Synchronous generator field loss fault protection method based on electrical quantity change characteristics

Abstract

The invention discloses a synchronous generator excitation loss fault protection method based on electrical quantity change characteristics, and belongs to the technical field of power system relay protection, and the method comprises the following steps: S1, obtaining a generator terminal negative sequence voltage at a stator side of a synchronous generator in real time, opening protection when the generator terminal negative sequence voltage is smaller than a negative sequence voltage locking threshold value, and entering S2; otherwise, the protection is locked; s2, acquiring the terminal voltage effective value, reactive power and power angle of the synchronous generator in real time, and calculating the variable quantity of each electrical quantity; and S3, based on the variation of each electrical quantity calculated in the step S2, calculating an excitation loss index at the current moment, judging whether the excitation loss index meets an excitation loss index criterion or not, and when the time that the excitation loss index continuously meets the excitation loss index criterion reaches a preset delay time, enabling the protection device to act at an exit. The method is not influenced by the load of the generator and the excitation loss degree, and the excitation loss fault can be detected reliably and quickly.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, specifically to a method for protecting synchronous generators from loss of excitation faults based on the characteristics of electrical quantity changes. Background Technology

[0002] Loss of excitation is a common type of fault in synchronous generators, and it is divided into complete loss of excitation faults and partial loss of excitation faults. Rotor grounding faults are a common type of fault in large units, and relevant national standards clearly stipulate that large-capacity synchronous generator sets must be equipped with loss of excitation protection. Currently, loss of excitation protection based on the impedance principle is the most widely used. The protection action area is set according to the measured impedance trajectory at the generator terminals after loss of excitation. If the measured impedance trajectory remains within the action area within the set delay, a loss of excitation fault is determined to have occurred. There are two criteria for loss of excitation impedance protection: 1) loss of excitation impedance criterion set by asynchronous boundary; 2) loss of excitation impedance criterion set by static stability boundary.

[0003] However, loss-of-magnetism protection based on impedance principle has the following drawbacks: (1) The protection method based on impedance principle has the risk of false tripping. It cannot effectively distinguish between loss of excitation and system stable power oscillation, causing the protection to malfunction under system oscillation.

[0004] (2) Protection methods based on impedance principle have the risk of poor speed of operation or failure to operate. Taking the loss-of-excitation impedance criterion set according to the static stability boundary as an example, the modified static stability boundary impedance circle is generally used as the operating zone in China. The modified static stability boundary impedance circle has a large deviation from the theoretical static stability boundary near the horizontal axis, which makes the relay operate slightly later when the generator loses excitation under heavy load conditions. Moreover, the larger the system interconnection reactance, the more significant the impact, resulting in poor protection speed of operation.

[0005] (3) When a synchronous generator experiences a partial loss of excitation fault, the power angle Compared to a complete demagnetization fault, the increase process is much slower. The trajectory of the measured impedance at the control terminal moves towards the operating region more slowly, and may even fluctuate at the operating boundary, causing the timer to be repeatedly reset, thus prolonging the operating time. The protection has poor speed response to partial demagnetization faults, and its speed response performance is further deteriorated under light load conditions, even causing the protection to fail to operate.

[0006] To address the problems of existing methods (loss of excitation protection methods based on impedance principle) such as slow response or even failure to operate under partial or light-load loss of excitation, and the risk of false operation under system oscillation, a new method for loss of excitation fault protection needs to be proposed. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for protecting synchronous generators from demagnetization faults based on the characteristics of electrical quantity changes, so as to reliably and quickly detect demagnetization faults without being affected by generator load and degree of demagnetization.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows.

[0009] A method for protecting synchronous generators from loss of excitation faults based on electrical quantity variation characteristics includes the following steps: S1. Real-time acquisition of the negative sequence voltage at the stator side of the synchronous generator. When the voltage is less than the negative sequence voltage blocking threshold, the protection is opened and proceed to step S2; otherwise, the protection is blocked. S2. Real-time acquisition of the effective value of the synchronous generator terminal voltage, reactive power, and power angle, and calculation of the changes in the above electrical quantities; S3. Based on the changes in each electrical quantity obtained in step S2, calculate the current demagnetization index and determine whether it meets the demagnetization index criterion. When the demagnetization index continuously meets the demagnetization index criterion for a preset delay time, the protection device outputs an action.

[0010] Preferably, the negative sequence voltage blocking threshold in step S1 is set by the following formula:

[0011] in, Rated phase voltage of the generator; proportional coefficient; The negative sequence voltage blocking threshold is set to [value]. This is twice the rated phase voltage of the generator.

[0012] Preferably, the proportionality coefficient .

[0013] Preferably, in step S2, calculating the change in each of the above electrical quantities specifically involves: calculating the change in each electrical quantity over one sampling period. The formula for calculating the per-unit change within the range is as follows: Per-unit variation of RMS terminal voltage ,in, The effective value of the synchronous generator terminal voltage in one sampling period The amount of change within; V B This is the reference value for the terminal voltage; Per-unit variation of reactive power ,in, The reactive power of a synchronous generator in one sampling period The amount of change within; This is the baseline value for reactive power; Power subscript perturbation change ,in, The power angle of the synchronous generator in one sampling period The amount of change within; This is the power angle reference value.

[0014] Preferably, the power subscript monotonicity change amount Standardize to 1 rad; the sampling period The value is set to 20ms.

[0015] Preferably, the formula for calculating the demagnetization index in step S3 is as follows:

[0016] in, This is an indicator of demagnetization. This is the gain coefficient.

[0017] Preferably, in step S3, the demagnetization index criterion specifically involves: judging the demagnetization index. Is it greater than the preset threshold? That is, satisfying .

[0018] Preferably, the setting threshold Determined by the following formula:

[0019] in, The maximum value of the demagnetization index obtained from the statistics of the protected generator during its historical normal operation; The reliability coefficient is used as the criterion.

[0020] Preferably, in step S3, the delay time is set using the system's lowest oscillation frequency, and its calculation formula is as follows:

[0021] in, This is the delay time; This is the lowest oscillation frequency of the system; This represents the time-delay reliability coefficient.

[0022] Preferably, the gain coefficient The value is 10 8 The reliability coefficient of the criterion The value is 100; the delay reliability coefficient The value is 1.1.

[0023] Due to the adoption of the above technical solutions, the technical progress achieved by this invention is as follows.

[0024] This invention is the first to propose using the differences in voltage, reactive power, and power angle changes during synchronous generator demagnetization and system oscillation to detect synchronous generator demagnetization faults. This solves the problems of existing methods, such as slow response or even failure to respond under partial or light-load demagnetization conditions, and the risk of false response under system oscillation conditions.

[0025] This invention has high reliability, better speed and sensitivity compared to existing methods, and good conditions for engineering application, which can meet the needs of synchronous generator set demagnetization protection in engineering.

[0026] This invention introduces a negative sequence voltage blocking mechanism, which effectively prevents the risk of protection malfunction when the system experiences an asymmetrical fault.

[0027] This invention effectively prevents the protection system from malfunctioning during system oscillation by setting the delay time based on the system's lowest oscillation frequency. Attached Figure Description

[0028] Figure 1 This is a flowchart of the present invention; Figure 2 This is a simulation circuit diagram of the present invention; Figure 3 The figures show a comparison of the loss-magnetization detection methods of the present invention and the traditional impedance principle loss-magnetization protection method in Case 1 and Case 2. Figure 3 (a) shows the demagnetization index waveform of the method of the present invention in Case 1. Figure 3 (b) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 1. Figure 3 (c) shows the demagnetization index waveform of the method of the present invention in Case 2. Figure 3 (d) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 2; Figure 4 The figures show a comparison of the loss-magnetization detection methods of the present invention and the traditional impedance principle loss-magnetization protection method in Cases 3 and 4. Figure 4 (a) shows the demagnetization index waveform of the method of the present invention in Case 3. Figure 4 (b) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 3. Figure 4 (c) shows the demagnetization index waveform of the method of the present invention in Case 4. Figure 4 (d) shows the impedance trajectory of the traditional impedance principle loss-of-magnetism protection method in Case 4; Figure 5 These are comparison figures of the loss-of-magnetism detection of the present invention and the traditional impedance principle loss-of-magnetism protection method in Cases 5 and 6. Figure 5 (a) shows the demagnetization index waveform of the method of the present invention in Case 5. Figure 5 (b) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 5. Figure 5(c) shows the demagnetization index waveform of the method of the present invention in Case 6. Figure 5 (d) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 6; Figure 6 These are comparison figures of the loss-magnetization detection of the present invention and the traditional impedance principle loss-magnetization protection method in Cases 7 and 8. Figure 6 (a) shows the demagnetization index waveform of the method of the present invention in Case 7. Figure 6 (b) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 7. Figure 6 (c) shows the demagnetization index waveform of the method of the present invention in Case 8. Figure 6 (d) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 8; Figure 7 This is a comparison diagram of the loss-magnetization detection of the present invention and the traditional impedance principle loss-magnetization protection method in Case 9. Figure 7 (a) shows the demagnetization index waveform of the method of the present invention in Case 9. Figure 7 (b) shows the impedance trajectory of the traditional impedance-based loss-of-magnetism protection method in Case 9; Figure 8 This is a simulation result diagram of the operation of the present invention during low-frequency system oscillation, wherein, Figure 8 (a) is a waveform diagram of the generator power angle during low-frequency oscillation. Figure 8 (b) is the response waveform diagram of the demagnetization index of the present invention under the corresponding operating conditions. Figure 8 (c) is the waveform diagram of the trip signal output of the protection device under the corresponding operating condition (it is always 0, indicating that there is no false tripping); Figure 9 This is a simulation result diagram of the operation of the present invention during high-frequency system oscillation, wherein, Figure 9 (a) is a waveform diagram of the generator power angle during high-frequency oscillation. Figure 9 (b) is the response waveform diagram of the demagnetization index of the present invention under the corresponding operating conditions. Figure 9 (c) is the waveform of the trip signal at the output of the protection device under the corresponding operating condition (it is always 0, indicating that there is no false trip). Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0030] A method for protecting synchronous generators from loss of excitation faults based on electrical quantity variation characteristics, combined with... Figure 1 As shown, it includes the following steps: S1. Real-time acquisition of negative sequence voltage at the generator terminals on the stator side of the synchronous generator. When the voltage is less than the negative sequence voltage blocking threshold, the protection is opened and proceeds to step S2; otherwise, the protection is blocked.

[0031] Since the loss of excitation fault is a symmetrical fault, and considering that the proposed protection method may have the risk of false tripping when the system experiences an asymmetrical fault, a negative sequence voltage blocking mechanism is introduced. The protection method is only enabled when the negative sequence voltage is less than the negative sequence voltage blocking threshold.

[0032] The negative sequence voltage blocking threshold is set by the following formula:

[0033] in, This refers to the generator's rated phase voltage; This is the proportionality coefficient; The negative sequence voltage blocking threshold is set to [value]. This is twice the rated phase voltage of the generator.

[0034] Specifically, the proportionality coefficient ,Right now .

[0035] when When the protection is deemed open, proceed to step S2; when If the condition is determined to be a protective lockout, return to re-execute step S1.

[0036] S2. Real-time acquisition of the effective value of the synchronous generator terminal voltage, reactive power, and power angle, and calculation of the changes in the above electrical quantities.

[0037] Real-time acquisition of the effective value of the generator terminal voltage reactive power and power angle Calculate the changes in each of the above electrical quantities, specifically: calculate the changes in each electrical quantity over one sampling period. The per-unit change within.

[0038] The calculation formula is as follows: Per-unit variation of RMS terminal voltage ,in, The effective value of the synchronous generator terminal voltage in one sampling period The amount of change within; V B This is the reference value for the terminal voltage; Per-unit variation of reactive power ,in, The reactive power of a synchronous generator in one sampling period The amount of change within; This is the baseline value for reactive power; Power subscript perturbation change ,in, The power angle of the synchronous generator in one sampling period The amount of change within; This is the power angle reference value.

[0039] Specifically, the power subscript unitization change Standardize to 1 rad; sampling period The value is set to 20ms, which corresponds to one cycle duration of a 50Hz power frequency system.

[0040] S3. Based on the changes in each electrical quantity obtained in step S2, calculate the current demagnetization index and determine whether it meets the demagnetization index criterion. When the demagnetization index continuously meets the demagnetization index criterion for a preset delay time, the protection device outputs an action.

[0041] (1) The formula for calculating the demagnetization index is as follows:

[0042] in, This is an indicator of demagnetization. This is the gain factor, used to improve the clarity of the demagnetization indicator. Specifically, the gain factor... The value is set to 10. 8 .

[0043] (2) The specific criteria for judging the demagnetization index are as follows: judging the demagnetization index Is it greater than the preset threshold? That is, satisfying .

[0044] Specifically, setting the threshold Determined by the following formula:

[0045] in, The value is the maximum value of the demagnetization index obtained from the historical normal operation of the protected generator. The value may vary for different units and must be obtained from the normal operation data of the protected unit. This is the reliability coefficient of the criterion. Specifically, the reliability coefficient of the criterion... The value is 100.

[0046] (3) The delay time is set using the system's lowest oscillation frequency, as follows: To prevent the protection method from malfunctioning during system oscillations, a time delay is introduced. This is because the periodic changes in all electrical quantities directly follow the power angle. The periodic changes in electrical quantities mean that the oscillation period of each electrical quantity is related to the power angle. The oscillation periods are completely consistent. Demagnetization index. The maximum period of the oscillation (i.e., the maximum period of the power angle oscillation) is the reciprocal of the system's lowest oscillation frequency, that is:

[0047] in, This represents the maximum period of oscillation in the demagnetization index. The minimum oscillation frequency of the system can be obtained from engineering experience or based on a full-parameter simulation of the system (usually provided by the provincial network dispatching department). Therefore, the delay time is tuned as follows:

[0048] in, This is the delay time; This refers to the time-delay reliability coefficient. Specifically, the time-delay reliability coefficient... The value is 1.1.

[0049] The working principle of this invention is as follows: When a synchronous generator experiences a loss of excitation fault, the effective value of the generator terminal voltage... Decrease, reactive power From positive reactive power to absorbed reactive power (i.e.) (reduced or even turned negative), power angle The three electrical quantities all change significantly, and the direction of change is consistent. This invention constructs a demagnetization index by monitoring the changes in these three electrical quantities in real time and multiplying them. When a demagnetization fault occurs, All show the same direction of change, making If the value increases sharply and exceeds the set threshold, the protection mechanism will activate after a delay.

[0050] When the system experiences power oscillations, although the various electrical quantities also change periodically, The direction and amplitude of the change are fundamentally different from those of a demagnetization fault, and the oscillation period is relatively long. This invention addresses this by reasonably adjusting the delay time. This effectively prevents malfunctions under oscillation conditions.

[0051] When an asymmetrical fault occurs in the system, the negative sequence voltage at the machine terminals... The voltage will increase significantly. At this time, the negative sequence voltage blocking circuit in step S1 will be blocked to prevent maloperation under asymmetrical faults.

[0052] The effectiveness of the proposed protection method will be verified through simulation below.

[0053] The simulation circuit was built using PSCAD / EMTDC. The specific simulation circuit is as follows: Figure 2As shown, the system includes a synchronous generator G, a transformer T, transmission lines (line one and line two), a line resistance-reactance-inductance equivalent model RRL, and an infinite AC power grid. The generator terminals of the synchronous generator G are connected to the beginning of line one and the beginning of line two via the transformer T. The end of line one is connected to the infinite AC power grid via the line resistance-reactance-inductance equivalent model RRL. A load is installed at the end of line two, with a load power of... ,and Active power The imaginary unit represents the orthogonality between reactive and active components. Fault simulation point A is set at the beginning of lines one and two (i.e., near the transformer side) to apply a three-phase short-circuit fault to simulate system oscillation conditions. Relevant parameters are shown in Table 1 below: Table 1 Simulation Parameters

[0054] Protection delay time T set Set to 1.5s; for ; K rel Set it to 100.

[0055] For synchronous generators operating under both normal and light load conditions, full and partial loss-excitation faults were simulated to compare the difference in operating time between the proposed synchronous generator loss-excitation fault protection method based on electrical quantity variation characteristics and the traditional impedance-based loss-excitation protection method. The impedance-based loss-excitation protection uses an asynchronous boundary impedance circle as the protection operating zone, with a delay time set to 1.5s. Specific data for each simulation case are shown in Table 2 below. Table 2 Specific data for each case

[0056] Comparison of loss of magnetization detection using the two protection methods in various cases, such as Figures 3 to 7 As shown, in each case, the horizontal axis of the demagnetization detection graph of the method proposed in this invention represents time (in seconds), and the vertical axis represents the demagnetization index; in each case, the horizontal axis of the demagnetization detection graph of the traditional impedance principle demagnetization protection method represents resistance. R The vertical axis represents reactance. X All units are per unit (pu). The simulation action time (time from loss of excitation to tripping) for each case is shown in Table 3 below: Table 3. Action time of various loss-of-magnetism protections under different cases It can be observed that when partial loss of excitation occurs under light load on a synchronous generator, the operation time of loss of excitation protection based on the impedance principle is significantly prolonged, exhibiting a pattern where the lighter the load and the lower the degree of loss of excitation, the longer the operation time, and even failure to operate occurs. In contrast, the synchronous generator loss of excitation fault protection method based on electrical quantity change characteristics proposed in this invention has very stable operation performance for loss of excitation faults, unaffected by generator load conditions or degree of loss of excitation. It can operate quickly and reliably in all test cases without any failure to operate. Moreover, compared with the traditional impedance principle protection method, the operation time is shortened by at least 58.6%, and it has good discrimination performance for loss of excitation faults.

[0057] The operation of the synchronous generator loss-of-excitation fault protection method based on electrical quantity change characteristics proposed in this invention is simulated under low-frequency and high-frequency system oscillations. t =4s at Figure 2 A three-phase short circuit was applied at point A, with fault durations set to 0.2s and 0.3s respectively to simulate low-frequency and high-frequency system oscillations. The simulation results are as follows: Figures 8 to 9 As shown, the protection system did not maloperate during either low-frequency or high-frequency system oscillations. The synchronous generator loss-of-excitation protection based on electrical quantity variation characteristics exhibits excellent anti-maloperation performance against system oscillations.

Claims

1. A method for protecting synchronous generators from loss of excitation faults based on electrical quantity variation characteristics, characterized in that: Includes the following steps: S1. Real-time acquisition of the negative sequence voltage at the stator side of the synchronous generator. When the voltage is less than the negative sequence voltage blocking threshold, the protection is opened and proceed to step S2; otherwise, the protection is blocked. S2. Real-time acquisition of the effective value of the synchronous generator terminal voltage, reactive power, and power angle, and calculation of the changes in the above electrical quantities; S3. Based on the changes in each electrical quantity obtained in step S2, calculate the current demagnetization index and determine whether it meets the demagnetization index criterion. When the demagnetization index continuously meets the demagnetization index criterion for a preset delay time, the protection device outputs an action.

2. The method for synchronous generator loss-of-excitation fault protection based on electrical quantity variation characteristics according to claim 1, characterized in that: The negative sequence voltage blocking threshold in step S1 is set by the following formula: in, This refers to the generator's rated phase voltage; This is the proportionality coefficient; The negative sequence voltage blocking threshold is set to [value]. This is twice the rated phase voltage of the generator.

3. The synchronous generator loss-of-excitation fault protection method based on electrical quantity change characteristics according to claim 2, characterized in that: The proportionality coefficient .

4. The method for protecting synchronous generators from excitation failure based on electrical quantity variation characteristics according to claim 1, characterized in that: In step S2, the change in each of the above electrical quantities is calculated, specifically by calculating the change in each electrical quantity over one sampling period. The formula for calculating the per-unit change within the range is as follows: Per-unit variation of RMS terminal voltage ,in, The effective value of the synchronous generator terminal voltage in one sampling period The amount of change within; V B This is the reference value for the terminal voltage; Per-unit variation of reactive power ,in, The reactive power of a synchronous generator in one sampling period The amount of change within; This is the baseline value for reactive power; Power subscript perturbation change ,in, The power angle of the synchronous generator in one sampling period The amount of change within; This is the power angle reference value.

5. The synchronous generator loss-of-excitation fault protection method based on electrical quantity change characteristics according to claim 4, characterized in that: The power subscript perturbation change Standardize to 1 rad; the sampling period The value is set to 20ms.

6. The method for protecting synchronous generators from excitation failure based on electrical quantity variation characteristics according to claim 4, characterized in that: The formula for calculating the demagnetization index in step S3 is as follows: in, This is an indicator of demagnetization. This is the gain coefficient.

7. The method for protecting synchronous generators from excitation failure based on electrical quantity variation characteristics according to claim 6, characterized in that: In step S3, the criteria for determining the demagnetization index are specifically as follows: determining the demagnetization index. Is it greater than the preset threshold? That is, satisfying .

8. The method for protecting synchronous generators from excitation failure based on electrical quantity variation characteristics according to claim 7, characterized in that: The set threshold Determined by the following formula: in, This represents the maximum value of the demagnetization index obtained statistically during the historical normal operation of the protected generator. The reliability coefficient is used as the criterion.

9. A method for protecting synchronous generators from excitation failure based on electrical quantity variation characteristics according to claim 8, characterized in that: In step S3, the delay time is set using the system's lowest oscillation frequency, and its calculation formula is as follows: in, This is the delay time; This is the lowest oscillation frequency of the system; This represents the time-delay reliability coefficient.

10. A method for protecting synchronous generators from excitation failure based on electrical quantity variation characteristics according to claim 9, characterized in that: The gain coefficient The value is 10 8 The reliability coefficient of the criterion The value is 100; the delay reliability coefficient The value is 1.1.

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