Intelligent detection instrument for secondary alternating current loop of current transformer

Abstract

According to the invention, open circuit, virtual connection, grounding and multi-point connection problems of all loops of the secondary loop of the current transformer can be comprehensively detected, a sampling module is added, logic judgment is carried out through software programming, all branches of the current transformer are detected, instrument intelligence is realized through an algorithm, and the detection accuracy is improved. The method can prompt debugging personnel that a branch has a problem, better serves power debugging personnel, shortens debugging time, ensures stable operation of a power system, and can be popularized in the whole power industry.

Description

Technical Field

[0001] At present, there are two methods used in this technical field to energize the CT circuit, namely, testing the secondary circuit from the CT primary circuit and energizing the CT secondary circuit. The entire power industry uses this method for power-on detection, and the shortcomings of both methods have not been solved. This invention patent of the current transformer secondary AC circuit intelligent detection instrument completely solves all the problems existing in the CT secondary circuit, can provide a basis for CT secondary circuit fault analysis, and can be promoted as a standard detection method in the power industry. Background Art

[0002] The following consequences may occur if the secondary circuit of the current transformer (CT) is open, loosely connected, ungrounded, or grounded at multiple points: Consequences of an open CT circuit: 1) The secondary circuit of the current transformer must not be open during operation, because when the secondary side is open, its current is zero, so no magnetic flux can be generated to offset the effect of the primary side magnetic flux. The secondary side can sense a voltage of about 1000V, which is a danger to people and equipment. The safety of the current transformer and the iron core of the current transformer itself will also heat up and burn out.

[0003] 2) The transformer core is saturated, the measurement is inaccurate, the CT ratio difference and angle difference increase, the protection fails to operate properly, and the accident expands.

[0004] There are many secondary circuit branches of the current transformer in the substation. When the secondary circuit wiring is in poor contact, the metering, measurement and protection sampling values ​​will be too small. When a short circuit accident occurs, it is easy to cause the protection to malfunction, causing the accident to expand. The false connection for a long time may develop into an open circuit.

[0005] One end of the secondary side of the current transformer must be grounded to prevent the high voltage on the primary side from flowing to the secondary side when the insulation between the primary and secondary coils breaks down, endangering the safety of people and equipment.

[0006] 1) The basic functions of current transformers are generally signal conversion and isolation. Because of the isolation, the secondary circuit is a floating system before grounding, that is, its potential has no fixed relationship with the ground potential. Single-point grounding is equivalent to giving grounding A reference potential is set at a point, which does not form a loop, short circuit, or affect the measurement. If multiple points are grounded, and there is a potential difference between these grounding points before grounding, then a loop will be formed after grounding, which is equivalent to a short circuit, causing errors in the measurement results. It is a safety requirement to ground the secondary circuit of the current transformer at one point.

[0007] 2) Multiple-point grounding may cause short circuit. The grounding grid of the switch field is not an ideal equipotential surface. When a large grounding current is injected into the ground grid, a large power frequency potential difference will appear between different grounding points. When grounding, if the grounding points are located on both sides of the current relay line, when a ground short circuit occurs, the ground potential difference between the two grounding points will generate additional current in the current coil. Especially for electrically connected circuits (such as differential protection), for example: there are two points of grounding, when a ground fault occurs externally, the ground potential difference will cause the current on one side to increase or decrease, which will lead to an increase in the differential circuit current and cause the protection device to malfunction.

[0008] Conclusion: How to efficiently detect open circuit and virtual connection of the secondary circuit of the current transformer, whether it is grounded and whether there are multiple grounding points, ensure the correct wiring of the current transformer circuit, and ensure that there are no safety hazards after the current transformer is put into operation has become an urgent problem to be solved. Summary of the invention

[0009] 1.1 Prior art related to the present invention Existing technology: 1 CT is powered on to detect the secondary circuit. A large current generator is used to connect the instrument output to P1 and P2 respectively. Windings 1S and 2S have outputs at the same time. This method is used to check the secondary circuit of the current transformer.

[0010] Figure 1 Schematic diagrams and physical drawings of related equipment.

[0011] Existing technology: 2 Check the secondary circuit of the current transformer through the relay protection instrument. The instrument wiring method is as follows: Figure 2 As shown in the figure, most samples are tested using this method. Use a relay protection instrument to pass current through IA, IB, and IC from the root of CT. The amplitude is 1A, 2A, and 3A. The protection (metering, measurement, etc.) device displays the current size as 1A for phase A, 2A for phase B, and 3A for phase C. Use this result to determine whether the circuit is correct. Figure 2 The schematic diagram and wiring diagram are shown in the attached figure.

[0012] Currently, there are only two methods for detecting the power-on of the secondary circuit of a current transformer (CT), namely the primary current method and the secondary current method.

[0013] 1. Disadvantages of testing the secondary circuit from the CT primary power supply (two testing points) 1) If the CT circuit is open, it is easy to give electric shock to the commissioning personnel; 2) The instruments and equipment required for a single flow are large and difficult to carry; 3) When the power is on, it is impossible to find out whether the CT circuit is reliably grounded or whether there are multiple grounding points; 4) The function of each winding cannot be determined by a single current, and all windings have current at the same time (metering, measurement, protection, bus differential, etc.).

[0014] 5) It is not possible to find that there is a false connection in the CT secondary circuit branch because the CT transformation ratio is too large.

[0015] 2. Disadvantages of CT secondary circuit flow method (4 detection points) 1) It is not possible to find that there is a false connection in the CT secondary circuit branch. The reason is that the output of the relay protection device (capacity is 150VA) is controlled by a single-chip microcomputer, and the problem of increased loop impedance cannot be found.

[0016] 2) Use a relay protection device to energize the CT terminal: It can only detect whether the wiring of the secondary circuit is correct, but cannot guarantee the integrity of the entire circuit. Figure 2 As shown, it is not possible to find that NA401 and NC401 are open circuited) 3) When the power is on, it is impossible to find out whether the CT circuit is reliably grounded or whether there are multiple grounding points; 1.2 Technical solution of the present invention (invention solution) Technical solution (11 detection points): Detection method principle: The patent of this invention uses Ohm's law and node current and voltage method to detect the sensitivity of current to impedance. The principle of high impedance is that when current flows through parallel branches, the current value of each branch is the same under the premise of the same impedance. If there is an open circuit or virtual connection in the circuit, the impedance value increases, the current value of this branch will decrease significantly, and the current values ​​of other branches will increase. The current flowing into and out of each branch is the same and the voltage of IN is measured by the current output IA, IB, IC, respectively, to achieve current Conduct a comprehensive inspection of the transformer secondary circuit to ensure that all branch circuits are wired correctly.

[0017] 1 If the CT circuit is wired correctly, use a relay protection instrument to pass currents of 1A, 2A, and 3A through IA, IB, and IC from the root of the CT. The protection (metering, measurement, etc.) device will display the current size as 1A for phase A, 2A for phase B, and 3A for phase C. Under ideal normal circuit conditions (each loop impedance is equal), NA401, NB401, NC401, and NN401 (two grounding ammeters) all show a current of 1.5A.

[0018] 2 If the ammeter and voltage indications are inaccurate, the corresponding analysis is as follows: 2.1 If the current indication of a branch of the current transformer is zero, it means that the branch is open. Figure 2 When the power is turned on in the wiring state, the readings of NA401 and NC401 are 0A, while the readings of NB401 and NN401 are 3A. It is found in time that the A phase in the circuit There is an open circuit problem with the C phase CT.

[0019] 2.2 If the current indication of a branch of the current transformer decreases and is not zero, while the current of other branches increases, it means that there is a virtual connection in the branch circuit.

[0020] 2.3 Measure the voltage of current output IA, IB, IC to IN respectively. The voltage meets the condition of UA<UB<UC. If this condition is not met, there is a virtual connection in the branch with higher voltage.

[0021] 2.4 If the grounding ammeter has no indication, it means that the CT circuit is not grounded. If the indications of the two grounding ammeters are not equal, it means that there are multiple grounding points in the CT circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 : Physical diagram and principle diagram of CT primary side power supply instrument Figure 2 : CT secondary side schematic and wiring diagram Figure 3 : This time Fang Ming’s schematic diagram and actual pictures of the instruments.

Claims

1. Intelligent detection instrument for current transformer secondary AC circuit: This patented invention can effectively detect all hidden dangers of open circuit, false connection, ungrounded and multi-point grounding in the transformer secondary circuit.

2. Intelligent detection instrument for current transformer secondary AC circuit: Through software algorithms, all 19 alarms of current transformer secondary AC circuit can be predicted, and new national standards can be formulated.

Images