Method for preventing three-phase imbalance protection misoperation of electric motor for nuclear power medium-voltage plant

Abstract

The invention relates to the technical field of motors, in particular to a method for preventing three-phase imbalance protection misoperation of a nuclear power medium-voltage auxiliary motor. The method comprises the following steps: measuring three-phase current of the low-voltage side of each high-voltage station transformer in real time through a current transformer; carrying out harmonic analysis on the three-phase current signal of the current transformer to obtain the spectrum distribution of the low-voltage side current of the high-voltage station transformer; extracting secondary harmonics in the three-phase current of the low-voltage side of the high-voltage plant transformer, and judging whether a main transformer no-load closing process occurs or not according to the amplitude change of the secondary harmonics of each phase of current; when the second harmonic of at least one current exceeds a set threshold value, judging that the main transformer is in a no-load switching-on process, and immediately sending an electrical signal to all motor protection devices of the high-voltage station transformer through a signal line so as to lock the three-phase current unbalance protection of each motor; and when the three-phase secondary harmonics are all lower than a set threshold value, judging that the influence caused by no-load closing of the main transformer is greatly reduced, and immediately sending electrical signals to all motor protection devices of the transformer for the high-voltage plant through a signal line, so that the three-phase current unbalance protection of each motor is unlocked. Protection maloperation events are prevented, and safe and stable operation of the power plant is ensured.

Description

Technical Field

[0001] This invention relates to the field of electric motor technology, and in particular to a method for preventing the malfunction of three-phase imbalance protection for electric motors used in medium-voltage nuclear power plants. Background Technology

[0002] High-voltage motors are important power loads in power plants. A power plant typically has dozens or hundreds of high-voltage motors used for water supply, ventilation, coal grinding, etc. The reliable operation of these high-voltage motors is an important guarantee for the normal operation of the power plant.

[0003] In recent years, several cases have occurred in domestic power plants where the three-phase imbalance protection of high-voltage motors under adjacent main transformers has tripped when the main transformer is closed under no-load conditions. This has caused some motors to suddenly shut down, posing a significant threat to the operational safety of power plants. Therefore, it is crucial to intervene in and suppress the malfunction of the three-phase imbalance protection of high-voltage motors caused by the no-load closing of the main transformer in power plants, in order to mitigate the safety risks posed by such incidents to power plants.

[0004] Currently, most high-voltage motors calculate their three-phase unbalance using two expressions based on the maximum current condition:

[0005] 1) When the maximum value of the three-phase current is less than the rated value, the criterion is generally to divide the difference between the effective value of the maximum phase current and the effective value of the minimum phase current by the rated current.

[0006] 2) When the maximum value of the three-phase current is greater than the rated value, the criterion is generally to divide the difference between the effective value of the maximum phase current and the effective value of the minimum phase current by the maximum phase current.

[0007] Protection typically employs inverse time characteristics, meaning that the greater the unbalanced current, the shorter the tripping time of the protection action.

[0008] This type of protection does not have a lockout mechanism. This means that regardless of whether the power plant is in normal or abnormal condition, as long as the high-voltage motor reaches the unbalanced protection's operating threshold, the protection will activate after a set time. In practice, during the no-load closing process of the main transformer, the inrush current can cause a surge current in adjacent main transformers, further affecting the downstream high-voltage transformers. This results in three-phase asymmetry in the high-voltage transformers, leading to three-phase imbalance in the motors and malfunctioning protection systems. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for preventing the maloperation of the three-phase imbalance protection of the medium-voltage plant motor in nuclear power plants. This method can lock out the three-phase imbalance protection of the high-voltage motor during the no-load closing process of the main transformer, prevent the protection from maloperating, and ensure the safe and stable operation of the power plant.

[0010] This invention provides a method for preventing the malfunction of three-phase imbalance protection for medium-voltage plant auxiliary motors in nuclear power plants, comprising the following steps:

[0011] Step 1: Measure the three-phase current on the low-voltage side of each high-voltage plant service transformer in real time using a current transformer;

[0012] Step 2: Perform harmonic analysis on the three-phase current signal of the current transformer to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer;

[0013] Step 3: Extract the second harmonic from the three-phase current on the low-voltage side of the high-voltage plant service transformer. Based on the amplitude changes of the second harmonic of each phase current, determine whether the no-load closing process of the main transformer transformer has occurred.

[0014] When the second harmonic of at least one current exceeds the set threshold, it is determined that the main transformer is in the no-load closing process. An electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage station service transformer, so that the three-phase current imbalance protection of each motor is blocked.

[0015] When the second harmonics of all three phases are below the set threshold, it is determined that the impact caused by the no-load closing of the main transformer has been greatly reduced. Then, an electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage plant service transformer, so that the three-phase current imbalance protection of each motor is released from the lockout.

[0016] In one specific embodiment of the present invention, the data length for harmonic analysis of the three-phase current signal of the current transformer is no greater than 0.5s.

[0017] In one specific embodiment of the present invention, the second harmonic setting threshold does not block the three-phase unbalanced protection of the motor when the main transformer is closed under no-load conditions, but reliably blocks the three-phase unbalanced protection of the motor when the main transformer is closed under no-load conditions.

[0018] In one specific embodiment of the present invention, in step S3, the set threshold is 10% to 18% of the real-time current.

[0019] In a specific embodiment of the present invention, in step 2, the time-domain current waveform of the three-phase current of the current transformer is converted into a frequency-domain signal to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer.

[0020] In one specific embodiment of the present invention, when it is determined that the main transformer is in the no-load closing process, a high-level signal is sent from the high-voltage plant service transformer to all motor protection devices connected to the high-voltage plant transformer through the signal line, thereby blocking the three-phase current imbalance protection of each motor through the high-level signal.

[0021] In one specific embodiment of the present invention, when it is determined that the main transformer is not in the no-load closing process or the impact caused by the no-load closing of the main transformer is greatly reduced, a low-level signal is sent from the high-voltage plant service transformer to all motor protection devices connected to the high-voltage plant transformer through the signal line, thereby unlocking the three-phase current imbalance protection of each motor through the low level.

[0022] In one specific embodiment of the present invention, when the second harmonic of two or more phases of current exceeds the set threshold, it is determined that the main transformer is in the no-load closing process, and an electrical signal is immediately sent to the protection devices of all motors connected to the high-voltage station service transformer through the signal line, so that the three-phase current imbalance protection of each motor is blocked.

[0023] Compared with the prior art, the method for preventing the malfunction of the three-phase imbalance protection of the medium-voltage plant motor in nuclear power plants according to the present invention has the following beneficial effects:

[0024] (1) The judgment of no-load closing of the main transformer is determined by the second harmonic current on the low-voltage side of the high-voltage plant service transformer. The judgment is placed on the level above the motor, and the judgment is more focused.

[0025] (2) By judging the second harmonic, reliable blocking of the three-phase unbalance protection of all high-voltage motors downstream of the high-voltage plant transformer is realized. There is no need to set the blocking conditions for each motor separately, which simplifies the blocking setting and can effectively prevent the three-phase unbalance protection of high-voltage motors from malfunctioning. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the primary electrical wiring of a power plant, illustrating a method for preventing maloperation of the three-phase unbalanced protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions, according to the present invention.

[0027] Figure 2 This is a schematic diagram of the primary electrical circuit of a high-voltage plant transformer and its connected high-voltage motor, which is part of a method for preventing the maloperation of the three-phase imbalance protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions, according to the present invention.

[0028] Figure 3 This invention provides a logic diagram for the three-phase imbalance protection of a high-voltage motor, which is a method for preventing maloperation of the three-phase imbalance protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions.

[0029] Figure 4 This is a primary main wiring diagram of a power plant, illustrating a method for preventing maloperation of the three-phase unbalanced protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions, according to the present invention.

[0030] Figure 5The present invention provides a method for preventing maloperation of the three-phase unbalanced protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions. The diagram shows the inrush current during the no-load closing of the main transformer.

[0031] Figure 6 The inrush current diagram of adjacent main transformers is provided for a method to prevent maloperation of the three-phase unbalanced protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions, according to the present invention.

[0032] Figure 7 The present invention relates to a method for preventing maloperation of the three-phase unbalanced protection of a high-voltage motor when the main transformer of a power plant is closed under no-load conditions, and includes a three-phase current diagram of the motor.

[0033] Figure 8 The current waveform diagram of the low-voltage side of the high-voltage station service transformer under the adjacent main transformer is shown in the figure below, which is a method for preventing the maloperation of the three-phase unbalance protection of the high-voltage motor when the main transformer of a power plant is closed under no-load conditions according to the present invention.

[0034] Figure 9 The diagram shows the effective values ​​of the fundamental and second harmonic currents of the low-voltage side A-phase current of the high-voltage station service transformer under the adjacent main transformer, which is part of the method for preventing the maloperation of the three-phase unbalanced protection of the high-voltage motor when the main transformer of a power plant is closed under no-load conditions according to the present invention.

[0035] Figure 10 The diagram shows the effective values ​​of the fundamental and second harmonic currents of the low-voltage side B-phase current of the high-voltage station service transformer under the adjacent main transformer, which is part of the method for preventing the maloperation of the three-phase unbalanced protection of the high-voltage motor when the main transformer of a power plant is closed under no-load conditions according to the present invention.

[0036] Figure 11 The diagram shows the effective values ​​of the fundamental and second harmonic currents of the low-voltage side C-phase current of the high-voltage station service transformer under the adjacent main transformer, which is part of the method for preventing the maloperation of the three-phase unbalanced protection of the high-voltage motor when the main transformer of a power plant is closed under no-load conditions according to the present invention.

[0037] In the diagram, 1-high voltage power grid, 2-first main transformer, 3-second main transformer, 4-generator #1, 5-generator #2, 6-high voltage plant auxiliary transformer #6, 7-motor, 8-current transformer, 9-harmonic analysis and inrush current detection device, 10-generator #3, 11-generator #4, 12-generator #5, 13-generator #6;

[0038] I 2A This represents the second harmonic current value of phase A on the low-voltage side of the high-voltage plant service transformer; I 2B This represents the second harmonic current value of phase B on the low-voltage side of the high-voltage plant service transformer; I 2C This represents the second harmonic current value of phase C on the low-voltage side of a high-voltage plant service transformer; I 2.op This indicates that a threshold value is set. Detailed Implementation

[0039] To further understand the present invention, embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the present invention.

[0040] An embodiment of the present invention discloses a method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants, comprising the following steps:

[0041] Step 1: Measure the three-phase current on the low-voltage side of each high-voltage plant service transformer in real time using a current transformer;

[0042] Step 2: Perform harmonic analysis on the three-phase current signal of the current transformer to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer;

[0043] In this step, the time-domain current waveform of the three-phase current of the current transformer is converted into a frequency-domain signal to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer.

[0044] The data length of the three-phase current signal of the current transformer during harmonic analysis is no greater than 0.5s.

[0045] Step 3: Extract the second harmonic from the three-phase current on the low-voltage side of the high-voltage plant service transformer. Based on the amplitude changes of the second harmonic of each phase current, determine whether the no-load closing process of the main transformer transformer has occurred.

[0046] The second harmonic setting threshold does not block the three-phase unbalanced motor protection when the main transformer is closed under no-load conditions, but reliably blocks the three-phase unbalanced motor protection when the main transformer is closed under no-load conditions.

[0047] The set threshold is 10% to 18% of the real-time current.

[0048] When the second harmonic of at least one current exceeds the set threshold, it is determined that the main transformer is in the no-load closing process. An electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage station service transformer, so that the three-phase current imbalance protection of each motor is blocked.

[0049] Preferably, the main transformer is determined to be in the no-load closing process only when the second harmonic of two or more phases of current exceeds the set threshold. An electrical signal is then sent to the protection devices of all motors connected to the high-voltage station service transformer via the signal line to lock out the three-phase current imbalance protection of each motor.

[0050] When it is determined that the main transformer is in the no-load closing process, a high-level signal is sent from the high-voltage station service transformer to the protection devices of all motors connected to the high-voltage station transformer through the signal line, and the three-phase current imbalance protection of each motor is blocked by the high level.

[0051] When the second harmonics of all three phases are below the set threshold, it is determined that the impact caused by the no-load closing of the main transformer has been greatly reduced. Then, an electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage plant service transformer, so that the three-phase current imbalance protection of each motor is released from the lockout.

[0052] Specifically, when it is determined that the main transformer is not in the no-load closing process or the impact caused by the no-load closing of the main transformer is greatly reduced, a low-level signal is sent from the high-voltage station service transformer to all motor protection devices connected to the high-voltage station transformer via the signal line, thereby unlocking the three-phase current imbalance protection of each motor through the low level.

[0053] To further understand the present invention, the following detailed description of the method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant motor in nuclear power plants provided by the present invention is provided in conjunction with the embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0054] Example 1

[0055] A small power plant has two generator sets, such as Figure 1 As shown, each generator is connected to the high-voltage grid via a main transformer. On the low-voltage side of the main transformer, there are two high-voltage plant service transformers, each of which carries several high-voltage motor loads.

[0056] To prevent the three-phase imbalance protection of the motor connected to the high-voltage station service transformer under the adjacent main transformer from malfunctioning during the no-load closing process of one of the main transformers, a current harmonic analysis and inrush current discrimination device needs to be installed on the low-voltage side of each high-voltage station service transformer. Figure 2 As shown in the figure, this device acquires the phase currents of each phase on the low-voltage side of the high-voltage plant service transformer in real time and performs real-time harmonic analysis.

[0057] When the device determines that the second harmonic of any phase current on the low-voltage side of the high-voltage plant service transformer exceeds a set threshold, it immediately sends a blocking signal to the three-phase unbalanced protection of the downstream high-voltage motor to prevent the motor's three-phase unbalanced protection from malfunctioning. Since the three-phase unbalanced protection delay of the high-voltage motor is typically several seconds, the blocking signal can reliably block the three-phase unbalanced protection of the high-voltage motor before it operates. The logic diagram of the high-voltage motor three-phase unbalanced protection is as follows: Figure 3 As shown.

[0058] Example 2

[0059] Taking a nuclear power plant as an example, this plant has a total of 6 large-capacity 1000MW generators. Each generator is stepped up to the 500kV power grid via a double-winding main transformer. Figure 4As shown. Each main transformer is connected to two three-winding high-voltage station service transformers on its low-voltage side, and each high-voltage station service transformer drives several high-voltage motors on its low-voltage side.

[0060] During the no-load closing process on the high-voltage side (500kV side) of main transformer #5, the waveform of its inrush current is as follows: Figure 5 As shown, the current in each phase is biased to one side of the time axis, exhibiting significant inrush current characteristics. During the no-load closing process of main transformer #5, the adjacent main transformer #1 generated a corresponding inrush current, such as... Figure 6 As shown, an unbalanced current is superimposed on the original load current, resulting in an asymmetry in the current of the No. 1 main transformer.

[0061] Figure 7 The three-phase current waveform of a high-voltage motor connected to a high-voltage plant service transformer shows a significant asymmetry. The three-phase imbalance of the high-voltage motor exceeds 20%, reaching the protection activation condition. However, due to the long protection operation delay (40 seconds for a 20% three-phase imbalance), if it can be determined within this timeframe that the high-voltage motor's three-phase current is caused by the no-load closing of the main transformer, a blocking signal can be issued for the motor's three-phase imbalance protection. (Refer to...) Figure 3 This effectively prevents the three-phase imbalance protection from malfunctioning.

[0062] Further observation of the low-voltage side current waveform of the high-voltage station service transformer upstream of the high-voltage motor is shown below. Figure 8 It is evident that the low-voltage side current of the high-voltage plant service transformer is also asymmetrical. A windowed Fourier analysis of the three-phase current on the low-voltage side of the high-voltage plant service transformer was performed to obtain the fundamental and second harmonic currents, as shown in the results. Figures 9-11 As shown.

[0063] As can be seen, before the No. 5 main transformer was closed under no-load conditions, the low-voltage side current of the high-voltage plant service transformer had almost no second harmonic. During the process of closing the No. 5 main transformer under no-load conditions, the second harmonic of each phase current on the low-voltage side of the high-voltage plant service transformer increased significantly. Therefore, the threshold for the second harmonic current was set at 50A.

[0064] Step 1: Measure the three-phase current on the low-voltage side of each high-voltage plant service transformer in real time using a current transformer;

[0065] Step 2: Convert the time-domain current waveform of the three-phase current of the current transformer into a frequency-domain signal to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer.

[0066] The data length of the three-phase current signal of the current transformer during harmonic analysis is no greater than 0.5s.

[0067] Step 3: Extract the second harmonic from the three-phase current on the low-voltage side of the high-voltage plant service transformer. Based on the amplitude changes of the second harmonic of each phase current, determine whether the no-load closing process of the main transformer transformer has occurred.

[0068] The set threshold is 50A;

[0069] When the second harmonic of two or more phases of current exceeds the set threshold, it is determined that the main transformer is in the no-load closing process. A high-level signal is sent from the high-voltage station service transformer to the protection devices of all motors connected to the high-voltage station transformer through the signal line. The three-phase current imbalance protection of each motor is blocked by the high-level signal.

[0070] When the second harmonics of all three phases are below the set threshold, it is determined that the impact caused by the no-load closing of the main transformer has been greatly reduced. A low-level signal is sent from the high-voltage plant service transformer to all motor protection devices connected to the high-voltage plant transformer through the signal line. The three-phase current imbalance protection of each motor is unlocked by the low level.

[0071] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0072] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants, characterized in that, Includes the following steps: Step 1: Measure the three-phase current on the low-voltage side of each high-voltage plant service transformer in real time using a current transformer; Step 2: Perform harmonic analysis on the three-phase current signal of the current transformer to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer; Step 3: Extract the second harmonic from the three-phase current on the low-voltage side of the high-voltage plant service transformer. Based on the amplitude changes of the second harmonic of each phase current, determine whether the no-load closing process of the main transformer transformer has occurred. When the second harmonic of at least one current exceeds the set threshold, it is determined that the main transformer is in the no-load closing process. An electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage station service transformer, so that the three-phase current imbalance protection of each motor is blocked. When the second harmonics of all three phases are below the set threshold, it is determined that the impact caused by the no-load closing of the main transformer has been greatly reduced. Then, an electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage plant service transformer, so that the three-phase current imbalance protection of each motor is released from the lockout.

2. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 1, characterized in that, The data length for harmonic analysis of the three-phase current signal of the current transformer is no greater than 0.5s.

3. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 1, characterized in that, The second harmonic setting threshold does not block the three-phase unbalanced motor protection when the main transformer is closed under no-load conditions, but reliably blocks the three-phase unbalanced motor protection when the main transformer is closed under no-load conditions.

4. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 5, characterized in that, In step S3, the set threshold is 10% to 18% of the real-time current.

5. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 1, characterized in that, In step 2, the time-domain current waveform of the three-phase current of the current transformer is converted into a frequency-domain signal to obtain the spectral distribution of the low-voltage side current of the high-voltage plant service transformer.

6. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 1, characterized in that, When it is determined that the main transformer is in the no-load closing process, a high-level signal is sent from the high-voltage station service transformer to the protection devices of all motors connected to the high-voltage station transformer through the signal line, and the three-phase current imbalance protection of each motor is blocked by the high level.

7. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 1, characterized in that, When it is determined that the main transformer is not in the no-load closing process or the impact caused by the no-load closing of the main transformer is greatly reduced, a low-level signal is sent from the high-voltage plant service transformer to all motor protection devices connected to the high-voltage plant transformer through the signal line, thereby unlocking the three-phase current imbalance protection of each motor through the low level.

8. The method for preventing maloperation of the three-phase imbalance protection of the medium-voltage plant auxiliary motor in nuclear power plants according to claim 1, characterized in that, When the second harmonic of two or more phases of current exceeds the set threshold, it is determined that the main transformer is in the no-load closing process. An electrical signal is immediately sent through the signal line to the protection devices of all motors connected to the high-voltage station service transformer, so that the three-phase current imbalance protection of each motor is blocked.

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