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MANAGING POWER QUALITY:

Solutions for assuring power quality and energy efficiency in automated facilities

As manufacturers automate their facilities, they are faced with many new challenges. More automation means more demand on the power supply. Control systems and the plant power system must deal with the increased electricity needed to power equipment, the non-linear, or harmonic-generating, character of these loads, and pressures from utilities to manage power factor, power quality and consumption.

Integrating the power system with facility controls can help facilities manage consumption, optimize energy use and monitor power quality, ultimately protecting valuable equipment and reducing electrical costs. Power quality usage issues that cost facilities the most money can often be addressed in a simple and cost effective manner with integrated power monitoring and programmable logic controllers. Gathering power supply information with Allen-Bradley Power Monitors enables a facility to make the adjustments necessary to reduce electrical operation costs.

Trends and Issues In power quality

Increased consumption: When manufacturers increase automation levels and their information systems infrastructure to strive for higher machinery efficiency and productivity, they increase stress on the power system. Overstress on the power system can cause unanticipated interruptions and unacceptable power quality. This in turn can lead to disruption of information and power-sensitive Systems. Proper monitoring and management of electrical consumption allows a facility to take full advantage of the benefits of automation.

Non-linear loads: Factory automation means accelerated use of electronic control devices, including motor controls, adjustable frequency drives, computer and electronic equipment, and energy efficient lights. These are "nonlinear loads," which typically cause power quality problems and often have a "chopped" current waveform with time (phase) delays, or waveforms that look substantially different from the customary sine voltage waveform.

Most energy-efficient electronic devices utilize a rectifier charging a capacitor in the primary power supply. The rectifier, in turn,. powers a converter. This configuration is common in automation equipment and generates harmonics and other power quality problems, such as transients, in the system.

At the same time, automated facilities are able to maximize benefit from input power. Managing when and how to use power and coordinating use with production and other facility needs, just as we do for other process inputs, will improve the electrical load profile and lower costs.

Utility pressures: Faced with the uncertainties of deregulation, competition from independent power producers (IPPs) and cogeneration and environmental regulations, utilities are concerned about profitability. Thus, they are holding back on capital expenditures for equipment and maintenance pending the outcome of their industry's realignment. This has an effect on the utility's ability to deliver adequate power at optimum quality.

At the same time, electrical costs for users have risen dramatically. At one time, electricity was purchased as a commodity; it was cheap. and plentiful. Facilities allocated electricity charges by the square foot, a woefully imprecise measure of use. Today, the industrial trend is toward using real "cost accounting" methods. Utilities charge facilities with demand penalties and power factor penalties to balance generation and distribution costs with actual usage patterns, providing both a penalty and incentive for "good energy management"

An informal survey of power consumers found that the biggest problems, in terms of excess electrical costs, are demand penalties and intermittent problems.

Allen-Bradley has found that these problems can be solved efficiently using integrated power monitors in a load management architecture. In

Fact we have found that the greatest successes can involve small, non-critical adjustments to the way a facility uses power, and the return on investment can be less than one year

Power quality problems can come in a number of unpleasant shapes and sizes: failure of power factor correction apparatus (for example: exploded kVAR compensation capacitors) and failure of harmonic correction apparatus.

Poor power quality is caused primarily by non-linear loads and transient loads from sources both internal and external to the facility, such as lightning, electric arc furnaces, arc welding and discharge machines.

Transient loads can induce unexpected control signals in other equipment, thereby creating additional disruptions or poor system operation.

These myths about power quality have an impact on how problems are addressed:

Myth: Wye Delta transformers cancel harmonic triplines. They do if the voltage is balanced perfectly and the loads are linear, but this is becoming a rare case.

Myth: Transformers and motors don't cause harmonics. Three-phase magnetics cause harmonics when saturated.

Myth: The maximum magnitude of a given harmonic on a real power system is 1/n times the fundamental. The maximum magnitude of a given harmonic is affected by voltage imbalance and the crest factor of the associated load current wave shape.

True resolution of harmonic problems requires an active, real-time load linearization solution. Much investigation is being done in this area. Reasonable, conventional measures for minimizing harmonic disturbances include:

  • Run equipment at the lowest possible voltage that meets the requirements of the connected loads.
  • Run separate sources for "dirty" loads and filter on the high side of the point of common coupling (PCC).
  • Check for and eliminate saturation effects in power transformers and synchronous machines.
  • Size the motors properly for the intended load.
  • Avoid local low voltage power factor correction capacitors.
  • Automate switching so it occurs at the most appropriate time.
  • Run the lowest possible voltage that still meets power delivery requirements of connected hardware.

Hunting for the intermittent, scenario-dependent or "random" fault is very difficult to pursue effectively. Part of the problem is that approval for dedicated troubleshooting hardware is tough to get because it is difficult to calculate its return on investment. Power monitoring suppliers must strive to provide a cost-efficient solution that justifies the investment in monitoring and troubleshooting aids for users.

The Power monitor can be used to alarm when a process is using more power than normal, or reduce speed of a process when the load drops below a certain level. Additionally, facility managers can assess machine condition by comparing power consumption under identical load conditions, and make adjustments based on optimal machine loading.

Conclusion

For automation to reach its potential in providing efficient operations, improved control and optimized energy use, facility engineers should take a plant-wide, long-term look at their power systems. In order to maintain a "clean" electrical supply and keep operating costs down, the perspective on electrical consumption must change from "electricity as a bulk commodity" to "electricity as a process variable."

As a process variable, electrical use can be accurately monitored, logged and analyzed over time. Automation of the energy management function through tools such as power monitoring gives facility managers a clear picture of electrical use, and guidelines for making the adjustments necessary for quality, cost-efficient operations.