Seems like there is a new power-quality instrument being advertised in the trade journals each month-or at least devices that claim such capabilities in their ads. For PQ purists who have “lived” the power quality monitoring-instrument evolution from the first-generation products in the mid-1970s to the fifth generation of the present day, it is a bit dismaying that every instrument that samples the voltage and/or current waveforms is now classified as having PQ capabilities.
Sure, each of these devices has a value to someone in some application. But the power-quality phenomenon encompasses such a wide range of parameters and disturbances to measure, usually simultaneously since they often interact with each other. It can be misleading to the user and confound the search for the source of the problem if the instrument you are using is “playing with only half the deck.”
Despite attempts by standards generating groups, such as the IEEE and IEC, to produce documents that define what and how to measure, there is still a large amount of “wiggle” room and even ambiguity in the standards that allows instruments to measure and record at the same monitoring point and produce different results. Take the simple term “volt-amperes” or VA. Which formula that is used to derive such from the voltage and current waveforms can generate significantly different numbers.
There is the most common definition, Vrms ¥ Irms: Read the voltmeter, read the current meter and multiply the results. However, in today's electrical environment of distorted and unbalanced voltage and current, there are several other ways to compute VA, including arithmetic VA, vector VA and effective VA. Knowing which one to use in which situation, and knowing which one is used in the instrument that you are using, can add more to the confusion, especially when trying to understand what is really happening on the electrical system.
In past articles, we have discussed how to use Kirchoff's and Ohm's Laws to understand what causes sags and swells as well as harmonics and transients. These rules apply to just about every monitoring situation. These same rules can be used to determine if what you are seeing in the recorded data makes sense and passes the test of reasonableness or if some other explanation needed.
As a quick reminder, Ohm's Law states the voltage is equal to the current multiplied by the impedance. Impedance is a combination of the resistance, inductance and capacitance. The latter two are frequency dependent. For inductors, the impedance goes up as the frequency increases, such as from harmonic currents. This is a factor in why the losses in electromechanical devices, such as motors and transformers, increase with harmonics, resulting in the need to derate them in the presence of such.
Kirchoff's Laws state that the sum of the currents entering and leaving any node or connection point must equal zero, and the sum of the voltages around a closed loop or circuit must equal zero. At the service entrance or point of common coupling (PCC), the current supplied by the electric utility is equal to the current distributed through the facility through the various distribution panels and circuits. It must go somewhere, even if diverted into ground currents. If there are harmonic currents from the facility being fed back to the grid, then these also must be equal. A small caveat here-sometimes the harmonic currents will be out of phase with each other and end up canceling each other out at the PCC. For example, if the third harmonic from one load is 180 degrees out of phase with another load, the net result will be no harmonic current flow at the connection point. This has been found to be particularly true with the higher-order harmonics.
On the voltage side, the voltage provided by the generator must equal the drops in the wiring and transformers and other in-line type impedances (referred to as the source impedance), plus the voltage across the loads. If the current from the loads increases significantly, then there will be additional current flowing through the source impedance, which will result in a larger voltage drop across that, leaving less voltage for the load (assuming the generator is stiff enough to provide more current with no appreciable voltage decrease).
So if you are recording data and find things happening-for example, the voltages make a sudden phase shift or drop out without any change in the current-you want to look twice at your data, connections, setups and even the monitor itself. To suddenly change voltage without any effect on the current would be somewhat unusual. Sometimes it is a bad measurement lead connection, which can vibrate with passing trucks or large motors in the facility, and produce open circuits in the measurement leads, rather than the electrical circuits themselves. Or there can be a large burst of high-frequency current transients, yet no change in the voltage waveform. Check the environment you are monitoring. If it is a room with lots of large HP ASDs, the electromagnetic fields from such may be coupling into the probes or the wiring connecting the probes to the instrument, producing false data.
If you have data that tends to require you to rewrite the laws of physics to explain it, apply the test of reasonableness first, before trying to get your name in lights and capture the Nobel Prize. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.