Method for Judging Slow Fusing of Excitation System PT Based on Inverse Time Characteristic Principle of Negative-Sequence Voltage

Abstract

Method for judging slow fuse of excitation system PT based on inverse time principle of negative sequence voltage, including the steps: Step 1, set a threshold value; Step 2, measure the secondary side voltage of PT and perform a new type of decomposition, extract the negative sequence voltage component U_, judge the range interval of the value of the negative sequence voltage component U_ and start timing, and enter Step 3 when the timing time is reached; Step 3, detect the value of the negative sequence stator current component I_, and judge whether a slow fuse phenomenon occurs. Multiple groups of negative sequence voltage thresholds are set, and an action delay is set for each group of negative sequence voltage thresholds. Moreover, the greater the negative sequence voltage, the shorter the action delay. On the one hand, it can shorten the time for reporting slow fuse of PT, achieve early detection and timely response, and prevent the main set from continuously increasing the excitation before the negative sequence voltage reaches the voltage criterion threshold in the "PT difference coefficient judgment method" and the "negative sequence voltage judgment method", which can greatly reduce the accident rate of generator mis-excitation and overvoltage; on the other hand, it can flexibly configure the action time of PT disconnection under different negative sequence voltages, with strong applicability.

Description

Technical Field

[0001] The present invention relates to the technical field of measurement and control of hydro-generators, and particularly to a method for judging slow melting of excitation system PT based on the inverse time principle of negative sequence voltage. Background Art

[0002] The slow fusing of the outgoing voltage transformer of a hydro-generator (abbreviated as PT slow melting) is a fault with a relatively high incidence rate in recent years. It is mainly manifested as the loosening of the fuse on the high-voltage side of a certain phase voltage transformer (abbreviated as PT) and the increase in the resistance of the slow-fusing fuse. The secondary voltage value of this phase PT will slowly decrease until the high-voltage fuse is completely blown. Since the excitation system regulator performs closed-loop regulation on the stator voltage of the generator based on the effective values of the A, B, and C phase voltages sampled by the PT at the generator outlet, the slow melting of the PT causes the effective value of the stator voltage collected by the excitation system to slowly decrease. As the degree of slow melting gradually deepens, the excitation system will continuously increase the excitation to achieve closed-loop control of the generator stator voltage, resulting in too high generator voltage or too high system bus voltage, and in severe cases, leading to accident trip and shutdown.

[0003] At present, there are mainly the following two judgment methods for the excitation system to deal with PT disconnection:

[0004] Method 1: PT difference coefficient judgment method. That is, the difference between the effective values of the stator voltages sampled by the double-set excitation regulators (|PT1 - PT2|) is used to judge whether the high-voltage fuse on the primary side of the PT is blown. The judgment logic is as Figure 1 shown. When |PT1 - PT2| > △Uset (△Uset is the set PT voltage difference coefficient), the excitation system determines that the PT has slow melting and acts after a delay of T (usually 60 ms).

[0005] Method 2: Negative sequence voltage judgment method. That is, the excitation system decomposes the PT secondary side voltage collected, extracts the negative sequence voltage component, and the judgment logic is as Figure 2 shown. U_ is the negative sequence voltage component; U_set is the negative sequence voltage judgment threshold; T is the negative sequence voltage judgment action time; I_ is the negative sequence stator current component; I_set is the negative sequence stator current judgment threshold; the negative sequence voltage component is compared with the set threshold. When the negative sequence voltage component exceeds the parameter set value, it is considered that the PT has slow melting and acts after a delay of T (usually 60 ms). This method needs to block the negative sequence stator current criterion.

[0006] However, the above two methods have their drawbacks. Affected by factors such as low measurement accuracy or inaccurate algorithms, in order to prevent the adverse effects caused by the misoperation of PT disconnection, the PT voltage difference coefficient △Uset and the negative sequence voltage judgment threshold U_set cannot be set too small. In engineering, it is usually set to 5% of the rated stator voltage. When a disconnection occurs in one or several phases of the secondary side of the PT, the effective value of the stator voltage sampled by the excitation significantly decreases, and the above two judgment methods can easily identify the PT disconnection fault; when a slow fuse occurs on the secondary side of the PT and the secondary side voltage of the PT slowly decreases but does not reach the voltage threshold of the PT disconnection criterion, the excitation system will always operate in the fault channel. According to the measured stator voltage, the fault channel continuously increases the excitation current to raise the stator voltage to the voltage given value. At this time, over-excitation or overvoltage is likely to occur, and even cause the protection to operate.

[0007] There are also existing technologies for judging PT disconnection in the excitation system. For example, Chinese Patent Document CN109541469 A records a method for judging PT disconnection in a generator excitation system. By using a dual-set excitation regulator: the difference between the effective values of the stator voltages sampled by Set A excitation regulator and Set B excitation regulator, or the difference between the stator voltage of the excitation on-line control regulator and the per-unit value of the anode voltage, to judge whether a slow fuse fault occurs in the high-voltage fuse on the primary side of the PT. However, such a judgment method is only suitable for detecting disconnection, cannot give an early warning of PT slow melting in a timely manner, and cannot solve the aforementioned technical problems. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for judging PT slow melting in an excitation system based on the inverse time principle of negative sequence voltage, increasing multiple groups of negative sequence voltage starting thresholds, and respectively configuring inverse time characteristics, adopting a hierarchical control strategy, so that the larger the negative sequence voltage, the shorter the action delay time, and the smaller the negative sequence voltage, the longer the action delay time, so as to enable the excitation system to also make a correct response when the secondary side voltage of the PT slowly decreases, and will not malfunction due to external disturbances.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] A method for judging PT slow melting in an excitation system based on the inverse time principle of negative sequence voltage, comprising the following steps:

[0011] Step 1: Set multiple groups of increasing negative sequence voltage thresholds U_set1 - U_setn and their corresponding decreasing action delay time thresholds T1 - Tn, and set the negative sequence stator current judgment threshold I_set;

[0012] Step 2: The generator excitation system collects the secondary voltage of the voltage transformer at the generator outlet, i.e., PT, and performs new decomposition to extract the negative sequence voltage component U_, and compares the negative sequence voltage U_ with the negative sequence voltage thresholds U_set1 - U_setn to determine the range interval of the value of the negative sequence voltage U_. Select the action delay time threshold corresponding to the range interval of the value of the negative sequence voltage U_ and start timing. If the value of the negative sequence voltage U_ has been within the current range interval during this timing period, proceed to Step 3. If the value of the negative sequence voltage U_ crosses into other range intervals during this timing period, re-select the action delay time threshold corresponding to the range interval of the value of the negative sequence voltage U_ and re-start timing according to this time threshold. When the timing time reaches the action delay time threshold, enter Step 3;

[0013] Step 3: Detect the negative sequence stator current component I_, and compare it with the negative sequence stator current determination threshold I_set value. When I_ < I_set, it is determined that a slow fuse phenomenon occurs in the voltage transformer at the generator outlet, and a slow fuse alarm is issued.

[0014] In the preferred solution, the generator excitation system in Step 2 has two regulators, namely the main excitation regulator A and the hot standby excitation regulator B. The main excitation regulator A collects the secondary voltage of the voltage transformer at the generator outlet and extracts the negative sequence voltage component UA_. The hot standby excitation regulator B collects the secondary voltage of the voltage transformer at the generator outlet and extracts the negative sequence voltage component UB_. In Step 2, first compare the value of the negative sequence voltage component UA_ with the negative sequence voltage thresholds U_set1 - U_setn. When a certain action delay time threshold timing reaches, the main excitation regulator A issues a PT disconnection fault signal and switches to the hot standby excitation regulator B for operation. When the value of the negative sequence voltage component UB_ is compared with the negative sequence voltage thresholds U_set1 - U_setn and also reaches the action delay time threshold corresponding to the current value of the negative sequence voltage component UB_, enter Step 3.

[0015] The number of groups of the above-mentioned increasing negative sequence voltage thresholds and decreasing action delay time thresholds is 5.

[0016] The above-mentioned increasing negative sequence voltage thresholds U_set1 - U_set5 are 2%, 2.5%, 3%, 4%, and 5% respectively; the decreasing action delay time thresholds T1 - T5 are 20S, 10S, 5S, 0.4S, and 0.06S respectively.

[0017] A method for judging slow melting of excitation system PT based on the inverse time principle of negative sequence voltage. By setting multiple groups of negative sequence voltage thresholds and setting an action delay for each group of negative sequence voltage thresholds, and the greater the negative sequence voltage, the shorter the action delay. On the one hand, it can shorten the time for reporting slow melting of PT, detect it in advance and respond in time, so as to prevent the main set from continuously increasing the excitation before the negative sequence voltage reaches the voltage criterion threshold in the "PT difference coefficient judgment method" and the "negative sequence voltage judgment method", and can greatly reduce the accident rate of generator mis-excitation and overvoltage; on the other hand, it can flexibly configure the action time of PT disconnection under different negative sequence voltages, with strong applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described below with reference to the drawings and embodiments:

[0019] Figure 1 It is a schematic diagram of the judgment logic of the PT difference coefficient judgment method;

[0020] Figure 2 It is a schematic diagram of the judgment logic of the negative sequence voltage judgment method;

[0021] Figure 3 It is a schematic diagram of the judgment logic of the inverse time judgment method of negative sequence voltage of the present invention;

[0022] Figure 4 It is a schematic diagram for measuring the action time of PT slow melting alarm when the test negative sequence voltage is 2%;

[0023] Figure 5 It is a schematic diagram for measuring the action time of PT slow melting alarm when the test negative sequence voltage is 2.5%;

[0024] Figure 6 It is a schematic diagram for measuring the action time of PT slow melting alarm when the test negative sequence voltage is 3%;

[0025] Figure 7 It is a schematic diagram for measuring the action time of PT slow melting alarm when the test negative sequence voltage is 5%. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] The technical solution of the present invention will be described in detail below with reference to the drawings and embodiments.

[0027] As Figure 3 shown in this way, the method for judging slow melting of excitation system PT based on the inverse time principle of negative sequence voltage includes the following steps:

[0028] Step 1: Set multiple groups of increasing negative sequence voltage thresholds U_set1-U_setn and their corresponding decreasing action delay time thresholds T1-Tn, and set the negative sequence stator current judgment threshold I_set;

[0029] Step 2: The generator excitation system collects the secondary side voltage of the voltage transformer at the generator outlet, i.e., PT, and performs a new type of decomposition to extract the negative sequence voltage component U_, and compares the negative sequence voltage U_ with the negative sequence voltage thresholds U_set1 - U_setn to determine the range interval of the value of the negative sequence voltage U_. Select the action delay time threshold corresponding to the range interval of the value of the negative sequence voltage U_ and start timing. If the value of the negative sequence voltage U_ remains within the current range interval during this timing period, proceed to Step 3. If the value of the negative sequence voltage U_ crosses into other range intervals during this timing period, re-select the action delay time threshold corresponding to the range interval of the value of the negative sequence voltage U_ and re-start timing according to this time threshold. When the timing time reaches the action delay time threshold, proceed to Step 3;

[0030] Step 3: Detect the negative sequence stator current component I_, and compare it with the negative sequence stator current determination threshold I_set value. When I_ < I_set, it is determined that a slow fuse phenomenon occurs in the voltage transformer at the generator outlet, and a slow fuse alarm is issued.

[0031] In a preferred solution, the generator excitation system in Step 2 has two regulators, namely the main excitation regulator A and the hot standby excitation regulator B. The main excitation regulator A collects the secondary side voltage of the voltage transformer at the generator outlet and extracts the negative sequence voltage component UA_. The hot standby excitation regulator B collects the secondary side voltage of the voltage transformer at the generator outlet and extracts the negative sequence voltage component UB_. In Step 2, first compare the value of the negative sequence voltage component UA_ with the negative sequence voltage thresholds U_set1 - U_setn. When the timing of a certain action delay time threshold reaches, the main excitation regulator A issues a PT disconnection fault signal and switches to the hot standby excitation regulator B for operation. When the value of the negative sequence voltage component UB_ is compared with the negative sequence voltage thresholds U_set1 - U_setn and also reaches the action delay time threshold corresponding to the current value of the negative sequence voltage component UB_, proceed to Step 3.

[0032] By using the main excitation regulator A and the hot standby excitation regulator B, it is possible to prevent misoperation caused by large errors in the PT value when a reading, conversion, or connection failure occurs in one set of excitation regulators.

[0033] The number of groups of the above-mentioned increasing negative sequence voltage thresholds and decreasing action delay time thresholds is 5.

[0034] The above-mentioned increasing negative sequence voltage thresholds U_set1 - U_set5 are 2%, 2.5%, 3%, 4%, and 5% respectively; the decreasing action delay time thresholds T1 - T5 are 20S, 10S, 5S, 0.4S, and 0.06S respectively.

[0035] The above method for determining slow fuse of PT in the excitation system based on the inverse time principle of negative sequence voltage sets five groups of negative sequence voltage thresholds and sets an action delay for each group of negative sequence voltage thresholds. As shown in Table 1, when the main set of the excitation regulator measures that the negative sequence voltage is greater than the minimum point U_set1 of the negative sequence criterion voltage, the regulator starts timing, looks up the action time according to the magnitude of the negative sequence voltage, and when the timing reaches the action time, it issues a PT disconnection fault signal and switches the excitation regulator to the slave set. The principle of parameter configuration is that the greater the negative sequence voltage, the shorter the action delay, and the smaller the negative sequence voltage, the longer the action delay.

[0036] Table 1 Setting values table of the inverse time determination method of negative sequence voltage

[0037] Negative sequence voltage Set value (percentage value) Delay value Set value (s) Minimum point U_set1 of negative sequence criterion voltage 2％ Minimum point T1 of negative sequence criterion time 20 Second point U_set2 of negative sequence criterion voltage 2.5％ Second point T2 of negative sequence criterion time 10 Third point U_set3 of negative sequence criterion voltage 3％ Third point T3 of negative sequence criterion time 5 Fourth point U_set4 of negative sequence criterion voltage 4％ Fourth point T4 of negative sequence criterion time 0.4 Maximum point U_set5 of negative sequence criterion voltage 5％ Maximum point T5 of negative sequence criterion time 0.06

[0038] The excitation system usually has two sets of regulators, one is the main one and the other is the hot standby. Taking the parameters listed in Table 1 as an example, the action principle of the inverse time determination method of negative sequence voltage is described. Assume that regulator set A is the main set and regulator set B is the standby. The negative sequence stator voltage measured by regulator set A is UA_.

[0039] When UA_ > 2%, the regulator starts timing. If UA_ is stable between 2% and 2.5% (excluding 2.5%), then when the timing reaches 20 seconds, the regulator issues a PT disconnection fault signal and switches to regulator set B for main operation;

[0040] If UA_ rapidly increases to 2.5% (less than 3%) due to slow fuse of PT, then an alarm signal is issued when the timing reaches 10s and it switches to regulator set B for main operation. Before the timing reaches, if UA_ increases to 3%, then the alarm signal is issued when the timing only reaches 5s, and so on;

[0041] If UA_ is less than 2%, the timing stops and resets.

[0042] Similarly, when regulator set B detects the negative sequence stator voltage, the action principle is the same.

[0043] This method can shorten the time for reporting slow fuse of PT, preventing the main set from continuously increasing the excitation when the negative sequence voltage does not reach the voltage criterion thresholds in the PT difference coefficient determination method and the negative sequence voltage determination method, and can greatly reduce the accident rates of generator mis-excitation and overvoltage; it can also flexibly configure the action time of PT disconnection under different negative sequence voltages and make personalized configurations according to the actual situation of the power plant, with strong applicability.

[0044] Example:

[0045] Test preparation:

[0046] The test equipment uses the NARI NES5100 excitation system. Before the test, the PT slow-fuse judgment criteria written according to the above logic are written into the excitation program, and the test parameters such as the negative sequence voltage threshold and action delay are set according to Table 1. The test instrument uses the relay protection tester produced by Haomai Power, which can provide two sets of independent three-phase voltage U A , U B , U C and U a , U b , U c , respectively simulate the secondary side voltage of the voltage transformer at the generator output end to the main excitation regulator A and the hot standby excitation regulator B, and introduce the excitation system switch alarm signal "PT slow fuse alarm" into the relay protection tester as the alarm action timing signal.

[0047] Test method:

[0048] (1) The excitation system is running with A as the main set and no-load, and the relay protection tester outputs the voltage U A =U B =U C =U a =U b =U c =57.74V, (rated voltage of the secondary side voltage of the voltage transformer at the generator output end), frequency is 50HZ, phase Ψ A =Ψ a =0°,Ψ B =Ψ b =-120°,Ψ C =Ψ c =120°;

[0049] (2) The test simulates the slow melting of the generator output A phase PT collected by the main excitation regulator A, that is, maintaining U B =U C =U a =U b =U c =57.74V unchanged, keep the frequency and phase unchanged, according to Table 2 "U A Set value", preset the A phase voltage value on the relay protection tester and lock it, unlock it after the inspection is ready, so that the A phase voltage on the relay protection tester is quickly adjusted to the preset value, wait for the "PT slow fuse alarm" of the excitation system to act, observe and record the action time (milliseconds) on the relay protection tester, and the test results are shown in Figures 4 to 7 ;

[0050] (3) After each set of tests is completed, the alarm signal must be reset, the regulator main set must be switched back to set A, and the A phase voltage on the relay protection tester must be restored. Then, steps (1) and (2) must be repeated to perform the second set of tests.

[0051] Table 2 "PT Slow Fuse Alarm" Test Data

[0052]

[0053] Test Conclusion:

[0054] This test was carried out by simulating the excitation under no-load conditions, that is, the negative-sequence stator current component I_ is 0. The comparison of the negative-sequence stator current component is consistent with the principle of negative-sequence voltage comparison, and there is no essential difference between the two. Therefore, the stator current component I_ being 0 does not affect the accuracy of the test. The focus of this test is to verify the correctness of the negative-sequence voltage start threshold hierarchical control strategy in the present invention.

[0055] As can be seen from Test Serial Number 1, when the negative-sequence voltage value does not reach the set threshold, the "PT slow fuse alarm" does not operate, and the operation result is correct; as can be seen from Test Serial Numbers 2 to 6, when the negative-sequence voltage value reaches the set threshold, the "PT slow fuse alarm" operates with a time delay, and the time delay value is generally consistent with the set value of the excitation system. The excitation system switches from Set A as the main to Set B as the main for operation. Due to the differences in the timing accuracy of the excitation system and the test instrument and the influence of signal transmission, the error is within a reasonable range.

[0056] From the above tests, it can be seen that the negative-sequence voltage start threshold hierarchical control strategy in the present invention is correct and feasible. The greater the negative-sequence voltage, the shorter the operation time delay, and the smaller the negative-sequence voltage, the longer the operation time delay. Thus, when the PT secondary-side voltage drops slowly, the excitation system can also make a correct response. Since it is a time-delay operation, the excitation system will not malfunction due to external short-term disturbances.

Claims

1. A method for judging slow melting of the excitation system PT based on the inverse time characteristic principle of negative sequence voltage, characterized in that, It includes the following steps: Step 1: Set multiple groups of increasing negative-sequence voltage thresholds U_set1 - U_setn and their corresponding decreasing action delay time thresholds T1 - Tn, and set the negative-sequence stator current determination threshold I_set; Step 2: The generator excitation system collects the secondary-side voltage of the voltage transformer at the generator outlet, i.e., PT, and decomposes it to extract the negative-sequence voltage component U_. Then compare the negative-sequence voltage U_ with the negative-sequence voltage thresholds U_set1 - U_setn to determine the range interval of the negative-sequence voltage U_ value, select the action delay time threshold corresponding to the range interval of the negative-sequence voltage U_ value and start timing. If the negative-sequence voltage U_ value remains within the current range interval during this timing, go to Step 3. If the negative-sequence voltage U_ value crosses into other range intervals during this timing, re-select the action delay time threshold corresponding to the range interval of the negative-sequence voltage U_ value and re-start timing according to this time threshold. When the timing reaches the action delay time threshold, go to Step 3; Step 3: Detect the negative-sequence stator current component I_, and compare it with the negative-sequence stator current determination threshold I_set value. When I_ < I_set, it is determined that a slow fuse phenomenon occurs at the voltage transformer at the generator outlet, and a slow fuse alarm is issued.

2. The method for determining slow fuse of excitation system PT based on the inverse time limit principle of negative sequence voltage according to claim 1, characterized in that The generator excitation system in Step 2 has two regulators, namely the main excitation regulator A and the hot standby excitation regulator B. The main excitation regulator A collects the secondary-side voltage of the voltage transformer at the generator outlet and extracts the negative-sequence voltage component UA_. The hot standby excitation regulator B collects the secondary-side voltage of the voltage transformer at the generator outlet and extracts the negative-sequence voltage component UB_. In Step 2, first compare the negative-sequence voltage component UA_ value with the negative-sequence voltage thresholds U_set1 - U_setn. When the timing of a certain action delay time threshold reaches, the main excitation regulator A issues a PT disconnection fault signal and switches to the hot standby excitation regulator B for operation. When the negative-sequence voltage component UB_ value is compared with the negative-sequence voltage thresholds U_set1 - U_setn and also reaches the action delay time threshold corresponding to the current negative-sequence voltage component UB_ value, go to Step 3.

3. The method for judging slow fuse of excitation system PT based on the inverse time principle of negative sequence voltage according to claim 1, characterized in that The number of groups of the increasing negative-sequence voltage thresholds and the decreasing action delay time thresholds is 5.

4. The method for determining slow fuse of excitation system PT based on the inverse time principle of negative sequence voltage according to claim 3, characterized in that The increasing negative-sequence voltage thresholds U_set1 - U_set5 are 2%, 2.5%, 3%, 4%, and 5% respectively; the decreasing action delay time thresholds T1 - T5 are 20S, 10S, 5S, 0.4S, and 0.06S respectively.

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