CN108776283A - A kind of CT not exclusively lower power distribution network single-phase disconnection fault judgment method of configuration with judge system - Google Patents

Abstract

The invention discloses a single-phase disconnection fault judgment method and a judgment system of a distribution network under incomplete CT configuration. The judgment method includes a fault start criterion and a disconnection fault detection criterion; Module, voltage/voltage change module, working power supply module, digital-to-analog conversion module, data calculation and processing module, control module, switch input and output module, communication interface module and human-computer interaction module; It constitutes a disconnection fault criterion, which can comprehensively consider the changes of current and voltage of each phase, not only can judge the disconnection fault when the CT is not fully configured, but also can reliably and accurately distinguish the disconnection fault from the CT disconnection and load Asymmetry and other conditions ensure the reliability of the disconnection fault protection criterion.

Description

A single-phase disconnection fault judgment method and judgment of distribution network under incomplete configuration of CT system

technical field

The invention relates to the technical field of power equipment detection, in particular to a method and system for judging a single-phase disconnection fault of a power distribution network under incomplete CT configuration.

Background technique

The frequent occurrence of disconnection faults in distribution lines has gradually attracted people's attention. Several literatures at home and abroad have theoretically analyzed the change of electrical quantities caused by disconnection faults of distribution lines, especially the change characteristics of each sequence component, and summarized the changing laws of related electrical quantities; The method of disconnection fault judgment and location, but in the distribution network, the CT incomplete configuration is common, and the acquisition and calculation of negative sequence components have great limitations; some literatures propose a disconnection fault judgment and location based on wavelet transform However, this type has high requirements for data sampling frequency and calculation processing accuracy, and is not suitable for large-scale promotion and application in distribution networks.

The current distribution network is not equipped with a dedicated disconnection fault protection device. After a disconnection fault occurs, the change law of the line current and bus voltage is very different from that of a short-circuit fault. The traditional short-circuit protection principle and device that reacts to an increase in current and a decrease in voltage cannot provide sensitive and reliable protection functions for disconnection faults. , when there is no corresponding protective measure after the disconnection fault occurs, there is a risk of faulty operation in the power grid. In addition, considering that the current distribution network adopts incomplete CT configuration, in general, A and C two-phase configuration CT, B phase without CT can not provide a complete three-phase current, assuming that A-phase disconnection occurs Fault, at this time, the current of phase A decreases from the normal load current to 0. If the branch of the distribution network is considered, the current of phase A will not decrease to 0, but it will decrease significantly. The C-phase current will also decrease, but the decrease is not obvious, and the minimum decrease is that of the normal load current. times. If the C-phase disconnection occurs, the obtained current change law is similar to that of the A-phase disconnection. At this time, only relying on the current amount to form the criterion cannot ensure the reliability of the disconnection fault judgment, because if load asymmetry occurs, A-phase or C-phase CT disconnection faults, etc., the change law of the current amount is similar. It is easy to misjudge the above situation as a single-phase disconnection fault. Therefore, it is necessary to propose a single-phase disconnection fault judgment system and method for distribution network considering incomplete CT configuration, so as to provide sensitive and reliable disconnection fault protection function for distribution network.

Contents of the invention

In order to solve the above problems, the object of the present invention is to provide a single-phase disconnection fault judgment method and judgment system of distribution network under incomplete configuration of CT.

In order to achieve the above object, the present invention is achieved through the following technical solutions:

A method for judging a single-phase disconnection fault of a distribution network under incomplete configuration of CTs, including a fault start criterion and a disconnection fault detection criterion;

The fault start criterion is

In the formula, Δi _φ is the variation of the current sampling value of phase A or phase C, and It is the current sampling value of phase A or phase C at the time before and at the time before 1 power frequency cycle, is the sum of k consecutive |i _φ |, 1<k≤N/2, I _N is the effective value of the rated current of the line, k _set is the setting threshold coefficient, and the value of k _set is 0.1~0.2;

The disconnection fault detection criterion includes the B-phase disconnection fault criterion, the A-phase or C-phase disconnection fault criterion;

Among them, the fault criterion of phase B disconnection is

in, Indicates that the amplitude of phase A current and phase C current is larger; Indicates that the amplitude of phase A current and phase C current is smaller; Indicates the phase difference between phase A current and phase C current; Indicates that the voltage amplitude of phase A and phase C is smaller;

A-phase or C-phase disconnection fault criterion is

in, Indicates that the amplitude of phase A current and phase C current is larger; Indicates that the amplitude of phase A current and phase C current is smaller; Indicates that the phase A voltage and the phase C voltage amplitude are larger; Indicates that the amplitude of phase A voltage, phase B voltage and phase C voltage is smaller.

A preferred method for judging a single-phase disconnection fault in a distribution network under incomplete CT configuration includes the following steps:

①The disconnection protection device continuously detects the A-phase and C-phase currents during normal operation. When the current sampling amount of any one of the two phases meets the fault start criterion, the disconnection protection starts and enters step ②;

②Judge the magnitude of the A-phase and C-phase currents, and then use the larger value of the two to compare the smaller value to determine whether the ratio is greater than 1.2. If it is greater than 1.2, go to step ③, otherwise go to step ④;

③If the current of phase A is greater than the current of phase C, use the voltage amplitude of phase C to compare the voltage amplitude of phase A and phase B, and judge whether the ratio is greater than 1.2. If it is greater than 1.2, it indicates that a power failure of phase C has occurred. Otherwise, there is no disconnection fault, and the process ends; similarly, if the current of phase C is greater than the current of phase A, use the voltage amplitude of phase A to be smaller than that of phase B and phase C to determine whether the ratio is greater than 1.2, if greater than 1.2, indicating that a phase A power failure has occurred, otherwise there is no disconnection fault, and the process is ended;

④Judge whether the phase angle difference between phase A current and phase C current is between [130°, 180°], and then compare the voltage amplitude of phase B with the smaller voltage amplitude of phase A and phase C to determine the ratio Whether it is greater than 1.2; if the above conditions are met at the same time, it indicates that the B-phase disconnection fault occurs, otherwise no disconnection fault occurs, and the process ends.

Preferably, the value of k _set is 0.15.

The present invention also includes a single-phase disconnection fault judgment system of distribution network under incomplete CT configuration, which consists of a current/voltage conversion module, a voltage/voltage change module, a working power supply module, a digital-to-analog conversion module, and a data calculation and processing module, Control module, switch value input and output module, communication interface module and human-computer interaction module, in which the digital-to-analog conversion module is connected with the current/voltage conversion module, voltage/voltage change module and data calculation and processing module respectively, and the control module is respectively connected with the data calculation module The processing module, the man-machine interaction module, the communication interface module are connected with the switching value input and output module.

Preferably, the input current signals are only A-phase and C-phase currents, and the input voltage information is A, B, and C three-phase voltages. The current signal is converted into a weak voltage signal suitable for the work and processing of the microcomputer system through the current/voltage conversion module, and the voltage signal is converted into a weak voltage signal suitable for the work and processing of the microcomputer system, with a peak-to-peak value of ±5V~±15V; It becomes a discrete digital signal, which is then calculated and processed by the data calculation and processing module to complete the calculation of the amplitude and phase of the voltage and current;

The control module performs logical comparison and judgment on the calculation results from the data calculation and processing module, and judges whether the current electrical quantity relationship satisfies the criterion. If the criterion is satisfied, it indicates that a disconnection fault has occurred on the line, and the control switch input and output module sends a trip command to control the circuit breaker to trip; action;

The digital input and output module is affected by the control module, which can send trip commands and alarm signals; it can also receive external circuit breaker position status signals or other digital information;

The working power supply module provides DC operating voltages of different amplitudes for the entire protection device;

The human-computer interaction module includes a keyboard and a liquid crystal display to complete basic human-computer interaction functions;

The communication interface module is used to complete the information interaction function between the disconnection protection device and the substation automation system. The disconnection protection device needs to upload the measurement, calculation and criterion results to the substation automation system, and also needs to receive various information from the substation automation system. remote command.

Preferably, the peak-to-peak value is ±10V.

Preferably, the data calculation and processing module is a TMS320 series DSP chip.

Preferably, the control module is a STM32F single-chip microcomputer chip.

Preferably, the DC operating voltages with different amplitudes are 220V, ±12V, 5V and 3.3V.

Compared with the prior art, the present invention has the following advantages:

The principle of the invention is clear, easy to implement, no additional equipment is needed, the cost is low, and the workload of operation and maintenance is not increased. According to the high enough sensitivity, it is allowed to start the criterion action when there is no disconnection fault, but not to start when the disconnection fault is not allowed. After the fault start criterion action, it enters the disconnection fault detection criterion. The requirement for this criterion is that it has high reliability and can reliably distinguish disconnection faults from non-disconnection faults. The reliability of the judgment, the present invention introduces the voltage and current together to constitute the disconnection fault criterion, which can comprehensively consider the changes in the current and voltage of each phase, not only the disconnection fault can be judged when the CT is not fully configured, but also It can reliably and accurately distinguish disconnection faults from CT disconnection and load asymmetry, ensuring the reliability of disconnection fault protection criteria.

Description of drawings

Fig. 1 is a flowchart of a method for judging a single-phase disconnection fault in a distribution network under incomplete CT configuration;

Figure 2 is a schematic diagram of the distribution network structure of a typical 10kV system;

Figure 3 is a schematic diagram of the change law of each phase current before and after the fault under the CT incomplete configuration;

Fig. 4 is a schematic diagram of the voltage change curve at A;

Figure 5 is a schematic diagram of the voltage change curve at B;

Figure 6 is a schematic diagram of the structural connection of the single-phase disconnection fault judgment system of the distribution network under the incomplete configuration of CT.

Detailed ways

The purpose of the present invention is to provide a single-phase disconnection fault judgment method and judgment system of distribution network under incomplete configuration of CT, which is realized by the following technical solutions:

The incomplete configuration of CT in the distribution network means that phase A and phase C are equipped with CT, and phase B is not equipped with CT. Therefore, for secondary equipment such as protection devices, only the current of phase A and phase C can be obtained, and the current of phase C cannot be obtained. Phase B current. For the busbar PT, it is generally equipped with a three-phase five-column PT, which can provide complete A, B, C three-phase voltage and zero-sequence voltage. The present invention proposes a corresponding single-phase disconnection protection criterion based on the configuration of the above-mentioned transformers and the corresponding electrical quantities.

Assuming that a phase A disconnection fault occurs, the current of phase A decreases from the normal load current to 0. If the branch of the distribution network is considered, the current of phase A will not decrease to 0, but will decrease significantly. The C-phase current will also decrease, but the decrease is not obvious, and the minimum decrease is the normal load current times. If the C-phase disconnection occurs, the obtained current change law is similar to that of the A-phase disconnection. At this time, only relying on the current amount to form the criterion cannot ensure the reliability of the disconnection fault judgment, because if load asymmetry occurs, A-phase or C-phase CT disconnection faults, etc., the change law of the current amount is similar. It is easy to misjudge the above situation as a single-phase disconnection fault. In order to improve the reliability of disconnection fault judgment under incomplete CT configuration, the present invention introduces the voltage quantity, which together with the current quantity constitutes the disconnection fault criterion.

When a phase A disconnection fault occurs, according to the change law of voltage, the voltage of phase A will increase obviously at this time, and the voltage of the other two non-fault phases will decrease, and the system will have a certain amplitude of zero-sequence voltage. When a phase A CT disconnection fault occurs, the three-phase voltage of the system is basically symmetrical, and the zero-sequence voltage amplitude will not be very large; when there is an asymmetrical three-phase load, although there will be a certain amplitude of the zero-sequence voltage, But the amplitude is still not very large, and the three-phase voltage of the system is basically symmetrical. It can be seen that considering the changes of the current and voltage of each phase comprehensively, not only can the disconnection fault be judged when the CT is not completely configured, but also can reliably and accurately distinguish the disconnection fault from the CT disconnection and load asymmetry, ensuring The reliability of the disconnection fault protection criterion is improved.

When a C-phase disconnection fault occurs, the corresponding electrical quantity is different except for the phase, but the change rule is the same, so I won't repeat it here.

When a phase B disconnection fault occurs, the currents of phase A and phase C will decrease slightly, but the phase difference will increase; at least one phase (faulty phase) of the three-phase voltage will increase significantly, and the remaining two phases will have Reduced, a certain magnitude of the zero-sequence voltage appears.

To sum up: when there is a non-B-phase disconnection fault, the current of the faulty phase will decrease obviously, and the current of the other phase will decrease slightly; when the B-phase disconnection fault occurs, the current of both phases will decrease slightly, but the phase difference will increase big. No matter which phase is disconnected, the disconnected phase of the three-phase voltage will increase significantly, and the other two phases will decrease slightly, and a certain amplitude of zero-sequence voltage will appear.

The disconnection fault protection criterion includes two parts: the first part is the fault start criterion, and the second part is the disconnection fault detection criterion. Among them, the requirement of the fault starting criterion is to have a high enough sensitivity to allow the start criterion action when there is a non-disconnection fault, but not to start when a disconnection fault is not allowed. After the fault start criterion is activated, it enters the disconnection fault detection criterion. The requirement for this criterion is that it has high reliability and can reliably distinguish disconnection faults from non-disconnection faults.

Start criteria:

In the formula, is the variation of phase A or phase C current sampling value, and It is the current sampling value at the time before phase A or phase C and at the time before 1 power frequency cycle, for consecutive k Summing, 1<k≤N/2, I _N is the RMS value of the rated current of the line, k _set is the coefficient of the setting threshold value, which can be 0.1~0.2 according to the empirical value.

The first condition in the starting criterion expression is that the reaction current decreases, and the second condition requires that the magnitude of the current decrease must meet certain requirements, and the disconnection protection is started after these two conditions are met at the same time.

For the starting criterion, it is important to ensure sufficient sensitivity, so as long as the current value decreases, the starting condition of the disconnection protection will be satisfied. Compared with other specific situations, it is necessary to make further distinctions with the help of disconnection fault detection criteria.

Disconnection fault detection criterion:

For the B-phase disconnection fault, the criterion is:

In the formula, Indicates that the amplitude of phase A current and phase C current is larger; Indicates that the amplitude of phase A current and phase C current is smaller; Indicates the phase difference between phase A current and phase C current; Indicates that the voltage amplitude of phase A and phase C is smaller. The ratio threshold 1.2 is determined based on empirical values, and the ranges of 130° and 180° are jointly determined based on theoretical analysis and calculation results.

When the B-phase disconnection fault occurs, the A and C two-phase currents will decrease, so the disconnection protection starting criterion can be started reliably. At this time, the current amplitudes of the two phases A and C are basically equal, so the first current amplitude ratio criterion will be satisfied; the phase difference between the two-phase currents of A and C will increase, up to 180°, so the second current amplitude criterion will be satisfied. Phase difference criterion; at this time, the voltage of phase B will increase, and at least one of the voltages of phases A and C will decrease, so the third criterion of voltage amplitude ratio will be satisfied.

For A-phase or C-phase disconnection, the criterion is:

In the formula, Indicates that the amplitude of phase A current and phase C current is larger; Indicates that the amplitude of phase A current and phase C current is smaller; Indicates that the phase A voltage and the phase C voltage amplitude are larger; Indicates that the amplitude of phase A voltage, phase B voltage and phase C voltage is smaller. The ratio threshold 1.2 is determined based on empirical values.

When a phase A or C phase disconnection fault occurs, the current of the fault phase will decrease significantly, so the disconnection protection start criterion can be reliably activated. At this time, the non-fault phase current is larger than the fault current, so the ratio of the two amplitudes will be larger, which meets the first current amplitude ratio criterion; at this time, the fault phase voltage rises, and the other two non-fault At least one of the phase voltages will decrease, so the ratio of the fault phase voltage to the minimum value of the non-fault phase voltage will be larger, satisfying the second amplitude voltage ratio criterion.

Analysis of the impact of abnormal operating conditions on the above criteria:

1) CT disconnection: The characteristics of electrical quantity change on the secondary side when the CT is disconnected are similar to the line disconnection fault, but there are also obvious differences. When the CT is disconnected, the phase current of the disconnected phase decreases obviously or becomes 0, which will satisfy the disconnection fault starting criterion, and may also meet the ratio criterion of the current amplitude, but at this time the amplitude and phase of the non-faulted phase current The difference is unchanged, and the three-phase voltage is basically in a symmetrical state, so the non-fault phase current phase angle difference criterion will not be satisfied, and the voltage amplitude ratio criterion will not be satisfied. Based on this, CT disconnection and line disconnection fault Make a reliable distinction. In addition, when a line disconnection fault occurs, it will generally cause similar changes in the current at multiple measuring points at the same time, and a CT disconnection will only cause a change in the current at a certain point. Disconnection faults are further distinguished.

2) Non-full-phase operation: In the distribution network, the non-full-phase operation of the line is mostly caused by switching operations except for disconnection faults. The switches of the distribution network are basically three-phase operation, and generally there will be no long-term non-full-phase operation state. If the line does not run in full phase for a long time due to switch failure and other reasons, it can be regarded as a disconnection fault. For short-term non-full-phase operation, an appropriate action delay can be set to avoid misjudgment of disconnection protection that may be caused by short-term non-full-phase operation.

3) Asymmetrical load: Load asymmetry is mostly caused by single-phase power consumption, and it is easy to appear on the load branch line. When the three-phase load asymmetry reaches a certain level, the three-phase current will appear similar to the disconnection fault, but the three-phase voltage of the system is still basically symmetrical when the load asymmetry occurs, and will not meet the above-mentioned voltage amplitude ratio criterion. According to this, asymmetrical load operation and single-phase disconnection fault can be distinguished.

The present invention will be further described below in conjunction with specific embodiments.

Example 1

As shown in Figure 1, the flow chart of the single-phase disconnection fault judgment method of the distribution network under the incomplete configuration of CT includes the following steps:

①The disconnection protection device continuously detects the A-phase and C-phase currents during normal operation. When the current sampling amount of any one of the two phases meets the fault start criterion, the disconnection protection starts, and enters step ②The disconnection fault judgment process;

②Judge the magnitude of the A-phase and C-phase currents, and then use the larger value of the two to compare the smaller value to determine whether the ratio is greater than 1.2. If it is greater than 1.2, go to step ③, otherwise go to step ④;

③If the current of phase A is greater than the current of phase C, use the voltage amplitude of phase C to compare the voltage amplitude of phase A and phase B, and judge whether the ratio is greater than 1.2. If it is greater than 1.2, it indicates that a power failure of phase C has occurred. Otherwise, there is no disconnection fault, and the process ends; similarly, if the current of phase C is greater than the current of phase A, use the voltage amplitude of phase A to be smaller than that of phase B and phase C to determine whether the ratio is greater than 1.2, if greater than 1.2, indicating that a phase A power failure has occurred, otherwise there is no disconnection fault, and the process is ended;

④Judge whether the phase angle difference between phase A current and phase C current is between [130°, 180°], and then compare the voltage amplitude of phase B with the smaller voltage amplitude of phase A and phase C to determine the ratio Whether it is greater than 1.2; if the above conditions are met at the same time, it indicates that the B-phase disconnection fault occurs, otherwise no disconnection fault occurs, and the process ends.

Example 2

As shown in Figure 2, a typical distribution network structure diagram, the 10kV system adopts the operation mode of ungrounded neutral point and the radial network structure of single power supply. There are n outgoing lines (L1~Ln) on the bus, and each outgoing line contains several load branches. Taking L1 as the research object, the branch situation is shown in the figure, in which the rectangular frame represents the circuit breaker, the dot represents the section switch, the arrow represents the load, and the rest of the lines L2~Ln adopt a simplified equivalent structure.

Taking the occurrence of a single-phase disconnection fault on the main feeder line L1 as an example, assuming that the disconnection fault point is located between the circuit breaker A and the section switch C in the figure, the change law of each phase current before and after the fault is shown in Figure 3. In the figure, due to the incomplete configuration of CT, there is no B-phase current curve;

After phase A is disconnected, the currents of both phases A and C decrease, and the faulty phase A has the most obvious change, with a change range of 71.42%, which meets the requirements of the start-up criterion and reliably starts the disconnection protection. At this time, the current amplitude of phase C The ratio to the current amplitude of phase A is about 3.2, which is greater than 1.2.

At this time, the voltage at A represents the voltage of each measurement point between the fault point and the power supply, and the voltage at C represents the voltage of each measurement point between the fault point and the load. The voltage change curve at A is shown in Figure 4, and B As shown in Figure 5, the voltage variation curve of the fault phase A is the highest at this time, about 7.5kV, and the voltage amplitudes of the two non-fault phases are basically equal, about 5.5kV, and the ratio between the two is about 1.38, which is greater than 1.2.

Based on the above changes in current and voltage, it can be seen that when a phase A disconnection fault occurs, the fault phase current is significantly reduced, and the disconnection protection can be started reliably; the ratio of the non-fault phase current to the fault phase current amplitude is greater than 1.2, and the fault phase voltage The ratio to the amplitude of the non-fault phase voltage is also greater than 1.2, which can reliably determine the occurrence of a phase A disconnection fault. For the C-phase disconnection fault, the rules are similar and will not be repeated here.

When the B-phase disconnection fault occurs, the B-phase current cannot be obtained because there is no CT installed in the B-phase. At this time, the currents of the two non-fault phases will also decrease. Since the starting criterion has high sensitivity, the disconnection protection can be started reliably. At this time, the current amplitudes of phase A and phase C are almost the same, so the ratio of the two amplitudes will not be greater than 1.2, and the phase difference between the two will increase to about 145°. The ratio of phase C voltage amplitude is about 1.4, which is greater than 1.2, so the criterion of phase B disconnection fault is satisfied. It can be seen that when phase B lacks CT, the disconnection of phase B can still be judged by means of the variation law of non-fault phase current and three-phase voltage.

Example 3

As shown in Figure 6, the structural connection diagram of the single-phase disconnection fault judgment system of the distribution network under the incomplete configuration of CT, the single-phase disconnection fault judgment system of the distribution network under the incomplete CT configuration consists of the current/voltage conversion module, Voltage/voltage change module, working power supply module, digital-to-analog conversion module, data calculation and processing module, control module, switch input and output module, communication interface module and human-computer interaction module, in which the digital-to-analog conversion module is connected with the current/voltage conversion module respectively The module, the voltage/voltage change module are connected to the data calculation and processing module, and the control module is respectively connected to the data calculation and processing module, the human-computer interaction module, the communication interface module and the switch input and output module;

Since the CT is not fully configured, the input current signals are only A-phase and C-phase currents, and the input voltage information is A, B, and C three-phase voltages. The current signal is converted into a weak voltage signal suitable for the work and processing of the microcomputer system through the current/voltage conversion module, and the voltage signal is converted into a weak voltage signal suitable for the work and processing of the microcomputer system. After the above-mentioned weak voltage signal is processed by the digital-to-analog conversion module, it becomes a discrete digital signal, and then it is calculated and processed by the data calculation and processing module to complete the calculation of the amplitude and phase of the voltage and current; the data calculation and processing module generally uses DSP chips, the typical ones are TMS320 series;

The control module performs logical comparison and judgment on the calculation results from the data calculation and processing module, and judges whether the current electrical quantity relationship meets the criterion. The control module uses a single-chip microcomputer chip, such as an STM32F single-chip microcomputer chip. If the criterion is satisfied, it indicates that a disconnection fault has occurred on the line, and the control switch input and output module sends a trip command to control the circuit breaker to trip; action;

The digital input and output module is affected by the control module, which can send trip commands and alarm signals; it can also receive external circuit breaker position status signals or other digital information;

The working power supply module provides DC operating voltages of different amplitudes for the entire protection device, including 220V, ±12V, 5V, 3.3V, etc.;

The human-computer interaction module includes a keyboard and a liquid crystal display to complete basic human-computer interaction functions;

The communication interface module is used to complete the information interaction function between the disconnection protection device and the substation automation system. The disconnection protection device needs to upload the measurement, calculation and criterion results to the substation automation system, and also needs to receive various information from the substation automation system. remote command.

Claims (9)

1. A method for judging a single-phase disconnection fault of a distribution network under an incomplete configuration of CT, characterized in that: a fault start criterion and a disconnection fault detection criterion are included; The fault start criterion is In the formula, Δi _φ is the variation of the current sampling value of phase A or phase C, and i _φ (n) and i _φ (nN) are the current sampling values of phase A or phase C at the corresponding time before and at the time before 1 power frequency cycle , is the sum of k consecutive |i _φ |, 1<k≤N/2, I _N is the effective value of the rated current of the line, k _set is the setting threshold coefficient, and the value of k _set is 0.1~0.2; The disconnection fault detection criterion includes the B-phase disconnection fault criterion, the A-phase or C-phase disconnection fault criterion; Among them, the fault criterion of phase B disconnection is in, Indicates that the amplitude of phase A current and phase C current is larger; Indicates that the amplitude of phase A current and phase C current is smaller; Indicates the phase difference between phase A current and phase C current; Indicates that the voltage amplitude of phase A and phase C is smaller; A-phase or C-phase disconnection fault criterion is in, Indicates that the amplitude of phase A current and phase C current is larger; Indicates that the amplitude of phase A current and phase C current is smaller; Indicates that the phase A voltage and the phase C voltage amplitude are larger; Indicates that the amplitude of phase A voltage, phase B voltage and phase C voltage is smaller. 2. The method for judging a single-phase disconnection fault of a distribution network under incomplete configuration of a CT according to claim 1, characterized in that: comprising the following steps: ①The disconnection protection device continuously detects the A-phase and C-phase currents during normal operation. When the current sampling amount of any one of the two phases meets the fault start criterion, the disconnection protection starts and enters step ②; ②Judge the magnitude of the A-phase and C-phase currents, and then use the larger value of the two to compare the smaller value to determine whether the ratio is greater than 1.2. If it is greater than 1.2, go to step ③, otherwise go to step ④; ③If the current of phase A is greater than the current of phase C, use the voltage amplitude of phase C to compare the voltage amplitude of phase A and phase B, and judge whether the ratio is greater than 1.2. If it is greater than 1.2, it indicates that a power failure of phase C has occurred. Otherwise, there is no disconnection fault, and the process ends; similarly, if the current of phase C is greater than the current of phase A, use the voltage amplitude of phase A to be smaller than that of phase B and phase C to determine whether the ratio is greater than 1.2, if greater than 1.2, indicating that a phase A power failure has occurred, otherwise there is no disconnection fault, and the process is ended; ④Judge whether the phase angle difference between phase A current and phase C current is between [130°, 180°], and then compare the voltage amplitude of phase B with the smaller voltage amplitude of phase A and phase C to determine the ratio Whether it is greater than 1.2; if the above conditions are met at the same time, it indicates that the B-phase disconnection fault occurs, otherwise no disconnection fault occurs, and the process ends. 3. The method for judging a single-phase disconnection fault of a distribution network under incomplete CT configuration according to claim 1, wherein the value of k _set is 0.15. 4. A single-phase disconnection fault judgment system for a distribution network under incomplete configuration of CT, characterized in that it consists of a current/voltage conversion module, a voltage/voltage change module, a working power supply module, a digital-to-analog conversion module, and data calculation and processing Module, control module, switch input and output module, communication interface module and human-computer interaction module, in which the digital-to-analog conversion module is connected with the current/voltage conversion module, voltage/voltage change module and data calculation and processing module respectively, and the control module is connected with the The data calculation and processing module, the human-computer interaction module, the communication interface module and the switching value input and output module are connected. 5. The single-phase disconnection fault judgment system of distribution network under incomplete configuration of CT according to claim 4, characterized in that, the input current signal only has A-phase and C-phase current, and the input voltage information is A , B, C three-phase voltage. The current signal is converted into a weak voltage signal suitable for the work and processing of the microcomputer system through the current/voltage conversion module, and the voltage signal is converted into a weak voltage signal suitable for the work and processing of the microcomputer system, with a peak-to-peak value of ±5V~±15V; It becomes a discrete digital signal, which is then calculated and processed by the data calculation and processing module to complete the calculation of the amplitude and phase of the voltage and current; The control module performs logical comparison and judgment on the calculation results from the data calculation and processing module, and judges whether the current electrical quantity relationship satisfies the criterion. If the criterion is satisfied, it indicates that a disconnection fault has occurred on the line, and the control switch input and output module sends a trip command to control the circuit breaker to trip; action; The digital input and output module is affected by the control module, which can send trip commands and alarm signals; it can also receive external circuit breaker position status signals or other digital information; The working power supply module provides DC operating voltages of different amplitudes for the entire protection device; The human-computer interaction module includes a keyboard and a liquid crystal display to complete basic human-computer interaction functions; The communication interface module is used to complete the information interaction function between the disconnection protection device and the substation automation system. The disconnection protection device needs to upload the measurement, calculation and criterion results to the substation automation system, and also needs to receive various information from the substation automation system. remote command. 6 . The single-phase disconnection fault judgment system of distribution network under incomplete CT configuration according to claim 5 , wherein the peak-to-peak value is ±10V. 7. A system for judging single-phase disconnection faults of distribution network under incomplete CT configuration according to claim 5, characterized in that the data calculation and processing module is a TMS320 series DSP chip. 8 . The single-phase disconnection fault judgment system of distribution network under incomplete CT configuration according to claim 5 , wherein the control module is a STM32F single-chip microcomputer chip. 9. A system for judging single-phase disconnection faults of distribution network under incomplete CT configuration according to claim 5, characterized in that the DC operating voltages of different amplitudes are 220V, ±12V, 5V and 3.3V.