Despite the industry's trend toward continuous power quality monitoring, not every facility or circuit has permanent equipment. Fortunately, there are a wide variety of portable, handheld meters available. And even if your facility has permanent monitoring equipment in place, these meters can help you track down problems. So let's take a look at the different types of handheld meters and their roles in power quality investigations. We'll start with the simplest ones and work our way up to the more sophisticated offerings.
You can use multimeters to check voltages and currents throughout your system. This is often enough to uncover problems with voltage regulation, circuit loading, neutral conductor overloading, or ground loops.
Be sure to use a multimeter that provides a true rms reading for the measured voltage or current. The reason is because some multimeters calculate the rms value based on other methods (the peak or rectified average of the measured signal). These methods, which assume the measured signal is sinusoidal, will give inaccurate readings for most facilities because a high percentage of loads have current waveforms that include significant harmonic components. Table 1, on page 54, shows how different methods for a variety of signals can produce errors. Most digital models on the market today provide true rms readings.
Multimeters also are ideal for finding ground loop problems. Ground loops occur when a system has more than one neutral-to-ground connection, which allows current to flow within the ground system. Normally, current should not flow in the ground system, and the return load current should flow only in the neutral (or grounded) conductor. Ground-system currents can find many paths for returning to the source, including building steel, communication wiring, and computer networks. All of these have the potential to cause interference and misoperation of equipment.
With the multimeter, start at the service entrance and check for currents in the green wire ground circuits. If there is load current flowing in these circuits, work your way to the appropriate sub-panels and/or individual loads until you find the neutral-to-ground connection causing the problem.
To obtain detailed harmonic information about measured waveforms, you'll need a portable harmonics meter (Photo). These meters allow you to check waveform characteristics for different types of loads in the facility and surrounding voltage distortion levels.
Other situations lend themselves to harmonics meters. For example, harmonic distortion concerns are often related to the application of capacitor banks. Use these meters to check the impact of capacitors or harmonic filters on distortion levels and the harmonic duty for capacitors or filters.
For facilities where the load current through the transformer includes significant harmonics levels, derating the transformers may be necessary. With a harmonics meter, you can check the loading and calculate the transformer derating requirements based on the harmonic content.
Many commercial facilities that have 3-phase circuits with single-phase electronic loads find neutral conductor overloading a significant problem. These loads have waveforms that include high levels of third harmonic, which can lead to higher neutral currents than phase currents (Fig. 1). You can employ handheld harmonics meters to identify this problem and evaluate the effectiveness of solutions, such as zigzag transformers.
Oscilloscopes are handheld instruments that record waveforms with high-frequency transients (see small photo on page 52). The waveforms are then viewed on the oscilloscope's display window or downloaded to a PC for analysis. You can use these meters to identify transient characteristics, which will help locate the transient's source and evaluate possible solutions.
Handheld oscilloscopes also are ideal for investigating high-frequency disturbances caused by the normal operation of equipment. For example, electronic equipment sometimes causes notching of the voltage waveform (Fig. 4). These notches include high-frequency components that can lead to communication interference or other problems.
The output waveform from modern, adjustable-speed motor drives that use pulse-width modulation is another concern. This output waveform has pulses with high rates of rise (Fig. 5), which may produce insulation degradation in motors that are not designed to handle voltage stress. Even the switching power supplies for PCs and other small electronic equipment can sometimes cause transients that interfere with sensitive loads. An oscilloscope conveniently records measurements directly at the equipment or device causing the disturbance.
In the past, oscilloscopes were large laboratory instruments. Today, thanks to advances in microelectronics, the handheld designs have more capability than many bench-top oscilloscopes.
Some handheld meters perform the full range of power quality monitoring functions. The large photo on page 52 shows one example. You can use it as a simple multimeter, or you can program it to record voltage variations, distortion levels, and transients.
This type of meter is probably more than you need to walk around the facility and track down wiring and grounding problems, but it can come in handy when you need to leave the meter in place over an extended period to characterize power quality variations.
Full-range meters also handle all 3-phase voltages and currents for simultaneous measurements, which is not possible with most handheld oscilloscopes or harmonics meters. Software is available to help you process the measurement results and present them in a report.
Handheld meters are available for almost any power quality problem. For a convenient summary of the different meters and the corresponding problems they can evaluate, see Table 2.
Like many industry experts, I advocate permanent monitoring. This approach allows you to continuously track the power quality characterizations of your facility and access pertinent information when problems arise. However, it is often necessary to supplement permanent monitoring with checks of voltages and currents on individual circuits and at individual loads to determine the causes and impacts of power quality variations. Handheld meters can be ideal for this purpose.