A drag cable leakage fault monitor

Abstract

The present invention relates to a leakage fault monitor for a dragged cable, which includes a current detection circuit, a controller, a zero-sequence current transformer, and a resistor R5; the zero-sequence current transformer detects the phasor sum of the three-phase currents of the dragged cable and outputs a zero-sequence current from the secondary side inductance L; the current detection circuit includes a current-to-voltage conversion circuit, an electronic control pulse voltage source, and a signal amplification circuit. The current-to-voltage conversion circuit converts the zero-sequence current into a voltage signal and outputs it to the signal amplification circuit, and forms a charge-discharge circuit with the secondary side inductance L and the electronic control pulse voltage source. One end of the resistor R5 is connected to the output end of the current-to-voltage conversion circuit, and the other end is grounded. The output of the signal amplification circuit is connected to the controller, and the electronic control pulse voltage source sends a pulse signal to the voltage signal; the controller separates and obtains the leakage current data and the pulse voltage source data from the output signal of the signal amplification circuit, and distinguishes the normal, short-circuit, open states of the transformer and the magnitude of the leakage current according to the obtained data.

Description

Technical Field

[0001] The present invention relates to a safety protection device in the field of power electronics, and particularly to a drag cable leakage fault monitor. Background Art

[0002] Drag cables are used for power supply operations on large mobile equipment such as mining shovels, quay-side cranes, stacker reclaimers, portable conveyors, and shore-to-ship. During operation, the failure rate of the dragged rubber-sheathed cable is relatively high, and insulation damage often occurs. In the light case, the insulation resistance decreases, and in the severe case, leakage occurs. Once leakage occurs, the personnel on the mobile equipment are prone to casualties due to continuous electric shock. Therefore, it is necessary to monitor the power drag cable in real time, detect leakage, and immediately disconnect the power supply. The drag cable leakage monitor belongs to the safety protection monitoring device, and has extremely high requirements for reliability and safety. When the power cable of the protected device leaks, the power supply should be accurately disconnected, and the fault should be reported. It needs to be sensitive but not misoperate.

[0003] As Figure 1 shown, in the three-phase three-wire circuit detection, the phasor sum of the three-phase currents is equal to zero, that is, Ia + Ib + Ic = 0. At this time, the induced current of a current transformer HB1 connected in the three-phase three-wire is zero. When an electric shock or leakage fault occurs in the circuit, a leakage current flows through the loop. At this time, the phasor sum of the three-phase currents passing through the current transformer is not equal to zero, and its phasor sum is: Ia + Ib + Ic = I, where I is the leakage current, that is, the zero-sequence current. In this way, there is an induced current in the secondary coil of the transformer HB1. After this current is applied to the electronic amplification circuit of the detection part in the monitor, corresponding current data is obtained. When compared with the predetermined action current value of the monitor device, if it is greater than the action current, the monitor controls the relay to act, and the main circuit trips. The current transformer connected here is called a zero-sequence current transformer. When the phasor sum of the three-phase currents is not equal to zero, the generated current is the zero-sequence current, which is also the leakage current.

[0004] Figure 2 This is the traditional leakage current detection principle. Its principle is that when the output current of the current transformer HB1 is very small (not exceeding the predetermined action current value) or zero, it indicates normal insulation and no leakage. When the output line of the current transformer is disconnected or short-circuited, or even when the detection circuit (such as current-to-voltage, signal amplification circuit, etc.) is damaged, the current detected by the monitor is also zero or very small, and the monitor always considers the insulation normal and no leakage accident. If leakage actually occurs at this time, the monitor will not alarm, nor will it disconnect the main circuit power supply, leaving a major safety hazard.

[0005] As Figure 2 and Figure 3 shown, when the circuit is connected normally and there is no leakage current, the output current of the current transformer HB1 is 0A, then the voltage U at the test point TS1TS1 is 0V, and the output voltage U after passing through the signal amplification circuit LCS_AN1 = 0V. When the MCU detects a 0V voltage, it considers the insulation normal and there is no leakage fault. When the output line I+ or I- of the current transformer is disconnected, the R5 resistor in the current-to-voltage circuit plays a pulling-down role, and the voltage U at the test point TS1 TS1 is 0V, and the output voltage U after passing through the signal amplification circuit LCS_AN1 = 0V. When the MCU detects a 0V voltage, it still considers the insulation normal and there is no leakage. When the output line of the current transformer is short-circuited, the voltage U at the test point TS1 TS1 is 0V, and the output voltage U after passing through the signal amplification circuit LCS_AN1 = 0V. When the MCU detects a 0V voltage, it still considers the insulation normal and there is no leakage. It can be seen from this that the MCU cannot normally distinguish between the disconnection or short-circuit of the output of the current transformer and the non-leakage fault.

[0006] In view of this, the monitor of the present invention should not only be able to monitor the leakage situation well, but also be able to perform real-time online monitoring on the current transformer, the output lines (I+, I-) of the transformer, and the current detection circuit, etc., and correctly and reliably judge the short-circuit and disconnection conditions of the transformer, completely eliminating potential safety hazards. Summary of the Invention

[0007] The present invention provides a dragging cable leakage fault monitor to improve the deficiencies of the prior art.

[0008] The described dragging cable leakage fault monitor includes a current detection circuit, a controller, a zero-sequence current transformer, and a resistor R5; the zero-sequence current transformer is used to detect the phasor sum of the three-phase currents of the dragging cable, and its secondary-side inductor L outputs a zero-sequence current; the current detection circuit includes a current-to-voltage circuit, an electronic control pulse voltage source, and a signal amplification circuit. The current-to-voltage circuit is used to convert the zero-sequence current into a voltage signal and output it to the signal amplification circuit, and forms a charge-discharge circuit with the secondary-side inductor L and the electronic control pulse voltage source. One end of the resistor R5 is connected to the output end of the current-to-voltage circuit, and the other end is grounded. The output of the signal amplification circuit is connected to the controller, and the electronic control pulse voltage source is controlled by the controller to send a pulse signal to the voltage signal; the controller separates and obtains the leakage current data and the pulse voltage source data from the output signal of the signal amplification circuit, and distinguishes the normal, short-circuit, and disconnection states of the transformer and the magnitude of the leakage current according to the obtained data.

[0009] Adopting the monitor architecture and method of the present invention, first, materials are obtained locally, and a charge-discharge circuit is formed by using the secondary-side excitation inductor of the current transformer and the pulse voltage source in the circuit to achieve the purpose of online detection of the working state of the transformer; second, the method of mixing the pulse power signal and the leakage current signal and then designing a filter in the MCU program to extract useful data improves the encryption of the monitoring system and also reduces the cost.

[0010] As an improved solution, the drag cable leakage fault monitor may further include a relay output circuit and a power circuit switch KJ1; the power circuit switch KJ1 is connected in series in the three-phase line of the drag cable, and the relay output circuit is controlled by the controller to drive the power circuit switch KJ1 to be disconnected or conducted. The abnormal power-off protection of the drag cable is implemented through the relay output circuit and the power circuit switch KJ1, where the relay is used for isolation between strong and weak electricity to protect the safety of the control side.

[0011] As an improved solution, the controller separates and obtains the leakage current data and the pulse voltage source data from the output signal of the signal amplification circuit, and further includes: using the data obtained by passing the output signal of the signal amplification circuit through the first band-pass filter as the pulse voltage source data, and the band-pass range of the first band-pass filter is configured as the frequency of the pulse signal; and / or using the data obtained by passing the output signal of the signal amplification circuit through the second band-pass filter as the leakage current data, and the band-pass range of the second band-pass filter is configured as the frequency of the detected leakage current. For example, the frequency band range of one filter is set to 49 Hz to 51 Hz to specifically extract the leakage current data; the frequency band range of another filter is set to 24 Hz to 26 Hz to specifically extract the pulse voltage source data. More preferably, to simplify the circuit and thus reduce costs, both the first band-pass filter and the second band-pass filter are implemented by programs.

[0012] To further reduce costs, the current-to-voltage circuit is set as resistor R1 and capacitor C1. One end of resistor R1 is grounded, and the other end is connected to the signal amplification circuit as the output. The secondary-side inductor L and capacitor C1 are respectively connected in parallel with resistor R1, greatly simplifying the structure. More preferably, on this basis, the electronically controlled pulse voltage source is set with triode Q1, resistor R7, and resistor R6. The B pole of triode Q1 is used to receive the square wave signal input by the controller, the E pole is connected to the negative voltage, the C pole is connected to the voltage signal through resistor R6, and resistor R7 is connected across the B pole and the E pole of triode Q1. In this way, the hardware cost only needs to add one triode and two small resistors to form a pulse charge and discharge power supply, achieving the optimization of costs and facilitating the realization of large-scale detection. Description of the Drawings

[0013] Figure 1 Shows the diagram of the traditional tow cable leakage fault monitoring system.

[0014] Figure 2 Shows the schematic diagram of the traditional leakage current detection.

[0015] Figure 3 Shows Figure 2 The voltage waveform output by the circuit when there is a leakage current in the circuit architecture.

[0016] Figure 4Shows the functional block diagram of the leakage current monitor of the present invention.

[0017] Figure 5 Shows the schematic diagram of reliable leakage current detection of the present invention.

[0018] Figure 6 Shows Figure 5 The output waveform after adding a 25Hz pulsed power supply to the circuit.

[0019] Figure 7 Shows Figure 5 The output waveform after adding a 25Hz pulsed power supply after the disconnect of the mutual inductor of

[0020] Figure 8 Shows Figure 5 The output waveform after adding a 25Hz pulsed power supply when the leakage current is not zero under the Specific embodiments

[0021] The technical solution of the present invention will be further described below in conjunction with the drawings and specific embodiments.

[0022] As Figure 4 shown, the monitor mainly consists of a current detection circuit 5, a relay output circuit 4, a controller 3, an operation panel 6, etc. The current detection module 5 controls the pulsed voltage source to continuously emit pulses, and at the same time, it real-time detects the output current signal of the current sensor in the main power circuit, and after processing, it sends it to the controller 3; the controller 3 filters the current signal through two band-pass filters to obtain the leakage current data and the pulsed voltage source data respectively. The DSP judges whether the sensor and the corresponding circuit are working normally online according to the data obtained after filtering. If an abnormality is found, it immediately controls the relay output circuit 4 to disconnect the power circuit switch KJ1 of the power circuit and cut off the power supply of the tow cable and the electrical equipment.

[0023] After the monitor is turned on, the monitor first controls the KJ1 switch in the main power circuit to close through the contactor control circuit 4. At this time, the tow cable and the electrical mobile equipment are connected to the power supply. At the same time, the current detection module 4 also starts to work, processes the current output by the sensor and sends it to the controller 3 for filtering calculation, decomposition and real-time judgment. After an abnormality is found, it immediately controls the contactor control circuit 4 to disconnect the main power circuit KJ1.

[0024] As Figure 5 shown, for situations such as the disconnection and short circuit of the current mutual inductor, the monitor regards them as serious blind spots of normal insulation and no leakage. The monitor of the present invention adds a pulsed voltage source to the Figure 2 circuit, uses the secondary side inductance L of the current mutual inductor to form a complete charge and discharge circuit, so that the voltage signal at the output end LCS_AN1 can effectively and reliably distinguish the normal, short circuit, disconnection, etc. of the mutual inductor circuit.

[0025] The frequency of the pulse voltage source is taken as 25 Hz. When the triode Q1 is turned on, the inductor L on the secondary side of the mutual inductor HB1 (the inductance value of the secondary side of the current transformer is relatively large, above 1 mH) starts to store energy, and the current flows from pin 2 to pin 1. At this time, the voltage U at the test point TS1 TS1 < 0 V; when the triode Q1 is turned off, the current in the inductor L on the secondary side of the mutual inductor continues to flow from pin 2 to pin 1, and the voltage U at the test point TS1 TS1 > 0 V. This cycle repeats, and at the output end LCS_AN1 of the detection circuit, a 25 Hz positive and negative pulse waveform is output, as Figure 6 shown.

[0026] If the current transformer is disconnected, because there is no energy storage and freewheeling effect of the large inductor in the pulse power supply circuit, only a 25 Hz negative pulse waveform is output at the output end LCS_AN1 of the detection circuit, as Figure 7 shown.

[0027] If the current transformer is short-circuited, the voltage at the test point TS1 is always 0 V, and only a 0 V voltage waveform is output at the output end LCS_AN1 of the detection circuit.

[0028] It can be seen that by introducing a pulse voltage source into the circuit, using the exciting inductance on the secondary side of the mutual inductor, and through the voltage waveform output by the detection circuit, the short-circuit, disconnection, and normal working states of the mutual inductor can be effectively distinguished. As long as the MCU judges the voltage waveform output by the detection circuit, it can identify the working state of the current leakage monitoring system. If the waveform detected by the MCU is a 25 Hz positive and negative pulse waveform, it means that the system and the line are normal; if only a 25 Hz negative pulse is detected, it means that the mutual inductor is disconnected; if a 0 V level is detected, it means that the mutual inductor is short-circuited.

[0029] The above analysis of the state signals of the leakage monitoring system is all carried out under the condition that the leakage current is 0 A. If the leakage current is added to the signal, the output of the detection circuit is no longer a standard 25 Hz pulse, but a mixed signal containing the leakage current. As Figure 8 shown, when the monitoring system circuit is normal and the leakage current is not zero amperes, the output waveform of the detection circuit at a frequency of 50 Hz. The frequency of the leakage current is the same as the power grid voltage frequency. The power grid frequency in our country is 50 Hz, so the leakage current frequency is taken as 50 Hz.

[0030] In this regard, the MCU needs to perform band-pass filtering on the signal output by the leakage current detection circuit to extract the desired signal. Two band-pass filters are designed in the program. The frequency band range of one filter is set to 49 Hz to 51 Hz, which is specifically used to extract the leakage current signal. If the data passing through this filter is 0, it means the leakage current is 0 A. The frequency band range of the other filter is set to 24 Hz to 26 Hz, which is specifically used to extract the pulse voltage source signal. The data obtained after passing through this filter is the pulse data when the leakage current is 0 A as described above.

[0031] By judging the data after the two filters, the MCU can know the status of the monitoring system (such as short circuit, disconnection, normal, etc.) and the magnitude of the leakage current, and execute the corresponding action logic to complete the control.

[0032] Advantages of the present invention:

[0033] 1. Using local materials, an excitation inductor on the secondary side of the current transformer and a pulse voltage source in the circuit are used to form a charge-discharge circuit, achieving the purpose of on-line detecting the working state of the transformer.

[0034] 2. Low cost. The hardware cost only needs to add a triode and two small resistors to form a pulse charge-discharge power supply.

[0035] 3. The method of mixing the pulse power supply signal and the leakage current signal and then designing a filter in the MCU program to extract useful data improves the encryption of the monitoring system and also reduces the cost.

[0036] The above specific embodiments are only several preferred embodiments of the present invention. Based on the technical solution of the present invention and the relevant revelations of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A drag cable leakage fault monitor, characterized in that: It includes a current detection circuit, a controller, a zero-sequence current transformer, and a resistor R5; The zero-sequence current transformer is used to detect the phasor sum of the three-phase currents of the drag cable, and outputs a zero-sequence current from its secondary-side inductor L; The current detection circuit includes a current-to-voltage conversion circuit, an electronic control pulse voltage source, and a signal amplification circuit. The current-to-voltage conversion circuit is used to convert the zero-sequence current into a voltage signal and output it to the signal amplification circuit, and forms a charge-discharge circuit with the secondary-side inductor L and the electronic control pulse voltage source. One end of the resistor R5 is connected to the output end of the current-to-voltage conversion circuit, and the other end is grounded. The output of the signal amplification circuit is connected to the controller, and the electronic control pulse voltage source is controlled by the controller to send a pulse signal to the voltage signal; The controller separates and obtains leakage current data and pulse voltage source data from the output signal of the signal amplification circuit, and distinguishes the normal, short-circuit, and open states of the transformer and the magnitude of the leakage current according to the obtained data; The electronic control pulse voltage source includes a triode Q1, a resistor R7, and a resistor R6. The B pole of the triode Q1 is used to receive the square wave signal input by the controller, the E pole is connected to a negative voltage, the C pole is connected to the voltage signal through the resistor R6, and the resistor R7 is connected across the B pole and the E pole of the triode Q1.

2. The drag cable leakage fault monitor according to claim 1, characterized in that: It includes a relay output circuit and a power circuit switch KJ1; The power circuit switch KJ1 is connected in series in the three-phase line of the drag cable, and the relay output circuit is controlled by the controller to drive the power circuit switch KJ1 to be disconnected or conducted.

3. The drag cable leakage fault monitor according to claim 1, characterized in that, The controller separates and obtains leakage current data and pulse voltage source data from the output signal of the signal amplification circuit, and further includes: Taking the data obtained by passing the output signal of the signal amplification circuit through a first band-pass filter as the pulse voltage source data, and the band-pass range of the first band-pass filter is configured as the frequency of the pulse signal; and / or Taking the data obtained by passing the output signal of the signal amplification circuit through a second band-pass filter as the leakage current data, and the band-pass range of the second band-pass filter is configured as the frequency of the detected leakage current.

4. The drag cable leakage fault monitor according to claim 3, characterized in that: Both the first band-pass filter and the second band-pass filter are implemented by programs.

5. The drag cable leakage fault monitor according to claim 1, characterized in that: The current-to-voltage conversion circuit includes a resistor R1 and a capacitor C1. One end of the resistor R1 is grounded, and the other end is connected as an output to the signal amplification circuit. The secondary-side inductor L and the capacitor C1 are respectively connected in parallel with the resistor R1.

6. The drag cable leakage fault monitor according to claim 1, characterized in that: If the controller detects that the pulse voltage source data is a positive and negative pulse waveform, it indicates that the transformer and the line are normal; if it detects that the pulse voltage source data has only a negative pulse, it indicates that the transformer is open; if it detects a 0V level, it indicates that the transformer is short-circuited.

7. The drag cable leakage fault monitor according to any one of claims 1-6, characterized in that: The inductance value of the secondary side of the zero-sequence current transformer is above 1 mH.

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