Anti-interference technology to improve the functional quality of power grid
1 Introduction With the rapid development of power electronics (PE) technology, people have put forward higher and higher requirements for the reliability, safety and quality of power supply of power systems. However, there are a large number of non-linear loads and impact loads in the power grid, including chemical, metallurgical, coal mines and household appliances, especially high-power converter devices, thyristor rectifiers, electric arc furnaces, etc., resulting in transient shock and reactive power in the power grid. Problems such as higher harmonics and three-phase unbalance are becoming more and more serious, causing pollution to the power grid, increasing energy loss, and degrading the quality of power supply, which is not conducive to the safety and economic operation of power supply, supply and use equipment. In particular, the interference of higher harmonics has constituted a major "public hazard" affecting power quality in the current power grid. Therefore, solving the harmonic suppression and reactive power compensation of the power system and ensuring the quality of the power supply has become a hot topic of concern to everyone.
2 Hazards of higher harmonics and requirements of modern control systems The voltage output from three-phase alternators in power systems is basically sinusoidal, that is, there are approximately no DC and higher harmonic components in the waveform. In the case of a fundamental wave, it is a symmetrical component, and the sum of the three-phase vectors is zero, and no electromagnetic field is formed externally. However, the harmonic current component is not zero due to the sum of the three-phase vectors, and can form a strong magnetic field, which has various harmful effects on the power grid.
2.1 Influence on power quality The nonlinear load is a harmonic source that injects harmonic current components that are multiples of the fundamental frequency into the grid. These harmonic currents create harmonic voltage drops on the grid, causing waveform distortion of the grid voltage and current, resulting in degradation of power quality.
2.2 Influence on the distribution network In the non-ferrous metal conductor, the distribution of the fundamental current can be approximated to be uniform throughout the entire section. When the harmonic current is passed, since the skin effect current concentrates on the thin surface of the conductor surface, the resistance of the harmonic current loop is increased, and the effective resistance of the conductor is increased, resulting in an increase in power loss and energy loss of the power grid. Higher harmonics can also cause voltage resonance in the power system, causing high voltage on the line, possibly breaking through the insulation of the line equipment.
2.3 Influence on power factor of power system Since the actual power factor of the equipment is less than the power factor of the equipment under ideal conditions, the higher harmonics increase the power consumed by the power equipment and reduce the power factor of the system.
2.4 The requirements of the variable frequency speed control system The frequency converter of the variable frequency speed control transmission system becomes an important part of the AC transmission because of its high efficiency and energy saving characteristics, but the rectifier bridge of the frequency converter is a non-linear load to the power grid, and its inverter Most of them use PWM technology. When working in switching mode and switching at high speed, a large amount of coupling noise will be generated. The EMI is serious, causing the inverter to operate in a harsh electromagnetic environment. The voltage and current on the input and output sides are more. Higher harmonics. Therefore, the inverter should be operated to prevent external interference, and to prevent it from interfering with the outside world, that is, to achieve so-called electromagnetic compatibility (EMC).
2.5 Requirements for Modern AC Motor Control Systems As the topology of new PE converters continues to emerge, the required computational and control functions are greatly increased. With the development of high-voltage and large-capacity PE devices, the application of DSP (digital signal processor) control technology will become more and more extensive. However, the electromagnetic environment of PE systems and motor control systems tends to be complex, and because of the high operating frequency, the anti-jamming capability of DSP is usually weaker than that of microprocessors. Therefore, improving the anti-interference ability of DSP and peripheral circuits is closely related to ensuring the reliable operation of the system. The "purification" of the power grid is an important prerequisite for the development and application of modern PE systems and AC motor control systems.
3 Main indicators for suppressing higher harmonics
3.1 Installation of AC filter device (passive filter)
In the power distribution system, the traditional method of harmonic suppression and reactive power compensation is to connect the passive power filter in parallel with the non-linear load to be compensated, to provide a low-resistance path for the harmonics, and also to provide the required load. Power, this is the most common and practical method. The device utilizes an inductor and a capacitor energy storage element. According to the resonance principle, the higher harmonics that need to be eliminated are tuned by the filter circuit to cause resonance. In order to obtain the characteristic of minimum impedance at the time of resonance, the harmonics of the specified number of times are effectively eliminated, and the harmonic current is absorbed locally in the harmonic source accessory, so that it is not injected into the power grid. The device has the advantages of low investment, high efficiency and simple structure. It is reliable in operation and easy to maintain, and has low operating cost. It not only filters, but also performs reactive power compensation. Therefore, passive filters are an important means of suppressing harmonics and reactive power compensation that are widely used at present. However, the compensation characteristics of this method are affected by the impedance, frequency and operating conditions of the power grid. They can only suppress the fixed frequency harmonics of a certain number of times, and it is likely to amplify other subharmonics and overload the filter. Even burned. In addition, the LC filter circuit will cause a parallel resonance problem with the system due to changes in the system impedance parameters, with serious consequences and consequences.
3.2 Application of Active Power Filter APF is a new type of PE device that can dynamically suppress harmonics. The filtering method is: firstly detecting the harmonic current from the compensation object, and then using the controllable power semiconductor device (compensation device) to inject the harmonic component (I or U) of the harmonic source with the same amplitude and opposite phase. The harmonic component (I or U) makes the total harmonic of the power supply zero, achieving the purpose of real-time compensation of harmonics. It has been proved by experience that APF is an ideal and flexible feasible solution for suppressing harmonics and compensating for reactive power, which will be highlighted below.
4 Active Power Filter (APF)
APF is the most effective PE device for suppressing grid harmonics and compensating reactive power and improving grid power quality. Most APF topologies utilize voltage source inverters, and typically use capacitors as energy storage devices as shown in Figure 1. The DC voltage is converted to an AC voltage by appropriately strobing a controllable power semiconductor switch. Although a single pulse per half cycle can be applied to the composite AC voltage, pulse width modulation (PWM) is commonly used today for the dynamic performance required in most applications.
Figure 1 APF topology of a voltage source inverter
To generate an arbitrary waveform of AC voltage, PWM technology is applied to the voltage source inverter of the DC bus voltage chopping. There are many PWM technologies that can form a sine wave or an arbitrary waveform. Using PWM technology, not only can the inverter be controlled as a voltage source, but also can be used as a current source to control the AC output of the filter. Figure 2 shows the most common triangular carrier (TC) technique utilized. This is the simplest linear control method that compares the output current error of a fixed amplitude with a fixed triangle. The output voltage Va during the switching period is forced to be limited to the carrier period of Vcar and equal to the average amplitude of the modulated wave Varef. The resultant voltage of a sinusoidal modulated wave contains a sinusoidal fundamental component Va(f) and unwanted harmonic components, which can be minimized by using the highest possible frequency carrier, but this depends on the semiconductor power. The maximum switching frequency of the device (IGBT, GTO or IGCT).
Figure 2 PWM carrier technology (triangular carrier)
Compared with traditional passive LC filters, APF has the following advantages:
(1) It can effectively suppress each harmonic and fractional harmonics, and can improve the power factor;
(2) As a high harmonic current source, it is not affected by the system impedance;
(3) Without resonance, changes in the structure, impedance and frequency of the system will not affect the compensation effect;
(4) In principle, it is better than LC filter. It can complete the compensation of each harmonic and fundamental reactive power with one device, and can also suppress flicker. It has the characteristics of one machine and multi-energy, and the cost performance is reasonable;
(5) Even if the frequency of the higher harmonics changes, it can be accurately compensated;
(6) Since the device itself can complete the output limitation, it will not be overloaded when the amount of higher harmonics is increased. Its main feature is that it can track and compensate harmonics whose frequency and amplitude are changed, and the compensation characteristics are not affected by the system impedance, and have an adaptive function. At the same time, there is a better expected compensation effect on the varying reactive power. Therefore, APF is the preferred solution for harmonic suppression in the future.
APF can be roughly divided into parallel type and series type, series type is suitable for compensating voltage type harmonic source load; parallel type is only suitable for compensating current type harmonic source load. In addition, there is a parallel-serial combination of parallel and series, and in many cases it is also used in combination with passive LC filters for better overall results.
4.1 Parallel APF
Figure 3 shows the compensation current generated by the parallel APF. Three different modulation techniques of the current source inverter are used, which are periodic sampling (PS), triangular carrier (TC) and hysteresis control (HB). The PS method is a power transistor switching operation of the APF when a square wave clock shift of a fixed (sampling) frequency is performed. The TC method uses a triangular wave and a higher harmonic to compare the switching states of the inverter at different times. This method has a faster response, but the switching frequency is not fixed and high, resulting in noise and large switching loss and high frequency distortion. The HB rule is given an allowable tolerance band, as long as the magnitude of the higher harmonics exceeds this tolerance band, the inverter switches. However, the switching frequency, loss and control accuracy are affected by the tolerance band width. The smaller the tolerance bandwidth, the higher the control accuracy. Of course, the switching frequency and switching losses are also increased. In general, HB can obtain better control performance, and it is widely used because of its combination of quick response and simple operation. Figure 3 shows that the HB method is the best method for this particular waveform and application, but the TC method is better when sinusoidal waveforms are required.
Figure 3 Current waveforms under different modulation techniques
A parallel APF with an auto-controlled DC bus has a topology similar to a static compensator (STATCOM), which is used to compensate reactive power in a power system. Parallel APFs compensate for harmonics of the load current by injecting equal and opposite harmonic currents. At this time, the parallel APF operates as a current source that injects harmonic components, and the harmonic components are generated by the load, but have been phase shifted by 180o. Figure 4 is a wiring diagram of the parallel APF, and Figure 5 shows the working principle of the APF compensation. In order to output the IF of the filter current waveform shown in Fig. 5, the control circuit of Fig. 4 needs to be set to generate the Vfab pulse width modulation voltage map shown in Fig. 6.
Figure 4 Topology of a parallel APF
Figure 5 generated filter current used to compensate load current harmonics
In Figure 5: is the grid current;
iL is the load current;
iF is the compensation current;
iL = fundamental component iL1 + higher harmonic component iLh + reactive component iLq.
In Figure 5, each current satisfies the relationship: is=iF+iL. If the compensation current iF=-iLh-iLq provided by the APF, then is=iL1, that is, the grid current only contains the fundamental component, which acts as a filter. Parallel APF is mainly suitable for the cancellation of current-type nonlinear load harmonic current and compensation for reactive power and three-phase unbalance.
Figure 6 shows the current waveform and PWM voltage for compensating load harmonics.
4.2 Series APF
The tandem APF was not referenced to the grid from 1980 and works primarily as a voltage regulator and a harmonic isolator between the grid and the non-linear load. Figure 7 is a wiring diagram of the series APF. The APF is connected in series between the power supply and the load through a matching transformer to eliminate voltage harmonics, balance or adjust the terminal voltage of the load, and ensure the voltage quality of the user's power supply, especially suitable for compensating for AC. Voltage imbalance and voltage sags in power and low power applications. Because there is no need for energy storage (battery), the total amount of components is small, which is more economical and effective for UPS. The series APF injects a voltage component in series with the power supply voltage, so it can be regarded as a controllable voltage source, compensating for voltage sags and convexities on the load side. However, the series APF loss is large, and various protection circuits are also complicated, so they are rarely used alone. It is often used to form a hybrid APF with a passive LC filter network. As shown in Figure 7, the passive LC filter is connected in parallel with the load. The series APF works like the same harmonic isolator, forcing the load current harmonics to circulate mainly through the passive filter without passing through the power distribution system. The advantage of this solution is that the rated power of the series APF is only a small fraction of the load rating (KVA), typically 5%. However, in voltage compensation, the apparent power rating of the series APF may increase. Fig. 8 is a schematic diagram showing the operation of the series filter to compensate the load side voltage harmonics. Series APF can also be used to combat interference from fundamental voltages. Figure 9 shows the effect of the series APF when the supply voltage drops by chance. As shown in Fig. 8, the load voltage is almost constant, and only a small instability and oscillation occur at the beginning and the end of the power supply voltage drop.
Figure 7 Series APF topology with parallel passive filter
Figure 8 Filter voltage for compensating for voltage disturbances
Figure 9 Series APF compensation function when power supply voltage is disturbed
4.3 String-Parallel APF
The serial-parallel APF is a combination of a series APF and a parallel APF, and FIG. 10 is its combined topology. Parallel APF is configured on the load side and can be used to compensate for load harmonics. The series APF is placed on the power supply side to provide harmonic filtering. This topology is also known as the Universal APF or Unified Power Quality Regulator (UPQC). The series section compensates for power supply voltage harmonics and voltage imbalance, acts as a latching filter for harmonics, and suppresses oscillations in the power system. The parallel section compensates for load current harmonics, reactive power, and load current imbalance. In addition, it regulates the capacitor voltage of the DC line. The power supplied or absorbed by the parallel sections is the power required by the series compensator and the power required to compensate for the losses. The main problem of this type of APF is that the control is complicated and the cost is high.
Figure 10 Unified Power Quality Conditioner (UPQC)
4.4 New Topology Using Multi-Level Inverters Multi-level inverters that have been in the research phase have recently been used in APF topologies. Figure 11 shows parallel APFs with three-level inverters, today in most inverters. In the field of application, three-level inverters have become more and more popular, such as power factor compensators. The advantage of the multi-level converter is that it reduces the harmonic content generated by the APF. Compared with ordinary inverters, it can output more (>2 levels) voltage, which is beneficial to reduce the harmonics generated by the filter itself. Another advantage is the ability to reduce the rating of the semiconductor voltage or current and to reduce the switching frequency required.
Figure 11 Parallel APF with 3-level inverter
Multi-level inverters should be able to establish multiple levels of voltage, so the quality of the output voltage is better. The "H" type converter based on multi-segment connection is equipped with three different DC voltage sources, which is the latest method to generate many levels of voltage. With this technology, a very good voltage waveform can be obtained with only a few series converters. At the same time of pulse width modulation, it can adjust the amplitude. As shown in Fig. 12, the amplitude modulation of the 81-level voltage can be generated only by four "H" converters per phase (four-stage inverter), so that the APF characteristic of "no harmonics" can be realized. Figure 13 shows the "4-stage 81-stage" shunt active power filter completed in the laboratory.
5 Conclusion
Anti-interference technology using active power filter (APF) is extremely important for suppressing high-order harmonics of power systems and improving power quality. The paper introduces the anti-jamming technology using active power filter (APF), and lists various novel and practical topologies. With the rapid development of power electronics (PE) technology, this technology is still developing.
Figure 12 "4-stage 81-level" inverter (single-phase) capable of amplitude modulation
Figure 13 Parallel Active Power Filter
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