The word "harmonic" comes from acoustics. The mathematical analysis of harmonics has laid a good foundation in18th century and19th century. The harmonic analysis method proposed by Fourier et al. is still widely used today. The harmonic problem of power system has attracted people's attention as early as the 1920s and 1930s. At that time, in Germany, the voltage and current waveforms were distorted due to the use of static mercury arc converters. J.C.Read's paper on converter harmonics published in 1945 is a classic paper on early harmonic research. In 1950s and 1960s, due to the development of HVDC transmission technology, a large number of papers were published on harmonic problems caused by converters in power systems. Since 1970s, due to the rapid development of power electronic technology, various power electronic devices have been widely used in power system, industry, transportation and home, and the harm caused by harmonics has become increasingly serious. All countries in the world attach great importance to harmonic problems. Many international academic conferences on harmonics have been held, and many countries and international academic organizations have formulated standards and regulations to limit harmonics in power systems and electrical equipment.
The definition of harmonics in power supply system is to decompose periodic non-sinusoidal electric quantity by Fourier series, and get a series of components which are the same as the fundamental frequency of power grid, which are called harmonics. The ratio of harmonic frequency to fundamental frequency (n=fn/f 1) is called harmonic order. Sometimes, non-integer multiples of harmonics appear in the power grid, which is called anharmonic or fractional harmonics. Harmonic is actually a kind of interference, which has "pollution" to the power grid. The field of electrical technology mainly studies the generation, transmission, measurement, harm and suppression of harmonics, and its frequency range is generally 2≤n≤40. In the industrial and domestic electricity load, inductive load accounts for a large proportion. Asynchronous motor, transformer, fluorescent lamp, etc. Are typical perceptual loads. The reactive power consumed by asynchronous motors and transformers accounts for a large proportion of the reactive power provided by the power system. Reactors and overhead lines in the power system will also consume some reactive power. Resistive load must absorb reactive power to work normally, which is determined by its own nature.
Nonlinear devices such as power electronic devices also consume reactive power, especially various phased devices. Such as phase-controlled rectifier, phase-controlled AC power regulating circuit, periodic converter, etc., when working, the fundamental current lags behind the grid voltage and consumes a lot of reactive power. In addition, these devices will generate a lot of harmonic current, and the harmonic source will consume reactive power. The fundamental current phase of diode rectifier circuit is almost the same as the grid voltage, so it basically does not consume fundamental reactive power. But it also produces a lot of harmonic current, so it also produces a certain amount of reactive power.
In recent 30 years, the application of power electronic devices has been increasingly extensive, which also makes power electronic devices the largest harmonic source. Among all kinds of power electronic devices, rectifier devices account for the largest proportion. Almost all commonly used rectifier circuits adopt thyristor phase-controlled rectifier circuits or diode rectifier circuits, among which three-phase bridge rectifier circuits and single-phase bridge rectifier circuits are the most. Harmonic pollution and power factor lag caused by resistive load rectifier circuit are well known. The diode rectifier circuit with capacitor filter on DC side is also a serious harmonic pollution source. The phase of the fundamental component of the input current in this circuit is almost the same as that of the power supply voltage, so the fundamental power factor is close to 1. But the harmonic component of its input current is very large, which causes serious pollution to the power grid and makes the total power factor very low. In addition, power electronic devices such as AC power regulating circuit and periodic converter with phase control mode will also produce a large number of harmonic currents on the input side.
(1) Poor generator quality will produce harmonics.
Because it is difficult to achieve absolute symmetry in the production of three-phase winding and absolute uniformity of iron core, the power supply will produce some harmonics, but generally speaking, it is rare.
(2) Power transmission and distribution system produces harmonics
In power transmission and distribution system, harmonics are mainly generated by power transformers. Because of the saturation of transformer core, the nonlinearity of magnetization curve and the consideration of economy when designing transformer, the working magnetic field density is selected in the near saturation section of magnetization curve, which makes the magnetization current show a peak waveform, thus containing odd harmonics. Its size is related to the structure of magnetic circuit and the saturation of iron core. The higher the saturation of the iron core, the farther the transformer operating point deviates from linearity, and the greater the harmonic current, of which the third harmonic current can reach 0.5% of the rated current.
(3) Harmonics generated by electrical equipment:
Thyristor rectifier equipment. Thyristor rectifiers are widely used in electric locomotives, aluminum reduction cells, charging devices, switching power supplies and many other aspects, causing a lot of harmonics to the power grid. As we know, the thyristor rectifier device adopts phase-shifting control to absorb the sine wave with missing angle from the power grid, thus leaving another part of the sine wave with missing angle in the power grid, which obviously contains a lot of harmonics. If the rectifier device is a single-phase rectifier circuit, it will contain odd harmonic current when connected to the inductive load, and the content of third harmonic can reach 30% of the fundamental wave; When connected with capacitive load, it contains odd harmonic voltage, and its harmonic content increases with the increase of capacitance value. If the rectifier device is a three-phase fully controlled bridge 6-pulse rectifier, the primary side of the transformer and the power supply line contain five or more odd harmonic currents; If it is a 12 pulse rectifier, there are also odd harmonic currents of 1 1 and above. According to statistics, the harmonic generated by rectifier device accounts for nearly 40% of all harmonics, and it is the largest harmonic source.
Frequency conversion device. Frequency conversion devices are often used in fans, pumps, elevators and other equipment. Because of phase control, harmonic components are very complex, including integer harmonics and fractional harmonics. The power of this kind of equipment is usually high. With the development of variable frequency speed regulation, more and more harmonics are caused to the power grid.
Arc furnace, calcium carbide furnace. Because it is difficult for the three-phase electrodes of the electric furnace to contact uneven burden at the same time when heating raw materials, the combustion is unstable, resulting in unbalanced three-phase load and harmonic current, which is injected into the power grid through the triangular connecting coil of the transformer. Among them, the 27th harmonic is the main one, with an average of 8% to 20% of the fundamental wave and a maximum of 45%.
Gas discharge electric light source. Fluorescent lamps, high-pressure mercury lamps, high-pressure sodium lamps and metal halide lamps belong to gas discharge electric light sources. Through the analysis and measurement of the volt-ampere characteristics of this kind of electric light sources, it is known that their nonlinearity is very serious, and some of them also contain negative volt-ampere characteristics, which will cause odd harmonic current to the power grid.
Household appliances. Television, video recorder, computer, dimming lamp, temperature regulating cooker, etc. With the voltage regulating and rectifying device, deep odd harmonics will be generated. In washing machines, electric fans, air conditioners and other equipment with windings, the waveform can also be changed due to the change of unbalanced current. Although these household appliances are small in power, they are huge in size and are also one of the main sources of harmonics.
The voltage provided by the ideal public power grid should be a single fixed frequency and a specified voltage amplitude. The appearance of harmonic current and harmonic voltage is a kind of pollution to the public power grid, which worsens the environment in which it uses electrical equipment and also affects the surrounding electrical equipment. Before power electronic equipment was widely used, people had done some research on harmonics and its harm, and had a certain understanding, but at that time, harmonic pollution had not attracted enough attention. In recent 30 or 40 years, with the rapid development of various power electronic devices, the harmonic pollution of public power grid has become increasingly serious, and various faults and accidents caused by harmonics have also occurred constantly. The seriousness of harmonic harm has attracted great attention. Harm of harmonics to public power grid and other systems is roughly as follows.
(1) harmonics will cause additional harmonic losses to components in the public power grid, thus reducing the efficiency of power generation, transmission and electrical equipment. When a large number of third harmonics flow through the neutral line, it will overheat the line and even cause a fire.
(2) Harmonics affect the normal work of various electrical equipment. The influence of harmonics on the motor will not only cause extra losses, but also produce mechanical vibration, noise and overvoltage, which will cause local overheating of the transformer in severe cases. Harmonics make capacitors, cables and other equipment overheat, insulation aging, life shortening and even damage.
(3) Harmonics will cause local parallel resonance and series resonance in public power grid, thus amplifying harmonics, greatly increasing the hazards of (1) and (2) above, and even causing serious accidents.
(4) Harmonics will lead to misoperation of relay protection and automatic devices, making the measurement of electrical measuring instruments inaccurate.
(5) Harmonics will interfere with the adjacent communication systems, generate noise under light, and reduce the communication quality; In the worst case, the residence is lost, which makes the communication system unable to work normally. Harmonic simply means that when the voltage or current of a certain frequency acts on a nonlinear load, it will produce sinusoidal voltage or current of other frequencies different from the original frequency.
Ripple refers to the AC component superimposed on the DC stability of DC voltage or current.
Although they are not the same thing conceptually, they are related. For example, the additional ripple on the power supply can easily produce harmonics of various frequencies on the electrical appliance; The existence of various frequency harmonics in the power supply will undoubtedly lead to the increase of ripple components in the power supply.
In addition to the situation that we need to generate harmonics in the circuit, it mainly has the following main hazards:
1, causing power grid resonance, causing overcurrent or overvoltage and causing accidents;
2. Increase additional losses and reduce the efficiency and equipment utilization rate of power generation, transmission and electricity consumption equipment;
3. Manufacturing electrical equipment (such as rotating electrical machines, capacitors, transformers, etc.). ) abnormal operation accelerates insulation aging, thus shortening its service life;
4, make the relay protection, automatic device, computer system and many electrical equipment work abnormally or can't normal action or operation;
5, make the measurement and measuring instruments can't indicate or measure correctly;
6, interfere with the communication system, reduce the quality of signal transmission, destroy the normal transmission of signals, and even damage communication equipment.
Harm of ripple:
1, it is easy to produce harmonics to electrical appliances, and harmonics will cause greater harm;
2, the efficiency of the power supply is reduced;
3. Strong ripple will cause surge voltage or current, which will cause electrical appliances to burn out;
4. It will interfere with the logical relationship of digital circuits and affect their normal work;
5, will bring noise interference, so that the image equipment, audio equipment can not work normally.
In a word, their appearance in places we don't need is harmful and needs to be avoided. There are many ways to suppress and eliminate harmonics and ripples, but it seems very difficult to completely eliminate them. We can only control the environment and equipment within the allowable range without affecting them.
The gradual increase of nonlinear loads in power grid is the same trend all over the world, such as frequency conversion drive or thyristor rectifier DC drive equipment, computers, uninterruptible power supply (UPS) used by important loads, energy-saving fluorescent lamp systems and so on. These nonlinear loads will lead to power grid pollution, power quality degradation, power supply equipment failure, and even serious fire accidents.
Power pollution and power quality deterioration are mainly manifested in the following aspects: voltage fluctuation, surge impact, harmonics, three-phase imbalance and so on.
1. Harm of power pollution
Power pollution will cause serious harm to electrical equipment, mainly including:
Interfere with the normal work of electronic equipment such as communication equipment and computer system, resulting in data loss or collapse.
It affects the performance of radio transmission system, radar system, nuclear magnetic vibration and other equipment, resulting in noise interference and image confusion.
Causing misoperation of electric automatic device and even serious accident.
Overheating of electrical equipment, increased vibration and noise, accelerated insulation aging, shortened service life, and even failure or burning.
It causes the fluctuation (flicker) of the light brightness and affects the work efficiency.
Resulting in an increase in power loss of the power supply system. Voltage fluctuation and flicker
Voltage fluctuation refers to the peak value of multiple sine waves that exceed (fall below) the standard voltage value in a period of time, from about half a cycle to hundreds of cycles, that is, from 10MS to 2.5 seconds, including overvoltage fluctuation and undervoltage fluctuation. Ordinary lightning arresters and overvoltage protectors can't eliminate overvoltage fluctuations at all, because they are used to eliminate transient pulses. Ordinary lightning arresters have considerable resistance when operating at limited voltage. Considering their rated heat capacity (Joule), these devices are easy to burn out and cannot provide future protection functions. This situation is often easily overlooked and is the main reason for the failure or shutdown of computers, control systems and sensitive equipment.
Another opposite situation is undervoltage fluctuation, which means that the peak values of multiple sine waves are lower than the standard voltage value in a period of time, or as usual: jitter or decline. Long-term low voltage may be caused by the overload of power supply companies or users, which may be an accident or a planned arrangement. What is more serious is the voltage loss, which is mostly caused by the separation and combination of heavy loads in the distribution network, such as large motors, central air-conditioning systems, electric arc furnaces, switching arcs, fuse blowing, circuit breakers tripping and so on. These are the reasons that usually lead to voltage distortion.
Frequent startup of large electrical equipment leads to periodic voltage fluctuation, such as welding machine, punch, crane, elevator, etc. These devices need short-term impulse power, mainly reactive power. Voltage fluctuation leads to unstable equipment power and decreased product quality; The flashing of lights causes eye fatigue and reduces work efficiency.
Surge impact
Surge surge refers to the short-term over (low) voltage in the system, that is, the voltage instantaneous pulse with the time not exceeding 1 millisecond. This pulse can be positive, negative, series or oscillating. They are also commonly called: spikes, gaps, interferences, burrs or abrupt changes.
The switching of large equipment (motors, capacitors, etc.) may lead to surge impact in the power grid. ) or the switching of large-scale thyristors in the power grid and the invasion of external lightning waves. Surge impact is easy to cause damage to electronic equipment components and insulation breakdown of electrical equipment; At the same time, it is also easy to cause data errors or crashes in computers and other equipment.
harmonic wave
The working current waveform of linear load, such as pure resistance load, is the same as the sine wave of input voltage, while the working current of nonlinear load, such as chopped DC load, is non-sine. The current/voltage of the traditional linear load only contains the fundamental wave (50Hz) and has no or only a small harmonic component, while the nonlinear load will generate considerable harmonics in the power system.
The superposition of harmonics and fundamental waves in power system causes waveform distortion, and the degree of distortion depends on the frequency and amplitude of harmonic current. The nonlinear load produces steep pulse current instead of smooth sine wave current, and the harmonic current in this pulse causes the voltage distortion of the power grid and forms harmonic components, which in turn causes other loads connected to the power grid to produce more harmonic currents.
A computer is such a nonlinear load. Computers, like most office electronic equipment, are equipped with diode/capacitor power supply, which only generates current at the peak of AC sine wave voltage, thus generating a large amount of third harmonic current (150Hz). Other devices that generate harmonic current mainly include: motor variable frequency governor, solid state heater, and other devices that generate non-sinusoidal current.
Fluorescent lighting system is also an important harmonic source. In the ordinary electromagnetic rectification lighting circuit, the typical value of the third harmonic is about 13%-20% of the fundamental wave (50Hz). In the lighting circuit of electronic ballast, the harmonic component is even as high as 80%.
Harmonic current generated by nonlinear load will affect many working links of power system, including transformers, neutral wires, motors, generators and capacitors. Harmonic current will seriously increase the operating temperature (K parameter) of transformers, motors and standby generators. Overcurrent on neutral wire (caused by harmonics and unbalance) will not only increase the temperature of conductor and cause insulation damage, but also produce circulating current in the coil of three-phase transformer, which will lead to overheating of transformer. Reactive power compensation capacitor will overheat due to harmonic distortion of power grid voltage, which will lead to serious overcurrent;
In addition, the capacitor will form a resonant circuit with the inductive elements in the power system, which will lead to a significant increase in the voltage across the capacitor, leading to serious failures. The starting capacitor of lighting device is also very sensitive to overheating caused by high frequency current, and the frequent damage of starting capacitor shows the influence of harmonics in power grid. Harmonics will also reduce the transmission efficiency and increase the loss of distribution lines, and interfere with the work of power carrier communication systems, such as power management system (ems) and clock system. In addition, harmonics will increase the measurement errors of power meter, active demand meter and watt-hour meter.
Three-phase imbalance
Three-phase unbalance will produce overcurrent (caused by harmonics and unbalance) on the neutral line, which will not only increase the temperature of the conductor, but also cause serious fire accidents.
Three-phase unbalanced current generally exists in power grid, especially in urban civil power grid and agricultural power grid, because there are a large number of single-phase loads. For three-phase unbalanced current, there is almost no effective solution except distributing the load as reasonably as possible. It is precisely because we can't find an effective way to solve the problem that people don't pay attention to it and few people study it.
The unbalanced current in the power grid will increase the copper loss of lines and transformers, increase the iron loss of transformers, reduce the output of transformers, and even affect the safe operation of transformers, resulting in three-phase voltage imbalance, thus reducing the quality of power supply.
, even affect the accuracy of the watt-hour meter and cause measurement loss.
Theoretical research proves that the copper loss of transformer and line is the smallest when the three-phase current is balanced, which means that the three-phase imbalance phenomenon increases the copper loss of transformer and line.
Influence of unbalanced current on copper loss of system
Let the total resistance of three-phase lines and transformer windings of a system be R. If the three-phase currents are balanced, IA= 100A, IB= 100A, IC= 100A, and total copper loss =1002r+1000r.
If the three-phase current is unbalanced, IA=50A, IB= 100A and IC= 150A, the total copper loss is = 502r+102r+1502r = 35,000r, which is higher than the copper loss in the equilibrium state/kl.
In a more serious state, if IA=0A, IB= 150A, IC= 150A, the total copper loss =1502r+1502r = 45,000r, which is 50% higher than that in the equilibrium state.
In the most serious state, if IA=0A, IB=0A, IC=300A, the total copper loss =3002R=90000R, which is three times higher than that in the equilibrium state.
Influence of Unbalanced Current on Transformer
The existing 10/0.4KV low-voltage distribution transformers are mostly three-phase three-column core transformers with Yyn0 connection. This type of transformer, when the secondary side load is unbalanced and there is neutral point current, the neutral point current is zero sequence current, while in
Because there is no neutral lead on the primary side, the zero-sequence current cannot flow, so the zero-sequence current cannot be balanced by ampere-turns. For the iron core, there is an excitation zero-sequence current, which is controlled by zero-sequence excitation impedance. According to the design of magnetic circuit, this zero sequence
The excitation impedance is large, the zero sequence current affects the symmetry of phase voltage, and the neutral point will shift. According to the calculation, when the neutral current is 25% of the rated current, the neutral displacement is about 7% of the rated voltage. National standard GB50052-
Article 6.08 stipulates: "When selecting three-phase transformer of Yyn0 wiring group, the current caused by single-phase unbalanced load shall not exceed 25% of the rated current of low-voltage winding, and the one-phase current shall not exceed the rated power when fully loaded.
Flow value. "Due to the above regulations, the capacity of Yyn0 tie line distribution transformer connected to single-phase load is limited, and the full utilization of transformer equipment capacity is also affected.
Moreover, for the three-phase three-column magnetic circuit, the zero-sequence magnetic flux can not form a loop in the magnetic circuit, but must form a loop in the fuel tank wall and fasteners, and the magnetic flux in the fuel tank wall and fasteners will produce a large eddy current loss, which will cause changes.
The iron loss of the press increases. When the zero-sequence current is too large, the zero-sequence magnetic flux is large, and the neutral point drift is too large, resulting in magnetic saturation of the iron core, sharp increase in iron loss and overheating of fasteners.
It is possible that any one phase current is not overloaded and the transformer is damaged due to local overheating.
Because the zero-sequence excitation impedance between distribution transformer and Yyn0 wiring group is large, the neutral current will cause great voltage change, resulting in serious three-phase voltage imbalance, which not only affects single-phase users, but also is used for three-phases.
Family has a greater influence.
Harm of three-phase load imbalance
Influence on distribution transformer
(1) unbalanced three-phase load will increase the loss of transformer;
The loss of transformer includes no-load loss and load loss. In general, the operating voltage of transformer is basically unchanged, that is, the no-load loss is a constant. The load loss varies with the working load of the transformer and is proportional to the square of the load current. When the three-phase load is unbalanced, the load loss of the transformer can be regarded as the sum of the load losses of three single-phase transformers.
From the mathematical theorem, we know that if the numbers of A, B and c 3 are all greater than or equal to zero, then a+b+c≥33√abc.
When a=b=c, algebra and a+b+c take the minimum value: a+b+c=33√abc.
Therefore, we can assume that the three-phase losses of the transformer are Qa=Ia2 R, Qb= Ib2 R and Qc =Ic2 R respectively, where Ia, Ib and Ic are the secondary load phase currents of the transformer and R is the phase resistance of the transformer. The loss expression of transformer is as follows:
QA+q b+ Qc≥33√(ia2r)(ib2r)(ic2r)と
Therefore, under the condition of constant transformer load, when Ia=Ib=Ic, that is, when the three-phase load reaches balance, the transformer loss is the smallest.
Transformer loss:
When the transformer is in three-phase balanced operation, namely Ia=Ib=Ic=I, QA+QB+QC = 3I2r;
The transformer operates at the maximum unbalance, that is, Ia=3I and Ib=Ic=0, QA = (3i) 2r = 9i2r = 3 (3i2r);
In other words, the loss at maximum unbalance is three times that at equilibrium.
(2) Unbalanced three-phase load may cause serious consequences of burning transformer:
When the above imbalance occurs, the phase current of heavy load is too large (increased to 3 times) and overload is too much, which may cause overheating of winding and transformer oil. The winding is overheated and the insulation aging is accelerated; Overheating of transformer oil will cause deterioration of oil quality, quickly reduce the insulation performance of transformer, shorten the service life of transformer (the service life will be reduced by half for every 8℃ temperature increase), and even burn the winding.
(3) Unbalanced operation of three-phase load will cause excessive zero-sequence current of transformer and increase the temperature rise of local metal parts;
Under the unbalanced operation of three-phase load, the transformer will inevitably produce zero-sequence current, and the existence of zero-sequence current in the transformer will produce zero-sequence magnetic flux in the iron core, forming a loop in the oil tank wall or other metal parts of the transformer. However, when designing the distribution transformer, these metal elements are not considered as magnetic elements, so the hysteresis and eddy current loss caused by them will make these elements heat up, which will lead to abnormal temperature rise of local metal parts of the transformer, and even lead to transformer operation accidents in serious cases.
3.2 Impact on High Voltage Lines
(1) Increase the loss of high-voltage lines:
When the three-phase load on the low voltage side is balanced, the 6 ~ 10 kV high voltage side is also balanced. Let each phase current of the high-voltage line be I, and its power loss is Δ p1= 3i2r.
The three-phase load imbalance of low-voltage power grid will be reflected to the high-voltage side. When the maximum unbalance occurs, the corresponding phase of high voltage is 1.5I, and the other two phases are all 0.75i, and the power loss is:
δP2 = 2(0.75 I)2R+( 1.5I)2R = 3.375 i2r = 1. 125(3I2R)；
That is, the power loss on the high-voltage line increased by 12.5%.
(2) Increase the tripping times of high-voltage lines and reduce the service life of switchgear;
We know that the fault of high-voltage line passing current accounts for a considerable proportion, and the reason is that the current is too large. Unbalanced three-phase load in low-voltage power grid may lead to a high-voltage phase current being too large, which may lead to high-voltage line tripping and power failure, and then lead to a large-scale power failure accident. At the same time, frequent tripping of substation switchgear will reduce the service life.
3.3 Impact on distribution boards and low-voltage lines
(1) unbalanced three-phase load will increase line loss;
In the three-phase four-wire power supply line, the load is evenly distributed to three phases, each phase current is I, the neutral current is zero, and its power loss is Δ p1= 3i2r.
When the maximum imbalance occurs, that is, one phase is 3I, the other two phases are zero, the neutral current is 3I, and the power loss is:
δP2 = 2(3I)2R = 18I2R = 6(3I2R)；
In other words, the power loss at the maximum imbalance is 6 times that at the balance. In other words, if the monthly loss is 1200 kwh at the maximum imbalance and only 200 kwh at the balance, we can know the loss reduction potential of adjusting the three-phase load.
(2) Unbalanced three-phase load may lead to serious consequences of burning lines and switchgear:
When the above imbalance occurs, the phase current of heavy load is too large (increased to 3 times) and the overload is too much. Because the calorific value Q=0.24I2Rt, if the current is increased by 3 times, the calorific value will increase by 9 times, which may lead to a linear increase in the temperature of the conductor in this phase, leading to burning out. Moreover, because the cross section of neutral conductor should be 50% of the cross section of phase conductor, some of them are often too small when choosing, and the quality of joint is not good, which increases the resistance of conductor. The probability of neutral wire burning is high.
Similarly, on the switchboard, the heavy-duty phase of the switch is burnt out, and the heavy-duty phase of the contact is burnt out, thus causing serious consequences such as damage to the whole machine.
3.4 Impact on power supply enterprises
Power supply enterprises directly manage households, and the loss of low-voltage power grid is large, which will reduce the economic benefits of power supply enterprises and even cause power supply enterprises to operate at a loss. The line loss in the contract area of agricultural electrician is high, the bonus of agricultural electrician is deducted, and even the salary can't be paid, which will inevitably affect the mood of agricultural electrician, from passive work to illegal crime in order to get money.
On the one hand, the burning of transformers, lines and switch cabinets will increase the power supply cost of power supply enterprises; On the other hand, power outage maintenance and power purchase replacement will lead to long-term power outage and reduced power supply, which will not only reduce the economic benefits of power supply enterprises, but also affect their reputation.
3.5 Impact on users
Unbalanced three-phase load and abnormal weight of one phase or two phases will definitely increase line voltage drop, reduce power quality and affect the use of consumer appliances.
Burnout of transformers, circuits and switchgear will affect the power supply of users, ranging from inconvenience to greater economic losses, such as the death of cultured animals and plants due to power failure, or being punished for not supplying power according to the contract. The burning of neutral wire may also cause a large number of low-voltage appliances to be burned by users.