CN104993456B - The method for preventing D.C. magnetic biasing from causing CT saturation to cause transformer differential protection malfunction - Google Patents
The method for preventing D.C. magnetic biasing from causing CT saturation to cause transformer differential protection malfunction Download PDFInfo
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Abstract
A kind of method for preventing D.C. magnetic biasing from causing CT saturation to cause transformer differential protection malfunction; judge whether differential protection starts according to difference of phase currents start-up criterion; the secondary harmonic component of the phase difference current is calculated after startup, the Blocking Differential Protection if more than 15%；Otherwise judge whether to meet second harmonic trend criterion, current transformer partial transient saturation is judged to if meeting, using revised ratio-restrained characteristic curve；Otherwise traditional ratio-restrained characteristic curve is still used.The present invention can identify the current transformer partial transient saturation as caused by D.C. magnetic biasing; and it is modified by contrast ratio braking characteristic curve; to prevent because current transformer partial transient saturation causes differential protection malfunction, solve the problems, such as that malfunction easily occurs based on traditional ratio restraint characteristic curve.
Description
Technical Field
The invention belongs to the technical field of power system relay protection, and particularly relates to a novel method for taking into account the influence of direct current magnetic biasing on a current transformer and preventing the current transformer from working near a saturation point after a sympathetic inrush current of the transformer disappears, so that local transient state saturation of the current transformer is caused, and differential protection malfunction is caused.
Background
In recent years, the power development of China is very rapid, and the interconnection among power grids in different regions of China is increasingly tight, so that the structure of the whole power system tends to be large, the reliability and the quality of electric energy can be ensured only by increasing the transmission capacity and the transmission distance of the system, and higher requirements are provided for the safety and the stability of the power grid at the present stage of China.
Two transformers are connected in parallel, when one transformer is switched on in a no-load state, the other transformer can generate sympathetic inrush current, and the problem that the sympathetic inrush current causes differential protection misoperation is always concerned. At present, most of the mechanisms of Transformer generation and inrush Current are researched, and the differential protection misoperation possibly caused by Current Transformer (CT) saturation due to the inrush Current is usually ignored. After the transformer generates the sympathetic inrush current, the primary current of the transformer is multiple times of the normal operation current, the value is large, the transformer often contains an aperiodic component attenuated in an exponential form, after the transformer flows into the CT, the excitation current of the CT also contains the aperiodic component, although the aperiodic component does not generate variable magnetic flux, the operating condition of the iron core can be changed, the iron core magnetic flux is closely related to the integral value of the aperiodic component of the primary current to time, when the direction of the aperiodic component is certain, the integral value continuously rises, so that the iron core magnetic flux is increased, the iron core magnetic flux operates near a saturation point, and further the CT is subjected to local transient state saturation. In addition, if there is remanence in the CT in the same direction as the magnetic flux generated by the excitation current required to transfer the non-periodic component of the primary current, the saturation of the CT is accelerated and aggravated. When the sympathetic inrush current disappears, the residual magnetism accumulated by the CT is possibly large, and the CT works on a hysteresis curve near a saturation point when the CT normally operates on the load current, so that the waveform is distorted and the actual primary current of the system can not be really reflected any more.
At present, the ultra-high voltage transmission industry is also rapidly developed, and a three-vertical three-horizontal ultra-high voltage backbone net rack is built, wherein more than 20 ultra-high voltage direct current transmission projects are built and built. When the operation mode of the Direct Current transmission system adopts a monopolar earth loop or bipolar loop unbalanced operation mode, the earth is equivalent to a resistor, potential differences exist between different grounding points, so that the earth and the power grid form a loop, Direct Current flowing through the earth flows into the alternating Current power grid through a transformer grounding neutral point, and the caused Direct Current magnetic biasing phenomenon is called High-voltage Direct Current (HVDC) type Direct Current magnetic biasing. The current of a HVDC type direct current bias is in the order of a few to a few tens of amperes. The direct current flows into the alternating current network through the neutral point of the transformer, so that the direct current is superposed on the power frequency current of the normal operation of the system current. Consistent with the analysis of the influence of the sympathetic inrush current on the CT saturation, if the magnetic flux generated by the direct-current magnetic biasing current is continuously accumulated and the direction is consistent with the CT residual magnetism direction, the CT saturation is accelerated, so that the secondary side waveform of the CT is distorted. At this time, if the ac system is affected by dc magnetic bias and another transformer is switched on in no-load, so that the transformer in operation generates and responds to inrush current, the accuracy of the CT transmission characteristics configured for the transformer will face serious examination. After the sympathetic inrush current disappears, under the influence of the direct current magnetic bias, the CT may operate in a local magnetic hysteresis loop near a saturation point, slight distortion of waveforms on the primary side and the secondary side of the CT is mainly reflected on the amplitude and the phase, the calculated differential current may be greater than the minimum action current of the rate braking characteristic, the braking current should be the load current of the line operation, and at this time, the braking current may possibly fall into the action region of the rate braking characteristic. Therefore, aiming at considering the influence of the direct current magnetic bias on the current transformer, the differential protection is easy to malfunction after the sympathetic inrush current disappears, corresponding precautionary measures are provided, important theoretical significance and practical value are achieved, and research results are helpful for improving the rapidity and the reliability of the power grid relay protection.
Disclosure of Invention
The invention discloses a method for preventing differential protection maloperation caused by current transformer saturation caused by direct current magnetic biasing, aiming at solving the technical problem that when the current transformer is influenced by direct current magnetic biasing, after a sympathetic inrush current of a transformer disappears, the current transformer still works near a saturation point to cause local transient state saturation of the current transformer and cause differential protection maloperation.
The invention specifically adopts the following technical scheme:
a method for preventing a current transformer from saturation caused by direct current magnetic biasing to cause transformer differential protection maloperation is characterized in that:
the method judges whether the current transformer is locally saturated in transient state or not based on the secondary harmonic content of the differential current of the transformer and the variation trend of the secondary harmonic content, so as to determine whether the differential protection of the transformer is carried out by adopting the corrected ratio braking characteristic curve or not.
A method for preventing a current transformer from saturation caused by direct current magnetic biasing to cause transformer differential protection misoperation is characterized by comprising the following steps:
(1) collecting secondary current values of mutual inductors on two sides of a transformer;
(2) judging whether the differential protection is started or not according to a phase current sudden change starting criterion, and if any phase current sudden change at two sides is larger than a set starting current IQDIf yes, differential protection is started;
(3) calculating three-phase differential current and braking current of the transformer;
(4) if any phase difference current of the transformer is larger than the preset minimum action current, calculating the second harmonic content of the phase differential current, if the second harmonic content in the differential current is higher than the preset value of the second harmonic content, locking differential protection, otherwise, entering the step (5);
(5) correcting the ratio braking characteristic curve;
(6) judging whether the phase difference current meets the second harmonic trend criterion, and if so, performing transformer differential protection by adopting a corrected ratio braking characteristic curve; otherwise, the differential protection of the transformer is still carried out by adopting the traditional ratio braking characteristic curve before correction.
The invention further comprises the following preferred embodiments:
in the step (2), the phase current sudden change starting criterion refers to whether the sudden change of any phase current of the transformer is larger than the set starting current IQDAnd if the differential protection value is larger than the preset value, the differential protection is started. The starting current IQD＝0.2Ie,In which IeThe rated current of the transformer.
In the step (4), the second harmonic content of the differential current is the ratio of the second harmonic to the fundamental wave of the phase difference current, and the preset value of the second harmonic content is 15%.
In step (5), i.e. the conventional ratio brake curve, of the ratio brake curve before correctionop.minFor minimum operating current, take Iop.min＝0.2Ie；Ires.1For the first knee braking current, take Ires.1＝Ie；Ires.2For the second knee braking current, take Ires.2＝3Ie(ii) a With the first inflection point o as the center, r is 0.15IeMaking a circle for the radius, intersecting the traditional ratio braking characteristic curve at two points b and c, and replacing a bo-co section in the traditional ratio braking characteristic curve with an arc bc as a corrected ratio braking characteristic curve; wherein IeThe rated current of the transformer.
In step (6), the second harmonic trend criterion is whether the second harmonic content reduction value of the phase difference current in a half cycle is smaller than a preset trend threshold value. The trend threshold is preferably 1% to 3%.
The invention has the beneficial effects that:
the invention provides a novel method for preventing differential protection misoperation caused by local transient state saturation of a current transformer after a sympathetic inrush current of the transformer disappears by taking the influence of direct current magnetic biasing on the current transformer into account. Under the combined action of the direct-current magnetic biasing direct current and the transformer sympathetic inrush current, the current transformer works near a saturation point, after the sympathetic inrush current disappears, the current transformer is locally saturated in a transient state, the amplitudes of the currents on the primary side and the secondary side are unequal and have phase offset, so that the amplitude of the calculated differential current is larger, and the minimum action current of ratio braking is achieved; at the moment, the braking current is equal to the load current and is easy to fall into an action area, so that the differential protection is in misoperation. The invention provides a method for identifying the local transient saturation of a current transformer caused by direct current magnetic biasing by utilizing the descending trend of the content of second harmonic, and correcting a ratio braking characteristic curve to prevent the false operation caused by the local transient saturation of the current transformer, thereby solving the problem that the false operation is easy to occur based on the original ratio braking characteristic curve, and meanwhile, when the current transformer is in normal operation (without direct current magnetic biasing) and has a fault in a generation region, the judgment is still carried out according to the traditional ratio braking characteristic curve of differential protection, the sensitivity of the operation is not influenced, the accuracy and the reliability are ensured, and the method has certain engineering practical significance.
Drawings
FIG. 1 is a conventional rate brake characteristic for differential protection;
FIG. 2 is a modified differential protection ratio braking characteristic; FIG. 3 is a simulation diagram of the embodiment (T2 closes at 1 s);
fig. 4 is a schematic diagram (per unit value) of a phase a and an inrush current waveform generated by T1 when T2 is switched on for 1s under the condition of no dc bias;
FIG. 5 is a schematic diagram (per unit) of the A-phase differential current waveform of T1 in a section of 7.2s to 7.4s after the sympathetic inrush current disappears under the condition of no DC magnetic bias;
FIG. 6 is a schematic diagram showing the second harmonic content of the phase A differential current wave of T1 in the 0 s-8 s section under the condition of no DC bias;
FIG. 7 is a schematic diagram (per unit value) of the A-phase and the corresponding inrush current waveform generated by T1 when T2 is switched on for 1s under the condition of DC magnetic bias;
FIG. 8 is a schematic diagram (per unit) of the A-phase differential current waveform of T1 in a section of 7.2s to 7.4s after the sympathetic inrush current disappears in the presence of DC bias;
FIG. 9 is a diagram showing the second harmonic content of the phase A differential current wave of T1 in the 0 s-8 s range under the condition of DC bias;
fig. 10 is a schematic diagram showing a secondary side current comparison (per unit value, secondary side current is converted to primary side) of a Y-side a-phase current transformer of a section T1 of 7.2s to 7.4s after a sympathetic inrush current disappears in the presence of dc magnetic biasing;
fig. 11 is a hysteresis curve of the a-phase core of T1 under the condition of dc bias, the left graph is a hysteresis curve of the a-phase core of T1 in the 1 s-8 s segment, and the right graph is a hysteresis curve of the a-phase core of T1 in the 7.2 s-7.4 s segment;
fig. 12 is a schematic flow chart of a new method for preventing differential protection malfunction due to local transient saturation of a current transformer after occurrence of a transformer and disappearance of a sympathetic inrush current by accounting for influences of a direct current magnetic bias on the current transformer in the embodiment of the present invention.
Detailed Description
The technical scheme of the invention is further described in detail by combining the drawings and the specific embodiments in the specification.
A new method for considering the influence of direct current magnetic bias on a current transformer and preventing the local transient state saturation of the current transformer when the transformer generates sympathetic inrush current so as to cause differential protection malfunction.
Fig. 12 is a flowchart of a method for preventing the differential protection malfunction of the transformer caused by the saturation of the current transformer due to the dc magnetic bias according to the present invention, which prevents the local transient saturation of the current transformer when the transformer generates and responds to the inrush current. The following detailed description takes the example of the wiring mode of the transformer as Y/d11, and the method comprises the following steps:
(1) acquiring a current value instantaneous value of a secondary side of a star-side three-phase current transformer and a current value instantaneous value of a secondary side of an angle-side three-phase current transformer of a transformer;
(2) calculating the variable quantity delta I of any phase current of the star side and the angular side of the transformer in real timeφIf the variable quantity of any phase current of the star side and the angular side of the transformer meets the criterion of sudden variable quantity starting of the transformer, starting the differential protection, and entering the step (3);
set phase current variation Δ IφThe mutation starting criterion is as follows:
ΔIφ＞IQD(1)
wherein the phase current variation amount DeltaIφAnd a starting current IQDThe calculation formula of (2) is as follows:
wherein, a cycle 24 point sampling is set, phi refers to A, B and C three phases, iφ(k) Instantaneous value of phase current for phase phi of k sampling points, IeFor conversion to rated current, i, on the star side of the transformerφ(k)、IeThe current values of the secondary sides of the current transformers are all current values;
(3) calculating to obtain a current phasor value of the secondary side of the star-side three-phase current transformer according to the current instantaneous value of the secondary side of the star-side three-phase current transformer of the transformer acquired in the step (1) and the current instantaneous value of the secondary side of the angle-side three-phase current transformer And the current phasor value of the secondary side of the angle side three-phase current transformerCalculating three-phase differential current I of transformerd(A)，Id(B)，Id(C) And three-phase braking current I of transformerr(A)，Ir(B)，Ir(C)：
Wherein,the current value of the secondary side of the star-side three-phase current transformer of the transformer is obtained;the current value of the secondary side of the three-phase current transformer on the angular side of the transformer is obtained;
(4) if any phase difference current of the transformer is larger than the preset minimum action current, calculating the second harmonic content of the differential current of the phase, wherein the formula is as follows:
wherein id.second(φ,k)、id.fundmental(phi, k) are the values of the second harmonic and fundamental wave of the phase difference current of the kth sampling point phi respectively, Iop.minFor minimum operating current, take Iop.min＝0.2Ie；
Comparing the second harmonic content of the phase, and locking differential protection if the second harmonic content is higher than 15%; if the content is less than 15%, entering the step (5);
(5) the conventional rate brake characteristic curve shown in fig. 1, in which I is shown in fig. 1, is modifiedop.minFor minimum operating current, take Iop.min＝0.2Ie；Ires.1For the first knee braking current, take Ires.1＝Ie；Ires.2For the second knee braking current, take Ires.2＝3Ie(ii) a Using the first inflection point as the center o，r＝0.15IeThe radius is made into a circle, the circle intersects with the original rate brake characteristic curve at two points b and c, and the curve abcde is the corrected rate brake characteristic curve, as shown in fig. 2.
(6) After the protection device detects that the second harmonic content of the phase is less than 15%, judging whether the descending trend of the second harmonic content of the phase in a half cycle is smooth or not, namely whether the formula (6) is satisfied or not:
P2nd(φ,k-12)-P2nd(φ,k)＜1％ (6)
if the 4 sampling points are judged to meet the formula (6), the second harmonic trend criterion is met, the fact that the second harmonic content is reduced due to local transient saturation of the current transformer caused by direct-current magnetic biasing is considered, the corrected ratio braking characteristic curve is opened, and whether the differential protection enters an action area or not is judged according to the corrected ratio braking characteristic curve; otherwise, if the formula (6) is not satisfied, the content of the second harmonic is considered to be obviously reduced, the harmonic current is judged to be reduced, or the differential current second harmonic content is reduced sharply due to the in-zone fault or the out-zone fault, the ratio braking characteristic curve is not opened and corrected, and whether the differential protection enters the action zone or not is judged according to the traditional ratio braking characteristic curve (shown in fig. 1).
The technical scheme of the method for preventing the differential protection misoperation of the transformer caused by the saturation of the current transformer due to the direct-current magnetic biasing is further described by a specific embodiment.
The saturation degree of the current transformer is related to the parameter setting thereof, in this case, the current transformer parameters of B, C two phases are set so that the current transformer does not have a saturation condition (i.e. the differential protection can be reliably locked), and only the detailed analysis on the saturation of the current transformer of the phase a is made, and the simplified analysis does not affect the reliability and the correctness of the application of the invention. By adopting the new method provided by the invention and correcting the ratio braking characteristic curve, the influence of the direct current magnetic bias on the current transformer is taken into account, and the judgment condition of the differential protection action after the transformer generates the sympathetic inrush current is as follows:
1) a simulation model of a single-ended system is constructed, as shown in FIG. 3, the wiring modes of the two transformers T1 and T2 are both Y/d11, and the rated voltage is 220kV:35 kV. Firstly, when no direct current bias current invades at a neutral point, T2 is switched on at 1s, a sympathetic inrush current is generated at T1, and a schematic diagram of the A-phase differential current waveform of T1 is shown in FIG. 4; FIG. 5 is a schematic diagram showing the A-phase differential current waveform of T1 in a section of 7.2 s-7.4 s after the sympathetic surge current disappears; the trend of the second harmonic content of the phase a differential current is shown in fig. 6.
2) When a neutral point of T1 is connected with a voltage source to simulate DC bias current to flow into an AC system, the magnitude of the DC bias current of each phase is 14A. The T2 is switched on at 1s, the T1 generates sympathetic inrush current, and the schematic diagram of the A-phase differential current waveform of the T1 is shown in FIG. 7; FIG. 8 is a schematic diagram showing the A-phase differential current waveform of T1 in a section of 7.2 s-7.4 s after the sympathetic surge current disappears; the trend of the second harmonic content of the phase a differential current is shown in fig. 9; the comparison shows that when no direct current magnetic bias current invades, the differential current of the A phase gradually attenuates after the sympathetic inrush current disappears, and the amplitude is close to 0; and after the direct current magnetic bias current invades, the amplitude of the differential current is still larger than the minimum action current (0.2p.u.), and the content of the second harmonic wave is lower than the threshold value of 15%.
3) After the sympathetic inrush current disappears, a comparison schematic diagram of the current of the secondary side of the current transformers 7.2 s-7.4 s on the phase a and the phase Y of T1 is shown in fig. 10 (the secondary side current is converted to the primary side), and it can be clearly found that after the sympathetic inrush current disappears, the secondary side current changes in amplitude and phase, which results in that the differential current is not attenuated to 0, which is approximately equal to 0.25 p.u.; the braking current is equal to the load current, approximately at 1 times the rated current. At this point, the rate brake characteristic shown in fig. 2 is adopted, and the brake falls into the action region exactly, so that the protection is not moved.
4) Based on the condition that a direct current magnetic bias invades an alternating current system, after a sympathetic surge current disappears, the differential current amplitude is still higher than the minimum action current, the content of the second harmonic wave is lower than 15%, but the change is gentle, the change amplitude in a half cycle is far smaller than 1%, namely, the P is met by continuously judging 4 sampling points2nd(φ,k-12)-P2nd(phi, k) < 1%. Therefore, the temperature of the molten metal is controlled,the current transformer is judged to be subjected to local transient saturation due to direct current magnetic biasing, so that amplitude difference and phase difference exist in primary and secondary side currents, the magnitude of differential current is larger than the minimum action current, waveform distortion is not obvious, and the content of secondary harmonic waves is low. Fig. 11 is a hysteresis curve of the a-phase core of T1, the left graph is a hysteresis curve of the a-phase core of T1 in a range of 1s to 8s, and the right graph is a hysteresis curve of the a-phase core of T1 in a range of 7.2s to 7.4 s.
5) And opening the corrected ratio brake characteristic curve, wherein the differential current amplitude is 0.25p.u., and the brake current amplitude is 0.9p.u., and the differential protection does not act when the differential brake characteristic curve is judged to fall into a brake area according to the corrected ratio brake characteristic curve.
The applicant has described and illustrated the embodiments of the present invention in detail with reference to the drawings and tables of the specification, but it should be understood by those skilled in the art that the fault information score values and the influencing factor correction coefficients in the above embodiments are only preferred embodiments of the present invention, and those skilled in the art may reasonably select or modify the fault information score values and the influencing factor correction coefficients according to the specific generator set excitation system model and actual operating condition under the inventive concept of the present invention. In conclusion, the detailed description of the present application is only for the purpose of helping the reader to better understand the spirit of the present invention and is not intended to limit the scope of the present invention, but rather, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention.
Claims (6)
1. A method for preventing a current transformer from saturation caused by direct current magnetic biasing to cause transformer differential protection misoperation is characterized by comprising the following steps:
(1) collecting secondary current values of mutual inductors on two sides of a transformer;
(2) judging whether the differential protection is started or not according to a phase current sudden change starting criterion, and if any phase current sudden change at two sides is larger than a set starting current IQDIf yes, differential protection is started;
(3) calculating three-phase differential current and braking current of the transformer;
(4) if any phase difference current of the transformer is larger than the preset minimum action current, calculating the second harmonic content of the phase differential current, if the second harmonic content in the differential current is higher than the preset value of the second harmonic content, locking differential protection, otherwise, entering the step (5);
(5) correcting the ratio braking characteristic curve; in the ratio braking curve before correction, i.e. in the conventional ratio braking curve, Iop.minFor minimum operating current, take Iop.min＝0.2Ie；Ires.1For the first knee braking current, take Ires.1＝Ie；Ires.2For the second knee braking current, take Ires.2＝3Ie(ii) a With the first inflection point o as the center, r is 0.15IeMaking a circle for the radius, intersecting the traditional ratio braking characteristic curve at two points b and c, and replacing a bo-co section in the traditional ratio braking characteristic curve with an arc bc as a corrected ratio braking characteristic curve; wherein IeRated current for the transformer;
(6) judging whether the phase difference current meets the second harmonic trend criterion, and if so, performing transformer differential protection by adopting a corrected ratio braking characteristic curve; otherwise, the differential protection of the transformer is still carried out by adopting the traditional ratio braking characteristic curve before correction.
2. The method for preventing the malfunction of the differential protection of the transformer caused by the saturation of the current transformer due to the DC magnetic bias as claimed in claim 1, wherein:
in the step (2), the phase current sudden change starting criterion refers to whether the sudden change of any phase current of the transformer is larger than the set starting current IQDAnd if the differential protection value is larger than the preset value, the differential protection is started.
3. The method for preventing the malfunction of the differential protection of the transformer caused by the saturation of the current transformer due to the DC magnetic bias as claimed in claim 2, wherein:
the starting current IQD＝0.2IeIn which IeTo becomeThe transformer is rated for current.
4. The method for preventing the malfunction of the differential protection of the transformer caused by the saturation of the current transformer due to the DC magnetic bias as claimed in claim 1, wherein:
in the step (4), the second harmonic content of the differential current is the ratio of the second harmonic to the fundamental wave of the phase difference current, and the preset value of the second harmonic content is 15%.
5. The method for preventing the malfunction of the differential protection of the transformer caused by the saturation of the current transformer due to the DC magnetic bias as claimed in claim 1, wherein:
in step (6), the second harmonic trend criterion is whether the second harmonic content reduction value of the phase difference current in a half cycle is smaller than a preset trend threshold value.
6. The method for preventing the malfunction of the differential protection of the transformer caused by the saturation of the current transformer due to the DC magnetic bias as claimed in claim 5, wherein:
the trend threshold is 1% -3%.
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| CN106410750A (en) * | 2016-09-14 | 2017-02-15 | 北京四方继保自动化股份有限公司 | Differential protection method based on adaptive ratio braking curve |
| CN109375131B (en) * | 2018-07-31 | 2022-03-04 | 中国电力科学研究院有限公司 | Method and system for identifying saturation speed and saturation depth of current transformer |
| CN110635456A (en) * | 2019-09-02 | 2019-12-31 | 华北电力大学 | Method for preventing differential protection from refusing action caused by fault inrush current at converter transformer valve side |
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| JP3697337B2 (en) * | 1997-06-12 | 2005-09-21 | 東京電力株式会社 | Digital type protective relay |
| CN101710689B (en) * | 2009-12-18 | 2012-06-13 | 南京中德保护控制系统有限公司 | Method for protecting strong feed-in line current |
| CN102761106B (en) * | 2012-07-10 | 2014-10-22 | 清华大学 | Quick starting and vector calculation method for intelligent substation integrated protection |
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| CN103259250B (en) * | 2013-04-27 | 2016-03-02 | 广东电网公司电力科学研究院 | Based on the method for intelligent substation station domain information identification transformer sympathetic inrush |
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