To put it simply, harmonics are the phenomenon that when a certain frequency of voltage or current acts on a non-linear load, a sinusoidal voltage or current of other frequencies different from the original frequency will be generated.

Ripple refers to the AC component superimposed on the DC stability in the DC voltage or current. Although they are not the same in concept, there is a connection between them. For example, the additional ripple on the power supply can easily produce harmonics of various frequencies on the electrical appliances; the existence of harmonics of each frequency in the power supply will undoubtedly lead to the increase of the ripple component in the power supply.

       In addition to the situation where we need to generate harmonics in the circuit, it mainly has the following main hazards：

        1. Cause resonance in the power grid to cause overcurrent or overvoltage to cause an accident;

        2. Increase additional losses and reduce the efficiency and utilization of power generation, transmission and power equipment;

        3. Make electrical equipment (such as rotating motors, capacitors, transformers, etc.) run abnormally, accelerate insulation aging, and shorten their service life;

        4. Make the relay protection, automatic device, computer system and many electrical equipment operate abnormally or fail to operate or operate normally;

        5. Make measurement and measuring instruments and meters unable to indicate or measure correctly;

        6. Interfere with the communication system, reduce the transmission quality of the signal, disrupt the normal transmission of the signal, and even damage the communication equipment.

       The harm of ripple:

       1. It is easy to produce harmonics on electrical appliances, and harmonics will cause more harm;

       2. Reduce the efficiency of the power supply;

       3. Strong ripple will cause surge voltage or current to be generated, resulting in burning of electrical appliances;

       4. It will interfere with the logical relationship of the digital circuit and affect its normal operation;

       5. It will cause noise interference and make the image equipment and audio equipment not work normally.

       In short, they are harmful when we don't need them, and we need to avoid them. There are many ways to suppress and remove harmonics and ripples, but it seems to be difficult to completely eliminate them. We only have to control them within an allowable range, and even if they do not affect the environment and equipment For our purpose.

       In recent years, the gradual increase of non-linear loads in the power grid is a common trend all over the world, such as variable frequency drives or thyristor rectified DC drive equipment, computers, uninterruptible power supplies (UPS) used by important loads, energy-saving fluorescent lamp systems, etc., these non-linear loads It will lead to 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 fluctuations, surges, harmonics, and three-phase imbalance.

       1. The hazards of power pollution

       Power pollution will cause serious harm to electrical equipment, mainly:

       Interfering with the normal operation of communication equipment, computer systems and other electronic equipment, causing data loss or crashes.

       Affect the performance of radio transmission systems, radar systems, nuclear magnetic resonance and other equipment, causing noise interference and image disorder.

       Causes the electrical automatic device to malfunction, or even a serious accident.

       Overheating of electrical equipment, increased vibration and noise, accelerated insulation aging, shortened service life, and even malfunctioned or burned.

       Cause the fluctuation (flicker) of the brightness of the light and affect the work efficiency.

       This leads to an increase in power loss in the power supply system.

       2.Types of power pollution

       2.1 Voltage fluctuation and flicker

       Voltage fluctuation refers to the peak value of multiple sine waves that exceed (below) the standard voltage value within a period of time, from half a cycle to several hundred cycles, that is, from 10MS to 2.5 seconds, including overvoltage fluctuations and undervoltage fluctuations. Ordinary lightning arresters and overvoltage protectors cannot eliminate overvoltage fluctuations at all, because they are used to eliminate transient pulses. Ordinary lightning arresters have a considerable resistance value during the voltage limiting action. Considering their rated heat capacity (Joule), these devices are easily burned out and cannot provide future protection functions. This situation is often easy to overlook, which is the main cause of failure or downtime of computers, control systems, and sensitive equipment.

       The other opposite situation is under-voltage fluctuation, which refers to the peak value of multiple sine waves, which is lower than the standard voltage value for a period of time, or as commonly said: shaking or falling. The long-term low-voltage situation may be caused by the power supply company or due to user overload. This situation may be an accident or planned arrangement. More serious is the loss of voltage, which is mostly caused by the opening and closing of heavy loads in the distribution network, such as the start and stop of large motors, central air-conditioning systems, electric arc furnaces, etc., as well as switching arcs, blown fuses, and tripping of circuit breakers. These are the common causes of voltage distortion.

       The frequent start-up of large-scale electrical equipment causes periodic voltage fluctuations, such as welding machines, punching machines, cranes, elevators, etc. These equipment require short-term impact power, mainly reactive power. Voltage fluctuations lead to unstable equipment power and lower product quality; light flicker causes eye fatigue and reduces work efficiency.

       2.2 Surge

       Surge impact refers to a short-term (low) voltage that occurs in the system, that is, a voltage instantaneous pulse with a time of no more than 1 millisecond. This pulse can be positive or negative, and can have a series or oscillating nature. They are also commonly called: spikes, gaps, interferences, glitches or mutations.

       Surge impact in the power grid can be caused by the switching of large-scale equipment (motors, capacitors, etc.) in the power grid or the opening of large thyristors, or the intrusion of external lightning waves. Surge impact can easily cause damage to electronic equipment components and electrical equipment insulation breakdown; at the same time, it can easily lead to computer and other equipment data errors or crashes.

       2.3 谐波

       Linear loads, such as purely resistive loads, have the same working current waveform as the sinusoidal waveform of the input voltage. Non-linear loads, such as chopper DC loads, have a non-sinusoidal waveform. The current/voltage of the traditional linear load only contains the fundamental wave (50Hz), with no or very small harmonic components, while the nonlinear load will generate considerable harmonics in the power system.

       Harmonic superimposes with the fundamental wave in the power system, causing waveform distortion. The degree of distortion depends on the frequency and amplitude of the harmonic current. Non-linear loads produce steep pulse-shaped currents instead of smooth sine wave currents. The harmonic currents in this pulse cause distortion of the grid voltage and form harmonic components, which in turn causes other loads connected to the grid to produce more harmonics. Wave current.

       The computer is one of such non-linear loads. Like most office electronic equipment, the computer is equipped with a diode/capacitor type power supply. This type of power supply only generates current at the peak of the AC sine wave voltage, so a large amount of it is generated. The third harmonic current (150Hz). Other devices that generate harmonic currents mainly include: motor frequency converters, solid state heaters, and other devices that generate non-sine wave currents.

       Fluorescent lighting systems are also an important source of harmonics. In ordinary electromagnetic rectifier lighting circuits, the typical value of the third harmonic is about 13%-20% of the fundamental (50Hz) value. In the electronic rectifier light circuit, the harmonic component is even as high as 80%.

       Harmonic currents generated by nonlinear loads will affect many working links of the power system, including transformers, neutral wires, motors, generators, and capacitors. Harmonic currents will cause a severe increase in the operating temperature (K parameter) of transformers, motors and backup generators. Overcurrent (caused by harmonics and unbalance) on the neutral wire will not only increase the temperature of the wire and cause insulation damage, but also generate a circulating current in the three-phase transformer coil, causing the transformer to overheat. The reactive power compensation capacitor will overheat due to the harmonic distortion of the grid voltage, and the harmonic will cause serious overcurrent;

       In addition, the capacitor will also form a resonant circuit with the inductive components in the power system, which will cause the voltage across the capacitor to increase significantly and cause serious failures. The starting capacitor of the lighting device is also very sensitive to overheating caused by high-frequency current. The frequent damage of the starting capacitor shows the influence of harmonics in the power grid. Harmonics can also cause the transmission efficiency of distribution lines to decrease, increase losses, and interfere with the work of power carrier communication systems, such as electrical energy management systems (EMS) and clock systems. Moreover, harmonics will also increase the measurement errors of power meters, active demand meters and watt-hour meters.

       2.4 Three-phase unbalance

       Three-phase unbalance will generate overcurrent on the neutral wire (caused by harmonics and unbalance), which will not only increase the temperature of the wire and cause insulation damage, but also generate circulating current in the three-phase transformer coil, causing the transformer to overheat, and even Cause a serious fire accident, etc.

       3.Power pollution control

       For the power pollution in the existing power supply network or the power grid to be built, it is necessary to conduct a careful analysis. There are usually two solutions: one is to partially reorganize the grid structure, and to separate or isolate the equipment that produces power pollution; the other is to use power purification and filtering For equipment to control, voltage harmonics are usually generated by current harmonics, and effective suppression of current harmonics will make the voltage distortion reach the required range. Many companies at home and abroad have begun to pay attention to the treatment of power pollution, investing in the installation of power purification and filtering devices, and have achieved the dual effects of improving power quality and energy saving.

       There are mainly the following methods to control power pollution: series reactor, active filter compensation, passive filter compensation, increase the number of phases of rectifier equipment, install various surge absorption protection devices, such as lightning arresters, etc.

       At present, passive filter compensation is the most practically used, effective, and low-cost solution. It includes three basic forms: series filtering, parallel filtering and low-pass filtering (series-parallel hybrid). Among them, the series filter is mainly suitable for the treatment of the third harmonic; the low-pass filter is mainly suitable for the treatment of the higher harmonics; the parallel filter is a comprehensive device that can filter out multiple harmonics and provide the reactive power of the system at the same time. It is the most widely used power purification filter device.

       In recent years, with the development of power electronics technology, active filter compensation technology has become increasingly mature and has been widely used. Compared with the traditional passive filter compensation system, it has the advantages of multiple functions, good adaptability and fast response speed. As the price continues to drop, the application will become increasingly common. The active filter compensation system has a very good application effect in many important places.

       The harm of unbalanced current

       Unbalanced currents between three phases are common in the power grid. Due to the existence of a large number of single-phase loads in urban civil power grids and agricultural power grids, the current imbalance between three phases is particularly serious. For the three-phase unbalanced current, there is almost no effective solution except to distribute the load as reasonably as possible. It is precisely because they can't find an effective way to solve the problem, so they are not valued by people, and few people conduct research.

       The unbalanced current in the power grid will increase the copper loss of the line and the transformer, increase the iron loss of the transformer, reduce the output of the transformer, and even affect the safe operation of the transformer. It will cause the three-phase voltage imbalance and thus reduce the quality of power supply, and even affect the electric energy meter. The accuracy caused by the measurement loss.

       Theoretical research proves that under the condition of outputting the same power, the copper loss of the transformer and the line is the smallest when the three-phase current is balanced, that is to say: the three-phase unbalance phenomenon increases the copper loss of the transformer and the line.

       The influence of unbalanced current on the copper loss of the system

       Suppose the total resistance of the three-phase circuit and transformer winding of a certain system is R. If the three-phase current is balanced, IA=100A, IB=100A, and IC=100A, then the total copper loss is=1002R+1002R+1002R=30000R.

       If the three-phase current is unbalanced, IA=50A, IB=100A, and IC=150A, the total copper loss=502R+1002R+1502R=35000R, which is an increase of 17% compared to the balanced copper loss.

       In a more serious state, if IA=0A, IB=150A, and IC=150A, the total copper loss=1502R+1502R=45000R, which is 50% more than the copper loss in the balanced state.

       In the most severe state, if IA=0A, IB=0A, and IC=300A, the total copper loss=3002R=90000R, which is 3 times the copper loss in the balanced state.

       The influence of unbalanced current on transformer

       The existing 10/0.4KV low-voltage distribution transformers are mostly Yyn0-connected three-phase three-leg core transformers. With this type of transformer, when the secondary side load is unbalanced and there is a neutral current, the neutral current is the zero sequence current, and on the primary side because there is no neutral lead wire, the zero sequence current cannot flow, so the zero sequence current Ampere-turn balance is not possible. For the core, there is an excitation zero-sequence current, which is controlled by the zero-sequence excitation impedance. According to the design of the magnetic circuit, this zero-sequence excitation impedance is large, and the zero-sequence current affects the symmetry of the phase voltage. , The neutral point will shift. According to calculations, when the neutral current is 25% of the rated current, the neutral point shift is about 7% of the rated voltage. Article 6.08 of the national standard GB50052-95 stipulates: "When a three-phase transformer of the Yyn0 connection group is selected, the current caused by the single-phase unbalanced load shall not exceed 25% of the rated current of the low-voltage winding, and the current of one of the phases shall not exceed the rated current value at full load." The above regulations limit the capacity of the single-phase load connected to the Yyn0 junction distribution transformer, and also affect the full utilization of the transformer equipment capacity. Moreover, for the three-phase three-column magnetic circuit, the zero sequence magnetic flux cannot be looped in the magnetic circuit, and a loop must be formed in the tank wall and fasteners, and the magnetic flux in the tank wall and fasteners will generate The large eddy current loss increases the iron loss of the transformer. When the zero-sequence current is too large and the zero-sequence magnetic flux is too large, some phase voltages will be too high due to the excessive drift of the neutral point, which will lead to the magnetic saturation of the iron core, which will increase the iron loss sharply. There will be accidents in which no one-phase current is overloaded but the transformer is damaged due to local overheating. Since the zero sequence magnetizing impedance of the distribution transformer of the Yyn0 connection group is relatively large, the zero line current will cause a large voltage change and form a relatively serious three-phase voltage imbalance, which not only affects single-phase users, but also affects three-phase users. The user's influence is greater.

       3 Harm of unbalanced three-phase load

       3.1 Impact on distribution transformers

       (1)The unbalanced three-phase load will increase the loss of the transformer:

       Transformer loss includes no-load loss and load loss. Under normal circumstances, the operating voltage of the transformer is basically unchanged, that is, the no-load loss is a constant. The load loss changes with the change of the transformer's operating load 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 loss of the three single-phase transformers.

       From the mathematical theorem, we know: assuming that the numbers of a, b, and c are all greater than or equal to zero, then a+b+c≥33√abc.

       When a=b=c, the algebra and a+b+c achieve 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, Qc=Ic2R, where Ia, Ib, and Ic are the transformer secondary load phase currents, and R is the transformer's phase resistance. Then the loss expression of the transformer is as follows:

       Qa+Qb+Qc≥33√〔(Ia2 R)(Ib2 R)(Ic2 R)〕

       It can be seen that when the load of the transformer remains unchanged, when Ia=Ib=Ic, that is, when the three-phase load reaches a balance, the loss of the transformer is the smallest.

       Then the transformer loss:

       When the transformer is in three-phase balanced operation, that is, when Ia=Ib=Ic=I, Qa+Qb+Qc=3I2R;

       When the transformer is operating at maximum unbalance, that is, Ia=3I, Ib=Ic=0, Qa=(3I)2R=9I2R=3(3I2R);

       That is, the loss at the time of maximum imbalance is 3 times that at the time of balance.

      (2)Unbalanced three-phase load may cause serious consequences of burning the transformer:

      When the above-mentioned unbalance, the heavy load phase current is too large (increased by 3 times), and the overload is too much, which may cause the winding and the transformer oil to overheat. Overheating of windings accelerates insulation aging; overheating of transformer oil causes deterioration of oil quality, rapidly reduces the insulation performance of the transformer, reduces the life of the transformer (every 8°C increase in temperature, the service life will be reduced by half), and even burns the windings.

      (3)The unbalanced operation of the three-phase load will cause the transformer zero sequence current to be too large, and the temperature rise of local metal parts will increase:

      The transformer under the unbalanced operation of the three-phase load will inevitably produce zero-sequence current, and the existence of zero-sequence current inside the transformer will produce zero-sequence magnetic flux in the iron core, and these zero-sequence magnetic fluxes will be on the tank wall of the transformer. Or other metal components form a loop. However, when the distribution transformer is designed, these metal components are not considered as magnetic components. The hysteresis and eddy current loss caused by this will cause these components to heat up, causing the temperature of the local metal parts of the transformer to rise abnormally, which will lead to accidents in the transformer operation in severe cases.

      3.2 Impact on high-voltage lines

      (1)Increase high-voltage line loss:

       When the three-phase load on the low-voltage side is balanced, the 6-10k V high-voltage side is also balanced. Set the current of each phase of the high-voltage line as I, and its power loss is： ΔP1 = 3I2R

       The three-phase load imbalance of the low-voltage power grid will be reflected to the high-voltage side. At the maximum imbalance, the high-voltage corresponding phase is 1.5I, the other two phases are both 0.75I, and the power loss is：

       ΔP2 = 2(0.75I)2R+(1.5I)2R = 3.375I2R =1.125(3I2R);

       That is, the power loss on the high-voltage line increases by 12.5%.

       (2)Increase the number of trips of high-voltage lines and reduce the service life of switchgear:

       We know that over-current faults on high-voltage lines account for a considerable proportion, and the reason is that the current is too large. The unbalanced three-phase load of the low-voltage power grid may cause excessive current in a certain phase of the high-voltage, resulting in a high-voltage line over-current tripping and blackout, causing a large-scale blackout. At the same time, frequent tripping of the switchgear of the substation will reduce the service life.

       3.3 Impact on power distribution panels and low-voltage lines

       (1)The unbalanced three-phase load will increase the line loss:

       Three-phase four-wire power supply line, the load is evenly distributed to the three phases, the current of each phase is I, the current of the neutral line is zero, and the power loss is： ΔP1 = 3I2R

       At the maximum unbalance, that is, one phase is 3I, the other two phases are zero, the neutral line current is also 3I, and the power loss is：

       ΔP2 = 2(3I)2R = 18I2R = 6(3I2R);

       That is, the power loss during the maximum imbalance is 6 times that of the balance. In other words, if 1200 kWh is lost per month during the maximum imbalance, then only 200 kWh is lost during the balance. This shows the loss reduction potential of adjusting the three-phase load.

       (2)Unbalanced three-phase load may cause serious consequences such as circuit breakage and switchgear damage：

       When the above-mentioned unbalance, the heavy load phase current is too large (increased by 3 times), and the overload is too much. Since the heating value Q=0.24I2Rt, and the current is increased by 3 times, the heating value is increased by 9 times, which may cause the temperature of the phase wire to rise linearly, which may lead to burning. And because the cross section of the neutral wire should generally be 50% of the cross section of the phase wire, but when choosing, some are often too small, and the quality of the joint is not good, which increases the resistance of the wire. The neutral wire is more likely to be blown.

       In the same way, on the power distribution panel, the heavy load phase of the switch will be burned out, and the heavy load phase of the contactor will be burned out, and the whole machine will be damaged.

       3.4 对供电企业的影响

       The power supply companies directly manage the households, and the low-voltage power grid has large losses, which will reduce the economic benefits of the power supply companies, and even cause the power supply companies to operate at a loss. Rural electrical workers contract line loss in the area, and the line loss is high. Agricultural electrical workers’ bonuses are deducted, and even their wages are not obtained. This will inevitably affect the mood of rural electrical workers. From the negative side of the work, to the crime in order to get money.

       Burned out transformers, broken lines, and burned switchgear will increase the cost of power supply for power supply companies. On the other hand, power outages and repairs and replacements will cause long-term power outages and reduce power supply. This will reduce the economic benefits of power supply companies and also affect them. The reputation of the power supply company.

       3.5 Impact on users

       The three-phase load is unbalanced and one or two phases are abnormally heavy, which will inevitably increase the voltage drop in the line, reduce the power quality, and affect the user's electrical appliances.

       Burned transformers, broken lines, and burned switchgear will affect the power supply of users, causing inconvenience to the light, and greater economic loss in the worst, such as the death of animals and plants that are cultivated due to power outages, or the failure to supply the goods in accordance with the contract is punished. The blown neutral line may also cause a large number of users to burn down the low-voltage electrical appliances.