In the electrical industry, Power Factor is widely known as a bit of a dark art. Over the last few years, advances in technology have brought new types of correction systems to the market along with a range of off the shelf cheap products that can be ordered online and promise the world but deliver little.
The below paper was written by Allan Ramson (NZCE, BEng, MBT), General Manager of kVArCorrect Ltd and provides an insight into the pros an cons of active versus passive power factor correction for different applications. For a full copy of this paper please click here.
History of Static VAr Generators (SVG) Technology in New Zealand
Metalect Industries combined with the Engineering School at Canterbury University to fund and direct research into Active Systems for 3 phase applications in 1996-1998. This culminated in 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. Whilst successful in electrical terms, the systems could not be economically manufactured due to component limitations of 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 many sites around New Zealand.
Conventional Capacitor Based Power Factor Systems
- Proven technology when correctly designed
- Larger switchboard builders and specialist companies know what they are doing
- Utilises available switchgear
- Contactors, MCB’s, fuses
- Simple to maintain
- Ensure air flow as designed, capacitor current within specifications, temperature within specification
- Simple to repair
- Replace contactors, capacitors, MCB’s, fuses, etc. All done by any Electrician without system shutdown (if designed correctly)
- Low cost test equipment
- Current meter, temperature measurement. Capacitance meter not required (if the current is correct, so is the capacitance)
- Easily expanded
- Add more capacitance in very small lumps as site grows
- Capacitor failure does not take whole system offline. Saves customer money in penalties by continuing to partially operate
- Cannot control leading power factor
- There are very few sites where this is required.
- Can be slow to react (but not always)
- Often not required. However, there are capacitor-based systems that are specifically designed to response sub-second
- Old capacitors may be prone to leaking
- Modern capacitors are optionally fire-retardant resin filled.
- Generate significant heat
- True, but active systems generate even more. If the available cooling cannot handle a capacitor system, it absolutely cannot cool an active system
- Can produce system resonance issues
- This is so rare; we have only ever documented one case in NZ (but can be easily fixed)
- Considered to be Old Fashioned
- A matter of image only
- Risk of fire in the event of capacitor failure
- Modern capacitors, combined with correct design, completely mitigate this.
Active Power Factor Systems (SVGs)
- Fast and accurate correction of power factor (leading or lagging)
- When working as designed, there is no doubt that the power factor correction delivered is excellent
- Often physically smaller than capacitor systems
- Wall mount options are lower cost than the rack mounted large SVGs
- More reliable than poorly designed conventional systems
- Certainly not true for properly designed capacitor-based systems
- Higher cost of installation than conventional capacitor-based systems
- Made up of unit purchase price plus 3 x CT’s (and may require air conditioning.) Even without air conditioning in the switch room, the cost of SVGs is higher
- Expensive to expand because the units come in big lumps. EG: if the system is 5kVAr short of achieving target, minimum step is 30kVAr at >$5-7k installed
- Contrast a capacitor-based system where an extra 5kVAr may be a few hundred dollars only
- All the eggs are in one basket, causing potential large penalty tariffs to the user
- If the unit has a fault and goes ‘off- line’ all power factor correction is unavailable. If a capacitor fails in a conventional system, then the rest can continue
- Generates almost twice the heat as a capacitor-based system and is specified for lower ambient temperatures to start with.
- Capacitor based systems have a higher ambient temperature specification. This is critical in non-air conditioned rooms over summer months
- Usually very high MTTR (mean time to repair) times – recommend 100% complete spare system backup to avoid 0% power factor control to the site (maximum exposure to penalties)
- When the whole system is offline, the full penalty tariffs will be incurred by the end user. One way around this is to use multiple 30kVAr units rather than a single larger unit, although this is a huge cost disadvantage
- Units have more capacitance internally than conventional systems. Worse, these capacitors are electrolytic type with corrosive acids inside
- True, and the lifetime of electrolytics is known to be significantly less than high quality MPP caps as used in capacitive type power factor systems. See kVArCorrect’s papers on Design Problems in Power Electronics
- More susceptible to dusty and humid environment compared to capacitor-based systems
- Modern capacitor-based systems can fail too, but the MTTR is significantly lower and can be fixed by local electricians without ‘return to base’ or very specific skill sets
A Combination of the two Technologies – THE HYBRID SYSTEM
Potentially, a Hybrid system that combines the two technologies can mitigate the cons of both technologies whilst accentuating the pros. The scenario would be, for a 100kVAr requirement, to provide 70kVAr of capacitor based modules with a 30kVAr SVG. In every case where the author has investigated the case for control of leading power factor, it has been found that the amount of leading kVAr required is less than 30% of the total requirement. For example, we’ve documented sites with 20-50kVAr leading at certain times and 200-300kVAr lagging at other times. This Hybrid system is undoubtedly more cost effective than a full 300kVAr of SVG, in addition to not having all of the negative points shown in the tables above. The 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.
The comparative pros and cons of the three technologies are summarised in the following table – the third column relates to kVArCorrect’s Hybrid system, which was developed specifically to overcome the limitations of both capacitor-based systems and active systems. It uses a traditional capacitor-based approach for bulk power factor correction, with a smaller active system to handle high speed as well as leading power factor requirements. The Hybrid system is designed to have the best of both technologies whilst offering superior reliability.
While fully active systems can provide exact kVAr requirements for both leading and lagging power factor in near- to real time, they can be extremely expensive, and are normally return-to-base in the event of electronics failure. Clients are often shocked to discover the cost of expanding a fully active system could be as high as the original installation.
Hybrid systems only rely on the active electronics for less than 25% of the overall available corrective kVAr’s, meaning 75% or more of the power factor correction is still available to mitigate the potential penalties, should the electronics require repair or servicing. Hybrid systems combine the speed and control benefits of a fully active system, with the maintainability and reliability of a capacitor-based system.
About the author
Allan Ramson is the owner of kVArCorrect Limited and has worked extensively in the Australasian Power Factor market for over ten years. Allan and other engineers at kVArCorrect are ex-employees of Ampcontrol and Metalect Industries in Rotorua, and have been involved with Active System technology in New Zealand for many years.
Also having been closely associated with few hundreds of power factor correction systems installed, it is with significant experience in the market that this document has been written. kVArCorrect designs and manufactures capacitor based power factor systems, Hybrid Capacitor/SVG systems, a range of power quality controllers, and SVG add-ons. Additionally, kVArCorrect sells SVG systems in 30kVAr, 50kVAr, 100kVAr and above sizes.