Flexible direct output system multi-converter station reactive power control method based on power operation domain

Abstract

The invention discloses a multi-converter-station reactive power control method for a flexible direct-current output system based on a power operation domain, and belongs to the technical field of offshore wind power flexible direct-current power transmission. The method comprises the following steps: calculating the reactive power demand of a receiving-end converter station according to grid-connected point voltage, a reactive power initial value before reactive power fluctuation of the converter station and other information; then stable operation constraint conditions of the receiving-end converter station are analyzed, the constraint conditions comprise converter station apparent power constraint, converter station alternating current limit constraint and converter station alternating current bus voltage constraint, and the steady-state power operation domain of the receiving-end converter station is determined by solving the boundary of the constraint conditions; and finally, based on a reactive power demand and the steady-state power operation domain, designing an active rapid transfer control method considering reactive fluctuation, and realizing power mutual aid and cooperative control among multiple converter stations. According to the method, the offshore wind power flexible direct output system can be always maintained in a steady-state power operation domain, and the reactive power supporting capacity and the operation stability of the system are improved.

Description

Technical Field

[0001] This invention relates to a reactive power control method for multiple converter stations in a flexible DC transmission system based on the power operating domain, belonging to the field of offshore wind power flexible DC transmission technology. Background Technology

[0002] With the large-scale integration of offshore wind power, the voltage immunity of onshore power grids is weakened. The flexible reactive power support capability of flexible DC transmission systems can effectively improve grid stability. However, when reactive power fluctuations occur due to onshore AC faults or load changes, if the receiving-end converter station cannot implement timely reactive power control, it can lead to problems such as AC voltage deviation exceeding limits, system DC overvoltage, or even system shutdown. If the receiving-end converter station provides reactive power support, the output active power may be reduced due to various system constraints. To transmit as much active power as possible from wind farms and improve operational economy, the surplus active power output capacity of other converter stations can be used to replace the reduced active power. Given the requirements for reactive power support capability and operational economy in flexible DC transmission systems, there is an urgent need for a rapid active power conversion control method suitable for multiple converter stations in flexible DC transmission systems, considering reactive power fluctuations, to ensure system stability and economy. Summary of the Invention

[0003] To address the aforementioned technical problems, the purpose of this invention is to propose a reactive power control method for multiple converter stations in a flexible DC transmission system based on a power operating domain. By calculating the reactive power demand of the receiving-end converter station and constructing a power operating domain including apparent power constraints, AC current limit constraints, and AC bus voltage constraints of the converter station, a fast active power conversion control method considering reactive power fluctuations is designed to ensure stable operation of the converter station and achieve power mutual assistance and coordinated control among multiple converter stations.

[0004] To achieve the above objectives, the technical solution adopted by this invention is as follows: a reactive power control method for multiple converter stations in a flexible DC transmission system based on the power operating domain. The offshore wind power flexible DC transmission system includes an offshore wind farm, multiple sending-end converter stations, an onshore collection switch station, a DC transmission submarine cable, a DC transmission land cable, a high-voltage DC overhead line, multiple receiving-end converter stations, and a receiving-end power grid. The method includes the following steps: Step 1: Calculate the reactive power demand of the receiving-end converter station based on information such as the grid connection point voltage and the reactive power value before the reactive power fluctuation of the converter station; Step 2: Analyze the stable operation constraints of the receiving-end converter station. The constraints include the apparent power constraint of the converter station, the AC current limit constraint of the converter station, and the AC bus voltage constraint of the converter station. The steady-state power operating domain of the receiving-end converter station is determined by solving the constraint boundary. Step 3: Based on the reactive power demand and the steady-state power operating domain, design an active power rapid conversion control method that considers reactive power fluctuations to achieve power mutual assistance and coordinated control among multiple converter stations.

[0005] In a preferred embodiment of the present invention, the reactive power requirement of the receiving-end converter station in step 1 is as follows:

[0006] In the formula, QC,re represents the current reactive power required by the receiving-end converter station; QC0 represents the initial reactive power value of the converter station before the reactive power fluctuation; ΔQC represents the reactive power fluctuation demand of the converter station; Δ I Cq This refers to the change in reactive current at the converter station, i.e., the change in the q-axis component of the current. k r The reactive current proportionality coefficient; Δ U C This represents the change in AC voltage at the converter station, and its value is equal to the current AC voltage value. U C Initial AC voltage value before reactive power fluctuation U C0 difference; I CN This refers to the rated AC current of the converter station.

[0007] In a preferred embodiment of the present invention, the apparent power constraint expression for the receiving-end converter station in step 2 is as follows:

[0008] In the formula, SC is the apparent power output of the receiving-end converter station; PC is the active power output of the receiving-end converter station; and QC is the reactive power output of the receiving-end converter station. k T This represents the maximum overload factor of the receiving-end converter station. S CN This represents the apparent power rating of the receiving-end converter station.

[0009] The expression for the AC bus voltage constraint of the receiving-end converter station is:

[0010] In the formula, U S This is the effective value of the receiving-end grid line voltage; X S This is the equivalent impedance of the receiving-end power grid, ignoring the small line resistance.

[0011] The expression for the AC current limit constraint of the receiving-end converter station is:

[0012] In the formula, I Cmax This represents the maximum AC current at the receiving-end converter station.

[0013] Combining all the above constraints, the power operating domain of the receiving-end converter station can be obtained. If the first... i The power of the receiving-end converter station is operating at point ( P Ci , Q Ci When ), its maximum output active power limit within the power operating domain is . P Cimax The maximum limit for output reactive power is... Q Cimax If the reactive power fluctuates, the first... i The power of the receiving-end converter station is operating at point ( P’ Ci , Q Ci,re When ), its maximum output active power limit within the power operating domain is . P’ Cimax The maximum limit for output reactive power is... Q’ Cimax .

[0014] As a preferred embodiment of the present invention, step 3, based on reactive power demand and the steady-state power operating domain, designs an active power rapid transition control method that considers reactive power fluctuations, including: Assumption N There are 1 receiving-end converter stations I The converter station experienced reactive power fluctuations, while J ( I + J = N No reactive power fluctuations were observed at any of the converter stations. The reactive power required by the receiving-end converter station can be divided into two segments: reactive power segment one and reactive power segment two. It is assumed that the total wind power of the wind farm remains constant during the reactive power fluctuation process.

[0015] Reactive power segment one: Q Ci,re ≤ Q’ Cimax and P Cimax ≤ P’ Cimax At this time, the reactive power required by the receiving-end converter station is within the maximum limit of the output reactive power in the power operating domain. I The output active power of each receiving-end converter station remains constant, which can be expressed as:

[0016] Reactive power segment two: Q Ci,re ≤ Q’ Cimax and P Cimax > P’ Cimax At this time, in order to prioritize meeting the reactive power demand of the receiving-end converter station and ensure that it continues to operate within the power operating range, I The output active power of each receiving-end converter station must be reduced to the maximum active power limit in the power operating domain, which can be expressed as:

[0017] but I The reduction in active power at one receiving-end converter station requires replacement by other converter stations. The change in active power at all converter stations can be expressed as:

[0018] In the formula, k j For the first j The active power conversion ratio coefficient of each converter station is determined based on the converter station's ratio with... I The electrical distance between converter stations and I The reactive power fluctuation level of each converter station is given; Δ P C This represents the change in active power at the receiving-end converter station after reactive power fluctuations. P w This indicates the output power of the wind farm; This indicates the active power of the receiving-end converter station after the change.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention constructs a reactive power control method for multiple converter stations in a flexible direct transmission system based on the power operating domain, thereby maximizing the system's ability to transmit active power while ensuring system operational stability.

[0020] First, the reactive power demand of the receiving-end converter station is calculated based on information such as the grid connection point voltage and the initial value of reactive power before reactive power fluctuations at the converter station.

[0021] Secondly, by analyzing the stable operation constraints of the receiving-end converter station, including the apparent power constraint, AC current limit constraint, and AC bus voltage constraint, the power operating domain of the receiving-end converter station is determined to ensure that the receiving-end converter station always operates within this power operating domain, thus guaranteeing the safe and stable operation of the converter station.

[0022] Finally, based on power demand and the aforementioned steady-state power operating domain, an active power rapid conversion control method considering reactive power fluctuations is designed to achieve power mutual assistance and coordinated control among multiple converter stations.

[0023] Compared with traditional reactive power support methods, this invention comprehensively considers reactive power support requirements and power operating domain under multiple constraints. While meeting reactive power support requirements, it fully utilizes the active power transmission capacity of the system, providing key technical support for the safe, stable, and economical operation of the flexible DC transmission system. Attached Figure Description

[0024] Figure 1 This invention presents a reactive power control method for multiple converter stations in a flexible DC transmission system based on the power operating domain. Figure 2 This is the equivalent circuit diagram of the AC side of the receiving-end converter station used in this invention; Figure 3 This is a topology diagram of the flexible direct transmission system used in this invention; Figure 4 This is a schematic diagram of the power operating domain calculated according to an embodiment of the present invention. Detailed Implementation

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] This embodiment provides a reactive power control method for multiple converter stations in a flexible direct power transmission system based on the power operating domain.

[0027] Figure 2 This is the equivalent circuit diagram of the AC side of the receiving-end converter station used in this invention.

[0028] Figure 3 The diagram shows the topology of the flexible direct transmission system used in this invention, which includes a wind farm, a sending-end converter station, an onshore switch station, a receiving-end converter station, and a receiving-end power grid connected in sequence.

[0029] like Figure 1 As shown, the method used in this embodiment includes the following steps: Step 1: Calculate the reactive power demand of the receiving-end converter station based on information such as the grid connection point voltage and the reactive power value before the reactive power fluctuation of the converter station; Step 2: Analyze the stable operation constraints of the receiving-end converter station. The constraints include the apparent power constraint of the converter station, the AC current limit constraint of the converter station, and the AC bus voltage constraint of the converter station. The steady-state power operating domain of the receiving-end converter station is determined by solving the constraint boundary. Step 3: Based on the reactive power demand and the steady-state power operating domain, design an active power rapid conversion control method that considers reactive power fluctuations to achieve power mutual assistance and coordinated control among multiple converter stations.

[0030] In a preferred embodiment, the reactive power requirement of the receiving-end converter station in step 1 is as follows:

[0031] In the formula, Q C,re This represents the reactive power currently required by the receiving-end converter station. Q C0 The initial value of reactive power at the converter station before reactive power fluctuation; Δ Q C To meet the reactive power fluctuation requirements of the converter station; Δ I Cq This refers to the change in reactive current at the converter station, i.e., the change in the q-axis component of the current. k r The reactive current proportionality coefficient; Δ U C This represents the change in AC voltage at the converter station, and its value is equal to the current AC voltage value. U C Initial AC voltage value before reactive power fluctuation U C0 difference; I CN This refers to the rated AC current of the converter station.

[0032] In a preferred embodiment, the apparent power constraint expression for the receiving-end converter station in step 2 is as follows:

[0033] In the formula, S C The apparent power output of the receiving-end converter station; P C The active power output by the receiving-end converter station; Q C The reactive power output of the receiving-end converter station; k T This represents the maximum overload factor of the receiving-end converter station. S CN This represents the apparent power rating of the receiving-end converter station.

[0034] The expression for the AC bus voltage constraint of the receiving-end converter station is:

[0035] In the formula, U S This is the effective value of the receiving-end grid line voltage;X S This is the equivalent impedance of the receiving-end power grid, ignoring the small line resistance.

[0036] The expression for the AC current limit constraint of the receiving-end converter station is:

[0037] In the formula, I Cmax This represents the maximum AC current at the receiving-end converter station.

[0038] Combining all the above constraints, the power operating domain of the receiving-end converter station can be obtained. If the first... i The power of the receiving-end converter station is operating at point ( P Ci , Q Ci When ), its maximum output active power limit within the power operating domain is . P Cimax The maximum limit for output reactive power is... Q Cimax If the reactive power fluctuates, the first... i The power of the receiving-end converter station is operating at point ( P’ Ci , Q Ci,re When ), its maximum output active power limit within the power operating domain is . P’ Cimax The maximum limit for output reactive power is... Q’ Cimax .

[0039] As a preferred embodiment, step 3, based on reactive power demand and the steady-state power operating domain, designs an active power rapid transition control method that considers reactive power fluctuations, including: Assumption N There are 1 receiving-end converter stations I The converter station experienced reactive power fluctuations, while J ( I + J = N No reactive power fluctuations were observed at any of the converter stations. The reactive power required by the receiving-end converter station can be divided into two segments: reactive power segment one and reactive power segment two. It is assumed that the total wind power of the wind farm remains constant during the reactive power fluctuation process.

[0040] Reactive power segment one: Q Ci,re ≤ Q’ Cimax and P Cimax ≤P’ Cimax At this time, the reactive power required by the receiving-end converter station is within the maximum limit of the output reactive power in the power operating domain. I The output active power of each receiving-end converter station remains constant, which can be expressed as:

[0041] Reactive power segment two: Q Ci,re ≤ Q’ Cimax and P Cimax > P’ Cimax At this time, in order to prioritize meeting the reactive power demand of the receiving-end converter station and ensure that it continues to operate within the power operating range, I The output active power of each receiving-end converter station must be reduced to the maximum active power limit in the power operating domain, which can be expressed as:

[0042] but I The reduction in active power at one receiving-end converter station requires replacement by other converter stations. The change in active power at all converter stations can be expressed as:

[0043] In the formula, k j For the first j The active power conversion ratio coefficient of each converter station is determined based on the converter station's ratio with... I The electrical distance between converter stations and I The reactive power fluctuation level of each converter station is given; Δ P C This represents the change in active power at the receiving-end converter station after reactive power fluctuations. P w This indicates the output power of the wind farm; This indicates the active power of the receiving-end converter station after the change.

[0044] Example 1, referring to Figure 4 This is the first embodiment of the present invention, which provides a reactive power control method for multiple converter stations in a flexible DC transmission system based on the power operating domain.

[0045] In this embodiment, the effective value of the receiving end grid line voltage U S =1p.u., equivalent impedance of the receiving-end power grid X S =0.3pu, the maximum allowable overload factor of the receiving-end converter station. kT =1, maximum AC current at the receiving-end converter station I Cmax =1p.u. At this time, the apparent power constraint expression of the receiving-end converter station is:

[0046] The expression for the AC bus voltage constraint of the receiving-end converter station is:

[0047] The expression for the AC current limit constraint of the receiving-end converter station is:

[0048] Considering the above constraints, the power operating domain of the receiving-end converter station in this embodiment is as follows: Figure 4 As shown, the receiving-end converter station needs to operate within this operating domain.

[0049] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0050] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A reactive power control method for a multi-converter station flexible DC transmission system based on power operating domain, characterized in that, The flexible DC transmission system includes an offshore wind farm, multiple sending-end converter stations, an onshore collecting switch station, a DC submarine transmission cable, a DC land transmission cable, a high-voltage DC overhead line, multiple receiving-end converter stations, and a receiving-end power grid. The method includes the following steps: Step 1: Calculate the reactive power demand of the receiving-end converter station based on the grid connection point voltage and the reactive power value before the reactive power fluctuation of the converter station; Step 2: Analyze the stable operation constraints of the receiving-end converter station. The constraints include the apparent power constraint of the converter station, the AC current limit constraint of the converter station, and the AC bus voltage constraint of the converter station. The steady-state power operating domain of the receiving-end converter station is determined by solving the constraint boundary. Step 3: Based on the reactive power demand and the steady-state power operating domain, design an active power rapid conversion control method that considers reactive power fluctuations to achieve power mutual assistance and coordinated control among multiple converter stations.

2. The reactive power control method for a multi-converter station flexible direct transmission system based on power operating domain as described in claim 1, characterized in that, The reactive power requirement of the receiving-end converter station in step 1 is as follows: ， In the formula, Q C,re This represents the reactive power currently required by the receiving-end converter station. Q C0 The initial value of reactive power at the converter station before reactive power fluctuation; Δ Q C To meet the reactive power fluctuation requirements of the converter station; Δ I Cq This refers to the change in reactive current at the converter station, i.e., the change in the q-axis component of the current. k r This is the reactive current proportionality coefficient; Δ U C This represents the change in AC voltage at the converter station, and its value is equal to the current AC voltage value. U C Initial value of AC voltage before reactive power fluctuation U C0 difference; I CN This refers to the rated AC current of the converter station.

3. The reactive power control method for a multi-converter station flexible direct transmission system based on power operating domain as described in claim 1, characterized in that, The apparent power constraint expression for the receiving-end converter station in step 2 is: ， In the formula, S C The apparent power output of the receiving-end converter station; P C The active power output by the receiving-end converter station; Q C The reactive power output of the receiving-end converter station; k T This represents the maximum overload factor of the receiving-end converter station. S CN The apparent power rating of the receiving-end converter station; The expression for the AC bus voltage constraint of the receiving-end converter station is: ， In the formula, US is the effective value of the receiving end grid line voltage; XS is the equivalent impedance of the receiving end grid, ignoring small values ​​of line resistance; The expression for the AC current limit constraint of the receiving-end converter station is: ， In the formula, ICmax is the maximum AC current of the receiving-end converter station; Combining all the above constraints, the power operating domain of the receiving-end converter station is obtained. If the first... i The power of the receiving-end converter station is operating at point ( P Ci , Q Ci When ), its maximum output active power limit within the power operating domain is . P Cimax ; The maximum limit of output reactive power is Q Cimax If the reactive power fluctuates, the first i The power of the receiving-end converter station is operating at point ( P’ Ci , Q Ci,re When ), its maximum output active power limit within the power operating domain is . P’ Cimax ; The maximum limit of output reactive power is Q’ Cimax。 4. The reactive power control method for a multi-converter station flexible direct transmission system based on power operating domain as described in claim 1, characterized in that, Step 3, based on reactive power demand and the steady-state power operating domain, designs a rapid active power shift control method that considers reactive power fluctuations, including: Assumption N There are 1 receiving-end converter stations I The converter station experienced reactive power fluctuations, while J ( I + J = N No reactive power fluctuations were observed at any of the converter stations; the reactive power range required by the receiving-end converter station was divided into two segments, including reactive power segment one and reactive power segment two, assuming that the total wind power of the wind farm remained unchanged during the reactive power fluctuation process. Reactive power segment one: Q Ci,re ≤ Q’ Cimax and P Cimax ≤ P’ Cimax At this time, the reactive power required by the receiving-end converter station is within the maximum limit of the output reactive power in the power operating domain. I The output active power of each receiving-end converter station remains constant, expressed as: ， Reactive power segment two: Q Ci,re ≤ Q’ Cimax and P Cimax > P’ Cimax At this time, in order to prioritize meeting the reactive power demand of the receiving-end converter station and ensure that it continues to operate within the power operating range, I The output active power of each receiving-end converter station must be reduced to the maximum active power limit in the power operating domain, expressed as: ， but I The reduction in active power at one receiving-end converter station requires other converter stations to perform a power conversion. The change in active power at all converter stations is expressed as follows: ， In the formula, k j For the first j The active power conversion ratio coefficient of each converter station is determined based on the converter station's ratio with... I The electrical distance between converter stations and I The reactive power fluctuation level of each converter station is given; Δ P C This represents the change in active power at the receiving-end converter station after reactive power fluctuations. P w This indicates the output power of the wind farm; This indicates the active power of the receiving-end converter station after the change.

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