectively, and if computer  40  determines that single-phase generators  110 ,  120  are out of synchronization, computer  40  activates opening operation signal  46 . Since the configuration and operation of computer  40  are identical to those in Embodiment 1, the description will not be repeated. Also in the case of a single-phase AC electric power system as described above, a transient voltage generated after the current is broken by circuit breaker  30 A can be suppressed by the method described in Embodiment 1. 
     It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the scope of the claims, rather than the above description, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims.