AC transient voltage regulation control method and system for flexible DC system

Abstract

The invention relates to an AC transient voltage regulation control method and system for a flexible DC system, and belongs to the field of flexible DC power transmission. When the AC voltage exceeds a normal operation range but does not enter an AC high-low voltage ride-through range, the system enters AC transient voltage regulation control, and upper and lower limit values of transient voltage regulation are used as voltage reference values of transient voltage regulation to obtain a reactive power reference value under the transient voltage regulation; adding the reactive power reference value in the alternating-current voltage mode and the reactive power reference value in the alternating-current voltage control mode to obtain a final reactive power reference value, and subtracting the reactive power reference value under transient voltage regulation from a set reactive power reference value in the reactive power control mode to obtain the final reactive power reference value. And finally, controlling the bridge arm voltage by using the final reactive power reference value. The reactive power can be dynamically adjusted in real time according to the upper and lower limit values of AC transient voltage regulation to suppress further deterioration of the AC voltage, and the stability of the system is improved.

Description

Technical Field

[0001] The invention relates to an AC transient voltage regulation control method and system for a flexible direct current system, belonging to the field of flexible direct current power transmission. Background Art

[0002] Flexible DC grid is a power network composed of multiple interconnected voltage source converters that transmit electric energy in a DC manner. Flexible DC grid not only has the technical characteristics of fast, flexible and controllable double-terminal flexible DC system, but also can maximize the integration of various new energy sources and load distribution characteristics for multi-complementarity. Therefore, it has become a research hotspot in the current power system.

[0003] The AC voltage in the flexible DC system generally operates in the range of 95% to 105% of the rated voltage. Within this AC voltage operating range, the flexible DC control system does not make any adjustments. When the AC voltage is abnormal, the detection basis of the existing technology is that the AC voltage can only be detected after it drops to a certain level (such as lower than 85% of the rated voltage), and then the AC voltage measurement point is replaced or the control system is switched to respond. These control strategies are not applicable to abnormal AC voltage control when the AC voltage is greater than 85% of the rated voltage.

[0004] When the AC voltage operates between 85% and 95% of the rated voltage or 105% and 115% of the rated voltage, the flexible DC control system has not yet entered the AC low voltage and high voltage ride-through control. At this time, whether in the reactive power control mode or the AC voltage control mode, how to use the AC voltage or reactive power deviation control to actively adjust the reactive power output of the converter to suppress the further deterioration of the AC voltage and improve the stability of the system is an urgent problem to be solved in the flexible DC control system. Summary of the invention

[0005] The purpose of the present invention is to provide a flexible DC system AC transient voltage regulation control method and system to solve the problem in the prior art that when the AC voltage exceeds the normal operating range but has not yet entered the AC low voltage ride through control and high voltage ride through control ranges, the reactive power output by the converter cannot be actively adjusted, resulting in abnormal AC voltage.

[0006] To achieve the above object, the solution of the present invention includes:

[0007] A method for controlling AC transient voltage regulation in a flexible DC system according to the present invention comprises:

[0008] When the AC voltage in the flexible DC system exceeds the upper and lower limit values ​​of the AC voltage transient voltage regulation and does not exceed the lower limit value of the AC voltage high voltage ride-through and the upper limit value of the AC voltage low voltage ride-through, the flexible DC system enters the AC transient voltage regulation control. During the AC transient voltage regulation control, the upper and lower limit values ​​of the AC voltage transient voltage regulation are used as the voltage reference values ​​of the AC transient voltage regulation, thereby obtaining the first reactive power reference value;

[0009] When the flexible DC system adopts the AC voltage control mode, the first reactive power reference value obtained is superimposed with the second reactive power reference value obtained in the AC voltage control mode, and the superposition result is used as the final reactive power reference value;

[0010] When the flexible DC system adopts the reactive power control mode, the first reactive power reference value is subtracted from the third reactive power reference value adopted in the reactive power control mode, and the result of the subtraction is used as the final reactive power reference value; the third reactive power reference value is the set value;

[0011] Finally, the final reactive power reference value is used to obtain the q-axis current component reference value to control the reference voltage of the bridge arm.

[0012] Furthermore, both the AC transient voltage regulation control and the AC voltage control adopt droop control, and the droop control coefficient of the AC transient voltage regulation is greater than the droop control coefficient adopted by the AC voltage control.

[0013] Furthermore, the AC transient voltage regulation control also imposes amplitude limits on the values ​​obtained by the droop control, wherein the lower amplitude limit is the maximum value of reactive power absorbed by the AC transient voltage regulation control, and the upper amplitude limit is the maximum value of reactive power emitted by the AC transient voltage regulation control.

[0014] Furthermore, the second reactive power reference value Q 1ref Calculated by the following formula:

[0015]

[0016] In the formula, Q 1ref is the second reactive power reference value, in Mvar; U represents the actual value of AC voltage; U ref is the set AC voltage reference value; ΔU1 and ΔU2 represent the upper and lower dead zones of AC voltage operation, respectively, in kV; k represents the AC voltage control droop coefficient, in Mvar / kV.

[0017] The present invention also provides an AC transient voltage regulation control system for a flexible DC system, comprising a reactive power controller and a current inner loop controller, and also comprising a transient voltage regulation controller, the transient voltage regulation controller being used to output a first reactive power reference value using the upper and lower limit values ​​of the AC voltage transient voltage regulation as voltage reference values ​​of the AC transient voltage regulation controller when the AC voltage exceeds the upper and lower limit values ​​of the AC voltage transient voltage regulation and does not exceed the lower limit value of the AC voltage high voltage crossing and the upper limit value of the AC voltage low voltage crossing;

[0018] When the flexible DC system adopts the AC voltage control mode, the reactive power controller adopts the reactive power reference value output by the AC voltage controller as the second reactive power reference value, superimposes the first reactive power reference value and the second reactive power reference value, and uses the superimposed result as the final reactive power reference value;

[0019] When the flexible DC system adopts the reactive power control mode, the reactive power controller adopts the set reactive power reference value as the third reactive power reference value, subtracts the first reactive power reference value from the third reactive power reference value, and uses the subtracted result as the final reactive power reference value;

[0020] The current inner loop controller is used to control the reference voltage of the bridge arm using the q-axis current component reference value obtained by the final reactive power reference value.

[0021] Furthermore, both the AC transient voltage regulating controller and the AC voltage controller adopt droop control, and the droop control coefficient of the AC transient voltage regulating controller is greater than the droop control coefficient adopted by the AC voltage controller.

[0022] Furthermore, the AC transient voltage regulator controller also limits the amplitude of the value obtained by the droop control, the lower amplitude limit is the maximum value of the reactive power absorbed by the AC transient voltage regulator controller, and the upper amplitude limit is the maximum value of the reactive power emitted by the AC transient voltage regulator controller.

[0023] Furthermore, the second reactive power reference value Q 1ref Calculated by the following formula:

[0024]

[0025] In the formula, Q 1ref is the second reactive power reference value, in Mvar; U represents the actual value of AC voltage; U ref is the set AC voltage reference value; ΔU1 and ΔU2 represent the upper and lower dead zones of AC voltage operation, respectively, in kV; k represents the AC voltage control droop coefficient, in Mvar / kV.

[0026] The beneficial effects of the present invention are as follows: as an improved invention, when the AC voltage in the flexible DC system exceeds the normal operating range but has not yet entered the AC low voltage ride-through control and high voltage ride-through control range, the flexible DC system enters the AC transient voltage regulation control, and uses the upper and lower limit values ​​of the AC transient voltage regulation as the voltage reference value of the AC transient voltage regulation to obtain the reactive power reference value under the transient voltage regulation, and the reactive power reference value is added to the reactive power reference value obtained in the AC voltage control mode in the AC voltage mode to obtain the final reactive power reference value, and in the reactive power control mode, the reactive power reference value set by the dispatching department minus the reactive power reference value under the transient voltage regulation is used as the final reactive power reference value, and finally the q-axis current component reference value is obtained by using the final reactive power reference value to control the reference voltage of the bridge arm. Therefore, the present invention can dynamically adjust the reactive power output of the converter in real time according to the upper and lower limit values ​​of the AC transient voltage regulation to suppress the further deterioration of the AC voltage and improve the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the internal and external loop controller of the flexible DC system;

[0028] Figure 2 It is the control characteristic of the flexible DC system;

[0029] Figure 3 It is a reactive type controller;

[0030] Figure 4 It is the transient voltage regulation control block diagram under AC voltage control mode;

[0031] Figure 5 It is the reactive power characteristic under AC voltage control mode;

[0032] Figure 6 This is the transient voltage regulation control block diagram under reactive power control mode;

[0033] Figure 7 It is the reactive power characteristic in reactive power control mode. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail in a clear and complete manner in conjunction with the accompanying drawings and embodiments.

[0035] System Example:

[0036] The concept of the present invention is that it includes an active power controller, a reactive power controller, a transient voltage regulation controller and a current inner loop controller. When the AC voltage exceeds the upper and lower limit values ​​of the transient voltage regulation and does not exceed the lower limit value of the high voltage crossing and the upper limit value of the low voltage crossing, the flexible DC system enters the transient voltage regulation control, and uses the upper and lower limit values ​​of the transient voltage regulation as the voltage reference value of the AC transient voltage regulation controller, and outputs the reactive power reference value of the transient voltage regulation; when the system is in the AC voltage control mode, the final reactive power reference value is the sum of the reactive power reference value output by the transient voltage regulation controller and the reactive power reference value output by the AC voltage controller; when the system is in the reactive power control mode, the final reactive power reference value is the reactive power reference value output by the reactive power controller minus the reactive power reference value output by the transient voltage regulation controller; finally, the final reactive power reference value is used to obtain the q-axis current component reference value to control the current inner loop controller.

[0037] like Figure 1 As shown, the flexible DC system AC transient voltage regulation control system adopts inner and outer loop controllers, including an outer loop power controller and an inner loop current controller, wherein the outer loop power controller includes an active power controller and a reactive power controller, the active power controller can adopt the method of fixed DC voltage control, fixed active power control and frequency control, and the reactive power controller can adopt the method of fixed AC voltage control and fixed reactive power control. The outer loop power controller calculates the current d-axis component reference value i required by the inner loop current controller through the active power controller and the reactive power controller respectively. dref and the current q-axis component reference value i qref , and then output the bridge arm reference voltage through the current inner loop controller.

[0038] like Figure 2 The figure shows the control characteristic diagram of the flexible DC system. Different control strategies correspond to different AC voltage operating ranges. rated_max with U rated_min When the AC voltage is between U rated_max with U max Between or U rated_min with U min The system automatically enters transient voltage regulation control; when the AC voltage exceeds U max or below U min When U rated , U rated_min , U rated_max , U max , U min Respectively expressed as the rated value of AC voltage, the lower limit of AC voltage transient regulation (95% U rated), AC voltage transient voltage regulation upper limit (105% U rated ), AC voltage high voltage ride-through lower limit (115% U rated ) and AC voltage low voltage ride-through upper limit (85% U rated ).

[0039] Specifically, when the AC voltage is at U rated_max with U rated_min When the reactive power controller is between Figure 3 As shown, the reactive power controller can adopt the AC voltage control mode and the reactive power control mode. When the AC voltage control mode is adopted, the reactive power controller adopts the reactive power reference value Q obtained by the AC voltage controller. 1ref (which can be called the second reactive power reference value) as the final reactive power reference value Q ref Here, the AC voltage controller generates a reactive power reference value Q by using the AC voltage reference value and the AC voltage actual value through droop control. 1ref ; When the reactive power control mode is adopted, the reactive power controller adopts the set reactive power reference value Q 0ref (also called the third reactive power reference value, which is generally issued by the dispatching department) is the final reactive power reference value Q ref , the final reactive power reference value Q ref The PI controller is used to calculate the current q-axis component reference value i required by the inner loop current controller. qref On this basis, the present invention adds an AC transient voltage regulator controller, which can dynamically adjust the reactive power output by the converter in real time according to the upper and lower limit values ​​of the AC transient voltage regulation to suppress further deterioration of the AC voltage and improve the stability of the system. The implementation of each controller is described in detail below.

[0040] 1. AC voltage controller

[0041] The AC voltage controller obtains the reactive power reference value Q through droop control according to the AC voltage reference value and the current AC voltage actual value. 1ref , as shown in the following formula, where the AC voltage reference value is the set AC voltage reference value U ref The sum or difference of the normal fluctuation dead zone values ​​ΔU1 and ΔU2 of the AC voltage.

[0042]

[0043] In the formula, Q 1ref It indicates the reactive power reference value calculated by the AC voltage controller in the AC voltage control mode, in Mvar; U indicates the actual value of the AC voltage, U refIt indicates the set AC voltage reference value. ΔU1 and ΔU2 respectively indicate the upper and lower dead zone values ​​of the AC voltage during operation. Usually, ΔU1 and ΔU2 are set to be consistent. The unit is kV and the value range is [0kV, 10kV]. k indicates the AC voltage control droop coefficient, in Mvar / kV.

[0044] When the AC voltage actual value U is within the set reference value U ref The operating dead zone is within (U ref -ΔU2)≤U≤(U ref +ΔU1), the reactive power reference value calculated by the AC voltage controller is always 0.

[0045] When the actual value of the AC voltage U is higher than the set reference value U ref When the upper dead zone ΔU1 is set, that is, (U ref +ΔU1)≤U≤U rated_max The reactive power reference value calculated by the AC voltage controller is k[U-(U ref +△U1)], the flexible DC system generates reactive power.

[0046] When the AC voltage actual value U is lower than the reference value U ref When the set running dead zone △U2, that is, U rated_min ≤U≤(U ref -ΔU2), the reactive power reference value calculated by the AC voltage controller is k[U-(U ref -△U2)], the flexible DC system absorbs reactive power.

[0047] 2. AC transient voltage regulator controller

[0048] When the AC voltage of the flexible DC system exceeds the upper and lower limits of transient voltage regulation and does not exceed the lower limit of AC voltage high voltage ride-through U max And AC voltage low voltage through the upper limit value U min , that is, the AC voltage U is greater than U rated max and less than U max Or U is less than U rated_min and greater than U min The AC transient voltage regulator controller of the present invention adopts droop control. The transient voltage regulator controller can operate in the AC voltage control mode or in the reactive power control mode.

[0049] Specifically,

[0050] 1) When the flexible DC system is in AC voltage control mode, the control logic is as follows: Figure 4As shown, it is necessary to combine the reactive power reference value (also called the first reactive power reference value) output by the AC transient voltage regulator controller and the reactive power reference value output by the AC voltage controller. When the output value of the AC transient voltage regulator controller is not equal to zero, the output value of the current AC voltage controller is immediately locked. At this time, the final reactive power reference value Q ref is the output value Q of the currently locked AC voltage controller 1ref The sum of the output value of the AC transient voltage regulation controller. Figure 4 In the figure, k1 represents the droop coefficient of AC transient voltage regulation, in Mvar / kV. k1 is greater than the droop coefficient k of the AC voltage controller, which makes the flexible DC system adjust the absorbed or emitted reactive power more slowly and suppresses the further deterioration of the AC voltage. After the droop control, the AC transient voltage regulation controller also includes a limiting module, where Q LL1 and Q LU1 They represent the maximum value of reactive power absorbed and emitted by the AC transient voltage regulator controller, respectively, through Q LL1 and Q LU1 The output result of droop control is limited to upper and lower amplitudes.

[0051] Reactive power characteristics in AC voltage control mode, such as Figure 5 As shown, Q rated_max and Q rated_min They represent the limit values ​​of reactive power emitted and absorbed by the AC voltage controller of the flexible DC system in the AC voltage control mode, Q max and Q min They respectively represent the maximum value of reactive power emitted and absorbed by the flexible DC system.

[0052] 2) When the flexible DC system is in reactive power control mode, the control logic is as follows: Figure 6 As shown, the final reactive power reference value Q ref The reactive power reference value Q issued by the dispatching department 0ref Subtract the output value of the AC transient voltage regulator controller. In the figure, k2 represents the droop coefficient of the AC transient voltage regulation, the unit is Mvar / kV, k2 is greater than the droop coefficient k of the AC voltage controller; Q LL2 and Q LU2 They represent the maximum limit values ​​of reactive power absorbed and emitted by the AC transient voltage regulator controller. rated_max When the AC voltage U is less than U rated_min When the voltage is higher than the load, the flexible DC system increases the reactive power generated to suppress the further deterioration of the AC voltage.

[0053] Reactive power characteristics in reactive power control mode, such as Figure 7 As shown, Q max and Qmin They respectively represent the maximum value of reactive power emitted and absorbed by the flexible DC system in reactive power mode.

[0054] Finally, the reactive power controller uses the final reactive power reference value Q ref and the actual value of reactive power Q, the PI controller is used to calculate the current q-axis component reference value i required by the inner loop current controller qref , as shown below,

[0055]

[0056] In the formula, i qref It represents the reference value of the current q-axis component of the inner loop current controller, in kA; Q ref It represents the reactive power reference value; Q represents the actual reactive power value of the flexible DC system, in Mvar; kp represents the proportional coefficient of the PI control link of the reactive power controller; ki represents the integral coefficient of the PI control link of the reactive power controller, which is dimensionless.

[0057] In summary, the final reactive power reference value Q ref It is divided into the following situations:

[0058] The AC voltage of the flexible DC system is within the normal operating range, that is, the AC voltage is within the range of U rated_max with U rated_min When the AC voltage control mode is adopted, the final reactive power reference value Q ref The reactive power reference value Q obtained by the AC voltage controller 1ref ; When the reactive power control mode is adopted, the final reactive power reference value Q ref The reactive power reference value Q set for the reactive power controller 0ref ;

[0059] The AC voltage of the flexible DC system is in the transient voltage regulation range, that is, the AC voltage is greater than U rated_max and less than U max Or U is less than U rated_min and greater than U min When the AC voltage control mode is adopted, the final reactive power reference value Q ref is the output value Q of the AC voltage controller 1ref and the output value of the AC transient voltage regulator controller; when the reactive power control mode is adopted, the final reactive power reference value Q ref The reactive power reference value Q issued by the dispatching department 0ref Subtract the output value of the AC transient voltage regulator controller.

[0060] Method Example:

[0061] The present invention provides an AC transient voltage regulation control method for a flexible DC system, which includes active power control, reactive power control, current inner loop control and transient voltage regulation control. The AC transient voltage regulation control method for a flexible DC system has been clearly introduced in the system embodiment and will not be repeated here.

Claims

1. A method for controlling AC transient voltage regulation in a flexible DC system, characterized in that: The method includes: When the AC voltage in the flexible DC system exceeds the upper and lower limit values ​​of the AC voltage transient voltage regulation and does not exceed the lower limit value of the AC voltage high voltage ride-through and the upper limit value of the AC voltage low voltage ride-through, the flexible DC system enters the AC transient voltage regulation control. During the AC transient voltage regulation control, the upper and lower limit values ​​of the AC voltage transient voltage regulation are used as the voltage reference values ​​of the AC transient voltage regulation, thereby obtaining the first reactive power reference value; When the flexible DC system adopts the AC voltage control mode, the first reactive power reference value obtained is superimposed with the second reactive power reference value obtained in the AC voltage control mode, and the superposition result is used as the final reactive power reference value; When the flexible DC system adopts the reactive power control mode, the first reactive power reference value is subtracted from the third reactive power reference value adopted in the reactive power control mode, and the result of the subtraction is used as the final reactive power reference value; the third reactive power reference value is the set value; Finally, the final reactive power reference value is used to obtain the q-axis current component reference value to control the reference voltage of the bridge arm.

2. The method for controlling AC transient voltage regulation of a flexible DC system according to claim 1, characterized in that: Both AC transient voltage regulation control and AC voltage control adopt droop control, and the droop control coefficient of AC transient voltage regulation is greater than the droop control coefficient adopted by AC voltage control.

3. The method for controlling AC transient voltage regulation of a flexible DC system according to claim 2, characterized in that: The AC transient voltage regulation control also performs amplitude limitation on the value obtained by the droop control, the lower amplitude limitation is the maximum reactive power absorbed by the AC transient voltage regulation control, and the upper amplitude limitation is the maximum reactive power emitted by the AC transient voltage regulation control.

4. The method for controlling AC transient voltage regulation of a flexible DC system according to claim 1, characterized in that: The second reactive power reference value Q 1ref Calculated by the following formula: In the formula, Q 1ref is the second reactive power reference value, in Mvar; U represents the actual value of AC voltage; U ref is the set AC voltage reference value; ΔU1 and ΔU2 represent the upper and lower dead zones of AC voltage operation, respectively, in kV; k represents the AC voltage control droop coefficient, in Mvar / kV.

5. A flexible DC system AC transient voltage regulation control system, comprising a reactive power controller and a current inner loop controller, characterized in that: It also includes a transient voltage regulation controller, which is used to output a first reactive power reference value using the upper and lower limit values ​​of the AC voltage transient voltage regulation as voltage reference values ​​of the AC transient voltage regulation controller when the AC voltage exceeds the upper and lower limit values ​​of the AC voltage transient voltage regulation and does not exceed the lower limit value of the AC voltage high voltage crossing and the upper limit value of the AC voltage low voltage crossing; When the flexible DC system adopts the AC voltage control mode, the reactive power controller adopts the reactive power reference value output by the AC voltage controller as the second reactive power reference value, superimposes the first reactive power reference value and the second reactive power reference value, and uses the superimposed result as the final reactive power reference value; When the flexible DC system adopts the reactive power control mode, the reactive power controller adopts the set reactive power reference value as the third reactive power reference value, subtracts the first reactive power reference value from the third reactive power reference value, and uses the subtracted result as the final reactive power reference value; The current inner loop controller is used to control the reference voltage of the bridge arm using the q-axis current component reference value obtained by the final reactive power reference value.

6. The flexible DC system AC transient voltage regulation control system according to claim 5, characterized in that: Both the AC transient voltage regulating controller and the AC voltage controller adopt droop control, and the droop control coefficient of the AC transient voltage regulating controller is greater than the droop control coefficient adopted by the AC voltage controller.

7. The AC transient voltage regulation control system of a flexible DC system according to claim 6, characterized in that: The AC transient voltage regulator controller also limits the amplitude of the value obtained by the droop control, the lower amplitude limit is the maximum reactive power absorbed by the AC transient voltage regulator controller, and the upper amplitude limit is the maximum reactive power emitted by the AC transient voltage regulator controller.

8. The AC transient voltage regulation control system of a flexible DC system according to claim 5, characterized in that: The second reactive power reference value Q 1ref Calculated by the following formula: In the formula, Q 1ref is the second reactive power reference value, in Mvar; U represents the actual value of AC voltage; U ref is the set AC voltage reference value; ΔU1 and ΔU2 represent the upper and lower dead zones of AC voltage operation, respectively, in kV; k represents the AC voltage control droop coefficient, in Mvar / kV.

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