CN117117789B - Transformer protection method and device - Google Patents
Transformer protection method and device Download PDFInfo
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- CN117117789B CN117117789B CN202311379012.5A CN202311379012A CN117117789B CN 117117789 B CN117117789 B CN 117117789B CN 202311379012 A CN202311379012 A CN 202311379012A CN 117117789 B CN117117789 B CN 117117789B
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Classifications
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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
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
- G01R29/18—Indicating phase sequence; Indicating synchronism
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0092—Details of emergency protective circuit arrangements concerning the data processing means, e.g. expert systems, neural networks
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Abstract
The invention discloses a transformer protection method, which belongs to the technical field of relay protection and comprises the following steps: respectively obtaining an effective value and a phase angle of a second harmonic of a differential current of the transformer; transmitting the effective value data to an effective value identification module in the direct current magnetic bias identification module, and judging whether the three-phase effective values are equal; detecting whether the three-phase second harmonic phase angle arrangement is in a positive phase sequence or a negative phase sequence by utilizing a phase angle identification module in the direct current magnetic bias identification module; the output of the effective value identification module and the output of the phase angle identification module are connected through a NAND logic circuit to form an output signal of the direct current magnetic bias identification module; the output signal of the second harmonic braking module is connected with the output signal of the direct current magnetic bias identification module through NAND logic to form a braking signal; when the tripping output signal of any phase is 1, the differential protection is started, and the breaker is opened. The invention realizes accurate identification of direct current magnetic bias and solves the problem of differential protection refusal caused by direct current magnetic bias.
Description
Technical Field
The invention relates to the technical field of relay protection, in particular to a transformer protection method and device.
Background
The ultra-high voltage direct current transmission is an effective way for solving the difficult problem of ultra-long distance transmission, and has a wide application prospect in the construction and development of a power grid due to the advantages of large power transmission capacity, wide transmission range, small transmission loss and the like. When the extra-high voltage line is maintained or fails, the transmission line runs in an asymmetric mode, and at the moment, leakage current of the direct current grounding electrode enters an alternating current power system through a neutral point of a transformer of the star-shaped connection winding, so that performances of a near-area alternating current transformer and a power grid are affected, namely, a direct current magnetic bias phenomenon is caused.
DC magnetic bias can cause problems of increased noise, aggravated vibration, local overheating, harmonic distortion and the like of the transformer, and seriously jeopardize the power transmission and the service life of the transformer. In addition, the direct current magnetic bias also has a certain influence on the differential protection of the transformer, and when the direct current property current exists in the transformer, the waveform of the exciting current can change, and positive and negative half waves are asymmetric. A large amount of second harmonic appears in the differential current, a second harmonic braking device is started by mistake, and a differential relay is locked, so that the differential protection of the transformer is disabled.
Therefore, the identification or inhibition measures of direct current magnetic bias are researched, the influence of magnetic bias current on differential protection of the transformer is eliminated, and the method has important significance for improving the safety and stability of a power grid. The traditional thinking is to add hardware equipment to inhibit bias current, and the invention is from the direct current bias identification angle, does not need to install additional hardware equipment, and economically and effectively solves the problem of differential protection rejection caused by direct current bias by a soft protection mode.
Disclosure of Invention
The invention aims to provide a transformer protection method, which aims to realize the purposes, and provides the following technical scheme:
a protection method of a transformer mainly comprises the following steps:
s1, calculating differential current based on secondary side data of a CT for protection, respectively obtaining an effective value and a phase angle of a second harmonic of the differential current of a transformer, and recording the data;
s2, transmitting the effective value data to an effective value identification module in the direct current magnetic bias identification module, and judging whether the three-phase effective values are equal. If the three-phase effective value equality degree is smaller than the appointed threshold value, the effective value identification module transmits a signal of 1;
s3, transmitting phase angle data to a phase angle identification module in the direct current magnetic bias identification module, and detecting whether the three-phase second harmonic phase angle arrangement is in a positive phase sequence (ABC, BCA or CAB) or a negative phase sequence (ACB, CBA or BAC). If the phase sequence is negative, the phase angle identification module transmits a signal 1;
s4, connecting the outputs of the effective value identification module and the phase angle identification module through a NAND logic circuit to form an output signal of the direct current magnetic bias identification module;
s5, the output signal of the second harmonic braking module is connected with the output signal of the direct-current magnetic bias identification module through NAND logic, so that a braking signal is formed;
s6, the braking signal and the output signal of the differential protection tripping module are connected through an AND gate to form a tripping output signal. When the ABC arbitrary phase tripping output signal is detected to be 1, the differential protection is started, and the circuit breaker is opened.
Based on the above steps, the invention can finally and obviously improve the differential protection performance. When the excitation surge occurs, the second harmonic braking device is started to lock the differential relay, so that the differential protection misoperation during the excitation surge is effectively prevented. When the DC magnetic bias occurs, the second harmonic of the system is increased, and the DC magnetic bias identification module sends out a signal to lock the second harmonic braking device. The second harmonic braking device is prevented from affecting the differential relay, and differential protection rejection in the direct-current magnetic bias period is effectively prevented.
Preferably, in the step S1, when the effective value and the phase angle data of the second harmonic of the differential current are recorded, ABC three-phase data are acquired, and only one phase of data cannot be selected as the direct-current magnetic bias identification basis. The effective value calculation should be divided according to different periods of the differential current fundamental wave, and each period calculates the effective value respectively.
Preferably, in step S2, the dc bias magnetic discrimination formula of the effective value discrimination module is discriminated by at least one or two discrimination modules as follows:
1) The effective value identification module formula 1 judges whether the effective values of the second harmonic of the differential current of the ABC three phases are equal or not:
in the method, in the process of the invention,the effective value of the second harmonic of the differential current of the ABC three phases in each fundamental wave period is respectively,is a similarity threshold. Preferably, the->It can be 1.2-1.8.
2) The effective value identification module formula 2 judges whether a serious second harmonic phenomenon occurs in the system or not:
in the method, in the process of the invention,the effective value of the second harmonic of the differential current of the ABC three phases in each fundamental wave period is respectively,is->Differential current fundamental effective value of same period, < +.>To represent the threshold of the second harmonic duty cycle. Preferably, the->It can be 0.08-0.15.
When the ABC phases of the three-phase currents all meet the effective value discrimination formula, the output signal of the effective value identification module is set to be 1 at the moment, which indicates that direct current magnetic bias possibly exists in the system at the moment.
Preferably, the direct current bias magnetic discrimination formula of positive and negative phase sequences in the step S3 is as follows:
wherein,the second harmonic phase angles of the differential currents of the ABC three phases are respectively. When any one of the three formulas is established, it can be determined that the second harmonic phase angle arrangement is in negative phase sequence, and the output signal of the phase angle identification module is set to 1 at this time, which means that there may be DC magnetic bias in the system at this time.
Preferably, the signal of the effective value recognition module or the phase recognition module is not abrupt. When the direct current magnetic bias discrimination formula of the effective value discrimination module or the phase discrimination module is established, the signal should be kept at 0 continuously for the time t1, and if the discrimination formula is still established after the time t1, the signal is changed into 1; when the direct current magnetic bias discrimination formula of the effective value discrimination module or the phase discrimination module is not established, the signal should be kept at 1 continuously for the time t2, and if the discrimination formula is still established after the time t2, the signal becomes 0.
Preferably, in step S4, the logic circuit has to meet the following requirements: only when the signals of the effective value identification module and the phase angle identification module are set to be 1, the system is judged to have direct-current magnetic bias, and the NAND gate outputs a 0-potential signal.
Preferably, in steps S5 and S6, when the system generates a magnetizing inrush current, the second harmonic braking module outputs a signal 1, and the dc bias recognition module outputs a braking signal 0 because the dc bias output signal 1 is not recognized. Because the brake signal and the differential protection tripping module output signal are AND logic, no matter the differential protection tripping module output signal is 1 or 0, the final output result is changed to 0 potential by the brake signal. The differential protection cannot trip, and differential protection locking during excitation surge is realized.
In steps S5 and S6, when the system generates dc bias, the second harmonic braking module outputs the signal 1, and the dc bias recognition module recognizes the dc bias output signal 0 and the nand logic circuit outputs the braking signal 1. Since the brake signal and the differential protection trip module output signal are AND logic. The final output result is not affected by the brake signal 1, and can be completely controlled by the output signal of the differential protection tripping module, so that the normal operation of differential protection in the direct current magnetic bias period is realized.
A transformer protection device comprising: the transformer protection device comprises a processor and a memory, wherein the processor is used for executing a control program stored in the memory, and the transformer protection method is realized when the control program is executed.
In the technical scheme, the transformer protection method provided by the invention has the following beneficial effects:
1. the invention adopts a soft protection idea, and does not need to install additional hardware equipment. The installation cost and the labor cost of the hardware equipment are effectively reduced, and the potential negative influence on the power system caused by the installation of the additional hardware equipment is prevented.
2. The invention cuts in from the relay protection angle, can be summarized as a relay protection secondary equipment setting method after the direct current magnetic bias enters the system, and the traditional method uses primary side equipment to restrain the direct current magnetic bias from entering the system. Therefore, the invention provides a new idea for eliminating the influence of direct current magnetic bias on differential protection.
Drawings
FIG. 1 is a schematic diagram of the present invention;
FIG. 2 is a graph showing transformer ratio differential protection in an alternative embodiment of the present invention;
FIG. 3 shows the variation of the second harmonic effective value of the differential current after the transformer is affected differently;
FIG. 4 shows the phase sequence relationship of the second harmonic phase angle of the differential current after the transformer is affected differently;
FIG. 5 is a logic flow diagram of a second embodiment;
FIG. 6 is a logic flow diagram of third and fourth embodiments;
FIG. 7 shows the change of the effective value of the fourth harmonic of the differential current after the transformer is affected differently;
FIG. 8 is a fourth harmonic phase angle phase sequence relationship of differential current after the transformer is affected differently;
Detailed Description
In order to make the technical scheme of the present invention better understood by those skilled in the art, the present invention will be further described in detail with reference to the accompanying drawings.
Referring to fig. 1, a transformer protection method includes a dc bias identification module, a second harmonic braking module, a differential protection trip module, and a logic circuit connecting the above modules. According to the actual situation of the local direct current magnetic bias invasion transformer, the parameters of each module can be flexibly adjusted so as to achieve the effect that the differential protection does not malfunction during the magnetizing inrush current and the differential protection does not refusal during the direct current magnetic bias.
In this embodiment, the data that the dc bias identification module needs to transfer is the effective value and phase angle of the differential current and its second harmonic. The method for obtaining the harmonic wave in the invention adopts Fourier transformation to decompose differential current into a series of sine wave components, defines the sine wave with the frequency of 50hz after decomposition as a fundamental wave, and defines the sine wave with the frequency of twice the fundamental wave as a second harmonic, and the definition of third and higher harmonic waves is based on the definition. The fundamental wave and each subharmonic are sine waves and have different frequencies, effective values and phase angles.
The DC magnetic bias identification module at least comprises an effective value identification module with output of S AM And the output of the phase angle identification module is S PA The output of the second harmonic braking module is S H The output of the differential protection tripping module is DP, and the output of the direct current magnetic bias identification module is S DC ，S AM And S is PA Output S via NAND gate logic DC ，S DC And S is H The signal output by the NAND gate logic circuit and the DP signal are connected through the AND gate logic circuit. The logic circuit connection is not limited, and the DC bias recognition module may further include a positive and negative half-wave asymmetric recognition module S as follows as ,S AM 、S PA And S is as The three outputs S through a NAND gate logic circuit DC . The specific working principle of the circuit of the invention is as follows:
working condition 1: when the transformer works normally, the second harmonic in the system is less, and the second harmonic braking module does not act. The direct-current magnetic bias identification module is not operated because no magnetic bias current affects the direct-current magnetic bias identification module. At this time, the differential protection can normally operate when the transformer fails in the region. The logic circuit is described as follows: s is S AM And S is PA The outputs are 0,S DC The output is 1; output signal S of second harmonic braking module H 0,S of a shape of 0,S DC And S is H The nand output of (2) is 1 so that the differential protection trip module is not latched.
Working condition 2: when the transformer is affected by exciting inrush current, a large amount of second harmonic waves appear in the system to enable the second harmonic wave braking module to be started, and the direct current magnetic bias identification module is not operated due to the fact that no magnetic bias current affects the transformer, so that differential protection of the transformer is locked. The logic circuit is described as follows: s is S AM And S is PA The outputs are 0,S DC The output is 1; output signal S of second harmonic braking module H Is 1, S DC And S is H And the nand output of (c) is 0, thereby latching the differential protection trip module.
Working condition 3: when the transformer is affected by direct current magnetic bias, the direct current magnetic bias identification module identifies that direct current magnetic bias is started, and even if a large amount of second harmonic waves occur in the system due to direct current magnetic bias, the direct current magnetic bias identification module sends out signals to enable the second harmonic wave braking module to be locked. At this time, if the internal fault of the transformer occurs, the differential protection can work normally. The logic circuit is described as follows: s is S AM And S is PA The outputs are all 1, S DC The output is 0; output signal S of second harmonic braking module H Is 1, S DC And S is H The nand output of (2) is 1 so that the differential protection trip module is not latched.
Only if both sub-modules recognize a dc bias S AM And S is PA The output is 1, and the direct current magnetic bias identification module can send out an action signal 0 to indicate that the direct current magnetic bias problem occurs in the system at the moment. The core problem solved by the identification module is how to distinguish two different conditions of exciting surge and direct current magnetic bias, and the reason for selecting two different modules can avoid misoperation of a single module and obviously improve the system compatibility of the identification module.
Alternatively, the whole dc bias recognition module may output 1 when either one of the effective value recognition module and the phase angle recognition module is operated.
Referring to fig. 1, in the present embodiment, when the dc bias discrimination formula of the effective value discrimination module or the phase discrimination module is established, the signal should be kept at 0 for the time t1, and if the discrimination formula is still established after the time t1, the signal becomes 1. The reason is to prevent the false identification of DC magnetic bias caused by transient interference of the transformer.
In this embodiment, when the dc bias discrimination formula of the effective value discrimination module or the phase discrimination module is not established, the signal should be kept at 1 for the time t2, and if the discrimination formula is still established after the time t2, the signal becomes 0. The reason is that when the transformer suffers from direct current magnetic bias interference, the differential current increases, the fundamental wave may be far greater than the second harmonic, and these potential interference terms may cause the direct current magnetic bias identification module to be inaccurate, resulting in signal malfunction of the effective value identification module or the phase identification module, so that the signal 1 jumps to the signal 0. If the time delay of t2 is set at this time, the differential protection is ensured to be tripped before the possible misoperation of the direct current magnetic bias identification module occurs.
In the embodiment of the invention, the differential protection tripping module can select ratio differential protection, taking a double-winding transformer as an example, and the action equation of the differential protection of the transformer is as follows:
wherein,indicates the differential protection operation amount, < >>，/>The amount of differential protection braking is indicated, the differential protection curves of the transformer are shown in fig. 2, which are the current phase values of the two sides converted to the primary side. In fig. 2, K represents a proportional differential, which is taken to be 0.4 in the present embodiment; />Representing the minimum operating current, taking 0.2 times the rated current in this example; />The inflection point current is represented, which in this embodiment may be 0.8 times the rated current, and all currents should be noted using phasor values.
In the embodiment of the invention, the second harmonic braking element is adopted to avoid the excitation surge current of the transformer, namely, if the second harmonic content in the differential current is detected to be larger than a setting value, the differential relay is locked, so that the protection misoperation during the excitation surge current is prevented. The criteria for the second harmonic braking element in this embodiment are as follows:
wherein,brake ratio of second harmonic brake module, +.>Representing differential electricityFlow fundamental component amplitude, ++>Representing the differential current second harmonic component amplitude. In this embodiment, <' > a->Optionally 15% -20%. In addition, the reference scheme of the second harmonic braking is three-phase or gate braking, namely, the second harmonic of any one phase in three-phase current exceeds the braking ratio, and all three-phase differential relays are locked, so that the differential protection of the transformer is disabled.
The principle and formula of the effective value recognition module and the phase angle recognition module are described in the form of PSCAD example simulation:
and PSCAD simulation is carried out on the effective value identification module, so that the distinction between direct-current magnetic bias and excitation surge current can be judged. The data selected for effective value simulation are 180MVA of transformer capacity, 50Hz of system frequency, 220kV of rated voltage of primary side of the transformer and 35kV of rated voltage of secondary side of the transformer. The difference between the dc bias and the second harmonic effective value of the excitation surge can be seen in fig. 3. As can be observed from FIG. 3, when the excitation surge current occurs in the system, the effective values of the second harmonic of the differential current in the ABC three phases are obviously different. When the system has the DC magnetic bias phenomenon, the effective values of the second harmonic waves of the differential currents in the ABC three phases are equal. Therefore, whether the effective values of the second harmonic of the differential current of the ABC three phases are equal or not can be judged, namely the ratio of the effective values of any two phases of ABC second harmonic is smaller than the similarity threshold valueTo distinguish between magnetizing inrush current and dc bias. In addition, since the second harmonic content is greatly increased when DC magnetic bias or excitation surge occurs, it is possible to determine whether or not the ratio of the second harmonic effective value to the fundamental effective value exceeds a prescribed threshold +.>To identify the DC bias or the excitation surge current, thereby eliminating the interference of the two phenomena and other factors.
PSCAD simulation is carried out on the phase angle identification module, and the distinction between direct-current magnetic bias and excitation surge current can be judged. The data selected by the phase angle identification simulation is 180MVA of transformer capacity, 50Hz of system frequency, 220kV of rated voltage of the primary side of the transformer and 35kV of rated voltage of the secondary side of the transformer. According to the simulation result of the embodiment, when the transformer is affected by the excitation surge current, the second harmonic phase angle of the ABC three-phase differential current meets the positive phase sequence relation (ABC, BCA or CAB). When the transformer is affected by dc bias, the second harmonic phase angle of the ABC three-phase differential current satisfies the negative phase sequence relationship (ACB, CBA or BAC), and reference is made specifically to fig. 4. Therefore, the magnetizing inrush current and the direct current magnetic bias can be distinguished according to the positive and negative phase sequence of the second harmonic phase angle of the differential current.
Compared with the traditional direct-current magnetic bias identification or suppression method, the method has the following innovation and advantages:
1. the method adopts a soft optimization strategy, so that the cost of the device is saved;
2. because the primary side current voltage is relatively large, the invention cuts in research from the low-voltage secondary side, thereby improving the safety of the power system and the device;
3. the direct-current magnetic bias identification method is combined and applied to the field of transformer protection, and the safety and reliability of the transformer are improved.
In a second embodiment, the logic flow diagram may refer to fig. 5:
the difference between the method and the embodiment I is that the method is used for identifying the effective value by supplementing the asymmetry of the positive half wave and the negative half wave, namely the direct current magnetic bias identification module can also comprise the asymmetry identification module of the positive half wave and the negative half wave, and then identifying the asymmetry of the positive half wave and the negative half wave, and when the two are 1, the output of the direct current magnetic bias identification module is 1, so that the reliability of the device and the method is improved.
The distinguishing process of the positive half-wave and negative half-wave asymmetric identification module comprises the following steps:
the method comprises the steps of obtaining exciting current data of a transformer, wherein in each fundamental wave period, 10 sampling points are respectively taken by positive and negative half waves, and the sampling points of the positive half waves are marked as followsThe negative half-wave sampling point is marked as +.>. Sampling Point->And->The half fundamental period is required to be 0.01 second apart, namely:
(unit: seconds)
In the method, in the process of the invention,and->Representing the sampling point +.>And sample point->Is used for sampling the time instants of the sample. Positive and negative half-wave asymmetry->The calculation formula of (2) is as follows:
in the method, in the process of the invention,and->Representing the sampling point +.>And sample point->Corresponding sampling timeExciting current. />Representation->Maximum value of>Representation->Is a minimum of (2).
Defining half-wave asymmetry thresholdWhen->And when the excitation current is in the positive and negative half-wave asymmetry state, the output of the positive and negative half-wave asymmetry recognition module is 1. Preferably, in this embodiment +.>Can be set to 1.3-1.5.
Alternatively, in the second embodiment, the effective value identification module and/or the phase identification module in the method in implementation one may be used for dc bias determination, and then the positive and negative half-wave asymmetric identification modules are used for determination at preset intervals to balance transient impact, where the preset time may be 1-10 fundamental wave periods.
In the third embodiment, the logic flow diagram may refer to fig. 6:
the difference between the two embodiments is that at least one or two or three modules of the effective value identification module, the positive and negative half-wave asymmetric identification module or the phase angle identification module output is 1, and the direct current magnetic bias identification module output is 1.
In the fourth embodiment, the logic flow diagram may refer to fig. 6:
the difference between the method and the third embodiment is that the effective value identification module and the phase angle identification module do not analyze based on the second harmonic any more, and the effective values and the phase angles of other higher harmonics are adopted for direct current magnetic bias identification. The reason why the effective value and the phase angle of the second harmonic are not adopted in the fourth embodiment is that the direct current magnetic bias phenomenon can obviously increase the content of each subharmonic including the second harmonic in the system, and the second harmonic is frequently and largely appeared and used in other occasions in the fields of power systems and relay protection, so that the error recognition probability of direct current magnetic bias can be effectively reduced by adopting higher subharmonic, and the method is better matched with various relay protection criteria.
The module of the embodiment mainly comprises an effective value identification module, a positive and negative half-wave asymmetric identification module, a phase angle identification module, a harmonic braking module, a differential protection tripping module and a logic circuit connected with the modules. According to the actual situation of the local DC magnetic bias invasion transformer, the parameters of the modules can be flexibly adjusted, and the false start of the harmonic braking module caused by the DC magnetic bias can be eliminated, so that the failure in the transformer area is kept from refusing the differential protection. Preferably, the higher harmonics in this embodiment are the fourth harmonics.
In this embodiment, the required data is the effective value and phase angle of the differential current and its fourth harmonic. The specific data acquisition method is to acquire CT secondary side data for protection, decompose differential current into a series of sine wave components based on Fourier transform theory, acquire sine wave with the frequency of 200hz, and calculate the effective value and phase angle of the sine wave in each fundamental wave frequency.
The output of the direct current magnetic bias identification module including the effective value identification module is S AM And the output of the phase angle identification module is S PA And a positive and negative half-wave asymmetric recognition module S as . The output of the harmonic braking module is S H The output of the differential protection tripping module is DP, and the output of the direct current magnetic bias identification module is S DC 。S AM ，S PA And S is as The three outputs S through a NOR gate logic circuit DC ，S DC And S is H The signal output by the NAND gate logic circuit and the DP signal are connected through the AND gate logic circuit. The specific working principle of the circuit of the invention is as follows:
working condition 1: when the transformer is in normal operation,the harmonic braking module is not active. Because no bias current affects the three sub-modules (the effective value recognition module, the phase angle recognition module and the positive and negative half-wave asymmetric recognition module) of the direct current bias recognition module, the direct current bias recognition module does not act. At this time, the differential protection can normally operate when the transformer fails in the region. The logic circuit is described as follows: s is S AM ，S PA ，S as The outputs are 0,S DC The output is 1; output signal S of harmonic braking module H 0,S of a shape of 0,S DC And S is H The nand output of (2) is 1 so that the differential protection trip module is not latched.
Working condition 2: when the transformer is affected by exciting inrush current, the harmonic braking module is started, and the direct-current magnetic bias identification module does not act due to the fact that no magnetic bias current affects the transformer, and differential protection of the transformer is locked. The logic circuit is described as follows: s is S AM ，S PA ，S as The outputs are 0,S DC The output is 1; output signal S of harmonic braking module H Is 1, S DC And S is H And the nand output of (c) is 0, thereby latching the differential protection trip module.
Working condition 3: when the transformer is affected by direct current magnetic bias, the direct current magnetic bias identification module identifies that the direct current magnetic bias is started, and the direct current magnetic bias identification module sends out a signal to enable the harmonic brake module to be locked. At this time, if the internal fault of the transformer occurs, the differential protection can work normally. The logic circuit is described as follows: s is S AM ，S PA ，S as At least one of the modules outputs 1, S DC The output is 0; output signal S of harmonic braking module H Is 1, S DC And S is H The nand output of (2) is 1 so that the differential protection trip module is not latched.
In the present embodiment, when at least one of the three sub-modules recognizes DC bias, S AM ，S PA ，S as At least one is 1, and the direct current magnetic bias identification module can send out an action signal 0 to indicate that the direct current magnetic bias problem occurs in the system. Compared with the first embodiment, the sensitivity of the direct current magnetic bias identification is improved by adopting OR logic for each submodule, but the reliability is slightly affected.
In the invention, three direct current magnetic bias identification sub-modules cannot be simply understood as parallel relation, and each sub-module can also adopt other logic circuit connection modes to meet the requirements of relay protection configuration, and the invention is not limited by the requirements. For those skilled in the art, the importance level of relay protection four can be fully weighed in the direct current magnetic bias identification problem according to the field condition of transformer protection, so as to select the best embodiment method. Alternatively, the above three sub-modules may be connected through an and gate to enhance reliability, and connected through an or gate to enhance sensitivity, and only the two sub-modules (phase angle recognition module, positive and negative half-wave asymmetric recognition module) having the fastest response speed are used to enhance speediness.
In this embodiment, the discrimination formulas of the three sub-modules (the effective value recognition module, the phase angle recognition module, and the positive and negative half-wave asymmetric recognition module) of the dc bias recognition module are as follows:
1) The effective value identification module formula 1 judges whether the effective values of the four harmonics of the differential currents of the ABC three phases are equal or not:
in the method, in the process of the invention,the four-harmonic effective value of the differential current of the ABC three phases in each fundamental wave period is respectively,is a similarity threshold. Preferably, the->It can be 1.2-1.8.
2) The effective value identification module formula 2 judges whether a serious fourth harmonic phenomenon occurs in the system or not:
in the method, in the process of the invention,the four-harmonic effective value of the differential current of the ABC three phases in each fundamental wave period is respectively,is->Differential current fundamental effective value of same period, < +.>To represent the threshold of the fourth harmonic duty cycle. Preferably, the->It can be 0.08-0.15.
When the ABC phases of the three-phase currents all meet the effective value discrimination formula, the output signal of the effective value identification module is set to be 1 at the moment, which indicates that direct current magnetic bias possibly exists in the system at the moment.
The direct current magnetic bias discrimination formula of positive and negative phase sequences is as follows:
wherein,the four harmonic phase angles of the differential current of ABC three phases respectively. When any one of the three formulas is established, it can be determined that the fourth harmonic phase angle arrangement is in positive phase sequence, and the output signal of the phase angle identification module is set to 1 at this time, which indicates that there may be dc bias in the system at this time.
The discrimination formula of the positive and negative half-wave asymmetric recognition module is the same as that of the second embodiment.
In this embodiment, the signals of the three sub-modules (the effective value recognition module, the phase angle recognition module, and the positive and negative half-wave asymmetric recognition module) of the dc bias recognition module are not abrupt, for the same reasons and in the same specific operation manner as those in the first embodiment.
In the embodiment of the invention, the differential protection tripping module can select ratio differential protection, and the specific formula and preferred parameters of the differential protection are the same as those of the first embodiment.
In this embodiment, the harmonic braking module still uses second harmonic braking, although the effective value and phase angle identification module uses fourth harmonic data. The reason is that the second harmonic braking method is adopted to avoid the excitation surge current of the transformer, and the protection of the transformer is consistent with that in the actual situation. The criterion formula and the preferred parameters of the second harmonic braking in this embodiment are the same as those of the first embodiment.
The reason why the effective value recognition module and the phase angle recognition module adopt fourth harmonic is described in the form of PSCAD example simulation:
the data selected by simulation is 180MVA of transformer capacity, 50Hz of system frequency, 220kV of rated voltage of primary side of the transformer and 35kV of rated voltage of secondary side of the transformer. In the simulation process, effective values and phase angles of three-time, four-time, five-time and six-time harmonic data are calculated in sequence, and whether the effective values and the phase angles of the harmonic data are enough to distinguish the direct current magnetic bias process from the excitation surge process is analyzed. The reason why the seven or more higher harmonics are not selected is that the seven or more higher harmonics occupy too low proportion (effective value of harmonic/effective value of fundamental) of fundamental, and the effective value identification module has larger numerical error when analyzing the severity index of harmonic.
According to the simulation result, each subharmonic effective value recognition module can work normally, namely the ABC three-phase harmonic effective values are equal when the DC magnetic bias phenomenon occurs, and the three-phase harmonic effective value difference is larger when the excitation inrush phenomenon occurs. And the phase angle identification module only meets the requirement of fourth harmonic, and the simulation result is shown in table 1.
TABLE 1 phase angle identification Module subharmonic simulation data during DC bias
In the above-mentioned table of the drawings,the phase angles of the subharmonic ABC three phases are respectively shown. Based on the data in Table 1, it can be analyzed that the phase angles of the third harmonic and the sixth harmonic are +.>The values are close, and the phase angle identification module is mainly used for judging +.>The magnitude relation is used for identifying positive and negative phase sequences so as to distinguish direct-current magnetic bias from excitation surge. Too close +.>The phase angle identification module is likely to malfunction, so third harmonic and sixth harmonic are not selectable.
In addition, based on the phase sequence data and the analysis of the five-harmonic excitation surge simulation result in table 1, when the direct-current magnetic bias and the excitation surge occur, the five-harmonic is negative, and the direct-current magnetic bias and the excitation surge cannot be distinguished. And combining the factors, and finally selecting fourth harmonic as the discrimination data of the direct current magnetic bias recognition module in the fourth embodiment. The effective value of the fourth harmonic and the phase angle PSCAD simulation results are shown in FIGS. 7 and 8. From the simulation graph, it can be analyzed that when the excitation surge phenomenon occurs, the effective values of the four harmonics of the ABC three phases are greatly different, and the ABC three phases are in negative phase sequence relation (ACB, CBA or BAC); when DC magnetic bias occurs, the effective values of four harmonics of the three phases ABC are equal, and the three phases ABC are in positive phase sequence relation (ABC, BCA or CAB); therefore, the fourth harmonic can be used as the basis for discriminating the dc bias recognition module in the fourth embodiment.
While certain exemplary embodiments of the present invention have been described above by way of illustration only, it will be apparent to those of ordinary skill in the art that modifications may be made to the described embodiments in various different ways without departing from the spirit and scope of the invention, the above embodiments may be combined or combined in any way, and the application is not limited thereto. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive of the scope of the invention, which is defined by the appended claims.
Claims (10)
1. A method for protecting a transformer, comprising the steps of:
s1, calculating differential current based on secondary side data of a CT for protection, respectively obtaining an effective value and a phase angle of a second harmonic of the differential current of a transformer, and recording the data;
s2, transmitting the effective value data to an effective value identification module in the direct current magnetic bias identification module, judging whether the three-phase effective values are equal in size, and if the three-phase effective values meet a specified threshold value, enabling an effective value identification module to transmit a signal to be 1;
s3, transmitting phase angle data to a phase angle identification module in the direct current magnetic bias identification module, detecting whether the three-phase second harmonic phase angle arrangement is in a positive phase sequence (ABC, BCA or CAB) or a negative phase sequence (ACB, CBA or BAC), and if the phase sequence is the negative phase sequence, enabling the phase angle identification module to transmit a signal 1;
s4, connecting the outputs of the effective value identification module and the phase angle identification module through a NAND logic circuit to form an output signal of the direct current magnetic bias identification module;
s5, the output signal of the second harmonic braking module is connected with the output signal of the direct-current magnetic bias identification module through NAND logic, so that a braking signal is formed;
and S6, connecting the brake signal and the output signal of the differential protection tripping module through an AND gate to form a tripping output signal, and starting differential protection when detecting that any one phase tripping output signal of the three phases is 1, and opening the circuit breaker.
2. The method according to claim 1, characterized by comprising the following parts: the device comprises a direct current magnetic bias identification module, a second harmonic braking module, a differential protection tripping module and a logic circuit connected with the modules.
3. The method according to claim 2, wherein the method comprises the following steps when the transformer works in different conditions, the working method of each module and the final action condition of the transformer are as follows:
working condition 1: when the transformer works normally, the second harmonic braking module does not act, the direct current magnetic bias identification module does not act, and when the transformer fails in an area, the differential protection normally acts;
working condition 2: when the transformer is affected by excitation surge current, the second harmonic braking module is started, the direct-current magnetic bias identification module does not act, and the differential protection of the transformer is locked;
working condition 3: when the transformer is affected by direct current magnetic bias, the direct current magnetic bias identification module identifies that the direct current magnetic bias is started, and the direct current magnetic bias identification module sends out a signal to enable the second harmonic braking module to be locked, and when the internal fault of the transformer occurs, the differential protection works normally.
4. The method of claim 1, wherein the effective value identification module dc bias discrimination formulas are divided into any one or two of the following groups:
1) Judging the effective value of the second harmonic of the differential current of the ABC three phases:
in the method, in the process of the invention,differential current second harmonic effective value of ABC three phases in each fundamental wave period respectively, +.>Is a similarity threshold;
2) Judging the ratio of the effective value of the second harmonic to the effective value of the fundamental wave in the system:
in the method, in the process of the invention,respectively for each fundamental wave periodThe effective value of the second harmonic of the differential current of the three phases ABC,is->Differential current fundamental effective value of same period, < +.>To represent the threshold of the second harmonic duty cycle.
5. The method of claim 1, wherein the dc bias discrimination formula of the phase angle identification module is as follows:
wherein,the phase angles of the second harmonic of the differential current of ABC three phases are respectively; when any one of the three formulas is established, the second harmonic phase angle arrangement is judged to be in a negative phase sequence, at the moment, the output signal of the phase angle identification module is set to be 1, and direct current magnetic bias exists in the system.
6. The method according to claim 4 or 5, wherein when the dc bias discrimination formula of the effective value discrimination module or the phase discrimination module is established, the signal should be kept at 0 for the time t1, and if the discrimination formula is still established after the time t1, the signal becomes 1; when the direct-current magnetic bias discrimination formula of the effective value discrimination module or the phase discrimination module is not established, the signal should be kept at 1 continuously for the time t2, and if the direct-current magnetic bias discrimination formula is still established after the time t2, the signal becomes 0.
7. The method of claim 6, wherein the logic circuit is required to satisfy the following requirements: when the signals of the effective value identification module and the phase angle identification module are both set to be 1, the system is judged to have direct-current magnetic bias, and the NAND gate outputs a 0-potential signal.
8. The method according to claim 1, characterized in that when a dc bias occurs, the three-phase currents are negative phase sequences (ACB, CBA or BAC) when the effective value of the three-phase second harmonic is within a threshold range.
9. The method of claim 1, wherein the transformer inrush current is avoided using a second harmonic braking element, the second harmonic braking module criteria:
in the method, in the process of the invention,brake ratio of second harmonic brake module, +.>Representing the magnitude of the fundamental component of the differential current,/and>representing the differential current second harmonic component amplitude.
10. A transformer protection device, comprising: a processor and a memory, the processor being configured to execute a control program stored in the memory, the control program when executed implementing the method of any one of claims 1 to 9.
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