A method for reactive power control of an entire station in an ultra-high voltage flexible DC transmission system

Abstract

This invention discloses a method for controlling the reactive power of an entire station in an ultra-high voltage flexible direct current (UHVDC) transmission system, belonging to the technical field of DC transmission. The method employs a load balancing principle to distribute the reactive power output demand of the entire station to each converter valve in the converter station. Specifically, the reactive power output demand is distributed to the corresponding converter valve according to the ratio of the rated capacity of each operating converter valve to the sum of the rated capacities of all operating converter valves. The technical solution of this application distributes the reactive power output demand of the entire station to the four valve groups of the UHVDC converter station, thereby reducing reactive power circulation between converter valves and maintaining similar operating conditions for each converter valve.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission technology, specifically relating to a method for controlling the reactive power of an entire station in an ultra-high voltage flexible DC power transmission system. Background Technology

[0002] Flexible DC transmission uses voltage source converters, which can independently adjust active and reactive power output, improve the transmission capacity of AC systems, and easily form multi-terminal DC transmission systems. It has significant competitiveness in applications such as renewable energy generation and grid connection, power supply for isolated cities, and AC system interconnection. With advancements in power electronic devices and control technologies, the capacity and voltage levels of flexible DC transmission are becoming increasingly higher.

[0003] To meet the requirements of high-capacity power transmission, it is necessary to increase the number of sub-modules to improve their voltage level. However, cascading too many sub-modules increases the difficulty of valve control equipment. Therefore, a bipolar structure is adopted, and each pole consists of two converter valves connected in series on the DC side, thus realizing the flexible DC ultra-high voltage project.

[0004] Ultra-high voltage (UHV) flexible direct current (UHVDC) transmission systems offer higher reliability, but their control is also more complex. Each pole has three operating modes: single-valve group, two-valve group, and shutdown. A total of nine operating modes can be achieved through bipolar arrangements. Research on reactive power optimization control under different modes is limited. To fully leverage the structural advantages of UHVDC flexible direct current transmission systems, an effective method for reactive power control across the entire station is needed. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a method for controlling the reactive power of an entire station in an ultra-high voltage flexible DC transmission system. This method enables coordinated control and optimization of reactive power across multiple converter units in the ultra-high voltage flexible DC transmission system based on their operating modes and converter valve capacities.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A method for controlling reactive power across an ultra-high voltage flexible direct current (UHVDC) transmission system is disclosed. The UHVDC converter station comprises two converter units: a positive unit and a negative unit. Each converter unit includes two converter valves connected in series on the DC side. The method employs a load balancing principle to allocate the reactive power output demand of the entire station to each converter valve. Specifically, the reactive power output demand of the entire station is allocated to the corresponding converter valve according to the ratio of the rated capacity of each operating converter valve to the sum of the rated capacities of all operating converter valves.

[0008] Furthermore, each converter valve in the ultra-high voltage flexible DC transmission system can be independently controlled to be in an operating or locked state.

[0009] Furthermore, the method specifically includes the following steps:

[0010] The operating status of the four converter valves in the entire station is collected, and the operating switching coefficients K1, K2, K3, and K4 of the four converter valves are set according to the operating status. When the converter valve is in the operating state, the operating switching coefficient is 1, and when it is in the locked state, the operating switching coefficient is 0.

[0011] Calculate the sum of the rated capacities of all operating converter valves: SNORM = S1NORM×K1 + S2NORM×K2 + S3NORM×K3 + S4NORM×K4, where S1NORM, S2NORM, S3NORM, and S4NORM are the rated powers of the four converter valves in the entire station.

[0012] The reactive power output coefficient of the entire station is calculated as Kall = Qref / SNORM, where Qref is the reactive power command of the entire station;

[0013] The required reactive power output for each converter valve is calculated as follows:

[0014] Q1 = S1NORM × K1 * Kall

[0015] Q2 = S2NORM × K2 * Kall

[0016] Q3 = S3NORM × K3 * Kall

[0017] Q4 = S4NORM × K4 * Kall

[0018] Q1, Q2, Q3, and Q4 represent the reactive power output of the four converter valves in the entire station.

[0019] Furthermore, the method specifically includes the following steps:

[0020] The operating status of the four converter valves in the entire station is collected, and the operating switching coefficients K1, K2, K3, and K4 of the four converter valves are set according to the operating status. When the converter valve is in the operating state, the operating switching coefficient is 1, and when it is in the locked state, the operating switching coefficient is 0.

[0021] Calculate the sum of the rated capacities of all operating converter valves: SNORM = S1NORM×K1 + S2NORM×K2 + S3NORM×K3 + S4NORM×K4, where S1NORM, S2NORM, S3NORM, and S4NORM are the rated powers of the four converter valves in the entire station.

[0022] Calculate the percentage of rated capacity for each converter valve:

[0023] KS1 = S1NORM × K1 / SNORM;

[0024] KS2 = S2NORM × K2 / SNORM;

[0025] KS3 = S3NORM × K3 / SNORM;

[0026] KS4 = S1NORM × K4 / SNORM;

[0027] Among them, KS1, KS2, KS3, and KS4 are the percentages of the rated capacity of the four converter valves in the entire station;

[0028] The required reactive power output for each converter valve is calculated as follows:

[0029] Q1 = Qref × KS1;

[0030] Q2 = Qref × KS2;

[0031] Q3 = Qref × KS3;

[0032] Q4 = Qref × KS4;

[0033] Where Qref is the reactive power command for the entire station; Q1, Q2, Q3, and Q4 are the reactive power outputs of the four converter valves for the entire station.

[0034] The beneficial effects of this application are: The reactive power control method of the entire station of the UHV flexible DC transmission system proposed in this application, based on the principle of load balancing, distributes the reactive power output demand of the entire station to the four valve groups of the UHV flexible DC converter station, thereby reducing the reactive power circulation between the converter valves and maintaining each converter valve under similar operating conditions. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the converter station of an ultra-high voltage flexible DC transmission system;

[0036] Figure 2 This application provides a method for controlling the reactive power of an ultra-high voltage flexible DC transmission system across the entire station.

[0037] Figure 3 This is another method for controlling the reactive power of an ultra-high voltage flexible DC transmission system at the entire station, provided in the embodiments of this application. Detailed Implementation

[0038] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] like Figure 1The diagram shows a converter station of an ultra-high voltage flexible direct current (UHVDC) transmission system, comprising two converter units (positive and negative poles), each including two converter valves connected in series on the DC side. This application provides a method for controlling the reactive power of the entire UHVDC flexible direct current transmission system. This method uses a load balancing principle to distribute the reactive power output demand of the entire station to each converter valve. Specifically, the reactive power output demand of the entire station is distributed to the corresponding converter valve according to the ratio of the rated capacity of each operating converter valve to the sum of the rated capacities of all operating converter valves.

[0040] In the above embodiments, each converter valve of the ultra-high voltage flexible DC transmission system can be independently controlled to be in an operating or locked state.

[0041] In optional embodiments, such as Figure 2 As shown, a method for controlling the reactive power of an entire station in an ultra-high voltage flexible DC transmission system specifically includes the following steps:

[0042] S110: Collect the operating status of the four converter valves in the entire station, and set the operating switching coefficients K1, K2, K3, and K4 of the four converter valves according to the operating status. When the converter valve is in the operating state, the operating switching coefficient is 1, and when it is in the locked state, the operating switching coefficient is 0.

[0043] S120: Calculate the sum of the rated capacities of all converter valves in operation: SNORM = S1NORM×K1 + S2NORM×K2 + S3NORM×K3 + S4NORM×K4, where: S1NORM, S2NORM, S3NORM, and S4NORM are the rated powers of the four converter valves in the entire station.

[0044] S130: Calculate the total reactive power output coefficient as Kall = Qref / SNORM, where Qref is the total reactive power command;

[0045] S140: Calculate the reactive power output required for each converter valve:

[0046] Q1 = S1NORM × K1 * Kall

[0047] Q2 = S2NORM × K2 * Kall

[0048] Q3 = S3NORM × K3 * Kall

[0049] Q4 = S4NORM × K4 * Kall

[0050] Q1, Q2, Q3, and Q4 represent the reactive power output of the four converter valves in the entire station.

[0051] In optional embodiments, such as Figure 3As shown, a method for controlling the reactive power of an entire station in an ultra-high voltage flexible DC transmission system specifically includes the following steps:

[0052] S210: Collect the operating status of the four converter valves in the entire station, and set the operating switching coefficients K1, K2, K3, and K4 of the four converter valves according to the operating status. When the converter valve is in the operating state, the operating switching coefficient is 1, and when it is in the locked state, the operating switching coefficient is 0.

[0053] S220: Calculate the sum of the rated capacities of all converter valves in operation: SNORM = S1NORM×K1 + S2NORM×K2 + S3NORM×K3 + S4NORM×K4, where: S1NORM, S2NORM, S3NORM, and S4NORM are the rated powers of the four converter valves in the entire station.

[0054] S230: Calculate the percentage of rated capacity for each converter valve:

[0055] KS1 = S1NORM × K1 / SNORM;

[0056] KS2 = S2NORM × K2 / SNORM;

[0057] KS3 = S3NORM × K3 / SNORM;

[0058] KS4 = S1NORM × K4 / SNORM;

[0059] Among them, KS1, KS2, KS3, and KS4 are the percentages of the rated capacity of the four converter valves in the entire station;

[0060] S240: Calculate the reactive power output required for each converter valve:

[0061] Q1 = Qref × KS1;

[0062] Q2 = Qref × KS2;

[0063] Q3 = Qref × KS3;

[0064] Q4 = Qref × KS4;

[0065] Where Qref is the reactive power command for the entire station; Q1, Q2, Q3, and Q4 are the reactive power outputs of the four converter valves for the entire station.

[0066] The control method of this application will be introduced below with a specific case.

[0067] In the converter station of the UHV flexible DC transmission system, one converter valve is in operation at the positive pole and two converter valves are in operation at the negative pole. The target reactive power of the entire station is 600 MVar, and the rated power of the four converter valves is 1000 MW.

[0068] One method for controlling reactive power across the entire power station includes the following steps:

[0069] Step 310: Collect the operating status of the four converter valves in the entire station, and determine their operating switching coefficients as K1=1, K2=0, K3=1, and K4=1 based on the operating conditions of the four converter valves.

[0070] Step 320: The actual total power is SNORM = S1NORM × K1 + S2NORM × K2 + S3NORM × K3 + S4NORM × K4 = 3000MW.

[0071] Step 330: The reactive power output coefficient for the entire station is Kall = 600 / 3000 = 0.2.

[0072] Step 340: The output reactive power of each converter valve is as follows:

[0073] Q1=S1NORM×K1*Kall=200Mvar

[0074] Q2 = S2NORM × K2 * Kall = 0 Mvar

[0075] Q3=S3NORM×K3*Kall=200Mvar

[0076] Q4=S4NORM×K4*Kall=200Mvar

[0077] If converter valve #2 is subsequently put into operation in the system, the reactive power distribution will then become as follows, according to the method in this embodiment:

[0078] The reactive power output coefficient for the entire station is Kall = 600 / 4000 = 0.15, and the reactive power output of each converter valve is as follows:

[0079] Q1=S1NORM×K1*Kall=150Mvar

[0080] Q2=S2NORM×K2*Kall=150Mvar

[0081] Q3=S3NORM×K3*Kall=150Mvar

[0082] Q4=S4NORM×K4*Kall=150Mvar

[0083] The second method for reactive power control of the entire power station includes the following steps:

[0084] Step 410: Collect the operating status of the four converter valves in the entire station, and determine their operating switching coefficients as K1=1, K2=0, K3=1, and K4=1 based on the operating conditions of the four converter valves.

[0085] Step 420: The actual total power is SNORM = S1NORM × K1 + S2NORM × K2 + S3NORM × K3 + S4NORM × K4 = 3000MW.

[0086] Step 430: Calculate the rated capacity percentage of each converter valve:

[0087] KS1=S1NORM×K1 / SNORM=1 / 3;

[0088] KS2 = S2NORM × K2 / SNORM = 0;

[0089] KS3=S3NORM×K3 / SNORM=1 / 3;

[0090] KS4=S1NORM×K4 / SNORM=1 / 3;

[0091] Step 440: Calculate the reactive power output required for each converter valve:

[0092] Q1 = Qref × KS1 = 200 Mvar;

[0093] Q2 = Qref × KS2 = 0Mvar;

[0094] Q3 = Qref × KS3 = 200 Mvar;

[0095] Q4 = Qref × KS4 = 200 Mvar;

[0096] If converter valve #2 is subsequently put into operation in the system, the reactive power distribution will then become as follows, according to the method in this embodiment:

[0097] KS1 = 0.25, KS2 = 0.25, KS3 = 0.25, KS4 = 0.25, and the reactive power output of each converter valve is as follows:

[0098] Q1 = Qref × KS1 = 150 Mvar

[0099] Q2 = Qref × KS2 = 150 Mvar

[0100] Q3 = Qref × KS3 = 150 Mvar

[0101] Q4 = Qref × KS4 = 150 Mvar

[0102] Using the scheme of this application embodiment, for the total reactive power command Qref determined at each moment, the sum of the reactive power output of the four converter valves is equal to Qref, and the ratio of the reactive power output of each converter valve to its rated capacity is equal.

[0103] Finally, it should be noted that the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting them. They cannot be used to limit the scope of protection of the present invention. Any modifications made based on the technical ideas proposed in the present invention are covered within the scope of the claims of the present invention.

Claims

1. A method for controlling the reactive power of an entire station in an ultra-high voltage flexible direct current (UHVDC) transmission system, wherein the converter station of the UHVDC transmission system comprises two converter units: a positive unit and a negative unit, and each converter unit further comprises two converter valves connected in series on the DC side, characterized in that: The method uses the principle of load balancing to allocate the reactive power output demand of the entire station to each converter valve in the converter station. That is, according to the ratio of the rated capacity of each converter valve in operation to the sum of the rated capacities of all converter valves in operation, the reactive power output demand of the entire station is allocated to the corresponding converter valve according to the ratio. The method specifically adopts any of the following schemes: Method 1 includes the following steps: The operating status of the four converter valves in the entire station is collected, and the operating switching coefficients K1, K2, K3, and K4 of the four converter valves are set according to the operating status. When the converter valve is in the operating state, the operating switching coefficient is 1, and when it is in the locked state, the operating switching coefficient is 0. Calculate the sum of the rated capacities of all operating converter valves: SNORM = S1NORM×K1 + S2NORM×K2 + S3NORM×K3 + S4NORM×K4, where S1NORM, S2NORM, S3NORM, and S4NORM are the rated powers of the four converter valves in the entire station. The reactive power output coefficient of the entire station is calculated as Kall = Qref / SNORM, where Qref is the reactive power command of the entire station; The required reactive power output for each converter valve is calculated as follows: Q1 = S1NORM × K1 * Kall Q2 = S2NORM × K2 * Kall Q3 = S3NORM × K3 * Kall Q4 = S4NORM × K4 * Kall Q1, Q2, Q3, and Q4 are the reactive power outputs of the four converter valves in the entire station, respectively. Method 2 includes the following steps: The operating status of the four converter valves in the entire station is collected, and the operating switching coefficients K1, K2, K3, and K4 of the four converter valves are set according to the operating status. When the converter valve is in the operating state, the operating switching coefficient is 1, and when it is in the locked state, the operating switching coefficient is 0. Calculate the sum of the rated capacities of all operating converter valves: SNORM = S1NORM×K1 + S2NORM×K2 + S3NORM×K3 + S4NORM×K4; Calculate the percentage of rated capacity for each converter valve: KS1 = S1NORM × K1 / SNORM; KS2 = S2NORM × K2 / SNORM; KS3 = S3NORM × K3 / SNORM; KS4 = S1NORM × K4 / SNORM; Among them, KS1, KS2, KS3, and KS4 are the percentages of the rated capacity of the four converter valves in the entire station; The required reactive power output for each converter valve is calculated as follows: Q1 = Qref × KS1; Q2 = Qref × KS2; Q3 = Qref × KS3; Q4 = Qref × KS4.

2. The method for controlling the reactive power of the entire station in an ultra-high voltage flexible DC transmission system as described in claim 1, characterized in that: Each converter valve in the ultra-high voltage flexible DC transmission system can be independently controlled to be in operation or locked state.

Images