16 factors to consider when specifying or purchasing a shunt active power filter [Part 2/2: Conclusion]

After the introduction of active power filters, this second article will continue discussing the factors to consider when specifying or purchasing a shunt active power filter for low or high voltage applications.

6- Rated output

APFs offer an instantaneous, continuous, stepless and seamless output that is not affected by grid voltage fluctuation. Their capacity and rated output can be selected to be exactly what the application requires.?

This is a big difference compared with conventional solutions like capacitor banks, shunt reactors or passive harmonic filters that are usually oversized to better adjust to changing demands of the equipment that has to be compensated. Another disadvantage of these conventional solutions is that they continuously over and undercompensate the electric power system as their output is injected into the system in steps of a certain size.

Selecting an APF that delivers the exact output demanded by the application helps reducing the costs of the whole solution. In the market can be found modular devices with an output as small as +/-30 kvar for SVGs or 25 A for AHFs to adjust to the needs of modern buildings or water irrigation systems for example. There can also be found modular devices with an output as large as +/-150 kvar for SVGs or 200 A for AHFs to adjust to the needs of modern manufacturing plants or renewable generation plants for example.

SVG output compared to conventional solutions

7- Controller and redundancy

The increased sensitivity of most facilities and processes to power quality problems turns the availability of electric power with good quality a crucial factor for the development of the electric power system. Ensuring complete system redundancy is a major issue in many applications nowadays, especially for critical process industries and critical process facilities.

A very safe design that ensures system redundancy is to use modular type APFs with an independent controller design (master/master arrangement). With this design, if any module of the APF fails, the rest will continue in operation without damaging equipment or interrupting processes.

Redundant modular type shunt APF

8- Electromagnetic compatibility (EMC)

In some countries there are strict guidelines regarding EMC. To be sure that the APF is not causing any interference it must be fitted with a properly designed EMC filter. Typical EMC standards required for APFs are IEC 61000-6-2 (immunity) and IEC 61000-6-4 (emission).

9- Harmonic compensation capacity

Harmonics can be seen in the odd and even harmonic orders. Common compensation capacity for APFs in the market is being able to mitigate up to the 50th harmonic order (odd and even). Sometimes there is a claim by some manufacturers of being able to mitigate the 51st harmonic order or above, which has little value as these harmonic orders do not cause problems or usually appear in electric power systems.

An important feature that APFs can offer is the possibility of selecting which harmonic order to compensate. For some devices, it is possible to select the whole harmonic spectrum (2nd to 50th, odd and even), but for some others only few harmonic orders can be selected. Depending on the application, the capacity to compensate a certain harmonic order is a critical issue affecting the performance of the whole system.

10- Derating according to harmonic order

The rating of an APF is usually defined at nominal load (at 50/60 Hz). As the APF works further up the harmonics its capacity compared to nominal starts to derate. For example, a derating of 50% at the 13th harmonic order means that a APF with a current output rating of 100 A has only the capability to compensate 50 A at the 13th harmonic order.

Derating is a matter of how robustly the APF is designed. This capability is more dependent on the change rate of the current than just the frequency and magnitude of the current (all different frequencies, their magnitude and their phase have an effect). Because of this, a derating curve cannot show the capability of a certain APF. The only way to verify the real compensating capability of a device is to check its di/dt capacity. This compensating capability is clearly better in 3-level NPC inverter topology APFs compared to 2-level devices.

11- Interharmonics

Interharmonics are usually caused by synchronization issues or the operation of equipment like cycloconverters, induction furnaces or some wind turbine generators. If the installation includes sources of interharmonics, the manufacturer should be consulted as not all APFs can deal with them.

12- Mounting arrangement

Most shunt APF suppliers offer several installation alternatives:?

• Cubicle type devices: A full rating APF is built into a cabinet that is fixed to the ground.
• Wall mounted devices: A full rating APF is built into a light and compact cabinet that is fixed to a wall.
• Rack-mounted devices: One or several APF racks mounted in an specially designed cabinet.
• Loose module devices: One or several APF modules can be installed inside an existing or a new cubicle.

A modular APF design allows end users to adapt to potential changes in future power quality and energy efficiency improvement needs or grid code requirements. Modular design means that it is possible to add easily extra capacity to the APF’s capacity within the existing configuration, saving both costs and space.

13- Losses

Depending on their design and topology, APFs can have higher or lower losses. Checking the losses is important as low losses will reduce the life cycle cost (LCC) on the investment.?

Usually, APFs have about 2-3% losses (depending on rated power). APFs built on 3-level NPC inverter topology have lower losses than 2-level ones. Depending on user profile, losses mean a potential for considerable financial savings if the LCC is calculated over a few years period.

14- HMI and commissioning software

There are different HMI setups for APFs. Some offer a very simple interface while others have a built-in power quality analyser to calculate the required compensation that includes graphs showing the current and voltage waveforms and many extra functions in different languages. A great added value for any HMI is the possibility of connection to any industrial IoT (IIoT) software platform.

Commissioning and service of APFs without proper tools can be time consuming. Some suppliers provide software for this. Minimum required functionality should be that the system performs a self-check of voltage and CT phase order, CT polarity check, self-diagnosis and self-calibration. Such features will find installation errors before they can cause problems and will shorten the commissioning time. If the APF does not have this type of software the commissioning becomes more complex and might require external support adding to the system costs.

15- Built-in protection functions

Modern APFs have several built-in protection functions to ensure safe and reliable operation during abnormal system conditions. Some of the most common built-in protection functions are:

• Internal short circuit protection.
• RMS and peak value overcurrent.
• AC-system and DC electrolytic capacitor over and undervoltage.
• Ripple circuit overloading and ripple circuit failure.
• IGBT switches and enclosure overtemperature.

16- Control of detuned capacitor banks

Very often APFs are installed at sites together with existing or new contactor or thyristor switched detuned capacitor banks. Some APF suppliers offer the possibility to control the steps of these banks directly from the APF’s control system through dedicated digital outputs in the APF. By doing this it is possible to use the comprehensive power quality monitoring and reporting features of APFs to accurately monitor all the parameters of the installation and manage the total power quality improvement needs.?

Together with an optimal system integration, this feature brings efficient operation, cost savings on the control system, and the possibility to build a hybrid var compensator (HVC) using an existing or a new detuned capacitor bank.

APFs are a high performance, flexible, compact, modular and cost-effective type of power electronics solutions that provide an instantaneous and effective response in low or high voltage electric power systems. They enable longer equipment lifetime, higher process reliability, improved power system capacity and stability, and reduced energy losses, complying with most demanding power quality and energy efficiency standards and grid codes.

Their versatility, instantaneous response time and numerous benefits compared to conventional solutions together with their price decrease in recent years make that nowadays APFs are considered as a natural replacement for conventional solutions in many applications. Their correct specification and purchasing process is a critical aspect when designing new systems or considering their use in existing ones.

If you would like to receive any of my publications on the topic or to explore how #ActivePowerFilters can benefit your application, feel free to reach me at [email protected].?

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.