Using Handheld Meters to Track PQ Problems

Dec 1, 2002 12:00 PM, By Mark McGranaghan, Electrotek Concepts, Inc.

Despite the industry trend toward continuous power quality monitoring, not every facility or circuit is equipped with permanent equipment.

Most industry experts advocate permanently installed power quality monitoring because it can continuously track the power quality characterizations of a facility and access pertinent information when problems arise. However, it may also be necessary to check voltage and current on individual circuits and at individual loads to determine the cause and effect of power quality variations. This is where handheld meters come into play.

Let's take a look at the different types of handheld meters and their role in power quality investigations. We'll start with the simplest models and work our way up to the more sophisticated offerings.

Multimeters.

You can use a multimeter to check voltages and currents and 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. Some multimeters calculate the rms value based on other methods, such as the peak or rectified average of the measured signal. These methods assume the measured signal is sinusoidal and will give inaccurate readings for most facilities because a high percentage of loads have current waveforms that include significant harmonic components. Most digital models on the market today provide true rms readings.

Multimeters can also help you find 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 shouldn't flow in the ground system, and the return load current should flow only in the neutral conductor. Ground-system currents can find many paths for returning to the source, including building steel, communications wiring, and computer networks. All of these have the potential to cause interference and misoperation of equipment.

Using the multimeter, start at the service entrance and check for currents in the ground circuits. If you find 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.

Harmonics meters.

To obtain detailed harmonic information about measured waveforms, you'll need a portable harmonics meter. You can use the meter to check waveform characteristics for different types of loads in the facility and surrounding voltage distortion levels.

There's more to harmonics meters than just harmonics, though. For example, harmonic distortion concerns are often related to the application of capacitor banks. You can use these meters to check the effect of capacitors or harmonic filters on distortion levels and the harmonic duty for capacitors or filters.

In facilities where the load current through the transformer includes significant harmonics levels, de-rating the transformers may be necessary. With a harmonics meter, you can check the loading and calculate the transformer de-rating requirements based on the harmonic content.

Many commercial facilities that have 3-phase circuits with single-phase electronic loads experience significant problems with the overloading of neutral conductors. These loads have waveforms that include high levels of third harmonic, which can lead to higher neutral currents than phase currents. You can employ handheld harmonics meters to identify this problem and evaluate the effectiveness of possible solutions like zigzag transformers.

Oscilloscopes.

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. Oscilloscopes record waveforms with high-frequency transients, identify their characteristics, and help you locate their source and evaluate possible solutions. You can then view the waveforms on the oscilloscope's display window or download them to a PC for analysis.

Handheld oscilloscopes are also well-suited to investigating high-frequency disturbances caused by the normal operation of equipment. For example, electronic equipment sometimes causes notching of the voltage waveform. These notches include high-frequency components that can lead to communications 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, which may produce insulation degradation in motors that aren't 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 records measurements directly at the equipment or device causing the disturbance.

Full-range meters.

Some handheld meters perform the full range of power quality monitoring functions. They can serve as a simple multimeter, or can be programmed to record voltage variations, distortion levels, and transients. Some even allow you to add function and capabilities, such as inrush or flicker, by simply changing a PC card.

This type of meter is probably more than you need to 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 of time to characterize power quality variations. It's also essential for diagnosing intermittent power quality problems that can affect or even be caused by equipment like adjustable speed drives, microprocessor-based production equipment, or today's advanced computer systems.

Full-range meters also handle all 3-phase voltages and currents for simultaneous measurements, which isn't possible with most handheld oscilloscopes or harmonics meters. Software is available to help you process the measurement results and present them in a report.

Keeping tabs on your system is a full-time job, and if you want the most accurate and up-to-date information, you need a permanently installed monitoring system. But such a tool isn't always the optimal choice for getting the job done. When it comes to getting to those hard-to-reach places, your best bet is choosing from the range of handheld testers now available.

McGranaghan is vice president of Electrotek Concepts, Knoxville, Tenn.