CN108962573B

CN108962573B - Replaceable low-voltage current transformer in electrified state and electrified replacement method thereof

Info

Publication number: CN108962573B
Application number: CN201810509085.4A
Authority: CN
Inventors: 黄国贞; 戴燎原; 蔡加鑫; 陈跃辉; 吴月黎
Original assignee: Quanzhou Power Supply Co of State Grid Fujian Electric Power Co Ltd
Current assignee: Quanzhou Power Supply Co of State Grid Fujian Electric Power Co Ltd
Priority date: 2018-05-24
Filing date: 2018-05-24
Publication date: 2023-05-26
Anticipated expiration: 2038-05-24
Also published as: CN108962573A

Abstract

The invention provides a low-voltage current transformer capable of being replaced in a charged state and a charged replacement method thereof, wherein the low-voltage current transformer comprises a primary coil, a secondary coil, an electromagnetic iron core, a display instrument, a three-phase current type AC/DC/AC circuit and a PWM control chip; the primary coil is wound on the primary side of the electromagnetic iron core and is connected to the A-phase circuit, the B-phase circuit and the C-phase circuit; the secondary coil is wound on the secondary side of the electromagnetic iron core and is connected with a display instrument; the three-phase current type AC/DC/AC circuit and the electromagnetic iron core are connected to the A-phase line, the B-phase line and the C-phase line in parallel, and used as a load transfer circuit for replacing the low-voltage current transformer, and the PWM control chip controls and conducts the three-phase current type AC/DC/AC circuit. The invention adopts the parallel three-phase current type AC/DC/AC circuit as the load transfer circuit, thereby realizing the safe replacement of the low-voltage current transformer under the conditions of live working and uninterrupted load.

Description

Replaceable low-voltage current transformer in electrified state and electrified replacement method thereof

Technical Field

The invention relates to the field of electrical equipment design, in particular to a replaceable low-voltage current transformer in an electrified state and an electrified replacement method thereof.

Background

With the rapid development of power systems, in the power systems, a current transformer for a low voltage is often used in the process of connecting a primary system and a secondary system. The low-voltage current transformer can provide accurate and standard information for metering and relay protection of the power system. The low-voltage current transformer is an indispensable component in a protection system of a power distribution cabinet and a switch cabinet. The insulated switchgear (GIS) with the low-voltage current transformer has the advantages of small occupied area, small volume, convenience, flexibility, high insulation level and the like, and is widely applied to a power system.

In real life, because the public and special transformers and the public distribution transformer load of the district are seasonal mutation and the service life of the transformer is expired, the transformer and the transformer are in fault, burn and other reasons to cause line loss distortion and electric quantity loss, and the replacement of the current transformer needs to be powered off. The operation of replacing the current transformer reduces the reliability of power supply and the continuity of power consumption, can cause customer dissatisfaction complaints, and reduces corresponding electricity sales.

At present, no low-voltage current transformer capable of being replaced in a live mode exists in China, so that the stable power supply of a load can be affected when the low-voltage current transformer is replaced every time.

Disclosure of Invention

In order to solve the defects in the prior art, the invention adopts equipment such as an alternating current contact electromagnetic relay, a three-phase current type AC/DC/AC circuit, a DSP chip and the like to carry out new circuits and related control designs, provides a replaceable low-voltage current transformer in an electrified state and an electrified replacement method thereof, and realizes the replacement of the low-voltage current transformer in the electrified state so as to overcome the defects in the prior art.

In order to achieve the above purpose, the invention provides a replaceable low-voltage current transformer in a charged state, which comprises a primary coil, a secondary coil, an electromagnetic iron core, a display instrument, a three-phase current type AC/DC/AC circuit and a PWM control chip; the three-phase circuit of the secondary coil is sequentially detached and wound on the secondary side of the electromagnetic iron core; and the three-phase outlet terminals a3, b3 and c3 of the secondary coil are sequentially and electrically connected with the display instrument connection port A, B, C and are used for displaying current data; the three-phase circuit of the primary coil is sequentially wound on the primary side of the electromagnetic iron core, and six connecting pins a1, a2, b1, b2, c1 and c2 are led out of the primary coil; the pins a1, a2, B1, B2, C1 and C2 are respectively connected with a main line joint and an auxiliary line joint led out by the main line joint, the main line joints of the pins a1 and a2 are connected with an A-phase line, the main line joints of the pins B1 and B2 are connected with a B-phase line, and the main line joints of the pins C1 and C2 are connected with a C-phase line for forming a current loop of the current transformer; the auxiliary line connectors of the pins a1, b1 and c1 are sequentially and electrically connected with three positive pins f1, f2 and f3 of a three-phase current type AC/DC/AC circuit, the auxiliary line connectors of the pins a2, b2 and c2 are sequentially and electrically connected with three negative pins f4, f5 and f6 of the three-phase current type AC/DC/AC circuit, twelve control pins of the three-phase current type AC/DC/AC circuit are correspondingly and electrically connected with twelve trigger pins of a PWM control chip, and the auxiliary line connectors are used for forming a load transfer loop for replacing the three-phase current type AC/DC/AC circuit.

Through the technical scheme, the PWM control chip controls and conducts the three-phase current type AC/DC/AC circuit, so that the three-phase current type AC/DC/AC circuit is connected with the low-voltage transformer in parallel and is used as a load transfer circuit, and uninterrupted power of a load is realized; the PWM control chip controls the three-phase current type AC/DC/AC circuit to change the current passing through the three-phase current type AC/DC/AC circuit, and the current data passing through the low-voltage current transformer displayed on the display instrument is in a safe range, so that the low-voltage current transformer is replaced, and the safety under the condition of live working is ensured.

As a further explanation of the low-voltage current transformer according to the present invention, it is preferable that the low-voltage current transformer further includes a first ac electromagnetic relay, a second ac electromagnetic relay, and an ac relay controller; the three normally-closed contacts d1, d2 and d3 of the first alternating-current electromagnetic relay are sequentially and electrically connected between the main line connector and the pins a1, b1 and c1, and the three normally-open contacts d4, d5 and d6 of the first alternating-current electromagnetic relay are sequentially and electrically connected between the auxiliary line connector and the positive pins f1, f2 and f3; three normally-closed contacts e1, e2 and e3 of the second alternating-current electromagnetic relay are sequentially and electrically connected between the main line joint and the pins a2, b2 and c2, and three normally-open contacts e4, e5 and e6 of the second alternating-current electromagnetic relay are sequentially and electrically connected between the auxiliary line joint and the negative pins f4, f5 and f6; the alternating current relay controller is electrically connected with the first alternating current electromagnetic relay and the second alternating current electromagnetic relay respectively.

Through the technical scheme, the first alternating current electromagnetic relay and the second alternating current electromagnetic relay are adopted to conduct the low-voltage current transformer and disconnect the three-phase current type AC/DC/AC circuit, so that electric quantity loss is avoided; when the low-voltage current transformer needs to be replaced, the three-phase current type AC/DC/AC circuit is conducted and the low-voltage current transformer is disconnected under the control of the AC relay controller, so that the current passing through the low-voltage current transformer is smaller, and the personal safety of operators is ensured to a greater extent.

As a further explanation of the current transformer according to the present invention, it is preferable that the main line connector and the sub line connector are both two-way connectors, and the two-way connectors are externally made of an insulating material.

Through the technical scheme, when the low-voltage current transformer is replaced, only the wiring end of the old low-voltage current transformer is pulled out, the main line connector is still connected to the power supply end, and the conduction of the auxiliary line connector and the three-phase current type AC/DC/AC circuit is not affected at the moment, and the replacement of power failure is not needed.

As a further explanation of the current transformer of the present invention, it is preferable that the three-phase current type AC/DC/AC circuit includes IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, IGBT7, IGBT8, IGBT9, IGBT10, IGBT11, IGBT12, filter capacitor C1, filter capacitor C2, filter capacitor C3, filter capacitor C4, filter capacitor C5, and filter capacitor C6; wherein, 1 pin of IGBT1, 1 pin of IGBT2 and 1 pin of IGBT3 are connected to positive pin g+; the pin 2 of the IGBT1 is electrically connected with the pin 1 of the IGBT4 and leads out the positive pin f1, the pin 2 of the IGBT2 is electrically connected with the pin 1 of the IGBT5 and leads out the positive pin f2, and the pin 2 of the IGBT3 is electrically connected with the pin 1 of the IGBT6 and leads out the positive pin f3; the pin 2 of the IGBT4, the pin 2 of the IGBT5 and the pin 2 of the IGBT6 are electrically connected to the negative pin g-and are used for forming an AC/DC conversion circuit for converting input alternating current into direct current; the 1 pin of the IGBT7, the 1 pin of the IGBT8 and the 1 pin of the IGBT9 are electrically connected with a positive pin h+ which is electrically connected with the positive pin g+; the pin 2 of the IGBT7 is electrically connected with the pin 1 of the IGBT10 and leads out the negative pin f4, the pin 2 of the IGBT8 is electrically connected with the pin 1 of the IGBT11 and leads out the negative pin f5, and the pin 2 of the IGBT9 is electrically connected with the pin 1 of the IGBT12 and leads out the negative pin f6; the pin 2 of the IGBT10, the pin 2 of the IGBT11 and the pin 2 of the IGBT12 are electrically connected with a negative pin h-which is electrically connected with the negative pin g-and is used for forming a DC/AC conversion circuit for converting direct current into output alternating current; the filter capacitor C1, the filter capacitor C2 and the filter capacitor C3 are respectively connected to the circuits of the positive pins f1, f2 and f3 in parallel; the filter capacitor C4, the filter capacitor C5 and the filter capacitor C6 are respectively connected in parallel to the lines of the negative pins f4, f5 and f 6.

Through the technical scheme, the three-phase current type AC/DC/AC circuit formed by 12 IGBTs is used to be matched and connected with 12 control pins of the PWM control chip, so that the current is ensured to be output as is, and the three-phase current type AC/DC/AC circuit is also suitable for being connected into a high-voltage power supply circuit.

As a further illustration of the current transformer of the present invention, the PWM control chip preferably employs a TMS320 series DSP chip.

Through the technical scheme, the PWM signal generated by the DSP chip is used for controlling the on-off of the three-phase current type AC/DC/AC circuit, so that the control is faster and more reliable and is convenient.

As a further explanation of the low-voltage current transformer according to the present invention, preferably, the first ac electromagnetic relay and the second ac electromagnetic relay are ac contact relays, and the ac hertz of the ac contact relays is 50Hz, the rated insulation voltage is 690V, and the rated operating voltage is 380V.

Through above-mentioned technical scheme, adopt contact relay to realize switching on and disconnection of low-voltage current transformer and power supply line, realize simply.

In order to achieve another object, the present invention further provides a live replacement method of the current transformer, the live replacement method comprising the steps of: 1) When the display instrument displays the secondary side current dissimilarity, the PWM control chip works to generate a PWM control signal; 2) The PWM control signals are input to twelve control pins of the three-phase current type AC/DC/AC circuit, and the three-phase current type AC/DC/AC circuit is conducted; or 3) the alternating current relay controller works, normally-closed contacts d1, d2 and d3 of the first alternating current relay are controlled to be opened, normally-open contacts d4, d5 and d6 of the second alternating current relay are controlled to be closed, normally-closed contacts e1, e2 and e3 of the second alternating current relay are controlled to be opened, normally-open contacts e4, e5 and e6 are controlled to be closed, so that the load transfer loop is conducted, and a current loop of the low-voltage current transformer is opened; 4) The PWM control chip controls the current increase passing through the three-phase current type AC/DC/AC circuit, and checks the display instrument to determine the current passing through the low-voltage transformer; 5) And the current data displayed by the display instrument is in a safety range, the old low-voltage current transformer is pulled out from the main line joint, and the new low-voltage current transformer is inserted.

The beneficial effects of the invention are as follows:

1) The invention adopts the three-phase current type AC/DC/AC circuit to be connected in parallel to the power supply circuit of the low-voltage current transformer as a load transfer circuit, controls the three-phase current type AC/DC/AC circuit to be conducted through the PWM control chip, and controls the current size of the three-phase current type AC/DC/AC circuit to play a role of shunting, so that the replacement of the low-voltage current transformer is safer under the condition of live working, the load is not powered off, and the normal use of a power supply system is not influenced;

2) The invention further adopts the first alternating current electromagnetic relay and the second alternating current electromagnetic relay to conduct the low-voltage current transformer and disconnect the three-phase current type AC/DC/AC circuit so as to avoid electric quantity loss; when the low-voltage current transformer needs to be replaced, the three-phase current type AC/DC/AC circuit is conducted and the low-voltage current transformer is disconnected under the control of the AC relay controller, so that the current passing through the low-voltage current transformer is smaller, and the personal safety of operators is ensured to a greater extent;

3) The invention further utilizes a three-phase current type AC/DC/AC circuit formed by 12 IGBTs to be matched and connected with 12 control pins of the PWM control chip, thereby ensuring the original output of current and being applicable to being connected into a high-voltage power supply circuit;

4) According to the live replacement method, the three-phase current type AC/DC/AC circuit is controlled to be conducted through the PWM control chip and the current is increased, so that the current passing through the low-voltage current transformer is reduced, and the live replacement of the low-voltage current transformer by means of the display instrument is realized under the condition that a load is not powered off.

Drawings

Fig. 1 is a circuit configuration diagram of a low-voltage current transformer of the present invention;

fig. 2 is a circuit configuration diagram of an ac relay controller with a relay according to the present invention;

fig. 3 is a circuit configuration diagram of the three-phase current type AC/DC/AC circuit of the present invention.

Detailed Description

For a further understanding of the structure, features, and other objects of the invention, reference should now be made in detail to the accompanying drawings of the preferred embodiments of the invention, which are illustrated in the accompanying drawings and are for purposes of illustrating the concepts of the invention and not for limiting the invention.

Example 1

As shown in fig. 1, fig. 1 is a circuit configuration diagram of a low-voltage current transformer of the present invention; the low-voltage current transformer comprises a primary coil 1, a secondary coil 2, an electromagnetic iron core 3, a display instrument 4, a three-phase current type AC/DC/AC circuit 8 and a PWM control chip 9.

The three-phase circuit of the secondary coil 2 is sequentially disassembled and wound on the secondary side of the electromagnetic iron core 3; and the three-phase outlet terminals a3, b3 and c3 of the secondary coil 2 are sequentially and electrically connected with the connecting port A, B, C of the display instrument 4 and are used for displaying current data, whether the low-voltage current transformer needs to be replaced or not can be judged through the current data, and when the low-voltage current transformer needs to be replaced, whether the current data is in a safety range or not can be determined through the display instrument, so that the personal safety of operators can be ensured when the low-voltage current transformer is replaced.

The three-phase circuit of the primary coil 1 is sequentially wound on the primary side of the electromagnetic iron core 3, and six connecting pins a1, a2, b1, b2, c1 and c2 are led out of the primary coil 1; the pins a1, a2, B1, B2, C1 and C2 are respectively connected with a main line joint 11 and a secondary line joint 12 led out from the main line joint 11, the main line joints of the pins a1 and a2 are connected with an A-phase line, the main line joints of the pins B1 and B2 are connected with a B-phase line, and the main line joints of the pins C1 and C2 are connected with a C-phase line for forming a current loop of the low-voltage current transformer; the auxiliary line connectors of the pins a1, b1 and c1 are sequentially and electrically connected with three positive pins f1, f2 and f3 of the three-phase current type AC/DC/AC circuit 8, and the auxiliary line connectors of the pins a2, b2 and c2 are sequentially and electrically connected with three negative pins f4, f5 and f6 of the three-phase current type AC/DC/AC circuit 8, so that a load transfer loop for replacing the low-voltage current transformer is formed, the load transfer loop has the shunting function, and the low-voltage current transformer is safer to replace.

Twelve control pins of the three-phase current type AC/DC/AC circuit 8 are correspondingly and electrically connected with twelve trigger pins of the PWM control chip 9, the PWM control chip adopts a TMS320 series DSP chip, and PWM signals are generated by the DSP chip to control the on-off of the three-phase current type AC/DC/AC circuit, so that the control is faster and more reliable and convenient; the on and off of the three-phase current type AC/DC/AC circuit is controlled by the PWM control chip so as to ensure that the three-phase current type AC/DC/AC circuit does not consume electric quantity when the low-voltage current transformer is normally used, and the three-phase current type AC/DC/AC circuit 8 is connected in parallel to the current loop of the low-voltage current transformer when the low-voltage current transformer is replaced, so that the uninterrupted power supply of the load is realized; at this time, the PWM control chip controls the three-phase current type AC/DC/AC circuit to increase the current passing through the three-phase current type AC/DC/AC circuit, reduce the current passing through the low-voltage current transformer, and view the current data passing through the low-voltage current transformer displayed on the display instrument, and perform the operation of replacing the low-voltage current transformer when the current data is within a safe range, so as to safely replace the low-voltage current transformer under the condition of live working.

Therefore, the three-phase current type AC/DC/AC circuit and the DSP chip are utilized to realize the replacement of the low-voltage current transformer in the electrified state. The step of the live replacement method is as follows.

First, when the display instrument displays the secondary side current abnormal, the PWM control chip works to generate PWM control signals.

Secondly, the PWM control signal is input to twelve control pins of the three-phase current type AC/DC/AC circuit, and turns on the three-phase current type AC/DC/AC circuit.

Again, the PWM control chip controls the increase in current passing in the three-phase current-type AC/DC/AC circuit and looks at the display meter to determine the magnitude of the current passing through the low-voltage transformer.

And finally, displaying that the current data displayed by the instrument is in a safety range, pulling out the old low-voltage current transformer from the main line joint, and inserting the old low-voltage current transformer into the new low-voltage current transformer.

The main line connector and the auxiliary line connector are both bidirectional plug connectors, the outside of the bidirectional plug connectors is made of insulating materials, when the low-voltage current transformer is replaced, only the wiring end of the old low-voltage current transformer is pulled out, the main line connector is still connected to the power supply end, at the moment, the conduction of the auxiliary line connector and the three-phase current type AC/DC/AC circuit is not affected, and the replacement of the three-phase current type AC/DC/AC circuit is not needed to be powered off; in addition, the three-phase current type AC/DC/AC circuit can be pulled out from the junction of the auxiliary line and then used for other current transformers without being provided with the three-phase current type AC/DC/AC circuit at each current transformer, so that the manufacturing budget is saved.

Referring to fig. 3, fig. 3 is a circuit configuration diagram of a three-phase current type AC/DC/AC circuit of the present invention; a three-phase current type AC/DC/AC circuit composed of 12 IGBTs is utilized to conform to the matching connection with 12 control pins of a PWM control chip.

Specifically, pin 1 of IGBT1, pin 1 of IGBT2, and pin 1 of IGBT3 are electrically connected to positive pin g+; the pin 2 of the IGBT1 is electrically connected with the pin 1 of the IGBT4 and leads out the positive pin f1, the pin 2 of the IGBT2 is electrically connected with the pin 1 of the IGBT5 and leads out the positive pin f2, and the pin 2 of the IGBT3 is electrically connected with the pin 1 of the IGBT6 and leads out the positive pin f3; the pin 2 of the IGBT4, the pin 2 of the IGBT5 and the pin 2 of the IGBT6 are electrically connected to the negative pin g-and are used for forming an AC/DC conversion circuit for converting input alternating current into direct current; the 1 pin of the IGBT7, the 1 pin of the IGBT8 and the 1 pin of the IGBT9 are electrically connected with a positive pin h+ which is electrically connected with the positive pin g+; the pin 2 of the IGBT7 is electrically connected with the pin 1 of the IGBT10 and leads out the negative pin f4, the pin 2 of the IGBT8 is electrically connected with the pin 1 of the IGBT11 and leads out the negative pin f5, and the pin 2 of the IGBT9 is electrically connected with the pin 1 of the IGBT12 and leads out the negative pin f6; the 2 pins of the IGBT10, the 2 pins of the IGBT11 and the 2 pins of the IGBT12 are electrically connected with a negative pin h-which is electrically connected with the negative pin g-and is used for forming a DC/AC conversion circuit for converting direct current into output alternating current, so that the direct current is ensured to be output as the original.

In order to filter out higher harmonics generated in the alternating current conversion process, the lines of the pins f1, f2, f3, f4, f5 and f6 are respectively connected with a filter capacitor, and the filter capacitor C1, the filter capacitor C2, the filter capacitor C3, the filter capacitor C4, the filter capacitor C5 and the filter capacitor C6 are connected with the lines in parallel.

Example 2

As shown in fig. 2, fig. 2 is a circuit configuration diagram of the ac relay controller with a relay according to the present invention; the low-voltage current transformer further comprises a first alternating current electromagnetic relay 5, a second alternating current electromagnetic relay 6 and an alternating current relay controller 7 on the basis of a connecting circuit of the primary coil 1, the secondary coil 2, the electromagnetic iron core 3, the display instrument 4, the three-phase current type AC/DC/AC circuit 8 and the PWM control chip 9. An alternating current electromagnetic relay is arranged to ensure that when the low-voltage current transformer is replaced, the current loop is completely disconnected and the load transfer loop is connected, so that the replacement of the low-voltage current transformer is safer. An alternating-current contact relay is selected as a first alternating-current electromagnetic relay and a second alternating-current electromagnetic relay, the alternating-current hertz of the alternating-current contact relay is 50Hz, the rated insulation voltage is 690V, and the rated working voltage is 380V.

Specifically, the connection double-position relay is used for controlling the connection and disconnection of the main line joint and the auxiliary line joint, three normally-closed contacts d1, d2 and d3 of the first alternating-current electromagnetic relay 5 are sequentially and electrically connected between the main line joint and the pins a1, b1 and c1, and three normally-open contacts d4, d5 and d6 of the first alternating-current electromagnetic relay 5 are sequentially and electrically connected between the auxiliary line joint and the positive pins f1, f2 and f3; three normally closed contacts e1, e2, e3 of the second ac electromagnetic relay 6 are electrically connected between the main line connector and the pins a2, b2, c2 in sequence, and three normally open contacts e4, e5, e6 of the second ac electromagnetic relay 6 are electrically connected between the auxiliary line connector and the negative pins f4, f5, f6 in sequence; at this time, the current loop of the low-voltage current transformer is turned on, the load transfer loop is completely turned off, no current passes through the three-phase current type AC/DC/AC circuit 8, the use of the low-voltage current transformer is not affected, and the electric quantity is not consumed.

The first alternating current relay 5 and the second alternating current relay 6 are connected to the alternating current relay controller 7, the alternating current relay controller 7 is utilized to control the first alternating current relay 5 and the second alternating current relay 6, when the low-voltage current transformer is replaced, the three normally-closed contacts d1, d2 and d3 are disconnected, the normally-open contacts d4, d5 and d6 are conducted, the load transfer loop is connected to the current loop of the low-voltage current transformer in parallel, the load is not powered off, the low-voltage current transformer is disconnected from the power supply, and the low-voltage current transformer can be safely replaced.

Therefore, the first alternating current electromagnetic relay and the second alternating current electromagnetic relay are adopted to conduct the low-voltage current transformer and disconnect the three-phase current type AC/DC/AC circuit, so that electric quantity loss is avoided; when the low-voltage current transformer needs to be replaced, the three-phase current type AC/DC/AC circuit is conducted and the low-voltage current transformer is disconnected under the control of the AC relay controller, so that the current passing through the low-voltage current transformer is smaller, the personal safety of operators is ensured to a greater extent, and the live replacement method comprises the following steps:

Secondly, the AC relay controller works, normally-closed contacts d1, d2 and d3 of the first AC electromagnetic relay are controlled to be opened, normally-open contacts d4, d5 and d6 of the first AC electromagnetic relay are controlled to be closed, normally-closed contacts e1, e2 and e3 of the second AC electromagnetic relay are controlled to be opened, normally-open contacts e4, e5 and e6 of the second AC electromagnetic relay are controlled to be closed, so that the load transfer loop is conducted, and a current loop of the current transformer of the AC current transformer is opened.

The main line connector and the auxiliary line connector are still bidirectional plug connectors, the outside of the bidirectional plug connectors is made of insulating materials, when the low-voltage current transformer is replaced, only the wiring end of the old low-voltage current transformer is pulled out, the main line connector is still connected to the power supply end, at the moment, the conduction of the auxiliary line connector and the three-phase current type AC/DC/AC circuit is not affected, and the replacement of the auxiliary line connector and the three-phase current type AC/DC/AC circuit is not required to be powered off

It should be noted that the foregoing summary and the detailed description are intended to demonstrate practical applications of the technical solution provided by the present invention, and should not be construed as limiting the scope of the present invention. Various modifications, equivalent alterations, or improvements will occur to those skilled in the art, and are within the spirit and principles of the invention. The scope of the invention is defined by the appended claims.

Claims

1. The replaceable low-voltage current transformer in the electrified state is characterized by comprising a primary coil (1), a secondary coil (2), an electromagnetic iron core (3), a display instrument (4), a three-phase current type AC/DC/AC circuit (8) and a PWM control chip (9); wherein,,

the three-phase circuit of the secondary coil (2) is sequentially wound on the secondary side of the electromagnetic iron core (3); the three-phase outlet ends a3, b3 and c3 of the secondary coil (2) are sequentially and electrically connected with a connecting port A, B, C of the display instrument (4) and are used for displaying current data;

the three-phase circuit of the primary coil (1) is sequentially wound on the primary side of the electromagnetic iron core (3), and six connecting pins a1, a2, b1, b2, c1 and c2 are led out of the primary coil (1); the pins a1, a2, B1, B2, C1 and C2 are respectively connected with a main line joint (11) and a secondary line joint (12) led out by the main line joint (11), the main line joints of the pins a1 and a2 are connected with an A-phase line, the main line joints of the pins B1 and B2 are connected with a B-phase line, and the main line joints of the pins C1 and C2 are connected with a C-phase line for forming a current loop of the low-voltage current transformer;

the auxiliary line connectors of the pins a1, b1 and c1 are sequentially and electrically connected with three positive pins f1, f2 and f3 of a three-phase current type AC/DC/AC circuit (8), the auxiliary line connectors of the pins a2, b2 and c2 are sequentially and electrically connected with three negative pins f4, f5 and f6 of the three-phase current type AC/DC/AC circuit (8), twelve control pins of the three-phase current type AC/DC/AC circuit (8) are correspondingly and electrically connected with twelve trigger pins of a PWM control chip (9), and the auxiliary line connectors are used for forming a load transfer loop for replacing the three-phase current transformer;

the low-voltage current transformer further comprises a first alternating-current electromagnetic relay (5), a second alternating-current electromagnetic relay (6) and an alternating-current relay controller (7); wherein,,

three normally closed contacts d1, d2 and d3 of the first alternating current electromagnetic relay (5) are sequentially and electrically connected between the main line joint and the pins a1, b1 and c1, and three normally open contacts d4, d5 and d6 of the first alternating current electromagnetic relay (5) are sequentially and electrically connected between the auxiliary line joint and the positive pins f1, f2 and f3;

three normally closed contacts e1, e2 and e3 of the second alternating current electromagnetic relay (6) are sequentially and electrically connected between the main line joint and the pins a2, b2 and c2, and three normally open contacts e4, e5 and e6 of the second alternating current electromagnetic relay (6) are sequentially and electrically connected between the auxiliary line joint and the negative pins f4, f5 and f6;

the alternating current relay controller (7) is respectively and electrically connected with the first alternating current electromagnetic relay (5) and the second alternating current electromagnetic relay (6);

the first alternating current electromagnetic relay (5) and the second alternating current electromagnetic relay (6) are used for conducting the low-voltage current transformer and disconnecting the three-phase current type AC/DC/AC circuit (8); when the low-voltage current transformer needs to be replaced, a three-phase current type AC/DC/AC circuit (8) is conducted and the low-voltage current transformer is disconnected under the control of an alternating current relay controller (7).

2. A current transformer according to claim 1, characterized in that the main line connector (11) and the secondary line connector (12) are both two-way connectors, the exterior of which is made of an insulating material.

3. The current transformer according to claim 1, wherein the three-phase current type AC/DC/AC circuit (8) comprises IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, IGBT7, IGBT8, IGBT9, IGBT10, IGBT11, IGBT12, filter capacitor C1, filter capacitor C2, filter capacitor C3, filter capacitor C4, filter capacitor C5 and filter capacitor C6; wherein,,

the 1 pin of the IGBT1, the 1 pin of the IGBT2 and the 1 pin of the IGBT3 are electrically connected to the positive pin g+; the pin 2 of the IGBT1 is electrically connected with the pin 1 of the IGBT4 and leads out the positive pin f1, the pin 2 of the IGBT2 is electrically connected with the pin 1 of the IGBT5 and leads out the positive pin f2, and the pin 2 of the IGBT3 is electrically connected with the pin 1 of the IGBT6 and leads out the positive pin f3; the pin 2 of the IGBT4, the pin 2 of the IGBT5 and the pin 2 of the IGBT6 are electrically connected to the negative pin g-and are used for forming an AC/DC conversion circuit for converting input alternating current into direct current;

the 1 pin of the IGBT7, the 1 pin of the IGBT8 and the 1 pin of the IGBT9 are electrically connected with a positive pin h+ which is electrically connected with the positive pin g+; the pin 2 of the IGBT7 is electrically connected with the pin 1 of the IGBT10 and leads out the negative pin f4, the pin 2 of the IGBT8 is electrically connected with the pin 1 of the IGBT11 and leads out the negative pin f5, and the pin 2 of the IGBT9 is electrically connected with the pin 1 of the IGBT12 and leads out the negative pin f6; the pin 2 of the IGBT10, the pin 2 of the IGBT11 and the pin 2 of the IGBT12 are electrically connected with a negative pin h-which is electrically connected with the negative pin g-and is used for forming a DC/AC conversion circuit for converting direct current into output alternating current;

the filter capacitor C1, the filter capacitor C2 and the filter capacitor C3 are respectively connected to the circuits of the positive pins f1, f2 and f3 in parallel; the filter capacitor C4, the filter capacitor C5 and the filter capacitor C6 are respectively connected in parallel to the lines of the negative pins f4, f5 and f 6.

4. The low-voltage current transformer of claim 1, wherein the PWM control chip employs a DSP chip of TMS320 series.

5. A low-voltage current transformer according to claim 1, characterized in that the first ac electromagnetic relay (5) and the second ac electromagnetic relay (6) are ac contact relays, the ac hertz of which is 50Hz, the rated insulation voltage is 690V, and the rated operating voltage is 380V.

6. A live replacement method for a current transformer according to any one of claims 1 to 5, comprising the steps of:

1) When the display instrument (4) displays the secondary side current abnormal, the PWM control chip (9) works to generate a PWM control signal;

2) The PWM control signals are input to twelve control pins of the three-phase current type AC/DC/AC circuit (8) and conduct the three-phase current type AC/DC/AC circuit (8); or,

3) The alternating current relay controller (7) works, normally-closed contacts d1, d2 and d3 of the first alternating current relay (5) are controlled to be opened, normally-open contacts d4, d5 and d6 are controlled to be closed, normally-closed contacts e1, e2 and e3 of the second alternating current relay (6) are controlled to be opened, normally-open contacts e4, e5 and e6 are controlled to be closed, so that the load transfer loop is conducted, and a current loop of the low-voltage current transformer is opened;

4) The PWM control chip (9) controls the current passing through the three-phase current type AC/DC/AC circuit (8) to increase, and checks the display instrument (4) to determine the current passing through the low-voltage transformer;

5) The current data displayed by the display instrument (4) is in a safety range, the old low-voltage current transformer is pulled out from the main line joint (11), and a new low-voltage current transformer is inserted.