A VSC-MTDC frequency modulation method and device considering dynamic constraints

Abstract

The application provides a VSC-MTDC frequency modulation method and device considering dynamic constraints, and relates to the technical field of power system frequency control. The method comprises the following steps: a frequency response model of a VSC-MTDC containing new energy is established; boundary constraints of the frequency response model in each period during the response process are determined according to predetermined requirements; when a load disturbance occurs, the VSC and energy storage of each region are adjusted in response when it is detected that the frequency deviation of the VSC-MTDC in each region exceeds a threshold value η 0 or exceeds the boundary constraints in the corresponding period; VSC driving events of each region are determined; after the VSC driving events occur, adaptive droop control of the VSC is performed in each period and in each region according to the boundary constraints of each period; energy storage driving events of each region are determined; after the energy storage driving events occur, the droop coefficient of the energy storage is adjusted in each period according to the period in which the energy storage is currently located. The application can make the frequency meet the dynamic constraints, achieve good frequency modulation effect, and effectively control the frequency recovery to meet the fine frequency modulation requirements.

Description

Technical Field

[0001] This invention relates to the field of power system frequency control technology, and specifically to a VSC-MTDC frequency regulation method and apparatus that takes into account dynamic constraints. Background Technology

[0002] Voltage Source Converter Based Multi-Terminal Direct Current (VSC-MTDC) systems enhance the flexibility of power transmission and sharing between power grids, and are widely used in large-scale grid integration of renewable energy. However, with the large-scale integration of frequency-sensitive equipment into the grid, the overall frequency regulation capability of the system decreases. In the event of high-power disturbances such as DC blocking, the frequency drop depth is easily aggravated, and even frequency security-damaging accidents may occur. In VSC-MTDC interconnection systems, this impact is even more pronounced across multiple regions.

[0003] To determine the permissible range of frequency deviation, according to relevant industry standards, the frequency offset is generally ±0.2Hz, which can be relaxed to ±0.5Hz when the system capacity is small. Furthermore, the relevant industry standards clearly state that "the assessment of frequency deviation should consider both duration and frequency of occurrence. For short-lived, occasional frequency deviations, if the system can recover quickly and does not affect the normal operation of the equipment, it can be considered to meet the standard requirements."

[0004] For example, some industry standards stipulate that "when a sudden load change causes a momentary frequency exceedance, the frequency deviation is allowed to be within ±0.5Hz, but the duration should be ≤5 seconds," and "when a load surge causes a momentary frequency exceedance of ±0.2Hz, if it recovers to the acceptable range within 10 seconds, it is considered a normal fluctuation." These industry standards indicate that a relatively large frequency deviation caused by a load surge is allowed to exist for a certain period of time, and if it recovers to within the steady-state threshold within the specified time, it is considered to meet the frequency modulation conditions. Therefore, the dynamic constraints of the frequencies in each region of VSC-MTDC are as follows: Figure 1 As shown, during frequency fluctuations, the frequency trajectory falling within the blue area satisfies the condition. Figure 1 The dynamic constraints shown will better meet the requirements for safe operation of the power grid.

[0005] However, current research does not have specific requirements for frequency modulation recovery. The currently known allowable range of frequency deviation is ±0.2~0.5Hz, and the frequency deviation evaluation index is relatively general. Furthermore, frequency modulation is also relatively general, and it is uniformly based on a single index that its value does not exceed ±0.2Hz, without considering the differences in the tolerable frequency deviation in different time ranges. Summary of the Invention

[0006] The purpose of this invention is to provide a VSC-MTDC frequency regulation method and apparatus that considers dynamic constraints. This invention addresses the problems of existing power grid frequency regulation methods, which uniformly use a single index of frequency deviation not exceeding ±0.2Hz without considering the differences in the frequency deviation that the power grid can tolerate within different time ranges, resulting in poor frequency regulation performance. This invention enables the frequency to meet dynamic constraints, achieves good frequency regulation performance, effectively controls frequency recovery to meet the requirements of refined frequency regulation, and has important reference value for the frequency security of power grids at all ends.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a VSC-MTDC frequency modulation method considering dynamic constraints, comprising:

[0008] Establish a frequency response model for VSC-MTDC with new energy sources; VSC-MTDC is a multi-terminal flexible DC transmission system based on voltage source converters;

[0009] Determine the boundary constraints of the frequency response model at each time point during the response process according to the predetermined requirements;

[0010] When a load disturbance occurs, if the frequency deviation of each region of the VSC-MTDC is detected to exceed the threshold, η If the boundary constraints are exceeded within the corresponding time period, the VSC and energy storage response of each region will be adjusted accordingly.

[0011] Determine the VSC driving events for each region. When a VSC driving event occurs, perform time-segmented and region-specific adaptive droop control based on the boundary constraints of each time period.

[0012] Identify the energy storage-driven events in each region. When an energy storage-driven event occurs, adjust the energy storage droop coefficient according to the current time period.

[0013] According to the present invention, a VSC-MTDC frequency regulation method considering dynamic constraints is provided. The VSC-MTDC includes at least one new energy generator set regional terminal and multiple thermal power unit regional terminals. Each new energy generator set regional terminal includes a new energy generator set and a VSC. Each thermal power unit regional terminal includes a thermal power unit, energy storage, a VSC and a controller.

[0014] According to the VSC-MTDC frequency modulation method considering dynamic constraints provided by the present invention, when a load disturbance occurs, if the rate of change of frequency deviation is greater than a threshold... η 1. Adjust the droop coefficient of the corresponding area to change the VSC from traditional droop control to improved droop control; the expression for the frequency deviation change rate is:

[0015]

[0016] In the formula, This refers to the frequency variation of the AC power grid. H The inertia coefficient, The load disturbance experienced by this area, This refers to the power change of a VSC connected to the AC power grid. The change in power of the connected energy storage; The power-frequency gain of the speed controller. The threshold value for the speed controller; is the time constant of the steam turbine.

[0017] According to the present invention, a VSC-MTDC frequency modulation method considering dynamic constraints is provided. When a load disturbance occurs, if the VSC participation in a certain area is detected to be less than a threshold, the method will be implemented accordingly. η When the value is 2, the VSC output power equalization process is performed in that area.

[0018] According to the VSC-MTDC frequency modulation method considering dynamic constraints provided by the present invention, the expressions for VSC drive events and energy storage drive events are as follows:

[0019] (9)

[0020] In the formula, For the last driving moment, This is the moment of this drive; n The total number of VSCs. , These are VSC drive events and energy storage drive events, respectively. for t Time of the first i Rate of change of frequency deviation of each VSC; For the first j The absolute value of the difference between the participation of each VSC and the average participation of all VSCs. = / For the first j The participation rate of each VSC For the first time after load disturbance j The unbalanced power shared by each VSC This is the first time before this disturbance. j Power margin of each VSC This represents the average participation rate across all VSCs.

[0021] According to the VSC-MTDC frequency modulation method considering dynamic constraints provided by the present invention, the calculation formula for the frequency droop control coefficient in the improved droop control is as follows:

[0022] (10)

[0023] In the formula, Let be the frequency droop control coefficient for the i-th VSC improved droop control at time t. β i For the i-th VSC conventional droop control, the frequency droop control coefficient is ; For time t, the first i Power sharing factor of each VSC For time t, the first i Frequency deviation factor for each region; For the first i Power margin of each VSC; For the first i The maximum allowable frequency deviation value for each region; This indicates the deviation of the actual operating AC frequency from the rated value; and This is a user-defined constant.

[0024] According to the VSC-MTDC frequency modulation method considering dynamic constraints provided by the present invention, the energy storage constraints are as follows:

[0025] (11)

[0026] (12)

[0027] (13)

[0028] In the formula, This represents the change in energy storage output power. This represents the maximum change in output power of the energy storage. This represents the initial state of charge of the energy storage. P is the rated capacity of energy storage. B (t) represents the energy storage output power at time t; SOC (t) represents the state of charge of the stored energy at time t. This represents the minimum state of charge allowed for energy storage. This represents the maximum state of charge allowed for energy storage.

[0029] According to the present invention, a VSC-MTDC frequency regulation method considering dynamic constraints determines the energy storage drive events in each region, including:

[0030] set up The moment when the load disturbance occurs. For the current moment, , and These are the required frequency safety thresholds for different time periods after a load disturbance occurs. , and Restore the frequency to , and The above are the maximum allowed times;

[0031] Divide the time after the load disturbance occurs into 0~ , ~ , ~ , ~ Four time periods;

[0032] Assumption The time period after the load disturbance occurs ~ Inside, The frequency measured at time is Then the frequency deviation Represented as:

[0033] (14)

[0034] Thus obtain t k Recovery time deviation at any moment Represented as:

[0035] (15)

[0036] In the formula, ROCOF ( t c () represents the rate of change of frequency deviation at the moment the load disturbance occurs;

[0037] Therefore, when equation (16) is satisfied, the energy storage-driven event occurs;

[0038] (16).

[0039] According to the present invention, a VSC-MTDC frequency modulation method considering dynamic constraints is provided for time-sharing adjustment of the energy storage droop coefficient, including:

[0040] when After a load disturbance occurs at a certain time, at the current time In +(0~ During the specified time period, the energy storage will not operate; at the current moment... In +( ~ During the time period, if The system detects at all times that the energy storage drive event does not meet the requirements of the dynamic constraints, as shown in equation (19):

[0041] (19)

[0042] In the formula, for The difference between the frequency constraint threshold and the actual value at any given time;

[0043] This indicates that emergency power support is needed for energy storage, and the energy storage droop coefficient needs to be adjusted, as shown in equation (20):

[0044] (20)

[0045] In the formula, 0~ after the load disturbance occurs Adjustment coefficient for time period After load disturbance ~ Adjustment coefficient for time period; current time In +( ~ If detected during the specified time period Not exceeding the threshold If the adjustment coefficient remains unchanged, then the adjustment coefficient remains unchanged; if it is detected Threshold exceeded If so, the adjustment coefficient needs to be increased, as shown in equation (21):

[0046] (twenty one)

[0047] In the formula, After load disturbance ~ Adjustment coefficient for time period; current time In +( ~ If detected during the specified time period Not exceeded If the adjustment coefficient remains unchanged, then the adjustment coefficient remains unchanged; if it is detected Threshold exceeded If so, the adjustment coefficient needs to be increased, as shown in equation (22):

[0048] (twenty two)

[0049] In the formula, After load disturbance ~ Adjustment coefficient for time period.

[0050] In a second aspect, the present invention provides a VSC-MTDC frequency modulation device considering dynamic constraints, comprising:

[0051] A modeling unit is established to build a frequency response model of a VSC-MTDC system incorporating new energy sources; VSC-MTDC is a multi-terminal flexible DC transmission system based on voltage source converters.

[0052] The determining unit is used to determine the boundary constraints of the frequency response model at each time period during the response process according to predetermined requirements;

[0053] The response unit is used to respond when a load disturbance occurs and the frequency deviation of each region of the VSC-MTDC exceeds a threshold. η If the boundary constraints are exceeded within the corresponding time period, the VSC and energy storage response of each region will be adjusted accordingly.

[0054] The first frequency modulation unit is used to determine the VSC driving events in each region. When a VSC driving event occurs, it performs time-segmented and region-segmented adaptive droop control of VSC according to the boundary constraints of each time period.

[0055] The second frequency regulation unit is used to determine the energy storage driving events in each region. When an energy storage driving event occurs, the energy storage droop coefficient is adjusted according to the current time period.

[0056] This invention has at least the following technical effects:

[0057] This invention provides a VSC-MTDC frequency regulation method and apparatus considering dynamic constraints. It sets the frequency safety domain based on the magnitude of the power system frequency deviation and its tolerable time constraints. Then, based on threshold settings, a more refined frequency regulation control method is formulated. VSC driving events are designed based on the frequency deviation change rate of each VSC and its participation in sharing unbalanced power. When a VSC driving event occurs, the VSC adopts droop control with time-sharing and region-sharing adaptive parameters that meet boundary constraints. When the disturbance is large enough to reach the established BESS driving event, emergency energy is supplied to the system from energy storage for time-sharing auxiliary frequency regulation. This method has advantages such as high reliability and frequency compliance with dynamic constraints. Based on the set boundary conditions, a good frequency regulation effect is achieved through the combination of different energy sources. Compared with other existing methods, this invention effectively controls frequency recovery to meet the requirements of refined frequency regulation, providing important reference value for the frequency security of the power grid at each end of the system. Attached Figure Description

[0058] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0059] In the attached diagram:

[0060] Figure 1 Dynamic constraint diagrams for the frequencies of each region of VSC-MTDC;

[0061] Figure 2 This is a schematic diagram of the structure of the VSC-MTDC of the present invention;

[0062] Figure 3 This is a block diagram of the dynamic drive control of the VSC-MTDC of the present invention;

[0063] Figure 4 This is a frequency evolution trajectory diagram under different scenarios of the present invention;

[0064] Figure 5 This is a per-unit frequency dynamic constraint diagram according to a specific embodiment of the present invention;

[0065] Figure 6 This is a frequency variation curve under small perturbations according to a specific embodiment of the present invention;

[0066] Figure 7 This is a power variation curve under small disturbances according to a specific embodiment of the present invention;

[0067] Figure 8 This is a frequency variation curve under a large disturbance according to a specific embodiment of the present invention;

[0068] Figure 9 This is a power variation curve under a large disturbance according to a specific embodiment of the present invention;

[0069] Figure 10 The flowchart shows the VSC-MTDC frequency modulation method considering dynamic constraints in this invention. Detailed Implementation

[0070] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0071] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0072] To prevent excessive frequency drops and equipment disconnection (converter station overload shutdown), the power grid urgently needs a precise and stringent frequency recovery criterion that considers dynamic constraints for frequency recovery after disturbances. Therefore, this invention establishes dynamic frequency boundary constraints to ensure that each region of the VSC-MTDC can safely traverse specified frequency deviations after a power shortage.

[0073] This invention, under frequency regulation requirements, is designed based on the reliability and safety regulations for power systems described in the relevant industry standards, taking into account the frequency thresholds and allowable ranges of steady-state frequency deviations in each standard. The design objective of the dynamic constraints is: after a fault, the system frequency must recover to above the required threshold within an allowable time period, and during the time-segmented recovery period, it must not exceed the upper threshold. Since the system withstand time varies under different frequency deviations—a smaller frequency deviation allows for a longer allowable adjustment time, while a larger frequency deviation requires a shorter allowable adjustment time—different frequency constraints need to be set during frequency regulation. The designed time-segmented dynamic boundary constraints are as follows: Figure 2 The blue area is shown in the image. Specifically, the boundary settings for dynamic constraints are: frequency recovery to 49.25Hz within 5 seconds after a fault, to 49.5Hz within 10 seconds, and to 49.8Hz within 30 seconds; the upper limit of the frequency threshold is 50.4Hz within 10 seconds after a fault, and 50.2Hz after 10 seconds, with the final frequency stabilizing between 49.8 and 50.2Hz. Figure 1 As shown.

[0074] Please see Figure 10 This invention provides a VSC-MTDC frequency modulation method considering dynamic constraints, comprising:

[0075] Step 1: Establish a frequency response model for VSC-MTDC including new energy sources;

[0076] Step 2: Determine the boundary constraints of the frequency response model for each time period during the response process according to the predetermined requirements;

[0077] It should be noted that current research on achieving mutual support for frequency regulation among different regional power grids in VSC-MTDC mainly focuses on VSC master-slave control, voltage margin control, and DC voltage droop control with additional frequency. However, under such control strategies, the droop coefficient of the converter station is designed according to its capacity, and its parameters are fixed and cannot be updated according to the real-time status of the system. Furthermore, when large unbalanced power fluctuations occur, there may be inconsistent participation, leading to serious consequences such as some converter stations reaching full load and being shut down, causing the frequency trajectory of the region to directly jump out of the designed boundary constraints. Therefore, this problem needs to be addressed.

[0078] With the increasing penetration of new energy sources, the frequency characteristics of the system are becoming more complex. Relying solely on regional thermal power units for frequency regulation is insufficient under certain circumstances. Considering that battery energy storage (BESS) offers rapid response and convenient installation, it is one of the most widely adopted and effective measures in mainstream frequency recovery control. Therefore, BESS is used as an auxiliary frequency regulation energy source for emergency frequency support. When BESS participates in grid frequency regulation, virtual droop and virtual inertial control are currently the main mainstream methods. Based on these two methods, to achieve better frequency regulation performance, some studies combine virtual inertial and droop control to participate in grid frequency regulation in a comprehensive control manner, optimizing frequency response performance. However, in existing frequency regulation strategies, BESS operates at its maximum output capacity at any given time, i.e., a full compensation strategy. This not only shortens the lifespan of BESS and increases the installed capacity, but also makes it easy for the BESS's State of Charge (SOC) to reach its upper and lower limits, leading to blockage and inability to output power.

[0079] This invention takes a multi-port VSC-MTDC system with new energy sources as the research object. The VSC-MTDC includes at least one new energy generator set regional terminal and multiple thermal power unit regional terminals. Each new energy generator set regional terminal includes a new energy generator set and a VSC. Each thermal power unit regional terminal includes a thermal power unit, energy storage, a VSC and a controller. Figure 2 In this context, new energy is represented by wind power. The system has one wind turbine area terminal, denoted as S1, and multiple wind turbine area terminals, denoted as S2~S1. n This indicates that wind power operates in a fixed MPPT (Maximum Power Point Tracking) mode, supplying electricity to the grid. When load disturbances occur at other terminals, they collectively regulate the frequency. Each thermal power unit is equipped with an auxiliary control device at its regional end, including a controller and energy storage, such as... Figure 2 As shown.

[0080] Step 3: When a load disturbance occurs, if the frequency deviation of each region of the VSC-MTDC is detected to exceed the threshold... ηIf the boundary constraints are exceeded within the corresponding time period, the VSC and energy storage response of each region will be adjusted accordingly.

[0081] Specifically, when a load disturbance occurs, the grid dispatching and control system (GDACS) detects the frequency deviation in the frequency changes at each terminal of the VSC-MTDC system. Does it exceed the threshold? η 0. The determination of whether the main frequency regulation system is started is shown in equation (1).

[0082] (1)

[0083] If Δ for each region f None exceeded the threshold η If 0 = 0.033Hz, then no operation is performed; if If the frequency exceeds the threshold by 0.033Hz, it indicates that a load disturbance has occurred in a certain area. At this time, it is necessary to activate the pre-designed drive event control so that VSC and BESS can participate in frequency regulation in a time-sharing and segmented strategy, as shown in equation (2):

[0084] (2)

[0085] In the formula, To compensate for the power output of the system in response to load disturbances, and These are the weighting factors for VSC and BESS, respectively. The power output of the converter station to cope with load disturbances. This refers to the power output of the energy storage station to cope with load disturbances. Considering that the control of the converter station is more direct and convenient, the power output control of the VSC is carried out throughout the entire frequency regulation process. Considering the cost of energy storage installation and the operating capacity, if it is put into operation in all stages, the required capacity is large, while if the capacity is small, putting it into operation in all stages will result in a deep charging and discharging of the energy storage, thus affecting its service life. Therefore, in order to reduce costs and achieve better regulation effect, the energy storage only provides emergency power support when the boundary constraints are not met during the frequency recovery phase after a disturbance.

[0086] Step 4: Determine the VSC driving events for each region. When a VSC driving event occurs, perform time-segmented and region-specific adaptive droop control based on the boundary constraints of each time period.

[0087] Step 5: Determine the energy storage driving events in each region. When an energy storage driving event occurs, adjust the energy storage droop coefficient according to the current time period.

[0088] Specifically, in order to meet the set boundary constraints during frequency modulation, the control strategy of the MTDC's main frequency modulation system needs to be designed more precisely and specifically after startup. The logic block diagram for fine-tuning the control strategy is as follows: Figure 3 As shown. Figure 3 In the middle, there is n Each region, drive controller 2~( n +1) Real-time detection of signals such as frequency and power deviation at each station and updating of data in memory, followed by event-driven conditional judgment. When a frequency deviation exceeding a threshold is detected... η When the boundary constraints are exceeded at time 0 or within the corresponding time period, the response adjustments for VSC and BESS are performed respectively. The VSC of each region is controlled by a unified drive controller 1. When the drive conditions of VSC are met, the virtual switch... K i The original traditional droop control was switched to an improved droop control, using time-sharing and segmented adaptive droop control to meet the frequency modulation requirements of boundary constraints; otherwise, traditional droop control was used to save resources and achieve the control objective as quickly as possible. The BESS for each region is controlled by drive controllers 2~( n +1) Perform independent control; after the drive conditions are met, the virtual switch... K Bi The system is closed, and energy storage provides support for emergency frequencies. The specific designs of each drive controller are shown below:

[0089] 1. Design of VSC-driven events

[0090] 1) Setting Driving Conditions: Considering boundary constraints, the driving conditions are designed. Figure 3 After modeling the system shown, the relationships between the variables during frequency regulation in the AC power grid are shown in equations (3) to (5):

[0091]

[0092] In the formula, This refers to the frequency variation of the AC power grid. H The inertia coefficient, D The damping coefficient is... This represents the change in mechanical power input to the generator. The load disturbance experienced by this area, This refers to the power variation of a converter station (VSC) connected to an AC power grid. The change in power of the connected energy storage; This represents the change in the position deviation of the speed governor. For generator power deviation, The power-frequency gain of the speed controller. The threshold value for the speed controller; is the time constant of the steam turbine.

[0093] After simplifying equations (3) to (5), we obtain the load disturbance. With frequency change The relationship is

[0094] (6)

[0095] Pair both sides of equation (6) t Taking the derivative, we obtain the rate of change of frequency deviation. ROCOF ) for:

[0096] (7)

[0097] From the observation of equation (4), we can see that ROCOF With Δ P L They are directly proportional, therefore they can be obtained through ROCOF The magnitude reflects the load power fluctuation in each region. When the system detects the i-th region at time t, ROCOF i(t) Exceeding the set threshold η When the threshold is 1 (within which the boundary constraint will definitely not be exceeded, as obtained through experiments), as shown in formula (5),

[0098] (8)

[0099] If a significant disturbance occurs in the system, the traditional droop control will not meet the boundary constraints. In this case, the controller needs to immediately calculate and update the data in the storage area, start the controller 1, close the virtual switch, and switch to the improved droop control strategy. Based on the boundary constraints and the real-time operation of the system, the droop coefficient is dynamically adjusted to meet the boundary conditions.

[0100] Secondly, considering that the system is a multi-terminal flexible DC system, when one VSC becomes fully loaded and goes out of operation, it will greatly increase the regulation pressure on the entire system, and it is very likely that some areas will directly jump out of the boundary constraints. Therefore, in order to ensure that the system meets the boundary conditions at each stage, the droop coefficient must be adjusted in time periods according to the maximum allowable frequency deviation in each time period, while ensuring that the converter station does not become fully loaded during frequency regulation. Regarding the problem of VSCs potentially becoming fully loaded and jumping out of the boundary constraints, considering that the reason why some VSCs in the MTDC system are prone to full load is that after a disturbance, the participation of each VSC in bearing the unbalanced power is inconsistent, that is, with a fixed droop coefficient set according to the VSC capacity, VSCs with less available power headroom will share more unbalanced power, while VSCs with more available power headroom will share less unbalanced power. Therefore, a threshold is set. η 2. When the converter station participation rate is detected to be less than the threshold η If condition 2 is met, the driving condition is satisfied, and the output power of the converter station is balanced to avoid problems such as some converter stations being fully loaded and deteriorating the frequency regulation performance of the system, which could lead to the failure to break the boundary constraints.

[0101] Set the conditions for the occurrence of the two driving events mentioned above: 1 for driving when a driving event occurs, and 0 for not driving when no driving event occurs; the total number of driving events equals the sum of the two events. If the sum is greater than 0, the controller will perform a driving operation; if the sum is 0, no driving operation will be performed.

[0102] Meanwhile, let the last driving time be... t (k-1) The timing of this drive is t (k) The driving events in the frequency modulation stage of the design system are shown in equation (9):

[0103] (9)

[0104] In equation (9), n This represents the total number of converter stations. f i(t) , g j(t) These are VSC drive events and energy storage drive events, respectively. ROCOF (t) VSCi for t Time of the first i Frequency deviation variation rate of each converter station η 1 is the frequency change rate threshold for switching control strategies at the converter station; φ j(t) For the first j The absolute value of the difference between the participation rate of an individual converter station and the average participation rate of all converter stations. η 2 is the threshold for determining whether the participation of each VSC is consistent. Φ j(t) = P j(t) / P AvHR,j,t(k) For the first j The participation rate of each converter station, among which P j(t) For the perturbation of the first j The unbalanced power shared by each converter station P AvHR,j,t(k) The first disturbance stored in the drive controller before this disturbance j The power margin of each converter station Z (t) This represents the average participation rate across all converter stations.

[0105] When the disturbance is small enough to satisfy the boundary constraints and the participation of each converter station is high Φ 1(t)， Φ 2(t)… Φ n(t) When the difference is small, no driving operation is performed; when the disturbance is large, the boundary constraints will not be satisfied and the participation of each converter station will be affected. Φ 1(t)， Φ 2(t)… Φ n(t) If the differences are small, the second driving condition will not be met. Whether to perform a driving operation depends on the first condition. ROCOF i(t) Whether the driving conditions for detection are met; when the disturbance is small and the participation of individual converter stations is low. Φ i(t) When the participation rate differs significantly from the overall average participation rate of converter stations, only that converter station is driven, and corresponding operations are performed; when the disturbance is large and the participation rate of each converter station is low... Φ 1(t)， Φ 2(t)… Φ n(t) If the difference is significant, the second driving condition is met, and the driving operation is performed directly. Furthermore, if the power margin of a certain area does not meet the standard for sharing the unbalanced power, meaning the converter station is already operating at maximum power output, the area will not participate in sharing the unbalanced power burden and will switch to constant power control to maintain stable operation of the area.

[0106] 2) Improved design of droop control after the occurrence of a driving event

[0107] After a VSC drive event occurs, based on the boundary constraints of each time period and considering the actual operating conditions such as the capacity and power margin of each converter station and frequency changes in each region, time-sharing and segmented adaptive VSC droop control is implemented. The formula for calculating the frequency droop control coefficient of the improved strategy is as follows:

[0108] (10)

[0109] In the formula, Let be the frequency droop control coefficient for the i-th VSC improved droop control at time t. β i For the i-th VSC conventional droop control, the frequency droop control coefficient is ; For time t, the first i Power sharing factor of each VSC For time t, the first i Frequency deviation factor for each region; = Pref,i(t) -| P i(t) |, for the first i Power margin of each VSC; Δ f max i For the first i The maximum allowable frequency deviation for each region is determined based on the frequency setting value used during boundary constraint operation; here, Δ is taken as the value. f max1 =(50-49.25)Hz; Δ f max2 =(50-49.5)Hz; Δ f max3 =(50-49.8)Hz; while the upper limit of the frequency dynamic constraint Δ f max_on =(50-50.2)Hz; Δ f i(t) This indicates the deviation of the actual operating AC frequency from the rated value; α 1 and λ i These are custom constants used to correct the adaptive droop coefficients corresponding to the respective factors, ensuring that their deviation from the initial droop coefficients in steady state is not too large.

[0110] As can be seen from equation (10), the improved strategy takes into account the boundary constraints and the real-time operating status of the converter station to adjust the droop coefficient, so that the period with large frequency deviation and the converter station with low power margin have a higher droop coefficient. This ensures that the converter station that is already very close to the operating limit will not try to share the power load required by the disturbed converter station, while the converter station with high power margin will bear more unbalanced power, so that the operation of the frequency curve is within the boundary constraints and the safety of the system is guaranteed.

[0111] 2. Design of BESS-driven events

[0112] 1) BESS's event-driven architecture: Based on Figure 1 The dynamic constraint standard shown takes into account frequency deviation. and frequency recovery time These can respectively reflect the degree of deviation and the recovery ability of frequency evolution, therefore, they are selected here. and To assess the safety of the frequency dynamic trajectory after disturbance and use it as input to the energy storage controller; considering that the energy storage battery achieves frequency regulation power control by changing its charging and discharging process, common control methods include virtual inertial control and virtual droop control, which are effective in suppressing initial frequency degradation and reducing steady-state frequency deviation, respectively. Since the energy storage here is mainly for emergency power support, the droop coefficient, which is closely related to the steady-state operation of the energy storage, is selected as the output control for the energy storage battery. The constraints that this energy storage must satisfy during frequency regulation are:

[0113] (1) Constraints of energy storage batteries:

[0114] (11)

[0115] In the formula, This represents the change in energy storage output power. This represents the maximum change in output power for energy storage.

[0116] (2) Energy storage SOC constraint: Energy storage system SOC The state calculation formula is as follows:

[0117] (12)

[0118] In the formula, This represents the initial state of charge of the energy storage. P is the rated capacity of energy storage. B (t) represents the stored energy output power at time t. SOC (t) represents the state of charge of the energy storage at time t. During actual operation of the energy storage system, it is necessary to ensure... Within a suitable range, that is:

[0119] (13)

[0120] In the formula, This represents the minimum state of charge allowed for energy storage. This represents the maximum state of charge allowed for energy storage.

[0121] Figure 4 These represent typical frequency evolution trajectories under different scenarios. For example... Figure 4 As shown, the light blue area represents the allowable range of frequency recovery based on dynamic constraints. The moment when the load disturbance (fault) occurs; The current time; , and These are the required frequency safety thresholds for different time periods after a load disturbance occurs. , and Restore the frequency to , and Based on the above maximum allowable time, the time after a load disturbance occurs is divided into 0~ , ~ , ~ , ~ Four time periods; and by Figure 4 As can be seen, trajectory 1 represents an unstable frequency phenomenon where the frequency remains at a low level after the fault and cannot be recovered; trajectory 2 represents a delayed frequency recovery situation where the frequency is below the boundary for certain periods of time; trajectory 3 is a safe frequency recovery process that meets the frequency recovery criterion (FRC). What energy storage needs to do is to restore trajectory 2 and trajectory 3 to within the blue threshold.

[0122] Assumption The time period after the load disturbance occurs ~ Within that range, the measured frequency was Then the frequency deviation It can be represented as:

[0123] (14)

[0124] according to Moment and its first-order trajectory ,get t k Recovery time deviation at any moment It can be represented as:

[0125] (15)

[0126] In the formula, and For the process quantity being calculated, ROCOF ( t c () represents the rate of change of frequency deviation at the moment the load disturbance occurs. It should be noted that when... The time is within the time period 0~ During this period, due to its relatively short duration, a large frequency deviation is generally permissible, therefore energy storage is not controlled during this time period; when Time in time period ~ and ~ When there is a segmentation between them, according to the segmentation characteristics of the boundary constraints, and Should be based on , and , calculate.

[0127] and It can be used to determine the severity of the current frequency trajectory evolution state. >0 and A value >0 indicates that the current frequency is below the specified safety threshold and the predicted recovery time exceeds the allowed recovery time. Therefore, the necessary condition for triggering the energy storage drive event is set as follows: and As shown in equation (16):

[0128] (16)

[0129] When all of the above conditions are met, the controller's triggering mechanism is activated to generate and send instructions to the corresponding energy storage controller.

[0130] 2) Design of energy storage droop coefficient after the driving event occurs

[0131] When the i When the energy storage controller in each area is triggered, its output power is:

[0132] (17)

[0133] In the formula, For the first i Inertia coefficient of each region For the first i The energy storage droop coefficient for each region. Considering that energy storage is an emergency frequency regulation support during the frequency recovery phase, the segmented adjustment is only performed on the energy storage droop coefficient, as shown in equation (18):

[0134] (18)

[0135] In the formula, These are the piecewise adjustment coefficients set according to the constraints. This is the initial droop coefficient.

[0136] when After a load disturbance occurs at a certain time, at the current time In +(0~ During the specified time period, energy storage will not operate; at the current time... In +( ~ During the specified time period, if the energy storage controller operates within that time period... The system detects at all times that the energy storage drive event does not meet the requirements of the dynamic constraints, as shown in equation (19):

[0137] (19)

[0138] This indicates that energy storage needs to be deployed for emergency power support. According to The difference between the frequency constraint threshold and the actual value at any given time The droop coefficient is adjusted based on the difference between the time exceeding the threshold and the allowed time, as shown in equation (20).

[0139] (20)

[0140] In the formula, 0~ after the load disturbance occurs Adjustment coefficient for time period After load disturbance ~ Adjustment factor for the time period. Current time. In +( ~ During the specified time period, if the controller detects... Not exceeding the threshold If the controller detects... Threshold exceeded In other words, the energy storage capacity is insufficient. + When the frequency measurement value reaches the boundary that satisfies the dynamic constraint, the adjustment coefficient needs to be increased, as shown in equation (21):

[0141] (twenty one)

[0142] In the formula, After load disturbance ~ Adjustment coefficient for the time period. Similarly, for the current moment... In +( ~ If the controller detects during a certain time period... Not exceeded If the controller detects... Threshold exceeded In other words, the energy storage capacity is insufficient. + When the frequency measurement value reaches the boundary that satisfies the dynamic constraint, the adjustment coefficient needs to be increased, as shown in equation (22):

[0143] (twenty two)

[0144] In the formula, After load disturbance ~ Adjustment coefficient for time period.

[0145] At this point, BESS control in one round of frequency regulation ends, and energy storage... Recovery is performed to prepare for the next disturbance and provide auxiliary support. It should be noted that the energy storage controller's control in the above process is based on the condition that the dynamic constraints are not met. If the conditions are met, the controller will not be triggered at this moment, but will be postponed to the next time segment. If the frequency in this area meets the boundary requirements of the dynamic constraints throughout the entire frequency regulation period, the BESS controller will not be activated, and there is no need to put the BESS into operation.

[0146] Based on the same inventive concept, another embodiment of the present invention provides a VSC-MTDC frequency modulation device considering dynamic constraints. This device corresponds to the method of the foregoing embodiment and includes:

[0147] A modeling unit is established to build a frequency response model of a VSC-MTDC system incorporating new energy sources; VSC-MTDC is a multi-terminal flexible DC transmission system based on voltage source converters.

[0148] The determining unit is used to determine the boundary constraints of the frequency response model at each time period during the response process according to predetermined requirements;

[0149] The response unit is used to respond when a load disturbance occurs and the frequency deviation of each region of the VSC-MTDC exceeds a threshold. η If the boundary constraints are exceeded within the corresponding time period, the VSC and energy storage response of each region will be adjusted accordingly.

[0150] The first frequency modulation unit is used to determine the VSC driving events in each region. When a VSC driving event occurs, it performs time-segmented and region-segmented adaptive droop control of VSC according to the boundary constraints of each time period.

[0151] The second frequency regulation unit is used to determine the energy storage driving events in each region. When an energy storage driving event occurs, the energy storage droop coefficient is adjusted according to the current time period.

[0152] The following is a specific embodiment of the present invention.

[0153] To verify the correctness and effectiveness of the frequency modulation method of this invention, a simulation platform was built based on MATLAB / Simulink. Figure 3 The diagram shows a simulation model of a multi-port VSC-MTDC system. Different frequency constraints are set during frequency modulation, such as… Figure 2 As shown. Considering that the simulation uses per-unit quantities and the change values ​​of each variable, the frequency is normalized to 50Hz to obtain the normalized boundary constraints as follows. Figure 5 As shown.

[0154] In setting region 2, a small load disturbance of 0.1 pu and a large load disturbance of 0.25 pu occur at time 0, respectively. The frequency and power change curves under the traditional frequency modulation strategy and the frequency modulation method of this invention are shown below. Figures 6-9 As shown.

[0155] Depend on Figure 6 , Figure 7 As shown, when a small disturbance occurs, the maximum frequency deviation is only -2.0 × 10⁻⁶. -3 The frequency limit (pu) is within the allowed range of the frequency boundary constraint, meaning that the entire frequency modulation process is within the blue constraint and within the safe range. Therefore, in order to save resources and improve economic efficiency, the converter station and energy storage are not started in the driving strategy of this invention, so the output frequency and output power are the same as those of the traditional strategy.

[0156] Depend on Figure 8 , Figure 9 It can be seen that when a large load disturbance occurs, observation Figure 8 The traditional FM method using the blue curve has a maximum frequency deviation of approximately 27 × 10⁻⁶. -3 The frequency deviation pu has exceeded the maximum allowable range, i.e., it exceeds the blue boundary constraint; however, using the method of this invention, the disturbance is detected at the moment of occurrence. ROCOF The frequency drop exceeded the threshold, so VSC auxiliary control was activated to slow down the frequency drop rate. During the frequency recovery period, because the frequency trajectory would exceed the light blue boundary constraint at t=5 s and 10 s, the BESS driver was activated to provide emergency power support, while keeping the frequency within the boundary constraint. Figure 9 As can be seen from the power output, the traditional frequency modulation method outputs less power after the disturbance, while the method of this invention outputs significantly more power at the beginning of the disturbance and at 5 s and 10 s. This is because the auxiliary power support of VSC and BESS is put into use, thus keeping the frequency modulation effect of the system within a safe boundary constraint.

[0157] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A VSC-MTDC frequency modulation method considering dynamic constraints, characterized in that, include: A frequency response model for a VSC-MTDC system incorporating new energy sources is established; the VSC-MTDC is a multi-terminal flexible DC transmission system based on a voltage source converter. Determine the boundary constraints of the frequency response model at each time point during the response process according to predetermined requirements; When a load disturbance occurs, if the frequency deviation of each region of the VSC-MTDC is detected to exceed a threshold... η If the boundary constraints are exceeded within the corresponding time period, the response adjustment of VSC and energy storage in each region shall be carried out. Determine the VSC driving events for each region. When a VSC driving event occurs, perform time-segmented and region-specific adaptive droop control based on the boundary constraints of each time period. Identify the energy storage-driven events in each region. When an energy storage-driven event occurs, adjust the energy storage droop coefficient according to the current time period. The expressions for the VSC driving event and the energy storage driving event are as follows: (9) In the formula, For the last driving moment, This is the moment of this drive; n The total number of VSCs. , These are VSC drive events and energy storage drive events, respectively. for t Time of the first i Rate of change of frequency deviation of each VSC; For the first j The absolute value of the difference between the participation of each VSC and the average participation of all VSCs. = / For the first j The participation rate of each VSC For the first time after load disturbance j The unbalanced power shared by each VSC This is the first time before this disturbance. j Power margin of each VSC This represents the average participation rate across all VSCs.

2. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 1, characterized in that, The VSC-MTDC includes at least one new energy generator unit regional terminal and multiple thermal power unit regional terminals. Each new energy generator unit regional terminal includes a new energy generator unit and a VSC, and each thermal power unit regional terminal includes a thermal power unit, energy storage, a VSC, and a controller.

3. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 2, characterized in that, When a load disturbance occurs, if the rate of change of frequency deviation is greater than a threshold η 1. Adjust the droop coefficient of the corresponding region to change the VSC from traditional droop control to improved droop control; wherein, the expression for the frequency deviation change rate is: In the formula, This refers to the frequency variation of the AC power grid. H The inertia coefficient, The load disturbance experienced by this area, This refers to the power change of a VSC connected to the AC power grid. The change in power of the connected energy storage; The power-frequency gain of the speed controller. The threshold value for the speed controller; is the time constant of the steam turbine.

4. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 3, characterized in that, When a load disturbance occurs, if the VSC participation in a certain area is detected to be less than a threshold... η When the value is 2, the VSC output power equalization process is performed in that area.

5. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 3, characterized in that, In the improved droop control, the formula for calculating the frequency droop control coefficient is: (10) In the formula, Let be the frequency droop control coefficient for the i-th VSC improved droop control at time t. β i For the i-th VSC conventional droop control, the frequency droop control coefficient is ; For time t, the first i Power sharing factor of each VSC For time t, the first i Frequency deviation factor for each region; For the first i Power margin of each VSC; For the first i The maximum allowable frequency deviation value for each region; This indicates the deviation of the actual operating AC frequency from the rated value; and This is a user-defined constant.

6. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 3, characterized in that, The constraints for the energy storage are: (11) (12) (13) In the formula, This represents the change in energy storage output power. This represents the maximum change in output power of the energy storage. This represents the initial state of charge of the energy storage. P is the rated capacity of energy storage. B (t) represents the energy storage output power at time t; SOC ( t ( ) represents the state of charge of the stored energy at time t. This represents the minimum state of charge allowed for energy storage. This represents the maximum state of charge allowed for energy storage.

7. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 6, characterized in that, The determination of energy storage-driven events in each region includes: set up The moment when the load disturbance occurs. For the current moment, , and These are the required frequency safety thresholds for different time periods after a load disturbance occurs. , and Restore the frequency to , and The above are the maximum allowed times; Divide the time after the load disturbance occurs into 0~ , ~ , ~ , ~ Four time periods; Assumption The time period after the load disturbance occurs ~ Inside, The frequency measured at time is Then the frequency deviation Represented as: (14) Thus obtain t k Recovery time deviation at any moment Represented as: (15) In the formula, ROCOF ( t c () represents the rate of change of frequency deviation at the moment the load disturbance occurs; Therefore, when equation (16) is satisfied, the energy storage-driven event occurs; (16)。 8. The VSC-MTDC frequency modulation method considering dynamic constraints according to claim 7, characterized in that, The time-segmented adjustment of the energy storage droop coefficient includes: when After a load disturbance occurs at a certain time, at the current time In +(0~ During the specified time period, the energy storage will not operate; at the current moment... In +( ~ During the time period, if The system detects at all times that the energy storage drive event does not meet the requirements of the dynamic constraints, as shown in equation (19): (19) In the formula, for The difference between the frequency constraint threshold and the actual value at any given time; This indicates that emergency power support is needed for energy storage, and the energy storage droop coefficient needs to be adjusted, as shown in equation (20): (20) In the formula, 0~ after the load disturbance occurs Adjustment coefficient for time period After the load disturbance occurs ~ Adjustment coefficient for time period; current time In +( ~ If detected during the specified time period Not exceeding the threshold If the adjustment coefficient remains unchanged, then the adjustment coefficient remains unchanged; if it is detected Threshold exceeded If so, the adjustment coefficient needs to be increased, as shown in equation (21): (21) In the formula, After the load disturbance occurs ~ Adjustment coefficient for time period; current time In +( ~ If detected during the specified time period Not exceeded If the adjustment coefficient remains unchanged, then the adjustment coefficient remains unchanged; if it is detected Threshold exceeded If so, the adjustment coefficient needs to be increased, as shown in equation (22): (22) In the formula, After the load disturbance occurs ~ Adjustment coefficient for time period.

9. A VSC-MTDC frequency modulation device considering dynamic constraints, characterized in that, include: A modeling unit is used to establish a frequency response model of a VSC-MTDC system incorporating new energy sources; the VSC-MTDC is a multi-terminal flexible DC transmission system based on a voltage source converter. The determining unit is used to determine the boundary constraints of the frequency response model at each time period during the response process according to predetermined requirements; A response unit is used to respond when a load disturbance occurs and the frequency deviation of each region of the VSC-MTDC exceeds a threshold. η If the boundary constraints are exceeded within the corresponding time period, the response adjustment of VSC and energy storage in each region shall be carried out. The first frequency modulation unit is used to determine the VSC driving events in each region. When a VSC driving event occurs, it performs time-segmented and region-segmented adaptive droop control of VSC according to the boundary constraints of each time period. The second frequency regulation unit is used to determine the energy storage driving events in each region. When an energy storage driving event occurs, the energy storage droop coefficient is adjusted according to the current time period. The expressions for the VSC driving event and the energy storage driving event are as follows: (9) In the formula, For the last driving moment, This is the moment of this drive; n The total number of VSCs. , These are VSC drive events and energy storage drive events, respectively. for t Time of the first i Rate of change of frequency deviation of each VSC; For the first j The absolute value of the difference between the participation of each VSC and the average participation of all VSCs. = / For the first j The participation rate of each VSC For the first time after load disturbance j The unbalanced power shared by each VSC This is the first time before this disturbance. j Power margin of each VSC This represents the average participation rate across all VSCs.

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