This application note details the procedures for using Fluke power quality analyzers for average watt measurements and converting those to watt-hours.
New technology, new challengesHarmonics are the byproducts of modern electronics. They are especially prevalent wherever there are large numbers of personal computers, adjustable speed drives, and other types of equipment that draw current in short pulses.
Getting to the root of the problemFinding the problem is relatively easy once you know what to look for and where to look. Harmonics symptoms are usually anything but subtle. This application note provides some basic pointers on how to find harmonics and some suggestions of ways to address the problems they create.
Sources of Harmonics
Defining the problemHarmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. For example, if the fundamental frequency is 60 Hz, then the second harmonic is 120 Hz, the third is 180 Hz, etc.
Harmonics are created by non-linear loads that draw current in abrupt pulses rather than in a smooth sinusoidal manner. These pulses cause distorted current wave shapes which in turn cause harmonic currents to flow back into other parts of the power system.
The inside storyThis phenomenon is especially prevalent with equipment that has diode-capacitor input power supplies; i.e., personal computers, printers and medical test equipment.
Electrically what happens is the incoming ac voltage is diode rectified and is then used to charge a large capacitor. After a few cycles, the capacitor is charged to the peak voltage of the sine wave (e.g., 170 V for a 120 V ac line). The electronic equipment then draws current from this high dc voltage to power the rest of the circuit.
Effects of Harmonics CurrentsSymptoms of harmonics usually show up in the power distribution equipment that supports the non-linear loads. There are two basic types of non-linear loads: single-phase and threephase. Single-phase, non-linear loads are prevalent in offices, while three-phase loads are widespread in industrial plants.
Each component of the power distribution system manifests the effects of harmonics a little differently, yet all are subject to damage and inefficient performance if not designed to handle electronic loads.
Neutral conductorsIn a three-phase, four-wire system, neutral conductors can be severely affected by nonlinear loads connected to the 120 V branch circuits. Under normal conditions for a balanced linear load, the fundamental 60 Hz portion of the phase currents will cancel in the neutral conductor.
In a four-wire system with single-phase, non-linear loads, certain odd-numbered harmonics called triplens - odd multiples of the third harmonic: 3rd, 9th, 15th, etc - do not cancel, but rather add together in the neutral conductor.
Excessive current in the neutral conductor can also cause higher-than-normal voltage drops between the neutral conductor and ground at the 120 V outlet.
Circuit breakersCommon thermal-magnetic circuit breakers use a bi-metallic trip mechanism that responds to the heating effect of the circuit current. They are designed to respond to the true-rms value of the current waveform and will trip when the trip mechanism gets too hot. This type of breaker has a good chance of protecting against harmonic current overloads.
A peak-sensing, electronic trip circuit breaker responds to the peak of current waveform. As a result, it won't always respond properly to harmonic currents.
Since the peak of the harmonic current is usually higher than normal, this type of circuit breaker may trip prematurely at a low current. If the peak is lower than normal, the breaker may fail to trip when it should.
Bus bars and connecting lugsNeutral bus bars and connecting lugs are sized to carry the full value of the rated phase current. They can become overloaded when the neutral conductors are overloaded with the additional sum of the triplen harmonics.
Electrical panelsPanels that are designed to carry 60 Hz currents can become mechanically resonant to the magnetic fields generated by higher frequency harmonic currents. When this happens, the panel vibrates and emits a buzzing sound at the harmonic frequencies.
TelecommunicationsTelecommunications systems often give you the first clue to a harmonics problem because the cable can be run right next to power cables. To minimize the inductive interference from phase currents, telecommunications cables are run closer to the neutral wire.
Triplens in the neutral conductor commonly cause inductive interference, which can be heard on a phone line. This is often the first indication of a harmonics problem and gives you a head start in detecting the problem before it causes major damage.
Finding HarmonicsA harmonic survey will give you a good idea if you have a problem and where it is located. Here are a few guidelines to follow.
- Load inventory.
- Tranformer heat check.
- Transformer secondary current
- Sub-Panel neutral current check
- Receptacle neutral-to-ground voltage check
Troubleshooting ToolsTo determine whether you have a harmonics problem you need to measure the true-rms value and the instantaneous peak value of the wave shape. For single applications, you need either a clamp meter like the Fluke 335, 336 or 337 or a multimeter like those in the Fluke 80, 170 and 180 Series that makes true-rms measurements. For three-phase applications, you'll need a power quality analyzer like the Fluke 430 Series.
"True-rms" refers to the root-mean-square, or equivalent heating value of a current or voltage wave shape. "True" distinguishes the measurement from those taken by "average responding" meters. The vast majority of low-cost, portable meters are average responding. These instruments give correct readings for pure sine waves only and will typically read low by as much as 50 percent when confronted with a distorted current waveform. True-rms meters give correct readings for any wave shape within the instrument's crest factor and bandwidth specifications.