Current transformer selection method and system for accessing million unit into power grid of 220kV and below

Abstract

The invention provides a current transformer selection method suitable for a million unit connected to a power grid of 220kV and below. The method comprises the following steps: 1, acquiring parameters; 2, calculating a comprehensive primary time constant; and 3, selecting a corresponding current transformer according to the comprehensive primary time constant. Compared with the prior art, the current transformer selection method has the advantages that the comprehensive primary time constant is calculated and compared with the reference value, so that a scientific and effective current transformer selection method is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to a current transformer, in particular to a current transformer selection method for a million unit access to 220kV and below power grid. BACKGROUND

[0002] The existing technology mainly has the following technical problems in the selection of traditional current transformers:

[0003] 1. Lack of adaptability standard in traditional CT selection The existing specification does not propose differentiated standards for CT selection of large-capacity million units accessing medium and low voltage power grid (220kV and below), resulting in selection depending on experience and easy to appear protection misoperation or insufficient measurement accuracy. 2. Insufficient matching of transient characteristics and power grid parameters The short-circuit current decay time constant of large-capacity units is large, and conventional CTs are easy to saturate, while the existing TPY level CT selection does not combine the influence of line impedance ratio (per unit) on the primary time constant. 3. Difficulty in balancing economy and reliability The existing technology does not establish a correlation model of short-circuit current amplitude, duration and CT saturation characteristics, resulting in over-provisioning of high-cost TPY level CT or risk of failure of ordinary CT protection.

[0004] Specifically, the existing standards such as DL / T 866-2015 do not establish special CT selection standards for million units accessing 220kV power grid, and engineering practice is forced to follow 220kV system selection specification or 500kV system selection specification. That is, in the existing technology, million units are connected to 500kv power grid, and units below million are connected to 220kv power grid. However, the situation of connecting million units to 220kv power grid has occurred, and under this condition, no existing standard describes or applies to this condition, so experience must be used, resulting in a technical gap. This will cause the technical personnel in the field to be unable to determine whether the transient characteristic margin of the CT will be excessive if the TPY level CT is used when the million units access the 220kV power grid. Also, it is unable to determine the probability of CT saturation under fault conditions if the 5P level CT is used when the million units access the 220kV power grid.

[0005] Therefore, there is an urgent need in the existing technology for a current transformer selection method and standard that can be used when million units access 220kV and below power grid. SUMMARY

[0006] The purpose of the present application is to provide a current transformer selection method and standard that can be used when million units access 220kV and below power grid.

[0007] The first aspect of the present application discloses a current transformer selection method for million units accessing 220kV and below power grid, characterized in that the method comprises the following steps:

[0008] (1) obtaining parameters

[0009] According to the requirement, parameters of corresponding unit and power grid are obtained, which include: short-circuit current provided by single million unit connected to corresponding power grid, voltage of the power grid is less than or equal to 220kv; primary time constant of single million unit; primary time constant and number of million units of the power grid line obtained according to specification reference value;

[0010] (2) calculating comprehensive primary time constant

[0011] In the case of assuming that short-circuit current at fault point is provided by unit side and system side, the comprehensive primary time constant is calculated, and the calculation formula is as follows:

[0012]

[0013] Wherein, t1 is the comprehensive primary time constant, x3 is the number of million units, x1 is the primary time constant of single million unit;

[0014] (3) selecting mutual inductor according to calculation result

[0015] The comprehensive primary time constant obtained in the step (2) is compared with the reference value, if the comprehensive primary time constant is greater than the reference value, TPY level mutual inductor is selected, if the comprehensive primary time constant is less than the reference value, 5P level mutual inductor is selected.

[0016] In a preferred embodiment, the step (1) further comprises obtaining primary time constant of the power grid line calculated according to per unit value.

[0017] In a preferred embodiment, the step (2) further comprises calculating per unit comprehensive primary time constant.

[0018] In a preferred embodiment, the calculation formula of the per unit comprehensive primary time constant is as follows:

[0019]

[0020] Wherein, t2 is the per unit comprehensive primary time constant, x3 is the number of million units, x1 is the primary time constant of single million unit, x2 is the primary time constant obtained according to per unit value of the power grid line.

[0021] In a preferred embodiment, the step (3) further comprises the following steps:

[0022] The per unit comprehensive primary time constant is compared with the reference value, as long as one of the per unit comprehensive primary time constant and the comprehensive primary time constant is greater than the reference value, TPY level mutual inductor is adopted, otherwise 5P level mutual inductor is used.

[0023] In a preferred embodiment, the reference value in step (3) can be adjusted to 80-120 ms.

[0024] In a preferred embodiment, the reference value includes two different reference values for comparing the integrated time constant and the unit time constant, respectively, and if one of them does not meet the standard, the TPY level current transformer is used for safety.

[0025] In a preferred embodiment, step (1) further includes calculating the overall time constant in the combined state of multiple million units.

[0026] In a preferred embodiment, the combined state includes parallel and series.

[0027] The second aspect of the application discloses a current transformer selection system, characterized in that the system comprises a data acquisition module, a calculation module, a selection module and an output module, wherein:

[0028] The data acquisition module is configured to collect data of the million unit and the power grid and input it into the calculation module;

[0029] The calculation module calculates the integrated time constant and the unit time constant according to the data calculated by the data acquisition module and compares them with the reference value;

[0030] The selection module selects the current transformer according to the comparison result of the calculation module and outputs the result to the output module;

[0031] The output module outputs the result to an output device, and the output device includes a display.

[0032] Compared with the prior art, the application has the following beneficial effects:

[0033] (1) The application proposes a selection criterion for current transformers for the special scenario of million unit access to 220kV and below power grid, which is different from the existing technology of reasoning to select current transformers from similar cases. The application proposes a specific selection method, which is characterized by calculating corresponding data, and further, according to the above, the application also calculates the integrated time constant, which reduces the error compared with the traditional experience method, effectively avoids the risk of misselecting 5P level current transformers to cause saturation or overcautious use of TPY level current transformers to cause cost increase.

[0034] (3) Designed for million unit access to 220kV and below voltage level,

[0035] A large number of technical features are described in the specification of the present application, which are distributed in various technical solutions. If all possible combinations of technical features (i.e. technical solutions) of the present application are listed, the specification will be too long. In order to avoid this problem, each technical feature disclosed in the above summary of the application, each technical feature disclosed in the following various embodiments and examples, and each technical feature disclosed in the drawings can be freely combined with each other to form various new technical solutions (which should be regarded as having been described in the specification), unless such combination of technical features is technically infeasible. For example, features A+B+C are disclosed in one example, features A+B+D+E are disclosed in another example, features C and D are equivalent technical means that play the same role, and can only be used at a time, and feature E can be combined with feature C technically, then the scheme of A+B+C+D should not be regarded as having been described because it is technically infeasible, and the scheme of A+B+C+E should be regarded as having been described. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 A flowchart of a method for selecting a current transformer suitable for a million-unit group connected to a 220kV and below power grid. DETAILED DESCRIPTION

[0037] The present inventors have developed, through in-depth research and a large number of screenings, a method for selecting a current transformer suitable for a million-unit group connected to a 220kV and below power grid. Compared with the prior art, the present application provides a more scientific and effective method for selecting a current transformer by calculating a comprehensive primary time constant and comparing the comprehensive primary time constant with a reference value.

[0038] To make the purpose, technical solutions and advantages of the present application clearer, the embodiments of the present application will be described in further detail below with reference to the drawings.

[0039] TERMS

[0040] PRIMARY TIME CONSTANT

[0041] The primary time constant is an important parameter of the saturation characteristic of the current transformer, which reflects the time required for the magnetic flux of the current transformer to reach the saturation state under the action of short-circuit current. The primary time constant has a strong correlation with the saturation degree of the current transformer, and is an important basis for judging the selection of CT type.

[0042] TPY-CLASS TRANSFORMER

[0043] TPY class current transformer is a current transformer with specific transient characteristics, which is designed to meet the transient protection requirements of power system under short-circuit fault conditions. TPY class current transformer has faster response speed and smaller saturation tendency, and is suitable for protection of high-voltage and large-capacity power system.

[0044] Short-circuit current

[0045] Short-circuit current refers to the abnormal current between phases or between phase and ground in power system due to some reasons such as insulation damage, misoperation, etc. The size and duration of short-circuit current have important influence on the stability of power system and the safety of equipment.

[0046] Per unit value

[0047] Per unit value is a common representation method in power system analysis, which divides the actual value by a certain reference value to obtain a relative value, so as to facilitate comparison and analysis between different voltage levels and capacity systems. In this patent, per unit value is used to calculate the primary time constant of the line.

[0048] Embodiment

[0049] The specific process of the current transformer selection method suitable for the access of million unit to 220kV and below power grid according to the present application is shown in Figure 1 , which includes the following steps:

[0050] (1) Obtain parameters

[0051] According to the requirements, obtain the parameters of the corresponding unit and power grid, including: the short-circuit current provided by a single million unit connected to the corresponding power grid, the voltage of the power grid is less than or equal to 220kv; the primary time constant of a single million unit; the primary time constant of the power grid line obtained according to the specification reference value, the number of million units and the primary time constant of the power grid line calculated according to the per unit value.

[0052] (2) Calculate the comprehensive primary time constant

[0053] The comprehensive primary time constant is calculated under the assumption that the short-circuit current at the fault point is provided by the unit side and the system side, and the calculation formula is as follows:

[0054]

[0055] Wherein, t1 is the comprehensive primary time constant, x3 is the number of million units, x1 is the primary time constant of a single million unit;

[0056] (3) Select the current transformer according to the calculation result

[0057] comparing the integrated primary time constant obtained in the step (2) with a reference value, if the integrated primary time constant is greater than the reference value, selecting a TPY level transformer, if the integrated primary time constant is less than the reference value, selecting a 5P level transformer.

[0058] Further, the method further comprises obtaining a primary time constant of the power grid line in per unit value.

[0059] Optionally, in an embodiment, the step (2) further comprises calculating a per unit integrated primary time constant.

[0060] Optionally, in an embodiment, the calculation formula of the per unit integrated primary time constant is as follows:

[0061]

[0062] wherein t2 is the per unit integrated primary time constant, x3 is the number of the million units, x1 is a primary time constant of a single million unit, and x2 is a primary time constant of the power grid line obtained in per unit value.

[0063] Optionally, in an embodiment, the step (3) further comprises the following steps:

[0064] comparing the per unit integrated primary time constant with a reference value, if one of the per unit integrated primary time constant and the integrated primary time constant is greater than the reference value, adopting a TPY level transformer, otherwise, using a 5P level transformer.

[0065] Optionally, in an embodiment, the reference value in the step (3) can be adjusted to 80-120 ms.

[0066] Optionally, in an embodiment, the step (3) comprises a first reference value and a second reference value, wherein the first reference value is used for comparison with the integrated primary time constant, and the second reference value is used for comparison with the per unit integrated primary time constant, so as to improve the accuracy of the selection of the current transformer.

[0067] Optionally, in an embodiment, the step (1) further comprises calculating an overall primary time constant in a combined state of multiple million units, that is, the selection of the current transformer when multiple million units are connected to the power grid together.

[0068] Optionally, in an embodiment, the combined state comprises parallel connection and series connection.

[0069] Optionally, in one embodiment, the reference value includes two different reference values ​​for respectively comparing the integrated primary time constant with the per-unit primary time constant. If one of them does not meet the standard, a TPY-grade mutual inductor is used for safety reasons.

[0070] This embodiment also includes a current transformer selection system, characterized in that the system includes: a data acquisition module, a calculation module, a selection module and an output module, wherein:

[0071] The data acquisition module is configured to collect data of the million units and the power grid and input the data into the calculation module; the calculation module calculates the integrated primary time constant and the per-unit primary time constant based on the data calculated by the data acquisition module and compares them with the reference value;

[0072] The selection module selects a current transformer according to the comparison result of the calculation module and outputs the result to the output module;

[0073] The output module outputs the result to an output device, which includes a display.

Claims

1. A method for selecting current transformers for connecting millions of units to a 220kV or lower power grid, characterized in that: The method comprises the following steps: (1) Get parameters Obtain the parameters of the corresponding unit and grid according to the requirements, including: the short-circuit current provided by a single MW unit connected to the corresponding grid, the voltage of the grid being less than or equal to 220kV; the primary time constant of a single MW unit; the primary time constant of the grid line obtained according to the standard reference value and the number of MW units; (2) Calculate the comprehensive primary time constant The comprehensive primary time constant is calculated assuming that the short-circuit current at the fault point is provided by both the unit side and the system side. The calculation formula is as follows: Wherein, t1 is the comprehensive primary time constant, x3 is the number of the million-unit units, and x1 is the primary time constant of a single million-unit unit; (3) Select the transformer based on the calculation results Compare the integrated primary time constant obtained in step (2) with the reference value. If the integrated primary time constant is greater than the reference value, select a TPY-level mutual inductor. If the integrated primary time constant is less than the reference value, select a 5P-level mutual inductor.

2. The method according to claim 1, characterized in that The step (1) further includes obtaining a primary time constant of the power grid line calculated as a per-unit value.

3. The method according to claim 2, characterized in that The step (2) further includes calculating a per-unit integrated primary time constant.

4. The method according to claim 3, characterized in that The calculation formula of the per-unit integrated primary time constant is as follows: Among them, t2 is the per-unit comprehensive primary time constant, x3 is the number of the million-unit units, x1 is the primary time constant of a single million-unit unit, and x2 is the primary time constant of the power grid line calculated according to the per-unit value.

5. The method according to claim 4, characterized in that The step (3) further comprises the following steps: The per-unit integrated primary time constant is compared with a reference value. As long as one of the per-unit integrated primary time constant and the integrated primary time constant is greater than the reference value, a TPY-grade mutual inductor is used; otherwise, a 5P-grade mutual inductor is used.

6. The method according to claim 5, characterized in that The reference value in step (3) can be adjusted to 80-120ms.

7. The method according to claim 8, characterized in that The reference value in step (3) may include two different reference values ​​for respectively comparing the integrated primary time constant with the per-unit primary time constant.

8. The method according to claim 1, characterized in that The step (1) also includes calculating the overall primary time constant under the combined state of multiple million-unit units.

9. The method according to claim 7, characterized in that The combination states include parallel connection and series connection.

10. A current transformer selection system applicable to the method according to any one of claims 1 to 9, characterized in that: The system includes: a data acquisition module, a calculation module, a selection module and an output module, wherein: The data acquisition module is configured to collect data of the million units and the power grid and input the data into the calculation module; The calculation module calculates the comprehensive primary time constant and the per-unit primary time constant based on the data calculated by the data acquisition module and compares them with the reference value; The selection module selects a current transformer according to the comparison result of the calculation module and outputs the result to the output module; The output module outputs the result to an output device, which includes a display.

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