A method for shutting down a multi-terminal hybrid DC transmission system

Abstract

The present invention discloses a method for shutting down a multi-terminal hybrid direct current (HVDC) power transmission system. Under the DC pole minimum power level, the trigger angle of the thyristor converter on the rectifier side is first increased to greater than 90 degrees, after which the thyristor converter is locked. The active power control inverter-side voltage source converter is then locked. Finally, the DC voltage reference value of the DC voltage control inverter-side voltage source converter is reduced from the shutdown initial value to 0 at a set slope. After a set delay, the DC voltage control inverter-side voltage source converter is locked. This method can achieve a smooth shutdown of the multi-terminal hybrid HVDC power transmission system and rapid discharge of the DC line, meeting the shutdown and maintenance needs of the multi-terminal hybrid HVDC power transmission system.

Description

Technical Field

[0001] The present invention belongs to the technical field of direct current (DC) transmission, and in particular relates to a method for shutting down a DC transmission system. Background Art

[0002] High-voltage direct current (HVDC) transmission systems can be divided into two types: conventional direct current transmission (LCC-HVDC) systems based on thyristor converters (SCRs) and flexible direct current transmission (VSC-HVDC) systems based on fully controlled voltage source converters (VSCs). Conventional direct current transmission (CDC) systems offer low cost, low losses, and mature operating technology. The vast majority of DC transmission systems currently in operation worldwide are LCC-HVDC systems. However, conventional direct current transmission (CDC) systems suffer from disadvantages such as susceptibility to commutation failures on the inverter side, strong dependence on the AC system, the need to absorb large amounts of reactive power, and the large footprint of converter stations. The new generation of flexible direct current transmission (VSCs) systems offer advantages such as decoupled control of active and reactive power, the ability to supply power to passive networks, a compact structure with a small footprint, and no commutation failures. However, they also have the disadvantage of higher costs. Therefore, combining the advantages of conventional and flexible direct current transmission (CDC) transmission, hybrid direct current transmission systems using thyristor converters at the sending-end converter station and voltage source converters at the receiving-end converter station have promising engineering application prospects.

[0003] A multi-terminal hybrid DC transmission system is a DC transmission system consisting of at least three converter stations connected by high-voltage DC transmission lines. It can realize the interconnection of multiple AC power grids with different transmission and absorption capabilities, while saving transmission line corridors. It is a more flexible hybrid DC transmission method. The common three-terminal hybrid DC transmission system consists of two DC poles, positive and negative, such as Figure 1 As shown, each DC pole is a complete DC transmission circuit including a rectifier end connected to the sending-end AC grid, two inverter ends connected to different receiving-end AC grids, a DC transmission line connecting the rectifier end and the inverter end, and a grounding pole. The rectifier end uses a thyristor converter and the inverter end uses a voltage source converter.

[0004] for Figure 1 The multi-terminal hybrid DC transmission system shown in the figure generally selects one terminal as the DC voltage control inverter terminal to control the DC voltage of the multi-terminal DC system, and the remaining terminals as active power control inverter terminals to control the DC power of the local terminal. For multi-terminal hybrid DC systems, the conventional shutdown method is to first lock the thyristor converter on the rectifier side and then lock the voltage source converter on the inverter side. This shutdown method has a significant impact on the power grid system, and the DC line voltage remains at a high level after the shutdown. It takes a long time to complete the natural discharge of the DC line, which is not conducive to the rapid implementation of power outage maintenance work. Therefore, it is necessary to combine the characteristics of the multi-terminal hybrid DC transmission system to develop a shutdown method that can achieve smooth shutdown of the multi-terminal hybrid DC transmission system and rapid discharge of the DC line. Summary of the Invention

[0005] In order to solve the technical problems mentioned in the above background technology, the present invention proposes a shutdown method for a multi-terminal hybrid direct current transmission system.

[0006] In order to achieve the above technical objectives, the technical solution of the present invention is:

[0007] A method for shutting down a multi-terminal hybrid direct current (DC) power transmission system. Any DC pole of the multi-terminal hybrid DC power transmission system includes a rectifier terminal connected to a sending-end AC power grid, at least two inverter terminals connected to a receiving-end AC power grid, and a DC transmission line connecting the rectifier terminal and the inverter terminal. The rectifier terminal utilizes a thyristor converter, and the inverter terminal utilizes a voltage source converter. One of the inverter terminals is selected as a DC voltage-controlled inverter terminal, and the remaining terminals are selected as active power-controlled inverter terminals. The method comprises the following steps:

[0008] After the DC pole shutdown command is issued, the rectifier end and the active power control inverter end reduce their respective DC power to the minimum power level;

[0009] According to the DC voltage control target of the DC pole at the minimum power level, the DC voltage reference value U of the voltage source converter at the DC voltage control inverter end is obtained. dVref The initial shutdown value U dVref-Stop ;

[0010] The rectifier end increases the trigger angle of the thyristor converter at the local end to a state greater than 90 degrees and then locks the thyristor converter, and sends the rectifier end locking signal to each inverter end through the inter-end communication channel;

[0011] The active power control inverter end immediately blocks the local voltage source converter after receiving the blocking signal from the rectifier end, and sends the blocking signal to the DC voltage control inverter end through the inter-end communication channel;

[0012] After the DC voltage control inverter detects that the blocking signals of the rectifier and active power control inverter appear, the DC voltage reference value U dVref By the initial shutdown value U dVref-Stop The voltage drops to 0 at the set slope, and the voltage source converter at this end is locked after the set delay time T1, and the DC pole shutdown process ends.

[0013] Furthermore, the DC voltage reference value U of the voltage source converter at the DC voltage control inverter end is obtained according to the DC voltage control target of the DC pole at the minimum power level. dVref The initial shutdown value U dVref-Stop The process is as follows:

[0014] According to the DC voltage control target of the DC pole at the minimum power level, the DC voltage reference value U of the DC pole at the DC voltage control inverter end is obtained. dcref ;

[0015] When the DC voltage control inverter of the DC pole includes a voltage source converter, U dcref The shutdown initial value U as the DC voltage reference value of the voltage source converter dVref-Stop ;

[0016] When the DC voltage control inverter end of the DC pole includes two or more voltage source converters running in series, U dcref According to the total number M of voltage source converters running in series, the shutdown initial value U is used as the DC voltage reference value of each operating voltage source converter. dVref-Stop ,in M is a positive integer.

[0017] Furthermore, for the DC voltage control inverter end, the DC voltage reference value of the voltage source converter at this end is The bridge arm reference voltage is calculated as the DC bias of the bridge arm voltage of the voltage source converter.

[0018] Furthermore, the calculation method of the bridge arm reference voltage is as follows:

[0019] u pj =0.5U dVref -u jo-ref

[0020] u nj =0.5U dVref +u jo-ref

[0021] Among them, u pj is the upper arm reference voltage of phase j, u nj is the lower arm reference voltage of phase j, u jo-ref is the j-phase AC reference voltage.

[0022] Furthermore, for the DC voltage control inverter end, the DC voltage reference value U dVref and the DC voltage measurement value U of the voltage source converter dV The difference after subtraction is adjusted by PI to obtain the d-axis current reference value, which realizes the control of the DC voltage of the voltage source converter.

[0023] Furthermore, when the DC voltage-controlled inverter end includes two or more voltage source converters operating in series, one of the voltage source converters is selected as the master converter, and the other voltage source converters are all used as slave converters. Each slave converter synchronously maintains its DC voltage reference value consistent with the DC voltage reference value of the master converter.

[0024] Compared with the prior art, the present invention has the following significant effects:

[0025] During the outage process, by sequentially locking each end and controlling the DC voltage to 0 before locking the DC voltage control inverter end, rapid discharge and zero-power level locking of the DC line can be achieved. This makes the outage process smoother and can effectively reduce the impact on the power grid system, better meeting the outage and maintenance needs of the multi-terminal hybrid DC transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the main circuit of a multi-terminal hybrid DC transmission system;

[0027] Figure 2 is a schematic diagram of the voltage source converter topology;

[0028] Figure 3 It is a flow chart of a method for shutting down a multi-terminal hybrid HVDC system;

[0029] Figure 4 It is a schematic diagram of the topology of two or more voltage source converters operating in series;

[0030] Figure 5 This is the structure diagram of the voltage source converter controller. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0032] In the multi-terminal hybrid direct current (DC) power transmission system of the present invention, any DC pole includes a rectifier terminal connected to a sending-end AC grid, at least two inverter terminals connected to a receiving-end AC grid, and a DC transmission line connecting the rectifier and inverter terminals. The rectifier terminals utilize thyristor converters, and the inverter terminals utilize voltage source converters. One of the inverter terminals is selected as a DC voltage-controlled inverter terminal, and the remaining terminals are selected as active power-controlled inverter terminals. This achieves smooth shutdown of the multi-terminal hybrid DC power transmission system and rapid discharge of the DC line, meeting the shutdown and maintenance needs of the multi-terminal hybrid DC power transmission system.

[0033] like Figure 1 The main circuit diagram of the multi-terminal hybrid DC transmission system provided by this embodiment is shown. It consists of two DC poles, positive and negative. Each DC pole is a complete DC transmission circuit, including a rectifier-side converter, two inverter-side converters, a DC line, and a grounding electrode. This embodiment uses a thyristor converter on the rectifier side and a voltage source converter on the inverter side. The voltage source converter adopts a modular multilevel structure and includes submodules of one or both of the following types:

[0034] The first type of submodule is a submodule that can output positive, negative, and zero levels in a non-latching state, such as a full-bridge submodule (FBSM).

[0035] The second type of sub-module is a sub-module that can only output positive and zero levels in a non-blocking state, such as a half-bridge sub-module (HBSM) and a quasi-full-bridge sub-module (SFBSM).

[0036] The topology diagram of the voltage source converter is as follows: Figure 2 As shown, where U ap 、U bp 、U cp are the upper arm voltages of the three phases a, b, and c of the voltage source converter, U an 、U bn 、U cn are the lower arm voltages of phases a, b, and c respectively; L0 is the inductance of the arm reactor.

[0037] The bridge arm submodule configuration of the voltage source converter includes the following two methods:

[0038] Method 1: The upper and lower bridge arms of each phase are composed of cascaded first-type submodules;

[0039] In the second mode, both the upper and lower bridge arms of each phase are hybrid bridge arms composed of two types of sub-modules, namely, the first type sub-module and the second type sub-module, which are cascaded, and the number of the two types of sub-modules in each bridge arm is configured in the same ratio.

[0040] for Figure 1 The multi-terminal hybrid DC transmission system shown in the figure generally selects one inverter terminal as the DC voltage-controlled inverter terminal, and the remaining terminals as active power-controlled inverter terminals. The rectifier terminal and active power-controlled inverter terminal control the DC power, while the DC voltage-controlled inverter terminal controls the DC voltage. After the rectifier terminal is first blocked, the DC side of the multi-terminal hybrid DC transmission system will have no power input. The DC line voltage is determined by the DC voltage-controlled inverter terminal. If the DC line voltage is controlled to drop to 0 and then the DC voltage-controlled inverter terminal is blocked, rapid discharge of the line can be achieved. For the DC voltage-controlled inverter terminal voltage source converter, its DC voltage is controlled based on the DC voltage reference value.

[0041] Based on the above analysis, this embodiment proposes a method for shutting down a multi-terminal hybrid DC transmission system, such as Figure 3 As shown, the steps are as follows:

[0042] S100: After the DC pole shutdown command is issued, the rectifier end and the active power control inverter end respectively reduce their respective DC powers to the minimum power level.

[0043] S200: Obtaining a DC voltage reference value U of the DC voltage control inverter voltage source converter according to the DC voltage control target of the DC pole at the minimum power leveldVref The initial shutdown value U dVref-Stop .

[0044] Regarding step S200, specifically, when the DC voltage-controlled inverter terminal of the DC pole is a single operating voltage source converter, the corresponding method is:

[0045] S211: Obtaining a DC voltage reference value U of the DC voltage control inverter terminal according to the DC voltage control target of the DC voltage at the minimum power level dcref ;

[0046] S212: When the DC voltage control inverter of the DC pole includes a voltage source converter, U dcref The shutdown initial value U as the DC voltage reference value of the voltage source converter dVref-Stop .

[0047] For step S200, specifically, when the following Figure 4 For the topology shown, when the DC voltage-controlled inverter end of the DC pole includes two or more voltage source converters operating in series, the corresponding method is:

[0048] S221: Obtaining a DC voltage reference value U of the DC voltage control inverter terminal according to the DC voltage control target of the DC voltage at the minimum power level dcref ;

[0049] S222: Control the DC voltage at the DC pole of the inverter end to obtain a DC voltage reference value U dcref According to the total number M of voltage source converters running in series, the shutdown initial value U is used as the DC voltage reference value of each operating voltage source converter. dVref-Stop ,in M is a positive integer.

[0050] S300: The rectifier end increases the trigger angle of the thyristor converter at the local end to a state greater than 90 degrees and then locks the thyristor converter, and sends a rectifier end locking signal to each inverter end through the inter-end communication channel.

[0051] S400: After receiving the blocking signal from the rectifier end, the active power control inverter end immediately blocks the local voltage source converter and sends the local blocking signal to the DC voltage control inverter end through the inter-end communication channel.

[0052] S500: After the DC voltage control inverter receives the blocking signals from the rectifier and active power control inverter, the DC voltage reference value U of the voltage source converter at the local end is set. dVref By the initial shutdown value U dVref-Stop The voltage drops to 0 at the set slope, and the local voltage source converter is locked after the set delay time T1, and the DC pole shutdown process ends.

[0053] Figure 5 The DC voltage control inverter end voltage source converter controller structure diagram provided by the present invention. In order to realize the control of the DC voltage control inverter end voltage source converter, preferably, the DC voltage reference value U dVref of The bridge arm reference voltage is calculated as the DC bias of the bridge arm voltage of the voltage source converter. The calculation formula of the bridge arm reference voltage is as follows:

[0054] u pj =0.5U dVref -u jo-ref

[0055] u nj =0.5U dVref +u jo-ref

[0056] (j=a,b,c)

[0057] Where: u pj is the upper arm reference voltage of phase j, u nj is the lower arm reference voltage of phase j, u jo-ref is the j-phase AC reference voltage.

[0058] In this embodiment, preferably, the voltage source converter DC voltage reference value U dVref and the DC voltage measurement value U of the voltage source converter dV The difference after subtraction is adjusted by PI to obtain the d-axis current reference value I sd-ref , to realize the control of the DC voltage of the voltage source converter.

[0059] In this embodiment, preferably, when the DC voltage-controlled inverter end of the DC pole includes two or more voltage source converters operating in series, one of the voltage source converters is selected as the master converter, and the other voltage source converters are all slave converters, and each slave converter synchronously maintains its DC voltage reference value consistent with the DC voltage reference value of the master converter.

[0060] The embodiments are only for illustrating the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for shutting down a multi-terminal hybrid direct current (HVDC) power transmission system, wherein any DC pole of the multi-terminal hybrid HVDC power transmission system includes a rectifier terminal connected to a sending-end AC grid, at least two inverter terminals connected to a receiving-end AC grid, and a DC transmission line connecting the rectifier terminal and the inverter terminal; the rectifier terminal uses a thyristor converter, and the inverter terminal uses a voltage source converter; one of the inverter terminals is selected as a DC voltage-controlled inverter terminal, and the remaining terminals are selected as active power-controlled inverter terminals; the method is characterized in that: The shutdown method comprises the following steps: After the DC pole shutdown command is issued, the rectifier end and the active power control inverter end reduce their respective DC power to the minimum power level; According to the DC voltage control target of the DC pole at the minimum power level, the DC voltage reference value U of the voltage source converter at the DC voltage control inverter end is obtained. dVref The initial shutdown value U dVref-Stop ; The rectifier end increases the trigger angle of the thyristor converter at the local end to a state greater than 90 degrees and then locks the thyristor converter, and sends the rectifier end locking signal to each inverter end through the inter-end communication channel; The active power control inverter end immediately blocks the local voltage source converter after receiving the blocking signal from the rectifier end, and sends the blocking signal to the DC voltage control inverter end through the inter-end communication channel; After the DC voltage control inverter detects that the blocking signals of the rectifier and active power control inverter appear, the DC voltage reference value U dVref By the initial shutdown value U dVref-Stop The voltage drops to 0 at the set slope, and the voltage source converter at this end is locked after the set delay time T1, and the DC pole shutdown process ends.

2. The method for shutting down a multi-terminal hybrid direct current transmission system according to claim 1, characterized in that: According to the DC voltage control target of the DC pole at the minimum power level, the DC voltage reference value U of the voltage source converter at the DC voltage control inverter end is obtained. dVref The initial shutdown value U dVref-Stop The process is as follows: According to the DC voltage control target of the DC pole at the minimum power level, the DC voltage reference value U of the DC pole at the DC voltage control inverter end is obtained. dcref ; When the DC voltage control inverter of the DC pole includes a voltage source converter, U dcref The shutdown initial value U as the DC voltage reference value of the voltage source converter dVref-Stop ; When the DC voltage control inverter end of the DC pole includes two or more voltage source converters running in series, U dcref According to the total number M of voltage source converters running in series, the shutdown initial value U is used as the DC voltage reference value of each operating voltage source converter. dVref-Stop ,in M is a positive integer.

3. The method for shutting down a multi-terminal hybrid direct current transmission system according to claim 1, characterized in that: For the DC voltage control inverter end, the DC voltage reference value U dVref of The bridge arm reference voltage is calculated as the DC bias of the bridge arm voltage of the voltage source converter.

4. The method for shutting down a multi-terminal hybrid direct current transmission system according to claim 3, characterized in that: The calculation method of the bridge arm reference voltage is as follows: in pj =0.5U dVref -in jo-ref in nj =0.5U dVref +in jo-ref Among them, u pj is the upper arm reference voltage of phase j, u nj is the lower arm reference voltage of phase j, u jo-ref is the j-phase AC reference voltage.

5. The method for shutting down a multi-terminal hybrid direct current transmission system according to claim 1, characterized in that: For the DC voltage control inverter end, the DC voltage reference value U dVref The DC voltage measurement value U of the voltage source converter dV The difference after subtraction is adjusted by PI to obtain the d-axis current reference value, which realizes the control of the DC voltage of the voltage source converter.

6. The method for shutting down a multi-terminal hybrid direct current transmission system according to claim 1, characterized in that: When the DC voltage-controlled inverter end includes two or more voltage source converters operating in series, one of the voltage source converters is selected as the master converter, and the other voltage source converters are all used as slave converters. Each slave converter synchronously maintains its DC voltage reference value consistent with the DC voltage reference value of the master converter.

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