Converter control method for improving power transmission stability of flexible dc interconnection system

Abstract

The application discloses a converter control method for improving power transmission stability of a flexible DC interconnection system, relates to the technical field of flexible interconnection system control strategies, and solves the power oscillation problem of the existing flexible DC interconnection device control method.The technical solution points are that the control modes of the converters on the two sides of the flexible interconnection system are adaptively switched according to the energy flow direction of the system.The technology realizes the control by coordinating the DC voltage, the power control outer ring, the limiter, the current control inner ring and other control links, superimposes the power control current feedback instruction after the limiter and the DC voltage control current feedback instruction as the current inner ring control instruction, and controls the operation of the power electronic converter through the current inner ring controller and the PWM modulation instruction.The inner ring current instruction value of the converters on the two sides of the flexible DC interconnection system is adaptively adjusted with the change of the system power flow direction, can be the control of the DC voltage or the control of the system power flow direction, and the control mode of the unilateral converter is not limited to the fixed DC voltage control or the fixed power control.Under the premise of ensuring the reliable and stable switching of the control modes of the two-side converters in the system, the system stability problem caused by the power flow direction under the fixed control mode of the unilateral converter is solved.

Description

Technical Field

[0001] This application relates to the field of control strategy technology for flexible DC interconnect systems, and in particular to a converter control method for improving the power transmission stability of flexible DC interconnect systems. Background Technology

[0002] Flexible DC interconnection systems can decouple AC systems and achieve DC interconnection mode. This not only effectively controls system short-circuit current and system power flow, but also provides dynamic voltage support during abnormal system operation, eliminates inrush current and circulating current caused by loop closure, ensures safe operation of the loop, and significantly improves the reliability of power distribution network supply. However, flexible DC interconnection devices consist of two voltage source converters (VSCs) connected in series via a DC link. Typically, one converter uses constant DC voltage control, and the other uses constant power control. Figure 1 As shown, when power is transferred from the DC line to the converter using constant power control, the converter acts as a constant power load, exhibiting a negative DC-side impedance characteristic. Under high power conditions, this easily leads to power oscillations within the system, severely reducing the stability of the DC bus. However, when the power direction changes, the DC-side impedance characteristic becomes positive, contributing to the stability of the DC bus. Clearly, under changing power flow conditions, the power oscillation problem of traditional control methods cannot be avoided, seriously threatening the system's stability.

[0003] Optimizing the control method of the converters on both sides of the flexible DC interconnection system can effectively avoid the power oscillation problem caused by a single control method when the power flow direction changes, which is conducive to improving the stability of the system. Therefore, it is of great significance to carry out research in this regard. Summary of the Invention

[0004] This application provides a converter control method to improve the power transmission stability of a flexible DC interconnection system. The technical objective is to solve the unavoidable power oscillation problem under single control of dual-sided converters in existing flexible DC interconnection systems. The method enables the control mode of both converters to adaptively adjust with the power flow direction of the system, and can switch between constant DC voltage control mode and constant power control mode. This solves the problem of system stability deterioration caused by internal power oscillation when the power flow direction changes.

[0005] The above-mentioned technical objective of this application is achieved through the following technical solution:

[0006] A converter control method for improving power transmission stability in a flexible DC interconnect system is disclosed. This method is implemented through a control system, and the controlled object is a dual-ended converter, comprising a first converter and a second converter. The control systems for both the first and second converters include a voltage outer loop control unit, a power outer loop control unit, a current inner loop control and PWM unit, a d-axis control module, and a q-axis control module. Each d-axis control module includes a voltage controller, a limiter, a power controller, and a first current inner loop controller. The q-axis control module includes a second current inner loop controller. The limiter for the d-axis control module of the first converter includes a first limiter and a second limiter, while the limiter for the d-axis control module of the second converter includes a second limiter and a third limiter.

[0007] The control method for the first converter includes:

[0008] i dacA * The calculated current command and the given minimum current command value i, which serve as the power outer loop control unit. dmin * The difference is calculated, and the difference is processed by the second limiter to obtain the current command i of the power outer loop control unit. d2A * ;

[0009] will i d2A * Multiply by the gain factor K and then combine with the initial DC voltage setpoint U in the outer voltage loop control unit. dc * The sum is used as the final DC voltage setpoint of the outer loop control unit. This final DC voltage setpoint is compared with the actual DC voltage feedback value U. dcA The difference is input to the voltage controller, and the initial voltage outer loop output current command i is obtained through the first limiter. d1A * ;

[0010] Finally, i d1A * With the power outer loop control unit current command i d2A * The summation yields the first current inner loop controller given current command i. dA * The second current inner loop controller is given a current command i. qA * The value is 0, and the first converter is operated by the inner current loop control and PWM unit control.

[0011] The control method for the second converter includes:

[0012] i dacB *The calculated current command and the given minimum current command value i, which serve as the power outer loop control unit. dmin * The sum is processed by the second limiter to obtain the current command i of the power outer loop control unit. d2B * ;

[0013] will i d2B * Multiply by the gain factor K and then combine with the initial DC voltage setpoint U in the outer voltage loop control unit. dc * The sum is used as the final DC voltage setpoint for the outer loop of voltage control. This final DC voltage setpoint is compared with the actual DC voltage feedback value U. dcB The difference is input to the voltage controller, and the initial voltage outer loop output current command i is obtained through the third limiter. d1B * ;

[0014] Finally, i d1B * With the power outer loop control unit current command i d2B * The summation yields the first current inner loop controller given current command i. dB * The second current inner loop controller is given a current command i. qB * The value is 0, and the second converter is operated through the inner current loop control and PWM unit control.

[0015] Furthermore, the limiting value of the first limiter is -i dmax * to i dmin * The limiting value of the second limiter is from 0 to i dmax * The limiting value of the third limiter is -i dmax * to -i dmin * ; where i dmax * with i dmin * These are the maximum and minimum d-axis current command values, respectively.

[0016] Furthermore, the minimum d-axis current command value i dmin * The selection can be freely adjusted based on the actual control system.

[0017] Furthermore, the gain coefficient K is a positive value, used to increase the DC voltage command value, and can be freely adjusted according to the actual control system.

[0018] Furthermore, the control method includes d-axis control and q-axis control.

[0019] Furthermore, the voltage outer loop control unit is used to control the DC bus voltage, and the power outer loop control unit is used to control the DC line power transmission. The voltage outer loop control unit, the power outer loop control unit, and the limiter work together to control the switching of the control modes of the first converter and the second converter.

[0020] Furthermore, the control methods include constant voltage control mode and constant power control mode.

[0021] The beneficial effects of this application are as follows:

[0022] (1) When the power flow direction of the system remains unchanged, the power flow direction is controlled by a constant voltage at the receiving end and controlled by a constant power at the other end. The DC side output impedance has a positive impedance characteristic, which is conducive to the stable operation of the system.

[0023] (2) When the power flow direction of the system changes, the control mode of the converters on both sides of the flexible DC interconnection system is adaptively switched to keep the power flow direction receiving end always in constant voltage control mode, and the system stability under complex working conditions is greatly improved.

[0024] (3) When the system transmission power is too high, the limiter saturates, which prevents the output current of the converter from increasing, thus giving the converter overload protection capability. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the control method for converters on both sides of a flexible DC interconnection system in the prior art.

[0026] Figure 2 This is a schematic diagram of the control method for the converters on both sides of the flexible DC interconnection system described in this application;

[0027] Figures 3a to 3c This is a schematic diagram of the simulation waveform of the switching control between the two converters when the flow direction changes under high power using the control method of this application.

[0028] Figures 4a to 4c This is a simulation waveform diagram of the switching control between the two converters when the initial power is small and the flow is positive, and then the power increases and the flow direction changes, using the control method of this application. Detailed Implementation

[0029] The technical solution of this application will be described in detail below with reference to the accompanying drawings.

[0030] Figure 2This is a schematic diagram of the control system in the converter control method for improving power transmission stability in flexible DC interconnect systems described in this application. The control method described in this application uses... Figure 2 The control system shown is used to implement this. This control system controls the system's two-ended converter, which is as follows: Figure 2 VSC-A and VSC-B are described in the diagram. The control systems of both VSC-A and VSC-B include a voltage outer loop control unit, a power outer loop control unit, a current inner loop control and PWM unit, a d-axis control module, and a q-axis control module. Each d-axis control module includes a voltage controller, a limiter, a power controller, and a first current inner loop controller. The q-axis control module includes a second current inner loop controller. The limiter for the d-axis control module of the first converter includes a first limiter and a second limiter, while the limiter for the d-axis control module of the second converter includes a second limiter and a third limiter.

[0031] Preferably, the control method includes d-axis control and q-axis control.

[0032] Preferably, the voltage outer loop control unit is used to control the DC bus voltage, and the power outer loop control unit is used to control the DC line power transmission. The voltage outer loop control unit, the power outer loop control unit, and the limiter work together to control the switching of the control modes of the first converter and the second converter.

[0033] As a specific embodiment, the control method for the converter VSC-A includes:

[0034] i dacA * The calculated current command and the given minimum current command value i, which serve as the power outer loop control unit. dmin * The difference is calculated, and the difference is processed by limiter 2 to obtain the current command i of the power outer loop control unit. d2A * ;

[0035] will i d2A * Multiply by the gain factor K and then combine with the initial DC voltage setpoint U in the outer voltage loop control unit. dc * The sum is used as the final DC voltage setpoint of the outer loop control unit. This final DC voltage setpoint is compared with the actual DC voltage feedback value U. dcA The difference is input to the voltage controller, and the initial voltage outer loop output current command i is obtained through limiter 1. d1A * ;

[0036] Finally, i d1A * With the power outer loop control unit current command id2A * The summation yields the first current inner loop controller given current command i. dA * The second current inner loop controller is given a current command i. qA * The value is 0, and the first converter is operated by the inner current loop control and PWM unit control.

[0037] The control methods for the VSC-B converter include:

[0038] i dacB * The calculated current command and the given minimum current command value i, which serve as the power outer loop control unit. dmin * The sum is processed by limiter 2 to obtain the current command i of the power outer loop control unit. d2B * ;

[0039] will i d2B * Multiply by the gain factor K and then combine with the initial DC voltage setpoint U in the outer voltage loop control unit. dc * The sum is used as the final DC voltage setpoint for the outer loop of voltage control. This final DC voltage setpoint is compared with the actual DC voltage feedback value U. dcB The difference is input to the voltage controller, and the initial voltage outer loop output current command i is obtained through limiter 3. d1B * ;

[0040] Finally, i d1B * With the power outer loop control unit current command i d2B * The summation yields the first current inner loop controller given current command i. dB * The second current inner loop controller is given a current command i. qB * The value is 0, and the second converter is operated through the inner current loop control and PWM unit control.

[0041] Preferably, the limiting value of limiter 1 is -i dmax * to i dmin * The limiting value of limiter 2 is from 0 to i dmax * The limiting value of limiter 3 is -i dmax * to -i dmin * ; where i dmax* with i dmin * These are the set maximum and minimum d-axis current command values, respectively. The addition of each limiter provides overload protection for the converter when the transmission power is high. Furthermore, the coordination of various limit values ​​ensures that the d-axis current command value of the inner loop control of the input current is always either the constant voltage control current command value or the constant power control current command value, thus ensuring that the converters on both sides of the system always switch between the two fixed control modes.

[0042] Preferably, the minimum current command value i dmin * The selection can be freely adjusted according to the actual system. The addition of this instruction value specifies the initial control mode of the two converters when the current is small during system startup, while avoiding the problem of frequent mode switching of the two converters when the power is small under this control mode.

[0043] Preferably, the gain coefficient K is a positive value, used to increase the DC voltage command value. It can be freely adjusted according to the actual system. Its function is that when the power controller output current command value is not in a saturated state, the addition of K rapidly increases the difference between the DC voltage command value and the feedback value, so that the voltage controller quickly saturates. When the power reverses, the dynamic response process is optimized, and the dual-sided converter control mode is quickly switched.

[0044] Among them, U dcA U dcB These are the DC-side voltages of VSC-A and VSC-B, respectively; U dc * P is the DC voltage setpoint. acA * and P acB * These are the AC-side active power setpoints for VSC-A and VSC-B, respectively; U dA and U dB These are the AC side d-axis voltage components of VSC-A and VSC-B, respectively; i dacA * and i dacB * These are the d-axis current setpoints corresponding to the AC side active power setpoints of VSC-A and VSC-B, respectively; i dmin * This is the minimum d-axis current command value, and also the current limit for low-power transmission; i dmax * This is the maximum d-axis current command value; i d1A * i d2A * and i d1B* i d2B * These are the two d-axis current reference values ​​output from the limiters of VSC-A and VSC-B, respectively; K is the i d2A * and i d2B * Gain coefficient; i dA * i dB * and i qA * i qB * These are the final current setpoints for the d-axis and q-axis current inner loop controllers of VSC-A and VSC-B, respectively.

[0045] i dmin * The addition of this feature restricts the control methods of both VSC-A and VSC-B converters during startup. During low-current startup, both VSC-A and VSC-B operate in constant-voltage control mode. This also avoids the problem of frequent control switching between the two converters during low-power operation, limiting the system current flow at lower power levels. This means that during low-power operation, the power flow to the receiving converter may use constant-power control while the other end uses constant-voltage control—a situation that is unavoidable. Since power oscillations mostly occur under high-power conditions, this does not contradict the principle of suppressing system power oscillations.

[0046] The direction of power flow from VSC-A to VSC-B is defined as the positive direction. The following analysis examines how power operates when flowing in the forward and reverse directions from both VSC-A and VSC-B perspectives, and how the switching occurs when power transitions from forward to reverse flow.

[0047] (1) Power forward flow

[0048] Give VSC-A a positive power setpoint P acA * The corresponding d-axis current setpoint i is obtained. dacA * Subtract i dmin * After that, we get i d2A * The power setpoint given to VSC-B is P. acB * And P acB * =-P acA * This allows us to obtain a d-axis current setpoint i. dacB* =-i dacA * , plus i dmin * After that, we get i d2B * .

[0049] when i dacA * >i dmin * When, the obtained i d2B * =0, U was not included dcB When the given value is pulled high, VSC-B operates in constant voltage control mode to maintain the DC voltage. Simultaneously, the obtained i... d2A * =i dacA * -i dmin * i d2A * Add to U dcA The given value pulls the original given value higher, causing the voltage controller to enter saturation, and the output value i d1A * =i dmin * Finally, i is obtained dA * =i dacA Therefore, VSC-A operates in constant power control mode.

[0050] When 0 dacA * dmin * When, the obtained i d2A * =0, U was not included dcA The given value is pulled high, therefore VSC-A operates in constant voltage control mode. Meanwhile, the obtained i... d2B * =i dmin * +i dacB * i d2B * Add to U dcB The given value increases the original given value, causing the voltage controller to enter saturation, and the output value is -i. dmin * Finally, i is obtained dB * =i dacB * Therefore, VSC-B operates in constant power control mode.

[0051] ​​(2) Power reverse flow

[0052] Give VSC-B a positive power setpoint P acB * This is equivalent to giving VSC-A a negative power setpoint P. acA * P acA * =-P acB * Therefore, i can be obtained dacA * =-i dacB * i dacA * Subtract i dmin * After that, we get i d2A * The obtained i d2A * =0, U was not included dcA The setpoint is pulled high, so VSC-A operates in constant voltage control mode.

[0053] For the d-axis current reference value i obtained in VSC-B dacB * , plus i dmin * After that, we get i d2B * The obtained i d2B * =i dmin * +i dacB * i d2B * Add to U dcB The given value pulls the original given value higher. Since VSC-A operates in constant voltage control mode, the voltage controller enters saturation, and the output value is -i. dmin * Finally, i is obtained dB * =i dacB * Therefore, VSC-B operates in constant power control mode.

[0054] (3) Power switching from forward flow to reverse flow

[0055] When power flows in the forward direction, and i dacA * >i dmin *As mentioned above, at this time, the voltage controller of VSC-A is in saturation, and VSC-A is in constant power control mode; the voltage controller of VSC-B is not in saturation, and VSC-B is in constant voltage control mode. When power suddenly flows in the reverse direction, due to i d2B * =i dmin * +i dacB * i d2B * Add to U dcB The given value rapidly increases the original given value, causing the voltage controller of VSC-B to enter saturation, ultimately obtaining i. dB * =i dacB * VSC-B operates in constant power control mode, and the operating mode switches. Simultaneously, due to reverse power, i... dacA * In the opposite direction, with i dmin * After difference calculation, the output i is limited by the limiter d2A * =0, U dcA The setpoint remains unchanged, so the voltage controller of VSC-A begins to desaturate and operates in constant voltage control mode to maintain the DC voltage. The voltage controller of VSC-B enters saturation and the voltage controller of VSC-A desaturates simultaneously.

[0056] When power flows in the forward direction, and 0 dacA * dmin * In other words, under low-power operating conditions, as mentioned above, the voltage controller of VSC-A is not in saturation, while the voltage controller of VSC-B is in saturation. When power suddenly flows in the reverse direction, due to i d2B * =i dmin * +i dacB * i d2B * Add to U dcB The given value will still pull the original given value higher, so that the voltage controller of VSC-B remains in saturation, ultimately resulting in i. dB * =i dacB * VSC-B still operates in constant power control mode. Meanwhile, i d2A * Still 0, U dcA ​​The given value remains unchanged, but due to the reverse transmission of VSC-B, U dc As the voltage rises, VSC-A continues to operate in constant voltage control mode to maintain the DC side voltage.

[0057] Table 1 shows the control methods for the two converters corresponding to different power flow directions.

[0058] Table 1 Control methods for two converters with different power flow directions.

[0059]

[0060] Figures 3a to 3c as well as Figures 4a to 4c The simulation results demonstrate that when the power flow direction of the system changes, the converters on both sides can adaptively adjust their control methods to avoid the problem of the DC side impedance characteristic exhibiting negative impedance characteristics when the power flow direction changes, thereby improving the DC voltage stability.

[0061] Figures 3a to 3c It operates under high current conditions, i. dacA * >i dmin * When power flows in the forward direction, i d1A * =i dmin * =20A, the VSC-A voltage controller has reached saturation, and VSC-A is operating in constant power control mode; at the same time, i d2B * =0, i dB * =i d1B * VSC-B operates in constant voltage control mode. When power flows in reverse, i d2A * =0, i dA * =i d1A * VSC-A operates in constant voltage control mode; meanwhile, i d1B * =-i dmin * =-20A, the VSC-B voltage controller has reached saturation, and VSC-B is operating in constant power control mode. The control modes of the two converters have been switched.

[0062] Figures 4a to 4c Under initial conditions, the system operates with low power in the forward direction, i.e., 0. dacA * dmin * ​​Subsequently, the power increases and the flow direction reverses. When the power flows in the forward direction, i d2A * =0, i dA * =i d1A * VSC-A operates in constant voltage control mode; meanwhile, i d1B * =-i dmin * When the voltage reaches -20, the VSC-B voltage controller reaches saturation and operates in constant power control mode. Even when power flows in the reverse direction, the VSC-B voltage controller remains in saturation and operates in constant power control mode. VSC-A continues to operate in constant voltage control mode. The simulation results are consistent with the control process analyzed above.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A converter control method for improving the power transmission stability of a flexible DC interconnect system, characterized in that, This control method is implemented through a control system. The controlled object is a dual-ended converter, which includes a first converter and a second converter. The control systems of both the first and second converters include a voltage outer loop control unit, a power outer loop control unit, a current inner loop control and PWM unit, a d-axis control module, and a q-axis control module. The d-axis control module includes a voltage controller, a limiter, a power controller, and a first current inner loop controller. The q-axis control module includes a second current inner loop controller. The limiter for the d-axis control module of the first converter includes a first limiter and a second limiter. The limiter for the d-axis control module of the second converter includes a second limiter and a third limiter. The control method for the first converter includes: i dacA * The calculated current command and the given minimum current command value i, which serve as the power outer loop control unit. dmin * The difference is calculated, and the difference is processed by the second limiter to obtain the current command i of the power outer loop control unit. d2A * ; will i d2A * Multiply by the gain factor K and then combine with the initial DC voltage setpoint U in the outer voltage loop control unit. dc * The sum is used as the final DC voltage setpoint of the outer loop control unit. This final DC voltage setpoint is compared with the actual DC voltage feedback value U. dcA The difference is input to the voltage controller, and the initial voltage outer loop output current command i is obtained through the first limiter. d1A * ; Finally, i d1A * With the power outer loop control unit current command i d2A * The summation yields the first current inner loop controller given current command i. dA * The second current inner loop controller is given a current command i. qA * The value is 0, and the first converter is operated by the inner current loop control and PWM unit control. The control method for the second converter includes: i dacB * The calculated current command and the given minimum current command value i, which serve as the power outer loop control unit. dmin * The sum is processed by the second limiter to obtain the current command i of the power outer loop control unit. d2B * ; will i d2B * Multiply by the gain factor K and then combine with the initial DC voltage setpoint U in the outer voltage loop control unit. dc * The sum is used as the final DC voltage setpoint for the outer loop of voltage control. This final DC voltage setpoint is compared with the actual DC voltage feedback value U. dcB The difference is input to the voltage controller, and the initial voltage outer loop output current command i is obtained through the third limiter. d1B * ; Finally, i d1B * With the power outer loop control unit current command i d2B * The summation yields the first current inner loop controller given current command i. dB * The second current inner loop controller is given a current command i. qB * The value is 0, and the second converter is operated through the inner current loop control and PWM unit control.

2. The converter control method as described in claim 1, characterized in that, The limiting value of the first limiter is -i dmax * to i dmin * The limiting value of the second limiter is from 0 to i dmax * The limiting value of the third limiter is -i dmax * to -i dmin * ; where i dmax * with i dmin * These are the maximum and minimum d-axis current command values, respectively.

3. The converter control method as described in claim 2, characterized in that, The minimum d-axis current command value i dmin * The selection can be freely adjusted based on the actual control system.

4. The converter control method as described in claim 1, characterized in that, The gain coefficient K is a positive value, used to increase the DC voltage command value, and can be freely adjusted according to the actual control system.

5. The converter control method as described in claim 1, characterized in that, The control method includes d-axis control and q-axis control.

6. The converter control method as described in claim 1, characterized in that, The voltage outer loop control unit is used to control the DC bus voltage, and the power outer loop control unit is used to control the DC line power transmission. The voltage outer loop control unit, the power outer loop control unit, and the limiter work together to control the switching of the control modes of the first converter and the second converter.

7. The converter control method as described in claim 6, characterized in that, The control methods include constant voltage control mode and constant power control mode.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by the processor, the program implements the converter control method for improving the power transmission stability of a flexible DC interconnect system as described in any one of claims 1 to 7.

9. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps in the converter control method for improving the power transmission stability of a flexible DC interconnect system as described in any one of claims 1 to 7.

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