Direct current voltage reverse mapping grid frequency flexible direct current active support control method

Abstract

The present application relates to a kind of DC voltage reverse mapping power grid frequency flexible direct active support control method, belong to power transmission and distribution technical field.The present application is when the power grid frequency reduces in the scene of load surge at receiving end, by the reverse mapping of power grid frequency-DC voltage, when flexible transmission power increases, it lifts DC voltage, improves the upper limit of flexible transmission power under frequency support demand.The core technical scheme of the present application is: by increasing the number N of each phase bridge arm sub-module input, realizes the injection of additional equivalent active power ΔP SM , in DC side embodies the characteristics of DC voltage rise;The lifting of DC voltage improves the upper limit of flexible transmission power while reflecting the information of power grid frequency drop to sending end;Sending end converter station reflects the change of power grid frequency to new energy station according to certain proportion, and the proportion can be artificially set by adjusting control parameter, and new energy station can respond to inertia according to the frequency change of sending end converter station output, to provide frequency support for receiving end power grid.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power transmission and distribution, and particularly relates to a VSC-HVDC active support control method for reverse mapping of DC voltage to grid frequency. BACKGROUND

[0002] The energy structure in China is accelerating the transformation to clean and low-carbon, and the new energy base in the west lacks local supporting power sources. Therefore, it has become a development trend to access the power grid through the VSC-HVDC technology which has the ability to actively build stable AC voltage and frequency.

[0003] The isolation effect of VSC-HVDC makes it impossible for new energy units to actively respond to the demand for grid frequency support, which will significantly reduce the grid inertia. It has become an urgent need to ensure the safe and stable operation of high-proportion new energy power systems to use new energy units to provide effective frequency support for the grid through VSC-HVDC. In the existing research on new energy sent through VSC-HVDC, the frequency of the receiving end grid is often transmitted to the new energy side through the decrease of DC voltage when the grid frequency decreases. In the case of unchanged IGBT current-carrying capacity, the decrease of DC voltage limits the growth space of DC transmission power, which affects the frequency support effect. For the method of transmitting frequency information through DC bus voltage, especially for the control of DC voltage showing an inverse correlation with the grid frequency, there is currently no research involved.

[0004] To solve the above problems, the application provides a VSC-HVDC active support control method for reverse mapping of DC voltage to grid frequency. SUMMARY

[0005] The purpose of the application is to provide a VSC-HVDC active support control method for reverse mapping of DC voltage to grid frequency to solve the problem that the isolation effect of VSC-HVDC makes it impossible for new energy units to actively respond to the demand for grid frequency support, which will significantly reduce the grid inertia.

[0006] To achieve the above purpose, the application adopts the following technical solutions:

[0007] The VSC-HVDC active support control method for reverse mapping of DC voltage to grid frequency comprises the following steps:

[0008] Step 1: Increase the number N of sub-module bridge arms under the premise that the sub-module capacitor voltage remains unchanged, and realize injection of additional equivalent active power ΔP to the bridge arm SM The DC voltage on the DC side shows the characteristics of rising.

[0009] Step 2: Reflect the information of power grid frequency drop to sending end by lifting of direct current voltage, and increase the upper limit of direct current transmission power under frequency support demand.

[0010] Step 3: The sending end converter station reflects the change of power grid frequency to new energy station according to a certain proportion, the proportion can be set artificially by adjusting control parameters, and the new energy station carries out inertia response according to the change of frequency output by the sending end converter station.

[0011] Compared with the prior art, the present application has the following beneficial effects:

[0012] The present application realizes the reverse mapping of power grid frequency-direct current voltage, lifts the direct current voltage when the direct current transmission power increases, and improves the upper limit of direct current transmission power under frequency support demand.

[0013] The present application realizes the reverse mapping of power grid frequency-direct current voltage, lifts the direct current voltage when the direct current transmission power increases, and improves the upper limit of direct current transmission power under frequency support demand. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The figure is a schematic diagram of a photovoltaic island through a flexible direct current transmission system sending out a system;

[0015] Figure 2 The figure is a sending end variable frequency control block diagram; in the figure, U dc is an actual value of direct current voltage, U dc_N is a rated value of direct current voltage; ω0 is a rated angular velocity; K B is a proportional coefficient;

[0016] Figure 3 The figure is a photovoltaic unit control strategy block diagram; in the figure, f s is an actual value of sending end system frequency, f0 is a rated value of frequency; V PVref0 is a direct current voltage reference value under load shedding operation in rated working condition; K d represents a virtual inertia coefficient; K p represents a frequency difference-direct current voltage droop coefficient. DETAILED DESCRIPTION

[0017] A direct current voltage reverse mapping power grid frequency active support control method related to the present application will be described in detail below. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope of the present application and its applications.

[0018] The technical problem to be solved by the present application is that, based on the existing method of transmitting frequency information by using DC bus voltage, the method actively controls the DC voltage to present anti-correlation changes with the grid frequency, actively raises the DC voltage when the receiving end frequency drops, improves the upper limit of the flexible DC transmission power under the frequency support demand, and makes the sending end converter station reflect the grid frequency changes to the photovoltaic field in a certain proportion, and cooperates with the new energy unit to complete the inertia response. The DC voltage anti-mapping grid frequency flexible DC active support control method proposed by the present application will be described below in combination with relevant drawings and specific examples, which specifically includes the following contents.

[0019] Embodiment 1

[0020] Please refer to Figure 1 The present application proposes a DC voltage anti-mapping grid frequency flexible DC active support control method, which is applied to a photovoltaic island through a flexible DC transmission system sending system. The system controls the DC voltage and the grid frequency to present anti-correlation changes through link ①. During the DC voltage rising process, the sending and receiving end converter stations maintain the sub-module energy unchanged through link ②. The sending end converter station reflects the grid frequency change information to the photovoltaic field through link ③. The photovoltaic field performs inertia response through link ④. Based on the above contents, the control strategy proposed specifically includes the following contents:

[0021] Step 1: Increase the number of sub-modules N input to each phase bridge arm under the premise that the sub-module capacitor voltage is unchanged, and realize the injection of additional equivalent active power ΔP to the bridge arm SM The DC voltage rising characteristics are embodied on the DC side, which specifically includes:

[0022] Under the premise that the sub-module capacitor voltage is unchanged, the number of sub-modules input to each phase bridge arm N is increased according to the degree of DC voltage rise. The DC bus voltage presents the dynamic characteristics shown in formula (1):

[0023]

[0024] In the formula: U dc is the DC voltage; P dc is the DC power; P s is the grid-connected power of the converter station; C' eq is the equivalent capacitance on the DC side after the number of input sub-modules is increased; ΔP SM is the power equivalent to the bridge arm injected due to the increase in the number of input sub-modules.

[0025] In the process of controlling the DC voltage rise, the expression of the total number of sub-modules N input to each phase bridge arm is shown in formula (2):

[0026]

[0027] In the formula: the reference value U crefThe submodule capacitor voltage rating U cn .

[0028] Therefore, the DC side equivalent capacitor C' eq And the equivalent injected power ΔP SM Can be represented by formula (3), formula (4) respectively:

[0029]

[0030] In the formula: C is the submodule capacitor value. After substituting formula (3) into formula (1), formula (5) is obtained:

[0031]

[0032] By comparing formula (4) and formula (5), it can be seen that by increasing the number of submodule inputs N of each phase bridge arm, additional equivalent active power ΔP SM Can be realized to inject into the bridge arm, which can reflect the characteristics of the DC voltage rise on the DC side.

[0033] Step 2: Reflect the information of the decrease of the grid frequency to the sending end through the lifting of the DC voltage, and increase the upper limit of the HVDC transmission power under the frequency support requirement, which specifically includes:

[0034] When the receiving end frequency decreases, modify the DC voltage setting value as shown in formula (6):

[0035] U dcref = U dc_N + K A (f ref -f m ) (6)

[0036] In the formula: U dcref is the DC voltage setting value, U dc_N is the DC voltage rating; f m is the actual value of the receiving end grid frequency, f ref is the rated frequency; K A is the control parameter of the DC voltage control link, which can be artificially selected according to the DC voltage rise degree.

[0037] Step 3: The sending end converter station reflects the change of the grid frequency to the new energy station in a certain proportion, which can be artificially set by adjusting the control parameter, and the new energy station responds to the inertia according to the change of the frequency output by the sending end converter station, which specifically includes:

[0038] The frequency control link of the sending end converter station adds a correction amount to the rated angular velocity ω0 based on the deviation ΔU dc Of the DC voltage, which can be specifically represented by formula (7):

[0039]

[0040] ω0+ K send ω0+ K B ω0+ K dc_N ω0+ K PV ω0+ K PVref ω0+ K

[0041]

[0042] ω0+ K PVref0 ω0+ K d ω0+ K p ω0+ K

[0043] It should be further explained that the steps 1, 2 and 3 as a whole are the invention content, so that the HVDC system can actively raise the DC voltage to improve the power transmission capacity when the receiving end frequency drops, and the frequency information can be transmitted to the sending end system, and finally the new energy unit can realize the frequency support for the receiving end system.

[0044] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this, any person skilled in the art can easily think of the changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A flexible DC active support control method that maps DC voltage inversely to grid frequency, characterized in that, By using the inverse mapping between grid frequency and DC voltage, the DC voltage is increased when the power of flexible DC transmission increases, thereby raising the upper limit of flexible DC transmission power under frequency support requirements; the control method specifically includes the following steps: Step 1: Increase the number of submodules deployed per phase bridge arm while keeping the submodule capacitor voltage constant. N This enables the injection of additional equivalent active power into the bridge arm. P SM On the DC side, it exhibits the characteristic of DC voltage rise; Step 2: By raising the DC voltage, the information about the decrease in grid frequency is reflected to the sending end, and the upper limit of flexible DC transmission power is increased under the frequency support requirement; Step 3: The sending-end converter station reflects the grid frequency change to the renewable energy power station according to a certain proportion. This proportion is manually set by adjusting control parameters. The renewable energy power station performs an inertial response based on the change in the output frequency of the sending-end converter station, which specifically includes the following: The frequency control circuit of the sending-end converter station is based on the DC voltage offset. U dc At rated angular velocity ω A correction factor is added to the base of 0, specifically represented by equation (7): (7) In the formula, ω send The output angular velocity of the sending-end converter station; ω 0 represents the rated angular velocity; K B This is the proportionality coefficient; U dc_N Rated DC voltage; By incorporating droop control and virtual inertia control based on grid frequency detection into the traditional control of photovoltaic converters, the photovoltaic converter can actively adjust its droop control according to changes in grid frequency. V PV Reference value; DC voltage reference value for photovoltaic converter V PVref Represented as equation (8): (8) In the formula, V PVref0 This is the reference value for DC voltage under rated operating conditions with reduced load. K d Indicates the virtual inertia coefficient; K p This represents the frequency difference-DC voltage droop coefficient. f This is the difference between the rated frequency and the actual frequency.

2. The flexible DC active support control method for DC voltage reverse mapping to grid frequency according to claim 1, characterized in that, Step 1 specifically includes the following: Assuming the submodule capacitor voltage remains constant, the total number of submodules added to each phase bridge arm is increased according to the degree of DC voltage rise. N This causes the DC bus voltage to exhibit the dynamic characteristics shown in equation (1): (1) In the formula, U dc It is a DC voltage; C ' eq The DC-side equivalent capacitance after the number of sub-modules is increased; P dc DC power; P s This refers to the grid-connected power of the converter station; P SM This represents the equivalent power injected into the bridge arm due to the increased number of sub-modules. During the process of controlling the rise of DC voltage, the total number of submodules engaged in each phase bridge arm is... N The expression for is shown in equation (2): (2) In the formula, the reference value U cref The rated voltage of the submodule capacitor U cn ; DC side equivalent capacitance C ' eq and equivalent injected power P SM Equations (3) and (4) represent the following respectively: (3) (4) In the formula, C This refers to the capacitance value of the submodule. Substituting equation (3) into equation (1), we get: (5) Comparing equations (4) and (5), by increasing the number of sub-modules deployed per phase bridge arm... N This enables the injection of additional equivalent active power into the bridge arm. P SM On the DC side, it exhibits the characteristic of DC voltage rise.

3. The flexible DC active support control method for DC voltage reverse mapping to grid frequency according to claim 2, characterized in that, In step 2, when the receiving end frequency decreases, the DC voltage setting value is modified as shown in equation (6): (6) In the formula, U dcref This is the DC voltage setting value; U dc_N This is the rated DC voltage. K A These are the control parameters for the DC voltage control circuit. f ref This is the frequency rating; f m This represents the actual frequency of the receiving-end power grid.

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