CN104993456B - The method for preventing D.C. magnetic biasing from causing CT saturation to cause transformer differential protection malfunction - Google Patents

Abstract

A method to prevent DC bias from causing current transformer saturation to cause misoperation of transformer differential protection. According to the starting criterion of phase current mutation, it is judged whether the differential protection is started. After starting, the second harmonic content of the differential current of the phase is calculated. If it is greater than 15%, the differential protection will be blocked; otherwise, it will be judged whether the second harmonic trend criterion is satisfied. If it is satisfied, it will be judged as partial transient saturation of the current transformer, and the modified ratio braking characteristic curve will be used; otherwise, the traditional one will still be used. Ratio braking characteristic curve. The invention can identify the local transient saturation of the current transformer caused by the DC bias, and correct the ratio braking characteristic curve to prevent the differential protection from malfunctioning due to the local transient saturation of the current transformer, and solve the problem based on the traditional The ratio braking characteristic curve is prone to misoperation.

Description

Prevent DC bias from causing saturation of current transformers and misoperation of transformer differential protection Methods

technical field

The invention belongs to the technical field of power system relay protection, and particularly relates to a method that takes into account the influence of DC bias magnetism on a current transformer to prevent the current transformer from still working near the saturation point after the inrush current of the transformer disappears, causing its local Transient saturation, a new method of causing misoperation of differential protection.

Background technique

In recent years, my country's electric power has developed very rapidly, and the interconnection between power grids in different regions of the country has become increasingly close, making the structure of the entire power system tend to become larger. It is necessary to increase the system transmission capacity and transmission distance to ensure the reliability and quality of electric energy. At this stage, the security and stability of the power grid put forward higher requirements.

Two transformers are connected in parallel, and when one transformer is switched on with no load, the other transformer will generate an inrush current, and the problem of differential protection misoperation caused by the inrush current has also attracted much attention. At present, most of the research is on the mechanism of transformer inrush and inrush current, which often ignores the inrush current that makes the current transformer (Current Transformer, CT) saturated and may lead to differential protection misoperation. After the transformer generates and responds to the inrush current, its primary current is several times the normal operating current, and the value is relatively large, and it often contains an aperiodic component that decays exponentially. After flowing into the CT, the excitation current of the CT also contains a non-periodic component. Although the non-periodic component does not produce changing magnetic flux, it can change the operating conditions of the iron core, and because the magnetic flux of the iron core is closely related to the integral value of the non-periodic component of the primary current with respect to time, when the direction of the non-periodic component is constant, The integral value will continue to rise, causing the core flux to increase, running near the saturation point, and then causing partial transient saturation of the CT. In addition, if there is residual magnetism in the CT, and the direction of the residual magnetism is the same as the direction of the magnetic flux generated by the excitation current required to transfer the non-periodic component of the primary current, it will accelerate and intensify the saturation of the CT. When the inrush current disappears, the CT may accumulate a large residual magnetism, and when it normally operates at the load current, it works on the hysteresis curve near the saturation point, which makes the waveform distorted and can no longer truly reflect the actual primary current of the system.

At present, the UHV power transmission business is also developing rapidly, and the "three vertical and three horizontal" UHV backbone grids are being built, of which more than 20 UHV DC transmission projects have been completed or are under construction. The UHV DC power grid will have a certain impact on the conventional AC power grid. When the operation mode of the DC transmission system adopts the unbalanced operation mode of the single pole ground loop or the bipolar loop, the ground is equivalent to a resistance, and the distance between different grounding points There is a potential difference, so that the earth and the grid form a loop, and the DC current flowing through the earth flows into the AC grid through the grounded neutral point of the transformer, and the resulting DC bias phenomenon is called HVDC (High-voltage Direct Current) type DC bias. The current of the HVDC type DC bias is about a few amps to tens of amps. The DC current flows into the AC network through the neutral point of the transformer, so that the system current superimposes the DC current on the normal operating power frequency current. Consistent with the above analysis of the influence of the inrush current on CT saturation, if the magnetic flux generated by the DC bias current is continuously accumulated, and the direction is consistent with the direction of the CT residual magnetism, the CT saturation will be accelerated and the CT secondary side waveform will be distorted. At this time, if the AC system is affected by the DC bias and at the same time another transformer is switched on with no load, causing the running transformer to generate an inrush current, the accuracy of the CT transmission characteristics configured by the transformer will be severely affected. severe test. After the inrush current disappears, affected by the DC bias, the CT may run in a local hysteresis loop near the saturation point, and the slight distortion of the waveform on the primary and secondary sides of the CT is mainly reflected in the amplitude and phase. The calculated difference The dynamic current may be greater than the minimum operating current of the ratio braking characteristic, and the braking current should be the load current of the line operation. At this time, it is likely to fall into the action area of the ratio braking characteristic. Therefore, considering the influence of DC bias on the current transformer, the differential protection is prone to misoperation after the inrush current disappears, and the corresponding preventive measures are put forward, which has important theoretical significance and practical value, and the research results will help to improve Rapidity and reliability of power grid relay protection.

Contents of the invention

In order to solve the technical problem that when the current transformer is affected by the DC bias, after the inrush current of the transformer disappears, the current transformer still works near the saturation point, which will cause its local transient saturation and cause the differential protection to malfunction , The invention discloses a method for preventing differential protection maloperation caused by current transformer saturation caused by DC bias.

The present invention specifically adopts the following technical solutions:

A method for preventing misoperation of transformer differential protection caused by saturation of current transformer caused by DC bias, characterized in that:

The method judges whether the current transformer is partially transiently saturated based on the magnitude of the second harmonic content of the differential current of the transformer and the change trend of the second harmonic content, so as to determine whether to use the corrected ratio braking characteristic curve for the transformer Differential Protection.

A method for preventing misoperation of transformer differential protection caused by saturation of a current transformer caused by DC bias, characterized in that the method includes the following steps:

(1) Collect the secondary current value of the transformer on both sides of the transformer;

(2) Judging whether the differential protection is started according to the starting criterion of the sudden change of phase current, if the sudden change of any phase current on both sides is greater than the set starting current I _QD , then the differential protection starts;

(3) Calculate the three-phase differential current and braking current of the transformer;

(4) If the differential current of any phase of the transformer is greater than the preset minimum operating current, calculate the second harmonic content of the differential current of the phase, if the second harmonic content in the differential current is higher than the second harmonic content At the preset value, block the differential protection, otherwise enter step (5);

(5) Correct the ratio braking characteristic curve;

(6) Judging whether the differential current of this phase satisfies the second harmonic trend criterion, if it is satisfied, the modified ratio braking characteristic curve is used for transformer differential protection; otherwise, the traditional ratio braking characteristic curve before modification is still used for protection Transformer differential protection.

The present invention further includes the following preferred solutions:

In step (2), the start-up criterion for sudden change of phase current refers to whether the sudden change of any phase current of the transformer is greater than the set start-up current I _QD , and if it is greater, the differential protection is started. The starting current I _QD =0.2I _e, , where I _e is the rated current of the transformer.

In step (4), the second harmonic content of the differential current refers to the ratio between the second harmonic and the fundamental wave of the differential current of the phase, and the preset value of the second harmonic content is 15%.

In step (5), in the ratio braking curve before correction, that is, in the traditional ratio braking curve, I _op.min is the minimum operating current, and I _op.min = 0.2I _e ; I _res.1 is the first Inflection point braking current, take I _res.1 = I _e ; I _res.2 is the second inflection point braking current, take I _res.2 = 3I _e ; take the first inflection point o as the center of the circle, r = 0.15I _e Make a circle with a radius, intersect the traditional ratio braking characteristic curve at points b and c, replace the bo-co segment in the traditional ratio braking characteristic curve with the arc bc as the modified ratio braking characteristic Curve; where I _e is the rated current of the transformer.

In step (6), the second harmonic trend criterion is whether the drop value of the second harmonic content of the phase differential current within half a cycle is smaller than a predetermined trend threshold. The trend threshold is preferably 1%-3%.

Beneficial effects of the present invention:

The present invention proposes a new method for preventing differential protection maloperation caused by partial transient saturation of the current transformer after the summation inrush current of the transformer disappears, taking into account the influence of the DC bias magnetic field on the current transformer. Under the joint action of DC bias DC and transformer and inrush current, the current transformer works near the saturation point. After the inrush current disappears, the current transformer is partially transiently saturated, and the current amplitudes of the primary and secondary sides are not equal. And there is a phase offset, which leads to a large amplitude of the calculated differential current, reaching the minimum operating current of ratio braking; at this time, the braking current is equal to the load current, which is easy to fall into the action area, causing the differential protection to malfunction. The present invention proposes to use the descending trend of the second harmonic content to identify the local transient saturation of the current transformer caused by the DC bias, and correct the ratio braking characteristic curve to prevent the malfunction caused by the local transient saturation of the current transformer, which solves the problem Based on the problem that the original ratio braking characteristic curve is prone to misoperation, at the same time, in normal operation (no DC bias) and in-zone faults, it is still judged according to the traditional ratio braking characteristic curve of differential protection, which does not affect the operation. Sensitivity, to ensure accuracy and reliability, has a certain engineering practical significance.

Description of drawings

Figure 1 is the traditional ratio braking characteristic curve of differential protection;

Fig. 2 is the differential protection ratio braking characteristic curve after correction; Fig. 3 is the emulation schematic diagram of embodiment (T2 closes at 1s);

Figure 4 is a schematic diagram of the A-phase and inrush current waveform (per unit value) generated by T1 when T2 is closed in 1s without DC bias;

Figure 5 is a schematic diagram of the A-phase differential current waveform of T1 in the 7.2s to 7.4s section after the inrush current disappears without DC bias (per unit value);

Figure 6 is a schematic diagram of the second harmonic content of the A-phase differential current wave of T1 in the 0s to 8s section without DC bias;

Figure 7 is a schematic diagram of the A-phase and inrush current waveform (per unit value) generated by T1 when T2 is closed in 1s under the condition of DC bias;

Figure 8 is a schematic diagram of the A-phase differential current waveform of T1 in the 7.2s to 7.4s section after the inrush current disappears in the case of DC bias (per unit value);

Figure 9 is a schematic diagram of the second harmonic content of the A-phase differential current wave of T1 in the 0s to 8s section in the case of DC bias;

Figure 10 is a schematic diagram of the comparison of the primary and secondary side currents of the Y-side A-phase current transformer of the Y-side A-phase current transformer at 7.2s to 7.4s after the inrush current disappears in the case of DC bias (per unit value, the secondary side current is converted to the primary side );

Figure 11 is the hysteresis curve of the A-phase iron core of T1 under the condition of DC bias. Hysteresis curve of phase A iron core;

Fig. 12 is a schematic flowchart of a new method for preventing partial transient saturation of the current transformer after the occurrence of the transformer and the disappearance of the inrush current, resulting in differential protection misoperation, taking into account the influence of the DC bias on the current transformer in the embodiment of the present invention.

detailed description

The technical solutions of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

A new method that takes into account the influence of DC bias magnetic field on the current transformer and prevents partial transient saturation of the current transformer when inrush current occurs in the transformer, resulting in misoperation of differential protection.

As shown in Fig. 12, it is a flow chart of the method for preventing DC bias from causing current transformer saturation to cause misoperation of transformer differential protection. . The following takes the transformer wiring mode as Y/d11 as an example to introduce in detail, and the method includes the following steps:

(1) Collect the instantaneous current value of the secondary side of the three-phase current transformer on the star side of the transformer and the instantaneous value of the current value on the secondary side of the three-phase current transformer on the corner side;

(2) Real-time calculation of the variation ΔI _φ of any phase current on the star side and the corner side of the transformer. If the variation of any phase current on the star side and the corner side of the transformer satisfies the start-up criterion of the sudden change of the transformer, the differential protection starts and enters step (3);

Set the sudden start criterion of the phase current variation ΔI _φ :

ΔI _φ >I _QD (1)

The calculation formulas of phase current variation ΔI _φ and starting current I _QD are:

Among them, it is assumed that one cycle is sampled at 24 points, φ refers to the three phases A, B, and C, i _φ (k) is the instantaneous value of the phase current of k sampling points φ phase, and I _e is the rated current converted to the star side of the transformer , i _φ (k), I _e are the current value of the secondary side of the current transformer;

(3) Calculate the star-side three-phase current mutual inductance according to the instantaneous current value of the secondary side of the three-phase current transformer on the star side of the transformer collected in step (1) and the instantaneous value of the current value on the secondary side of the three-phase current transformer on the corner side The current phasor value on the secondary side of the And the current phasor value of the secondary side of the three-phase current transformer on the corner side Calculate transformer three-phase differential current I _d (A), I _d (B), I _d (C) and transformer three-phase brake current I _r (A), I _r (B), I _r (C):

in, is the current value of the secondary side of the three-phase current transformer on the star side of the transformer; is the current value of the secondary side of the three-phase current transformer on the corner side of the transformer;

(4) If the differential current of any phase of the transformer is greater than the preset minimum operating current, calculate the second harmonic content of the differential current of this phase, the formula is as follows:

Among them, i _d.second (φ,k), i _d.fundmental (φ,k) are the values of the second harmonic and fundamental wave of the k-th sampling point φ-phase differential current respectively, and I _op.min is the minimum action Current, take I _op.min = 0.2I _e ;

Compare the second harmonic content of this phase, if it is higher than 15%, then block the differential protection; if it is lower than 15%, then enter step (5);

(5) Modify the traditional ratio braking characteristic curve shown in Figure 1, as shown in Figure 1, in the traditional ratio braking curve, I _op.min is the minimum operating current, and I _op.min = 0.2I _e ; I _res.1 is the braking current of the first inflection point, take I _res.1 = I _e ; I _res.2 is the braking current of the second inflection point, take I _res.2 = 3I _e ; take the first The inflection point is the center o, r=0.15I _e is the radius to make a circle, intersect with the original ratio braking characteristic curve at points b and c, and the curve abcde is the modified ratio braking characteristic curve, as shown in Figure 2.

(6) After the protection device detects that the second harmonic content of the phase is less than 15%, it judges whether the downward trend of the second harmonic content of the phase within half a cycle is gentle, that is, whether it satisfies formula (6):

P _2nd (φ,k-12)-P _2nd (φ,k)＜1% (6)

If all four sampling points in a row satisfy the formula (6), then the second harmonic trend criterion is satisfied, and it is considered that the local transient saturation of the current transformer caused by the DC bias causes the second harmonic content to decrease. After opening the correction According to the ratio braking characteristic curve, it is judged whether the differential protection has entered the action area according to the modified ratio braking characteristic curve; The inrush current subsides, or the internal/external fault causes the second harmonic content of the differential current to drop sharply, and the ratio braking characteristic curve is not opened. According to the traditional ratio braking characteristic curve (shown in Figure 1), it is judged whether the differential protection is Enter the action zone.

The following further introduces the technical solution of the method disclosed by the present invention for preventing the current transformer saturation caused by the DC bias magnetic field and causing the maloperation of the transformer differential protection through a specific embodiment.

The degree of saturation of the current transformer is related to its parameter setting. In this case, the parameters of the current transformers of phase B and C are set so that no saturation occurs (that is, the differential protection can be reliably blocked), and only the current transformer saturation of phase A is made Detailed analysis shows that this simplified analysis does not affect the reliability and correctness of the application of the present invention. By adopting the new method proposed by the present invention and correcting the ratio braking characteristic curve, considering the influence of the DC bias on the current transformer, the judgment of the differential protection action after the transformer generates and responds to the inrush current is as follows:

1) Construct the simulation model of the single-ended system, as shown in Figure 3, the wiring modes of the two transformers T1 and T2 are both Y/d11, and the rated voltage is 220kV:35kV. First, when there is no DC bias current intrusion at the neutral point, T2 closes in 1s, and T1 generates a summation inrush current. The schematic diagram of the A-phase differential current waveform of T1 is shown in Fig. 4; Fig. 5 is 7.2s after the summation inrush current disappears The schematic diagram of the phase A differential current waveform of T1 in the ~7.4s section; the schematic diagram of the second harmonic content trend of the phase A differential current is shown in Figure 6.

2) When the T1 neutral point is connected to the voltage source to simulate the DC bias current flowing into the AC system, the amplitude of the DC bias current of each phase is 14A. T2 closes in 1s, and T1 generates a summation inrush current. The schematic diagram of the A-phase differential current waveform of T1 is shown in Figure 7; The schematic diagram of the second harmonic content trend of the phase differential current is shown in Figure 9; the comparison shows that when there is no DC bias current intrusion, the differential current of phase A gradually decays after the inrush current disappears, and the amplitude is close to 0 ; And when the DC bias current invades, the amplitude of the differential current is still greater than the minimum operating current (0.2p.u.), and the second harmonic content is lower than the threshold of 15%.

3) After the inrush current disappears, the current comparison diagram of the primary and secondary side of the current transformer on the Y side of phase A of T1 from 7.2s to 7.4s is shown in Figure 10 (the current on the secondary side is converted to the primary side), and it can be clearly seen that It is found that after the inrush current disappears, the secondary side current changes in amplitude and phase, which causes the differential current not to decay to 0, which is approximately equal to 0.25p.u.; the braking current is equal to the load current, which is approximately 1 times the rated current current. At this time, if the ratio braking characteristic shown in Figure 2 is adopted, it will just fall into the action area, causing the protection not to move.

4) Based on the fact that there is a DC bias magnetic intrusion into the AC system, and after the inrush current disappears, the differential current amplitude is still higher than the minimum operating current, and the second harmonic content is lower than 15%, but its change is gentle, within half a cycle The amplitudes are all far less than 1%, that is, the continuous judgment of the four sampling points all satisfies P _2nd (φ,k-12)-P _2nd (φ,k)<1%. Therefore, it is judged that the local transient saturation of the current transformer is caused by the DC bias, resulting in the amplitude difference and phase difference of the primary and secondary side currents, resulting in the differential current being greater than the minimum operating current, and the waveform distortion is not obvious, making Its second harmonic content is low. Figure 11 is the hysteresis curve of the A-phase iron core of T1, the left figure is the hysteresis curve of the A-phase iron core of T1 from 1s to 8s, and the right figure is the hysteresis curve of the A-phase iron core of T1 from 7.2s to 7.4s curve.

5) Open the modified ratio braking characteristic curve, the differential current amplitude is 0.25p.u., and the braking current amplitude is 0.9p.u. According to the modified ratio braking characteristic curve, it is judged to fall into the braking area, and the differential protection does not operate .

The applicant has explained and described the embodiments of the present invention in detail in conjunction with the drawings and tables of the description, but those skilled in the art should understand that the fault information scores and correction coefficients of the influencing factors in the above embodiments are only preferred implementations of the present invention Under the inventive idea of the present invention, those skilled in the art can reasonably select or modify the fault information score and the correction coefficient of the influencing factors according to the specific generator set excitation system model and actual working conditions. In a word, the detailed description of this application is only to help readers better understand the spirit of the present invention, but not to limit the protection scope of the present invention. On the contrary, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention. within the scope of protection.

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 I_QDIf 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, I_op.minFor minimum operating current, take I_op.min＝0.2I_e；I_res.1For the first knee braking current, take I_res.1＝I_e；I_res.2For the second knee braking current, take I_res.2＝3I_e(ii) a With the first inflection point o as the center, r is 0.15I_eMaking 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 I_eRated 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 I_QDAnd 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 I_QD＝0.2I_eIn which I_eTo 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%.