Power Factor Correction
(PFC)
Introduction
Power factor correction (PFC) is the technology that restores the power factor to as close to unity as is economically viable. This is usually accomplished by the addition of capacitors to an electrical network, therefore a compensation for the reactive power demand of the inductive load, reducing the extra duty on the supply and the electricity bill.
Implementing PFC is usually installed in the form of capacitors and therefore adding capacitance onto a network. The capacitors are used to offset as much as possible of the typically found inductive load commonly found on networks.
Inductive loads - Transformers, electric motors, coils, etc.
Quick Lesson
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The instantaneous voltage across a pure resistor is 'in-phase' with current
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The instantaneous voltage across a pure inductor 'leads' the current by 90 degrees
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The instantaneous voltage across a pure capacitor 'lags' the current by 90 degrees
Therefore the voltage across a pure inductor and voltage across a pure capacitor are in exact opposition to each other and are 180 degrees out-of-phase.
Knowing as detailed above its quite simple to summarise that whenever reactive power is present (always) we use caps to offset inductive loads and inductors to offset capacitive loads. Please note that it's 99% of the time always capacitive equipment to be installed.
What's the Score
Typically the corrected Power Factor (PF) is aimed to be around 0.95 and actually, certain power distributors offer incentives for operating with a greater PF of 0.9, penalising customers with poorer than 0.9 PF. There are various ways that this is monitored/metered however the net result is that in order to reduce wasted energy and therefore money the customer is encouraged to apply PFC with all the desire to achieve as close to unity (1.0) as possible therefore what is being supplied is what is being used.
What's to specify
Essentially you have two options
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Allow a breaker for Future for Power Factor Correction
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Allow a breaker and the Power Factor Correction Installed
Both are viable and common practice, however, typically it is more common to size and design the PFC solution into your schemes which is pretty simple.... kind of.
Overall Size
Firstly as a rule of thumb its always been the number of capacitor banks required is 1/5 of the incoming supply. Can I just stress this is a rough guide and every system has its own requirements of PFC, some will be less and some will be more. I would also like to add this rule is a bit old hat now, equipment is much more sophisticated and it's generally a lot less than this so if you are struggling it's a comfortable rule in my opinion.
Step Size
Typically PFC is available in steps of 25kVar & 50kVar, others will provide variations but this is common. Obviously, if you require more than 2 banks I would always have at least 2No. of the smallest and the rest in 50kVar steps, if required, the controller will do the fancy bit of bringing them in for you.
To Detune or Non-Detuned
Detuned filtering is a technique to correct the PF avoiding the risk of resonance condition performed by shifting the resonance frequency to lower values where no harmonic currents are present. This achieved by modifying capacitors circuit formed by the transformer and the capacitor bank, introducing a filter reactor in series with the capacitors, This way it’s not possible to have a real resonance condition.
Essentially if you have dangerous harmonics on a system, harmonic filtration will be used and whenever it is, used detuned filters on your capacitor banks, pretty much a rule of thumb. If all is well, ignore this part.
Control
Control is simple, it's implemented and monitored by a PLC device, typically supplied by the capacitor manufacturer, this monitors the incoming supply and controls the bringing in and out of the PFC, simples.
Connection
Connection and therefore the install is simple, the capacitors are contactor controlled and the contactor is connected via a 3/4 pole breaker directly coupling them onto the bus when closed. The controller or PLC would at the very most have a single phase supply and neutral for power, and also a single phase CT connected at the switchboard incoming supply on L1 to monitor the PF.
Location
Pretty much always on the Main LV Switchboard, where the supply comes in, in more extreme cases, it can be local to equipment, also dotted around for particular areas or one for each building if supplying several, however again its quite unique but for most applications, main switchboard, job done.