A reactive power compensation and AC filtering system applied to high voltage direct current transmission

Abstract

The application discloses a reactive power compensation and AC filter device system applied to high-voltage direct current transmission, wherein active and passive reactive power compensation and AC filter devices arranged reasonably at the valve side of a converter transformer and the grid side can effectively improve the reactive power compensation and AC filter performance of a converter station, greatly reduce the harmonic current of the converter transformer and the adjusting frequency of a tap switch, improve the overall performance of the equipment of the converter station, reduce the land occupation area of the converter station, and make the traditional high-voltage direct current transmission technology better adapt to the environment of a new power system.

Description

Technical Field

[0001] This invention belongs to the field of power systems and power electronics, and specifically relates to a reactive power compensation and AC filtering system for high voltage direct current transmission. Background Technology

[0002] The core equipment of a high-voltage direct current (HVDC) transmission system is the converter, which converts AC to DC. The converter works by controlling the thyristors to conduct at a fixed angle within the power frequency cycle, thus converting AC to DC. During this process, the converter absorbs a large amount of reactive power from the grid and injects a significant amount of harmonics into the grid. Therefore, conventional HVDC projects require a large number of reactive power compensation and AC filtering devices. Currently, most converter stations adopt a solution of installing passive reactive power compensation and AC filtering equipment on the converter transformer side. Passive reactive power compensation and AC filtering equipment is composed of passive components such as power capacitors, reactors, and resistors. It exhibits capacitive reactive power characteristics for the fundamental frequency, achieving reactive power compensation, and forms a low-impedance path for specific harmonics, achieving filtering. Passive equipment has a relatively simple structure and high reliability, but it also has some significant drawbacks: 1) The equipment consists of a large number of groups, requiring a large area. 2) Single-group equipment uses mechanical switches for switching, which is slow and reactive power cannot be continuously adjusted. Switching under weak system conditions will cause large system voltage fluctuations. Especially for new energy power transmission scenarios in new power system environments, the overvoltage problem is very prominent and may lead to a large number of new energy sources being disconnected from the grid in severe cases. 3) The installation location is on the converter transformer grid side, and its effect on voltage support on the converter transformer valve side is limited in the event of a fault; 4) The filtering performance is greatly affected by the harmonic impedance of the AC system. When the actual system conditions change beyond the design range, the filtering performance deteriorates. 5) Equipment stress is greatly affected by background harmonics. When the background harmonics of the system increase beyond the design range, it can easily cause the filter equipment to overload and trip, affecting the normal operation of the station.

[0003] With the technological advancements in power electronic switching devices, Static Synchronous Compensators (STATCOMs) have been widely adopted in power systems. A STATCOM is a new type of power electronic device used for dynamically suppressing harmonics and compensating for reactive power. It can compensate for harmonics and reactive power of varying magnitudes. Its basic principle is to detect harmonic and reactive currents from the object being compensated, and then generate a compensating current of equal magnitude but 180° out of phase, thereby canceling out the harmonics and reactive power.

[0004] The core device of STATCOM is a controllable converter valve, whose output voltage and current are highly controllable. Compared with passive devices, it has the following advantages: 1) A single STATCOM unit can flexibly handle reactive power and various harmonics, and perform selective control as needed, requiring fewer equipment groups and occupying less space; 2) The reactive power and harmonic output can be continuously and smoothly adjusted with fast adjustment speed. When the DC operating conditions change, STATCOM has a stronger dynamic support capability for AC system voltage and is more suitable for application scenarios where new energy is transmitted through DC. 3) STATCOM is not sensitive to changes in AC system voltage and impedance, frequency and passive RLC component parameters. Through the regulation of the control system, the reactive power compensation and filtering effect are basically unaffected. 4) When the background voltage and harmonics exceed the design range, the control system can take limiting measures to prevent the STATCOM converter valve from overloading.

[0005] Based on the excellent characteristics of STATCOM, some DC transmission projects have configured STATCOM on the converter substation side for dynamic reactive power compensation. However, the capacity of the STATCOM used is relatively small, and most of the reactive power compensation in the converter station still uses passive equipment. Therefore, its actual effect is extremely limited, and it is difficult to overcome the inherent shortcomings of passive equipment. Moreover, the single grid-side configuration method is also difficult to give full play to the advantages of STATCOM. Therefore, how to better utilize STATCOM and passive equipment in DC transmission projects is an urgent problem to be solved in practical applications. Summary of the Invention

[0006] The purpose of this application is to propose a reactive power compensation and AC filtering system for high voltage direct current transmission. By rationally arranging reactive power compensation and AC filtering equipment on the converter transformer valve side and the grid side, the reactive power compensation and AC filtering performance of the converter station can be effectively improved.

[0007] To achieve the above objectives, this application adopts the following technical solution: A reactive power compensation and AC filtering system for high-voltage direct current (HVDC) transmission includes a reactive power compensation and AC filtering branch on the converter transformer valve side and a reactive power compensation and AC filtering branch on the converter transformer grid side, wherein: The reactive power compensation and AC filter branch on the converter transformer valve side is installed on the AC lead on the converter transformer valve side, adopts STATCOM form, and is configured in units of six-pulse converters. Each converter transformer valve side is configured with at least one parallel STATCOM branch. The reactive power compensation and AC filter branch on the converter transformer side is installed on the AC bus on the converter transformer side. It adopts a combination of STATCOM and passive filter. According to the unified configuration of the whole station, the converter transformer side with the common bus is configured with at least one parallel STATCOM branch and at least one parallel passive filter branch.

[0008] According to some embodiments, the reactive power compensation and AC filtering system is characterized in that the converter valve topology of the STATCOM branch on the converter valve side adopts a modular multilevel structure, the three-phase converter valve adopts a single-star connection, double-star connection, or delta connection with the neutral point not grounded, and the sub-module connection adopts a single-phase full-bridge converter structure.

[0009] According to some embodiments, the STATCOM branch on the converter transformer side dynamically compensates for the reactive power of each six-pulse converter, with a reactive power capacity Q. STATCOM_V Equal to the maximum reactive power Q of the thyristor converter valve LCC6 and the maximum reactive power Q of the corresponding converter transformer TR6 The sum of the values; the STATCOM branch on the converter valve side uses an active filter to filter out the low-frequency harmonic current generated by a single six-pulse converter, and the design value of each harmonic current is equal to the maximum value of the harmonic current of the six-pulse converter.

[0010] According to some embodiments, the converter valve topology of the STATCOM branch on the converter substation side adopts a modular multilevel structure, the three-phase converter valve adopts a single-star connection method or a double-star connection method with the neutral point directly grounded, and the sub-module connection adopts a single-phase full-bridge converter structure.

[0011] According to some embodiments, the STATCOM branch on the converter substation side dynamically compensates for the reactive power on the converter substation side to support the AC system voltage.

[0012] According to some embodiments, when any STATCOM branch on the converter transformer valve side fails to operate due to a fault, resulting in insufficient reactive power and low-frequency harmonic current on the valve side, the STATCOM branch on the converter transformer grid side compensates for the reactive power and low-frequency harmonic current of the corresponding twelve-pulse converter. The reactive power capacity Q of the STATCOM branch on the converter transformer grid side is... STATCOM_G Equal to the grid-side reactive power demand value Q AC_GRID Maximum reactive power Q of a single twelve-pulse converter LCC12 The sum of these values ​​equals the design value of each harmonic current, which is equal to the maximum harmonic current of a single twelve-pulse converter.

[0013] According to some embodiments, the passive filter branch on the converter transformer side has high-pass characteristics and adopts a high-pass damping filter or parallel capacitor form to filter out the high-frequency harmonic currents generated by all thyristor converter valves. The design value of each harmonic current is equal to the sum of the maximum harmonic currents of all DC converters in the entire station.

[0014] Compared with the prior art, the beneficial effects of this application are: 1) The use of STATCOM replaces most passive static var compensators and AC filter equipment, reducing the equipment's footprint; 2) Compared with conventional grid-side reactive power compensation technology, compensating the reactive power of the six-pulse converter through valve-side STATCOM can stabilize the valve-side voltage of the converter transformer, so that the firing angle remains almost unchanged during the rise and fall of DC power, thereby greatly reducing the number of adjustments to the converter transformer tap changer and improving the reliability of converter transformer operation. 3) Compared with conventional grid-side filtering technology, the low-frequency harmonic current of the six-pulse converter can be greatly reduced by compensating the low-frequency harmonic current of the converter transformer through the valve-side STATCOM, thereby reducing the heat generation and vibration of the converter transformer and further improving the reliability of the converter transformer operation. 4) A combination of active and passive filtering methods is adopted. STATCOM active filtering is used for low frequencies and passive filtering is used for high frequencies, giving full play to their respective advantages, resulting in higher filtering efficiency and stronger performance. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the reactive power compensation and AC filtering system for high-voltage direct current transmission using a single twelve-pulse converter provided in this embodiment; Figure 2 This is a schematic diagram of a STATCOM single-star connection.

[0016] Figure 3 This is a schematic diagram of a STATCOM dual-star connection.

[0017] Figure 4 This is a schematic diagram of the STATCOM triangle connection method.

[0018] Figure 5 This is the main wiring diagram of a second-order high-pass damped filter and a parallel capacitor. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0020] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection, an electrical connection, or a connection that allows for mutual communication; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two elements or the interaction between two elements. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances. In the description of this application, "multiple" means two or more, unless otherwise expressly and specifically limited. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more features.

[0021] The following disclosure provides many different implementations or examples for carrying out different structures of this application. To simplify the disclosure of this application, the components and arrangements of specific examples are described below. Of course, these are merely examples and are not intended to limit this application.

[0022] This application provides a reactive power compensation and AC filtering system for high-voltage direct current (HVDC) transmission, including a reactive power compensation and AC filtering branch on the converter transformer valve side and a reactive power compensation and AC filtering branch on the converter transformer network side. The reactive power compensation and AC filtering branch on the converter transformer valve side is installed on the AC lead of the converter transformer valve side, using a STATCOM configuration, and is configured on a six-pulse converter unit basis, with at least one parallel STATCOM branch configured on each converter transformer valve side. The reactive power compensation and AC filtering branch on the converter transformer network side is installed on the AC bus of the converter transformer network side, using a combination of STATCOM and passive filters, configured uniformly throughout the station, with at least one parallel STATCOM branch and at least one parallel passive filter branch configured on the converter transformer network side sharing the bus.

[0023] like Figure 1The illustrated embodiment is a schematic diagram of a reactive power compensation and AC filtering system for a twelve-pulse converter high-voltage direct current transmission. Each twelve-pulse converter consists of a Y / Y bridge six-pulse converter and a Y / D bridge six-pulse converter. The reactive power compensation and AC filtering system of this invention is shown in the dashed box. This system includes reactive power compensation and AC filtering branches on the converter transformer valve side and on the converter transformer grid side. A STATCOM branch is configured on the AC lead between the Y / Y bridge converter transformer and the Y / Y bridge converter valve, and another STATCOM branch is configured on the AC lead between the Y / D bridge converter transformer and the Y / D bridge converter valve. The reactive power compensation and AC filtering branches on the converter transformer grid side are installed on the AC bus on the converter transformer grid side, using a combination of STATCOM and passive filters. Following a unified configuration for the entire station, this embodiment configures one parallel STATCOM branch and one parallel passive filter branch. The embodiments of this application, by rationally arranging active and passive reactive power compensation and AC filtering equipment on the valve side and grid side of the converter transformer, can effectively improve the reactive power compensation and AC filtering performance of the converter station, greatly reduce the harmonic current of the converter transformer and the number of tap changer adjustments, improve the overall performance of the converter station equipment, reduce the footprint of the converter station, and make traditional high-voltage direct current transmission technology better adapt to the new power system environment.

[0024] The main equipment of both the valve-side STATCOM and the grid-side STATCOM consists of STATCOM converter valves and connecting reactors. The STATCOM converter valve topology adopts a modular multilevel structure, with each phase valve group consisting of N sub-modules linked in series. The number of sub-modules is determined by the voltage level of the connected AC system and the rated voltage of each sub-module. The sub-module links adopt a single-phase full-bridge converter structure.

[0025] The STATCOM three-phase converter valve on the valve side adopts a single-star connection, a double-star connection, or a delta connection with the neutral point not grounded.

[0026] Unlike the valve-side STATCOM, the grid-side STATCOM three-phase converter valve adopts a single-star connection or a double-star connection with the neutral point directly grounded.

[0027] STATCOM single star connection method as follows Figure 2 As shown, the neutral point of the STATCOM on the valve side is not grounded, while the neutral point of the STATCOM on the grid side is directly grounded.

[0028] STATCOM dual-star connection method as follows Figure 3 As shown, the neutral point of the STATCOM on the valve side is not grounded, while the neutral point of the STATCOM on the grid side is directly grounded.

[0029] STATCOM triangle connection method as follows Figure 4 As shown.

[0030] In some embodiments, the valve-side STATCOM dynamically compensates for the reactive power of each six-pulse converter, with a reactive power capacity Q. STATCOM_V Equal to the maximum reactive power Q of the thyristor converter valve LCC6 and the maximum reactive power Q of the corresponding converter transformer TR6 The sum of the two; secondly, the valve-side STATCOM branch uses an active filter to filter out the low-frequency harmonic current generated by a single six-pulse converter, and the design value of each harmonic current is equal to the maximum value of the harmonic current of the six-pulse converter.

[0031] In some embodiments, the grid-side STATCOM dynamically compensates for reactive power on the grid side of the converter transformer to support the AC system voltage. Furthermore, when any STATCOM branch on the valve side of the converter transformer fails to operate, resulting in insufficient reactive power and low-frequency harmonic current on the valve side, the grid-side STATCOM branch compensates for the reactive power and low-frequency harmonic current of the corresponding twelve-pulse converter. Its reactive power capacity Q... STATCOM_G Equal to the grid-side reactive power demand value Q AC_GRID Maximum reactive power Q of a single twelve-pulse converter LCC12 The sum of these values ​​equals the design value of each harmonic current, which is equal to the maximum harmonic current of a single twelve-pulse converter. By jointly configuring the STATCOM branch on the grid side and the valve side, the grid-side STATCOM serves as a backup in the event of a valve-side STATCOM failure, which is more reliable than configuring compensation only on the valve side.

[0032] For LCC DC converter valves, the characteristic harmonic of the valve-side current is 6k±1, and the characteristic harmonic of the grid-side current is 12k±1. The higher the frequency, the smaller the amplitude of the harmonic current. Typically, STATCOM active filters perform well in filtering low-frequency harmonics below the 50th order. However, for higher-frequency harmonics, the filtering effect of STATCOM deteriorates due to sampling and control errors. Passive filters, on the other hand, are much easier to use for filtering high-frequency harmonics. Because high-frequency components are far from the fundamental frequency and have small amplitudes, only a small-capacity passive filter is needed to achieve good filtering results. Therefore, using STATCOM and passive filters for high- and low-frequency coordinated filtering is an efficient solution.

[0033] In some embodiments, the grid-side passive filter branch has high-pass characteristics and can be in the form of a high-pass damping filter or a parallel capacitor to filter out the high-frequency harmonic currents generated by all thyristor converter valves. The design value of each harmonic current is equal to the sum of the maximum harmonic currents of all DC converters in the entire station.

[0034] The main wiring diagram of the second-order high-pass damped filter and parallel capacitor is as follows: Figure 5 As shown.

[0035] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A reactive power compensation and AC filtering system for high-voltage direct current transmission, characterized in that, This includes reactive power compensation and AC filtering branches on the converter transformer valve side and reactive power compensation and AC filtering branches on the converter transformer grid side, wherein: The reactive power compensation and AC filter branch on the converter transformer valve side is installed on the AC lead on the converter transformer valve side, adopts STATCOM form, and is configured in units of six-pulse converters. Each converter transformer valve side is configured with at least one parallel STATCOM branch. The reactive power compensation and AC filter branch on the converter transformer side is installed on the AC bus on the converter transformer side. It adopts a combination of STATCOM and passive filter. According to the unified configuration of the whole station, the converter transformer side with the common bus is configured with at least one parallel STATCOM branch and at least one parallel passive filter branch. When any STATCOM branch on the converter transformer valve side fails to operate, resulting in insufficient reactive power and low-frequency harmonic current on the valve side, the STATCOM branch on the converter transformer grid side compensates for the reactive power and low-frequency harmonic current of the corresponding twelve-pulse converter. The reactive power capacity Q of the STATCOM branch on the converter transformer grid side is... STATCOM_G Equal to the grid-side reactive power demand value Q AC_GRID Maximum reactive power Q of a single twelve-pulse converter LCC12 The sum of these values ​​equals the design value of each harmonic current, which is equal to the maximum harmonic current of a single twelve-pulse converter. The twelve-pulse converter consists of a Y / Y bridge six-pulse converter and a Y / D bridge six-pulse converter.

2. The reactive power compensation and AC filtering system as described in claim 1, characterized in that, The converter valve topology of the STATCOM branch configured on the converter transformer valve side adopts a modular multilevel structure. The three-phase converter valve of the STATCOM branch configured on the converter transformer valve side adopts a single star connection, double star connection, or delta connection with a neutral point not grounded. The submodule link adopts a single-phase full-bridge converter structure.

3. The reactive power compensation and AC filtering system as described in claim 1, characterized in that, The STATCOM branch configured on the converter valve side dynamically compensates for the reactive power of each six-pulse converter, with a reactive power capacity Q. STATCOM_V Equal to the maximum reactive power Q of the thyristor converter valve LCC6 and the maximum reactive power Q of the corresponding converter transformer TR6 The sum of the values; the STATCOM branch configured on the converter valve side uses an active filtering method to filter out the low-frequency harmonic current generated by a single six-pulse converter, and the design value of each harmonic current is equal to the maximum value of the harmonic current of the six-pulse converter.

4. The reactive power compensation and AC filtering system as described in claim 1, characterized in that, The converter valve topology of the STATCOM branch configured on the converter substation side adopts a modular multilevel structure. The three-phase converter valve of the STATCOM branch configured on the converter substation side adopts a single-star connection method or a double-star connection method with the neutral point directly grounded. The sub-module link adopts a single-phase full-bridge converter structure.

5. The reactive power compensation and AC filtering system as described in claim 1, characterized in that, The STATCOM branch configured on the converter substation side dynamically compensates for the reactive power on the converter substation side to support the AC system voltage.

6. The reactive power compensation and AC filtering system as described in claim 1, characterized in that, The passive filter branch configured on the converter transformer side has high-pass characteristics and adopts the form of a high-pass damping filter or parallel capacitor to filter out the high-frequency harmonic currents generated by all thyristor converter valves. The design value of each harmonic current is equal to the sum of the maximum harmonic currents of all six-pulse converters in the entire station.

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