This article mainly introduces the design of the three-phase power filter and the design process of the three-phase power filter.
Power filterThe power filter is a filter circuit composed of capacitors, inductors and resistors, also known as "power EMI filter" or "EMI power filter", a passive two-way network, one end of which is the power supply and the other end is the load . The principle of the power filter is one kind-impedance adaptation network: the greater the impedance adaptation between the input and output sides of the power filter and the power and load sides, the more effective the attenuation of electromagnetic interference. The filter can effectively filter out the frequency point of a specific frequency in the power line or the frequency other than the frequency point to obtain a power signal of a specific frequency, or eliminate a power signal of a specific frequency.
The power filter is an electrical equipment that effectively filters out the frequency point of a specific frequency in the power line or the frequency other than the frequency point. The function of the power filter is to obtain a power signal of a specific frequency by connecting the power filter to the power line, or to eliminate the power signal of a specific frequency.
Using this characteristic of the power supply filter, a square wave group or compound noise after passing the power supply filter can be turned into a sine wave of a specific frequency.
High-power power filters such as Satons, UBS, inverters, etc. will generate a large amount of harmonic currents. This type of filter requires active power filter APF. APF can filter out the 2~50th harmonic current.
Three-phase power filter design This article mainly takes three-phase active power filter as an example to discuss related issues in the design process of three-phase power filter.
The three-phase active power filter can filter out the harmonics in real time, eliminate the harmonic currents in the nonlinear load, or eliminate the harmonic currents generated on the grid side, thereby effectively reducing the system voltage distortion rate; and realizing dynamic noise Power compensation can generate capacitive reactive power or inductive reactive power in time, which can effectively improve the power factor of the system; it can achieve the purpose of reducing consumption and energy saving, effectively reducing line loss and transformer loss, and can effectively alleviate the problem of equipment heating while extending Equipment application time, and to ensure stable and reliable operation of the power system. Three-phase active power filter has great practical significance to the development of modern power system, but the design level of three-phase active power filter is low. Therefore, discussing the design of three-phase active power filter has great significance for the effective operation of power system. Realistic meaning.
1. Brief Discussion on Three-phase Active Power Filter
1. Active power filter
Power electronic equipment and non-linear loads are now widely used. At this time, harmonic currents and reactive currents are injected into the power grid in large quantities, which threatens the operation and normal use of power grids and electrical equipment. Active power filter is an equipment device that dynamically suppresses harmonics and compensates for reactive power. This type of power electronic equipment can effectively compensate for frequency and size changes, harmonics, and reactive power. It is a very ideal device for compensating harmonics. Ideal equipment for compensating harmonics. The active power filter has extremely high controllability, its response speed is very fast, and it can track and compensate each harmonic and the required reactive power in time, and this characteristic will not be affected by the system, and it can be reasonably amplified without harmonics. Its volume and weight are small. The cost, technology, and capacity of a single active power filter are restricted. The research on large-capacity active power filters has achieved many achievements. Three-phase active power filters are widely used in power systems, but the structure of this method is very complicated. , The control procedure is also very cumbersome, and the design work is difficult.
2. Three-phase active power filter
The three-phase active power filter has many good performance characteristics, and the range of harmonics that can be filtered is very wide, usually 2-50 harmonics can be filtered, which can be filtered to remove characteristic harmonics, and can also filter non-characteristics Filtering; harmonic filtering efficiency is high, the harmonic current removal rate under rated power is 95%; the response speed is extremely fast, and the response time of harmonic compensation current exceeds 10MS; it can dynamically inject compensation current into the power system in a single phase, thereby Effectively improve the system three-phase imbalance problem; can automatically eliminate resonance to ensure the safe operation of facilities and equipment; can set the output and maximum 100% current limit output to ensure long-term stable operation of the equipment; have filtering and reactive power compensation functions , Such as inductive and capacitive, so as to achieve filtering or compensation functions; its operation interface is simple and convenient; the interface can display voltage, current, harmonics and other parameters in real time, and the menu settings are flexible and reasonable, and the harmonic elimination mode and reactive power compensation mode can be selected It can achieve simultaneous compensation mode with harmonic reactive power, and has target power factor and output current, and can record real-time faults in time. The design and selection are simple and reasonable, and installation, operation and maintenance are also very simple; 10 units can be extended in parallel operation; The protection measures are simple and complete, with system voltage overvoltage protection and undervoltage protection, output overcurrent protection, overheat protection, and control voltage undervoltage protection. Its working principle is mainly to detect the load current in the power grid in real time, so as to reasonably separate the harmonic current components, and issue a control command based on the magnitude of the harmonic current to achieve real-time generation of equal and opposite compensation currents, and inject the compensation current into the power grid. At this time, the purpose of instantaneous cancellation and filtering of harmonic currents can be achieved, and reactive power compensation can be achieved.
2. Three-phase active power filter main circuit parameter selection
The main circuit parameters of the three-phase active power filter and its system model will seriously affect the system control effect. Choosing appropriate main circuit parameters is critical to the stable operation of the filter and is also an important condition for effective control performance. Analysis of several aspects: 1. Selection of AC side inductance
The harmonics and transient currents in the command current of the active power filter are very critical, and the actual output current is required to have a good tracking ability. When the compensation object of the active power filter is determined, the main circuit parameter selection of the active power filter and its Performance and efficiency are closely related. The inductance passing low frequency and blocking high frequency current will cause the active filter compensation current to be harmonic currents. The inductance value design is closely related to the current tracking performance and its compensation effect. The main circuit parameters are mutually restricted, and the inductance value cannot be selected separately. Stable current tracking ability is an important part of harmonic compensation in active power filter operation, and is closely related to the final harmonic compensation effect. When the grid voltage and the filter DC side capacitor voltage are determined, the compensation current tracking effect is related to the main circuit power device. The switching frequency and the instantaneous rate of change of the compensation current are closely related.
(1) Device switching frequency
The switching frequency of the device is very important to the operation of the filter. The purpose of the active power filter is to generate harmonics. At this time, it is emphasized that the system has a higher current bandwidth. If the switching frequency is too low, it means that the main circuit current bandwidth is low, and it is difficult to output high-frequency current components. At this time, the system lacks the corresponding harmonic compensation capability, which leads to low compensation effects.
(2) Current waveform
As far as the current waveform is concerned, a low switching frequency will cause the ripple component in the compensation current to continue to increase, and the subsequent system loss will increase, and the current tracking effect will deteriorate accordingly; if the switching frequency is high, It will increase the switching power loss, thereby increasing the system loss, and the system efficiency will continue to decrease.
(3) Compensation current
The instantaneous rate of change of compensation current will face two contradictory issues. The main circuit in the active power filter should have a higher rate of change of compensation current to ensure that the compensation has a greater rate of change of current, even in non-linear loads. Generate compensation current to achieve the purpose of harmonic compensation. The compensation current waveform is closely related to the compensated load current waveform, and when the compensated load current change rate is large, the requirements for the filter will be higher; and the compensation current change rate should not be too high to adapt to the compensation current ripple The size requirements. If the rate of change in the compensation current is too large, the output current of the filter will be over-modulated, and a large ripple burr will appear in the compensation current, making the compensation effect ineffective. When designing the main circuit parameters, under the condition that the grid voltage and the filter DC capacitor voltage are determined, the filter change rate is generated by the AC side interface inductance. The mutual correlation and restrictive relationship between the main circuit parameters are very important, and the inductance parameters must be acceptable. Ensure that the system has high current bandwidth, dynamic performance, and low switching loss to ensure safe and stable operation of the filter.
(4) Series inductance
When designing series inductance, if the current crosses zero, the current change rate is large. At this time, the inductance must be small to better meet the requirements of fast tracking current; the output current ripple of the peak sinusoidal current is extremely serious, and at this time The inductance should be large to meet the requirements of the switch to suppress harmonic currents. 2. Selection of DC bus rated voltage
It is necessary to comprehensively analyze the working process of the three-phase active power filter under the control of the space vector, and determine the connection between the main circuit parameter selections, and not only pay attention to one of them. The DC side capacitor voltage of the three-phase active power filter will be limited due to external factors. At this time, the actual situation of the specific nonlinear load to be compensated should be comprehensively analyzed, and the DC side capacitor voltage should be reasonably selected. The selection of the DC side capacitor voltage of the active power filter is closely related to the specific compensated nonlinear load, and is inseparable from the problem of the AC side interface inductance and the grid voltage vector. The selection of the critical value of the capacitor voltage on the DC side of the active power filter and its compensation current are specific to the harmonic distribution characteristics, and the inductance parameters of the AC side interface are also closely related to it, and it is not a simple fixed value.
3. DC bus capacitor selection
The active power filter is the process of charging and discharging the DC side capacitor, during which the filter performance and the DC side capacitor voltage under steady-state operation should remain unchanged. The size of the capacitance will seriously affect the voltage fluctuation. If the capacitance is large, the voltage fluctuation is small, and the active filter has a good filtering effect; but if the capacitance is large, the cost will increase accordingly. The purpose of the DC side capacitor is to provide a corresponding voltage reference for the converter. The DC voltage must remain stable during the actual operation of the converter. The switching devices in the converter will experience losses due to operation. At this time, a stable DC voltage is very The key is to absorb active current in the system to achieve the purpose of maintaining the stability of the DC voltage. DC side capacitors are mostly used to stabilize voltage source converters, which are stable DC voltage sources. When negative sequence and harmonic currents are on the DC capacitor side, corresponding energy pulsations will occur, and due to the action of switches and active power filtering There are many problems with energy pulsation caused by inductive energy storage on the AC side of the converter, and these require reasonable buffering by the capacitor on the DC side.
4. Design of AC side output filter
The purpose of the active power filter is to deal with the power quality problems caused by the harmonic current caused by the nonlinear load in the power grid. The power electronic equipment can detect the load-side current to obtain the distorted current component, and then generate it with the inverter. The pulse width modulated wave is filtered through the output low-pass filter to remove the switching ripple, and then merged into the power grid to generate the corresponding compensation current and then injected into the power grid to fully offset the load harmonic current to ensure the power side current and sine wave Close, effectively improving the power quality of the power grid. There are harmonic currents of the 50th order and below in the output current, and all the harmonics above the 50th order should be filtered out. Usually LCL filter is used. The size of the inductor and capacitor of this filter is reasonable. When the filter is at high frequency, it can get good attenuation, and the system impedance has little effect on the output current.
Three, three-phase active power filter realization impact analysis
The realization of three-phase active power filter mainly includes harmonic analysis, current inner loop tracking control, DC bus voltage stabilization, and driving signal generation. If the bus voltage is lower than the set value, the active power filter can stably absorb energy in the power grid; if the bus voltage is higher than the set value, the active power filter will release the functional energy in the power grid to maintain reasonable The DC bus voltage is stable. Most of the active power filter drive signal generation is achieved by SPMW modulation, and the switching frequency should be 125kHz. This article analyzes the following analysis of the impact of three-phase active power filter implementation:
1. The impact of sudden load changes on the DC bus voltage
The full harmonic detection algorithm is implemented by reducing the output current of the pass filter by the load current. The inherent time delay of the low pass filter output is that it will cause the fundamental wave residue to be generated in the DC current when the load changes suddenly. When the load drops suddenly, the active power filter can absorb the corresponding active energy from the grid, prompting the active power filter's bus voltage to increase rapidly; otherwise, the bus voltage will suddenly drop. The new harmonic detection algorithm does not need to reduce the output of the filter by the load current. The harmonic command can be extracted in a simple way. There is no fundamental wave in the command current when the load changes suddenly, and it will not cause the DC bus of the active power filter. The voltage has a wide range of changes. The full harmonic algorithm can effectively maintain the stability of the DC bus voltage when the load changes suddenly, while the DC bus voltage fluctuates greatly, and the new harmonic algorithm has no fluctuations in the DC bus voltage. It can be said that the new harmonic algorithm is helpful to the stable operation of the system.
2. The influence of phase error on reactive current detection
The dp method under the instantaneous reactive power theory has good real-time performance and not much calculation. When the voltage in the power grid is distorted, it can accurately detect the harmonics and reactive currents in the current, and it can quickly detect the current . There will be a phase difference between the sine of the grid voltage and the sine of the coordinate change in the system, which will cause measurement errors in the detection results. Generally, the phase difference is caused by two reasons. One is that there is a negative sequence voltage between the asymmetry of the grid voltage. , Zero-crossing detection phase lock mode coordinate transformation The phase of positive and cosine signals is determined by the system voltage, which is also the same as the sum of positive sequence voltage and negative sequence voltage components, but the expected sine signal should be in phase with the positive sequence component; the second is The phase is shifted due to inaccurate phase lock. The phase difference of the sinusoidal component in the coordinate transformation causes errors in the fundamental active current detection, and at this time, the harmonic and reactive current cannot be detected correctly. The voltage when the load current distortion is asymmetrical is also unbalanced, or the error of its key link causes a large phase difference between them, and there are corresponding errors in harmonics and reactive currents, which will also seriously affect the control. The active component of the DC voltage also affects the phase difference. And this problem is dealt with by dp coordinate closed-loop phase-locking method, zero-crossing detection and correction phase-locking method, power supply voltage vector synchronization reference coordinate method and other methods.
3. The influence of low-pass filter on detection error
DC extraction is achieved through a low-pass filter. The performance of the filter will directly affect the detection error. It is usually expected to filter out all AC components to obtain pure DC. At this time, it is difficult for a low-pass filter to achieve pure DC filtering. Larger.
With the rapid development of the social economy, the power industry pays more attention to the importance of higher harmonic suppression and emphasizes the reduction of harmonic pollution. Many new active filter systems have also emerged at the same time, and the facilities and equipment are different, and the operation of the power system is also It is required to accurately and quickly compensate for grid harmonic currents or harmonic voltages. If the active filter needs to achieve such effects, the system circuit design must be scientific and reasonable, but this mostly depends on the design of the three-phase active power filter . Active power filters should have good voltage control functions and high-performance current control functions. The voltage control performance must have good steady-state accuracy, and safe start control with reasonable dynamic performance. This article briefly discusses the three-phase active power filter, discusses the selection of the main circuit parameters of the three-phase active power filter, and analyzes the impact of the realization of the three-phase active power filter, and is designed for the three-phase active power filter. Provide reference basis.
How to detect the quality of three-phase power filterConnect a signal generator, input sinusoidal alternating current, measure the voltage ratio between the DC component and the AC component in the input current through a filter and an oscilloscope. If the ratio is small, the filter is abnormal. If the ratio is large, it is the proportion of DC. Larger, it means that the filter has filtered the AC, that is, the filter is intact.
ConclusionThis is the end of the introduction about the design of the three-phase power filter. This article is intended to be used as a reference and I hope it will be helpful to you.
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