DIRECT-CURRENT POWER TRANSMISSION PROTECTION DEVICE, CONVERTER AND PROTECTION METHOD

Abstract

The present invention discloses a direct-current power transmission protection device, including a resistor unit and a bidirectional circulation current switch unit, and the resistor unit and the bidirectional circulation current switch unit being connected in parallel to form the protection device, where at least one resistor is cascaded to form the resistor unit, and at least one bidirectional circulation current switch is cascaded to form the bidirectional circulation current switch unit. In addition, the present invention also provides a converter including the protection device, and at least one protection device is connected in series in each phase unit. The present invention provides the protection device, the converter and a protection method for flexible direct-current power transmission, which can rapidly and effectively suppress direct-current short circuit current and damp current oscillation, and not only can better protect equipment safety, but also can greatly shorten fault current attenuation time, thus shortening direct-current shutdown time and further decreasing economic loss and system destabilization risk caused by shutdown to the max. Moreover, the protection device is structurally simple and low in cost, and has good implementability and economic efficiency.

Claims

1. A direct-current power transmission protection device, characterized by comprising a resistor unit and a bidirectional circulation current switch unit, and the resistor unit and the bidirectional circulation current switch unit being connected in parallel to form the protection device, wherein at least one resistor is cascaded to form the resistor unit, and at least one bidirectional circulation current switch is cascaded to form the bidirectional circulation current switch unit.

2. The protection device of claim 1, characterized in that a diode unit and a switching transistor unit are connected in parallel to form the bidirectional circulation current switch, wherein an anode of the switching transistor unit, a cathode of the diode unit and one end of the resistor unit are connected together, defined as a positive terminal of the protection device; a cathode of the switching transistor unit, an anode of the diode unit and the other end of the resistor unit are connected together, defined as a negative terminal of the protection device; and at least one diode is cascaded to form the diode unit, and at least one switching transistor is cascaded to form the switching transistor unit.

3. The protection device of claim 2, characterized in that the switching transistor is a power semiconductor device with a turn-off function.

4. The protection device of claim 2, characterized in that an IGBT is adopted as the bidirectional circulation current switch, a collector of the IGBT serves as an anode of the bidirectional circulation current switch, and an emitter of IGBT serves as a cathode of the bidirectional circulation current switch; or at least one IGCT or at least one GTO is adopted to form the bidirectional circulation current switch, an anode of the IGCT or the GTO serves as an anode of the bidirectional circulation current switch, and a cathode of the IGCT or the GTO serves as a cathode of the bidirectional circulation current switch; or an MOSFET is adopted to form the bidirectional circulation current switch, a drain of the MOSFET serves as an anode of the bidirectional circulation current switch, and a source of the MOSFET serves as a cathode of the bidirectional circulation current switch.

5. The protection device of claim 1, characterized by also being provided with a corresponding cooling device.

6. The protection device of claim 1, characterized by also comprising a voltage protection element connected in parallel between the anode and the cathode of the protection device.

7. The protection device of claim 1, characterized by also comprising a bypass switch element connected in parallel between the anode and the cathode of the protection device.

8. The protection device of claim 1, characterized in that a value range of the resistor R is between 0.1 ohm and 100 ohm.

9. A converter comprising the protection device of claim 1, wherein the converter comprises three phases, each phase comprises an upper bridge arm and a lower bridge arm, and a reactor unit and at least one submodule is connected in series to form each bridge arm; the upper bridge arm and the lower bridge arm of each phase are combined together to form a phase unit, a joint of the upper bridge arm and the lower bridge arm is a midpoint, and leading-out terminals of the three upper bridge arms are connected together to serve as a positive terminal of the converter; the leading-out terminals of the three lower bridge arms are connected together to serve as a negative terminal of the converter; and the converter is characterized in that the at least one protection device is connected in series in each phase unit.

10. The converter of claim 9, characterized in that connecting the at least one protection device in series specifically means that the at least one protection device is connected in series between the reactor and the submodule, or is connected in series between the reactor and the midpoint, or is connected in series between the two submodules, or is connected in series between the submodule and the positive terminal of the converter, or is connected in series between the submodule and the negative terminal of the converter, or is connected in series at all the foregoing positions on the upper bridge arm and the lower bridge arm.

11. The converter of claim 9, characterized in that the numbers of the protection devices connected in series in each phase units are equal.

12. The converter of claim 9, characterized in that the converter is applicable to a voltage source topology, and is a modularized multi-level flexible direct-current system, a two-level flexible direct-current system or a three-level flexible direct-current system.

13. A protection method of the converter of claim 9, characterized by comprising the following steps: detecting and determining whether a direct-current side short circuit fault occurs, and if so, applying a turn-off signal to the bidirectional circulation current switches of all the protection devices connected in series in the bridge arms; and after fault current attenuation is complete, recovering system operation.

14. The converter of claim 10, characterized in that the numbers of the protection devices connected in series in each phase units are equal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a modularized multi-level flexible direct-current power transmission topology;

[0031] FIG. 2 is a unit structure diagram of a protection device;

[0032] FIG. 3 is a structural schematic diagram of a converter comprising the protection devices; and

[0033] FIG. 4 is a schematic diagram of a short circuit fault of the converter comprising the protection devices.

DETAILED DESCRIPTION OF THE INVENTION

[0034] In reference to the drawings, the technical solution of the present invention is described in detail below.

[0035] FIG. 1 shows the topological structure of a converter. The converter includes three phases, each phase comprises an upper bridge arm and a lower bridge arm, and the converter comprises six bridge arms in total. A reactor L and N submodules SM are connected in series to form each bridge arm, and the upper bridge arm and lower bridge arm of each phase are combined together to form a phase unit. A joint of the upper bridge arm and the lower bridge arm is a midpoint. Leading-out terminals of the three upper bridge arms are connected together to serve as a positive terminal of the converter; and the leading-out terminals of the three lower bridge arms are connected together to serve as a negative terminal of the converter.

[0036] The present invention provides a protection device, which comprises a resistor unit and a bidirectional circulation current switch unit, and the resistor unit and the bidirectional circulation current switch unit are connected in parallel to form the protection device. The resistor unit may be one resistor, or may be formed by a plurality of resistors cascaded in a way of serial connection or parallel connection. Likewise, the bidirectional circulation current switch unit may also be formed by cascading at least one bidirectional circulation current switch.

[0037] Further, a diode unit and a switching transistor unit are connected in parallel to form the bidirectional circulation current switch. An anode of the switching transistor unit, a cathode of the diode unit and one end of the resistor unit are connected together, defined as a positive terminal of the protection device. A cathode of the switching transistor unit, an anode of the diode unit and the other end of the resistor unit are connected together, defined as a negative terminal of the protection device. At least one diode is cascaded to form the diode unit, and at least one switching transistor is cascaded to form the switching transistor unit. The switching transistor is a power semiconductor device with a turn-off function.

[0038] An IGBT also may be adopted as the bidirectional circulation current switch, a collector of the IGBT serves as an anode of the bidirectional circulation current switch, and an emitter of the IGBT serves as a cathode of the bidirectional circulation current switch.

[0039] Alternatively, at least one IGCT or at least one GTO plus other auxiliary devices is adopted to form the bidirectional circulation current switch, an anode of the IGCT or gate-turn-off thyristor (GTO) serves as an anode of the bidirectional circulation current switch, and a cathode of the IGCT or GTO serves as a cathode of the bidirectional circulation current switch.

[0040] Alternatively, the bidirectional circulation current switch may also be composed of an MOSFET plus other auxiliary devices, a drain of the metal-oxide-semiconductor field-effect transistor (MOSFET) serves as an anode of the bidirectional circulation current switch, and a source of the MOSFET serves as a cathode of the bidirectional circulation current switch.

[0041] In addition, the current of the bridge arms may flow through the switching transistors of the protection devices, so the protection device may also be provided with a corresponding cooling device. Preferably, water cooling radiation may be adopted.

[0042] In addition, a voltage protection element may also be connected in parallel between the anode and cathode of the protection device to suppress the over-voltage of both ends of the switch.

[0043] In addition, a bypass switch element may also be connected in parallel between the anode and cathode of the protection device in order to conveniently cut off the protection device.

[0044] In addition, a value range of the resistor R in the protection device is between 0.1 ohm and 100 ohm.

[0045] FIG. 2 shows a preferred embodiment of the protection device, which includes a resistor, a switching transistor and a diode. An anode of a switching transistor T, a cathode of a diode and one end of a resistor are connected together, defined as a positive terminal of the protection device, and a cathode of the switching transistor T, an anode of the diode and the other end of the resistor are connected together, defined as a negative terminal of the protection device.

[0046] It should be noted that the resistor unit may be one resistor, or may be formed by a plurality of resistors constructed in a cascading way such as serial connection and parallel connection, but is not limited to an actual resistor. Likewise, the switching transistor may be formed by a plurality of cascaded switching transistors, and the diode may be formed by a plurality of cascaded diodes. FIG. 2 is an equivalent circuit diagram of the embodiment of the protection device.

[0047] The present invention also provides a converter including the protection device. The converter includes three phases, each phase includes an upper bridge arm and a lower bridge arm, and a reactor unit and at least one submodule are connected in series to form each bridge arm. The upper bridge arm and the lower bridge arm of each phase are combined together to form a phase unit, a joint of the upper bridge arm and the lower bridge arm is a midpoint, and leading-out terminals of the three upper bridge arms are connected together to serve as a positive terminal of the converter; the leading-out terminals of the three lower bridge arms are connected together to serve as a negative terminal of the converter; and the converter is characterized in that at least one protection device is connected in series in each phase unit.

[0048] Preferably, the numbers of the protection devices connected in series in each phase units are equal.

[0049] The protection devices may be connected in series at any positions in the phase units, and for example, the protection devices may be connected in series between the reactor and the submodules or is connected in series between the reactor and the midpoint, or is connected in series between the two submodules, or is connected in series between the submodule and the positive terminal of the converter, or is connected in series between the submodule and the negative terminal of the converter.

[0050] Moreover, the protection devices may be separately connected in series in the upper bridge arms or the lower bridge arms, or may be connected in series at symmetric or asymmetric positions in the upper bridge arms and the lower bridge arms.

[0051] The protection devices connected in series in the phase units may be connected in series at corresponding or non-corresponding positions.

[0052] The converter may be a modularized multi-level flexible direct-current system, a two-level flexible direct-current system or a three-level flexible direct-current system.

[0053] A preferred embodiment as shown in FIG. 3 means that the protection devices are connected in series in the converter given by FIG. 1, and one protection device is connected in series between the submodule and the reactor of each of the three upper bridge arms.

[0054] The serial connection mode of the protection devices in the converter provided in the present invention is a serial connection at the alternating-current side of the converter. Thus, the beneficial effects are as follows: (1) The protection device and the converter may be designed integrally, having good engineering implementability and saving spaces. The protection device may adopt a modularized design concept and may readily adopt standardized design and assembly together with the submodule of the converter. The shape, size and wiring of the protection device are consistent with those of the submodule, the protection device is directly installed in the converter, and therefore does not occupy an extra space, and this advantage is very important in flexible direct-current power transmission projects highly stressing the compact design. (2) Separate energy access is not required. The switching transistor in the protection device and a control loop thereof need proper power supply, and are installed at the alternating-current side, and the protection devices may share an energy access loop with a converter valve. (3) The protection device may share a cooling device with other devices rather than be equipped with a separate cooling device. The high-power power electronic device in the protection device needs to be cooled by water cooling circulation, and is installed at the alternating-current side of the converter valve, and the protection device may share a water cooling device with the converter valve. (4) The structure is simple, the size is small, and the cost is low. Compared with the direct-current side, the effective current value of the alternating-current side is small, and the switching transistor of the same model in the converter, rather than a multi-switching transistor parallel structure adopted to increase the rated current value, may be chosen as the switching transistor of the protection device. (5) The effect of suppressing current oscillation is better. Theoretical analysis and an experiment result show that the protection device connected in series at the alternating-current side of the converter has a good suppression effect on current oscillation at the initial stage of a fault, and can effectively suppress the fault current peak of the bridge arms and decrease the current stress of the submodules of the whole bridge arms.

[0055] For the converter including the protection device provided in the present invention, when the positive terminal of the converter is short-circuited with the transmission line or other equipment connected with the cathode, high short circuit current may flow through the respective bridge arms, plus influences by capacitors and inductors in the whole loop, the short circuit current may further be oscillated, and as a result, the safety of the entire system equipment is endangered severely. FIG. 4 is a schematic diagram of the occurring short-circuiting.

[0056] Aiming at this problem, the present invention also provides a protection method for suppressing direct-current short circuit current and damping current oscillation by utilizing the protection device, which includes the following steps:

[0057] (1) Detect and determine whether a direct-current side short circuit fault occurs, and if so, go to step (2).

[0058] (2) Apply a turn-off signal to bidirectional circulation current switches of all protection devices connected in series in the bridge arms.

[0059] (3) Detect and determine whether fault current attenuation is complete, and if so, go to step (4).

[0060] (4) Recover the operation of the flexible direct-current power transmission system.

[0061] Cascading connection called in the present invention includes connection modes such as serial connection and parallel connection.

[0062] The foregoing embodiments are merely intended to illustrate the technical idea of the present invention, and cannot limit the protection scope of the present invention, and any modifications which are made on the basis of the technical solution according to the technical concept put forward by the present invention shall fall into the protection scope of the present invention.