The use of highly energy-dependent machinery is very common in commercial and industrial settings. Keeping the machinery running at optimum efficiency—with an eye on operating
costs—is a constant concern for plant managers and technicians.
One factor that can have important consequences for maintaining the optimal performance from machinery is power quality. Power quality is an often overlooked component of
troubleshooting or routine maintenance.
Low power quality costs money, both in terms of higher energy costs as well as the toll it takes on equipment. When energy is being poorly utilized it results in excess power
usage and can lead utilities to impose financial penalties for poor power factor or high peak demands. Low power quality also increases the cost of maintenance and repair. The
increased risk of equipment failure or damage due to poor power quality also adds cost of replacing equipment, diagnosis and labor.
Better understanding the nature of electricity, the effect it can have on equipment, and how to identify problems with power quality is an important step towards maximizing
plant efficiency, preventing disruptions to manufacturing, and containing costs.
About Electricity
When a piece of equipment is plugged in, it is, in effect, connected to a generating facility, transmission and distribution lines, a meter, and the internal wiring of the plant
in which the equipment is located. It is a very complicated system which provides ample opportunity for the quality of power to be compromised due to variations in weather, generation,
demand and other factors.
Ideally, the AC voltage generated and supplied by the utility perfectly matches the amplitude and frequency as established by national standards and has an impedance of zero ohms at all
frequencies. In the real world, though, no power source is ideal. Power disturbances can involve voltage, current, or frequency and typically manifest as dips, swells, harmonic
distortion, unbalance, flicker, and transients.
Though power disturbances can originate anywhere along the route from generation to usage, the fact is that over 80% of power quality issues come from within the end user’s facility.
Though lightning strikes, accidents, and weather conditions can cause problems with power quality from outside the facility, the odds are significantly higher that any issues are
caused by improper wiring and grounding, overloaded circuits, harmonics or just the simple act of starting up and shutting down large pieces of machinery.
What Is Power Quality?
Power quality describes the relationship between the electrical power and the connected equipment. If every piece of equipment reliably functions under normal operating conditions,
we assume the power is clean. Once equipment starts to malfunction or prematurely fail under standard conditions, poor power quality may be the culprit. As facilities expand
production capabilities or adapt to new technologies, there can be changes to the overall power requirements. Every new installation or upgrade also increases the likelihood of
a power problem (bad ground, loose connection, unbalancing a load, introducing harmonics…) A power quality issue can be described as any deviation from a nominal voltage source.
Although the power utility company is generally the first to be suspected, most power quality issues actually originate from within a facility.
Power quality determines the suitability of electrical power to drive motors, machines and other end user devices. Poor power quality affects the ability of these devices to operate
properly which may cause malfunctions, premature failure or may cause them to not work at all. The general terms given to describe some of the variables in electrical service include
nominal voltage fluctuation, unbalanced loads, transient voltages and currents, and harmonic distortion in the waveforms for AC power.
Particularly important to power quality is the synchronization of the voltage frequency and phase which allows electrical systems to function in their intended manner without
significant loss of performance. With traditional “linear” loads, the wave shape of the steady-state current follows the wave shape of the applied voltage. This means the load will
neither distort the shape of the voltage sine wave, nor cause non-sinusoidal currents to flow in the circuit.
On the other hand, the impedance of “non-linear” loads changes with the applied voltage so that the current drawn by the non-linear load will not be sinusoidal even when it is connected
to a sinusoidal voltage. These non-sinusoidal currents contain harmonic currents that create voltage distortion.
AC power systems which have excessive harmonics can result in a number of power quality issues such as reduced electric motor horsepower output, overheated transformers, very high neutral
conductor currents in three-phase systems, and electromagnetic noise which can interfere with sensitive electrical equipment.
Not long ago, non-linear loads were usually found in heavy industrial applications only. The harmonics they generated were generally localized. This no longer holds true. Increasingly,
we find electrical loads controlled by non-linear components. For example, new power conversion technologies such as the Switch-Mode Power Supply (SMPS) can be found in virtually every
electronic device making them a substantial portion of the total load in most commercial buildings. The highly non-linear loads are rich in harmonics and all the potential problems
associated with them.
Understanding the causes and effects of poor power quality is the first step towards identifying and improving the condition.
Assessing Power Quality
There are a number of tools and techniques available to assess power quality. Oscilloscopes, for example, allow observation of the waveform of AC voltage. Anything other than a clean
sine wave could be an indication of trouble.
Another way to assess power quality, this time without sophisticated equipment, is to use two voltmeters— one an averaging type and the other a true-RMS type—and compare their
readings. Averaging meters are designed to work only with sine waves and will not register proper readings if not a sine wave. True-RMS meters will work with all waveforms. A power
system with good quality power should generate equal voltage readings between the two meters. The greater the difference between the two meters, the greater it’s harmonic content
and the lower the power quality.
Though these methods can give technicians some indication of the quality of their power, for real qualitative analysis there is no substitute for an instrument designed specifically
to assess power quality.
Power quality meters/analyzers are the class of instruments designed to identify issues with power quality. They work by very quickly sampling the AC voltage at many different
points along the waveform shape, digitizing those points of information, and using a microprocessor to perform a numerical analysis to arrive at harmonic frequency magnitudes.
With the ability to monitor current as well, power quality analyzers can also calculate/display common power values. Most power quality analyzers simultaneously monitor multiple
phases to quickly get the whole picture of the system.
Power quality meters/analyzers often have a large digital display and are capable of displaying multiple measurements and graphically representing waveforms. The amount of information
these meters present, as well as the manner in which the information is displayed is very useful in the hands of a skilled technician since different types of nonlinear loads tend
to generate different spectrum “signatures” which help identify the source of the problem.
Like any class of instruments, there is a considerable range of features found within the class. Some types of measurement features found in power quality meters include:
Total harmonic distortion
Total harmonic distortion, or THD is a common measurement of the level of harmonic distortion present in power systems. THD is defined as the ratio of total harmonics to the value
at fundamental frequency and is caused by any non-linear equipment or electronics. Distortion factor is a closely related term, sometimes used as a synonym. %DF is the THD in reference
to the total RMS signal, which is never greater than 100%. Hamonics can wreak havoc on the entire electrical system as the higher frequencies create additional voltage and/or current.
Unplanned circuit tripping and dangerous heat are the most common traits of harmonics.
Power factor
Power factor is the ratio of the real power flowing to the load and the apparent power that is supplied to the circuit. The power factor can range in value from 0 to 1. In an electric
power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase
the energy lost in the distribution system, and require larger wires and other equipment. Electrical utilities will usually charge a higher cost to industrial or commercial customers
where there is a low power factor due to the cost of wasted energy and equipment requirements.
Balance
Three-phase electric power is commonly used for generation, transmission, and distribution of electric power. It is also commonly used to power large motors and heavy loads. The
appeal of three-phase systems stems from them being more efficient and requiring less conductor material.
A three-phase system uses three conductors each which carry an alternating current of the same frequency and voltage amplitude relative to a common reference but with a phase
difference of one third the period. This phase delay gives constant power transfer to a balanced linear load.
In general, it is practical to distribute the loads as evenly as possible across each of the three phases. In practice, however, systems rarely have perfectly balanced loads,
currents, voltages and impedances in all three phases. Generally, the difference between the highest and the lowest voltages should not exceed 4%. Imbalances greater than this may
lead the components, especially motors, to overheat and motor controllers to shut down. In addition, many solid-state motor controllers and inverters include components that are
especially sensitive to voltage imbalances.
Unbalanced loads are also inefficient for the entire electrical system. The supply of electricity must be enough to provide the power required by the highest loaded phase. An
unbalanced load leaves unused capacity which costs money and may lead to fines from the electrical utility.
Phase angle
In AC electrical circuits, the relationship between a voltage and a current sine wave within the same circuit is very important. When capacitors or inductors are included in the
circuit, the current and voltage do not peak at the same time. The distance the waveform has shifted from a certain reference point along the horizontal zero axis is expressed
in degrees and referred to as the phase shift or phase angle. Many analyzers can also display the phasor diagram to graphically show the relationship between the voltages and
corresponding current measurements.
A large phase angle is a sign of inefficiency and lowers the power factor.
Energy cost calculations
Poor power quality can lead to wasted energy. A power factor less than 1 means energy is being wasted. How much does it actually cost to inefficiently run a facility? Certain meters
have the ability to monetarily quantify these energy losses.
Things to consider when selecting a power quality meter:
- Total Power of your system. Single phase? Three phase? Is there any solar involved?
- What Voltage Events do you need to capture? (Transients, Dips & swells, harmonics, flicker)
- What needs to be displayed graphically? (Phasor diagram, harmonics, waveform capture)
- Other calculations or parameters? (Monetize the energy loss, Crest factor, Transformer K factor, power inverter efficiency)
- Are there specific logging or connectivity requirements?
- Does you meter need to meet any particular standard? IEC 61000-4-30 Class, for example?
If you have any questions regarding power quality meters, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
