Preface
Currently, computers, green lighting gas discharge lighting source, inverter air conditioning, inverter washing machines and so on popularized to tens of thousands of households, intelligent buildings abound, to people's living, learning, office to create a comfortable, efficient, fast living environment, this kind of to bring many benefits to people. Energy-saving, intelligent products are more or less by the combination of non-linear loads, making the low-voltage distribution network of non-linear load capacity is increasing the proportion of the resulting low-voltage distribution network voltage distortion and harmonic currents on the power grid, non-linear equipment itself, with the network of other power-using equipment hazards, some serious impact on the safety of the equipment operation, greatly shorten the service life of the equipment, the intelligent network system is paralyzed, Even cause a fire.
The harmonic hazards of power supply and distribution networks and the urgent need to prevent and control in the field of engineering design has been commonly recognized, and in the national, industry norms and standards gradually through mandatory standards to limit the size of the harmonic current, in the project power supply and distribution design should be taken in the harmonic prevention and control measures to ensure that the power distribution network equipment is not due to the harmonic hazards of the serious consequences.
1 harmonic analysis
1.1 harmonic mechanism and harm
In the ideal power system, voltage and voltage are pure sinusoidal wave. In fact, when the current flows through the power supply and the applied voltage is not a linear relationship between the load, the formation of non-sinusoidal current. These nonlinear loads are collectively known as harmonic source loads.
Period of the non-sinusoidal current Fourier analysis, you will get a series of different frequencies of the superposition of sinusoidal current, in addition to the fundamental frequency of the power, its frequency for the fundamental frequency of the integer multiple of the current is called harmonic.
The number of harmonics is the ratio of the harmonic frequency to the fundamental frequency (n=fn/f1), e.g. 150Hz is called the 3rd harmonic and 250Hz is called the 5th harmonic ......
Harmonic currents can cause problems within the power supply system as well as within the device, and the hazards are manifested in the following ways: amplification of harmonic currents causing resonance, voltage distortion is too high; over-zero noise; increase the neutral line current, causing fire; transformer and induction motor overheating, jeopardizing the normal operation of the motor and transformer; circuit breaker malfunction; correct power factor capacitor overload damage; skin effect, harmonic resistance is greater than the fundamental wave resistance, increasing the loss of power supply lines; damage to some sensitive electronic equipment, resulting in the loss of communication data.
1.2 Harmonic analysis of various industries
The author has long been engaged in the engineering field of electrical design work, through the design of the project back to visit, and for harmonic problems in the basic data collection, in the various power equipment suppliers and production unit equipment management personnel *** with the efforts of seasonal, multi-temporal units of the main power supply power supply point of the current waveform data acquisition and analysis of the various industries, the harmonic characteristics have a basic understanding. Harmonic characteristics have a basic understanding of the industry distribution network harmonic sources and harmonic characteristics of the performance is as follows:
1.2.1 Office buildings
Currently, the degree of intelligence of office buildings is increasingly high, the number of non-linear loads in the power equipment and the proportion of electrical equipment are rapidly increasing, the power quality of the city grid caused by the pollution is becoming increasingly serious.
Inside these large amounts of automation equipment, common non-linear loads are: video display equipment (CRT and LCD type display equipment), computer equipment, air conditioning, various types of energy-saving lighting equipment (fluorescent lamps, a variety of high-pressure discharge lamps, dimmable lamps, etc.), office class of electrical equipment (printers, copiers, scanners, projectors, etc.), the speed drive (inverter pumps, air conditioning compressors, large elevators). compressors, large elevators). All of these will become the harmonic distortion disturbance source of the low-voltage power supply system.
The power distribution in the building is characterized by many single-phase equipments and high proportion of non-linear loads, which causes power quality problems:
1) A large number of zero-sequence third harmonic currents are injected into the grid, which makes the third harmonic voltage of the LV power supply bus exceed the limit value of the national standard, and affects the safe and normal operation of LV power equipments, especially the harmonic-sensitive equipments.
2) Neutral line on the current is too high (zero sequence current will be superimposed on the neutral line, mainly the third harmonic current), which leads to the neutral line heat, line loss increases, and the neutral line in the design of the selected conductor is thin, so it is easy to cause a fire due to high temperature.
The harmonic characteristics of office buildings are shown in the following table:
Harmonics main number of times Harmonic content (THDi)
3, 5, 7 Pollution procedure in general: 10% to 15%
Pollution procedure in serious: 15% to 20%
1.2.2 Hospitals
Medical equipment is the focus of the harmonic sources: mainly NMR*** * vibration equipment and gas pedals.
Nuclear magnetic *** vibration equipment: the spectrum of harmonics is very complex, the typical spectrum is concentrated in the 3rd, 5th, 7th and 9th, and the harmonic spectrum ranges from about 3 to 43rd.
Accelerators: the typical spectrum is concentrated in the 3rd, 5th, 7th, 9th, 11th and 13th, the spectrum is wide, and the harmonic spectrum ranges from about 3 to 49th.
The test of a hospital's equipment, analyzed by the test data found that: gas pedal, X-ray machine, gastrointestinal machine and other equipment to produce THDi about 50% to 60%; CT (computed tomography), magnetic *** vibration, DSA (digital subtraction angiography machine), etc., in about 30%; electronic testing equipment, operating room, gamma knife, etc., in the 10% to 15%; frequency conversion equipment at about 35%, etc.
Harmonic currents in hospital power distribution systems are characterized as follows:
1) The harmonic currents generated by harmonic sources have a wide frequency spectrum.
2) The harmonic current distortion rate is very high and the natural power factor of the equipment is very high.
3) There is a lot of electronic and medical equipment in hospitals, which are sensitive to harmonics.
4) A large number of harmonic currents and harmonic voltages can affect the normal operation of relay protection equipment in the distribution system and the continuity of power supply in the distribution system.
The harmonic characteristics of medical buildings are shown in the following table:
Harmonics main number Harmonic content (THDi)
3, 5, 7, 9 Pollution program in general: 15% to 20%
Pollution program in serious: 20% to 25%
1.2.3 Communication room
From the safeguarding of the communication power in the use of high-reliability equipment, UPS is an indispensable part. In today's large and medium-sized UPS production, more 6-pulse thyristor rectifier UPS design solutions. UPS can provide the load with a regulated voltage and high precision, frequency stabilization, waveform distortion of small high-quality power.
The use of UPS and DC switching power supply in the communication room, on the one hand, a good solution to the voltage instability, power supply continuity and other general users are very concerned about the power quality problems, but due to the UPS and DC switching power supply on the input side of the rectifier circuits, so the two have become the main nonlinear loads in the power grid.
The main power quality problems are:
1) Generation of a large amount of harmonic currents, which pollute the grid and at the same time may transfer harmonics to the IT equipment, causing a very large interference or even harm to the communication system. Harmonics are very sensitive to these systems, there may be, for example, computer system paralysis or dead, stagnation, distortion and system control malfunction, which will cause a lot of damage. And it should be especially pointed out that harmonic interference hides the disaster of the collapse of the whole system, which can happen at any time.