Novel four-quadrant power flow control device of high-voltage power transmission network

Abstract

The invention discloses a novel four-quadrant power flow control device of a high-voltage power transmission network, and relates to the technical field of novel power system power flow control devices. The control device comprises a four-quadrant power flow control transformer PFCT and a unified power flow controller UPFC, the PFCT comprises a three-phase four-winding excitation unit and a three-phase four-winding series unit, the UPFC comprises a parallel converter, a series converter and a DC capacitor, the UPFC and the PFCT are connected by sharing a primary winding of the excitation unit of the PFCT, and the DC capacitor is connected with the excitation unit of the PFCT by sharing a secondary winding of the excitation unit of the PFCT. The winding of the PFCT provides an input voltage for the UPFC parallel converter, and the UPFC series converter is connected with the series unit winding of the PFCT and is used for injecting a controllable continuous voltage into a line and adjusting the power flow in the line. The control device has the characteristics of low maintenance cost, high reliability and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of novel power system flow control devices, and in particular to a novel four-quadrant flow control device for a high-voltage transmission network. Background Art

[0002] With the construction of UHV power grid and new power system in my country, the 220kV and above high-voltage transmission network is prone to power flow blocking problems, with some lines overloaded and some lines inefficiently utilized, which in turn affects the economy and stability of the power system. Therefore, it is urgent to fully tap the potential of the existing power grid and effectively control the power flow and voltage of the high-voltage power grid. The control device of the flexible AC transmission system (FACTS) can effectively solve the problems of power flow blocking and electromagnetic circulation in the power grid, and has good application and development prospects in improving the power flow transmission capacity of the line and enhancing the stability of the system.

[0003] At present, for high-voltage and large-capacity applications, the commonly used power flow control devices are mainly unified power flow controllers (UPFC) and dual-core phase-shifting transformers (PST). UPFC has the advantages of fast response speed and can independently adjust the grid voltage, active power and reactive power, but it has disadvantages such as high manufacturing and operation and maintenance costs, low reliability, and large footprint. The dual-core phase-shifting transformer has the advantages of low manufacturing and operation and maintenance costs and high reliability, but because it cannot independently adjust the grid voltage amplitude and phase, it cannot achieve four-quadrant operation of the working point. In the field of distribution networks, Sen transformers (ST) can independently adjust active power and reactive power like UPFC, and their economy and reliability are better than UPFC, but because the tap switch is directly connected in series in the line, it limits its application in high-voltage and large-capacity power grids. Therefore, for high-voltage transmission networks, the development of power flow control devices with low operation and maintenance costs, high reliability, and four-quadrant operation of the working point has very good engineering application prospects. Summary of the invention

[0004] The technical problem to be solved by the present invention is how to provide a novel four-quadrant power flow control device for high-voltage transmission networks with low maintenance cost and high reliability.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a new four-quadrant power flow control device for a high-voltage transmission network, including: a four-quadrant power flow control transformer PFCT and a unified power flow controller UPFC, the PFCT includes a three-phase four-winding excitation unit and a three-phase four-winding series unit, the UPFC includes a parallel converter, a series converter and a DC capacitor, the UPFC and the PFCT are connected by sharing the primary winding of the PFCT excitation unit, the winding of the PFCT provides input voltage for the UPFC parallel converter, and the UPFC series converter is connected to the series unit winding of the PFCT, which is used to inject a controllable continuous voltage into the line to regulate the power flow in the line.

[0006] The beneficial effects of the above technical solution are as follows: in the present application, the PFCT realizes the output of discrete series compensation voltage by adjusting the ordinary on-load voltage regulator, and the UPFC realizes the output of continuous series compensation voltage by injecting a voltage with adjustable amplitude and phase into the device through its series converter. The discrete voltage output by the PFCT is superimposed on the continuous voltage output by the UPFC to realize the function of controllable amplitude of the output continuous voltage and flexible adjustable phase angle from 0 to 360°. The combination of these two modules effectively controls the power flow of the device.

[0007] The series units in the PFCT can control the amplitude and phase of the compensation voltage to achieve four-quadrant operation of the working point. UPFC has multiple control functions, which can not only control active power, but also compensate for the reactive power of the system. In the FHPFC system, the PFCT is responsible for most of the regulation of active and reactive power flows, while the UPFC uses its smaller capacity for precise power control, and the remaining capacity is used for reactive power compensation. The device (FHPFC) adopts a mode of combining small-capacity UPFC with large-capacity PFCT, which not only covers the control range of traditional large-capacity UPFC, but also has the advantages of low cost, high efficiency, excellent electromagnetic compatibility, low harmonic content, and easy operation. It effectively solves the high cost, which is a key problem that hinders the widespread promotion of UPFC. As a cost-effective and efficient control device, it shows its great potential in future applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 The overall circuit topology diagram of the device according to the embodiment of the present invention;

[0009] Figure 2 A UPFC model in the device according to an embodiment of the present invention;

[0010] Figure 3 is a phasor diagram of the compensation voltage output by the FHPFC in an embodiment of the present invention;

[0011] Figure 4aA schematic diagram of a power transmission system in a power flow regulation process of the device according to an embodiment of the present invention;

[0012] Figure 4b A schematic diagram of voltage phasor in an embodiment of the present invention;

[0013] Figure 5 The gear adjustment principle when adjusting the voltage in the device described in the embodiment of the present invention;

[0014] Figure 6 This is the gear adjustment principle during tidal flow control in the device described in the embodiment of the present invention. DETAILED DESCRIPTION

[0015] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0016] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0017] like Figure 1 As shown, an embodiment of the present invention discloses a four-quadrant power flow control device for a high-voltage transmission network. The device includes a large-capacity new-type topology transformer as a main body and is equipped with a small-capacity unified power flow controller UPFC.

[0018] Specifically, the device includes: 1) a new topology power flow control transformer PFCT, the PFCT includes a three-phase four-winding excitation unit and a three-phase four-winding series unit. 2) a small-capacity unified power flow controller UPFC, the UPFC includes a parallel converter (converter 1), a series converter (converter 2) and a DC capacitor. The UPFC and the PFCT form the device by sharing the primary winding of the PFCT excitation unit, and the fourth winding a E3 -x E3 , the fourth winding b E3 -y E3 , the fourth winding c E3 -z E3 Provide input voltage for UPFC parallel converter, and UPFC series converter is connected to the fourth winding a of the series unit of PFCT. S3 -x S3 , the fourth winding b S3 -yS3 , the fourth winding c S3 -z S3 Connection used to inject a controllable continuous voltage into the line, thereby regulating the power flow in the line.

[0019] The excitation unit in the PFCT comprises:

[0020] 1) The primary winding part (first winding), such as Figure 1 As shown, the first winding is a YN type connection, including the first winding A E -X E (A phase winding), first winding B E -Y E (B phase winding) and the first winding C E -Z E (C phase winding); the first winding of the excitation unit is a YN type connection, the first winding A E -X E The first end of the A-phase winding is connected to the first end of the A-phase transmission line. E -X E The end of the first winding B is grounded; E -Y E The first end of the B-phase winding is connected to the first end of the B-phase transmission line. E -Y E The end of the first winding C is grounded; E -Z E The first end of the C phase winding is connected to the first end of the C phase transmission line. E -Z E The end of the ground.

[0021] 2) Secondary winding part (secondary winding) of the excitation unit with switch and on-load tap changer:

[0022] like Figure 1 As shown, the second winding is yn connected, including the second winding a E1 -x E1 , the second winding b E1 -y E1 and the second winding c E1 -z E1 , including on-load tap changer; the second winding a E1 -x E1 The switch connects the second winding a of the series unit s1 -x s1 The first end of the second winding a E1 -x E1 The neutral point tap changer is grounded; the second winding b E1 -y E1The switch connects the second winding b of the series unit s1 -y s1 The first end of the second winding b E1 -y E1 The neutral point tap changer is grounded; the third winding c E1 -z E1 The switch connects the second winding c of the series unit s1 -z s1 The first end of the second winding c E1 -z E1 The neutral point tap-changer is grounded.

[0023] 3) Secondary winding part (tertiary winding) of the excitation unit with switch and on-load tap changer:

[0024] like Figure 1 As shown, the third winding includes a third winding a E2 -x E2 , the third winding b E2 -y E2 And the third winding c E2 -z E2 ; The third winding a E2 -x E2 The switching switches are connected to the third winding b of the series unit s2 -y s2 The tail end and the third winding c of the series unit s2 -z s2 The first end of the third winding is connected E2 -x E2 The neutral point tap changer is grounded: the third winding b E2 -y E2 The switching switches are connected to the third winding c of the series unit s2 -z s2 The tail end and the third winding a of the series unit s2 -x s2 The first end of the third winding b E2 -y E2 The neutral point tap changer is grounded; the third winding c E2 -z E2 The switching switches are connected to the third winding a of the series unit s2 -x s2 The tail end and the third winding b of the series unit s2 -y s2 The first end of the third winding c E2 -z E2 The neutral point tap-changer is grounded.

[0025] 4) Secondary winding part (fourth winding), including the fourth winding aE2 -x E3 , the fourth winding b E3 -y E3 And the fourth winding c E3 -z E3 , connected to the parallel converter.

[0026] The series connection unit in the PFCT comprises:

[0027] 1) If Figure 1 As shown, the primary winding portion includes a first winding, and the first winding includes a first winding A S -X S , the first winding B S -Y S And the first winding C S -Z S , the first winding A S -X S Connected in series with the A phase of the transmission line; the first winding B S -Y S Connected in series with the B phase of the transmission line; the first winding C S -Z S Connected in series with phase C of the transmission line.

[0028] 2) Secondary winding part (second winding):

[0029] like Figure 1 As shown, the secondary winding part includes a second winding, the second winding is yn connected, and the second winding includes a second winding a s1 -x s1 , the second winding b s1 -y s1 and the second winding c s1 -z s1 , the second winding a s1 -x s1 The first end is connected to the second winding a of the excitation unit E1 -x E1 The switch of the second winding a s1 -x s1 The end of the second winding b is grounded; s1 -y s1 The first end is connected to the second winding b of the excitation unit E1 -y E1 The switch of the second winding b s1 -y s1 The end of the second winding c is grounded; s1 -z s1 The first end is connected to the second winding c of the excitation unit E1 -z E1 The switch of the second winding c s1-z s1 The end of the ground;

[0030] 3) Secondary winding part (third winding):

[0031] like Figure 1 As shown, the secondary winding part includes a third winding, the third winding is connected in a triangle, and the connection method is ax(b)-y(c)-z(a). The third winding includes a third winding a s2 -x s2 , the third winding b s2 -y s2 And the third winding c s2 -z s2 , the third winding a s2 -x s2 The first end is divided into two paths, the first path is connected to the third winding b in the excitation unit E2 -y E2 The switch is connected to the second winding and the third winding c s2 -z s2 The tail end of the third winding a s2 -x s2 The tail end of the third winding b s2 -y s2 The first end of the third winding b s2 -y s2 The first end of the excitation unit and the third winding c E2 -z E2 The switch connection of the third winding b s2 -y s2 The tail end is divided into two paths, the first path and the third winding c s2 -z s2 The first end is connected to the first end, and the second end is connected to the third winding a in the excitation unit E2 -x E2 toggle switch connection.

[0032] 4) Secondary winding part (fourth winding)

[0033] like Figure 1 As shown, the secondary winding part includes a fourth winding, and the fourth winding includes: a fourth winding a s3 -x s3 , the fourth winding b s3 -y s3 And the fourth winding c s3 -z s3 , connected to the series converter.

[0034] Unified Power Flow Controller (UPFC)

[0035] like Figure 1As shown, the UPFC parallel converter (converter 1) is connected to the fourth winding of the PFCT excitation unit to realize the function of rectifying the AC voltage into a DC voltage. The UPFC series converter (converter 2) is connected to the fourth winding of the PFCT series unit to realize the function of inverting the DC voltage into an AC voltage. The UPFC includes the following parts:

[0036] 1) Parallel converter (converter 1):

[0037] like Figure 2 As shown, the fourth winding a of the parallel converter and the excitation unit E3 -x E3 、The fourth winding b E3 -y E3 or the fourth winding c E3 -z E3 The parallel converter uses power electronic devices (such as IGBT or GTO) to perform power conversion between AC and DC to achieve reactive power compensation and voltage control on the parallel side.

[0038] 2) Series converter (converter 2):

[0039] like Figure 2 As shown, the fourth winding a of the series converter and the series unit s3 -x s3 、The fourth winding b s3 -y s3 or the fourth winding c s3 -z s3 connected and connected to the shunt converter through a DC bus; the function of the series converter is to convert DC power into AC power and inject a compensation voltage through the series unit to control the active and reactive power flow of the transmission line.

[0040] 3) DC capacitor (C):

[0041] like Figure 2 As shown, there is a DC bus between the parallel converter and the series converter, and a capacitor (marked as C) is connected in it. This capacitor is used to smooth the DC voltage and reduce fluctuations to ensure stable power transmission between the converters.

[0042] 4) Inverter module:

[0043] like Figure 2 As shown, the power electronic switches (such as diodes and thyristors) in the converter module are used to control the direction and size of current flow. These switches can accurately adjust the output voltage and current through control signals to achieve the purpose of controlling power flow and regulating voltage.

[0044] The UPFC parallel converter (converter 1) is connected to the fourth winding of the PFCT excitation unit to realize the function of rectifying the AC voltage into a DC voltage. The UPFC series converter (converter 2) is connected to the fourth winding of the PFCT series unit to realize the function of inverting the DC voltage into an AC voltage.

[0045] The following is a detailed description of the above steps:

[0046] Voltage Regulation Process:

[0047] The voltage phasor at the head end of the power flow control device is: The amplitude is U S , the phase is 0°, and the output voltage phasor of the series side of FHPFC (Four-quadrant High-voltage Power Flow Controller, FHPFC, i.e. the power flow control device) is The amplitude is U FHPFC , the phase is θ FHPFC ; The output voltage phasor of the UPFC series side is The amplitude is U UPFC , the phase is θ UPFC ; The output voltage phasor of the PFCT series side is The amplitude is U PFCT , the phase is θ PFCT Taking the PFCT regulation level m=1 as an example, the steps of PFCT continuous voltage regulation are described in detail. Figure 3 The FHPFC output voltage phasor diagram is shown.

[0048] like Figure 3 As shown in the figure, the continuous compensation voltage range that FHPFC can output is: all voltage phasors starting from point O and ending at any point in the small circle. Figure 3 It is shown that PFCT can output discrete voltage phasors with O as the starting point and 9 circle centers as the end points, while the output range of UPFC is the voltage phasor with the end point of the PFCT output voltage phasor as the starting point and any point on the edge of the circle or in the small circle as the end point. After the phasors of the two are superimposed, the voltage phasor of the FHPFC formed covers all the small circle areas. As shown in M ​​in the figure, the voltage phasor output by PFCT is The voltage phasor output by UPFC is The two voltage phasors are superimposed to synthesize the voltage phasor output by FHPFC. At this time, the voltage at the head end of the transmission line is given by the voltage phasor and Synthetic

[0049] Analysis of the flow regulation process:

[0050] Figure 4a-4b The schematic diagram of the power transmission system connected to the FHPFC device and its voltage phasor, where the FHPFC series compensation voltage is represented by the phasor Indicates that the amplitude is U FHPFC , the phase is θ FHPFC The two ends of the transmission line are simplified to an ideal single-machine infinite system. Before compensation, the voltage at the head end of the system is Indicates that its amplitude is U S , the phase is 0°, the system end voltage is Indicates that its amplitude U R , the phase is -δ°, that is, the phase difference between the head and the end of the system is δ°. After FHPFC compensation, Figure 4b As shown, the voltage phasor at the head end of the system is Indicates that its amplitude is U S1 , at this time the phase difference between the beginning and the end of the system is δ1. Ignoring the resistance and capacitance of the transmission line, the equivalent reactance of the transmission line is X L .

[0051] Depend on Figure 4a It can be seen that the power transmitted on the transmission line is Then the power at the end of the transmission line can be obtained The expression is:

[0052]

[0053] When the FHPFC device is not connected to the system, the output voltage of the FHPFC series side is At this time, the power at the end of the transmission line is Then the power at the end of the transmission line can be obtained The expression is:

[0054]

[0055] The phasor expressions of the voltage at the head and end of the transmission line and the compensation voltage on the FHPFC series side are as follows:

[0056]

[0057]

[0058] Substituting equation (3) into equation (1) and (2) respectively, we can obtain the active and reactive power at the end of the transmission system after FHPFC is connected to the system:

[0059]

[0060] Without FHPFC compensation device, the active and reactive power at the end of the system is:

[0061]

[0062] From equations (4)-(7), it can be obtained that the regulation amount of the FHPFC device on the active and reactive power at the end of the system is:

[0063]

[0064] Under the condition of steady-state operation of the system, it can be seen from the above formula that by adjusting the switching position of the FHPFC tap switch, the amplitude and phase of the output voltage on the FHPFC series side can be effectively changed, so as to flexibly regulate the active and reactive power at the end of the system and realize the power flow regulation of the transmission line, so as to achieve the purpose of regulating the steady-state power flow and voltage phase, balancing the power flow of the multi-line transmission system, improving the cross-section transmission capacity, and increasing the emergency transmission capacity of the backup line. When the FHPFC is adjusted, the end power P and Q of the transmission line are continuous with the change of the phase shift angle θ. Since the UPFC in the FHPFC can output a continuous compensation voltage, it makes up for the defect of the discrete regulation of the PFCT, thus realizing the continuous regulation of the series compensation voltage.

[0065] Control strategy

[0066] 1) Gear adjustment control during voltage regulation

[0067] Figure 5 This is a gear adjustment schematic diagram for voltage regulation, showing the process of deciding whether to increase or decrease the gear by comparing the target voltage and the actual voltage. The following will introduce the functions of each part step by step:

[0068] 1. Comparison between target voltage upper limit and actual voltage

[0069] exist Figure 5 In the upper part of the circuit, there is a comparator circuit that compares the upper limit of the target voltage with the actual voltage. If the actual voltage exceeds the upper limit of the target voltage, the comparator will output a signal to trigger the pulse generator.

[0070] 2. Pulse generator and gear boost

[0071] When the comparator outputs a signal, the pulse generator is triggered to generate a pulse signal. The pulse signal is output to the gear boost after passing through a multiplier (which may be combined or modulated with the signal). The purpose of this process is to reduce the actual voltage through the gear boost when the actual voltage exceeds the upper limit, so as to control the voltage within the target range.

[0072] 3. Comparison between target voltage lower limit and actual voltage

[0073] In the lower part of the figure, there is also a comparator circuit that compares the lower limit of the target voltage with the actual voltage. If the actual voltage is lower than the lower limit of the target voltage, the comparator outputs a signal to trigger the pulse generator below.

[0074] 4. Pulse generator and gear reduction

[0075] When the comparator outputs a signal, the pulse generator below will generate a pulse signal. This pulse signal also passes through a multiplier and is output to the gear reduction. The purpose of this is to increase the actual voltage through the gear reduction when the actual voltage is lower than the lower limit, so as to keep the voltage within the target range.

[0076] Summarize: Figure 5 The logic of gear adjustment in the voltage regulation process is described. When the actual voltage exceeds the preset upper and lower limits, the system will automatically trigger the pulse generator and control the voltage within the target range by adjusting the gear (upshift or downshift), thereby achieving stable voltage control.

[0077] 2) Gear adjustment control during flow regulation

[0078] Figure 6 The principle of gear adjustment in power flow control is shown. In power flow control systems, gear adjustment is used to keep the active power of the system within the target range in order to meet the needs of the power system. The following is a detailed description of each part of the figure:

[0079] 1. Comparison between target active power and actual active power

[0080] Target active power: This is the active power value that the system hopes to achieve, usually set by the system demand or operator.

[0081] Actual active power: This is the actual active power value of the current system.

[0082] Error calculation: The difference between the target active power and the actual active power is calculated through an adder to obtain an error signal, which represents the deviation between the current power and the target value.

[0083] 2. Dead zone processing

[0084] A "dead zone" is set here, which allows a small error range and does not trigger the gear adjustment. If the error is within this dead zone, the system considers that the power deviation is within an acceptable range and does not adjust the gear up or down. The setting of the dead zone can avoid frequent small adjustments, maintain the stability of the system and reduce unnecessary switching operations.

[0085] 3. Hysteresis comparator

[0086] If the error signal exceeds the deadband range, the error signal will enter the hysteresis comparator. The hysteresis comparator is used to determine the polarity and magnitude of the error to determine whether an upshift or downshift is required. When the error is positive and exceeds the upper limit of the deadband, the hysteresis comparator will output a signal indicating that the gear needs to be increased. When the error is negative and exceeds the lower limit of the deadband, the hysteresis comparator will output a signal indicating that the gear needs to be decreased.

[0087] 4. Pulse generator and gear adjustment

[0088] The output signal of the hysteresis comparator triggers the pulse generator to generate a modulation pulse. In the upper path, if the gear needs to be increased, the pulse generator generates a pulse signal, which is output to the gear increase unit after passing through a multiplier (for signal modulation or transmission). In the lower path, if the gear needs to be decreased, the pulse generator generates a corresponding pulse signal, which is output to the gear decrease unit after passing through the multiplier.

[0089] 5. Gear up and down control

[0090] According to the pulse signal, the system will increase or decrease the gear accordingly to adjust the active power close to the target value. The adjustment of the gear can affect the output power of the system to meet the target demand. This adjustment process will be repeated until the actual active power reaches the target active power and remains within the dead zone, and the system will stop further adjustment.

[0091] Summarize: Figure 6 An automatic adjustment system is presented, which determines whether to adjust the gear position by comparing the target active power and the actual active power. Frequent adjustment is avoided by setting a dead zone, and the hysteresis comparator and pulse generator are used to control the increase or decrease of the gear position, thereby achieving stable control of the system active power.

[0092] In summary, when the control device is connected to the high-voltage transmission line, the target value is adjusted according to the system flow to obtain the target amplitude and phase angle of the compensation voltage. By adjusting the on-load tap changer of its excitation unit, the two orthogonal compensation voltages injected into the transmission line by the series unit are changed, thereby realizing the regulation of the voltage amplitude and phase at the line sending end. On this basis, the UPFC outputs a continuous voltage to reduce the error between the final compensation voltage and the target value, thereby realizing four-quadrant continuous operation of the working point. The compensation voltage amplitude output by the FHPFC of the present invention is continuously controllable, and the phase angle can be flexibly adjusted from 0 to 360°. The main body adopts a transformer with a new topology, which can improve the reliability of the device operation and significantly reduce the operating cost of the system. Combined with a small-capacity UPFC, it can improve the flow control accuracy, realize the continuous adjustment of the system flow, and improve the safety and stability of the transmission line.

Claims

1. A novel four-quadrant power flow control device for a high-voltage transmission network, characterized in that include: A four-quadrant power flow control transformer PFCT and a unified power flow controller UPFC, wherein the PFCT includes a three-phase four-winding excitation unit and a three-phase four-winding series unit, and the UPFC includes a parallel converter, a series converter and a DC capacitor. The UPFC and the PFCT are connected by sharing the primary winding of the PFCT excitation unit. The winding of the PFCT provides input voltage for the UPFC parallel converter, and the UPFC series converter is connected to the winding of the series unit of the PFCT to inject a controllable continuous voltage into the line to regulate the power flow in the line.

2. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: The excitation unit in the PFCT comprises: 1) Primary winding part, including the first winding A E -X E , the first winding B E -Y E And the first winding C E -Z E ; 2) Secondary winding part, including the second winding a E1 -x E1 , the second winding b E1 -y E1 and the second winding c E1 -z E1 , including on-load tap-changer; 3) Secondary winding part, including the third winding a E2 -x E2 , the third winding b E2 -y E2 And the third winding c E2 -z E2 , including on-load tap-changer; 4) Secondary winding part, including the fourth winding a E2 -x E3 , the fourth winding b E3 -y E3 And the fourth winding c E3 -z E3 , connected to the parallel converter.

3. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: The series units in the PFCT include: 1) Primary winding part, including the first winding A S -X S , the first winding B S -Y S And the first winding C S -Z S , respectively connected in series with the A, B, and C phases of the transmission line; 2) Secondary winding part, including the second winding a s1 -x s1 , the second winding b s1 -y s1 and the second winding c s1 -z s1 , connected to the corresponding second winding in the excitation unit; 3) Secondary winding part, including the third winding a s2 -x s2 , the third winding b s2 -y s2 And the third winding c s2 -z s2 , connected to the corresponding third winding in the excitation unit; 4) Secondary winding part, including the fourth winding a s3 -x s3 , the fourth winding b s3 -y s3 And the fourth winding c s3 -z s3 , connected to the series converter.

4. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: The first winding of the excitation unit is a YN type connection. E -X E The first end of the first winding A is connected to the first end of the A-phase transmission line. E -X E The end of the first winding B is grounded; E -Y E The first end of the first winding B is connected to the first end of the B phase transmission line. E -Y E The end of the first winding C is grounded; E -Z E The first end of the first winding C is connected to the first end of the C phase transmission line. E -Z E The end of the ground.

5. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: The second winding of the excitation unit is yn connected: The second winding a E1 -x E1 The switch connects the second winding a of the series unit s1 -x s1 The first end of the second winding a E1 -x E1 The neutral point tap changer is grounded; The second winding b E1 -y E1 The switch connects the second winding b of the series unit s1 -y s1 The first end of the second winding b E1 -y E1 The neutral point tap changer is grounded; The third winding c E1 -z E1 The switch connects the second winding c of the series unit s1 -z s1 The first end of the second winding c E1 -z E1 The neutral point tap-changer is grounded.

6. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: The third winding of the excitation unit is yn connected: The third winding a E2 -x E2 The switching switches are connected to the third winding b of the series unit s2 -y s2 The tail end and the third winding c of the series unit s2 -z s2 The first end of the third winding is connected E2 -x E2 Neutral point tap changer grounding: The third winding b E2 -y E2 The switching switches are connected to the third winding c of the series unit s2 -z s2 The tail end and the third winding a of the series unit s2 -x s2 The first end of the third winding b E2 -y E2 Neutral point tap changer grounding: The third winding c E2 -z E2 The switching switches are connected to the third winding a of the series unit s2 -x s2 The tail end and the third winding b of the series unit s2 -y s2 The first end of the third winding c E2 -z E2 The neutral point tap-changer is grounded.

7. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: The series unit part: 1) The first winding A S -X S Connected in series in the A-phase transmission line, the first winding B S -Y S Connected in series in the B-phase transmission line, the first winding C S -Z S Connected in series in the C-phase transmission line; 2) The second winding is yn connected, the second winding a s1 -x s1 The first end is connected to the second winding a of the excitation unit E1 -x E1 The switch of the second winding a s1 -x s1 The end of the second winding b is grounded; s1 -y s1 The first end is connected to the second winding b of the excitation unit E1 -y E1 The switch of the second winding b s1 -y s1 The end of the second winding c is grounded; s1 -z s1 The first end is connected to the second winding c of the excitation unit E1 -z E1 The switch of the second winding c s1 -z s1 The end of the ground; 3) The third winding is connected in a triangle, the connection method is ax(b)-y(c)-z(a), the third winding a s2 -x s2 The first end is divided into two paths, the first path is connected to the third winding b in the excitation unit E2 -y E2 The switch is connected to the second winding and the third winding c s2 -z s2 The tail end of the third winding a s2 -x s2 The tail end of the third winding b s2 -y s2 The first end of the third winding b s2 -y s2 The first end of the excitation unit and the third winding c E2 -z E2 The switch connection of the third winding b s2 -y s2 The tail end is divided into two paths, the first path and the third winding c s2 -z s2 The first end is connected to the first end, and the second end is connected to the third winding a in the excitation unit E2 -x E2 toggle switch connection.

8. The novel four-quadrant power flow control device for a high-voltage transmission network according to claim 1, characterized in that: Each of said UPFCs comprises: 1) Parallel converter: Parallel converter and the fourth winding a of the excitation unit E3 -x E3 、The fourth winding b E3 -y E3 or the fourth winding c E3 -z E3 The parallel converter is connected to the series converter through a DC bus, and the parallel converter uses power electronic devices to convert power between AC and DC to achieve reactive power compensation and voltage control on the parallel side; 2) Series converter: The fourth winding a of the series converter and the series unit s3 -x s3 、The fourth winding b s3 -y s3 or the fourth winding c s3 -z s3 The series converter is used to convert direct current into alternating current and inject a compensation voltage through the series unit to control the active and reactive power flow of the transmission line; 3) DC capacitor: There is a DC bus between the parallel converter and the series converter, and a DC capacitor is connected in it. The DC capacitor is used to smooth the DC voltage; The power electronic switches in the parallel converter and the series converter are used to control the direction and magnitude of current flow, and the output voltage and current of the power electronic switches are adjusted by control signals.

9. The novel four-quadrant power flow control device for a high-voltage transmission network as claimed in claim 8, characterized in that: The parallel converter is connected to the fourth winding of the excitation unit of the PFCT and is used to rectify the AC power into DC power; The series converter is connected to the fourth winding of the series unit of the PFCT and is used for inverting the direct current into alternating current.

10. The novel four-quadrant power flow control device for a high-voltage transmission network as claimed in claim 1, characterized in that: 1) After the device is connected to the transmission line, according to the actual needs of the power system, the target values ​​of the active power flow and reactive power flow at the receiving end of the line are obtained, and the target values ​​are converted into the amplitude and phase of the compensation voltage by calculation; 2) The excitation unit of the PFCT provides input voltage, and the series unit of the PFCT injects a controllable voltage on the transmission line; by adjusting the switching switch and the on-load tap changer of the second winding of the excitation unit, the compensation voltage of the series unit is changed, thereby changing the amplitude of the voltage at the line sending end; by adjusting the switching switch and the on-load tap changer of the third winding of the excitation unit, the compensation voltage of the series unit is changed, thereby changing the phase of the voltage at the line sending end; the compensation voltages provided by the two windings are superimposed to obtain voltages of different amplitudes and phases, which are injected into the transmission line, so as to control the voltage level and power flow distribution of the line; 3) When the parallel converter of UPFC works in the rectification state, the parallel converter converts the AC power collected from the line into DC power, and then flows to the series converter through the DC capacitor. At this time, the corresponding series converter operates in the inverter state, converting the DC power into AC power, and then the output voltage is connected to the transmission line through the series transformer.

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