CN112787308B - Differential protection method, system and medium for transformer based on differential current duty ratio - Google Patents
Differential protection method, system and medium for transformer based on differential current duty ratio Download PDFInfo
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- CN112787308B CN112787308B CN202110098186.9A CN202110098186A CN112787308B CN 112787308 B CN112787308 B CN 112787308B CN 202110098186 A CN202110098186 A CN 202110098186A CN 112787308 B CN112787308 B CN 112787308B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/04—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for transformers
- H02H7/045—Differential protection of transformers
Abstract
The invention discloses a differential protection method, a differential protection system and a differential protection medium of a transformer based on differential current duty ratio, wherein the method comprises the following steps: setting a differential current duty cycle; constructing a first protection criterion from the break angle of a differential current waveform according to the duty ratio of the differential current; constructing a second protection criterion according to the duty ratio of the differential current and from the width of the differential current waveform; constructing a third protection criterion according to the duty ratio of the differential current; and configuring a first protection criterion, a second protection criterion, a third protection criterion and a second harmonic braking criterion through three logic gates to construct a final protection scheme. The invention constructs three protection criteria reflecting the waveform characteristics of the differential current according to the duty ratio of the differential current, and combines the three criteria with the existing second harmonic braking criteria to form a differential protection scheme for the power transformer, thereby further improving the safety of the transformer and being widely applied to the technical field of relay protection of power systems.
Description
Technical Field
The invention relates to the technical field of relay protection and signal processing of a power system, in particular to a differential protection method, a differential protection system and a differential protection medium of a transformer based on a differential current duty ratio.
Background
The transformer is an important primary device in a power system, the safe and stable operation of the transformer is important, and the misoperation or the failure of the transformer protection device caused by disturbance can cause great economic loss to a power company. However, during the normal operation of the power transformer, the power transformer inevitably suffers from various disturbance currents, such as inrush current and unbalanced current caused by different saturation degrees of current transformers at two ends of the transformer. Therefore, unlike a general line differential protection device, a differential protection device provided in the main protection of a transformer also needs to be able to make a correct operation judgment under various disturbance conditions.
At present, the existing transformer differential protection scheme generally improves the safety of the transformer by being equipped with a second harmonic braking criterion. Although the problem of misoperation of a protection device caused by inrush current can be solved effectively by matching a second harmonic braking criterion, the existing transformer differential protection scheme has the following defects along with continuous expansion of the scale of a power grid and continuous improvement of transformer core materials: 1. the existing solutions may not be able to effectively identify the inrush current generated by a transformer under high remanence conditions. 2. The current transformers at the two ends of the transformer may cause a fault in a delayed cutting area of the protection device under the condition of different degrees of transient saturation. 3. The existing protection scheme can not effectively identify the unbalanced current generated by the current transformers at the two ends of the transformer under the condition of local saturation. 4. In the case of a turn-to-turn fault in the transformer, the turn-to-turn fault may be removed in a delayed manner by the protection device.
Disclosure of Invention
To solve at least one of the technical problems in the prior art to a certain extent, an object of the present invention is to provide a method, a system and a medium for differential protection of a transformer based on a differential current duty ratio.
The technical scheme adopted by the invention is as follows:
a differential protection method of a transformer based on differential current duty ratio comprises the following steps:
setting a differential current duty cycle;
constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio;
constructing a second protection criterion according to the differential current duty ratio and from the width of a differential current waveform;
constructing a third protection criterion for solving the identification of symmetric inrush current according to the differential current duty ratio;
configuring the first, second, third, and second harmonic braking criteria through logic gates to construct a final protection scheme.
Further, the setting the differential current duty cycle includes:
for the differential current signal in a period, converting the differential current signal into a square wave signal through a preset formula;
defining a duty cycle of the square wave signal as the differential current duty cycle;
the expression of the preset formula is as follows:
wherein f (T) represents the input differential current signal, SW (T) represents the output square wave signal, ThIndicating a set threshold.
Further, the constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty cycle includes:
obtaining a differential current I in a cyclerInput signal as criterion and calculating the mean value mid of the current1；
The differential current IrThe waveform of (A) is shifted to the upper side of the time axis, and the current is represented as IsAnd calculating the current IsMean value mid of2；
According to the differential current duty ratio and the current IsMean value mid1And mean value mid2Obtaining a first duty cycle DR1And a second duty cycle DR2；
According to the first duty ratio DR1And the second duty cycle DR2Obtaining a deviation value characterizing the differential current IrThe degree of deviation from the time axis;
and constructing and obtaining a first protection criterion according to the deviation value and a first set threshold.
Further, the constructing a second protection criterion according to the differential current duty ratio and based on the width of the differential current waveform includes:
according to the second duty ratio DR2Constructing a second protection criterion for reflecting the waveform width criterion by using a second set threshold;
the expression of the second protection criterion is:
DR2<DR2th
wherein, DR2thA threshold value is set for the second.
Further, the third protection criterion, which is constructed to solve the identification of symmetric inrush current according to the differential current duty cycle, includes:
will input signal IrTaking the absolute value and recording as IaCalculating IaMean value mid of3；
According to the duty ratio of the differential current signal and the current IaAnd mean value mid3Obtaining a third duty cycle DR3；
According to the third duty ratio DR3And obtaining a third protection criterion by using a third preset threshold value, wherein the third protection criterion is used for identifying symmetrical inrush current.
Further, the logic gate comprises two OR gates and an AND gate;
said second protection criterion being an input to a first or gate and said third protection criterion being another input to said first or gate;
the output of the first OR gate is used as one input of the AND gate, and the second harmonic braking criterion is used as the other input of the AND gate;
the output of the AND gate serves as one input of a second OR gate, and the first protection criterion serves as the other input of the second OR gate.
Further, the expression of the deviation value is:
where Dev is the offset value.
The other technical scheme adopted by the invention is as follows:
a differential current duty cycle based transformer differential protection system comprising:
a setting module for setting a differential current duty cycle;
the first judgment module is used for constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio;
the second judgment module is used for constructing a second protection criterion according to the differential current duty ratio and from the width of a differential current waveform;
the third judgment module is used for constructing a third protection criterion for solving the identification of the symmetric inrush current according to the differential current duty ratio;
and the logic combination module is used for configuring the first protection criterion, the second protection criterion, the third protection criterion and the second harmonic braking criterion through logic gates so as to construct a final protection scheme.
The other technical scheme adopted by the invention is as follows:
a differential current duty cycle based transformer differential protection system comprising:
at least one processor;
at least one memory for storing at least one program;
when executed by the at least one processor, cause the at least one processor to implement the method described above.
The other technical scheme adopted by the invention is as follows:
a storage medium having stored therein processor-executable instructions for performing the method as described above when executed by a processor.
The invention has the beneficial effects that: the invention constructs three protection criteria reflecting the waveform characteristics of the differential current according to the duty ratio of the differential current, and combines the three criteria with the existing second harmonic braking criteria to form a differential protection scheme for the power transformer, thereby further improving the safety of the transformer.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following description is made on the drawings of the embodiments of the present invention or the related technical solutions in the prior art, and it should be understood that the drawings in the following description are only for convenience and clarity of describing some embodiments in the technical solutions of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a diagram of an output square wave and its corresponding duty cycle for different signal waveforms under different threshold conditions according to an embodiment of the present invention;
FIG. 2 is a diagram of the logical arrangement of three protection criteria and the existing second harmonic braking criteria in an embodiment of the present invention;
FIG. 3 is a schematic diagram illustrating the protection effect of the transformer under the condition of high remanence in the embodiment of the present invention;
fig. 4 is a schematic diagram of a protection effect under the condition of a single-phase ground fault in a transformer generation area when the current transformer has different degrees of transient saturation in the embodiment of the present invention;
FIG. 5 is a schematic diagram illustrating a protection effect of a transformer switch-on in case of an inter-turn fault in the embodiment of the present invention;
fig. 6 is a schematic diagram of a protection effect when a current transformer is partially saturated when a fault occurs outside a transformer area in the embodiment of the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the accompanying drawings are illustrative only for the purpose of explaining the present invention, and are not to be construed as limiting the present invention. The step numbers in the following embodiments are provided only for convenience of illustration, the order between the steps is not limited at all, and the execution order of each step in the embodiments can be adapted according to the understanding of those skilled in the art.
In the description of the present invention, it should be understood that the orientation or positional relationship referred to in the description of the orientation, such as the upper, lower, front, rear, left, right, etc., is based on the orientation or positional relationship shown in the drawings, and is only for convenience of description and simplification of description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention.
In the description of the present invention, the meaning of a plurality of means is one or more, the meaning of a plurality of means is two or more, and larger, smaller, larger, etc. are understood as excluding the number, and larger, smaller, inner, etc. are understood as including the number. If the first and second are described for the purpose of distinguishing technical features, they are not to be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated or implicitly indicating the precedence of the technical features indicated.
In the description of the present invention, unless otherwise explicitly limited, terms such as arrangement, installation, connection and the like should be understood in a broad sense, and those skilled in the art can reasonably determine the specific meanings of the above terms in the present invention in combination with the specific contents of the technical solutions.
The embodiment provides a differential protection method of a transformer based on a differential current duty ratio, which comprises the following steps:
and S1, setting the differential current duty ratio.
As shown in fig. 1(a) and 1(b), the half-cycle sinusoidal waveform and the full-cycle sinusoidal waveform are processed by equation (2) to obtain a square wave, with the average value and the half-average value of the signal in one cycle as thresholds; fig. 1(a) shows a duty ratio of a half-wave sine wave, fig. 1(b) shows a duty ratio of a full-wave sine wave, and τ in fig. 1 shows a waveform duty ratio. As can be seen from fig. 1, setting the threshold values in the same way for different waveforms may result in different duty cycle values; and different duty cycle values will be obtained by setting the threshold values in different ways for the same waveform. It follows that the duty cycle of the waveform can indirectly reflect the characteristics of the waveform. The duty cycle of the differential current defined in this embodiment includes the following steps:
s11, the duty ratio of the related signal in one period in the conventional electronic communication field can be expressed as:
where τ represents the duty cycle of the signal, tupIndicating the activation duration of the signal, tsTo representThe period of the signal.
S12, similarly, for the differential current signal in one period, the differential current signal can be first converted into a square wave by equation (2), and then the duty ratio obtained by the square wave is defined as the duty ratio of the differential current waveform:
in the formula, f (T) represents the input differential current, SW (T) represents the output square wave signal, ThIndicating a set threshold.
And S2, constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio.
Consider the ideal case of the differential current for transformer inrush and internal fault conditions as half-sine and full-sine waveforms as shown in fig. 1. A first protection criterion is constructed from a break angle of a differential current waveform based on the definition of the differential current duty ratio, and the specific steps and processes are as follows:
s21, firstly, obtaining the differential current I in one periodrInput signal as criterion and calculating the mean value of the current as mid1；
S22, and then applying the differential current IrMoves above the time axis, and the current is denoted as IsAnd calculate IsMean value mid of2；
S23, then applying current IsAs the input signal of the formula (2), the threshold values of the formula (2) are respectively set to mid1And mid2Accordingly, different duty ratios DR can be obtained1And DR2；
S24, DR-based Final1And DR2Construction of equation (3) to characterize differential Current IrDegree of deviation from the time axis:
s25, setting threshold DthTo discriminate between disturbances and internal faults, the first protection criterion is represented by the equation (4), where DthThe value of (d) is set to 0.15.
Dev<Dth (4)
And S3, constructing a second protection criterion according to the differential current duty ratio and the width of the differential current waveform.
Consider the ideal case of the differential current for transformer inrush and internal fault conditions as half-sine and full-sine waveforms as shown in fig. 1. Constructing a second protection criterion based on the definition of the differential current duty ratio and starting from the width of a differential current waveform, wherein the specific steps and processes are as follows:
s31, defining a criterion for reflecting the width of the waveform in order to make up for the deficiency of the first criterion. According to DR in the step S22The solution process of (a) constructs a second protection criterion, denoted as (5);
DR2<DR2th (5)
in the formula, DR2thThe value is set to 0.48 for a preset threshold.
S4, constructing a third protection criterion for solving the identification of the symmetrical inrush current according to the differential current duty ratio.
Consider the ideal case of the differential current for transformer inrush and internal fault conditions as half-sine and full-sine waveforms as shown in fig. 1. Based on the definition of the differential current duty ratio, a third protection criterion is constructed for solving the problem of symmetrical inrush current identification, and the specific steps and processes are as follows:
s41, firstly, inputting the signal IrTaking the absolute value and recording the result as Ia；
S42, then calculating IaMean value mid of3；
S43, then applying current IaIs an input signal of the formula (2), and the threshold value of the formula (2) is set to be 0.5 times mid3Then DR can be obtained accordingly3；
S44, finally, setting a threshold DR3thSymmetric inrush current was identified and is denoted (6):
DR3<DR3th (6)
in the formula, DR3thThe value of (d) is set to 0.48.
S5, configuring the first protection criterion, the second protection criterion, the third protection criterion and the second harmonic braking criterion through logic gates to construct a final protection scheme.
Three protection criteria and the existing second harmonic braking criteria are combined into a logic configuration circuit with a three-layer structure through two logic OR gates and one logic AND gate to form the final protection scheme. As an alternative embodiment, a second protection criterion is provided as an input of the first or gate, and a third protection criterion is provided as another input of the first or gate; the output of the first OR gate is used as one input of the AND gate, and the second harmonic braking criterion is used as the other input of the AND gate; the output of the AND gate serves as one input of a second OR gate, and the first protection criterion serves as the other input of the second OR gate.
To illustrate the present embodiment, the effect of the above-described transformer differential protection scheme based on differential current duty cycle is described. FIG. 3 shows the protection effect of this scheme under the condition of high remanence of transformer, where FIG. 3(a) shows the waveform of differential current, FIG. 3(b) shows the second harmonic content and Dev value of differential current, and FIG. 3(c) shows the DR of differential current2And DR3A value of (d); the dotted lines in the figure represent the threshold values for the criterion, which in fig. 3(b) and 3(c) are 0.15 and 0.48, respectively. As shown by the solid line in fig. 3(b), in this case the differential current second harmonic content will be below a given threshold (minimum up to 8.9%), and a protection device equipped with only the existing second harmonic braking criterion will inevitably malfunction. As can be seen from the solid line and the point (b) in fig. 3, when the Dev value of the differential current is much smaller than the threshold, the first protection criterion C1 will output a braking signal, and it can be known from the logical relationship in table 1 that the relay protection device will send out a braking signal. Therefore, the first criterion C1 in this embodiment will help to improve the reliability of the transformer differential protection device in this case.
TABLE 1
Note: c1, C2, C3 and R respectively represent three protection criteria and a second harmonic braking criterion in the present embodiment, and "x" in table 1 represents either a logic signal "1" or "0", where the logic signal "1" is a braking signal.
FIG. 4 is a graph showing the protection effect of the present embodiment under the condition of single-phase ground fault in the transformer generation region when the current transformer is subjected to different degrees of transient saturation, where FIG. 4(a) is the differential current waveform, FIG. 4(b) is the second harmonic content and Dev value of the differential current, and FIG. 4(c) is the differential current DR2And DR3The value of (c). As shown by the solid line and the dot in fig. 4(b), when the Dev value of the differential current is much larger than the threshold, further determination is made to identify whether the current is a disturbance current. However, as shown by the solid line in fig. 4(b), due to the influence of transient saturation of the current transformer on the fault current waveform, the second harmonic content of the differential current is lower than the given threshold value after about 71ms of the fault, so that the relay protection device configured with only the existing second harmonic braking criterion will cause a larger trip delay in such a case. As shown in fig. 4(C), after the fault occurs for about one power frequency period, the second criterion C2 and the third criterion C3 in this embodiment are both greater than a given threshold, and therefore both criteria will release the trip path for protection. According to the logic relation in the table 1, the relay protection device finally releases the protection device, and then the fault tripping operation is executed. Therefore, the embodiment can effectively shorten the fault trip time to one cycle in this case.
FIG. 5 shows the protection effect of the present embodiment in case of a transformer switching on and off fault, where FIG. 5(a) shows the differential current waveform, FIG. 5(b) shows the second harmonic content and Dev value of the differential current, and FIG. 5(c) shows the differential current DR2And DR3The value of (c). As shown by the solid line and the dot in FIG. 5(b), when the Dev value of the differential current is much larger than the threshold value, further determination is made to determine whether the current is a disturbance current. As shown by the solid line in fig. 5(b), the inrush current affects the fault current waveform, and the differential voltage is generated when the transformer is closed at an inter-turn faultThe second harmonic content of the stream rises above the set threshold, so a relay protection device configured only with the existing second harmonic braking criterion will in such cases cause a trip delay of more than 625 ms. As shown in fig. 5(C), after the transformer is switched on for a power frequency period of turn-to-turn fault, both the second criterion C2 and the third criterion C3 in this embodiment are greater than a given threshold, and these two criteria will release the trip path for protection. According to the logic relation in the table 1, the relay protection device can finally release the protection device to trip. Therefore, the embodiment can greatly shorten the fault trip time under the condition.
FIG. 6 shows the protection effect of the present embodiment when the current transformer is partially saturated during the fault outside the transformer cut-out area, where FIG. 6(a) shows the differential current waveform, FIG. 6(b) shows the second harmonic content and Dev value of the differential current, and FIG. 6(c) shows the differential current DR2And DR3The value of (c). As shown by the solid line in fig. 6(b), in such a case the second harmonic braking criterion will fail at 233ms due to the second harmonic content of the differential current being below a given threshold. As shown in fig. 6(b), the point-solid line shows, during the period when the current transformer has a local saturation transformer and generates an out-of-range fault to generate a large disturbance current, the Dev value of the differential current is much smaller than the threshold, the first criterion C1 in this embodiment will output a braking signal, and it can be known from the logical relationship in table 1 that the relay protection device will send out the braking signal. Therefore, the reliability of the relay protection device can be effectively improved under the conditions.
In summary, compared with the prior art, the present embodiment has the following advantages and beneficial effects:
1. the embodiment can effectively brake the action of the protection device caused by inrush current under the condition that the transformer has high residual magnetism.
2. The embodiment can effectively shorten the time of fault tripping of the protection device under the condition that the current transformers at two ends of the transformer are subjected to transient state saturation of different degrees.
3. According to the embodiment, the time of fault tripping of the protection device can be effectively shortened under the condition that the transformer is switched on in turn-to-turn fault.
4. The embodiment can effectively brake the action of the protection device caused by unbalanced current under the condition that the current transformers at two ends of the transformer are locally saturated.
The present embodiment further provides a differential protection system for a transformer based on differential current duty ratio, including:
a setting module for setting a differential current duty cycle;
the first judgment module is used for constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio;
the second judgment module is used for constructing a second protection criterion according to the differential current duty ratio and from the width of a differential current waveform;
the third judgment module is used for constructing a third protection criterion for solving the identification of the symmetric inrush current according to the differential current duty ratio;
and the logic combination module is used for configuring the first protection criterion, the second protection criterion, the third protection criterion and the second harmonic braking criterion through logic gates so as to construct a final protection scheme.
The differential protection system of the transformer based on the differential current duty ratio of the embodiment can execute the differential protection method of the transformer based on the differential current duty ratio provided by the embodiment of the method of the invention, can execute any combination implementation steps of the embodiment of the method, and has corresponding functions and beneficial effects of the method.
The present embodiment further provides a differential protection system for a transformer based on differential current duty ratio, including:
at least one processor;
at least one memory for storing at least one program;
when executed by the at least one processor, cause the at least one processor to implement the method described above.
The differential protection system of the transformer based on the differential current duty ratio of the embodiment can execute the differential protection method of the transformer based on the differential current duty ratio provided by the embodiment of the method of the invention, can execute any combination implementation steps of the embodiment of the method, and has corresponding functions and beneficial effects of the method.
The embodiment of the application also discloses a computer program product or a computer program, which comprises computer instructions, and the computer instructions are stored in a computer readable storage medium. The computer instructions may be read by a processor of a computer device from a computer-readable storage medium, and the computer instructions executed by the processor cause the computer device to perform the method as described above.
The present embodiment further provides a storage medium, which stores an instruction or a program for executing the differential protection method for a transformer based on differential current duty ratio provided in the embodiment of the method of the present invention, and when the instruction or the program is executed, the step can be implemented by any combination of the embodiments of the method.
In alternative embodiments, the functions/acts noted in the block diagrams may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Furthermore, the embodiments presented and described in the flow charts of the present invention are provided by way of example in order to provide a more thorough understanding of the technology. The disclosed methods are not limited to the operations and logic flows presented herein. Alternative embodiments are contemplated in which the order of various operations is changed and in which sub-operations described as part of larger operations are performed independently.
Furthermore, although the present invention is described in the context of functional modules, it should be understood that, unless otherwise stated to the contrary, one or more of the described functions and/or features may be integrated in a single physical device and/or software module, or one or more functions and/or features may be implemented in a separate physical device or software module. It will also be appreciated that a detailed discussion of the actual implementation of each module is not necessary for an understanding of the present invention. Rather, the actual implementation of the various functional modules in the apparatus disclosed herein will be understood within the ordinary skill of an engineer, given the nature, function, and internal relationship of the modules. Accordingly, those skilled in the art can, using ordinary skill, practice the invention as set forth in the claims without undue experimentation. It is also to be understood that the specific concepts disclosed are merely illustrative of and not intended to limit the scope of the invention, which is defined by the appended claims and their full scope of equivalents.
The functions, if implemented in the form of software functional units and sold or used as a stand-alone product, may be stored in a computer readable storage medium. Based on such understanding, the technical solution of the present invention may be embodied in the form of a software product, which is stored in a storage medium and includes instructions for causing a computer device (which may be a personal computer, a server, or a network device) to execute all or part of the steps of the method according to the embodiments of the present invention. And the aforementioned storage medium includes: a U-disk, a removable hard disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), a magnetic disk or an optical disk, and other various media capable of storing program codes.
The logic and/or steps represented in the flowcharts or otherwise described herein, e.g., an ordered listing of executable instructions that can be considered to implement logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. For the purposes of this description, a "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.
More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic device) having one or more wires, a portable computer diskette (magnetic device), a Random Access Memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory.
It should be understood that portions of the present invention may be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, the various steps or methods may be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, any one or combination of the following techniques, which are known in the art, may be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application specific integrated circuit having an appropriate combinational logic gate circuit, a Programmable Gate Array (PGA), a Field Programmable Gate Array (FPGA), or the like.
In the foregoing description of the specification, reference to the description of "one embodiment/example," "another embodiment/example," or "certain embodiments/examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
While embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that: various changes, modifications, substitutions and alterations can be made to the embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.
While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (5)
1. A differential protection method of a transformer based on differential current duty ratio is characterized by comprising the following steps:
setting a differential current duty cycle;
constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio;
constructing a second protection criterion according to the differential current duty ratio and from the width of a differential current waveform;
constructing a third protection criterion for solving the identification of symmetric inrush current according to the differential current duty ratio;
configuring the first protection criterion, the second protection criterion, the third protection criterion and a second harmonic braking criterion through logic gates to construct a final protection scheme;
the setting the differential current duty cycle comprises:
for the differential current signal in a period, converting the differential current signal into a square wave signal through a preset formula;
defining a duty cycle of the square wave signal as the differential current duty cycle;
the expression of the preset formula is as follows:
wherein f (T) represents the input differential current signal, SW (T) represents the output square wave signal, ThIndicating a set threshold;
constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio comprises the following steps:
obtaining a differential current I in a cyclerInput signal as criterion and calculating the mean value mid of the current1(ii) a The differential current IrThe waveform of (A) is shifted to the upper side of the time axis, and the current is represented as IsAnd calculating the current IsMean value mid of2；
According to the differential current duty ratio and the current IsMean value mid1And mean value mid2Obtaining a first duty cycle DR1And a second duty cycle DR2(ii) a The first duty cycle DR1To average value mid1As a threshold value ThWhile the current IsDuty ratio of the square wave signal corresponding to the input signal; the second duty cycle DR2To average value mid2As a threshold value ThWhile the current IsDuty ratio of the square wave signal corresponding to the input signal;
according to the first duty ratio DR1And the second duty cycle DR2Obtaining a deviation value characterizing the differential current IrThe degree of deviation from the time axis;
constructing and obtaining a first protection criterion according to the deviation value and a first set threshold;
constructing a second protection criterion from the width of a differential current waveform according to the differential current duty ratio comprises the following steps:
according to the second duty ratio DR2Constructing a second protection criterion for reflecting the waveform width criterion by using a second set threshold;
the expression of the second protection criterion is:
DR2<DR2th
wherein, DR2thSetting a threshold for the second;
the third protection criterion constructed to address identification of symmetric inrush current according to the differential current duty cycle includes:
will be a differential current IrTaking the absolute value and recording as IaCalculating IaMean value mid of3；
According to the duty ratio of the differential current signal and the current IaAnd mean value mid3Obtaining a third duty cycle DR3(ii) a The third duty cycle DR3To average value mid3As a threshold value ThWhile the current IaDuty ratio of the square wave signal corresponding to the input signal;
according to the third duty ratio DR3Obtaining a third protection criterion by using a third preset threshold value, wherein the third protection criterion is used for identifying symmetrical inrush current;
the expression of the deviation value is as follows:
where Dev is the offset value.
2. The differential protection method of the transformer based on the differential current duty ratio of claim 1, wherein the logic gates comprise two or gates and an and gate;
said second protection criterion being an input to a first or gate and said third protection criterion being another input to said first or gate;
the output of the first OR gate is used as one input of the AND gate, and the second harmonic braking criterion is used as the other input of the AND gate;
the output of the AND gate serves as one input of a second OR gate, and the first protection criterion serves as the other input of the second OR gate.
3. A differential protection system for a transformer based on differential current duty cycle, comprising:
a setting module for setting a differential current duty cycle;
the first judgment module is used for constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio;
the second judgment module is used for constructing a second protection criterion according to the differential current duty ratio and from the width of a differential current waveform;
the third judgment module is used for constructing a third protection criterion for solving the identification of the symmetric inrush current according to the differential current duty ratio;
the logic combination module is used for configuring the first protection criterion, the second protection criterion, the third protection criterion and the second harmonic braking criterion through logic gates so as to construct a final protection scheme;
the setting the differential current duty cycle comprises:
for the differential current signal in a period, converting the differential current signal into a square wave signal through a preset formula;
defining a duty cycle of the square wave signal as the differential current duty cycle;
the expression of the preset formula is as follows:
wherein f (T) represents the input differential current signal, SW (T) represents the output square wave signal, ThIndicating a set threshold;
constructing a first protection criterion from the break angle of the differential current waveform according to the differential current duty ratio comprises the following steps:
obtaining a differential current I in a cyclerInput signal as criterion and calculating the mean value mid of the current1(ii) a The differential current IrThe waveform of (A) is shifted to the upper side of the time axis, and the current is represented as IsAnd calculating the current IsMean value mid of2；
According to the differential current duty ratio and the current IsMean value mid1And mean value mid2Obtaining a first duty cycle DR1And a second duty cycle DR2(ii) a The first duty cycle DR1To average value mid1As a threshold value ThWhile the current IsDuty ratio of the square wave signal corresponding to the input signal; the second duty cycle DR2To average value mid2As a threshold value ThWhile the current IsDuty ratio of the square wave signal corresponding to the input signal;
according to the first duty ratio DR1And the second duty cycle DR2Obtaining a deviation value characterizing the differential current IrThe degree of deviation from the time axis;
constructing and obtaining a first protection criterion according to the deviation value and a first set threshold;
constructing a second protection criterion from the width of a differential current waveform according to the differential current duty ratio comprises the following steps:
according to the second duty ratio DR2Constructing a second protection criterion for reflecting the waveform width criterion by using a second set threshold;
the expression of the second protection criterion is:
DR2<DR2th
wherein, DR2thSetting a threshold for the second;
the third protection criterion constructed to address identification of symmetric inrush current according to the differential current duty cycle includes:
will be a differential current IrTaking the absolute value and recording as IaCalculating IaMean value mid of3；
According to the duty ratio of the differential current signal and the current IaAnd mean value mid3Obtaining a third duty cycle DR3(ii) a The third duty cycle DR3To average value mid3As a threshold value ThWhile the current IaAs input signalsDuty cycle of the corresponding square wave signal;
according to the third duty ratio DR3Obtaining a third protection criterion by using a third preset threshold value, wherein the third protection criterion is used for identifying symmetrical inrush current;
the expression of the deviation value is as follows:
where Dev is the offset value.
4. A differential protection system for a transformer based on differential current duty cycle, comprising:
at least one processor;
at least one memory for storing at least one program;
when executed by the at least one processor, cause the at least one processor to implement a differential current duty cycle based transformer differential protection method as claimed in any one of claims 1-2.
5. A storage medium having stored therein a program executable by a processor, wherein the program executable by the processor is adapted to perform the method of any one of claims 1-2 when executed by the processor.
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| GB1400054A (en) * | 1971-09-24 | 1975-07-16 | Unisearch Ltd | Protection of transformers |
| CN100595989C (en) * | 2007-02-14 | 2010-03-24 | 国网南京自动化研究院 | High-speed sampling interruptive angle locking differential protection numerical method |
| CN103675415B (en) * | 2012-08-31 | 2017-03-01 | 西门子公司 | Excitation surge current detection method, braking magnetizing inrush current method and its device |
| CN104078934B (en) * | 2014-07-15 | 2017-01-18 | 国家电网公司 | Transformer differential current protection device mis-operation preventing relay protection method |
| CN105071344B (en) * | 2015-07-17 | 2017-11-28 | 许继集团有限公司 | A kind of transformer excitation flow recognition method |
| CN106655097B (en) * | 2017-01-09 | 2018-08-17 | 湖南大学 | Power transformer excitation surge current recognition methods |
| CN109066588A (en) * | 2018-08-29 | 2018-12-21 | 南京理工大学 | A kind of excitation flow recognition method based on improvement waveform dissymmetry coefficient |
| CN110994553B (en) * | 2019-12-17 | 2021-02-12 | 华南理工大学 | Transformer differential protection constant value adjusting method based on waveform decay characteristic |
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