As last month’s article indicated, verification that voltage sags were still the most common power quality disturbance couldn’t be determined. This doesn’t mean we don’t need to be concerned about voltage sags, because they can have a significant financial impact or even cause life safety issues. Using the journalistic tool of asking what, where, when, why and how can help us better understand sags and determine whether voltage sags still hold the crown.

The standards groups have defined sags (or dips) with significant detail and testing procedures that allow data from different PQ instruments to report the same information about the same disturbance. Skipping the technical details, a voltage sag is a reduction in the rms voltage from the nominal voltage level that lasts from a fraction of the fundamental power frequency to up to one minute. They fall into the general category of rms variations, with voltage swells being the counterpart as an increase in rms voltage. 

These rms variations happen continually in most electrical systems but are too small to cause any problems, though they may be an indication of a future problem. If the remaining or residual voltage fall below the threshold (usually 90% of nominal voltage), then the disturbance is labeled a voltage sag. That doesn’t mean all equipment experiencing such will malfunction. Over time, enough equipment will experience issues, so it’s a common reference point defined in most standards.

How many?

The remaining voltage magnitude is the first clue to be aware of. The second is how many of the voltage phases are affected in a polyphase circuit. A single-phase line to ground fault usually has a different source and effect on the load than a three-phase disturbance. Based on the wiring configuration of the transformer feeding the facility, what happens on the utility side can be reflected differently on the facility side, especially if there is a delta-wye or wye-delta transformation. Knowledge of such can help determine what the source is if it occurred on the source side of a transformer.

What data? 

The last “what” data needed is how long it lasted, referred to as the duration. Again, the standards have spelled out how it is accurately and repeatably identified. It’s basically determined from when the rms voltage of a cycle went below the limit until it either returns back above or until it goes even lower, below the interruption voltage limit. A disturbance will change from a voltage sag to a voltage interruption if the origin is on the distribution system feeding the facility, and the substation breaker opens to attempt to clear the fault. 

All those downstream from the breaker see the interruption, while those on parallel feeders from the substation would see the sag terminate and the voltage go back to normal. This is typically the case when the duration of the voltage sag is 6 to 10 cycles long, which is a common fault-clearing time for a distribution system. Transmission protection systems operate faster, as more energy is involved, and it is best to clear it faster. Distribution breaker protection schemes want to make sure it is not a momentary situation, such as the wind causing the wire to make a self-clearing ground fault, before opening the circuit.

But why?

Why they occur is determined by where the origin is and the how of Kirchhoff’s and Ohm’s laws. The electric grid operators try to keep the voltage within tight limits, compensating for the amount of current consumers are drawing from the system. A sudden increase in current will result in a voltage loss through the source impedances, leaving less for the loads. 

The sudden increase in current when a large-horsepower motor is being energized within the facility will result in a voltage sag. The inrush current can be 6 to 10 times larger than the running current, causing a decrease in the voltage for the duration of the inrush current. After that time, the voltage will be slightly lower than before the motor started since there is still a voltage loss from the current during the steady-state operation of the motor until voltage regulation equipment can adjust for such (if present). Knowing when the voltage sag occurred is a helpful data point in determining causality, and it is easier when the facility has a permanently installed PQ monitoring system. 

There are some PQ monitoring instruments and systems that can record all the data needed to give you the information and answers as to the what, when, where, why and how questions. This doesn’t remove the need for the electricians and facility managers to understand these things, as, in the end, they are the ones that need to fix the problems to keep them from the financial losses from lost production and damaged equipment from voltage sags.

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