A zero-sequence current transformer secondary circuit disconnection detection device and method

Abstract

The present invention discloses a zero-sequence current transformer secondary circuit disconnection detection device and method, including a DC generator, a voltage and current collector, a central processing unit, a display setting unit and two current lines, wherein the DC generator is used to generate DC voltage and DC current; the voltage and current collector converts the DC voltage and DC current analog quantities into digital quantities; the central processing unit obtains the corresponding resistance value according to the digital quantities of the DC voltage and DC current, and judges whether the CT secondary circuit is disconnected by the DC current magnitude and resistance value; the display setting unit displays the magnitude and judgment result of the measured DC voltage and DC current, and is used to set the amplitude and length of the pulse; one end of the two current lines are connected to the DC generator, and the other end are inserted into the third current terminal and the fourth current terminal of the zero-sequence current transformer secondary circuit. The present invention can regularly check the zero-sequence circuit in operation, timely discover the CT disconnection fault, and avoid the accident of protection refusal.

Description

Technical Field

[0001] The invention relates to a current transformer secondary circuit detection device, and in particular to a zero-sequence current transformer secondary circuit disconnection detection device and method. Background Art

[0002] With the increase of capacitive current in the distribution network, the low-voltage side of the substation in coastal areas has been transformed from arc suppression coil grounding to low-resistance grounding. Zero-sequence current transformers (abbreviated as zero-sequence CT) are installed on each outgoing line to connect the zero-sequence current to the protection device. The protection device is also upgraded to have the function of zero-sequence overcurrent tripping. Since the zero-sequence current is basically zero during normal operation, all current protection devices cannot detect whether the zero-sequence circuit is broken. If the secondary side of the zero-sequence CT is broken, when a single-phase grounding occurs in the line, it will cause the outgoing line protection to refuse to operate, and the main transformer protection will be over-operated, which is a serious consequence. Therefore, it is necessary to develop a device that can detect whether the secondary circuit of the zero-sequence CT is broken, regularly check the zero-sequence circuit in operation, promptly detect the CT break fault, and avoid the accident of protection refusal. Summary of the invention

[0003] The object of the present invention is to provide a zero-sequence current transformer secondary circuit disconnection detection device and method, which can regularly check the running zero-sequence circuit, timely discover the CT disconnection fault, and avoid the accident of protection failure.

[0004] The technical scheme for realizing the object of the present invention is: a zero-sequence current transformer secondary circuit disconnection detection device, wherein the zero-sequence current transformer secondary circuit comprises a zero-sequence CT, a first current terminal, a second current terminal, a third current terminal, a fourth current terminal, a first current connecting piece, a second current connecting piece and a protection device, wherein the secondary terminal of the zero-sequence CT is connected to the first current terminal through a secondary cable, the first current terminal and the second current terminal are connected through a first current connecting piece, the second current terminal is connected to the protection device, the protection device is connected to the fourth current terminal, the third current terminal and the fourth current terminal are connected through a second current connecting piece, and the third current terminal is connected to another secondary terminal of the zero-sequence CT through a secondary cable, wherein the device comprises a DC generator, a voltage and current collector, a central processing unit, a display setting unit and two current lines, wherein:

[0005] The DC generator is used to generate DC voltage and DC current, and output them to the voltage and current collector;

[0006] The voltage and current collector is used to convert the DC voltage and DC current analog quantities into digital quantities and output them to the central processing unit;

[0007] The central processor obtains the corresponding resistance value according to the digital values ​​of the DC voltage and the DC current, and determines whether the CT secondary circuit is disconnected according to the DC current and the resistance value;

[0008] The display setting unit is used to display the magnitude and judgment result of the measured DC voltage and DC current, and to set the amplitude and length of the pulse;

[0009] One end of the two current lines is connected to the DC generator, and the other end is directly inserted into the third current terminal and the fourth current terminal of the secondary circuit of the zero-sequence current transformer.

[0010] Furthermore, judging whether the CT secondary circuit is disconnected by the DC current and the resistance value is as follows: if the DC current and the resistance value are both within a set range, it is considered that the secondary circuit is normal; otherwise, the secondary circuit is disconnected.

[0011] Furthermore, it also includes an alarm device connected to the central processing unit, and the alarm device issues an alarm when the secondary circuit is disconnected.

[0012] Furthermore, the voltage and current acquisition accuracy of the voltage and current collector are within ±2mV and ±2mA respectively.

[0013] Furthermore, the DC generator generates a DC voltage of 10mV to 30mV.

[0014] Furthermore, the AC resistance in the DC generator is less than 1 ohm and can withstand an AC current of 5A.

[0015] A detection method based on the zero-sequence current transformer secondary circuit disconnection detection device comprises the steps of:

[0016] Insert the other ends of the two current wires of the zero-sequence current transformer secondary circuit break detection device into the current terminals at both ends of the first current connecting piece or the second current connecting piece of the zero-sequence current transformer secondary circuit respectively;

[0017] Start the device, self-test is passed, and AC current flows through the device;

[0018] Open the first current connection piece or the second current connection piece to start testing;

[0019] Record the test results displayed on the display setting unit;

[0020] After the test is completed, the first current connecting piece or the second current connecting piece is closed, and the two current wires of the device plugged into the current terminals at both ends of the first current connecting piece or the second current connecting piece are unplugged.

[0021] Compared with the prior art, the present invention has the following significant effects: the present invention detects the conduction state of the secondary circuit by adding a pulse component to the zero-sequence secondary circuit at the current terminal block. The present invention does not require power outage of the primary equipment and can detect in a powered state. At the same time, it does not affect the operation of the zero-sequence tripping function of the protection device. According to the detection result, the disconnection defect of the zero-sequence secondary circuit can be discovered in time, avoiding the refusal of the protection at this level to operate when a ground fault occurs in the system. The present invention has a good application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the module of the device of the present invention.

[0023] Figure 2 This is a connection diagram of normal zero-sequence CT and protection device.

[0024] Figure 3 This is the wiring diagram during the test. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] The existing zero-sequence CT (zero-sequence current transformer) and the protection device are connected as follows: Figure 2 The secondary circuit of the zero-sequence current transformer comprises a zero-sequence CT, a first current terminal A1, a second current terminal A2, a third current terminal B1, a fourth current terminal B2, a first current connecting piece, a second current connecting piece and a protection device. The secondary terminal S1 of the zero-sequence CT is connected to the first current terminal A1 through a secondary cable, the first current terminal A1 and the second current terminal A2 are connected through a first current connecting piece, the second current terminal A2 is connected to the protection device, the protection device is connected to the fourth current terminal B2, the third current terminal B1 and the fourth current terminal B2 are connected through a second current connecting piece, and the third current terminal B1 is connected to another secondary terminal S2 of the zero-sequence CT through a secondary cable;

[0027] Depend on Figure 2 It can be seen that the flow path of the zero-sequence CT secondary current is:

[0028] Zero-sequence CT secondary S1 terminal - secondary cable - first current connecting piece - secondary wiring in the cabinet - protection device - secondary wiring in the cabinet - second current connecting piece - secondary cable - zero-sequence CT secondary S2 terminal.

[0029] For the secondary circuit of the zero-sequence current transformer, the secondary current of the zero-sequence current transformer is zero during normal operation. The existing protection device cannot detect whether the secondary circuit of the zero-sequence current transformer is disconnected. The basic principle of the zero-sequence current transformer secondary circuit disconnection detection of the present invention is to measure the magnitude of the pulse current by adding a pulse DC voltage to the secondary circuit, and then obtain the conduction state of the circuit. Figure 1 , a zero-sequence current transformer secondary circuit disconnection detection device, including a DC generator, a voltage and current collector, a central processing unit, a display setting unit and external wiring, wherein:

[0030] To avoid the influence of DC on zero-sequence CT, the DC generator should have a small DC value. The DC generator needs to generate a DC voltage of 10mV to 30mV. Considering that the DC generator is connected in series in the zero-sequence secondary circuit, the line may be grounded during the detection process. The AC resistance of the DC generator should be less than 1 ohm when viewed from the outside, and it can withstand an AC current of 5A for a short time.

[0031] The voltage and current collector can collect the DC voltage and output DC current generated by the DC generator and provide them for calculation by the central processor. The collection accuracy is required to be within ±2mV and ±2mA.

[0032] The central processor receives the data from the voltage and current collector, obtains the corresponding resistance value according to the digital value of the DC voltage and DC current, and analyzes the data. If the DC current and resistance value are within the set range, the secondary circuit is considered normal, otherwise the secondary circuit is disconnected. If the collected current is zero or very small, it is judged that the zero-sequence CT secondary circuit is disconnected. If the measured current is normal and the corresponding resistance value is within a reasonable range, it is judged that there is no problem with the CT secondary circuit.

[0033] The display setting unit is a human-machine interactive interface, which is used to display the measured voltage and current, and can set the amplitude and length of the pulse. It can display the test results and whether the zero-sequence CT secondary circuit is broken, using an LCD display.

[0034] The external wiring consists of two current wires, one end of each current wire is connected to the DC generator, and the other end is a standard current terminal that can be directly inserted into the current terminal of the secondary circuit of the zero-sequence current transformer (on the substation switch cabinet).

[0035] The protection device determines whether a ground fault occurs in the line by collecting the magnitude of the zero-sequence current and issues a trip command.

[0036] A detection method based on the zero-sequence current transformer secondary circuit break detection device is:

[0037] Combination Figure 3, insert the two secondary wires of the device of the present invention into the third current terminal (B1) and the fourth current terminal B2 (or the first current terminal A1 and the second current terminal A2) respectively, and ensure that the connection is tight.

[0038] The device is powered on and passes the self-test, ensuring that AC current can flow through the device.

[0039] The second connecting piece between the third current terminal B1 and the fourth current terminal B2 is opened.

[0040] Turn on the test function of the device and obtain the test results.

[0041] Record the test results shown on the display setup unit.

[0042] Close the second connecting piece between the third current terminal B1 and the fourth current terminal B2.

[0043] Unplug the two current wires of the device of the present invention plugged into the third current terminal B1 and the fourth current terminal B2.

[0044] All testing is completed.

[0045] It should be emphasized that the present invention should avoid affecting the running secondary circuit during the detection process, that is, if a single-phase grounding fault occurs in the line during the detection process, the detection device will not affect the flow of zero-sequence current.

Claims

1. A zero-sequence current transformer secondary circuit disconnection detection device, the zero-sequence current transformer secondary circuit comprising a zero-sequence CT, a first current terminal (A1), a second current terminal (A2), a third current terminal (B1), a fourth current terminal (B2), a first current connecting piece, a second current connecting piece and a protection device, the secondary terminal (S1) of the zero-sequence CT is connected to the first current terminal (A1) through a secondary cable, the first current terminal (A1) and the second current terminal (A2) are connected through a first current connecting piece, the second current terminal (A2) is connected to the protection device, the protection device is connected to the fourth current terminal (B2), the third current terminal (B1) and the fourth current terminal (B2) are connected through a second current connecting piece, and the third current terminal (B1) is connected to another secondary terminal (S2) of the zero-sequence CT through a secondary cable; characterized in that The pulse current is measured by adding a pulse DC voltage to the secondary circuit, thereby obtaining the conduction state of the circuit. The device includes a DC generator, a voltage and current collector, a central processing unit, a display setting unit and two current lines, wherein: The DC generator is used to generate DC voltage and DC current, and output them to the voltage and current collector; The voltage and current collector is used to convert the DC voltage and DC current analog quantities into digital quantities and output them to the central processing unit; The central processing unit obtains the corresponding resistance value according to the digital values ​​of the DC voltage and the DC current, and determines whether the secondary circuit is disconnected according to the DC current and the resistance value; The display setting unit is used to display the magnitude and judgment result of the measured DC voltage and DC current, and to set the amplitude and length of the pulse; One end of the two current lines is connected to the DC generator, and the other end is inserted into the third current terminal (B1) and the fourth current terminal (B2) of the secondary circuit of the zero-sequence current transformer.

2. The zero-sequence current transformer secondary circuit disconnection detection device according to claim 1, characterized in that: The method of judging whether the secondary circuit is disconnected by the DC current and the resistance value is as follows: if the DC current and the resistance value are both within a set range, the secondary circuit is considered normal; otherwise, the secondary circuit is disconnected.

3. The zero-sequence current transformer secondary circuit disconnection detection device according to claim 1, characterized in that: It also includes an alarm device connected to the central processing unit, which issues an alarm when the secondary circuit is disconnected.

4. The zero-sequence current transformer secondary circuit disconnection detection device according to claim 1, characterized in that: The voltage and current acquisition accuracy of the voltage and current collector are within ±2mV and ±2mA respectively.

5. The zero-sequence current transformer secondary circuit disconnection detection device according to claim 1, characterized in that: The DC generator generates a DC voltage of 10mV to 30mV.

6. The zero-sequence current transformer secondary circuit disconnection detection device according to claim 5, characterized in that: The AC resistance in the DC generator should be less than 1 ohm and can withstand an AC current of 5A.

7. A detection method based on the zero-sequence current transformer secondary circuit disconnection detection device according to any one of claims 1 to 6, characterized in that: Includes steps: Insert the other ends of the two current wires of the zero-sequence current transformer secondary circuit break detection device into the current terminals at both ends of the first current connecting piece or the second current connecting piece of the zero-sequence current transformer secondary circuit respectively; Start the device, self-test is passed, and AC current flows through the device; Open the first current connection piece or the second current connection piece to start testing; Record the test results displayed on the display setting unit; After the test is completed, the first current connecting piece or the second current connecting piece is closed, and the two current wires of the device plugged into the current terminals at both ends of the first current connecting piece or the second current connecting piece are unplugged.

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