A multi-terminal direct current power transmission system inverter side alternating current fault ride-through control method

Abstract

The application discloses a kind of multi-terminal DC power transmission system inverter side AC fault ride-through control method, in rectifying station using fixed power / current control, one inverter station VSC selects fixed DC voltage control, the basic control strategy of the rest inverter station VSC selects fixed power / current control, for rectifying station configuration DC voltage control;For the DC overvoltage current rise control of each inverter station VSC of fixed power / current control, when the absolute value of DC voltage is higher than the design operating voltage range, according to overvoltage current rise control parameter, increase the DC power / current control reference value of this station.The application can reduce the DC overvoltage degree of multi-terminal DC power transmission system in the process of inverter station VSC AC fault ride-through of multi-terminal DC power transmission system, avoid the overvoltage tripping of DC system and overvoltage damage accident of DC equipment, so that multi-terminal DC power transmission system can safely and reliably ride through the serious fault of inverter station VSC AC side.

Description

Technical Field

[0001] This invention relates to a method for AC fault ride-through control on the inverter side of a multi-terminal DC transmission system, and particularly to a method for AC fault ride-through on the inverter side of a parallel multi-terminal DC transmission system in which all inverter stations adopt flexible DC converters (VSCs), belonging to the field of multi-terminal DC transmission. Background Technology

[0002] A parallel multi-terminal DC transmission system comprises three or more converter stations. The converters of the same pole at each converter station are connected between the ground and the DC transmission line of that pole, forming a DC transmission system with a parallel structure of multiple converter stations. Flexible DC converters (VSCs) can switch between rectification and inversion modes without changing the polarity of the DC voltage, and they do not suffer from commutation failure. They are particularly suitable for forming parallel multi-terminal DC transmission systems, and especially suitable as inverter converters in such systems. Figure 1 This is a typical topology diagram of a parallel three-terminal high-voltage direct current transmission system in which all inverter stations use flexible DC converters (VSCs).

[0003] In parallel multi-terminal DC transmission systems using VSC converters in inverter stations, a severe AC fault at the inverter station significantly limits the transmission power of the VSC converters, causing the DC-side voltage to rise continuously. This can easily lead to a shutdown of the multi-terminal DC system and even overvoltage breakdown of DC equipment such as the VSC converter valve submodules. Therefore, researching control methods for AC faults on the inverter side of parallel multi-terminal DC transmission systems using VSC converters is of significant practical importance for the safe and reliable operation of such systems.

[0004] Chinese patent CN105896585A, "A Method for AC Side Fault Ride-through Control in a Hybrid Multi-Terminal DC Transmission System," adopts a strategy of limiting the maximum voltage on the rectifier side and controlling the inverter station with constant DC current and configuring low-voltage current limiting control. This method has a good effect on AC fault control in rectifier stations, but for AC faults in inverter stations with constant DC voltage control, it can only achieve ride-through control under minor AC fault conditions, and there is a risk of failure to ride through the fault under severe AC fault conditions.

[0005] For parallel multi-terminal DC transmission systems using VSC converters in inverter stations, a method needs to be proposed to solve the ride-through control problem under severe AC fault conditions on the inverter side, based on the characteristic that AC faults on the inverter side cause DC voltage rise. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for controlling AC fault ride-through on the inverter side of a multi-terminal DC transmission system, which reduces the DC overvoltage level of the multi-terminal DC transmission system during the AC fault ride-through process of the inverter station VSC, so that it can safely and reliably ride through severe faults on the AC side of the inverter station VSC.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for AC fault ride-through control on the inverter side of a multi-terminal DC transmission system, wherein the multi-terminal DC transmission system includes at least one rectifier station and at least two inverter stations; the method for AC fault ride-through control on the inverter side is as follows:

[0009] Based on the basic control strategy of using constant power / current control in the rectifier station, constant DC voltage control in one inverter station, and constant power / current control in the remaining inverter stations, DC voltage control is configured for the rectifier station, and DC overvoltage boost control is configured for each inverter station with constant power / current control.

[0010] A DC voltage reference value is set for the rectifier station, which is greater than the voltage reference value of the inverter station under constant DC voltage control. When the rectifier station detects that the absolute value of the DC voltage is greater than the set DC voltage reference value, the DC voltage control of the rectifier station reduces the upper limit of the constant power / current control reference value of the rectifier station or increases the lower limit of the constant power / current control trigger angle command of the rectifier station to limit the rise of DC voltage.

[0011] An overvoltage boosting parameter is set for each inverter station under constant power / current control. When each inverter station under constant power / current control detects that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, it increases its respective power / current reference value according to the set overvoltage boosting parameter to stabilize the DC voltage.

[0012] As a preferred embodiment of the present invention, the overvoltage boosting parameter is set for each inverter station under constant power / current control, as follows:

[0013] When each inverter station under constant power / current control detects that the DC voltage is within the normal operating range, the power / current reference value of each inverter station under constant power / current control remains unchanged. When each inverter station under constant power / current control detects that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, an additional power / current reference value is added to the original power / current reference value. This additional power / current reference value has a positive linear relationship with the absolute value of the DC voltage. The minimum value between the original power / current reference value and the additional power / current reference value and the maximum allowable power / current delivery capacity is selected as the new power / current reference value for each inverter station under constant power / current control.

[0014] As a preferred embodiment of the present invention, the rectifier station adopts a DC converter LCC or a flexible DC converter VSC, and the inverter station adopts a flexible DC converter VSC.

[0015] As a preferred embodiment of the present invention, the inverter-side AC fault ride-through control method includes the following steps:

[0016] Step 1: Based on the basic control strategy of using constant power / current control in the rectifier station, constant DC voltage control in one inverter station, and constant power / current control in the other inverter stations, DC voltage control is configured for the rectifier station, and DC overvoltage boost control is configured for each inverter station under constant power / current control.

[0017] Step 2: Set a DC voltage reference value for the rectifier station. The DC voltage reference value for the rectifier station is greater than the voltage reference value for the inverter station controlled by the constant DC voltage.

[0018] Step 3: Set overvoltage boost parameters for each inverter station under constant power / current control;

[0019] Step 4: When an AC fault occurs at the inverter station controlled by constant DC voltage, the DC voltage of the multi-terminal DC transmission system rises. When each inverter station controlled by constant power / current detects that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, it increases its respective power / current reference value according to the overvoltage boosting parameters set in Step 3 to stabilize the DC voltage. When the rectifier station controlled by DC voltage detects that the absolute value of the DC voltage is greater than the DC voltage reference value set in Step 2, the rectifier station DC voltage control reduces the upper limit of the rectifier station constant power / current control reference value or increases the lower limit of the rectifier station constant power / current control firing angle command to limit the rise of DC voltage.

[0020] Step 5: When an AC fault occurs at an inverter station using constant power / current control, the DC voltage is controlled by an inverter station using constant DC voltage control. If the DC voltage continues to rise, and other inverter stations using constant power / current control that have not experienced AC faults detect that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, they will increase their respective power / current reference values ​​according to the overvoltage boosting parameters set in Step 3 to stabilize the DC voltage. If the DC voltage continues to rise and the rectifier station detects that the absolute value of the DC voltage is greater than the DC voltage reference value set in Step 2, the rectifier station's DC voltage control will decrease the upper limit of the rectifier station's constant power / current control reference value or increase the lower limit of the rectifier station's constant power / current control firing angle command to limit the rise in DC voltage.

[0021] Step 6: When the AC fault on the inverter side is cleared and the DC voltage is restored, the multi-terminal DC transmission system resumes its original normal operation control strategy, that is, the rectifier station adopts constant power / current control, one inverter station adopts constant DC voltage control, and the other inverter stations adopt constant power / current control.

[0022] Compared with the prior art, the present invention, employing the above technical solution, has the following technical effects:

[0023] This invention can reduce the DC overvoltage level of a multi-terminal DC transmission system during AC fault ride-through of the inverter station VSC, avoiding DC system overvoltage tripping and DC equipment overvoltage damage accidents, and enabling the multi-terminal DC transmission system to safely and reliably ride through serious faults on the AC side of the inverter station VSC. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a parallel three-terminal DC transmission system. Station 1 is a rectifier station, which uses an LCC (Line Commutated Converter) type converter. Stations 2 and 3 are inverter stations, which use VSC (Voltage Source Converter) type converters.

[0025] Figure 2 This is a flowchart of the AC fault ride-through control method on the inverter side of a three-terminal DC transmission system. Detailed Implementation

[0026] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0027] This invention discloses an AC fault ride-through control method for the inverter side of a multi-terminal DC transmission system. The multi-terminal DC transmission system includes at least one rectifier station and at least two inverter stations. The rectifier station can be a conventional DC converter (LCC) or a flexible DC converter (VSC). All inverter stations use flexible DC converters (VSC).

[0028] Based on the basic control strategy of using constant power / current control in the rectifier station, constant DC voltage control in one inverter station VSC, and constant power / current control in the remaining inverter station VSCs, DC voltage control is configured for the rectifier station, and its DC voltage reference value is set; DC overvoltage boost control is configured for each inverter station VSC with constant power / current control, and overvoltage boost control parameters are set. When the absolute value of the DC voltage is higher than the design operating voltage range, the DC power / current control reference value of this station is increased according to the overvoltage boost control parameters.

[0029] When the absolute value of the actual DC voltage is higher than the preset reference value, the DC voltage control configured in the rectifier station will reduce the upper limit of the reference value of the rectifier-side constant power / current control or increase the lower limit of the trigger angle command of the rectifier-side constant power / current control to limit the rise of DC voltage.

[0030] DC overvoltage current boost control is configured for each inverter station VSC with constant power / current control. The overvoltage current boost control parameters are set as follows: when the DC voltage is within the normal operating range, the power / current reference value of this station remains unchanged; when the absolute value of the DC voltage is higher than the design operating voltage range, an additional power / current reference value is added to the original power / current reference value of this station; this power / current reference value increase is positively linearly related to the absolute value of the DC voltage, and the power / current reference value increase is 0 when the DC voltage is within the normal operating range; the minimum value between the original power / current reference value of this station and the additional power / current reference value increase and the maximum allowable power / current transmission capacity of this station is selected as the new power / current reference value.

[0031] The DC voltage control configured in the rectifier station has a preset reference value that is higher than the voltage reference value of the inverter station VSC with constant DC voltage control.

[0032] To provide a detailed description of the technical solution of this invention, Figure 1 The three-terminal DC transmission system shown is an example, where station 1 is a rectifier station using an LCC converter, and stations 2 and 3 are inverter stations using VSC converters. In this example, the basic control strategy for multi-terminal DC is set as follows: station 1 uses constant power / current control, station 2 uses constant power / current control, and station 3 uses constant DC voltage control. For this example, the schematic flowchart of the inverter-side AC fault ride-through method of the present invention is as follows. Figure 2 As shown, it includes the following steps:

[0033] Step 1: Configure DC voltage control for the rectifier station; configure DC overvoltage boost control for each inverter station VSC with constant power / current control.

[0034] Step 2: Set the DC voltage reference value of the rectifier station. The preset reference value is higher than the voltage reference value of the inverter station VSC with constant DC voltage control. When the actual absolute value of the DC voltage is higher than the preset reference value, the DC voltage control configured in the rectifier station will reduce the upper limit of the constant power / current control reference value on the rectifier side or increase the lower limit of the constant power / current control trigger angle command on the rectifier side to limit the rise of DC voltage.

[0035] In this embodiment, station 3, which employs a constant DC voltage control strategy, maintains the DC voltage of the multi-terminal DC system at 1 times the system rated value, while the DC voltage reference value of station 1 is set to 1.03 times the system rated value. When the absolute value of the DC voltage of station 1 is higher than 1.03 times the system rated value, the DC voltage control of station 1 will increase the lower limit of the firing angle command to limit the rise of the DC voltage.

[0036] Step 3: Set overvoltage boost current control parameters for each inverter station VSC with constant power / current control: When the DC voltage is within the normal operating range, the power / current reference value of this station remains unchanged. When the absolute value of the DC voltage is higher than the design operating voltage range, an additional power / current reference value is added to the original power / current reference value of this station. This power / current reference value increase is positively linearly related to the absolute value of the DC voltage. When the DC voltage is within the normal operating range, this power / current reference value increase is 0. Select the minimum value between the original power / current reference value of this station and the additional power / current reference value increase, and the maximum allowable power / current transmission capacity of this station as the new power / current reference value.

[0037] In this embodiment, station 2 uses constant power control, and its overvoltage current boosting control circuit is set as follows:

[0038]

[0039] In the above formula, P ref U is the original power / current reference value for station 2. d Let U0 be the upper limit of the normal operating range of the DC voltage, and the above are normalized values ​​corresponding to the rated values. Assuming U0 is 1.01, when the absolute value of the DC voltage |U d When | ≥ 1.01, the overvoltage boosting parameter k is set to 7; otherwise, it is 0. The original power command P of station 2. ref After adding the overvoltage booster circuit, it is then connected to the station's maximum transmission power P. max Take the smaller value to obtain the new power / current reference value for station 2.

[0040] If there are multiple VSC inverter stations that use constant power / current control, the overvoltage boosting parameters k and U0 of each station can be selected with different values ​​as needed.

[0041] Step 4: When an AC fault occurs at a VSC inverter station with constant DC voltage control, the DC voltage of the multi-terminal DC transmission system will rise. When each VSC inverter station with constant power / current control detects that the absolute value of the DC voltage is higher than the normal operating range, it will increase its respective power / current reference value according to the set overvoltage boosting parameters to stabilize the DC voltage. When the actual absolute value of the DC voltage is higher than the preset reference value of the rectifier station's DC voltage control, the rectifier station's DC voltage control will reduce the upper limit of the constant power / current control reference value on the rectifier side or increase the lower limit of the constant power / current control firing angle command on the rectifier side to limit the rise in DC voltage.

[0042] For example, when an AC fault occurs at station 3, causing the absolute value of the DC voltage |U dWhen the power output reaches 1.02 times the rated value, Station 2 will output 0.07 times more rated power than the original power command without reaching its maximum transmission power, which helps to suppress the rise in DC voltage; when the AC fault at Station 3 is more severe, causing the absolute value of DC voltage to increase... d When the power output reaches 1.05 times the rated value, Station 2 will output 0.28 times more rated power than the original power command without reaching the maximum transmission power. In addition, the DC voltage control of Station 1 will increase the lower limit of the firing angle command to limit the transmission power of the rectifier station and prevent the DC voltage from rising further.

[0043] Step 5: When an AC fault occurs at a VSC inverter station with constant power / current control, the DC voltage is first controlled by the VSC inverter station with constant DC voltage control. If the DC voltage continues to rise, and other VSC inverter stations with constant power / current control detect that the absolute value of the DC voltage is higher than the normal operating range, they will increase their respective power / current reference values ​​according to the set overvoltage boost parameters to stabilize the DC voltage. When the actual absolute value of the DC voltage is higher than the preset reference value of the rectifier station's DC voltage control, the rectifier station's DC voltage control will reduce the upper limit of the constant power / current control reference value on the rectifier side or increase the lower limit of the constant power / current control firing angle command on the rectifier side to limit the rise in DC voltage.

[0044] Step Six: After the AC fault on the inverter side is cleared, the DC voltage gradually recovers, and each of the multi-terminal DC converter stations will resume its original normal operation control strategy. That is, the rectifier station adopts constant power / current control, one inverter station VSC adopts constant DC voltage control, and the other inverter station VSCs adopt constant power / current control.

[0045] 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 AC fault ride-through control on the inverter side of a multi-terminal DC transmission system, wherein the multi-terminal DC transmission system includes at least one rectifier station and at least two inverter stations; characterized in that, The inverter-side AC fault ride-through control method includes the following steps: Step 1: Based on the basic control strategy of using constant power / current control in the rectifier station, constant DC voltage control in one inverter station, and constant power / current control in the other inverter stations, DC voltage control is configured for the rectifier station, and DC overvoltage boost control is configured for each inverter station under constant power / current control. Step 2: Set a DC voltage reference value for the rectifier station. The DC voltage reference value for the rectifier station is greater than the voltage reference value for the inverter station controlled by the constant DC voltage. Step 3: Set overvoltage boost parameters for each inverter station under constant power / current control; Step 4: When an AC fault occurs at the inverter station controlled by constant DC voltage, the DC voltage of the multi-terminal DC transmission system rises. When each inverter station controlled by constant power / current detects that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, it increases its respective power / current reference value according to the overvoltage boosting parameters set in Step 3 to stabilize the DC voltage. When the rectifier station controlled by DC voltage detects that the absolute value of the DC voltage is greater than the DC voltage reference value set in Step 2, the rectifier station DC voltage control reduces the upper limit of the rectifier station constant power / current control reference value or increases the lower limit of the rectifier station constant power / current control firing angle command to limit the rise of DC voltage. Step 5: When an AC fault occurs at an inverter station using constant power / current control, the DC voltage is controlled by an inverter station using constant DC voltage control. If the DC voltage continues to rise, and other inverter stations using constant power / current control that have not experienced AC faults detect that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, they will increase their respective power / current reference values ​​according to the overvoltage boosting parameters set in Step 3 to stabilize the DC voltage. If the DC voltage continues to rise and the rectifier station detects that the absolute value of the DC voltage is greater than the DC voltage reference value set in Step 2, the rectifier station's DC voltage control will decrease the upper limit of the rectifier station's constant power / current control reference value or increase the lower limit of the rectifier station's constant power / current control firing angle command to limit the rise in DC voltage. Step 6: When the AC fault on the inverter side is cleared and the DC voltage is restored, the multi-terminal DC transmission system resumes its original normal operation control strategy, that is, the rectifier station adopts constant power / current control, one inverter station adopts constant DC voltage control, and the other inverter stations adopt constant power / current control.

2. The AC fault ride-through control method for the inverter side of a multi-terminal DC transmission system according to claim 1, characterized in that, The overvoltage boosting parameters are set for each inverter station under constant power / current control, as detailed below: When each inverter station under constant power / current control detects that the DC voltage is within the normal operating range, the power / current reference value of each inverter station under constant power / current control remains unchanged; When each inverter station under constant power / current control detects that the absolute value of the DC voltage is greater than the upper limit of the normal operating range, an additional power / current reference value is added to the original power / current reference value. This additional power / current reference value has a positive linear relationship with the absolute value of the DC voltage. The minimum value between the original power / current reference value and the additional power / current reference value and the maximum allowable power / current delivery capacity is selected as the new power / current reference value for each inverter station under constant power / current control.

3. The AC fault ride-through control method for the inverter side of a multi-terminal DC transmission system according to claim 1, characterized in that, The rectifier station uses either a DC converter (LCC) or a flexible DC converter (VSC), while the inverter station uses a flexible DC converter (VSC).

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