When setting out to monitor any power system, it is very important to determine what type of wiring configuration is used before making the connections. The primary and secondary sides of transformers are often different wiring configurations, as can be the loads from the source.
This affects not only how you would connect the voltage and current probes, but also what you will see in the data. Despite claims by some instrument vendors, the correct power (watts) data cannot always be created later from the RMS values of incorrectly connected voltage leads and current probes, and definitely not without the waveforms saved, which would require virtually infinite memory capacity.
Poly-phase circuits are often three-phase circuits in most industrial and commercial electrical distribution systems with wye and delta the most common. Loads in a wye circuits can be connected line-to-neutral as well as line-to-line, which can make the phase power flow calculations a bit confusing.
Most single-family residential dwellings have a configuration called “split phase,” which is really just a single-phase of a three-phase circuit with a site-created midpoint—the notorious neutral connection.
Checking for balanced current flow is easier here, as half of the residential load should be fed off one of these created single phase circuits, and the rest off the other, being 180 degrees apart from each other in phase, though there also are some loads that do span across circuits.
Failure to keep the load current reasonably balanced on the two single-phase circuits can actually “move” the neutral point, especially if the impedance of such to ground isn’t small enough.
Two additional parameters that are possible to mathematically calculate are the residual and net currents. The residual current is the vectorial sum of the phase currents for delta circuits and the net current, which is the vectorial sum of the phase and neutral currents in a wye. Both of these parameters can be a proxy for leakage or ground current in the respective circuits. This beats trying to put a gigantic current probe around a building or every possible ground conductor path and turns your four channel current instrument into a five channel at no extra cost.
A significant difference between a delta and wye circuit is relative to the current flow. Looking at a typical wye connected load, the current flowing down a phase conductor flows through one leg or element of the load into the center conductor or neutral and back to the source. This holds for each of the three phases.
In a balanced circuit with no harmonics, the 120-degree phase shift of each current results in a canceling effect in the neutral, and no current flow (see Figure 1). The measured phase shift between the voltage and current is an indication of the power factor of that leg. Any loads connected across the phases act as delta connected loads, with a different set of rules.
In a delta circuit, the current coming down the phase conductor will divide between the two legs that are connected at each corner. If the impedances of each leg were identical, then the current would divide evenly. Since this is often not the case, the ratio may not be known. If the load were purely resistive, there will be a 30-degree phase shift between the voltage and current (see Figure 2).
Any power factor calculation on an individual phase would be altered by this inherent phase shift. Often only the three-phase power factor is of any value. And quick calculation shortcuts that worked in the past aren’t often valid anymore, such as using the square root of three as a multiplier between line-to-neutral and line-to-line voltages as well as multiplying one-phase power by three to get the full three-phase power. These relationships aren’t valid with imbalances or harmonics present.
In the world of power quality, these factors about the type of circuit are important when selecting a monitor, connecting the monitor and analyzing the data from it.
For example, if the monitor does not have differential inputs on the voltage input channels, it is probably designed to monitor the voltage phases with reference to the “neutral” voltage input.
However, a delta circuit may not be referenced or connected to any grounded conductor. The voltage waveforms and measurements will be made relative to this “pseudo neutral,” which is not how the loads are connected. This means that what the monitor is seeing is not what the load is seeing, which is what really matters.
For example, if a rapid reduction of equal magnitude of the voltage occurs on two phases relative to the neutral, the load connected across those phases would not have experienced any disturbance.
Conversely, if a disturbance occurs that causes change or shift in the 120-degree phase relationship between two phases, the voltage measured between each of the phases and the pseudo neutral may remain the same (indicating that nothing has happened), yet the voltage between the two phases that is powering the load may be sharply reduced.
Therefore, it is important that the monitoring be done consistent with how the loads are connected—whether it is a delta or a wye really does matter. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.