A method and device for additional frequency coordination control of a VSC / LCC DC parallel hybrid system considering non-fault side stability

Abstract

The application discloses a VSC / LCC DC parallel hybrid system additional frequency coordination control method and device which takes into account non-fault side stability. The method monitors the frequency deviation of the fault side power grid and the frequency change rate of the non-fault side power grid in real time; when the frequency deviation exceeds the dead zone, the expected total power support is calculated; the anti-disturbance margin of the non-fault side is dynamically evaluated according to the frequency change rate, and the dynamic upper limit value of the power support is calculated; the expected support amount is compared with the dynamic upper limit value, and the smaller one is taken as the actual support amount for distribution and execution; when the frequencies on both sides are restored to the safe range, the support is exited. The application prevents the secondary frequency instability of the non-fault side while guaranteeing the frequency support of the fault side, and realizes the multi-region bidirectional frequency safety guarantee.

Description

Technical Field

[0001] This invention relates to the field of power system frequency stability control technology, specifically to an additional frequency coordination control method and device for a VSC / LCC DC parallel hybrid system that takes into account the stability of the non-fault side. Background Technology

[0002] High-voltage direct current (HVDC) transmission technology has been widely applied in my country. The landing points of both VSC (Flexible Direct Current) and LCC (Conventional Direct Current) technologies are becoming increasingly concentrated, forming a "hybrid multi-infeed" pattern where multiple HVDC lines land on the same AC grid and are very close in electrical distance. In these hybrid multi-infeed systems, when a frequency disturbance occurs in an AC grid due to generator tripping, DC blocking, or large load fluctuations, existing technologies mainly use the following methods for frequency coordination control:

[0003] (1) Additional frequency control for single DC: Introduce frequency-active power supplementary controllers (such as frequency droop control, virtual inertia control, frequency limit controller FLC, etc.) into VSC DC or LCC DC, so that the DC transmission power can respond to the frequency changes of the connected AC power grid and provide frequency support.

[0004] (2) Coordinated frequency control of multiple DC circuits: When there are multiple DC circuits in the system, the frequency support task is allocated among the DC circuits according to the preset priority order or allocation ratio.

[0005] (3) Coordinated control of DC frequency limit controller: By unifying the filtering links, parameter control and power change rate of multiple DC FLCs, the external characteristics of different types of DC FLCs are unified to achieve coordinated frequency control.

[0006] However, all of the above-mentioned existing technologies have a common, long-neglected safety hazard: they are essentially "fault-side demand-driven" control logics—the trigger signal and power support amount of frequency coordination control are determined solely by the frequency deviation amplitude of the fault-side grid, without considering the frequency security risks that the non-fault-side grid on the DC counterpart may bear when performing power support.

[0007] In a hybrid multi-feed system, VSC DC and LCC DC are connected to different AC grids (or different AC regions). When VSC DC (or LCC DC) significantly adjusts its DC power in response to frequency disturbances on the fault side, it also means that the non-faulty side of the DC grid is experiencing an equivalent power surge. When the non-faulty side grid has a low inertia level (such as in scenarios with a high proportion of renewable energy integration or partial unit maintenance), this "passively borne" power surge may cause its frequency deviation or frequency change rate to exceed the safety threshold, triggering secondary frequency instability.

[0008] Therefore, the existing "fault-side demand-driven" frequency coordination control strategy, while improving the frequency characteristics on the fault side, implicitly carries the risk of transferring frequency risks from the fault side to the non-fault side, creating a chain reaction. Summary of the Invention

[0009] The technical problem to be solved by this invention is: in a VSC / LCC DC parallel hybrid system, while providing effective frequency support for the fault-side grid, it prevents secondary frequency instability of the non-fault-side grid caused by excessive power support, thereby achieving bidirectional frequency security between multiple regions.

[0010] A frequency coordination control method for a VSC / LCC DC parallel hybrid system that considers the stability of the non-fault side, wherein the VSC / LCC DC parallel hybrid system includes at least one VSC DC converter station and at least one LCC DC converter station, the AC side of the VSC DC converter station and the AC side of the LCC DC converter station are connected in parallel with a relatively short electrical distance through an AC power grid, so that the AC power grids connected to each converter station can mutually influence each other's frequencies through DC power modulation, the method comprising:

[0011] Step S1: Monitor the frequency information of the AC power grid connected to each converter station in the parallel hybrid multi-feeder system in real time, and obtain the frequency deviation of the fault-side power grid where frequency disturbance occurs. and the frequency variation rate of at least one non-faulty side power grid ;

[0012] Step S2, when the frequency deviation of the faulty power grid is detected When the frequency control dead zone is exceeded, the coordination controller calculates the total expected power support required to suppress the frequency fluctuations of the fault-side power grid according to the preset frequency-active power supplementary control strategy. ;

[0013] Step S3, the coordination controller adjusts the real-time frequency change rate of the non-faulty side power grid. The current disturbance immunity margin of the non-faulty side power grid is dynamically assessed, and the dynamic upper limit of power support that the non-faulty side power grid can safely withstand is calculated based on the assessment results. ;

[0014] Step S4, the coordination controller will adjust the total desired power support. With the aforementioned power support dynamic upper limit value The two values ​​are compared, and the smaller value is taken as the actual power support amount. and the actual power support amount executed. The power is distributed between at least two DC transmission systems according to a preset allocation ratio to provide power support.

[0015] Step S5, when the frequency deviation of the fault-side power grid... The frequency change rate of the non-faulty side power grid is restored to within the preset frequency control dead zone. When the power level returns to within the preset safety threshold, the power support will gradually be deactivated.

[0016] 2. Furthermore, in step S3, the power support dynamic upper limit value... Calculate using the following formula:

[0017] ;

[0018] In the formula, H eq RoC represents the real-time equivalent inertial constant of the non-faulty side of the power grid. max f is the preset upper limit of the safety threshold for the rate of change of frequency. n Where K is the rated frequency, and K is an adjustment factor related to the DC system capacity.

[0019] Furthermore, the equivalent inertial constant H eq The results can be obtained through one of the following methods: weighted average calculation based on the inertia constant and rated capacity of each online generator unit in the non-faulty side power grid; online identification by monitoring the frequency change rate of the non-faulty side power grid under small power disturbances; or pre-setting based on the offline simulation calculation results of the non-faulty side power grid and updating the table according to changes in operating mode.

[0020] Furthermore, in step S1, the frequency deviation of the fault-side power grid... The real-time frequency of the fault-side power grid With rated frequency The difference between them; the rate of frequency change of the non-faulty side power grid It is the derivative of the real-time frequency of the non-faulty side power grid with respect to time.

[0021] Furthermore, in step S4, the at least two DC transmission systems include the VSC DC converter station and the LCC DC converter station; the preset allocation ratio is determined by: determining the power support allocation ratio according to the ratio of the short-circuit capacity of the VSC DC converter station and the LCC DC converter station connected to the power grid, or according to a preset priority order, prioritizing one of the DC lines to undertake the power support task, and supplementing it by the other DC line when its adjustable capacity is insufficient.

[0022] Furthermore, it also includes safety interlocking control: when any of the following conditions are detected, the power support dynamic upper limit limiting logic in steps S3 to S4 is interlocked, and the mode is switched to a preset fixed upper limit or full support mode: the frequency change rate of the non-faulty side power grid. The monitoring signal is abnormal or communication is interrupted; the frequency deviation of the fault-side power grid. If the preset emergency threshold is exceeded, the system enters emergency power support mode; or if the VSC DC converter station or the LCC DC converter station experiences a blocking or commutation failure fault.

[0023] A frequency coordination control device for a VSC / LCC DC parallel hybrid system that considers the stability of the non-fault side, wherein the VSC / LCC DC parallel hybrid system includes at least one VSC DC converter station and at least one LCC DC converter station, and the AC side of the VSC DC converter station and the AC side of the LCC DC converter station are connected in parallel through an AC power grid to meet a preset proximity condition. The device includes:

[0024] The frequency monitoring module is used to monitor the frequency information of the AC power grid connected to each converter station in the parallel hybrid multi-feeder system in real time, and to output at least the frequency deviation of the fault-side power grid where frequency disturbance occurs. and the frequency variation rate of at least one non-faulty side power grid ;

[0025] The expected support quantity calculation module is used to calculate the frequency deviation when the fault-side power grid is detected. When the preset frequency control dead zone is exceeded, the expected total power support required to suppress the frequency fluctuations of the fault-side power grid is calculated according to the preset frequency-active power supplementary control strategy. ;

[0026] The safety constraint calculation module is used to calculate the real-time frequency change rate of the non-faulty side power grid. The current disturbance immunity margin of the non-faulty side power grid is dynamically assessed, and the dynamic upper limit of power support that the non-faulty side power grid can safely withstand is calculated based on the assessment results. ;

[0027] The power limiting and allocation module is used to limit the total desired power support. With the aforementioned power support dynamic upper limit value The two values ​​are compared, and the smaller value is taken as the actual power support amount. and the actual power support amount executed. Power is allocated to at least two DC transmission systems according to a preset allocation ratio to provide power support.

[0028] The frequency recovery determination and exit module is used to determine the frequency deviation of the faulty power grid. The frequency change rate of the non-faulty side power grid is restored to within the preset frequency control dead zone. When the output power returns to within the preset safety threshold, the output power support will exit the command.

[0029] Furthermore, the safety constraint calculation module includes:

[0030] The equivalent inertia constant acquisition submodule is used to acquire the real-time equivalent inertia constant H of the non-faulty side power grid. eq ;

[0031] The upper limit calculation submodule is used to calculate the power support dynamic upper limit value according to the following formula:

[0032] ;

[0033] In the formula, H eq RoC represents the real-time equivalent inertial constant of the non-faulty side of the power grid. max f is the preset upper limit of the safety threshold for the rate of change of frequency. n Where K is the rated frequency, and K is an adjustment factor related to the DC system capacity.

[0034] Furthermore, it also includes a safety interlocking control module, used to interlock the power support dynamic upper limit limiting logic in the safety constraint calculation module and the power limiting and allocation module when any of the following conditions are detected, switching to a preset fixed upper limit or full support mode: the frequency change rate of the non-faulty side power grid. The monitoring signal is abnormal or communication is interrupted; the frequency deviation of the fault-side power grid. The preset emergency threshold is exceeded; the VSC DC converter station or the LCC DC converter station experiences a blocking or commutation failure fault.

[0035] Furthermore, the parallel connection between the VSC DC converter station and the LCC DC converter station can be any one or a combination of the following: the sending end or receiving end of the VSC DC converter station is connected to the same AC bus as the rectifier station or inverter station of the LCC DC converter station; the VSC DC converter station and the LCC DC converter station are connected to the same voltage level bus in the same AC hub substation; the VSC DC converter station and the LCC DC converter station are electrically connected through a short-distance AC tie line.

[0036] The present invention has the following beneficial effects:

[0037] Beneficial technical effects

[0038] (1) Achieve proactive safety protection for the non-faulty side power grid: For the first time, the real-time frequency change rate of the non-faulty side power grid is introduced as a hard constraint into the frequency coordination control closed loop, which fundamentally avoids the potential chain reaction of the existing "faulty side demand-driven" strategy that may transfer frequency risk from the faulty side to the non-faulty side.

[0039] (2) Dynamically adapt to changes in grid operation mode: The dynamic upper limit of power support is dynamically calculated based on the real-time equivalent inertial constant and frequency change rate of the non-faulty side grid, which can adapt to changes in grid operation mode (such as unit start-up and shutdown, and fluctuations in new energy output). Compared with the fixed limit method, it has better adaptability and safety.

[0040] (3) Maximize the support effect on the fault side within the safety constraints: This invention does not "not support" or "support less", but provides the maximum possible amount of power support under the premise of ensuring the safety of the non-fault side, thus achieving the optimal balance between the support effect on the fault side and the safety guarantee of the non-fault side.

[0041] (4) Compatible with existing frequency control strategies and achieves smooth embedding: This invention does not replace the existing frequency-active power supplementary control strategy, but serves as the upstream power command safety limiting link, which can be easily embedded into the existing frequency coordination control architecture with minimal modification to the original control strategy.

[0042] (5) The project is highly feasible: the key control variables required can all be obtained through mature online monitoring or identification technologies; the present invention is based on the existing VSC DC and LCC DC control and protection hardware platform, and can be implemented only through software upgrades without increasing the investment in primary equipment. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the VSC / LCC DC parallel hybrid system structure to which this invention applies.

[0044] Figure 2 This is a flowchart of the additional frequency coordination control method of the present invention that takes into account the stability of the non-fault side.

[0045] Figure 3 This is a schematic diagram illustrating the calculation principle of the dynamic upper limit of power support.

[0046] Figure 4 This is a comparison chart showing the effects of using the method of this invention with existing full support methods. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] like Figure 1As shown, this invention is applicable to VSC / LCC DC parallel hybrid multi-feed systems. The VSC / LCC DC parallel hybrid multi-feed system includes at least one VSC DC converter station and at least one LCC DC converter station. The AC side of the VSC DC converter station and the AC side of the LCC DC converter station are connected in parallel via an AC power grid, with an electrical distance satisfying a preset proximity condition. This allows the AC power grids connected to each converter station to mutually influence each other's frequencies through DC power modulation.

[0049] Specifically, the parallel connection between the VSC DC converter station and the LCC DC converter station is any one or a combination of the following: (1) the sending end or receiving end of the VSC DC converter station is connected to the same AC bus as the rectifier station or inverter station of the LCC DC converter station; (2) the VSC DC converter station and the LCC DC converter station are connected to the same voltage level bus in the same AC hub substation; (3) the VSC DC converter station and the LCC DC converter station are electrically connected through a short-distance AC tie line.

[0050] This embodiment uses the Chongqing-Hubei back-to-back power grid interconnection project as a prototype to construct a VSC / LCC DC parallel hybrid system. The VSC back-to-back converter station (±420kV, 1250MW per unit) connects to the equivalent power grids of Sichuan-Chongqing and Hubei; the LCC Longquan converter station (±500kV, 3000MW bipolar) connects to the equivalent power grids of Hubei and East China. The VSC Hubei-side receiving-end converter station and the LCC Longquan rectifier station share a 500kV AC bus, forming a parallel relationship with strong electrical coupling. The AC power grid adopts an equivalent model with no less than four cross-sections.

[0051] The VSC control system employs a dual-closed-loop current vector control strategy. The outer-loop power controller includes constant active power control, constant DC voltage control, and constant reactive power control modes. The LCC control system uses constant DC current control on the rectifier side and constant turn-off angle control on the inverter side, and is equipped with a low-voltage current limiting unit and a current deviation control unit.

[0052] like Figure 2 As shown in the figure, an embodiment of the present invention provides an additional frequency coordination control method for a VSC / LCC DC parallel hybrid multi-feed system that takes into account the stability of the non-fault side, including the following steps:

[0053] Step S1: Real-time monitoring of frequency information

[0054] Real-time monitoring of the frequency information of the AC power grid connected to each converter station in the parallel hybrid multi-feed system.

[0055] Specifically, real-time frequency signals of each AC power grid are continuously acquired at a sampling rate of not less than 100Hz by frequency measurement units (such as PMUs or frequency relays) deployed at the AC bus of each converter station. The frequency information includes at least the frequency deviation of the fault-side power grid where the frequency disturbance occurred. and the frequency variation rate of at least one non-faulty side power grid .

[0056] Among them, the frequency deviation of the fault-side power grid The real-time frequency of the fault-side power grid With rated frequency The difference between (typically 50Hz) and, i.e. = - The frequency variation rate of the non-faulty side power grid. The real-time frequency of the non-faulty side power grid is the derivative of time, i.e. = df / dt, which can be calculated by numerical differentiation and high-frequency noise can be eliminated by using a low-pass filter.

[0057] Step S2: Calculation of total expected power support

[0058] When the frequency deviation of the fault-side power grid is detected When the frequency exceeds the preset frequency control dead zone (e.g., ±0.05Hz), frequency coordination control is triggered.

[0059] The coordinating controller calculates the total expected power support required to suppress frequency fluctuations in the fault-side power grid based on a preset frequency-active power supplementary control strategy. .

[0060] The preset frequency-active power supplementary control strategy may adopt any one or more of the following combinations:

[0061] (1) Frequency droop control;

[0062] (2) Virtual inertia control;

[0063] (3) Frequency Limiting Controller (FLC): When the frequency deviation exceeds the dead zone, the power adjustment amount is output according to the preset power-frequency characteristic curve;

[0064] (4) The combined output of the above strategies.

[0065] The specific control strategy described above is existing technology, and this invention does not limit it, but the total amount of the desired power output is... This serves as an input for subsequent steps in this invention.

[0066] Step S3: Calculation of dynamic upper limit for power support

[0067] The coordination controller adjusts the frequency change rate based on the real-time frequency change rate of the non-faulty side of the power grid. The current disturbance immunity margin of the non-faulty side power grid is dynamically assessed, and the dynamic upper limit of power support that the non-faulty side power grid can safely withstand is calculated based on the assessment results. .

[0068] The core physical significance of this step lies in the fact that the non-faulty side of the power grid will experience an equal amount of power surge when providing power support, which will cause frequency changes. In order to prevent the frequency change rate of the non-faulty side power grid from exceeding its safety threshold, it is necessary to calculate its maximum tolerable power surge amplitude.

[0069] Specifically, the power support dynamic upper limit Calculate using the following formula:

[0070] .

[0071] The physical meaning and acquisition method of each parameter in the formula are as follows:

[0072] (1) H eq The real-time equivalent inertia constant of the non-faulty side power grid, expressed in seconds (s), characterizes the grid's resistance to frequency variations. eq A larger value indicates stronger grid inertia and greater resistance to disturbances. The equivalent inertia constant H... eq Obtained through one of the following methods:

[0073] Method A (preferred method): The weighted average calculation is performed based on the inertia constant and rated capacity of each online generator unit in the non-faulty side of the power grid. This method requires obtaining the real-time operating status information of all online generator units in the power grid, which can be obtained through the SCADA / EMS system.

[0074] Method B: Online identification is performed by monitoring the rate of frequency change of the non-faulty side power grid under minor power disturbances (such as normal load fluctuations within the system);

[0075] Method C: Based on the offline simulation calculation results of the non-faulty side power grid, pre-set H under multiple typical operating modes. eq The value is updated by looking up the table based on changes in the current actual operating mode.

[0076] (2) : The preset upper limit of the safe frequency change rate threshold, in Hz / s. This value is set according to the power grid operation procedures or actual operating experience, for example, it can be set to 0.5Hz / s or 1.0Hz / s. This threshold represents the maximum allowable frequency change rate of the non-faulty side of the power grid; exceeding this value is considered a threat to the frequency stability of the power grid.

[0077] (3) f n Rated frequency, the standard for China's power grid is 50Hz.

[0078] (4) K: An adjustment coefficient related to the DC system capacity, dimensionless, with a value range of 0.8 to 1.2. This coefficient is used to correct for the influence of factors such as the actual adjustability and response delay of the DC system when performing power support. When the DC system has sufficient adjustable capacity and fast response speed, K can take a larger value; otherwise, a smaller value is taken. The value of K can be preset according to the specific parameters of the DC system.

[0079] Figure 3 This is a schematic diagram illustrating the calculation principle of the aforementioned dynamic upper limit of power support. (See diagram below.) Figure 3 As shown, the coordination controller acquires the real-time frequency change rate of the non-faulty side of the power grid. and real-time equivalent inertial constant H eq Combined with the preset upper limit of the frequency change rate safety threshold RoC max Rated frequency f n and the adjustment factor K related to the DC system capacity, according to The dynamic upper limit of power support is dynamically calculated. This upper limit is dynamically adjusted according to the fluctuation of the real-time frequency change rate of the non-faulty side power grid: when the frequency change rate of the non-faulty side power grid is low, its disturbance immunity margin is large, and the dynamic upper limit of power support is correspondingly high; when the frequency change rate of the non-faulty side power grid is close to the safety threshold, the dynamic upper limit of power support is automatically reduced, thereby maximizing the available power support capability while ensuring the frequency safety of the non-faulty side.

[0080] Step S4: Power Support Limiting and Allocation

[0081] The coordination controller will support the total desired power. With the aforementioned power support dynamic upper limit value The two values ​​are compared, and the smaller value is taken as the actual power support amount. ,Right now:

[0082] = min( , )

[0083] The physical meaning of this step is as follows: when the expected support required by the faulty side does not exceed the upper limit that the non-faulty side can safely bear, the expected value is fully executed; when the expected support exceeds the safe bearing capacity of the non-faulty side, the actual support is limited to the safe upper limit, thereby providing the maximum possible power support while prioritizing the frequency safety of the non-faulty side itself.

[0084] Obtain the actual power support amount. Then, it is distributed among at least two DC transmission systems according to a preset allocation ratio to perform power support.

[0085] The at least two DC transmission systems include the VSC DC converter station and the LCC DC converter station. The preset allocation ratio is any one of the following:

[0086] The power support allocation ratio is determined based on the ratio of the short-circuit capacity of the VSC DC converter station and the LCC DC converter station connected to the power grid. For example, if the short-circuit capacity of the VSC DC connection point is S... VSC The short-circuit capacity of the LCC DC connection point is S. LCC， Then press S VSC :S LCC The power support capacity is allocated proportionally. The larger the short-circuit capacity, the stronger the power support capability of the converter station to the connected power grid, and the more support tasks it can undertake.

[0087] —Based on a pre-set priority order, one DC line is given priority to undertake the power support task, and the other DC line supplements the task when its adjustable capacity is insufficient. For example, the fast response capability of the VSC DC line can be used to undertake the initial support task, and when the required support exceeds the adjustable capacity limit of the VSC DC line, the remaining part is supplemented by the LCC DC line.

[0088] Step S5: Power Support Exit Determination

[0089] During power support execution, the frequency deviation of the fault-side power grid is continuously monitored. and the frequency change rate of the non-faulty side power grid .

[0090] When the frequency deviation of the fault-side power grid Return to within the preset frequency control dead zone (i.e., | >| ≤ dead zone value), and the frequency change rate of the non-faulty side power grid. Restored to within the preset safety threshold (i.e., | >| ≤ RoC max When this happens, the power support will gradually be deactivated.

[0091] The specific implementation method of the "gradual withdrawal" is as follows: gradually reduce the power support command of each DC transmission system according to the preset withdrawal rate (such as reducing the current support amount by 10% per second) until it reaches zero, so as to avoid the power support withdrawal from causing a secondary impact on the system.

[0092] Safety interlock control

[0093] During the execution of steps S3 to S4 above, safety interlocking control logic is also executed simultaneously. When any of the following conditions are detected, the power support dynamic upper limit limiting logic in steps S3 to S4 is immediately interlocked, switching to a preset fixed upper limit or full support mode:

[0094] (1) The frequency change rate monitoring signal of the non-faulty side power grid is abnormal or communication is interrupted—at this time, reliable data cannot be obtained. To prevent incorrect limiting due to constraint calculation failure, the measured value will automatically switch to a preset fixed upper limit value (such as 20% of the rated power) or directly switch to the full support mode.

[0095] (2) The frequency deviation of the fault-side power grid If the preset emergency threshold (e.g., ±0.5Hz) is exceeded, the emergency power support state is entered. At this time, the faulty side of the power grid faces a serious risk of frequency instability. Priority is given to ensuring the safety of the faulty side, and the safety constraints on the non-faulty side are temporarily lifted.

[0096] (3) When the VSC DC converter station or the LCC DC converter station experiences a blocking or commutation failure fault, the system enters a transient fault control mode. At this time, the DC system itself can no longer perform power support and blocking upper limit limit logic normally, and is taken over by other available DC or backup control strategies.

[0097] The above method implementation will be further explained below with reference to a specific operating scenario:

[0098] In a VSC / LCC DC parallel hybrid multi-feed system, the VSC DC connects to AC system 1 and AC system 2, and the LCC DC connects to AC system 2 and AC system 3, forming a three-region parallel hybrid multi-feed system. AC system 2 is the receiving-end system, and AC system 1 and AC system 3 are the sending-end systems.

[0099] At a certain moment, AC system 2 experienced a generator trip fault, and the frequency dropped from 50Hz to 49.6Hz.

[0100] Step S1: The frequency monitoring module detects the frequency deviation of AC system 2. = -0.4Hz (outside the ±0.05Hz dead zone), while simultaneously monitoring the real-time frequency change rate of AC system 1. = 0.3Hz / s (close to the preset safety threshold of 0.5Hz / s).

[0101] Step S2: The coordinating controller implements a preset frequency droop control strategy (droop coefficient K). droop = 2000MW / Hz), calculate the total expected power support. = 2000 × 0.4 = 800MW.

[0102] Step S3: The coordinating controller adjusts the frequency change rate of AC system 1 in real time. = 0.3Hz / s, combined with the current equivalent inertial constant H of AC system 1 eq =100000MW.s (obtained by weighted calculation of online unit information), take RoC max = 0.5Hz / s, f n = 50Hz, K= 1.0, calculate the dynamic upper limit of power support: = 1.0 × 10000 × (0.5 − 0.3) / 50 = 400MW. This calculation result shows that AC system 1 can only safely withstand a power surge of about 400MW under the current operating conditions. Exceeding this value will cause its frequency change rate to exceed the safe threshold of 0.5Hz / s.

[0103] Step S4: Coordination Controller Takes = min(800, 400) = 400MW is the actual power support amount implemented, and it is allocated according to the short-circuit capacity ratio of VSC DC and LCC DC (assumed to be 1:3), with VSC DC supporting 100MW and LCC DC supporting 300MW. This actual support amount is much smaller than the expected 800MW on the fault side, effectively preventing AC system 1 from experiencing its own frequency instability due to excessive support. This power support amount is allocated between VSC and LCC according to the short-circuit capacity ratio of the Sichuan-Chongqing-Hubei and East China-Hubei sides.

[0104] Step S5: As the generator set restarts and the frequency recovers, when the frequency of AC system 2 recovers to above 49.95Hz (within the dead zone), and AC system 1's... When the power rate drops back to 0.1 Hz / s (within the safety threshold), the power support gradually withdraws at a rate of 40 MW per second until it reaches zero.

[0105] Safety interlock control: When an abnormal frequency change rate signal is detected in the Sichuan-Chongqing power grid, it automatically switches to the fixed upper limit 600MW mode; when the frequency deviation of the Hubei power grid is detected to exceed the emergency threshold of 0.5Hz, it automatically switches to the full support mode; when a commutation failure is detected in the VSC or LCC, the third module automatically interlocks.

[0106] Figure 4This diagram compares the effects of the method of this invention with existing full support methods. Under the same fault scenario, although existing full support methods can restore the frequency of the faulty grid faster, the non-faulty grid experiences significant frequency shifts due to excessive power fluctuations. However, with the method of this invention, the actual amount of power support is limited to the dynamic upper limit that the non-faulty grid can safely withstand, the frequency shift of the non-faulty grid is controlled within a safe range, and the frequency of the faulty grid can still be effectively restored to a qualified range.

[0107] This method was validated in a VSC / LCC DC parallel hybrid simulation system built on the RTDS real-time digital simulation platform. The simulation step size was 50 microseconds, and the AC power grid adopted a four-level equivalent cross-section model.

[0108] Scenario A—Xiangfan Generator Unit Trip Fault in Hubei Power Grid: Simulates a loss of 635MW of generating power in the Hubei power grid due to a fault at the Xiangfan node. Simulation results comparing this method with the full support strategy show that: under the full support strategy, the frequency recovery of the Hubei power grid is faster, but the frequency of the Sichuan-Chongqing power grid shows a significant deviation of approximately 0.12Hz; the method of this invention limits the frequency deviation of the Sichuan-Chongqing power grid to within 0.08Hz. The frequency recovery speed of the Hubei power grid is slightly slower than that under the full support strategy, but it eventually recovers to 49.958Hz, which is still within the acceptable range.

[0109] Scenario B—Hubei Power Grid Load Shedding Fault: Simulates a 606MW load shedding in the Hubei power grid, leading to a frequency increase. Under the full support strategy, the Sichuan-Chongqing power grid experiences a significantly lower frequency due to a substantial power reduction. The method of this invention limits the VSC power reduction to control the frequency deviation of the Sichuan-Chongqing power grid within a smaller range.

[0110] Scenario C—Weak Inertia Operation Mode of the Sichuan-Chongqing Power Grid: This scenario simulates the Sichuan-Chongqing power grid operating in a low-inertia mode (equivalent inertia constant reduced to approximately 60% of normal value) due to maintenance of some generating units. In this mode, the disturbance rejection capability of the non-faulty side is significantly weakened. Full power support in this weak-inertia mode causes the frequency change rate of the Sichuan-Chongqing power grid to exceed the safety threshold of 0.5 Hz / s. The method of this invention automatically reduces the upper limit of power support, ensuring that the frequency change rate of the Sichuan-Chongqing power grid is always controlled within the safety threshold, successfully avoiding secondary frequency instability.

[0111] The simulation results above verify that the method of the present invention can still provide effective frequency support for the faulty side while taking into account the frequency safety of the non-faulty side, thus realizing bidirectional safety assurance in multi-region coordinated control.

[0112] This embodiment also provides an additional frequency coordination control device for a VSC / LCC DC parallel hybrid multi-feed system that takes into account the stability of the non-faulty side. This device embodiment corresponds to the above method embodiment; each step in the method embodiment is implemented by the corresponding functional module in this device embodiment. The device includes:

[0113] The frequency monitoring module is used to monitor the frequency information of the AC power grid connected to each converter station in the parallel hybrid multi-feeder system in real time, and to output at least the frequency deviation of the fault-side power grid where frequency disturbance occurs. and the frequency variation rate of at least one non-faulty side power grid ;

[0114] The expected support quantity calculation module is used to calculate the frequency deviation when the fault-side power grid is detected. When the preset frequency control dead zone is exceeded, the expected total power support required to suppress the frequency fluctuations of the fault-side power grid is calculated according to the preset frequency-active power supplementary control strategy. ;

[0115] The safety constraint calculation module is used to calculate the real-time frequency change rate of the non-faulty side power grid. The current disturbance immunity margin of the non-faulty side power grid is dynamically assessed, and the dynamic upper limit of power support that the non-faulty side power grid can safely withstand is calculated based on the assessment results. ;

[0116] The power limiting and allocation module is used to limit the total desired power support. With the aforementioned power support dynamic upper limit value The two values ​​are compared, and the smaller value is taken as the actual power support amount. and the actual power support amount executed. Power is allocated to at least two DC transmission systems according to a preset allocation ratio to provide power support.

[0117] The frequency recovery determination and exit module is used to determine the frequency deviation of the faulty power grid. The frequency change rate of the non-faulty side power grid is restored to within the preset frequency control dead zone. When the output power returns to within the preset safety threshold, the output power support will exit the command.

[0118] The method of this invention maintains frequency support on the faulty side while effectively preventing secondary frequency instability on the non-faulty side, achieving bidirectional safety assurance across multiple regions in a hybrid multi-feed system. Specifically, it has the following characteristics and effects:

[0119] (1) Achieve proactive safety protection for the non-faulty side power grid: For the first time, the real-time frequency change rate of the non-faulty side power grid is introduced as a hard constraint into the frequency coordination control closed loop, which fundamentally avoids the potential chain reaction of the existing "faulty side demand-driven" strategy that determines the power support amount only based on the frequency deviation of the faulty side, which may transfer the frequency risk from the faulty side to the non-faulty side.

[0120] (2) Dynamically adapt to changes in grid operation mode: The dynamic upper limit of power support is dynamically calculated based on the real-time equivalent inertial constant and frequency change rate of the non-faulty side grid, which can adapt to changes in grid operation mode (such as unit start-up and shutdown, and fluctuations in new energy output). Compared with the fixed limit method, it has better adaptability and safety.

[0121] (3) Maximize the support effect on the fault side within the safety constraints: By comparing the expected total power support with the dynamic upper limit of power support and taking the smaller value as the actual support amount, the maximum possible power support is provided under the premise of ensuring the safety of the non-fault side, thus achieving the optimal balance between the support effect on the fault side and the safety guarantee of the non-fault side.

[0122] (4) Compatible with existing frequency control strategies and highly feasible for engineering implementation: As the power command safety limiting link upstream of the existing frequency-active power supplementary control strategy, this invention can be easily embedded into the existing frequency coordination control architecture. Based on the existing VSC / LCC DC control and protection hardware platform, it can be implemented with only software upgrades and without increasing primary equipment investment.

[0123] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A frequency coordination control method for a VSC / LCC DC parallel hybrid system that takes into account the stability of the non-fault side, wherein the VSC / LCC DC parallel hybrid system includes at least one VSC DC converter station and at least one LCC DC converter station, the AC side of the VSC DC converter station and the AC side of the LCC DC converter station are connected in parallel with a relatively short electrical distance through an AC power grid, so that the AC power grids connected to each converter station can mutually influence each other's frequencies through DC power modulation, characterized in that... The method includes: Step S1: Monitor the frequency information of the AC power grid connected to each converter station in the parallel hybrid multi-feeder system in real time, and obtain the frequency deviation of the fault-side power grid where frequency disturbance occurs. and the frequency variation rate of at least one non-faulty side power grid ; Step S2, when the frequency deviation of the faulty power grid is detected When the frequency control dead zone is exceeded, the coordination controller calculates the total expected power support required to suppress the frequency fluctuations of the fault-side power grid according to the preset frequency-active power supplementary control strategy. ; Step S3, the coordination controller adjusts the real-time frequency change rate of the non-faulty side power grid. The current disturbance immunity margin of the non-faulty side power grid is dynamically assessed, and the dynamic upper limit of power support that the non-faulty side power grid can safely withstand is calculated based on the assessment results. ; Step S4, the coordination controller will adjust the total desired power support. With the aforementioned power support dynamic upper limit value The two values ​​are compared, and the smaller value is taken as the actual power support amount. and the actual power support amount executed. The power is distributed between at least two DC transmission systems according to a preset allocation ratio to provide power support. Step S5, when the frequency deviation of the fault-side power grid... The frequency change rate of the non-faulty side power grid is restored to within the preset frequency control dead zone. When the power level returns to within the preset safety threshold, the power support will gradually be deactivated.

2. The method according to claim 1, characterized in that, In step S3, the power support dynamic upper limit value Calculate using the following formula: ； In the formula, H eq RoC represents the real-time equivalent inertial constant of the non-faulty side of the power grid. max f is the preset upper limit of the safety threshold for the rate of change of frequency. n Where K is the rated frequency, and K is an adjustment factor related to the DC system capacity.

3. The method according to claim 2, characterized in that, The equivalent inertial constant H eq The results can be obtained through one of the following methods: weighted average calculation based on the inertia constant and rated capacity of each online generator unit in the non-faulty side power grid; online identification by monitoring the frequency change rate of the non-faulty side power grid under small power disturbances; or pre-setting based on the offline simulation calculation results of the non-faulty side power grid and updating the table according to changes in operating mode.

4. The method according to claim 1, characterized in that, In step S1, the frequency deviation of the fault-side power grid The real-time frequency of the fault-side power grid With rated frequency The difference between them; the rate of frequency change of the non-faulty side power grid It is the derivative of the real-time frequency of the non-faulty side power grid with respect to time.

5. The method according to claim 1, characterized in that, In step S4, the at least two DC transmission systems include the VSC DC converter station and the LCC DC converter station; the preset allocation ratio is determined by the ratio of the short-circuit capacity of the VSC DC converter station and the LCC DC converter station connected to the power grid, or by prioritizing one of the DC lines to undertake the power support task according to a preset priority order, and supplementing it by the other DC line when its adjustable capacity is insufficient.

6. The method according to claim 1, characterized in that, It also includes safety interlocking control: when any of the following conditions are detected, the power support dynamic upper limit limiting logic in steps S3 to S4 is interlocked, and the mode is switched to a preset fixed upper limit or full support mode: the frequency change rate of the non-faulty side power grid. The monitoring signal is abnormal or communication is interrupted; the frequency deviation of the fault-side power grid. If the preset emergency threshold is exceeded, the system enters emergency power support mode; or if the VSC DC converter station or the LCC DC converter station experiences a blocking or commutation failure fault.

7. A frequency coordination control device for a VSC / LCC DC parallel hybrid system that takes into account the stability of the non-fault side, wherein the VSC / LCC DC parallel hybrid system includes at least one VSC DC converter station and at least one LCC DC converter station, wherein the AC side of the VSC DC converter station and the AC side of the LCC DC converter station are connected in parallel via an AC power grid, and the electrical distance meets a preset proximity condition, characterized in that, The device includes: The frequency monitoring module is used to monitor the frequency information of the AC power grid connected to each converter station in the parallel hybrid multi-feeder system in real time, and to output at least the frequency deviation of the fault-side power grid where frequency disturbance occurs. and the frequency variation rate of at least one non-faulty side power grid ; The expected support quantity calculation module is used to calculate the frequency deviation when the fault-side power grid is detected. When the preset frequency control dead zone is exceeded, the expected total power support required to suppress the frequency fluctuations of the fault-side power grid is calculated according to the preset frequency-active power supplementary control strategy. ; The safety constraint calculation module is used to calculate the real-time frequency change rate of the non-faulty side power grid. The current disturbance immunity margin of the non-faulty side power grid is dynamically assessed, and the dynamic upper limit of power support that the non-faulty side power grid can safely withstand is calculated based on the assessment results. ; The power limiting and allocation module is used to limit the total desired power support. With the aforementioned power support dynamic upper limit value The two values ​​are compared, and the smaller value is taken as the actual power support amount. and the actual power support amount executed. Power is allocated to at least two DC transmission systems according to a preset allocation ratio to provide power support. The frequency recovery determination and exit module is used to determine the frequency deviation of the faulty power grid. The frequency change rate of the non-faulty side power grid is restored to within the preset frequency control dead zone. When the output power returns to within the preset safety threshold, the output power support will exit the command.

8. The apparatus according to claim 7, characterized in that, The security constraint calculation module includes: The equivalent inertia constant acquisition submodule is used to acquire the real-time equivalent inertia constant H of the non-faulty side power grid. eq ; The upper limit calculation submodule is used to calculate the power support dynamic upper limit value according to the following formula: ； In the formula, H eq RoC represents the real-time equivalent inertial constant of the non-faulty side of the power grid. max f is the preset upper limit of the safety threshold for the rate of change of frequency. n Where K is the rated frequency, and K is an adjustment factor related to the DC system capacity.

9. The apparatus according to claim 7, characterized in that, It also includes a safety interlocking control module, used to interlock the power support dynamic upper limit limiting logic in the safety constraint calculation module and the power limiting and allocation module when any of the following conditions are detected, switching to a preset fixed upper limit or full support mode: the frequency change rate of the non-faulty side power grid. The monitoring signal is abnormal or communication is interrupted; the frequency deviation of the fault-side power grid. Exceeding the preset emergency threshold; The VSC DC converter station or the LCC DC converter station experienced a lockout or commutation failure fault.

10. The apparatus according to claim 7, characterized in that, The parallel connection between the VSC DC converter station and the LCC DC converter station can be any one or a combination of the following: the sending end or receiving end of the VSC DC converter station is connected to the same AC bus as the rectifier station or inverter station of the LCC DC converter station; the VSC DC converter station and the LCC DC converter station are connected to the same voltage level bus in the same AC hub substation; the VSC DC converter station and the LCC DC converter station are electrically connected through a short-distance AC tie line.

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