Published In February 2007
From A to Z, power quality in the education market takes on added importance:Consider these power quality (PQ) definitions and their effect on the built environment:
ALWAYS follow proper safety procedures, including NFPA 70E, the Occupational Safety and Health Administration materials, the equipment manufacturer’s operator’s manuals and other applicable instructions. Shortcuts can be deadly.
BEGIN MONITORING at either the point of common coupling (where the electric utility and the building service meet) for a facility study or at an individual piece of equipment experiencing a power quality related problem, and then work toward the other end.
CORRECT CONNECTION of voltage leads and current transducers are very important for power measurements. The correct power-related parameters can’t be determined after the fact.
DELTA WIRING CONFIGURATIONS don’t have a path for the triplen harmonics (3, 6, 9, 12, 15, etc. ) to exit, so they circulate in the transformers, resulting in heating (see Y).
EYES are one of the most important power quality tools. They can spot the source of some problems, even without a PQ monitor.
FOURIER TRANSFORMS are mathematical processes that can take a sampled waveform and determine the harmonic and interharmonic content of such.
GROUNDING is often on the list of things to look for as causing power quality problems. However, it is a misconception that the path to ground must be nearly zero to prevent power quality problems. That is for safety. If it were true for power quality, then airplanes and ships wouldn’t operate without a very long wire. An equipotential grounding system matters more for power quality.
HARMONICS are integer multiples of the fundamental frequency, such as 2 × 60 = 120 Hz and 3 × 60 = 180 Hz, and they are usually caused by non-linear loads.
I—THE SYMBOL USED for current in most equations is the letter I, as in V = I × R. Though you can monitor for power quality disturbances without current, it is much easier to find the source of the problem if it is also monitored.
JUMPERING the neutral and ground (or grounded and grounding conductors) at other than the service-entrance bonding point or at a separately derived source can be both a safety and power quality problem.
KIRCHOFF’S AND OHM’S LAWS are the basis for solving most power quality problems. If you’re not familiar with them, do a search on the Internet for some tutorial material.
LOSSES in the source and load impedances are a common cause of power quality events. More current equals more voltage drop.
MOST POWER QUALITY PROBLEMS originate from within a facility, from large or harmonic generating loads turning on, or changes in the electrical distribution system. Find out from the facility managers what has happened recently if the problems suddenly start appearing.
NEUTRAL-TO-GROUND measurements can be an important part of monitoring a wye, split or single-phase system. If the measurement is zero and is not made at the bonding point, then there is probably an illegal neutral-to-ground bond in a panel or outlet. Likewise, if it is several volts or more, there is probably excessive current flow in the neutral or miswiring.
OHM’S LAW is the voltage is proportional to the current multiplied by the impedance (see K).
POWER has many different parameters, including watts (real power), VA (apparent power), VAR (reactive power), PF (power factor = W/VA) and DPF (displacement power factor or cosine of angle between V and I).
QUESTION the measurement process if you get results that would contradict the laws of physics, such as a large change in the voltage without any change in current.
RESISTIVE TYPE ONLY loads aren’t that common in today’s electric world, except in equipment such as electric heating elements.
SAGS are a decrease in the voltage below 90 percent of nominal, and they are usually the most common type of power quality event. They often result from a abnormal and large increase in current, either on the distribution system from a fault, or from a load starting up in a facility.
TRANSIENTS are very short duration disturbances, often measured in microseconds. They can be impulsive in nature (such as a lightning strike) or oscillatory (power capacitor switching).
U—IN SOME PARTS of the world, U is the parameter for voltage, rather than V. VOLTAGE QUALITY is often used as a substitute for power quality, especially in compliance type monitoring such as EN50160.
WATTS are the measure of real work being done by the voltage and current, resulting from the resistive part of the loads. Reactive power is the inductive and capacitive component and basically just goes back and forth in the distribution system without doing real work, but the system must be sized to handle it.
XMFR is a common abbreviation for transformers. In today’s world, xmfr units are often overloaded due to losses from harmonics and increased demands from additional equipment.
Y —WYE WIRING configurations have a grounded conductor or neutral that is supposed to have very little current flowing. Unbalanced current levels or significant triplen harmonics are additive in the neutral and can actually exceed the phase currents.
Z—IS THE LETTER USED to represent impedance, which is the combination of resistance, capacitance and inductance, the last two of which are frequency dependent.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.