Residential Power Quality Questions

David Sweetman | Dec 08, 2009

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While there have been a large number of papers and other research on the effects of power quality for commercial and industrial operations, there has been very little published on the effects of power quality in residential applications. This is becoming more of a concern to the residential user with the increased usage of electronics, e.g., computers, washing machines, entertainment equipment, in the home.

Since I live in a remote rural area, where the "modern" 55 kV transmission lines were installed in 1936 and the backup (55 kV copper lines) were installed in 1908, I have been subjected to a variety of residential power quality issues. Efforts to mitigate include voltage regulators (some with UPS) for all electronic equipment (except large items such as washing machines), a whole house surge protector, and whole house capacitors (size determined with help from our co-op based on actual current and typical sag duration). We also generate some of our electricity, with battery backup (which also mitigates some of the problems).

The problems I have observed include:

  • Power sags, typically voltage decreases by ~ 25% for 100-500 ms, 3-5 times per day.

  • Poor power factor, ~0.92 (large number of local motors for irrigation pumps), which apparently magnifies some of the power sags.
  • Frequent outages, e.g., ~1-5 minutes, once every 1-2 weeks. Longer outages of 1-6 hours only occur 2-3 times per year.
  • There appears to be very few surges or spikes and very little harmonic distortion.
  • Power sags are the most frequent problem and cause of failure of electrical or electronic components. When the voltage decreases, the component uses more current (i.e., to maintain the same performance/power consumption). While the momentary increase in current may not cause an immediate failure, the reliability of the component is reduced from the thermal and other stresses associated with the increased current. While larger motors are more likely to fail from extended power sags, small motors (e.g., in CD or DVD players) can also be damaged. We had a 1-year-old 5 hp motor (for the well pump) fail due to damage caused by the frequent over-current pulses. While there is no way to inspect the new motor, I believe the addition of the whole house capacitors has reduced the stress on the motor.

    The reported power factor for residences varies significantly, e.g., some report in the 60-75% range, others state more in the 85-95% range. I cannot find any studies that have quantified the actual power factor, either for rural or urban settings. A low power factor means there is significant reactive power (VAR). The kWh meter for the home records total power, i.e., reactive and resistive, so a reduction in reactive power will reduce the monthly bill. Commercial sites with large inductive loads (mostly motors) often add capacitors, so the capacitive reactance offsets the inductive reactance and one only pays for the resistive power. Capacitors are often added to pump motors (e.g., energy efficient refrigerators), both to reduce the inductive reactance and for "soft starts" (to reduce the large starting inductive current). For the home, adding capacitors costs for the capacitor and installation, as well as the ongoing operating cost (the capacitor consumes some power just to operate). In my case, I added 55 ?f capacitors to a double-pole breaker in the main breaker panel (where the grid power enters the property). These capacitors cost about $1/year to operate (at $0.11/kWh) and have significantly reduced the number of sags that initiate a response from my voltage regulators.

    In our area, power surges or spikes are not common; however, I still installed a whole-house surge protection device. While we do not have local lightening (although that does occur in the mountains over which the transmission lines traverse), there are some spikes when the large irrigation pump motors shut off. The device has never tripped.

    While the advent of "smart" meters may address some measuring concerns, there is little likelihood of those meters appearing in our area in the foreseeable future. I cannot tell even then if the smart meters will help with the power quality issues, to actually measure and record critical parameters, such as VAR (in addition to kW), power factor, sag magnitude and duration, or surge/spike magnitude and duration.

    Questions that need to be addressed include:

    • What is the typical residential power factor? I expect rural areas that use more motors (e.g., for well pumps) will have a lower power factor.

  • Is correcting the power factor at the residence economically viable? Are the VARs saved off the kWh bill worth the initial cost and operating cost of the capacitors?
  • What equipment should be put on voltage regulators (with or without UPS)? What settings for initiation should the voltage regulator be set to (how low can the voltage go before the regulator brings the voltage back to normal)?
  • Will someone provide a reasonably priced (< $250) power quality meter (power factor, VAR, sags, surges) with recording (e.g., flash card or downloadable via USB) that can easily be attached to the main breaker panel (e.g., on a breaker or via CTs)?
  • What else can be done to improve residential power quality?
  • The following graphs are examples of the actual voltage at our location.



    Chart showing typical outage, i.e., short duration.



    Chart showing typical voltage variations. Note, the sensitivity of the recording instrument in the UPS is not adequate to show the short duration sags that actually occurred during this period.

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    Comments

    The quality of household electric power is definately a serious problem and will likely become more serious in the future as demand for power increases and more power is generated from wind. Identifying the problem as David has done is the first step toward defining the problem and developing a range of strategies to manage the problem.

    A strange part of me wonders if there may be a future in DC service within the household. So many of our loads (in number if not overall quantity) seem to want DC. An occasional 12V plug might be useful. Could easily power LED lights as well, and interface nicely with solar PV. Methinks the existing system assumed such small draws from residences that power factor could be ignored. That may have worked in the '30s, but not so today with all the stuff we have plugged in.

    DC may be OK for off-grid applications, but will not help the majority of residences. Use of DC systems would also require batteries, which are expensive to maintain and replace, as well as reducing the efficiency of PV systems. While most charge controllers have adequate regulation for battery charging, I am not sure the level of regulation, especially when doing bulk or equalization charge, would be adequate for many DC appliances.

    I think the main impact on power factor is from the greatly increased use of motors, not only powered washing machines starting in the 50's, but all other conveniences in homes and businesses, e.g., HVAC, entertainment equipment, kitchen appliances (home & restaurants). With adequate metering, each location can then institute appropriate correction, rather than trying to have the utility or co-op try to find a universal solution (which is unlikely).

    It has been mentioned in the article that "The kWh meter for the home records total power, i.e., reactive and resistive, so a reduction in reactive power will reduce the monthly bill".
    My understanding is that the energy measured in these meters corresponds to resistive power only as given by the product of voltage and the component of the current in phase with the voltage which corresponds to resistive current.

    Can anyone help me with a solution to this problem? I had a new Samsung mini-split HVAC unit installed and apparently the control board fried when the power was connected. The contractor said the manufacture stated the voltage ready of 250 caused the board to fail. My power company supplies 120/240 voltage with a 5% variance, so 126/252 could be expected. The HVAC system is rated for 208/230v-1Ph-60Hz, so with a 10% variance, based on the mid point the unit can only hand 240 volts, the manufacture stated 245 is the max.

    What can I do to regulate the voltage to this unit? The unit is directly wired to the breaker box. I have see "voltage regulators" on line, but most are converting from 240 to 120. I need some specifics on the regulator I need and who needs to install it.

    My power company does not have a solution and my HVAC contractor is not an electrician.

    Any help is appreciated.