【 Weak current College 】 on building electronic equipment of harmonic problems 】

Summary: building components and equipment has become a low-voltage power grid in harmonic source. This article in accordance with the test data to several groups, the most common electronic devices and components in overtone and impact analysis, propose appropriate measures.

Keywords: Harmonic problems solutions for electronic equipment

With the community and building intelligent rise and popularity of the information processing technology, electronic computer, color television and electronic energy-saving lighting and other electronic devices and components have broad access to our learning, work and life. These components and equipment belonging to the non-linear load, in large amounts of concentrated use of buildings or residential area, the nonlinear resulting harmonic currents, if not control, makes the LV network of voltage and current waveforms to produce distortion, impact energy quality.
First, the electronic equipment of harmonic phenomenon and reasons
Electronic devices are generally the rectifier, only near Peak AC voltage, rectifiers are conduction with input current. Because a period breakover time is very short, it is essential to maintain the equipment normal work-current, so the input current is pulse shape. This pulse shape input current for fundamental content in small, but the harmonic content and operating current increases, the amplitude of the pulse, forming a serious distortion of current injection low voltage network, become impossible to ignore the harmonic source.
Electronic computers and TV content of harmonic currents, total harmonic distortion rate high. This high load harmonic currents in load using peak into low voltage network, will result in voltage and current THD rate increases, the impact on power quality, if you exceed the limit value, the standard also may cause harm.
According to the relevant information, in the household appliances (mainly television) set used by the residents of the community,on the quality of voltage and low voltage network have a noticeable effect. The peak load, voltage, total aberration and 3 times, 5 harmonics have met or exceeded the limits of national regulations, but also further increase the trend.
Second, the harmonic to the influence of the electric power system equipment
Harmonics making power waveform contamination, deterioration of the quality of power supply, additional losses increased transmission capacity, is the public power grid. Its systems and equipment of main performance in several areas.
1. on transformers and motors, harmonic voltage increase the core Eddy current losses, harmonic currents increase the copper loss, temperature rise, the accelerated ageing of insulation, reducing the efficiency and utilization, to shorten the service life. At present in order to suppress the usual 3-Harmonic Dyn11 wiring of transformers, make 3 harmonics in Delta Connection windings in circulation, try not to inject the power grid. It should be noted that when the harmonic content is large, the circulation may also cause overheating transformer windings.
2. the role of the harmonic voltage, the capacitor will generate additional power loss, accelerate the ageing of insulation. More seriously, a large number of harmonic currents is likely to cause capacitors and other components between parallel resonance or series resonance, resulting in a harmonic current amplification and harmonic voltage increase. The danger of harmonic overvoltage and overcurrent, would not only damage the capacitor overload, would make and capacitor joins distribution loop of all lines, equipment for flicker overpressure overload and damage. According to statistics, over 70 per cent of harmonic fault occurs in the capacitor.
3. on power cables and distribution lines, harmonic current frequency increase caused obvious skin effect, conductor resistance increases, line losses and increase the heating, insulation, premature aging, prone to groundfault, resulting in a potential fire hazard. At the same time, the harmonic balance between the three phase 3 load n line current increase significantly. In the main distribution circuit load is large amounts of concentrated use of electronic computers and massive adoption of electronic energy-saving gas lighting, N line current even reached phase line current twice, causing N lines overheat, burned, or even cause fire.
4. distribution circuit for harmonic current content makes the circuit breaker trip capacity reduction. This is because the distorted current zero-crossing, the arc current over time rate than current power frequency sine, arc voltage recovery much more quickly, so that the arc easily renewed. The fact that air electromagnetic circuit breaker does not trip its breaking capacity range waveform distortion rate exceeds 50 per cent of the fault current, also causes damage to the circuit breaker.
5. harmonic on power system relaying, measuring instruments and communication system equipment, signal interference and damage.
III. national limits for harmonic standards
In order to guarantee the harmonic pollution, power grid and electrical equipment for safe and economic operation, in recent years the country has developed a series of electromagnetic compatibility and safety of national standards, harmonic limits stated requirements. In the power quality public harmonics (GB/T14549-93) 0.38KV low voltage network harmonic voltage and harmonic current limits provided in table 3, table 4:
The implementation of these standards for electronic equipment production and testing, the design as well as for supply and distribution of electricity which provides monitoring and management.
IV. measures to reduce the influence of harmonics
1. in civil construction and low voltage distribution design, especially for electricity load for single-phase electricity supply of electric power distribution line, neutral (N) section should not be less than the cross-sectional area of the line. But on a large number of centralized use computers, televisions and other electronic equipment supply, TN system distribution circuit of N (PEN) line of the section should not be less than the line section 2 times to increase N line ampacity to avoid overloading the fever and damaged wire.
2. on the application of electronic equipment and components for more distribution line protection, should use a neutral switch electrical overcurrent protection, and should be appropriate to increase breaker breaking capacity, to prevent the short-circuit breaking capacity is insufficient due to damage to the switch and the device.
3. to prevent power capacitor on the amplification of harmonics, thus causing resonance or over current settings on the capacitor should note the following: ① appropriate adjustments to the installation location of the capacitor to change the network parameters. ② under possible number of harmonic resonance, determine the capacity of the capacitor, or adjust the capacity capacitor grouping to avoid resonance point. ③ in capacitor circuit-ShiWhen the reactor, the limits for harmonic current capacitor slip road. For example, to limit 3 ~ 5 harmonic current, you can install the equivalent of capacitor 4%-6% of series reactors.
4. installation of the system of parallel AC filter. AC filter with passive and active, since civil building load type changes, electronic equipment produced by the harmonic number relatively fixed, therefore the use of passive filter.
On the low frequency (13) the number of harmonics, due to the low content is large, you can set a single frequency of single-tuner passive filter to filter out. Single-tuner filter by capacitors in series harmonic reactor components, basic principle is the need to filter out frequency harmonics as ideal tuning point in this frequency filter produces the series resonance, the formation of low resistance pathway to absorb most of harmonic currents.
On a higher frequency (13 and over) harmonic because of their travel, optional a common high-pass filter to filter out. The most common high-pass filter is a second-order high-pass filter, reactor, resistor and capacitor hybrid connection. On a certain time (13) Harmonic frequencies above the harmonic, filter impedance is a less than its resistance value of low resistance pathway, so that the number of times higher harmonic currents are effectively absorbed.
Now some manufacturers (Nokia, Shenzhen Hai Yi da, etc.) can provide a source filter. The basic principle of the APF as a current source, and load harmonic source parallel to extremely fast response speed, send and load harmonic current amplitude value equality, the same phase, the current in the opposite direction, so that they offset each other, the supply side of total harmonic currents to zero. Filter can also be compensation of reactive power and electricity-phase asymmetry. As a result of higher prices, reimbursement capacity is small (single compensating current 100A), applies only to supply the high quality requirements (e.g. important buildings of the central monitoring system, computer system, etc) sites.
5. strengthen electronic production management, inspection and supervision, encourage manufacturers to adopt active power factor correction, and other new technologies, producing low harmonic value of electronic products. From the source to the harmonic pollution control, this is the most fundamental measures.