Debates within the organizations that develop the power quality standards will probably continue for years about why “power” isn’t really the subject of the standards. The majority of the standards present how to measure, monitor, characterize, categorize and set limits on voltage. Two of the best-known and used standards, IEEE Std 1159 and IEC 61000-4-30, give current only a passing reference (Now there are IEC standards about limiting current harmonics in equipment, and also, IEEE 519 gives equal attention to voltage and current, but it often gets misapplied as limits for equipment as well.)
However, since one can’t have power without voltage and current, measuring the current accurately to determine the characteristics of a power source and how the load equipment needs it to properly operate is important even in nonbilling-type applications. While direct current (DC) circuits have the current flowing only from the positive terminal of the voltage source to the negative, alternating current (AC) circuits have it flowing in each direction every other half cycle. This phenomenon allows for a number of other measurement tools that can be used to measure AC current that don’t work for DC circuits.
The use of shunt resistors or in-line current measurement through a digital multimeter inputs aren’t very practical in most power quality applications, which often require connecting to existing (and often energized) circuits that cannot readily be disconnected. To safely check current on such circuits requires current-measuring devices that are rated for the voltage and current levels of the circuit. Failure to properly select such a device can and has resulted in serious accidents. This is especially of concern for technicians using old inventory of current probes that were manufactured back when 600 volts (V) or lower was the common rating, versus the 1,000V CAT III applications today. As always, proper personal protective equipment and safety procedures should be followed.
There are three basic technologies used in current measurement that are “clamp-able” around a conductor. Until recently, the most common probe was the clamp-on current transformer (CT), which works on capturing the magnetic fields associated with AC current flow through wire windings around a split iron core. These CTs typically come in full-scale ranges of 1, 10, 30, 100, 300, 1,000 and 3,000 amperes (A). Since they have an iron core that must be sufficiently magnetized to work properly, they usually are accurate only down to 10 percent of full scale. They often can take over-range of two or three times full scale for a short time. However, refer to the manufacturer’s ratings, as sustained over-range conditions can cause overheating, which can result in meltdown and damage to probe and circuits. The high end of the usable bandwidth may be as low as 5 kilohertz, making them good for harmonics but not suitable for medium- and high-frequency transients. And they definitely don’t like DC current, just like any transformer.
The Rogowski coil (often referred to as “flex probe”) uses the rate of change of current captured by a helical coil of wire that is integrated through electronic circuits to a proportional voltage. The coil is encapsulated in a rubber tube-like housing that separates to allow the coil to go around a conductor. The flexible nature of the rubber allows it to be “snaked” around conductors, where the CT probe jaws can’t easily get around without moving a conductor, which can be hazardous with some old conductors. This flexibility, coupled with a wide operating range, has caused this type of tool to gain popularity. A single probe often has a switchable range to allow it to go from 30A to 300A to 3,000A, requiring less equipment to carry around, especially when you don’t know the full-scale current levels ahead of time. Flex probes also tend to have better low-amplitude response and higher bandwidths. However, they are basically antennas that pick up electrical fields, including stray fields from other conductors and sources. Using them in a room full of large horsepower adjustable speed drives is known to produce erroneous measurements. Hall-effect current probes measure the voltage difference across a Hall element that is subject to the magnetic field from current flow using a semiconductor device. This works for both AC and DC currents, which makes them unique compared with the other two technologies. The latter two have electronics that require power from a battery or other source to operate, which is also a factor when considering long-term monitoring.
In general, review the specs for operating ranges (current, voltage and bandwidth) and amplitude and phase accuracy before connecting to a circuit. Also, proper polarity orientation of the probe to match the direction of the current flow (source to load) is necessary, or the power number will be wrong. With properly connected current probes, you can analyze the full power quality spectrum, look for clues in the data as to what caused it and which direction it came from.