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.
GB/T 14549—93 harmonics of power quality in public power grid. The standard stipulates the allowable injection values of harmonics of different voltage levels and the limit value of harmonic voltage (phase voltage) of public power grid. Commonly used filters can be roughly divided into the following seven types:
Monotone harmonic filter
Single tuned filter is the most widely used type with narrow passband, good filtering effect, low loss and easy tuning.
Double tuned filter
Double-tuned filter can replace two single-tuned filters, and only one reactor (L 1) is needed to bear all the impulse voltage, but the wiring is complicated and the tuning is difficult, so it is only used in EHV systems.
First-order high pass filter
The first-order high-pass filter is generally not used because of the large fundamental loss.
Second-order high pass filter
The second-order high-pass filter has wide bandwidth, good filtering effect, adjustable resonance point and curve sharpness, and can prevent accidental vibration and amplification. Therefore, the second-order wide passband is also used as a low-order filter.
Third-order high pass filter
The third-order high-pass filter is usually filtered by electric arc furnace.
"C" type high-pass filter
The "C" type high-pass filter is used for electric arc furnace filtering, which is especially effective for the second harmonic.
passive filter
The traditional method of installing harmonic compensation device is to use LC tuning filter. This method can compensate both harmonics and reactive power, and its structure is simple, so it has been widely used. The main disadvantage of this method is that the compensation characteristics are affected by the impedance of the power grid and the operating state, and it is easy to have parallel resonance with the system, which leads to harmonic amplification and LC filter overload or even burning. In addition, it can only compensate harmonics with fixed frequency, and the compensation effect is not ideal.
Passive filters, also known as LC filters, can be divided into single-tuned filters, double-tuned filters and high-pass filters. In practical application, several groups of single-tuned filters and high-pass filters are often used to form a filtering device. Single tuned filter is also called single tuned filter circuit, which is mainly composed of controller, capacitor, reactor, switch, control circuit and protection circuit.
Whether it is high pressure or low pressure, it is the same. Single-tuned filter is very popular and widely used because of its few components and low cost.
The main voltage levels of low-voltage LC filter are 400V, 660V, 1000V, which mainly depends on the user's voltage level.
High-voltage filtering generally refers to the voltage classes of 6KV, 10 KV and 35 KV. Generally speaking, the main filtering is in 6 KV and 10 KV systems.
The difference between high-voltage filter and low-voltage filter is mainly that the components used are different in withstand voltage, withstand different currents, require different safety distances, and have different design and manufacturing difficulties.
The filtered current also depends on the harmonic current of the system to be filtered.
The number of harmonics filtered is different according to the actual situation of each project, which is generally 20% ~ 50% of the original harmonic content of the system. According to the specific situation of the project, several groups of filters can be set up, and the filtering effect can reach more than 70% of the original harmonic content, but this requires more efforts in the protection circuit, which is relatively complicated.
In a word, the final result of filtering is to make the harmonic content of the system meet the requirements of national standards or users' requirements for harmonics. As we know, capacitors compensate reactive power, and we also use capacitors in filter circuits. Its function is to filter at harmonic frequency, but to compensate reactive power at fundamental frequency. Therefore, the filter capacitor plays the role of reactive power compensation at the fundamental frequency.
Low voltage compensation filter device
The design of low-voltage filter compensation device should follow the relevant provisions of the state. Generally speaking, the low-voltage filter compensation device is installed in the cabinet. The design of the filter device and the evaluation of the effect mainly depend on whether the effect really filters out harmonics and whether it follows the national standards on harmonics. Each cabinet shell can be installed with 2~5 circuits, depending on the specific situation. 9.4, high voltage filter compensation device
The high-voltage filter compensation device can be installed in the cabinet or frame. LC single-tuned filter circuit is composed of filter capacitor and filter reactor, which can filter out certain harmonics. The reactor can be an iron core, and according to the applied power, the reactor can also be an air-core reactor. Because it is high voltage, it must comply with the relevant regulations of the state on high voltage devices. The difference between MKP and MPP technology lies in the connection mode of power capacitors in compensation system.
MKP(MKF MKK) capacitor
This technology is to directly plate metal on polypropylene film. Its size is smaller than that of the capacitor adopting MPP technology. Due to the low requirements on production technology, its manufacturing and raw material costs are much lower than MPP technology. MKP is the most common capacitor technology, and it has more advantages because of its miniaturization design and dielectric capacity.
MPP( MKV) capacitor
MPP technology uses cardboard coated with metal on both sides as electrode and polypropylene film as medium. This makes it larger in size than the capacitor using MKP technology. The production is very accurate, because vacuum drying technology must be used to remove all residual moisture in the capacitor winding, and insulating oil must be filled in the cavity. The main advantage of this technology is high temperature resistance.
self-healing
Both types of capacitors are self-healing. In the process of self-healing, the energy stored in the capacitor will produce a small arc at the fault perforation point. The arc will evaporate the fine metal near the perforation point, thus restoring the complete isolation of the medium. In the process of self-healing, the effective area of the capacitor will not be reduced to any practical extent. Each capacitor is equipped with an overvoltage breaker to prevent electrical or thermal overload. This test conforms to VDE 560 and IEC 70 and 70A standards.
Development of capacitors
Until about 1978, dielectric injection technology including PCB was still used to manufacture power capacitors. Later, it was found that PCB was toxic, and this toxic gas would be released when it was burned. These capacitors are no longer allowed and must be disposed of. They must be sent to a special waste incinerator or buried in a safe place.
The power loss of the capacitor with PCB is about 30 W/kvar. The capacitor itself is made of metal-plated cardboard.
Because the use of this kind of capacitor is forbidden, a new capacitor technology has been developed. In order to meet the requirements of energy-saving trend, the development of small power capacitors has become the goal of efforts.
The new capacitor is made by drying process or injecting a small amount of oil (vegetable oil). Using metallized plastic film instead of metallized cardboard fully embodies its environmental protection characteristics, and the power consumption is only 0.3 W/kvar. This shows that the power consumption is reduced to1100 after improvement. These capacitors are developed according to the conventional power grid conditions. In the process of energy crisis, people began to study the phase control technology. The result of phase control is to pollute the power grid and produce new faults.
Because the previous generation capacitor has high self-inductance, high-frequency current and voltage (harmonics) cannot be absorbed, while the new capacitor will absorb more harmonics.
Therefore, it is possible that when the old and new capacitors work on the same bus, the operating conditions and service life will show great differences, and it is also possible that the new capacitor will be damaged in a short time due to the above reasons.
Great changes have taken place in power grid conditions, so it is more and more important to choose the correct capacitor technology. The service life of capacitors will be shortened due to the following factors:
Harmonic load is +0.
9.5.4.2, higher grid voltage.
9.5.4.3, high ambient temperature.
The harmonic load in our distribution system is increasing. In the foreseeable future, only the combined reactance compensation system may be suitable for use.
Many power supply companies stipulate that only compensation systems with live reactance can be installed. Other companies must follow their rules.
If the user decides to continue to use the reactance-free compensation system, he should at least choose a capacitor with a higher rated voltage. This kind of capacitor can bear higher harmonic load, but it can't avoid resonance accident. Application occasions of power electronic devices
Today, many production processes are unimaginable without power electronic devices. At least the following electrical equipment is used in each factory:
9.6. 1. 1, lighting control system (brightness level)
9.6. 1.2, switching power supply (computer, TV)
9.6. 1.3, motor speed control equipment
9.6. 1.4, self-inductance saturated core
9.6. 1.5 uninterruptible power supply
9.6. 1.6, rectifier
9.6. 1.7 welding equipment
9.6. 1.8, electric arc furnace
9.6. 1.9, machine tool (numerical control)
9.6. 1. 10, electronic control mechanism
9. 6. 1. 1. 1 EDM machine
All these nonlinear electrical devices will produce harmonics, which will lead to the failure of the distribution system itself or the equipment connected to the system.
It may be wrong to consider only the root cause of equipment failure in power plants where the failure phenomenon occurs. Faults may also be caused by harmonics generated by neighboring factories that affect the public distribution network.
Before installing the power factor compensation system, it is very important to test the distribution system to determine the system structure suitable for you.
Tunable filter circuit and combined filter are well-known methods to solve harmonic problems. Another method is to use dynamic active filter.
9.6.2 Basic terms
9.6.2. 1, carrier (AF)
It is a high-frequency signal attached to the grid voltage, which is used to control street lamps, HT/NT conversion systems and energy storage heaters at night.
9.6.2.2 sum carrier (AF) detection circuit.
An element consisting of a primary choke and a parallel resonant circuit (secondary choke and capacitor) in parallel. The AF phase-locked circuit is used to detect the AF signal loaded by the power supply department.
9.6.2.3 reactance
The choke coil is connected in series in the capacitor circuit.
9.6.2.4, reactance coefficient
Percentage of choke inductance X L relative to capacitance inductance x c.
For example, the standard reactance coefficients are 5.5%, 7% and 14%.
Combined filter
Two loops with different reactance coefficients are connected in parallel to detect the clutter signal, which is used to clean the power grid quality at low cost. Cosφ power factor represents the phase difference between current and voltage. Inductive and capacitive cosφφ explains the quality characteristics of power supply. Cosφφ φ can be used to describe the reactive power component in the power grid.
fourier analysis
A non-sinusoidal function can be decomposed into its harmonic components by Fourier analysis. The waveform at the sinusoidal frequency ω 0 is called the fundamental component. Waveforms with a frequency of n ×ω 0 are called harmonic components.
9.6.5. Tuned Harmonic Absorber
A resonant circuit is composed of a choke coil and a capacitor in series, and the impedance to harmonic current is minimal after tuning. Tuned resonant circuit is used to accurately eliminate the main harmonic components in distribution network.
9.6.6. Untuned Harmonic Absorber
A resonant circuit consisting of a choke and a capacitor in series is tuned to a frequency lower than the lowest harmonic to prevent resonance.