Variable speed pumped storage unit generator side symmetrical loss of excitation protection method based on under-throw impedance circle

Abstract

This invention discloses a method for symmetrical loss-of-excitation protection of variable-speed pumped-storage units based on a downward-throwing impedance circle, belonging to the field of generator relay protection. The method includes: real-time acquisition of the measured voltage and current at the generator terminals of the variable-speed pumped-storage unit, and calculation of the measured impedance at the generator terminals; constructing a symmetrical loss-of-excitation power circle criterion on the operating power plane of the variable-speed pumped-storage unit; further transforming the symmetrical loss-of-excitation power circle criterion onto the impedance plane to obtain the downward-throwing impedance circle criterion; if the measured impedance at the generator terminals enters the downward-throwing impedance circle and remains within the circle for a time greater than t, then initiating an emergency shutdown of the unit. This invention can quickly detect and judge symmetrical loss-of-excitation faults on the generator side, such as three-phase winding open circuits and complete loss of pulses in the generator-side converter, and can also schedule timely shutdowns for maintenance of the variable-speed pumped-storage unit. Furthermore, this method has high reliability and can effectively ensure the safe operation of the power grid and the variable-speed pumped-storage unit, possessing significant engineering application value.

Description

Technical Field

[0001] This invention belongs to the field of generator relay protection, and more specifically, relates to a method for symmetrical loss of excitation protection of variable speed pumped-storage units based on a downward-throwing impedance circle. Background Technology

[0002] Loss of excitation is a frequent type of fault in generator sets, especially in large units with complex excitation systems. In current pumped storage power stations, both constant-speed and variable-speed pumped storage units often employ self-excited excitation systems. However, constant-speed pumped storage units use a DC excitation winding structure and a constant-speed control strategy, while variable-speed pumped storage units use a three-phase AC excitation winding structure and a variable-speed control strategy. These differences in excitation winding structure and control strategy result in significant differences in the excitation system composition of the two types of units, thus altering the form and characteristics of loss of excitation faults.

[0003] Currently, loss-of-excitation protection schemes for DC-excited synchronous generators (constant-speed pumped-storage units) are relatively mature, but research on loss-of-excitation protection for AC-excited doubly-fed induction generators (variable-speed pumped-storage units) is limited. Because variable-speed pumped-storage units employ a unique AC excitation structure, unlike the traditional classification of partial and complete loss-of-excitation faults for DC-excited synchronous generators, loss-of-excitation faults for variable-speed pumped-storage units are often categorized as symmetrical or asymmetrical. Symmetrical loss-of-excitation faults can be further classified into turbine-side and grid-side loss-of-excitation faults based on their location. Different fault types result in different loss-of-excitation phenomena in variable-speed pumped-storage units; therefore, technically, targeted loss-of-excitation protection can only be configured based on the specific characteristics of each fault.

[0004] When a variable-speed pumped-storage unit experiences a symmetrical loss-of-excitation fault on the generator side, such as a three-phase winding open circuit, excitation regulator malfunction, or other reasons leading to the complete loss of pulses in the generator-side converter, the rotor windings will completely lose excitation current. After loss of excitation, the variable-speed pumped-storage unit becomes uncontrolled and will absorb a large amount of reactive power from the power system. If the system's reactive power reserves are insufficient, this can easily cause a severe voltage drop, or even system voltage collapse, due to the large reactive power deficit. Furthermore, the variable-speed pumped-storage unit's speed increases rapidly after loss of excitation, easily leading to runaway speed, posing a significant threat to the unit's safety. Therefore, this invention addresses the need for symmetrical loss-of-excitation fault protection on the generator side of variable-speed pumped-storage units by proposing a method for symmetrical loss-of-excitation protection on the generator side based on a downward-throwing impedance circle. Summary of the Invention

[0005] To address the gaps in existing technologies, this invention provides a method for symmetrical loss of excitation protection on the generator side of a variable-speed pumped-storage unit based on a downward-throwing impedance circle. The purpose is to quickly detect symmetrical loss of excitation faults on the generator side of the variable-speed pumped-storage unit, such as three-phase winding disconnection and complete loss of pulses in the generator-side converter, and to promptly shut down the variable-speed pumped-storage unit for maintenance, thereby ensuring the safe operation of the power grid and the variable-speed pumped-storage unit.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for symmetrical loss-of-excitation protection of a variable-speed pumped-storage unit based on a downward-throwing impedance circle is provided, comprising the following steps:

[0007] S1. Real-time acquisition of terminal voltage measurement of variable speed pumped storage unit and terminal current measurement The terminal measurement impedance Z is calculated.

[0008] S2. Construct a symmetrical demagnetization power circle criterion on the operating power plane of the variable speed pumped storage unit;

[0009] S3. Transform the symmetrical demagnetization power circle criterion onto the impedance plane to obtain the downward throw impedance circle criterion;

[0010] S4. If the measured impedance at the generator terminal enters the downward impedance throwing circle and remains in the circle for a time greater than t, then the generator unit shall be shut down in an emergency.

[0011] Furthermore, in step S2,

[0012] The constructed criterion for the symmetrical demagnetization power circle is as follows:

[0013]

[0014] In the formula, all parameters are per-unit values, and the center of the power circle is the leading-phase operating power point of the variable-speed pumped-storage unit under rated voltage when there is a symmetrical loss-of-excitation fault on the generator side. P represents the active power generated by the variable-speed pumped-storage unit; Q represents the reactive power generated by the variable-speed pumped-storage unit; U N The rated voltage of the variable speed pumped storage unit; x s For the variable speed pumped-storage unit, L is the stator reactance, which is the sum of the main reactance and the stator winding leakage reactance; L is the radius of the demagnetization power circle.

[0015] Furthermore, in step S2,

[0016] The radius L of the constructed symmetrical demagnetization power circle shall be selected according to the following requirements:

[0017] S201: The selection of radius L needs to take into account that the range of the demagnetization power circle constructed cannot cover the normal operating state of the variable speed pumped storage unit, that is, the radius must be smaller than the minimum power output P. 发电.min (pu) and minimum pumping output P抽水.min (pu) limit. That is...

[0018] L < P 发电.min &L<P 抽水.min

[0019] S202: Since the de-excitation leading-phase operating power value of the variable-speed pumped-storage unit is proportional to the square of the terminal voltage, in order to ensure that the de-excitation leading-phase operating power point reliably enters the symmetrical de-excitation power circle, a radius of [missing information] is set.

[0020]

[0021] In the formula, all parameters are per-unit values, Q N.sc This represents the power output of the variable-speed pumped-storage unit under rated voltage during demagnetization and phase-advancing operation. U min This refers to the voltage drop at the generator terminal of a variable-speed pumped-storage unit under severe symmetrical loss of magnetization on the generator side.

[0022] Furthermore, step S3 specifically includes:

[0023] S301: Transform the symmetrical loss-of-magnetism power circle criterion onto the impedance plane to obtain the downward-throwing impedance circle criterion expressed in per-unit values.

[0024]

[0025] All parameters in the formula are per-unit values, and the center of the downward-throwing impedance circle is... radius is The action zone is within the downward impedance circle.

[0026] S302: Perform per-unit value conversion to obtain the downward throw impedance circle criterion expressed in nominal value.

[0027]

[0028] In the formula, R and X are named values, and L and x are named values. s Z is the per-unit value. N This is the impedance reference value.

[0029] Furthermore, in step S4, the dwell time threshold t is set according to the maximum dwell time of the impedance trajectory within the downward impedance circle during system oscillation.

[0030] Furthermore, in step S1,

[0031] The formula for measuring impedance Z at the computer terminal is:

[0032]

[0033] in, Measure the voltage at the terminal; R is the measured current at the terminal; R is the real axis coordinate of the measured impedance Z on the impedance plane; X is the imaginary axis coordinate of the measured impedance Z on the impedance plane.

[0034] Another aspect of the present invention provides a symmetrical loss-of-excitation protection system for a variable-speed pumped-storage unit based on a downward-throwing impedance circle, comprising: a computer-readable storage medium and a processor;

[0035] The computer-readable storage medium is used to store executable instructions;

[0036] The processor can be used to read executable instructions stored in the computer-readable storage medium and execute the above method.

[0037] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0038] This invention is the first to propose constructing a protection criterion using the power plane, and then using a downward-throwing impedance circle criterion obtained by transforming the power plane and impedance plane to effectively detect symmetrical demagnetization faults on the turbine side of variable-speed pumped-storage units. This solves the problems of false tripping and failure to trip that exist when the traditional impedance circle criterion is applied to variable-speed pumped-storage units. The downward-throwing impedance circle criterion proposed in this invention has high reliability and can effectively detect symmetrical demagnetization faults on the turbine side under various operating conditions of variable-speed pumped-storage units. It can meet the requirements for demagnetization protection of variable-speed pumped-storage units in engineering. Moreover, this invention does not require additional equipment on existing protection schemes, only changes to the program algorithm, which greatly saves development costs. Attached Figure Description

[0039] Figure 1 A schematic flowchart of a method for symmetrical loss-of-excitation protection of a variable-speed pumped-storage unit based on a downward-throwing impedance circle, provided in an embodiment of the present invention;

[0040] Figure 2 This is a schematic diagram of a variable-speed pumped-storage unit model provided in an embodiment of the present invention;

[0041] Figure 3 A power plan diagram of a variable-speed pumped-storage unit provided in an embodiment of the present invention;

[0042] Figure 4 The symmetrical demagnetization power circle constructed in the operating power plane provided in the embodiments of the present invention;

[0043] Figure 5 (a) and (b) in the embodiments of the present invention show the turbine terminal impedance measurement trajectories when the turbine side is symmetrically demagnetized under subsynchronous and supersynchronous operating conditions, respectively. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0045] This embodiment provides a method for symmetrical loss-of-excitation protection on the generator side of a variable-speed pumped-storage unit based on a downward-throwing impedance circle, such as... Figure 1 As shown, it includes:

[0046] S1. Real-time acquisition of terminal voltage measurement of variable speed pumped storage unit and terminal current measurement The terminal measurement impedance Z and its position on the impedance plane (RX plane) are calculated.

[0047] Specifically, the formula for measuring impedance Z at the computer terminal is:

[0048]

[0049] S2. Construct a symmetrical demagnetization power circle criterion on the operating power plane of the variable speed pumped storage unit;

[0050] Specifically, the constructed criterion for the symmetrical demagnetization power circle is as follows:

[0051]

[0052] In the formula, all parameters are per-unit values, and the center of the power circle is the leading-phase operating power point of the variable-speed pumped-storage unit under rated voltage when there is a symmetrical loss-of-excitation fault on the generator side. P represents the active power generated by the variable-speed pumped-storage unit; Q represents the reactive power generated by the variable-speed pumped-storage unit; U N The rated voltage of the variable speed pumped storage unit; x s For the variable speed pumped-storage unit, L is the stator reactance, which is the sum of the main reactance and the stator winding leakage reactance; L is the radius of the demagnetization power circle.

[0053] The radius L of the constructed symmetrical demagnetization power circle shall be selected according to the following requirements:

[0054] (1) The selection of radius L needs to take into account that the range of the demagnetization power circle constructed cannot cover the normal operating state of the variable speed pumped storage unit, that is, the radius should be smaller than the minimum power output P. 发电.mi n(pu) and minimum pumping output P 抽水.min (pu) limit. That is...

[0055] L < P 发电.min &L<P 抽水.min

[0056] (2) Since the de-excitation leading-phase operating power value of the variable-speed pumped-storage unit is proportional to the square of the terminal voltage, in order to ensure that the de-excitation leading-phase operating power point reliably enters the symmetrical de-excitation power circle, a radius of [missing information] can be set.

[0057]

[0058] In the formula, all parameters are per-unit values, Q N.sc This represents the power output of the variable-speed pumped-storage unit under rated voltage during demagnetization and phase-advancing operation. U min This refers to the voltage drop at the generator terminal of a variable-speed pumped-storage unit under severe symmetrical loss of magnetization on the generator side.

[0059] S3. Transform the symmetrical demagnetization power circle criterion onto the impedance plane to obtain the downward throw impedance circle criterion;

[0060] Specifically, step S3 includes:

[0061] S301: Transform the symmetrical loss-of-magnetism power circle criterion onto the impedance plane to obtain the downward-throwing impedance circle criterion expressed in per-unit values.

[0062]

[0063] All parameters in the formula are per-unit values, and the center of the downward-throwing impedance circle is... radius is The action zone is within the downward impedance circle.

[0064] S302: Perform per-unit value conversion to obtain the downward throw impedance circle criterion expressed in nominal value.

[0065]

[0066] In the formula, R and X are named values, and L and x are named values. s Z is the per-unit value. N This is the impedance reference value.

[0067] S4. If the measured impedance at the generator terminal enters the downward impedance circle (i.e., the position of the measured impedance at the generator terminal on the impedance plane is located within the downward impedance circle) and stays within the circle for a time greater than t, then the unit shall be started for emergency shutdown.

[0068] Specifically, in step S4, the dwell time threshold t is set according to the maximum dwell time of the impedance trajectory within the downward impedance circle during system oscillation.

[0069] Below, with Figure 2Taking the variable-speed pumped-storage unit model shown as an example, the technical details of the present invention will be explained. In the control strategy of this variable-speed pumped-storage unit, grid voltage-oriented vector control is adopted on the grid side, and stator flux linkage-oriented vector control is adopted on the generator side. In addition, the generator terminal of the variable-speed pumped-storage unit is connected to the infinite bus system via a 500kV main transformer. The specific parameters of the variable-speed pumped-storage unit in the embodiment are shown in Table 1.

[0070] Table 1 Parameters of Variable Speed ​​Pumped Storage Unit

[0071]

[0072] In the embodiments, the normal operating power range of the variable speed pumped storage unit is as follows: Figure 3 The shaded area in the diagram. Regarding electrical characteristic limitations, the power characteristics of variable-speed pumped-storage units are limited by their rated capacity, maximum rotor current, and maximum rotor voltage. The rated capacity limit is represented by a unit circle, such as... Figure 3 The dashed circle in the middle; the maximum rotor current limit is as follows: Figure 3 A single-dot dashed circle; maximum rotor voltage limit as shown. Figure 3 In the context of a double-dotted circle, if the unit and converter parameters are designed reasonably, the power operating range of a variable-speed pumped-storage unit is generally not limited by the maximum rotor voltage. Besides being limited by electrical characteristics, the power characteristics of a variable-speed pumped-storage unit are also limited by the output power limit of the prime mover and the hydraulic characteristics. Figure 3 ① is the maximum power output limit for power generation, with a power factor of 0.9; ② is the minimum power output limit for power generation, with a power factor of 0.3 pu; ③ is the minimum power output limit for pumping water, with a power factor of 0.7 pu; ④ is the maximum power output limit for pumping water, with a power factor of 0.98. Figure 3 The document also provides the transformation of the statically stable boundary impedance circle and asynchronous operating impedance circle used in traditional loss-of-excitation protection into the statically stable boundary power circle and asynchronous operating power circle in the power plane. The operating region of the statically stable boundary power circle is outside the circle, while the operating region of the asynchronous operating power circle is inside the circle. Figure 3 As can be seen, the normal operating power range of the variable-speed pumped-storage unit (shown in shaded areas) intersects with the static stability boundary power circle and the asynchronous operating power circle. Therefore, the variable-speed pumped-storage unit may malfunction under normal operating conditions. Furthermore, when a symmetrical loss-of-excitation fault occurs on the turbine side of the variable-speed pumped-storage unit, the leading power point is located at the tangent point between the static stability boundary power circle and the asynchronous operating power circle, making it difficult to guarantee the determinism of the protection action. Therefore, traditional loss-of-excitation protection schemes are unreliable when applied to variable-speed pumped-storage units. The following section further examines the downward impedance circle criterion proposed in this invention.

[0073] S1. Real-time acquisition of terminal voltage measurement of variable speed pumped storage unit and terminal current measurement The terminal measurement impedance Z and its position on the impedance plane (RX plane) are calculated.

[0074] In the embodiment, the turbine terminal impedance measurement trajectory after symmetrical loss of excitation on the turbine side under different operating conditions of the variable speed pumped storage unit is as follows: Figure 5 (a) and (b) in the figure. They are mostly irregular curves. After a symmetrical loss of excitation fault occurs on the machine side, the measured impedance trajectory at the machine terminal will drop rapidly from the normal operating state point to the vicinity of the loss of excitation leading phase operating power point through different motion trajectories.

[0075] S2. Construct a symmetrical demagnetization power circle criterion on the operating power plane of the variable speed pumped storage unit;

[0076]

[0077] In this embodiment, the center of the power circle is the leading power point of the variable-speed pumped-storage unit under rated voltage when there is a symmetrical loss of excitation fault on the generator side. According to the parameters of the variable-speed pumped-storage unit as shown in Table 1, x in the above formula s =x m +x σs ,

[0078]

[0079] Therefore, the center of the circle is (-0.2756, 0).

[0080] The value of the radius L of the symmetrical demagnetization power circle should be selected according to the following requirements.

[0081] (1) The selection of radius L needs to take into account that the range of the demagnetization power circle constructed cannot cover the normal operating state of the variable speed pumped storage unit, that is, the radius should be smaller than the minimum power output P. 发电.min (pu) and minimum pumping output P 抽水.min (pu) limitation. According to the output limitation of the variable-speed pumped-storage unit in the embodiment, i.e.

[0082] L < 0.3 pu & L < 0.7 pu

[0083] (2) Since the power value of the variable-speed pumped-storage unit during loss of excitation is proportional to the square of the terminal voltage, in order to ensure that the operating power reliably falls within the power circle when the variable-speed pumped-storage unit experiences symmetrical loss of excitation on the turbine side, the following can be taken:

[0084]

[0085] Q N.sc This refers to the power output of the variable-speed pumped-storage unit under rated voltage during demagnetization and phase-advancing operation; U minThis represents the voltage drop at the generator terminals of a variable-speed pumped-storage unit under extremely severe symmetrical loss of excitation on the generator side. In this embodiment, the power consumption of the variable-speed pumped-storage unit under rated voltage during symmetrical loss of excitation fault is 0.2756 pu. Considering that the voltage drop at the generator terminals caused by a single unit's loss of excitation is often limited, U can be set... min If the value is 0.8 pu, then the radius L of the symmetrical demagnetization power circle can be taken as...

[0086]

[0087] The value of L also satisfies the requirement of (1), therefore the symmetrical demagnetization power circle is obtained as follows:

[0088] [Q-(-0.2756)] 2 +P 2 <0.099 2

[0089] On the power plane, the constructed criterion for the symmetrical demagnetization power circle is as follows: Figure 4 As shown.

[0090] S3. Transform the symmetrical demagnetization power circle criterion onto the impedance plane to obtain the downward throw impedance circle criterion;

[0091] In this embodiment, the symmetrical loss-of-magnetism power circle criterion is first transformed onto the impedance plane to obtain the downward-throwing impedance circle criterion expressed in per-unit values.

[0092]

[0093] After substituting the corresponding parameters, we get

[0094] R 2 +(X-(-4.167)) 2 <(1.497) 2

[0095] Further per-unit value conversion yields the downward throw impedance circle criterion expressed in nominal value.

[0096]

[0097] After substituting the corresponding parameters, we get

[0098] R 2 +(X-(-3.075)) 2 <(1.104) 2

[0099] The position of the downward-thrown impedance circle in the impedance plane is seen in Figure 5 In (a) and (b), the traditional statically stable boundary impedance circle and asynchronous operating impedance circle criteria are given for comparison.

[0100] S4. If the measured impedance at the generator terminal enters the downward impedance throwing circle and remains in the circle for a time greater than t, then the generator unit shall be shut down in an emergency.

[0101] In this embodiment, the dwell time threshold t is set according to the maximum time the impedance trajectory stays within the downward impedance circle during system oscillation. In this embodiment, t is set to 0.5s.

[0102] according to Figure 5 In (a) and (b), when the VSPSU is operating in different power quadrants, after a symmetrical loss of excitation fault occurs on the machine side, the measured impedance trajectory at the machine terminal will rapidly drop from the normal operating point to near the power point of the loss of excitation leading phase operation via different trajectories. It can be seen that the final stable measured impedance point does not enter the traditional impedance circle, causing the protection to fail to operate. On the other hand, it can be seen that when the VSPSU is in a deeper leading phase operation state, its machine terminal measured impedance point easily enters the static stability boundary impedance circle or asynchronous operation impedance circle. Since it is in a stable operating state at this time, the traditional loss of excitation scheme will cause false tripping after a delay. However, the downward-throwing impedance circle criterion proposed in this invention (see...) Figure 5 (a) and (b) in the table can effectively cover the power point of demagnetization and advance phase operation, and can reliably operate under various operating conditions of the variable speed pumped storage unit. More test results under various operating conditions are shown in Table 2. On the other hand, the power range reflected by the downward impedance circle criterion proposed in this invention does not overlap with the normal operating condition of the variable speed pumped storage unit. Therefore, no maloperation will occur under the normal operating condition of the variable speed pumped storage unit, and the proposed scheme has good reliability.

[0103] Table 2 Results of demagnetization protection operation of variable speed pumped storage units under different operating conditions

[0104]

[0105] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for generator-side symmetrical loss-of-field protection of a variable speed pumped storage unit based on a lower throw impedance circle, characterized in that, Includes the following steps: S1. Real-time acquisition of measured voltage at the generator terminal of the variable-speed pumped storage unit and measured current at the generator terminal , to calculate the measured impedance at the generator terminal ; S2. Construct a symmetrical demagnetization power circle criterion on the operating power plane of the variable speed pumped storage unit; S3. Transform the symmetrical demagnetization power circle criterion onto the impedance plane to obtain the downward throw impedance circle criterion; S4. If the measured impedance at the generator terminal enters the downward impedance throwing circle and stays in the circle for a time greater than t, then the unit shall be shut down in an emergency. In step S2: The constructed criterion for the symmetrical demagnetization power circle is: In the formula, all parameters are per-unit values, and the center of the power circle is the leading-phase operating power point of the variable-speed pumped-storage unit under rated voltage when there is a symmetrical loss-of-excitation fault on the generator side. P represents the active power generated by the variable-speed pumped-storage unit; Q represents the reactive power generated by the variable-speed pumped-storage unit; U N The rated voltage of the variable speed pumped storage unit; x s For the variable speed pumped-storage unit, the stator reactance is the sum of the main reactance and the stator winding leakage reactance; L is the radius of the demagnetization power circle. Step S3 specifically includes: S301: Transform the symmetrical loss-of-magnetism power circle criterion onto the impedance plane to obtain the downward-throwing impedance circle criterion expressed in per-unit values. All parameters in the formula are per-unit values, and the center of the downward-throwing impedance circle is... , radius is The action zone is within the downward throwing impedance circle; S302: Perform per-unit value conversion to obtain the downward throw impedance circle criterion expressed in nominal value. In the formula, R and X are named values, and L and x are named values. s Per unit value, This is the impedance reference value.

2. The method for symmetrical loss-of-excitation protection of variable-speed pumped-storage units based on a downward-throwing impedance circle as described in claim 1, characterized in that, In step S2, the radius L of the constructed symmetrical demagnetization power circle is selected according to the following requirements: S201: The selection of radius L needs to take into account that the range of the constructed demagnetization power circle cannot cover the normal operating state of the variable speed pumped storage unit, that is, the radius must be smaller than the minimum power generation output. and minimum pumping output Limitation, unit is pu; that is S202: Since the de-excitation leading-phase operating power value of the variable-speed pumped-storage unit is proportional to the square of the terminal voltage, in order to ensure that the de-excitation leading-phase operating power point reliably enters the symmetrical de-excitation power circle, a radius of [missing information] is set. In the formula, all parameters are per-unit values, Q N.sc This represents the power output of the variable-speed pumped-storage unit under rated voltage during demagnetization and phase-advancing operation. ;U min This refers to the voltage drop at the generator terminal of a variable-speed pumped-storage unit under severe symmetrical loss of magnetization on the generator side.

3. The method for symmetrical loss-of-excitation protection of variable-speed pumped-storage units based on a downward-throwing impedance circle as described in claim 1, characterized in that, In step S4, t is set according to the maximum time that the impedance trajectory stays within the downward-throwing impedance circle during system oscillation.

4. The method for symmetrical loss-of-excitation protection of variable-speed pumped-storage units based on a downward-throwing impedance circle as described in claim 1, characterized in that, In step S1: The formula for measuring impedance Z at the computer terminal is: in, Measure the voltage at the terminal; R is the measured current at the terminal; R is the real axis coordinate of the measured impedance Z on the impedance plane; X is the imaginary axis coordinate of the measured impedance Z on the impedance plane.

5. A symmetrical loss-of-excitation protection system for a variable-speed pumped-storage unit based on a downward-throwing impedance circle, characterized in that, include: Computer-readable storage media and processors; The computer-readable storage medium is used to store executable instructions; The processor can be used to read executable instructions stored in the computer-readable storage medium and execute the method as described in any one of claims 1-4.

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