SWITCHGEAR

Abstract: A switchgear operable to conduct or shut off the a.c. cur­ rent to an a.c. load by use of a semiconductor switch. The switchgear is formed such that a common thyristor switch in serial connection with a switch is connected in parallel to a switch for each of capacitor banks, so that each capacitor bank is connected or disconnected to the a.c. power line in non-contact manner through the thyristor switch.

Description: The invention relates to a switchgear comprising: a semiconductor switch for controlling an a.c. current to a load by being operated to turn on in synchronism with the voltage phase of an a.c. power source so that no rush current flows to said load; and a first switch connected between said semiconductor switch and said load. Such a switchgear is known from GB-A-1241816. There have been proposed various switchgears for connecting capacitors to load circuit. Fig. 1 is a schematic diagram of the conventional capacitor switchgear for controlling reactive power using mechanical switches. In the figure, reference number 1 denotes an a.c. power line, 2a-2c are switches, and 3a-3c are a.c. load capacitors connected to the a.c. power line 1 through the switches 2a-2c, respectively. Next, the operation of the above arrangement will be described. In Fig. 1, when the power-factor of the a.c. power line 1 becomes lower due to inductive load, the switches 2a-2c are turned on sequentially so that the capacitors 3a-3c are connected to line in accordance with the value of power-factor. In this case, a rush current 6-10 times the rated current will flow in a certain phase relationship of throwing the switches 2a-2c, resulting in a significant distortion in the a.c. power line voltage, which adversely affects other facilities (e.g., thyristor converter) connected on the same power line. On this account, the capacitors cannot have a large unit bank capacitance, and an increased number of capacitor banks are needed. This results disadvantageously in a larger installation space and a higher construction cost. In addition, when the switches are operated continually, the transient phenomena at connecting or disconnecting the capacitors impair the work life of the switches 2a-2c and capacitors 3a-3c. To cope with these problems in the use of frequent switching operation, a capacitor switchgear is designed to connect or disconnect capacitors 3a-3c with thyristor switches 4a-4c, each made up of anti-parallel-connected thyristors as shown in Fig. 2, in place of the switches 2a-2c in Fig. 1, as disclosed for example in GB-A- 1241816. Fig. 3 shows a basic circuit arrangement for connecting or disconnecting a capacitor 3 to an a.c. power source 1 by means of the thyristor switch 4 shown in Fig. 2. Next, the operation of the above basic circuit arrangement will be described in connection with the waveform diagram of Figs. 4A, B and C. In Figs. 4A, B and C, the thyristor switch 4 is off up to a time point t₁, and a voltage E equal to the a.c. peak power voltage shown in Fig. 4A is applied across the thyristor switch 4 as shown in Fig. 4B. When the load capacitor 3 is to be connected to the power source, the thyristor switch 4 is controlled by a control signal provided by a control circuit (not shown) so that it is turned on in response to a zero voltage across it. Namely, the thyristor switch 4 is turned on at the time point t₁ in Figs. 4B and C. Then, a current shown in Fig. 4C flows thorugh the capacitor. To turn off the thyristor switch 4, the firing signal to the gate of the thyristors is removed, and the thyristor switch 4 becomes nonconductive at a time point t₂ when the capacitor current has fallen to zero as shown in Fig. 4C. The time point t₂ is coincident with the peak phase of the power voltage E, and the capacitor 3 is charged to the peak power voltage E at the time point t₂. Accordingly, when the thyristor switch 4 becomes off at t₂, the capacitor 3 is left charged to the voltage E, which is added to the voltage of the a.c. power source 1, resulting in the application of a 2E voltage across the thyristor switch 2 following the time point t₂ as shown in Fig. 4B. On this account, the thyristor devices used need to have a withstand voltage of at least 2E-3E. During the period when the capacitor 2 is connected to the power source, the thyristor switch 4 stays on, creating a significant power loss due to the forward voltage drop of the thyristors 4, resulting in a lower power efficiency and the need of a large cooling system that makes the switchgear expensive. The conventional capacitor switchgears are arranged as described above, and in the case of using a mechanical switch shown in Fig. 1, it cannot be used for a frequent switching operation and the capacitor bank cannot have a large unit capacitance due to the creation of a power voltage distortion by the rush current when the capacitors are connected. In another case of using a thyristor switch shown in Fig. 2, the forward voltage drop of each thyristor creates a large power loss and a thyristor switch is needed for each capacitor bank, resulting in a higher construction cost. A further known solution is disclosed in CA-A-1 229 380, in which a non-linear resistor is used in parallel to a thyristor switch which is adapted to close after the semiconductor switch. It is an object of the present invention to provide a switchgear which is less expensive as compared with conventional ones, has a low electrical loss during the conductive period and which may use semiconductor switching devices having a low break-down voltage. This object is being solved by a switchgear of the kind described in which a second switch is connected in parallel to the serial connection of said semiconductor switch and said first switch. Preferable embodiments are defined in the dependent claims. Fig. 1 is a schematic diagram of the conventional capacitor switchgear for controlling reactive power using mechanical switches; Fig. 2 is a schematic diagram of the conventional capacitor switchgear made up of a thyristor switch every capacitor bank; Fig. 3 shows a basic circuit arrangement for connecting a capacitor to an a.c. power source by means of the thyristor switch in Fig. 2; Figs. 4A-4C are waveform diagrams of the circuit of Fig. 3; Fig. 5 is a schematic diagram showing a first embodiment of a switchgear according to this invention; Fig. 6 is a schematic diagram showing the second embodiment of this invention; Figs. 7A-7F are waveform diagrams showing an operation of the circuit of Fig. 5 when the capacitor is connected to the power line; Figs. 8A-8F are waveform diagrams showing an operation of the circuit of Fig. 5 when the capacitor is disconnected to the power line; Figs. 9A-9F are waveform diagrams showing an operation of the circuit of Fig. 6 when the capacitor is disconnected to the power line; Fig. 10 is a detailed circuit diagram of the control circuit 8 in Fig. 6; Fig. 11 is a schematic diagram showing the third embodiment of the invention; and Figs. 12A-12F are waveform diagrams of the circuit of Fig. 11. Fig. 5 is a circuit diagram of the switchgear according to a first embodiment of this invention wherein the portions corresponding to the circuit of Fig. 1 are shown by the same reference numerals. In Fig. 5, reference numeral 5 illustrates a thyristor switch commonly used for a plurality of capacitor banks 3a-3c, and 6a-6c depict switches 6a-6c for the thyristor switch, inserted between the load terminal of the thyristor switch 4 and power supply terminals of capacitors 3a-3c. The operation of the above arrangement will be described with reference to Figs. 7A-7B. Initially, the mechanical switches 2a-2c and 6a-6c and the thyristor switch 5 are all in the off-state, and the capacitor banks 3a-3c are disconnected from the power line 1. For connecting the capacitor bank 3a to the a.c. power line 1, the switch 6a is turned on at an arbitrary time point t₁, while leaving the thyristor switch 5 in nonconductive state as shown in Fig. 7B. Consequently, the thyristor switch 5 is applied with a voltage equal to the peak power voltage E, as shown in Fig. 7D, and at the time point t₂ when the voltage across the thyristor switch 5 has become zero, the control circuit 8 provides a gate signal to turn on the thyristor switch 5. Then, the capacitor bank 3a is connected to the a.c. power line 1 without the creation of a rush current. The thyristor switch 5 thereafter conducts a current I T as shown in Fig. 7C. After the capacitor bank 3a has been connected to the power line, the switch 2a is turned on at a time point t₃ as shown in Fig. 7E, so that the current flowing through the thyristor switch 5 is taken by the switch 2a as shown in Fig. 7F. Then, the thyristor switch 5 is turned off and, thereafter, the switch 6a is turned off at a time point t₄. Next, for connecting the capacitor bank 3b, the switch 6b is turned on with the thyristor switch 5 being kept off, as in the previous case of connecting the capacitor bank 3a, and the thyristor switch 5 is turned on when the voltage across it has become zero. The capacitor bank 3b is thus connected to the power line 1 without the creation of a rush current. Subsequently, the switch 2b is turned on so that the current flowing through the thyristor switch 5 is shunted by the switch 2b. Then, the thyristor switch 5 is turned off and, thereafter, the switch 6b is turned off. The capacitor bank 3c be connected to the power line through the use of the thyristor switch 5 in the same procedure as described above. The thyristor switch 5 operates to connect capacitor loads 3a-3c to the power line, so that the creation of a rush current in the circuit is prevented, and the thyristor switch can have a short-time rating and causes less power loss, since the load current is taken by the switches 2a-2c immediately after the corresponding capacitor has been connected to the power line 1. Next, the operation for disconnecting the capacitor bank 3c will be described. In Fig. 8B, the switch 6c is turned on at a time point t₁, and the thyristor switch 5 is also turned on at the same time. Thereafter, the switch 2c is turned off at t₂ as shown in Fig. 8E so that the current flowing through the switch 2c is shunted to the thyristor switch 5 as shown in Figs. 8C and 8F. After the gate signal to the thyristor switch 5 is removed, the thyristor switch 5 is turned off at a time point t₃ when the current flowing through it falls to zero as shown in Fig. 8C, and the current in the circuit is shut off. At this time, the capacitor 3c is charged to the peak power voltage E, causing the thyristor switch 5 to be applied with a voltage of 2E. Thereafter, the switch 6c is turned off at a time point t₄, and the capacitor bank 3c is completely disconnected from the a.c. power line 1. The capacitor bank 3b is disconnected from the power line in the same way as of the capacitor bank 3c, and initially the switch 6b and the thyristor switch 5 are turned on at the same time. Thereafter, the switch 2b is turned off so that the current flowing through the switch 2b is shunted to the thyristor switch 5. After the gate signal to the thyristor switch 5 is removed, it is turned off when the current flowing in it falls to zero, and the current in the circuit is shut off. Thereafter, the switch 6b is turned off, and the capacitor bank 3b is completely disconnected from the a.c. power line 1. The same operations take place when the capacitor bank 3a is disconnected from the a.c. power line 1. As described above, when connecting or disconnecting the capacitor banks 3a-3c to the power line, the operation takes place in a noncontact fashion by the use of the thyristor switch 5 so as to prevent the occurrence of transient phenomena such as a rush current, whereby a voltage distortion is not created on the a.c. power line 1 when each capacitor is connected to the power line. Because of the absence of a rush current to the capacitor at the connecting operation, its work life is not impaired. Moreover, the switches 2a-2c and 6a-6c are turned on or off when no current flows in it or the current is bypassed by other circuit, whereby the work life of the switches is not impaired. These switches are not required to have a large current switching capacity, and common circuit breakers can be used for this purpose. Fig. 6 is a schematic diagram showing the second embodiment of this invention, in which components identical to those of Fig. 5 are referred to by the common symbols. The arrangement of Fig. 6 includes a nonlinear resistor 7 connected in parallel to the thyristor switch 5. The connecting operation for the capacitor banks 3a-3c in this embodiment is completely identical to the case of Fig. 5. Whereas, in disconnecting the capacitors 3a-3c from the power line in the embodiment of Fig. 5, the capacitors 3a-3c are charged to the peak power voltage E immediately after the thyristor switch 5 has been turned off at time point t₃ in Fig. 8, causing the thyristor switch 5 to be applied to a voltage of 2E as shown by the dashed line in Fig. 8D, and the thyristor switch 5 needs to have a withstand voltage of at least 2E, as mentioned previously. However, in this embodiment the nonlinear resistor 7 connected parallel to the thyristor switch 5 causes voltage across the turned-off thyristor switch 5 to fall to a level substantially equal to the peak power voltage E, as shown by the solid curve in Fig. 9D. The nonlinear resistor 7 is applied with the voltage only during a short period when the thyristor switch 5 is turned on or off, resulting in a small power dissipation by the resistor. On this account, it is possible to suppress the voltage applied across the thyristor switch 5 to a level substantially equal to the voltage E during the short transitional period. Accordingly, through the connection of the nonlinear resistor 7 in parallel to the thyristor switch 5 , the voltage across the thyristor switch 5 can be limited, whereby inexpensive switchgear can be realized. Fig. 10 is a detailed diagram of a control circuit 8 as one example used in the above embodiment. In Fig. 11 showing the third embodiment of this invention, the power system includes an a.c. power source 11 having a peak power voltage E, a thyristor switch 12 consisting of two anti-parallel connected thyristors 12a and 13b, and a capacitor load 13, all connected in series in this order. The switchgear further includes a nonlinear resistor 14 connected in parallel to the thyristor switch 12, a mechanical switch 15 used to open the circuit of the thyristor switch, a mechanical switch 16 used to shunt the thyristor switch, and a control circuit 18 for turning on or off the thyristors 12a and 12b. The operation of the foregoing inventive arrangement will be described with reference to the waveforms of Fig. 12A-12F. Initially, the thyristor switch 12 and mechanical switches 15 and 16 are all in the off-state, and the capacitor load 13 is disconnected from the a.c. power source 11. For connecting the capacitor 13 to the a.c. power source 11, the switch 15 is turned on at an arbitrary time point t₁₁ as shown in Fig. 12B, while the thyristor switch 12 is kept in the nonconductive state. Consequently, the thyristor switch 12 is applied with a voltage equal to the peak power voltage E as shown in Fig. 12F, and a thyristor switch current flows as shown in Fig. 12C. At a time point t₁₂ when the voltage across the thyristor switch 12 has fallen to zero, the control circuit 18 provides a gate signal to the thyristors 12a and 12b so that the thyristor switch 12 is turned on, thereby connecting the capacitor 13 to the a.c. power source 11 without creating a rush current. At a time point t₁₃, the switch 16 is turned on as shown in Fig. 12E so as to shunt the thyristor switch 12, and it is turned off. After the time point t₁₃, the existing current flows through the switch 16, and almost no electrical loss is created. The switch 15 is left on as shown in Fig. 12B. For disconnecting the capacitor 13 from the power source 11, the control circuit 18 provides a gate signal so that the thyristor switch 12 is turned on at a time point t₁₄ and, at the same time, the switch 16 is turned off. Then, the existing current flows through the switch 15 and thyristor switch 12, creating a thyristor switch current as shown in Fig. 12. Thereafter, the thyristor gate signal goes off, and the thyristor switch 12 is turned off at a time point t₁₅ when the thyristor switch current has fallen to zero. At the time point t₁₅, the capacitor 13 is charged to the peak power voltage E, that would cause the application of voltage 2E across the thyristor switch 12 as shown by the dashed curve in Fig. 12F, however in actual the nonlinear resistor 14 connected in parallel to the thyristor switch 12 acts to suppress the voltage to a level substantially equal to the peak power voltage E. The nonlinear resistor 14 is applied with the voltage only during a short period when the thyristor switch 12 is turned on or off, resulting in a lower power dissipation by the resistor, and accordingly a large voltage factor can be set. The nonlinear resistor 14 also serves to suppress external surge voltages penetrating to the thyristor switch 12 during the application of the power voltage, whereby the thyristor switch 12 suffices to have a withstand voltage as high as the peak power voltage. After the thyristor switch 12 has turned off, the switch 15 is opened at a time point t₁₆, and thereafter the thyristor switch 12 and nonlinear resistor 14 are relieved of the voltage application. The switches 15 and 16 are always operated in a state of zero current as shown in Fig. 12, and therefore they may be circuit breakers with less opening and closing capacities. Although in the foregoing embodiments the switchgear is used to connect or disconnect capacitor banks, it can be applied to any a.c. load besides capacitor banks. Although the foregoing embodiments employ circuit breakers for the switches, other switching devices can also be used. Although in the foregoing embodiment the thyristor switch is formed of anti-parallel connected thyristors, other semiconductor switching devices such as GTO thyristors can also be used to accomplish the same effects as described above. According to the foregoing embodiment, the switchgear is formed such that a common thyristor switch in serial connection with a switch is connected in parallel to a switch for each of capacitor banks, so that each capacitor bank is connected or disconnected to the a.c. power line in non-contact manner through the thyristor switch, whereby the voltage distortion on the power line when the capacitor bank is connected to the power line is prevented, the work life of the capacitors and switches is not impaired even under continual switching operations, and an inexpensive switchgear with less power loss during the conductive period can be realized. Moreover, the provision of a nonlinear resistor connected in parallel to the thyristor switch is effective for the suppression of an excessive voltage applied to the thyristor switch.

Claims: A switchgear comprising: a semiconductor switch (5; 12) for controlling an a.c. current to a load (3a, 3b, 3c; 13) by being operated to turn on in synchronism with the voltage phase of an a.c. power source (1; 11) so that no rush current flows to said load; and a first switch (6a, 6b, 6c; 15) connected between said semiconductor switch (5; 12) and said load (3a, 3b, 3c; 13) characterized by a second switch (2a, 2b, 2c; 16) connected in parallel to the serial connection of said semiconductor switch (5; 12) and said first switch (6a, 6b, 6c; 15). A switchgear according to claim 1, wherein said semiconductor switch (5; 12) comprises two anti-parallel connected thyristors (12a, 12b). A switchgear according to claim 1 or 2, characterized in that the first switch (6a, 6b, 6c; 15) is adapted to close in advance of the turn-on operation of said semiconductor switch (5; 12) when said load (3a, 3b, 3c; 13) is to be connected to said a.c. power source and adapted to open after said load has been disconnected from said a.c. power source by the operation of said semiconductor switch (5; 12) when said load is to be disconnected from said a.c. power source; and that the second switch (2a, 2b, 2c; 16) is adapted to close in advance of the turn-off operation of said semiconductor switch (5; 12) when said load (3a, 3b, 3c; 13) is to be connected to said a.c. power source and adapted to open after said semiconductor switch (5; 12) has turned on when said load is to be disconnected from said a.c. power source. A switchgear according to claim 1 or 2, characterized by only one semiconductor switch (5) for controlling a.c. currents to a plurality of loads (3a, 3b, 3c); a plurality of first switches (6a, 6b, 6c) each connected in series to the power supply line of each of said loads (3a, 3b, 3c) and adapted to close after said semiconductor switch (5) has turned-on so that the corresponding load is energized through said semiconductor switch (5) when said load is to be connected to said a.c. power source and adapted to open simultaneously to the turn-off operation of said semiconductor switch (5) when said load is to be disconnected from said a.c. power source; and a plurality of second switches (2a, 2b, 2c) each connected between said semiconductor switch (5) on the load side thereof and said load (3a, 3b, 3c) on the power supply side thereof and adapted to close initially and then open after said semiconductor switch (5) and corresponding first switch (6a, 6b, 6c) had once been conductive and subsequently said first switch has become open when said load is to be connected to said a.c. power source, and adapted to open after said first switch had opened and subsequently said semiconductor switch (5) has turned off to block a load current flowing therethrough when said load is to be disconnected from said a.c. power source. A switchgear according to any preceeding claim, further comprising: a nonlinear resistor (7) connected in parallel to said semiconductor switch (5).

Inventors: TAKEDA, MASATOSHI C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1985
Main classification: H01H9
IPC codes: H02H9, H01H9