Direct-current cable insulation monitoring device, system and application

Abstract

The invention discloses a direct-current cable insulation monitoring device and system and application, and belongs to the technical field of rail transit direct-current traction power cable insulation monitoring. A resistance loop composed of a first resistor, a second resistor and a third resistor is adopted, and voltage signals of key points are collected in real time in combination with an isolation transmitter; and fault judgment and tripping control are carried out through the protection device based on a set percentage threshold algorithm, so that the problems of low response speed, difficulty in accurately distinguishing fault types and dependence on complex and high-cost equipment in the existing direct-current cable insulation fault detection are solved; the real-time accurate monitoring of the insulation state is realized, additional signal injection equipment is not needed, the detection efficiency and reliability are remarkably improved, the system complexity and cost are reduced, and the system is particularly suitable for insulation safety protection of high-voltage scenes such as direct-current traction power supply and the like.

Description

TECHNICAL FIELD

[0001] The application relates to the field of rail transit DC traction power cable insulation monitoring, in particular to a DC cable insulation monitoring device, system and application. BACKGROUND

[0002] The DC devices in the traction power supply system are generally installed in an insulated manner. With the passage of time, the aging of the insulation material and the reduction of the insulation performance will have a great impact on the safety of the devices. Therefore, the insulation of the devices needs to be monitored. The external interface of the DC device needs to be connected with a DC cable. The rail transit DC traction power cable, as a part of the catenary, also needs to be subjected to insulation monitoring. The high-voltage DC power cable can be divided into four categories according to the different insulation media: viscous impregnated paper insulation DC cable, gas-filled DC cable, oil-filled DC cable and extruded insulation DC cable (also known as polymer insulated cable). The DC cable for rail transit is generally a 1500V or below extruded insulation DC cable. The insulation mixture usually uses EPR, HEPR, XLPE and the like. The insulation mixture is usually subjected to detection of electrical performance indexes such as dielectric constant, DC volume resistivity, dielectric strength and loss factor. In addition, the space charge test under different temperatures and field strengths and the insulation conductivity test under different temperatures and field strengths are also included to evaluate the temperature and conductivity and electric field characteristics of the insulation material. The factory detection of the cable cannot completely prevent the occurrence of insulation failure in the application scene. Although there is no mandatory requirement for the insulation monitoring of the DC cable under the operating conditions in China, more and more users have begun to require this in the export projects. The following methods are usually used for the detection of the cable, each of which has different shortcomings: partial discharge monitoring, mainly using high-frequency sensors or pulse current method, which can detect some signs in the early stage of insulation aging, but is mainly applied to AC systems; insulation resistance test, which is the most basic detection method, mainly using a megohm meter to measure the insulation resistance value of the cable; withstand voltage test: the insulation strength of the cable is detected by applying a high voltage, which has a certain damage to the cable and belongs to a lossy detection; and other auxiliary methods, which all lack continuous monitoring and cannot distinguish the insulation fault types. SUMMARY

[0003] In view of the deficiencies of the prior art, the present application provides a DC cable insulation monitoring device to solve the problems of the prior art that lack continuous monitoring and cannot distinguish the insulation fault types.

[0004] To achieve the above purpose, the technical scheme is as follows: The application relates to a direct-current cable insulation monitoring device, which is composed of a cable core, an inner insulation layer, a metal sheath and an outer insulation layer from inside to outside, and is characterized by comprising a resistance circuit, an isolation transmitter and a protection device; the resistance circuit comprises a first resistor, a second resistor and a third resistor connected in series, one end of the first resistor is connected to the cable core at one end of the resistance circuit, the other end of the first resistor is connected to the metal sheath, and the other end of the resistance circuit is connected to a negative bus; the isolation transmitter is connected in parallel across the third resistor, is used for collecting a voltage signal across the third resistor, and outputs the voltage signal after isolation and conversion; the protection device is connected to the output end of the isolation transmitter, is used for receiving the signal output by the isolation transmitter, and compares the value of the signal with a preset tripping threshold value to determine the occurrence and type of insulation fault, wherein the insulation fault type includes an insulation fault between the sheath and the ground and an insulation fault between the cable core and the sheath.

[0005] Preferably, the signal uses a voltage signal, the isolation transmitter is an isolation voltage transmitter, the tripping threshold value comprises a first threshold value and a second threshold value, when the voltage signal is greater than or equal to the first threshold value, the insulation fault is determined to occur and the insulation fault type is determined to be the insulation fault between the cable core and the sheath, and when the voltage signal is less than or equal to the second threshold value, the insulation fault is also determined to occur and the insulation fault type is determined to be the insulation fault between the sheath and the ground.

[0006] Preferably, the protection device further internally calculates the value of the voltage signal as a current value.

[0007] Preferably, the protection device outputs a tripping control command when the insulation fault is determined to occur.

[0008] Preferably, the first threshold value and the second threshold value are set according to percentage values of a full range.

[0009] Preferably, the percentage values are calculated by the formulae p X(%) =100x(U Xtrip -U X(0) )÷[(U X(100%) -U Xrip )+(U Xtrip -U X(0) )] and p Z(%) =100x(U Z(0) -U Ztrip )÷[(U Ztrip -U Z(100%) )+(U Z(0) -U Ztrip )], wherein p X(%) is the first threshold value percentage required when the insulation fault between the cable core and the sheath occurs, U XtripU Z(%) Second threshold percentage to be set for the insulation fault Z between the sheath and ground Ztrip U X(0) and U Z(0) =U X(0) =U Z(0) =R 213 xU nominal / (R 211 +R 212 +R 213 ) where U nominal is the voltage of the cable core in the absence of faults.

[0010] Based on the same inventive concept, the present solution also discloses a DC cable insulation monitoring system, comprising the aforementioned DC cable insulation monitoring device, further comprising a DC circuit breaker and a control center; the output of the protection device of the DC cable insulation monitoring device is connected to the corresponding DC circuit breaker of the corresponding DC cable, for issuing a trip command to the corresponding DC circuit breaker when determining that an insulation fault has occurred; the protection device is also connected to the control center through a communication network, for uploading monitoring data and fault information to the control center.

[0011] Preferably, the communication network uses Ethernet or optical fiber.

[0012] Preferably, the trip command takes into account a delay for handling transient and harmonic phenomena.

[0013] Based on the same inventive concept, the present solution also discloses an isolated transmitter and protection device as an application of DC cable insulation monitoring, connected to a resistance circuit to form the aforementioned DC cable insulation monitoring device, for monitoring the occurrence and type of insulation faults.

[0014] The beneficial effects of the scheme relative to the prior art are that: the provided scheme realizes monitoring through a pure hardware circuit and simple operation, is easy to integrate into an existing system, adopts percentage setting for the trip threshold, can adapt to different system voltages and different sensitivity requirements, has strong versatility, can clearly distinguish between the two types of insulation faults by setting a resistance circuit and monitoring the voltage of the resistance therein, and can continuously detect the insulation state of the DC cable, and by using the operation function of the protection device, the corresponding power supply circuit can be tripped under different insulation fault conditions by calculating and setting the trip threshold, and data can be transmitted to the control center through the network. The scheme solves the problems of slow response speed, difficulty in accurately distinguishing fault types, and dependence on complex and high-cost equipment in existing DC cable insulation fault detection through the cooperation of a hardware circuit and intelligent criteria, realizes real-time and accurate monitoring of the insulation state of the DC cable, does not require additional signal injection equipment, significantly improves detection efficiency and reliability, and reduces system complexity and cost, and is especially suitable for insulation safety protection in high-voltage scenarios such as DC traction power supply. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a schematic diagram of an embodiment of the DC cable insulation monitoring device of the present application; Figure 2 It is a schematic diagram of the framework of an embodiment of the DC cable insulation monitoring system of the present application; Figure 3 a is a schematic diagram of an insulation fault between the cable core and the sheath of the DC cable insulation monitoring device embodiment; Figure 3 b is an equivalent schematic diagram of the resistance circuit when an insulation fault occurs between the cable core and the sheath of the DC cable insulation monitoring device embodiment; Figure 4 a is a schematic diagram of an insulation fault between the sheath and the ground of the DC cable insulation monitoring device embodiment; Figure 4 b is an equivalent schematic diagram of the resistance circuit when an insulation fault occurs between the sheath and the ground of the DC cable insulation monitoring device embodiment; Figure 5 It is a schematic diagram of the circuit when the DC cable insulation monitoring device embodiment of the present application is applied to a 900VDC system; Figure 6 It is a schematic diagram of the circuit when another embodiment of the DC cable insulation monitoring device of the present application is applied to a 1800VDC system; Figure 7 It is a trip threshold trend graph of an embodiment of the protection device of the DC cable insulation monitoring device of the present application; Figure 8 It is a schematic diagram of the composition of the DC cable and the two types of insulation faults applied by the present application; Among them, 1: DC cable, 11: cable core, 12: metal sheath, 13: internal insulation layer, 14: external insulation layer, 2: DC cable insulation monitoring device, 21: resistance circuit, 211: first resistor, 2111: first series resistor, 2112: second series resistor, 212: second resistor, 213: third resistor, 22: isolation transmitter, 23: protection device, 3: control center, 4: DC circuit breaker, Z: insulation fault between sheath and ground, X: insulation fault between cable core and sheath, 6: XG terminal. Detailed Implementation

[0016] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0017] This embodiment provides a technical solution: a DC cable insulation monitoring device for monitoring the insulation of DC cables. Generally, a DC cable 1 used for rail transit traction is composed of a cable core 11, an inner insulation layer, a metal sheath 12, and an outer insulation layer from the inside out. In this embodiment, the inner insulation layer is an internal heat insulation layer 13, and the outer insulation layer is an external heat insulation layer 14. Both the internal heat insulation layer 13 and the external heat insulation layer use insulating materials. The internal heat insulation layer 13 usually uses an extruded solid medium such as cross-linked polyethylene XLPE. The external heat insulation layer 14 serves as the outer sheath, and can be polyethylene ST7, etc. The metal sheath 12 can be a corrugated aluminum sheath or other metal shielding structures.

[0018] The entire DC cable insulation monitoring device 2 includes a resistance circuit 21, an isolation transmitter 22, and a protection device 23; the resistance circuit includes a first resistor 211, a second resistor 212, and a third resistor 213 connected in series. One end of the first resistor 211, located at the end of the resistance circuit, is connected to the cable core 11, and its voltage is U. nominal The protection device 23 originally had an analog input that continuously collected the busbar voltage value. Cable core 11 was directly connected to the busbar, thus enabling the collection of U... nominal The other end of the first resistor 211 is connected to the metal sheath 12 via the XG terminal 6, and the other end of the resistor circuit is connected to the negative busbar; the input terminal of the isolation transmitter 22 is connected in parallel across the third resistor 213 to continuously acquire the voltage signal across the third resistor, and outputs an analog signal after isolating and converting the voltage signal; in this embodiment, the protection device 23 uses a sampling period of 20μs when receiving the signal output by the isolation transmitter 22, and the protection device 23 also internally calculates the voltage signal value as a current value; the voltage U across the third resistor 213This reflects the current flowing in the third resistor 213. Under fault-free conditions, this current has an equilibrium point. However, in the event of a fault, the current may increase significantly or disappear completely. The equilibrium point can be determined based on the cable core voltage U. nominal The value is continuously recalculated. The protection device 23 is connected to the output terminal of the isolation transmitter 22 to receive the signal output by the isolation transmitter 22 and compare the value of the signal with the preset trip threshold to determine the occurrence and type of insulation fault and output a trip control command. The insulation fault types include insulation fault Z between the sheath and the ground and insulation fault X between the cable core and the sheath.

[0019] In this scheme, the signal used is a voltage signal. The isolation transmitter 22 is an isolation voltage transmitter. The tripping threshold includes a first threshold and a second threshold. When the voltage signal is greater than or equal to the first threshold, an insulation fault is determined to have occurred and the type of insulation fault is determined to be insulation fault X between the cable core and the sheath. When the voltage signal is less than or equal to the second threshold, an insulation fault is also determined to have occurred and the type of insulation fault Z between the sheath and the ground. The signal transmitted by the isolation transmitter 22 is U. 213 U when there is no fault 213 The equilibrium point voltage is given by formula U. 213 =R 213 xU nominal / (R 211 +R 212 +R 213 )calculate.

[0020] In the extreme cases of the two insulation faults, when an insulation fault X occurs between the cable core and the sheath, the first resistor 211 is short-circuited, and the current in the third resistor will reach its maximum value; when an insulation fault Z occurs between the sheath and the ground, the second resistor 212 and the third resistor 213 are short-circuited, and the current in the third resistor is close to 0.

[0021] For traction power supply systems, the primary and secondary insulation isolation voltage is 10kV. High-voltage isolation type transmitters must be used to meet the safety isolation requirements of GB / T 17045 or EN61140 and the insulation requirements of GB 4793.1 or EN 61010-1. For example, the MIU10 transmitter from Saixuelong can be used.

[0022] When the insulation fault is detected, the protection device also outputs a trip control command. To adapt to different system voltages and sensitivity requirements, the protection device 23 sets the first and second thresholds as percentages of full scale: with full-scale faults defined as 100% and the balance point as 0. The percentages are calculated using the following formula: p X(%) =100×(UXtrip - U X(0) )÷[(U X(100%) -U Xrip )+(U Xtrip -U X(0) )] p Z(%) =100×(U Z(0) -U Ztrip )÷[(U Ztrip - UZ(100%) )+(U Z(0) -U Ztrip )] Where p X(%) U is the first threshold percentage that needs to be set when an insulation fault X occurs between the cable core and the sheath. Xtrip p is the expected trip voltage for an insulation fault X between the cable core and sheath. Z(%) The second threshold percentage that needs to be set when there is an insulation fault Z between the sheath and ground, U Ztrip The expected trip voltage is the voltage at which an insulation fault Z occurs between the sheath and ground.

[0023] Traction power supply systems typically use two voltage standards: 900VDC and 1800VDC. The application of this solution to these two systems is as follows.

[0024] In the embodiment of the 900VDC system, the first resistor 211, the second resistor 212, and the third resistor 213 are 82kΩ, 6.8kΩ, and 250Ω respectively. Under normal insulation conditions, U nominal 900V, U 213 The voltage is 2.53V, which is the equilibrium point, and there is no current leakage. When an insulation fault X occurs between the cable core and the sheath, U... 213 32V, I 213 It is 0.128A; when an insulation fault occurs between the sheath and ground, U 213 0V, I 213 It is 0A. That is, the equilibrium point voltage U. X(0) =U Z(0) =2.53V, the full-scale voltage U when an insulation fault X occurs between the cable core and the sheath. X(100%) =32V, the full-scale voltage U when an insulation fault Z occurs between the sheath and ground. Z(100%) =0V. In this embodiment, U Xtrip To set it to 20V, the final calculated p X(%) It is 59.28, U Ztrip To set it to 1.5V, the final calculated p Z(%) It is 40.71.

[0025] In this embodiment of the 1800VDC system, the first resistor 211, the second resistor 212, and the third resistor 213 are 164kΩ, 6.8kΩ, and 250Ω, respectively. In this embodiment, the first resistor 211 can be composed of two identical 82kΩ first resistors connected in series, a first series link 2111 and a second series link 2112. Under normal insulation conditions, U nominal 1800V, U 213 The voltage is 2.63V, which is the equilibrium point, and there is no current leakage. When an insulation fault X occurs between the cable core and the sheath, U... 213 64V, I 213 It is 0.256A; when an insulation fault occurs between the sheath and ground, U 213 0V, I 213 It is 0A. That is, the equilibrium point voltage U. X(0) =U Z(0) =2.63V, the full-scale voltage U when an insulation fault X occurs between the cable core and the sheath. X(100%) =64V, the full-scale voltage U when an insulation fault Z occurs between the sheath and ground. Z(100%) =0V. In this embodiment, U Xtrip To set it to 40V, the final calculated p X(%) It is 70.02, U Ztrip To set it to 1.5V, the final calculated p Z(%) It is 42.97.

[0026] In practical applications, using percentages is more convenient. For example, in both examples, the percentage is 60%, and in the 900V example, U... Xtrip That is, 20.212V, U Ztrip In the 1800V example, U is 1.012V. Xtrip It is 39.452V, U Ztrip It is 1.052V.

[0027] Based on the same inventive concept, this solution also discloses a DC cable insulation monitoring system, including the aforementioned DC cable insulation monitoring device 2, a DC circuit breaker 4, and a control center 3. The output terminal of the protection device 23 of the DC cable insulation monitoring device 2 is connected to the DC circuit breaker 4 corresponding to the corresponding DC cable 1, and is used to issue a trip command to the corresponding DC circuit breaker 4 when an insulation fault is detected. The protection device 23 is also connected to the control center 4 through a communication network, and is used to upload monitoring data and fault information to the control center 4 for analysis and processing. The communication network can use Ethernet RJ45 or fiber optic. The entire DC cable insulation monitoring system consists of multiple DC cables 1 whose cable cores 11 and metal sheaths 12 are respectively connected to the resistance circuit 21 of the DC cable monitoring device 2. Through the isolation conversion of the isolation transmitter 22 and the analog calculation of the protection device 23, the DC circuit breaker connected to the corresponding DC cable is tripped when a trip threshold is reached, thereby effectively responding to DC cable insulation faults. The trip command takes into account the delay for handling transient and harmonic phenomena. If a sudden abnormal value of the third resistor is detected within the specified delay time, a trip command will not be sent. Since the value measured by protection device 23 is very small, shielding interference only in the external circuit cannot completely avoid interference. Therefore, it is necessary to filter short-term faults within the protection device. When the transient fault value reaches the trip threshold for a very short time, such as a few milliseconds, and then disappears, it may be due to the accumulation of noise interference at some point in the measurement circuit, rather than a steady-state value reached by the actual fault value. Protection device 23 will filter out this situation through delay filtering to avoid frequently sending trip commands to the circuit breaker. For example, when the delay value is set to 50ms, if the duration of this fault value is less than 50 milliseconds, a trip command will not be sent.

[0028] Based on the same inventive concept, this solution also discloses an isolation transmitter 22 and a protection device 23 for the application of DC cable insulation monitoring. They are connected with the resistor circuit 21 to form the aforementioned DC cable insulation monitoring device 2, which is used to monitor the occurrence and type of insulation faults.

[0029] In the description of this application, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0030] The above description is only a preferred embodiment of the present solution, but the scope of protection claimed by the present solution is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in this application, based on the technical solution and inventive concept of this application, should be included within the scope of protection of this application.

Claims

1. A DC cable insulation monitoring device, wherein the DC cable is composed of a cable core (11), an inner insulation layer, a metal sheath (12), and an outer insulation layer from the inside out, characterized in that: The system includes a resistor circuit (21), an isolation transmitter (22), and a protection device (23). The resistor circuit includes a first resistor (211), a second resistor (212), and a third resistor (213) connected in series. One end of the first resistor (211) is connected to the cable core (11), and the other end of the first resistor (211) is connected to the metal sheath (12). The other end of the resistor circuit is connected to the negative busbar. The input terminal of the isolation transmitter (22) is connected in parallel across the third resistor (213) to collect the voltage signal across the third resistor and output the voltage signal after isolation and conversion. The protection device (23) is connected to the output terminal of the isolation transmitter (22) to receive the signal output by the isolation transmitter (22) and compare the value of the signal with a preset trip threshold to determine the occurrence and type of insulation fault. The insulation fault types include insulation faults between the sheath and the ground (Z) and insulation faults between the cable core and the sheath (X).

2. The DC cable insulation monitoring device according to claim 1, characterized in that: The signal uses a voltage signal, the isolation transmitter (22) is an isolation voltage transmitter, the trip threshold includes a first threshold and a second threshold, when the voltage signal is greater than or equal to the first threshold, the insulation fault is judged to have occurred and the insulation fault type is judged to be an insulation fault between the cable core and the sheath (X), when the voltage signal is less than or equal to the second threshold, the insulation fault is also judged to have occurred and the insulation fault type is judged to be an insulation fault between the sheath and the ground (Z).

3. The DC cable insulation monitoring device according to claim 2, characterized in that: The protection device (23) also internally calculates the voltage signal value as a current value.

4. The DC cable insulation monitoring device according to claim 2, characterized in that: When an insulation fault is detected, the protection device outputs a trip control command.

5. The DC cable insulation monitoring device according to claim 4, characterized in that: The first threshold and the second threshold are set as a percentage of full scale.

6. The three-station anti-misoperation interlocking device according to claim 5, characterized in that: The percentage is derived from the formula. Calculate, where p X(%) U is the first threshold percentage that needs to be set when an insulation fault X occurs between the cable core and the sheath. Xtrip p is the expected trip voltage for an insulation fault X between the cable core and sheath. Z(%) The second threshold percentage that needs to be set when there is an insulation fault Z between the sheath and ground, U Ztrip U is the expected trip voltage for an insulation fault Z between the sheath and ground. X(0) and U Z(0) The equilibrium point voltage is given by formula U. X(0) =U Z(0) =R 213 xU nominal / (R 211 +R 212 +R 213 ) calculate, where U nominal The voltage of the cable core (11) when there is no fault.

7. A DC cable insulation monitoring system, characterized in that: The device includes a DC cable insulation monitoring device (2) as described in any one of claims 1 to 6, and also includes a DC circuit breaker (4) and a control center (3); the output terminal of the protection device (23) of the DC cable insulation monitoring device (2) is connected to the DC circuit breaker (4) corresponding to the corresponding DC cable (1), and is used to issue a trip command to the corresponding DC circuit breaker (4) when an insulation fault is determined to occur; the protection device (23) is also connected to the control center (4) through a communication network, and is used to upload monitoring data and fault information to the control center (4).

8. The DC cable insulation monitoring system according to claim 7, characterized in that: The communication network uses Ethernet or fiber optics.

9. The DC cable insulation monitoring system according to claim 7, characterized in that: The trip command takes into account the delay for handling transient and harmonic phenomena.

10. An isolation transmitter and protection device for DC cable insulation monitoring, characterized in that: The DC cable insulation monitoring device (2) is connected to the resistor circuit (21) to form the DC cable insulation monitoring device (2) according to any one of claims 1 to 6, for monitoring the occurrence and type of insulation faults.

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