Power Quality

Power quality is the term used to describe the degree of consistency electrical energy is expected to maintain from generation to point of use.  This consistency, or quality level is affected by voltage stability, power factor, harmonics, voltage spikes and sub-cycle transients.

For the purpose of our discussion we will focus on voltage stability, power factor and harmonics because they can affect the operation and output of a home energy metering system.  Detecting very quick voltage spikes and sub-cycle transients requires far more sensitive, and expensive, power quality analyzer equipment.  This is beyond the scope of what residential metering systems can provide.

Before delving into the details of power quality lets clarify the meaning of the electrical terms voltage, current, power and energy.  Picture the metaphor of a firefighter spraying water on a fire...

• Current is the water flowing through the fire hose.
• Voltage is the pressure behind the water in the hose.
• Power is the speed at which the water exits the nozzle.
• Energy is the amount of water sprayed on the fire over time.

All energy metering devices require some type of voltage and current input to calculate power and energy.  Voltage is normally picked up through a direct connection to a spare breaker in the panel.  Some low cost monitors just use a preset voltage.  While they avoid having to make a connection to live wiring the accuracy of power and energy measurements will suffer.

Current is measured with a current transformer or CT which is nothing more than a small coil that surrounds the conductor being measured.  CT's can be split core or solid core.  Split core CT's can be opened and closed around the wire without having to disconnect it.  Solid core CT's, which are generally less expensive, require the wire to be disconnected and threaded through the CT for the connection.  Click here for more info about CT's.

Voltage Stability

In the U.S., utilities are required to deliver line-to-neutral voltage to customers within a tolerance of +/-10 percent as a power quality standard.  In other words, if your single phase line-to-neutral voltage is 120 volts, delivery can vary from 108 volts to 132 volts.  This fluctuation can be caused by a number of factors which may include living close to an industrial complex, utility capacitor bank or sub-station switching or incoming wiring problems.

If you find your voltage readings are outside of this range, contact your utility and ask them to test the power at your meter.  If their readings concur with yours ask them to take appropriate action to correct the voltage fluctuation problem.  If these voltage variances are left unchecked damage can occur to motors and sensitive electronic equipment.

It is normal for voltage to dip by slightly when there when there is a big increase in current.  This can occur when a heat pump compressor, clothes dryer or similar high load device turns on.  The reverse is also true as the voltage should return to its previous level when the load is removed.   A good home energy monitor should able to capture and record this voltage/current relationship within the graphing features of its software.

Power Factor Basics

Power factor is a measurement of how efficiently power is used.  It is expressed is the ratio of real power, in kilowatts (KW), to apparent power which is expressed in kilovolt-amperes (KVA).  Apparent power is made up of two components, real power (KW) and reactive power which is expressed in kilovolt amperes reactive (KVAR).  The relationship between these three components can be expressed graphically in what is known as the power triangle:

Real power (KW) performs physical work through resistive loads such as heating a burner element or an illuminating an incandescent light bulb.   Reactive power (KVAR) is used to sustain magnetic fields such as those found in the coils of electric motors or the power supplies of electronic equipment.

Reactive power does no physical work but circulates current between the generation point and the load.  This circulating current places a heavier load on the generator and the distribution system.  Click here for more info about the principles of power factor and techniques on how to improve it.

In the residential environment low power factors show up on large motor loads such as HVAC system compressors an pool pumps.  They are also present on smaller motor loads such as refrigerator compressors, wash machines and dish washer pumps.  However, due to the smaller size of these loads, the impact on the whole-house power factor is slight.

Most residential utility rates in the U.S. do not charge for low power factor, but large commercial and industrial customers can be affected.  Residential customers normally pay for kilowatt-hours which is the energy measurement based on real power only. If a utility rate charges for kilovolt amp-hours (KVAH) then both real (KW) and reactive (KVAR) power components are included.  This means you are billed according to length of the hypotenuse on the power triangle shown above instead of the shorter, real power leg.

Large commercial and industrial customers generally do pay a penalty for low power factor.   Although rate schedules vary, it is common to see assessment begin when power factor drops below 95 percent on these large accounts.

Power factor can be corrected with the installation of power factor correction capacitors.   However, these capacitors can introduce additional harmonics into the electrical distribution system.  A properly designed power factor correction system should raise the power factor to a desired target level, typically 95 to 98 percent, and provide adequate mitigation of harmonics.

If your home energy monitor system uses power line carrier technology or X-10, power factor correction capacitors may interfere with the communication signal.  Under certain conditions, these capacitors can act as a sink by absorbing much of the that signal.  If you suspect this to be the case when setting up your home energy monitor, disconnect any capacitors and see if the signal improves.

Harmonics

Harmonics can also affect the operation of power line carrier or X-10 controlled devices.   Click here for a technical discussion on harmonics. As people come to rely more and more on sophisticated electronic equipment in their homes, power quality will play a bigger and bigger role.  Power supplies for all digital equipment; whether it be TV's, computers and peripherals, home automation or home theater; all generate some level of harmonics.

Although residential customers generally do not pay a penalty for the for the efficiency losses stemming from poor power quality - yet, residential rates may adopt penalties in the future.   New energy signature technologies will be built into future electronic devices and appliances that will be used to help the utility manage loads, or so they say.  The information could also be used to generate additional revenue streams for the utility when power quality inefficiencies are detected and quantified.

Understanding the basics of power quality can help you to trouble shoot nagging problems inside and outside of your home electrical system and keep you alert to potential billing changes in the future.  For an expanded technical discussion please visit our Power Factor or Harmonics page.