Active power filter
Active power filters are a new type of dedicated equipment for power harmonic control, made using modern power electronics technology and digital signal processing technology based on high-speed DSP devices.
It consists of two main parts: a command current calculation circuit and a compensation current generation circuit. The command current calculation circuit monitors the current in the line in real time and converts the analog current signal into a digital signal, which is then sent to a high-speed digital signal processor (DSP) for processing. The DSP separates the harmonics from the fundamental frequency and sends a drive pulse to the compensation current generation circuit in the form of a pulse width modulation (PWM) signal. This drives the IGBT or IPM power module to generate a compensation current with the same amplitude but opposite polarity as the grid harmonic current, which is injected into the grid to compensate for or cancel the harmonic current and actively eliminate power harmonics.
How are harmonics generated?
Harmonics are generated when a sinusoidal voltage is applied to a nonlinear load, causing the current to become non-sinusoidal. This non-sinusoidal current creates a voltage drop across the grid impedance, resulting in a non-sinusoidal voltage waveform as well. Currently, most loads widely used in industries such as communications, semiconductors, petrochemicals, chemical fibers, steel furnaces, and automobile manufacturing are nonlinear loads, including variable frequency drives, rectifiers, uninterruptible power supplies, switching power supplies, electric arc furnaces, welding equipment, computers, elevators, variable frequency air conditioners, energy-saving lamps, and photocopiers. The large amount of harmonic current generated by these nonlinear loads flows into the power grid, causing voltage waveform distortion. This harmonic pollution poses a serious threat to the power grid and users.
The dangers of harmonics
1. Causes overheating of cables, motors, and transformers, leading to a reduction in their service life or damage;
2. Damage to sensitive equipment, leading to production or experimental interruptions and causing significant losses;
3. Causes circuit breaker malfunction and regional shutdown incidents;
4. Causing capacitor overload or damage due to malfunction;
5. This can lead to a large current in the neutral line, causing system failure.
6. Induced grid resonance;
7. Harmonics present in the power grid will reduce power supply efficiency;
8. If the harmonic pollution level is too high, the power supply department will not approve the grid connection.
To put it simply, in one sentence: APF appears in rectifiers, frequency converters, UPS, primary power supplies, and other similar applications.
What is SVG?
SVG stands for Static Var Generator, also known as a high-voltage dynamic reactive power compensation device or static synchronous compensator. It refers to a device that uses a self-commutated power semiconductor bridge converter to perform dynamic reactive power compensation. SVG is currently the best solution in the field of reactive power control. Compared to traditional methods such as synchronous condensers, capacitor banks, and traditional SVCs (Synchronous Dynamic Var Compensators) mainly represented by thyristor-controlled reactors (TCRs), SVG has unparalleled advantages.
Connections and differences
the difference
SVG (Static Var Compensator) is a device used to improve the power factor of a system; while active power filters (most commercially available active power filters are connected in parallel) are harmonic compensation devices used to eliminate harmonics within the system. This is the most important difference between the two.
connect
The two can be used together to compensate for reactive power and suppress harmonic currents in the system.
In simple terms, APF primarily functions as a filter, while SVG primarily compensates for reactive power. If harmonics are present, an SVG needs to be paired with a filter. The advantage of SVG is that its capacity can be relatively large. APFs are typically available in capacities of 50A, 75A, and 100A. APFs place higher demands on the system. However, if the system requires reactive power compensation, then more precise calculations and simulations are necessary.
passive filter
This circuit mainly consists of passive components R, L, and C.
passive filter device
This device consists of passive components such as capacitors, reactors, and sometimes resistors, to form a low-impedance path for a certain harmonic or higher harmonics, thereby suppressing higher harmonics. Since the adjustment range of the SVC needs to be expanded from the inductive region to the capacitive region, the filter is connected in parallel with the dynamically controlled reactor. This satisfies both reactive power compensation and power factor improvement, while also eliminating the influence of higher harmonics.
The types of filters widely used internationally include: single-tuned filters of various orders, double-tuned filters, second-order wideband and third-order wideband high-pass filters, etc.
Passive filtering generally refers to using inductors and capacitors to create a low-impedance parallel path (tuned filtering) for a specific harmonic current, thus preventing it from flowing into the system. The advantages of passive filtering include low cost, stable operation, relatively mature technology, and large capacity. The disadvantages are that the harmonic filtering efficiency is generally only 80%, and the reactive power compensation for the fundamental frequency is also limited.
Differences and connections between active and passive filters
Active filters are composed of integrated operational amplifiers (op-amps) and resistors (R) and capacitors (C). They offer advantages such as no inductor required, small size, and light weight. Integrated op-amps have high open-loop voltage gain and input impedance, and low output resistance. When used in active filter circuits, they also provide voltage amplification and buffering. However, integrated op-amps have limited bandwidth, making it difficult to achieve very high operating frequencies in current active filter circuits.
Active power filters are themselves harmonic sources! Relying on power electronic devices, they generate a set of harmonic vectors with equal amplitude but opposite phase to the system harmonics upon detection. This cancels out the system harmonics, transforming them into a sinusoidal waveform. Besides filtering harmonics, active power filters can also dynamically compensate for reactive power. Their advantages include rapid response, harmonic filtering exceeding 95%, and meticulous reactive power compensation. Disadvantages include high cost and small capacity. Due to the current immaturity of large-capacity silicon valve technology internationally, the capacity of commonly used active power filters currently does not exceed 600 kvar. Their operational reliability is also lower than that of passive filters.
Currently, in applications requiring large capacity and detailed compensation, a hybrid active and passive system is generally used, where the passive system performs large-capacity filtering and compensation, while the active system performs fine-tuning.
In principle, active filters can achieve very high Q values, but excessively high Q values are not conducive to stability. If the characteristic curve of an active filter is not good enough, it may be due to insufficient bandwidth of the op-amp you are using. In principle, the characteristics of an active or passive filter should be consistent. The main issue is fabrication.
Passive RC filters are not the same as active RC filters. Both active RC and passive LC filters can implement the Bottworth function, but implementing this function with passive RC is far from ideal, as its minimum attenuation value is extremely high (this point is little known). Therefore, passive RC functions are generally not used as filter approximation functions.
Moreover, calculations show that the quality factor of a passive low-pass second-order filter is very low, reaching a maximum of 0.5, but this is not achievable at all frequencies.
Although passive filters have advantages such as low investment, high efficiency, simple structure and convenient maintenance, and are widely used in power distribution networks at present, their characteristics are greatly affected by system parameters. They can only eliminate certain harmonics, while amplifying some harmonics and even causing resonance phenomena. With the development of power electronics technology, people have gradually shifted the research direction of filtering to active power filters (APF).
An APF (Automatic Power Filter) uses controllable power semiconductor devices to inject a current into the power grid with the same amplitude but opposite phase to the harmonic source current, thereby reducing the total harmonic current of the power supply to zero and achieving real-time harmonic current compensation. Compared with passive filters, it has the following characteristics:
1. It can not only compensate for harmonics, but also suppress flicker and compensate for reactive power. It has the characteristics of multiple functions and is relatively reasonable in terms of cost performance.
2. The filtering characteristics are not affected by system impedance, etc., which can eliminate the risk of resonance with system impedance;
3. It has an adaptive function and can automatically track and compensate for changing harmonics, which means it has the characteristics of high controllability and fast response.
Passive filter circuit: If the filter circuit consists only of passive components (resistors, capacitors, inductors).
Active filter circuit: If the filter circuit is composed not only of passive components, but also of active components (bipolar transistors, unipolar transistors, integrated operational amplifiers).
In simple terms, active circuits require components to have a power supply. Passive circuits, on the other hand, do not require a power supply.
Here, "source" can be understood as "power source".
In a passive circuit, the signal will eventually decay to zero without external signal supplementation. An active element is defined as a component that can provide an external circuit with an average power greater than zero, and this average power can last for an infinitely long time; conversely, it is a passive element. In this case, the source can be understood as the signal source provided by an active element.
Working principle of active filters
The current on the DC line is collected by a current transformer, sampled by an A/D converter, and processed by a harmonic separation algorithm to obtain a harmonic reference signal. This signal is used as the modulation signal for the PWM (Pulse Width Modulation) and compared with a triangular wave to obtain a switching signal. This switching signal is used to control the IGBT single-phase bridge. According to the principle of PWM technology, by reversing the switching signals of the upper and lower bridge arms, a harmonic current of equal magnitude but opposite direction to the harmonic signal on the line can be obtained, thus canceling out the harmonic current on the line. This is the feedforward control part. The harmonic components of the line current after the active filter connection point are then fed back as the input to the regulator to adjust the error of the feedforward control.
Passive/Active Comparison Table
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