Checking power line quality with test instruments

The average homeowner probably doesn’t think about the quality of the electrical power coming out of household wall sockets. But that’s not the case for large industrial facilities where capacitive, inductive, and non-linear electrical loads can be large enough to distort the ac waveform. In those cases, electrical power quality becomes an important measurement.

Electric utilities carefully monitor and regulate the RPM of their generators, because their rotational speed determines their frequency and voltage output. Speed deviations are important because the speed of synchronous motors in time-keeping applications is directly related to frequency. (For an interesting set of graphs depicting power-line frequency stability in different parts of the globe, check out the Wikipedia page on power quality.

AC power in all but the smallest back-up power plants is generated and distributed in three-phase format. Three-phase customers connect through step-down transformers to these lines. Power for single-phase customers derives from individual conductors, with line workers taking care to maintain load balance by alternating lines. Driving through a residential neighborhood with aerial services, you can see how this works.

The rotary nature of AC generators gives their voltage output the form of a sine wave. For resistive loads such as heaters and incandescent lighting, the exact shape of the sine wave is not critical. But synchronous and inductive motors, rectifiers at the front end of electronic equipment, and transformers work best when powered from a sine wave free of total harmonic distortion.

Unfortunately, some distribution systems and facility branch-circuit wiring can deliver ac power that is distorted due to the presence of harmonics and/or noise. Originally, the biggest offender was noise, introduced into the distribution system by arcing electro-mechanical relays and arcing brushes in dc motors. Today, a far more serious culprit is harmonic distortion, introduced by non-linear loads. They include fluorescent ballasts, variable-frequency drives and switching power supplies among others. These loads are powerful presences in offices and industrial facilities. They are close electrically and spatially to the equipment most affected by them.

When a motor is observed to run at an elevated temperature or when it cuts out excessively, there can be a variety of causes, such as aging electrical insulation, bad bearings, vibration or binding in the load. If a preliminary inspection does not reveal one of these causes, a good approach is to take a good hard look at power quality. You can begin by talking to the utility engineers. It is possible that other nearby customers are having similar problems, in which case the cause may be located in a neighboring facility and hopefully a solution is imminent. Otherwise, power quality measurements and mitigation are your job.

Assuming we are dealing with a three-phase supply, the first instrument to deploy in checking power quality is the ubiquitous multimeter. One often-used model is the Fluke 87V priced around $400. Useful for all types of electrical and electronic work, it is optimized for VFDs, plant automation and power distribution measurements in high-energy and challenging settings. Since it is a hand-held, battery-operated meter, the inputs are isolated from ground and from one another, so it is safe to take measurements where voltages are referenced to but float above ground potential. For this reason, it can be used to evaluate three-phase, Y-configured systems at 480-Vac VFD inputs and associated dc buses at around 678 V. Pressing the yellow button on the front panel activates an internal low-pass filter, permitting accurate measurements in VFD inverter circuits and at the motor terminals.

Other instruments that are relevant here are the Fluke 434-II, 435-II and 437-II three-phase power quality and energy analyzers. These power analyzers perform a comprehensive array of measurements pertaining to power systems encountered in industrial facilities. Measuring modes are:
• Phase voltages – They should be within 3% of one another and reasonably close to the nominal value, subject to utility parameters. Scope Waveform reveals the waveform shape, which should be a smooth sine wave free of distortion. Dips and Swells records abrupt voltage variations. Transients displays anomalies.
• Phase currents – Volts, Amps and Hertz as well as Dips and Swells compares voltage and current. Inrush Current quantifies motor and other load inrush currents. Variations from nominal should not be greater than 10%.
• Crest Factor pertains to waveform distortion and should not exceed 1.8. Harmonics Mode displays harmonics and total harmonic distortion.
• Flicker reveals short- and long-term rapid fluctuations, which can be recorded in Trend over a user-specified time interval.
• Dips and Swells records abrupt voltage fluctuations of one-half cycle or greater.
• Frequency rarely deviates from nominal. To display frequency, select Volts/Amps/Hz. This metric will be recorded in Trend.
• Unbalance pertains to phase voltage. Maximum deviation from average should be one percent, while current deviation may be 10%. Greater amounts may be caused by source or load. Scope Phase and Unbalance Modes provide details.
• Energy Loss Calculator locates energy loss and calculates resulting cost.
• Power Inverter Efficiency measures inverter output efficiency.
• Mains Signaling checks embedded control signals in the utility feed or branch circuitry.
• Logger provides a large memory for storing multiple readings at high resolution.
• Power Wave is a high-resolution eight-channel scope recorder.

The Fluke three-phase power quality and energy analyzer can log all measured values. Average, minimum and maximum values are logged with a user-specified average time, the default being one second. The logged data is saved on an SD card, and it is available by pressing the Memory key and Function key F1. A maximum of 150 readings can be logged.

Another instrument that performs power quality analysis is the Tektronix MDO3000 Series Oscilloscope. The MDO3PWR Power Analysis Module enables the oscilloscope to analyze power quality, switching loss, harmonics, safe operating area, modulation, ripple and slew rate (di/dt and dV/dt). To access these functions, with the module inserted in the oscilloscope, press Test. Then turn Multipurpose Knob a to select Power Analysis, and press the soft key associated with Analysis.

Power quality is the term generally used and understood to describe the combination of defects discussed above. But power quality is a misnomer because the parameter of interest is voltage. Power is the product of voltage and current. Current is almost exclusively determined by the connected load and should be an issue only insofar as it affects the voltage applied to other loads. In all cases, defective power quality is really a voltage issue and only indirectly pertains to current and power.

In the final analysis, power quality is a compatibility problem. Resistive loads such as electric heat and incandescent lighting are relatively unaffected by most power-quality events. They are primarily impacted by large spikes and generally over-current and lightning-protection devices are capable of preventing damage. Motors and transformers are far more sensitive. An induction motor, not VFD-powered, should not be run at less than rated voltage. A significant reduction will convert the internal windings to resistive heat elements and depending on duration, the electrical insulation will degrade, setting the stage for further heating.

Harmonics, which are at higher frequencies than the fundamental, make for reduced capacitive reactance and greater current flow in conductors that are insulated from one another. In audio and video systems, harmonics, noise and voltage fluctuations negatively impact performance. Electronic filters and reactors can help.