Power Quality | Common Issues Defined

Understanding Power Quality Terms

Power Quality? Voltage sags? Voltage swells? What are they? We Americans expect that when we plug into an outlet, we will receive 120 Volts all the time, every time. Outages are rare here in North America, so most of the time, 120 Volts (plus or minus 10%) are available.

The trouble is when this changes…

Terms to Know

Power Quality

Power Quality is the electrical engineering discipline. It is associated with quantifying the transient and short-term changes in delivered voltage, current, and frequency when the 120 Volts are not available.

Voltage Sags

Voltage sags are short-duration reductions of the delivered voltag. When the voltage is below 90% of normal for a period of 1/2 cycle (0.0333 seconds) to 1 minute.

Typical Voltage Sags plotted on a CBEMA/ITIC curve

Voltage Swells

Voltage swells are short-duration increases of the delivered voltage. When the voltage is above 110% of normal for a period of 1/2 cycle (0.0333 seconds) to 1 minute (60 seconds).

The voltage and the duration define both voltage sags and swells. Engineers use acceptance curves (i.e., CBEMAITIC, or SEMI F47 curves) to plot the magnitude (in percentage) of voltage sags or swells against the duration (time from start to recovery to normal).

Below is a real-world example from last year (2020) in the San Francisco bay area.

2020 PQ Events
SF Bay Area 2020 Power Quality ITIC Curve

If voltage sags or swells plot within the curve, electronic equipment will ride through without impact. These events are shown in green.

If voltage sags or swells plot outside the curve, it will likely impact electronic equipment. These events are shown in red.

Note that outages are voltage sags, too. They end up with zero voltage and typically last longer than one minute.

Power Quality | Key Points

Voltage sags and swells happen in the utility grid’s normal operation

Customers typically are not aware of the voltage sag or swell until their equipment is affected.

The impact on equipment from a voltage sag or swell depends on the magnitude and duration

It typically results in equipment shutdown from less severe sags/swells to actual equipment damage if the sag/swell is severe enough.

Electric utilities do not monitor these short-duration power quality events or the impact on their customers.

The utility relies on their 99.9% or better track record of delivering voltage to the customer. Moreover, they hope the consumer doesn’t do the math and realize 99.9% still means more than 8 hours of out-of-spec voltage delivered every year.

Whether you are at home or your business, voltage sags and swells are happening every day. If you want to understand what those changes are doing to your operation, the responsibility is on you to measure and record the power quality events.

Are you tracking voltage sags and swells at your location? 

Call APT. We provide the instruments, software, and services to help you track utility PQ events like voltage sags and swells that impact your operation.

Get this information to the people who need to know within your organization!

Andrew E Taylor, PE Chief Executive Officer APT