Comparison between active power filters and conventional solutions: [Part 4/5: Hybrid var compensators (HVC)]

Hybrid var compensators

Hybrid var compensators (HVC for short), also called hybrid active power filters (HAPF for short) have been around since the 1990s. Description of their topology and operating principle can be found as far back as 1996. They were developed as a special design of shunt active power filters (APF for short) for two reasons.

• Firstly, to take care economically of power quality problems and grid code and energy efficiency requirements of the electric power system that conventional passive solutions like mechanically switched capacitor banks (MSC for short), mechanically switched reactors (MSR for short) and passive harmonic filters (PHF for short) or conventional active solutions like thyristor switched capacitor banks (TSC for short) and thyristor switched reactors (TSR for short) could not handle.
• Secondly, to reduce overall project costs for applications that needed the real time response and the control functions of shunt APFs, and at the same time required large quantities of capacitive reactive power.

Features

The most typical features of HVCs that can be found nowadays in the market can be classified into the following categories.

Benefits

The most typical benefits of HVCs that can be found nowadays in the market can be classified into the following categories.

Comparison with conventional solutions

There are several solutions competing with hybrid var compensators that can be applied to a certain installation once that the power quality and energy efficiency problems and their magnitudes are identified. They provide different levels of problem mitigation depending on their technical complexity and the financial investment.

These solutions are connected in parallel with the problematic equipment or facility and can be divided into eight groups depending on the technology used (passive or active), the output provided by the device (capacitive, inductive or both), and their ability to mitigate harmonics.

Conventional solutions have a slow response time, typically 2 to 10 seconds when they are manufactured with electromechanical contactors or circuit breakers, and around 20 milliseconds when they are manufactured with thyristor switches. As a result of these response times, uncompensated reactive power demand by dynamic loads can produce unbalance, cause flicker, increase system losses, and cause overall poor power factor which will reduce the electric supply capacity. Installations also suffer often from harmonics, interharmonics and voltage variations that the conventional solutions are not able to mitigate properly.?

Comparison with shunt capacitor banks

Shunt capacitor banks are very sensitive to harmonic voltages present often in today’s installations since they have a lower impedance at higher frequencies. The operation of hybrid var compensators is not affected by existing system harmonics and does not present any risk of creating system resonances, which is always a possibility when operating shunt capacitor banks.

Comparison with shunt reactors

Hybrid var compensators are also capable of compensating capacitive installations, something that shunt capacitor banks are unable to do. Nowadays many facilities are becoming increasingly capacitive due to the increased use of electronics, such as LED lighting or data center servers, and the reduction of linear loads. In these cases, shunt reactor banks have been used. Hybrid var compensators are able to provide in real-time the inductive reactive power needed by these installations to operate properly.

Comparison with passive harmonic filters

Passive harmonic filters are designed to take care of installations’ harmonic distortion problems and can be connected directly to the low or high voltage electric power system. All passive harmonic filter configurations have a capacitive character as they are built with inductive, capacitive and resistive elements configured and tuned to control harmonics. The technical approach of these filters is to provide a low impedance path to harmonic currents at certain frequencies.

They are an efficient and economical solution for certain applications, for example, if specific harmonic frequencies usually produced by a specific piece of equipment need to be mitigated. But they are not efficient when working at partial loads and they possess the risk of causing resonances within the system if not properly sized.

Features comparison

Total cost of ownership comparison

Summary

Capacitor banks and shunt reactors are usually designed for applications just requiring reactive power support (capacitive or inductive). Passive harmonic filters are designed to take care mainly of harmonic distortion problems. Hybrid var compensators have many competitive advantages compared with the use of conventional solutions in installations including:

• Stepless and seamless output, no visible “steps” in the reactive power output.
• Only one current transformer signal input required from the system, no need for duplicated current transformers in the installation.
• Capacitive and inductive real-time reactive power compensation in one device. The SVG or AHF of the hybrid var compensator can also produce inductive reactive power. This ensures that there will be no system overcompensation from steps or loads.
• Stable control as all power quality problems and energy efficiency requirements of the installation are taken care of by a single controller. This avoids system hunting caused by the use of multiple controllers.
• Their output is independent from network impedance, curve form of the current to be compensated and the quality of the supply voltage.

It is very important to notice that the total cost of ownership of hybrid var compensators compared to the TCO of conventional solutions for a certain application it is very much dependent on the topology of the installation, the design of the whole system, the ratings of the device needed for taking care of the problems of the application and the requirements of the end user.

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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.