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There was a sign at my college job: “Engage brain before opening mouth,” which is similar to what my parents would often say: “Think before you speak.” The same basic concept applies before you take a power quality monitor into the field, connect it up and collect gigabytes of data.
The first basic question is whether your primary purpose to conduct a survey (also called an audit) for benchmark data, or are you troubleshooting a potential power quality-related problem? Second, is the survey or troubleshoot for the entire facility or a particular piece of equipment? The answer to both will determine if you start at the point-of-common-coupling (PCC) and work your way into the facility or if you start at a piece of equipment and work toward the PCC. In most facilities, the PCC is at the service entrance where the electric utility meter is located. This will help isolate what comes from the utility and what originates from within the facility.
Though there may be some redundancy in the processes of site power quality benchmark or compliance audits, facilitywide energy surveys, sitewide and equipment-specific troubleshooting, each has its own merits.
Site power quality audits, benchmark studies, preinstallation assessments, quality of supply compliance verifications, or baseline studies are all basically the same type of monitoring task. The audit process follows these steps: plan/prepare; monitor; analyze data for trends, compliance and potential problems; and repeat process periodically or when a significant change is made within facility.
Monitoring should be conducted for at least one week to get a more complete picture of the electrical supply, since different loads occur on different days of the week. In addition, it is preferable to collect data during both the heating and cooling seasons, as the steady-state voltage levels as well as harmonics and transient activity usually vary accordingly. A power quality monitor should be set to capture the full range of power quality phenomena, since by nature of the audit, whatever is out there or not out there is important information. Along with power quality disturbance triggers, the minimum subset of periodic or timed interval data recordings every 10 minutes should include the root mean square (rms) voltage, rms current, power, power factor, frequency, voltage total harmonic distortion, harmonic rms current, flicker, voltage and current imbalance, and the second through 15th individual harmonics for voltage and current.
Once the data is collected, the analysis begins. Compare the information to the acceptable limits of the mission-critical equipment in the facility. If that isn’t available, use the limits in the ANSI C84.1-2006: The American National Standard for Electrical Power Systems and Equipment—Voltage Ratings (60 hertz). Review the data to see which parameters appear near or exceed the usually acceptable limits for such. This may warrant moving the monitor downstream toward particular circuits that are suspected to be the major contributors for these potential problems. The data should be saved for comparison to subsequent reaudits, just like a doctor compares one’s medical history to look for potentially unhealthy changes.
An electrical energy audit at a facility is somewhat similar in process to the power quality audit, but it should start with a review of the electric bills from the utility for the past year. Few facility managers seem to know for what they are really paying, as they don’t realize the demand portion of the electric bill is often higher than the energy consumption; they tend to just focus on the kilowatt-hour readings.
The monitoring should be done preferably for a billing cycle, or at least for one business cycle (which is how often the process repeats itself). To correlate to an electric bill, the monitoring should be done from the first billing date of one month to the next month. Generally, the parameters that we are concerned with for an energy audit are different than the power quality audit, though harmonics can play a part in both types of audits. The demand, energy, power factor, imbalance and harmonic values should be collected and reviewed. The figure shows that the demand at an industrial site follows a similar pattern for five working days (note that one week had a holiday in it). The difference from workday to workday is mostly related to the outside temperature, as the heating, ventilating and air conditioning (HVAC) accounts for a significant portion of the variable demand.
A close examination of the daily power requirements, coupled with knowledge of how the facility operates, can negate the need to turn equipment on and off to figure out what each contributes. If necessary, this can be further refined down to the production line level, or even the equipment level, to determine the efficiency of the equipment versus its nameplate rating. It also can determine the payback for installing newer energy-efficient equipment and lighting. Wasted energy often is in the form of heat, which, in turn, puts a large load on the HVAC system.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.