Patent Application
20160322177
The present invention relates, in general, to a bidirectional direct current (DC) circuit breaker and, more particularly, to a bidirectional DC circuit breaker that breaks a bidirectional fault current flowing in a DC line for power transmission or distribution when a fault occurs on one side or the other side of the DC line.
Generally, a bidirectional DC circuit breaker is a switching device that may break current flowing through a high-voltage transmission line carrying 50 kV or more, such as a High-Voltage Direct Current (HVDC) transmission system. That is, the bidirectional DC circuit breaker is installed in a DC line and serves to break a fault current so as to prevent the supply thereof when a fault occurs on one side or the other side of the DC line. Of course, it may also be applied to an intermediate voltage DC distribution system for a DC voltage level that ranges approximately from 1 to 50 kV.
When a fault current occurs in a system, the bidirectional DC circuit breaker opens a main switch installed in a DC line so as to disconnect the circuit in which the fault occurred, whereby the fault current is interrupted. However, because there is no zero current point in a DC line, an arc, generated across the terminals of a main switch when the main switch is opened, is not extinguished. Accordingly, the fault current continuously flows through the arc, and thus the fault current cannot be interrupted.
In Japanese Patent Application Publication No. 1984-068128, illustrated in
However, according to this conventional art, because it is necessary to superpose a resonance current Ip, which is greater than the DC current Inc, the circuit rating must be more than two times that of the rated current. Also, in order to realize such a large resonance current Ip, multiple resonations are necessary, and thus the breaking speed is decreased. Also, the conventional DC circuit breaker has a problem in that it cannot interrupt a bidirectional fault current.
An object of the present invention is to provide a bidirectional DC circuit breaker that may completely break a fault current in a main switch even if the bidirectional DC circuit breaker does not apply a resonance current to the main switch.
Another object of the present invention is to provide a bidirectional DC circuit breaker that may break a fault current, flowing through an arc, by intentionally making a zero current point in order to eliminate the arc generated in a main switch when the main switch is opened.
A further object of the present invention is to provide a bidirectional DC circuit breaker that may break a bidirectional fault current in a high-voltage DC line.
In order to accomplish the above objects, a bidirectional DC circuit breaker according to the present invention includes a main switch, installed in the DC line, for breaking current of the DC line by being opened when a fault occurs on one side or the remaining side of the DC line; an L/C circuit, which is connected in parallel with the main switch (110) and includes a capacitor and a reactor, which are connected in series with each other; a switching element for charging, which is connected in series with the L/C circuit and is switched in order to supply a charging current for charging the capacitor; first and second switching elements for forming a first closed circuit between the L/C circuit and the main switch and switching a flow of current in a first direction; and third and fourth switching elements for forming a second closed circuit between the L/C circuit and the main switch and switching a flow of current in a second direction, wherein when the main switch is opened, current is supplied to the main switch in the first direction or the second direction by the voltage charged in the capacitor, whereby an arc generated in the main switch is extinguished.
In the present invention, a charging resistor (Rc), installed between the L/C circuit and a ground (GND), for charging the capacitor (131) using the charging current, is further included.
In the present invention, the switching element for charging and the first to fourth switching elements include a power semiconductor switch, turn-on/turn-off control of which is possible.
In the present invention, a diode, installed in the second closed circuit, for conducting current only in the second direction, is further included.
In the present invention, a nonlinear resistor connected in series with the diode in the second closed circuit is further included.
In the present invention, the first and second switching elements are connected with two ends of the L/C circuit and two ends of the main switch in the first closed circuit so as to conduct current in the first direction.
In the present invention, the third and fourth switching elements are connected with the two ends of the L/C circuit and the two ends of the main switch in the second closed circuit so as to conduct current in the second direction.
In the present invention, the switching element for charging is configured to conduct the charging current to the capacitor by being turned on in a state in which the main switch is closed, and to break the charging current from being supplied to the capacitor by being turned off when the main switch is opened due to a fault, which occurs on the one side or the remaining side.
In the present invention, if an arc is generated when the main switch is opened, the first and second switching elements are turned on, current is supplied to the main switch in the first direction by the voltage (Vc) charged in the capacitor, and the supplied current causes a zero current at the main switch, whereby the arc generated in the main switch is extinguished.
In the present invention, the current supplied to the main switch has a magnitude that is greater than that of a fault current and has a direction opposite to that of the fault current, which continuously flows through the arc in the main switch.
In the present invention, if an arc is generated when the main switch is opened, the third and fourth switching elements are turned on, current is supplied to the main switch in the second direction by the voltage (Vc) charged in the capacitor, and the supplied current causes a zero current at the main switch, whereby the arc generated in the main switch (110) is extinguished.
In the present invention, the current supplied to the main switch has a magnitude that is greater than that of a fault current and has a direction opposite to that of the fault current, which continuously flows through the arc in the main switch.
In the present invention, wherein when the arc is extinguished in the main switch, the switching element for charging is turned on so as to charge the capacitor, and when charging of the capacitor is completed, a closed circuit is formed by closing the main switch.
The present invention may quickly extinguish an arc that is generated when a main switch is switched in a bidirectional DC circuit breaker, whereby a fault current may be completely broken.
Also, the bidirectional DC circuit breaker according to the present invention may break a bidirectional fault current using a single circuit.
Also, according to the present invention, because a capacitor is discharged without reversing the polarity of voltage charged in the capacitor, the time taken to break a fault current may be reduced.
Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Also, detailed descriptions of known functions and configurations which have been deemed to make the gist of the present invention unnecessarily obscure will be omitted below.
Referring to
Here, a first nonlinear resistor 120 is connected in parallel with the main switch 110 in order to prevent excessive voltage, equal to or greater than a rated voltage, from being applied to the two ends of the bidirectional DC circuit breaker 100 when the main switch 110 is opened. When a high voltage equal to or greater than a preset reference voltage arises from a fault and is induced at the two ends of the bidirectional DC circuit breaker 100, the first linear resistor 120 is automatically turned on and consumes the high voltage. In the present embodiment, the nonlinear resistor 120 may be implemented as a varistor.
In the present embodiment, because a high voltage is induced in the DC line 10, a high current flows in the main switch 110. Accordingly, when the main switch 110 is opened due to the occurrence of a fault, an arc is generated across the switching terminals of the main switch 110, and a DC fault current continuously flows in the DC line 10 through the arc. Therefore, in the present invention, an additional device or circuit is required in order to completely break the fault current by extinguishing the arc.
Specifically, an L/C circuit 130 and a switching element for charging 140, first connected in series, are connected in parallel with the main switch 110. The L/C circuit 130 is configured to include a capacitor 131 and a reactor 132, which are connected in series. The switching element for charging 140 is switched in order to supply a charging current to the L/C circuit 130. Specifically, in a normal state in which the main switch 110 is closed, the switching element for charging 140 is turned on and supplies a charging current from A or B to the L/C circuit 130, whereby the capacitor 131 is charged. Then, when charging of the capacitor 131 is completed, the switch element for charging 140 is turned off because the current passing through it becomes zero. Accordingly, the charging current is prevented from being supplied to the capacitor 131. This is intended to supply the current, induced by the voltage +Vc already charged in the capacitor 131, to the main switch 110.
Meanwhile, first and second switching elements 150 and 160 are connected with the two ends of the L/C circuit 130, whereby a first closed circuit is formed through the L/C circuit 130 and the main switch 110. Here, the first and second switching elements 150 and 160 are switched in order for current to flow in the first direction along the first closed circuit by the voltage +Vc, charged in the capacitor 131 of the L/C circuit 130. Also, third and fourth switching elements 170 and 180 are connected with the two ends of the L/C circuit 130, whereby a second closed circuit is formed through the L/C circuit 130 and the main switch 110. The third and fourth switching elements 170 and 180 are switched in order for current to flow in the second direction along the second closed circuit by the voltage +Vc, charged in the capacitor 131. Another embodiment may further include a diode 190, installed in the second closed circuit in order to conduct current only in the second direction, and/or a second nonlinear resistor 200. Such a diode 190 conducts current in the second closed circuit only in the second direction but does not conduct current in the direction opposite thereto, whereby current is prevented from flowing to ground when the current flows in the first direction. Also, the second nonlinear resistor 200 is connected in series with the diode 190 in order to prevent energy at B from freewheelingly resonating after breaking a fault current in the main switch 110. Here, the first closed circuit differs from the second closed circuit. Also, both the first direction and the second direction mean the direction in which current flows, and they represent the flow of current in different directions.
The first and second switching elements 150 and 160 are installed in the same direction so that current flows in the first direction when they are turned on. Also, the third and fourth switching elements 170 and 180 are installed in the same direction so that current flows in the second direction when they are turned on. Here, the current, flowing in the first and second direction, is induced by the voltage +Vc charged in the capacitor 131 of the L/C circuit 130, and this current is supplied to the main switch 110 in the first direction or in the second direction so as to extinguish an arc generated in the main switch 110 when the main switch 110 is opened. To this end, this current must be supplied to the main switch 110 in the direction opposite that of the current that is flowing through the arc generated in the main switch 110. Therefore, the first direction or the second direction is selected depending on the direction in which the current flows through the arc in the main switch 110. In other words, when current needs to flow in the first direction, the first and second switching elements 150 and 160 are turned on, whereas when current needs to flow in the second direction, the third and fourth switching elements 170 and 180 are turned on. Of course, the turning on and off of the first and second switching elements 150 and 160 is implemented in the opposite way to the turning on and off of the third and fourth switching elements 170 and 180.
In the present embodiment, it is desirable for the first switching element 150 to be connected in parallel with the switching element for charging 140. Although not illustrated in the drawing, the (on/off) operations of the switching element for charging 140 and the first to fourth switching elements 150, 160, 170 and 180 are controlled by a control unit (not illustrated). These switching elements 140 to 180 may include, for example, power semiconductor switches. These power semiconductor switches are elements, the turn-on control of which is possible, and may be implemented as, for example, thyristors. Alternatively, as elements, the turn-on/turn-off control of which is possible, they may be implemented as, for example, a GTO, IGCT, IGBT, or the like.
Further, in the bidirectional DC circuit breaker 100 of the present embodiment, a charging resistor Rc is connected between the L/C circuit 130 and a ground GND. Through this charging resistor Rc, the capacitor 131 of the L/C circuit 130 is initially charged with a DC voltage Vc.
As shown in the drawing, the first and second switching elements 150 and 160 are connected with the two ends of the L/C circuit 130 and the two ends of the main switch 110 in the first closed circuit, thus conducting current in the first direction. Also, the third and fourth switching elements 170 and 180 are connected with the two ends of the L/C circuit and the two ends of the main switch 110 in the second closed circuit, thus conducting current in the second direction.
In
Therefore, as shown in (a), the current supplied from A flows along the DC line 10 and is delivered to B via the main switch 110. Also, the current supplied from A flows to the L/C circuit 130 and the charging resistor Rc through the switching element for charging 140. Accordingly, a DC voltage +Vc is charged in the capacitor 131 of the L/C circuit 130. For the convenience of description, the voltage charged in the capacitor 131 by the current supplied through the switching element for charging 140 is referred to as +Vc, as illustrated in (a) of
Further, in (b), because the main switch 110 is closed in the normal state, the current supplied from B flows along the DC line 10 and is delivered to A via the main switch 110. Here, the current supplied from B also flows to the L/C circuit 130 and the charging resistor Rc through the switching element for charging 140, which is turned on. Accordingly, the DC voltage +Vc is charged in the capacitor 131 of the L/C circuit 130.
As described above, because the switching element for charging 140 is turned on in the normal state, the capacitor 131 of the L/C circuit 130 is charged with the voltage +Vc by the current supplied from A or B.
First, in the normal state, as shown in
Here, in the state in which the third and fourth switching elements 170 and 180 are turned off, the first and second switching elements 150 and 160 are turned on, whereby the voltage +Vc charged in the capacitor 131 causes current to flow in the first direction through the first switching element 150, the main switch 110, and the second switching element 160. This current creates a zero current in the main switch 110, whereby the arc is extinguished. As described above, it is desirable for the current supplied to the main switch 110 in the first direction to have a magnitude that is greater than that of the fault current and to flow in the opposite direction to the fault current, which continuously flows through the arc in the main switch 110. To this end, the charging capacity of the capacitor may be determined.
After the fault current is breaked in the main switch 110 by extinguishing the arc generated in the main switch 110, the voltage at B rapidly increases compared to A. The voltage rapidly increased at B is consumed in the nonlinear resistor 120, which is connected in parallel with the main switch 110. At the same time, the switching element for charging 140 is turned on again, and some of the current is supplied to the L/C circuit 130 therethrough, whereby the capacitor 131 is recharged with +Vc.
Here, the bidirectional DC circuit breaker 100 of the present invention has a characteristic such that the operation of reclosing the main switch 110 is possible. In other words, when a fault at A is eliminated after opening the main switch 110, the control unit closes the main switch 110 so as to make a closed circuit again in the DC line 10. If the fault at A is not eliminated after the closed circuit is made by closing the main switch 110, the above-mentioned processes are repeated. This reclosing operation is possible because the capacitor 131 maintains a charged state with +Vc in the L/C circuit 130 after the arc is extinguished in the main switch 110.
As described above, the bidirectional DC circuit breaker 100 supplies current to the main switch 110 using the voltage charged in the capacitor 131 of the L/C circuit 130, whereby the arc generated in the main switch 110 is extinguished, and the fault current flowing through the arc is breaked. Meanwhile, when a fault occurs at B, the control unit detects the fault and opens the closed main switch 110, as illustrated in
Here, in the state in which the first and second switching elements 150 and 160 are turned off, the third and fourth switching elements 170 and 180 are turned on, whereby the voltage +Vc charged in the capacitor 131 causes current to flow in the second direction through the third switching element 170, the main switch 110, and the fourth switching element 180. In another embodiment, a diode 190 may be additionally installed in the second closed circuit, and when such a diode 190 is installed, the current flows in the second direction through the third switching element 170, the diode 190, the main switch 110, and the fourth switching element 180. The diode 190 prevents current from being conducted in the direction opposite to the second direction. This is intended to prevent current from flowing to the ground GND in the direction opposite to the second direction when the flow of current is generated in the first direction. The current supplied in the second direction causes a zero current in the main switch 110, whereby the arc is extinguished. As described above, it is desirable for the current supplied to the main switch 110 in the second direction to have a magnitude that is greater than that of the fault current and to have the opposite direction to the fault current, which continuously flows through the arc in the main switch 110.
After the fault current is breaked in the main switch 110 by extinguishing the arc generated in the main switch 110, the voltage at A rapidly increases compared to B. The voltage rapidly increased at A is consumed in the nonlinear resistor 120, which is connected in parallel with the main switch 110. At the same time, the switching element for charging 140 is turned on again, and some of the current is supplied to the L/C circuit 130 therethrough, whereby the capacitor 131 is recharged with +Vc. Also, the application of the nonlinear resistor 200, which is connected in series with the diode 190, may eliminate a problem in which energy at B freewheelingly resonates due to the diode 190 after the fault current in the main switch 110 is breaked.
Here, in the bidirectional DC circuit breaker 100 of the present invention shown in
As described above, the bidirectional DC circuit breaker 100 according to the present invention is characterized in that it uses a switching element for charging 140 rather than using the main switch CB, as in the conventional art, which is illustrated in
Although the present invention has been described above in connection with the preferred embodiments, the present invention is not limited to the above embodiments. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Accordingly, the scope of the present invention should be defined by the scope of the appended claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2013-0163719 | Dec 2013 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2014/012857 | 12/24/2014 | WO | 00 |
