Offshore wind power three-winding transformer impedance type selection method

Abstract

The invention discloses an impedance type selection method for an offshore wind power three-winding transformer, which comprises the following steps of: (1) selecting high-medium short-circuit impedance Zk1-2 of the three-winding transformer according to a standard value, and (2) selecting the high-low impedance Zk1-3 of the three-winding transformer by considering two factors: short-circuit current tolerance level and main transformer relay protection setting requirement, and (4) according to the relationship between the short-circuit current limit value of the high-voltage side of the three-winding transformer and the impedance of the three windings, deducing to obtain the value of medium-low impedance Zk2-3. The method systematically elaborates how to determine the value range of the high-low impedance Zk1-3 according to the short-circuit current level and the relay protection setting requirement, and fills the blank of the three-winding transformer station power side impedance value method in the offshore wind power transmission scene. Meanwhile, the values of the high-medium impedance Zk1-2, the high-low impedance Zk1-3 and the medium-low impedance Zk2-3 are calculated from the theoretical angle, a design basis is provided for offshore wind power engineering equipment type selection, and the method has high universality and applicability in engineering equipment type selection.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of offshore wind power equipment selection, and particularly relates to a method for selecting the impedance of a three-winding transformer of offshore wind power. BACKGROUND

[0002] With the increase of installed capacity of offshore wind power projects year by year, the voltage level of the power transmission scheme is also increased, and the voltage level of the collection line of the offshore wind farm is increased from 35 kV to 66 kV. Therefore, the voltage level of the low-voltage side of the main transformer is also increased from 35 kV to 66 kV. Since the 66 kV station transformer is mostly an oil transformer, the area and cost of the offshore booster station are increased. At present, a three-winding transformer is mostly used, and the third winding adopts a 10.5 kV voltage level and is led out as a power supply for station electricity.

[0003] For an offshore wind power project, the high / middle / low voltage sides of the three-winding transformer are usually 230 kV / 66 kV / 10.5 kV. The high-voltage side is connected to the onshore AC system through a submarine cable, the middle-voltage side is connected to the offshore wind farm, and the low-voltage side is used for station electricity. Since the voltage difference between the high-voltage side and the low-voltage side is large, in order to limit the short-circuit current of the low-voltage side, a high-impedance transformer is usually selected, and too high short-circuit impedance will cause difficulties in the setting of the relay protection.

[0004] So far, the published literature has not specified the selection method of the impedance of the three-winding transformer of offshore wind power, especially has not specified how the short-circuit current level and the setting requirement of the relay protection limit the selection range of the impedance. Random selection of the impedance value of the three-winding transformer may not meet the short-circuit current level requirement of the offshore wind power project, and causes difficulties in the setting of the relay protection. SUMMARY

[0005] The purpose of the application is to provide a selection and calculation method of the impedance of the three-winding transformer of offshore wind power, and to provide a design basis for the selection of the transformer equipment of offshore wind power projects. To this end, the application adopts the following technical solution:

[0006] A selection method of the impedance of the three-winding transformer of offshore wind power, comprising:

[0007] (1) High-middle short-circuit impedance Z of the three-winding transformer k1-2 Generally selected according to the standard value, about 16%.

[0008] (2) High-low impedance Z of the three-winding transformer k1-3 Two factors need to be considered in the selection: the short-circuit current tolerance level and the setting requirement of the main transformer relay protection, and the high-low impedance Z of the three-winding transformer is determined according to the method k1-3 The value range is:

[0009] (3) Based on the short-circuit current limit on the high-voltage side of the three-winding transformer and the relationship between the three winding impedances, the medium-low impedance Z is derived. k2-3 Values.

[0010] Furthermore, in step (2), Z is determined based on the short-circuit current withstand level. k1-3 The specific implementation process of the selection is as follows:

[0011] The short-circuit impedance ensures that the short-circuit current still meets the short-circuit current withstand level of the transformer equipment under the most severe fault. Therefore, it is necessary to calculate the maximum short-circuit current on the low-voltage side of the three-winding transformer to determine the minimum value for short-circuit impedance selection.

[0012] When the land-based AC system operates under normal conditions, ignoring system impedance and field line impedance, the fault type on the low-voltage side of the three-winding transformer is a three-phase-to-ground short-circuit fault. This is the maximum short-circuit current operating condition, and the total short-circuit current referred to the low-voltage side satisfies:

[0013]

[0014] Among them, S T U is the rated capacity of a three-winding transformer. B3 The rated voltage on the low-voltage side, k w This is the short-circuit current conversion factor for offshore wind farms. This is the short-circuit current limit on the low-voltage side of a three-winding transformer.

[0015] Therefore, the high-low impedance Z is determined based on the short-circuit current withstand level. k1-3 The range of values ​​is:

[0016]

[0017] Furthermore, in step (2), Z is determined according to the main transformer relay protection setting requirements. k1-3 The specific implementation process of the selection is as follows:

[0018] Excessive short-circuit impedance can lead to insufficient sensitivity of the differential protection on the high-voltage side of the main transformer, failing to meet the requirements of the relay protection setting technical specifications. Therefore, it is necessary to calculate the minimum short-circuit current on the high-voltage side of the three-winding transformer to determine the maximum value for short-circuit impedance selection. The sensitivity verification condition for the main transformer differential protection is as follows: the onshore AC system is operating under low-mode conditions, the offshore wind turbines are not generating electricity, and a two-phase metallic ground fault occurs on the low-voltage side.

[0019] Define k1 and k2 as the slopes of the longitudinal differential protection action characteristic curve, I op For the differential protection operating current, I op.min I is the minimum operating current value. r For the longitudinal differential protection braking current, I r0Z represents the braking current value at the transition point between the two curves. smax Z is the system impedance in the small mode. L U is the line impedance. B1 The rated voltage on the high-voltage side, k 接线系数 To convert the coefficient to the secondary measuring equipment, K sen0 This is the minimum sensitivity requirement.

[0020] When I r ≤I r0 At that time, I op Z falls on the curve with slope k1. k1-3 satisfy:

[0021]

[0022] Z needs to be substituted. k1-3 Calculate I r Check if I is satisfied r ≤I r0 conditions.

[0023] When I r >I r0 At that time, I op Z falls on the curve with slope k2. k1-3 satisfy:

[0024]

[0025] Z needs to be substituted. k1-3 Calculate I r Check if I is satisfied r >I r0 conditions.

[0026] Therefore, the high-low impedance Z is determined according to the main transformer relay protection setting requirements. k1-3 The range of values ​​is:

[0027]

[0028] Furthermore, in (2), considering the short-circuit current withstand level and the main transformer relay protection setting factors, the high-low impedance Z of the three-winding transformer is... k1-3 The range of values ​​is:

[0029]

[0030] Furthermore, the specific implementation process of (3) is as follows:

[0031] The impedance relationship of the three windings and the short-circuit current limit on the high-voltage side must meet the following constraints:

[0032]

[0033] Among them, the high-medium impedance Z of the three-winding transformer k1-2 High-low impedance Z k1-3 Rated current I on the high-voltage side B1 High-voltage side short-circuit current limit Given the quantities, the high-voltage side impedance Z of the three-winding transformer can be obtained by solving. k1 and medium-low impedance Z k2-3 .

[0034] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0035] (1) For offshore wind power transmission scenarios, this invention systematically explains how the short-circuit current level and relay protection setting requirements are used to determine the high-low impedance Z. k1-3 The range of values ​​fills the gap in the method for determining the impedance of the power supply side of a three-winding transformer station in the scenario of offshore wind power transmission.

[0036] (2) This invention proposes a method for selecting the impedance of a three-winding transformer. This method calculates the high-medium impedance Z from a theoretical perspective. k1-2 High-low impedance Z k1-3 Medium to low impedance Z k2-3 The value of provides a design basis for the selection of equipment in offshore wind power projects.

[0037] (3) The method of the present invention is simple to implement and has strong versatility and applicability in the selection of engineering equipment. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of a three-winding transformer for offshore wind power.

[0039] Figure 2 This is a schematic diagram of the longitudinal differential protection action characteristic curve of the relay protection device manufactured by the relay protection manufacturer. Detailed Implementation

[0040] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] like Figure 1 As shown, the three windings and voltage levels of the offshore wind turbine three-winding transformer are as follows: 230kV for the high-voltage side, 66kV for the medium-voltage side, and 10.5kV for the low-voltage side. The high-voltage side is connected to the onshore AC system via a submarine cable, the medium-voltage side is connected to the offshore wind farm, and the low-voltage side provides power to the station. The impedance selection method for the offshore wind turbine three-winding transformer includes:

[0042] (1) High-medium short-circuit impedance Z of three-winding transformer k1-2 The standard value is selected as 16%.

[0043] (2) High-low impedance Z of three-winding transformer k1-3 The selection of a transformer requires consideration of two factors: short-circuit current withstand level and main transformer relay protection setting requirements. Based on this method, the high-low impedance Z of the three-winding transformer is determined. k1-3 Range of values;

[0044] (4) Based on the short-circuit current limit on the high-voltage side of the three-winding transformer and the relationship between the three winding impedances, the medium-low impedance Z is derived. k2-3 Values.

[0045] In (2), Z is determined based on the short-circuit current withstand level. k1-3 The specific implementation process of the selection is as follows:

[0046] The short-circuit impedance ensures that the short-circuit current still meets the short-circuit current withstand level of the transformer equipment under the most severe fault. Therefore, it is necessary to calculate the maximum short-circuit current on the low-voltage side of the three-winding transformer to determine the minimum value for short-circuit impedance selection.

[0047] When the onshore AC system is operating under normal conditions, ignoring the system impedance and the line impedance within the field, the fault type on the low-voltage side of the three-winding transformer is a three-phase ground fault. This is the maximum short-circuit current condition. The short-circuit current calculation includes both the onshore AC system side and the offshore wind farm side.

[0048] When a three-phase ground fault occurs on the low-voltage side of a three-winding transformer, the short-circuit current on the land-based AC system side is:

[0049]

[0050] Among them, S T U is the rated capacity of a three-winding transformer. B3 The rated voltage on the low-voltage side, I B3 This is the rated current on the low-voltage side.

[0051] Because the short-circuit current supplied by the offshore wind farm side is limited by the grid-side converter of the wind turbine, this limiting value is generally 1.2 pu to 1.5 pu. Since the grid-side converter of the wind turbine is a non-ideal current source, it is affected by line impedance and transformer impedance. To improve system reliability and reduce calculation difficulty, when a three-phase ground fault occurs on the low-voltage side of the three-winding transformer, the short-circuit current on the offshore wind farm side is calculated according to the following relationship:

[0052]

[0053] Where, k w This is the short-circuit current conversion factor for offshore wind farms, used to characterize the relationship between the maximum short-circuit current and the rated current of the wind farm.

[0054] When a three-phase ground fault occurs on the low-voltage side of a three-winding transformer, the total short-circuit current referred to the low-voltage side satisfies:

[0055]

[0056] in, This is the short-circuit current limit on the low-voltage side of a three-winding transformer.

[0057] In summary, the high-low impedance Z is determined based on the short-circuit current withstand level. k1-3 The range of values ​​is:

[0058]

[0059] Furthermore, in step (2), Z is determined according to the main transformer relay protection setting requirements. k1-3 The specific implementation process of the selection is as follows:

[0060] Excessive short-circuit impedance will result in low sensitivity of the differential protection on the high-voltage side of the main transformer, which does not meet the requirements of the relay protection setting technical specifications. Therefore, it is necessary to calculate the minimum short-circuit current on the high-voltage side of the three-winding transformer to determine the maximum value of the short-circuit impedance selection.

[0061] The sensitivity verification conditions for the main transformer differential protection are as follows: the onshore AC system is operating in low mode, the wind turbines in the offshore wind farm are not generating electricity, and a two-phase metallic ground fault occurs on the low-voltage side.

[0062] The two-phase short-circuit current on the low-voltage side is:

[0063]

[0064] Among them, Z smax Z is the system impedance in the small mode. L This represents the line impedance.

[0065] The short-circuit current measured by the high-voltage side secondary equipment is:

[0066]

[0067] Where, k 接线系数 To convert the coefficients to secondary measuring equipment, U B3 U is the rated voltage on the low-voltage side. B1 This is the rated voltage on the high-voltage side.

[0068] Longitudinal differential protection braking current I r Generally, 0.5 times the short-circuit current is taken, that is...

[0069]

[0070] Figure 2This is the operating characteristic curve of the longitudinal differential protection device from the relay protection manufacturer. The curve mainly includes two segments with slopes k1 and k2. The area above the curve is the operating region of the longitudinal differential protection. op.min I is the minimum operating current value. r0 This represents the braking current value at the transition point between the two curve segments. (Through...) Figure 2 The curve can be obtained from the braking current I r The operating current I is derived. op The expression is:

[0071]

[0072] The sensitivity of the longitudinal differential protection should meet the following requirements:

[0073]

[0074] Among them, K sen0 This is the minimum sensitivity requirement. According to the technical specification "Guideline for Calculation of Relay Protection Settings for Large Generators and Transformers" (DL / T684-2012), the sensitivity of the longitudinal differential protection of the main transformer shall not be less than 1.5, i.e., K is taken as... sen0 =1.5.

[0075] When I r ≤I r0 At that time, I op If the curve lies on the slope k1, then the sensitivity of the differential protection satisfies the following:

[0076]

[0077] We can obtain:

[0078]

[0079] Substitute I r We can obtain:

[0080]

[0081] Finally, we need to substitute Z. k1-3 Calculate I r Check if I is satisfied r ≤I r0 conditions.

[0082] When I r >I r0 At that time, I op If the curve lies on the slope of k2, then the sensitivity of the differential protection satisfies the following:

[0083]

[0084] We can obtain:

[0085]

[0086] Substitute I r We can obtain:

[0087]

[0088] Finally, we need to substitute Z. k1-3 Calculate I r Check if I is satisfied r >I r0 conditions.

[0089] In summary, the high-low impedance Z is determined based on the main transformer relay protection setting requirements. k1-3 The range of values ​​is:

[0090]

[0091] Furthermore, in step (2), the high-low impedance Z of the three-winding transformer... k1-3 The range of values ​​is:

[0092]

[0093] The specific implementation process of step (3) is as follows:

[0094] The impedance relationship of the three windings and the short-circuit current limit on the high-voltage side must meet the following constraints:

[0095]

[0096] Among them, the high-medium impedance Z of the three-winding transformer k1-2 High-low impedance Z k1-3 Rated current I on the high-voltage side B1 High-voltage side short-circuit current limit Given the quantities, the high-voltage side impedance Z of the three-winding transformer can be obtained by solving. k1 and medium-low impedance Z k2-3 .

[0097] In this embodiment, the parameters of a certain offshore wind power project are shown in the table below:

[0098]

[0099] Assume I r ≤I r0 At this time, the operating current I op The high-low impedance Z of the three-winding transformer lies on the curve with slope k1. k1-3 The range of values ​​is

[0100] 71.2% ≤ Z k1-3≤179.3%

[0101] Substitute Z k1-3 The check does indeed satisfy I r ≤I r0 The condition, therefore Z k1-3 The range of values ​​is reasonable.

[0102] Select Z according to actual engineering requirements. k1-3 The value of Z can be determined by considering the impedance relationship of the three windings. k2-3 For example, when Z k1-3 =179.3% and Z k1-2 When = 16%, Z can be solved. k2-3 =165.5%.

[0103] Therefore, the impedance values ​​of the three windings of the three-winding transformer are completely determined. A calculation example illustrates the feasibility of the impedance selection method for offshore wind power three-winding transformers proposed in this invention.

[0104] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for selecting the impedance of a three-winding transformer for offshore wind power, characterized in that, (1) High-medium short-circuit impedance Z of three-winding transformer k1-2 Select according to the standard value; (2) High-low impedance Z of three-winding transformer k1-3 The selection of equipment needs to consider two factors: the short-circuit current withstand level and the setting requirements of the main transformer relay protection; (3) Based on the short-circuit current limit on the high-voltage side of the three-winding transformer and the relationship between the three winding impedances, the medium-low impedance Z is derived. k2-3 Values.

2. The impedance selection method for a three-winding transformer according to claim 1, characterized in that, The high-low impedance Z is determined based on the short-circuit current withstand level. k1-3 The range of values ​​is: Among them, S T U is the rated capacity of a three-winding transformer. B3 The rated voltage on the low-voltage side, k w This is the short-circuit current conversion factor for offshore wind farms. This is the short-circuit current limit on the low-voltage side of a three-winding transformer.

3. The impedance selection method for a three-winding transformer according to claim 1, characterized in that, The high-low impedance Z is determined according to the main transformer relay protection setting requirements in (1). k1-3 The range of values ​​is: Where k1 and k2 are the slopes of the longitudinal differential protection action characteristic curve, I op.min I is the minimum operating current value. r0 Z represents the braking current value at the transition point between the two curves. smax Z is the system impedance in the small mode. L U is the line impedance. B1 The rated voltage on the high-voltage side, k 接线系数 To convert the coefficients to secondary measuring equipment, K sen0 This is the minimum sensitivity requirement.

4. The impedance selection method for a three-winding transformer according to claim 1, characterized in that, In (2), the high-low impedance Z of the three-winding transformer k1-3 The range of values ​​is:

5. The impedance selection method for a three-winding transformer according to claim 1, characterized in that, The impedance relationship of the three windings and the short-circuit current limit on the high-voltage side must meet the following constraints: Among them, the high-medium impedance Z of the three-winding transformer k1-2 High-low impedance Z k1-3 Rated current I on the high-voltage side B1 High-voltage side short-circuit current limit Given the quantities, the low impedance Z of the three-winding transformer can be solved. k2-3 .

6. The impedance selection method for a three-winding transformer according to claim 1, characterized in that, The high-to-medium short-circuit impedance Z of the three-winding transformer k1-2 The selected value is 16%.

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