CN105703370B - Unified power flow controller combining series compensation and current converter - Google Patents

Abstract

The invention provides a unified power flow controller that combines series compensation and a converter. One end of the first converter is connected in parallel with one side winding of the series transformer, and the other end is connected in sequence to the second converter and the parallel transformer; The other side winding of the series transformer is connected to the transmission line; the reactor unit is connected in series with the first switch and then connected in parallel with the first converter; the capacitor unit is connected in series with the second switch and then connected in parallel with the first converter. Compared with the existing technology, the present invention provides a unified power flow controller that combines series compensation and an inverter, which can provide inductive fixed offset adjustment by a reactor and capacitive fixed offset adjustment by a capacitor. It can be used in transmission lines or distribution lines to perform capacitive or inductive adjustment, increase line transmission capacity, improve system stability, control line flow, enhance system damping, and reduce costs. And the reactor unit and capacitor unit are arranged on the converter side, and the insulation level of the two is relatively reduced.

Description

A unified power flow controller combining series compensation and converter

Technical field

The invention relates to the field of power electronics technology, and in particular to a unified power flow controller that combines series compensation and an inverter.

Background technique

With the vigorous development of the power system, problems such as large-scale access to new energy sources, increasingly complex grid structures, uneven power flow distribution, and insufficient voltage support capabilities have brought new challenges to the safe and stable operation of the power grid. Power supply bottlenecks have appeared in some areas and cannot meet the needs of load development. Judging from the actual situation of the power grid, uneven distribution of power flow is an important factor restricting the transmission capacity of the power grid. The traditional power grid lacks effective power flow regulation methods. It is a realistic and ideal choice to use the new FACTS (Flexible Alternative Current Transmis System) device to improve system operating conditions and increase power grid transmission capacity.

Unified Power Flow Controller (UPFC), as a representative of the third generation FACTS equipment, is the most comprehensive FACTS device to date. It can realize parallel compensation, series compensation, phase shifting, terminal voltage adjustment, etc. separately or simultaneously. a basic function. UPFC can not only realize power flow regulation in terms of power system stability, reasonably control active power and reactive power, improve the transmission capacity of lines, and achieve optimal operation; it can also dynamically support the voltage of access points through fast reactive power throughput. , improve the system voltage stability; it can also improve the system damping and improve the power angle stability.

Conventional unified power flow controller has flexible and excellent control functions, but its promotion and use in power systems does not have the capacity and price advantages. Traditional capacitors and reactors can be used as line series compensation, but their flexibility and action speed cannot meet the requirements for precise adjustment. The unified power flow controller requires more flexible function configuration, and series compensation needs to be combined with the unified power flow controller.

Contents of the invention

In order to meet the needs of the existing technology, the present invention provides a unified power flow controller that combines series compensation and an inverter.

The technical solution of the present invention is:

The unified power flow controller includes a reactor unit, a capacitor unit, a first converter, a second converter, a first switch, a second switch, a third switch, a series transformer and a parallel transformer;

One end of the first converter is connected in parallel with one side winding of the series transformer, the other end is connected to the second converter and the parallel transformer in sequence, and the other end of the parallel transformer is connected to the busbar or transmission line; The other side winding of the series transformer is connected to the transmission line, and the winding is connected in parallel with the third switch;

The reactor unit is connected in series with the first switch and then in parallel with the first converter, and the other end of the first switch is connected to the series transformer;

The capacitor unit is connected in series with the second switch and then in parallel with the first converter, and the other end of the second switch is connected to the series transformer.

The preferred embodiment further provided by the present invention is:

The capacitor unit includes one capacitor or a series capacitor group composed of multiple capacitors or a parallel capacitor group composed of multiple capacitors; the parallel capacitor group includes an automatic switching switch, and the automatic switching switch is used for the parallel capacitor group The capacitors are put in and out to realize the capacitance adjustment of the parallel capacitor bank.

The preferred embodiment further provided by the present invention is:

The first switch includes any one of a circuit breaker, an isolation switch and a power electronic switch or any two switches connected in parallel; the first switch is used to control the input and withdrawal of the reactor unit;

The second switch includes any one of a circuit breaker, an isolation switch and a power electronic switch or any two switches connected in parallel; the second switch is used to control the input and withdrawal of the capacitor unit.

The third switch includes any one of a circuit breaker, an isolation switch and a power electronic switch or any two switches connected in parallel; the third switch is used to control the input and withdrawal of the series transformer.

The preferred embodiment further provided by the present invention is:

When the first switch and the second switch include the power electronic switch, the power electronic switch includes a thyristor bidirectional switch, and the thyristor bidirectional switch includes an anti-parallel thyristor;

The thyristor bidirectional switch of the first switch is also used to adjust the equivalent impedance of the reactor unit by changing the firing angle of its thyristor.

A further preferred embodiment provided by the present invention is that the unified power flow controller includes a fixed capacitive offset structure and a fixed inductive offset structure.

The preferred embodiment further provided by the present invention is:

When the unified power flow controller is a fixed capacitive offset structure: it includes a capacitor unit, a first inverter, a second inverter, a second switch, a third switch, a series transformer and a parallel transformer;

One end of the first converter is connected in parallel with one side winding of the series transformer, the other end is connected to the second converter and the parallel transformer in sequence, and the other end of the parallel transformer is connected to the busbar or transmission line; The other side winding of the series transformer is connected to the transmission line, and the winding is connected in parallel with the third switch;

The capacitor unit is connected in series with the second switch and then in parallel with the first converter, and the other end of the second switch is connected to the series transformer.

The preferred embodiment further provided by the present invention is:

When the unified power flow controller is a fixed inductive offset structure: it includes a reactor unit, a first converter, a second converter, a first switch, a third switch, a series transformer and a parallel transformer;

One end of the first converter is connected in parallel with one side winding of the series transformer, the other end is connected to the second converter and the parallel transformer in sequence, and the other end of the parallel transformer is connected to the busbar or transmission line; The other side winding of the series transformer is connected to the transmission line, and the winding is connected in parallel with the third switch;

The reactor unit is connected in series with the first switch and then in parallel with the first converter, and the other end of the first switch is connected to the series transformer.

A further preferred embodiment provided by the present invention is: when the first switch is closed and the second switch and the third switch are opened, the unified power flow controller is used to realize inductive offset compensation; the second switch is closed, Turn off the first switch and the third switch, and the unified power flow controller is used to implement capacitive offset compensation; turn off the first switch, the second switch, and the third switch, and the unified power flow controller is used to implement No offset compensation.

The preferred embodiment further provided by the present invention is:

When the power electronic switch included in the first switch is a thyristor bidirectional switch, close the second switch to directly connect the capacitor unit in parallel with the first converter, and open the third switch to change the triggering of the thyristor in the thyristor bidirectional switch. angle, thereby performing capacitive compensation or inductive compensation on the transmission line:

When the firing angle of the thyristor in the thyristor bidirectional switch is changed, the equivalent reactance of the reactor unit is less than the equivalent capacitive reactance of the capacitor unit, then the unified power flow controller works in the capacitive compensation mode, and the Capacitive compensation is performed on the above transmission lines;

When the firing angle of the thyristor in the thyristor bidirectional switch is changed, the equivalent reactance of the reactor unit is greater than the equivalent capacitive reactance of the capacitor unit, then the unified power flow controller works in the inductive compensation mode, and the Transmission lines are inductively compensated.

A further preferred embodiment provided by the present invention is: the first converter and the second converter both include a two-level converter, a three-level converter, a diode clamped converter, and a flying capacitor. Any one or more of type converters, modular multi-level converters and H-bridge cascade multi-level converters.

Compared with the closest existing technology, the beneficial effects of the present invention are:

1. The invention provides a unified power flow controller that combines series compensation and a converter, which can be used in transmission lines or distribution lines to perform capacitive or inductive adjustment, increase line transmission capacity, improve system stability, and control Line power flow, enhanced system damping, solves the problem that the existing unified power flow controller cannot be adjusted continuously and quickly in a wide range, and reduces the cost of the unified power flow controller.

2. The invention provides a unified power flow controller that combines series compensation and a converter, which reduces the insulation level of the reactor and capacitor, improves the dynamic response performance, and at the same time reduces the capacity of the voltage source converter.

3. The present invention provides a unified power flow controller that combines series compensation with an inverter. The reactor unit can adjust the capacitance capacity through automatic switching. The thyristor bidirectional switch in parallel with the reactor unit can continuously adjust the reactance. It realizes hierarchical and continuous adjustment of equivalent capacitive reactance, focusing on steady-state control; the converter can provide continuous and rapid bidirectional adjustment capabilities, focusing on dynamic control.

Description of the drawings

Figure 1: Schematic structural diagram of a unified power flow controller combining series compensation and an inverter in an embodiment of the present invention;

Figure 2: Schematic structural diagram of another unified power flow controller combining series compensation and an inverter in an embodiment of the present invention;

Figure 3: Schematic structural diagram of yet another unified power flow controller combining series compensation and an inverter in the embodiment of the present invention;

Figure 4: Schematic diagram of the connection between the reactor unit and the capacitor unit in the embodiment of the present invention;

Figure 5: Schematic diagram of the equivalent impedance range of the converter in the embodiment of the present invention;

Figure 6: Schematic diagram of the compensation range of a unified power flow controller combining series compensation and an inverter in an embodiment of the present invention;

Among them, 101: reactor unit; 102: capacitor unit; 103: first converter; 104: series transformer; 105: second converter; 106: parallel transformer.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention.

A unified power flow controller combining series compensation and an inverter provided by an embodiment of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic structural diagram of a unified power flow controller that combines series compensation and an inverter in an embodiment of the present invention. As shown in the figure, the unified power flow controller that combines series compensation and an inverter in this embodiment includes:

Reactor unit 101, capacitor unit 102, first converter 103, series transformer 104, second converter 105, parallel transformer 106, first switch, second switch and third switch. in,

One end of the first converter 103 is connected in parallel with one side winding of the series transformer 104, and the other end is connected to the second converter 105 and the parallel transformer 106 in sequence. The other end of the parallel transformer 106 is connected to the busbar or transmission line. ; The other side winding of the series transformer 104 is connected to the transmission line, and the winding is connected in parallel with the third switch.

The reactor unit 101 is connected in series with the first switch and then in parallel with the first converter 103 , and the other end of the first switch is connected to the series transformer 104 .

The capacitor unit 102 is connected in series with the second switch and then in parallel with the first inverter 103. The other end of the second switch is connected to the series transformer 104.

In the embodiment of the present invention, the capacitor unit 102 includes one capacitor or a series capacitor group composed of multiple capacitors or a parallel capacitor group composed of multiple capacitors; the parallel capacitor group includes an automatic switching switch, and the automatic switching switch is used in the parallel capacitor group. Capacitors are put in and out to realize the capacitance adjustment of the parallel capacitor bank.

In the embodiment of the present invention, the first switch includes any one of a circuit breaker, an isolation switch and a power electronic switch or any two switches connected in parallel. The first switch is used to control the input and withdrawal of the reactor unit. When the power electronic switch is a thyristor bidirectional switch, which includes thyristors connected in reverse parallel, the second switch is closed to directly connect the capacitor unit in parallel with the first converter, and the third switch is opened to change the firing angle of the thyristor in the thyristor bidirectional switch. , adjust the equivalent impedance of the reactor unit 101, thereby performing capacitive compensation or inductive compensation on the transmission line:

When the firing angle of the thyristor in the thyristor bidirectional switch is changed and the equivalent reactance of the reactor unit 101 is less than the equivalent capacitive reactance of the capacitor unit 102, the unified power flow controller works in the capacitive compensation mode to perform capacitive compensation on the transmission line.

When the firing angle of the thyristor in the thyristor bidirectional switch is changed and the equivalent reactance of the reactor unit 101 is greater than the equivalent capacitive reactance of the capacitor unit 102, the unified power flow controller works in the inductive compensation mode to perform inductive compensation on the transmission line.

By controlling the first converter and the second converter, while realizing the capacitive or inductive compensation function, the series impedance can also be controlled to accurately adjust the phase angle of the line.

In the embodiment of the present invention, the second switch includes any one of a circuit breaker, an isolation switch and a power electronic switch or any two switches connected in parallel. The second switch is used to control the input and withdrawal of the capacitor unit. Among them, the two switches in parallel can be a circuit breaker and an isolating switch in parallel, a circuit breaker and a power electronic switch in parallel, and an isolating switch and a power electronic switch in parallel.

In the embodiment of the present invention, the third switch includes any one of a circuit breaker, an isolation switch and a power electronic switch or any two switches connected in parallel; the third switch is used to control the input and withdrawal of the series transformer.

In this embodiment, the unified power flow controller can be used to realize inductive offset compensation by closing the first switch, opening the second switch, and opening the third switch; closing the second switch, opening the first switch, and opening the third switch. switch, the unified power flow controller can be used to realize capacitive offset compensation; when the first switch is turned off, the second switch is turned off, and the third switch is turned off, the unified power flow controller can be used to realize offset-free compensation.

In the embodiment of the present invention, the first converter 103 and the second converter 105 include two-level converters, three-level converters, diode clamped converters, flying capacitor converters, Any one or more of modular multi-level converters and H-bridge cascade multi-level converters.

Figure 2 is a schematic structural diagram of a unified power flow controller combining series compensation and an inverter that is preferred in the embodiment of the present invention. As shown in the figure, in this embodiment, the reactor unit 101 includes a reactor L, and the capacitor unit 102 includes A capacitor C, the first converter 103 and the second converter 105 are converters VSC, and the series transformer 104 is a series transformer Tse. The reactor L is connected in series with the first switch and then connected in parallel to both ends of the first converter VSC. The capacitor C and the second switch are connected in series and also connected in parallel to both ends of the first converter VSC. The first switch and the second switch are connected respectively. Among the windings on one side of the series transformer Tse, the winding on the other side of the series transformer Tse is connected to the transmission line between system 1 and system 2, and a third switch is connected in parallel to both ends of the winding. The parallel transformer 106 is connected to the system side.

Figure 3 is a schematic structural diagram of a unified power flow controller combining series compensation and an inverter in an embodiment of the present invention. As shown in the figure, the reactor unit 101 in this embodiment includes a reactor L, and the capacitor unit 102 includes a The capacitor C, the converter 103 is the converter VSC, and the coupling transformer 104 is the coupling transformer Tse. The reactor L is connected in series with the first switch, the capacitor C is connected in parallel at both ends of the branch composed of the reactor L and the first switch, and the mixed branch composed of the reactor L, the first switch and the capacitor C is connected in series with the converter VSC The two ends of one side winding of the coupling transformer Tse are connected in parallel, and the other side winding of the coupling transformer Tse is connected to the transmission line between system 1 and system 2. A second switch is connected in parallel to both ends of the winding.

Figure 4 is a schematic diagram of the connection between the reactor unit 101 and the capacitor unit 102 in the embodiment of the present invention. As shown in the figure, the reactor unit 101 in this embodiment includes a reactor, and the capacitor unit includes a capacitor and a switching switch. One switch is a thyristor bidirectional switch. The reactor is connected in series with the thyristor bidirectional switch and then connected in parallel with the capacitor. The arrester MOV and the mechanical switch S are connected in parallel with the capacitor respectively. Among them, the mechanical switch S is used to bypass the reactor unit 101 and the capacitor unit 102 when the reactor unit 101 or the capacitor unit 102 fails.

Figure 5 is a schematic diagram of the equivalent impedance range of the first converter 103 in the embodiment of the present invention. As shown in the figure, the converter 103 in this embodiment adjusts the output voltage and the current amplitude and phase angle flowing through the transmission line. Relationship, it can be obtained that the equivalent impedance range of the converter is within the circle with Zinv as the radius, the reactance value range is [-X _inv , + X _inv ], and the capacitive compensation is set to forward compensation.

Figure 6 is a schematic diagram of the compensation range of the unified power flow controller combining series compensation and an inverter in an embodiment of the present invention. As shown in the figure, the unified power flow controller combining series compensation and an inverter includes a capacitive compensation mode and an inductive compensation mode. Compensation mode can realize compensation of different impedances. By controlling the first converter and the second converter, while realizing the capacitive or inductive compensation function, the series impedance can also be controlled to accurately adjust the phase angle of the line.

The unified power flow controller in the present invention includes a fixed capacitive offset structure and a fixed inductive offset structure. in,

1. Fixed capacitive offset structure

In this embodiment, the unified power flow controller includes a capacitor unit, a first inverter, a second inverter, a second switch, a third switch, a series transformer and a parallel transformer;

One end of the first converter is connected in parallel with one side winding of the series transformer, and the other end is connected to the second converter and the parallel transformer in sequence. The other end of the parallel transformer is connected to the busbar or transmission line; the other side of the series transformer The winding is connected to the transmission line, and the winding is connected in parallel with the third switch; the capacitor unit is connected in series with the second switch and then connected in parallel with the first converter, and the other end of the second switch is connected to the series transformer.

2. Fixed perceptual offset structure

In this embodiment, the unified power flow controller includes a reactor unit, a first converter, a second converter, a first switch, a third switch, a series transformer and a parallel transformer;

One end of the first converter is connected in parallel with one side winding of the series transformer, and the other end is connected to the second converter and the parallel transformer in sequence. The other end of the parallel transformer is connected to the busbar or transmission line; the other side of the series transformer The winding is connected to the transmission line, and the winding is connected in parallel with the third switch; the reactor unit is connected in series with the first switch and then connected in parallel with the first converter, and the other end of the first switch is connected to the series transformer.

1. Capacitive compensation mode

In this embodiment, the capacitive reactance of the capacitor unit ₁₀₂ is set to In the mode, the compensation range of the unified power flow controller combining series compensation and converter on the device side is the vector sum of the capacitive reactance X _c and the converter impedance Zinv.

In this embodiment, the thyristor firing angle of the first switch can be continuously adjusted, and the series impedance of the first inverter 103 can also be adjusted. The first inverter 103 has a faster response speed and can be used to enhance system damping and further improve The dynamic response capability of the system. By investing in different capacitor _capacities , the capacitive reactance

2. Perceptual compensation model

In this embodiment, the reactance of the _reactor is set to The compensation range of the controller on the device side is the vector sum of reactance X _L and converter impedance Zinv.

In this embodiment, if the transmission line fails, in order to reduce the short-circuit current of the transmission line and reduce the energy absorbed by the arrester, all the thyristors in the first switch are controlled to be turned on, and at the same time, the impedance of the first inverter 103 can be quickly adjusted. value and improve the dynamic response capability of the system. When the dynamic adjustment from capacitive compensation mode to inductive compensation mode is realized, the maximum inductive compensation on the device side of the unified power flow controller combining series compensation and converter is -X _L -Z _inv , and the maximum capacitive compensation is X _c + Z _inv .

The following describes the working process of the unified power flow controller combining series compensation and an inverter that is preferred in the embodiment of the present invention:

1. The unified power flow controller combining series compensation and converter is not connected to the system.

Before the unified power flow controller combining series compensation and converter is connected to the system, the second switch is closed, and the thyristor bidirectional switch of the first converter 103 and the first switch is locked. The unified power flow controller combining series compensation and converter In bypass state, it will not have any impact on the operating status of the system.

2. Unified power flow controller integrating series compensation and converter into the system

The unified power flow controller that combines series compensation and converter determines the impedance value that needs to be compensated based on upper-level dispatch instructions and actual operating conditions. When the reactance of long-distance transmission lines is large, it needs to operate in capacitive compensation mode to compensate for the reactance of the transmission lines and reduce transmission losses. When the transmission line is overloaded due to short-circuit fault, increased load, parallel line tripping, etc., it needs to operate in inductive impedance compensation mode to limit the current amplitude. By adjusting the thyristor firing angle of the thyristor bidirectional switch in the first switch, the unified power flow controller combining series compensation and converter works in the capacitive compensation mode or the inductive compensation mode, and at the same time, the first commutation is controlled according to the needs of system damping adjustment. The operation of the processor 103 improves the dynamic response of the system.

In the embodiment of the present invention, the protection measures of the unified power flow controller combining series compensation and converter in the event of system failure or device failure mainly include:

1. When a fault occurs on the system side, such as a single-phase ground fault on the line or overcurrent on the line, in order to protect the unified power flow controller that combines series compensation with the converter, the second switch needs to be closed and the first converter exited. 103 runs.

2. When a fault occurs inside the first converter 103, it is necessary to trigger pulse blocking of the switching device of the first converter 103, close the second switch, and exit the operation of the first converter 103. At this time, the reactor Unit 101 and capacitor unit 102 can still perform capacitive compensation or inductive compensation.

Those of ordinary skill in the art can understand that all or part of the processes in the methods of the above embodiments can be implemented by instructing relevant hardware through a computer program. The program can be stored in a computer-readable storage medium. The program can be stored in a computer-readable storage medium. During execution, the process may include the processes of the embodiments of each of the above methods. The storage medium may be a magnetic disk, an optical disk, a read-only memory (Read-Only, ROM) or a random access memory (Random Access Memory, RAM), etc. At the same time, the present invention does not indicate the lightning arresters, gaps and other equipment used to protect capacitors, series transformers, inverters and their switching devices. It does not mean that these devices do not exist when the device is designed and manufactured and the project is actually implemented. When the actual project is implemented, there will be many isolation switches, circuit breakers, current measuring equipment, and voltage measuring equipment that are not marked in the drawings of this embodiment. This does not mean that these devices will not exist when the project is actually implemented.

The unified power flow controller combining series compensation and converter in the embodiment of the present invention can be used in transmission lines or distribution lines to perform capacitive or inductive adjustment, increase line transmission capacity, improve system stability, and control line power flow. Enhanced system damping solves the problem that the existing unified power flow controller cannot be adjusted continuously and quickly in a wide range, and reduces the cost of the unified power flow controller.

Another embodiment of the present invention is shown in Figure 3. The reactor unit, capacitor unit and converter unit are connected in parallel and then connected to one side of the series transformer. By adjusting the thyristor firing angle of the thyristor bidirectional switch in the first switch and adjusting the reactance value of the incorporated reactor unit, the automatic switching of the capacitor bank can adjust the capacitance value of the incorporated capacitor, and the converter unit can realize the inverter unit with Zinv as the radius. impedance circle. The impedance injected into the system side can be controlled through the equivalent impedance adjustment of the reactor unit, capacitor unit and converter, thereby achieving dynamic adjustment of a wide range of impedance.

It should be noted that the case of connecting inductors and capacitors in series with the converter is not limited to the structure shown in the embodiment. Any situation in which large-scale series compensation is achieved by increasing or decreasing inductors and capacitors falls within the scope of the present invention. . The parallel side of the unified power flow control can be connected to the busbar of this line or to other lines, which all fall within the scope of the present invention.

Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the invention. In this way, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims (8)

1. The unified power flow controller is characterized by comprising a reactor unit, a capacitor unit, a first converter, a second converter, a first switch, a second switch, a third switch, a series transformer and a parallel transformer;

one end of the first converter is connected with one side winding of the series transformer in parallel, the other end of the first converter is sequentially connected with the second converter and the parallel transformer, and the other end of the parallel transformer is connected into a bus or a power transmission line; the winding on the other side of the series transformer is connected with a power transmission line, and the winding is connected with the third switch in parallel;

the reactor unit is connected with a first switch in series and then connected with the first converter in parallel, and the other end of the first switch is connected with the series transformer;

the capacitor unit is connected in series with a second switch and then connected in parallel with the first converter, and the other end of the second switch is connected into the series transformer;

when the power electronic switch included in the first switch is a thyristor bidirectional switch, the second switch is closed so as to directly connect the capacitor unit with the first converter in parallel, the third switch is opened, and the triggering angle of the thyristor in the thyristor bidirectional switch is changed, so that the capacitive compensation or the inductive compensation is carried out on the power transmission line;

when the triggering angle of the thyristor in the thyristor bidirectional switch is changed, the equivalent reactance of the reactor unit is smaller than the equivalent capacitance of the capacitor unit, and the unified power flow controller works in a capacitive compensation mode to perform capacitive compensation on the power transmission line;

when the triggering angle of the thyristor in the thyristor bidirectional switch is changed, the equivalent reactance of the reactor unit is larger than the equivalent capacitance of the capacitor unit, and the unified power flow controller works in an inductive compensation mode to perform inductive compensation on the power transmission line;

closing the first switch, and when the second switch and the third switch are opened, the unified power flow controller is used for realizing inductive offset compensation; closing the second switch, opening the first switch and the third switch, wherein the unified power flow controller is used for realizing capacitive offset compensation; the first switch, the second switch and the third switch are disconnected, and the unified power flow controller is used for realizing offset-free compensation;

the capacitance reactance of the capacitor unit is X _c The equivalent impedance range of the first converter is within a circle with Zinv as radius, and the capacitive compensation is forward compensation, and the compensation range of the unified power flow controller combined with the converter at the device side in the capacitive compensation mode is capacitive impedance X _c Vector sum with inverter impedance Zinv;

the reactor reactance of unit X _L The compensation range of the unified power flow controller combined by series compensation and the converter in the inductive compensation mode at the device side is reactance X _L And the current converter resistorVector sum of Zinv resistance;

in the dynamic adjustment of the capacitive compensation mode to the inductive compensation mode, the maximum inductive compensation of the unified power flow controller at the device side is-X _L -Zinv, maximum capacitive compensation is xc+zinv.

2. A unified power flow controller in combination with a series compensation and converter as claimed in claim 1, characterized in that,

the capacitor unit comprises one capacitor or a series capacitor bank consisting of a plurality of capacitors or a parallel capacitor bank consisting of a plurality of capacitors; the parallel capacitor bank comprises an automatic switching switch, and the automatic switching switch is used for switching in and switching out of the capacitors in the parallel capacitor bank, so that the capacitance value of the parallel capacitor bank is adjusted.

3. A unified power flow controller in combination with a series compensation and converter as claimed in claim 1, characterized in that,

the first switch comprises any one switch or any two switches connected in parallel of a breaker, an isolating switch and a power electronic switch; the first switch is used for controlling the input and the withdrawal of the reactor unit;

the second switch comprises any one switch or any two switches connected in parallel of a breaker, an isolating switch and a power electronic switch; the second switch is used for controlling the input and the withdrawal of the capacitor unit;

the third switch comprises any one switch or any two switches connected in parallel of a breaker, an isolating switch and a power electronic switch; the third switch is used for controlling the input and the output of the series transformer.

4. A unified power flow controller in combination with a series compensation and converter as claimed in claim 3,

when the first switch and the second switch comprise the power electronic switch, the power electronic switch comprises a thyristor bidirectional switch, and the thyristor bidirectional switch comprises thyristors which are connected in anti-parallel;

the thyristor bidirectional switch of the first switch is also used for adjusting the equivalent impedance of the reactor unit by changing the triggering angle of the thyristor.

5. A unified power flow controller in combination with a series compensation converter as claimed in claim 1 wherein the unified power flow controller comprises a fixed capacitive offset structure and a fixed inductive offset structure.

6. A unified power flow controller in combination with a series compensation and converter as claimed in claim 5, it is characterized in that the method comprises the steps of,

when the unified power flow controller is of a fixed capacity offset structure: the power supply comprises a capacitor unit, a first converter, a second switch, a third switch, a series transformer and a parallel transformer;

one end of the first converter is connected with one side winding of the series transformer in parallel, the other end of the first converter is sequentially connected with the second converter and the parallel transformer, and the other end of the parallel transformer is connected into a bus or a power transmission line; the winding on the other side of the series transformer is connected with a power transmission line, and the winding is connected with the third switch in parallel;

the capacitor unit is connected in series with the second switch and then connected in parallel with the first converter, and the other end of the second switch is connected into the series transformer.

7. A unified power flow controller in combination with a series compensation converter as claimed in claim 5 wherein,

when the unified power flow controller is of a fixed inductive offset structure: the power supply comprises a reactor unit, a first converter, a second converter, a first switch, a third switch, a series transformer and a parallel transformer;

one end of the first converter is connected with one side winding of the series transformer in parallel, the other end of the first converter is sequentially connected with the second converter and the parallel transformer, and the other end of the parallel transformer is connected into a bus or a power transmission line; the winding on the other side of the series transformer is connected with a power transmission line, and the winding is connected with the third switch in parallel;

the reactor unit is connected with the first switch in series and then connected with the first converter in parallel, and the other end of the first switch is connected with the series transformer.

8. A unified power flow controller in combination with a series compensation and converter as claimed in claim 1, it is characterized in that the method comprises the steps of, the first converter and the second converter comprise any one or more converters of a two-level converter, a three-level converter, a diode clamping converter, a flying capacitor converter, a modularized multi-level converter and an H-bridge cascading multi-level converter.