CN106124920A - Transformator neutral conductor virtual connection fault detection method - Google Patents

Abstract

The invention proposes a transformer neutral line virtual connection fault detection method, comprising: after the transformer fails, using a neutral line virtual connection detection device to measure the phase voltages of the three phases L1, L2 and L3 of the distribution network to be tested , the detection device includes a digital multimeter, an image recognition unit, a control unit, an alarm unit, and a display unit; the phase voltages of L1, L2, and L3 are detected multiple times by pulling the circuit breaker and neutral wire ground, and the detection device analyzes and compares the detection to obtain L1 , L2 and L3 phase voltage, analyze and determine whether the transformer has a neutral wire virtual connection fault, send a fault alarm through the alarm unit, display the display unit, and repair the neutral wire to eliminate the virtual connection fault. The fault detection method proposed by the invention is simple, convenient and easy to operate. The multimeter is clamped in the detection device, which is easy to maintain and replace. The automatic comparison of phase voltage data is realized through the control center, and the detection value and fault detection result can be displayed in real time.

Description

Fault Detection Method of Transformer Neutral Line Virtual Connection

Technical field :

The invention relates to the field of low-voltage power distribution, in particular to a transformer neutral line virtual connection fault.

Background technology :

In the low-voltage distribution network, the transmission line generally adopts a three-phase four-wire system, in which three lines represent the three phases of A, B, and C without splitting, and the other is the neutral line N. (Different from the neutral line, there are two lines in the single-phase transmission line entering the user, one we call the live line, and the other we call the neutral line, and the neutral line normally passes current to form the current in the single-phase line In the three-phase system, the three-phase self-contained circuit, under normal circumstances, the neutral line has no current). So it is called three-phase four-wire system. In the 380V low-voltage distribution network, the N line is set up to obtain 220V line-to-line voltage from 380V phase-to-phase voltage. That is to say, in the three-phase four-wire system, when the three-phase self-contained circuit is used in the three-phase system, the N line is usually called the neutral line. In a single-phase circuit, the N line is called the neutral line. The difference is that the neutral wire has no current and the neutral wire has current.

Whether the neutral point is grounded, also known as the neutral point system. The neutral point system can be roughly divided into two categories, namely the neutral point grounding system and the neutral point insulation system. According to the regulations of the International Electrotechnical Commission (IEC), the low-voltage power distribution system is divided into three types: IT, TT, and TN, and the TN system is further divided into three types: TN-C, TN-S, and TN-C-S.

In the IT system, the live part of the power supply end is insulated from the ground or grounded through high impedance, and the metal casing of the electrical equipment is directly grounded. The IT system is suitable for places with poor environmental conditions, prone to one-phase grounding or fire and explosion, such as coal mines, chemical plants, textile factories, etc., and can also be used in rural areas. However, the zero protection device cannot be installed, because the potential of the neutral line is not fixed during normal operation, and the neutral line should not be repeatedly grounded.

TT system, the neutral point of the power supply of this system is directly grounded, and the metal casing of the electrical equipment is connected to a grounding level that has nothing to do with the grounding point of the power supply end with a protective grounding wire, referred to as protective grounding or grounding system. When there are a large number of single-phase 220V electrical equipment in the power distribution system, and the line laying environment is likely to cause one phase to be grounded or the neutral line to be broken, thereby causing the zero potential to rise, the electrical equipment shell should not be connected to zero and the TT system should be used. The TT system is suitable for places such as cities and towns, rural residential areas, industrial enterprises and scattered civil buildings. When the load end and the first end of the line are equipped with a leakage switch, and the end of the main line is equipped with a zero-break protection, it can become a fully functional system.

TN system, the neutral point of the power supply terminal of the TN system is directly grounded, and the metal casing of the electrical equipment is connected to the central point with a protective zero line. This method is referred to as the protective zero connection or zero connection system. According to the combination of neutral line (working zero line) and protection line (protection zero line), the TN system is divided into the following three forms: (1) TN-C system. In this system, the working zero line and the protection zero line are shared (referred to as PEN), and this system is customarily called a three-phase four-wire system.

TN-S system, in this system, the working zero line N and the protection zero line PE are completely separated from the neutral point of the power supply end, and this system is customarily called a three-phase five-wire system.

The neutral point of the power supply terminal of the TN system is directly grounded, and the metal casing of the electrical equipment is connected to the central point with a protective zero line. This method is referred to as the protective zero connection or zero connection system. According to the combination of neutral line (working zero line) and protection line (protection zero line), the TN system is divided into the following three forms: (1) TN-C system. In this system, the working zero line and the protection zero line are shared (referred to as PEN), and this system is customarily called a three-phase four-wire system. (2) TN-S system. In this system, the working neutral line N and the protective neutral line PE are completely separated from the neutral point of the power supply end. This system is customarily called a three-phase five-wire system. (3) TN-C-S system. In this system, the working zero line and the protection zero line are partially shared, and this system is a local three-phase five-wire system.

The following points should be noted: ① TN-C system is suitable for industrial enterprises with single-phase 220V, portable and mobile electrical equipment, and single-phase 220V fixed electrical equipment is less, but does not need to be connected to zero; TN- S system is suitable for industrial enterprises, high-rise buildings and large-scale civil buildings; TN-C-S system is suitable for industrial enterprises; when the load end is equipped with leakage opening and the main line end is equipped with zero protection, it can also be used for new residential quarters. ②When TN-C, TN-S, and TN-C-S systems are in normal operation, the potential of the zero line can sometimes reach above 50V; the potential of the shell of the TN-C system is equal to the potential of the working zero line, and the potential of the shell of the TN-S system is zero. -C-S system shell potential is not zero, equal to working zero trunk line potential. ③When the electrical equipment collides with the shell, the short-circuit current of the TN system is relatively large. The potential of the shell at the touch point is ≥110V, as long as the design is reasonable, the time is relatively short; if the human body accidentally touches the live part, the danger is high; in the TN-C system, when the sensitivity of the phase-to-phase short circuit protection device is not enough, due to the equipment shell connected to the working zero Line N, and the equipment is not insulated from the ground. During normal operation, the leakage switch cannot work through the residual current, so the leakage switch cannot be installed, and zero-sequence overcurrent protection can only be used; TN-S system, because the equipment shell is connected to the protective zero line PE , when working normally, the leakage switch has no residual current, so when the sensitivity of the phase-to-phase short circuit protection device is not enough, a leakage switch can be installed to protect the single-phase short circuit; TN-C-S system, PE, N common trunk line section can not use leakage Protection, leakage protection can be used for the line segment separated by PE and N, and leakage protection can be used for electrical equipment. ④ When one phase of the line is grounded, the grounding short-circuit current of the TN-C system is small, which is usually not enough to activate the phase-to-phase short-circuit protection and zero-sequence protection device of the line, so that the zero position of the transformer and the shell of all zero-connected equipment will be charged for a long time. The smaller the resistance is, the more dangerous it is; the grounding device of the substation should use a ring-shaped pressure equalizing ring; the leakage protection cannot be installed at the head end of the main line, and the one-phase grounding fault of the line cannot be cut off. This is a major shortcoming of the TN-C system; TN-S system, In addition to having the same characteristics as the TN-C system, leakage protection switches can be installed at the first ends of the lines at all levels to cut off the faulty lines; the TN-C-S system, except having the same characteristics as the TN-C system, some Leakage protection can be installed. ⑤When the working zero line is disconnected, the 220V single-phase equipment may be burned due to the asymmetrical three-phase load after the zero point of the TN-C system is broken, and the zero position may be burned, and the shell of the electrical equipment is connected to zero, which makes the shell electrified and endangers personal safety . If the zero line in the single-phase circuit is broken, all 220V voltage will be applied to the equipment casing; the potential of the equipment casing will rise due to the zero break, and the leakage protection will not work; the disconnection of the zero trunk line in the three-phase circuit of the TN-S system will burn the equipment , but the shell is not charged, and there is no danger to the person; the neutral line in the single-phase circuit is disconnected, which does not endanger the safety of the person or the equipment; the PEN line of the TN-C-S system is disconnected, which is dangerous to the person, and there is no danger to the person when the N line is disconnected. Dangerous, but the disconnection of the working zero trunk line can cause the equipment to be burned. ⑥Regarding the problem of repeated grounding, the TN-C system should ground the neutral line repeatedly, no matter in the case of faults such as grounding of one phase of the line, disconnection of the neutral line, or one-phase contact with the shell, or under normal operating conditions where the loads of each phase are seriously asymmetric It can reduce the potential of the neutral line and the shell of electrical equipment, but it cannot eliminate the risk of electric shock; the working fragmentary line of the TN-C system should not be repeatedly grounded, but the protective neutral line can be grounded repeatedly if necessary. Because the repeated grounding of the working zero line has little effect on protecting personal safety, and the protective effect on safety after the zero is broken is not obvious; after the working zero line is grounded, the leakage protection cannot be used at the head end of the main line; repeated grounding of the protective zero line can reduce the impact on the shell The potential of the shell during short circuit; the PEN line in the TN-C-S system should be grounded repeatedly, and the N line should not be grounded repeatedly. ⑦In the TN system, a zero-break protection device is installed. Its function is: TN-C and TN-C-S systems can play multiple protection functions, which can prevent the use of electrical equipment from being charged and burned due to the disconnection of the zero line. Single-phase 220V electrical equipment; it can prevent the long-term electrification of the housing of the electrical equipment caused by one-phase grounding of the line; it can prevent the neutral line from being caused by load asymmetry, the existence of the third harmonic and unreasonable selection of the neutral line during normal operation. If the voltage drop is too large and the zero position of the transformer is shifted, the neutral line and the zero-connected equipment shell will generate high potential; when the electrical equipment touches the shell, and the short-circuit protection sensitivity is not enough, it can act as a backup protection to prevent large pieces of electrical equipment from being damaged. The shell is charged for a long time; for the TN-S system, it can prevent the single-phase 220V electrical equipment from being burned due to the disconnection of the working zero line; The potential of the working neutral line is too high during normal operation. ⑧ Among the above-mentioned IT systems, TT systems, and TN systems, it is recommended to use the TN-S system, continue to use the TN-C-S system, and stop promoting the use of the TN-C system. At present, the most commonly used low-voltage power distribution system in my country is the TN-C-S system.

During the operation of the low-voltage power distribution system, the transformer will inevitably have faults such as disconnection of the neutral line and virtual connection. If the fault of the virtual connection of the neutral line cannot be found and analyzed in time, it will bring a lot of harm to the distribution network. During the virtual connection of the neutral line of the distribution network transformer, if the L1 phase is grounded, current will flow through the grounding wire of the distribution transformer, which may easily lead to electric shock; the virtual connection of the neutral line of the distribution transformer In the middle, the three-phase load capacity will be unbalanced, resulting in a certain offset of the neutral point of the distribution voltage device, and the index of the grounding point potential exceeds zero, which will increase the voltage of some phases and easily damage the equipment. running.

The document "Troubleshooting of Faults Caused by Faulty Connection of Neutral Line of Equipment Power Supply" discloses the detection method of faulty neutral wire connection. Using the high and low temperature damp heat test box to over-temperature alarm fault, check and measure the electrical circuit of the equipment, and finally confirm that the cause of the fault is The virtual connection of the neutral line of the power supply of the equipment is used to analyze the impact of the virtual connection of the neutral line of the power supply on the equipment. Although it is possible to accurately determine the fault of the virtual connection of the neutral line, it requires a relatively complicated high and low temperature humidity and heat test chamber, which increases the cost.

Patent document CN 102801131B discloses a neutral line disconnection detection and protection method and device, which belong to the technical field of low-voltage system protection. The device is mainly composed of a current detection unit, a detection signal conversion unit, a signal processing unit, a protection execution unit and an audible and visual alarm unit. This method collects the current signals of the three-phase line and the neutral line, and then processes and analyzes the signals by the single-chip microcomputer, thereby judging whether the neutral line is broken. When the neutral line is broken, the single-chip microcomputer sends an action signal to disconnect the circuit and Alert. The components used in the invention are simple and easy to implement, have a high degree of automation, can effectively detect the disconnection of the neutral line and implement protection for it, and have good engineering applicability. However, the structure of the detection device is relatively complicated, and it cannot detect the virtual connection of the neutral line.

Invention content :

The invention proposes a transformer neutral line virtual connection fault detection method, which includes the following steps: 1) After the voltage transformer fails, use a neutral line virtual connection detection device to measure the three-phase L1, L2 and L3 of the distribution network to be tested The phase voltage of each phase, the detection device includes a digital multimeter, an image recognition unit, a control unit, an alarm unit and a display unit; 2) The detection device analyzes and compares and detects the phase voltages of L1, L2 and L3, if the phase voltages of L1, L2 and L3 If the phase voltages are different, the display unit shows that the transformer is faulty; 3) After the circuit breaker is opened, the voltage of the three-phase L1, L2 and L3 is detected. If the voltage of each phase is normal, and the shell of the distribution box is normally grounded, the neutral line Not grounded, the appearance of the insulated wire in the box is normal, and the connections of the contacts are tight, continue to check; 4) Ground the neutral wire at the distribution box, and use wires to make a good connection between the neutral wire and the box shell and ground flat steel. After switching on and sending power, the voltage of each phase is detected by the detection device. If the voltage of each phase is normal, after 4-8 hours of trial operation, the voltage of each phase is detected by the detection device. If the voltage of each phase is not the same, the transformer has a neutral state. In the event of a virtual connection fault, the alarm unit will issue a fault alarm, and the display unit will display that the transformer has a virtual connection fault of the neutral line; 5) Eliminate the virtual connection fault of the neutral line by repairing or replacing the neutral line.

Further, the digital multimeter is clamped into the detection device.

Further, the digital multimeter includes the functions of ammeter, voltmeter and ohmmeter.

Further, the accuracy of the digital multimeter is between ±(0.7%+1) and ±(0.1%+1) of reading.

Further, the image recognition unit is located above the digital multimeter, and is used to read the voltage figures of the multimeter.

Further, the detection device also includes a switch, and the control unit controls the action of the switch to make the digital multimeter detect the voltage of each phase of L1, L2 and L3 respectively.

Further, the alarm unit includes LED lights and speakers.

Further, the display unit includes a liquid crystal display or an LED display.

Further, in step 4), after running for 5 hours, the voltage of each phase is detected by the detection device.

The beneficial effects of the present invention are:

1. The detection device is used to detect the virtual connection fault of the neutral line of the transformer of the low-voltage distribution network, and the operation is simple and convenient.

2. Through the alarm unit and the display unit, it can be displayed that a neutral line virtual connection fault has occurred, which can play a vigilant effect;

3. The detection device uses the existing digital multimeter to detect the voltage of each phase of L1, L2 and L3, which is convenient and simple. The multimeter is snapped into the detection device, which is easy to maintain and replace.

4. The detection device controls the automatic switching of the switching switch through the control unit, detects the phase voltage values of the three phases respectively, and recognizes the digital universal readings through the digital image recognition unit, automatically compares them, and gives the results, which realizes automatic comparison.

5. The detection value and comparison results of each phase voltage and fault detection results can be displayed on the display unit, which can be viewed intuitively.

Description of drawings :

Figure 1 is a schematic diagram of the detection device.

Fig. 2 is a flow chart of the method for finding and analyzing faults of transformer neutral wire virtual connection in the present invention.

Specific implementation methods :

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

As shown in FIG. 1 , it is a schematic structural diagram of a neutral wire virtual connection detection device of the present invention, and the detection device includes a digital multimeter, an image recognition unit, a control unit, an alarm unit and a display unit. Digital multimeters are commonly used digital multimeters, including ammeter, voltmeter, and ohmmeter functions. The accuracy of the meter is between ±(0.7%+1) and ±(0.1%+1) of the reading. The digital multimeter is clamped in the detection device to detect the voltage of each phase of L1, L2 and L3, which is convenient and concise, and the multimeter is clamped in the detection device, which is easy to maintain and replace.

The fault detection method for transformer neutral line virtual connection proposed by the present invention is a specific flow chart as shown in Figure 2, including the following steps:

1) After the transformer fails, use a neutral line virtual connection detection device to measure the phase voltages of the three phases L1, L2 and L3 of the distribution network to be tested. The detection device includes a digital multimeter, an image recognition unit, a control unit, The alarm unit and display unit, and the image recognition unit are located above the digital multimeter, used to read the voltage figures of the multimeter, and transmit the read data to the control center.

2) The phase voltages of L1, L2 and L3 are obtained through analysis and comparison of the detection device. The control center of the detection device includes a single-chip microcomputer and a storage module, and the control center controls the switching of the switch, so that the multimeter detects the phase voltages of L1, L2 and L3 respectively. After receiving the data sent by the image recognition unit, the control center first sends the phase voltages of L1, L2 and L3 to the storage module for storage, and sends the phase voltage data of L1, L2 and L3 to the display unit for display. Then analyze and compare the phase voltage data of L1, L2, and L3. If the phase voltages of L1, L2, and L3 are not the same, the display unit will display that the transformer has failed.

3) After pulling the circuit breaker, check the voltage of the three-phase L1, L2 and L3. If the voltage of each phase is normal, that is, the phase voltage of L1, L2 and L3 is equal, check whether the shell of the distribution box passes through the grounded flat steel. Bolt connection, whether the neutral wire is grounded, check whether the appearance of the insulated wires in the box is abnormal, if the connections of the contacts are tight, the shell of the distribution box is normally grounded, the neutral wire is not grounded, the appearance of the insulated wires in the box is normal, and the connections of the contacts are tight , continue to the next check.

4) Ground the neutral wire at the distribution box, and use wires to connect the neutral wire to the box shell and the ground flat steel. After switching on and transmitting power, detect the voltage of each phase through the detection device. If the voltage of each phase is normal, try After running for 4-8 hours, preferably running for 5 hours, after switching on and sending power, try running for a period of time to make the system’s transformers and electrical appliances run in a stable state; if the running time is too short, the system’s faults cannot be fully displayed , If the running time is too long, if the system fails, it will bring harm to the transformer and distribution network. After that, the voltage of each phase is detected by the detection device again. If the voltages of each phase are not the same, it can be determined that the transformer has a neutral wire fault, and the fault alarm is issued through the alarm unit, and the display unit shows that the transformer has a neutral wire fault. Fault. The alarm unit includes LED lights and speakers, which can have a better alert effect. The display unit includes a liquid crystal display or an LED display, and the detection value and comparison result of each phase voltage and the fault detection result can be displayed on the display unit, which can be viewed intuitively.

5) After that, check and repair the neutral wire to eliminate the fault of neutral wire connection. If the neutral line is buried underground or sealed in the wall, it should be considered to lay a new line as the neutral line, so as to eliminate the fault of the neutral line virtual connection and ensure the normal operation of the low-voltage distribution network system and transformer.

Fault Finding Analysis

In the three-phase four-wire low-voltage system, there are a large number of single-phase equipment connected to each phase. Because of the asynchronous use of equipment and the randomness of equipment opening, three-phase load imbalance is inevitable. Due to construction technology, overcurrent heating, poor maintenance, external damage and other reasons, disconnection faults may occur. Once the neutral line is disconnected, the neutral point potential will shift after the breakpoint, causing the three-phase voltage to lose balance and the load to increase. The lower the large phase voltage, the smaller the load and the higher the phase voltage. This is because the single-phase equipment originally connected to different phases has been connected in series and then connected to the main line voltage. According to the principle of voltage division in series circuits, a group of equipment with large impedance (small load) has high voltage division, and will burn out in severe cases. The equipment even caused a fire, and a group of equipment with small impedance (large load) had a low partial voltage, and the equipment could not work normally.

As the phase electrical equipment with high voltage is burned out and the load withdraws, the impedance becomes larger, and the voltage of the phase becomes higher and higher, while the more equipment is put into the phase with low voltage, the impedance decreases and the voltage decreases. Such a vicious cycle leads to phase The voltage can be as low as tens of volts and as high as 300 volts. The initial multimeter measurements at the monitoring station are an extreme example. In addition, there was no obvious burning of equipment at the station at that time, which also easily led the testers to make wrong judgments.

After the neutral line is grounded at the user end, when the three-phase load is unbalanced, the unbalanced current flows back to the neutral point of the secondary side of the distribution transformer through the grounding point, thereby reducing the displacement of the neutral point after the breakpoint, so that the single The terminal voltage of the phase equipment will not deviate too much. After the equipment is in normal use, the unbalance of the three-phase load increases. Due to the relatively large impedance of the ground, although the neutral wire is well grounded, it is still impossible to ensure that the voltage of each phase is symmetrical, and the disconnection of the neutral wire will only affect the voltage on the load side. The phase voltage does not affect the line voltage.

Repeat the grounding of the neutral wire. The earth is used as the current path, because the impedance of the neutral line is measured in milliohms, and the impedance of the earth loop is measured in ohms, there is a huge difference between the two, so the three-phase voltage will still be seriously unbalanced after the neutral line is disconnected. Just to a lesser degree. Based on the analysis of previous similar cases, it is concluded that it is not a fundamental measure to restrict the asymmetry of the load phase voltage caused by the disconnection of the neutral wire by repeated grounding, and the technical effect produced is very limited.

According to the analysis, when measuring the voltage, due to the large internal resistance of the multimeter voltage block, the current flowing through the meter itself is very small, and the power consumed is also very small, so when the current passes through the virtual junction of the neutral line, the voltage generated The drop is very small, and the resistance at the virtual junction does not significantly affect the reading of the multimeter, so it creates the illusion that the voltage is sufficient. After the load is connected, the resistance at the virtual junction will cause a large voltage drop in the circuit, and even be in an open circuit state; It is easy to cause wrong judgments by maintenance personnel.

Claims (9)

1. the present invention proposes a kind of transformator neutral conductor virtual connection fault detection method, it is characterised in that comprise the steps: 1) Use each phase phase voltage of neutral conductor virtual connection detection measurement device power distribution network to be measured three-phase L1, L2 and L3, described detection device bag Include digital multimeter, image identification unit, control unit, alarm unit and display unit；2) detection device analysis comparison and detection Obtain the phase voltage of L1, L2 and L3, if the phase voltage of L1, L2 and L3 differs, occurred in that by display unit display transformator Fault；3) after pulling open chopper, again detecting the voltage of three-phase L1, L2 and L3, if each phase voltage is normal, and distribution box shell is just Often ground connection, the neutral conductor is unearthed, and in case, insulated conductor outward appearance is without exception, and each contact connects fastening, continues checking for；4) at distribution box The neutral conductor and case shell, grounding flat steel, by neutral earthing, are done good connection with wire by place, by detection device after combined floodgate power transmission Detect each phase voltage, if each phase voltage is normal, by test run after 4-8 hour, detect each phase voltage by detection device, if each phase Voltage differs, then this transformator there occurs neutral conductor virtual connection fault, sends fault warning by alarm unit, shows list simultaneously Unit's display transformator occurs in that neutral conductor virtual connection fault；5) by repairing the neutral conductor or changing the neutral conductor, neutral conductor virtual connection is eliminated Fault.

Fault detection method the most according to claim 1, it is characterised in that described digital multimeter is connected to detect device In.

Fault detection method the most according to claim 2, it is characterised in that described digital multimeter comprises ammeter, electricity Pressure table, ohmmeter function.

Fault detection method the most according to claim 2, it is characterised in that the precision of described digital multimeter is at reading Between ± (0.7%+1) and ± (0.1%+1).

Fault detection method the most according to claim 1, it is characterised in that described image identification unit is positioned at digital versatile Above table, for reading the voltage digital of circuit tester.

Fault detection method the most according to claim 1, it is characterised in that described detection device also includes switching switch, Control unit controls switching switch motion, makes digital multimeter detect the voltage of each phase of L1, L2 and L3 respectively.

Fault detection method the most according to claim 1, it is characterised in that described alarm unit includes LED and raises one's voice Device.

Fault detection method the most according to claim 1, it is characterised in that described display unit include liquid crystal display or Light-emitting diode display.

Fault detection method the most according to claim 1, it is characterised in that in described step 4) is after running 5 hours, logical Cross detection device and detect each phase voltage.