A method for monitoring open-phase of auxiliary transformers in nuclear power plants

Abstract

The present invention provides a method for monitoring the open-phase of auxiliary transformers in nuclear power plants, which includes the following steps: Step S1: The first auxiliary transformer and the second auxiliary transformer operate separately; all four voltage transformers are put into operation; Step S2: The open-phase monitoring range is the auxiliary power supply incoming line and the primary incoming line of the high-voltage side of the auxiliary transformer; Step S3: Collect the bus voltage of the high-voltage side of the auxiliary transformer, the bus voltage of the low-voltage side of the first auxiliary transformer, and the bus voltage of the low-voltage side of the second auxiliary transformer; Step S4: The phase angle and the effective value of the voltage are obtained by using the high-precision FFT algorithm technology to ensure sufficient measurement accuracy, and the open-phase monitoring is realized by calculating the phase angle difference and the voltage ratio error. The open-phase monitoring method provided by the present invention improves the reliability of the auxiliary power supply and thus improves the safety of the operation of the nuclear power plant.

Description

Technical Field

[0001] The present invention relates to the technical field of auxiliary transformer open-phase monitoring, and particularly relates to a method for monitoring open-phase of an auxiliary transformer in a nuclear power plant. Background Art

[0002] The unbalanced operation caused by faults such as poor primary contact, breaker operating mechanism and its electrical control circuit leads to overheating or burning accidents of station service motors and generators, which not only causes a major loss to the power plant, but also poses a great threat to the safe operation of the power system. The following are three open-phase events listed:

[0003] 1. Open-phase event of Byron nuclear power unit. Specifically, on January 30, 2012, due to the low bus voltage of the coolant pump (RCP pump) of Unit 2 of Byron Nuclear Power Plant, the reactor automatically shut down during full power operation. Byron is a pressurized water reactor nuclear power plant designed by Westinghouse. The station service power system has four sections of 6.9 kV non-safety-class buses, two sections of 4 kV non-safety-class buses and two sections of 4 kV safety-class buses. Two sections of 4 kV safety-class buses and two sections of 6.9 kV non-safety-class buses are powered by one of the auxiliary transformers and connected to the 345 kV switchyard. The remaining two sections of non-safety-class 6.9 kV buses and two sections of non-safety-class 4 kV buses are powered by station service transformers under normal operating conditions of the generator. Cause of the event: Due to the lead wire drop on the porcelain insulator sleeve of the 345 kV switchyard, a high-resistance grounding fault occurred on the high-voltage side of the auxiliary transformer, which caused single-phase disconnection of phase C of the two auxiliary transformers. Since it was a high-resistance grounding, the grounding current was not sufficient to cause the differential protection of the auxiliary transformer to trip. However, the loss of voltage in phase C caused an unbalanced voltage between phases A-C and B-C of the auxiliary transformer, and the reactor protection system correctly discriminated the unbalance of the 6.9 kV bus voltage, resulting in reactor shutdown. At the same time, several large motors powered by the auxiliary transformer SAT tripped due to overcurrent of the phase current, including important station service water pumps, equipment cooling water pumps and charging pumps. This event exposed that the equipment failure of the switchyard caused open-phase without alarm and was not detected in time.

[0004] 2. Phase loss incidents of the Vandellos nuclear power unit (in Vandellos, Spain). Specifically, the Vandellos Unit 2 is a three-loop pressurized water reactor designed by Westinghouse, with two off-site power supplies and two emergency diesel generators. In August 2006, the unit tripped completely without a phase loss alarm in the main control. The reason for the complete trip was that the cable connection at the upper part of the A-phase support insulator on the high-voltage side of the main transformer was loose, triggering the negative-sequence protection action of the main generator, causing the steam turbine to trip and the reactor to trip automatically. After the main generator tripped, since the healthy phase would still generate voltage through other phases of the transformer or the auxiliary transformer, the low-voltage relay could not detect the phase loss fault, and the installed electrical protection could not detect the phase loss situation after the main generator was disconnected, resulting in the tripping of some motor protections.

[0005] 3. Phase loss caused by the fracture at the connection between the down-lead wire and the bushing terminal of Tower No. 30 (terminal tower) of a 220 kV line at a domestic nuclear power plant. Specifically, Units 1-4 of a certain nuclear power plant were in the power operation state. To meet the grid requirements, the 220 kV Yu* Line and *Dun Line were operated in a loop through the 220 kV I / II section bus-tie 21M switch. The NCS in the network control building showed that the A-phase current of the 220 kV Yu* Line dropped instantaneously to 0. After receiving the notice from the main control, the maintenance personnel immediately organized an inspection. After confirmation, the sampling of the line protection device, measurement and control device, and fault recorder of the 220 kV line at the nuclear power plant was abnormal, and the line protection device gave a TA phase loss alarm. Further inspection found that the A-phase down-lead wire at Tower No. 30 of the 220 kV Yu* Line was fractured at the connection with the bushing, and the bushing lead-out terminal board was severely ablated. The Yu* Line was taken out of service urgently, the down-lead wire and the bushing terminal board were cleaned and polished, the lead terminal board of the lightning arrester was redrilled, and the bolts were replaced and tightened. After the treatment was completed, the Yu* Line was energized and put back into service. Cause analysis: The down-lead wire of the overhead line and the bushing terminal board were loosened due to the long-term influence of sea breeze and seaside salt spray corrosion, resulting in heating and finally ablation and phase loss.

[0006] Currently, the technologies adopted are to add dedicated phase loss protections, mainly including:

[0007] 1) Install high-precision optical fiber current transformer protections. Flexible optical fiber current transformers have high measurement accuracy, can monitor the no-load exciting current of the transformer, and have a certain degree of reliability and sensitivity.

[0008] 2) Injection protection at the grounded neutral point of the transformer. That is, a non-power frequency injection power supply is adopted at the grounded neutral point of the transformer in an active injection manner to judge the phase loss fault under the condition of no-load or light load of the transformer.

[0009] However, the existing technologies have the following disadvantages:

[0010] 1) Install high-precision optical fiber current transformer protection. Although it has certain reliability and sensitivity, it is necessary to install optical fiber current transformers and corresponding acquisition interface devices on site, which is difficult to implement. In addition, in the case of the direct connection installation method of GIS and transformer, it is difficult to select the installation position of the optical fiber current transformer, and this scheme is not suitable for implementation.

[0011] 2) Inject protection into the grounded neutral point of the transformer. That is, a non-power frequency injection power supply is adopted at the grounded neutral point of the transformer in an active injection manner. The injected non-power frequency voltage will cause harmonic interference to the transformer and the electrical system, affecting the normal operation of relay protection and electronic equipment. The on-site implementation risk is large, affecting the safe and reliable operation of the electrical system. At the same time, this protection cannot judge the specific phase of the broken phase.

[0012] The currently adopted technology is the invasive mode, which has certain impacts on the operation of the electrical system, is inconvenient to install and use, and requires certain investment and other deficiencies and defects. Summary of the Invention

[0013] The purpose of the present invention is to overcome the defects described in the prior art, so as to provide a method for monitoring the broken phase of the auxiliary transformer in a nuclear power plant. The method for monitoring the broken phase of the auxiliary transformer in a nuclear power plant improves the reliability of the auxiliary power supply, and further improves the safety of the operation of the nuclear power plant.

[0014] In order to achieve the above purpose, the present invention provides the following technical solutions:

[0015] A method for monitoring the broken phase of an auxiliary transformer in a nuclear power plant includes the following steps:

[0016] Step S1: The first auxiliary transformer and the second auxiliary transformer are in a separated state, operating under no-load or light load; all four voltage transformers are put into operation;

[0017] Step S2: The monitoring range of the broken phase is the auxiliary power supply inlet line and the primary inlet line of the high-voltage side of the auxiliary transformer;

[0018] Step S3: Collect the bus voltage of the high-voltage side of the auxiliary transformer, the bus voltage of the low-voltage side of the first auxiliary transformer, and the bus voltage of the low-voltage side of the second auxiliary transformer;

[0019] Step S4: The phase angle and the effective value of the voltage are obtained by using the high-precision FFT (Fast Fourier Transform) algorithm technology to ensure sufficient measurement accuracy. The broken phase monitoring is realized by calculating the phase angle difference and the voltage ratio error.

[0020] Both the first auxiliary transformer and the second auxiliary transformer adopt the YNyn0+d wiring method.

[0021] When the bus voltage of the high-voltage side is greater than 70% Ueh (rated phase voltage) and the bus voltage of the low-voltage side is greater than 70% Uex (Rated line voltage), the high-voltage side busbar U A Phase angle minus the low-voltage side busbar U AB Move 30 in the lagging direction 0 After the phase angle, calculate the phase angle difference. If the absolute value of the phase angle difference exceeds 0.1°, alarm after a 5s delay and prompt that the A-phase on the high-voltage side is disconnected.

[0022] Furthermore, when the high-voltage side busbar voltage is greater than 70%U eh And the low-voltage side busbar voltage is greater than 70%U ex At this time, the high-voltage side busbar U B Phase angle minus the low-voltage side busbar U BC Move 30 in the lagging direction 0 After the phase angle, calculate the phase angle difference. If the absolute value of the phase angle difference exceeds 0.1°, alarm after a 5s delay and prompt that the B-phase on the high-voltage side is disconnected.

[0023] Furthermore, when the high-voltage side busbar voltage is greater than 70%U eh And the low-voltage side busbar voltage is greater than 70%U ex At this time, the high-voltage side busbar U C Phase angle minus the low-voltage side busbar U CA Move 30 in the lagging direction 0 After the phase angle, calculate the phase angle difference. If the absolute value of the phase angle difference exceeds 0.1°, alarm after a 5s delay and prompt that the C-phase on the high-voltage side is disconnected.

[0024] The high-voltage side busbar U A And the low-voltage side busbar U AB / 1.732 voltages are both greater than 70%U eh At this time, calculate the A-phase voltage ratio error N A ; The high-voltage side busbar U B And the low-voltage side busbar U BC / 1.732 voltages are both greater than 70%U eh At this time, calculate the B-phase voltage ratio error N B ; The high-voltage side busbar U C And the low-voltage side busbar U CA / 1.732 voltages are both greater than 70%U eh At this time, calculate the C-phase voltage ratio error N C ; When the phase with the smallest absolute value of the three-phase voltage ratio errors is less than 50% of the absolute value of the voltage ratio error of any phase, alarm after a 5s delay, and the phase with the smallest absolute value of the voltage ratio error is the disconnected phase on the high-voltage side.

[0025] Both the first auxiliary transformer and the second auxiliary transformer adopt the YNd11 wiring method.

[0026] When the high-voltage side busbar voltage is greater than 70%Ueh and when the low-voltage side bus voltage is greater than 70%U ex the high-voltage side bus U A phase angle minus the low-voltage side bus U AC phase angle, perform phase angle difference calculation. If the absolute value of the phase angle difference exceeds 0.1°, delay for 5s to alarm and prompt that the A phase of the high-voltage side is broken; the high-voltage side bus takes U B phase angle minus the low-voltage side bus U BA phase angle, perform phase angle difference calculation. If the absolute value of the phase angle difference exceeds 0.1°, delay for 5s to alarm and prompt that the B phase of the high-voltage side is broken; the high-voltage side bus takes U C phase angle minus the low-voltage side bus U CB phase angle, perform phase angle difference calculation. If the absolute value of the phase angle difference exceeds 0.1°, delay for 5s to alarm and prompt that the C phase of the high-voltage side is broken. The high-voltage side bus U A voltage effective value is greater than 70%U eh and the low-voltage side bus U AC voltage effective value is greater than 70%U ex calculate the voltage ratio error N of phase A A ; the high-voltage side bus U B voltage effective value is greater than 70%U eh , the low-voltage side bus U BA voltage effective value is greater than 70%U ex calculate the voltage ratio error N of phase B B ; the high-voltage side bus U C voltage effective value is greater than 70%U eh , the low-voltage side bus U CB voltage effective value is greater than 70%U ex , calculate the voltage ratio error N of phase C C ; when the phase with the smallest absolute value of the three-phase voltage ratio error is less than 50% of the absolute value of the voltage ratio error of any phase, delay for 5s to alarm, and the phase with the smallest absolute value of the phase voltage ratio error is the broken phase of the high-voltage side.

[0027] Compared with the prior art, the phase break monitoring method of the auxiliary transformer in the nuclear power plant provided by the present invention has the following beneficial effects:

[0028] (1) The phase break monitoring method of the auxiliary transformer in the nuclear power plant of the present invention develops a monitoring logic, which can judge the broken phase and alarm in time when the equipment has a phase break fault, reminding the operator to deal with it in time and ensuring the safety and reliability of the power supply of the nuclear power plant.

[0029] (2) Without adding additional electrical quantity acquisition equipment and professional software, the fault diagnosis and intelligent analysis and judgment are realized through the existing equipment, improving the reliability of the electrical system of the power plant.

[0030] (3) The present invention solves the problem of monitoring the open phase on the high-voltage side under the no-load or light-load operation conditions of the auxiliary transformer, and realizes complete functions such as intelligent diagnosis and fault phase indication.

[0031] (4) The phase angle difference monitoring method and the voltage ratio error monitoring method of the present invention can be used independently or jointly, and the joint use can further improve the accuracy of judgment.

[0032] (5) It is not necessary to inject non-power frequency voltage, and it will not generate harmonic interference and pollution to the transformer and the electrical system, and will not pose a threat to the safety of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. 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 also be obtained based on these drawings.

[0034] Figure 1 It is the primary wiring diagram of the auxiliary transformer system of the nuclear power plant provided by the embodiment of the present invention;

[0035] Figure 2 It is the block diagram of the phase angle difference monitoring method under the YNyn0+d wiring method provided by the embodiment of the present invention;

[0036] Figure 3 It is the block diagram of the voltage ratio monitoring method under the YNyn0+d wiring method provided by the embodiment of the present invention;

[0037] Figure 4 It is the block diagram of the phase angle difference monitoring method under the YNd11 wiring method provided by the embodiment of the present invention;

[0038] Figure 5 It is the block diagram of the voltage ratio monitoring method under the YNd11 wiring method provided by the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] Although the open phase monitoring method of the auxiliary transformer of the present invention can be implemented in a variety of different ways, the exemplary embodiments will be described in detail herein with reference to the drawings, and it is not intended to limit the scope of the present invention to the exemplary embodiments. Therefore, in essence, the description of the drawings and the detailed description should be considered as illustrative rather than restrictive of the present invention.

[0040] The following will be further described in detail through specific embodiments.

[0041] For the convenience of understanding, the open phase of the auxiliary transformer will be analyzed first.

[0042] The structure of a transformer mainly consists of components such as an iron core, windings, an oil tank, and insulating bushings. The iron core is the magnetic circuit part of the transformer. According to the structural type, the iron core is divided into two types: core type and shell type. Since the core-type iron core structure is relatively simple and the arrangement and insulation of the windings are also relatively easy, power transformers mainly adopt the core-type iron core structure. Only some special transformers (such as arc furnace transformers) adopt the shell-type iron core structure. Since a transformer is a device that acts jointly by the magnetic circuit and the electric circuit, the magnetic circuit system of a three-phase transformer can be divided into two categories: those in which the magnetic circuits of each phase are independent of each other and those in which they are related to each other. The main types of magnetic circuits are independent, 3-column, 5-column, etc. The windings are the electric circuit part of the transformer. The commonly used connection groups of auxiliary transformers mainly have two wiring methods: YNyn0+d, YNd11, etc.

[0043] In the case of no-load or light load of the auxiliary transformer, a single-phase open circuit occurs on the high-voltage side. The severity of the current and voltage imbalance on the low-voltage side depends on (1) the type of grounding on the power supply side; (2) the high-voltage / primary winding configuration; (3) the low-voltage / secondary winding configuration; (4) the type of transformer iron core (independent, 3-column, 5-column).

[0044] For a transformer with a star-connected grounded high-voltage winding, its zero-sequence impedance circuit is formed through the low-voltage delta winding or through a high zero-sequence magnetic resistance circuit (3-column core type structure). During a single-phase open circuit of the transformer, if the other two phases are normal, then the winding voltage of the open phase will be reconstructed and remain basically unchanged. Since the open phase is reconstructed by the synthesis of the magnetic fluxes of the other intact phases, the phases on both sides of the open phase are the same, and the voltage ratio error is equal to the turns ratio. Due to the influence of leakage impedance and magnetizing impedance, the phases on both sides of the healthy phase are different, and the voltage ratio is not equal to the turns ratio, with a certain error.

[0045] As Figure 1 shown, the present invention provides a method for monitoring single-phase open circuit of an auxiliary transformer in a nuclear power plant, including the following steps:

[0046] Step S1: The first auxiliary transformer and the second auxiliary transformer are operated separately; the states of the circuit breakers and disconnectors are as follows:

[0047] Operating state of the equipment on the 220 kV side: Close the first circuit breaker (numbered 9LGR300JA), the second circuit breaker 9LGR100JA, and the third circuit breaker 9LGR200JA, close the first disconnector 9LGR302JS, the second disconnector 9LGR301JS, the third disconnector 9LGR101JS, the fourth disconnector 9LGR102JS, the fifth disconnector 9LGR202JS, and the sixth disconnector 9LGR201JS, and put the first voltage transformer 9LGR301TU and the second voltage transformer 9LGR001TU into operation;

[0048] Operating status of equipment on the 6kV side: Close the fourth circuit breaker 9LGR104JA and the fifth circuit breaker 9LGR204JA, open the sixth circuit breaker 9LGR102JA, the seventh circuit breaker 9LGR209JA and the eighth circuit breaker 9LGR202JA, and put the third voltage transformer 9LGR101TU and the fourth voltage transformer 9LGR201TU into operation;

[0049] Step S2: The monitoring range is the auxiliary power supply incoming line and the primary incoming line of the high-voltage side of the auxiliary transformer;

[0050] Step S3: Signal acquisition, that is, acquire the high-voltage side bus voltage, the low-voltage side bus voltage of the first auxiliary transformer, and the low-voltage side bus voltage of the second auxiliary transformer. Specifically, the high-voltage side bus voltage (220kV bus) is taken from the second voltage transformer 9LGR001TU; the low-voltage side bus (9LGR001TB) of the first auxiliary transformer (1# auxiliary transformer) is taken from the third voltage transformer 9LGR101TU; the low-voltage side bus (9LGR002TB) of the second auxiliary transformer (2# auxiliary transformer) is taken from the fourth voltage transformer 9LGR201TU.

[0051] Step S4: The phase angle and the effective value of the voltage are obtained by using the high-precision FFT (Fast Fourier Transform) algorithm technology, and sufficient measurement accuracy must be ensured.

[0052] For different transformer connection methods, there are slightly different implementation methods. The specific monitoring logic is as follows:

[0053] (1) Both auxiliary transformers are in the YNyn0+d connection method:

[0054] (1.1) Phase angle difference monitoring method

[0055] As Figure 2 shown, when the high-voltage bus voltage is greater than 70%U eh (rated phase voltage), and when the low-voltage bus voltage is greater than 70%U ex (rated line voltage):

[0056] Subtract the phase angle of the low-voltage side bus U AB from the phase angle of the high-voltage side bus U A after moving 30° in the lagging direction for phase angle difference calculation. When the absolute value of the phase angle difference exceeds 0.1° and delays for 5s to alarm, and prompt that the A phase of the high-voltage side is disconnected;

[0057] Subtract the phase angle of the low-voltage side bus U BC from the phase angle of the high-voltage side bus U B after moving 30° in the lagging direction for phase angle difference calculation. When the absolute value of the phase angle difference exceeds 0.1° and delays for 5s to alarm, and prompt that the B phase of the high-voltage side is disconnected;

[0058] The high-voltage side busbar takes U C Phase angle minus the U of the low-voltage side busbar CA After lagging and shifting by 30°, the phase angle is calculated for the phase angle difference. When the absolute value of the phase angle difference exceeds 0.1°, it alarms after a 5-second delay and prompts that the C phase of the high-voltage side is disconnected.

[0059] (1.2) Voltage ratio error monitoring method

[0060] As Figure 3 shown, when the voltages of the high-voltage side busbar U A and the low-voltage side busbar U AB / 1.732 are both greater than 70% of U eh (rated phase voltage), calculate the voltage ratio error N of phase A A ; when the voltages of the high-voltage side busbar U B and the low-voltage side busbar U BC / 1.732 are both greater than 70% of U eh , calculate the voltage ratio error N of phase B B ; when the voltages of the high-voltage side busbar U C and the low-voltage side busbar U CA / 1.732 are both greater than 70% of U eh , calculate the voltage ratio error N of phase C C ; when the absolute value of the voltage ratio error of the phase with the smallest absolute value among the three phase voltage ratio errors is less than 50% of the absolute value of the voltage ratio error of any phase, it alarms after a 5-second delay, and the phase with the smallest absolute value of the phase voltage ratio error is the phase disconnected on the high-voltage side.

[0061] It should be noted that the phase angle difference monitoring method and the voltage ratio error monitoring method are used simultaneously and verified with each other to further improve the reliability of the open-phase monitoring. When both auxiliary transformers are judged to be open-phase, it means that the incoming line has an open-phase, and it can be further distinguished whether it is an open-phase inside or outside the factory, with high regional selectivity.

[0062] (2) Both auxiliary transformers are in the YNd11 connection mode:

[0063] (2.1) Phase angle difference monitoring method

[0064] As Figure 4 shown, when the high-voltage busbar voltage is greater than 70% of U eh (rated phase voltage) and the low-voltage busbar voltage is greater than 70% of U ex (rated line voltage):

[0065] The phase angle of the high-voltage side busbar U A minus the phase angle of the low-voltage side busbar U AC is used to calculate the phase angle difference. When the absolute value of the phase angle difference exceeds 0.1°, it alarms after a 5-second delay and prompts that the A phase of the high-voltage side is disconnected; the high-voltage side busbar takes UB The phase angle minus the low-voltage side busbar U BA and the subsequent phase angle are used for phase angle difference calculation. When the absolute value of the phase angle difference exceeds 0.1°, an alarm is given after a 5-second delay, and it is prompted that the high-voltage side phase B is open-circuited; the high-voltage side busbar takes U C The phase angle minus the low-voltage side busbar U CB and the subsequent phase angle are used for phase angle difference calculation. When the absolute value of the phase angle difference exceeds 0.1°, an alarm is given after a 5-second delay, and it is prompted that the high-voltage side phase C is open-circuited.

[0066] (2.2) Voltage ratio error monitoring method

[0067] As Figure 5 shown, when the phase voltage U of the high-voltage side busbar A , the effective value is greater than 70% of U eh and the line voltage U of the low-voltage side busbar AC , the effective value is greater than 70% of U ex , calculate the voltage ratio error N of phase A A ; the high-voltage side busbar takes U B , the effective value is greater than 70% of U eh and the low-voltage side busbar U BA , the effective value is greater than 70% of U ex , calculate the voltage ratio error N of phase B B ; the high-voltage side busbar takes U C , the effective value is greater than 70% of U eh and the low-voltage side busbar U CB , the effective value is greater than 70% of U ex , calculate the voltage ratio error N of phase C C ; when the phase with the smallest absolute value of the three-phase voltage ratio error is less than 50% of the absolute value of the voltage ratio error of any phase, an alarm is given after a 5-second delay, and the phase with the smallest absolute value of the phase voltage ratio error is the open-circuited phase on the high-voltage side.

[0068] In addition, the phase angle difference monitoring method and the voltage ratio error monitoring method are used simultaneously and verified with each other to further improve the reliability of open-phase monitoring. When both auxiliary transformers are judged to be open-phase, it means that the incoming line has an open-phase fault, and it is possible to further distinguish whether it is an open-phase inside or outside the plant, with good regional selectivity.

[0069] The open-phase monitoring method for the auxiliary transformer of a nuclear power plant provided by the present invention is applicable to the open-phase monitoring and protection of the auxiliary transformer system of a nuclear power plant under no-load or light-load conditions, and can timely detect the open-phase fault of the primary system, which is a mature and inexpensive effective solution.

[0070] The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims described above.

Claims

1. A method for monitoring open-phase of auxiliary transformers in nuclear power plants, characterized in that, it includes the following steps: Step S1: The first auxiliary transformer and the second auxiliary transformer operate separately; all four voltage transformers are put into operation; Step S2: The open-phase monitoring range is the auxiliary power supply incoming line and the primary incoming line of the high-voltage side of the auxiliary transformer; Step S3: Collect the bus voltage of the high-voltage side of the auxiliary transformer, the bus voltage of the low-voltage side of the first auxiliary transformer, and the bus voltage of the low-voltage side of the second auxiliary transformer; Step S4: The phase angle and the effective value of the voltage are obtained by using the high-precision FFT algorithm technology to ensure sufficient measurement accuracy, and the open-phase monitoring is realized by calculating the phase angle difference and the voltage ratio error; When both the first auxiliary transformer and the second auxiliary transformer adopt the YNyn0+d wiring method; the voltage of the high-voltage side bus U A and the voltage of the low-voltage side bus U AB / 1.732 are both greater than 70%U eh ; when U eh is the rated phase voltage, calculate the voltage ratio error N A of phase A; when the voltage of the high-voltage side bus U B and the voltage of the low-voltage side bus U BC / 1.732 are both greater than 70%U eh ; calculate the voltage ratio error N B of phase B; when the voltage of the high-voltage side bus U C and the voltage of the low-voltage side bus U CA / 1.732 are both greater than 70%U eh ; calculate the voltage ratio error N C of phase C; when the absolute value of the voltage ratio error of the phase with the smallest absolute value among the three phase voltage ratio errors is less than 50% of the absolute value of the voltage ratio error of any phase, give an alarm after a 5s delay, and the phase with the smallest absolute value of the phase voltage ratio error is the broken-phase of the high-voltage side.

2. The method for monitoring open-phase of auxiliary transformers in nuclear power plants according to claim 1, characterized in that, When the high-voltage side bus voltage is greater than 70% U eh and the low-voltage side bus voltage is greater than 70% U ex at this time, U ex is the rated line voltage. The phase angle of the high-voltage side bus U A subtracts the phase angle after the phase angle of the low-voltage side bus U AB moves 30° in the lagging direction, and the phase angle difference is calculated. If the absolute value of the phase angle difference exceeds 0.1°, an alarm will be given after a 5-second delay, and it will be prompted that the A phase of the high-voltage side is disconnected.

3. The method for monitoring open-phase of auxiliary transformers in nuclear power plants according to claim 1, characterized in that, When the high-voltage side bus voltage is greater than 70% U eh and the low-voltage side bus voltage is greater than 70% U ex at this time, U ex is the rated line voltage, the phase angle of the high-voltage side bus U B subtracts the phase angle of the low-voltage side bus U BC after moving 30° in the lagging direction, and the phase angle difference is calculated. If the absolute value of the phase angle difference exceeds 0.1°, an alarm will be given with a 5-second delay, and it will be prompted that the B phase of the high-voltage side is disconnected.

4. The method for monitoring open-phase of auxiliary transformers in nuclear power plants according to claim 1, characterized in that, When the high-voltage side bus voltage is greater than 70%U eh and the low-voltage side bus voltage is greater than 70%U ex at this time, U ex is the rated line voltage. The phase angle of the high-voltage side bus U C subtracts the phase angle of the low-voltage side bus U CA after moving 30° in the lagging direction, and the phase angle difference is calculated. If the absolute value of the phase angle difference exceeds 0.1°, an alarm will be given after a 5-second delay, and it will be prompted that the C phase of the high-voltage side is disconnected.

5. The method for monitoring open-phase of auxiliary transformers in nuclear power plants according to claim 1, characterized in that, both the first auxiliary transformer and the second auxiliary transformer adopt the YNd11 wiring method.

6. The method for monitoring open-phase of auxiliary transformers in nuclear power plants according to claim 5, characterized in that, When the high-voltage side bus voltage is greater than 70%U eh and the low-voltage side bus voltage is greater than 70%U ex at this time, U ex is the rated line voltage, the phase angle of the high-voltage side bus U A subtracts the phase angle of the low-voltage side bus U AC and then calculates the phase angle difference. If the absolute value of the phase angle difference exceeds 0.1°, it will alarm after a 5-second delay and prompt that the A phase of the high-voltage side is disconnected; the high-voltage side bus takes the phase angle of U B subtracts the phase angle of the low-voltage side bus U BA and then calculates the phase angle difference. If the absolute value of the phase angle difference exceeds 0.1°, it will alarm after a 5-second delay and prompt that the B phase of the high-voltage side is disconnected; the high-voltage side bus takes the phase angle of U C subtracts the phase angle of the low-voltage side bus U CB and then calculates the phase angle difference. If the absolute value of the phase angle difference exceeds 0.1°, it will alarm after a 5-second delay and prompt that the C phase of the high-voltage side is disconnected.

7. The method for monitoring open-phase of auxiliary transformers in nuclear power plants according to claim 5, characterized in that, Phase voltage U of the high-voltage side bus A , with the effective value greater than 70% of U eh and the line voltage U of the low-voltage side bus AC , with the effective value greater than 70% of U ex When, U ex is the rated line voltage, calculate the voltage ratio error N of phase A A ; When the effective value of the voltage of the high-voltage side bus U B is greater than 70% of U eh , and the effective value of the voltage of the low-voltage side bus U BA is greater than 70% of U ex , calculate the voltage ratio error N of phase B B ; When the effective value of the voltage of the high-voltage side bus U C is greater than 70% of U eh , and the effective value of the voltage of the low-voltage side bus U CB is greater than 70% of U ex , calculate the voltage ratio error N of phase C C ; When the absolute value of the voltage ratio error of the phase with the smallest absolute value is less than 50% of the absolute value of the voltage ratio error of any phase, delay for 5 s to alarm, and the phase with the smallest absolute value of the voltage ratio error is the broken-phase of the high-voltage side.

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