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

Abstract

The invention discloses a converter control method for improving the power transmission stability of a flexible direct current interconnection system, relates to the technical field of flexible interconnection system control strategies, and solves the problem of power oscillation existing in an existing flexible direct current interconnection device control method. According to the technical scheme, the method is characterized in that converter control modes on the two sides of the flexible interconnection system are adaptively switched according to the system energy flow direction. The technology is realized by coordinating control links such as direct-current voltage, a power control outer loop, an amplitude limiter and a current control inner loop, and a power control current feedback instruction passing through the amplitude limiter and a direct-current voltage control current feedback instruction are superposed to serve as a current inner loop control instruction; the operation of the power electronic converter is controlled through a current inner loop controller and a PWM modulation instruction; inner loop current instruction values of the converters on the two sides of the flexible direct current interconnection system are adaptively adjusted along with the change of the power flow direction of the system, direct current voltage can be controlled, the power flow direction of the system can also be controlled, and the single-side converter control mode is not limited to constant direct current voltage control or constant power control. When the power of the system changes, on the premise of ensuring reliable and stable switching of the control modes of the converters on the two sides in the system, the system stability problem caused by the power flow direction problem under the fixed control mode of the single-side converter is solved.

Description

Technical Field

[0001] The present application relates to the technical field of control strategies for flexible DC interconnection systems, and in particular to a converter control method for improving power transmission stability of a flexible DC interconnection system. Background Art

[0002] The flexible DC interconnection system can decouple and interconnect the AC system and realize the DC interconnection mode, which can not only effectively control the system short-circuit current and system power flow, but also provide dynamic voltage support when the system is not operating normally, eliminate the impact current and circulating current caused by the closing loop, ensure the safe operation of the closing loop, and greatly improve the power supply reliability of the distribution network. However, the flexible DC interconnection device is composed of two voltage source converters (VSCs) connected in series through a DC link. Usually, the converter at one end adopts a constant DC voltage control, and the converter at the other end adopts a constant power control, such as Figure 1 As shown. When power is transmitted from the DC line to the converter using constant power control, the converter acts as a constant power load, and the DC side impedance characteristic is a negative impedance characteristic, which is easy to cause power oscillation in the system under high power, seriously reducing the stability of the DC bus. When the power direction changes, the DC side impedance characteristic presents a positive impedance characteristic, which helps to stabilize the DC bus. Obviously, under the condition of changing power flow direction, the power oscillation problem of the traditional control method cannot be avoided, which seriously threatens the stability of the system.

[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 beneficial to improving the stability of the system. Research on this is of great significance. Summary of the invention

[0004] The present application provides a converter control method for improving the power transmission stability of a flexible DC interconnection system. Its technical purpose is to solve the inevitable power oscillation problem under the single control of the dual-side converters of the existing flexible DC interconnection system, so that the control mode of the converters on both sides can be adaptively adjusted with the power flow direction of the system, and can switch between a constant DC voltage control mode and a constant power control mode, thereby solving the problem of system stability deterioration caused by internal power oscillation of the system when the power flow direction changes.

[0005] The above technical objectives of this application are achieved through the following technical solutions:

[0006] A converter control method for improving the power transmission stability of a flexible DC interconnection system, the control method is implemented by a control system, the control object is a system double-ended converter, the double-ended converter includes a first converter and a second converter, the control systems of the first converter and the second converter both 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 both include 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 of the d-axis control module for the first converter includes a first limiter and a second limiter, and the limiter of the d-axis control module for the second converter includes a second limiter and a third limiter;

[0007] The control method of the first converter comprises:

[0008] i dacA * The calculated current command of the power outer loop control unit is equal to the given minimum current command value i dmin * 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 it by the gain coefficient K and compare it with the initial given value U of the DC voltage in the voltage outer loop control unit. dc * The sum is used as the final DC voltage given value of the voltage outer loop control unit. The final DC voltage given value is equal to the actual DC voltage feedback value U dcA After the difference is made, it 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 * The current command i of the power outer loop control unit d2A * The sum is used to obtain the current command i given by the first current inner loop controller dA * , let the second current inner loop controller give a current command i qA * is 0, the first converter is controlled to operate through the current inner loop control and the PWM unit;

[0011] The control method of the second converter comprises:

[0012] i dacB *The calculated current command of the power outer loop control unit is equal to the given minimum current command value i dmin * The added value is processed by the second limiter to obtain the current instruction i of the power outer loop control unit d2B * ;

[0013] will i d2B * Multiply it by the gain coefficient K and compare it with the initial given value U of the DC voltage in the voltage outer loop control unit. dc * The final DC voltage given value of the voltage control outer loop is added together with the actual DC voltage feedback value U dcB After the difference is made, it 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 * The current command i of the power outer loop control unit d2B * The sum is used to obtain the current command i given by the first current inner loop controller dB * , let the second current inner loop controller give a current command i qB * is 0, and the second converter is controlled to operate through the current inner loop control and the PWM unit.

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

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

[0017] Furthermore, the gain coefficient K is a positive value, which is used to raise the DC voltage given 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 cooperate to control the switching of the control modes of the first converter and the second converter.

[0020] Furthermore, the control mode includes a constant voltage control mode and a constant power control mode.

[0021] The beneficial effects of this application are:

[0022] (1) When the power flow direction of the system remains unchanged, the power flow receiving end is under constant voltage control, and the other end is under constant power control. The DC side output impedance presents 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 to the receiving end always in a constant voltage control mode, greatly improving the system stability under complex working conditions.

[0024] (3) When the system transmission power is too large, the limiter is saturated, making it impossible for the output current of the converter to increase, so that the converter has overload protection capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of a method for controlling converters on both sides of a flexible DC interconnection system in the prior art;

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

[0027] Figure 3a to Figure 3c It is a schematic diagram of the simulation waveform of the control switching of the converters on both sides when the flow direction changes at a relatively high power when the control method of the present application is adopted;

[0028] Figures 4a to 4c This is a schematic diagram of the simulation waveform of the control switching of the converters on both sides when the initial power is small and flows in the forward direction, and then the power increases and the flow direction changes when the control method of the present application is adopted. DETAILED DESCRIPTION

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

[0030] Figure 2The schematic diagram of the structure of the control system in the converter control method for improving the power transmission stability of the flexible DC interconnection system described in this application is as follows: Figure 2 The control system is used to control the double-ended converter of the system. Figure 2 VSC-A and VSC-B in. The control systems of VSC-A and VSC-B each 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 each 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 of the d-axis control module for the first converter includes a first limiter and a second limiter, and the limiter of the d-axis control module for 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 cooperate to control the switching of the control modes of the first converter and the second converter.

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

[0034] i dacA * The calculated current command of the power outer loop control unit is equal to the given minimum current command value i dmin * 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 it by the gain coefficient K and compare it with the initial given value U of the DC voltage in the voltage outer loop control unit. dc * The sum is used as the final DC voltage given value of the voltage outer loop control unit. The final DC voltage given value is equal to the actual DC voltage feedback value U dcA After the difference is made, it 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 * The current command i of the power outer loop control unitd2A * The sum is used to obtain the current command i given by the first current inner loop controller dA * , let the second current inner loop controller give a current command i qA * is 0, the first converter is controlled to operate through the current inner loop control and the PWM unit;

[0037] The control method of the converter VSC-B includes:

[0038] i dacB * The calculated current command of the power outer loop control unit is equal to the given minimum current command value i dmin * The added value is processed by limiter 2 to obtain the current instruction i of the power outer loop control unit d2B * ;

[0039] will i d2B * Multiply it by the gain coefficient K and compare it with the initial given value U of the DC voltage in the voltage outer loop control unit. dc * The final DC voltage given value of the voltage control outer loop is added together with the actual DC voltage feedback value U dcB After the difference is made, it is input to the voltage controller and the initial voltage outer loop output current command i is obtained through the limiter 3. d1B * ;

[0040] Finally, I d1B * The current command i of the power outer loop control unit d2B * The sum is used to obtain the current command i given by the first current inner loop controller dB * , let the second current inner loop controller give a current command i qB * is 0, and the second converter is controlled to operate through the current inner loop control and the PWM unit.

[0041] Preferably, the limit value of limiter 1 is -i dmax * to i dmin * , the limit value of limiter 2 is 0 to i dmax * , the limit value of limiter 3 is -i dmax * to -i dmin * ; where i dmax* with i dmin * They are the set maximum d-axis current command value and the set minimum d-axis current command value respectively. The addition of each limiter, on the one hand, enables the converter to have overload protection capability when the transmission power is large, and on the other hand, through the coordination of various limit values, it is ensured that the d-axis current command value of the input current inner loop control is always a constant voltage control current command value or a constant power control current command value, 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 choice of can be freely adjusted according to the actual system. The addition of this command value stipulates the initial control mode of the converters on both sides when the current is small when the system starts, and at the same time avoids the frequent mode switching problem of the converters on both sides when the power is small under this control mode.

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

[0044] Among them, U dcA , U dcB are the DC side voltages of VSC-A and VSC-B respectively; U dc * is the given value of DC voltage; P acA * and P acB * are the AC side active power reference values ​​of VSC-A and VSC-B respectively; U dA and U dB are the d-axis voltage components of the AC side of VSC-A and VSC-B respectively; i dacA * and i dacB * are the d-axis current given values ​​corresponding to the AC side active power given values ​​of VSC-A and VSC-B respectively; i dmin * is the minimum d-axis current command value, and also the current given limit corresponding to low power transmission; i dmax * is the maximum d-axis current command value; i d1A * 、i d2A * and i d1B* 、i d2B * are the two d-axis current given values ​​output by VSC-A and VSC-B through the limiter; K is i d2A * and i d2B * The gain coefficient of i dA * 、i dB * and i qA * 、i qB * They are the final current reference values ​​of the current inner loop controller of the d-axis and q-axis of VSC-A and VSC-B respectively.

[0045] i dmin * The addition of , limits the control mode of the two converters VSC-A and VSC-B when starting. When the system starts with a small current, VSC-A is in constant voltage control mode, and VSC-B is in constant voltage control mode. At the same time, it avoids the problem of frequent switching of the control of the two converters when the system is running at a low power, and limits the current flow of the system at a relatively low power. This makes it inevitable that when the system is running at a low power, the converter at the receiving end of the power flow may adopt constant power control, and the other end adopts constant voltage control. The power oscillation problem mostly occurs under high power conditions, so it does not contradict the concept of suppressing system power oscillation.

[0046] The direction of power flow from VSC-A to VSC-B is defined as the positive direction. The following analyzes how the power operates when it flows forward and reversely from VSC-A and VSC-B, and how the power switches from forward flow to reverse flow.

[0047] (1) Power flows in the forward direction

[0048] Give VSC-A a positive power reference value P acA * , and obtain the corresponding d-axis current given value i dacA * , minus i dmin * After getting i d2A * The power given to VSC-B is P acB * , and P acB * =-P acA * , we can get a d-axis current given value i dacB* =-i dacA * , plus i dmin * After getting i d2B * .

[0049] when i dacA * >i dmin * When i d2B * = 0, U dcB The given value is raised, and VSC-B works in the constant voltage control mode to maintain the DC voltage. At the same time, the obtained i d2A * =i dacA * -i dmin * ,i d2A * Add to U dcA The given value pulls up the original given value, causing the voltage controller to enter saturation, and the output value i d1A * =i dmin * , and finally get i dA * =i dacA , so VSC-A works in constant power control mode.

[0050] When 0 dacA * dmin * When i d2A * = 0, U dcA The given value is pulled up, so VSC-A works in constant voltage control mode. At the same time, the obtained i d2B * =i dmin * +i dacB * ,i d2B * Add to U dcB The given value pulls up the original given value, causing the voltage controller to enter saturation and the output value to be -i dmin * , and finally get i dB * =i dacB * , so VSC-B works in constant power control mode.

[0051] ​​(2) Power Flow in Reverse Direction

[0052] Give VSC-B a positive power reference value P acB * , which is equivalent to giving VSC-A a negative power reference value P acA * , P acA * =-P acB * , so we can get i dacA * =-i dacB * ,i dacA * Subtract i dmin * After getting i d2A * , the obtained i d2A * = 0, U dcA The given value is pulled high, so VSC-A works in constant voltage control mode.

[0053] For the d-axis current given value i obtained in VSC-B dacB * , plus i dmin * After getting i d2B * , the obtained i d2B * =i dmin * +i dacB * ,i d2B * Add to U dcB The given value increases the original given value. Since VSC-A works in the constant voltage control mode, the voltage controller enters saturation and the output value is -i dmin * , and finally get i dB * =i dacB * , so VSC-B works in constant power control mode.

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

[0055] When power flows in the forward direction and i dacA * >i dmin *When the power suddenly flows in the opposite direction, due to the i d2B * =i dmin * +i dacB * ,i d2B * Add to U dcB The given value quickly increases the original given value, causing the voltage controller of VSC-B to enter a saturated state, and finally obtaining i dB * =i dacB * , VSC-B works in constant power control mode, and the working mode switches. At the same time, due to the reverse power, i dacA * Inverse, with i dmin * After the difference is made, the limiter outputs i d2A * =0,U dcA The given value remains the original given value, so the voltage controller of VSC-A begins to desaturate and works in the constant voltage control mode to maintain the DC voltage. The voltage controller of VSC-B enters the saturation state and the voltage controller of VSC-A desaturates at the same time.

[0056] When power flows in the forward direction and 0 dacA * dmin * , that is, under low power conditions, it can be seen from the above that at this time, the voltage controller of VSC-A is not in saturation, and the voltage controller of VSC-B is in saturation. When the power suddenly flows in the opposite direction, due to i d2B * =i dmin * +i dacB * ,i d2B * Add to U dcB The given value still pulls up the original given value, so that the voltage controller of VSC-B is still in saturation state, and finally i dB * =i dacB * , VSC-B still works in constant power control mode. d2A * Still 0, U dcA ​​The given value remains the original given value. Due to the reverse transmission of VSC-B, U dc Therefore, VSC-A still works in the constant voltage control mode to maintain the DC side voltage.

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

[0058] Table 1 Control methods of two converters corresponding to different power flow directions

[0059]

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

[0061] Figure 3a to Figure 3c It is operated under high current conditions, that is, i dacA * >i dmin * , when power flows in the forward direction, i d1A * =i dmin * =20A, the VSC-A voltage controller reaches saturation, and VSC-A works in constant power control mode; at the same time, i d2B * =0,i dB * =i d1B * , VSC-B works in constant voltage control mode. When power flows in reverse direction, i d2A * =0,i dA * =i d1A * , VSC-A works in constant voltage control mode; at the same time, i d1B * =-i dmin * =-20A, the VSC-B voltage controller reaches saturation, VSC-B works in constant power control mode, and the control modes of the two converters are switched.

[0062] Figures 4a to 4c Under the initial conditions, the system runs forward with low power, that is, 0 dacA * dmin * ​​, then the power increases and flows in the reverse direction. When the power flows in the forward direction, i d2A * =0,i dA * =i d1A * , VSC-A works in constant voltage control mode; at the same time, i d1B * =-i dmin * =-20, the VSC-B voltage controller reaches saturation, and VSC-B works in constant power control mode. When the power flows in the reverse direction, the VSC-B voltage controller is still in saturation and works in constant power control mode. VSC-A still works 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 principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A converter control method for improving power transmission stability of a flexible DC interconnection system, characterized in that: The control method is implemented by a control system, and the control object is a system double-ended converter, which includes a first converter and a second converter. The control systems of the first converter and the second converter both 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 modules both include a voltage controller, a limiter, a power controller and a first current inner loop controller, and the q-axis control module includes a second current inner loop controller. The limiters of the d-axis control module for the first converter include a first limiter and a second limiter, and the limiters of the d-axis control module for the second converter include a second limiter and a third limiter. The control method of the first converter comprises: i dacA * The calculated current command of the power outer loop control unit is equal to the given minimum current command value i dmin * 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 it by the gain coefficient K and compare it with the initial given value U of the DC voltage in the voltage outer loop control unit. dc * The sum is used as the final DC voltage given value of the voltage outer loop control unit. The final DC voltage given value is equal to the actual DC voltage feedback value U dcA After the difference is made, it 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 * The current command i of the power outer loop control unit d2A * The sum is used to obtain the current command i given by the first current inner loop controller dA * , let the second current inner loop controller give a current command i qA * is 0, the first converter is controlled to operate through the current inner loop control and the PWM unit; The control method of the second converter comprises: i dacB * The calculated current command of the power outer loop control unit is equal to the given minimum current command value i dmin * The added value is processed by the second limiter to obtain the current instruction i of the power outer loop control unit d2B * ; will i d2B * Multiply it by the gain coefficient K and compare it with the initial given value U of the DC voltage in the voltage outer loop control unit. dc * The final DC voltage given value of the voltage control outer loop is added together with the actual DC voltage feedback value U dcB After the difference is made, it 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 * The current command i of the power outer loop control unit d2B * The sum is used to obtain the current command i given by the first current inner loop controller dB * , let the second current inner loop controller give a current command i qB * is 0, and the second converter is controlled to operate through the current inner loop control and the PWM unit.

2. The converter control method according to claim 1, characterized in that: The limiting value of the first limiter is -i dmax * to i dmin * The second limiter has a limit value of 0 to i dmax * , the limiting value of the third limiter is -i dmax * to -i dmin * ; where i dmax * with i dmin * They are the set maximum d-axis current command value and the set minimum d-axis current command value respectively.

3. The converter control method according to claim 2, characterized in that: The minimum d-axis current command value i dmin * The choice can be freely adjusted according to the actual control system.

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

5. The converter control method according to claim 1, characterized in that: The control method includes d-axis control and q-axis control.

6. The converter control method according to 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 cooperate to control the switching of the control mode of the first converter and the second converter.

7. The converter control method according to claim 6, characterized in that: The control mode includes a constant voltage control mode and a constant power control mode.

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

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, the steps in the converter control method for improving the power transmission stability of a flexible DC interconnection system as claimed in any one of claims 1 to 7 are implemented.

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