Method and device for checking polarity of CT (current transformer) of main transformer based on magnetizing inrush current
By judging the CT polarity of the main transformer by using the excitation surge current, the problems of large workload and high cost of CT polarity verification in the power plant main transformer infrastructure project are solved, and more efficient and low-cost CT polarity verification is achieved.
Patent Information
- Application Number
- CN202510587735.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-08
- Publication Date
- 2025-08-08
AI Technical Summary
In the infrastructure or renovation project of main transformer of power plant, the workload and cost of CT polarity verification are relatively large, especially the investment cost of leasing fake loads is high, and the increase in capacitor load leads to increased difficulty in measuring secondary current.
By judging the phasor relationship between the secondary reactive power and the secondary current, the CT polarity of each side of the main transformer is verified, and methods and devices based on excitation surge current are used to reduce false load leasing and disassembly operations.
It reduces the workload and investment cost at the project site, reduces capacitor load leasing, simplifies the difficulty of secondary current measurement, and improves the efficiency of CT polarity verification.
Smart Images
Figure CN120446632A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the field of power system protection, and in particular relates to a method and device for checking the polarity of a main transformer CT. Background Art
[0002] For power plant main transformer capital construction projects or protection renovation projects, for transformer differential protection, capital construction projects usually use rental dummy loads to complete the polarity verification of the differential protection CT. Main transformer protection renovation projects use the generator zero-start current method to complete the polarity verification of the differential protection CT. The workload and cost are relatively large, especially for rental dummy loads, which have a very high investment cost. At the same time, the dummy load usually uses a capacitor bank. When the capacity increases, the fifth harmonic content will increase, which also brings certain difficulties to the on-site measurement of secondary current using a clamp ammeter. Summary of the Invention
[0003] The purpose of the present invention is to provide a method and device for checking the polarity of the main transformer CT based on the excitation inrush current, which is specifically designed to solve the problem of high workload and high cost in the construction or renovation of the main transformer of a power plant. The method uses the angular relationship of the excitation inrush current on each side of the main transformer and the positive and negative relationship of the reactive power when multiple transformers are simultaneously impacted to determine the correctness of the polarity of the main transformer differential protection CT.
[0004] In order to achieve the above object, the technical solution of the present invention is:
[0005] A method for checking the polarity of a main transformer CT based on magnetizing inrush current includes the following steps:
[0006] Step 1: Use the magnetizing inrush current of the main transformer and other transformers on the low-voltage side of the main transformer at the same time when the inrush current is applied, and verify the polarity of the CTs on each side of the main transformer that participate in the main transformer differential protection based on the secondary reactive power or the phasor relationship of the secondary current on each side of the main transformer.
[0007] Step 2: When the instantaneous current value of any phase of the CT participating in the differential protection on each side of the main transformer exceeds the current value threshold and the polarity check function pressure plate is in the ON state, the main transformer differential CT polarity check logic is entered;
[0008] Step 3: Calculate the reactive power of each phase based on the secondary voltage and secondary current of the voltage transformer (PT) on each side of the main transformer. By comparing the phasors of the reactive power, determine the correctness of the polarity of the CT on each side of the main transformer.
[0009] Step 4: Determine the correctness of the polarity of the CTs on each side of the main transformer based on the phasor angle relationship of the secondary currents of the CTs on each side of the main transformer.
[0010] As a further preferred embodiment of the present invention, the CTs in step 1 are located on each side of the main transformer, and each CT constitutes the main transformer differential protection. The CTs on each side adopt a 180° connection method, that is, the same-named terminals of the CTs on each side participating in the main transformer differential protection are far away from the main transformer.
[0011] As a further preferred embodiment of the present invention, the magnetizing inrush current of the main transformer and other transformers carried on the low-voltage side of the main transformer is utilized at the moment of simultaneous impact. When the instantaneous value of the current of any phase of the CT participating in the differential protection on each side of the main transformer is greater than the current value threshold, the polarity verification function pressure plate is put into the state to enter the main transformer differential CT polarity verification logic.
[0012] As a further preferred embodiment of the present invention, the reactive power of each phase is calculated based on the PT secondary voltage and CT secondary current on each side of the main transformer, and the correctness of the CT polarity on each side of the main transformer is determined by comparing the phasors of the reactive power.
[0013] As a further preferred embodiment of the present invention, under the premise that the instantaneous value of any phase current on any side of the main transformer is greater than the current threshold value, the correctness of the polarity of the CT on each side of the main transformer can also be judged based on the phasor angle relationship of the secondary current of the CT on each side of the main transformer.
[0014] As a further preferred solution of the present invention, the reactive power judgment logic and the secondary current phasor judgment criterion can be used to select whether to start or stop the system according to the actual situation on site.
[0015] The present invention also discloses a device for checking the polarity of a main transformer CT based on an excitation inrush current, comprising a sampling module, a calculation module, and a judgment module, wherein:
[0016] The sampling module is used to collect all branch currents involved in differential protection on the main transformer, the high-voltage side voltage of the main transformer, and the low-voltage side voltage to provide electrical quantity data for the protection device. The sampling module can select and support analog input or digital signal, which is suitable for conventional engineering and digital engineering.
[0017] The calculation module is used to calculate the data collected by the sampling module, and calculate the reactive power of each phase on each side, as well as the instantaneous value and effective value of the current on each side according to the sampling values of the voltage transformer and current transformer of each branch, for program judgment;
[0018] The judgment module is used to substitute the data of the calculation module into the polarity verification equation for judgment based on the result of the calculation module, and judge the correctness of the transformer CT polarity based on the angular relationship between the phase quantity of reactive power, the instantaneous value of current or the effective value of current.
[0019] The beneficial effects of the present invention are as follows: in view of the fact that the workload and cost of traditional transformer CT polarity verification work are relatively large, especially the need to rent dummy loads at infrastructure sites, the investment cost is very high, and as the capacitor load capacity increases, the difficulty of measuring secondary current on site using a clamp ammeter will increase. The present invention proposes a method and device for main transformer CT polarity verification based on excitation inrush current, which can effectively reduce the workload of CT polarity verification through generator zero-start current at the engineering site, while reducing the rental and disconnection work of capacitor loads in infrastructure projects, reducing investment costs, and at the same time, due to the reduction in capacitor load capacity, reducing the difficulty of on-site testing. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Attachment Figure 1 This is a schematic diagram of the present embodiment;
[0021] Attachment Figure 2 This is a schematic diagram of the phasors of the secondary voltage and secondary current of the main transformer;
[0022] Attachment Figure 3 A schematic diagram of the hardware module.
[0023] Attachment Figure 1 This is the principle diagram of this embodiment, taking the method of configuring the main transformer protection device of this embodiment as an example, wherein
[0024] CT1: Main transformer high-voltage side CT
[0025] CT2, CT3, CT4: CTs of each branch on the low-voltage side of the main transformer
[0026] The above four CTs are connected to the main transformer protection device to calculate the current amplitude and phase of each branch of the main transformer.
[0027] The reactive power of the main transformer at the moment of impact can be calculated using the high-voltage side PT and CT1; the reactive power of the three branch main transformers on the low-voltage side of the main transformer at the moment of impact can be calculated using the low-voltage side PT and the three CTs CT2, CT3, and CT4.
[0028] Attachment Figure 2 This is a schematic diagram of the secondary voltage and secondary current phasors of the main transformer. When the high and low voltage sides of the main transformer are connected in Ynd11 mode, when the main transformer is energized by impulse, the secondary voltage and secondary current phasors on the high voltage side and the secondary voltage and secondary current phasors on the low voltage side are shown.
[0029] Attachment Figure 3Figure 2 is a schematic diagram of a hardware module. When the main transformer protection is configured using the method described in this embodiment, the hardware module can be integrated into the main transformer protection device or an independent device. For the independent device, the PT circuit in the acquisition circuit is connected in parallel with the main transformer protection, and each phase is introduced into the independent device separately. The CT circuit in the acquisition circuit is connected using a clamp ammeter or the CT circuit of the independent device is connected in series with the CT circuit of the main transformer protection. DETAILED DESCRIPTION
[0030] The technical solution of the present invention is described in detail below with reference to the accompanying drawings.
[0031] The present invention provides a method for checking the polarity of a main transformer CT based on an excitation inrush current, which is characterized by comprising the following steps:
[0032] Step 1: Use the magnetizing inrush current of the main transformer and other transformers on the low-voltage side of the main transformer at the same time when the inrush current is applied, and verify the polarity of the CTs on each side of the main transformer that participate in the main transformer differential protection based on the secondary reactive power or the phasor relationship of the secondary current on each side of the main transformer.
[0033] Step 2: When the instantaneous current value of any phase of the CT participating in the differential protection on each side of the main transformer exceeds the current value threshold and the polarity check function pressure plate is in the ON state, the main transformer differential CT polarity check logic is entered;
[0034] Step 3: Calculate the reactive power of each phase based on the secondary voltage and secondary current of the voltage transformer (PT) on each side of the main transformer. By comparing the phasors of the reactive power, determine the correctness of the polarity of the CT on each side of the main transformer.
[0035] Step 4: Determine the correctness of the polarity of the CTs on each side of the main transformer based on the phasor angle relationship of the secondary currents of the CTs on each side of the main transformer.
[0036] In the above scheme, each CT in step 1 is located on each side of the main transformer, such as Figure 1As shown, CT1, CT2, CT3, and CT4 constitute the main transformer differential protection. CT1 is the CT on the high-voltage side of the main transformer, while CT2, CT3, and CT4 are CTs for the branches on the low-voltage side of the main transformer. Each CT3 and CT4 branch carries two transformers. Conventional methods for verifying the polarity of main transformer CTs require that before the first main transformer of an infrastructure project is commissioned, a capacitor load, commonly known as a dummy load, be installed at the lower ends of CT2, CT3, and CT4. The high-voltage side switch of the main transformer is closed and connected to the grid, and the polarity of the main transformer differential protection CTs is verified using the capacitor loads. However, this solution requires only one capacitor set, installed at the lower end of branch CT2. The CT3 and CT4 branches can then be used to shock transformers 1 and 2 together with the main transformer. Alternatively, after the polarity of CT3, CT4, and CT1 is verified, a high-voltage cable is used to connect transformer 1 to the lower end of CT2, and the main transformer is again shocked to verify the differential polarity of CT1 and CT2. No dedicated dummy load is required.
[0037] In the above scheme, in step 2, the excitation inrush current at the moment of simultaneous impact of the main transformer and other transformers carried on the low-voltage side of the main transformer is used. When the instantaneous current value of any phase of the CT participating in the differential protection on each side of the main transformer is greater than the current value threshold, the polarity verification function pressure plate is put into operation to enter the main transformer differential CT polarity verification logic. The reason for using the instantaneous current value of any phase on any side to be greater than the current value threshold is to increase the sensitivity of the protection judgment. The current value threshold is usually set to 0.06In, where In is the secondary rated current value of the group of CTs. The role of the polarity verification function pressure plate is to avoid the protection device always triggering the judgment logic during operation. It is only put into operation when the polarity of the main transformer CT needs to be verified in the initial stage of equipment commissioning. After the main transformer is put into formal operation, the pressure plate is in the withdrawal state.
[0038] In the above scheme, in step 3, the reactive power of each phase is calculated based on the PT secondary voltage and CT secondary current on each side of the main transformer. By comparing the phasors of the reactive power, the correctness of the CT polarity on each side of the main transformer is determined. Take the case where the main transformer carries two other transformers as an example. Figure 1 As shown in the figure, when the transformer receives power surges and has no internal faults, the power is mainly reflected in reactive power. For the main transformer, the high-voltage side CT1 common-name terminal is far away from the main transformer, and the reactive power measured by the main transformer high-voltage side PT and CT1 branch is positive. However, for CT3 and CT4, the excitation inrush current flows into the opposite-name terminal and out of the common-name terminal, and the reactive power measured by the main transformer low-voltage side PT and CT3 and CT4 branches is negative. Based on this feature, the specific judgment criteria are set as follows:
[0039]
[0040] U1>U 1.set &U2<U2.set
[0041] The meanings of the symbols are as follows:
[0042] The instantaneous current value of each phase on the high voltage side of the main transformer;
[0043] The instantaneous current value of each phase on the low-voltage side 1 of the main transformer;
[0044] The instantaneous current value of each phase on the low-voltage side 1 of the main transformer;
[0045] I set is the current starting value threshold;
[0046] Q set is the secondary power threshold; k is the reliability coefficient
[0047] Reactive power value of each phase on the high voltage side of the main transformer;
[0048] It is the reactive power value of low voltage side 1 of the main transformer;
[0049] It is the reactive power value of low voltage side 1 of the main transformer;
[0050] U1 is the positive sequence voltage value of the main transformer high voltage side; U 1.set is the positive sequence voltage threshold;
[0051] U2 is the negative sequence voltage value of the main transformer high voltage side; U 2.set is the negative sequence voltage threshold;
[0052] When the above three equations are all satisfied, it is determined that the polarity of the differential CTs on each side of the main transformer is normal.
[0053] For sites where there is no PT on the low voltage side or high voltage side and reactive power measurement cannot be used, the current phasor comparison method described in step 4 can be used, such as Figure 2 As shown, taking the main transformer with Ynd11 connection as an example, the high-voltage side voltage lags the low-voltage side voltage by 30°, and the primary current of the excitation inrush current on the high-voltage side lags the low-voltage side by approximately 30°. Since the polarity of the main transformer differential protection CT is connected in a 180° manner, the secondary current of the main transformer high-voltage side CT leads the secondary current of the low-voltage side CT by approximately 150°. Based on this characteristic, the specific judgment criteria are as follows:
[0054]
[0055] The meanings of the symbols are as follows:
[0056] The instantaneous current phasor value of each phase on the high voltage side of the main transformer;
[0057] The instantaneous current phasor value of each phase on the low-voltage side 1 of the main transformer;
[0058] It is the instantaneous current phasor value of each phase on the two sides of the low-voltage side of the main transformer.
[0059] When the above three equations are all satisfied, it is determined that the polarity of the differential CTs on each side of the main transformer is normal.
[0060] The above current phasor relationship is only a schematic representation of the angles on each side of the Ynd11 wiring method. For different transformer primary wiring methods, the angle relationship determined by current phasors can also be set accordingly. For example, if the primary wiring method of the transformer high and low voltage sides is Ynd1, the corresponding high and low voltage side angle relationship is between 120° and 300°, which is the correct CT polarity judgment range.
[0061] In addition, the present invention also provides a device for checking the polarity of the main transformer CT based on the excitation inrush current, including a sampling module, a calculation module, and a judgment module. Figure 3 As shown, where:
[0062] The sampling module is used to collect all branch currents participating in the differential protection on the main transformer, the high-voltage side voltage of the main transformer, and the low-voltage side voltage to provide electrical quantity data for the protection device. The sampling module can select and support analog input or digital signals to adapt to conventional projects and digital projects. For the method integrated into the main transformer protection device, the above-mentioned acquisition circuit is implemented by the main transformer protection itself. For independent devices, an independent external acquisition circuit is used. For example, the PT access circuit is connected in parallel with the main transformer protection PT circuit, and the CT access circuit uses a clamp ammeter or is connected in series with the main transformer CT circuit to obtain sampling data.
[0063] The calculation module is used to calculate the data collected by the sampling module, and calculate the reactive power of each phase on each side, as well as the instantaneous value and effective value of the current on each side according to the sampling values of the voltage transformer and current transformer of each branch, for program judgment;
[0064] The judgment module is used to substitute the data of the calculation module into the polarity verification equation for judgment based on the result of the calculation module, and judge the correctness of the transformer CT polarity based on the angular relationship between the phase quantity of reactive power, the instantaneous value of current or the effective value of current.
[0065] The above embodiments are only for illustrating the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical method in accordance with the technical idea proposed by the present invention fall within the protection scope of the present invention.
Claims
1. A method for checking the polarity of a main transformer CT based on magnetizing inrush current, characterized in that The steps include: Step 1: Use the magnetizing inrush current of the main transformer and other transformers on the low-voltage side of the main transformer at the same time when the inrush current is applied, and verify the polarity of the CTs on each side of the main transformer that participate in the main transformer differential protection based on the secondary reactive power or the phasor relationship of the secondary current on each side of the main transformer. Step 2: When the instantaneous current value of any phase of the CT participating in the differential protection on each side of the main transformer exceeds the current value threshold and the polarity check function pressure plate is in the ON state, the main transformer differential CT polarity check logic is entered; Step 3: Calculate the reactive power of each phase based on the secondary voltage and secondary current of the voltage transformer (PT) on each side of the main transformer. By comparing the phasors of the reactive power, determine the correctness of the polarity of the CT on each side of the main transformer. Step 4: Determine the correctness of the polarity of the CTs on each side of the main transformer based on the phasor angle relationship of the secondary currents of the CTs on each side of the main transformer.
2. The method for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 1, wherein: The CTs in step 1 are located on each side of the main transformer, and the CTs constitute the differential protection of the main transformer.
3. A method for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 1 or 2, characterized in that: By utilizing the magnetizing inrush current of the main transformer and other transformers carried on the low-voltage side of the main transformer at the same time of impact, the instantaneous value of the current of any phase of the CT participating in the differential protection on each side of the main transformer is greater than the current value threshold and the polarity verification function pressure plate is put into the state to enter the main transformer differential CT polarity verification logic.
4. A method for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 1 or 3, characterized in that: The reactive power of each phase is calculated based on the PT secondary voltage and CT secondary current on each side of the main transformer. The correctness of the CT polarity on each side of the main transformer is determined by comparing the phasors of the reactive power. Taking the impact of the main transformer on two other transformers as an example, the specific judgment criteria are as follows: U1>U 1.set &U2<U 2.set formula 3 The meanings of the symbols are as follows: The instantaneous current value of each phase on the high voltage side of the main transformer; The instantaneous current value of each phase on the low-voltage side 1 of the main transformer; The instantaneous current value of each phase on the low-voltage side 1 of the main transformer; I set is the current starting value threshold; Q set is the secondary power threshold; k is the reliability coefficient Reactive power value of each phase on the high voltage side of the main transformer; It is the reactive power value of low voltage side 1 of the main transformer; It is the reactive power value of low voltage side 1 of the main transformer; U1 is the positive sequence voltage value of the main transformer high voltage side; U 1.set is the positive sequence voltage threshold; U2 is the negative sequence voltage value on the high voltage side of the main transformer; U 2.set is the negative sequence voltage threshold; When equations 1 to 3 are satisfied, it is determined that the polarity of the differential CTs on each side of the main transformer is normal.
5. The method for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 1 or 4, characterized in that: Under the premise of satisfying Equation 1, the correctness of the polarity of the CTs on each side of the main transformer is judged based on the phasor angle relationship of the secondary currents of the CTs on each side of the main transformer. The specific criteria are as follows: The meanings of the symbols are as follows: The instantaneous current phasor value of each phase on the high voltage side of the main transformer; The instantaneous current phasor value of each phase on the low-voltage side 1 of the main transformer; It is the instantaneous current phasor value of each phase on the two sides of the low-voltage side of the main transformer. When equations 4 to 6 are satisfied, it is determined that the polarity of the differential CTs on each side of the main transformer is normal.
6. A device for checking the polarity of main transformer CT based on magnetizing inrush current, characterized in that It includes sampling module, calculation module and judgment module, among which: The sampling module is used to collect all branch currents involved in differential protection on the main transformer, the high-voltage side voltage of the main transformer, and the low-voltage side voltage to provide electrical quantity data for the protection device; The calculation module is used to calculate the data collected by the sampling module, and calculate the reactive power of each phase on each side, as well as the instantaneous value and effective value of the current on each side according to the sampling values of the voltage transformer and current transformer of each branch; The judgment module is used to substitute the result of the calculation module into the corresponding action equation to judge the polarity correctness of the CT on each side of the main transformer.
7. The device for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 6, characterized in that: The sampling module can support analog input and fiber optic digital signals according to the actual requirements of the project. For conventional projects, the sampling module is divided into two systems: an analog input module and an analog-to-digital conversion module; for digital projects, the sampling module is an optical signal processing module.
8. The device for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 6, characterized in that: The calculation module calculates the effective value and instantaneous value of each phase current and the reactive power of each phase based on the sampling values of all branch current transformers on each side of the main transformer and the sampling values of voltage transformers on both sides of the main transformer collected by the sampling module.
9. The device for checking the polarity of a main transformer CT based on magnetizing inrush current according to claim 6, characterized in that: The judgment module substitutes the data of the calculation module into the polarity verification equation for judgment, and judges the correctness of the transformer CT polarity according to the angular relationship between the phasor of reactive power, the instantaneous value of current or the effective value of current.