Converter station frequency modulation method and system based on flexible DC power transmission system, and medium

Abstract

The invention discloses a converter station frequency modulation method and system based on a flexible DC power transmission system, and a medium, and the method comprises the steps: obtaining a corresponding mechanical power variable quantity and an electromagnetic power variable quantity based on the obtained mechanical power and electromagnetic power of each unit in a power system, inputting the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model to obtain a maximum frequency deviation result; determining a frequency change rate corresponding to the power system, and based on the maximum frequency deviation result and the frequency change rate, obtaining an expected active power change amount of the converter station in a full time sequence process; and determining a corresponding DC voltage based on the expected active power variation, and controlling a converter to perform frequency modulation based on the DC voltage. According to the invention, the future state of the power system can be predicted, and the DC voltage of the converter can be adaptively adjusted according to the prediction result, so that more accurate frequency modulation control is realized.

Description

Technical Field

[0001] The present application relates to the field of power system measurement, and in particular to a frequency regulation method, system and medium for a converter station based on a flexible direct current transmission system. Background Art

[0002] In order to cope with climate change and environmental pollution, my country's eastern power grid is gradually expanding the scale of clean energy acceptance, resulting in a year-on-year decrease in the proportion of traditional thermal power and hydropower and other synchronous generators. This change directly caused a decline in the inertia and frequency regulation capabilities of the power grid, posing a severe challenge to the frequency stability of the power system. At the same time, the flexible direct current transmission technology with voltage source converter as the core has been widely used in many fields such as new energy grid connection by virtue of its flexible control mode, independent regulation capability of active and reactive power, island power supply function and no commutation failure.

[0003] Most of the existing converter frequency regulation control strategies use fixed frequency regulation parameters for frequency regulation. This strategy mainly relies on the information of the current commutation bus frequency deviation to adjust the output power of the modular multilevel converter (MMC). However, this method has limitations. On the one hand, since the fixed parameter control strategy cannot be flexibly adjusted according to the changes in the actual operating conditions of the system, its frequency regulation effect is often not ideal when facing different load fluctuations and power grid failures. On the other hand, the existing frequency regulation strategy lacks the ability to predict the future state of the system, and can often only passively respond to the frequency deviation that has already occurred, but cannot actively prevent potential frequency fluctuations. Therefore, it limits the frequency regulation effect and may even increase the risk of power grid operation.

[0004] Application Contents

[0005] The present application provides a frequency regulation method, system and medium for a converter station based on a flexible direct current transmission system, which can not only predict the future state of the power system, but also adaptively adjust the DC voltage of the converter according to the prediction results to achieve more accurate frequency regulation control.

[0006] In a first aspect, the present application provides a converter station frequency modulation method based on a flexible direct current transmission system, comprising:

[0007] Based on the obtained mechanical power and electromagnetic power of each unit in the power system, the corresponding mechanical power variation and electromagnetic power variation are obtained respectively, and the mechanical power variation and electromagnetic power variation are input into a preset frequency prediction model to obtain a maximum frequency deviation result;

[0008] Determine the frequency change rate corresponding to the power system, and obtain the expected active power change of the converter station in the full time sequence process based on the maximum frequency deviation result and the frequency change rate;

[0009] A corresponding DC voltage is determined based on the expected active power change, and a converter is controlled to perform frequency modulation based on the DC voltage.

[0010] The embodiment of the present application can accurately predict the future state of the power system by inputting the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model, and then accurately obtain the maximum frequency deviation result, which is convenient for the subsequent adaptive adjustment of the DC voltage of the converter according to the prediction result to achieve frequency modulation; by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power variation of the converter station in the full time sequence based on the maximum frequency deviation result and the frequency change rate, and then facilitate the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation; according to the expected active power variation, the DC voltage that can control the frequency modulation of the converter can be determined, and the frequency modulation control of the converter can be accurately performed according to the DC voltage. Compared with the prior art, the present application can not only predict the future state of the power system, but also adaptively adjust the DC voltage of the converter according to the prediction result to achieve more accurate frequency modulation control.

[0011] Furthermore, the frequency change rate corresponding to the power system is determined as follows:

[0012] Acquire the rated capacity and inertia time constant of each unit in the power system, and obtain the equivalent inertia time constant corresponding to the power system based on the rated capacity and the inertia time constant;

[0013] Based on the equivalent inertia time constant and the electromagnetic power change, a frequency change rate corresponding to the power system is obtained.

[0014] In this way, by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power change of the converter station in the whole timing process based on the maximum frequency deviation result and the frequency change rate, thereby facilitating the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation.

[0015] Furthermore, the calculation formula for obtaining the equivalent inertia time constant corresponding to the power system based on the rated capacity and the inertia time constant is specifically:

[0016]

[0017] In the formula, H ae is the equivalent inertia time constant, S Bi is the rated capacity of the i-th unit, H i is the inertia time constant of the ith unit.

[0018] Furthermore, based on the maximum frequency deviation result and the frequency change rate, the expected active power change amount of the converter station in the full time sequence process is obtained, which is specifically:

[0019] Obtaining a frequency ratio based on the maximum frequency deviation result and the frequency change rate;

[0020] Acquire the maximum power variation of the converter, and obtain the virtual inertia and the virtual damping coefficient based on the maximum frequency deviation result, the frequency ratio and the maximum power variation;

[0021] Based on the virtual inertia and the virtual damping coefficient, the expected active power change of the converter station in the full time sequence process is determined.

[0022] In this way, by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power change of the converter station in the whole timing process based on the maximum frequency deviation result and the frequency change rate, thereby facilitating the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation.

[0023] Furthermore, the calculation formula for obtaining the frequency ratio based on the maximum frequency deviation result and the frequency change rate is specifically:

[0024]

[0025] Wherein, η is the frequency ratio, Δf max is the maximum frequency deviation result; v R is the frequency change rate; H ae is the equivalent inertia time constant; D ae is the equivalent damping coefficient, S Bi is the rated capacity of the i-th unit, D i is the damping coefficient of the ith unit; R ae is the first parameter corresponding to the speed regulator; ω n is the fourth parameter, T pae is the second parameter corresponding to the speed regulator; ζ is the fifth parameter, T fae is the third parameter corresponding to the speed regulator; φ is the sixth parameter,

[0026] Furthermore, the virtual inertia and virtual damping coefficient are obtained based on the maximum frequency deviation result, the frequency ratio and the maximum power change, specifically:

[0027]

[0028] In the formula, H m is the virtual inertia, D m is the virtual damping coefficient, ΔP max is the maximum power variation, Δf max is the maximum frequency deviation result, η is the frequency ratio, v R is the frequency change rate.

[0029] Furthermore, the expected active power change of the converter station in the full time sequence process is determined based on the virtual inertia and the virtual damping coefficient, specifically:

[0030]

[0031]

[0032] In the formula, ΔP MMC is the expected active power change, H m is the virtual inertia, D m is the virtual damping coefficient, Δf is the bus frequency deviation, v c is the frequency change rate when the strategy is switched.

[0033] Furthermore, the calculation formula for determining the corresponding DC voltage based on the expected active power change is specifically:

[0034]

[0035] Where U dcref is the converter DC voltage reference value, H m is the virtual inertia, f is the current output frequency of the inverter, f0 is the rated frequency of the inverter, D m is the virtual damping coefficient, Δf is the bus frequency deviation, U dc0 is the rated DC voltage of the converter, C eq is the lumped capacitance parameter of the converter.

[0036] In a second aspect, the present application provides a converter station frequency regulation system based on a flexible direct current transmission system, comprising: a prediction module, a processing module and a frequency regulation module;

[0037] The prediction module is used to obtain the corresponding mechanical power variation and electromagnetic power variation based on the obtained mechanical power and electromagnetic power of each unit in the power system, and input the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model to obtain a maximum frequency deviation result;

[0038] The processing module is used to determine the frequency change rate corresponding to the power system, and obtain the expected active power change amount of the converter station in the full time sequence process based on the maximum frequency deviation result and the frequency change rate;

[0039] The frequency modulation module is used to determine the corresponding DC voltage based on the expected active power change, and control the converter to perform frequency modulation based on the DC voltage.

[0040] The embodiment of the present application can accurately predict the future state of the power system by inputting the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model, and then accurately obtain the maximum frequency deviation result, which is convenient for the subsequent adaptive adjustment of the DC voltage of the converter according to the prediction result to achieve frequency modulation; by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power variation of the converter station in the full time sequence based on the maximum frequency deviation result and the frequency change rate, and then facilitate the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation; according to the expected active power variation, the DC voltage that can control the frequency modulation of the converter can be determined, and the frequency modulation control of the converter can be accurately performed according to the DC voltage. Compared with the prior art, the present application can not only predict the future state of the power system, but also adaptively adjust the DC voltage of the converter according to the prediction result to achieve more accurate frequency modulation control.

[0041] In a third aspect, the present application also provides a computer-readable storage medium, which includes a stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute the converter station frequency regulation method based on the flexible direct current transmission system as described in the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a flow chart of an embodiment of a converter station frequency modulation method based on a flexible direct current transmission system provided by the present application;

[0043] Figure 2 It is a structural schematic diagram of the frequency prediction model provided by this application;

[0044] Figure 3 It is the IEEE 39-node system grid structure provided by this application;

[0045] Figure 4 It is the system frequency response curve under three working conditions provided in this application;

[0046] Figure 5 This is the MMC active power variation curve provided by this application under the frequency modulation strategy of this application;

[0047] Figure 6It is the active power change curve of MMC under the frequency modulation strategy of the present application and the virtual synchronous control strategy provided by the present application;

[0048] Figure 7 It is a structural schematic diagram of an embodiment of a converter station frequency modulation system based on a flexible direct current transmission system provided in the present application. DETAILED DESCRIPTION

[0049] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0050] It should be understood that the step numbers used in this article are only for the convenience of description and are not intended to limit the order in which the steps are executed.

[0051] It should be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an" and "the" are intended to include plural forms unless the context clearly indicates otherwise.

[0052] The terms “include” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0053] The term "and / or" means and includes any and all possible combinations of one or more of the associated listed items.

[0054] In order to cope with climate change and environmental pollution, my country's eastern power grid is gradually expanding the scale of clean energy acceptance, resulting in a decline in the proportion of traditional synchronous generators such as thermal power and hydropower, causing a decline in grid inertia and frequency regulation capabilities, which in turn poses a severe challenge to the frequency stability of the power system. Flexible DC transmission technology is widely used in renewable energy grid connection due to its advantages such as flexible control, independent power regulation, island power supply and no commutation failure. However, the existing converter frequency regulation strategy mostly uses fixed parameters for frequency regulation, relying on the current frequency deviation to adjust the MMC output power, which has limitations. On the one hand, the control strategy cannot be flexibly adjusted according to the system status, and the frequency regulation effect is not ideal; on the other hand, the existing frequency regulation strategy lacks the ability to predict the future state of the system, and only passively responds to the frequency deviation that has occurred, but cannot actively prevent potential frequency fluctuations. Therefore, the frequency regulation effect is limited and may even increase the operation risk.

[0055] Next, the nouns involved in this application are analyzed:

[0056] Modular multilevel converter, also known as Modular Multilevel Converter, or MMC for short, is a high-efficiency modular circuit for power conversion and control. It is a converter composed of multiple cascaded sub-modules (SM) with the same structure. Each sub-module contains a level and a power switch. The direction and size of the current are controlled by adjusting the state of the power switch. MMC controls the voltage and current by adjusting the level of each sub-module. During the power conversion process, MMC can adjust the state of the sub-module as needed to output the required voltage and current.

[0057] The full time series usually refers to the time series over the entire period of time starting from a certain initial moment until the end of the specific event or process of interest. In this context, it may cover all stages of MMC operation, including startup, stable operation, load changes, fault response, etc.

[0058] Based on this, the embodiment of the present application provides a converter station frequency regulation method and system based on a flexible direct current transmission system, which can not only predict the future state of the power system, but also adaptively adjust the DC voltage of the converter according to the prediction results to achieve more accurate frequency regulation control.

[0059] The embodiments of the present application provide a converter station frequency regulation method and system based on a flexible DC transmission system, which are specifically described through the following embodiments. First, the converter station frequency regulation method based on a flexible DC transmission system in the embodiments of the present application is described.

[0060] The frequency modulation method of a converter station based on a flexible direct current transmission system provided in an embodiment of the present application relates to the field of power system measurement. The frequency modulation method of a converter station based on a flexible direct current transmission system provided in an embodiment of the present application can be applied to a terminal, can also be applied to a server side, and can also be software running in a terminal or a server side. In some embodiments, the terminal can be a smart phone, a tablet computer, a laptop computer, a desktop computer, etc.; the server side can be configured as an independent physical server, or can be configured as a server cluster or a distributed system composed of multiple physical servers, and can also be configured as a cloud server that provides basic cloud computing services such as cloud services, cloud databases, cloud computing, cloud functions, cloud storage, network services, cloud communications, middleware services, domain name services, security services, CDN, and big data and artificial intelligence platforms; the software can be an application that implements the frequency modulation method of a converter station based on a flexible direct current transmission system, etc., but is not limited to the above forms.

[0061] The present application can also be used in numerous general or special computer system environments or configurations. For example: personal computers, server computers, handheld or portable devices, tablet devices, multiprocessor systems, microprocessor-based systems, set-top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments including any of the above systems or devices, etc. The present application can be described in the general context of computer executable instructions executed by a computer, such as program modules. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform specific tasks or implement specific abstract data types. The present application can also be practiced in distributed computing environments, in which tasks are performed by remote processing devices connected through a communication network. In a distributed computing environment, program modules can be located in local and remote computer storage media including storage devices.

[0062] Embodiment 1

[0063] Please refer to Figure 1 , Figure 1 It is a flowchart of an embodiment of a converter station frequency modulation method based on a flexible direct current transmission system provided by the present application, including steps S101 to S103;

[0064] Step S101, obtaining corresponding mechanical power variation and electromagnetic power variation respectively based on the obtained mechanical power and electromagnetic power of each unit in the power system, and inputting the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model to obtain a maximum frequency deviation result;

[0065] It can be understood that, firstly, the mechanical power of all the generator sets in the power system can be obtained in real time through sensors or remote monitoring systems installed on each generator set, and the electromagnetic power of all the generator sets in the power system can be obtained in real time through a data acquisition system, wherein the generator sets can include but are not limited to thermal power units and hydropower units; secondly, after the mechanical power and electromagnetic power of each unit are obtained, it is necessary to compare the mechanical power and electromagnetic power at the current moment with the previous moment (or a certain reference moment) to obtain the mechanical power change of each unit (such as the mechanical power change ΔP of the thermal power unit). t , Mechanical power variation of thermal power unit ΔP h ) and the electromagnetic power variation, and the mechanical power variation and electromagnetic power variation corresponding to each unit are summarized to obtain the mechanical power variation ΔP of the power system. mae and the change in electromagnetic power ΔP e ,in, In the formula, ΔP mae is the mechanical power change of the power system, ΔPti is the mechanical power variation of the i-th thermal power unit, ΔP hi is the mechanical power change of the i-th hydropower unit. Finally, due to the mechanical power change of the power system ΔP mae and the change in electromagnetic power ΔP e It will affect the frequency stability of the power system, thus causing frequency deviation. Therefore, the mechanical power change ΔP of the power system will be obtained. mae and the change in electromagnetic power ΔP e Input the preset frequency prediction model to predict the frequency deviation Δf s , and the frequency deviation Δf s The mechanical power change of each unit ΔP mae and the change in electromagnetic power ΔP e Therefore, it is necessary to continuously update the mechanical power change ΔP that affects the power system. mae and the change in electromagnetic power ΔP e , and based on the updated power system mechanical power change ΔP mae and the change in electromagnetic power ΔP e Continuously predict the frequency deviation Δf s By comparing multiple frequency deviation results, the maximum frequency deviation result Δf of the power system can be determined. max , wherein the frequency prediction model is established in advance based on the dynamic characteristics and frequency response mechanism of the power system, and can simulate the frequency response process of the power system under a given power change. The structural diagram of the frequency prediction model is as follows Figure 2 shown.

[0066] Step S102, determining the frequency change rate corresponding to the power system, and obtaining the expected active power change amount of the converter station in the full time sequence process based on the maximum frequency deviation result and the frequency change rate;

[0067] It can be understood that when the maximum frequency deviation result Δf is determined max After that, it is necessary to determine the frequency change rate v corresponding to the power system R , to facilitate the subsequent expected active power change ΔP MMC The calculation is as follows: First, obtain the rated capacity S of each unit in the power system Bi and the inertia time constant H i , and based on the rated capacity S Bi and the inertia time constant H i Get the equivalent inertia time constant H corresponding to the power system ae , wherein the equivalent inertia time constant H corresponding to the power system is obtained ae The specific calculation formula is: In the formula, Hae is the equivalent inertia time constant, S Bi is the rated capacity of the i-th unit, H i is the inertia time constant of the i-th unit. Then, based on the equivalent inertia time constant H ae and the electromagnetic power variation ΔP e , and obtain the frequency change rate v corresponding to the power system R , where the frequency change rate v corresponding to the power system is obtained R The calculation formula is: In the formula, v R is the rate of change of power system frequency, ΔP e is the electromagnetic power variation, H ae is the equivalent inertia time constant.

[0068] It should be noted that the rated capacity S of each unit in the power system Bi The inertia time constant H of each unit in the power system can be obtained by, but not limited to, consulting the unit technical documents, contacting the unit manufacturer or using simulation software. i The inertia time constant may be calculated by, but is not limited to, directly measuring the acceleration and torque of the generator rotor, or may be derived based on a mathematical model, which is not limited in this application.

[0069] In this way, by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power change of the converter station in the whole timing process based on the maximum frequency deviation result and the frequency change rate, thereby facilitating the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation.

[0070] It can be understood that when the frequency change rate v corresponding to the power system is obtained R After that, the expected active power change ΔP can be determined MMC Specifically, first, based on the maximum frequency deviation result Δf max and the frequency change rate v R , and the frequency ratio η is obtained, wherein the calculation formula for the frequency ratio η is obtained as follows:

[0071]

[0072] Wherein, η is the frequency ratio, Δf max is the maximum frequency deviation result; v R is the frequency change rate; H ae is the equivalent inertia time constant; D ae is the equivalent damping coefficient, S Bi is the rated capacity of the i-th unit, D iis the damping coefficient of the ith unit; R ae is the first parameter corresponding to the speed regulator; ω n is the fourth parameter, T pae is the second parameter corresponding to the speed regulator; ζ is the fifth parameter, T fae is the third parameter corresponding to the speed regulator; φ is the sixth parameter,

[0073]

[0074] The derivation process of the frequency ratio η formula is as follows: First, assume that there are n t Thermal power units, n h The parameters of the speed control systems of the thermal power units and the hydropower units are aggregated, and the differential equation of the speed control system of the aggregated units can be obtained as follows: In the formula, ΔP mae is the mechanical power variation of the power system, R ae is the first parameter corresponding to the speed regulator, T pae is the second parameter corresponding to the speed regulator, Δf s is the frequency deviation of the power system, T fae is the third parameter corresponding to the speed regulator, the first parameter R ae , the second parameter T pae and the third parameter T fae Satisfies the following formula: In the formula, R ae is the first parameter, R ti is the regulation coefficient of the ith thermal power unit, R hi is the adjustment coefficient of the ith hydropower unit, n t and n h are the number of thermal power units and hydropower units, T pae is the second parameter, ΔP mae is the change in mechanical power of the power system, T Ri is the reheating time constant of the ith thermal power unit, T Wi is the water hammer effect time constant of the i-th hydropower unit, ΔP ti is the mechanical power variation of the i-th thermal power unit, ΔP hi is the mechanical power variation of the i-th hydropower unit, F Hi is the power ratio of the high-pressure boiler of the i-th thermal power unit. Then, based on the differential equation of the speed control system of the unit, the system frequency response transfer function is obtained as follows: Where Δf is the bus frequency deviation, ΔP e is the electromagnetic power variation, R ae is the first parameter, Tpae is the second parameter, T fae is the third parameter, s is the complex frequency domain variable in Laplace transform, H ae is the equivalent inertia time constant, D ae is the system equivalent damping coefficient, S Bi is the rated capacity of the i-th unit, D i is the damping coefficient of the ith unit. Finally, the transfer function based on the system frequency response is: and the power system frequency change rate v R The calculation formula is: The frequency ratio η formula can be derived, namely

[0075] It should be noted that the above parameters can all be obtained through existing technologies, and the acquisition process is not the focus of this application, so it will not be repeated here.

[0076] Secondly, obtain the maximum power change ΔP of the converter max , and based on the maximum frequency deviation result Δf max , the frequency ratio η and the maximum power variation ΔP max Get the virtual inertia H m and virtual damping coefficient D m , where the relevant calculation formula is:

[0077]

[0078] In the formula, H m The virtual inertia of MMC adaptive m The virtual damping coefficient for MMC adaptation, ΔP max is the maximum power variation, Δf max is the maximum frequency deviation result, η is the frequency ratio, v R is the frequency change rate.

[0079] It should be noted that the virtual inertia H is adjusted m and virtual damping coefficient D m The size of the parameter can control the strategy switching point. In extreme cases, H m When D is close to 0, the MMC enters the virtual damping control mode at the initial operating point. m If it is close to 0, the MMC will enter the virtual damping control mode at the lowest frequency.

[0080] Finally, based on the virtual inertia H m and the virtual damping coefficient D m , determine the expected active power change ΔP of the converter station during the entire timing processMMC , where the relevant calculation formula is:

[0081]

[0082]

[0083] In the formula, ΔP MMC is the expected active power change, H m is the virtual inertia, D m is the virtual damping coefficient, Δf is the bus frequency deviation, v c is the frequency change rate when the strategy is switched.

[0084] In this way, by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power change of the converter station in the whole timing process based on the maximum frequency deviation result and the frequency change rate, thereby facilitating the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation.

[0085] Step S103: determining a corresponding DC voltage based on the expected active power variation, and controlling the converter to perform frequency modulation based on the DC voltage.

[0086] It is understandable that when the active power change ΔP is determined MMC After that, the frequency of the converter can be modulated. Specifically, if the fixed active power control method is adopted, the expected active power change of the MMC is ΔP MMC It is the active power reference value. Based on the active power reference value, the converter can be directly controlled. If the constant DC voltage control method is adopted, the virtual inertia H is required. m and the damping coefficient D m Relationship with DC voltage Calculation of DC voltage reference value U dcref Specifically, first, the DC voltage U dc The relationship with power P is approximately: In the formula, C eq is the lumped capacitance parameter of the converter, U dc is the DC voltage of the converter, P in is the active power flowing into MMC from the DC side, P out is the active power flowing out of the MMC on the AC side. Therefore, in order to make the active power P of the MMC out To show frequency modulation characteristics, the DC voltage U dc satisfy, In the formula, C eq is the lumped capacitance parameter of the converter, U dc is the DC voltage of the converter, H mis the virtual inertia, f is the current output frequency of the converter, and integrating both sides of the equation yields: Therefore, the DC voltage command value U of the converter is dcref The calculation formula is: Where U dcref is the converter DC voltage reference value, H m is the virtual inertia, f is the current output frequency of the inverter, f0 is the rated frequency of the inverter, D m is the virtual damping coefficient, Δf is the bus frequency deviation, U dc0 is the rated DC voltage of the converter, C eq is the lumped capacitance parameter of the converter.

[0087] The embodiment of the present application can accurately predict the future state of the power system by inputting the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model, and then accurately obtain the maximum frequency deviation result, which is convenient for the subsequent adaptive adjustment of the DC voltage of the converter according to the prediction result to achieve frequency modulation; by determining the frequency change rate corresponding to the power system, it is convenient to accurately determine the expected active power variation of the converter station in the full time sequence based on the maximum frequency deviation result and the frequency change rate, and then facilitate the subsequent adaptive adjustment of the DC voltage of the converter to achieve frequency modulation; according to the expected active power variation, the DC voltage that can control the frequency modulation of the converter can be determined, and the frequency modulation control of the converter can be accurately performed according to the DC voltage. Compared with the prior art, the present application can not only predict the future state of the power system, but also adaptively adjust the DC voltage of the converter according to the prediction result to achieve more accurate frequency modulation control.

[0088] The verification process of this method is given below. This method is verified in a modified IEEE 39-bus system. The simulation is based on PSS / E. The modified IEEE 39-bus system grid structure is as follows: Figure 3 As shown in Figure 1, the MMC-based flexible DC system is connected to node 7, and the main parameters of MMC-HVDC are shown in Table 1. The total installed capacity of the original calculation example of the IEEE 39-node system is about 8050MVA and the load is 6124MW. After the MMC-HVDC is connected, the unit capacity and output remain unchanged, and all loads increase proportionally to maintain the balance between supply and demand.

[0089] Table 1 Main parameters of single-pole MMC-HVDC

[0090]

[0091] Assume that the maximum short-term support power of MMC is 0.1pu, which is 200MW. First, three working conditions are selected for simulation calculation, namely, MMC does not participate in frequency modulation, MMC adopts the frequency modulation strategy of this application, and MMC adopts the virtual synchronous control strategy. Under the virtual synchronous control strategy, the inertia and damping coefficient of MMC are fixed, and the selection principle is to ensure that MMC will not be overloaded during the entire transient process. Figure 4 The system frequency response curves under three working conditions are given. It can be seen that after the MMC adopts the frequency modulation strategy of this application or the virtual synchronous control strategy, the lowest point of the system frequency can be effectively improved, thereby improving the frequency stability of the system. Compared with the virtual synchronous control strategy, the frequency modulation strategy of this application can further increase the lowest point of the system frequency and speed up the frequency recovery process. This is because in the process of system frequency recovery, inertia control plays an obstructive role, and the frequency modulation strategy of this application is transformed into damping control in this process, which is conducive to improving the system frequency recovery speed.

[0092] The MMC active power change curve under the frequency modulation strategy of this application is as follows: Figure 5 shown. Figure 5 The actual active power change of MMC and the calculation results of the two components of inertia and damping are shown. Under the frequency modulation method of this application, the actual active power change of MMC is actually the maximum value under inertia and damping control, that is, the upper limit. The active power change curve of MMC under the frequency modulation strategy and virtual synchronization control strategy of this application is shown in Figure 2. Figure 6 In comparison, the frequency modulation method of the present application can release the maximum power margin of the MMC at the maximum frequency deviation point, while the maximum power of the MMC under the virtual synchronization control strategy appears before the arrival of the lowest point, and afterwards, due to inertia control reasons, the output power of the MMC continues to decline, and the power support capacity of the MMC cannot be fully utilized. Therefore, the simulation results verify the effectiveness of the present application.

[0093] Embodiment 2

[0094] Please refer to Figure 6 , Figure 6 It is a structural schematic diagram of an embodiment of a converter station frequency regulation system based on a flexible direct current transmission system provided by the present application, including a prediction module 100, a processing module 200 and a frequency regulation module 300;

[0095] The prediction module 100 is used to obtain the corresponding mechanical power variation and electromagnetic power variation based on the obtained mechanical power and electromagnetic power of each unit in the power system, and input the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model to obtain a maximum frequency deviation result;

[0096] The processing module 200 is used to determine the frequency change rate corresponding to the power system, and obtain the expected active power change amount of the converter station in the full time sequence process based on the maximum frequency deviation result and the frequency change rate;

[0097] The frequency modulation module 300 is used to determine a corresponding DC voltage based on the expected active power variation, and control the converter to perform frequency modulation based on the DC voltage.

[0098] Since the information interaction, execution process, etc. between the modules in the above-mentioned converter station frequency regulation system based on the flexible DC transmission system are based on the same concept as the embodiment of the converter station frequency regulation method based on the flexible DC transmission system in the first aspect of the present invention, the technical effects achieved are basically the same. For specific contents, please refer to the description in the first embodiment of the method of the present invention, and will not be repeated here.

[0099] The device embodiments described above are merely illustrative, wherein the modules described as separate components may or may not be physically separated, i.e., may be located in one place, or may be distributed to multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the method of this embodiment.

[0100] The present invention also provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the converter station frequency regulation method based on the flexible direct current transmission system as described in the first embodiment above is implemented.

[0101] A person skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-monitorable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0102] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present application in detail. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the scope of protection of the present application.

[0103] It is particularly pointed out that for those skilled in the art, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this application should be included in the protection scope of this application.

Claims

1. A frequency modulation method for a converter station based on a flexible direct current transmission system, characterized in that: include: Based on the obtained mechanical power and electromagnetic power of each unit in the power system, the corresponding mechanical power variation and electromagnetic power variation are obtained respectively, and the mechanical power variation and electromagnetic power variation are input into a preset frequency prediction model to obtain a maximum frequency deviation result; Determine the frequency change rate corresponding to the power system, and obtain the expected active power change of the converter station in the full time sequence process based on the maximum frequency deviation result and the frequency change rate; A corresponding DC voltage is determined based on the expected active power change, and a converter is controlled to perform frequency modulation based on the DC voltage.

2. The frequency modulation method of a converter station based on a flexible DC transmission system according to claim 1, characterized in that: The determining of the frequency change rate corresponding to the power system is specifically: Acquire the rated capacity and inertia time constant of each unit in the power system, and obtain the equivalent inertia time constant corresponding to the power system based on the rated capacity and the inertia time constant; Based on the equivalent inertia time constant and the electromagnetic power change, a frequency change rate corresponding to the power system is obtained.

3. The frequency modulation method of a converter station based on a flexible DC transmission system according to claim 2, characterized in that: The calculation formula for obtaining the equivalent inertia time constant corresponding to the power system based on the rated capacity and the inertia time constant is specifically: In the formula, H ae is the equivalent inertia time constant, S Bi is the rated capacity of the i-th unit, H i is the inertia time constant of the ith unit.

4. The converter station frequency modulation method based on the flexible DC transmission system according to claim 1 is characterized in that: The expected active power change amount of the converter station in the full time sequence process is obtained based on the maximum frequency deviation result and the frequency change rate, specifically: Obtaining a frequency ratio based on the maximum frequency deviation result and the frequency change rate; Acquire the maximum power variation of the converter, and obtain the virtual inertia and the virtual damping coefficient based on the maximum frequency deviation result, the frequency ratio and the maximum power variation; Based on the virtual inertia and the virtual damping coefficient, the expected active power change of the converter station in the full time sequence process is determined.

5. The frequency modulation method of a converter station based on a flexible DC transmission system according to claim 4, characterized in that: The calculation formula for obtaining the frequency ratio based on the maximum frequency deviation result and the frequency change rate is specifically: Wherein, η is the frequency ratio, Δf max is the maximum frequency deviation result; v R is the frequency change rate; H ae is the equivalent inertia time constant; D ae is the equivalent damping coefficient, S Bi is the rated capacity of the i-th unit, D i is the damping coefficient of the ith unit; R ae is the first parameter corresponding to the speed regulator; ω n is the fourth parameter, T pae is the second parameter corresponding to the speed regulator; ζ is the fifth parameter, T fae is the third parameter corresponding to the speed regulator; φ is the sixth parameter, 6. The frequency modulation method of a converter station based on a flexible DC transmission system according to claim 4, characterized in that: The virtual inertia and virtual damping coefficient are obtained based on the maximum frequency deviation result, the frequency ratio and the maximum power change, specifically: In the formula, H m is the virtual inertia; D m is the virtual damping coefficient; ΔP max is the maximum power variation; Δf max is the maximum frequency deviation result; η is the frequency ratio; v R is the frequency change rate.

7. The frequency modulation method of a converter station based on a flexible DC transmission system according to claim 4, characterized in that: The method of determining the expected active power change of the converter station in the full time sequence process based on the virtual inertia and the virtual damping coefficient is specifically as follows: In the formula, ΔP MMC is the expected active power change; H m is the virtual inertia; D m is the virtual damping coefficient; Δf is the bus frequency deviation; v c is the frequency change rate when the strategy is switched.

8. The frequency modulation method of a converter station based on a flexible DC transmission system according to claim 4, characterized in that: The calculation formula for determining the corresponding DC voltage based on the expected active power change is specifically: Where U dcref is the converter DC voltage reference value; H m is the virtual inertia; f is the current output frequency of the converter; f0 is the rated frequency of the converter; D m is the virtual damping coefficient; Δf is the bus frequency deviation; U dc0 is the rated DC voltage of the converter; C eq is the lumped capacitance parameter of the converter.

9. A converter station frequency modulation system based on a flexible DC transmission system, characterized in that: include: Prediction module, processing module and frequency modulation module; The prediction module is used to obtain the corresponding mechanical power variation and electromagnetic power variation based on the obtained mechanical power and electromagnetic power of each unit in the power system, and input the mechanical power variation and the electromagnetic power variation into a preset frequency prediction model to obtain a maximum frequency deviation result; The processing module is used to determine the frequency change rate corresponding to the power system, and obtain the expected active power change amount of the converter station in the full time sequence process based on the maximum frequency deviation result and the frequency change rate; The frequency modulation module is used to determine the corresponding DC voltage based on the expected active power change, and control the converter to perform frequency modulation based on the DC voltage.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium includes a stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute the converter station frequency regulation method based on the flexible direct current transmission system according to any one of claims 1 to 8.

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