Hundreds of power quality measurements monitor electrical systems on your motors and drives. But these six, specific measurements can help uncover hidden issues in a plant’s energy use which often lead to additional costs, equipment damage, and even unscheduled downtime.
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In a balanced, three-phase system, the phase voltages should be equal or very close. An unbalance in these measurements can cause poor performance or premature failure. These large problems can be caused by smaller issues that are much easier to fix if caught during regular, preventive maintenance measurements, such as:
The major costs are associated with motor replacement and lost income due to circuit protection trips. Replacing equipment combined with the cost of labor for the work and the unplanned downtime can add up quickly.
To catch voltage unbalance issues, look at the inputs to motors, VFDs, and UPSs. It’s important to know how much of an unbalance should raise a red flag. According to the EN power quality standard for required voltage unbalance, as a ratio of negative to positive sequence components, it should be less than 2% at the point of common coupling. NEMA specs call for less than 1% for motor loads. Consult your user manuals for other equipment. NEMA MG 1 states the motor must be derated for unbalance greater than 1%.
Figure 1. Taking preventive maintenance measurements with a power quality analyzer, like the Fluke Power Quality Analyzer can help you catch energy issues before they become too large.
Total Harmonic Distortion (THD) is all harmonics on an asset combined. Some current distortion is normal, it’s a part of a power system supplying electronic loads such as computers, business machines, electronic lighting ballasts, and control systems. But anything more than 5% on any phase should be investigated further. At this level, or for longer periods of time, it can cause problems like:
The major costs of THD are associated with the life of motors and transformers shortening. If the equipment is part of production systems, income can be affected as well.
To find these kinds of issues, take measurements and track baseline normal for your motors, transformers, and neutral conductors serving electronic loads. Monitor current levels and temperature at transformers to be sure that they are not overstressed. Neutral current should not exceed the capacity of the neutral conductor.
As insulation deteriorates, it begins to leak. Loads will draw slightly higher current as they age, and they may send some of this leakage current into the grounding system. Faults within the equipment may also cause high ground current. These issues can lead to further damage, shortened asset life, unscheduled downtime, and unplanned cost if left for too long. Excessive phase currents can further damage insulation and overheat the load; over-current can cause protection devices to trip; and excessive ground current can create unsafe voltages on metal chassis, cabinets, and conduit.
With increasing phase current, the highest costs often come from premature motor failure and the lost income from over-current protection devices tripping. In order to catch these issues before they become too large, regularly check and monitor any critical load, but especially motors, VFDs, and transformers. The best way to check insulation is by periodically checking equipment with an insulation tester. You can also check equipment while it’s in service by measuring and tracking all the currents (phase, neutral, and ground) to make sure none of these are increasing significantly over time.
Measuring against the nameplate rating of the load should give you a baseline to raise red flags. The nameplate rating should never be exceeded. Track the phase current being drawn by a load over the months or years so you can get a sense whether the current is changing and take mitigating action before the issues are too big.
Figure 2. Power quality loggers, like the Fluke Power Quality Logger, can catch intermittent issues and gather information needed for peak demand and power factor calculations.
Voltage sags are momentary reductions in RMS voltage. Loads may be added without notifying plant management, and these loads may draw down system voltage, especially if they draw high inrush currents. Also, as electrical systems age, the impedance of the system may increase, making the system more prone to voltage sags. Most loads will operate at 90% of nominal voltage, but if voltage sags further or for extended periods of time, it can cause resets on electronic equipment or overcurrent protection trips. Sags on one or two phases of three-phase loads can cause the other phase(s) to draw higher current to compensate.
Voltage sags can lead to lost income when a computer randomly resets, control system resets, VFD trips, and shortened life of backup power system’s UPS due to frequent cycling. As part of your preventive maintenance program, tracking measurements on motors, VFDs, UPSs, or panels serving power to computer equipment or industrial controls should help catch issues as they arise. Taking the steps to mitigate the problems before they get too large can save you unplanned downtime and cost.
Utilities monitor the amount of power a facility consumes. Several times an hour, utilities will calculate the average demand for that interval. Peak demand is the highest average demand during all the intervals in a billing cycle, and is what the company will charge based on. Commercial and industrial customers can manage the high cost of peak demand rates by staggering load cycles to reduce total draw at any one time.
The potential savings here depends on the rate schedule of the utility. But, checking a few things can help you adjust assets and schedules to cut peak demand rate costs, such as:
Power factor is an equation to show energy efficiency in a facility. It’s the ratio of working power (measured in kW) to apparent power (measured in kVA). Apparent power, or demand, is how much power is used to run equipment and machinery. Apparent power is found through the total amount of working power (kW) and reactive power, or wasted power (kVAR), combined. Power factor is usually expressed as a percentage—and the lower the percentage, the less efficient power usage is.
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Utilities may charge higher rates or penalties for low power factor, penalizing facilities requiring higher reactive power. To avoid paying higher utility fees, power factor should be higher than .97. System capacity restrictions cause voltage drops and overheating, so capacitors may be applied on individual loads, at a confluence of several loads, or at the service entrance to improve power factor.
Looking into a few areas can help you reduce these costs. Understand if your utility rate plan charges for reactive demand or low power factor. Find out how the utility measures power factor. Are they looking at peak intervals or averages?
Finally, while running preventive maintenance routes through a facility, identify loads that are causing lagging reactive power. Once you know where the issues may be coming from, you can develop a strategy for power factor correction and eliminate as many utilities penalties as possible.
This story originally appeared in the May issue of Plant Services. Subscribe to Plant Services here.
About the Author: Jason AxelsonCihan Şenel
Technical Sales Engineer
Aktif Mühendislik
It is one of the biggest purposes of institutions and organizations that use, generate, transmit, distribute energy and aim efficiency. The more efficient usage of electric energy is possible by maximizing the power quality. Disturbances in power quality parameters negatively affect electrification systems. The power quality must be measured so that these adverse effects can be detected and removed through the solution. It is one of the most important factors that to make accurate measurements from the right points.
The majority of the power quality problems are directly related to production, transmission, distribution and consumer, as well as being burden-related problems at consumption points. Distribution and transmission companies providing electricity energy are obliged to supply qualified electricity to their subscriptions in accordance with the parameters and limits defined in TS EN standard. The same responsibility applies to the consumer and it is obliged to continue the operation continuity within the limits specified in this standard. One of the main problems for the companies providing distribution and transmission services is the determination of which subscriber originates the deterioration of the power quality parameters in the transmission and distribution of electricity energy. The quality of the power must be measured to determine the source of the problems.
The measurement of the quality of the power is the complete sum of the data collected after the measurements made according to standard to check that the electrical energy provided by the electricity network parameter limits defined in national and international standards. This data is analyzed, the problems identified are explained to the consumer or the provider organization in the simplest way, and the comments about the suggestions for the solution of the detected problems are called the power quality report.
It is suitable for electrical engineers to analyze the measurements of the power quality measurements, to convert the analyzed data into a report, to have experience and knowledge of the quality of the problem that can identify and solve the problems.
It is very important that the measurement of the quality of the power is done correctly and the safety of life and property in the environment are provided at the same time.
First, attention must be paid to the precautions to be taken for work safety at the point to be measured. The work safety equipment of the measuring person, for example; insulated gloves, outfit protective against arcs, insulated shoes etc. and other side safety equipment must be used correctly.
The measuring instruments to be used when measuring the quality of the power must be class A of IEC -4-30. These standard includes features of device’ features like sampling rate, sensitivity, which parameters should be measured.
If the measurement of the power quality needs to be done in order to determine a specific problem, a detailed information about how the problem is experienced should be shared. This is a very important issue to approach the solution of the problem. In this point, it is determined how long a measurement should be made at the point where the problem is experienced.
Power quality meters must be correctly connected to the measuring points. The current direction, current transformer and voltage transformer ratio must be entered correctly in the measuring device. The power values should be checked according to the case where the measured power is production or consumption point. Many power quality measurements are made incorrectly due to incorrect connection.
The measurement times specified in the standard at the point of measurement have high importance for the check to the parameter limits. Power quality measurements which are not taken in the right time leads to misinterpretations. For this reason, the correct time limits must be used for each measurement.
The determination of power quality problems and the success of the applications to solve these problems undoubtedly pass through the measurement and analysis of a power quality to be correctly performed. Following the correct power quality measurement, a report should be prepared that is interpreted in accordance with the standards and applications for resolution should be ordered. In this way the power quality measurement is completed correctly and the problem is solved successfully.