CN106405191A - Zero-sequence current sampling circuit capable of AC grounding precise judgment, and judgment method thereof - Google Patents

Abstract

The invention discloses a zero-sequence current sampling circuit for accurate judgment of AC grounding, which includes an A-phase sampling circuit, a B-phase sampling circuit, a C-phase sampling circuit, a zero-sequence current amplitude A/D conversion module, and a zero-sequence resistance phase preprocessing Module, intelligent microprocessor, A-phase sampling circuit, B-phase sampling circuit and C-phase sampling circuit respectively perform three-phase power supply current sampling and superimpose the three-phase current signals and send them to the zero-sequence current amplitude A/D conversion module and zero-sequence current amplitude A/D conversion module The sequence resistance phase preprocessing module correctly judges the magnitude and phase of the zero-sequence current for ground faults, alarms or outputs protection signals. The invention has the advantages of simple structure, reliable operation and high cost performance, and effectively guarantees the safe and economical operation of the power grid.

Description

A zero-sequence current sampling circuit for accurate judgment of AC grounding and its judgment method

technical field

The invention relates to a zero-sequence current sampling circuit and a grounding judgment method, in particular to a zero-sequence current sampling circuit and a judgment method for accurately judging AC grounding.

Background technique

The ground fault of the power grid is a common fault. If it occurs, it must be eliminated in time, otherwise it will affect the safe operation of the power grid. At present, each connection point of the power grid is generally equipped with a ground fault protection device, which will send an alarm signal when it detects a ground fault, and send a trip signal to disconnect the ground fault line after the set time, so as to avoid the expansion of the accident .

Due to the diversity of power grid structure and grounding methods, the existing power grid grounding protection devices generally have the problem of inaccurate grounding judgment, and often have false or missed grounding faults, which endanger the safe and economical operation of the power grid.

The main reason for the inaccurate judgment of the ground fault of the existing power grid grounding protection device products is the inaccurate sampling of the zero-sequence current generated by the ground fault, because the sampling of the zero-sequence current generated by the ground fault involves high-voltage electrical isolation, etc. Measures and small ground fault currents are mixed in the large load current, and it is difficult to obtain a signal that is linearly proportional to the actual ground fault current in amplitude and phase and can be read by an intelligent microprocessor circuit.

Contents of the invention

The technical problem to be solved by the present invention is to provide a zero-sequence current sampling circuit and its judging method for accurate judging of AC grounding.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A zero-sequence current sampling circuit for accurate judgment of AC grounding, characterized in that it includes an A-phase sampling circuit, a B-phase sampling circuit, a C-phase sampling circuit, a zero-sequence current amplitude A/D conversion module, and a zero-sequence resistance phase preprocessing Module, intelligent microprocessor, A-phase sampling circuit, B-phase sampling circuit and C-phase sampling circuit respectively perform three-phase power supply current sampling and superimpose the three-phase current signals and send them to the zero-sequence current amplitude A/D conversion module and zero-sequence current amplitude A/D conversion module The sequence resistance phase preprocessing module will correctly judge the magnitude and phase of the zero-sequence current for grounding faults, alarm or output protection signals.

Further, the structure of the A-phase sampling circuit is that the current transformer La1 is arranged on the A-phase line of the three-phase power supply to perform current sampling, and one end of the current transformer La1 is connected to one end of the switching resistor Ra1 and the voltage dividing adjustment potentiometer Ra2, and the current The other end of the transformer La1 is connected to the other end of the conversion resistor Ra1 and the other end of the voltage dividing resistor Ra3, the other end of the voltage dividing adjustment potentiometer Ra2 and the voltage dividing resistor Ra3 is connected to one end of the current limiting protection resistor Ra4, and the current limiting protection resistor Ra4 The other end is connected to the base of the signal following PNP transistor Da1 and one end of the phase feedback adjusting inductor La2, the other end of the phase feedback adjusting inductor La2 is connected to one end of the phase feedback limiting resistor Ra5, the other end of the phase feedback limiting resistor Ra5 is connected to the signal following PNP type The collector of the triode Da1 is connected to one end of the phase feedback proportional resistor Ra6, and the signal follows the emitter of the PNP transistor Da1. The signal limiting resistor Ra7 and one end of the signal following shunt resistor Ra8 are connected, and the other end of the signal following shunt resistor Ra8 is grounded, and the overvoltage protection device The positive electrode of Da2 is connected to the other end of the current transformer La1, and the negative electrode of the overvoltage protection device Da2 is connected to the base of the signal-following PNP transistor Da1;

The structure of the B-phase sampling circuit is that the current transformer Lb1 is arranged on the B-phase line of the three-phase power supply to perform current sampling, and one end of the current transformer Lb1 is connected with the switching resistor Rb1 and one end of the voltage dividing adjustment potentiometer Rb2, and the current transformer Lb1 The other end is connected to the other end of the conversion resistor Rb1 and the other end of the voltage dividing resistor Rb3, the other end of the voltage dividing adjustment potentiometer Rb2 and the voltage dividing resistor Rb3 is connected to one end of the current limiting protection resistor Rb4, and the other end of the current limiting protection resistor Rb4 is connected to The signal follows the base of the PNP transistor Db1 and one end of the phase feedback adjustment inductor Lb2 is connected, the other end of the phase feedback adjustment inductor Lb2 is connected to one end of the phase feedback limiting resistor Rb5, the other end of the phase feedback limiting resistor Rb5 is connected to the signal following the PNP transistor Db1 set One end of the electrode and phase feedback proportional resistor Rb6 is connected, the signal follows the emitter of the PNP transistor Db1, the signal limiting resistor Rb7 and one end of the signal following shunt resistor Rb8 are connected, the other end of the signal following shunt resistor Rb8 is grounded, and the positive electrode of the overvoltage protection device Db2 is connected to The other end of the current transformer Lb1 is connected, and the negative electrode of the overvoltage protection device Db2 is connected to the base of the signal-following PNP transistor Db1;

The structure of the C-phase sampling circuit is that the current transformer Lc1 is set on the three-phase power supply C line to perform current sampling, one end of the current transformer Lc1 is connected with the switching resistor Rc1 and one end of the voltage dividing adjustment potentiometer Rc2, and the current transformer Lc1 is connected to another end of the current transformer Lc1. One end is connected to the other end of the conversion resistor Rc1 and the other end of the voltage dividing resistor Rc3, the other end of the voltage dividing adjustment potentiometer Rc2 and the voltage dividing resistor Rc3 is connected to one end of the current limiting protection resistor Rc4, and the other end of the current limiting protection resistor Rc4 is connected to the signal Follow the base of the PNP transistor Dc1, connect one end of the phase feedback adjustment inductor Lc2, the other end of the phase feedback adjustment inductor Lc2 is connected to one end of the phase feedback limiting resistor Rc5, and the other end of the phase feedback limiting resistor Rc5 is connected to the collector of the signal follower PNP transistor Dc1 1. One end of the phase feedback proportional resistor Rc6 is connected, the signal follows the emitter of the PNP transistor Dc1 and the signal limiting resistor Rc7 and one end of the signal follows the shunt resistor Rc8 is connected, the other end of the signal follows the shunt resistor Rc8 is grounded, the positive electrode of the overvoltage protection device Dc2 is connected to the current The other end of the transformer Lc1 is connected, and the negative electrode of the overvoltage protection device Dc2 is connected to the base of the signal follower PNP transistor Dc1.

Further, one end of the voltage bias resistor R9 is connected to the working DC power supply VDD, the other end of the voltage bias resistor R9 is connected to the other end of the current transformer La1, the other end of the current transformer Lc1 is connected to one end of the voltage bias resistor R10, and the voltage bias resistor The other end of R10 is grounded, the positive pole of the overvoltage protection device Da2 is connected to the negative pole of the overvoltage protection device D3, the positive poles of the overvoltage protection device D3 and the overvoltage protection device Db2 are grounded, and the positive pole of the overvoltage protection device Dc2 is connected to the other end of the current transformer Lc1 .

Further, the other end of the phase feedback proportional resistor Ra6, the phase feedback proportional resistor Rb6 and the phase feedback proportional resistor Rc6 is connected to the working DC power supply VDD, the negative pole of the overvoltage protection device D4 and the collector of the zero-sequence current extraction PNP transistor D5, The positive pole of the overvoltage protection device D4 is grounded, the other ends of the signal limiting resistor Ra7, the signal limiting resistor Rb7 and the signal limiting resistor Rc7 are connected to the base of the zero-sequence current extraction PNP transistor D5, and the emitter of the zero-sequence current extraction PNP transistor D5 is connected to One end of the zero-sequence current load resistor R11, the input end of the zero-sequence current amplitude A/D conversion module and the input end of the zero-sequence resistance phase preprocessing module are connected, and the zero-sequence current amplitude A/D conversion module and the zero-sequence resistance phase preprocessing module are connected. The output terminal of the processing module is connected with the intelligent microprocessor.

A method for accurately judging AC grounding, characterized in that it comprises the following steps:

Accurate sampling of A-phase current: A-phase current is electrically isolated by the current transformer La1, and then converted from the conversion resistor Ra1 to the voltage signal at the two ends of the conversion resistor Ra1, and the voltage is divided and adjusted by the voltage divider adjustment potentiometer Ra2 and the voltage divider Ra3. The connection between the voltage divider adjustment potentiometer Ra2 and the voltage divider resistor Ra3 outputs a signal that is precisely proportional to the amplitude of the phase A current, and is sent to the base of the signal following the PNP transistor Da1 through the current limiting protection resistor Ra4, and the inductance is adjusted through phase feedback The phase ratio feedback and adjustment of La2, phase feedback limiting resistor Ra5, and phase feedback proportional resistor Ra6 are precisely proportional to the amplitude of the A-phase current at the junction of the signal following the transmitter stage of the PNP transistor Da1 and the signal following the shunt resistor Ra8 And the A-phase current sampling signal that is accurate and consistent in phase;

Accurate sampling of B-phase current: After the B-phase current is electrically isolated by the current transformer Lb1, it is converted from the conversion resistor Rb1 to the voltage signal at the two ends of the conversion resistor Rb1, and the voltage is divided and adjusted by the voltage division adjustment potentiometer Rb2 and the voltage division resistor Rb3. The connection between the voltage divider adjustment potentiometer Rb2 and the voltage divider resistor Rb3 outputs a signal that is precisely proportional to the amplitude of the B-phase current, and the signal is sent to the base of the PNP transistor Db1 through the current limiting protection resistor Rb4, and the inductance is adjusted through phase feedback Phase proportional feedback and adjustment of Lb2, phase feedback limiting resistor Rb5, and phase feedback proportional resistor Rb6, at the junction where the signal follows the transmitter stage of the PNP transistor Db1 and the signal follows the shunt resistor Rb8, the output is precisely proportional to the amplitude of the B-phase current And the B-phase current sampling signal that is accurate and consistent in phase;

Accurate sampling of C-phase current: After the current transformer Lc1 electrically isolates the C-phase current, it is converted from the conversion resistor Rc1 to the voltage signal at the two ends of the conversion resistor Rc1, and the voltage is divided and adjusted by the voltage division adjustment potentiometer Rc2 and the voltage division resistor Rc3. The connection between the voltage divider adjustment potentiometer Rc2 and the voltage divider resistor Rc3 outputs a signal that is precisely proportional to the amplitude of the C-phase current, and the signal is sent to the base of the PNP transistor Dc1 through the current limiting protection resistor Rc4, and the inductance is adjusted through phase feedback The phase proportional feedback and adjustment of Lc2, phase feedback limiting resistor Rc5, and phase feedback proportional resistor Rc6 are precisely proportional to the amplitude of the C-phase current at the connection between the signal following the transmitter stage of the PNP transistor Dc1 and the signal following the shunt resistor Rc8 And the C-phase current sampling signal that is accurate and consistent in phase;

Accurate sampling of zero-sequence current: The three-phase currents of A, B, and C are respectively collected by the signal-limited resistor Ra7, the signal-limited resistor Rb7, and the signal-limited resistor Rc7 to the base of the zero-sequence current extraction PNP transistor D5. Extract the zero-sequence current signal from the connection between the emitter of the PNP transistor D5 and the zero-sequence current load resistor R11, and its amplitude is digitized by the zero-sequence current amplitude A/D conversion module, and the phase is digitized by the zero-sequence resistor phase preprocessing module and then sent To the intelligent microprocessor, the intelligent microprocessor performs correct ground fault judgment, alarm or output protection signal according to the amplitude and phase of the precise zero-sequence current.

Further, the voltage bias resistor R9 and the voltage bias resistor R10 provide the bias voltage to ensure the normal sampling of the A-phase AC current, and the overvoltage protection device Da2/overvoltage protection device Db2/overvoltage protection device Dc2 and overvoltage protection device D3 limits the overvoltage in the front part to avoid damage to the circuit in the back part.

Further, the overvoltage protection device D4 limits damaging overvoltages introduced through the power supply.

Compared with the prior art, the present invention has the following advantages and effects: simple structure, reliable operation, high cost performance, accurately judge the AC grounding condition of the power grid grounding protection device product through zero-sequence current sampling, so that the grounding protection device product can protect the grounding fault Accurately send out the alarm signal or correctly send out the trip signal for disconnecting the ground fault line to ensure the safe and economical operation of the power grid.

Description of drawings

Fig. 1 is a circuit diagram of a zero-sequence current sampling circuit for accurate judgment of AC grounding according to the present invention.

detailed description

The present invention will be further described in detail below in conjunction with the accompanying drawings and examples. The following examples are explanations of the present invention and the present invention is not limited to the following examples.

As shown in the figure, a zero-sequence current sampling circuit for accurate judgment of AC grounding includes A-phase sampling circuit, B-phase sampling circuit, C-phase sampling circuit, zero-sequence current amplitude A/D conversion module 1, zero-sequence resistance phase Preprocessing module 2, intelligent microprocessor 3, A-phase sampling circuit, B-phase sampling circuit and C-phase sampling circuit respectively perform three-phase power supply current sampling and superimpose the three-phase current signals and send them to the zero-sequence current amplitude A/D The conversion module 1 and the zero-sequence resistance phase pre-processing module 2 make correct ground fault judgments, alarms or output protection signals based on the amplitude and phase of the zero-sequence current.

The structure of the A-phase sampling circuit is that the current transformer La1 is set on the A-phase line of the three-phase power supply for current sampling, one end of the current transformer La1 is connected with the conversion resistor Ra1 and one end of the voltage divider adjustment potentiometer Ra2, and the other end of the current transformer La1 Connect with the other end of the conversion resistor Ra1 and the other end of the voltage dividing resistor Ra3, the other end of the voltage dividing adjustment potentiometer Ra2 and the voltage dividing resistor Ra3 is connected with one end of the current limiting protection resistor Ra4, and the other end of the current limiting protection resistor Ra4 is connected to the signal follower The base of the PNP transistor Da1 is connected to one end of the phase feedback adjusting inductor La2, the other end of the phase feedback adjusting inductor La2 is connected to one end of the phase feedback limiting resistor Ra5, and the other end of the phase feedback limiting resistor Ra5 is connected to the signal follower PNP transistor Da1 collector, One end of the phase feedback proportional resistor Ra6 is connected, the signal follows the emitter of the PNP transistor Da1 and one end of the signal limiting resistor Ra7 and the signal follows the shunt resistor Ra8 are connected, the other end of the signal follows the shunt resistor Ra8 is grounded, and the positive electrode of the overvoltage protection device Da2 is connected to the current mutual inductance The other end of the device La1 is connected, and the negative electrode of the overvoltage protection device Da2 is connected to the base of the signal follower PNP transistor Da1;

The structure of the B-phase sampling circuit is that the current transformer Lb1 is set on the B-phase line of the three-phase power supply for current sampling, one end of the current transformer Lb1 is connected with the conversion resistor Rb1 and one end of the voltage divider adjustment potentiometer Rb2, and the other end of the current transformer Lb1 Connect with the other end of the conversion resistor Rb1 and the other end of the voltage dividing resistor Rb3, the other end of the voltage dividing adjustment potentiometer Rb2 and the voltage dividing resistor Rb3 is connected with one end of the current limiting protection resistor Rb4, and the other end of the current limiting protection resistor Rb4 is connected to the signal follower The base of the PNP transistor Db1 is connected to one end of the phase feedback adjustment inductor Lb2, the other end of the phase feedback adjustment inductor Lb2 is connected to one end of the phase feedback limiting resistor Rb5, and the other end of the phase feedback limiting resistor Rb5 is connected to the signal follower of the PNP transistor Db1 collector, One end of the phase feedback proportional resistor Rb6 is connected, the signal follows the emitter of the PNP transistor Db1 and one end of the signal limiting resistor Rb7 and the signal follows the shunt resistor Rb8 are connected, the other end of the signal follows the shunt resistor Rb8 is grounded, the positive electrode of the overvoltage protection device Db2 is connected to the current mutual inductance The other end of the device Lb1 is connected, and the negative electrode of the overvoltage protection device Db2 is connected to the base of the signal-following PNP transistor Db1;

The structure of the C-phase sampling circuit is that the current transformer Lc1 is set on the C line of the three-phase power supply for current sampling, one end of the current transformer Lc1 is connected with the conversion resistor Rc1 and one end of the voltage division adjustment potentiometer Rc2, and the other end of the current transformer Lc1 is connected with the The other end of the conversion resistor Rc1 is connected to the other end of the voltage dividing resistor Rc3, the other end of the voltage dividing adjustment potentiometer Rc2 and the voltage dividing resistor Rc3 is connected to one end of the current limiting protection resistor Rc4, and the other end of the current limiting protection resistor Rc4 is connected to the signal following PNP The base of the phase feedback regulating transistor Dc1 is connected to one end of the phase feedback adjusting inductor Lc2, the other end of the phase feedback adjusting inductor Lc2 is connected to one end of the phase feedback limiting resistor Rc5, and the other end of the phase feedback limiting resistor Rc5 is connected to the signal follower PNP type transistor Dc1 collector, phase One end of the feedback proportional resistor Rc6 is connected, the signal follows the emitter of the PNP transistor Dc1, the signal limiting resistor Rc7 and one end of the signal following shunt resistor Rc8 are connected, the other end of the signal following shunt resistor Rc8 is grounded, the positive pole of the overvoltage protection device Dc2 is connected to the current transformer The other end of Lc1 is connected, and the cathode of the overvoltage protection device Dc2 is connected to the base of the signal follower PNP transistor Dc1.

One end of the voltage bias resistor R9 is connected to the working DC power supply VDD, the other end of the voltage bias resistor R9 is connected to the other end of the current transformer La1, the other end of the current transformer Lc1 is connected to one end of the voltage bias resistor R10, and the other end of the voltage bias resistor R10 Grounding, the positive pole of the overvoltage protection device Da2 is connected to the negative pole of the overvoltage protection device D3, the positive poles of the overvoltage protection device D3 and the overvoltage protection device Db2 are grounded, and the positive pole of the overvoltage protection device Dc2 is connected to the other end of the current transformer Lc1. The phase feedback proportional resistor Ra6, the other end of the phase feedback proportional resistor Rb6 and the phase feedback proportional resistor Rc6 are connected to the working DC power supply VDD, the negative pole of the overvoltage protection device D4 and the collector of the zero-sequence current extraction PNP transistor D5, and the overvoltage protection device D4 The positive pole is grounded, the other end of the signal limiting resistor Ra7, the signal limiting resistor Rb7 and the signal limiting resistor Rc7 are connected to the base of the zero-sequence current extraction PNP transistor D5, the emitter of the zero-sequence current extraction PNP transistor D5 and the zero-sequence current load resistor One end of R11 is connected to the input end of the zero-sequence current amplitude A/D conversion module and the input end of the zero-sequence resistance phase preprocessing module, and the output end of the zero-sequence current amplitude A/D conversion module and the zero-sequence resistance phase preprocessing module Connect with intelligent microprocessor.

A method for accurately judging AC grounding, comprising the following steps:

Accurate sampling of A-phase current: A-phase current is electrically isolated by the current transformer La1, and then converted from the conversion resistor Ra1 to the voltage signal at the two ends of the conversion resistor Ra1, and the voltage is divided and adjusted by the voltage divider adjustment potentiometer Ra2 and the voltage divider Ra3. The connection between the voltage divider adjustment potentiometer Ra2 and the voltage divider resistor Ra3 outputs a signal that is precisely proportional to the amplitude of the phase A current, and is sent to the base of the signal following the PNP transistor Da1 through the current limiting protection resistor Ra4, and the inductance is adjusted through phase feedback The phase ratio feedback and adjustment of La2, phase feedback limiting resistor Ra5, and phase feedback proportional resistor Ra6 are precisely proportional to the amplitude of the A-phase current at the junction of the signal following the transmitter stage of the PNP transistor Da1 and the signal following the shunt resistor Ra8 and A-phase current sampling signal that is accurate and consistent in phase; voltage bias resistor R9 and voltage bias resistor R10 provide bias voltage to ensure normal sampling of A-phase AC current, overvoltage protection device Da2 and overvoltage protection device D3 limit The front part of the overvoltage avoids the damage of the rear part of the circuit

Accurate sampling of B-phase current: After the B-phase current is electrically isolated by the current transformer Lb1, it is converted from the conversion resistor Rb1 to the voltage signal at the two ends of the conversion resistor Rb1, and the voltage is divided and adjusted by the voltage division adjustment potentiometer Rb2 and the voltage division resistor Rb3. The connection between the voltage divider adjustment potentiometer Rb2 and the voltage divider resistor Rb3 outputs a signal that is precisely proportional to the amplitude of the B-phase current, and the signal is sent to the base of the PNP transistor Db1 through the current limiting protection resistor Rb4, and the inductance is adjusted through phase feedback Phase proportional feedback and adjustment of Lb2, phase feedback limiting resistor Rb5, and phase feedback proportional resistor Rb6, at the junction where the signal follows the transmitter stage of the PNP transistor Db1 and the signal follows the shunt resistor Rb8, the output is precisely proportional to the amplitude of the B-phase current and the B-phase current sampling signal that is accurate and consistent in phase; the voltage bias resistor R9 and the voltage bias resistor R10 provide bias voltage to ensure the normal sampling of the A-phase AC current, and the overvoltage protection device Db2 and overvoltage protection device D3 limit The front part of the overvoltage avoids the damage of the rear part of the circuit

Accurate sampling of C-phase current: After the current transformer Lc1 electrically isolates the C-phase current, it is converted from the conversion resistor Rc1 to the voltage signal at the two ends of the conversion resistor Rc1, and the voltage is divided and adjusted by the voltage division adjustment potentiometer Rc2 and the voltage division resistor Rc3. The connection between the voltage divider adjustment potentiometer Rc2 and the voltage divider resistor Rc3 outputs a signal that is precisely proportional to the amplitude of the C-phase current, and the signal is sent to the base of the PNP transistor Dc1 through the current limiting protection resistor Rc4, and the inductance is adjusted through phase feedback The phase proportional feedback and adjustment of Lc2, phase feedback limiting resistor Rc5, and phase feedback proportional resistor Rc6 are precisely proportional to the amplitude of the C-phase current at the connection between the signal following the transmitter stage of the PNP transistor Dc1 and the signal following the shunt resistor Rc8 And the phase C current sampling signal that is accurate and consistent in phase; the voltage bias resistor R9 and the voltage bias resistor R10 provide the bias voltage to ensure the normal sampling of the A phase AC current, and the overvoltage protection device Dc2 and the overvoltage protection device D3 limit the The front part of the overvoltage avoids the damage of the rear part of the circuit

Accurate sampling of zero-sequence current: The three-phase currents of A, B, and C are respectively collected by the signal-limited resistor Ra7, the signal-limited resistor Rb7, and the signal-limited resistor Rc7 to the base of the zero-sequence current extraction PNP transistor D5. Extract the zero-sequence current signal from the connection between the emitter of the PNP transistor D5 and the zero-sequence current load resistor R11, and its amplitude is digitized by the zero-sequence current amplitude A/D conversion module, and the phase is digitized by the zero-sequence resistor phase preprocessing module and then sent To the intelligent microprocessor, the intelligent microprocessor performs correct ground fault judgment, alarm or output protection signal according to the amplitude and phase of the precise zero-sequence current.

Wherein, the overvoltage protection device D4 limits the destructive overvoltage introduced by the power supply.

The above content described in this specification is only an illustration of the present invention. Those skilled in the technical field to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, as long as they do not deviate from the content of the present invention specification or exceed the scope defined in the claims, all should Belong to the protection scope of the present invention.

Claims (7)

1. a kind of AC earth precisely judges zero-sequence current sample circuit it is characterised in that：Comprise A phase sample circuit, B phase sample circuit, C phase sample circuit, zero-sequence current amplitude A/D modular converter, zero sequence resistance phase place pretreatment module, intelligent microprocessor, A phase sample circuit, B phase sample circuit and C phase sample circuit carry out three phase mains current sampling respectively and be delivered to zero-sequence current amplitude A/D modular converter after being superimposed three-phase current signal and the amplitude of zero-sequence current and phase place are carried out correct earth fault judgement, warning or output protection signal by zero sequence resistance phase place pretreatment module.

2. the zero-sequence current sample circuit that precisely judges according to a kind of AC earth described in claim 1 it is characterised in that：Described A phase sample circuit structure is，Current transformer La1 is arranged on and carries out current sampling on three phase mains A phase line，Current transformer La1 one end is connected with one end of transfer resistance Ra1 and partial pressure regulator potentiometer Ra2，The current transformer La1 other end is connected with the other end of the transfer resistance Ra1 other end and divider resistance Ra3，The other end of partial pressure regulator potentiometer Ra2 and divider resistance Ra3 is connected with one end of current limiting safeguard resistor Ra4，The current limiting safeguard resistor Ra4 other end and signal follow the base stage of PNP type triode Da1、One end that phase feedback adjusts inductance La2 connects，Phase feedback is adjusted the inductance La2 other end and is connected with phase feedback restriction resistance Ra5 one end，Phase feedback limits the resistance Ra5 other end and follows PNP type triode Da1 colelctor electrode with signal、Phase feedback proportion resistor Ra6 one end connects，Signal is followed one end that the emitter stage of PNP type triode Da1 follows shunt resistance Ra8 with signal limiting resistance Ra7 and signal and is connected，Signal follows shunt resistance Ra8 other end ground connection，Overvoltage protecting device Da2 positive pole is connected with the current transformer La1 other end，Overvoltage protecting device Da2 negative pole is followed PNP type triode Da1 base stage with signal and is connected；

Described B phase sample circuit structure is，Current transformer Lb1 is arranged on and carries out current sampling on three phase mains B phase line，Current transformer Lb1 one end is connected with one end of transfer resistance Rb1 and partial pressure regulator potentiometer Rb2，The current transformer Lb1 other end is connected with the other end of the transfer resistance Rb1 other end and divider resistance Rb3，The other end of partial pressure regulator potentiometer Rb2 and divider resistance Rb3 is connected with one end of current limiting safeguard resistor Rb4，The current limiting safeguard resistor Rb4 other end and signal follow the base stage of PNP type triode Db1、One end that phase feedback adjusts inductance Lb2 connects，Phase feedback is adjusted the inductance Lb2 other end and is connected with phase feedback restriction resistance Rb5 one end，Phase feedback limits the resistance Rb5 other end and follows PNP type triode Db1 colelctor electrode with signal、Phase feedback proportion resistor Rb6 one end connects，Signal is followed one end that the emitter stage of PNP type triode Db1 follows shunt resistance Rb8 with signal limiting resistance Rb7 and signal and is connected，Signal follows shunt resistance Rb8 other end ground connection，Overvoltage protecting device Db2 positive pole is connected with the current transformer Lb1 other end，Overvoltage protecting device Db2 negative pole is followed PNP type triode Db1 base stage with signal and is connected；

Described C phase sample circuit structure is，Current transformer Lc1 is arranged on and carries out current sampling on three phase mains C circuit，Current transformer Lc1 one end is connected with one end of transfer resistance Rc1 and partial pressure regulator potentiometer Rc2，The current transformer Lc1 other end is connected with the other end of the transfer resistance Rc1 other end and divider resistance Rc3，The other end of partial pressure regulator potentiometer Rc2 and divider resistance Rc3 is connected with one end of current limiting safeguard resistor Rc4，The current limiting safeguard resistor Rc4 other end and signal follow the base stage of PNP type triode Dc1、One end that phase feedback adjusts inductance Lc2 connects，Phase feedback is adjusted the inductance Lc2 other end and is connected with phase feedback restriction resistance Rc5 one end，Phase feedback limits the resistance Rc5 other end and follows PNP type triode Dc1 colelctor electrode with signal、Phase feedback proportion resistor Rc6 one end connects，Signal is followed one end that the emitter stage of PNP type triode Dc1 follows shunt resistance Rc8 with signal limiting resistance Rc7 and signal and is connected，Signal follows shunt resistance Rc8 other end ground connection，Overvoltage protecting device Dc2 positive pole is connected with the current transformer Lc1 other end，Overvoltage protecting device Dc2 negative pole is followed PNP type triode Dc1 base stage with signal and is connected.

3. the zero-sequence current sample circuit that precisely judges according to a kind of AC earth described in claim 2 it is characterised in that：Voltage bias resistance R9 one end is connected with working dc power VDD; the voltage bias resistance R9 other end is connected with the current transformer La1 other end, the current transformer Lc1 other end and voltage bias resistance R10 one end; the voltage bias resistance R10 other end is grounded; overvoltage protecting device Da2 positive pole is connected with overvoltage protecting device D3 negative pole; overvoltage protecting device D3 and the plus earth of overvoltage protecting device Db2, the positive pole of overvoltage protecting device Dc2 connects the other end of current transformer Lc1.

4. the zero-sequence current sample circuit that precisely judges according to a kind of AC earth described in Claims 2 or 3 it is characterised in that：Described phase feedback proportion resistor Ra6、The other end of phase feedback proportion resistor Rb6 and phase feedback proportion resistor Rc6 and working dc power VDD、Overvoltage protecting device D4 negative pole and zero-sequence current extract PNP type triode D5 colelctor electrode and connect，Overvoltage protecting device D4 plus earth，Signal limiting resistance Ra7、The other end of signal limiting resistance Rb7 and signal limiting resistance Rc7 connects the base stage that zero-sequence current extracts PNP type triode D5，Zero-sequence current extracts emitter stage and zero-sequence current load resistance R11 one end of PNP type triode D5、The input of the input of zero-sequence current amplitude A/D modular converter and zero sequence resistance phase place pretreatment module connects，The outfan of zero-sequence current amplitude A/D modular converter and zero sequence resistance phase place pretreatment module is connected with intelligent microprocessor.

5. a kind of accurate determination methods of AC earth are it is characterised in that comprise the steps of：

A phase current precisely samples：A phase current is changed into the voltage signal at transfer resistance Ra1 bis- end after current transformer La1 electrical isolation by transfer resistance Ra1，Adjust the signal in partial pressure regulator potentiometer Ra2 and the output of divider resistance Ra3 junction and A phase current accurate ratio in amplitude through partial pressure regulator potentiometer Ra2 and divider resistance Ra3 partial pressure and partial pressure，Deliver to the base stage that signal follows PNP type triode Da1 through current limiting safeguard resistor Ra4，Inductance La2 is adjusted by phase feedback、Phase feedback limits resistance Ra5、The phase ratio feedback of phase feedback proportion resistor Ra6 and regulation，Signal follow the emitting stage of PNP type triode Da1 and signal follow the junction output of shunt resistance Ra8 with A phase current accurate ratio and precisely consistent A phase current sampled signal in phase place in amplitude；

B phase current precisely samples：B phase current is changed into the voltage signal at transfer resistance Rb1 bis- end after current transformer Lb1 electrical isolation by transfer resistance Rb1，Adjust the signal in partial pressure regulator potentiometer Rb2 and the output of divider resistance Rb3 junction and B phase current accurate ratio in amplitude through partial pressure regulator potentiometer Rb2 and divider resistance Rb3 partial pressure and partial pressure，Deliver to the base stage that signal follows PNP type triode Db1 through current limiting safeguard resistor Rb4，Inductance Lb2 is adjusted by phase feedback、Phase feedback limits resistance Rb5、The phase ratio feedback of phase feedback proportion resistor Rb6 and regulation，Signal follow the emitting stage of PNP type triode Db1 and signal follow the junction output of shunt resistance Rb8 with B phase current accurate ratio and precisely consistent B phase current sampled signal in phase place in amplitude；

C phase current precisely samples：C phase current is changed into the voltage signal at transfer resistance Rc1 bis- end after current transformer Lc1 electrical isolation by transfer resistance Rc1，Adjust the signal in partial pressure regulator potentiometer Rc2 and the output of divider resistance Rc3 junction and C phase current accurate ratio in amplitude through partial pressure regulator potentiometer Rc2 and divider resistance Rc3 partial pressure and partial pressure，Deliver to the base stage that signal follows PNP type triode Dc1 through current limiting safeguard resistor Rc4，Inductance Lc2 is adjusted by phase feedback、Phase feedback limits resistance Rc5、The phase ratio feedback of phase feedback proportion resistor Rc6 and regulation，Signal follow the emitting stage of PNP type triode Dc1 and signal follow the junction output of shunt resistance Rc8 with C phase current accurate ratio and precisely consistent C phase current sampled signal in phase place in amplitude；

Zero-sequence current accurately samples：A, B, C three-phase current is respectively through signal limiting resistance Ra7, signal limiting resistance Rb7, signal limiting resistance Rc7 is collected to zero-sequence current and extracts addition at PNP type triode D5 base stage, extract PNP type triode D5 emitter stage in zero-sequence current and export zero sequence current signal with zero-sequence current load resistance R11 junction, its amplitude is through zero-sequence current amplitude A/D modular converter digitized, phase place delivers to intelligent microprocessor after zero sequence resistance phase place pretreatment module digitized, intelligent microprocessor carries out correct earth fault judgement according to the amplitude of accurately zero-sequence current and phase place, report to the police or output protection signal.

6. according to the accurate determination methods of a kind of AC earth described in claim 5 it is characterised in that：Voltage bias resistance R9 and voltage bias resistance R10 provides bias voltage, ensures the normal sampling of A cross streams electric current, and overvoltage protecting device Da2/ overvoltage protecting device Db2/ overvoltage protecting device Dc2 and overvoltage protecting device D3 places restrictions on its previous section overvoltage and avoids its rear part circuit to damage.

7. according to the accurate determination methods of a kind of AC earth described in claim 5 it is characterised in that：Overvoltage protecting device D4 places restrictions on the damageability overvoltage introducing by power supply.