Overcurrent protection circuit for three-phase AC circuit

Abstract

The invention discloses an overcurrent protection circuit for a three-phase AC circuit. The output ends of three current sampling circuit units are connected to the overcurrent protection unit. The overcurrent protection unit includes a three-phase rectifier circuit unit, a hysteresis comparison circuit unit and a trigger circuit unit. The three-phase rectifier circuit unit includes three in-phase proportional operation circuit units and a subtraction operation circuit unit. The input ends of the three in-phase proportional operation circuit units are respectively connected to the output ends of the three current sampling circuit units. The output end of the in-phase proportional operation circuit unit is connected to the first input end and the second input end of the subtraction operation circuit unit through a first double-switch diode. The final voltage output end of the subtraction operation circuit unit is electrically connected to the input end of the hysteresis comparison circuit unit. The output end of the hysteresis comparison circuit unit is electrically connected to the signal input end of the trigger circuit unit. The output end of the trigger circuit unit is used to connect to a protection action control circuit.

Description

Technical Field

[0001] The invention relates to the field of alternating current protection, and in particular to an overcurrent protection circuit for a three-phase alternating current circuit. Background Art

[0002] The extensive use of modern power electronic equipment and nonlinear loads has caused increasingly serious power pollution. The management of power quality has become a topic of increasing concern to the power sector and its users. Therefore, the research and development of power quality management equipment is of great significance. For the power quality management instruments that are increasingly widely used, they manage the reactive power and harmonics of the power grid by emitting compensation current. During the operation of the equipment, the biggest safety hazard is the overcurrent of the three-phase AC circuit, which causes damage to the power electronic equipment, directly threatening the safety of the equipment and the power grid, and even causing fire and other accidents in serious cases.

[0003] my country's power grid generally adopts a three-phase four-wire grid architecture. The three-phase four-wire grid architecture requires high-precision measurement of the current in the three-phase AC circuit while also performing overcurrent protection. That is, the collected signal is sent to the controller for processing and calculation, while the collected signal is compared with the preset voltage threshold to generate a corresponding fault signal to drive the hardware protection circuit to respond, thereby quickly eliminating the problems caused by overcurrent faults and protecting the safety and stability of equipment and power grids.

[0004] However, in a three-phase AC circuit, a traditional hardware overcurrent protection device needs to compare the voltage signal of each phase multiple times through multiple voltage comparison circuits, and then select the largest voltage signal to compare with a preset voltage protection threshold. Finally, based on the comparison result, it is determined whether the subsequent protection action circuit needs to be triggered. The circuit is complex, the calculation process is cumbersome, and the cost of components is high. Therefore, how to simplify the circuit structure, improve the system response speed, and reduce costs while improving the system reliability of the hardware overcurrent protection device is an urgent problem to be solved. Summary of the invention

[0005] The purpose of the present invention is to address the corresponding deficiencies of the prior art and provide an overcurrent protection circuit for a three-phase AC circuit. A three-phase rectifier circuit is used to combine multiple voltage signals of each phase of the three-phase AC circuit and compare them with a pre-set voltage protection threshold to determine whether a subsequent protection action circuit needs to be triggered. The circuit structure and control method are simplified, and the response speed of the entire circuit is improved. Not only the cost is reduced, but also the fluctuation of the fault signal near the protection threshold is avoided, and the reliability of the system is improved. The fault signal can be quickly transmitted to the subsequent protection action circuit to ensure that the subsequent protection action circuit protects the three-phase AC circuit in time when an overcurrent fault occurs.

[0006] The object of the present invention is to adopt the following scheme to achieve: an overcurrent protection circuit for a three-phase AC circuit, comprising three current sampling circuit units respectively connected to each phase line of the three-phase AC circuit, the output ends of the three current sampling circuit units are connected to an overcurrent protection unit including three voltage input ends, the overcurrent protection unit comprises a three-phase rectifier circuit unit, a hysteresis comparison circuit unit, and a trigger circuit unit, the hysteresis comparison circuit unit is used to compare and judge the output voltage of the three-phase rectifier circuit unit with a preset voltage threshold;

[0007] The three-phase rectifier circuit unit is provided with three input terminals and one output terminal, the three input terminals of the three-phase rectifier circuit unit are respectively connected to the output terminals of the three current sampling circuit units, the output terminal of the three-phase rectifier circuit unit is electrically connected to the input terminal of the hysteresis comparison circuit unit, the output terminal of the hysteresis comparison circuit unit is electrically connected to the signal input terminal of the trigger circuit unit, and the output terminal of the trigger circuit unit is used to connect to the protection action control circuit;

[0008] The three-phase rectification circuit unit includes three in-phase proportional operation circuit units and one subtraction operation circuit unit. The input ends of the three in-phase proportional operation circuit units are respectively connected to the output ends of the three current sampling circuit units. The output end of the in-phase proportional operation circuit unit is connected to the first input end and the second input end of the subtraction operation circuit unit through a first dual-switch diode. The final voltage output end of the subtraction operation circuit unit is electrically connected to the input end of the hysteresis comparison circuit unit.

[0009] Preferably, the in-phase proportional operational circuit unit includes a third operational amplifier, an input resistor is arranged between the in-phase input terminal of the third operational amplifier and the output terminal of the current sampling circuit unit, a pull-down resistor for preventing the generation of an erroneous operation signal in a suspended state is arranged between the input resistor and the output terminal of the current sampling circuit unit, and the inverting input terminal of the third operational amplifier is electrically connected to the output terminal of the third operational amplifier, so that the output voltage of the in-phase proportional operational circuit unit is fed back to the inverting input terminal, forming a voltage follower, increasing the input impedance, reducing the output impedance, and playing a buffering role;

[0010] The output end of the third operational amplifier is connected to the first input end and the second input end of the subtraction circuit unit through the first double-switch diode.

[0011] Preferably, the first dual-switch diode includes a first diode and a second diode, the anode of the first diode is connected to the first input terminal of the subtraction circuit unit, the cathode of the second diode is connected to the second input terminal of the subtraction circuit unit, and the cathode of the first diode and the anode of the second diode are both connected to the output terminal of the third operational amplifier.

[0012] Preferably, the subtraction circuit unit comprises a fourth operational amplifier, the inverting input terminal of the fourth operational amplifier is connected to the anode of the first dual-switch diode through the first resistor, a second resistor is connected in parallel between the anode of the first dual-switch diode and the first resistor, and the second resistor is grounded;

[0013] The non-inverting input terminal of the fourth operational amplifier is connected to the cathode of the first dual-switch diode through a third resistor, a fourth resistor is connected in parallel between the third resistor and the non-inverting input terminal of the fourth operational amplifier, and the fourth resistor is grounded;

[0014] The output terminal of the fourth operational amplifier is connected to the inverting input terminal of the fourth operational amplifier through a fifth resistor;

[0015] The output terminal of the fourth operational amplifier is connected to the input terminal of the hysteresis comparison circuit unit;

[0016] The resistance values ​​of the first resistor, the second resistor, the third resistor, the fourth resistor and the fifth resistor are equal.

[0017] Preferably, the hysteresis comparison circuit unit includes a comparator, a first comparison voltage input terminal of the comparator is used to connect a circuit that outputs a preset voltage threshold, a second comparison voltage input terminal of the comparator is electrically connected to the output terminal of the three-phase rectifier circuit unit through a filter circuit unit, the first comparison voltage input terminal of the comparator is electrically connected to the signal output terminal of the comparator through a hysteresis resistor, the second comparison voltage input terminal of the comparator is connected in parallel with a second dual-switch diode for voltage limiting, and the signal output terminal of the comparator is electrically connected to the signal input terminal of the trigger circuit unit.

[0018] The filter circuit unit is a first RC parallel circuit.

[0019] An anode of the second dual switch diode is grounded, and a cathode of the second dual switch diode is connected to a power supply providing a +5V voltage.

[0020] Preferably, the trigger circuit unit includes a D trigger, a data input terminal and a clock input terminal of the D trigger are connected to a 5V DC power supply through a sixth resistor for selecting an operating mode, a direct reset terminal is used to be connected to a circuit that outputs a signal to release the trigger latch, a direct set terminal is connected to a signal output terminal of a hysteresis comparison circuit unit, a first signal output terminal of the D trigger is used to be electrically connected to a signal input terminal of a software control module in a protection action control circuit, and a second signal output terminal of the D trigger is used to be electrically connected to a signal input terminal of a hardware control device in a protection action control circuit.

[0021] The direct setting terminal of the D flip-flop is connected to the output terminal of the hysteresis comparison circuit unit through a pull-up resistor.

[0022] The present invention has the following beneficial effects: the output ends of the three current sampling circuit units are connected to an overcurrent protection unit including three voltage input ends, the overcurrent protection unit includes a three-phase rectification circuit unit, a hysteresis comparison circuit unit, and a trigger circuit unit, the hysteresis comparison circuit unit is used to compare and judge the output voltage of the three-phase rectification circuit unit with a preset voltage threshold, and generate a low / high level signal to output to the trigger circuit unit.

[0023] The three-phase rectifier circuit unit is provided with three input terminals and one output terminal, the three input terminals of the three-phase rectifier circuit unit are respectively connected to the output terminals of the three current sampling circuit units, the output terminal of the three-phase rectifier circuit unit is electrically connected to the input terminal of the hysteresis comparison circuit unit, the output terminal of the hysteresis comparison circuit unit is electrically connected to the signal input terminal of the trigger circuit unit, and the output terminal of the trigger circuit unit is used to connect to the protection action control circuit;

[0024] The current of each phase of the three-phase AC circuit passes through three current sampling circuit units respectively, and is output to each input end of the three-phase rectifier circuit unit to be converted into a DC voltage signal, and then output to the hysteresis comparison circuit unit, and compared with the preset voltage threshold to generate a low / high level signal to output to the trigger circuit unit. If the hysteresis comparison circuit unit outputs a low level signal, the trigger circuit unit outputs a fault signal for starting the protection action control circuit; if the hysteresis comparison circuit unit outputs a high level signal, the trigger circuit unit does not output a fault signal, so the subsequent protection action control circuit will not be started.

[0025] The three-phase rectification circuit unit includes three in-phase proportional operation circuit units and one subtraction operation circuit unit. The input ends of the three in-phase proportional operation circuit units are respectively connected to the output ends of the three current sampling circuit units. The output end of the in-phase proportional operation circuit unit is connected to the first input end and the second input end of the subtraction operation circuit unit through a first dual-switch diode. The final voltage output end of the subtraction operation circuit unit is electrically connected to the input end of the hysteresis comparison circuit unit.

[0026] Preferably, the in-phase proportional operational circuit unit includes a third operational amplifier, an input resistor is arranged between the in-phase input terminal of the third operational amplifier and the output terminal of the current sampling circuit unit, a pull-down resistor for preventing the generation of an erroneous operation signal in a suspended state is arranged between the input resistor and the output terminal of the current sampling circuit unit, and the inverting input terminal of the third operational amplifier is electrically connected to the output terminal of the third operational amplifier, so that the output voltage of the in-phase proportional operational circuit unit is fed back to the inverting input terminal, forming a voltage follower, increasing the input impedance, reducing the output impedance, and playing a buffering role;

[0027] The in-phase proportional operation circuit unit converts the voltage signals output by the three current sampling circuit units into two voltage signals, namely, a maximum voltage signal and a minimum voltage signal, through a first double-switch diode.

[0028] Preferably, the subtraction circuit unit comprises a fourth operational amplifier, the inverting input terminal of the fourth operational amplifier is connected to the anode of the first dual-switch diode through the first resistor, a second resistor is connected in parallel between the anode of the first dual-switch diode and the first resistor, and the second resistor is grounded;

[0029] The non-inverting input terminal of the fourth operational amplifier is connected to the cathode of the first dual-switch diode through a third resistor, a fourth resistor is connected in parallel between the third resistor and the non-inverting input terminal of the fourth operational amplifier, and the fourth resistor is grounded;

[0030] The output terminal of the fourth operational amplifier is connected to the inverting input terminal of the fourth operational amplifier through a fifth resistor;

[0031] The output terminal of the fourth operational amplifier is connected to the input terminal of the hysteresis comparison circuit unit;

[0032] The resistance values ​​of the first resistor, the second resistor, the third resistor, the fourth resistor and the fifth resistor are equal.

[0033] A subtraction circuit unit is used to convert two voltage signals into voltage fluctuation amplitudes for comparison with a preset voltage threshold.

[0034] Preferably, the hysteresis comparison circuit unit includes a comparator, a first comparison voltage input terminal of the comparator is used to connect a circuit that outputs a preset voltage threshold, a second comparison voltage input terminal of the comparator is electrically connected to the output terminal of the three-phase rectifier circuit unit through a filter circuit unit, the first comparison voltage input terminal of the comparator is electrically connected to the signal output terminal of the comparator through a hysteresis resistor, the second comparison voltage input terminal of the comparator is connected in parallel with a second dual-switch diode for voltage limiting, and the signal output terminal of the comparator is electrically connected to the signal input terminal of the trigger circuit unit.

[0035] The filter circuit unit is a first RC parallel circuit.

[0036] An anode of the second dual switch diode is grounded, and a cathode of the second dual switch diode is connected to a power supply providing a +5V voltage.

[0037] Preferably, the trigger circuit unit includes a D trigger, a data input terminal and a clock input terminal of the D trigger are connected to a 5V DC power supply through a sixth resistor for selecting an operating mode, a direct reset terminal is used to be connected to a circuit that outputs a signal to release the trigger latch, a direct set terminal is connected to a signal output terminal of a hysteresis comparison circuit unit, a first signal output terminal of the D trigger is used to be electrically connected to a signal input terminal of a software control module in a protection action control circuit, and a second signal output terminal of the D trigger is used to be electrically connected to a signal input terminal of a hardware control device in a protection action control circuit.

[0038] The direct setting terminal of the D trigger is connected to the output terminal of the hysteresis comparison circuit unit through a pull-up resistor to prevent the fault signal from being triggered by mistake. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a structural schematic diagram of the present invention;

[0040] Figure 2 is a schematic diagram of an overcurrent protection unit of the present invention;

[0041] Figure 3 is a circuit diagram of an overcurrent protection unit of the present invention;

[0042] Figure 4 A schematic diagram of three current sampling circuit units used in an embodiment of the present invention;

[0043] Figure 5 This is a circuit diagram of three current sampling circuit units used in an embodiment of the present invention. DETAILED DESCRIPTION

[0044] like Figures 1 to 5 As shown, an overcurrent protection circuit for a three-phase AC circuit includes three current sampling circuit units 100 respectively connected to each phase line of the three-phase AC circuit, the output ends of the three current sampling circuit units 100 are connected to an overcurrent protection unit 200 including three voltage input ends, the overcurrent protection unit 200 includes a three-phase rectifier circuit unit 4, a hysteresis comparison circuit unit 5, and a trigger circuit unit 6, the hysteresis comparison circuit unit 5 is used to compare and judge the output voltage of the three-phase rectifier circuit unit 4 with a preset voltage threshold, if the output voltage of the three-phase rectifier circuit unit 4 is greater than the preset voltage threshold, a low level signal is generated and output to the trigger circuit unit 6.

[0045] The three-phase rectifier circuit unit 4 is provided with three input terminals and one output terminal, the three input terminals of the three-phase rectifier circuit unit 4 are respectively connected to the output terminals of the three current sampling circuit units 100, the output terminal of the three-phase rectifier circuit unit 4 is electrically connected to the input terminal of the hysteresis comparator circuit unit 5, the output terminal of the hysteresis comparator circuit unit 5 is electrically connected to the signal input terminal of the trigger circuit unit 6, and the output terminal of the trigger circuit unit 6 is used to connect the protection action control circuit;

[0046] The current of each phase of the three-phase AC circuit passes through three current sampling circuit units respectively, and is output to each input end of the three-phase rectifier circuit unit to be converted into a DC voltage signal, and then output to the hysteresis comparison circuit unit 5, and compared with the preset voltage threshold, to generate a low / high level signal output to the trigger circuit unit 6. If the hysteresis comparison circuit unit 5 outputs a low level signal, the trigger circuit unit 6 outputs a fault signal for starting the protection action control circuit; if the hysteresis comparison circuit unit 5 outputs a high level signal, the trigger circuit unit 6 does not output a fault signal, so the subsequent protection action control circuit will not be started.

[0047] The three-phase rectifier circuit unit 4 includes three in-phase proportional operation circuit units and one subtraction operation circuit unit. The input ends of the three in-phase proportional operation circuit units are respectively connected to the output ends of the three current sampling circuit units 100. The output end of the in-phase proportional operation circuit unit is connected to the first input end and the second input end of the subtraction operation circuit unit through a first dual-switch diode. The final voltage output end of the subtraction operation circuit unit is electrically connected to the input end of the hysteresis comparison circuit unit 5.

[0048] In this embodiment, the in-phase proportional operational circuit unit includes a third operational amplifier, that is, the third operational amplifier is an operational amplifier U4A / operational amplifier U4B / operational amplifier U5A, an input resistor is set between the in-phase input terminal of the third operational amplifier and the output terminal of the current sampling circuit unit 100, in this embodiment, the input resistor is resistor R10 / resistor R21 / resistor R29, and the resistance value of the input resistor is 10kΩ, and a pull-down resistor for preventing the generation of an erroneous operation signal in a suspended state is set between the input resistor and the output terminal of the current sampling circuit unit 100, in this embodiment, the pull-down resistor is resistor R13 / resistor R24 / resistor R30, and the resistance value of the pull-down resistor is 100kΩ, the inverting input terminal of the third operational amplifier is electrically connected to the output terminal of the third operational amplifier, so that the output voltage of the in-phase proportional operational circuit unit is fed back to the inverting input terminal, forming a voltage follower, increasing the input impedance, reducing the output impedance, and playing a buffering role;

[0049] The output end of the third operational amplifier is connected to the first input end and the second input end of the subtraction circuit unit through the first double-switch diode.

[0050] In this embodiment, the first dual-switch diode includes a first diode and a second diode, the anode of the first diode is connected to the first input end of the subtraction circuit unit, the cathode of the second diode is connected to the second input end of the subtraction circuit unit, and the cathode of the first diode and the anode of the second diode are both connected to the output end of the third operational amplifier.

[0051] The first dual switch diode is a dual switch diode D1 / dual switch diode D2 / dual switch diode D3, and the first dual switch diode is MMBD7000LT1.

[0052] In this embodiment, the subtraction circuit unit includes a fourth operational amplifier, which is an operational amplifier U5B. The inverting input terminal of the fourth operational amplifier is connected to the positive electrode of the first dual-switch diode through the first resistor R14. A second resistor R18 is connected in parallel between the positive electrode of the first dual-switch diode and the first resistor R14, and the second resistor R18 is grounded.

[0053] The in-phase input terminal of the fourth operational amplifier is connected to the cathode of the first dual-switch diode through a third resistor R25, a fourth resistor R28 is connected in parallel between the third resistor R25 and the in-phase input terminal of the fourth operational amplifier, and the fourth resistor R28 is grounded;

[0054] The output terminal of the fourth operational amplifier is connected to the inverting input terminal of the fourth operational amplifier through a fifth resistor R11;

[0055] The output terminal of the fourth operational amplifier is connected to the input terminal of the hysteresis comparator circuit unit 5;

[0056] The resistance values ​​of the first resistor R14, the second resistor R18, the third resistor R25, the fourth resistor R28 and the fifth resistor R11 are equal. In this embodiment, the resistance values ​​of the first resistor R14, the second resistor R18, the third resistor R25, the fourth resistor R28 and the fifth resistor R11 are all 10kΩ.

[0057] In this embodiment, the hysteresis comparison circuit unit 5 includes a comparator U7A, which is an LMV393 powered by a 5V single-end. The first comparison voltage input terminal of the comparator U7A is used to connect a circuit that outputs a preset voltage threshold. The second comparison voltage input terminal of the comparator U7A is electrically connected to the output terminal of the three-phase rectifier circuit unit 4 through a filter circuit unit. In this embodiment, the filter circuit unit is a first RC parallel circuit, which is composed of a seventh resistor R17 with a resistance of 10kΩ and a capacitor with a capacitance of 1nF in parallel. The first comparison voltage input terminal of the comparator U7A is electrically connected to the signal output terminal of the comparator U7A through a hysteresis resistor R26, and the second comparison voltage input terminal of the comparator U7A is connected in parallel with a second dual-switch diode for voltage limiting. The signal output terminal of the comparator U7A is electrically connected to the signal input terminal of the trigger circuit unit 6.

[0058] In this embodiment, the first comparison voltage input terminal of the comparator U7A is the second pin of LMV393, the second comparison voltage input terminal of the comparator U7A is the third pin of LMV393, the signal output terminal of the comparator U7A is the first pin of LMV393, and the resistance of the hysteresis resistor R26 is 30kΩ.

[0059] The anode of the second dual switch diode is grounded, and the cathode of the second dual switch diode is connected to a power supply providing a +5V voltage. In this embodiment, the second dual switch diode is a dual switch diode D4, and the dual switch diode D4 is MMBD7000LT1.

[0060] In this embodiment, the trigger circuit unit 6 includes a D flip-flop U8A, which is a 74HC74. The data input terminal D and the clock input terminal CP of the D flip-flop U8A are connected to a 5V DC power supply through a sixth resistor R23 for selecting the working mode. The resistance value of the sixth resistor R23 is 10kΩ. The direct reset terminal RD of the D flip-flop U8A is used to connect to a circuit that outputs a signal for releasing the trigger latch. The direct set terminal SD of the D flip-flop U8A is connected to the signal output terminal of the hysteresis comparison circuit unit 5. The first signal output terminal Q of the D flip-flop U8A is used to electrically connect to the signal input terminal of the software control module in the protection action control circuit. The second signal output terminal of the D flip-flop U8A is used to electrically connect to the signal input terminal of the software control module in the protection action control circuit. Used to be electrically connected to the signal input terminal of the hardware control device in the protection action control circuit.

[0061] The direct set terminal SD of the D flip-flop U8A is connected to the output terminal of the hysteresis comparator circuit unit 5 through a pull-up resistor R16 to prevent false triggering of a fault signal. The resistance value of the pull-up resistor R16 is 10 kΩ.

[0062] In this embodiment, the light emitting diode LED1 can also be used as a fault indicator light, through a current limiting resistor R27 and the second signal output terminal of the D trigger connect.

[0063] The current sampling circuit unit 100 described in the present invention is three common sampling circuit units for current sampling. In this embodiment, the current sampling circuit unit 100 includes a current transformer 1, a sampling circuit 2, and a conditioning circuit 3. The output end of the current transformer 1 is electrically connected to the input end of the sampling circuit 2, the output end of the sampling circuit 2 is electrically connected to the input end of the conditioning circuit 3, and the output end of the conditioning circuit 3 is electrically connected to the voltage input end of the three-phase rectifier circuit unit 4 of the overcurrent protection unit 200.

[0064] The first pin IP and the second pin IN of the current transformer 1 are used to connect any phase line of a three-phase AC circuit, the third pin iN of the current transformer 1 is an analog ground, the fourth pin iP of the current transformer 1 is electrically connected to the in-phase input terminal of the first operational amplifier U1A of the conditioning circuit 3, and the sampling circuit 2 is arranged between the third pin iN and the fourth pin iP of the current transformer 1;

[0065] The sampling circuit 2 includes a sampling resistor R3 and a filter capacitor C3. The sampling resistor R3 and the filter capacitor C3 are connected in parallel between the third pin iN and the fourth pin iP of the current transformer 1. In this embodiment, the sampling accuracy of the sampling resistor R3 is 0.1%, and the resistance is 1 kΩ.

[0066] The output end of the first operational amplifier U1A of the conditioning circuit 3 is connected to the in-phase input end of the second operational amplifier U1B, and the output end of the second operational amplifier U1B is connected to the voltage input end of the three-phase rectifier circuit unit 4. The conditioning circuit 3 is provided with a second RC parallel circuit for filtering interference signals. In this embodiment, the second RC parallel circuit is arranged between the first operational amplifier U1A and the second operational amplifier U1B of the conditioning circuit 3, and plays a role in attenuating high-frequency signals. After filtering out the high-frequency signal and the interference signal in the voltage signal output by the first operational amplifier of the conditioning circuit, the voltage amplitude is adjusted again by the second operational amplifier.

[0067] The output end of the first operational amplifier U1A is connected to the inverting input end of the first operational amplifier U1A; the output end of the second operational amplifier U1B is connected to the inverting input end of the second operational amplifier U1B, making it a voltage follower, increasing the input impedance, reducing the output impedance, and playing a buffering role.

[0068] In the current sampling circuit unit 100 of the present invention, the current of the phase line of the three-phase AC circuit passes through the current transformer 1 of the current sampling circuit unit 100. After the large current on the primary side of the current transformer 1 is converted into a small current on the secondary side, that is, the current of tens of amperes or even hundreds of amperes is converted into a small current of milliamperes. It is converted into a voltage signal through the sampling circuit 2, and then the voltage amplitude is adjusted by the first operational amplifier U1A and the second operational amplifier U1B of the conditioning circuit 3 in sequence, and then input into the input end of the three-phase rectifier circuit unit 4.

[0069] The above technical solution is used to make the following embodiments:

[0070] The present invention is used for power quality management equipment such as active power filters, static VAR generators, inverters, energy storage converters, etc. The present invention is used for sampling and overcurrent protection of 400V three-phase AC power, and the design current is 30A.

[0071] The three current sampling circuit units 100 are respectively connected to the A phase, the B phase, and the C phase of the three-phase current circuit. Since the circuit structures of the current sampling circuit units 100 are the same, the following is a detailed description taking the A phase of the three-phase current circuit as an example:

[0072] The transformation ratio of the current transformer 1 is 100A:5mA. The current of phase A in the three-phase current circuit passes through the current transformer 1 to obtain an output current signal iLa of -1.5mA to 1.5mA. The output current signal iLa is converted into a voltage signal of -1.5V to 1.5V after passing through the sampling resistor R3 with a sampling accuracy of 0.1% and a resistance of 1kΩ in the sampling circuit 2. The voltage signal is followed by the first operational amplifier U1A in the conditioning circuit 3 and then filtered by the second RC parallel circuit.

[0073] After filtering, the voltage signal is followed by the second operational amplifier U1B to obtain a voltage signal of -1.5V to 1.5V and sent to the overcurrent protection unit 200. Of course, the voltage signal output by the current sampling circuit unit 100 can also be sent to the analog-to-digital conversion circuit, but a dual switch diode should be used to limit the voltage signal to within the allowable range of the input voltage of the analog-to-digital conversion circuit.

[0074] In this embodiment, since the present invention is used for power quality management equipment, it is necessary to output current to compensate for harmonics and reactive power in the power grid, and control the current size to ensure the safety of the equipment. In power quality management, the higher the frequency, the less harmonic content. At present, the harmonic frequencies generally eliminated on the market are 2 to 51 times, and the maximum elimination frequency is 2.55kHz, so the design cutoff frequency needs to be greater than 4kHz to ensure the accuracy of the required sampling. Therefore, the cutoff frequency is set to 16kHz here. The second RC parallel circuit includes a resistor R1 with a resistance value of 10kΩ and a capacitor C4 with a value of 1nF to ensure that the obtained filter cutoff frequency is 16kHz.

[0075] According to the above example, the currents of phase A, phase B and phase C in the three-phase current circuit are converted into voltage signals IA_P, IB_P and IC_P through the current sampling circuit unit 100, and are voltage followed by the third operational amplifier. The operational amplifier U4A / operational amplifier U4B / operational amplifier U5A are rectified by the dual switch diode D1, the dual switch diode D2 and the dual switch diode D3 respectively. The dual switch diode uses MMBD7000LT1.

[0076] After rectification, two DC voltage signals Vmax and Vmin are obtained. It can be known that the calculation formula of the final voltage output terminal voltage V-final of the subtraction proportional circuit formed by the fourth operational amplifier U5B is as follows:

[0077]

[0078] Here, the resistance values ​​of R25, R28, R11, and R14 are set to be equal, all 10kΩ, then the calculation formula for the final voltage output terminal voltage V-final of the subtraction proportional circuit can be simplified as follows:

[0079] V-final=Vmax-Vmin

[0080] The final voltage output terminal voltage V-final of the subtraction proportional circuit is subjected to RC primary filtering through a resistor R17 and a capacitor C20 to filter out interference signals. Here, the resistor R17 with a resistance value of 10kΩ and the capacitor C20 with a capacitance value of 1nF are set, and the filtering cut-off frequency is 16kHz; the comparator U7 is powered by a 5V single-ended power supply LMV393 to prevent the power supply difference from affecting the comparator U7, and a limiting effect is performed at the dual switch diode D4 to limit the signal to between 0 and 5V; a hysteresis comparison circuit is formed in the comparator U7A, and a hysteresis resistor R26 with a resistance value of 30kΩ is set to prevent the output fault signal from jittering due to signal fluctuations, thereby causing misoperation.

[0081] The voltage V-final at the final voltage output terminal of the subtraction ratio circuit is compared with the preset voltage threshold VREF by the comparator U7A. After the comparison and judgment, the comparator U7A outputs a comparison result voltage signal V, that is, the comparator U7A makes the following comparison and judgment:

[0082] When V-final > VREF, the first pin of the comparator U7A outputs a low level, and the fifth pin of the D flip-flop U8A is triggered to generate a high-level fault signal OI_INT. The subsequent hardware protection device closes the relevant control function by driving the triode to output a drive signal. In this embodiment, that is, the PWM pulse for driving the IGBT is turned off, thereby turning off the current output of the inverter; and the sixth pin of the D flip-flop U8A generates a low-level signal OI to the controller circuit for signal processing. After receiving the low-level signal OI, the controller circuit takes protective measures for the circuit.

[0083] At this time, if a light-emitting diode LED1 is connected to the sixth pin of the D flip-flop U8A as a fault indicator, the light-emitting diode LED1 will emit light to indicate that an overcurrent fault has occurred;

[0084] When V-final < VREF, the first pin of the comparator U7A outputs a high level, and the D flip-flop U8A does not generate a fault signal; the fourth pin of the D flip-flop U8A uses a pull-up resistor R16 with a resistance value of 10 kΩ to prevent mis-triggering of the fault signal. The D flip-flop U8A is 74HC74 and is powered by 5V. The second and third pins of the D flip-flop U8A are connected to the 5V power supply through a sixth resistor R23 with a resistance value of 10 kΩ to select the working mode. The first pin is connected to the circuit that outputs the signal to解除触发器锁存的信号. When the overcurrent fault is eliminated and it is necessary to解除触发器锁存的信号, the circuit that outputs the signal to解除触发器锁存的信号 emits a low-level trigger reset signal RESETA to解除触发器锁存的信号 and reset the trigger.

[0085] It should be noted that before the voltage signals IA_P, IB_P, and IC_P are processed by the operational amplifier, they should first be voltage-dropped by a resistor R13 with a relatively large resistance value to prevent misoperation signals from being generated in the floating state. The resistance value of the resistor R13 is 100 kΩ;

[0086] The above-mentioned operational amplifiers are all precision operational amplifiers, and the precision operational amplifiers are all provided by a dual operational amplifier AD712J. Decoupling capacitors with a capacitance value of 0.1 uF are provided at the power input terminals of the dual operational amplifier AD712J.

[0087] The dual operational amplifier AD712J is powered by dual power supplies ±15V, and the output voltage range of the dual operational amplifier AD712J is -15V to 15V.

[0088] The present invention can be used in power quality management equipment such as active power filters and static VAR generators, as well as equipment such as inverters and energy storage converters, to ensure that the current of the output three-phase alternating current will not over-current.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification made to the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. An overcurrent protection circuit for a three-phase AC circuit, comprising three current sampling circuit units (100) for respectively connecting to each phase line of the three-phase AC circuit, characterized in that: The output ends of the three current sampling circuit units (100) are connected to an overcurrent protection unit (200) having three voltage input ends, the overcurrent protection unit (200) comprising a three-phase rectification circuit unit (4), a hysteresis comparison circuit unit (5), and a trigger circuit unit (6), the hysteresis comparison circuit unit (5) being used to compare and judge the output voltage of the three-phase rectification circuit unit (4) with a preset voltage threshold; The three-phase rectifier circuit unit (4) is provided with three input ends and one output end, the three input ends of the three-phase rectifier circuit unit (4) are respectively connected to the output ends of the three current sampling circuit units (100), the output end of the three-phase rectifier circuit unit (4) is electrically connected to the input end of the hysteresis comparison circuit unit (5), the output end of the hysteresis comparison circuit unit (5) is electrically connected to the signal input end of the trigger circuit unit (6), and the output end of the trigger circuit unit (6) is used to connect to the protection action control circuit; The three-phase rectification circuit unit (4) comprises three in-phase proportional operation circuit units and a subtraction operation circuit unit, the input ends of the three in-phase proportional operation circuit units are respectively connected to the output ends of the three current sampling circuit units (100), the output end of the in-phase proportional operation circuit unit is connected to the first input end and the second input end of the subtraction operation circuit unit via a first double switch diode, and the final voltage output end of the subtraction operation circuit unit is electrically connected to the input end of the hysteresis comparison circuit unit (5); The in-phase proportional operational circuit unit comprises a third operational amplifier, an input resistor is arranged between the in-phase input terminal of the third operational amplifier and the output terminal of the current sampling circuit unit (100), a pull-down resistor for preventing the generation of an erroneous operation signal in a suspended state is arranged between the input resistor and the output terminal of the current sampling circuit unit (100), the inverting input terminal of the third operational amplifier is electrically connected to the output terminal of the third operational amplifier, and the output terminal of the third operational amplifier is connected to the first input terminal and the second input terminal of the subtraction operational circuit unit via a first dual-switch diode.

2. The overcurrent protection circuit for a three-phase AC circuit according to claim 1, characterized in that: The first dual-switch diode includes a first diode and a second diode, the anode of the first diode is connected to the first input end of the subtraction circuit unit, the cathode of the second diode is connected to the second input end of the subtraction circuit unit, and the cathode of the first diode and the anode of the second diode are both connected to the output end of the third operational amplifier.

3. The overcurrent protection circuit for a three-phase AC circuit according to claim 1, characterized in that: The subtraction circuit unit comprises a fourth operational amplifier, the inverting input terminal of the fourth operational amplifier is connected to the positive electrode of the first double switch diode through the first resistor (R14), a second resistor (R18) is connected in parallel between the positive electrode of the first double switch diode and the first resistor (R14), and the second resistor (R18) is grounded; The in-phase input terminal of the fourth operational amplifier is connected to the cathode of the first dual-switch diode via a third resistor (R25), a fourth resistor (R28) is connected in parallel between the third resistor (R25) and the in-phase input terminal of the fourth operational amplifier, and the fourth resistor (R28) is grounded; The output terminal of the fourth operational amplifier is connected to the inverting input terminal of the fourth operational amplifier through a fifth resistor (R11); The output end of the fourth operational amplifier is connected to the input end of the hysteresis comparator circuit unit (5); The resistance values ​​of the first resistor (R14), the second resistor (R18), the third resistor (R25), the fourth resistor (R28), and the fifth resistor (R11) are equal.

4. The overcurrent protection circuit for a three-phase AC circuit according to claim 1, characterized in that: The hysteresis comparison circuit unit (5) comprises a comparator (U7A), a first comparison voltage input terminal of the comparator (U7A) being used to connect a circuit that outputs a preset voltage threshold, a second comparison voltage input terminal of the comparator (U7A) being electrically connected to an output terminal of a three-phase rectifier circuit unit (4) via a filter circuit unit, a first comparison voltage input terminal of the comparator (U7A) being electrically connected to a signal output terminal of the comparator (U7A) via a hysteresis resistor (R26), a second dual-switch diode for voltage limiting being connected in parallel to the second comparison voltage input terminal of the comparator (U7A), and a signal output terminal of the comparator (U7A) being electrically connected to a signal input terminal of a trigger circuit unit (6).

5. The overcurrent protection circuit for a three-phase AC circuit according to claim 4, characterized in that: The filter circuit unit is a first RC parallel circuit.

6. The overcurrent protection circuit for a three-phase AC circuit according to claim 4, characterized in that: An anode of the second dual switch diode is grounded, and a cathode of the second dual switch diode is connected to a power supply providing a +5V voltage.

7. The overcurrent protection circuit for a three-phase AC circuit according to claim 1, characterized in that: The trigger circuit unit (6) comprises a D flip-flop (U8A), wherein a data input terminal (D) and a clock input terminal (CP) of the D flip-flop (U8A) are connected to a 5V DC power supply via a sixth resistor (R23) for selecting an operating mode, a direct reset terminal (RD) of the D flip-flop (U8A) is connected to a circuit for outputting a signal for releasing a trigger latch, a direct set terminal (SD) of the D flip-flop (U8A) is connected to a signal output terminal of a hysteresis comparison circuit unit (5), a first signal output terminal (Q) of the D flip-flop (U8A) is electrically connected to a signal input terminal of a software control module in a protection action control circuit, and a second signal output terminal (Q) of the D flip-flop (U8A) is electrically connected to a signal input terminal of a hardware control device in the protection action control circuit.

8. The overcurrent protection circuit for a three-phase AC circuit according to claim 7, characterized in that: The direct setting terminal (SD) of the D flip-flop (U8A) is connected to the output terminal of the hysteresis comparison circuit unit (5) via a pull-up resistor (R16).

Images