Look Both Ways… Then Look Again: Connecting power quality monitors the right way

Diagram of wiring and load configurations | Richard P. Bingham
Wiring and load configurations | Richard P. Bingham

Though many never experienced this, my generation had pedestrian and bicycle training using projectors paired with vinyl records that went “BEEP!” to alert the projectionist to advance the film strip. Jiminy Cricket, the star of the show, said about crossing the street: “Look both ways … then look again.” This is also good advice when connecting a power quality monitor. Without an understanding of what is upstream (toward the source) and downstream (toward the loads), the data collected can be easily misinterpreted or even invalid.

The figure below shows a typical electrical system in an industrial/commercial facility. The electric utility provides voltage that is much higher than what’s needed within the facility, so it is stepped down using the transformer, which is usually located just outside the facility near where the electrical service enters the building to the main panel.

The transformers are typically either a delta-wye or delta-delta winding configuration. Delta configurations don’t allow the triplen harmonic currents (3, 6, 9, 12 …) to pass through from one side to the other. They do, however, circulate within the delta wiring and need to be compensated for. Wye configurations allow for a neutral (or grounded) conductor, though there are some high-leg or center-tapped delta secondaries still around that have such.

In the configuration shown here, there are three different types of loads. L1, L2 and L3 are single-phase loads connected from one of the phase conductors to the neutral conductor. Whereas the neutral conductor in the past had very little current flowing, that is not very common anymore with the proliferation of nonlinear loads. The triplen harmonics are additive in the neutral conductor, which can raise the Vn as the circuit goes further from the transformer. Knowing how far the monitoring location from the service panel is located can help determine if a Vn-Vg reading of 0.0, 0.5 or 2.5 volts (V) is considered a potential problem. A voltage of 0.0 should only occur very near the service panel where the bond between the grounding conductor and the grounded conductor or neutral is. A reading of 0.0 at a distance from the service panel likely means that there is an illegal neutral-ground bond in another location nearby. A reading of 0.5V for Vn-Vg is generally considered a reasonable value. Values over 1 require further investigation to find the source of the elevated voltage.

Monitoring a single phase load, such as L1, L2 or L3, is a straightforward connection process in most cases. Voltage leads connect the [+] channel to the phase voltage, the [-] channel to the neutral, and another [+] channel with its [-] channel connected to Vn and Vg, respectively. The current probe is connected around the phase conductor going to the load with the arrow pointing towards the load. If not diligent, the power parameters (watts [W], power factor [PF], kilowatt-hours [kWh], etc.) will not be correct.

Load L4 is an L-L load, connected between two of the phase conductors. There can be three L-L connections to the load or it can be just one, such as a residential oven or 14,000-Btu air conditioner. For a three-phase load, a current probe is connected to each of the phase conductors, again pointing the arrow towards the load. The voltage leads can be connected with one channel’s [+] going to Va and its [-] to Vb; the next [+] to Vb and [-] to Vc; and the last [+] to Vc and its [-] to Va. With some monitors, there are just the phase connections to each [+], and the [-] or common terminal goes to Vn.

Monitoring a delta load is perhaps the most challenging to understand. The current that comes from each phase voltage conductor is split between two internal loads. For example, part of L1 goes through M1 and part through M3. The same is true for the other phase currents. If one assumes that all three internal impedances (M1, M2 and M3) are identical, then the split would be the same. But this isn’t always true. There is often a voltage imbalance due to other loads in the system. This creates current imbalances. The old rule of using the square root of 3 to convert from L-L voltage to L voltage (and for phase currents) doesn’t hold up with today’s nonlinear and unbalanced systems. The phase current has a 30-degree phase shift from the L-L voltage that is not part of the power factor. It makes looking at phasor diagrams a bit challenging at first.

Lastly, the step-down transformer configuration also can make it challenging to determine what happened on the utility system. With a delta-wye transformer, a single-line-to-ground fault on the primary side will show up on the secondary as a sag to 88% for two L-L voltages and 33% on the third. The L-N voltages will be a sag to 58% on two, but no sag on the third.

Before you connect your power quality monitor, look both ways and see how you should connect the instrument and what will affect the data that you’re measuring so you can see what’s really happening.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

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