Control method and device for high-voltage direct-current power transmission system

Abstract

The invention provides a control method and device for a high-voltage direct-current power transmission system, and the power transmission system comprises a power grid commutation converter with a plurality of converter valves, a tap controller for an alternating-current transformer, and a synchronous static compensator. The synchronous static compensator adjusts the valve side alternating voltage of the power transmission system by outputting or absorbing reactive power of the power transmission system. The control method comprises the steps of obtaining operation parameters of the power transmission system; and in response to determining that the operation parameter deviates from the preset rated parameter, according to the deviation value of the operation parameter deviating from the rated parameter, utilizing the synchronous static compensator and selectively utilizing the tap controller to adjust the valve side alternating current voltage, so that the deviation value of the valve side alternating current voltage relative to the rated voltage is smaller than a voltage deviation threshold value.

Description

Technical Field

[0001] The present invention relates to the field of electric power technology, and in particular to a control method and device for a high voltage direct current transmission system. Background Art

[0002] Clean energy such as photovoltaic, wind power and hydrogenation is developing towards greater power generation and higher places. Therefore, long-distance power transmission requires larger high-voltage direct current transmission. So far, generally ±800kV / 5kA / 8GW high-voltage direct current stations are required to meet the requirements.

[0003] However, existing HVDC solutions cannot fully meet this requirement. So far, VSC valves cannot meet the high current requirements due to the limitations of IGBTs. In addition, even if IGBTs can achieve the above requirements, the cost of VSC stations is too high and the economic efficiency is poor. Although LCC HVDC can easily achieve the target transmission power, the high penetration of renewable energy into the grid will affect the operation of line-commutated converters.

[0004] Currently, relevant technicians have studied the system consisting of a line-commutated converter (LCC) HVDC and a STATCOM. So far, most of the existing research is based on the configuration where the STATCOM and LCC are connected to the common AC bus of the commutated converter. Summary of the invention

[0005] The present disclosure provides a control method and device for a high voltage direct current transmission system.

[0006] According to one aspect of the present disclosure, a control method for a high-voltage direct current transmission system is provided, wherein the transmission system includes a grid-commutated converter having a plurality of converter valves, a tap controller for an AC transformer, and a synchronous static compensator, the synchronous static compensator adjusting the valve-side AC voltage of the transmission system by outputting or absorbing reactive power of the transmission system, the control method comprising: acquiring operating parameters of the transmission system; and in response to determining that the operating parameters deviate from preset rated parameters, adjusting the valve-side AC voltage using the synchronous static compensator and selectively using the tap controller according to an offset value of the operating parameters from the rated parameters, so that the offset value of the valve-side AC voltage relative to the rated voltage is less than a voltage offset threshold.

[0007] According to another aspect of the present disclosure, there is provided a high-voltage direct current transmission system, comprising: a grid-commutated converter, comprising a plurality of converter valves, wherein the grid-commutated converter is electrically connected to a corresponding AC transformer; an AC transformer having a tap controller for adjusting the voltage-transformation ratio of the AC transformer; a static synchronous compensator, wherein terminals of the static synchronous compensator are electrically connected between the grid-commutated converter and the corresponding AC transformer; a detection device configured to obtain operating parameters of the transmission system; and a controller, wherein the controller is configured to, in response to determining that an operating parameter deviates from a preset rated parameter, adjust a valve-side AC voltage using the synchronous static compensator and selectively using the tap controller according to an offset value of the operating parameter deviating from the rated parameter, so that an offset value of the valve-side AC voltage relative to the rated voltage is less than a voltage offset threshold.

[0008] According to another aspect of the present disclosure, there is also provided a configuration method for a high-voltage direct current transmission system, wherein the transmission system comprises a grid-commutated converter having a plurality of converter valves, a tap controller for an AC transformer, and a synchronous static compensator, the synchronous static compensator regulating the valve-side AC voltage of the transmission system by outputting or absorbing the reactive power of the transmission system, the configuration method comprising: determining a maximum value, a minimum value, and a rated voltage value of the valve-side AC voltage of the transmission system; determining a maximum voltage transformation ratio of the AC transformer based on a ratio of the maximum value of the valve-side AC voltage to the rated voltage value; determining a minimum voltage transformation ratio of the AC transformer based on a ratio of the minimum value of the valve-side AC voltage to the rated voltage value; and configuring a plurality of tap contacts of the tap controller according to the maximum voltage transformation ratio and the minimum voltage transformation ratio.

[0009] According to one or more embodiments of the present disclosure, the LCC can always operate at the rated operating point to obtain the best system performance, and at the rated operating point, the valve-side AC voltage will be maintained at a preset rated voltage position.

[0010] It should be understood that the content described in this section is not intended to identify the key or important features of the embodiments of the present disclosure, nor is it intended to limit the scope of the present disclosure. Other features of the present disclosure will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments with reference to the following drawings.

[0012] In the accompanying drawings, unless otherwise specified, the same reference numerals throughout the multiple drawings represent the same or similar parts or elements. These drawings are not necessarily drawn to scale. It should be understood that these drawings only depict some embodiments disclosed in the present application and should not be regarded as limiting the scope of the present application.

[0013] Figure 1A circuit diagram of a high voltage direct current transmission system according to an embodiment of the present invention is shown; and

[0014] Figure 2 A schematic diagram of a three-phase circuit of an LCC (grid commutated converter) and an embedded STATCOM (static synchronous compensator) in a high-voltage direct current transmission system according to an embodiment of the present invention is shown;

[0015] Figure 3 A flow chart showing a control method for a high voltage direct current transmission system according to an embodiment of the present disclosure is shown;

[0016] Figure 4 A flow chart showing a control method for a high voltage direct current transmission system according to an embodiment of the present disclosure is shown;

[0017] Figure 5 A method for regulating valve-side AC voltage according to an example of the present disclosure is shown;

[0018] Figure 6 A structural block diagram of a high voltage direct current transmission system according to an embodiment of the present disclosure is shown;

[0019] Figure 7 A flow chart of a configuration method for a high voltage direct current transmission system according to an embodiment of the present disclosure is shown;

[0020] Figure 8 A flow chart showing a configuration method for a high voltage direct current transmission system according to another embodiment of the present disclosure is shown;

[0021] Fig. 9 The single-line diagram of the inverter side of the LCC with an embedded STATCOM is shown. DETAILED DESCRIPTION

[0022] In the following, only some exemplary embodiments are briefly described. As those skilled in the art will appreciate, the described embodiments may be modified in various ways without departing from the spirit or scope of the present disclosure. Therefore, the drawings and descriptions are considered to be exemplary and non-restrictive in nature.

[0023] Before introducing the method of the embodiment of the present disclosure in detail, a high voltage direct current transmission system is briefly introduced. Figure 1 is a circuit diagram of a high voltage direct current transmission system 100 according to the present disclosure and the present embodiment, such as Figure 1 As shown, the HVDC transmission system 100 mainly includes an LCC (grid commutated converter) 102 and a STATCOM (static synchronous compensator) 103 located on the same converter transformer branch in addition to the converter transformer. Figure 1As shown, the STATCOM 103 on the same converter transformer branch is arranged on the side of the converter transformer close to the LCC 102. This arrangement makes the installation positions of the STATCOM 103 and the LCC 102 closer, thereby providing the possibility of arranging the STATCOM 103 and the LCC 102 together (for example, in the same valve hall).

[0024] Figure 2 It is a three-phase circuit diagram of an LCC (grid commutation converter) and an embedded STATCOM (static synchronous compensator) in a high-voltage direct current transmission system according to the present disclosure and the present embodiment. The grid commutation converter includes a 6-pulse converter valve. In some other embodiments, the grid commutation converter may also include a 12-pulse converter valve. Taking the 6-pulse converter valve as an example, the 6-pulse converter valve is a three-phase bridge converter bridge, and each phase of the converter bridge includes an upper and lower half bridge, a total of six bridge arms, and each bridge arm is provided with a half-controlled thyristor valve. Among them, valves T1 and T2, T3 and T4, T5 and T6 respectively constitute three arm pairs, and the central terminal of each arm pair is correspondingly connected to the three phases a, b, and c of the converter transformer. STATCOM is connected to the AC system by direct hanging, that is, each phase of STATCOM is directly connected to the central terminal of each arm pair of the 6-pulse converter valve.

[0025] STATCOM is connected in parallel in the power grid, which is equivalent to a variable reactive current source. Its reactive current can be flexibly controlled to automatically compensate for the reactive power required by the system. On the one hand, it effectively solves the problem of harmonic interference in parallel capacitors. On the other hand, it can suppress or control harmonics according to actual requirements and improve power quality. STATCOM can be realized by connecting a voltage source inverter (VSC) in parallel to the power grid through a reactor or transformer. By adjusting the amplitude and phase of the output voltage on the AC side of the inverter, it can quickly absorb or output the required reactive power, thereby achieving the purpose of rapid dynamic reactive adjustment. As an active compensation device, STATCOM can not only track and compensate the impact current of impact loads, but also track and compensate harmonic currents.

[0026] The IGBT compensation device STATCOM has advantages over the traditional compensation methods represented by fixed capacitor compensation, mechanical switch capacitor switching, thyristor capacitor switching, etc. First of all, STATCOM has anti-harmonic function, which can better ensure the safety of the system. STATCOM is a controllable current source that only compensates for the fundamental reactive current. The system harmonic current will not cause damage to the compensation equipment, which prolongs its life and reduces the maintenance workload. At the same time, it avoids the harmonic amplification that may be caused by the capacitor bank and prevents other equipment and compensation equipment in the system from being damaged by harmonic overvoltage. In addition, STATCOM has dynamic continuous smooth compensation and higher response speed, which makes the compensation effect of voltage flicker better. STATCOM can follow the load changes, dynamically and continuously compensate the power factor, can output reactive power, and can also absorb reactive power, avoiding the situation of reactive reverse transmission. STATCOM can also solve the problem of load imbalance. STATCOM not only does not generate harmonics, but also can dynamically compensate harmonics while compensating reactive power. Finally, the current source characteristics and output reactive current of STATCOM are not affected by bus voltage, while the traditional SVC (static VAR compensation device) has impedance characteristics, and the output current decreases linearly with the bus voltage. STATCOM has great advantages when used for voltage control. The lower the system voltage, the more dynamic reactive power is needed to support the voltage. The output reactive current of STATCOM has nothing to do with the system voltage; and the lower the system voltage, the lower the ability of SVC to output reactive current.

[0027] The system consisting of LCC (line-commutated converter) and STATCOM has been widely studied in the past few years. The advantages of LCC HVDC with STATCOM include mitigation of commutation failure, enhanced stability, improved power quality, etc. So far, most of the related research projects are based on the configuration of STATCOM and LCC connected to the common AC bus of the converter. The inventors realized that since the LCC-HVDC system and the embedded STATCOM share the same converter transformer, the operation of the STATCOM will affect the action of the converter transformer tap controller, thereby affecting the operation of the HVDC system. Therefore, Figure 1 The topology shown requires coordinated operation of the tap controller and the STATCOM.

[0028] The present disclosure proposes a control method for coordinated operation of a STATCOM and a converter transformer tap controller for an LCC-HVDC system with an embedded STATCOM valve. According to one or more embodiments of the present disclosure, the LCC can always operate at a rated operating point to obtain optimal system performance, at which the valve-side AC voltage will remain at a rated voltage position. Figure 3 FIG. 3 is a flow chart showing a control method 300 for a high voltage direct current transmission system according to an embodiment of the present disclosure. Figure 3As shown, the method 300 includes:

[0029] Step 310, obtaining operating parameters of the power transmission system; and

[0030] In response to determining that the operating parameters deviate from the preset rated parameters, step 320 uses a synchronous static compensator and selectively uses a tap controller to adjust the valve-side AC voltage according to the offset value of the operating parameters from the rated parameters so that the offset value of the valve-side AC voltage relative to the rated voltage is less than a voltage offset threshold.

[0031] In step 310, the operating parameters of the power transmission system are parameters indicating the current operating state of the power transmission system, including but not limited to parameters such as valve-side AC voltage, trigger angle and / or DC current on the rectifier side, and shut-off angle and / or DC voltage on the inverter side. For the same power transmission system, these operating parameters are interrelated. For example, when the power transmission system has a certain valve-side AC voltage, the power transmission system should have a trigger angle and / or DC current on the rectifier side and a shut-off angle and / or DC voltage on the inverter side corresponding to the valve-side AC voltage. Therefore, it can be understood that when the valve-side AC voltage is offset relative to the rated voltage, other operating parameters will also be offset relative to their respective rated parameters, and the offset value of the valve-side AC voltage relative to the rated voltage can be indirectly determined based on the offset value of the operating parameter.

[0032] In step 320, the valve side AC voltage can be adjusted with reference to the offset value of the operating parameter. When the valve side AC voltage is adjusted so that the operating parameter is almost offset relative to its rated parameter (for example: the offset value of the operating parameter from the rated parameter is less than the preset parameter threshold), it can be determined that the offset value of the valve side AC voltage relative to the rated voltage has been made less than the voltage offset threshold, thereby ensuring the normal operation of the entire power transmission system. Take the trigger angle on the rectifier side as an example for explanation. For example, the angle difference between the trigger angle currently detected by the power transmission system and the rated trigger angle preset by the system is A (for example: π / 12). Then, the valve side AC voltage will be adjusted subsequently so that the current trigger angle changes, thereby reducing the above-mentioned angle difference A. When the above-mentioned angle difference A is adjusted to be close to 0, the corresponding offset value of the valve side AC voltage relative to the rated voltage is also basically close to 0.

[0033] In the above step 320, the synchronous static compensator is mainly used to adjust the valve side AC voltage. As mentioned above, STATCOM is equivalent to a variable reactive current source, whose reactive current can be flexibly controlled to automatically compensate for the reactive power required by the system, and indirectly adjust the valve side AC voltage by adjusting the compensated reactive power. However, in some cases, the offset value of the valve side AC voltage is large, exceeding the upper limit of the reactive power adjustment of the synchronous static compensator. In this case, the tap controller needs to be used to adjust the valve side AC voltage at the same time.

[0034] In some embodiments, the synchronous static compensator may have a maximum output value and a maximum absorption value for reactive power. Using the synchronous static compensator and selectively using the tap controller to adjust the valve-side AC voltage includes: in response to determining that when the synchronous static compensator is adjusted to have the maximum output value or the maximum absorption value, the offset value of the valve-side AC voltage relative to the rated voltage is still greater than the voltage offset threshold, and simultaneously adjusting the valve-side AC voltage using the synchronous static compensator and the tap controller.

[0035] Generally speaking, the nominal AC voltage should correspond to the zero reactive power output of the synchronous static compensator (STATCOM) and the zero tap position of the converter transformer. When the system AC voltage changes, the embedded STATCOM first outputs / absorbs reactive power to maintain the valve-side AC voltage. As long as the valve-side AC voltage remains at the rated voltage value, the tap controller can operate at the rated operating point, that is, the rectifier side maintains a constant DC current and trigger angle at the same time, or maintains a constant turn-off angle and DC voltage on the inverter side at the same time, which is quite beneficial to the system performance.

[0036] However, as mentioned above, considering the wide range of AC voltage variation and the capacity limitation of STATCOM, a coordinated tap control is required. When the STATCOM capacity is fully used, the tap controller starts to work to make the valve-side AC voltage close to the rated voltage value, and then the STATCOM is used for fine adjustment. Since the tap controller has a wide adjustment range and can cope with the entire AC voltage variation range of the system, the system can always operate at the rated operating point under steady-state conditions.

[0037] Figure 4 The flowchart of the control method 400 for a high voltage direct current transmission system according to an embodiment of the present disclosure is shown. The tap controller includes a plurality of tap contacts. In the method 400, the valve side AC voltage is regulated by using the synchronous static compensator and the tap controller at the same time. The method 400 is executed after determining that the offset value of the valve side AC voltage exceeds the reactive power regulation upper limit of the synchronous static compensator. Figure 4 As shown, the method 400 includes:

[0038] Step 410, adjusting the tap contact currently connected to the tap controller so that the valve-side AC voltage is close to the rated voltage; and

[0039] Step 420: Adjust the reactive power output or absorbed by the synchronous static compensator so that the offset value of the valve-side AC voltage relative to the rated voltage is smaller than a voltage offset threshold.

[0040] The transformer in the high-voltage direct current transmission system is a static electrical device that converts AC power of one voltage level into AC power of another voltage level of the same frequency through electromagnetic induction between coils and using the law of electromagnetic induction. Generally, several taps, also known as taps, are drawn from the high-voltage winding of the transformer. The tap controller is a device used to control the switching of taps. The tap controller changes the voltage-to-transformation ratio of the transmission system by switching the taps connected to the transmission system, thereby adjusting the AC voltage on the valve side.

[0041] It can be understood that in some embodiments, when it is determined that the STATCOM and the tap controller need to be adjusted simultaneously, the STATCOM can be adjusted first, and then the tap controller can be adjusted. For example, as described above, after determining that the offset value of the valve-side AC voltage exceeds the reactive power adjustment upper limit of the synchronous static compensator, the tap controller is adjusted. However, in other embodiments, the tap controller can be adjusted first, and then the STATCOM can be adjusted. For example, in response to determining that the adjustment of the tap controller reduces the offset value of the valve-side AC voltage relative to the rated voltage, the tap controller can be roughly adjusted first, and then the STATCOM can be used for fine adjustment. In short, the present disclosure does not limit the operating sequence of the STATCOM and the tap controller. The "simultaneous adjustment" mentioned above should be understood as operating both the STATCOM and the tap controller, and should not be understood as the two must be operated at the same time or there is other operating time sequence that must be met.

[0042] Figure 5 A flow chart of a method 500 for adjusting a valve side AC voltage according to an example of the present disclosure is shown. Figure 5 As shown, the method 500 includes:

[0043] Step 510, it is detected that the AC voltage changes, and before the change occurs, the HVDC transmission system operates at a nominal operating point, the output / absorbed reactive power Qs of the STATCOM is 0, and the tap controller is controlled at the position of the initial tap (hereinafter also referred to as 0 tap);

[0044] Step 520, STATCOM outputs / absorbs reactive power Qs to adjust the valve side AC voltage;

[0045] Step 530, determining whether the STATCOM has reached the maximum regulation capacity;

[0046] Step 540, if the determination result of step 530 is yes, maintaining the adjusted valve-side AC voltage at the rated voltage; and

[0047] Step 550 , if the determination result of step 530 is no, the tap controller of the AC transformer moves one step to make the valve-side AC voltage close to the rated voltage.

[0048] The above steps 520, 530 and 550 may be loop steps and may be performed multiple times until the offset value of the valve-side AC voltage is less than the voltage offset threshold. In this case, the tap controller may move multiple steps.

[0049] In some embodiments, the synchronous static compensator is located at the rectifier side of the power transmission system and is used to adjust the valve-side AC voltage of the rectifier side. In this case, the operating parameters include a firing angle and / or a DC current, and adjusting the reactive power output or absorbed by the synchronous static compensator includes: adjusting the reactive power output or absorbed by the synchronous static compensator based on the firing angle and / or the DC current of the power transmission system input to the synchronous static compensator.

[0050] In some embodiments, the synchronous static compensator is located at the inverter side of the power transmission system and is used to adjust the valve side AC voltage of the inverter side. In this case, the operating parameters include a cut-off angle and / or a DC voltage, and adjusting the reactive power output or absorbed by the synchronous static compensator includes: adjusting the reactive power output or absorbed by the synchronous static compensator based on the cut-off angle and / or the DC voltage of the power transmission system input to the synchronous static compensator.

[0051] In the present application, a new control function is also developed in STATCOM. STATCOM does not have to refer only to the valve side AC voltage for voltage regulation, and it can also refer to other relevant operating parameters to adjust the output or absorbed reactive power. For example, in the above embodiment, STATCOM can refer to the trigger angle of the LCC rectifier side or the DC voltage of the LCC inverter side to adjust the output or absorbed reactive power. These operating parameters can be input into the built-in PI controller of STATCOM in real time for PI feedback regulation, so as to finally stabilize the operating parameters near the rated parameters.

[0052] According to another aspect of the present disclosure, a high voltage direct current power transmission system is also provided. Figure 6 FIG. 6 shows a structural block diagram of a high voltage direct current transmission system 600 according to an embodiment of the present disclosure. Figure 6As shown, the system 600 includes: a grid-commutated converter 610, including a plurality of converter valves, wherein the grid-commutated converter is electrically connected to a corresponding AC transformer; an AC transformer having a tap controller 620 for adjusting the voltage-transformation ratio of the AC transformer; a static synchronous compensator 630, the terminals of which are electrically connected between the grid-commutated converter and the corresponding AC transformer; a detection device 640, configured to obtain operating parameters of the transmission system; and a controller 650, the controller being configured to, in response to determining that an operating parameter deviates from a preset rated parameter, adjust the valve-side AC voltage using the synchronous static compensator and selectively using the tap controller according to an offset value of the operating parameter deviating from the rated parameter, so that the offset value of the valve-side AC voltage relative to the rated voltage is less than a voltage offset threshold.

[0053] In some embodiments, the synchronous static compensator has a maximum output value and a maximum absorption value for reactive power, and wherein the controller 650 is further configured to: in response to determining that when the synchronous static compensator is adjusted to have a maximum output value or a maximum absorption value, the offset value of the valve-side AC voltage relative to the rated voltage is still greater than a voltage offset threshold, simultaneously adjust the valve-side AC voltage using the synchronous static compensator and the tap controller.

[0054] In some embodiments, the synchronous static compensator has a maximum output value and a maximum absorption value for reactive power, and wherein the controller 650 is further configured to: in response to determining that the adjustment of the tap controller reduces the offset value of the valve-side AC voltage relative to the rated voltage, simultaneously adjust the valve-side AC voltage using the synchronous static compensator and the tap controller.

[0055] In some embodiments, the tap controller includes multiple tap contacts, and wherein the controller 650 is further configured to: adjust the tap contact currently connected to the tap controller so that the valve side AC voltage is close to the rated voltage; and adjust the reactive power output or absorbed by the synchronous static compensator so that the offset value of the valve side AC voltage relative to the rated voltage is less than the voltage offset threshold.

[0056] In some embodiments, the synchronous static compensator is located on the rectifier side of the transmission system and is used to adjust the valve-side AC voltage on the rectifier side. The operating parameters include a trigger angle and / or a DC current, and wherein the controller 650 is further configured to adjust the reactive power output or absorbed by the synchronous static compensator based on the trigger angle and / or the DC current of the transmission system input to the synchronous static compensator.

[0057] In some embodiments, the synchronous static compensator is located on the inverter side of the transmission system, and is used to adjust the valve side AC voltage on the inverter side. The operating parameters include the shutdown angle and / or the DC voltage, and wherein the controller 650 is further configured to: adjust the reactive power output or absorbed by the synchronous static compensator based on the shutdown angle and / or the DC voltage of the transmission system input to the synchronous static compensator.

[0058] In order to meet the coordinated operation of STATCOM and transformer tap controller, the present disclosure also proposes a configuration method for a high-voltage direct current transmission system. The transmission system includes a grid-commutated converter with multiple converter valves, a tap controller for an AC transformer, and a synchronous static compensator, which adjusts the valve-side AC voltage of the transmission system by outputting or absorbing the reactive power of the transmission system. Figure 7 A flow chart of a configuration method 700 for a high voltage direct current transmission system according to an embodiment of the present disclosure is shown. Figure 7 The method is used to determine the configuration of the tap controller. Figure 7 As shown, the above configuration method 700 includes:

[0059] Step 710, determining the maximum value, minimum value and rated voltage value of the valve-side AC voltage of the power transmission system;

[0060] Step 720, determining the maximum voltage transformation ratio of the AC transformer based on the ratio of the maximum value of the valve-side AC voltage to the rated voltage value;

[0061] Step 730, determining the minimum voltage transformation ratio of the AC transformer based on the ratio of the minimum value of the valve-side AC voltage to the rated voltage value; and

[0062] Step 740: configure a plurality of tap contacts of the tap controller according to the maximum voltage transformation ratio and the minimum voltage transformation ratio.

[0063] In step 710, the maximum and minimum values ​​of the valve-side AC voltage can be actually detected during the normal operation of the relevant high-voltage direct current transmission system, so as to determine the configuration of the tap contacts of the transformer in the future; or the maximum and minimum values ​​of the valve-side AC voltage can be detected by simulating the model of the relevant high-voltage direct current transmission system through calculation. In steps 720 and 730, the maximum voltage transformation ratio and the minimum voltage transformation ratio of the AC transformer are determined so that the adjustment of the tap can cover the range of the valve-side AC voltage of the entire high-voltage direct current transmission system. After obtaining the voltage transformation ratio of the transformer, the specific configuration of multiple tap contacts (such as the number of tap contacts and the position of each contact on the transformer, etc.) can be further determined in step 740.

[0064] Figure 8FIG. 8 is a flow chart of a configuration method 800 for a high voltage direct current transmission system according to another embodiment of the present disclosure. The method 800 is used to determine the configuration of a STATCOM, such as Figure 8 As shown, the configuration method 800 includes:

[0065] Step 810, determining a first reactive power consumed by a grid-commutated converter of a power transmission system;

[0066] Step 820, determining a second reactive power between an AC bus of the power transmission system and a grid-commutated converter;

[0067] Step 830, determining a maximum output value and a maximum absorption value of the static synchronous compensator for reactive power according to the first reactive power and the second reactive power; and

[0068] Step 840 , configuring the static synchronous compensator based on the maximum output value and the maximum absorption value.

[0069] In step 810, determining the first reactive power includes: determining the first reactive power according to the number of converter valves of the grid commutation converter, the trigger angle / turn-off angle and the commutation overlap angle of the power transmission system, the DC current and the no-load DC voltage of the power transmission system.

[0070] In step 820 , determining the second reactive power includes determining the second reactive power according to the resistance and inductance of the equivalent circuit and a voltage difference between the valve-side AC voltage of the power transmission system and the AC bus voltage.

[0071] The method 800 will be described below by taking the inverter side as an example. Fig. 9 The single-line diagram of the inverter side of the LCC with an embedded STATCOM is shown. Where Uv is the valve-side AC voltage with a phase angle of θ relative to the AC bus voltage Uac, normalized to the transformer valve side. Qs is the reactive power provided by the STATCOM. Qf is the reactive power provided by the AC shunt capacitor represented by the reactance Xf. Qd is the reactive power consumption of the LCC. R, X are the impedances between the HVDC valve and the AC bus. P, Q are the active power and reactive power before the AC bus, and Pac, Qac are the active power and reactive power from the DC system to the AC grid.

[0072] The reactive power of LCC (i.e., the first reactive power) is defined as:

[0073]

[0074] Where n is the number of converter valves; γ is the shut-off angle; μ is the commutation overlap angle; I d is the DC current; U di0 It is the ideal no-load DC voltage. di0 With valve side AC voltage Uv The relationship is:

[0075]

[0076] Ideally, the valve side AC voltage U v The size of U is controlled by STATCOM. The LCCHVDC system also transmits active power to the AC system, U v Ahead of U ac , the phase angle is θ. Therefore, P, Q (that is, the second reactive power) can be expressed as:

[0077]

[0078]

[0079] in,

[0080] Here, U v and U ac is normalized relative to the transformer valve side voltage. Assume that U ac0 is the actual AC bus voltage, then:

[0081]

[0082] Among them, Tr is the transformer ratio.

[0083] The reactive power provided by the AC shunt capacitor can be regarded as:

[0084]

[0085] Therefore, the reactive power provided by STATCOM can be calculated as:

[0086] Q s =Q d +Q (8)

[0087] At the nominal operating point, the nominal AC voltage corresponds to zero reactive power output of the STATCOM and zero tap position of the converter transformer. The LCC DC transmission system delivers rated active power, and its reactive consumption is met only by the AC shunt capacitors. By solving the above equations with the nominal system parameters, the nominal transformer ratio can be determined.

[0088] When the AC bus voltage changes, the STATCOM will output / absorb reactive power to maintain the valve-side AC voltage. Assuming there is no transformer tap controller, the STATCOM capacity will be set quite large and will not be able to meet the requirements of the entire AC voltage variation range, which will drive up costs. Through the above-mentioned coordinated control between the STATCOM and the transformer tap changer, the required STATCOM capacity can be significantly reduced. In this way, the LCC HVDC system can always operate at the optimal operating point, which is very beneficial to the operation of the entire system. In addition, compared with the ordinary LCC HVDC system, the number of required transformer taps can also be reduced. The number of tap steps mentioned above refers to the number of selectable transformer ratios.

[0089] In a normal LCC HVDC system, the required tap changer range is calculated as follows:

[0090]

[0091]

[0092] Among them, U acmax , U acmin , U acN are the maximum, minimum and rated AC voltage respectively; U di0N It's U di0 Rated value; U di0max OLTC , U di0min OLTC is the maximum and minimum U for the tap changer range di0 . Maximum and minimum transformer ratio n max and n min It will be further used to determine the number of steps of the tap controller.

[0093] By utilizing the proposed coordinated control, the second term in the above equations (9) and (10) is always equal to 1. Therefore, the number of required tap changer steps can be greatly reduced. Coordinated control helps to avoid frequent tap changer operations, thereby improving system reliability.

[0094] It should be understood that in this specification, the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships or dimensions that are based on the orientations or positional relationships or dimensions shown in the accompanying drawings, and these terms are used only for the convenience of description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the scope of protection of the present disclosure.

[0095] In addition, the terms "first", "second", "third", etc. are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", "third" may explicitly or implicitly include one or more of the feature. In the description of the present disclosure, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0096] In the present disclosure, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present disclosure can be understood according to specific circumstances.

[0097] In the present disclosure, unless otherwise expressly specified and limited, a first feature being “above” or “below” a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being “above”, “above”, and “above” a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being “below”, “below”, and “below” a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0098] This specification provides many different embodiments or examples that can be used to implement the present disclosure. It should be understood that these different embodiments or examples are purely exemplary and are not intended to limit the scope of protection of the present disclosure in any way. Those skilled in the art can conceive of various changes or replacements based on the disclosure of the present specification, which should all be included in the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure should be based on the scope of protection defined by the attached claims.

Claims

1. A control method for a high voltage direct current transmission system, wherein: The power transmission system comprises a grid-commutated converter having a plurality of converter valves, a tap controller for an AC transformer, and a synchronous static compensator, wherein the synchronous static compensator adjusts the valve-side AC voltage of the power transmission system by outputting or absorbing reactive power of the power transmission system, and the control method comprises: obtaining operating parameters of the power transmission system; and In response to determining that the operating parameter deviates from a preset rated parameter, the valve-side AC voltage is adjusted using the synchronous static compensator and selectively using the tap controller according to the offset value of the operating parameter deviating from the rated parameter so that the offset value of the valve-side AC voltage relative to the rated voltage is less than a voltage offset threshold.

2. The control method according to claim 1, wherein: The synchronous static compensator has a maximum output value and a maximum absorption value for reactive power, and wherein, in response to determining that the operating parameter deviates from a preset rated parameter, adjusting the valve-side AC voltage using the synchronous static compensator and selectively using the tap controller according to an offset value of the operating parameter from the rated parameter comprises: In response to determining that when the synchronous static compensator is adjusted to have a maximum output value or a maximum absorption value, the offset value of the valve-side AC voltage relative to the rated voltage is still greater than the voltage offset threshold, the valve-side AC voltage is adjusted using the synchronous static compensator and the tap controller.

3. The control method according to claim 1, wherein: The synchronous static compensator has a maximum output value and a maximum absorption value for reactive power, and wherein, in response to determining that the operating parameter deviates from a preset rated parameter, adjusting the valve-side AC voltage using the synchronous static compensator and selectively using the tap controller according to an offset value of the operating parameter from the rated parameter comprises: In response to determining that the tap controller is adjusted so that the offset value of the valve-side AC voltage relative to the rated voltage is reduced, the valve-side AC voltage is adjusted using the synchronous static compensator and the tap controller at the same time.

4. The control method according to claim 2 or 3, wherein: The tap controller includes a plurality of tap contacts, and wherein simultaneously using the synchronous static compensator and the tap controller to adjust the valve-side AC voltage includes: adjusting the tap contact currently connected to the tap controller so that the valve-side AC voltage approaches the rated voltage; and The reactive power output or absorbed by the synchronous static compensator is adjusted so that an offset value of the valve-side AC voltage relative to the rated voltage is smaller than a voltage offset threshold.

5. The control method according to claim 4, wherein: The synchronous static compensator is located at the rectifier side of the power transmission system and is used to adjust the valve side AC voltage on the rectifier side. The operating parameters include a trigger angle and / or a DC current. The step of adjusting the reactive power output or absorbed by the synchronous static compensator includes: The reactive power output or absorbed by the synchronous static compensator is adjusted based on the firing angle and / or the direct current of the power transmission system input to the synchronous static compensator.

6. The control method according to claim 4, wherein: The synchronous static compensator is located at the inverter side of the power transmission system and is used to adjust the valve side AC voltage of the inverter side. The operating parameters include a cut-off angle and / or a DC voltage. The step of adjusting the reactive power output or absorbed by the synchronous static compensator includes: The reactive power output or absorbed by the synchronous static compensator is adjusted based on the cut-off angle and / or the DC voltage of the power transmission system input to the synchronous static compensator.

7. A high voltage direct current transmission system, comprising: A grid-commutated converter, comprising a plurality of converter valves, wherein the grid-commutated converter is electrically connected to a corresponding AC transformer; The AC transformer has a tap controller for adjusting the voltage transformation ratio of the AC transformer; A static synchronous compensator, the terminals of which are electrically connected between the grid-commutated converter and the corresponding AC transformer; a detection device configured to obtain operating parameters of the power transmission system; and A controller, wherein the controller is configured to, in response to determining that the operating parameter deviates from a preset rated parameter, adjust the valve-side AC voltage using the synchronous static compensator and selectively using the tap controller according to the offset value of the operating parameter deviating from the rated parameter, so that the offset value of the valve-side AC voltage relative to the rated voltage is less than a voltage offset threshold.

8. The power transmission system according to claim 7, wherein: The synchronous static compensator has a maximum output value and a maximum absorption value for reactive power, and wherein the controller is further configured to: In response to determining that when the synchronous static compensator is adjusted to have a maximum output value or a maximum absorption value, the offset value of the valve-side AC voltage relative to the rated voltage is still greater than the voltage offset threshold, the valve-side AC voltage is adjusted using the synchronous static compensator and the tap controller.

9. The power transmission system according to claim 7, wherein: The synchronous static compensator has a maximum output value and a maximum absorption value for reactive power, and wherein the controller is further configured to: In response to determining that the tap controller is adjusted so that the offset value of the valve-side AC voltage relative to the rated voltage is reduced, the valve-side AC voltage is adjusted using the synchronous static compensator and the tap controller at the same time.

10. The power transmission system according to claim 8 or 9, wherein: The tap controller comprises a plurality of tap contacts, and wherein the controller is further configured to: adjusting the tap contact currently connected to the tap controller so that the valve-side AC voltage approaches the rated voltage; and The reactive power output or absorbed by the synchronous static compensator is adjusted so that an offset value of the valve-side AC voltage relative to the rated voltage is smaller than a voltage offset threshold.

11. The power transmission system according to claim 10, wherein: The synchronous static compensator is located at the rectifier side of the power transmission system and is used to adjust the valve side AC voltage of the rectifier side. The operating parameters include a trigger angle and / or a DC current. The controller is further configured to: The reactive power output or absorbed by the synchronous static compensator is adjusted based on the firing angle and / or the direct current of the power transmission system input to the synchronous static compensator.

12. The power transmission system according to claim 10, wherein: The synchronous static compensator is located at the inverter side of the power transmission system and is used to adjust the valve side AC voltage of the inverter side. The operating parameters include a cut-off angle and / or a DC voltage. The controller is further configured to: The reactive power output or absorbed by the synchronous static compensator is adjusted based on the cut-off angle and / or the DC voltage of the power transmission system input to the synchronous static compensator.

13. A configuration method for a high voltage direct current transmission system, wherein: The power transmission system comprises a grid-commutated converter having a plurality of converter valves, a tap controller for an AC transformer, and a synchronous static compensator, wherein the synchronous static compensator adjusts the valve-side AC voltage of the power transmission system by outputting or absorbing reactive power of the power transmission system, and the configuration method comprises: Determining a maximum value, a minimum value and a rated voltage value of a valve-side AC voltage of the power transmission system; Determining a maximum voltage transformation ratio of the AC transformer based on a ratio of a maximum value of the valve-side AC voltage to the rated voltage value; Determining a minimum voltage transformation ratio of the AC transformer based on a ratio of a minimum value of the valve-side AC voltage to the rated voltage value; and A plurality of tap contacts of the tap controller are configured according to the maximum voltage transformation ratio and the minimum voltage transformation ratio.

14. The configuration method according to claim 13, further comprising: determining a first reactive power consumed by a grid-commutated converter of the power transmission system; determining a second reactive power between an AC bus of the power transmission system and the grid-commutated converter; Determining a maximum output value and a maximum absorption value of reactive power by the static synchronous compensator according to the first reactive power and the second reactive power; as well as The static synchronous compensator is configured based on the maximum output value and the maximum absorption value.

15. The configuration method according to claim 14, wherein: Determining a first reactive power consumed by a grid-commutated converter of the power transmission system comprises: The first reactive power is determined according to the number of converter valves of the grid commutation converter, the trigger angle or the shutdown angle and the commutation overlap angle of the power transmission system, the DC current and the no-load DC voltage of the power transmission system.

16. The configuration method according to claim 14, wherein: Determining a second reactive power in an equivalent circuit between an AC bus of the power transmission system and the grid-commutated converter comprises: The second reactive power is determined according to the resistance and inductance of the equivalent circuit and the voltage difference between the valve-side AC voltage of the power transmission system and the AC bus voltage.

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