Grounding protection method and device for a three-machine brushless excitation system

Abstract

The present invention relates to a grounding protection method and device for a three-machine brushless excitation system. The grounding protection method includes: detecting in real time the shaft current of the generator shaft with respect to the ground; detecting in real time the three-phase voltages of the stator winding of the sub-exciter, and determining whether a stator grounding fault occurs in the sub-exciter according to the three-phase voltages and the shaft current; injecting square-wave alternating current into the stator winding of the main exciter in real time, and detecting in real time the value of the resistance of the stator winding of the main exciter with respect to the ground, and determining whether a stator grounding fault occurs in the main exciter according to the value of the resistance with respect to the ground and the shaft current; when a stator grounding fault occurs in the sub-exciter or a stator grounding fault occurs in the main exciter, controlling the outlet switch of the generator to act. Through this technical solution, timely alarm or tripping can be achieved, preventing damage to the main and sub-exciters, avoiding the deterioration of the fault to an interphase fault when a single-phase grounding fault occurs in the main exciter or the sub-exciter, and even causing damage to major equipment such as the main and sub-exciters.

Description

Technical Field

[0001] The invention relates to the field of nuclear power protection, and in particular to a grounding protection method and device for a three-machine brushless excitation system. Background Art

[0002] Many nuclear power plants use a three-machine brushless excitation system, which consists of a main exciter, an auxiliary exciter, and a generator rotor. Figure 1 As shown, the auxiliary exciter is a permanent magnet machine with magnetic steel that rotates to form a rotating magnetic field, thereby forming a three-phase voltage on the stator winding 11 of the auxiliary exciter. The auxiliary exciter voltage is sent to the AVR system 12 for rectification, and the rectified DC voltage is output to the stator winding 13 of the main exciter. The main exciter rotor winding 14 has a rotating three-phase winding inside, which generates three-phase AC power after rotation and is sent to the coaxial main exciter three-phase rotating diode rectifier 15 for full-wave rectification. The rectified DC power is given to the generator rotor winding 16 to form a generator rotating magnetic field. The generator stator winding 17 emits three-phase AC power under the rotating magnetic field and is transmitted to the power grid.

[0003] Since the main exciter rotor winding 14 rotates with the generator shaft, it is difficult to obtain electrical parameters such as the excitation current and excitation voltage of the main exciter rotor winding 14 and the generator stator winding 17. Moreover, the existing protection mechanism does not have grounding protection for the auxiliary exciter and the main exciter. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a grounding protection method and device for a three-machine brushless excitation system in view of the defect that the auxiliary exciter and the main exciter in the prior art have no grounding protection.

[0005] The technical solution adopted by the present invention to solve the technical problem is: constructing a grounding protection method for a three-machine brushless excitation system, comprising:

[0006] Step S10. Real-time detection of the shaft current of the main shaft of the generator to the ground;

[0007] Step S20. Real-time detection of the three-phase voltage of the stator winding of the auxiliary exciter, and judging whether a stator grounding fault occurs in the auxiliary exciter according to the three-phase voltage and the shaft current;

[0008] Step S30. Injecting square wave AC into the stator winding of the main exciter in real time, and detecting the resistance of the stator winding of the main exciter to ground in real time, and judging whether the main exciter has a stator grounding fault according to the resistance of the stator winding to ground and the shaft current;

[0009] Step S40. When a stator grounding fault occurs in the auxiliary exciter or a stator grounding fault occurs in the main exciter, the output switch of the generator is controlled to operate.

[0010] Preferably, it further includes:

[0011] When a stator grounding fault occurs in the pilot exciter or a stator grounding fault occurs in the main exciter, an alarm signal is output.

[0012] Preferably, the step S20 includes:

[0013] Step S21. Real-time detect the three-phase phase voltages of the stator winding of the pilot exciter;

[0014] Step S22. Respectively compare each phase voltage of the stator winding of the pilot exciter with a first threshold value, and determine whether at least one phase voltage among the three-phase phase voltages is less than the first threshold value;

[0015] Step S23. Calculate the three-phase line voltages of the stator winding of the pilot exciter according to the three-phase phase voltages, and determine whether the three-phase line voltages change;

[0016] Step S24. Determine whether the shaft current is greater than a second threshold value;

[0017] Step S25. Determine whether a stator grounding fault occurs in the pilot exciter according to the judgment results of the step S22, the step S23 and the step S24.

[0018] Preferably, the step S20 further includes:

[0019] Step S26. Determine whether the detection circuit for detecting the three-phase phase voltages is normal;

[0020] Moreover, the step S25 includes:

[0021] Determine whether a stator grounding fault occurs in the pilot exciter according to the judgment results of the step S22, the step S23, the step S24 and the step S26.

[0022] Preferably, in the step S30, injecting square-wave alternating current into the stator winding of the main exciter in real time and detecting the ground resistance of the stator winding of the main exciter in real time includes:

[0023] Step S31. Control the coupling unit as a power generator to generate square-wave alternating current in real time, and connect one end of the power generator between the negative pole of the stator winding circuit of the main exciter and the ground, wherein two external resistors connected in parallel are connected between the negative pole of the stator winding circuit of the main exciter and the ground, and the resistance values of the two external resistors are equal;

[0024] Step S32. Obtain the voltage of the sampling resistor, wherein the sampling resistor is connected between the other end of the power generator and the ground;

[0025] Step S33. Calculate the resistance value of the ground resistance of the stator winding of the main exciter according to the voltage of the square-wave alternating current, the resistance value of the external resistor, the voltage and resistance value of the sampling resistor.

[0026] Preferably, in the step S30, determining whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the shaft current includes:

[0027] Step S34. Determine whether the resistance value of the ground resistance is less than a third threshold;

[0028] Step S35. Determine whether the shaft current is greater than a second threshold;

[0029] Step S36. Determine whether the main exciter has a stator grounding fault according to the judgment results of the step S34 and the step S35.

[0030] The present invention also constructs a grounding protection device for a three-machine brushless excitation system, including:

[0031] A shaft current detection module for detecting in real time the shaft current of the generator shaft to the ground;

[0032] A first fault judgment module for detecting in real time the three-phase voltages of the stator winding of the sub-exciter and determining whether the sub-exciter has a stator grounding fault according to the three-phase voltages and the shaft current;

[0033] A second fault judgment module for injecting square-wave alternating current into the stator winding of the main exciter in real time, detecting in real time the resistance value of the ground resistance of the stator winding of the main exciter, and determining whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the shaft current;

[0034] A protection module for controlling the action of the outlet switch of the generator when the sub-exciter has a stator grounding fault or the main exciter has a stator grounding fault.

[0035] Preferably, it further includes:

[0036] An alarm module for outputting an alarm signal when the sub-exciter has a stator grounding fault or the main exciter has a stator grounding fault.

[0037] Preferably, the first fault judgment module includes:

[0038] A phase voltage detection unit for detecting in real time the three-phase phase voltages of the stator winding of the sub-exciter;

[0039] The phase voltage judgment unit is configured to compare each phase voltage of the stator winding of the sub-exciter with a first threshold respectively, and determine whether at least one phase voltage among the three-phase voltages is less than the first threshold, and generate a first intermediate signal according to the judgment result;

[0040] The line voltage judgment unit is configured to calculate the three-phase line voltages of the stator winding of the sub-exciter according to the three-phase voltages, and determine whether the three-phase line voltages change, and generate a second intermediate signal according to the judgment result;

[0041] The detection circuit judgment unit is configured to judge whether the detection circuit is normal, and generate a third intermediate signal according to the judgment result, wherein the detection circuit is used to detect the three-phase voltages;

[0042] The shaft current judgment unit is configured to judge whether the shaft current is greater than a second threshold, and generate a fourth intermediate signal according to the judgment result;

[0043] The first operation unit is configured to perform a logical AND operation on the first intermediate signal, the second intermediate signal, the third intermediate signal, and the fourth intermediate signal to generate a protection signal for the stator grounding fault of the sub-exciter.

[0044] Preferably, the second fault judgment module includes:

[0045] The alternating current injection unit is configured to control the coupling unit as a power generator to generate a square-wave alternating current in real time, and connect one end of the power generator between the negative pole of the stator winding circuit of the main exciter and the ground, wherein two external resistors connected in parallel are connected between the negative pole of the stator winding circuit of the main exciter and the ground, and the resistance values of the two external resistors are equal;

[0046] The voltage sampling unit is configured to obtain the voltage of the sampling resistor, wherein the sampling resistor is connected between the other end of the power generator and the ground;

[0047] The resistor calculation unit is configured to calculate the resistance value of the resistance to the ground of the stator winding of the main exciter according to the voltage of the square-wave alternating current, the resistance value of the external resistor, the voltage of the sampling resistor, and the resistance value;

[0048] The resistor judgment unit is configured to judge whether the resistance value of the resistance to the ground is less than a third threshold, and generate a fifth intermediate signal according to the judgment result;

[0049] The second operation unit is configured to perform a logical AND operation on the fifth intermediate signal and the fourth intermediate signal to generate a protection signal for the stator grounding fault of the main exciter.

[0050] In the technical solution provided by the present invention, according to the voltage state of the auxiliary exciter and the ground insulation state of the main exciter monitored in real time, combined with the shaft current of the generator shaft to the ground, it is judged in real time whether the main and auxiliary exciters have grounding faults, and an alarm or tripping can be given in time to prevent damage to the main and auxiliary exciters, avoid the deterioration of the fault to an interphase fault when a single-phase grounding fault occurs in the main exciter or the auxiliary exciter, and even cause damage to major equipment such as the main and auxiliary exciters. Moreover, since the shaft current is added as a judgment parameter, the reliability of the protection is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the embodiments of the present invention, the drawings required for describing the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings. In the drawings:

[0052] Figure 1 is a schematic structural diagram of a three-machine brushless excitation system;

[0053] Figure 2 is a flowchart of Embodiment 1 of the grounding protection method for the three-machine brushless excitation system of the present invention;

[0054] Figure 3 is a partial structural schematic diagram of the grounding protection device of the three-machine brushless excitation system and Embodiment 2 of the three-machine brushless excitation system of the present invention;

[0055] Figure 4 is a partial structural schematic diagram of the grounding protection device of the three-machine brushless excitation system and Embodiment 2 of the three-machine brushless excitation system of the present invention;

[0056] Figure 5 is Figure 4 a partial equivalent circuit diagram;

[0057] Figure 6A is a waveform diagram of the square-wave AC voltage and the voltage of the sampling resistor when the main exciter has no stator grounding fault;

[0058] Figure 6B is a waveform diagram of the square-wave AC voltage and the voltage of the sampling resistor when the main exciter has a stator grounding fault;

[0059] Figure 7 is a logic structure diagram of Embodiment 1 of the grounding protection device of the three-machine brushless excitation system of the present invention;

[0060] Figure 8 is Figure 7 the logic structure diagram of the first fault judgment module in

[0061] Figure 9 Yes Figure 7 It is the logic structure diagram of the first embodiment of the second fault judgment module. Specific implementation manner

[0062] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0063] Figure 2 It is the flowchart of the first embodiment of the grounding protection method for the three-machine brushless excitation system of the present invention. The grounding protection method of the three-machine brushless excitation system in this embodiment is applied to a grounding protection device. The grounding protection device includes, for example, a protection device with the model number 7UM62, which can be used to protect the stator grounding faults of the main exciter and the auxiliary exciter. Moreover, the grounding protection method in this embodiment specifically includes:

[0064] Step S10. Real-time detect the shaft current of the generator shaft to the ground. For example, the current of the generator rotor grounding carbon brush can be detected through the measurement transmitter terminal on the grounding protection device.

[0065] Step S20. Real-time detect the three-phase voltage of the stator winding of the auxiliary exciter, and judge whether the auxiliary exciter has a stator grounding fault according to the three-phase voltage and the shaft current;

[0066] Step S30. Inject square-wave alternating current into the stator winding of the main exciter in real time, and real-time detect the resistance value of the ground resistance of the stator winding of the main exciter, and judge whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the shaft current;

[0067] Step S40. When the auxiliary exciter has a stator grounding fault, or the main exciter has a stator grounding fault, control the outlet switch of the generator to act.

[0068] In the technical solution of this embodiment, according to the voltage state of the auxiliary exciter and the ground insulation state of the main exciter monitored in real time, combined with the shaft current of the generator shaft to the ground, it is judged in real time whether the main and auxiliary exciters have grounding faults, which can alarm or trip in time to prevent damage to the main and auxiliary exciters, and avoid the deterioration of the fault to an interphase fault when the main exciter or the auxiliary exciter has a single-phase grounding fault, and even cause damage to major equipment such as the main and auxiliary exciters. Moreover, since the shaft current is added as a judgment parameter, the reliability of the protection is improved.

[0069] Further, the grounding protection method for the three-machine brushless excitation system of the present invention further includes: when a stator grounding fault occurs in the pilot exciter or in the main exciter, an alarm signal is output. For example, when a grounding fault occurs, an alarm can be given in the form of sound, light, and electricity.

[0070] Further, in an alternative embodiment, step S20 includes:

[0071] Step S21. Real-time detect the three-phase phase voltages of the stator winding of the pilot exciter;

[0072] Regarding this step, in a specific embodiment, in combination with Figure 3 , a voltage transformer is built in the grounding protection device 100, and the three-phase phase voltages of the stator winding 11 of the pilot exciter can be detected through this voltage transformer.

[0073] Step S22. Compare each phase voltage of the stator winding of the pilot exciter with a first threshold respectively, and determine whether at least one phase voltage among the three-phase phase voltages is less than the first threshold;

[0074] In this step, the first threshold can be 50%U, where U is the phase voltage of the stator winding of the pilot exciter under no-fault condition. Additionally, it should be noted that the pilot exciter is a neutral-point non-grounded system. Therefore, when a single-phase grounding fault occurs in the stator winding, the neutral point will shift, and the voltage of the faulty phase to ground is zero, which is obviously lower than the first threshold.

[0075] Step S23. Calculate the three-phase line voltages of the stator winding of the pilot exciter according to the three-phase phase voltages, and determine whether the three-phase line voltages change;

[0076] In this step, it should be noted that since the pilot exciter is a neutral-point non-grounded system, even if a single-phase grounding fault occurs in the stator winding, the three-phase line voltages are still the same as those under no-fault condition, that is, they are all 1.732U.

[0077] Step S24. Determine whether the shaft current is greater than a second threshold;

[0078] In this step, it should be noted that if a grounding fault occurs in the stator winding of the pilot exciter, there will definitely be a phenomenon of high shaft current. Therefore, the shaft current parameter can be added as another criterion for this protection to improve the reliability of the protection and prevent mis-tripping.

[0079] Step S25. Determine whether a stator grounding fault occurs in the pilot exciter according to the judgment results of step S22, step S23, and step S24.

[0080] In this step, when a stator grounding fault occurs in the pilot exciter, the phase voltage of the faulty phase will drop to 0 (less than the first threshold), the phase voltage of the non-faulty phase will rise to 1.732 times that in normal conditions, and the three-phase line voltage will remain unchanged. At the same time, the shaft current of the generator shaft to the ground will increase (greater than the second threshold). Therefore, it can be determined whether a stator grounding fault has occurred in the pilot exciter based on this.

[0081] Further, step S20 further includes:

[0082] Step S26. Determine whether the detection circuit for detecting the three-phase phase voltage is normal;

[0083] Regarding this step, in a specific embodiment, in combination with Figure 3 , it can be determined whether the detection circuit is normal by judging whether the fuse FU in the detection circuit of each phase voltage is blown. For example, if the fuse of the detection circuit of a certain phase voltage is blown, it can be confirmed that the detection circuit of this phase voltage is abnormal; it can also be determined whether the detection circuit is normal by comparing the detected value of the phase voltage and the detected value of the phase current. For example, for a certain phase stator winding, if the detected phase current decreases significantly while the phase voltage is normal, this situation can also be considered that the detection circuit of this phase voltage is abnormal;

[0084] Moreover, step S25 includes:

[0085] According to the judgment results of step S22, step S23, step S24 and step S26, determine whether a stator grounding fault has occurred in the pilot exciter.

[0086] In this embodiment, the PT break lock in the grounding protection device 100 is used to prevent misoperation of the protection caused by abnormal voltage sampling circuit.

[0087] Further, in an alternative embodiment, the step of injecting square-wave alternating current into the stator winding of the main exciter in real time and detecting the resistance value of the ground resistance of the stator winding of the main exciter in step S30 specifically includes:

[0088] Step S31. Control the coupling unit as a power generator to generate square-wave alternating current in real time, and connect one end of the power generator between the negative pole of the stator winding circuit of the main exciter and the ground. Among them, two external resistors connected in parallel are connected between the negative pole of the stator winding circuit of the main exciter and the ground, and the resistance values of the two external resistors are equal;

[0089] Step S32. Obtain the voltage of the sampling resistor, where the sampling resistor is connected between the other end of the power generator and the ground;

[0090] Step S33. Calculate the resistance value of the ground resistance of the stator winding of the main exciter according to the voltage of the square-wave alternating current, the resistance value of the external resistor, the voltage and resistance value of the sampling resistor.

[0091] Further, in an optional embodiment, in step S30, determining whether the main exciter has a stator ground fault according to the resistance value of the ground resistance and the shaft current specifically includes:

[0092] Step S34. Determine whether the resistance value of the ground resistance is less than a third threshold;

[0093] Step S35. Determine whether the shaft current is greater than a second threshold;

[0094] Step S36. Determine whether the main exciter has a stator ground fault according to the judgment results of step S34 and step S35.

[0095] In a specific embodiment, regarding step S30, as Figure 4 shown, a voltage control module is built in the ground protection device 100. The voltage control module is used to output a control voltage Ucontrol, and the control voltage Ucontrol can control the coupling unit 200 to generate a square-wave alternating current. For example, the square-wave alternating current is a square-wave alternating current with an amplitude of 50V and the positive and negative polarities changing 1 - 4 times per second. Specifically, the conversion frequency depends on the frequency setting value of the power generator. Moreover, the control voltage Ucontrol is proportional to the injected 50V alternating square-wave voltage in both amplitude and frequency. At the same time, two external resistors Rv are connected in series between the negative pole of the stator winding circuit of the main exciter and the ground, and the generated square-wave alternating current is injected into the connection point of the two external resistors Rv, that is, symmetrically injected between the negative pole of the main excitation circuit and the ground.

[0096] As Figure 4 shown, the sampling resistor Rm in the coupling unit 200 is connected between the other end of the square-wave alternating current and the ground, and the resistance value of the sampling resistor is a known value. In addition, since the resistance value of the stator winding 13 of the main exciter is very small and can be ignored, the ground resistance of the stator winding circuit cable (positive cable) can be equivalent to Re, and the ground capacitance of the stator winding circuit cable (positive cable) can be equivalent to Ce. Based on this, the equivalent circuit is as Figure 5 shown. The square-wave alternating current passes through two externally connected resistors Rv in parallel, then through the ground resistance Re and the ground capacitance Ce in parallel, and finally forms a loop through the sampling resistor Rm. The current of this loop is Ie.

[0097] Combined with Figure 4 and Figure 5, the voltage Um across the sampling resistor is connected for monitoring through the measurement transmitter terminal on the ground protection device 100. Since the voltage of the square-wave alternating current, the resistance value of the external resistor Rv, and the resistance value of the sampling resistor Rm are all known, and the voltage Um of the sampling resistor Rm can be obtained through detection, therefore, the resistance value of the ground resistance Re of the stator winding loop cable (positive cable) can be calculated based on the above-known quantities. Specifically, the current Ie in the stator winding loop of the main exciter can be reflected by the voltage drop across the series-connected sampling resistor Rm with a low resistance value. Whenever the polarity of the injected AC voltage Ue flips, a charging current Ie will be generated and flow through the two external resistors and then into the equivalent ground capacitance Ce. This charging current forms a voltage drop Um across the sampling resistor Rm in a certain proportion. Once the ground capacitance Ce of the stator winding loop is fully charged, the charging current Ie will become zero, and the voltage Um across the sampling resistor Rm will also become zero, as Figure 6A shown. If a ground fault occurs in the stator winding loop, then a continuous ground current will be formed in the loop, and the voltage Um across the sampling resistor Rm will also be maintained at a relatively high voltage, such as 0.75V, as Figure 6B shown. Therefore, the magnitude of the current Ie in the loop depends on the size of the ground resistance Re.

[0098] Of course, in other embodiments, the resistance value of the ground resistance Re can also be calculated through a formula according to the proportional relationship between the resistance value of the ground resistance Re, the resistance value of the sampling resistor Rm, and their respective voltages. The specific formula is:

[0099]

[0100] where, R E is the resistance value of the ground resistance Re, U H is the amplitude of the square-wave alternating current Ue, Um is the voltage across the sampling resistor Rm, R M is the resistance value of the sampling resistor Rm, and R V is the resistance value of the external resistor Rv.

[0101] Figure 71 is a logical structure diagram of the first embodiment of the grounding protection device of the three-machine brushless excitation system of the present invention. The grounding protection device of the three-machine brushless excitation system of the embodiment comprises: an axis current detection module 110, a first fault judgment module 120, a second fault judgment module 130, and a protection module 140, wherein the axis current detection module 110 is used to detect the axis current of the main shaft of the generator to the ground in real time; the first fault judgment module 120 is used to detect the three-phase voltage of the stator winding of the auxiliary exciter in real time, and judge whether the auxiliary exciter has a stator grounding fault according to the three-phase voltage and the axis current; the second fault judgment module 130 is used to inject square wave alternating current into the stator winding of the main exciter in real time, and detect the resistance value of the stator winding of the main exciter to the ground in real time, and judge whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the axis current; the protection module 140 is used to control the output switch action of the generator when the stator grounding fault occurs in the auxiliary exciter or the stator grounding fault occurs in the main exciter.

[0102] Furthermore, the grounding protection device of the present invention further comprises an alarm module, which is used to output an alarm signal when a stator grounding fault occurs in the auxiliary exciter or a stator grounding fault occurs in the main exciter.

[0103] Furthermore, the first fault judgment module includes: a phase voltage detection unit, a phase voltage judgment unit, a line voltage judgment unit, a detection loop judgment unit, a shaft current judgment unit and a first operation unit. Among them, the phase voltage detection unit is used to detect the three-phase phase voltage of the stator winding of the auxiliary exciter in real time; the phase voltage judgment unit is used to compare the phase voltage of each phase of the stator winding of the auxiliary exciter with the first threshold value respectively, and judge whether there is at least one phase voltage of the three-phase phase voltage that is less than the first threshold value, and generate a first intermediate signal according to the judgment result; the line voltage judgment unit is used to calculate the three-phase line voltage of the stator winding of the auxiliary exciter according to the three-phase phase voltage, and judge whether the three-phase line voltage changes, and generate a second intermediate signal according to the judgment result; the detection circuit judgment unit is used to judge whether the detection circuit is normal, and generate a third intermediate signal according to the judgment result, wherein the detection circuit is used to detect the three-phase phase voltage; the shaft current judgment unit is used to judge whether the shaft current is greater than the second threshold value, and generate a fourth intermediate signal according to the judgment result; the first operation unit is used to perform a logical AND operation on the first intermediate signal, the second intermediate signal, the third intermediate signal, and the fourth intermediate signal to generate a protection signal for the stator grounding fault of the auxiliary exciter.

[0104] In a specific embodiment, in combination Figure 8, after the three-phase phase voltages (Ua, Ub, Uc) of the stator winding of the auxiliary exciter are detected in real time, the phase voltages of each phase can be respectively compared with the first threshold (50%U), and it is judged by the OR gate 121 whether at least one of the three-phase phase voltages is less than the first threshold, and a first intermediate signal S1 is generated according to the judgment result. If at least one of the three-phase phase voltages is less than the first threshold, then S1 is at a high level, otherwise it is at a low level.

[0105] The three-phase line voltages of the stator winding of the auxiliary exciter are also calculated based on the detected three-phase phase voltages, and it is judged whether the three-phase line voltages change, that is, it is judged whether the three-phase line voltages are all 1.732U, and a second intermediate signal S2 is generated according to the judgment result. Moreover, if the three-phase line voltages are all 1.732U, then S2 is at a high level, otherwise it is at a low level.

[0106] It is also judged whether the detection circuit is normal, and a third intermediate signal S3 is generated according to the judgment result. Moreover, if it is normal, then S3 is at a high level, otherwise it is at a low level.

[0107] It is also judged whether the shaft current is greater than the second threshold, and a fourth intermediate signal S4 is generated according to the judgment result. Moreover, if the shaft current is greater than the second threshold, then S4 is at a high level, otherwise it is at a low level.

[0108] Moreover, in this embodiment, comprehensive judgment is carried out through two logic AND gates 122 and 123. First, the first intermediate signal S1, the second intermediate signal S2, and the third intermediate signal S3 are sent into the logic AND gate 122. The output signal of the logic AND gate 122 is sent into one input terminal of the logic AND gate 123, and the fourth intermediate signal is input into the other input terminal of the logic AND gate 123. The output signal of the logic AND gate 123 is the protection signal for the stator ground fault of the auxiliary exciter.

[0109] In a specific embodiment, in combination with Figure 8, the second fault judgment module includes: an alternating current injection unit, a voltage sampling unit, a resistance calculation unit, a resistance judgment unit, and a second operation unit. Among them, the alternating current injection unit is used to control the coupling unit as a power generator to generate a square-wave alternating current in real time, and connect one end of the power generator between the negative pole of the stator winding circuit of the main exciter and the ground. Among them, two externally connected resistors connected in parallel are connected between the negative pole of the stator winding circuit of the main exciter and the ground, and the resistance values of the two externally connected resistors are equal; the voltage sampling unit is used to obtain the voltage of the sampling resistor, where the sampling resistor is connected between the other end of the power generator and the ground; the resistance calculation unit is used to calculate the resistance value of the ground resistance of the stator winding of the main exciter according to the voltage of the square-wave alternating current, the resistance value of the externally connected resistor, the voltage and resistance value of the sampling resistor; the resistance judgment unit is used to judge whether the resistance value of the ground resistance is less than a third threshold value, and generate a fifth intermediate signal according to the judgment result; the second operation unit is used to perform a logical AND operation on the fifth intermediate signal and the fourth intermediate signal to generate a protection signal for the stator ground fault of the main exciter.

[0110] In a specific embodiment, in combination with Figure 9 , Figure 4 and Figure 5 , the current Ie in the circuit can be calculated according to the resistance value of the sampling resistor and the detected voltage of the sampling resistor, and then the voltage Ue of the square-wave alternating current and the current Ie are sent to the calculation unit 131. This calculation unit 131 calculates the resistance value of the ground resistance Re of the stator winding of the main exciter according to the voltage Ue of the square-wave alternating current, and judges whether the resistance value of the ground resistance Re is less than a third threshold value, and generates a fifth intermediate signal. Moreover, if the resistance value of the ground resistance Re is less than the third threshold value, the fifth intermediate signal S5 is at a high level, otherwise it is at a low level. Then the fifth intermediate signal S5 and the fourth intermediate signal S4 are respectively sent to the logic AND gate 132, and the output signal of the logic AND gate 132 is the protection signal for the stator ground fault of the main exciter.

[0111] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims

1. A grounding protection method for a three-machine brushless excitation system, characterized in that, Including: Step S10. Real-time detect the shaft current of the generator's large shaft to the ground; Step S20. Real-time detect the three-phase voltages of the stator winding of the sub-exciter, and determine whether the sub-exciter has a stator grounding fault according to the three-phase voltages and the shaft current; Step S30. Real-time inject square-wave alternating current into the stator winding of the main exciter, and real-time detect the resistance value of the ground resistance of the stator winding of the main exciter, and determine whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the shaft current; Step S40. When the sub-exciter has a stator grounding fault, or the main exciter has a stator grounding fault, control the outlet switch of the generator to act; Wherein, the step S20 includes: Step S21. Real-time detect the three-phase phase voltages of the stator winding of the sub-exciter; Step S22. Respectively compare each phase voltage of the stator winding of the sub-exciter with a first threshold, and determine whether at least one phase voltage in the three-phase phase voltages is less than the first threshold; Step S23. Calculate the three-phase line voltages of the stator winding of the sub-exciter according to the three-phase phase voltages, and determine whether the three-phase line voltages change; Step S24. Determine whether the shaft current is greater than a second threshold; Step S25. Determine whether the sub-exciter has a stator grounding fault according to the judgment results of the step S22, the step S23 and the step S24; In the step S30, determining whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the shaft current includes: Step S34. Determine whether the resistance value of the ground resistance is less than a third threshold; Step S35. Determine whether the shaft current is greater than a second threshold; Step S36. Determine whether the main exciter has a stator grounding fault according to the judgment results of the step S34 and the step S35.

2. The grounding protection method for the three-machine brushless excitation system according to claim 1, characterized in that, Also including: When the sub-exciter has a stator grounding fault, or the main exciter has a stator grounding fault, output an alarm signal.

3. The grounding protection method for the three-machine brushless excitation system according to claim 1, characterized in that, The step S20 further includes: Step S26. Determine whether the detection circuit for detecting the three-phase phase voltages is normal; Moreover, the step S25 includes: Determine whether the sub-exciter has a stator grounding fault according to the judgment results of the step S22, the step S23, the step S24 and the step S26.

4. The grounding protection method for the three-machine brushless excitation system according to claim 1, characterized in that, In the step S30, real-time injecting square-wave alternating current into the stator winding of the main exciter, and real-time detecting the resistance value of the ground resistance of the stator winding of the main exciter, includes: Step S31. Control the coupling unit as a power generator to generate square-wave alternating current in real time, and connect one end of the power generator between the negative pole of the stator winding circuit of the main exciter and the ground, wherein two external resistors connected in parallel are connected between the negative pole of the stator winding circuit of the main exciter and the ground, and the resistance values of the two external resistors are equal; Step S32. Obtain the voltage of the sampling resistor, wherein the sampling resistor is connected between the other end of the power generator and the ground; Step S33. Calculate the resistance value of the ground resistance of the stator winding of the main exciter according to the voltage of the square-wave alternating current, the resistance value of the external resistor, the voltage and resistance value of the sampling resistor.

5. A grounding protection device for a three-machine brushless excitation system, characterized in that, Including: An axial current detection module for detecting the axial current of the generator shaft to the ground in real time; A first fault judgment module for detecting the three-phase voltage of the stator winding of the auxiliary exciter in real time and judging whether the auxiliary exciter has a stator grounding fault according to the three-phase voltage and the axial current; A second fault judgment module for injecting square-wave alternating current into the stator winding of the main exciter in real time, detecting the resistance value of the ground resistance of the stator winding of the main exciter in real time, and judging whether the main exciter has a stator grounding fault according to the resistance value of the ground resistance and the axial current; A protection module for controlling the outlet switch of the generator to act when the auxiliary exciter has a stator grounding fault or the main exciter has a stator grounding fault; The first fault judgment module includes: A phase voltage detection unit for detecting the three-phase phase voltages of the stator winding of the auxiliary exciter in real time; A phase voltage judgment unit for respectively comparing each phase voltage of the stator winding of the auxiliary exciter with a first threshold value, judging whether at least one phase voltage among the three-phase phase voltages is less than the first threshold value, and generating a first intermediate signal according to the judgment result; A line voltage judgment unit for calculating the three-phase line voltages of the stator winding of the auxiliary exciter according to the three-phase phase voltages, judging whether the three-phase line voltages change, and generating a second intermediate signal according to the judgment result; A detection loop judgment unit for judging whether the detection loop is normal and generating a third intermediate signal according to the judgment result, where the detection loop is used to detect the three-phase phase voltages; An axial current judgment unit for judging whether the axial current is greater than a second threshold value and generating a fourth intermediate signal according to the judgment result; A first operation unit for performing a logical AND operation on the first intermediate signal, the second intermediate signal, the third intermediate signal, and the fourth intermediate signal to generate a protection signal for the stator grounding fault of the auxiliary exciter; The second fault judgment module includes: A resistance judgment unit for judging whether the resistance value of the ground resistance is less than a third threshold value and generating a fifth intermediate signal according to the judgment result; A second operation unit for performing a logical AND operation on the fifth intermediate signal and the fourth intermediate signal to generate a protection signal for the stator grounding fault of the main exciter.

6. The grounding protection device of the three-machine brushless excitation system according to claim 5, characterized in that, It further includes: An alarm module for outputting an alarm signal when the auxiliary exciter has a stator grounding fault or the main exciter has a stator grounding fault.

7. The grounding protection device of the three-machine brushless excitation system according to claim 5, characterized in that, The second fault judgment module includes: An alternating current injection unit for controlling the coupling unit as a power generator to generate square-wave alternating current in real time, and connecting one end of the power generator between the negative pole of the stator winding circuit of the main exciter and the ground. There are two external resistors connected in parallel between the negative pole of the stator winding circuit of the main exciter and the ground, and the resistance values of the two external resistors are equal; A voltage sampling unit for obtaining the voltage of a sampling resistor, where the sampling resistor is connected between the other end of the power generator and the ground; A resistance calculation unit for calculating the resistance value of the ground resistance of the stator winding of the main exciter according to the voltage of the square-wave alternating current, the resistance value of the external resistor, the voltage and resistance value of the sampling resistor.

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