Power Quality

Your questions answered: Evolution of Power Factor Correction in Low-Voltage Distribution Networks

Electrical engineers should understand what power factor correction is, why engineers should correct power factor, the evolution from power factor to true power factor and various power factor correction techniques

By Remi Bolduc October 6, 2021
Courtesy: Schneider Electric

Electrical engineers should understand what power factor correction is, why engineers should correct power factor, the evolution from power factor to true power factor and various power factor correction techniques. The presenter will also examine applications that involve power factor correction in harmonic-rich networks and how it can be used to mitigate flicker. The discussion will conclude by looking at the future of power factor correction by exploring electronic VAR compensation. Watch the full webcast here.

Questions answered by: Remi Bolduc, product application engineer for digital power, Schneider Electric 

Question: I like the example shown for the power factor calculations for a 6-pulse variable frequency drive (VFD_. Do you have a power factor calculation example for active front in VFDs?

Bolduc: AFE VFDs normally have a THDi at maximum load in the range of 3 to 5%. So, you can replace the 1 (100% THDi) or the 0.4 (40% THDi) with 0.05 (5% THDi) in the DF formula. You will find that TPF is almost equal to DPF since the harmonic distortion is low.

Question: Could be mitigate the high power hp motor starting with high current inrush (as PF is very low) with classic methods (capacitor banks for starting transition)? If not, what could be the solutions other than soft starter and VFD (if exist)?

Bolduc: The short answer is no, simply because standard automatic capacitor banks would be too slow to respond. To compensate inrush current of rapidly fluctuation inductive load, like motor start, you need a reactive compensator system that can respond with 5 to 10 milliseconds. The HVC presented in the webcast would be a suitable solution to support the voltage during motor start, it’s been used before for such application.

Question: How do you easily measure owner factor?

Bolduc: Power factor can be measured via portable or fixed power meters. Example; using a Fluke portable power meter or a fixed PM5000 from Schneider Electric.

When we add power factor correction to our power distribution system (i.e., MCC) should we use fuses or circuit breaker between the bus and the power factor correction?

Bolduc: Power factor correction capacitor banks need a short circuit protection like any other loads. Both protection options are acceptable, 1) Fused disconnect 2) circuit breaker. Note: NEC asked the capacitor bank protection device to be rated for a minimum of 1.35 x In (capacitor bank nominal current).

What are the power losses across an input line reactor that is added for filtering?

Bolduc: Assuming we are talking about a line reactor added at the input of a VFD, losses should be less than 1% (Compared to the VFD kW rating).

For a system that has a low power factor due to induction motor deficiencies (repairs needed), what happens when you attempt to correct Power Factor with capacitors? What happens when repairs / replacements of motors are done on a system that has been corrected with capacitors?

Bolduc: When you correct PF at an induction motor terminal, the current is reduced upstream (due to kVA reduction) of the point of connection (Motor and capacitor bank). The induction motor itself draws the same current. Fixed capacitor bank are normally only installed on Direct Online Motor, consult motor manufacturer for more details.

Is LED lighting having an impact on THD in buildings?

Bolduc: Yes, because the LED driver (ac to dc converter) draw harmonic current. The THDi varies based on the LED driver design, normally cheaper LED lights have higher THDi.

Could you explain further about reducing costs by reducing I2R losses? Are you saying that the savings is from lower demand costs because of lower kVA or are you saying it reduces energy used? If reducing energy, how does that work if the utility meters are kilowatt-hour meters (kilowatt-hour, not kVAh) for the energy part of the bill?

Bolduc: Let’s agree that the reduction in kW/h is small, but it’s there. The reduction comes from the fact that you are reducing the amount of current (due to reduction in kVA) flowing through your network. Less current flowing through your network impedance equals less I2R losses. Inside a facility the losses normally vary between 3 to 5%, if you correct PF the losses may be reduced by 1 to 2% (varies based on the original PF and the targeted PF, plus the PFC location). Keep in mind that this reduction of current is reflected from the power generation point (power plant) all the way down to the consumers. I have heard some utilities mentioned a reduction of 4% in kW/h when PF is corrected by their customers.

Courtesy: Schneider Electric

Courtesy: Schneider Electric

Question: The issue I have to deal with is clocked power supplies which are producing 3rd harmonic on neutral – any suggestion for that?

Bolduc: Most manufacturers use 3-phase 4-wire active harmonic filter to reduce the flow of zero sequence harmonics (3rd, 6th, 9th, 12th, 15th H) in the low voltage distribution systems. Alternatively, the clock manufacturer can embed filtering inside the clock itself.

Question: Can you provide an example of a 50-horsepower 6-pulse drive? Compare the cost of single active harmonic filter versus six line reactors.

Bolduc: Line reactors or dc bus choke are you first line of defense against harmonic and are the cheapest solution to take the THDi from 80 to 100% to approximately 35 to 45%, however it’s not enough to meet the IEEE 519-2014 harmonic standard. You normally have to add an active harmonic filter along with the line reactor or dc bus choke to bring the THDi at full load (TDD) to 5 or 8% (Most cases that varies based on the short circuit of the source). Normally if you have 3 VFDs or more AHF is cheaper than low harmonic emissions VFDs (12 or 18 pulses, VFD with passive filter or AFE VFD’s).

Question: For a facility that has an active PFC protection device, what is your advice when they have to run a standby generator due to utility outages? Due to the slower response of the PFC equipment (30-50 seconds as you mentioned), there is a risk of the PFC equipment causing leading power factor when larger motors cycle off, which will cause the generator protection system to shut down.

Bolduc: Advance PFC relays (capacitor bank controller) have a function that can turn off all stages as soon as a leading PF is detected. So, you could use such function. If you don’t have that function you can turn off the PFC during gen set operation using a dry contact from the generator that turn OFF the capacitor bank controller. For more advanced application AHF or EVC are used to correct PF in real time so there is no risk of leading PF during generator operation.

Question: What is linear load and its type also define difference b/w linear and nonlinear load please

Bolduc: Linear loads draw linear current, example; induction motor, incandescent light bulb, heater, capacitor. Nonlinear loads draw nonlinear current, example; VFD, rectifier, dc drive. Basically, loads that convert ac voltage to dc voltage with the use of semi-conductor like Diode, SCR’s, IGBT’s.

Question: Can you explain why choose 4.2 rather than a number like 4.8, which is closer to 5 since we will also need to filter the 5th if 6 pulse drives are installed?

Bolduc: Tuning of capacitor banks in 3 phases industrial network vary between 3.8 to 4.8 p.u. H, the tuning frequency will influence how much harmonic will be absorbed by the capacitor bank. In order to reduce stress on the capacitor bank, often tuning ranging from 3.8 to 4.2 are used. When a capacitor bank is tuned to 4.8 H, it’s recommended to do a harmonic study and simulation in order to avoid overloading your passive filter (4.8 H tuning).

Question: How helpful are K-rated transformers to mitigate harmonics? Any implications when used with PF capacitors?

Bolduc: K rating means the transformer can handle a define amount of stress caused by harmonics. In Delta-Wye auxiliary transformers with 480 V primary with a 120/208 V secondary, normally the zero sequence harmonics (3rd H and its multiple) are trapped in the DELTA winding so they don’t travel upstream on the 480 V network. There is no change (no filtering) of harmonic flow on the 120/208 V network.

Question: On slide 34, ACR with switching: An SCR switch is shown in Lines 1 and 2, but not line 3. Would you please elaborate.

Bolduc: Since the capacitors are connected in DELTA, we only need to switch OFF two phases to stop the flow of current, this configuration reduces the cost of the solid-state switch.

Question: When would I see a delta vs a wye connected capacitor bank? Would there be grounding issues with either of the connections?

Bolduc: Normally either type of connections is connected to ground. Even when you have WYE connected capacitors, they are kept ungrounded. However, if the WYE connection is connected to ground, there is a risk of third harmonics flowing into the capacitor bank (sometime seen on MV cap banks).

Question: Which is the power range for EVC (100kVA or more)?

Bolduc: The existing range is from 50 kVAR to 250 kVAR, in the future they will be more powerful. Keep in mind that several EVC can be connected in parallel to achieve higher kVAR ratings.

Question: Concerning EVC, what is an example of a system requiring adjustment for a “leading” power factor condition? What would specifically cause that?

Bolduc: Fixed capacitor banks or large concentration of compensated Switch Mode Power Supply (SMPS) can cause leading PF. In Data Center, leading PF has been observed that was caused by SMPS that caused issue with the backup generators. Another example if leading PF may exist as well.

Question: I am not an electrical person. But how is all this applicable to health care facilities?

Bolduc: In health care, PF compensation may reduce your utility bill by reducing your demand charges (varies from Utility to Utility) and it will help reduce losses and help regulate your supply voltage. A stable supply voltage helps sensitive loads found in Hospitals. Harmonics should also be kept within the IEEE 519-2014 standard limits in hospitals.

Question: What kind of price range of EVCs currently targeting?

Bolduc: To be defined.

Question: Is there a good method to estimate the size of a power factor correction system on a new facility where equipment to be used is not fully defined?

Bolduc: You can estimate to use a capacitor bank in the range of 20 to 30% of your supply transformer rating. For a 1000 kVA XFR, you could use a 200 to 300 kVAR capacitor bank.

Question: Is there a relationship with higher power factor and reduction in utility spend?

Bolduc: It varies from utility to utility, however normally PF would play a role on the utility demand charges. Some utilities ask for a 90% PF some others ask 95% and some others gives bonuses when your PF is maintained near 100%. Contact your utility to find out their demand rate structure.

Question: EVC?

Bolduc: EVC = Electronic Var Compensator. You can see some more details in the presentation.

Question: Is there any overlap in industry between EVC and active harmonic filters?

Bolduc: Yes, however EVC should be cheaper than active harmonic filter.

Question: What is the best way to plan for power factor when you do initial designs when motor PF is not known are there common PFs based on facility types?

Bolduc: LV induction motor PF is normally in the range of 80 to 85% lagging. For detailed discussions, it’s best to discuss with your local Schneider representative.

Question: HFC?

Bolduc: I am guessing that this acronym should be HVC instead of HFC. HVC = Hybrid Var Compensator used to compensate rapidly fluctuating inductive loads in real time (within 5 milliseconds).

Question: Helpful information, thank you! Can you summarize which types of power factor correction technology you see trending as successful in different types sites. How about K-12 schools, light manufacturing, office buildings, colleges?

Bolduc: K-12 schools, light manufacturing, office buildings, colleges: In these types of applications automatic de-tuned type capacitor banks are currently used. We are starting to see active harmonic filter being used, as well. In the future we will see EVC being used at these sites.

Question: Is unity power factor better than 0.95 lagging power factor for a utility?

Bolduc: Yes, it further reduces the kVA demand and the losses in their electrical distribution system.

Question: How is power factor different from displacement power factor?

Bolduc: Power Factor (PF) and Displacement Power Factor (DPF) are the same, they are both equal to kW/kVA or Cos phi.

Question: Where is the preferred location to connect the EVC? at the main substation or downstream of the main substation near the load center?

Bolduc: The most cost-effective location will be at the main distribution switchboard. However, installation at the MCC level will yield better voltage regulation and better losses reduction.

Question: What is the cost difference between the solution expose by the specialist?

Bolduc: Generally speaking: Fixed capacitors are the cheapest, automatic capacitor bank would be in midrange in price and EVC would be the most expensive.

Question: Please comment on Y-Y transformers vs Delta-Y xfmrs in transmitting harmonics from customer to utility.

Bolduc: In a Y-Y XFR configuration, all the harmonics will flow back to the Utility network.    In a DELTA-Y configuration, the zero sequence harmonics (3rd, 6th, 9th, 12th, 15th harmonic) will be trapped in the DELTA, therefore they won’t propagate back to the Utility network. All other harmonics (Positive and negative sequence) will flow back to the Utility network.

What kind of graphic display is available for the user while it is in service?

Bolduc: On PFC relays (PF controller), LCD displays are used with various parameters and icons. On EVC or AHF, touch screen displays are normally used which have icons, bar graph, phasor diagrams, plain text and oscilloscope functions.

How is EVC sized? How large of a system can an individual unit work on? (Size as kVA or MVA of transformer. . .)

Bolduc: EVC or HVC vary in size from 50 kVAR to 20 MVAR, these systems are sized based on the PQ objectives (PF, voltage regulation, flicker) A transient study or load flow study may need to be performed for complex applications. For basic PF application this formula can be used: KVAR = KW x (Tan Cos-1(Present PF) – Tan Cos-1(Desired PF)).

Considering the various types of power factor correction, and with consideration to resonance and related issues, what type of PFC would you usually recommend for higher pulse order VFDs with low harmonic injection back in to the distribution system?

Bolduc: For PF correction being the main objective, even if the dominant harmonics are 11th & 13th or 17th & 19th harmonic order, we would still recommend de-tuned type capacitor banks tuned at the fifth harmonic order (4.2 H p.u.) to avoid any chance of resonance condition. If harmonic filtering is required, then we would recommend the use of active harmonic filters.

Question: How do mitigate the harmonics at LV level supplying power to EV chargers?

Bolduc: To mitigate harmonic on EV chargers, we would recommend the use of active harmonic filters.

Question: Are there any issues with generators running in parallel using an active filter? E.g.: 2500kVA transformer with 1500kW generator with a 300kVAR Accusine with 480v system?

Bolduc: No. However if you are running in co-generation (Utility + Gen Set) we would recommend to have sensing CTs on both source of power complete with a summation CT in order for the AccuSine to sense the current flowing to both power source. If more details are required feel free to contact the Schneider Tech support.

Question: How do you compensate the inductive losses of the MT/LV transformers with de EVC connect to the LV side? With capacitors I could overcompensate the low voltage controller… but with EVC … is the same?

Bolduc: The same concept applies to EVC, simply set a higher PF target to compensate for reactive power from upstream inductive loads (EX: XFR).

Question: Is there a rule of thumb for the PF range below which we should be concerned and look for PFC solution?

Bolduc: From the utility point of view, any customers that have a load greater than 50 kW can have a PF penalty if their PF is below a certain level (Example: below 90% lag). That requirement will vary based on the utility billing structure.

Question: Can increasing wire temperature rating and size extend life of equipment susceptible to harmonic issues?

Bolduc: It will not help the loads that are susceptible to harmonics, it will only help the LV distribution network.

Question: So, how do you safely use PF correction capacitors with VSDs on the bus?

Bolduc: The basic approach is to use De-Tuned type automatic capacitor banks. When harmonic is too high, then use an active harmonic filter (AHF) to mitigate harmonics and also correct PF. Advanced AHF have three modes of compensation: 1) Harmonic mitigation, 2) PF correction and 3) load current balancing.

Question: Is this EVC available from Eaton for medium voltage applications?

Bolduc: Not at this point in time. In the future, maybe.

Question: Are owners that are simply adding power factor correction capacitors to an electrical distribution system, without a study first, doing themselves any good? For example, at a 9-12 school with 277/480V electrical distribution, and conventional 4-pipe chilled water HVAC system.

Bolduc: If they are using simple fixed or standard automatic capacitor banks, they could cause harmonic issues on the LV network. At a minimum, in these networks I would recommend to use detuned type capacitor banks to correct PF. Alternatively, active harmonic filters can be used to mitigate harmonic and also do PF correction in these sites.

Question: What are the benefits of using EVC technology versus traditional capacitor technologies?

Bolduc: 1) Precise reactive power control with infinite resolution, 2) Real time reactive Power injection (5 to 10 milliseconds), 3) Better voltage regulation on the LV network, 4) 1 phase Reactive power injection (1 phase versus 3 phase injection), 5) Load current balancing included, 6) Power Monitoring and communication with BMS included, 7) If sized appropriately can help mitigate flicker.

Question: Is it better to specify VFD drives with high power factor and low harmonics rather than trying to fix the problem after the fact?

Bolduc: If you have two VFDs or less, AFE VFDs are a good solution. If you have more than 2 VFD’s, it’s more cost effective to use standard 6 pulses VFD’s combined with active harmonic filter to control harmonics in your LV network.

Question: Is power factor related to PUE?

Bolduc: Not that I know of.

Question: I am a pure mechanical person, and always have difficulty understanding reactive, apparent and active powers. Can you provide a simpler explanation of these three concepts?

Bolduc: Basically, in the power triangle, Pythagoras law applies. Where: kVA = √ (kW² + kVAR²). In other words, your power source must supply the vector sum of the real power (kW) plus the reactive power (kVAR) which is equal to the Apparent power measured in kVA. By correcting PF (kW/kVA) would reduce the amount of reactive power (kVAR) in the electrical distribution, therefore reducing the amount of apparent power (kVA) supplied by the source.

Remi Bolduc
Author Bio: Remi Bolduc, Competency Center Manager for Digital Power, Schneider Electric