A STARTING METHOD AND STOPPING METHOD FOR A STATIC SYNCHRONOUS SERIES COMPENSATOR

Abstract

A starting and stopping method for a static synchronous series compensator (SSSC) is provided. A starting process includes: first, connecting a converter (4) to a shunt transformer (2), and closing an incoming line switch (1) of the shunt transformer to charge the current converter until a direct-current voltage is stable; then, opening the incoming line switch of the shunt transformer, and connecting the converter to a series transformer (5) through a change-over switch (3); deblocking the converter, and controlling a current of a bypass switch (6) of the series transformer to approach zero; further, opening the bypass switch of the series transformer, and enabling the converter to enter a normal operation mode after a line is stable; and in stopping the SSSC, first, switching a control mode of the converter to make the current of the bypass switch approaches zero when closed the bypass switch, then controlling a current of the series transformer to be gradually reduced to zero to make the series transformer out of service, and then blocking the converter. By means of the method, an overvoltage and an overcurrent of a converter when a series transformer is put into use and exits can be effectively suppressed, and current oscillation of an line and impact on the series transformer can be reduced, thereby achieving stable starting and stopping of an SSSC.

Claims

1. A starting method for a static synchronous series compensator (SSSC), the SSSC comprising a converter, a shunt transformer, a series transformer, and at least one bypass switch of the series-side transformer, the starting method comprising the following steps: (a1) setting an initial status of starting the SSSC; (a2) connecting the converter to the shunt transformer, closing an incoming line switch of the shunt transformer, and charging the converter, entering step (a3) after the charging ends; (a3) disconnecting the converter from the shunt transformer, and connecting the converter to the series transformer, entering step (a4) after stabilization; (a4) deblocking the converter in a zero current control mode of the bypass switch; (a5) controlling a current of the bypass switch of the series transformer to be gradually reduced below a current threshold, and opening the bypass switch of the series transformer; and (a6) after a line is stably operated, enabling the SSSC to enter a normal operation mode, thereby completing a starting process.

2. The starting method for an SSSC according to claim 1, wherein setting the initial status in step (a1) comprises: disconnecting the converter from both the shunt transformer and the series transformer, closing the bypass switch of the series transformer, and opening the incoming line switch of the shunt transformer.

3. The starting method for an SSSC according to claim 1, wherein the connection is implemented by using a mechanical switch or a switch that is formed of power electronics.

4. The starting method for an SSSC according to claim 1, wherein if a start circuit is connected in series on a valve side of the shunt transformer, in the starting step (a2), the start circuit is first put in use before closing the incoming line switch of the shunt transformer, and the start circuit then exits after the charging ends.

5. The starting method for an SSSC according to claim 4, wherein the start circuit connected in series comprises a resistor and a switch that are connected in parallel.

6. The starting method for an SSSC according to claim 1, wherein the starting method is applicable to the following voltage source converter topologies, comprising, a two-level topology, a three-level topology, a multi-transformer topology, or a modular multi-level topology.

7. The starting method for an SSSC according to claim 1, wherein the zero current control mode of the bypass switch in the starting step (a4) is used to control the current that flows through the bypass switch to approach zero.

8. The starting method for an SSSC according to claim 1, wherein in the starting steps (a4) and (a5), the bypass switch of the series transformer is a mechanical switch or a switch that is formed of power electronics.

9. The starting method for an SSSC according to claim 1, wherein the current threshold in the starting step (a5) is an allowable maximum current value for the bypass switch of the series transformer to be normally opened.

10. The starting method for an SSSC according to claim 1, wherein the starting method is applicable to an interline power flow controller, at least one converter is connected to a shunt transformer, and starting is performed according to the foregoing steps.

11. A stopping method for a static synchronous series compensator (SSSC), the SSSC comprising a converter, a series transformer, and at least one bypass switch of the series-side transformer, the stopping method comprising the following steps: (b1) switching the converter to a zero current control mode of the bypass switch, entering step (b2) after stabilization; (b2) closing the bypass switch of the series transformer, entering step (b3) after stabilization; (b3) controlling a current of the series transformer to be gradually reduced below a current threshold, entering step (b4) after stabilization; and (b4) blocking the converter, and making the series transformer out of service from an alternating-current line, thereby completing a stopping process.

12. The stopping method for an SSSC according to claim 11, wherein the starting and stopping method is applicable to the following voltage source converter topologies, comprising, a two-level topology, a three-level topology, a multi-transformer topology, or a modular multi-level topology.

13. The stopping method for an SSSC according to claim 11, wherein the zero current control mode of the bypass switch in the stopping step (b1) is used to control the current that flows through the bypass switch to approach zero.

14. The stopping method for an SSSC according to claim 11, wherein in the stopping steps (b1) and (b2), the bypass switch of the series transformer is a mechanical switch or a switch that is formed of power electronics.

15. The stopping method for an SSSC according to claim 11, wherein the current threshold in the stopping step (b3) is an allowable maximum current value for the bypass switch of the series transformer to be normally opened.

16. The stopping method for an SSSC according to claim 11, wherein the stopping method is applicable to an interline power flow controller, at least one converter is connected to a shunt transformer, and stopping is performed according to the foregoing steps.

17. The starting method for an SSSC according to claim 2, wherein the connection is implemented by using a mechanical switch or a switch that is formed of power electronics.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a single-phase equivalent structural diagram of an SSSC according to an embodiment of the present invention;

[0031] FIG. 2 is a single-phase equivalent structural diagram of an SSSC after a start circuit is added according to an embodiment of the present invention; and

[0032] FIG. 3 is a flowchart of starting and stopping according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The following detailed description of the present invention is made with reference to the accompanying drawings.

[0034] The present invention provides a starting and stopping method for an SSSC. The SSSC is shown in FIG. 1. In FIG. 1, 1 represents an incoming line switch of a shunt transformer, 2 represents the shunt transformer, 3 represents a change-over switch, 4 represents a converter, 5 represents a series transformer, 6 represents a bypass switch connected in parallel to a line-side winding of the series transformer, and 7 represents a reactance such as a line value. An alternating-current side of the converter 4 is respectively connected to the shunt transformer 2 and the series transformer 5 through the change-over switch 3.

[0035] FIG. 3 is a flowchart of starting and stopping modes an SSSC according to the present invention.

[0036] The starting method has the steps of: first, connecting the converter 4 to an alternating-current system through the shunt transformer 2 for charging, then connecting the converter 4 to the series transformer 5, deblocking the converter 4, and then putting the series transformer 5 in use. The starting steps are as follows:

[0037] (a1) setting an initial status of starting the SSSC: the converter 4 is disconnected from both the shunt transformer 2 and the series transformer 5, the bypass switch 6 of the series transformer is closed, and the incoming line switch 1 of the shunt transformer is opened;

[0038] (a2) connecting the converter 4 to the shunt transformer 2, closing the incoming line switch 1 of the shunt transformer, and charging the converter 4 in an uncontrolled manner, entering step (a3) after the charging ends;

[0039] as shown in FIG. 2 according to another embodiment of the present invention, a start circuit 8 may be connected in series on a valve side of the shunt transformer 2, the start circuit 8 is first put in use before closing the incoming line switch 1 of the shunt transformer, so as to reduce impact on the converter 4 when the shunt transformer 2 is put in use, and the start circuit 8 exits after the shunt transformer 2 is put in use; the start circuit 8 that is connected in series includes a resistor and a switch that are connected in parallel;

[0040] (a3) disconnecting the converter 4 from the shunt transformer 2, and connecting the converter 4 to the series transformer 5, entering step (a4) after stabilization;

[0041] (a4) deblocking the converter 4 in a zero current control mode of the bypass switch;

[0042] (a5) controlling a current of the bypass switch of the series transformer 5 to be gradually reduced below a current threshold, and opening the bypass switch 6 of the series transformer; and

[0043] (a6) after the line is stably operated, enabling the SSSC to enter a normal operation mode, thereby completing a starting process.

[0044] The stopping method includes the following steps:

[0045] (b1) switching the converter 4 to a zero current control mode of the bypass switch, entering step (b2) after stabilization;

[0046] (b2) closing the bypass switch 6 of the series transformer, entering step (b3) after stabilization;

[0047] (b3) controlling a current of the series transformer 5 to be gradually reduced below a current threshold, entering step (b4) after stabilization; and

[0048] (b4) blocking the converter 4, and making the series transformer 5 out of service from an alternating-current line, thereby completing a stopping process.

[0049] The foregoing connection is implemented by using a mechanical switch or a switch that is formed of power electronics, for example, the change-over switch 3 shown in FIG. 1 and FIG. 2.

[0050] The foregoing starting and stopping method is applicable to an SSSC using various voltage source converter topologies such as a two-level topology, a three-level topology, a multi-transformer topology, or a modular multi-level topology.

[0051] The zero current control mode of the bypass switch in the foregoing starting step (a4) and stopping step (b1) is used to control the current that flows through the bypass switch to approach zero.

[0052] In the foregoing starting steps (a4) and (a5) and stopping steps (b1) and (b2), the bypass switch 6 of the series transformer may be a mechanical switch or a switch that is formed of power electronics.

[0053] In this embodiment, the current threshold in the starting step (a5) and the stopping step (b3) is an allowable maximum current value for the bypass switch 6 of the series transformer to be normally opened. For different projects or different bypass switches, the current thresholds are different, which is well-known in the art and a description thereof will be omitted.

[0054] The foregoing starting and stopping method is applicable to an interline power flow controller, at least one converter is connected to a shunt transformer, and starting and stopping are performed according to the foregoing steps.

[0055] It should be noted that the elements such as reactors, resistors, and switches in the present invention mostly refer to equivalent elements. That is, equivalent elements in a circuit structure may be separate elements, or may be formed of multiple same elements that are cascaded (in series or in parallel). For any equivalent element in the embodiments of the present invention, any equivalent circuit that can implement the same function should fall within the protection scope of the embodiments of the present invention.

[0056] Finally, it should be noted that the foregoing embodiments are only used to describe the technical solutions of the present invention rather than to limit the present invention. It should be understood that changes or equivalent substitutions can be made to the detailed description of the present invention by a person skilled in the art. However, these changes or variations all fall within the scope of the claimed patent.