Secondary overvoltage suppression method and system in receiving end fault recovery period

Abstract

The invention discloses a secondary overvoltage suppression method and system in a receiving end fault recovery period, and relates to the technical field of safe operation of a power system, and the method comprises the steps: obtaining a current instruction value, and carrying out the calculation, and obtaining the slope of the current instruction value; the slope of the current instruction value is judged, and the fault recovery stage of the receiving end is distinguished; and controlling a slope amplitude limiter based on the receiving end fault recovery stage to realize secondary overvoltage suppression. The secondary overvoltage phenomenon generated in the fault recovery period under the novel control strategy of the wind power flexible direct output system can be solved with low cost, the control strategy belongs to an additional control strategy, safe and stable operation of a power system is facilitated, the method is suitable for the scene of coordinated control of multiple PI controllers, and the method is suitable for popularization and application. And an effective solution is provided for a transient problem of a power system caused by a supersaturation phenomenon in multi-PI coordination control.

Description

Technical Field

[0001] The present invention relates to the technical field of safe operation of electric power systems, and in particular to a method and system for suppressing secondary overvoltage during fault recovery at a receiving end. Background Art

[0002] Driven by the "dual carbon" goals, new energy technologies represented by wind power generation have been widely used and developed rapidly. As the proportion of wind power in the power system continues to grow rapidly, the transmission method of wind power sent through a flexible direct current transmission system has been more widely used. At present, wind power through a flexible direct current transmission system is usually constructed in accordance with the phase-locked synchronous control of wind turbines, the flexible direct current adopts a modular multilevel converter MMC, and the sending end adopts constant voltage / frequency control, that is, V / F control, and the receiving end adopts a constant DC voltage control method. However, this combination of control methods has some significant shortcomings. For example, when the flexible direct current receives When a three-phase metallic grounding fault occurs at the end, the active power at the sending end cannot be actively adjusted, which will lead to a more serious DC overvoltage phenomenon. This overvoltage will not only cause potential damage to the system equipment, but may also cause the system protection device to malfunction, affecting the stability of the entire power system. Regarding the DC overvoltage problem when there is a fault at the receiving end of the wind power transmission system via flexible direct current, new research has shown that when the direct-drive wind turbine adopts grid-type control, the flexible direct current sending end adopts fixed DC voltage control, and the receiving end adopts a constant power control structure, the flexible direct current sending end converter can automatically respond to the receiving end fault and reduce the sending end power, thereby suppressing the overvoltage on the DC side.

[0003] However, the current common solutions also have many shortcomings. First, after the response capability of the sending-end controller is fully called upon, the coordination of multiple controllers to participate in the fault ride-through process becomes more complicated, and even secondary overvoltage occurs during fault recovery. The amplitude of the secondary overvoltage may even exceed the amplitude of the primary overvoltage, seriously endangering the safe and stable operation of the power system. Secondly, the current research on the secondary overvoltage problem is still in its infancy, and the existing solutions have many limitations. For example, if hardware energy-consuming devices are added, the system investment cost will increase, and the switching strategy of the energy-consuming devices is more complicated. At the same time, the corresponding software control strategies are rarely proposed and have not received widespread attention. Therefore, there is an urgent need for an effective method to solve the secondary overvoltage phenomenon. Summary of the invention

[0004] In view of the fact that the research on the secondary overvoltage problem in the prior art is still in its infancy when suppressing the secondary overvoltage phenomenon during the recovery of the receiving-end fault, if a hardware energy-consuming device is added, the system investment cost will increase, and the switching strategy of the energy-consuming device is relatively complicated. At the same time, the corresponding software control strategy is rarely proposed and has not received widespread attention. Therefore, the present invention is proposed.

[0005] Therefore, the problem to be solved by the present invention is how to provide a method for suppressing secondary overvoltage during the receiving end fault recovery period under the control strategy of a new wind power flexible direct current transmission system, and a method for solving the DC secondary overvoltage problem during the fault recovery period under the control strategy of a new wind power flexible direct current transmission system.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0007] In the first aspect, an embodiment of the present invention provides a method for suppressing secondary overvoltage during receiving end fault recovery, which includes obtaining a current command value and calculating it to obtain the slope of the current command value; judging the slope of the current command value to distinguish the receiving end fault recovery stage; and controlling the slope limiter based on the receiving end fault recovery stage to achieve secondary overvoltage suppression.

[0008] As a preferred solution of the secondary overvoltage suppression method during the receiving end fault recovery period of the present invention, wherein: the current command value is obtained by detecting the d-axis current command value output by the receiving end MMC fixed active power control in the controller, and the slope of the current command value is obtained, which includes the following steps: using a first-order low-pass filter to process the receiving end d-axis current command value to eliminate the influence of high-frequency noise on the slope of the current command value during steady-state operation of the system; calculating the slope of the current command value every selected time period T; the calculation formula of the slope of the current command value is as follows:

[0009]

[0010] Where k is the slope of the d-axis current command value at the receiving end; T is the selected time period; i sdref[t] is the d-axis current command value at the receiving end during the previous calculation; i sdref[t+T] is the receiving end d-axis current command value after the selected time period T.

[0011] As a preferred solution of the method for suppressing secondary overvoltage during the receiving-end fault recovery period of the present invention, the receiving-end fault recovery stage includes a slope limiting stage, an integral recovery stage and a controller recovery stage; the distinguishing the receiving-end fault recovery stage includes the following steps: judging the slope k of the receiving-end d-axis current command value, if the slope k of the receiving-end d-axis current command value is ≤ the set threshold K, then judging that the receiving-end MMC fixed active power control is in a normal working state; if the slope k of the receiving-end d-axis current command value is greater than the set threshold K, then judging that the receiving-end MMC fixed active power control is not in a normal working state, and entering the slope limiting stage; after entering the slope limiting stage, judging the receiving-end active power and the sending-end active power, if the receiving-end active power is less than the sending-end active power, then judging that entering the integral recovery stage; if the receiving-end active power is ≥ the sending-end active power, then judging that it is in the slope limiting stage; after entering the integral recovery stage, judging the slope k of the receiving-end d-axis current command value, if the slope k of the receiving-end d-axis current command value is less than the slope limiting maximum value K max , it is determined that the receiving-end MMC active power control has resumed normal working state and entered the controller recovery phase; if the slope k of the receiving-end d-axis current command value ≥ the maximum slope limit K max , it is determined that the secondary overvoltage has not been completely suppressed and is in the integral recovery stage.

[0012] As a preferred scheme of the method for suppressing secondary overvoltage during the receiving-end fault recovery period described in the present invention, the slope limiting stage includes: if the slope k of the receiving-end d-axis current command value is ≤ the set threshold K, it is determined that the receiving-end MMC fixed active power control is in a normal working state, the slope limiter does not work, and the control link directly uses the receiving-end d-axis current command value generated by the receiving-end MMC fixed active power control; if the slope k of the receiving-end d-axis current command value is greater than the set threshold K, it is determined that the receiving-end MMC fixed active power control is not in a normal working state, the slope limiter is started, and the slope of the receiving-end d-axis current command value is limited, so that the receiving-end d-axis current command value increases according to the slope k=K of the receiving-end d-axis current command value.

[0013] As a preferred solution of the method for suppressing secondary overvoltage during receiving-end fault recovery of the present invention, the calculation formula of the fixed active power in the normal working state is as follows:

[0014]

[0015] Among them, i sdref is the fixed active power in normal working state; k psd P is the proportional parameter of the proportional integral PI link of the active power control at the receiving end; vref is the command value of the active power at the receiving end; P v is the active power of the MMC receiving end; k isdis the integral parameter of the receiving end active power control; s is the Laplace operator; after the slope limiter is started, the receiving end d-axis current command value satisfies the following formula:

[0016] i sdrefmax =kt<1.1pu

[0017] Among them, i sdrefmax is the maximum value of the d-axis current command value at the receiving end; k is the slope of the d-axis current command value at the receiving end; t is the duration of the limit.

[0018] As a preferred solution of the secondary overvoltage suppression method during the receiving end fault recovery period of the present invention, the integral recovery stage includes: limiting the active power of the receiving end based on the slope limit, so that the active power of the receiving end is less than the active power of the sending end, and the active power of the receiving end and the active power of the sending end satisfy the following formula:

[0019] P v =u sd i sd <P w

[0020] Among them, P v is the active power of the MMC receiving end; P w is the active power of the MMC sending end; u sd is the output voltage of the d-axis at the receiving end of the MMC; i sd is the output current of the d-axis at the receiving end of the MMC; when the active power at the receiving end is less than the active power at the sending end, the DC side voltage rises rapidly, accelerating the saturation recovery of the d-axis current at the sending end, canceling the low voltage stage of the DC side voltage, and suppressing the secondary overvoltage; the calculation formula of the active power at the receiving end is as follows:

[0021] P v =u sa i sa +u sb i sb +u sc i sc

[0022] Among them, P v is the active power of the MMC receiving end; u sa is the AC voltage of phase a at the receiving end; u sb is the AC voltage of phase b at the receiving end; u sc is the c-phase AC voltage at the receiving end; i sa is the AC current of phase a at the receiving end; i sb is the AC current of phase b at the receiving end; i sc is the c-phase AC current at the receiving end.

[0023] As a preferred solution of the secondary overvoltage suppression method during the receiving end fault recovery period of the present invention, the controller recovery stage includes: judging the slope k of the receiving end d-axis current command value, if the slope k of the receiving end d-axis current command value is less than the slope limit maximum value K max , it is determined that the receiving-end MMC active power control has resumed normal working state and the slope limiter is turned off; if the slope k of the receiving-end d-axis current command value ≥ the maximum slope limit value K max , it is determined to be in the integral recovery stage and the secondary overvoltage suppression continues.

[0024] In the second aspect, in order to further solve the safety problems existing in the safe operation of the power system, the embodiment of the present invention provides a secondary overvoltage suppression system during the fault recovery period of the receiving end, which includes: a slope calculation module, which is used to obtain the d-axis current command value output by the receiving-end MMC fixed active power control and calculate to obtain the slope of the d-axis current command value of the receiving end; a stage distinction module, which is used to judge the slope of the d-axis current command value of the receiving end, and divide the receiving-end fault recovery stage into a slope limiting stage, an integral recovery stage or a controller recovery stage; a voltage suppression module, which is used to process according to the slope limiting stage, the integral recovery stage and the controller recovery stage respectively, and control the slope limiter, the receiving-end active power and the sending-end active power to achieve secondary overvoltage suppression.

[0025] In a third aspect, an embodiment of the present invention provides a computer device, comprising a memory and a processor, wherein the memory stores a computer program, wherein: when the computer program is executed by the processor, any step of a method for suppressing secondary overvoltage during receiving end fault recovery as described in the first aspect of the present invention is implemented.

[0026] In a fourth aspect, an embodiment of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein: when the computer program is executed by a processor, it implements any step of a method for suppressing secondary overvoltage during receiving end fault recovery as described in the first aspect of the present invention.

[0027] The beneficial effects of the present invention are as follows: the present invention proposes a method for suppressing secondary overvoltage during the receiving-end fault recovery period under a new wind power flexible direct current transmission system control strategy, which belongs to an additional control strategy and can solve the secondary overvoltage phenomenon generated during the fault recovery period under the new control strategy of the wind power flexible direct current transmission system at a lower cost, which is beneficial to the safe and stable operation of the power system, is suitable for scenarios of coordinated control of multiple PI controllers, and provides an effective solution to the transient problem of the power system caused by oversaturation in the multi-PI coordinated control. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. Among them:

[0029] Figure 1 This is a simulation system topology diagram of the new wind power flexible direct current transmission system in Example 1.

[0030] Figure 2 This is a detailed control block diagram of each link of the new wind power flexible direct current transmission system in Example 1.

[0031] Figure 3 This is a diagram of the overall operation architecture of the system in Example 1.

[0032] Figure 4 This is a block diagram of the secondary overvoltage suppression strategy in Example 1.

[0033] Figure 5 This is a logical schematic diagram of the secondary overvoltage suppression strategy in Example 1.

[0034] Figure 6 Schematic diagram of the simulation waveforms of overvoltage and secondary overvoltage and the effect of the control strategy in Example 2. DETAILED DESCRIPTION

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0036] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0037] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0038] Example 1

[0039] Reference Figure 1 to Figure 5 , which is the first embodiment of the present invention, provides a method for suppressing secondary overvoltage during fault recovery at a receiving end, comprising the following steps:

[0040] S1: Obtain the current command value and perform calculation to obtain the slope of the current command value.

[0041] Specifically, Figure 1 The figure shows the simulation system topology of the new wind power transmission system. In the new wind power transmission system, the wind farm is composed of direct-drive wind turbines and adopts grid-type control. The detailed control process of each link is as follows: Figure 2 As shown, the flexible DC transmission system adopts a pseudo-bipolar connection mode, the sending end of the flexible DC transmission system is connected to the direct-drive wind farm, and a phase-locked loop is used to achieve synchronization with the wind farm. A constant DC voltage control is used to generate an output d-axis current command value, and a constant reactive power is used to generate an output q-axis current command value; the receiving end of the flexible DC transmission system is connected to the large AC power grid, and a phase-locked loop is used to achieve synchronization with the power grid. A constant active power control is used to generate an output d-axis current command value, and a constant reactive power is used to generate an output q-axis current command value.

[0042] Further, such as Figure 3 The overall operation architecture of the system is shown in Figure 1, where the active power command value P of the virtual synchronous loop of the direct-drive wind turbine is gref After a 50ms delay, it is used as the active power command value P of the receiving end fixed power control vref , because when the new wind power is proved to have a receiving-end fault through the flexible direct current transmission system control strategy, the sending-end converter responds automatically, significantly reducing the sending-end power to suppress the overvoltage on the DC side. Due to the influence of losses, during normal operation, the active power of the sending end is greater than the active power of the receiving end. However, in the initial stage of fault recovery, the active power recovery speed of the sending end is lower than the active power recovery speed of the receiving end, resulting in secondary overvoltage on the DC side. Therefore, a secondary overvoltage suppression strategy during fault recovery is proposed.

[0043] Specifically, Figure 4 and Figure 5 FIG. 1 shows a block diagram of a secondary overvoltage suppression strategy and a logic diagram of a secondary overvoltage suppression strategy, wherein the receiving-end d-axis current command value output by the receiving-end MMC fixed active power control is detected and the slope of the receiving-end d-axis current command value is calculated. When a fault occurs at the receiving end, the receiving-end d-axis current command value output by the receiving-end MMC fixed active power outer loop PI controller rises sharply, and the slope limiter is started to control the receiving-end d-axis current command value to rise according to the maximum slope K, and the receiving-end d-axis current command value I is limited during the fault duration and fault recovery period. sdrefmax <1.1pu, in the later stage of fault recovery, the slope of the d-axis current command value of the receiving end is judged. If the slope of the d-axis current command value of the receiving end drops below the maximum slope K, the MMC fixed active power control of the receiving end resumes the normal working state, and the slope limiter is turned off to realize the suppression of the secondary overvoltage.

[0044] Preferably, the current command value is obtained by detecting the d-axis current command value output by the receiving-end MMC fixed active power control in the controller to obtain the receiving-end d-axis current command value.

[0045] Preferably, obtaining the slope of the current command value includes the following steps: using a first-order low-pass filter to process the receiving end d-axis current command value to eliminate the influence of high-frequency noise on the slope of the current command value during steady-state operation of the system. The specific formula of the first-order low-pass filter is as follows:

[0046]

[0047] Wherein, H(s) is the transfer function of the first-order low-pass filter; τ is the first-order low-pass filter time constant; and s is the Laplace operator.

[0048] The slope of the current command value is calculated every selected time period T.

[0049] Specifically, the calculation formula of the slope of the current command value is as follows:

[0050]

[0051] Where k is the slope of the d-axis current command value at the receiving end; T is the selected time period; i sdref[t] is the d-axis current command value at the receiving end during the previous calculation; i sdref[t+T] is the receiving end d-axis current command value after the selected time period T.

[0052] Preferably, by introducing a first-order low-pass filter to process the d-axis current command value at the receiving end, the influence of high-frequency noise is effectively eliminated and the accuracy of slope calculation is improved; by considering the characteristics of the wind farm and the characteristics of the flexible DC transmission system, the dynamic control strategy based on the slope of the current command value and the secondary overvoltage suppression in the fault recovery process are combined, and this is achieved through slope calculation and limiting, which cleverly utilizes the dynamic characteristics of the system, realizes refined control of the fault recovery process, and can more effectively suppress the secondary overvoltage.

[0053] S2: Determine the slope of the current command value to distinguish the receiving end fault recovery stage.

[0054] Preferably, the receiving end fault recovery phase includes a slope limiting phase, an integral recovery phase and a controller recovery phase.

[0055] Specifically, distinguishing the receiving end fault recovery stage includes the following steps: judging the slope k of the receiving end d-axis current command value; if the slope k of the receiving end d-axis current command value ≤ the set threshold K, it is determined that the receiving end MMC fixed active power control is in a normal working state, the slope limiter does not work, and the control link directly uses the receiving end d-axis current command value generated by the fixed active power control.

[0056] If the slope k of the d-axis current command value at the receiving end is greater than the set threshold K, it is determined that the active power control of the MMC at the receiving end is not in a normal working state, and the slope limiting stage is entered. The slope limiter is started to perform slope limiting on the d-axis current command value at the receiving end, so that the d-axis current command value at the receiving end increases according to the slope k=K of the d-axis current command value at the receiving end.

[0057] After entering the slope limiting stage, the active power of the receiving end and the active power of the sending end are judged. If the active power of the receiving end is less than the active power of the sending end, it is determined to enter the integral recovery stage, so that the DC side voltage rises rapidly, accelerates the saturation recovery of the d-axis current at the sending end, cancels the low voltage stage of the DC side voltage, and suppresses the secondary overvoltage. In the initial stage of fault recovery, the active recovery speed of the sending end is lower than the active recovery speed of the receiving end, and in normal operation, the active power of the sending end is greater than the active power of the receiving end. Therefore, by performing slope limiting on the d-axis current command value of the receiving end in the slope limiting stage, the recovery of the active power of the sending end is faster than that of the receiving end, thereby promoting the system to recover to a normal state faster.

[0058] If the active power at the receiving end is ≥ the active power at the sending end, it is determined to be in the slope limiting stage, and the start of the slope limiter is maintained unchanged.

[0059] After entering the integral recovery phase, the slope k of the receiving end d-axis current command value is judged. If the slope k of the receiving end d-axis current command value is less than the maximum slope limit K max , it is determined that the active power control of the receiving MMC has resumed normal working state, the controller recovery phase is entered, and the slope limiter is turned off.

[0060] If the slope k of the receiving end d-axis current command value ≥ the maximum slope limit K max , it is determined that the secondary overvoltage has not been completely suppressed and is in the integral recovery stage.

[0061] Preferably, by dividing the fault recovery process into a slope limiting stage, an integral recovery stage and a controller recovery stage based on the slope of the current command value, the control strategy can be optimized according to the characteristics of different recovery stages, avoiding the problems that may be caused by traditional static control strategies, and introducing a comparative judgment of the active power at the receiving end and the active power at the sending end in the integral recovery stage, which can not only effectively solve the secondary overvoltage problem that is difficult to deal with in traditional methods, but also promote the system to recover to normal faster.

[0062] Specifically, for the threshold value K used to determine whether to enter the slope limiting stage, the slope variation range during normal operation is understood by collecting and analyzing the historical data of the system's long-term operation, and the initial threshold is set in combination with the safety margin. The system is simulated under various operating conditions, and the change in the slope of the receiving-end d-axis current command value is observed. Then, the initial threshold is adjusted to obtain the threshold value K that can accurately determine whether the system has a fault; for the maximum slope limiting value K used to determine whether to enter the controller recovery stage, max , by adjusting the threshold according to the rated capacity and overload capacity of the system, combined with the current bearing capacity of various devices and the system's requirements for fault recovery speed, and determining the optimal exit conditions for fault recovery through a large number of simulation tests, the normal operation and safety of the system are ensured.

[0063] S3: Control the slope limiter based on the receiving end fault recovery stage to achieve secondary overvoltage suppression.

[0064] Specifically, the slope limiting stage includes: if the slope k of the receiving-end d-axis current command value is less than or equal to the set threshold value K=0.5, it is determined that the receiving-end MMC fixed active power control is in a normal working state, the slope limiter does not work, and the control link directly uses the receiving-end d-axis current command value generated by the receiving-end MMC fixed active power control.

[0065] If the slope k of the d-axis current command value at the receiving end is greater than the set threshold value K=0.5, it is determined that the active power control of the MMC at the receiving end is not in a normal working state, and the slope limiting stage is entered. The slope limiter is started to limit the slope of the d-axis current command value at the receiving end, so that the d-axis current command value at the receiving end increases according to the slope k=K of the d-axis current command value at the receiving end.

[0066] Furthermore, the calculation formula of the fixed active power in normal working state is as follows:

[0067]

[0068] Among them, i sdref is the fixed active power in normal working state; k psd P is the proportional parameter of the proportional integral PI link of the active power control at the receiving end; vref is the command value of the active power at the receiving end; P v is the active power of the MMC receiving end; k isd is the integral parameter of the active power control at the receiving end; s is the Laplace operator.

[0069] Further, after the slope limiter is started, the receiving end d-axis current command value during the fault period and the fault recovery period satisfies the following formula:

[0070] i sdrefmax =kt<1.1pu

[0071] Among them, i sdrefmax is the maximum value of the d-axis current command value at the receiving end; k is the slope of the d-axis current command value at the receiving end; t is the duration of the limit.

[0072] Preferably, in the slope limiting stage, by judging the system state based on a set threshold and starting the slope limiter, sudden changes in the current command value can be effectively prevented, thereby avoiding system shocks, which can not only suppress secondary overvoltages but also improve the overall stability of the system.

[0073] Specifically, the integral recovery stage includes: limiting the active power of the receiving end based on the slope limit, so that the active power of the receiving end is less than the active power of the sending end, and the active power of the receiving end and the active power of the sending end in the middle stage of fault recovery meet the following formula:

[0074] P v =u sd i sd <P w

[0075] Among them, P v is the active power of the MMC receiving end; P w is the active power of the MMC sending end; u sd is the output voltage of the d-axis at the receiving end of the MMC; i sd is the output current of the d-axis at the receiving end of the MMC.

[0076] When the active power at the receiving end is less than the active power at the sending end, the DC side voltage rises rapidly, accelerating the saturation recovery of the d-axis current at the sending end, canceling the low voltage stage of the DC side voltage, and suppressing the secondary overvoltage. The rising speed of the d-axis current command value at the receiving end is controlled by the slope limiter, which indirectly limits the recovery speed of the active power at the receiving end, so that the active power at the receiving end is always less than the active power at the sending end, avoiding the secondary overvoltage caused by the too fast recovery of the active power at the receiving end.

[0077] Furthermore, the calculation formula of the receiving end active power is as follows:

[0078] P v =u sa i sa +u sb i sb +u sc i sc

[0079] Among them, P v is the active power of the MMC receiving end; u sa is the AC voltage of phase a at the receiving end; u sb is the AC voltage of phase b at the receiving end; u sc is the c-phase AC voltage at the receiving end; i sais the AC current of phase a at the receiving end; i sb is the AC current of phase b at the receiving end; i sc is the c-phase AC current at the receiving end.

[0080] Preferably, in the integral recovery stage, the inherent characteristics of the system are cleverly utilized to promote the system to recover to normal state faster by temporarily allowing power imbalance, controlling the active power at the receiving end to be less than the active power at the sending end, accelerating the saturation recovery of the d-axis current at the sending end, and accelerating the rising speed of the DC side voltage, thereby suppressing the generation of secondary overvoltage and effectively shortening the low voltage stage of the DC side voltage, promoting the recovery of the system, and breaking the inherent concept of always pursuing power balance in traditional control.

[0081] Specifically, the controller recovery stage includes: in the later stage of fault recovery, judging the slope of the d-axis current command value of the receiving end, if the slope k of the d-axis current command value of the receiving end is less than the maximum slope limit K max , it is determined that the active power control of the receiving MMC has resumed normal working state and the slope limiter is turned off.

[0082] If the slope k of the receiving end d-axis current command value ≥ the maximum slope limit K max , it is determined that the secondary overvoltage has not been completely suppressed and is in the integral recovery stage, and the secondary overvoltage suppression continues.

[0083] Preferably, by combining the characteristics of the wind farm and the characteristics of the flexible direct current transmission system, the fault recovery process is divided into a slope limiting stage, an integral recovery stage and a controller recovery stage, so as to achieve refined control of the fault recovery process, and by formulating an adaptive control mechanism, dynamically determine whether the slope of the current command value is lower than the maximum slope limit value, so as to accurately determine the fault stage of the system, and then control the switch of the slope limiter and the active power of the receiving and sending ends, which can effectively suppress the secondary overvoltage at the critical moment of fault recovery, improve the reliability and stability of the new wind power transmission system through flexible direct current, and have strong adaptability.

[0084] The present embodiment also provides a secondary overvoltage suppression system during the receiving-end fault recovery period, including: a slope calculation module, used to obtain the d-axis current command value output by the receiving-end MMC fixed active power control and calculate it to obtain the slope of the receiving-end d-axis current command value; a stage distinction module, used to judge the slope of the receiving-end d-axis current command value, and divide the receiving-end fault recovery stage into a slope limiting stage, an integral recovery stage or a controller recovery stage; a voltage suppression module, used to process according to the slope limiting stage, the integral recovery stage and the controller recovery stage respectively, control the slope limiter, the receiving-end active power and the sending-end active power, and realize the suppression of secondary overvoltage.

[0085] This embodiment also provides a computer device, which is applicable to a method for suppressing secondary overvoltage during a receiving end fault recovery period, and includes: a memory and a processor; the memory is used to store computer executable instructions, and the processor is used to execute computer executable instructions to implement a method for suppressing secondary overvoltage during a receiving end fault recovery period as proposed in the above embodiment.

[0086] The computer device may be a terminal, and the computer device includes a processor, a memory, a communication interface, a display screen and an input device connected via a system bus. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The communication interface of the computer device is used to communicate with an external terminal in a wired or wireless manner, and the wireless manner can be achieved through WIFI, an operator network, NFC (near field communication) or other technologies. The display screen of the computer device may be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device may be a touch layer covering the display screen, or a key, trackball or touchpad provided on the housing of the computer device, or an external keyboard, touchpad or mouse, etc.

[0087] This embodiment also provides a storage medium on which a computer program is stored. When the program is executed by a processor, a secondary overvoltage suppression method during receiving end fault recovery as proposed in the above embodiment is implemented; the storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random access memory (Static Random Access Memory, referred to as SRAM), electrically erasable programmable read-only memory (Electrically Erasable Programmable Read-Only Memory, referred to as EEPROM), erasable programmable read-only memory (Erasable Programmable Read Only Memory, referred to as EPROM), programmable read-only memory (Programmable Red-Only Memory, referred to as PROM), read-only memory (Read-Only Memory, referred to as ROM), magnetic memory, flash memory, magnetic disk or optical disk.

[0088] In summary, the present invention proposes a method for suppressing secondary overvoltage during the receiving-end fault recovery period under a new wind power flexible direct current transmission system control strategy. The method belongs to an additional control strategy and can solve the secondary overvoltage phenomenon generated during the fault recovery period under the new control strategy of the wind power flexible direct current transmission system at a lower cost. The method is beneficial to the safe and stable operation of the power system and is suitable for scenarios of coordinated control by multiple PI controllers. The method provides an effective solution to the transient problem of the power system caused by oversaturation in the multi-PI coordinated control.

[0089] Example 2

[0090] Reference Figure 6 , which is the second embodiment of the present invention. This embodiment is different from the first embodiment in that, in order to verify its beneficial effects, experimental comparison data between the present invention and the prior art are provided.

[0091] This example simulates the connection between the receiving end of a new type of wind power transmission system and a large AC power grid, and considers the fault duration of 100ms for the case of a three-phase metallic grounding fault at the receiving end. The overvoltage and secondary overvoltage generation process of the new type of wind power transmission system includes the fault period, the initial stage of fault recovery, the middle stage of fault recovery and the secondary overvoltage stage.

[0092] During the fault period, the receiving end power P v The sharp drop in the receiving end inner ring current i sdref The actual d-axis current command value output When the limiter limit value is reached, the voltage amplitude of the DC side overvoltage will be suppressed due to the action of the DC voltage controller at the sending end.

[0093] In the initial stage of fault recovery, due to the receiving terminal voltage u s Rapid recovery, receiving end power P v It will also recover quickly until the receiving end power P v >Receiving end active power command value P vref , the inner loop current i sdref The power of the sending end will also recover, but the sending end power P w <Receiver power P v , DC side voltage U dc Further drop to meet the DC side voltage U dc <Sending end voltage U dcref Therefore, the DC voltage at the sending end is controlled by the inner loop i cdref Finally, under the action of the limiter, the actual d-axis current command value input to the current loop Reaching limit value -I cdmax .

[0094] In the middle of fault recovery, the sending end power is P w =u cd I cdmax , the receiving end power is P v =u sd I sdmax , until the current i sdref The anti-saturation process ends, where the DC side voltage U dc Maintaining a low voltage level, the change is not large, so the DC voltage control inner loop of the sending end is i cdref Still decreasing.

[0095] In the secondary overvoltage stage, the sending end power is still P w =u cd I cdmax , the receiving end power is restored under the control of the fixed active power control, so the power relationship satisfies the sending end power P w >Receiver power P v Therefore, the DC side voltage U dc When the DC voltage U dc >Sending end voltage U dcref After that, the DC voltage at the sending end is controlled by the inner loop i cdref The DC side voltage U dc It reaches its maximum value, forming a secondary overvoltage with a large amplitude.

[0096] Therefore, since the power of the receiving end and the power of the sending end are not much different in the middle of the fault recovery, the DC side voltage is in the low voltage stage for a long time, and the saturation phenomenon of the inner loop current of the PI control at the sending and receiving ends is too long, resulting in the generation of secondary overvoltage. Then, by adding the slope limiting control strategy proposed in the present invention to the outer loop of the active power control at the receiving end, the d-axis current and active power of the receiving end are limited, the time that the DC side voltage is in the low voltage process is reduced, and the desaturation process of the inner loop current of the PI control at the sending end is accelerated, thereby avoiding the generation of secondary overvoltage.

[0097] like Figure 6 The simulation waveforms of overvoltage and secondary overvoltage and the schematic diagram of the control strategy effect are shown. The traditional control strategy, the new wind machine network and MMC grid following strategy and the new strategy are compared with the method proposed in the present invention. In the method proposed in the present invention, after a fault occurs at the receiving end, the receiving end control will detect that the outer loop output d-axis current command value is out of the steady-state value, and the slope limiter will be started. By limiting the rising rate of the d-axis current at the receiving end, the power recovery speed and the maximum value of the receiving end are limited. When the desaturation process of the inner loop current of the sending and receiving ends is completed, the slope limiter is closed, and the receiving end power P v Able to quickly track the command value P of the receiving end active power vrefAs can be seen from the figure, compared with the new wind power grid and MMC grid following strategy, the present invention can effectively suppress the secondary overvoltage. At the same time, the present invention is an additional control strategy, and its purpose is to solve the secondary overvoltage phenomenon in the process of the new wind power flexible direct current transmission system control strategy. Compared with the traditional control strategy, the two have little difference in the suppression effect of secondary overvoltage, but in the overvoltage stage, the existing technology adopted by the present invention can achieve better results, indicating that the present invention can solve the secondary overvoltage phenomenon generated during fault recovery under the new control strategy of the wind power flexible direct current transmission system with less cost, which is beneficial to the safe and stable operation of the power system.

[0098] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for suppressing secondary overvoltage during fault recovery at a receiving end, characterized in that: include: Obtaining a current command value and performing calculations to obtain a slope of the current command value; Determining the slope of the current command value to distinguish the receiving end fault recovery stage; The slope limiter is controlled based on the receiving end fault recovery stage to achieve secondary overvoltage suppression.

2. The method for suppressing secondary overvoltage during receiving-end fault recovery as claimed in claim 1, characterized in that: The current command value is obtained by detecting the d-axis current command value output by the receiving-end MMC fixed active power control in the controller; The step of obtaining the slope of the current command value comprises the following steps: The d-axis current command value at the receiving end is processed by a first-order low-pass filter to eliminate the influence of high-frequency noise on the slope of the current command value during steady-state operation of the system; Calculate the slope of the current command value every selected time period T; The calculation formula of the slope of the current command value is as follows: Where k is the slope of the d-axis current command value at the receiving end; T is the selected time period; i sdref[t] is the d-axis current command value at the receiving end during the previous calculation; i sdref[t+T] is the receiving end d-axis current command value after the selected time period T.

3. The method for suppressing secondary overvoltage during receiving-end fault recovery as claimed in claim 2, characterized in that: The receiving end fault recovery stage includes a slope limiting stage, an integral recovery stage and a controller recovery stage; The distinguishing receiving end fault recovery phase comprises the following steps: The slope k of the d-axis current command value of the receiving end is judged. If the slope k of the d-axis current command value of the receiving end is less than or equal to the set threshold value K, it is determined that the active power control of the MMC at the receiving end is in a normal working state; If the slope k of the receiving-end d-axis current command value is greater than the set threshold K, it is determined that the receiving-end MMC fixed active power control is not in a normal working state and enters the slope limiting stage; After entering the slope limiting stage, the active power of the receiving end and the active power of the sending end are judged. If the active power of the receiving end is less than the active power of the sending end, it is determined to enter the integral recovery stage; If the active power at the receiving end ≥ the active power at the sending end, it is determined to be in the slope limiting stage; After entering the integral recovery phase, the slope k of the receiving end d-axis current command value is judged. If the slope k of the receiving end d-axis current command value is less than the maximum slope limit K max , it is determined that the active power control of the receiving MMC has resumed normal working state and enters the controller recovery phase; If the slope k of the receiving end d-axis current command value ≥ the maximum slope limit K max , it is determined that the secondary overvoltage has not been completely suppressed and is in the integral recovery stage.

4. The method for suppressing secondary overvoltage during receiving-end fault recovery as claimed in claim 3, characterized in that: The slope limiting stage includes: If the slope k of the receiving-end d-axis current command value is less than or equal to the set threshold K, it is determined that the receiving-end MMC fixed active power control is in a normal working state, the slope limiter does not work, and the control link directly uses the receiving-end d-axis current command value generated by the receiving-end MMC fixed active power control; If the slope k of the d-axis current command value at the receiving end is greater than the set threshold K, it is determined that the active power control of the MMC at the receiving end is not in a normal working state, and the slope limiter is started to limit the slope of the d-axis current command value at the receiving end, so that the d-axis current command value at the receiving end increases according to the slope k=K of the d-axis current command value at the receiving end.

5. The method for suppressing secondary overvoltage during receiving-end fault recovery as claimed in claim 4, characterized in that: The calculation formula of the fixed active power in normal working state is as follows: Among them, i sdref is the fixed active power in normal working state; k psd P is the proportional parameter of the proportional integral PI link of the active power control at the receiving end; vref is the command value of the active power at the receiving end; P v is the active power of the MMC receiving end; k isd is the integral parameter of the active power control at the receiving end; s is the Laplace operator; After starting the slope limiter, the receiving end d-axis current command value satisfies the following formula: i sdrefmax =kt<1.1p.u. Among them, i sdrefmax is the maximum value of the d-axis current command value at the receiving end; k is the slope of the d-axis current command value at the receiving end; t is the duration of the limit.

6. The method for suppressing secondary overvoltage during receiving-end fault recovery as claimed in claim 5, characterized in that: The integral recovery phase includes: The active power of the receiving end is limited based on the slope limit, so that the active power of the receiving end is less than the active power of the sending end, and the active power of the receiving end and the active power of the sending end satisfy the following formula: P v =u sd i sd <P w Among them, P v is the active power of the MMC receiving end; P w is the active power of the MMC sending end; u sd is the output voltage of the d-axis at the receiving end of the MMC; i sd is the output current of the d-axis at the receiving end of the MMC; When the active power at the receiving end is less than the active power at the sending end, the DC side voltage rises rapidly, accelerating the saturation recovery of the d-axis current at the sending end, canceling the low voltage stage of the DC side voltage, and suppressing the secondary overvoltage; The calculation formula of the receiving end active power is as follows: P v =in sa and sa +in sb and sb +in sc and sc Among them, P v is the active power of the MMC receiving end; u sa is the AC voltage of phase a at the receiving end; u sb is the AC voltage of phase b at the receiving end; u sc is the c-phase AC voltage at the receiving end; i sa is the a-phase AC current at the receiving end; i sb is the AC current of phase b at the receiving end; i sc is the c-phase AC current at the receiving end.

7. The method for suppressing secondary overvoltage during receiving-end fault recovery as claimed in claim 6, characterized in that: The controller recovery phase includes: The slope k of the receiving end d-axis current command value is judged. If the slope k of the receiving end d-axis current command value is less than the maximum slope limit K max , it is determined that the active power control of the receiving-end MMC has resumed normal operation and the slope limiter is turned off; If the slope k of the receiving end d-axis current command value ≥ the maximum slope limit K max , it is determined to be in the integral recovery stage and the secondary overvoltage suppression continues.

8. A secondary overvoltage suppression system during receiving end fault recovery, based on a secondary overvoltage suppression method during receiving end fault recovery according to any one of claims 1 to 7, characterized in that: include, The slope calculation module is used to obtain the d-axis current command value output by the receiving-end MMC fixed active power control and calculate it to obtain the slope of the d-axis current command value of the receiving-end; A stage distinguishing module is used to judge the slope of the d-axis current command value of the receiving end, and distinguish the receiving end fault recovery stage into a slope limiting stage, an integral recovery stage or a controller recovery stage; The voltage suppression module is used to process the slope limiting stage, the integral recovery stage and the controller recovery stage respectively, control the slope limiter, the receiving end active power and the sending end active power, and realize the suppression of the secondary overvoltage.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of a method for suppressing secondary overvoltage during receiving-end fault recovery as described in any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of a method for suppressing secondary overvoltage during receiving-end fault recovery as described in any one of claims 1 to 7 are implemented.

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