How to specify or buy shunt active power filter-EE Times Asia

2021-11-22 04:59:49 By : Mr. Jeremy CH

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APF is one of the fastest-growing technologies to solve power quality issues and meet grid codes and energy efficiency requirements.

Most of today’s installations face a large number of power quality issues and challenges to comply with grid codes and energy efficiency requirements because they are not designed to support non-linear, unbalanced and variable loads and generators that make up a large proportion of modern electricity of. Power Systems.

These problems and challenges stem from variable speed drives (VSD), welding machines, transformers, furnaces, lighting, double conversion UPS systems, high dynamic loads, single-phase loads, battery chargers, railway electrification systems, fossil fuel generators, and renewable energy And other equipment. Generate sources, just to name a few. Events such as capacitor switching (from existing capacitor banks or passive harmonic filters), automatic reclosing operations of transmission and distribution lines, or starting of large motors can also cause these problems and challenges.

Active power filter (APF) is one of the fastest-growing power electronic technologies used to solve power quality problems and meet the grid specifications and energy efficiency requirements of various market segments and applications. Modern APFs can be controlled as current or voltage generators. They can be connected in parallel or in series with the power system. Parallel APFs are called parallel APFs or current-fed APFs, and they are the most widely used APFs today. APF connected in series is also called a voltage-fed APF. Parallel APF based on voltage source converter (VSC) is by far the most common type due to its well-known topology, reasonable cost and simple installation procedure.

Traditional solutions such as capacitor banks, shunt reactors, and passive harmonic filters composed of inductance, capacitance, and resistance elements do not always respond correctly to the dynamic characteristics required by modern power systems. Although the structure is simple and sometimes highly cost-effective depending on the application, they inherit some shortcomings.

Parallel APF was developed and commercialized as an economical solution to solve all these shortcomings and provide performance independent of the characteristics of the power system, while being able to meet the real-time requirements of modern loads and generators.

Figure 1: The basic architecture of offloading APF.

A shunt APF is a device that acts as a controlled current source and can provide any type of current waveform in real time. They are equipped with energy storage elements, IGBT bridges and control systems, enabling the equipment to inject the required current into the power system. They can be installed at any point of the system (low pressure or high pressure) in parallel with equipment that causes problems or needs to meet certain requirements. Their operation has nothing to do with the network impedance, the curve form of the current to be compensated, and the quality of the power supply voltage.

Over the years, the shunt APF has been designed to provide specific functions. They can compensate current-based distortions, such as current harmonics and current imbalances, and voltage-based distortions, such as voltage harmonics, voltage fluctuations, voltage changes, and voltage imbalances.

They can also support the development of clean energy through power factor correction and reduction of energy loss in the power system.

Custom APF solutions currently available on the market include active harmonic filters (AHF), static reactive power generators (SVG), active load balancers (ALB) and two special designs, hybrid reactive power compensators ( HVC) and low harmonic drive (LHD).

Figure 2: Types of shunt APF.

2. Specify or purchase shunt active power filter

When specifying or purchasing a parallel APF for low or high voltage applications, several factors should be carefully considered. The following factors are some of the most important factors.

In principle, parallel APF can correct a variety of power quality problems, such as harmonic distortion (any phase sequence), fundamental frequency reactive power (non-unit displacement factor), negative sequence fundamental component (unbalanced component), zero-sequence Basic components (neutral current) and voltage fluctuations.

Parallel APF can also support the development of clean energy through power factor correction and reduction of energy loss in the power system.

It is easy to think that just adding control functions to the basic parallel APF inverter and control system can solve all kinds of power quality problems and grid code requirements. However, each corrective action will affect the volt-ampere rating of the inverter, thereby increasing the cost of the equipment. Since the energy flow represented by the instantaneous power peak may be very large, the compensation of imbalance and voltage fluctuation also has an impact on the DC side energy storage.

It is important to study the application in detail and clearly define the functions required by the APF to deal with the questions or requirements raised by the end user. According to the required function, the correct type of shunt APF can be specified or selected.

Figure 3: Typical functions that APF can provide.

Some power quality phenomena happen very quickly, and some grid codes and energy efficiency requirements require real-time response. It is important to evaluate the response time and overall response time of the APF to ensure that the device meets the needs of the application.

Response speed plays an important role in determining the control principles implemented in the required APF. Generally speaking, the cost of any particular APF is related to the response speed of the implementation.

Figure 4: Typical transient response of SVG.

Shunt APFs provide a variety of voltages, the most common being 200 V to 690 V, because they are built using low voltage IGBT switches. Some manufacturers provide devices that can be directly connected to the power system within this voltage range.

Some other manufacturers design the APF for a certain voltage level (usually 400 V), and then connect the APF to the 200-690 V power system by using a step-down or step-up transformer. This approach increases the scale, cost, and loss of the entire solution.

Figure 5: A typical direct low voltage connection for APF.

A suitable step-up transformer can be used to connect the APF to a high voltage (over 1 kV) system. In these cases, the current transformer that provides the current signal is located near the high-voltage equipment that needs to be compensated.

Figure 6: A typical high voltage connection for APF.

When using transformers to connect APFs, these transformers should be carefully studied during the project design phase. As the impedance between the APF and the power system increases, the step-up or step-down transformer may reduce the compensation performance.

Most modern parallel APFs are based on the latest VSC topology, the 3-level Neutral Point Clamped (NPC) inverter topology. Compared with the APF based on the traditional 2-level topology, it has many advantages.

In the three-level topology, the switching frequency and voltage stress are distributed between the IGBT switches to obtain better output voltage spectrum performance. The reduced stress can extend the service life of power electronic equipment. It also achieves higher efficiency, smaller output current ripple, lower loss, lower noise level, as well as a more compact design and smaller layout. These advantages greatly reduce the total cost of ownership (TCO).

Figure 7: Comparison of efficiency between 3-level NPC inverter topology and 2-level.

Streaming APF using Industrial Internet of Things (IIoT) technology can accurately and consistently capture and transmit data in real time. APF's adoption of IIoT benefits from the increased availability and affordability of sensors and processors. IIoT allows machine learning and big data technology to be integrated into devices.

Figure 8: IIoT calculation requirements for offloaded APF.

Some manufacturers offer the possibility to connect all APFs on the site through a web-based architecture. Then, the operator can overview the status of all connected APFs and record them. This makes it possible to record events that may cause production disturbances and to monitor individual APFs.

The market's demand for smart devices and wireless connection technologies for industrial equipment continues to grow. Many APFs on the market provide the possibility of remote asset connection, big data processing and analysis by using the IIoT software platform. This can improve the operational efficiency of the facility through predictive maintenance and remote management (for example, improve uptime and asset utilization).

APF provides instantaneous, continuous, stepless and seamless output that is not affected by grid voltage fluctuations. Their capacity and rated output can be selected according to the needs of the application.

This is a big difference compared to traditional solutions such as capacitor banks, shunt reactors or passive harmonic filters, which are usually too large to better adapt to the changing needs of the equipment that must be compensated . Another shortcoming of these traditional solutions is that when their output is injected into the system in steps of a certain size, they constantly overcompensate and undercompensate the power system.

Choosing an APF that can provide the precise output required by the application can help reduce the cost of the overall solution. Modular equipment with output as low as SVG/-30 kvar or AHF 25 A can be found on the market to meet the needs of modern buildings or irrigation systems. You can also find modular equipment with output up to SVG/-150 kvar or AHF 200 A to meet the needs of modern manufacturing plants or renewable power plants.

The increased sensitivity of most facilities and processes to power quality issues makes the availability of high-quality power a key factor in the development of power systems. Ensuring complete system redundancy is a major issue in many applications today, especially for critical process industries and critical process facilities.

A very safe design to ensure system redundancy is to use a modular APF with an independent controller design (main/main arrangement). With this design, if any module of the APF fails, the remaining modules will continue to operate without damaging the equipment or interrupting the process.

Figure 9: Redundant modular shunt APF.

In some countries/regions, there are strict guidelines regarding EMC. To ensure that APF does not cause any interference, it must be equipped with appropriately designed EMC filters. The typical EMC standards required by APF are IEC 61000-6-2 (Immunity) and IEC 61000-6-4 (Emission).

Harmonics can be seen in odd and even harmonics. The common compensation capabilities of APF on the market can reduce up to 50 harmonics (odd and even). Sometimes, some manufacturers claim to be able to reduce the harmonic order of 51 times or more, but it is of little value because these harmonic orders do not cause problems or usually appear in the power system.

An important function that APF can provide is to select the harmonic order to be compensated. For some devices, the entire harmonic spectrum (2nd to 50th, odd and even) can be selected, but for some other devices, only a few harmonic orders can be selected. Depending on the application, the ability to compensate for a specific harmonic order is a key issue that affects the performance of the entire system.

Derating according to harmonic order

The rated value of APF is usually defined as the rated load (50/60 Hz). As the APF further increases the harmonics, its capacity begins to be derated compared to the nominal value. For example, the 13th harmonic derating by 50% means that an APF with a current output rating of 100 A can only compensate for the 13th harmonic by 50 A.

The derating depends on the robustness of the APF design. This ability depends more on the rate of change of the current, not just the frequency and amplitude of the current (all different frequencies, amplitudes and phases will have an effect). Therefore, the derating curve cannot show the capability of a certain APF. The only way to verify the actual compensation capability of the equipment is to check its di/dt capacity. Compared with 2-level devices, this compensation capability is significantly better in 3-level NPC inverter topology APF.

Interharmonics are usually caused by synchronization problems or the operation of equipment such as cycloconverters, induction furnaces, or certain wind turbine generators. If the installation includes interharmonic sources, the manufacturer should be consulted, as not all APFs can handle them.

Most parallel APF suppliers offer multiple installation options:

The modular APF design enables end users to adapt to future power quality and energy efficiency improvement requirements or potential changes in grid code requirements. The modular design means that additional capacity can be easily added to the APF's capacity within the existing configuration, thereby saving cost and space.

Depending on its design and topology, APF can have higher or lower losses. It is important to check for losses, because low losses will reduce the life cycle cost (LCC) of the investment.

Generally, the loss of APF is about 2-3% (depending on the rated power). The APF based on the 3-level NPC inverter topology has lower losses than the 2-level. According to the user's situation, the loss means that if the LCC is calculated within a few years, it is possible to save a lot of money.

Figure 11: Loss comparison between 3-level NPC inverter topology and 2-level.

APF has different HMI settings. Some provide a very simple interface, while others have a built-in power quality analyzer to calculate the required compensation, which includes graphs showing current and voltage waveforms and many additional features in different languages. For any HMI, a huge added value is that it can be connected to any IIoT software platform.

Without the right tools, APF debugging and service can be very time-consuming. Some vendors provide software for this. The minimum required function should be the system to perform voltage and CT phase sequence self-check, CT polarity check, self-diagnosis and self-calibration. This type of feature will detect installation errors before problems occur and will reduce debugging time. If APF does not have this type of software, debugging will become more complicated and external support may be required, which will increase system costs.

Modern APF has a variety of built-in protection functions to ensure safe and reliable operation under abnormal system conditions. Some of the most common built-in protection features are:

Control of detuning capacitor bank

APF is usually installed on site together with existing or new contactor or thyristor switch detuning capacitor bank. Some APF vendors offer the possibility to control the steps of these groups directly from APF's control system through dedicated digital outputs in APF. By doing so, APF's comprehensive power quality monitoring and reporting functions can be used to accurately monitor all parameters of the equipment and manage overall power quality improvement needs.

Together with the best system integration, this feature brings high-efficiency operation, control system cost savings, and the possibility of using existing or new detuned capacitor banks to construct a hybrid reactive power compensator (HVC).

Pedro Esteban is the Asia Pacific Director of Merus Power Plc. Since 2002, he has extensive global experience in sustainable energy innovation and transformation, including renewable energy, power electronic solutions, energy storage, microgrid and its smart grid integration. He has served as a leading expert in various marketing, strategic planning, business development and communications positions at Areva Transmission and Distribution, Alstom Grid and General Electric. He has been based in Singapore since 2012.

To contact Pedro, you can contact him at or

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