Low-excitation limiter, unit and low-excitation limiting control method

Abstract

The invention relates to the technical field of electric power, and discloses a low-excitation limiter, a unit and a low-excitation limitation control method.The low-excitation limiter comprises an interpolation function module, a first subtraction module and a low-excitation limitation amplification module which are electrically connected in sequence; the interpolation function module is used for outputting a reactive power reference value based on an input active power value of a motor; the first subtraction module is used for subtracting a reactive power reference value from the reactive power value of the motor to obtain a reactive power difference value; and the low-excitation limiting amplification module is used for converting the reactive power difference value into low-excitation limiting output voltage. The problems that in the prior art, low-excitation limitation switching oscillation of generator excitation is difficult to avoid, and low-excitation limitation adaptability is insufficient are solved.

Description

Technical Field

[0001] This invention relates to the field of power technology, specifically to a low excitation limiter, a generator set, and a low excitation limiting control method. Background Technology

[0002] Currently, asynchronous power equipment accounts for an increasingly large proportion of the power system. Asynchronous power sources consume less energy, but their peak-shaving and bidirectional controllability are insufficient, leading to increasing peak-shaving pressure on thermal power units. How to rebalance the stability design of thermal power units under deep peak-shaving conditions is a topic that the thermal power industry needs to study and develop in the long term. In recent years, thermal power units have needed to achieve deep peak-shaving while simultaneously requiring generator leading phase. Current low excitation limits are mostly determined based on leading phase test results in the 50%-100% rated power range, and the leading phase test results in the deep peak-shaving range have not been accurately confirmed.

[0003] In recent years, the paper "Liang Hao, Xie Huan, Wang Cong, et al. Characteristic Analysis of Accelerated Power System Stabilizer under Deep-Advanced Phase Condition of Generator [J]. Automation of Electric Power Systems, 2022, 46 (22): 192-199." studied the oscillation event analysis of an accelerated PSS under deep-advanced phase condition, revealing the importance of optimizing power system stabilizer parameters under deep-advanced phase condition. The paper "AJP Ramos, LRLins, EHD Fittipaldi, L. Monteah, Performance of Under-Excitation Limiter of Synchronous Machines for System Critical Disturbances, IEEE Trans. on Power Systems, Vol.12, No.4, Nov.1997" reported an event of voltage oscillation caused by mismatch between the under-excitation limit parameter and the main loop control parameter. The paper "Power System Stability and Generator Excitation Control [M]. Liu Qu. China Electric Power Press. 2006" proposed increasing the time response of the under-excitation limit to avoid overlap with the control frequency domain of the voltage main loop, thus preventing the recurrence of oscillation events.

[0004] In recent years, with the increasing complexity of the power grid operating environment, small voltage oscillations have occurred frequently. When the system voltage experiences small oscillations, and active power is adjusted, generating units enter the low-excitation limiting operation zone. Within 30 minutes, there are as many as 50 low-excitation limiting operation times, creating new disturbances to the system. Currently, there is no stable system specifically designed to suppress frequent low-excitation limiting during voltage oscillations, and the adaptability of low-excitation limiting is insufficient. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides a low excitation limiter, a generator unit, and a low excitation limit control method, which solves the problems of unavoidable generator excitation low excitation limit switching oscillation and insufficient adaptability of low excitation limit in the prior art.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: An underexcitation limiter includes an interpolation function module, a first subtraction module, and an underexcitation limit amplification module connected in sequence. The interpolation function module outputs a reactive power reference value based on the input active power value of the motor. The first subtraction module subtracts the reactive power value of the motor from the reactive power reference value to obtain a reactive power difference. The underexcitation limit amplification module converts the reactive power difference into a underexcitation limit output voltage. The method for outputting the reactive power reference value based on the active power value of the motor is as follows: a characteristic curve of the interpolation function is formed by setting active power points and corresponding reactive power points. If the active power value of the motor is not equal to the active power value corresponding to the set active power point on the characteristic curve, interpolation is performed on the two active power points on the characteristic curve closest to the active power value of the motor to calculate and output the reactive power reference value.

[0007] The beneficial effects of this invention are: The interpolation function module outputs a reactive power reference value. After interpolating the reactive power reference value and the reactive power value, the output of the low excitation limit is obtained. The difference between the generator terminal voltage and the low excitation limit output are compared and the maximum value of the two is taken. After being added to the PSS output, it is input to the AVR stage to obtain the excitation voltage. The magnitude of the excitation voltage is adjusted to control the change of the generator's output reactive power.

[0008] Based on the above technical solution, the present invention can be further improved as follows.

[0009] As a preferred technical solution, the interpolation function module adopts the PQ interpolation function.

[0010] The beneficial effects of adopting the above-mentioned further solutions are: The PQ interpolation function can use a finite number of feature points to represent all features within the range covered by the feature points. When the input point exceeds the interval represented by the feature point, an input error alarm can be issued. The PQ interpolation function achieves continuous adjustment of all active power points within the normal operating range of the motor with a small amount of storage.

[0011] As a preferred technical solution, the characteristic curve of the PQ interpolation function is obtained through a phase advance test at N points; where N is an integer and N≥2.

[0012] The beneficial effects of adopting the above-mentioned further solutions are: It is easy to operate, consumes few resources, and is highly efficient.

[0013] A generator unit including the aforementioned low-excitation limiter.

[0014] The beneficial effects of this invention are: The interpolation function module outputs a reactive power reference value. After interpolating the reactive power reference value and the reactive power value, the output of the low excitation limit is obtained. The difference between the generator terminal voltage and the low excitation limit output are compared and the maximum value of the two is taken. After being added to the PSS output, it is input to the AVR stage to obtain the excitation voltage. The magnitude of the excitation voltage is adjusted to control the change of the generator's output reactive power.

[0015] As a preferred technical solution, a motor electrically connected to a low-excitation limiter is included.

[0016] The beneficial effects of adopting the above-mentioned further solutions are: This facilitates the implementation of low excitation limit control for the unit.

[0017] As a preferred technical solution, the motor is one or more of the following: synchronous condenser, generator, pumped storage unit, synchronous motor.

[0018] The beneficial effects of adopting the above-mentioned further solutions are: It is easy to apply in scenarios such as synchronous condensers, generators, pumped storage units, and synchronous motors, and has a wide range of applications.

[0019] As a preferred technical solution, the motor is a generator, including an automatic voltage regulation module electrically connected to the generator.

[0020] The beneficial effects of adopting the above-mentioned further solutions are: The motor terminal voltage is transmitted to the AVR and LEL modules. After processing by the AVR and LEL modules, the excitation voltage is output. The excitation voltage changes the magnitude of the motor's reactive power, achieving the purpose of accurately adjusting the reactive power. When the motor terminal voltage drops, the voltage deviation is transmitted to the AVR input stage, and after being transmitted to the AVR, a larger excitation voltage is output, quickly adjusting the motor's reactive power.

[0021] As a preferred technical solution, it includes a second subtraction module, a third subtraction module, and a maximum value function module. The generator, the second subtraction module, the maximum value function module, the third subtraction module, and the automatic voltage regulation module are electrically connected in sequence. The generator, the underexcitation limiter, and the maximum value function module are also electrically connected in sequence.

[0022] The beneficial effects of adopting the above-mentioned further solutions are: By combining the low excitation limiter, generator, second subtraction module, maximum value function module, third subtraction module, and automatic voltage regulation module, the system achieves the functions of avoiding generator excitation low excitation limit switching oscillation and wide adaptability of low excitation limit.

[0023] A low-excitation limit control method for controlling a low-excitation limiter includes the following steps: S1, Detect whether the low excitation limit is engaged: if yes, proceed to step S2A; if no, proceed to step S2B; S2A checks whether the reactive power value at the motor terminal is less than M times the reactive power value output by the interpolation function: if yes, exit the low excitation limit; if no, maintain the status quo; where M is set in advance. S2B checks whether the reactive power value at the motor terminal is less than the reactive power value output by the interpolation function: if yes, maintain the status quo; if no, activate the low excitation limit.

[0024] The beneficial effects of this invention are: Frequent activation and deactivation of the low-excitation limit are avoided by designing a difference between the activation and deactivation values. The activation value is based on the set value of the low-excitation limit. Since the generator phase-leading test determines the boundary of the low-excitation limit, under the reactive power phase-leading value set by the low-excitation limit, it can ensure that the generator can achieve deep phase-leading while maintaining sufficient stability margins for the bus voltage, power angle, and generator core temperature within the station. This ensures stable operation of the power station even when subjected to external disturbances during phase-leading.

[0025] As a preferred technical solution, M ranges from 0.95 to 0.99.

[0026] The beneficial effects of adopting the above-mentioned further solutions are: Designing low-excitation limits can optimize the configuration of appropriate low-excitation limit input and output values ​​during voltage or reactive power oscillations in the system. This improves the adaptability of generator low-excitation limits to complex voltage and reactive power oscillation conditions without reducing the generator's leading phase capability.

[0027] Compared with the prior art, the present invention has the following advantages: (1) The interpolation function module of the present invention outputs a reactive power reference value. After interpolating the reactive power reference value and the reactive power value, the output of the low excitation limit is obtained. After comparing the difference calculated by the generator terminal voltage and the output of the low excitation limit, the maximum value of the two is taken, added to the PSS output, and then input to the AVR stage to obtain the excitation voltage. The magnitude of the excitation voltage is adjusted to control the change of the generator output reactive power. (2) The present invention determines the activation and deactivation of the low excitation limiter based on the active power status of the power station; (3) The structure described in this invention can achieve long-term stable operation of synchronous motor equipment without increasing equipment investment, and achieves stable system operation at a lower cost, and has enhanced anti-voltage oscillation capability; (4) The present invention can be applied to a variety of occasions, such as synchronous condensers, generators, pumped storage units, synchronous motors and any combination of stations containing the above motors. Attached Figure Description

[0028] Figure 1 A schematic diagram of an excitation thermal power unit with a low excitation limiter; Figure 2 A schematic diagram of a low-excitation limiting model including a generator; Figure 3 Control structure diagram for low-excitation limit input; Figure 4 The control structure diagram for low-excitation limit exit; Figure 5 A flowchart for the improved low-incentive-limited input and output logic. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0030] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0031] Example 1 like Figures 1 to 5 As shown, a low-excitation limiter includes an interpolation function module, a first subtraction module, and a low-excitation limit amplification module connected in sequence. The interpolation function module outputs a reactive power reference value based on the input active power value of the motor. The first subtraction module subtracts the reactive power value of the motor from the reactive power reference value to obtain a reactive power difference. The low-excitation limit amplification module converts the reactive power difference into a low-excitation limit output voltage. The method for outputting the reactive power reference value based on the active power value of the motor is as follows: a characteristic curve of the interpolation function is formed by setting active power points and corresponding reactive power points. If the active power value of the motor is not equal to the active power value corresponding to the set active power point on the characteristic curve, interpolation is performed on the two active power points on the characteristic curve closest to the active power value of the motor to calculate and output the reactive power reference value.

[0032] The beneficial effects of this invention are: The interpolation function module outputs a reactive power reference value. After interpolating the reactive power reference value and the reactive power value, the output of the low excitation limit is obtained. The difference between the generator terminal voltage and the low excitation limit output are compared and the maximum value of the two is taken. After being added to the PSS output, it is input to the AVR stage to obtain the excitation voltage. The magnitude of the excitation voltage is adjusted to control the change of the generator's output reactive power.

[0033] Based on the above technical solution, the present invention can be further improved as follows.

[0034] As a preferred technical solution, the interpolation function module adopts the PQ interpolation function.

[0035] The beneficial effects of adopting the above-mentioned further solutions are: The PQ interpolation function can use a finite number of feature points to represent all features within the range covered by the feature points. When the input point exceeds the interval represented by the feature point, an input error alarm can be issued. The PQ interpolation function achieves continuous adjustment of all active power points within the normal operating range of the motor with a small amount of storage.

[0036] As a preferred technical solution, the characteristic curve of the PQ interpolation function is obtained through a phase advance test at N points; where N is an integer and N≥2.

[0037] The beneficial effects of adopting the above-mentioned further solutions are: It is easy to operate, consumes few resources, and is highly efficient.

[0038] A generator unit including the aforementioned low-excitation limiter.

[0039] The beneficial effects of this invention are: The interpolation function module outputs a reactive power reference value. After interpolating the reactive power reference value and the reactive power value, the output of the low excitation limit is obtained. The difference between the generator terminal voltage and the low excitation limit output are compared and the maximum value of the two is taken. After being added to the PSS output, it is input to the AVR stage to obtain the excitation voltage. The magnitude of the excitation voltage is adjusted to control the change of the generator's output reactive power.

[0040] As a preferred technical solution, a motor electrically connected to a low-excitation limiter is included.

[0041] The beneficial effects of adopting the above-mentioned further solutions are: This facilitates the implementation of low excitation limit control for the unit.

[0042] As a preferred technical solution, the motor is one or more of the following: synchronous condenser, generator, pumped storage unit, synchronous motor.

[0043] The beneficial effects of adopting the above-mentioned further solutions are: It is easy to apply in scenarios such as synchronous condensers, generators, pumped storage units, and synchronous motors, and has a wide range of applications.

[0044] As a preferred technical solution, the motor is a generator, including an automatic voltage regulation module electrically connected to the generator.

[0045] The beneficial effects of adopting the above-mentioned further solutions are: The motor terminal voltage is transmitted to the AVR and LEL modules. After processing by the AVR and LEL modules, the excitation voltage is output. The excitation voltage changes the magnitude of the motor's reactive power, achieving the purpose of accurately adjusting the reactive power. When the motor terminal voltage drops, the voltage deviation is transmitted to the AVR input stage, and after being transmitted to the AVR, a larger excitation voltage is output, quickly adjusting the motor's reactive power.

[0046] As a preferred technical solution, it includes a second subtraction module, a third subtraction module, and a maximum value function module. The generator, the second subtraction module, the maximum value function module, the third subtraction module, and the automatic voltage regulation module are electrically connected in sequence. The generator, the underexcitation limiter, and the maximum value function module are also electrically connected in sequence.

[0047] The beneficial effects of adopting the above-mentioned further solutions are: By combining the low excitation limiter, generator, second subtraction module, maximum value function module, third subtraction module, and automatic voltage regulation module, the system achieves the functions of avoiding generator excitation low excitation limit switching oscillation and wide adaptability of low excitation limit.

[0048] A low-excitation limit control method for controlling a low-excitation limiter includes the following steps: S1, Detect whether the low excitation limit is engaged: if yes, proceed to step S2A; if no, proceed to step S2B; S2A checks whether the reactive power value at the motor terminal is less than M times the reactive power value output by the interpolation function: if yes, exit the low excitation limit; if no, maintain the status quo; where M is set in advance. S2B checks whether the reactive power value at the motor terminal is less than the reactive power value output by the interpolation function: if yes, maintain the status quo; if no, activate the low excitation limit.

[0049] The beneficial effects of this invention are: Frequent activation and deactivation of the low-excitation limit are avoided by designing a difference between the activation and deactivation values. The activation value is based on the set value of the low-excitation limit. Since the generator phase-leading test determines the boundary of the low-excitation limit, under the reactive power phase-leading value set by the low-excitation limit, it can ensure that the generator can achieve deep phase-leading while maintaining sufficient stability margins for the bus voltage, power angle, and generator core temperature within the station. This ensures stable operation of the power station even when subjected to external disturbances during phase-leading.

[0050] As a preferred technical solution, M ranges from 0.95 to 0.99.

[0051] The beneficial effects of adopting the above-mentioned further solutions are: Designing low-excitation limits can optimize the configuration of appropriate low-excitation limit input and output values ​​during voltage or reactive power oscillations in the system. This improves the adaptability of generator low-excitation limits to complex voltage and reactive power oscillation conditions without reducing the generator's leading phase capability.

[0052] Example 2 like Figures 1 to 5 As shown, based on Example 1, this example provides a more detailed implementation method.

[0053] This invention is mainly aimed at the current scenario of thermal power units with voltage oscillation. Existing systems do not have a stable system for suppressing voltage oscillation and frequent low excitation limits. This invention proposes a method to avoid oscillation switching due to low excitation limits of generator excitation, while being able to adapt to voltage oscillation conditions. This enables the system to cope with small disturbance conditions and has good adaptability.

[0054] This invention proposes a method to avoid oscillations during generator excitation under-excitation limit switching, encompassing the control system of thermal power units and the under-excitation limit stage. By fully utilizing the existing excitation design framework, a low-excitation limiter for voltage oscillation is designed, providing a feasible technical route for practically solving the problem of insufficient adaptability to low-excitation limits.

[0055] This invention provides a method to avoid generator excitation low excitation limit switching oscillation, solving the problem that the low excitation limiter in the existing control scheme cannot adapt to voltage oscillation, and providing a feasible control approach for improving the adaptability of thermal power plants with low excitation limiters.

[0056] Figure 1 The excitation thermal power unit with a low excitation limiter was demonstrated.

[0057] The unit mainly consists of a generator, an automatic voltage regulator (AVR), an underexcitation limiter (LEL), a rectifier module, a main transformer, an excitation transformer, an equivalent power grid, and a physical transformer (PT). The PCC is the point of common coupling, and the PT is the generator terminal voltage transformer. The rectifier module converts the low-voltage AC voltage of the excitation transformer into DC voltage, enabling continuous adjustment of the generator rotor winding voltage. The main transformer converts the generator terminal voltage into a high voltage, achieving low-loss power transmission. The excitation transformer converts the generator terminal voltage into a low excitation AC voltage, ensuring stable and controllable excitation system under various operating conditions. The PT converts the generator terminal voltage into a 100V low voltage, enabling continuous voltage monitoring.

[0058] Figure 1 In this system, the power station includes a generator and an excitation system. The process of the generator adjusting the voltage is as follows: the energy generated by the generator is sent to the PCC through the main transformer, and the PCC collects the energy from each user and sends it to the equivalent power grid.

[0059] The excitation system is the voltage regulation system within a power plant, mainly comprising the excitation transformer, PT, AVR, LEL, excitation circuit, and rectifier module. The excitation circuit refers to the coil windings that can generate a magnetic field, i.e., the windings of the generator used to regulate reactive power. Its function is to adjust the generator terminal voltage and reactive power.

[0060] First, the PT collects the generator terminal voltage. Simultaneously, the generator terminal voltage is stepped down to the low voltage required by the excitation system through the excitation transformer. The generator terminal voltage collected by the PT is transmitted to the AVR and LEL modules after digital processing. After processing by the AVR and LEL modules, the excitation voltage is output. The excitation voltage has an effect in the excitation circuit, changing the magnitude of the generator reactive power and achieving the purpose of accurately adjusting the reactive power.

[0061] When the PCC voltage drops, the generator terminal voltage will also drop through the transmission action of the main transformer. The drop in generator terminal voltage is transmitted to the AVR input circuit through the PT. After the voltage deviation is transmitted to the AVR, a large excitation voltage output is generated, which quickly adjusts the generator reactive power and adjusts the PCC voltage to quickly return to normal.

[0062] A method to avoid generator excitation low excitation limit switching oscillation includes three aspects: first, the construction of a low excitation limit model; second, the design of low excitation limit logic; and third, the coordination between low excitation limit and automatic voltage regulation.

[0063] 1. Construction of the low-excitation-constraint model: Figure 2 The structure of a low-excitation limiting model containing a generator is shown.

[0064] Figure 2 The model includes a low-excitation limiting model for thermal power units, where P is the active power at the generator terminal, K is the low-excitation limiting amplification module, Q is the reactive power at the generator terminal, Max is the maximum value function, and PSS is the power system stabilizer.

[0065] Figure 2 and Figure 1 The main relationships are: Figure 2 yes Figure 1 This is derived from the refinement of the LEL and AVR parts and the simplification of the generator model.

[0066] The generator model inputs active power P and terminal voltage U signals. Terminal voltage U is input to the voltage control loop, and the difference between terminal voltage U and the voltage reference value is calculated to obtain the calculated terminal voltage difference. Active power P is input to the PQ interpolation function. The interpolation curve of the PQ interpolation function is generally obtained from the phase-advancing test results at 30%, 50%, 75%, and 100% points, and these results are written into the interpolation function module. The reactive power limit values ​​for other power points are obtained through numerical interpolation (linear or nonlinear interpolation) at the four existing key points. The PQ interpolation function module outputs a reactive power reference value. The difference between the reactive power reference value and the actual reactive power value is multiplied by K to obtain the low-excitation limit output. The calculated terminal voltage difference and the low-excitation limit output are compared, and the maximum value is taken. This value is added to the PSS output and input to the AVR loop to obtain the excitation voltage. Adjusting the excitation voltage controls the change in the generator's output reactive power.

[0067] exist Figure 2 In the process, when the low excitation limit is activated, the input of the voltage control loop will be shielded, and the excitation control will enter a closed-loop system with the low excitation limit as the main control link. When the reactive power value is pulled out of the low excitation limit range, it will re-enter the voltage closed-loop control range.

[0068] 2. Low-excitation limiting logic design: As described in the construction section of the low excitation limit model, the low excitation limit is a function set to prevent the generator from entering the demagnetization protection zone due to excessive leading phase. The design purpose is to quickly pull the generator from the excessive leading phase zone to the non-excitation zone. Currently, it is generally set as a curve. However, when the system voltage fluctuates, the generator reactive power also fluctuates at a certain frequency, causing the generator low excitation limit to frequently activate and deactivate.

[0069] The logical structure of low-incentive-limited input is as follows: Figure 3 As shown.

[0070] Control structure for low-excitation limit exit, such as Figure 4 As shown.

[0071] Improved low-incentive limit input and output logic flow, such as Figure 5 As shown.

[0072] The core problem that this invention needs to solve is how to ensure the generator's operating capacity while preventing the low excitation limit from frequently being activated and deactivated during voltage fluctuations.

[0073] This invention avoids frequent activation and deactivation of the low-excitation limit by designing a difference between the activation and deactivation values. The activation value is based on the set value of the low-excitation limit. Since the generator phase-advancing test determines the boundary of the low-excitation limit, under the reactive power phase-advancing value set by the low-excitation limit, it ensures that the generator can achieve deep phase-advancing while maintaining sufficient stability margins for the bus voltage, power angle, and generator core temperature within the station. This also ensures stable operation of the power station even when subjected to external disturbances during phase-advancing.

[0074] The design of the exit value needs to consider the following factors: first, to ensure the overall stability of the power station; second, to distinguish it from the input value; and third, to not reduce the generator's leading phase capability.

[0075] To achieve the above three objectives, firstly, setting values ​​lower than the low excitation limit are excluded. While a lower low excitation limit can improve the overall phase advance capability of the station, it also compromises the station's operational stability and has not been verified by phase advance tests. Secondly, exit values ​​identical to the input value are excluded, as setting the same exit value as the input value cannot prevent frequent input / output. Finally, considering all the above factors, a lower low excitation limit is selected. This invention sets the low excitation limit at 0.98 (0.95-0.99 times) times the input value, thus differentiating it from the input value and avoiding frequent input / output of the low excitation limit due to high-frequency, small fluctuations.

[0076] The activation and deactivation of the low excitation limit are closely related to the active power output of the power station. When the active power is relatively small (less than 30%-40% of the rated active power value), the reactive power value entering the low excitation limit is relatively deep; when the active power is relatively large (greater than 70%-80% of the rated active power value), the reactive power value entering the low excitation limit is relatively shallow.

[0077] The logic for the low excitation limit is that the low excitation limit is activated when the generator reactive power is detected to reach the limit value of the low excitation limit curve.

[0078] The exit logic of the low excitation suppressor is as follows: when the generator reactive power is detected to reach 0.95-0.99 times the limit value of the low excitation limit curve (based on the active power value P as the input variable, it is necessary to determine whether the condition is met based on the active power value P), the low excitation limiter exits.

[0079] 3. Coordination between low excitation limiting and AVR control: After designing the low excitation limit activation and deactivation logic, this invention changes the original low excitation limit action range. Therefore, it is necessary to verify, within 0.95-0.99 times the low excitation limit range, whether the overall low excitation trend of the generator after the low excitation limit is activated and replaces the main loop generator terminal voltage control, shows a significant decrease in low excitation compared to the original scheme.

[0080] The low-excitation-limited input and output logic designed in this invention involves switching logic during input and output. Therefore, it is necessary to verify the stability during the switching period through small disturbance tests under the input and output logic.

[0081] A specific example is: A low-excitation limit control method for controlling a low-excitation limiter includes the following steps: S1, Detect whether the low excitation limit is engaged: if yes, proceed to step S2A; if no, proceed to step S2B; S2A checks whether the reactive power value at the motor terminal is less than M times the reactive power value output by the interpolation function (based on the active power value P as the input variable, it is necessary to determine whether the condition is met based on the active power value P): if yes, exit the low excitation limit; if no, maintain the status quo; where M is set in advance. S2B checks whether the reactive power value at the motor terminal is less than the reactive power value output by the interpolation function: if yes, maintain the status quo; if no, activate the low excitation limit.

[0082] The present invention differs from other technical solutions in the following ways: (1) The present invention determines the activation and deactivation of the low excitation limiter based on the active power status of the power station.

[0083] (2) The structure described in this invention can achieve long-term stable operation of synchronous motor equipment without increasing equipment investment, and achieves stable system operation at a lower cost, and has enhanced anti-voltage oscillation capability.

[0084] (3) The present invention can be applied to a variety of occasions, such as synchronous condensers, generators, pumped storage units, synchronous motors, and any combination of stations containing the above motors.

[0085] As described above, the present invention can be implemented well.

[0086] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0087] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0088] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0089] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0090] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0091] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

[0092] All features disclosed in all embodiments of this specification, or steps in all methods or processes implied in the disclosure, may be combined and / or extended or replaced in any way, except for mutually exclusive features and / or steps.

[0093] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Based on the technical essence of the present invention, any simple modifications, equivalent substitutions, and improvements made to the above embodiments within the spirit and principles of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A low excitation limiter characterized by, The low-field limiter comprises an interpolation function module, a first subtraction module and a low-field limiting amplification module connected in sequence; the interpolation function module is configured to output a reactive power reference value based on an input active power value of the motor; the first subtraction module is configured to subtract the reactive power reference value from a reactive power value of the motor to obtain a reactive power difference value; and the low-field limiting amplification module is configured to convert the reactive power difference value into a low-field limiting output voltage; wherein the method of outputting the reactive power reference value based on the active power value of the motor comprises: setting an active power point and a corresponding reactive power point to form a characteristic curve of the interpolation function, and if the active power value of the motor is not equal to the active power value corresponding to the set active power point on the characteristic curve, interpolating two active power points on the characteristic curve closest to the active power value of the motor to calculate and output the reactive power reference value.

2. A low excitation limiter according to claim 1, characterized in that The interpolation function module adopts a P-Q interpolation function.

3. A low excitation limiter according to claim 2, characterized in that When the interpolation function module is in operation, the characteristic curve of the P-Q interpolation function is obtained through N phase advance tests; wherein N is an integer and N≥2.

4. A machine unit, characterized in that The low-field limiter comprises any one of the low-field limiters according to claims 1 to 3.

5. A unit as claimed in claim 4, characterised in that The motor is electrically connected to the low-field limiter.

6. A unit as claimed in claim 5, characterised in that The motor is one or more of the following: a motor, a generator, a pumped storage unit and a synchronous motor.

7. A unit as claimed in claim 6, characterised in that The motor is a generator, and the automatic voltage regulating module is electrically connected to the generator.

8. A unit as claimed in claim 7, characterised in that The low-field limiter comprises a second subtraction module, a third subtraction module and a maximum value taking function module, and the generator, the second subtraction module, the maximum value taking function module, the third subtraction module and the automatic voltage regulating module are connected in sequence.

9. A low excitation limit control method characterized by, The low-field limiter comprises any one of the low-field limiters according to claims 1 to 3. The method for controlling the low-field limiter comprises the following steps: S1, detecting whether the low-field limiter is in operation: if yes, entering step S2A; if no, entering step S2B; S2A, detecting whether the motor terminal reactive power value is less than M times the reactive power value output by the interpolation function: if yes, exiting the low-field limiter; if no, maintaining the current state; wherein M is set in advance; 10. The low excitation limit control method according to claim 9, wherein S2B, detecting whether the motor terminal reactive power value is less than the reactive power value output by the interpolation function: if yes, maintaining the current state; if no, operating the low-field limiter. The range of M is 0.95-0.99.

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