1. Hazards to distribution transformer:
For the distribution transformer with the wiring group of yyno, during the three-phase four wire system wiring, the third harmonic current generates harmonic voltage in the original and auxiliary windings of the distribution transformer and causes the neutral point voltage to rise. In addition, because the odd harmonics have the same phase, their phasors on the neutral line are superimposed, so that the phasors of the zero sequence current on the neutral line are superimposed. Therefore, the medium linearity is heated due to the harmonic current.
During three-phase three wire connection, the ACSR Cable sequence harmonic forms the neutral point voltage due to the same phase, which changes the phase voltage.
For the distribution transformer with dyno wiring group, the zero sequence harmonic current forms a circulating current in the winding to heat the distribution transformer winding and reduce the service life of the distribution transformer.
2. Hazards to power cables:
Due to the high frequency rise of harmonic number and the larger cross-sectional area of cable conductor, the skin effect is more obvious, resulting in the increase of AC resistance of conductor and the reduction of allowable current of cable. In addition, the resistance of the cable and the inductive reactance of the line are connected in series with the system, and the capacitor for improving the power factor and the capacitive reactance of the line are connected in parallel with the system. A certain value shows that the inductance and capacitance may vibrate.
3. Hazards to power capacitors:
When there are harmonics in the power grid, when the capacitor is input, the terminal voltage increases, the current passing through the capacitor increases, and the loss power of the capacitor increases. The capacitive reactance of the capacitor is inversely proportional to the frequency. The higher the frequency, the smaller the impedance. Therefore, the harmonic voltage can greatly increase the current of the capacitor. If the allowable conditions of the capacitor are exceeded, the capacitor will be overcurrent and overload, resulting in abnormal heating. Especially when the capacitor is put into the power grid with distorted voltage, it may also aggravate the harmonic of the power grid, that is, harmonic expansion. Partial discharge is induced in the medium. Due to the large voltage change rate and high partial discharge intensity, it can accelerate the aging of the insulating medium, so as to shorten the service life of the capacitor（ http://www.diangon.com/ Copyright) generally speaking, for every 10% increase in voltage, the service life of the capacitor will be shortened by about 1 / 2. When the harmonic is matched with the parameters of the capacitor, it will produce resonance and make the harmonic square. When the harmonic is serious, it will cause bulging, breakdown, group explosion and greater loss of the capacitor.
4. Damage to incandescent lamps:
Incandescent bulb is a widely used lighting fixture. Its life is closely related to the thermal effect of harmonics. The AAC Cable of distortion coefficient shortens the life of bulb, and changing the fundamental voltage has a greater impact than changing the distortion coefficient.
5. Damage to motor:
The damage of harmonic to motor is mainly caused by additional loss, mechanical vibration, noise and harmonic overvoltage.
The distorted wave can increase the magnetic saturation of the motor core, especially the magnetic saturation of the core clamp winding, and reduce the fundamental short-circuit impedance. It is generally believed that the magnitude of the Nth harmonic current of the three-phase asynchronous motor can be calculated by the following formula:
6. Impact on electric energy meter:
(1) mechanical watt hour meter is an inductive mechanism. When measuring electric energy, it can be divided into the following three situations:
① The distorted wave power supply provides linear load. At this time, the electric energy meter measures the fundamental wave electric energy and some harmonic electric energy, which will deteriorate the performance of user equipment. In this way, users not only suffer from harmonic pollution, but also pay more electricity charges.
② The sine wave power supply supplies the nonlinear load. At this time, the electric energy meter measures the fundamental wave electric energy minus part of the harmonic electric energy. This shows that users pollute the power grid and pay less electricity bills.
③ The distorted wave power supply supplies nonlinear loads, which is more complicated. The fundamental current flows to the load, while the harmonic current is not necessarily. It may flow to the load or to the power supply, depending on the specific situation. The most direct method is to measure with an instrument.
⑵ the electronic watt hour meter uses an integral circuit, whether the power grid provides harmonic current to the load or negative current
The harmonic current provided by the load to the power grid will be integrated and measured together, which will cause measurement error.
7. Hazards to low voltage switchgear:
For the circuit breaker for power distribution, the full electromagnetic circuit breaker is easy to be affected by the harmonic current, which increases the iron consumption and generates heat. At the same time, it is difficult to trip due to the influence on the electromagnet and eddy current, and the higher the harmonic number, the greater the influence; The thermal magnetic circuit breaker generates heat due to the skin effect of conductor and the increase of iron consumption, resulting in the reduction of rated current and tripping current; For electronic circuit breakers, harmonics should also reduce their rated current, especially for electronic circuit breakers that detect peak values. AAAC Cable can be seen that the above three distribution circuit breakers may misoperate due to harmonics.
For the leakage circuit breaker, because the harmonic will affect the leakage current, the circuit breaker may be abnormally heated and misoperate or refuse to operate. For electromagnetic contactors, the harmonic current increases the temperature rise of magnet components, and the increase of coil temperature reduces the rated current. For the relay, the current should also be reduced due to the influence of harmonic current. They may cause misoperation during operation.
8. Interference to weak current system equipment:
For weak current equipment such as computer network, communication, cable TV, alarm and building automation, harmonics in power system are coupled to these systems through electromagnetic induction, electrostatic induction and conduction to produce interference. The coupling strength of electromagnetic induction and electrostatic induction is directly proportional to the interference frequency, and the conduction is coupled through the common grounding. A large amount of unbalanced current flows into the grounding electrode to interfere with the weak current system.
9. Impact on communication lines:
Generally, the working frequency of audio channel is about 200-400hz, and many harmonics of power supply system are within this frequency range. Because the power levels of power lines and communication lines are very different, the harmonics in the power supply system will cause perceptible, sometimes even unacceptable interference noise.
10. Impact on rectifier, inverter and logic control circuit:
Rectifier devices and thyristor circuits are widely used in various electrical equipment, ranging from rectifier and inverter devices for DC transmission, DC power supply for urban trams, to frequency conversion equipment, television, microcomputer, AC regulated power supply, battery charger and uninterruptible power supply (UPS), https://www.diangon.com/m234091.html On the one hand, these devices open and close different circuits according to a certain law, resulting in harmonic current injected into the power grid and become a harmonic source. On the other hand, the distorted waveform affects the normal operation of them and their loads. The distortion wave will cause the error of the control circuit, cause the ignition angular displacement, and the excessive current change rate and voltage change rate or overheating of the rectifier will cause equipment failure and damage to the rectifier components. These will bring harm to the converter. At the same time, the logic control circuit of some rectifier devices often leads to operation error due to distorted voltage.
11. Influence on fuse melt:
Overheating of the harmonic current in the melt will cause the displacement of its ampere second characteristic curve. Therefore, for the fault with low current, special attention should be paid to the selection of fuse to prevent accidental fusing and power failure, affecting the power supply.
12. Influence of harmonic skin effect:
The alternating current generates alternating magnetic flux through the conductor, and the alternating magnetic flux generates induced electromotive force with the conductor, so that the current density inside the conductor increases from the center to the outside along the conductor section, and the central current density is the smallest, which is commonly known as skin effect. Skin effect not only reduces the current carrying capacity of the conductor, but also increases the loss of the conductor, and this skin effect becomes more serious with the increase of current frequency. In general, the loss caused by skin effect is small at power frequency (50 Hz), while when the harmonic frequency is high, the loss and impact caused by skin effect such as harmonic with fifth harmonic frequency above 250 Hz are very significant. Therefore, the skin effect of harmonics should be considered when selecting conductors, and the conductor section should be increased appropriately.
13. Harmonics are also harmful to human body:
From the perspective of human physiology, when human cells are stimulated and excited, they will fluctuate rapidly or flip reversibly on the basis of cell membrane resting potential. If its frequency is close to the harmonic frequency, the electromagnetic radiation of power grid harmonic will directly affect human brain and heart.
Harm of harmonics to cables
As we all know, harmonic refers to the electric quantity whose frequency is an integral multiple of the fundamental wave contained in the current.
Generally, harmonic generation comes from three aspects: harmonic generation due to low quality of power generation, harmonic generation due to transmission and distribution system and harmonic generation due to electrical equipment. The third point is the main cause of harmonics, such as nonlinear loads, such as rectifier, switching power supply, UPS, frequency converter, inverter, etc. As shown in Figure 1, the harmonic generated by the equipment.
Figure 1 harmonics generated by equipment
The harmonics generated by the above three points have a serious impact on our electrical equipment. For the Xi’an Metro cable mentioned at the beginning of this article. Let’s focus on the harm of harmonics to cables.
Harmonic pollution will increase the dielectric loss, transmission loss, leakage current, temperature rise and partial discharge of dry-type cable, and increase the possibility of single-phase grounding fault.
Because the distributed capacitance of power cable can amplify the harmonic current, when the system load is low, the system voltage increases and the harmonic voltage increases accordingly. The higher the rated voltage level of the cable, the greater the risk of cable medium instability caused by harmonics, and the more prone to failure. As shown in Figure 2, it is cable combustion caused by harmonics.
Figure 2 cable combustion caused by harmonics
For the problem cables of Xi’an metro, the impact of harmonics will be more prominent. Unqualified cables and heating caused by harmonics are prone to accidents. This will be more serious in a relatively closed and crowded subway station. Of course, the problem with cable is not just about harmonics.
Therefore, for the safety of our transportation and other aspects, we should not only require from the transmission medium, but also check from the source of power quality. At this time, a device is needed to accurately capture and measure all parameters of power quality, and then analyze them to timely investigate possible potential safety hazards.