A method for handling an ac side fault of a hybrid dc transmission system

Abstract

The application discloses a kind of hybrid DC power transmission system AC side fault processing method, when serious AC fault of receiving end AC power grid appears, the DC voltage reference value of inverter end voltage source converter is quickly reduced to 0 from normal value, and the trigger angle of rectifier end thyristor converter is quickly increased to greater than 90 degrees state;When serious AC fault of receiving end AC power grid disappears, the DC voltage reference value of inverter end voltage source converter is quickly restored to normal value from 0, and the trigger angle of rectifier end thyristor converter is quickly restored to normal angle state.The application can quickly eliminate the difference between inverter end voltage source converter AC, DC side power after receiving end AC power grid fault, avoid the continuous rise of sub-module capacitor voltage and DC voltage, realize hybrid DC power transmission system when serious AC fault of receiving end AC power grid occurs Fault crossing, meet the processing needs of hybrid DC power transmission system AC side fault.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for handling AC side faults of a power transmission system, in particular to a method for handling AC side faults of a hybrid HVDC power transmission system. BACKGROUND

[0002] High voltage direct current (HVDC) power transmission systems can be divided into two types: conventional HVDC power transmission system (LCC-HVDC) based on thyristor converters and flexible HVDC power transmission system (VSC-HVDC) based on voltage source converters. The conventional HVDC power transmission system has low cost, small loss and mature operation technology, and most of the HVDC power transmission systems in operation in the world are LCC-HVDC systems. However, the conventional HVDC power transmission system has the disadvantages of easy commutation failure at the inverter side, strong dependence on the AC system, need to absorb a large amount of reactive power, and large land occupation of the converter station. The new generation of flexible HVDC power transmission system has the advantages of decoupling control of active power and reactive power, power supply to passive networks, compact structure, small land occupation, and no commutation failure problem, but has the disadvantage of high cost. Therefore, a hybrid HVDC power transmission system with one end of the converter station using thyristor converters and the other end of the converter station using voltage source converters has a good engineering application prospect by combining the advantages of the conventional HVDC power transmission system and the flexible HVDC power transmission system.

[0003] A common two-terminal hybrid HVDC power transmission system is composed of positive and negative DC poles, as shown in FIG. 1. Each DC pole is a complete DC power transmission loop including a rectifier end connected to the sending AC power grid, an inverter end connected to the receiving AC power grid, a DC power 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. Figure 1

[0004] For the two-terminal hybrid HVDC power transmission system, the sending AC power grid is connected to the rectifier end of the positive DC pole and the rectifier end of the negative DC pole, and the receiving AC power grid is connected to the inverter end of the positive DC pole and the inverter end of the negative DC pole. Figure 1 ​The hybrid DC transmission system shown has its AC and DC power balanced under normal operating conditions. The voltage of the submodule capacitors and the DC voltage of the DC pole of the voltage source converter remain stable. When a fault occurs in the AC grid at the receiving end, the drop in AC voltage will limit the power output of the inverter voltage source converter and cause a difference between the AC and DC power. The power difference will be stored in the submodule capacitors in the form of capacitor energy storage, causing the voltage of the submodule capacitors and the DC voltage to rise. If the voltage rises too high, it will cause the DC system to shut down. Therefore, it is necessary to take necessary measures to handle AC faults. The commonly used approach is to increase the firing angle of the thyristor converter after detecting a DC voltage rise to a certain level at the rectifier end, while keeping the DC voltage control target at the inverter end unchanged. This suppresses the rise in submodule capacitor voltage and DC voltage by limiting the power input to the DC system on the rectifier side. However, this method is less effective at suppressing voltage rise when a severe AC fault occurs in the receiving-end AC grid, and may still lead to the shutdown of the DC system due to overvoltage. Therefore, it is necessary to study corresponding handling methods for severe AC faults in the receiving-end AC grid of a hybrid DC transmission system in order to achieve fault ride-through. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a method for handling AC-side faults in a hybrid DC transmission system that enables fault ride-through when a severe AC fault occurs in the receiving-end AC grid.

[0006] Technical solution: A method for handling AC side faults in a hybrid DC transmission system, comprising the following steps:

[0007] (1) Obtain the reference value U of the DC voltage of the inverter-end voltage source converter based on the DC voltage control target of the DC pole. dVref Normal value U dVref-Nom ;

[0008] (2) When the inverter detects a severe AC fault signal from the receiving-end AC grid, after a set delay of T1, the DC voltage reference value U of the inverter voltage source converter is... dVref From normal value U dVref-Nom The slope is reduced to 0 according to the set slope, and the severe AC fault signal of the AC power grid at the receiving end is sent to the rectifier end through the inter-end communication channel;

[0009] (3) When the rectifier detects a severe AC fault signal in the receiving AC power grid, the firing angle of the thyristor converter at the rectifier end is rapidly increased to a state greater than 90 degrees.

[0010] (4) When the inverter detects the disappearance of the severe AC fault signal of the AC grid at the receiving end, the DC voltage reference value U of the inverter voltage source converter is adjusted. dVref The value rises from 0 to the normal value U at a set slope.dVref-Nom ;

[0011] (5) When the severe AC fault signal of the AC power grid at the receiving end is detected to disappear at the rectifier end, the firing angle of the thyristor converter at the rectifier end is quickly restored to the normal angle state, and the AC fault handling process ends.

[0012] Furthermore, the normal value U in step (1) dVref-Nom The acquisition method is as follows:

[0013] When the inverter terminal of the DC pole is a single operating voltage source converter, the implementation steps are as follows:

[0014] (111) Obtain the reference value U of the DC voltage at the DC pole of the voltage source converter based on the DC voltage control target of the DC pole. dcref ;

[0015] (112) When the inverter terminal of the DC pole contains a voltage source converter, the DC voltage reference value U dcref The normal value U, which serves as the reference value for the DC voltage of the voltage source converter. dVref-Nom ;

[0016] When the inverter terminal of the DC pole contains two or more voltage source converters operating in series, the implementation steps are as follows:

[0017] (121) Obtain the reference value U of the DC voltage at the DC pole of the voltage source converter based on the DC voltage control target of the DC pole. dcref ;

[0018] (122) The DC voltage reference value U dcref The voltage source converters are allocated according to the total number M of series-connected voltage source converters, and the normal value U is used as the DC voltage reference value for each operating voltage source converter. dVref-Nom ,in M is a positive integer.

[0019] Furthermore, each voltage source converter on the inverter side is equipped with a voltage source converter controller in its control device. The voltage source converter controller will input the DC voltage reference value U of the voltage source converter. dVref of The arm reference voltage is calculated as the DC bias of the bridge arm voltage in the voltage source converter.

[0020] u pj =0.5U dVref -u jo-ref

[0021] u nj =0.5U dVref +u jo-ref

[0022] Where j = a, b, c; u pj U is the reference voltage for the upper arm of phase j. nj U is the reference voltage for the lower arm of phase j. jo-ref This is the AC reference voltage for phase j.

[0023] Furthermore, the control method for the DC voltage of the voltage source converter is as follows: The reference value U of the DC voltage of the voltage source converter is... dVref DC voltage measurement value U of voltage source converter dV The difference after subtraction is used to obtain the d-axis current reference value after being adjusted by the DC voltage outer loop controller PI; then the d-axis current reference value is used as input and adjusted by the current inner loop controller to obtain the AC reference voltage.

[0024] Furthermore, when the inverter terminal of the DC pole contains two or more voltage source converters operating in series, one of the voltage source converters is selected as the master control converter, and the remaining voltage source converters are slave control converters. Each slave control converter synchronously maintains its DC voltage reference value consistent with the DC voltage reference value of the master control converter.

[0025] Furthermore, the detection method for the occurrence of a severe AC fault signal in the receiving-end AC power grid is any one or two of the following:

[0026] Method 1: Detect that the positive sequence modulus of the three-phase AC voltage collected by the inverter station at the receiving end of the AC grid is lower than the voltage threshold value U. set1 ;

[0027] Method 2: Detect the phase voltage amplitude of each phase of the three-phase AC grid collected by the inverter station, and set the phase AC voltage amplitude below the voltage threshold value U. set2 The sum of the quantities of each phase is greater than or equal to a set value N, where N is a positive integer and 1 ≤ N ≤ 3.

[0028] Positive sequence magnitude of three-phase AC voltage The expression is:

[0029]

[0030] in, The positive sequence d-axis component of the AC voltage. This represents the positive-sequence q-axis component of the AC voltage.

[0031] The amplitude of the AC voltage of this phase is defined as the absolute value of the peak value of the instantaneous AC voltage of this phase within this cycle.

[0032] Compared with the prior art, the present invention has the following significant effects: After a severe AC fault occurs in the receiving-end AC grid, by rapidly reducing the DC voltage of the inverter-end voltage source converter and coordinating the adjustment of the firing angle of the rectifier-end thyristor converter, it can limit the power input to the DC system at the rectifier end, rapidly reduce the power flowing into the inverter-end voltage source converter, and eliminate the power difference between the AC and DC sides of the inverter-end voltage source converter. This effectively avoids the continuous rise of the submodule capacitor voltage and DC voltage, realizes fault ride-through of the hybrid DC transmission system when a severe AC fault occurs in the receiving-end AC grid, and meets the needs of handling AC side faults in the hybrid DC transmission system. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the main circuit of a hybrid DC transmission system with two ends.

[0034] Figure 2 This is a schematic diagram of the voltage source converter topology of the present invention;

[0035] Figure 3 This is a flowchart of the AC side fault handling method of the present invention;

[0036] Figure 4 This is a schematic diagram of the topology of two or more voltage source converters operating in series in this invention;

[0037] Figure 5 The present invention provides a structural diagram of a voltage source converter controller. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0039] The hybrid DC transmission system of this invention includes, at any DC pole, a rectifier terminal connected to the sending-end AC grid, an inverter terminal connected to the receiving-end AC grid, and a DC transmission line connecting the rectifier terminal and the inverter terminal. The rectifier terminal employs a thyristor converter, and the inverter terminal employs a voltage source converter, enabling fault ride-through of the hybrid DC transmission system in the event of a severe AC fault in the receiving-end AC grid, thus meeting the requirements for handling AC-side faults in the hybrid DC transmission system.

[0040] like Figure 1 The diagram shows the main circuit of a two-terminal hybrid DC transmission system, consisting of positive and negative DC poles. Each DC pole is a complete DC transmission circuit including a rectifier-side converter, an inverter-side converter, DC lines, and a grounding electrode. The rectifier-side uses a thyristor converter, and the inverter-side uses a voltage source converter. Figure 2 The diagram shows a voltage source converter topology.

[0041] The voltage source converter adopts a modular multilevel structure and includes sub-modules of one or more of the following types:

[0042] The first type of submodule is a submodule that can output positive, negative and zero levels in the non-locked state, such as the full bridge submodule (FBSM).

[0043] The second type of submodule is one that can only output positive and zero levels in the non-locked state, such as half-bridge submodule (HBSM) and full-bridge submodule (SFBSM).

[0044] like Figure 2 As shown, where U ap U bp U cp These are the upper arm voltages of phases A, B, and C of the voltage source converter, U. an U bn U cn These are the lower bridge arm voltages for phases A, B, and C, respectively; L0 is the inductance value of the bridge arm reactor.

[0045] The bridge arm submodules of the voltage source converter can be configured in the following two ways:

[0046] Method 1: Each phase's upper and lower bridge arms are composed of cascaded first-type sub-modules;

[0047] Method 2: Each phase upper and lower bridge arm is a hybrid bridge arm composed of two types of sub-modules, namely the first type and the second type, cascaded together, with the same ratio of the two types of sub-modules in each bridge arm.

[0048] for Figure 1 The hybrid DC transmission system shown generally adopts the following basic control mode: the thyristor converter at the rectifier end controls the DC current, and the voltage source converter at the inverter end controls the DC voltage.

[0049] For an inverter-side voltage source converter, its DC-side power P dV AC side power P s They can be represented as follows:

[0050] P dV =U dV I dc (1)

[0051] P s =1.5u sd i sd (2)

[0052] In the formula, U dV I is the DC voltage of the voltage source converter. dc For direct current; u sd For the d-axis component of the AC voltage of the voltage source converter, i sd This represents the d-axis component of the AC current in the voltage source converter.

[0053] Under normal operating conditions, ignoring losses, the AC and DC power of the inverter-side voltage source converter is in balance, and the submodule capacitor voltage and DC voltage of the voltage source converter remain stable. When a fault occurs in the receiving-end AC grid, the amplitude of the inverter-side AC voltage drops, and the AC power P of the voltage source converter decreases. s As the current decreases, under the condition of maintaining a constant DC current at the rectifier end, a difference will appear between the AC and DC power of the receiving-end voltage source converter. The power difference will be stored in the submodule capacitor in the form of capacitor energy storage and will cause the submodule capacitor voltage and DC voltage to rise. The DC voltage will continue to rise before the AC and DC power of the receiving-end voltage source converter reaches balance.

[0054] As shown in equation (1), under the condition of maintaining a constant DC current at the rectifier end, if the DC voltage of the inverter-side voltage source converter is rapidly reduced to 0, the DC power of the inverter-side voltage source converter can be rapidly reduced to 0, thereby eliminating the power difference between the AC and DC sides of the voltage source converter after a fault in the AC grid at the receiving end, and preventing the submodule capacitor voltage and DC voltage from rising to excessively high levels, which would lead to DC shutdown. For the inverter-side voltage source converter, its DC voltage is obtained by target control based on the DC voltage reference value.

[0055] Based on the above analysis, the overall flowchart of the AC side fault handling method of the present invention is as follows: Figure 3 As shown, the implementation steps are as follows:

[0056] S100, obtains the DC voltage reference value U of the inverter-side voltage source converter based on the DC voltage control target of the DC pole. dVref Normal value U dVref-Nom .

[0057] When the inverter terminal of the DC pole is a single operating voltage source converter, the specific methods include:

[0058] S111, Obtain the reference value U of the DC voltage at the DC pole of the voltage source converter terminal based on the DC voltage control target of the DC pole. dcref ;

[0059] S112, when the inverter terminal of the DC pole includes a voltage source converter, U dcref The normal value U, which serves as the reference value for the DC voltage of the voltage source converter. dVref-Nom .

[0060] When adopting such Figure 4 In the topology shown, when the inverter terminal of the DC pole contains two or more voltage source converters operating in series, the specific configurations include:

[0061] S121, Obtain the reference value U of the DC voltage at the DC pole of the voltage source converter terminal based on the DC voltage control target of the DC pole. dcref ;

[0062] S122, the DC voltage reference value U at the DC terminal of the voltage source converter is... dcref The voltage source converters are allocated according to the total number M of series-connected voltage source converters, and the normal value U is used as the DC voltage reference value for each operating voltage source converter. dVref-Nom ,in,

[0063]

[0064] In equation (3), M is a positive integer.

[0065] S200, when the inverter detects a severe AC fault signal from the receiving-end AC grid, after a set delay of T1, the DC voltage reference value U of the inverter voltage source converter is released. dVref From normal value U dVref-Nom The slope is reduced to 0 according to the set slope, and the severe AC fault signal of the AC power grid at the receiving end is sent to the rectifier end through the inter-end communication channel.

[0066] S300: When the rectifier detects a severe AC fault signal in the receiving AC power grid, it rapidly increases the firing angle of the thyristor converter at the rectifier end to a state greater than 90 degrees.

[0067] S400, when the inverter detects the disappearance of the severe AC fault signal from the receiving-end AC grid, the DC voltage reference value U of the inverter voltage source converter is changed. dVref The value rises from 0 to the normal value U at a set slope. dVref-Nom .

[0068] S500: When the rectifier detects the disappearance of the severe AC fault signal of the receiving AC power grid, it quickly restores the firing angle of the thyristor converter at the rectifier end to the normal angle state, and the AC fault handling process ends.

[0069] for Figure 1 The hybrid DC transmission system shown has two ends, and each voltage source converter at the inverter end is equipped with a control device as follows: Figure 5 The voltage source converter controller is shown. In the voltage source converter control steps described above, the DC voltage reference value U of the voltage source converter is... dVref of The arm reference voltage is calculated as the DC bias of the bridge arm voltage in the voltage source converter. The formula for calculating the bridge arm reference voltage is as follows:

[0070] u pj =0.5U dVref -u jo-ref (4)

[0071] u nj =0.5U dVref +u jo-ref (5)

[0072] (j = a, b, c)

[0073] In the formula, u pj U is the reference voltage for the upper arm of phase j. nj U is the reference voltage for the lower arm of phase j. jo-ref This is the AC reference voltage for phase j.

[0074] At the same time, the DC voltage reference value U of the voltage source converter will be... dVref DC voltage measurement value U of voltage source converter dV The difference after subtraction is used to obtain the d-axis current reference value after being adjusted by the DC voltage outer loop controller PI; then the d-axis current reference value is used as input and adjusted by the current inner loop controller to obtain the AC reference voltage.

[0075] When the inverter terminal of the DC pole contains two or more voltage source converters operating in series, one of the voltage source converters is selected as the master converter, and the other converters are slave converters. Each slave converter synchronously maintains its DC voltage reference value consistent with the DC voltage reference value of the master converter.

[0076] The detection method for a severe AC fault signal in the receiving-end AC power grid can be any one or two of the following:

[0077] Method 1: Detect that the positive sequence modulus of the three-phase AC voltage collected by the inverter station at the receiving end of the AC grid is lower than the voltage threshold value U. set1 ;

[0078] Method 2: Detect the phase voltage amplitude of each phase of the three-phase AC voltage collected by the inverter station from the receiving-end AC grid, and set the phase AC voltage amplitude below the voltage threshold value U. set2 The sum of the quantities of each phase is greater than or equal to a set value N, where N is a positive integer and 1 ≤ N ≤ 3.

[0079] Among them, the positive sequence magnitude of the three-phase AC voltage The expression is: The positive sequence d-axis component of the AC voltage. The positive sequence q-axis component of the AC voltage; the amplitude of the AC voltage of this phase is defined as the absolute value of the peak value of the instantaneous AC voltage of this phase within this cycle.

[0080] 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 method for handling AC-side faults in a hybrid DC transmission system, characterized in that, The steps include the following: (1) Obtain the reference value U of the DC voltage of the inverter-end voltage source converter based on the DC voltage control target of the DC pole. dVref Normal value U dVref-Nom ; (2) When the inverter detects a severe AC fault signal from the receiving-end AC grid, after a set delay of T1, the DC voltage reference value U of the inverter voltage source converter is... dVref From normal value U dVref-Nom The slope is reduced to 0 according to the set slope, and the severe AC fault signal of the AC power grid at the receiving end is sent to the rectifier end through the inter-end communication channel; (3) When the rectifier detects a severe AC fault signal in the receiving AC power grid, the firing angle of the thyristor converter at the rectifier end is rapidly increased to a state greater than 90 degrees. (4) When the inverter detects the disappearance of the severe AC fault signal of the AC grid at the receiving end, the DC voltage reference value U of the inverter voltage source converter is adjusted. dVref The value rises from 0 to the normal value U at a set slope. dVref-Nom ; (5) When the severe AC fault signal of the AC power grid at the receiving end is detected to disappear at the rectifier end, the firing angle of the thyristor converter at the rectifier end is quickly restored to the normal angle state, and the AC fault handling process ends. The normal value U in step (1) dVref-Nom The acquisition method is as follows: When the inverter terminal of the DC pole is a single operating voltage source converter, the implementation steps are as follows: (111) Obtain the reference value U of the DC voltage at the DC pole of the voltage source converter based on the DC voltage control target of the DC pole. dcref ; (112) When the inverter terminal of the DC pole contains a voltage source converter, the DC voltage reference value U dcref The normal value U, which serves as the reference value for the DC voltage of the voltage source converter. dVref-Nom ; When the inverter terminal of the DC pole contains two or more voltage source converters operating in series, the implementation steps are as follows: (121) Obtain the reference value U of the DC voltage at the DC pole of the voltage source converter based on the DC voltage control target of the DC pole. dcref ; (122) The DC voltage reference value U dcref The voltage source converters are allocated according to the total number M of series-connected voltage source converters, and the normal value U is used as the DC voltage reference value for each operating voltage source converter. dVref-Nom ,in M is a positive integer; Each voltage source converter on the inverter side is equipped with a voltage source converter controller in its control device. The voltage source converter controller transmits the DC voltage reference value U of the voltage source converter. dVref of The arm reference voltage is calculated as the DC bias of the bridge arm voltage in the voltage source converter. in pj =0.5U dVref -in jo-ref in nj =0.5U dVref +in jo-ref Where j = a, b, c; u pj U is the reference voltage for the upper arm of phase j. nj U is the reference voltage for the lower arm of phase j. jo-ref This is the AC reference voltage for phase j.

2. The method for handling AC side faults in a hybrid DC transmission system according to claim 1, characterized in that: The control method for the DC voltage of the voltage source converter is as follows: The reference value U of the DC voltage of the voltage source converter is... dVref DC voltage measurement value U of voltage source converter dV The difference after subtraction is used to obtain the d-axis current reference value after being adjusted by the PI controller of the DC voltage outer loop. Then, the d-axis current reference value is used as input, and after adjustment by the current inner loop controller, the AC reference voltage is obtained.

3. The method for handling AC side faults in a hybrid DC transmission system according to claim 1, characterized in that: When the inverter terminal of the DC pole contains two or more voltage source converters operating in series, one of the voltage source converters is selected as the master control converter, and the remaining voltage source converters are slave control converters. Each slave control converter synchronously maintains its DC voltage reference value consistent with the DC voltage reference value of the master control converter.

4. The method for handling AC side faults in a hybrid DC transmission system according to claim 1, characterized in that, The detection method for the occurrence of a severe AC fault signal in the receiving-end AC power grid is any one or two of the following: Method 1: Detect that the positive sequence modulus of the three-phase AC voltage collected by the inverter station at the receiving end of the AC grid is lower than the voltage threshold value U. set1 ; Method 2: Detect the phase voltage amplitude of each phase of the three-phase AC grid collected by the inverter station, and set the phase AC voltage amplitude below the voltage threshold value U. set2 The sum of the quantities of each phase is greater than or equal to a set value N, where N is a positive integer and 1 ≤ N ≤ 3. Positive sequence magnitude of three-phase AC voltage The expression is: in, The positive sequence d-axis component of the AC voltage. This represents the positive-sequence q-axis component of the AC voltage. The amplitude of the AC voltage of this phase is defined as the absolute value of the peak value of the instantaneous AC voltage of this phase within this cycle.

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