CN102005744B - Method for forming differential current based on current slow attenuation of differential current zero crossing identification CT (computed tomography) - Google Patents

Abstract

The method for identifying the slow decay of CT current to form a differential current based on the zero crossing of the differential current includes the following steps: 1) Calculate the three-point integral value S(n)=x(n)+x(n-1)+x( n-2), and compare it with the three-point integral value S(n-1) of the previous point difference flow. If S(n)*S(n-1)<0, it is judged that the current point is a zero-crossing point; 2) If there are more than two zero-crossing points within the time T, it is judged as an internal fault, otherwise the differential protection is blocked. The invention realizes that the differential protection does not malfunction when the slow decay of the CT current causes differential current when the external fault is removed.

Description

The method of identifying the slow decay of CT current to form a differential current based on the zero crossing of the differential current

technical field

The invention relates to a method for identifying differential current formed by slow decay of CT current in a power system.

Background technique

At present, there are two main types of CTs used for relay protection in domestic power systems: ordinary P-level CTs and TP-level CTs considering transient characteristics.

P-level CT adopts closed iron core without air gap, and the manufacturing material is usually cold-rolled silicon steel sheet, which has a small magnetic permeability. This type of CT can only guarantee the steady-state error, and its accurate characteristics are determined by the composite error when the primary current is the steady-state current. When a short-circuit fault occurs in the transient process of a system, non-periodic components will appear in the excitation current of CT, and the amplitude of this current can be up to dozens of times the amplitude of the steady-state excitation current. Under the action of this current, the iron core of the CT will quickly enter a saturated state after a period of time. At this time, the CT cannot truly transmit the primary current, and its secondary current waveform will be distorted. For differential protection, a large differential current will be generated, which will cause differential protection to malfunction. In addition, in order to reduce the core loss of CT, the iron core material generally adopts silicon steel sheets with narrow hysteresis loops, but the remanence of such silicon steel sheets is usually still relatively large. After the primary system cuts off the short-circuit current, the residual magnetism of the human body remains in the iron core; after the faulty branch recloses, if the short circuit occurs again, and the direction of the residual magnetic flux is the same as that generated by the non-periodic component, the iron core will If it reaches saturation more quickly, the secondary current will be seriously distorted, and the differential protection is more likely to malfunction.

In high-voltage and ultra-high-voltage systems, due to the large capacity of the single machine and the large cross-section of the transmission line conductor, the time constant of the primary system is relatively large, even up to 0.25--0.5 seconds, resulting in slow attenuation of the non-periodic component of the short-circuit current, and the amplitude of the short-circuit current It is usually larger and can reach more than 30 times the rated current of the CT. Due to the above-mentioned shortcomings of the P-class CT, the TPY-class CT is mostly used in systems above 500kV.

However, the use of TPY-level CT will have the following problems: TPY-level CT has better transient characteristics than P-level CT. In order to achieve the above indicators, there is an air gap in the TPY-level CT core, so its remanence is much smaller than that of P-level CT. It is generally stipulated that the residual magnetism of TPY class CT should not exceed 10% of the saturation flux. When an out-of-area fault occurs in the system, the circuit breaker generally cuts off the fault at the zero-crossing point of the primary current, and at this time the CT magnetic flux is generally the largest. Since the residual magnetism of the TPY-level CT is small, according to the principle of magnetic flux continuity, the magnetic flux cannot be mutated. Due to the large attenuation time constant, the magnetic flux will decay slowly, so a DC component that decays slowly will also be generated in the secondary circuit. , 2005. The attenuated DC component is the differential current for the differential protection, which may cause the differential protection to malfunction.

Contents of the invention

The technical problem to be solved by the present invention is: to solve the problem that the slowly attenuating DC component on the secondary side of the CT when the primary current of the TPY level CT is cut off may cause a malfunction of the differential protection.

The technical solution adopted by the present invention to solve its technical problems is as follows:

The method for identifying the slow decay of CT current to form a differential current based on the zero crossing of the differential current includes the following steps:

1) AC sampling: obtain each current sampling value of differential protection;

2) Differential filtering is performed on each current participating in the differential protection calculation. The specific calculation formula is y(n)=x(n)-x(n-2), x(n) represents the current sampling value at the current point, and x(n -2) is the current sampling value at 2 points before the current point, and y(n) represents the output after differential filtering;

3) Calculate the three-phase differential current by using the differential current of each branch;

4) Calculate the three-point integral value S(n)=x _d (n)+x _d (n-1)+x _d (n-2) of the current point difference flow, and compare it with the three-point integral value S of the previous point difference flow (n-1) for comparison, such as S(n)*S(n-1)<0, it is judged that the current point is a zero-crossing point; where x _d (n) represents the differential current value of the current point, x _d (n- 1) Indicates the differential current value of a point before the current point, x _d (n-2) represents the differential current value of two points before the current point;

5) If there are more than two zero-crossing points within the time T, it is judged as an internal fault, otherwise the differential protection is blocked.

Step 5) The time T is 17ms.

The beneficial effects of the present invention are as follows: the present invention provides a method for distinguishing faults in the power system area from differential currents caused by slow decay of CT current based on differential current crossing zero and intermittent characteristics, so as to ensure that the slow decay of CT current causes differential current when the external fault is removed. The differential protection does not malfunction during flow.

Description of drawings

Figure 1 shows the non-periodic components generated on the secondary side of the CT when the primary current of the TPY-level CT is cut off.

Detailed ways

As shown in Figure 1, when the primary current of a TPY-level CT is cut off, the slowly decaying DC component generated by the secondary side of the CT is also a slowly decaying DC component in the differential current, and its main feature is that the differential current is always biased by time On the shaft side, the differential current has no zero-crossing point, and the waveform of the differential current is discontinuous after passing through the differential. When an internal fault occurs, no matter whether there is branch CT saturation, the differential current is continuous, there is a zero-crossing point, and the waveform after the difference is uninterrupted, that is, there is a differential current around the zero-crossing point, so the present invention is mainly based on the above-mentioned characteristics To identify the differential current formed by the slow attenuation generated when the primary current is cut off, so as to avoid the misoperation of the differential protection.

Differential filtering is used to filter out the DC component of the differential current.

By scanning the three-point integral value of the differential current sampling value, when the three-point integral value of the current differential current sampling value has a different sign from the three-point integral value of the previous differential current sampling value, it is judged that there is a zero-crossing point.

Considering that the attenuation time constant of the fault in the zone is 120ms, when the difference current is full offset, there will be 2 zero-crossing points in the longest continuous 17ms of the difference current, so if there are more than 2 zero-crossing points in the continuous 17ms, it is judged as a fault in the zone , otherwise the differential protection is blocked.

Below in conjunction with example the present invention is described in further detail. However, the invention is not limited to the examples given.

1) AC sampling: obtain each current sampling value of differential protection.

2) Differential filtering is performed on each current participating in the differential protection calculation. The specific calculation formula is y(n)=x(n)-x(n-2), x(n) represents the current sampling value at the current point, and x(n -2) is the current sampling value at 2 points before the current point, and y(n) represents the output after differential filtering.

3) Calculate the three-phase differential current by using the differential current of each branch.

4) Calculate the three-point integral value S(n)=x _d (n)+x _d (n-1)+x _d (n-2) of the current point difference flow, and compare it with the three-point integral value S of the previous point difference flow (n-1) for comparison, such as S(n)*S(n-1)<0, it is judged that the current point is a zero-crossing point; where x _d (n) represents the differential current value of the current point, x _d (n- 1) represents the differential current value of a point before the current point, and x _d (n-2) represents the differential current value of two points before the current point.

5) Continuously scan the differential current, if there are more than 2 zero-crossing points in the 17ms data window, it is judged as an internal fault, otherwise the differential protection will be blocked.

Claims (2)

1. The method for identifying the slow decay of CT current to form a differential current based on differential current crossing zero, is characterized in that, comprising the following steps: 1) AC sampling: obtain each current sampling value of differential protection; 2) Differential filtering is performed on each current participating in the differential protection calculation. The specific calculation formula is y(n)=x(n)-x(n-2), x(n) represents the current sampling value at the current point, and x(n -2) is the current sampling value at 2 points before the current point, and y(n) represents the output after differential filtering; 3) Calculate the three-phase differential current by using the differential current of each branch; 4) Calculate the three-point integral value S(n)=x _d (n)+x _d (n-1)+x _d (n-2) of the current point difference flow, and compare it with the three-point integral value S of the previous point difference flow (n-1) for comparison, such as S(n)*S(n-1)<0, it is judged that the current point is a zero-crossing point; where x _d (n) represents the differential current value of the current point, x _d (n- 1) Indicates the differential current value of a point before the current point, x _d (n-2) represents the differential current value of two points before the current point; 5) If there are more than two zero-crossing points within the time T, it is judged as an internal fault, otherwise the differential protection is blocked. 2 . The method for identifying the slow decay of CT current to form a differential current based on the differential current crossing zero according to claim 1 , wherein the time T in step 5) is 17 ms.

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