Patent Publication Number: US-7711502-B2

Title: Power switching control apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a power switching control apparatus, which is constituted to arrange phase switches at individual phases of a three-phase AC power circuit and to control the individual phase switches independently of one another. 
   2. Description of Related Art 
   A synchronous switching apparatus is disclosed in International Laid-Open WO000/04564. In this synchronous switching apparatus, phase switches arranged at the individual phases of a three-phase AC power circuit are controlled independently of one another, and the individual phase switches are so made at the set phases as to suppress the generation of the inrush current or surge voltage which is severe against the system device such as the transformer, shunt reactor, power lines or capacitor banks of the three-phase AC power circuit. 
   Generally speaking, however, the contacts of the phase switch are erroded by the arcs, and the drive mechanism of the moving contacts disperses and has its driving characteristics varied according to the surrounding environment such as the ambient temperature. JP2001-135205A has disclosed a one-phase power switching apparatus. In this power switching apparatus, on the basis of the waveform of a phase current and the pre-arcing time of a switch, the making operation time of the switch is detected and is reflected on the control of the next making instant of the switch. This making operation time of the switch is the action time from the time when the making command of the switch is fed to the time when the contact is actually connected. 
   In case the individual phase switches of the three-phase AC power circuit are made independently of one another and in set phases, no phase current flows in the state where the first phase switch is made, if the load is of a non-grounded neutral point type. In case the load is of the type, in which it has a common core of the grounded neutral point type, the phase switch to be finally made has two preceding phases made so that it is made in the substantial “0” voltage between contact. In the constitution where the making operation time of the phase switch is detected on the basis of the phase current waveform containing pre-arcs, therefore, the phase current waveforms containing the pre-arcs cannot be obtained when all the phase switches are made to raise a problem that it is impossible to detect the making operation times of all the phase switches precisely. 
   SUMMARY OF THE INVENTION 
   This invention contemplates to provide a power switching control apparatus, which is improved to solve that problem. 
   According to a first aspect of the invention, there is provided a power switching control apparatus having an A-phase switch, a B-phase switch and a C-phase switch connected with the A-phase, B-phase and C-phase of a three-phase AC power circuit, respectively, for controlling the individual phase switches independently of one another. The power switching control apparatus comprises making operation time information detecting means for outputting making operation time information ITA, ITB and ITC representing the in individual making operation times of said A-phase switch, said B-phase switch and said C-phase switch, respectively, and switching control means for controlling the making instants of said A-phase switch, said B-phase switch and said C-phase switch on the basis of said making operation time information ITA, ITB and ITC. Said making operation time information detecting means outputs the making operation time information of at least one of said individual phase switches on the basis of the motion of the movable contact of said phase switch, and outputs the making operation time information of at least another one of said individual phase switches on the basis of the phase current of said phase switch. 
   According to a second aspect of this invention, there is provided a power switching control apparatus having an A-phase switch, a B-phase switch and a C-phase switch connected with the A-phase, B-phase and C-phase of a three-phase AC power circuit, respectively, for controlling the individual phase switches independently of one another. The power switching control apparatus comprises making operation time information detecting means for outputting making operation time information ITA, ITB and ITC representing the individual making operation times of said A-phase switch, said B-phase switch and said C-phase switch, respectively, and switching control means for controlling the making instants of said A-phase switch, said B-phase switch and said C-phase switch on the basis of said making operation time information ITA, ITB and ITC. Three contact operation sensors for detecting the motions of the individual movable contacts of said A-phase switch, said B-phase switch and said C-phase switch, and three phase current sensors for detecting the individual phase currents of said A-phase, said B-phase and said C-phase are connected with said making operation time information detecting means. Said making operation time information detecting means outputs the making operation time information ITA, ITB and ITC of said A-phase switch, said B-phase switch and said C-phase switch individually on the basis of one of the detected output of said contact operation sensor and the detected output of said phase current sensor. 
   In the power switching control apparatus according to the first aspect of the invention, the making operation time information detecting means outputs the making operation time information of at least one of the individual phase switches on the basis of the motion of the movable contact of the phase switch, and outputs the making operation time information of at least another of the individual phase switches on the basis of the phase current of the phase switch. In the phase where the making operation time information is outputted on the basis of the motion of the movable contact of the phase switch, the making operation time information is obtained on the basis of the motion of the movable contact even if the making operation time information based on the phase current is not obtained. As a result, the making operation time information of the phase switches can be obtained more precisely. 
   In the power switching control apparatus according to the second aspect of the invention, three contact operation sensors for detecting the motions of the individual movable contacts of the A-phase switch, the B-phase switch and the C-phase switch, and three phase current sensors for detecting the individual phase currents of the A-phase, the B-phase and the C-phase are connected with the making operation time information detecting means. The making operation time information detecting means outputs the making operation time information ITA, ITB and ITC of the A-phase switch, the B-phase switch and the C-phase switch individually on the basis of one of the detected output of the contact operation sensor and the detected output of the phase current sensor. Even if the making operation time information based on the phase current is not obtained on each of the A-phase switch, the B-phase switch and the C-phase switch, the making operation time information can be obtained on the basis of the motions of the movable contact. As a result, the making operation time information of the phase switches can be obtained more precisely. 
   The foregoing and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present of invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing Embodiment 1 of a power switching control apparatus according to this invention; 
       FIG. 2  is an explanatory diagram of making timings of individual phase switches of Embodiment 1; 
       FIG. 3  is a block diagram showing Embodiment 2 of a power switching control apparatus according to this invention; 
       FIG. 4  is an explanatory diagram of making timings of individual phase switches of Embodiment 2; and 
       FIG. 5  is a block diagram showing Embodiment 3 of a power switching control apparatus according to this invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Several embodiments of the present invention are described in the following with reference to the accompanying drawings. 
   Embodiment 1 
     FIG. 1  is a block diagram showing Embodiment 1 of a power switching control apparatus according to this invention. The power switching control apparatus of Embodiment 1 includes a three-phase AC power circuit  10 , a switching apparatus  20  and a control unit  30 . 
   The three-phase AC power circuit  10  is a transmission system or a distribution system for a commercial AC voltage, for example. This three-phase AC power circuit  10  includes A-phase, B-phase and C-phase phase lines  11 A,  11 B and  11 C of A-phase, B-phase and C-phase, and a load  15 A connected with the phase lines. In Embodiment 1, the load  15 A is a load of the type having a non-grounded neutral point, and is specified by a delta-connected three-phase capacitor bank  16 . The phase voltages of the individual phase lines  11 A,  11 B and  11 C on the input sides of individual phase switches  21 A,  21 B and  21 C are designated by VA, VB and VC, and the phase currents on the load sides of the individual phase switches  21 A,  21 B and  21 C are designated by IA, IB and IC. 
   The switching apparatus  20  switches the individual phase lines  11 A,  11 B and  11 C. This switching apparatus  20  includes the A-phase switch  21 A, the B-phase switch  21 B and the C-phase switch  21 C. The A-phase switch  21 A is connected with the phase line  11 A, and the B-phase switch  21 B and the C-phase switch  21 C are connected with the phase lines  11 B and  11 C, respectively. The individual phase switches  21 A,  21 B and  21 C are exemplified by power breakers, and are arranged either at substations of a power transmission line or at distributors at a transmission line. 
   The individual phase switches  21 A,  21 B and  21 C are so constituted that they can be controlled independently of one another. The individual phase switches  21 A,  21 B and  21 C are so turned ON at preset phase angles as to suppress the generation of the inrush current or surge voltage severe for the system device of the three-phase AC power circuit  10 . The A-phase switch  21 A is fed with a making command signal SA from the control unit  30  so that the A-phase switch  21 A makes connection of the movable contact with the fixed contact on the basis of that making command signal SA. Similarly, the B-phase switch  21 B and the C-phase switch  21 C are fed with the making command signals SB and SC, respectively, so that the phase switches  21 B and  21 C make connection of their individual movable contacts with the fixed contacts on the basis of the making command signals SB and SC. 
   In the making operations, the individual phase switches  21 A,  21 B and  21 C perform the making operations for making operation times TA, TB and TC. These making operation times TA, TB and TC are operation periods from the making command signals SA, SB and SC to the connections of the movable contacts of the phase switches  21 A,  21 B and  21 C with the fixed contacts. These making operation times TA, TB and TC are dependent on the characteristics of the making mechanisms of the individual phase switches  21 A,  21 B and  21 C but independent of one another, and change with time because the movable contacts and the fixed contacts are consumed by the arc. These making operation times TA, TB and TC also change dependent on the control voltages of the individual phase switches  21 A,  21 B and  21 C on the making mechanisms and on the environmental conditions such as the temperature. 
   The control unit  30  includes switching control means  31  and making operation time information detecting means  33 A. The control unit  30  is constituted by using a microcomputer, for example, and the switching control means  31  and the making operation time information detecting means  33 A are also constituted of the operation device, the storage device and so on of the microcomputer. As a matter of fact, the control unit  30  is equipped with not only the making operation time information detecting means  33 A but also control voltage detecting means of the switch and environment information detecting means such as the ambient temperature. However, this invention is characterized by the control relating to the making operation times TA, TB and TC, so that the control voltage detecting means and the environment information detecting means are omitted from the description of the invention. 
   The switching control means  31  generates and feeds the making command signals SA, SB and SC to the individual phase switches  21 A,  21 B and  21 C. The switching control means  31  stores, for the individual phase switches  21 A,  21 B and  21 C, making operation time information ITA, ITB and ITC representing the past making operation times TA, TB and TC, in the storage device of the microcomputer, and generates the making command signals SA, SB and SC with reference to the stored information of the past making operation time information ITA, ITB and ITC so that the individual phase switches  21 A,  21 B and  21 C may be made at the set phases even if their individual making operation times might change. The making command signals SA, SB and SC for the individual phase switches  21 A,  21 B and  21 C are fed to the making operation time information detecting means  33 A, too, so as to detect the making operation time information ITA, ITB and ITC indicating the new making operation times TA, TB and TC based thereon. 
   The making operation time information detecting means  33 A includes first detecting means  35 A and second detecting means  37 A. In Embodiment 1, the first detecting means  35 A is coupled to the switching control means  31  and a contact operation sensor  36 A arranged in the A-phase switch  21 A. This first detecting means  35 A receives the making command signal SA for the A-phase switch  21 A from the switching control means  31 , and receives a contact operation signal SATR indicating the motion of the movable contact of the A-phase switch  21 A, from the contact operation sensor  36 A. The contact operation sensor  36 A is a pulse generator for generating, when the movable contact of the A-phase switch  21 A is made to the fixed contact on the basis of the making command signals SA, pulse signals sequentially each time the movable contact turns a unit angle in response to the motion of that movable contact. This pulse signal is fed as the contact operation signal SATR to the first detecting means  35 A. 
   The first detecting means  35 A counts, in response to the making command signal SA, the contact operation signal SATR, and counts the lapse time till the counted value reaches the set count which is assumed as the connection between the movable contact and the fixed contact. This lapse time represents the making operation time TA of the A-phase switch  21 A. The first detecting means  35 A feeds the making operation time information ITA representing the making operation time TA to the switching control means  31 . The making operation time information ITA of the A-phase switch  21 A is stored at the switching control means  31  in the storage device of the microcomputer, and is used for determining the generation timing of the making command signal SA for the A-phase switch  21 A of the next and subsequent times. 
   In Embodiment 1, the second detecting means  37 A is coupled to the switching control means  31  and the B-phase and C-phase phase current sensors  38 B and  38 C. This second detecting means  37 A receives the making command signals SB and SC to the B-phase switch  21 B and the C-phase switch  21 C, from the switching control means  31 , and receives phase current signals SIB and SIC of the B-phase switch  21 B and the C-phase switch  21 C, from the phase current sensors  38 B and  38 C. The phase current sensor  38 B is coupled to the B-phase line  11 B between the B-phase switch  21 B and the load  15 A, and generates the phase current signal SIB according to a phase current IB of the B-phase switch  21 B. Likewise, the phase current sensor  38 C is coupled to the C-phase line  11 C between the C-phase switch  21 C and the load  15 A, and generates the phase current signal SIC according to the phase current IC of the C-phase switch  21 C. 
   The second detecting means  37 A feeds the making operation time information ITA and ITC of the B-phase switch  21 B and the C-phase switch  21 C to the switching control means  31 . The making operation time information ITB is the sum of the B-phase making lapse time and the B-phase pre-arcing time. The B-phase making lapse time is calculated on the basis of the making command signal SB for the B-phase switch  21 B and the phase current signal SIB from the phase current sensor  38 B. Specifically, the B-phase making lapse time is calculated as the lapse time from the reception of the making command signal SB to the B-phase conduction starting instant which is determined on the basis of the waveform of the phase current signal SIB. The B-phase pre-arcing time is calculated by dividing the instantaneous value of the B-phase voltage VB at the B-phase conduction starting time, by the rate of change of the insulating characteristics between the movable contact and the fixed contact in the making process of the B-phase switch  21 B. 
   Likewise, the making operation time information ITC is the sum of the C-phase making lapse time and the C-phase pre-arcing time. The C-phase making lapse time is calculated on the basis of the making command signal SC for the C-phase switch  21 C and the phase current signal SIC from the phase current sensor  38 C. Specifically, the C-phase making lapse time is calculated as the lapse time from the reception of the making command signal SC to the C-phase conduction starting instant which is determined on the basis of the waveform of the phase current signal SIC. The C-phase pre-arcing time is calculated by dividing the instantaneous value of the C-phase voltage VC at the C-phase conduction starting time, by the rate of change of the insulating characteristics between the movable contact and the fixed contact in the making process of the C-phase switch  21 C. 
   The sum of the B-phase making lapse time and the B-phase pre-arcing time, and the sum of the C-phase making lapse time and the C-phase pre-arcing time represent the making operation times TB and TC of the phase switches  21 B and  21 C, respectively, so that the making operation time information ITB and ITC represent the making operation times TB and TC, respectively. These making operation time information ITB and ITC of those B-phase switch  21 B and the C-phase switch  21 C are stored at the switching control means  31  in the storage device of the microcomputer and are used for determining the generation timings of the making command signals SB and SC for the B-phase switch  21 B and the C-phase switch  21 C of the next and subsequent times. 
   Now, in Embodiment 1, the making instants TAON, TBON and TCON for the A-phase switch  21 A, the B-phase switch  21 B and the C-phase switch  21 C are set, as shown in  FIG. 2 , by the switching control means  31 , for example. These making instants TAON, TBON and TCON are individually set to suppress the inrush current or surge voltage severe against the system device connected with the three-phase AC power circuit  10 . As shown in  FIG. 2 , more specifically, the making instant TAON for the A-phase switch  21 A is set at an arbitrary timing at and before making another phase, e.g., +60 degrees of the reference phase of the A-phase voltage VA in the A-phase line  11 A. The making instant TBON for the B-phase switch  21 B is set at +150 degrees of the reference phase, for example, and making instant TCON for the C-phase switch  21 C is set at +240 degrees of the reference phase, for example. In other words, the making instant TAON precedes the making instant TBON, and the making instant TBON precedes the making instant TCON. 
   In Embodiment 1, the load  15 A is a load of the non-grounded neutral point type. At the making instant TAON of the first A-phase switch  21 A, the B-phase switch  21 B and the C-phase switch  21 C are OFF. In the ON contact of the A-phase switch  21 A, therefore, the phase current IA of the A-phase switch  21 A does not flow. In Embodiment 1, however, the contact operation sensor  36 A is arranged in the A-phase switch  21 A. Even without the flow of the phase current IA, the making operation time information ITA representing the making operation time TA of the A-phase switch  21 A can be outputted from the first detecting means  35 A on the basis of the contact operation signal SATR of the contact operation sensor  36 A. At the making instants TBON and TCON for the B-phase switch  21 B and the C-phase switch  21 C, the phase currents IB and IC of the individual phase switches  21 B and  21 C flow when the contacts of the switches  21 B and  21 C are turned ON. On the basis of the phase current signals SIB and SIC from the phase current sensors  38 B and  38 C, therefore, the making operation time information ITB and ITC representing the making operation times TB and TC of the B-phase switch  21 B and the C-phase switch  21 C can be outputted from the second detecting means  37 A. In Embodiment 1, therefore, the making operation time information ITA, ITB and ITC representing the making operation times TA, TB and TC of all the phase switches  21 A,  21 B and  21 C can be obtained more precisely. 
   Here in Embodiment 1, even if the phase of the making instant TAON for the A-phase switch  21 A changes from the set phase of  FIG. 2 , the phase current IA does not flow with similar effects, although the A-phase switch  21 A is always made, so long as the making instant TAON precedes the making instants TBON and TCON for the B-phase switch  21 B and the C-phase switch  21 C. 
   Embodiment 2 
     FIG. 3  is a block diagram showing Embodiment 2 of a power switching control apparatus according to this invention, and  FIG. 4  is an explanatory view of making instants TAON, TBON and TCON in Embodiment 2. 
   In this Embodiment 2, the load  15 A of Embodiment 1 is replaced by a load  15 B. Moreover, the making instants TAON, TBON and TCON for the individual phase switches  21 A,  21 B and  21 C are so changed, as shown in  FIG. 4 . Accordingly, in Embodiment 2, the making operation time information detecting means  33 A of Embodiment 1 is replaced by making operation time information detecting means  33 B. This making operation time information detecting means  33 B includes first detecting means  35 B and second detecting means  37 B. The first detecting means  35 B is constituted to receive the making command signal SC from the switching control means  31  and to receive a contact operation signal SCTR from a contact operation sensor  36 C arranged in the C-phase switch  21 C. Moreover, the second detecting means  37 B is constituted to receive the making command signals SA and SB from the switching control means  31 , and to receive phase current signals SIA and SIB, respectively, from a phase current sensor  38 A coupled to the A-phase line  11 A and a phase current sensor  38 B coupled to the B-phase line  11 B. The remaining constitutions are identical to those of Embodiment 1. 
   In this Embodiment 2, the load  15 B is the load of the grounded neutral point type with the phase-shared core. This load  15 B is formed into a cored reactor or transformer connected in a star shape. The load  15 B has a core  17  shared among the individual phases, and this core  17  is wounded by a reactor  18  connected with the individual phase lines  11 A,  11 B and  11 C. The reactor  18  is connected in a star shape, and has its neutral point connected with the earth point E. 
   In Embodiment 2, moreover, the making instants TAON, TBON and TCON for the individual phase switches  21 A,  21 B and  21 C are so set by the switching control means  31  as are shown in  FIG. 4 . The making instant TAON of the A-phase switch  21 A is set at +90 degrees, for example, with respect to the reference phase of the phase voltage VA; the making instant TBON of the B-phase switch  21 B is set at +150 degrees, for example, with respect to the reference phase; and the making instant TCON of the C-phase switch  21 C is set at +210 degrees, for example, with respect to the reference phase. 
   In Embodiment 2, the first detecting means  35 B receives the contact operation signal SCTR indicating the motion of the movable contact of the C-phase switch  21 C from the contact operation sensor  36 C. Specifically, the contact operation sensor  36 C is a pulse generator for generating, when the movable contact of the C-phase switch  21 C is made toward the fixed contact on the basis of the making command signal SC, pulse signals sequentially in response to the motion of that movable contact each time the movable contact turns a unit angle. This pulse signal is fed as the contact operation signal SCTR to the first detecting means  35 B. 
   The first detecting means  35 B counts the contact operation signal SCTR when it receives the making command signal SC, and counts the lapse time, till the counted value reaches the set count, at which the movable contact and the fixed contact are made. This lapse time indicates the making operation time TC of the C-phase switch  21 C. The first detecting means  35 B feeds the making operation time information ITC indicating that making operation time TC, to the switching control means  31 . The making operation time information ITC of the C-phase switch  21 C is stored at the switching control means  31  in the storage device of the microcomputer, and is used for determining the generation timing of the making command signal SC for the C-phase switch  21 C of the next and subsequent times. 
   In Embodiment 2, the second detecting means  37 B receives the making command signals SA and SB to the A-phase switch  21 A and the B-phase switch  21 B, from the switching control means  31 , and receives phase current signals SIA and SIB of the A-phase switch  21 A and the B-phase switch  21 B, from the phase current sensors  38 A and  38 B. The phase current sensor  38 A is coupled to the A-phase line  11 A between the A-phase switch  21 A and a load  13 , and generates the phase current SIA according to a phase current IA of the A-phase switch  21 A. The phase current sensor  38 B is coupled, as in Embodiment 1, to the B-phase line  11 B between the B-phase switch  21 B and a load  13 A, and generates the phase current signal SIB according to the phase current IB of the B-phase switch  21 B. 
   The second detecting means  37 B feeds the making operation time information ITA and ITB of the A-phase switch  21 A and the B-phase switch  21 B to the switching control means  31 . The making operation time information ITA is the sum of the A-phase making lapse time and the A-phase pre-arcing time. The A-phase making lapse time is calculated on the basis of the making command signal SA for the A-phase switch  21 A and the phase current signal SIA from the phase current sensor  38 A. Specifically, the A-phase making lapse time is calculated as the lapse time from the reception of the making command signal SA to the A-phase conduction starting instant which is determined on the basis of the waveform of the phase current signal SIA. The A-phase pre-arcing time is calculated by dividing the instantaneous value of the A-phase voltage VA at the A-phase conduction starting time, by the rate of change of the insulating characteristics between the movable contact and the fixed contact in the making process of the A-phase switch  21 A. 
   Like Embodiment 1, the making operation time information ITB is the sum of the B-phase making lapse time and the B-phase pre-arcing time. The B-phase making lapse time is calculated on the basis of the making command signal SB for the B-phase switch  21 B and the phase-current signal SIB from the phase current sensor  38 B. Specifically, the B-phase making lapse time is calculated as the lapse time from the reception of the making command signal SB to the B-phase conduction starting instant which is determined on the basis of the waveform of the phase current signal SIB. The B-phase pre-arcing time is calculated by dividing the instantaneous value of the B-phase voltage VB at the B-phase conduction starting time, by the rate of change of the insulating characteristics between the movable contact and the fixed contact in the making process of the B-phase switch  21 B. 
   The sum of the A-phase making lapse time and the A-phase pre-arcing time, and the sum of the B-phase making lapse time and the B-phase pre-arcing time represent the making operation times TA and TB of the phase switches  21 A and  21 B, respectively, so that the making operation time information ITA and ITB represent the making operation times TA and TB, respectively. These making operation time information ITA and ITB of those A-phase switch  21 A and the B-phase switch  21 B are stored at the switching control means  31  in the storage device of the microcomputer and are used for determining the generation timings of the making command signals SA and SB for the A-phase switch  21 A and the B-phase switch  21 B of the next and subsequent times. 
   In Embodiment 2, the load  15 B is a load of the grounded neutral point type with the common core. At the making instant TAON of the first A-phase switch  21 A and at the making instant TBON of the next B-phase switch  21 B, the phase currents IA and IB flow when those contacts are ON. On the basis of the phase current signals SIA and SIB of the phase current sensors  38 A and  38 B, therefore, the making operation time information ITA and ITB indicating the making operation times TA and TB of the A-phase switch  21 A and the B-phase switch  21 B can be outputted from the second detecting means  37 B. 
   In this Embodiment 2, at the making instant TCON for the C-phase switch  21 C, the A-phase switch  21 A and the B-phase switch  21 B are made beforehand. Therefore, the voltage to be induced in the reactor  18  connected with the C-phase line  11 C is equal to the C-phase voltage VC so that the voltage between those contacts is made in the substantial “0” voltage on the C-phase switch  21 C. In the C-phase switch  21 C, therefore, the pre-arc does not occur before the contact is turned ON. From this phase current IC, the making operation time information TC cannot be determined as in the other A-phase and B-phase. In this Embodiment 2, however, the contact operation sensor  36 C is arranged in the C-phase switch  21 C. Even if the pre-arc does not occur at the making instant of the C-phase switch  21 C, the making operation time information ITC indicating the making operation time TC of the C-phase switch  21 C can be outputted from the first detecting means  35 B on the basis of the contact operation signal SCTR of the contact operation sensor  36 C. As a result, it is possible to make more precise the making operation time information ITA, ITB and ITC indicating the making operation times TA, TB and TC of all the phase switches  21 A,  21 B and  21 C. 
   Here in Embodiment 2, so long as the phase of the making instant TCON with respect to the C-phase switch  21 C is after the making instants TAON and TBON for the A-phase switch  21 A and the B-phase switch  21 B even if it changes from the set phase of  FIG. 4 , the C-phase switch  21 C is made with similar effects in the substantially “0” voltage between contacts. Even at the making instants TAON, TBON and TCON shown in  FIG. 2 , similar effects can be obtained because the making instant TCON for the C-phase switch  21 C occurs after the making instants TAON and TBON for the A-phase switch  21 A and the B-phase switch  21 B. 
   Embodiment 3 
     FIG. 5  is a block diagram showing Embodiment 3 of a power switching control apparatus according to this invention. In this power switching control apparatus of Embodiment 3, the load  15 A in Embodiment 1 is replaced by an arbitrary three-phase load  15 , the making instants TAON, TBON and TCON for the individual phase switches  21 A,  21 B and  21 C are arbitrarily set. Accordingly, in Embodiment 3, the making operation time information detecting means  33 A of Embodiment 1 is replaced by making operation time information detecting means  33 . This making operation time information detecting means  33  includes first detecting means  35 , second detecting means  37  and comparing-selecting means  40 . The first detecting means  35  is constituted to receive the making command signals SA, SB and SC from the switching control means  31  and to receive the contact operation signals SATR, SBTR and SCTR, respectively, from contact operation sensors  36 A,  36 B and  36 C arranged in the phase switches  21 A,  21 B and  21 C, respectively. On the other hand, the second detecting means  37  is constituted to receive the making command signals SA, SB and SC from the switching control means  31  and to receive the phase current signals SIA, SIB and SIC of the phase switches  21 A,  21 B and  21 C from the phase current sensors  38 A,  38 B and  38 C coupled to the individual phase lines  11 A,  11 B and  11 C, respectively. The comparing-selecting means  40  includes comparing means  41  and selecting means  42 . The remaining constitutions are similar to those of Embodiment 1. 
   The load  15  of this Embodiment 3 is an arbitrary three-phase load, which can be used as any of the load  15 A of the non-grounded neutral point type, as shown in  FIG. 1 , the common core load  15 B of the grounded neutral point type, as shown in  FIG. 3 , or another three-phase load. Moreover, the making instants TAON, TBON and TCON for the individual phase switches  21 A,  21 B and  21 C are also arbitrarily set to those of  FIG. 2  or  FIG. 4  or other timings. 
   The contact operation sensors  36 A,  36 B and  36 C arranged at the individual phase switches  21 A,  21 B and  21 C are pulse generators for generating pulse signals sequentially each time the movable contacts of the corresponding phase switches  21 A,  21 B and  21 C turn by unit angles in response to the motion of the movable contacts when the movable contacts are made toward the fixed contacts on the basis of the making command signals SA, SB and SC. These pulse signals are fed as the contact operation signals SATR, SBTR and SCTR to the first detecting means  35 . 
   In response to the individual making command signals SA, SB and SC, the first detecting means  35  counts the individual contact operation signals SATR, SBTR and SCTR, and generates the contact ON signals SAON, SBON and SCON when the counted value reaches the set value, at which the movable contacts and the fixed contacts of the corresponding phase switches  21 A,  21 B and  21 C are made. In this Embodiment 3, the contact ON signals SAON, SBON and SCON are outputted from the first detecting means  35  to the comparing means  41  of the comparing-selecting means  40 . In response to the individual making command signals SA, SB and SC, moreover, the first detecting means  35  counts the individual contact operation signals SATR, SBTR and SCTR, individually, and counts the lapse times till the reach of the set counts, at which it is imagined that the movable contacts and the fixed contacts of the corresponding phase switches  21 A,  21 B and  21 C are made. These individual lapse times are the first information representing the making operation times TA, TB and TC of the individual phase switches  21 A,  21 B and  21 C, and the first detecting means  35  outputs the individual lapse times as first making operation time information ITA 1 , ITB 1  and ITC 1  from the first detecting means  35  to the selecting means  42 . 
   The individual phase current sensors  38 A,  38 B and  38 C are coupled to the individual phase lines  11 A,  11 B and  11 C between the phase switches  21 A,  21 B and  21 C and the load  15 , and generate the phase current signals SIA, SIB and SIC according to the phase currents IA, IB and IC flowing through the phase switches  21 A,  21 B and  21 C, respectively. At the making instants of the individual phase switches  21 A,  21 B and  21 C, the pre-arc may occur or not, depending upon the kind of the load  15  and the settings of the making instants TAON, TBON and TCON. 
   The phase current signals SIA, SIB and SIC are individually fed to the second detecting means  37 . In Embodiment 3, at the timings of the phase current signals SIA, SIB and SIC to flow, current flow starting signals SAS, SBS and SCS indicating the current flow starts are generated by the second detecting means  37  and are fed to the comparing means  41 . In case the pre-arcs occur, these current flow starting signals SAS, SBS and SCS indicate the starting points of the pre-arcs, at which the flows start before the contacts of the individual phase switches are turned ON. In the absence of the pre-arcs, the current flow starting signals SAS, SBS and SCS indicate the flow starts of the phase currents which start to flow after the contacts of the corresponding phase switches were turned ON. 
   On the basis of the individual making command signals SA, SB and SC and the individual phase current signals SIA, SIB and SIC, moreover, the second detecting means  37  generates second making operation time information ITA 2 , ITB 2  and ITC 2  of the individual phase switches  21 A,  21 B and  21 C, and feeds the second making operation time information ITA 2 , ITB 2  and ITC 2  to the selecting means  42  of the comparing-selecting means  40 . The second making operation time information ITA 2 , ITB 2  and ITC 2  are effective in case the corresponding phase switches  21 A,  21 B and  21 C are followed by the pre-arcs. In case the making of the corresponding phase switches  21 A,  21 B and  21 C is not followed by the pre-arcs, the second making operation time information ITA 2 , ITB 2  and ITC 2  are the signals considering the pre-arcs which do not really exist, so that they are ineffective. 
   The comparing means  41  of the comparing-selecting means  40  receives contact ON signals SAON, SBON and SCON from the first detecting means  35  and the current flow starting signals SAS, SBS and SCS from the second detecting means  37 . This comparing means  41  compares the contact ON signals SAON, SBON and SCON and the current flow starting signals SAS, SBS and SCS to decide the effectiveness of the second making operation time information ITA 2 , ITB 2  and ITC 2 , and outputs select signals SSA, SSB and SSC representing the effectiveness to the selecting means  42 . This selecting means  42  is fed with the first making operation time information ITA 1 , ITB 1  and ITC 1  from the first detecting means  35 , and with the second making operation time information ITA 2 , ITB 2  and ITC 2  from the second detecting means  37 . On the basis of the select signals SSA, SSB and SSC, the selecting means  42  selects either the first making operation time information ITA 1 , ITB 1  and ITC 1  and the second making operation time information ITA 2 , ITB 2  and ITC 2 , and outputs the making operation time signals ITA, ITB and ITC to the switching control means  31 . 
   The making operation time information ITA is selected, on the basis of the select signal SSA, from either of the first and second making operation time information ITA 1  and ITA 2 . When the comparing means  41  decides that the second making operation time information ITA 2  is effective, the select signal SSA instructs the selecting means  42  to select the second making operation time signal ITA 2 , so that the select means  42  outputs the second making operation time information ITA 2  as the making operation time information ITA. When the comparing means  41  decides that the second making operation time information ITA 2  is ineffective, the selecting means  42  outputs the first making operation time information ITA 1  as the making operation time information ITA. Likewise, the making operation time information ITB is selected, on the basis of the select signal SSB, from either of the first and second making operation time information ITB 1  and ITB 2 . When the comparing means  41  decides that the second making operation time information ITB 2  is effective, the select signal SSB instructs the selecting means  42  to select the second making operation time signal ITB 2 , so that the select means  42  outputs the second making operation time information ITB 2  as the making operation time information ITB. When the comparing means  41  decides that the second making operation time information ITB 2  is ineffective, the selecting means  42  outputs the first making operation time information ITB 1  as the making operation time information ITB. Likewise, the making operation time information ITC is selected, on the basis of the select signal SSC, from either of the first and second making operation time information ITC 1  and ITC 2 . When the comparing means  41  decides that the second making operation time information ITC 2  is effective, the select signal SSC instructs the selecting means  42  to select the second making operation time signal ITC 2 , so that the select means  42  outputs the second making operation time information ITC 2  as the making operation time information ITC. When the comparing means  41  decides that the second making operation time information ITC 2  is ineffective, the selecting means  42  outputs the first making operation time information ITC 1  as the making operation time information ITC. 
   The effectiveness of the second making operation time information ITA 2 , ITB 2  and ITC 2  by the comparing means  41  is decided on the individual contact ON signals SAON, SBON and SCON. By using the generation timings of the individual contact ON signals SAON, SBON and SCON as the reference timings, it is decided whether or not the current flow starting signals SAS, SBS and SCS are present for a constant period at and before the reference timing containing the reference timing. If the current flow starting signals are present for the aforementioned individual predetermined periods, it is decided that the current flow starting signals indicate the starting points of the pre-arcs, and that the corresponding second making operation time information is effective. Otherwise, it is decided that the current flow starting signals are not the starting points of the pre-arcs, and that the corresponding second making operation time information is ineffective. 
   For example, the load  15  is the load  15 A of the non-grounded neutral point type, as shown in  FIG. 1 , the A-phase switch  21 A of the individual phase switches  21 A,  21 B and  21 C may be made earlier than the remaining B-phase switch  21 B and the C-phase switch  21 C. In this case, the phase current IA does not flow in the A-phase switch  21 A made first. The current flow starting signal SAS flows after the B-phase switch  21 B is made after the contact ON signal SAON so that the second making operation time information ITA 2  corresponding to the phase current signal SIA is made ineffective. When the B-phase switch  21 B and the C-phase switch  21 C are made, on the other hand, the phase currents IB and IC flow from the starting points of the pre-arcs. By using the generating timings of the contact ON signals SBON and SCON as the reference timings, therefore, the current flow starting signals SBS and SCS exist for the constant time period at and before the reference timing including that reference timings. It is, therefore, decided that the second making operation time information ITB 2  and ITC 2  corresponding to those phase current signals SIB and SIC are effective. 
   Moreover, the load  15  is the load  15 B with the common core of the grounded neutral point type, as shown in  FIG. 3 , and the C-phase switch  21 C of the individual phase switches  21 A,  21 B and  21 C is finally made at the making instant TCON after the A-phase switch  21 A and the B-phase switch  21 B. In this case, the C-phase switch  21 C is made with the voltage between the contacts being substantially 0 voltage, so that its phase current IC flows after the contact of the C-phase switch  21 C is turned ON. It is, therefore, decided that the second making operation time information ITC 2  corresponding to the phase current signal IC is ineffective. When the A-phase switch  21 A and the B-phase switch  21 B are made, on the other hand, the phase currents IA and IB flow from the starting points of the pre-arcs. By using the generating timings of the contact ON signals SAON and SBON as the reference timings, therefore, the current flow starting signals SAS and SBS exist for the constant time period at and before the reference timing including that reference timings. It is, therefore, decided that the second making operation time information ITA 2  and ITB 2  corresponding to those phase current signals SIA and SIB are effective. 
   Thus according to Embodiment 3, irrespective of the kind of the load  15  and the making instants TAON, TBON and TCON of the individual phase switches  21 A,  21 B and  21 C, either of the first making operation time information ITA 1 , ITB 1  and ITC 1  and the second making operation time information ITA 2 , ITB 2  and ITC 2  can always be selected to detect all the making operation time information ITA, ITB and ITC more precisely. 
   The power switching control apparatus according to this invention is utilized as the switching control apparatus for the three-phase AC power circuit. 
   Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this is not limited to the illustrative embodiments set forth herein.