Reducing Hidden Costs in Electrical Systems

Reducing Hidden Costs in Electrical Systems

Low power factor and high levels of voltage and current harmonic distortion, are a significant but often hidden cost in industrial operations.

Without corrective measures, these factors increase electrical utility charges, impair equipment health, and prevent efficient industrial expansion.

Further complicating its impact, devices that correct a low power factor can introduce negative interaction with harmonic-distortion producing loads, causing electronic equipment malfunctions and related downtime. Facility managers should take four steps to improve power efficiency and quality, and increase their return on energy investments.

Power Factor Basics

Defined as the ratio between real power measured in kilowatts (kW) and apparent power measured in kilovolt-amperes (kVA), power factor is a dimensionless number between 0 and 1, often expressed as a per cent.

It measures how effectively a specific load consumes electrical energy to produce work. The higher the power factor, the more effective the use of electrical capacity, the converse is also true.

For example, imagine a facility operating at 5 MW with a power factor of 0.5. To get 5 MW of real power at 0.5 power factor, 10 MVA of apparent power needs to be transferred (5 MW ÷ 0.5 = 10 MW).

Clearly, low power factor has real energy cost consequences. More energy costs more money. In addition, electrical utilities will often add penalty charges to users that have low power factor below a certain threshold. These charges add up over time, resulting in significant amounts of wasted money every year.

Step One: Examine the Electrical Utility Charges

The first step facility managers should take to correct the power factor is to examine the utility bill, which will state exactly how much extra the utility charges each month for a low power factor.

Managers should review how much they are paying for power in kilowatt hours. Then they should see if they are charged for kilovar demand and if a power-factor penalty is included. These charges may be labeled under apparent power demand, reactive energy surcharge or kilovar demand.

Step Two: Integrate Power Monitoring and Corrective Devices

Once they understand the system-wide financial costs, they should use power monitors to continuously assess the facility's power factor to determine if they have a low power factor.

Monitoring needs to be continuous because the power factor changes over time, as loads turn on and off. If the facility manager uncovers a consistent, low power factor, he can calculate the size of a fixed or static capacitor bank to install and correct the power factor.

He may uncover a variable power factor, in which parts of the facility that use motors powered across the line have a low power factor but other parts, that use VFD's, have a high power factor. If so, he should consider installing power control with removable levels of capacitance, depending on how and when conditions change.

Dynamic control is critical in variable power factor. Facilities have many loads that turn on and off at different times, and managers need to be able to respond to the resulting changes in power factor.

The opposite of lagging power factor is a leading power factor, meaning that the facility returns reactive power to the electrical utility while still being charged for it.

For example, leaving capacitors on while electrical loads are turned off can cause leading power factor.

In this case, the utility may reduce the power-factor penalty, but the facility still wastes money. Facility managers need to keep their power factor on middle ground to have the right balance of efficiency – with right-size equipment, no penalty charges and no power returning to the utility company.

Step Three: Minimize Harmonic Distortions

Once managers have examined the facility's power factor and integrated capacitance, they need to evaluate any related non-linear loads that create harmonic current and voltage distortion.

Combating a low power factor by adding capacitance is effective. However, non-linear loads, such as variable frequency drives (VFDs), and even computer equipment with switched-mode power supplies can interact with power factor capacitance and create a resonance condition, resulting in severe voltage and current distortion.

Without proper mitigation, harmonic currents and voltage distortion can lead to equipment malfunctions, including power transformer and power factor capacitor overheating and failure, and equipment overvoltage faults.

Facility managers can monitor the level of harmonics with power-monitoring devices and examine the total harmonic distortion or the individual harmonics on the system.

Data from power monitors can include available amperage and the demand for amperes in the facility. If consumption is significantly close to the total available, harmonic distortion could be affecting the facility's electrical equipment or ability to expand. It also could be adding to the electrical utility's surcharge for the power factor.

Once the level of harmonic distortion is identified, facility managers need to decide how impactful of an issue it is.

Reducing distortion can not only reduce harmonics but also help correct the power factor. Depending on the gravity of the problem, facility managers can mitigate harmonic currents with harmonic filters added to the incoming power, including 18-pulse transformers, passive filters and active filters.

Step Four: Continue to Monitor

The levels of power factor and harmonic distortion will determine systemwide efficiency and production. Facility managers should continue to monitor both after correction and mitigation.

They can add or remove capacitance or filters to reach the middle ground that is ideal for their facility. With these four steps, facility managers can identify and rectify hidden costs.

Learn more about energy-management and energy-monitoring solutions.

The JOURNAL

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The JOURNAL from Rockwell Automation and Our PartnerNetwork™ is a bimonthly magazine, published by Putman Media, Inc., designed to educate engineers about leading-edge industrial automation methods, trends and technologies.