CN101726660A - Identification method of transformer internal faults based on leakage magnetic field model - Google Patents
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Abstract
The invention discloses an identification method of transformer internal faults based on a leakage magnetic field model, belonging to the technical field of the major equipment relaying protection of electric systems. The identification method of transformer internal faults comprises the following steps of: calculating the difference of active power winding resistance consuming power infused to the leakage magnetic field model; forming protection criterion by combining with a threshold value; and identifying whether a transformer generates internal faults or not. The transformer internal fault identification method has small calculated amount without being influenced by a wiring mode, is adapted to various operation conditions of the transformer and has reliability, sensitivity, high redundancy rate and property obviously superior to the traditional power differential protection.
Description
Technical Field
The invention belongs to the technical field of relay protection of main equipment of a power system, and particularly relates to a transformer internal fault identification method based on a leakage magnetic field model.
Background
Regardless of traditional analog protection or digital protection generally adopted at present, transformer differential protection based on kirchhoff current law is one of main protection of power transformers. Although the differential protection applied to the transformer protection has certain defects in principle, the differential protection cannot be replaced as the main protection for operating the transformer. Therefore, how to improve the reliability and sensitivity of the protection operation is an important matter of the current transformer differential protection research. The transformer differential protection is influenced by adverse factors such as matching error of transformation ratio of a current transformer, transformer tap, CT transformation error, high-resistance grounding single-phase short circuit, turn-to-turn short circuit with small turns, excitation surge current and the like. The first five adverse factors can be solved through the self characteristics of differential protection, and the magnetizing inrush current needs to be additionally provided with a locking link to prevent misoperation. Therefore, the most critical and difficult problem of the transformer differential protection is how to prevent the differential protection from misoperation caused by the excitation inrush current of the transformer.
In recent years, researchers at home and abroad are dedicated to the research of a new principle of transformer protection, and a new principle and a new method for judging the magnetizing inrush current are provided. These principles and methods can be broadly divided into two categories: one type is to judge only by current magnitude and take the identification of current waveform as the main basis; the other is to perform discrimination by using both the voltage amount and the current amount. The traditional identification technology cannot avoid the problem of magnetizing inrush current, so many scholars propose a new transformer protection principle by using voltage and current, such as a magnetic flux characteristic principle, an equivalent circuit parameter identification method, a method based on a transformer loop equation, a method based on instantaneous excitation inductance and a power differential method. Particularly, the criterion based on the power differential principle is studied more deeply, however, the scheme still has the following 3 problems to be solved: (1) adverse effects caused by excitation inrush current cannot be avoided, and the charging process of the 1 st cycle during inrush current needs to be avoided, so that the judgment delay is caused; (2) during inrush current, copper loss is difficult to accurately calculate, iron loss is increased, and setting is difficult; (3) for the Y/delta connection transformer, the copper loss can not be determined due to the fact that the current in the delta side winding can not be obtained, and the protection sensitivity is reduced.
Disclosure of Invention
The invention aims to provide a transformer internal fault identification method based on a leakage magnetic field model, aiming at the problems of the prior transformer protection described in the background technology. The method is characterized by comprising the following steps:
the method comprises the following steps: protection is started according to current sudden change:
calculating the abrupt change amount delta i of each phase currentk＝||ik-ik-N|-|ik-N-ik-2NL, wherein k is the serial number of the sampling point, and N is the number of the sampling points in each period;
when the sudden change of a certain phase current satisfies Δ ik＞0.2IeAnd then after one period of data is acquired after 20ms of delay, starting protection to calculate the injection power of the leakage magnetic field model, wherein IeIs the volume of a transformerFixing current;
step two: forming a leakage magnetic field model:
judging the wiring form of the transformer, reducing all winding parameters of the transformer to the same side, eliminating mutual inductance parameters to construct a new equivalent loop balance equation, and constructing a leakage magnetic field model only related to leakage inductance and winding resistance through the equivalent loop balance equation;
step three: calculating the injection active power of the leakage magnetic field model:
the instantaneous power absorbed by the leakage magnetic field model is as follows: sg(t)＝uab(t)*ida(t) decomposition into sg(t)＝+s′g(t) form, wherein the direct current componentActive power absorbed by the leakage magnetic field model;
step four: calculating a difference value P between the injected active power of the leakage magnetic field model and the active power consumed by the normal winding resistance;
step five: the method for distinguishing the fault and the non-fault state of the transformer and distinguishing the fault in the transformer area from the fault outside the transformer area comprises the following steps:
1) setting a threshold value xi;
2) the power difference value P calculated by the step fouriAnd the threshold value xi forms a protection criterion, when any group P meets the criterion PiIf yes, judging that the transformer has internal fault;
3) when the power difference value P of each phase calculated in the step four simultaneously meets the criterion PiWhen the voltage is less than xi, the transformer is judged not to have a fault.
Step two, forming a leakage magnetic field model for a three-phase three-winding wiring form, such as delta/Y0Wiring, the parameters of the transformer are reduced to delta side, wherein:
and eliminating mutual inductance flux linkage and delta side winding current in a winding voltage loop equation, and combining each defined phase difference current to deduce an equivalent loop balance equation when the transformer has no internal fault:
for a two-winding wire form transformer, the following equivalent loop balance equations are formed:
wherein,
and constructing a leakage magnetic field model only containing leakage inductance and winding resistance through an equivalent loop balance equation.
Fourthly, calculating the difference value P between the active powers according to the following formula for the three-winding transformer:
for a two-winding transformer, then the calculation is done by:
where T is the sampling period.
And fifthly, taking the threshold as xi being 0.05P in transformer internal fault recognition0In which P is0Is the no-load loss of the transformer.
The invention constructs a leakage magnetic field model only related to leakage inductance and winding resistance by utilizing the measured current of the transformer, and provides a criterion for protecting the transformer by calculating the difference value of the injected active power of the leakage magnetic field model and the active power consumed by normal winding resistance. Adverse effects caused by iron loss and copper loss of the transformer are thoroughly eliminated; the transformer is suitable for various wiring forms such as a single-phase transformer, a three-phase two-winding transformer, a three-phase three-winding transformer and the like; the calculated amount is small, the influence of a Y/delta connection mode is avoided, and leakage inductance parameters of the transformer do not need to be known; the differential protection circuit is suitable for various operation conditions of the transformer, and is obviously superior to the current differential protection principle in performance.
Drawings
FIG. 1: a wiring diagram of a three-phase three-winding transformer;
FIG. 2: a leakage magnetic field model diagram of the three-phase three-winding transformer;
FIG. 3: a wiring diagram of a three-phase two-winding transformer;
FIG. 4: model diagram of leakage magnetic field of three-phase two-winding transformer;
FIG. 5: the transformer in the embodiment operates the wiring diagram;
FIG. 6-a: a tap position diagram of a fault winding in the primary side of the transformer;
FIG. 6-b: a position diagram of a fault winding tap inside a secondary side of the transformer;
FIG. 7: a three-phase differential current oscillogram and a difference value change curve chart of three-phase active power under the condition that the transformer is normally airdropped;
FIG. 8: the method comprises the following steps that a oscillogram of three-phase differential current and a difference value change curve chart of three-phase active power are obtained under the condition that a transformer is in an air-drop mode and is connected with a Y side B and the ground;
FIG. 9: the method comprises the following steps that a oscillogram of three-phase differential current and a difference value change curve chart of three-phase active power are obtained under the condition that a transformer is airdropped in a small turn ratio fault;
FIG. 10: the method comprises the following steps that in the normal operation of the transformer, a oscillogram of three-phase differential current and a difference value change curve chart of three-phase active power under the condition that a fault occurs when a phase B at a Y side is grounded;
FIG. 11: the method comprises the following steps that (1) a oscillogram of three-phase differential current and a difference value change curve chart of three-phase active power under the condition that a small turn ratio fault occurs in the normal operation of a transformer;
FIG. 12: the existing power differential principle under various operating conditions and a moving model experiment calculation result chart of the transformer internal fault identification method based on the leakage magnetic field model;
FIG. 13: the program flow diagram of the protection scheme in this embodiment.
Detailed Description
The preferred embodiments will be described in detail below with reference to the accompanying drawings. It should be emphasized that the following description is merely exemplary in nature and is not intended to limit the scope of the invention or its application.
The invention provides a transformer internal fault identification method based on a leakage magnetic field model. The key technology is that a leakage magnetic field model only containing leakage inductance of a unilateral winding and related to winding resistance is constructed, a criterion of transformer protection is formed by using a power difference value of injected active power of the leakage magnetic field model and power consumed by normal winding resistance, and whether the transformer has internal faults or not is judged.
Shown in FIG. 1 as Δ/Y0In a wired three-winding transformer, all parameters have been reduced to the delta side. Wherein u isa1、ub1、uc1The voltage of each phase of the delta side winding; u. ofa2、ub2、uc2And ua3、ub3、uc3Are respectively Y and Y0The voltage of each phase of the side winding; r is1、r2、r3Is Δ, Y, Y0Resistance values of the side windings; l is1、L2、L3Leakage inductance corresponding to self-leakage magnetic flux; m is12、m13、m21、m23、m31、m32Leakage inductance corresponding to the mutual leakage flux. The self-leakage magnetic flux and the mutual leakage magnetic flux are both closed mainly through air or transformer oil, the corresponding leakage reactance is constant, and m is12＝m21，m13＝m31，m23＝m32。
Fig. 2 is a schematic diagram of a leakage magnetic field model having a three-phase three-winding transformer structure, where the terminal voltages of the leakage magnetic field model are:
the direction of the arrow is the direction of the voltage drop; the current injected into the leakage magnetic field model is as follows: i.e. ida＝iLa1+ia2-ib2+ia3-ib3. Terminal voltage u of leakage magnetic field model during no-load closingab12＝ub1-ua1+ua2-ub2The current injected into the leakage magnetic field model is as follows: i.e. ida＝ia2-ib2。
The wiring diagram of the Y/delta wired three-phase two-winding transformer shown in fig. 3, all parameters have been reduced to the Y-side. Wherein ia，ib，icIs the current in the delta winding, iA，iB，iCIs the current in the Y winding, ua，ub，ucIs the voltage in the delta winding, uA，uB，uCIs the voltage in the Y winding, iLa，iLb，iLcIs current of three phases A, B and C outside the delta winding, LA，LB，LCIs leakage inductance in the Y winding, La，Lb，LcIs the leakage inductance in the delta winding, R is the resistance in the Y winding, and R is the resistance in the delta winding.
Fig. 4 is a schematic diagram of a certain group of Y-side leakage magnetic field models of a three-phase two-winding transformer structure, where the terminal voltages of the leakage magnetic field models are:
the direction of the arrow is the direction of the voltage drop; the current injected into the leakage magnetic field model is as follows: i.e. idA＝iLa+iA-iB. When no-load closing, the terminal voltage of the leakage magnetic field model is as follows: u. ofab2＝ub-ua+uA-uBThe current injected into the leakage magnetic field model is as follows: i.e. idA＝iA-iB。
Fig. 5 is a diagram of the operation wiring of the transformer in the present embodiment, which is a three-phase transformer composed of three single-phase DMB-10 type transformers, and the wiring groups are Yn, d 11. The relevant parameters are: (1) capacity: 10 kVA; (2) low-side rated voltage: 380V; (3) low-side rated current: 25.3A; (4) high-side rated voltage: 1000V; (5) high-side rated current: 10A; (6) no-load current: 1.45 percent; (7) no-load loss: 100W; (8) short-circuit loss: 0.35 percent; (9) short-circuit voltage: 9 to 15 percent. (10) Sum of inductances of primary and secondary windings (reduced to the primary side): 0.0038H; (11) sum of resistances of primary and secondary windings (reduced to primary side): 0.16610 omega.
Fig. 6 is a diagram of the tap positions of the primary and secondary side internal fault windings of the transformer, and various operation states of the transformer are simulated according to the conditions of the experimental system, including normal operation, no-load switching-on, internal ground fault, internal inter-turn fault, airdrop-in internal ground fault, airdrop-in internal inter-turn fault and the like. The influences of different moments, different phases and different fault positions are fully considered, and multiple experiments are carried out on each fault type.
FIG. 7 shows phase-difference current i when the transformer is normally unloaded and the magnetizing inrush current occurs during no-load closingdA、idBAnd idCThe waveform of (a) is shown by a double-dashed line, a solid line and a dashed-dotted line. And (b) shows a change curve of the injected active power of each phase leakage magnetic field model and the difference value of the consumed power of the normal winding resistance. The transformer is normally airdropped, and the active power difference cannot be zero due to the estimation error of the winding resistance and the measurement errors of the voltage transformer and the current transformer, and as can be seen from the diagram (b), the maximum active power difference value Pimax1.98W. Xi 0.05P05W, i.e. the power difference P of each phase simultaneously satisfies PiThe criterion of < xi, the transformer protection is locked, reliable and not misoperated.
FIG. 8 shows the fault condition of transformer air-drop on Y-side B phase to ground, with phase difference of current idA、idBAnd idCThe waveform of (a) is shown by a double-dashed line, a solid line and a dashed-dotted line. And (b) shows a change curve of the injected active power of each phase leakage magnetic field model and the difference value of the consumed power of the normal winding resistance. When becomingWhen the transformer has an internal fault, the active power difference of the fault phase is increased due to a series of reactions such as arc discharge heating and the like. From the graph (b), P can be seenimax＝668.6W，P1＞ξ，P2Xi, two power differences satisfy PiAnd if yes, judging that the transformer has an internal fault, and quickly and reliably acting.
Fig. 9 shows the case of transformer air-drop at a small turn ratio fault: before closing, the phase A on the Y side has turn-to-turn fault of 2.4 percent of turn ratio. Phase difference current idA、idBAnd idCThe waveform of (a) is shown by a double-dashed line, a solid line and a dashed-dotted line. And (b) shows a change curve of the injected active power of each phase leakage magnetic field model and the difference value of the consumed power of the normal winding resistance. When a small turn ratio fault occurs, the current change of a leakage field model of the transformer is small, but the current change in a short-circuit turn is large. The short circuit part can be equivalent to the 3 rd winding of the transformer to be in fault, the value of the power difference value is large, and P can be seen from the graph (b)imax＝35.3W，P1＞ξ，P3Xi, two groups of power difference values satisfy PiAnd if yes, judging that the transformer has an internal fault, and quickly and reliably acting. It is worth mentioning that the graph (a) shows that the second harmonic content in the three-phase current accounts for more than 36% of the fundamental component, and if the second harmonic braking ratio is 15% (the common value), the protection will be delayed for a longer time.
FIG. 10 shows the phase difference current i when the fault condition of the phase B on the Y side occurs during the normal operation of the transformerdA、idBAnd idCThe waveform of (a) is shown by a double-dashed line, a solid line and a dashed-dotted line. And (b) shows a change curve of the injected active power of each phase leakage magnetic field model and the difference value of the consumed power of the normal winding resistance. From the graph (b), P can be seenimax＝560.2W，P1＞ξ，P2Xi, two power differences satisfy PiAnd if yes, judging that the transformer has an internal fault, and quickly and reliably acting. Fig. 8 and 10 show that the method can be suitable for the conditions of no-load closing and grounding fault of the transformer in the normal operation process, the criterion is accurate, the redundancy is high, and the protection is realizedCan be reliably operated.
Fig. 11 shows a case where a small turn ratio fault occurs during normal operation of the transformer, and a turn-to-turn fault of 2.4% of the turn ratio occurs in the phase a on the Y side. Phase difference current idA、idBAnd idCThe waveform of (a) is shown by a double-dashed line, a solid line and a dashed-dotted line. And (b) shows a change curve of the injected active power of each phase leakage magnetic field model and the difference value of the consumed power of the normal winding resistance. From the graph (b), P can be seenimax＝29.8W，P1＞ξ，P3Xi, two groups of power difference values satisfy PiAnd if yes, judging that the transformer has an internal fault, and quickly and reliably acting. Fig. 9 and 11 show that the method is simultaneously suitable for the conditions of no-load closing and small turn ratio faults in the normal operation process, and has reliable performance and high redundancy.
Fig. 12 shows the calculation results of the dynamic model experiment of the existing power differential principle and the transformer internal fault identification method based on the leakage magnetic field model in each case. For the existing power differential principle, if P is used as normalcIf the threshold is set to 2090W at 1.5 times the maximum value, 8 cases with numbers 3, 4, 12, 13, 14, 15, 23, and 24 will not be recognized. For the purposes of the present invention, P is normally the casemMaximum value of (D) and fault condition PmIs different by more than 10 times, according to xi being 0.05P0And 5W is taken. FIG. 12 shows that the method is suitable for various operating conditions, can correctly distinguish the internal and external faults of the subarea, has great redundancy, and has obviously better reliability and sensitivity than the existing power differential principle.
FIG. 13 is a flowchart of the protection scheme in this embodiment, first obtaining the voltage and current of each side of the transformer; then constructing a leakage magnetic field model, and calculating the voltage and the current of the leakage magnetic field model; then calculating the difference value of the injected active power of the leakage magnetic field model and the consumed power of the normal winding resistance to form the criterion of transformer protection; and finally, judging whether the transformer fails according to whether the power difference is larger than a threshold value.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and those skilled in the art can easily conceive the changes or substitutions within the technical scope of the present invention, and the present invention shall be covered thereby. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (4)
1. A transformer internal fault identification method based on a leakage magnetic field model is characterized by comprising the following steps:
the method comprises the following steps: protection is started according to current sudden change:
calculating the abrupt change amount delta i of each phase currentk＝||ik-ik-N|-|ikN-ik-2NL, wherein k is the serial number of the sampling point, and N is the number of the sampling points in each period;
when the sudden change of a certain phase current satisfies Δ ik＞0.2IeThen, one period of data is acquired after 20ms delayAnd then starting protection to calculate the injection power of the leakage magnetic field model, wherein IeThe rated current of the transformer;
step two: forming a leakage magnetic field model:
judging the wiring form of the transformer, reducing all winding parameters of the transformer to the same side, eliminating mutual inductance parameters to construct a new equivalent loop balance equation, and constructing a leakage magnetic field model only related to leakage inductance and winding resistance through the equivalent loop balance equation;
step three: calculating the injection active power of the leakage magnetic field model:
the instantaneous power absorbed by the leakage magnetic field model is as follows: sg(t)＝uab(t)*ida(t) decomposition into sg(t)＝+s′g(t) form, wherein the direct current componentActive power absorbed by the leakage magnetic field model;
step four: calculating a difference value P between the injected active power of the leakage magnetic field model and the active power consumed by the normal winding resistance;
step five: the method for distinguishing the fault and the non-fault state of the transformer and distinguishing the fault in the transformer area from the fault outside the transformer area comprises the following steps:
1) setting a threshold value xi;
2) the power difference value P calculated by the step fouriAnd the threshold value xi forms a protection criterion, when any group P meets the criterion PiIf yes, judging that the transformer has internal fault;
3) when the power difference value P of each phase calculated in the step four simultaneously meets the criterion PiWhen the voltage is less than xi, the transformer is judged not to have a fault.
2. The method for identifying the internal fault of the transformer based on the leakage magnetic field model according to claim 1, wherein the leakage magnetic field model is formed in the second step, and for the three phases and the three phasesWinding wiring patterns, e.g. delta/Y0Wiring, the parameters of the transformer are reduced to delta side, wherein:
and eliminating mutual inductance flux linkage and delta side winding current in a winding voltage loop equation, and combining each defined phase difference current to deduce an equivalent loop balance equation when the transformer has no internal fault:
for a two-winding wire form transformer, the following equivalent loop balance equations are formed:
wherein,
and constructing a leakage magnetic field model only containing leakage inductance and winding resistance through an equivalent loop balance equation.
3. The transformer internal fault identification method based on the leakage magnetic field model according to claim 1, wherein the difference P between the active powers is calculated by the following formula for a three-winding transformer:
for a two-winding transformer, then the calculation is done by:
where T is the sampling period.
4. The method according to claim 1, wherein the threshold is ξ ═ 0.05P in transformer internal fault recognition0In which P is0Is the no-load loss of the transformer.
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| CN103762554A (en) * | 2014-02-18 | 2014-04-30 | 国家电网公司 | Fault detecting method for side windings of three-phase three-winding transformer |
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