Residual voltage detection circuit and line protection device

Abstract

The invention relates to the technical field of power fault detection, and mainly relates to a residual voltage detection circuit and a line protection device.The residual voltage detection circuit comprises a sampling circuit and a signal holding circuit, the signal holding circuit adopts double coils and comprises a first coil and a second coil, and a loop where the first coil is located is provided with a first controllable switch tube in series; the first controllable switch tube is connected when the sampling circuit collects residual voltage, a second controllable switch tube is arranged in a loop where the second coil is located in series, the control end of the second controllable switch tube is used for being connected with a feeder terminal, and the signal holding circuit further comprises a residual voltage output branch used for being connected with the feeder terminal. The residual voltage output branch is provided with a first contact of the double coils in series. A sampling circuit is used for obtaining a residual voltage signal generated at the moment of closing of a faulty power grid line, and a signal holding circuit formed by double coils is used for transmitting a sampling signal to a feeder terminal, so that the feeder terminal can still receive the residual voltage signal when the feeder terminal loses power after power supply is recovered.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power failure detection, and mainly relates to a residual voltage detection circuit and a line protection device. BACKGROUND

[0002] With the continuous upgrading of the power grid system, higher and higher requirements are put forward for the reliability, safety and economy of the power grid. In the process of power grid operation, sudden failures may occur, causing the power grid to trip. The fault causing the power grid to trip can be divided into transient fault and permanent fault, wherein the transient fault may be caused by a tree branch or an animal briefly touching the tree branch, when the power supply is cut off by tripping, the electric arc will extinguish itself, and the circuit will resume normal operation, the permanent fault may be caused by damage to key devices, broken wires and other reasons that the circuit cannot recover automatically. Therefore, after the current power grid trips, the protection device will try to close again within a set time (generally 0.5 seconds), if the type of fault occurring in the power grid is a transient fault, the power grid will restore connection and resume power supply, and the power grid will resume normal operation; if the type of fault occurring in the power grid is a permanent fault, the protection device will trip again and be locked, and the terminal records that the permanent protection occurs in the line, and does not perform automatic closing again until the line is repaired.

[0003] Permanent faults are generally judged by residual voltage detection, but in the existing power system, when a permanent fault occurs, the feeder terminal may lose power and the backup power supply may also lose power at the same time, at the moment of reclosing, the feeder terminal is still in a power loss state, at this time the feeder terminal cannot work, and therefore cannot record the residual voltage signal at the moment of reclosing, when the feeder terminal restores power supply after reclosing, since no residual voltage signal is recorded, the feeder terminal cannot generate a lock signal, at this time the protection device will close again in the permanent fault circuit, causing tripping again, and even further damaging the circuit, causing the power outage range to further expand, threatening the safe and stable operation of the power grid. SUMMARY

[0004] The purpose of the present application is to provide a residual voltage detection circuit and a line protection device to solve the problem that the feeder terminal cannot record whether a residual voltage is generated in the power loss state in the existing residual voltage detection method.

[0005] The application provides a residual voltage detection circuit to solve the above technical problems, which comprises a sampling circuit and a signal holding circuit, the signal holding circuit adopts a double coil, comprising a first coil and a second coil, a first controllable switch tube is connected in series in a loop of the first coil, the first controllable switch tube is turned on when the sampling circuit collects residual voltage, so that the first coil is turned on, a second controllable switch tube is connected in series in a loop of the second coil, a control end of the second controllable switch tube is used for being connected with a feeder terminal, and the second controllable switch tube is turned on when a reset signal sent by the feeder terminal after the power supply of a power grid line is restored is received, the signal holding circuit further comprises a residual voltage output branch used for being connected with the feeder terminal, a first contact point of the double coil is connected in series on the residual voltage output branch, the first contact point is actuated when the first coil is turned on, and the actuation is maintained until the second coil is turned on, and the sampling circuit is provided with a second contact point, the second contact point is used for being disconnected when the first coil is turned on and being connected when the second coil is turned on, so that the reset of the residual voltage detection circuit is realized.

[0006] Further, a control contact point provided in the sampling circuit is further included, a coil of the control contact point is arranged in the power grid line, so that the coil of the control contact point loses power when the power grid line fails, the control contact point is closed, and the residual voltage detection circuit is connected to the failed power grid line.

[0007] Further, the sampling circuit adopts a voltage dividing circuit, two ends of the voltage dividing circuit are used for being connected with the power grid line, so as to collect a voltage signal at the moment when the power grid line is closed, a voltage dividing point of the voltage dividing circuit is connected with the control end of the first controllable switch tube, and the first controllable switch tube is turned on when the voltage signal of the voltage dividing circuit reaches a residual voltage threshold.

[0008] Further, the residual voltage detection circuit further comprises a third controllable switch tube, the third controllable switch tube is arranged in the voltage dividing circuit, a control end of the third controllable switch tube is connected with a line in which the first coil is arranged, so as to be turned on when the first coil is electrified, and the first controllable switch tube is saturated and turned on.

[0009] Further, the sampling circuit is connected to the power grid line through an isolation rectifier circuit, the isolation rectifier circuit comprises a mutual inductor and a rectifier circuit, a high-voltage end of the mutual inductor is used for being connected with the power grid line, a low-voltage end of the mutual inductor is connected with an alternating current end of the rectifier circuit, and a direct current end of the rectifier circuit is connected with the sampling circuit.

[0010] Further, the first coil and the second coil are both connected in parallel with diodes, so as to consume residual current of the first coil or the second coil when the first coil or the second coil loses power.

[0011] Further, the first controllable switch tube, the second controllable switch tube and the third controllable switch tube are all triodes or MOS tubes.

[0012] Further, the resistance in the voltage dividing circuit comprises a controllable resistance.

[0013] Further, a voltage stabilizing diode is arranged in the loop where the second coil is located.

[0014] The line protection device comprises a feeder terminal and a residual voltage detection circuit. The residual voltage detection circuit comprises a sampling circuit and a signal holding circuit. The signal holding circuit comprises a double coil, including a first coil and a second coil. A first controllable switch tube is arranged in series in the loop where the first coil is located. The first controllable switch tube is turned on when the sampling circuit collects residual voltage, so as to turn on the first coil. A second controllable switch tube is arranged in series in the loop where the second coil is located. The control end of the second controllable switch tube is used to be connected with the feeder terminal, and is turned on when receiving the reset signal sent by the feeder terminal after the power supply of the power grid line is restored. The signal holding circuit further comprises a residual voltage output branch used to be connected with the feeder terminal. A first contact point of the double coil is arranged in series in the residual voltage output branch. The first contact point is actuated when the first coil is turned on, and maintains actuation until the second coil is turned on. The sampling circuit is provided with a second contact point. The second contact point is used to be disconnected when the first coil is turned on, and is closed when the second coil is turned on, so as to realize the reset of the residual voltage detection circuit.

[0015] Further, a control contact point is arranged in the sampling circuit. The coil of the control contact point is arranged in the power grid line, so that the coil of the control contact point loses power when the power grid line fails, the control contact point is closed, and the residual voltage detection circuit is connected to the failed power grid line.

[0016] Further, the sampling circuit adopts a voltage dividing circuit. The two ends of the voltage dividing circuit are used to be connected with the power grid line, so as to collect the voltage signal at the moment when the power grid line is closed. The voltage dividing point of the voltage dividing circuit is connected with the control end of the first controllable switch tube. The first controllable switch tube is turned on when the voltage signal of the voltage dividing circuit reaches the residual voltage threshold value.

[0017] Further, the residual voltage detection circuit further comprises a third controllable switch tube. The third controllable switch tube is arranged in the voltage dividing circuit. The control end of the third controllable switch tube is connected with the line where the first coil is located, so as to turn on the first controllable switch tube when the first coil is electrified, and make the first controllable switch tube saturated.

[0018] Further, the sampling circuit is connected to the power grid line through an isolation rectifier circuit. The isolation rectifier circuit comprises a mutual inductor and a rectifier circuit. The high-voltage end of the mutual inductor is used to be connected with the power grid line. The low-voltage end of the mutual inductor is connected with the alternating current end of the rectifier circuit. The direct current end of the rectifier circuit is connected with the sampling circuit.

[0019] Further, the first coil and the second coil are both connected in parallel with a diode, so as to consume the residual current of the first coil or the second coil when the first coil or the second coil loses power.

[0020] Further, the first controllable switch tube, the second controllable switch tube and the third controllable switch tube are all triodes or MOS tubes.

[0021] Furthermore, the resistors in the voltage divider circuit include adjustable resistors.

[0022] Furthermore, a Zener diode is also provided in the circuit containing the second coil.

[0023] The beneficial effects of this invention are as follows: As an improved invention, this invention utilizes a sampling circuit to acquire the residual voltage signal generated at the moment the faulty power grid line is closed, and transmits the sampled signal to the feeder terminal through a signal holding circuit formed by two coils. When residual voltage is generated at the moment the faulty power grid line is closed, the voltage of the sampling circuit can turn on the first coil. At this time, the first contact in the residual voltage output branch is activated, and the state of the first contact can represent the residual voltage signal. After the faulty power grid line is closed, the feeder terminal is energized. At this time, the feeder terminal controls the second coil to conduct, and the first contact will remain activated until the second coil is activated. Therefore, through the cooperation of the first coil and the second coil, the state of the first contact representing the residual voltage signal is maintained until the feeder terminal is energized, ensuring that the feeder terminal can still receive the residual voltage signal when it lost power after power is restored. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the residual voltage detection circuit. Detailed Implementation

[0025] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0026] This invention utilizes a sampling circuit to acquire the residual voltage signal generated at the moment the faulty power grid line is closed, and transmits the sampled signal to the feeder terminal through a signal holding circuit formed by two coils, ensuring that the feeder terminal can still receive the residual voltage signal when it lost power after power is restored.

[0027] Implementation of residual voltage detection circuit This invention proposes a residual voltage detection circuit, which includes a sampling circuit and a signal holding circuit. The sampling circuit is mainly used to collect the residual voltage signal in the power grid line, and the signal holding circuit is used to hold the sampled signal until the feeder terminal is energized.

[0028] Specifically, the signal holding circuit uses a dual-coil configuration, which includes a first coil and a second coil. A first controllable switch is connected in series in the circuit containing the first coil. The first controllable switch turns on when the sampling circuit acquires residual voltage, thus turning on the first coil. A second controllable switch is connected in series in the circuit containing the second coil. The control terminal of the second controllable switch is used to connect to the feeder terminal. The second controllable switch turns on when it receives a reset signal sent by the feeder terminal after the power grid line has been restored.

[0029] The signal holding circuit also includes a residual voltage output branch for connection to the feeder terminal. A first contact with a dual coil is connected in series on the residual voltage output branch. The first contact actuates when the first coil is turned on and remains actuated until the second coil is turned on. The sampling circuit has a second contact, which is used to open when the first coil is turned on and close when the second coil is turned on, thus resetting the residual voltage detection circuit. The residual voltage detection circuit also includes a third controllable switch. The control terminal of the third controllable switch is connected to the line containing the first coil, so that it conducts when the first coil is energized, causing the first controllable switch to saturate and conduct.

[0030] The sampling circuit employs a voltage divider circuit. The two ends of the voltage divider circuit are connected to the power grid line to collect the voltage signal at the moment the power grid line is switched on. The voltage division point of the voltage divider circuit is connected to the control terminal of the first controllable switch. When the voltage signal on the voltage divider circuit reaches the residual voltage threshold, the first controllable switch is turned on. As one embodiment, the voltage divider circuit can be composed of multiple voltage divider resistors connected in series. By setting the resistance ratio of the first and second voltage divider resistors, the residual voltage threshold can be set. To facilitate adjustment of the residual voltage threshold, one of the resistors in the voltage divider circuit can be an adjustable resistor. In this invention, the voltage divider circuit consists of two voltage divider resistors connected in series, and the connection point of the first and second voltage divider resistors is the voltage division point of the voltage divider circuit.

[0031] Since the voltage of the circuit being tested is relatively high, while the operating voltage of the devices in the residual voltage detection circuit is relatively low, the sampling circuit is connected to the power grid line through an isolation rectifier circuit. The isolation rectifier circuit includes a current transformer and a rectifier circuit. The high-voltage end of the current transformer is used to connect to the power grid line, the low-voltage end of the current transformer is connected to the AC end of the rectifier circuit, and the DC end of the rectifier circuit is connected to the sampling circuit.

[0032] To ensure that the residual voltage detection circuit is only connected to the faulty power grid line when the circuit being detected fails, a control contact coil in the sampling circuit is incorporated into the power grid line. When the power grid line fails, the control coil loses power, the corresponding control contact closes, and the residual voltage detection circuit is connected to the faulty power grid line. When power is restored to the power grid line, the control coil is powered again, the control contact opens, and the residual voltage detection circuit is disconnected from the power grid line.

[0033] As one embodiment, the first, second, and third controllable switching transistors in this invention can be transistors or MOSFETs. The following description uses an example where all three switching transistors are transistors and the voltage divider circuit consists of two series-connected voltage divider resistors to illustrate the residual voltage detection circuit. The specific circuit structure is as follows: Figure 1 As shown.

[0034] Specifically, the sampling circuit in the residual voltage detection circuit consists of a first voltage divider resistor R1 and a second voltage divider resistor R3 connected in series. The first coil in the signal holding circuit is the operating coil RL1A shown in the figure, and the first controllable switch is Q2. The collector and emitter of Q2 are connected in the circuit of the operating coil RL1A. The base of Q2 is connected to one end of the second voltage divider resistor R3 through the current-limiting resistor R4, and the other end of the second voltage divider resistor R3 is grounded. The end of the second voltage divider resistor R3 connected to the current-limiting resistor R4 is also connected to the collector of Q1 (i.e., the third controllable switch). A voltage drop resistor R2 is connected to the emitter of Q1, and the base of Q1 is connected in the circuit of the first coil and connected to the collector of Q2.

[0035] The first contact is contact RL1D in the diagram. One end of contact RL1D is connected to a 3.3V power supply, and the other end (CY1 in the diagram) is used for the feeder terminal connection. The second coil is the reset coil RL1B in the diagram. The reset coil RL1B is connected to the collector of Q3 (i.e., the second controllable switch). The emitter of Q3 is grounded, and the base of Q3 is connected to one end of resistor R5. The other end of resistor R5 (CY_FG in the diagram) is connected to the feeder terminal. The base of Q3 is also grounded through resistor R6 to provide a conduction voltage to the base when Q3 is turned on. The reset coil RL1B is also grounded through TVS1.

[0036] The second contact is RL1C in the diagram, and the control contact is RL2B. Contacts RL1C and RL2B are connected in series to connect the sampling circuit and the isolation rectifier circuit. The current transformer in the isolation rectifier circuit is PT1 in the diagram, and the rectifier circuit is ZL1, which can be a full-bridge or half-bridge rectifier. The coil of contact RL2B is RL2A in the diagram. One end of RL2A is grounded, and the other end is connected to the power supply (i.e., the mains line).

[0037] Since the coil is inductive and the current cannot change abruptly, there will still be current in the coil after the external voltage is disconnected. Therefore, in this invention, diodes are connected in parallel across the ends of coils RL1A, RL1B, and RL2A. This allows the current in the coil to form a closed loop through the diodes after the external voltage is disconnected, thus consuming the residual current. To filter the residual input voltage, capacitor C1 is connected in parallel across the sampling circuit, and capacitor C2 is connected in parallel across the second voltage divider resistor R3.

[0038] In operation, if a permanent fault occurs in the power grid, the control coil RL2A is de-energized, the control contact RL2B closes, and the detection circuit connects to the power grid. At the moment of reclosing, the sampling circuit samples the residual voltage through the isolation rectifier circuit. The first voltage divider resistor R1 and the second voltage divider resistor R3 divide the sampled residual voltage, causing the base voltage of Q2 to rise and Q2 to conduct. The first coil, connected in series with Q2, then conducts. At this time, the voltages across the first coil are almost equal, and the emitter voltage of Q1 is higher than its base voltage, causing Q1 to conduct. The parallel connection of R1 and R2 increases the voltage drop across resistor R3, putting Q2 in a saturated conduction state. At this time, the first contact RL1D closes and the second contact RL1C opens, connecting the residual voltage output branch. CY1 is then at a high level (i.e., 3.3V), serving as the residual voltage signal. After reclosing is completed, the power grid line is restored. At this time, the feeder terminal is energized and can receive the CY1 signal. Simultaneously, the feeder terminal outputs a CY_FG to control Q3 to conduct, energizing the reset coil and opening RL1C to complete the detection. At the same time, contact RL1C closes.

[0039] The dual coils used in the signal holding circuit maintain the original state of the contacts even when the coils are de-energized. Therefore, when the first coil is de-energized, the first contact will remain closed and continuously send a residual voltage signal to the feeder terminal, ensuring that the feeder terminal can still receive the residual voltage signal after the power supply is restored.

[0040] When the faulty power grid line is repaired and power is restored, the feeder terminal first sends a reset signal to the circuit where the second coil is located. At this time, the second transistor conducts under the action of the reset signal, the second coil conducts, the first contact is disconnected, and the second contact is closed to restore the residual voltage detection circuit to its initial state.

[0041] Implementation methods of line protection devices This invention proposes a line protection device, including a feeder terminal and a residual voltage detection circuit. The residual voltage detection circuit includes a sampling circuit and a signal holding circuit. The sampling circuit is mainly used to collect the residual voltage signal in the power grid line, and the signal holding circuit is used to hold the sampled signal until the feeder terminal is energized.

[0042] Specifically, the signal holding circuit uses a dual-coil configuration, which includes a first coil and a second coil. A first controllable switch is connected in series in the circuit containing the first coil. The first controllable switch turns on when the sampling circuit acquires residual voltage, thus turning on the first coil. A second controllable switch is connected in series in the circuit containing the second coil. The control terminal of the second controllable switch is used to connect to the feeder terminal. The second controllable switch turns on when it receives a reset signal sent by the feeder terminal after the power grid line has been restored.

[0043] The signal holding circuit also includes a residual voltage output branch for connection to the feeder terminal. A first contact with a dual coil is connected in series on the residual voltage output branch. The first contact actuates when the first coil is turned on and remains actuated until the second coil is turned on. The sampling circuit has a second contact, which is used to open when the first coil is turned on and close when the second coil is turned on, thus resetting the residual voltage detection circuit. The residual voltage detection circuit also includes a third controllable switch. The control terminal of the third controllable switch is connected to the line containing the first coil, so that it conducts when the first coil is energized, causing the first controllable switch to saturate and conduct.

[0044] The sampling circuit employs a voltage divider circuit. The two ends of the voltage divider circuit are connected to the power grid line to collect the voltage signal at the moment the power grid line is switched on. The voltage division point of the voltage divider circuit is connected to the control terminal of the first controllable switch. When the voltage signal on the voltage divider circuit reaches the residual voltage threshold, the first controllable switch is turned on. As one embodiment, the voltage divider circuit can be composed of multiple voltage divider resistors connected in series. By setting the resistance ratio of the first and second voltage divider resistors, the residual voltage threshold can be set. To facilitate adjustment of the residual voltage threshold, one of the resistors in the voltage divider circuit can be an adjustable resistor. In this invention, the voltage divider circuit consists of two voltage divider resistors connected in series, and the connection point of the first and second voltage divider resistors is the voltage division point of the voltage divider circuit.

[0045] Since the voltage of the circuit being tested is relatively high, while the operating voltage of the devices in the residual voltage detection circuit is relatively low, the sampling circuit is connected to the power grid line through an isolation rectifier circuit. The isolation rectifier circuit includes a current transformer and a rectifier circuit. The high-voltage end of the current transformer is used to connect to the power grid line, the low-voltage end of the current transformer is connected to the AC end of the rectifier circuit, and the DC end of the rectifier circuit is connected to the sampling circuit.

[0046] To ensure that the residual voltage detection circuit is only connected to the faulty power grid line when the circuit being detected fails, a control contact coil in the sampling circuit is incorporated into the power grid line. When the power grid line fails, the control coil loses power, the corresponding control contact closes, and the residual voltage detection circuit is connected to the faulty power grid line. When power is restored to the power grid line, the control coil is powered again, the control contact opens, and the residual voltage detection circuit is disconnected from the power grid line.

[0047] As one embodiment, the first, second, and third controllable switching transistors in this invention can be transistors or MOSFETs. The following description uses an example where all three switching transistors are transistors and the voltage divider circuit consists of two series-connected voltage divider resistors to illustrate the residual voltage detection circuit. The specific circuit structure is as follows: Figure 1 As shown.

[0048] Specifically, the sampling circuit in the residual voltage detection circuit consists of a first voltage divider resistor R1 and a second voltage divider resistor R3 connected in series. The first coil in the signal holding circuit is the operating coil RL1A shown in the figure, and the first controllable switch is Q2. The collector and emitter of Q2 are connected in the circuit of the operating coil RL1A. The base of Q2 is connected to one end of the second voltage divider resistor R3 through the current-limiting resistor R4, and the other end of the second voltage divider resistor R3 is grounded. The end of the second voltage divider resistor R3 connected to the current-limiting resistor R4 is also connected to the collector of Q1 (i.e., the third controllable switch). A voltage drop resistor R2 is connected to the emitter of Q1, and the base of Q1 is connected in the circuit of the first coil and connected to the collector of Q2.

[0049] The first contact is contact RL1D in the diagram. One end of contact RL1D is connected to a 3.3V power supply, and the other end (CY1 in the diagram) is used for the feeder terminal connection. The second coil is the reset coil RL1B in the diagram. The reset coil RL1B is connected to the collector of Q3 (i.e., the second controllable switch). The emitter of Q3 is grounded, and the base of Q3 is connected to one end of resistor R5. The other end of resistor R5 (CY_FG in the diagram) is connected to the feeder terminal. The base of Q3 is also grounded through resistor R6 to provide a conduction voltage to the base when Q3 is turned on. The reset coil RL1B is also grounded through TVS1.

[0050] The second contact is RL1C in the diagram, and the control contact is RL2B. Contacts RL1C and RL2B are connected in series to connect the sampling circuit and the isolation rectifier circuit. The current transformer in the isolation rectifier circuit is PT1 in the diagram, and the rectifier circuit is ZL1, which can be a full-bridge or half-bridge rectifier. The coil of contact RL2B is RL2A in the diagram. One end of RL2A is grounded, and the other end is connected to the power supply (i.e., the mains line).

[0051] To filter the input residual voltage, capacitor C1 is connected in parallel with the sampling circuit, and capacitor C2 is connected in parallel with the second voltage divider resistor R3.

[0052] In operation, if a permanent fault occurs in the power grid, the control coil RL2A is de-energized, the control contact RL2B closes, and the detection circuit connects to the power grid. At the moment of reclosing, the sampling circuit samples the residual voltage through the isolation rectifier circuit. The first voltage divider resistor R1 and the second voltage divider resistor R3 divide the sampled residual voltage, causing the base voltage of Q2 to rise and Q2 to conduct. The first coil, connected in series with Q2, then conducts. At this time, the voltages across the first coil are almost equal, and the emitter voltage of Q1 is higher than its base voltage, causing Q1 to conduct. The parallel connection of R1 and R2 increases the voltage drop across resistor R3, putting Q2 in a saturated conduction state. At this time, the first contact RL1D closes and the second contact RL1C opens, connecting the residual voltage output branch. CY1 is then at a high level (i.e., 3.3V), serving as the residual voltage signal. After reclosing is completed, the power grid line is restored. At this time, the feeder terminal is energized and can receive the CY1 signal. Simultaneously, the feeder terminal outputs a CY_FG to control Q3 to conduct, energizing the reset coil and opening RL1C to complete the detection. At the same time, contact RL1C closes.

[0053] The dual coils used in the signal holding circuit maintain the original state of the contacts even when the coils are de-energized. Therefore, when the first coil is de-energized, the first contact will remain closed and continuously send a residual voltage signal to the feeder terminal, ensuring that the feeder terminal can still receive the residual voltage signal after the power supply is restored.

[0054] When the faulty power grid line is repaired and power is restored, the feeder terminal first sends a reset signal to the circuit where the second coil is located. At this time, the second transistor conducts under the action of the reset signal, the second coil conducts, the first contact is disconnected, and the second contact is closed to restore the residual voltage detection circuit to its initial state.

Claims

1. A residual voltage detection circuit characterized by comprising: The residual voltage detection circuit comprises a sampling circuit and a signal holding circuit, the signal holding circuit adopts a double coil, comprising a first coil and a second coil, a first controllable switch tube is connected in series in a loop in which the first coil is located, the first controllable switch tube is turned on when the sampling circuit collects the residual voltage, so that the first coil is turned on, a second controllable switch tube is connected in series in a loop in which the second coil is located, a control end of the second controllable switch tube is used to be connected with a feeder terminal, and the second controllable switch tube is turned on when receiving a reset signal sent by the feeder terminal after the power supply of the power grid line is restored, the signal holding circuit further comprises a residual voltage output branch used to be connected with the feeder terminal, a first contact point of the double coil is connected in series on the residual voltage output branch, the first contact point is actuated when the first coil is turned on, and the actuation is maintained until the second coil is turned on, the sampling circuit is provided with a second contact point, the second contact point is used to be disconnected when the first coil is turned on and is used to be closed when the second coil is turned on, so that the reset of the residual voltage detection circuit is realized.

2. The residual voltage detection circuit according to claim 1, characterized by The control contact point is further arranged in the sampling circuit, a coil of the control contact point is arranged in the power grid line, so that the coil of the control contact point loses power when the power grid line fails, the control contact point is closed, and the residual voltage detection circuit is connected to the failed power grid line.

3. The residual voltage detection circuit according to claim 1, characterized by The sampling circuit adopts a voltage dividing circuit, two ends of the voltage dividing circuit are used to be connected with the power grid line, so as to collect a voltage signal at the moment when the power grid line is closed, a voltage dividing point of the voltage dividing circuit is connected with the control end of the first controllable switch tube, and the first controllable switch tube is turned on when the voltage signal of the voltage dividing circuit reaches a residual voltage threshold.

4. The residual voltage detection circuit according to claim 3, characterized by The residual voltage detection circuit further comprises a third controllable switch tube, the third controllable switch tube is arranged in the voltage dividing circuit, a control end of the third controllable switch tube is connected with a line in which the first coil is located, so as to be turned on when the first coil is electrified, and the first controllable switch tube is saturated and turned on.

5. The residual voltage detection circuit according to claim 1, wherein The sampling circuit is connected to the power grid line through an isolation rectifier circuit, the isolation rectifier circuit comprises a mutual inductor and a rectifier circuit, a high-voltage end of the mutual inductor is used to be connected with the power grid line, a low-voltage end of the mutual inductor is connected with an alternating current end of the rectifier circuit, and a direct current end of the rectifier circuit is connected with the sampling circuit.

6. The residual voltage detection circuit according to claim 3, wherein The first coil and the second coil are both connected in parallel with diodes, so as to consume residual current of the first coil or the second coil when the first coil or the second coil loses power.

7. The residual voltage detection circuit according to claim 1, wherein The first controllable switch tube, the second controllable switch tube and the third controllable switch tube are all triodes or MOS tubes.

8. The residual voltage detection circuit according to claim 3, wherein The resistance in the voltage dividing circuit comprises a controllable resistance.

9. The residual voltage detection circuit according to claim 1, wherein The loop in which the second coil is located is further provided with a voltage stabilizing diode.

10. A line protection device, comprising a feeder terminal and a residual voltage detection circuit, characterized in that, The residual voltage detection circuit adopts the residual voltage detection circuit in any one of claims 1-9.

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