CN105021920A - Multiple characteristic identification method of excitation inrush current of main transformer - Google Patents

Abstract

The invention relates to a multi-feature identification method for main transformer excitation inrush current, belonging to the technical field of electric power system relay protection. The multi-feature identification method for main transformer excitation inrush current of the present invention is used to discriminate the second harmonic, discontinuity angle and CT saturation characteristics of differential current waveform. The content will be very high, which can be blocked by the second harmonic; the discontinuity angle is relatively small, and the second harmonic content is relatively low. At this time, the CT saturation feature recognition is added. The present invention uses the above-mentioned multi-feature recognition method to identify the excitation inrush current, so that the differential protection It does not malfunction when the main transformer is airdropped normally, and acts quickly when the faulty transformer is airdropped, improving the reliability of the main transformer protection device.

Description

A multi-feature identification method for main transformer excitation inrush current

technical field

The invention relates to a multi-feature identification method for main transformer excitation inrush current, belonging to the technical field of electric power system relay protection.

Background technique

The relay protection device requires that the faulty equipment can be reliably removed when the system fails to ensure the safe and stable operation of the system, but the damage caused by the incorrect action (including refusal and misoperation) of the protection device is also huge.

In recent years, there has been a phenomenon of misoperation of the main transformer differential protection when the 500kV autotransformer is air-dropped. Through the analysis of the recorded wave form when the main transformer is air-dropped, it is found that the second harmonic content of the inrush current is very small, only about 12%, and the discontinuity angle is about 90 degrees. This situation usually occurs when the main transformer is overhauled and put into operation. The reason is that after detecting the DC resistance of the main transformer winding, the main transformer iron core is not degaussed according to the maintenance regulations, resulting in a large residual magnetism in the main transformer iron core. If at the same time Airdropping the main transformer near the voltage zero-crossing point, and the increase direction of the magnetic flux is consistent with the direction of the residual magnetism, will cause a large excitation inrush current, and the waveform is close to a sine wave, and the second harmonic content is very low.

The way to prevent misoperation of differential protection when airdropping the main transformer is to require the maintenance personnel to demagnetize the main transformer iron core after maintenance in strict accordance with the maintenance procedures; Identify the excitation inrush current waveform, so as to prevent the differential protection from malfunctioning when the main transformer is air-dropped. At present, the identification method of excitation inrush current mostly adopts the 2nd harmonic identification or waveform identification criterion. Under low remanence, the excitation inrush current can be correctly identified and the protection will be blocked. However, under the condition of high remanence, the content of 2nd harmonic is relatively low. At the same time The waveform is close to a sine wave, and there is a risk of malfunction of the excitation inrush protection.

Contents of the invention

The purpose of the present invention is to provide a multi-feature identification method for main transformer excitation inrush current, so as to solve the problem of malfunction of the main transformer protection device when the main transformer is air-dropped with high remanence.

In order to solve the above technical problems, the present invention provides a multi-feature identification method for main transformer excitation inrush current. The identification method includes the following steps:

1) Collect the current on each side of the transformer, and synthesize the differential current, calculate the current amplitude of the differential current of each phase, and judge whether the differential current amplitude of each phase is greater than the criterion start threshold;

2) If yes, calculate the second harmonic content of the differential current of the phase, and judge whether the second harmonic content is greater than the maximum set value or less than the minimum set value, if not, perform a differential operation on the differential current;

3) Calculate the discontinuity angle of the current waveform according to the differential current after the difference, judge whether the discontinuity angle meets the requirements, and judge whether the CT is saturated when the requirements are met, and if the CT is not saturated, determine the excitation inrush current, and block the differential protection. Otherwise, open the differential protection.

Described step 3) the discriminating of discontinuity angle is to realize by judging how many sample points have to satisfy discontinuity angle discriminant formula, discontinuity angle discriminant formula is:

Δi _K ＜0.2min(|i _max |,I _n )

Among them, Δi _K is the current sampling value after the difference, i _max is the maximum value of the current cycle difference, I _n is the rated current of the CT, for the sampling of N points of the cycle cycle, if the continuous 13N/72 sampling points satisfy the above formula, then the judgment is satisfied for the discontinuity angle.

CT saturation in said step 3) is realized by judging whether the sign of the waveform maximum point is the same as that of the previous sampling point. If they are the same, it is judged as CT unsaturated, otherwise, it is considered as CT saturation.

In the step 3), if the discontinuity angle of the differential current satisfies and does not meet the CT saturation criterion, it is judged as an exciting inrush current and the differential protection of this phase is blocked; otherwise, it is judged as a non-exciting inrush current and the differential protection of this phase is enabled.

In the step 2), if the second harmonic content is greater than the maximum set value, it is considered that the phase current is inrush current and the differential protection of this phase is blocked; if the second harmonic content is less than the minimum set value, the current of this phase is considered normal and Open the real phase differential protection.

In the step 2), the highest setting value is 30%, and the lowest setting value is 7.5%.

The difference equation that described step 2) adopts is:

Δi _K ＝C(i _K -i _K-2 )

Among them, i _K is the sampling value of the current point, i _K-2 is the sampling value of the first two points, Δi _K is the current sampling value after difference, C is the coefficient, which is related to the number of sampling points N, C=N/4π.

If the amplitude of the differential current in the step 1) is not greater than the threshold for starting the criterion, then the current of this phase is considered normal and the differential protection of this phase is enabled.

The beneficial effects of the present invention are: the main transformer excitation inrush multi-feature identification method of the present invention is used to discriminate the second harmonic, discontinuity angle and CT saturation characteristics of the differential current waveform, when excitation inrush occurs, the inrush waveform discontinuity angle When the second harmonic content is relatively large, the second harmonic content will be very high, which can be blocked by the second harmonic; the second harmonic content is relatively low when the discontinuity angle is relatively small. At this time, the discontinuity angle and CT saturation feature recognition are increased. The present invention adopts the above-mentioned multi-feature recognition The method is used to identify the excitation inrush current, so that the differential protection will not malfunction when the main transformer is normally air-dropped, and it will act quickly when the faulty transformer is air-dropped, so as to improve the reliability of the main transformer protection device.

Description of drawings

Figure 1 is an airdrop current distribution diagram on the high voltage side of an autotransformer.

Fig. 2 is a flow chart of a multi-feature identification method for main transformer excitation inrush current;

Figure 3 is the excitation inrush current waveform and its differential waveform when the main transformer is airdropped with high remanence;

Figure 4 is the fault CT saturation waveform and its differential waveform diagram in the time zone when one side of the main transformer has power supply.

Detailed ways

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

The multi-feature identification method of the main transformer excitation inrush current is mainly used in the high-voltage relay protection device of the smart grid. The second harmonic of the time-varying excitation inrush current is too small to cause misoperation of the differential protection and improve the reliability of the main transformer protection device. The differential protection current of the main transformer is taken from the high-voltage side CT, the medium-voltage side CT and the low-voltage side CT of the transformer. The current distribution of the high-voltage side during airdrop is shown in Figure 1.

As an independent functional module, the multi-feature recognition method for main transformer excitation inrush current is integrated in the main transformer protection device, and this functional module is always on. First judge the magnitude of the differential current, if the differential current is greater than the judgment threshold, carry out the second harmonic judgment, otherwise open the differential protection; judge whether the second harmonic content is less than 30%, if it is not satisfied, directly block the differential protection, if it meets the judgment Whether the second harmonic content is greater than 7.5, if not, open the differential protection directly, otherwise proceed to the next step; perform differential processing on the differential current waveform to determine the discontinuity angle of the current waveform, if the discontinuity angle is greater than the set angle (this embodiment is 65 degrees), and at the same time the sign of the maximum point of the waveform is the same as that of the previous sampling point (in this embodiment, it refers to the symbol of the 4th point in the front), it is determined to be the excitation inrush current, and the differential protection is blocked, otherwise the differential protection is opened. The feature of this criterion is that it can distinguish the characteristics of CT saturation and excitation inrush current. Although both CT saturation and excitation inrush current have discontinuity angles, the difference between the two is that the secondary current rapidly drops to near 0 when CT is saturated, and the secondary current after entering steady-state saturation The subharmonic content is extremely small; when the excitation inrush current has a large remanence and the closing angle is near the voltage zero-crossing point, the continuous part of its waveform is close to a sine wave, and there will be no instantaneous change of the current waveform slope, which is related to the system impedance and transformer winding. related to reactance.

This functional module judges the differential current waveform discontinuity angle and CT saturation characteristics after the difference, as shown in Fig. 3 and Fig. 4 . Since the excitation inrush current waveform is discontinuous when the transformer is air-dropped, and the maximum point of the current change rate is far from the minimum point, which is greater than the interval of 4 sampling points, the fourth sampling point in front is selected to improve the reliability of the algorithm when judging the CT saturation state, because CT After saturation, the current will change suddenly, but due to the number of sampling points per cycle, CT secondary leakage reactance and sampling filter circuit combined with the factors of the differential algorithm itself, the 1-2 points in front of the maximum value of the differential waveform may not necessarily be inverse signs, so consider With a certain margin, the reliability of the algorithm can be improved without negative impact on the algorithm by selecting the fourth sampling point in front. When the CT is saturated, the current quickly drops to 0, and the sign of the slope at the fourth point before the maximum rate of change is opposite to that of the point with the maximum slope. Therefore, these characteristics can be used to identify whether it is an inrush current for empty charging.

The specific steps of the multi-feature identification method for main transformer excitation inrush current of the present invention are as follows:

1. First collect and calculate the differential current amplitudes of A, B, and C three-phase currents, and judge whether the differential current amplitudes of each phase are greater than the start-up threshold. The discriminant equation used is:

|I _dΦ |＞0.16I _B

Among them, I _dΦ is the magnitude of any phase current of differential current A, B, C, and I _B is the reference side current of the main transformer. If it is not greater than the starting threshold, the differential protection of this phase is opened.

2. If the amplitude of the differential current is greater than the start-up threshold, then calculate the second harmonic content of the differential current of this phase, and judge whether the second harmonic content is greater than the maximum set value or less than the minimum set value, the highest set value in this embodiment The setting value is 30%, and the minimum setting value is 7.5%. When the second harmonic content of the differential current is greater than 30%, the differential protection of this phase will be blocked directly. When the second harmonic content of the differential current is less than When 7.5%, open the differential protection directly.

3. If the second harmonic content of the differential current is between 7.5% and 30%, the differential current waveform is differentiated, and the differential equation used is:

Δi _K ＝C(i _K -i _K-2 )

In the above formula, i _K is the sampling value of the current point, i _K-2 is the sampling value of the first two points, Δi _K is the current sampling value after the difference, C is a coefficient, which is related to the number of sampling points N, C=N/4π.

4. For the differential current after the difference, the discontinuity angle is judged and whether there is a CT saturation feature. The judgment of the discontinuity angle is realized by judging how many sampling points satisfy the discontinuity angle discrimination formula. The discontinuity angle discrimination formula is:

i _K ＜0.2min(|i _max |,I _n )

In the above formula, i _max is the maximum value of the cycle wave difference current, I _n is the CT rated current, for the cycle wave N point sampling, if the continuous 13N/72 sampling points satisfy the above formula, it is judged that the discontinuity angle is satisfied; for 80 For point sampling, 14 consecutive sampling points should satisfy the discontinuity angle discriminant formula.

The method of discriminating the non-CT saturation feature is: judging whether the maximum value i _max of the cycle wave difference current is consistent with the sign of the 4th point i _x in front of it, the formula is as follows:

i _x i _max ＞0

If the above formula is satisfied, it is judged not to be CT saturation, otherwise, it is considered to be CT saturation.

5. Continuously compare the sampling points of one cycle (80 sampling points per cycle in this embodiment), if the above conditions are met, it will be judged as excitation inrush current, and the differential protection will be blocked; otherwise, the differential protection will be opened.

The multi-feature identification method of main transformer excitation inrush current conducts phase-separated discrimination on the second harmonic, discontinuity angle, and CT saturation characteristics of the differential current waveform. When the current waveform of a certain phase of the differential current meets the blocking condition, only the differential protection of this phase is blocked, and other phases are not blocked. Phase differential protection. When the differential current amplitude is less than the criterion start threshold, the discrimination of steps 2 to 5 is not performed; if the second harmonic content is less than 7.5% or greater than 30%, the discrimination of steps 3 to 5 is not performed, In order to reduce the computational workload of the protective device.

Claims (8)

1. a main transformer excitation surge current multi-feature recognition method, is characterized in that, this recognition methods comprises the following steps:

1) gather the electric current of each side of transformer, and synthesize difference current, calculate the current amplitude of each phase difference current, judge whether the difference current amplitude of each phase is greater than criterion and starts threshold;

2) if so, calculate the secondary harmonic component of this phase difference current, judge whether its secondary harmonic component is greater than the highest setting value or is less than minimum setting value, if not, then calculus of differences is carried out to difference current;

3) calculate current waveform interval angle according to differentiated difference current, judge whether interval angle meets the demands, and determine whether that when meeting the demands CT is saturated; if CT is unsaturated; then be defined as excitation surge current, Blocking Differential Protection, otherwise open differential protection.

2. main transformer excitation surge current multi-feature recognition method according to claim 1, is characterized in that, described step 3) in interval angle differentiation by differentiate have how many sampled points to meet interval angle discrimination formula to realize, interval angle discrimination formula is:

Δi _K＜0.2min(|i _max|,I _n)

Wherein Δ i _kfor differentiated current sampling data, i _maxfor this cycle difference stream maximal value, I _nfor CT rated current, for every cycle N point sampling, if 13N/72 sampled point meets above formula continuously, be then judged to interval angle and meet.

3. main transformer excitation surge current multi-feature recognition method according to claim 2, is characterized in that, described step 3) in CT saturated be realize by judging that whether waveform maximum point identical with the symbol of preceding sample point, if identical, then be judged as that CT is unsaturated, otherwise, think that CT is saturated.

4. main transformer excitation surge current multi-feature recognition method according to claim 3, is characterized in that, described step 3) if in difference stream interval angle meet and do not meet the saturated criterion of CT, be judged to excitation surge current and this phase of locking differential protection; Otherwise be judged to non-excitation surge current, open this phase differential protection.

5. main transformer excitation surge current multi-feature recognition method according to claim 1; it is characterized in that; described step 2) if in secondary harmonic component be greater than the highest setting value; then think this phase excitation surge current this phase of locking differential protection; if secondary harmonic component is less than minimum setting value, then think that this phase current normally also opens this phase differential protection.

6. main transformer excitation surge current multi-feature recognition method according to claim 5, is characterized in that, described step 2) in the highest setting value be 30%, minimum setting value is 7.5%.

7. main transformer excitation surge current multi-feature recognition method according to claim 1, is characterized in that, described step 2) difference equation that adopts is:

Δi _K＝C(i _K-i _K-2)

Wherein i _kfor current point sampled value, i _k-2for front 2 point sampling values, Δ i _kfor differentiated current sampling data, C is coefficient, relevant with sampling number N, C=N/4 π.

8. main transformer excitation surge current multi-feature recognition method according to claim 1, is characterized in that, described step 1) in difference current amplitude be not more than criterion and start threshold, then think that this phase current is normally and open this phase differential protection.