Power fluctuation suppression method and system based on LCC and MMC

Abstract

The invention discloses a power fluctuation suppression method and system based on an LCC and an MMC. The method comprises the following steps: collecting operation state data of a receiving end converter and electrical operation data of a system in real time; according to the operation state data of the current converter at the receiving end, whether commutation failure occurs at the receiving end is judged; when it is judged that commutation failure occurs, joint calculation is performed based on system electrical operation data and a preset sending end hybrid cascade analysis model, and a reactive compensation instruction for the MMC and a current compensation instruction for the LCC are generated; and the reactive compensation instruction is sent to the MMC at the rectification side, the current compensation instruction is sent to the LCC at the rectification side, and transient active power fluctuation suppression of the DC system is realized through cooperative regulation and control of the MMC and the LCC. According to the invention, the transient stability and power transmission capability of the DC system after the commutation failure are obviously improved, and the problem of active power transmission drop of the DC system after the commutation failure of the sending-end hybrid cascade DC power transmission system is solved.

Description

Technical Field

[0001] This invention relates to a power fluctuation suppression method and system based on LCC and MMC, belonging to the field of high voltage direct current transmission technology. Background Technology

[0002] With the rapid development of high-voltage direct current (HVDC) transmission technology, hybrid cascaded HVDC transmission systems, combining the technological advantages of LCC (Line-Commutated Converter) and MMC (Modular Multilevel Converter), have gradually become an important solution for long-distance, high-capacity power transmission. LCCs are characterized by low construction costs and large transmission capacity, while MMCs offer advantages such as decoupled control of active and reactive power and fault ride-through capabilities. The combination of these two technologies effectively improves system flexibility, stability, and renewable energy transmission capacity, better meeting power transmission demands and ensuring system stability and economy. However, hybrid cascaded systems face numerous challenges in actual operation. Especially when the receiving-end AC system fails, LCCs are prone to commutation failure. The interaction between LCCs and MMCs in hybrid cascaded systems makes transient processes more complex, and power coupling characteristics exacerbate system instability, severely affecting the stable transmission of DC power. Therefore, in-depth research on the transient active-reactive interaction mechanism of hybrid cascaded systems after commutation failure, and the proposal of effective DC active power support strategies, are of great significance for improving the system's large-scale renewable energy transmission capacity and ensuring the safe and stable operation of the power grid.

[0003] Existing technological foundations include analyzing the fault recovery characteristics of strong AC systems, revealing the strong coupling relationship between AC voltage dynamic response and DC power regulation, pointing out the need to establish a collaborative recovery mechanism between AC and DC systems to improve recovery efficiency during the fault recovery phase, proposing an improved control architecture integrating DC current feedback to enhance system power transmission capability, revealing the modulation law of DC control parameters on power oscillations from the perspective of dynamic interaction mechanism, and establishing a suppression strategy with both time-domain and frequency-domain characteristics by comparing and analyzing the differences in dynamic characteristics between quasi-synchronous control and traditional power angle oscillations. Overall, existing research on DC power transmission focuses primarily on the control optimization of AC systems and single-type converters, lacking a holistic consideration of the coordinated operation of multiple types of converters in hybrid systems. It fails to effectively analyze the dynamic response characteristics and coupling effects of LCC and MMC, and cannot effectively reflect the active power characteristics and fluctuation mechanism of the system after commutation failure. Research on control strategies for hybrid cascaded systems has limitations; the influencing factors of DC active power drop in the sending-end AC system are not yet clear, and the self-regulation capabilities of hybrid cascaded systems are not fully utilized. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a power fluctuation suppression method and system based on LCC and MMC. Based on the coordinated control of MMC and LCC, the rapid reactive power regulation capability of MMC is fully utilized for transient buffering and the steady-state current regulation capability of LCC, which significantly improves the transient stability and power transmission capability of DC system after commutation failure. It solves the problem of active power transmission drop in DC system after commutation failure in hybrid cascaded DC transmission system at the sending end.

[0005] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0006] On the one hand, this invention discloses a power fluctuation suppression method based on LCC and MMC, comprising the following steps:

[0007] Real-time acquisition of operating status data of the receiving-end converter and system electrical operation data;

[0008] Based on the operating status data of the receiving-end converter, determine whether a commutation failure has occurred at the receiving end;

[0009] When a commutation failure is determined, based on the system electrical operation data, with the control objective of suppressing the increase in active power fed into the sending-end AC system, joint calculations are performed based on the preset sending-end hybrid cascade analysis model to generate reactive power compensation instructions for MMC and current compensation instructions for LCC.

[0010] The reactive power compensation command is sent to the MMC on the rectifier side, and the current compensation command is sent to the LCC on the rectifier side. Through the coordinated control of the MMC and the LCC, the transient active power fluctuation of the DC system is suppressed.

[0011] Furthermore, the system electrical operating data includes voltage data of the sending-end AC bus and current and voltage data of the DC system.

[0012] Furthermore, the receiving-end converter operating status data includes the receiving-end converter bus voltage;

[0013] The determination of whether a commutation failure has occurred at the receiving end includes the following steps:

[0014] When the voltage drop of the receiving-end commutator bus exceeds a first preset threshold and the duration of the drop exceeds a second preset threshold, a commutation failure is determined to have occurred.

[0015] Furthermore, the sending-end hybrid cascade analysis model includes:

[0016] The MMC calculation unit is used to calculate the MMC reactive power adjustment amount on the rectifier side based on the electrical operation data of the system and a preset MMC reactive power compensation calculation formula.

[0017] LCC calculation unit; used to calculate the rectifier-side LCC DC current regulation based on the rectifier-side MMC reactive power regulation and system electrical operation data, and based on a preset LCC current compensation calculation formula.

[0018] The MMC instruction unit is used to generate reactive power compensation instructions for MMC based on the reactive power adjustment amount of the rectifier-side MMC.

[0019] The LCC instruction unit is used to generate a current compensation instruction for the LCC based on the DC current adjustment amount of the rectifier-side LCC.

[0020] Furthermore, the expression for the MMC reactive power compensation calculation formula is as follows:

[0021] ;

[0022] ;

[0023] In the formula, This indicates the reactive power regulation of the MMC on the rectifier side; This indicates the reactive power consumed by the MMC; This indicates the reactive power consumed by the LCC;

[0024] This indicates the rated reactive power input from the AC system to the DC system; This indicates the reactive power input to the reactive power compensation device; Indicates the system's short-circuit capacity; This indicates the transient voltage of the AC bus at the sending end.

[0025] Furthermore, the expression for the LCC current compensation calculation formula is as follows:

[0026] ;

[0027] In the formula, This indicates the DC current adjustment amount of the LCC on the rectifier side; This indicates the reactive power regulation of the MMC on the rectifier side; Indicates the rated DC current; This indicates the turns ratio of the receiving-end converter transformer at time t; Indicates the AC bus voltage at the sending end; Indicates the DC side voltage of the LCC;

[0028] Furthermore, the construction of the sending-end hybrid cascade analysis model relies on a preset sending-end hybrid cascade system topology, the structure of which is as follows:

[0029] The high-voltage side of the rectifier uses a set of 12-pulse LCCs with constant current control; the low-voltage side uses three sets of parallel MMCs with constant reactive power control.

[0030] The high-voltage LCC and the low-voltage MMC are connected in parallel to the sending-end AC bus via a transformer;

[0031] The inverter side uses two sets of constant current controlled 12-pulse LCCs.

[0032] On the other hand, this invention discloses a power fluctuation suppression system based on LCC and MMC, applicable to the aforementioned power fluctuation suppression method based on LCC and MMC, including:

[0033] The data acquisition module is used to collect real-time operating status data of the receiving-end converter and system electrical operation data;

[0034] The status judgment module is used to determine whether a commutation failure has occurred at the receiving end based on the operating status data of the receiving-end converter.

[0035] The instruction generation module is used to generate reactive power compensation instructions for MMC and current compensation instructions for LCC based on the electrical operation data of the system when a commutation failure is determined. The module aims to suppress the increase in active power fed into the AC system at the sending end and performs joint calculations based on a preset hybrid cascade analysis model at the sending end.

[0036] The instruction execution module is used to send the reactive power compensation instruction to the MMC on the rectifier side and the current compensation instruction to the LCC on the rectifier side. Through the coordinated control of the MMC and the LCC, the transient active power fluctuation of the DC system is suppressed.

[0037] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0038] The power fluctuation suppression method and system based on LCC and MMC of the present invention firstly uses the receiving-end commutation failure as a signal to actively trigger the control action of the sending-end rectifier station, realizing cross-end linkage of information and control; secondly, based on the coordinated regulation of MMC and LCC, the rapid reactive power regulation capability of MMC is fully utilized for transient buffering and the steady-state current regulation capability of LCC, which significantly improves the transient stability and power transmission capability of DC system after commutation failure, and solves the problem of active power transmission drop in DC system after commutation failure in the sending-end hybrid cascaded DC transmission system. Attached Figure Description

[0039] Figure 1 This is a flowchart illustrating the power fluctuation suppression method based on LCC and MMC provided in Embodiment 1 of the present invention.

[0040] Figure 2This is the topology of the hybrid cascaded high-voltage direct current transmission system provided in Embodiment 1 of the present invention;

[0041] Figure 3 This is a schematic diagram of the hybrid cascade system for the feed end after commutation failure provided in Embodiment 1 of the present invention;

[0042] Figure 4 This is a schematic diagram of the steady-state operation curve of the CIGER standard system provided in Embodiment 1 of the present invention;

[0043] Figure 5 This is a diagram showing the relationship between active and reactive power in the AC feed system provided in Embodiment 1 of the present invention.

[0044] Figure 6 This is a block diagram of the DC active power support strategy based on a hybrid cascaded system provided in Embodiment 1 of the present invention;

[0045] Figure 7 This is a graph showing the change in active power of the sending-end DC system in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0046] Figure 8 This is a graph showing the change in active power of the receiving-end DC system in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0047] Figure 9 This is a graph showing the change in active power of the receiving-end AC system in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0048] Figure 10 This is a graph showing the change of the DC transient current at the sending end in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0049] Figure 11 This is a graph showing the change in transient voltage of the sending-end AC bus in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0050] Figure 12 This is a graph showing the change in transient voltage of the receiving-end AC bus in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0051] Figure 13 This is a graph showing the transient voltage change of the DC bus at the sending end in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention.

[0052] Figure 14 This is a graph showing the transient voltage change of the receiving-end DC bus in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0053] Figure 15 This is the curve showing the change in the shut-off angle in the simulation results of the commutation failure duration of 0.1s provided in Embodiment 1 of the present invention;

[0054] Figure 16 This is the curve showing the change in DC current command value in the simulation results of a commutation failure duration of 0.1s provided in Embodiment 1 of the present invention. Detailed Implementation

[0055] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0056] Example 1

[0057] This embodiment 1 provides a power fluctuation suppression method based on LCC and MMC, such as Figure 1 As shown, it includes the following steps:

[0058] Real-time acquisition of operating status data of the receiving-end converter and system electrical operation data;

[0059] Based on the operating status data of the receiving-end converter, determine whether a commutation failure has occurred at the receiving end;

[0060] When a commutation failure is determined, based on the system electrical operation data, with the control objective of suppressing the increase in active power fed into the sending-end AC system, joint calculations are performed based on the preset sending-end hybrid cascade analysis model to generate reactive power compensation commands for MMC and current compensation commands for LCC.

[0061] The reactive power compensation command is sent to the MMC on the rectifier side, and the current compensation command is sent to the LCC on the rectifier side. Through the coordinated control of the MMC and LCC, the transient active power fluctuation of the DC system can be suppressed.

[0062] The technical concept of this invention is as follows: First, by using the receiving-end commutation failure as a signal, the control action of the sending-end rectifier station is actively triggered, realizing cross-end linkage of information and control; Second, based on the coordinated regulation of MMC and LCC, the rapid reactive power regulation capability of MMC is fully utilized for transient buffering and the steady-state current regulation capability of LCC, which significantly improves the transient stability and power transmission capability of the DC system after commutation failure, and solves the problem of active power transmission drop in the DC system after commutation failure in the sending-end hybrid cascaded DC transmission system.

[0063] The specific steps are as follows:

[0064] Step 1: Collect real-time operating status data of the receiving-end converter and system electrical operating data.

[0065] The system's electrical operating data includes voltage data for the sending-end AC bus and current and voltage data for the DC system.

[0066] Step 2: Based on the operating status data of the receiving-end converter, determine whether a commutation failure has occurred at the receiving end.

[0067] The receiving-end converter operating status data includes the receiving-end converter bus voltage;

[0068] Determining whether a commutation failure has occurred at the receiving end includes the following steps:

[0069] When the voltage drop of the receiving-end commutator bus exceeds the first preset threshold and the duration of the drop exceeds the second preset threshold, a commutation failure is determined to have occurred.

[0070] Step 3: When a commutation failure is determined, based on the system electrical operation data, with the control objective of suppressing the increase in active power fed into the sending-end AC system, joint calculations are performed based on the preset sending-end hybrid cascade analysis model to generate reactive power compensation commands for MMC and current compensation commands for LCC.

[0071] 3.1 The construction of the hybrid cascade analysis model at the sending end depends on the pre-defined hybrid cascade system topology at the sending end.

[0072] like Figure 2 As shown, the topology of the sending-end hybrid cascaded system is as follows:

[0073] The high-voltage side of the rectifier uses a set of 12-pulse LCCs with constant current control; the low-voltage side uses three sets of parallel MMCs with constant reactive power control.

[0074] The high-voltage LCC and the low-voltage MMC are connected in parallel to the sending-end AC bus via a transformer;

[0075] The inverter side uses two sets of constant current controlled 12-pulse LCCs.

[0076] During normal system operation, all active power output from the AC system is transferred to the DC system, while all reactive power provided by the AC system and the reactive power compensation device is absorbed by the rectifier. The balance between active and reactive power is as follows:

[0077] (1)

[0078] (2)

[0079] In the formula, This represents the active power input from the AC system to the DC system. Indicates the active power of the DC system at the sending end; Indicates the DC current at the sending end; Indicates the DC voltage at the sending end;

[0080] This represents the reactive power input from the AC system to the DC system. This indicates the reactive power input to the reactive power compensation device; Indicates the reactive power of the sending-end DC system; This indicates the reactive power consumed by the LCC; This indicates the reactive power consumed by the MMC.

[0081] reactive power input to the reactive power compensation device It can be represented as:

[0082] (3)

[0083] In the formula, Indicates the rated AC bus voltage; This indicates the rated reactive power input to the reactive power compensation device.

[0084] Under steady-state operating conditions, the reactive power generated by an LCC typically reaches 40%-60% of its transmitted active power. The reactive power characteristics of an LCC in a converter station can be described by the following mathematical model:

[0085] (4)

[0086] In the formula, This indicates the reactive power consumed by the LCC; Indicates direct current; Indicates the DC open-circuit voltage of the LCC; This indicates the DC side voltage of the LCC.

[0087] LCC DC voltage parameter U under no-load condition LCC0 This directly affects the accuracy of reactive power calculations, and its value has a functional relationship with the operating parameters of the converter transformer and the AC bus voltage.

[0088] (5)

[0089] In the formula, Indicates the turns ratio of the LCC converter transformer; Indicates the AC bus voltage at the sending end;

[0090] After a commutation failure in the receiving-end system, the power balance of the sending-end hybrid cascaded system is disrupted, resulting in a large amount of active and reactive power surplus being fed into the AC system, such as... Figure 3 As shown, it is represented as

[0091] (6)

[0092] (7)

[0093] In the formula, This represents the increment of active power fed into the AC system. This indicates the rated active power input from the AC system to the DC system; Indicates the active power of the DC system at the sending end;

[0094] This represents the increase in reactive power fed into the AC system; This indicates the rated reactive power input from the AC system to the DC system; This indicates the reactive power input to the reactive power compensation device; This indicates the reactive power of the sending-end DC system.

[0095] Simultaneously, affected by the active and reactive power fed into the AC system, a transient overvoltage occurs on the sending-end AC bus, which can be expressed as:

[0096] (8)

[0097] In the formula, Indicates the AC bus voltage at the sending end; Indicates the rated AC bus voltage;

[0098] Represents the longitudinal component of the transient voltage of the AC bus; This represents the increase in reactive power fed into the AC system; Represents the transverse component of the AC bus transient voltage; This represents the active power fed into the AC system. This represents the equivalent reactance of the sending-end AC system;

[0099] (9)

[0100] In the formula, This indicates the system's short-circuit capacity.

[0101] Substituting equation (9) into equation (8), we can obtain the per-unit value of the AC bus voltage after commutation failure:

[0102] (10)

[0103] In the formula, Indicates the transient voltage of the sending-end AC bus; Indicates the system's short-circuit capacity; This represents the increment of active power fed into the AC system. This represents the increase in reactive power fed into the AC system;

[0104] Substituting equations (6) and (7) into equation (10) and rearranging, we obtain the active power P of the sending-end DC system after commutation failure. dr It can be represented as:

[0105] (11)

[0106] In the formula, Indicates the active power of the DC system at the sending end; This indicates the rated active power input from the AC system to the DC system;

[0107] Indicates the calculation factor; Indicates the reactive power of the sending-end DC system; This indicates the rated reactive power input from the AC system to the DC system; Indicates the system's short-circuit capacity;

[0108] This indicates the rated reactive power input to the reactive power compensation device; Indicates the transient voltage of the sending-end AC bus; This indicates the rated active power input from the AC system to the DC system;

[0109] As shown in equation (11), after a commutation failure occurs, the reactive power Q of the rectifier can be adjusted. dr This aims to improve the transmission of active power in DC systems. However, the mathematical model is complex and its dynamic characteristics are unclear, failing to intuitively reveal the time-domain evolution of active power and its correlation with reactive power. Therefore, it is necessary to analyze the time-domain fluctuation mechanism of DC active power and propose a composite control strategy that coordinates the reactive power compensation capability of the MMC (Multi-Mode Control) and the current command value of the LCC (Limited Control Classification). By adjusting the reactive power ΔQ fed into the AC system, the active power of the DC system can be effectively improved.

[0110] The DC power model established by Equation (11) has limitations such as complex structure and unclear dynamic characteristics, which restrict the optimization design of the control strategy. Therefore, by analyzing Equation (1), the mechanism of the influence of DC current and DC voltage on the active power transmitted by the DC system is revealed, and the dynamic sensitivity relationship shown in Equation (12) is established based on partial derivative analysis:

[0111] (12)

[0112] From equation (12), it can be seen that the active power fluctuation of DC transmission is affected by U dr I dr dI dr / dt、dU dr / dt impact.

[0113] To further investigate the active power P of the sending-end DC system dr The fluctuation mechanism requires clarifying the DC current I at the sending end after commutation failure. dr and the DC voltage U at the sending end dr From the changing characteristics, we can obtain:

[0114] (13)

[0115] In the formula, This represents the equivalent resistance of a DC system.

[0116] A short-circuit fault in the receiving-end AC system may cause commutation failure in the inverter. The system can be divided into three stages: the commutation failure fault stage, the commutation failure recovery stage, and the AC system fault recovery stage. A schematic diagram of the DC current versus sending-end DC voltage operating curves is shown below. Figure 4 As shown.

[0117] In the first stage, the inverter's DC voltage U di It will drop to 0, thus causing the DC current I to... dr Increase, satisfying dI dr / dt>0. After the fault, the MMC controlled by constant DC voltage does not block and operates normally. Therefore, the DC side voltage U of the MMC after the fault will remain unchanged. MMC It remains near the rated value, but will be affected by the DC current I at the sending end. dr The increased impact decreased slightly, and the system transitioned from steady-state operating point A to transient point B. The rectifier-side constant current controller rapidly increased the commutation angle α. r This makes the DC side voltage U of the LCC LCC Rapidly decrease to -U MMC DC voltage at the sending end U dr Decrease, dU dr / dt<0, then send the DC current I dr Reduce, dI dr When dt < 0, the system moves from point B to point C. Therefore, the DC current I at the sending end... dr During the rising phase, from point A to point B, the DC active power P dr A brief increase, followed by a rise in the DC current I at the sending end. dr With the DC voltage U at the sending end dr During the phase of the decline, i.e., from point B to point C in the diagram, dP dr / dt is always less than 0, P dr The DC active power decreases.

[0118] AC voltage U at the receiving end Li With DC voltage U di The relationship between them can be expressed as

[0119] (14)

[0120] In the formula, N represents the number of six-pulse rectifiers; T i Indicates the turns ratio of the receiving-end converter transformer; β represents the inverter turn-off angle; X i This represents the commutation inductance of the inverter.

[0121] In the second stage, the inverter can commutate normally. According to equation (14), due to the AC bus voltage U at the receiving end... Li The influence of the DC voltage U at the receiving end di It begins to increase rapidly. Because the AC system fault has not yet been cleared, the receiving-end DC voltage U... di It will reach fault steady state. With the receiving-end DC voltage U di Rapid increase, U MMC Due to the constant change in voltage and the delay time of the PI controller, the rectifier control cannot immediately adjust U. LCC DC current at the sending end I dr First, the system will decrease from transient point C to transient point D. Then, as the rectifier-side LCC constant current controller reduces the commutation angle α... r U LCC Increase the DC current I at the sending end. dr A brief rise. During this process, according to equation (10), dI dr / dt<0 and dU dr / dt>0, due to the hysteresis of DC voltage changes, dP will be affected. dr / dt<0, which will result in the active power P of the sending-end DC system. dr Further drops. Meanwhile, in cases of severe faults, the receiving-end AC bus voltage U... Li It will reach a lower value, according to equation (14), the DC voltage U at the receiving end di It will also reach a lower value. And because the DC current is relatively small, the active power P of the sending-end DC system... dr It will also reach a lower value. Therefore, the more severe the AC system fault, the lower the active power transmission value of the DC system.

[0122] In the third stage, when the AC system fault is cleared, the receiving-end AC bus voltage U Li It will quickly recover to the rated value, receiving-end DC voltage U di Increase to the rated DC voltage U at the receiving end diN Because the LCC rectifier control cannot reduce the commutation angle α in time. r DC voltage at the sending end U dr Unable to increase immediately, this may lead to U di dr DC current I at the sending end dr The decrease occurs when the system transitions from transient point D to transient point E, causing dI to...​dr / dt<0 and dU dr / dt>0, due to the hysteresis of DC voltage changes, dP will be affected. dr / dt<0, which will lead to an increase in the active power P of the sending-end DC system. dr This reduction will lead to a third active power drop in the DC transmission system.

[0123] In summary, the three-stage drop in DC active power caused by commutation failure stems from the interaction between the receiving-end voltage collapse, current dynamic response, and control regulation delay during the transient process. Particularly noteworthy is the strong nonlinear coupling between the rectifier reactive power and the AC bus voltage in equation (11), which makes it difficult for traditional constant current control to compensate for the reactive power deficit during the fault period in a timely manner. Figure 4 As shown, when commutation failure occurs on the inverter side, it triggers a DC voltage U at the receiving end. di During a sudden drop, the deep coupling between reactive power and voltage at the receiving-end converter station will exacerbate the DC voltage U at the receiving end through equation (14). di The dynamic drop in voltage, and thus the active power fluctuation is amplified through equation (12). This coupling effect is more pronounced in the second and third stages: when the AC fault is not completely cleared, the AC bus voltage U at the receiving end... Li The sustained low level is limited by equation (14) to control the DC voltage U at the receiving end. di Recovery speed is affected, but the rectifier-side LCC is controlled by the lag response of the constant current stage, and its commutation angle adjustment cannot quickly offset the DC voltage U at the sending end caused by reactive power-voltage coupling. dr Descent inertia.

[0124] The above analysis of the transient characteristics and fluctuation mechanism of active power shows that the active power drop caused by commutation failure is essentially due to the dual constraints of strong reactive-voltage coupling and the lag in the dynamic response of the controller. The analysis of active power in this paper clarifies the inherent limitations of traditional constant current control under the nonlinear constraint of equation (11) - the inability to synchronously compensate for the reactive power of the rectifier station and the lag in regulating the DC voltage, highlighting the necessity of constructing a collaborative control strategy based on the structural characteristics of the hybrid cascaded system. Therefore, the active power support strategy proposed in Part III is based on the "reactive-voltage-active" coupling mechanism revealed by this time-domain analysis, and breaks through the bottleneck of active power support in the existing control under multivariable coupling scenarios by actively regulating the dynamic characteristics of the reactive power of the rectifier.

[0125] The above analysis shows that the DC active power drop caused by commutation failure is closely related to the system transient characteristics, control response, and reactive power-voltage coupling. In particular, the strong coupling between the rectifier reactive power and the converter bus voltage in equation (11) further increases the complexity of active power support control. To this end, based on the structural advantages of the hybrid cascaded system, this invention proposes a control strategy with the active power dynamic characteristics characterized by equations (6), (7), and (10) as the core. By reducing the active power ΔP fed into the AC system at the sending end, the active power transmission capability of the DC system is actively improved.

[0126] After commutation failure, the active power ΔP, reactive power ΔQ, and transient voltage U fed into the AC system... Lrpu The relation is

[0127] (15)

[0128] In the formula, Indicates the system's short-circuit capacity; Indicates the transient voltage of the sending-end AC bus;

[0129] This represents the increment of active power fed into the AC system. This represents the increase in reactive power fed into the AC system.

[0130] According to equation (15), when the short-circuit capacity S Cr =12000MVA, transient voltage U Lrpu The diagrams showing the relationship between the active power ΔP and reactive power ΔQ fed into the AC system when the values ​​are 1.05, 1.1, 1.15, and 1.2 are as follows: Figure 5 As shown.

[0131] based on Figure 5 The analysis results of equation (15) show that under different converter bus transient voltage conditions, the reactive power ΔQ and active power ΔP fed into the AC system exhibit a positive correlation. Within a certain range, by synergistically increasing the reactive power consumption Q of the rectifier-side LCC and MMC, dr This can suppress the negative impact of ΔP on DC transmission power and provide a theoretical basis for constructing an active power support strategy.

[0132] According to equations (7) and (15), the expression for the reactive power consumed by the converter is:

[0133] (16)

[0134] To achieve the goal of making the active power ΔP fed into the AC system approach zero, the reactive power regulation ΔQ of the rectifier-side MMC is... MMC for:

[0135] (17)

[0136] (18)

[0137] In the formula, This indicates the reactive power regulation of the MMC on the rectifier side; This indicates the reactive power consumed by the MMC; This indicates the reactive power consumed by the LCC;

[0138] This indicates the rated reactive power input from the AC system to the DC system; This indicates the reactive power input to the reactive power compensation device; Indicates the system's short-circuit capacity; This indicates the transient voltage of the AC bus at the sending end.

[0139] Meanwhile, in the operation of AC / DC hybrid transmission systems, the reactive power dynamic regulation margin of the MMC converter is limited by the rated voltage constraint of the AC network and the capacity design parameters of the rectifier. Therefore, the system needs to reduce power loss through the coordinated optimization of the LCC constant current control loop. Based on the multi-controller simultaneous solution method, the dynamic adjustment amount ΔI of the DC current command value of the rectifier station is... dr The mathematical model established by equations (6), (7), (17), and (18) can be used to derive the following:

[0140] (19)

[0141] In the formula, This indicates the DC current adjustment amount of the LCC on the rectifier side; This indicates the reactive power regulation of the MMC on the rectifier side; Indicates the rated DC current; This indicates the turns ratio of the receiving-end converter transformer at time t; Indicates the AC bus voltage at the sending end; Indicates the DC side voltage of the LCC;

[0142] In summary, based on the analysis of equations (15) to (19), this invention proposes a DC active power support strategy, the control block diagram of which is shown below. Figure 6 As shown. The core of this strategy lies in regulating the reactive power ΔQ of the rectifier-side MMC. MMC With the DC current regulation amount ΔI of the rectifier side LCC dr The coordinated regulation of the system aims to reduce the active power ΔP fed into the AC system to zero, thereby reducing the impact of power disturbances in the sending-end AC system on DC active power transmission. The multi-converter coordination capability of the hybrid cascaded system is fully utilized, significantly improving the transient stability and power transmission capability of the DC system after commutation failure.

[0143] 3.2 Specifically, the hybrid cascade analysis model at the sending end in this embodiment includes:

[0144] The MMC calculation unit is used to calculate the MMC reactive power regulation on the rectifier side based on the system electrical operation data and the preset MMC reactive power compensation calculation formula.

[0145] LCC calculation unit; used to calculate the DC current regulation of the rectifier-side LCC based on the reactive power regulation of the rectifier-side MMC and the electrical operation data of the system, and based on the preset LCC current compensation calculation formula.

[0146] The MMC instruction unit is used to generate reactive power compensation instructions for MMC based on the reactive power adjustment amount of the MMC on the rectifier side.

[0147] The LCC instruction unit is used to generate current compensation instructions for the LCC based on the DC current adjustment of the rectifier-side LCC.

[0148] The expression for MMC reactive power compensation calculation is as follows:

[0149] ;

[0150] ;

[0151] In the formula, This indicates the reactive power regulation of the MMC on the rectifier side; This indicates the reactive power consumed by the MMC; This indicates the reactive power consumed by the LCC;

[0152] This indicates the rated reactive power input from the AC system to the DC system; This indicates the reactive power input to the reactive power compensation device; Indicates the system's short-circuit capacity; This indicates the transient voltage of the AC bus at the sending end.

[0153] The expression for calculating LCC current compensation is as follows:

[0154] ;

[0155] In the formula, This indicates the DC current adjustment amount of the LCC on the rectifier side; This indicates the reactive power regulation of the MMC on the rectifier side; Indicates the rated DC current; This indicates the turns ratio of the receiving-end converter transformer at time t; Indicates the AC bus voltage at the sending end; Indicates the DC side voltage of the LCC;

[0156] Step 4: Send the reactive power compensation command to the MMC on the rectifier side and the current compensation command to the LCC on the rectifier side. Through the coordinated control of the MMC and LCC, the transient active power fluctuation of the DC system can be suppressed.

[0157] This embodiment further provides a simulation model of a hybrid DC transmission system built on the PSCAD / EMTDC simulation platform, with system parameters shown in Table 1. The model employs a series structure on the rectifier side with a 12-pulse LCC at the high-voltage end and three parallel MMC valve groups at the low-voltage end, while the inverter side is configured with a dual 12-pulse LCC structure. The effectiveness of the proposed control strategy is verified by setting an 80Ω single-phase ground fault on the receiving-end AC bus, with an initiation time of 3 seconds and commutation failure durations of 0.1 seconds and 0.2 seconds, respectively.

[0158] Table 1. Some simulation parameters of the DC system

[0159]

[0160] Based on the system simulation parameters of this embodiment and equations (17), (18), and (19), the simulation results of the control strategy proposed in this invention are as follows: Figure 7-16 As shown in the figure, P dr P represents the active power of the sending-end DC system. di P represents the active power of the receiving-end DC system. ad Indicates the active power of the receiving-end AC system; I drpu Indicates the DC transient current at the sending end; U Lrpu U represents the transient voltage of the sending-end AC bus; Lipu U represents the transient voltage of the receiving-end AC bus; drpu U represents the transient voltage of the DC bus at the sending end; dipu Represents the transient voltage of the receiving-end DC bus; γ represents the turn-off angle; I dref This indicates the commanded value for DC current.

[0161] Simulation results show that after a commutation failure, the original system's DC active power exhibits a typical three-stage change characteristic—a brief power surge followed by a continuous drop in the initial stage of the fault. After adopting the control strategy proposed in this paper, the system's dynamic characteristics are significantly improved: during a 3-second fault, the switching angle of the receiving-end inverter station changes, and the DC active power recovers from 1000MW to 2500MW. Both the sending-end and receiving-end DC active power, as well as the receiving-end AC active power, are significantly enhanced. Simultaneously, the transient overvoltage of the sending-end AC bus is effectively suppressed, the DC voltage support capability is strengthened, and the DC current fluctuation is significantly reduced. Comparing operating conditions with different durations, the proposed control strategy demonstrates good adaptability.

[0162] In summary, this invention proposes a method for suppressing and controlling transient active power fluctuations in a hybrid cascaded DC system using LCC and MMC, by coordinating the adjustment of the reactive power regulation ΔQ of the MMC on the rectifier side. MMC With the DC current regulation amount ΔI of the rectifier side LCC dr This significantly improves the DC active power transmission capacity of the system after commutation failure, suppresses overvoltage on the sending-end AC bus, reduces DC system voltage dips, lowers DC current fluctuations, and effectively improves system stability. Furthermore, this strategy can adapt to different transient voltage conditions, providing a theoretical basis and simulation analysis reference for the safe and efficient operation of hybrid cascaded DC transmission systems with large-scale renewable energy integration.

[0163] Example 2

[0164] This embodiment 2 provides a power fluctuation suppression system based on LCC and MMC, applicable to the power fluctuation suppression method based on LCC and MMC in embodiment 1, including:

[0165] The data acquisition module is used to collect real-time operating status data of the receiving-end converter and system electrical operation data;

[0166] The status judgment module is used to determine whether a commutation failure has occurred at the receiving end based on the operating status data of the receiving-end converter.

[0167] The instruction generation module is used to generate reactive power compensation instructions for MMC and current compensation instructions for LCC when a commutation failure is determined. Based on the system electrical operation data, with the control objective of suppressing the active power increment fed into the AC system at the sending end, it performs joint calculations based on the preset hybrid cascade analysis model at the sending end.

[0168] The instruction execution module is used to send reactive power compensation instructions to the MMC on the rectifier side and current compensation instructions to the LCC on the rectifier side. Through the coordinated control of the MMC and LCC, transient active power fluctuations in the DC system can be suppressed.

[0169] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0170] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0171] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0172] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0173] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A power fluctuation suppression method based on LCC and MMC, characterized in that, Includes the following steps: Real-time acquisition of operating status data of the receiving-end converter and system electrical operation data; Based on the operating status data of the receiving-end converter, determine whether a commutation failure has occurred at the receiving end; When a commutation failure is determined, based on the system electrical operation data, with the control objective of suppressing the increase in active power fed into the sending-end AC system, joint calculations are performed based on the preset sending-end hybrid cascade analysis model to generate reactive power compensation instructions for MMC and current compensation instructions for LCC. The reactive power compensation command is sent to the MMC on the rectifier side, and the current compensation command is sent to the LCC on the rectifier side. Through the coordinated control of the MMC and the LCC, the transient active power fluctuation of the DC system is suppressed.

2. The power fluctuation suppression method based on LCC and MMC according to claim 1, characterized in that, The system's electrical operating data includes voltage data for the sending-end AC bus and current and voltage data for the DC system.

3. The power fluctuation suppression method based on LCC and MMC according to claim 1, characterized in that, The receiving-end converter operating status data includes the receiving-end converter bus voltage; The determination of whether a commutation failure has occurred at the receiving end includes the following steps: When the voltage drop of the receiving-end commutator bus exceeds a first preset threshold and the duration of the drop exceeds a second preset threshold, a commutation failure is determined to have occurred.

4. The power fluctuation suppression method based on LCC and MMC according to claim 1, characterized in that, The sending-end hybrid cascade analysis model includes: The MMC calculation unit is used to calculate the MMC reactive power adjustment amount on the rectifier side based on the electrical operation data of the system and a preset MMC reactive power compensation calculation formula. LCC calculation unit; used to calculate the rectifier-side LCC DC current regulation based on the rectifier-side MMC reactive power regulation and system electrical operation data, and based on a preset LCC current compensation calculation formula. The MMC instruction unit is used to generate reactive power compensation instructions for MMC based on the reactive power adjustment amount of the rectifier-side MMC. The LCC instruction unit is used to generate a current compensation instruction for the LCC based on the DC current adjustment amount of the rectifier-side LCC.

5. The power fluctuation suppression method based on LCC and MMC according to claim 4, characterized in that, The expression for the MMC reactive power compensation calculation formula is as follows: ； ； In the formula, This indicates the reactive power regulation of the MMC on the rectifier side; This indicates the reactive power consumed by the MMC; This indicates the reactive power consumed by the LCC; This indicates the rated reactive power input from the AC system to the DC system; This indicates the reactive power input to the reactive power compensation device; Indicates the system's short-circuit capacity; This indicates the transient voltage of the AC bus at the sending end.

6. The power fluctuation suppression method based on LCC and MMC according to claim 4, characterized in that, The expression for the LCC current compensation calculation formula is as follows: ； In the formula, This indicates the DC current adjustment amount of the LCC on the rectifier side; This indicates the reactive power regulation of the MMC on the rectifier side; Indicates the rated DC current; This indicates the turns ratio of the receiving-end converter transformer at time t; Indicates the AC bus voltage at the sending end; This indicates the DC side voltage of the LCC.

7. The power fluctuation suppression method based on LCC and MMC according to claim 1, characterized in that, The construction of the hybrid cascade analysis model for the sending end relies on a pre-defined hybrid cascade system topology for the sending end. The structure of the hybrid cascade system topology for the sending end is as follows: The high-voltage side of the rectifier uses a set of 12-pulse LCCs with constant current control; the low-voltage side uses three sets of parallel MMCs with constant reactive power control. The high-voltage LCC and the low-voltage MMC are connected in parallel to the sending-end AC bus via a transformer; The inverter side uses two sets of constant current controlled 12-pulse LCCs.

8. A power fluctuation suppression system based on LCC and MMC, applicable to the power fluctuation suppression method based on LCC and MMC as described in any one of claims 1-7, characterized in that, include: The data acquisition module is used to collect real-time operating status data of the receiving-end converter and system electrical operation data; The status judgment module is used to determine whether a commutation failure has occurred at the receiving end based on the operating status data of the receiving-end converter. The instruction generation module is used to generate reactive power compensation instructions for MMC and current compensation instructions for LCC based on the electrical operation data of the system when a commutation failure is determined. The module aims to suppress the increase in active power fed into the AC system at the sending end and performs joint calculations based on a preset hybrid cascade analysis model at the sending end. The instruction execution module is used to send the reactive power compensation instruction to the MMC on the rectifier side and the current compensation instruction to the LCC on the rectifier side. Through the coordinated control of the MMC and the LCC, the transient active power fluctuation of the DC system is suppressed.

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