Active versus Passive Power Factor Correction

Capacitor-based Low Voltage (LV) Power Factor Correction systems have been the accepted technology for correcting power factor for at least the past 50 years. There have been many improvements in capacitor designs over the years. When properly designed, these systems can be reliable and rugged. However, there are many capacitor-based systems installed that struggle to cope with harmonics, leading power factor, system response time, and temperature issues.

While most of the challenges above can be catered for with correct design, correct component selection, and quality manufacture, there are alternate technologies available that should be considered.

Since 2016, Active Systems or Static VAr Generators (SVGs) have become available for LV applications and are challenging conventional power factor correction equipment. Like many technologies, proponents of SVG systems are very quick to point out the advantages of SVGs over conventional capacitor-based power factor systems, but sometimes fail to discuss limitations and disadvantages of the newer generation SVGs. In the following paragraphs, we will discuss the pros and cons of both systems, so you can make an informed decision on the best solution for your application.

History of SVG Technology in New Zealand

Metalect Industries worked with the Engineering School at Canterbury University to fund and direct research into Active Systems for 3 phase applications in 1996-1998. This resulted in a publication of a PhD thesis by Edward Arnold Memelink in 1999. From there, single phase prototypes were built and tested. Metalect Industries and a technical team from Canterbury then extended the design to 3 phase, obtained government research grants, and units were built and site tested, notably on the Queenstown Gondolas. While successful in electrical terms, the systems could not be economically manufactured due to component limitations at the time, and by 2006, the projects were abandoned. Several related products were spun off as ongoing products for Metalect Industries, and several of the ZVX units (Zero Crossing By-Pass units) have been installed in sites around New Zealand.

Conventional Capacitor Based Power Factor Systems

There are numerous capacitor based systems available, but the systems and technology are not complicated. A controller measures the parameter, calculates how many kVAr are required to correct a lagging power factor, and switches the capacitors onto the bus accordingly. Conceptually, very simple. Depending on the particular site conditions, harmonic blocking reactors may be required to prevent harmonic currents causing overheating in the capacitors. Almost without fail, fan forced air cooling is required, and regular inspection and maintenance is highly recommended. This type of power factor correction equipment has been built for many years, and many switchboard builders, panel manufacturers, electrical contracting companies, and individual sites have built systems with varying success, dependent on their level of appreciation of the issues. Unfortunately, many systems fail early due to a variety of problems. Experienced designers and manufacturers can, and do, design systems that have working lives of over 15 years, even in very arduous site conditions.

Capacitor systems can be built with thyristor switches instead of conventional contactors. This can make them a fast-acting solution, operating within 30milliseconds, still not as fast as an SVG, but fast enough for most applications. Thyro switches also switch on at an optimum time, resulting in no inrush current in the capacitor, and prevents premature aging of the capacitors.

The fact is that not all Capacitor Based Systems are created equal, and designs at a lower cost typically exhibit severe system limitations. In fact, some designs have been so poor that capacitors have in the past caught on fire and caused major damage.

Active Power Factor Systems (SVGs)

Although SVGs have been available for several years from traditional suppliers (Schneider, ABB, Siemens, Frako, etc) their pricing has often been prohibitive. More recently, Asian manufacturers have released products into the Australasian market. In reality, there are actually less than five Asian manufacturers, with some of them allowing their product to be brand labeled something different, so presumably there is a greater variety of SVGs on the market than there actually is. This is the case in New Zealand, where the same units are available from different sources with different names on them. Conceptually, SVGs are simple, but require significant finesse in the electronic design due to an assortment of individual components. Flexibility is excellent, and there is no doubt that as electronics become simpler and more reliable, these systems will thrive. However, there are some facets of these systems that need to be considered carefully to achieve reliability, sustainability, and still be cost effective.

A Combination of the two Technologies – THE HYBRID SYSTEM

A Hybrid system that combines the two technologies can mitigate the shortcomings of both technologies, while accentuating thier respective advantages. The scenario would be, for a 100kVAr requirement, to provide 70kVAr of capacitor-based modules with a 30kVAr SVG. In every situation where the author has investigated a case of control of the leading power factor, it has been found that the amount of leading kVAr needed is less than 30% of the total requirement. For example, in a data center with ambient temperature, a leading power factor of about 25kVAr is common. However, when the temperatures are outside this temperature range, and require significant air conditioning units to operate, the power factor is typically 150-300kVAr lagging. In this general example, 275kVAr of capacitive and 30kVAr of SVG can cater for all situations. This Hybrid system is undoubtedly more cost effective than a full 300kVAr of SVG. The Hybrid system will produce far less heat and will not be completely off-line due to an SVG electronic malfunction, as there would be significant capacity in the capacitor-based section of the system to avoid the bulk of penalties.

Furthermore, if the hybrid system is modular, such as kVAr Solutions’ systems, there would be an option of a redundancy module as a secondary backup, or a solution to the rare event of an SVG failure. What would happen is a capacitor-based module of the same size as the SVG can be substituted while the SVG is away being repaired. In this situation, the client will not have penalty tariffs imposed.

kVAr Solutions’ Hybrid systems have been specifically engineered to overcome the limitations of both capacitor-based systems and active systems. kVAr Solutions uses a traditional capacitor-based approach for bulk power factor correction, in conjunction with a smaller active system to handle high speed and leading power factor requirements. The Hybrid system is designed to be a cost-effective solution, utilising the best of both technologies while offering superior reliability.

Summary

Fully active systems are only price competitive for smaller systems, as they come in fixed minimum sizes. When choosing an active system, it is important that the end user looks at the cost of expanding the system if required. For example, if you start with a 100kVAr active system, and would like to expand by 20kVAr, a complete 100kVAr module will need to be purchased, as opposed to increasing the size of the existing system or even purchasing a smaller module.

In general, fully active systems are expensive to expand in this way, compared to capacitor-based and Hybrid systems. Fully active systems are completely electronic, and if the electronics fail in any way, the whole unit shuts down, and most will require repair and/or replacement at the supplier’s factory. While the system is being repaired off-site, the customer is exposed to full power factor penalty tariffs and peak demand charges from the electricity supplier.

In comparison, capacitor-based and Hybrid systems can be repaired on-site by electricians using readily available components and equipment. More importantly, these units will still function as a capacitor-only system if the power electronics fail, meaning the customer is still protected against power factor penalty tariffs and peak demand charges from the electricity supplier.

Fully active system suppliers often criticise and exaggerate the heat load of capacitor-based systems, and the frequency of capacitors overheating. In reality, a fully active system will generate more heat per kVAr than a capacitor-based system, and effective cooling is even more critical – with many fully active systems requiring installation in an air-conditioned room. Furthermore, active system vendors often criticise capacitor-based systems for having many capacitors. While this does mean the traditional units take up more space, the power factor capacitors used are safer and more reliable. This is because in a fully active system, electrolytic capacitors are hidden from sight to smooth the DC bus within the unit. These capacitors are filled with very corrosive acids to increase the microfarads of capacitance (allowing them to be much smaller physically).

Recommendations

It is inevitable that designers, consultants, and engineers will have a preferred solution, due to thier past experiences or personal preferences, as with any technical system. All three solutions discussed in this comparison have certain applications in which they perform thier best, and other applications where they aren't the right tool for the job. It is complicated to provide ‘broad brush’ industry recommendations, as applications are so varied, but we have the following guidelines.

Capacitor Based Power Factor Correction:

Are ideal in situations where there are motors and pumps that run consistently and need continuous correction at a fairly stable level. Manufacturing plants, pump stations, quarries.

SVG, Electronic Power Factor Correction:

Used in situations where there is a leading power factor issue, where instantaneous correction is required, or if there is a significant harmonic problem. These may include buildings with a preference for LED lights, solar inverters, variable speed drives, and large HVAC systems.

Hybrid Power Factor Correction – Capacitor banks plus SVG module:

This system is the most comprehensive solution, perfect for sites where there is a need for leading and lagging power factor correction and harmonic mitigation. With a capacitor system that does the ‘heavy lifting’ the SVG unit can tidy up the sine wave and deal with any leading power factor or power factor ‘spikes’ cause when a motor is put under load. i.e. as a sawblade hits a log, or grain is dumped into an auger.

Engineered Solutions:

kVAr Solutions has a team of experienced design engineers who can work through a design tailored for your site. This can include setting up a data logger and monitoring the loads, re-engineering existing systems, and providing advice on setup and install.