Control optimization method for improving fault ride-through capability of hybrid commutation converter

Abstract

The invention discloses a control optimization method for improving the fault ride-through capability of a hybrid commutation converter, and the method comprises the steps: judging the severity of a fault when an inverter side AC system is detected to have a fault, and adjusting a DC instruction if the fault drop degree enables the hybrid commutation converter to be in a critical commutation failure risk interval. And if the hybrid commutation converter has commutation failure after the inverter side AC system has a fault, the instructions of the DC current and the DC voltage are respectively adjusted. According to the invention, the reliability of the commutation failure resistance capability of the hybrid commutation converter can be further improved; and meanwhile, the recovery speed of the direct-current power transmission system can be improved and the risk of continuous commutation failure can be reduced under extreme conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current transmission, and particularly to a control optimization method for improving the fault ride-through ability of a hybrid commutation converter. Background Art

[0002] Conventional high-voltage direct current transmission (LCC-HVDC) has the advantages of large transmission capacity, fast power controllability, relatively low cost, etc., and has been widely used in long-term large-scale power transmission across regions. However, commutation failure easily caused by faults on the receiving-end AC side has always been a difficult problem troubling the industry. In actual engineering, functions such as commutation failure prevention control and low-voltage current limiting auxiliary control are equipped to reduce the probability of commutation failure, but the improvement effect of the control link is limited. Therefore, efforts have been made at home and abroad to transform the topology of the inverter side of LCC-HVDC to better improve the ability to resist commutation failure.

[0003] Multiple schemes for topology transformation have been proposed. The early HVDC based on capacitor commutation converters was applied in a demonstration project in Brazil. It provides additional commutation voltage by connecting a capacitor in series between the transformer and the converter outlet. However, if commutation failure occurs, the capacitor charging process is uncontrollable, and problems such as capacitor overvoltage will occur. For this reason, a commutation converter with a controllable capacitor was proposed. The controllable capacitor commutation converter connects the capacitor in series between the AC bus and the commutation transformer, and uses thyristors to regulate the capacitor voltage. Considering that more rapid and flexible control of the series voltage can increase the commutation voltage area during the first commutation process of the valve group after an AC grid fault, fully controlled power electronic switch devices replace the thyristors in the full-bridge sub-module, and a series voltage commutation converter and an enhanced grid commutation converter are proposed. Although the above transformation schemes involve fully controlled devices, their purpose is still to use capacitors for auxiliary commutation, and the commutation failure suppression effect is still limited to a certain extent.

[0004] Reforming the converter using fully controlled power electronic devices and forcibly turning off the current during abnormal commutation to complete commutation has more advantages in completely solving commutation failure. The rapid development of fully controlled power electronic devices has laid the foundation for the implementation of this technical route. Some scholars have proposed replacing some thyristors in the bridge arm with IGBTs in antiparallel diodes. There is also research proposing a controllable commutation converter structure based on the parallel connection of thyristor and IGBT double branches. The bridge arm of this converter is divided into two sub-branches. The thyristor and IGBT are connected in series to form the main branch to carry large currents; another auxiliary branch composed of IGBTs is connected in parallel with the main branch to transfer current and turn off the commutation current during abnormal commutation. Considering that IGCT is a fully controlled device developed on the basis of thyristors and has the characteristics of high surge current withstand capacity and low loss, the researcher proposed a new type of hybrid line-commutated converter that replaces thyristors in the bridge arm with RB-IGCTs in proportion. This type of converter with both natural commutation and forced commutation capabilities can be collectively referred to as a hybrid commutated converter (HCC).

[0005] Currently, the relevant research mainly focuses on the hybrid commutation principle and turn-off characteristics of HCC, while there is less research on the control system level of HCC-HVDC. Most still continue to use the technical framework of the LCC DC system and do not change the overall control protection strategy. Since HCC has a strong ability to resist commutation failure and is in normal steady-state operation and transient operation without commutation failure in most cases, it is currently the first choice to select a constant voltage controller as the main controller to ensure the stability of DC system voltage and power transmission. However, there are the following deficiencies in using a constant voltage controller for HCC that need to be solved urgently: (1) During transient operation, the regulation ability is insufficient, and there are problems of high forced commutation frequency and critical risk. (2) After extreme situations occur, the regulation ability is insufficient, and continuous commutation failure is likely to occur. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a control optimization method for improving the fault ride-through ability of a hybrid commutated converter in view of the deficiencies of the prior art, improve the regulation ability of the constant voltage controller, improve the ability of the hybrid line-commutated converter to suppress commutation failure, and enhance the reliability of the commutation failure resistance ability of HCC; at the same time, it can also improve the recovery speed of the DC transmission system and reduce the risk of continuous commutation failure in extreme situations.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is: a control optimization method for improving the fault ride-through ability of a hybrid commutated converter, including the following steps:

[0008] If a fault occurs in the AC system on the inverter side, it is judged whether the degree of fault drop exceeds the set threshold. If so, it is determined that the hybrid commutation converter is in the critical commutation failure risk interval, and the DC current command of the DC power transmission system is adjusted; if commutation failure occurs in the hybrid commutation converter after a fault occurs in the AC system on the inverter side, the commands of the DC current and the DC voltage are adjusted respectively.

[0009] The expression for the degree of fault drop D is: where U N is the rated AC bus voltage value, U ac is the actual AC bus voltage value, U acmin is the minimum AC voltage value of the receiving-end commutation bus determined according to the design target of the hybrid commutation converter.

[0010] The specific implementation process of adjusting the DC current command of the DC power transmission system includes: setting the expected reactive power Q acref exchanged between the converter station and the AC system as the rated value Q acn under normal conditions, and using the reactive power balance formula and the expression of the reactive power consumed by the inverter station to obtain the current command I acref corresponding to Q ord , and taking the current command I ord as the reference value and inputting it into the constant current controller on the rectifier side, thereby reducing the DC current value I d ; the reactive power balance formula is: Q i =Q f +Q ac +Q c ; Q ac is the reactive power exchanged between the converter station and the AC system, Q f is the reactive power provided by the reactive power compensation equipment already put into operation under the current state, Q i is the reactive power consumed by the converter, Q c is the additional reactive power consumption caused by the AC side fault.

[0011] The expression between Q acref and the corresponding current command I ord is: where I ord represents the DC current command value, N p represents the number of poles, X represents the equivalent short-circuit impedance of the secondary side of the commutation transformer, k represents the commutation transformer turns ratio, γ min represents the minimum turn-off angle on the inverter side, U ac represents the commutation bus voltage, B c represents the capacity of the reactive power compensation equipment of the converter station.

[0012]

[0013] The specific implementation process of separately adjusting the commands for DC current and DC voltage includes: setting Q acref =Q acn +Q c , and using the reactive power balance formula and the expression of the reactive power consumed by the inverter station to obtain the current command I acref corresponding to Q ord . Input the current command I ord as a reference value into the constant current controller on the rectifier side, thereby reducing the DC current value I d ; Q c is the increased reactive power consumption caused by the AC side fault;

[0014] Using the expression between the DC voltage U d and the minimum turn-off angle Double the value of the minimum turn-off angle in it, keep other parameters at their rated values, and use the obtained U d value as the reference value U ord of the constant voltage controller on the inverter side, thereby adjusting the firing angle; where X r2 is the commutation reactance of each phase of the inverter station, and α is the firing angle.

[0015] The method of the present invention further includes: introducing a ramp function. When it is detected that the AC bus voltage rises and reaches the threshold U th during the fault, make the reference value U ord of the constant voltage controller on the inverter side and the current command I ord return to their rated values; the expression of the ramp function is:

[0016]

[0017] where N is the number of equal divisions, U ordN and I ordN are the rated command values of DC voltage and DC current respectively, and U' ord and I' ord are the DC voltage and DC current command values output by the ramp function respectively.

[0018] As an inventive concept, the present invention also provides a control optimization method for improving the fault ride-through ability of a hybrid commutation converter, including a memory, a processor, and a computer program stored on the memory; the processor executes the computer program to implement the steps of the above method.

[0019] As an inventive concept, the present invention also provides a computer-readable storage medium, on which a computer program / instructions are stored; when the computer program / instructions are executed by a processor, the steps of the above method are implemented.

[0020] As an inventive concept, the present invention also provides a computer program product, including a computer program / instructions; when the computer program / instructions are executed by a processor, the steps of the above method are implemented.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention solves the problem of insufficient adjustment ability of HCC in two transient working conditions after adopting a constant voltage controller. In the transient case without commutation failure, since the constant voltage controller can better ensure the stability of the DC voltage, the DC current cannot be adjusted through the existing VDCOL link to improve the commutation process. Therefore, if the AC side fault is relatively serious, HCC will always be under the regulation of high risk and high turn-off stress. After commutation failure occurs in HCC due to extreme conditions, due to the clamping voltage effect after the arrester conducts, the control system cannot significantly adjust the DC current through control switching or the VDCOL link to improve the commutation process; at the same time, the constant voltage controller will continuously increase the firing angle to stabilize the DC voltage, further deteriorating the commutation process. The adjustment strategy of the present invention to obtain the corresponding DC current command value through the reactive power expectation value and dynamically adjust the controller reference value well solves the above problems and can timely adjust and improve the commutation process. Therefore, the reliability of HCC to resist commutation failure is improved; at the same time, in extreme cases, the recovery speed of the DC transmission system can also be increased and the risk of consecutive commutation failures can be reduced. Description of the Drawings

[0022] Figure 1 It is a structural diagram of a high-voltage DC transmission system adopting HCC;

[0023] Figure 2 It is a schematic diagram of the reactive power exchange of the converter station during a fault;

[0024] Figure 3 It is the newly added part in the high-voltage DC transmission control system according to the embodiment of the present invention;

[0025] Figure 4 It is the comparison of the simulation waveforms of the original control system and the present invention under single-phase fault without commutation failure; (a) comparison of current commands, (b) comparison of turn-off currents, (c) comparison of firing angles, (d) comparison of DC powers;

[0026] Figure 5 It is the comparison of the simulation waveforms of the original control system and the present invention after commutation occurs under single-phase fault; (a) comparison of firing angles, (b) comparison of current commands, (c) comparison of DC voltages, (d) comparison of DC currents, (e) comparison of DC powers, (f) comparison of AC bus voltages;

[0027] Figure 6For the commutation process of the half-valve arm current, (a) the original control system, (b) the embodiment of the present invention;

[0028] Figure 7 For the comparison of the simulation waveforms between the original control system and the present invention after commutation failure occurs in two-phase and three-phase faults; (a) comparison of DC voltage and current under two-phase faults, (b) comparison of DC voltage and current under three-phase faults. Specific embodiments

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0030] Embodiment 1

[0031] Based on the CIGRE standard model in the PSCAD / EMTDC software, the present invention replaces some thyristors with RB-IGCTs to construct an HCC topology structure, such as Figure 1 the structure diagram of the high-voltage DC transmission system adopting HCC. At the same time, the fixed turn-off angle controller on the inverter side of the standard model is changed to a fixed voltage controller to verify the control system instruction adaptive adjustment method proposed in the embodiments of the present invention.

[0032] (1) Set a single-phase 0.15H grounding fault on the AC bus, and its duration is 3.0s - 3.1s. In the operation method of the embodiments of the present invention, when a fault in the inverter-side AC system is detected, the severity D of the fault will be determined. Assuming that D > 85% is detected under the current fault, it is determined at this time that HCC is in the risk range of critical commutation failure. Therefore, the first part of this method is executed to adjust the DC current command. For the CIGRE standard model, the rated value of the reactive power exchanged between the converter station and the AC system under normal conditions is -60 Mvar. Therefore, the expected value Q of the reactive power exchanged between the converter station and the AC system is set acref to be -60 Mvar. At this time, during the fault period, the reactive power exchange situation of the converter station is as Figure 2 shown, and the reactive power balance formula can be expressed by Equation (1):

[0033] Q i = Q f + Q ac + Q c (1)

[0034] Substitute Q acref= -60 Mvar, Q f and Q c measured value, the corresponding Q i value can be obtained. Then, using the reactive power consumption expression of the inverter station:

[0035]

[0036] get Q acref and the corresponding current command I ord expression between:

[0037]

[0038]

[0039] The newly added part in the HVDC transmission control system in the embodiments of the present invention is as Figure 3 shown. At this time, execute the first part of the embodiments of the present invention, and send the obtained current command I ord into the control link on the rectifier side of the HVDC transmission control system, so that the constant current controller on the rectifier side adjusts the DC current on the DC transmission line. The above approach is equivalent to providing Q c equivalently by the reactive power change amount of the AC system. Therefore, Q i is equal to the reduction amount of Q f during the fault, so that I ord is appropriately reduced, realizing the improvement of the commutation process. Figure 4 is the comparison of simulation waveforms. The red represents the result of the original control system, and the blue represents the result after adding the embodiments of the present invention. Figure 4 as shown in (a) of d . Due to the reduction of I Figure 4 in (b) of Figure 4 it can be seen that during the fault, the peak value of the HCC fault current using the method of the embodiments of the present invention is significantly lower than that in the original controller. And the maximum number of turn-offs is positively correlated with the peak value of the turn-off current. Therefore, this helps to reduce the risk of the IGCT junction temperature rising and failing due to multiple large current turn-offs or the energy accumulation and failure of the arrester. And the α output by the constant voltage controller on the inverter side is almost the same, as shown in (c) of

[0040] (2) Set a single-phase 0.15H grounding fault with a duration of 3.0s - 3.1s. In an embodiment of the present invention, taking the example of reducing the reference voltage of the arrester in one valve arm by 15% to simulate the extreme situation where the reference voltage decreases due to aging and leads to commutation failure, the effectiveness of the present invention is demonstrated. After a fault occurs in the AC system on the inverter side under this working condition, the hybrid commutation converter experiences commutation failure, and then the first and second parts of the embodiment of the present invention are executed to adjust the commands for DC current and DC voltage respectively. First, set Q acref = Q acn + Q c , and obtain the corresponding current command I acref for Q ord in the same way and reduce I d . At the same time, using the expression between the DC voltage U d and the minimum turn-off angle:

[0041]

[0042] change the value of the minimum turn-off angle to twice the rated value, that is, 30°, keep other parameters at the rated value, and then use the obtained U d value as the reference value U ord of the constant voltage controller on the inverter side, thereby adjusting the trigger angle to avoid the occurrence of continuous commutation failure caused by the continuous increase of the trigger angle, as in Figure 3 of the second part. Figure 5 is the comparison of simulation waveforms. The red represents the result of the original control system, and the blue represents the result after adding the embodiment of the present invention. After the embodiment of the present invention starts to execute, first, I ord starts to decrease significantly to adjust I d , reducing the required commutation area. At the same time, due to the change of the reference value of the constant voltage controller, the output trigger angle is also adjusted in the decreasing direction, increasing the supply commutation area. As shown by the blue curves in (a) and (b) of Figure 5 . Due to the above adjustments, HCC does not experience continuous commutation failure during the fault. Therefore, whether it is U d , I d , or the DC power transmission gradually recovers, all of which are better than the original control system, as shown in (c), (d), and (e) of Figure 5 . And due to the decrease in the reactive power demand of HCC and the adjustment effect during the fault process, Uac will rise after dropping to a certain extent. When the set threshold is reached, I ord and U ord gradually transition to the rated value, thus accelerating the recovery speed of the DC system after commutation failure and also solving the overvoltage problem generated after the AC system fault is cleared, as shown in (f) of Figure 5 . Figure 6It is a comparison of the commutation process of the half-valve bridge arm current, which can more intuitively show that under the regulation of the embodiments of the present invention, the commutation process of HCC has been improved and the normal commutation has been gradually restored. This also helps to prevent the occurrence of failures caused by the continuous commutation failure, such as the rise of the IGCT junction temperature failure or the energy accumulation failure of the arrester.

[0043] Under the same conditions, two-phase 0.2H grounding faults and three-phase 0.25H grounding faults are respectively set, and their duration is 3.0s - 3.1s, U d and I d The change results are as Figure 7 shown. Figure 7 In (a) is the result under two-phase grounding fault; Figure 7 In (b) is the result under three-phase grounding fault. The HCC with the original control system also had continuous commutation failures during the fault, while the addition of the embodiments of the present invention avoided the occurrence of the above phenomenon. Therefore, the method of the embodiments of the present invention is effective for different fault types.

[0044] Embodiment 2

[0045] Embodiment 2 of the present invention provides a terminal device corresponding to Embodiment 1 above. The terminal device can be a processing device for a client, such as a mobile phone, a notebook computer, a tablet computer, a desktop computer, etc., to execute the method of the above embodiment.

[0046] The terminal device of this embodiment includes a memory, a processor, and a computer program stored on the memory; the processor executes the computer program on the memory to implement the steps of the method of Embodiment 1 above.

[0047] In some implementations, the memory can be a high-speed random access memory (RAM: Random Access Memory), and may also include non-volatile memory, such as at least one disk memory.

[0048] In other implementations, the processor can be a central processing unit (CPU), a digital signal processor (DSP), or various types of general-purpose processors, which are not limited here.

[0049] Embodiment 3

[0050] Embodiment 3 of the present invention provides a computer-readable storage medium corresponding to Embodiment 1 above, on which computer programs / instructions are stored. When the computer programs / instructions are executed by a processor, the steps of the method of Embodiment 1 above are implemented.

[0051] A computer-readable storage medium can be a tangible device that retains and stores instructions for use by an instruction execution device. A computer-readable storage medium can be, for example, but is not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any combination of the foregoing.

[0052] Those skilled in the art will understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The solutions in the embodiments of the present application can be implemented in various computer languages. For example, object-oriented programming languages such as Java and interpreted scripting languages such as JavaScript.

[0053] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram, as well as the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices produce means for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0054] These computer program instructions can also be loaded onto a computer or other programmable data processing device, such that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0055] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications once they learn the basic creative concepts. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present application.

[0056] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalent technologies, this application is also intended to cover these modifications and variations.

Claims

1. A control optimization method for improving the fault ride-through ability of a hybrid commutation converter, characterized in that Including the following steps: If a fault occurs in the inverter-side AC system, determine whether the degree of fault drop exceeds a set threshold. If so, it is determined that the hybrid commutation converter is in the critical commutation failure risk interval, and the DC current command of the DC transmission system is adjusted; if commutation failure occurs in the hybrid commutation converter after a fault occurs in the inverter-side AC system, the commands of the DC current and the DC voltage are adjusted respectively.

2. The control optimization method for improving the fault ride-through ability of a hybrid commutation converter according to claim 1, wherein The expression for the fault drop degree D is as follows: Where, U N is the rated AC bus voltage value, U ac is the actual AC bus voltage value, and U acmin is the minimum AC voltage value of the receiving-end commutation bus determined according to the design goal of the hybrid commutation converter.

3. The control optimization method for improving the fault ride-through ability of the hybrid commutation converter according to claim 1, characterized in that, The specific implementation process for adjusting the DC current command of a DC transmission system includes: setting the expected reactive power Q exchanged between the converter station and the AC system acref as the rated value Q under normal conditions acn , and using the reactive power balance formula and the expression of the reactive power consumed by the inverter station to obtain the current command I acref corresponding to Q ord , and inputting the current command I ord as the reference value into the constant current controller on the rectifier side, thereby reducing the DC current value I d ; the reactive power balance formula is: Q i =Q f +Q ac +Q c ; Q ac is the reactive power exchanged between the converter station and the AC system, Q f is the reactive power provided by the reactive power compensation equipment already put into operation under the current state, Q i is the reactive power consumed by the converter, and Q c is the increased reactive power consumption caused by the AC side fault.

4. The control optimization method for improving the fault ride-through capability of the hybrid commutation converter according to claim 3, characterized in that, Q acref The expression between it and the corresponding current command I ord is as follows: Where I ord represents the DC current command value, N p represents the number of poles, X represents the equivalent short-circuit impedance on the secondary side of the converter transformer, k represents the turns ratio of the converter transformer, γ min represents the minimum turn-off angle on the inverter side, U ac represents the converter bus voltage, B c represents the reactive power compensation equipment capacity of the converter station.

5. The control optimization method for improving the fault ride-through ability of the hybrid commutation converter according to claim 3, characterized in that 6. The control optimization method for improving the fault ride-through ability of the hybrid commutation converter according to claim 1, wherein The specific implementation process of separately adjusting the commands for DC current and DC voltage includes: setting Q acref = Q acn + Q c , and using the reactive power balance formula and the expression of the reactive power consumed by the inverter station to obtain the corresponding current command I acref of Q ord . Input the current command I ord as the reference value into the constant current controller on the rectifier side, thereby reducing the DC current value I d ; Q c is the increased reactive power consumption caused by the AC side fault; Using the DC voltage U d and the expression with the minimum turn-off angle Double the value of the minimum turn-off angle among them, keep other parameters at their rated values, and use the obtained U d value as the reference value U ord of the constant-voltage controller on the inverter side to adjust the firing angle; where X r2 is the commutation reactance per phase of the inverter station, and α is the firing angle.

7. The control optimization method for improving the fault ride-through ability of the hybrid commutation converter according to claim 1, characterized in that It further includes: Introduce a ramp function. When it is detected that the AC bus voltage rises and reaches the threshold U during a fault th , set the reference value U ord of the constant voltage controller on the inverter side and the current command I ord to return to the rated value; the expression of the ramp function is: Among them, N is the number of equal divisions, U ordN and I ordN are the rated command values of DC voltage and DC current respectively, and U’ ord and I’ ord are the DC voltage and DC current command values output by the ramp function respectively.

8. A control optimization system for improving the fault ride-through ability of a hybrid commutation converter, comprising a memory, a processor, and a computer program stored on the memory; characterized in that, The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 7.

9. A computer-readable storage medium having a computer program / instruction stored thereon; characterized in that, When the computer program / instructions are executed by the processor, the steps of the method according to any one of claims 1 to 7 are implemented.

10. A computer program product, comprising a computer program / instructions; characterized in that, When the computer program / instructions are executed by the processor, the steps of the method according to any one of claims 1 to 7 are implemented.

Images