Pump storage unit loss-of-excitation protection method based on internal potential and reactive power characteristics

Abstract

The application discloses a pumped storage unit loss-of-excitation protection method based on internal potential and reactive power characteristics, and belongs to the technical field of generator relay protection, and comprises the following steps: S1. collecting three-phase voltage, three-phase current and stator side reactive power signals of the pumped storage unit, and calculating dq axis voltage components and dq axis current components through coordinate transformation; S2. constructing an internal potential analytical expression based on a transient model of the pumped storage unit; S3. constructing a starting criterion based on terminal voltage variation, and constructing an action criterion based on internal potential variation characteristics and reactive power variation characteristics; S4. when the starting criterion is satisfied, if the action criterion continuously satisfies a preset action criterion delay time, a loss-of-excitation protection action signal is output; if the action criterion does not satisfy the action condition, the starting criterion is reset. The application can effectively distinguish the loss-of-excitation fault from system oscillation, thereby improving the reliability and rapidity of the loss-of-excitation protection of the pumped storage unit.

Description

Technical Field

[0001] This invention relates to the field of generator relay protection technology, specifically to a method for loss-of-excitation protection of pumped-storage units based on internal potential and reactive power characteristics. Background Technology

[0002] Pumped storage units play crucial roles in power systems, including peak shaving, frequency regulation, and emergency backup. Their safe and stable operation is essential for ensuring grid reliability. During operation, excitation system anomalies can lead to de-excitation faults in the unit. After a de-excitation fault occurs, the unit's reactive power reverses, shifting from providing reactive power support to the system to absorbing a large amount of reactive power from the system. This results in a drop in terminal voltage, and in severe cases, may trigger a power system voltage collapse. Therefore, configuring reliable de-excitation protection devices is crucial for ensuring the safe and stable operation of the unit and the power system.

[0003] Currently, loss-of-excitation protection methods based on terminal impedance measurement are widely used in engineering. This method identifies faults by constructing impedance trajectories and setting operating zones. However, existing technologies still have the following problems: under complex operating conditions such as power system oscillations, the terminal impedance measurement trajectory may inadvertently enter the pre-set protection operating zone, leading to malfunctions of the protection device and affecting the normal operation of the system. Therefore, how to accurately identify loss-of-excitation faults and effectively distinguish between loss-of-excitation faults and system oscillations under complex operating conditions such as system oscillations is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for loss of excitation protection of pumped-storage units based on internal potential and reactive power characteristics, so as to effectively distinguish between loss of excitation faults and system oscillations, thereby improving the reliability and speed of loss of excitation protection of pumped-storage units.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows.

[0006] A method for loss-of-excitation protection of pumped-storage units based on internal potential and reactive power characteristics includes the following steps: S1. Collect the three-phase voltage, three-phase current and stator-side reactive power signals of the pumped-storage unit, and calculate the dq-axis voltage component and dq-axis current component through coordinate transformation; S2. Construct an analytical expression of the internal potential based on the transient model of the pumped-storage unit; S3. Construct a start-up criterion based on the change in terminal voltage, and construct an action criterion based on the characteristics of internal potential change and reactive power change; S4. When the start criterion is met, if the action criterion continues to meet the preset action criterion delay time, the loss of excitation protection action signal is output; if the action criterion does not meet the action conditions, the start criterion is reset.

[0007] Preferably, step S2 specifically includes: Based on the pumped-storage unit voltage equation and flux linkage equation, and neglecting stator resistance and high-frequency transient components, the analytical expression for the transient internal potential of the pumped-storage unit is obtained:

[0008] in, Internal potential, This is the q-axis stator voltage. For d-axis transient reactance, This represents the d-axis stator current.

[0009] Preferably, in step S3, the activation criterion includes any one of the following: The terminal voltage is lower than the preset voltage threshold and continues for the preset start-up criterion delay time; The voltage change at the machine terminal is lower than the preset voltage change threshold and continues for the preset start-up criterion delay time. If any of the above conditions are met, the start criterion is deemed satisfied. The change in terminal voltage is obtained by subtracting the terminal voltage values ​​corresponding to two sampling points spaced apart by a differential interval.

[0010] Preferably, the voltage threshold is a per-unit value, specifically 0.95 pu; the voltage change threshold is specifically 0; and the start-up criterion delay time is 100 ms.

[0011] Preferably, in step S3, the action criterion is:

[0012] And only when <0 and When <0, the action criterion is satisfied; in, The steady-state per-unit value of the internal potential before the activation criterion is first satisfied. For the first Per-unit potential value within each sampling point For the first The change in internal electric potential at a point. The stator reactive power steady-state per-unit value before the trigger criterion is first met. For the first per-unit value of stator reactive power at each sampling point For the first The change in stator reactive power at a given point. The sampling sequence number is incremented by differential intervals.

[0013] Preferably, the and All values ​​are taken as the average value of the wave one week before the start-up.

[0014] Preferably, in step S4, the preset action criterion delay time is set to 1.5 seconds.

[0015] Due to the adoption of the above technical solutions, the technical progress achieved by this invention is as follows.

[0016] This invention boasts high reliability, effectively distinguishes between demagnetization faults and system oscillations, and significantly enhances its anti-interference capability. The core innovation of this invention lies in its elimination of the drawback of traditional impedance protection, which indirectly reflects loss of excitation, and the introduction of "internal potential," a physical quantity directly characterizing the generator's excitation state, into loss of excitation protection. The internal potential is obtained in real-time from the generator terminal electrical quantities, and its physical meaning is clear: the essence of a loss of excitation fault is a decrease in excitation current, which leads to a corresponding decrease in internal potential; however, during system oscillation, the trends of internal potential and reactive power change are inconsistent. Based on this, this invention constructs a joint action criterion of "decreasing internal potential and decreasing reactive power," providing dual confirmation from both the root cause of loss of excitation (weakened excitation) and its external manifestation (reactive power reversal). This mechanism fundamentally solves the problem of maloperation caused by the non-faulty entry of the impedance trajectory into the operating zone under system oscillation conditions in traditional impedance protection, significantly improving the reliability and anti-interference capability of the protection.

[0017] This invention has good responsiveness and fast action speed: The internal potential used in this invention is calculated in real time from the electrical quantities at the generator terminals, without relying on external system parameters or long delays, resulting in a fast response speed. Simulation results show that under different load levels and different degrees of loss of excitation, the protection action time of this invention is significantly better than that of traditional impedance-type loss of excitation protection methods, enabling faster fault clearing and ensuring the safety of the generator unit and the power grid.

[0018] This invention has clear logic and is easy to implement: This invention employs a two-level criterion structure of "voltage-based activation + characteristic quantity action". The activation criterion quickly senses system disturbances, while the action criterion accurately identifies demagnetization faults. The logic hierarchy is clear, and the physical meaning of each criterion threshold is clear. It is easy to implement on existing digital relay protection devices and has high engineering application value. Attached Figure Description

[0019] Figure 1 This is a flowchart of the present invention; Figure 2 These are simulation results of the protection response under the condition of complete de-excitation of the unit under different load levels in this invention example, wherein... Figure 2 (a) shows the changes in terminal voltage and internal potential before and after a loss-of-excitation fault occurs under a 100% load condition. Figure 2(b) shows the changes in terminal voltage and internal potential before and after the occurrence of a demagnetization fault under a load level of 80%. Figure 2 (c) shows the changes in terminal voltage and internal potential before and after the occurrence of a demagnetization fault under a load level of 60%. Figure 2 (d) shows the changes in terminal voltage and internal potential before and after the occurrence of a demagnetization fault under a load level of 40%. Figure 2 (e) is the response process of the start-up criterion under a load level of 100%. Figure 2 (f) shows the response process of the start-up criterion under a load level of 80%. Figure 2 (g) is the response process of the start-up criterion under a load level of 60%. Figure 2 (h) is the response process of the start-up criterion under a load level of 40%; Figure 2 (i) is the response process of the action criterion under the condition of 100% load level. Figure 2 (j) represents the response process of the action criterion under a load level of 80%. Figure 2 (k) represents the response process of the action criterion under a load level of 60%. Figure 2 (l) is the response process of the action criterion under a load level of 40%; Figure 2 (m) represents the response process of the traditional impedance criterion under a 100% load condition. Figure 2 (n) represents the response process of the traditional impedance criterion under a load level of 80%. Figure 2 (o) represents the response process of the traditional impedance criterion under a load level of 60%. Figure 2 (p) shows the response process of the traditional impedance criterion under a load level of 40%; Figure 3 The following are simulation results of the unit's loss-of-excitation protection response under different loss-of-excitation conditions in the examples of this invention. Figure 3 (a) shows the changes in terminal voltage and internal potential of the pumped-storage unit before and after a fault occurs under operating conditions with a 100% loss of magnetization. Figure 3 (b) shows the changes in terminal voltage and internal potential of the pumped-storage unit before and after a fault occurs, under operating conditions with a 75% loss of magnetization. Figure 3 (c) shows the changes in terminal voltage and internal potential of the pumped-storage unit before and after a fault occurs under operating conditions with a 50% loss of magnetization. Figure 3 (d) shows the changes in terminal voltage and internal potential of the pumped-storage unit before and after a fault occurs under the condition of 25% demagnetization. Figure 3 (e) shows the response process of the start-up criterion under the condition of 100% demagnetization. Figure 3 (f) shows the response process of the start-up criterion under the condition of 75% demagnetization. Figure 3 (g) represents the response process of the start-up criterion under a 50% demagnetization condition. Figure 3(h) represents the response process of the start-up criterion under a 25% demagnetization condition; Figure 3 (i) is the response process of the action criterion under the condition of 100% demagnetization. Figure 3 (j) represents the response process of the action criterion under the condition of 75% demagnetization. Figure 3 (k) represents the response process of the action criterion under the condition of 50% demagnetization. Figure 3 (l) is the response process of the action criterion under the condition of 25% demagnetization; Figure 3 (m) represents the response process of the traditional impedance criterion under a 100% demagnetization condition. Figure 3 (n) represents the response process of the traditional impedance criterion under a 75% demagnetization condition. Figure 3 (o) represents the response process of the traditional impedance criterion under a 50% loss of magnetism condition. Figure 3 (p) shows the response process of the traditional impedance criterion under the condition of 25% demagnetization. Figure 4 This is the loss-of-excitation protection response of the pumped-storage unit under system oscillation conditions in an example of the present invention, wherein, Figure 4 (a) shows the changes in terminal voltage and internal potential before and after a single-phase ground fault occurs. Figure 4 (b) shows the changes in terminal voltage and internal potential before and after the fault occurs under a two-phase-to-ground short-circuit condition. Figure 4 (c) shows the changes in terminal voltage and internal potential before and after the fault occurs under two-phase short-circuit conditions. Figure 4 (d) shows the changes in terminal voltage and internal potential before and after the fault occurs under three-phase short-circuit conditions; Figure 4 (e) shows the response process of the starting criterion under a single-phase-to-ground short-circuit condition. Figure 4 (f) shows the response process of the starting criterion under the two-phase-to-ground short-circuit condition. Figure 4 (g) represents the response process of the starting criterion under two-phase short-circuit conditions. Figure 4 (h) represents the response process of the starting criterion under three-phase short-circuit conditions; Figure 4 (i) is the response process of the action criterion under the single-phase ground fault condition. Figure 4 (j) represents the response process of the action criterion under the two-phase-to-ground short-circuit condition. Figure 4 (k) represents the response process of the action criterion under two-phase short-circuit conditions. Figure 4 (l) is the response process of the action criterion under three-phase short circuit conditions; Figure 4 (m) represents the response process of the traditional impedance criterion under a single-phase ground fault condition. Figure 4 (n) represents the response process of the traditional impedance criterion under a two-phase-to-ground short-circuit condition. Figure 4 (o) represents the response process of the traditional impedance criterion under two-phase short-circuit conditions. Figure 4(p) represents the response process of the traditional impedance criterion under three-phase short-circuit conditions. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0021] A method for loss-of-excitation protection of pumped-storage units based on internal potential and reactive power characteristics, combined with Figure 1 As shown, it includes the following steps: S1. Collect the three-phase voltage, three-phase current, and stator-side reactive power signals of the pumped-storage unit, and calculate the dq-axis voltage component and dq-axis current component through coordinate transformation.

[0022] The three-phase voltage and three-phase current at the pumped-storage unit terminals are acquired in real time using voltage and current transformers, while the reactive power signal on the stator side is also acquired. The electrical quantities in the three-phase stationary coordinate system are converted to the synchronous rotating dq coordinate system using Park transformation, and the d-axis (direct axis) voltage component, q-axis (quadrature axis) voltage component, d-axis current component, and q-axis current component are calculated.

[0023] S2. Construct an analytical expression of the internal potential based on the transient model of the pumped-storage unit.

[0024] The core of this step lies in using the internal potential to directly characterize the excitation state of the unit. Its working principle is as follows: During normal operation, the terminal voltage of the pumped-storage unit is jointly determined by the internal potential and armature reaction; when a loss-of-excitation fault occurs, the excitation current decreases, causing the internal potential to decrease accordingly. Therefore, the internal potential is the most direct physical quantity reflecting the excitation state.

[0025] Based on the pumped-storage unit's voltage and flux linkage equations, and neglecting stator resistance and high-frequency transient components, this step derives the analytical expression for the transient internal potential of the pumped-storage unit:

[0026] in, This is the internal potential (per unit value). This is the q-axis stator voltage (per unit). The transient reactance of the d-axis (per unit value, known parameters of the unit). This represents the d-axis stator current (per unit value).

[0027] This step transforms the internal potential, which is difficult to measure directly, into a parameter that can be calculated in real time from the electrical quantities at the machine terminal.

[0028] S3. Construct a start-up criterion based on the change in terminal voltage, and construct an action criterion based on the characteristics of internal potential change and reactive power change.

[0029] (1) Triggering Criterion: The activation criterion is used to quickly detect system disturbances and trigger subsequent action criterion determination. In this step, the activation criterion includes any one of the following, and the activation criterion is deemed satisfied if any one of the following conditions is met: ① Terminal voltage The voltage is below a preset threshold and remains below the preset start-up criterion delay time, specifically:

[0030] in, For the first The per-unit value of the terminal voltage at each sampling point.

[0031] The voltage threshold is a per-unit value, specifically 0.95 pu.

[0032] ② The voltage change at the machine terminal is lower than the preset voltage change threshold and continues for a preset start-up criterion delay time, specifically:

[0033] in, For the first The terminal voltage value corresponding to each sampling point For the first The terminal voltage difference component corresponding to each sampling point For the first Per-unit voltage value at each sampling point The differential interval is used. Specifically, the change in terminal voltage is obtained by subtracting the terminal voltage values ​​corresponding to two sampling points separated by a differential interval. =Sampling frequency / power system frequency / 4. In this embodiment, the sampling frequency is 1200Hz and the power system frequency is 50Hz, so the differential interval n = 6; the voltage change threshold is specifically 0.

[0034] The startup criterion delay time is set to 100ms.

[0035] When any of the above conditions are met, the criterion outputs a "met" signal, and the protection enters the action criterion discrimination stage.

[0036] (2) Action criteria: The action criterion is used to accurately identify whether a fault is a genuine loss of excitation. Its working principle is as follows: the essential characteristic of a loss of excitation fault is a decrease in internal potential due to weakened excitation, while the unit changes from generating reactive power to absorbing reactive power, i.e., a decrease in reactive power. However, under non-loss-of-excitation disturbances such as system oscillations, the trends of internal potential and reactive power changes are often inconsistent. Therefore, this embodiment constructs an action criterion based on a joint judgment of changes in internal potential and reactive power.

[0037] In this step, the action criterion is:

[0038] And only when <0 and When <0, the action criterion is satisfied; in, The steady-state per-unit value of the internal potential before the activation criterion is first satisfied. For the first Per-unit potential value within each sampling point For the first The change in internal electric potential at a point. The stator reactive power steady-state per-unit value before the trigger criterion is first met. For the first per-unit value of stator reactive power at each sampling point For the first The change in stator reactive power at a point; It is a sampling sequence number that increases in differential intervals, rather than a consecutive sampling point number.

[0039] Specifically, and All values ​​are taken as the average value of the wave one week before the start-up.

[0040] This logic double-checks the root cause of the loss of excitation fault (the decrease in internal potential indicates weakened excitation) and the external manifestation (the decrease in reactive power indicates reactive power reversal) to ensure the accuracy of the judgment.

[0041] S4. When the start criterion is met, if the action criterion continues to meet the preset action criterion delay time, the loss of excitation protection action signal is output; if the action criterion does not meet the action conditions, the start criterion is reset.

[0042] In this step, the preset action criterion delay time is set to 1.5 seconds.

[0043] During the action criterion determination process, if at any moment occurs ≥0 or If the value is ≥0 (i.e., the internal potential or reactive power does not show a continuous decreasing trend), the action criterion is determined to be unmet, the start criterion is immediately reset, the timer is cleared, and the protection returns to its initial state. This mechanism can effectively prevent maloperation of the protection during transient processes such as system oscillations.

[0044] Simulation verification and effect description: In this embodiment of the invention, simulation verification was performed on the following three operating conditions, and the results were compared with those of the traditional loss-of-excitation protection method: Case 1: Protection response when the pumped-storage unit experiences a complete loss-of-excitation fault; Case 2: Protection response when the pumped-storage unit experiences loss-of-excitation faults of varying degrees; Case 3: Response performance of the pumped-storage unit's loss-of-excitation protection under system oscillation conditions. Simulation results show that the proposed method's initiation and action criteria exhibit good discrimination performance under the above operating conditions, demonstrating significant engineering application value.

[0045] Taking a specific variable-speed pumped-storage unit as an example, the basic parameters of the unit are shown in Table 1.

[0046]

[0047] To verify the protection response performance of the proposed method under complete demagnetization faults at different load levels, four simulation conditions were set with load levels of 100%, 80%, 60%, and 40%, respectively, and all were... t A complete demagnetization fault is applied at 4s. The parameters for each simulation condition are shown in Table 2.

[0048]

[0049] Simulation results are as follows Figure 2 In Figure 2 (a)- Figure 2 (d), corresponding to Cases 1 to 4 respectively. Statistical analysis starting from the time of fault occurrence shows that the proposed method quickly identifies system disturbances under the above four operating conditions using the startup criteria, such as... Figure 2 (e)- Figure 2 (h); the protection action times of the proposed method under the above four working conditions are 1.52s, 1.54s, 1.55s, and 1.57s, respectively. Figure 2 (i)- Figure 2 (l); The required protection action times for the loss-of-excitation protection method based on impedance criterion are 3.61s, 4.58s, 4.89s, and 5.13s, respectively. Figure 2 (m)- Figure 2 As shown in (p). Simulation results demonstrate that, under different load levels, the proposed method can detect and activate loss-of-excitation faults based on internal potential characteristics. Compared with traditional loss-of-excitation protection methods based on impedance criteria, the proposed method exhibits superior performance in terms of activation speed.

[0050] To verify the protection response performance of the proposed method under different demagnetization levels, four simulation conditions with demagnetization levels of 100%, 75%, 50%, and 25% were set, and... t Simulation verification was performed by applying a corresponding degree of demagnetization fault at 4s. The parameters for each simulation condition are shown in Table 3.

[0051]

[0052] Simulation results are shown below Figure 3 In Figure 3 (a)- Figure 3 (d), corresponding to Cases 1 to 4 respectively. Statistical analysis starting from the time of fault application shows that the proposed method quickly identifies system disturbances under the above four operating conditions using the startup criterion, such as... Figure 3 (e)- Figure 3 (h); the protection action times of the proposed method under the above four working conditions are 1.52s, 1.53s, 1.55s and 1.56s, respectively. Figure 3 (i)- Figure 3 (l); The required protection action times for the loss-of-excitation protection method based on impedance criterion are 2.61s, 2.82s, 3.11s, and 3.34s, respectively. Figure 3 (m)- Figure 3 (p). Simulation results show that the proposed method can detect and operate on demagnetization faults under different degrees of demagnetization, and exhibits good speed of operation; compared with the traditional demagnetization protection method based on impedance criterion, its operation time is shorter.

[0053] To further evaluate the reliability of the proposed protection criterion and verify its ability to distinguish between demagnetization faults and system oscillations, a system oscillation simulation was conducted. System oscillation was simulated by setting different types of short-circuit faults and rapidly clearing them. The short-circuit types included single-phase-to-ground short circuits, two-phase-to-ground short circuits, two-phase short circuits, and three-phase short circuits, and none of these conditions included demagnetization factors. t =4s sets the corresponding system oscillation condition, and the parameters of each simulation condition are shown in Table 4.

[0054]

[0055] Simulation results are as follows Figure 4 In Figure 4 (a)- Figure 4 (d), corresponding to Cases 1 to 4 respectively. Statistics based on the fault application time show that the proposed method failed to determine the occurrence of system disturbances under the starting criteria in the first three operating conditions mentioned above. Figure 4 (e)- Figure 4 (g) Under the fourth operating condition, the start criterion determines whether a system disturbance has occurred within 3.5 seconds, such as... Figure 4 (h); The proposed method failed to provide protection in all four of the above operating conditions, such as Figure 4 (i)- Figure 4 (l); all loss-of-excitation protection methods based on impedance criteria have entered the operating zone, such as Figure 4 (m)- Figure 4(p), and under the system oscillation condition induced by a three-phase short-circuit fault and its rapid clearing, the impedance trajectory repeatedly enters the operating region, with the longest duration within the operating region being 0.431s, such as Figure 4 (p) is close to the setting delay time of traditional impedance-type loss-of-excitation protection. t =0.5s. If the oscillation frequency is further reduced, the single continuous dwell time will exceed the 0.5s set delay time, thus triggering a malfunction of the traditional impedance-type loss-of-excitation protection. The results show that the proposed action criterion was never met under all operating conditions, and the protection device did not operate. This indicates that the proposed method can maintain the lockout state under system oscillation conditions, avoid malfunctions, and effectively distinguish between loss-of-excitation faults and system oscillations.

Claims

1. A method for loss-of-excitation protection of pumped-storage units based on internal potential and reactive power characteristics, characterized in that: Includes the following steps: S1. Collect the three-phase voltage, three-phase current and stator-side reactive power signals of the pumped-storage unit, and calculate the dq-axis voltage component and dq-axis current component through coordinate transformation; S2. Construct an analytical expression of the internal potential based on the transient model of the pumped-storage unit; Step S2 specifically includes: Based on the pumped-storage unit voltage equation and flux linkage equation, and neglecting stator resistance and high-frequency transient components, the analytical expression for the transient internal potential of the pumped-storage unit is obtained: in, Internal potential, This is the q-axis stator voltage. For d-axis transient reactance, This refers to the d-axis stator current. S3. Construct a start-up criterion based on the change in terminal voltage, and construct an action criterion based on the characteristics of internal potential change and reactive power change; In step S3, the activation criterion includes any one of the following: The terminal voltage is lower than the preset voltage threshold and continues for the preset start-up criterion delay time; The voltage change at the machine terminal is lower than the preset voltage change threshold and continues for the preset start-up criterion delay time. If any condition in the initiation criterion is met, the initiation criterion is deemed satisfied. The change in terminal voltage is obtained by subtracting the terminal voltage values ​​corresponding to two sampling points spaced apart by a differential interval. =Sampling frequency / (4 × power system frequency); In step S3, the action criterion is: And only when <0 and When <0, the action criterion is satisfied; in, The steady-state per-unit value of the internal potential before the activation criterion is first satisfied. For the first Per-unit potential value within each sampling point For the first The change in internal electric potential at a point. The stator reactive power steady-state per-unit value before the trigger criterion is first met. For the first per-unit value of stator reactive power at each sampling point For the first The change in stator reactive power at a given point. The sampling sequence number is incremented by differential intervals; The and All values ​​are taken as the average value of the previous week before startup; S4. When the start criterion is met, if the action criterion continues to meet the preset action criterion delay time, the loss of excitation protection action signal is output; if the action criterion does not meet the action conditions, the start criterion is reset, the timer is cleared, and the protection returns to the initial state.

2. The method for loss-of-excitation protection of pumped-storage units based on internal potential and reactive power characteristics according to claim 1, characterized in that: The voltage threshold is a per-unit value, specifically 0.95 pu; the voltage change threshold is specifically 0; and the start-up criterion delay time is 100 ms.

3. The method for loss-of-excitation protection of pumped-storage units based on internal potential and reactive power characteristics according to claim 1, characterized in that: In step S4, the preset action criterion delay time is set to 1.5 seconds.

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