Preserving power quality in changing times
Power quality is an important plant issue in 2015. This is true even though both electric utility system reliability and immunity of plant processes and equipment to power-related disruptions have generally improved in the last 20 years. It will remain important in the coming years as a proliferation of distributed energy resources like solar and wind continues to comprise a larger portion of the energy generation mix. So how can you ensure that your facility is unaffected by these changes? Here are a few tips to consider as you revisit your power quality priorities.
Pay attention to the basics
Facility managers need to recognize that most power disturbances result from degradation of or damage to the power wiring systems, especially to the bonding and grounding systems in an industrial facility. Regardless of the energy-supply mix, effective grounding systems will continue to be vital to the safe and reliable operation of the power distribution system.
Normally, managers don’t think of an effective ground-fault return path as even necessary, much less half of the power circuit, but that is the case during any phase-to-ground fault. An effective bonding and grounding system ensures that employees are safe, equipment damage is limited, and the trouble is confined to the smallest possible part of the electrical system.
Deterioration within the power wiring system is more widely accepted as a power quality concern that justifies a portion of the maintenance budget. Power conductors carry current 24/7 to enable electrical loads to operate correctly, so loose connections in these systems receive careful attention-in the manner of infrared scanning to identify loose (hot) connections, and active power monitoring and control systems.
Grounding systems, however, transmit high currents only during fault conditions or surge events, so degradation is harder to track and easier to neglect. Like an airbag or parachute, however, it would be really nice to know that the grounding system will work as intended when required. Fortunately, there are ways to determine the state of a grounding system that don’t involve the equivalent of crashing into a tree or jumping from an airplane.
Determine if your system needs attention
There is a fourfold approach to grounding system assessments that are recommended:
- Review the grounding system design; IEEE Standards 3003.1 (System Grounding) and 3003.2 (Equipment Grounding) are useful here.
- Inspect to determine the condition and adherence to the design, especially at key points like substations, transformers, generators, and transfer switches.
- Test any portions of the grounding system that are suspect or cannot be inspected; see IEEE Standard 81-2012, for example.
- Correct any deficiencies or areas of concern or uncertainty.
The grounding-system design review starts with power equipment and generators. Are these systems intended to be solidly grounded? Resistance or impedance grounded? Or ungrounded? Each system type has different requirements with regard to the magnitude and handling of ground-fault current, protective relaying, ground-fault detection, transient-voltage surge suppression, and, of course, the types of loads that can be operated effectively from each system type.
Whatever the power system type, at least three aspects of grounding should be consistent:
- All grounding electrodes, ground buses, conduit, cable trays, metallic structures, and electrical equipment at the facility shall be bonded together to minimize voltage potential differences between any of that equipment. (Actually, "shall" is the correct word-it is a requirement of the National Electrical Code) This includes other utilities like water piping, gas lines, and fueling equipment.
- This bonded network should be connected to earth in a manner that ensures a low impedance path at all points in the system.
- A lightning abatement system should be considered based on the lightning frequency and density data for the region. See NFPA 780, Standard for the Installation of Lightning Protection Systems.
Final points about grounding
Not sure if that metallic conduit constitutes an effective ground-fault return path? A qualified expert with a direct or two-point resistance method can test the integrity. Will the grounding assessment and testing require a shutdown? Unfortunately, yes. Most likely some portion of the system will need to be de-energized so internal inspections and testing of power-system components can be performed safely and effectively.
What are the common corrective actions required? Mitigation for grounding deficiencies generally involves additional grounding conductors, replacement of grounding straps or connectors, additional connections to the grounding/bonding system, and additional or replacement grounding electrodes (ground rods-but never "isolated" and always bonded to the rest of the grounding system). Often, the overcurrent protective devices like circuit breakers may require ground-fault settings to be changed.
These improvements can be low-cost insurance against the disruptions caused by grounding anomalies.
The "other half" of power distribution
Earlier, I mentioned loose connections, or points in the power wiring where inadequate connection points have been introduced or exist due to deterioration. These are so common as to support an entire industry of on-site testing using sophisticated cameras that can detect the infrared energy emitted by faulty connections. Newer technologies exist as well, like remote temperature sensors that can be affixed to connection points in the electrical distribution system and wirelessly transmit temperature data.
In addition, a properly placed and programmed power monitoring system can detect the erratic voltage and current signatures that result from insufficient connections. While the discontinuities in the current waveform may be harder to distinguish from load current among industrial equipment, voltage sags and transients on the load side of faulty connections can be easier to detect.
In fact, one troublesome power quality problem at an automotive plant was finally resolved when we captured voltage sags on a feeder breaker when none appeared on the main breaker in the same switchgear lineup. The only possible explanation was a loose connection between the main and feeder. Sure enough, a shutdown and inspection revealed busbar lugs that were improperly terminated inside the switchgear, yet these were hidden from view of infrared scanning equipment.
Electric utility reliability
Maintaining reliability is a concern that keeps utilities up at night and cause sleeplessness among plant engineers. Distributed energy resources, declining infrastructure, and changing weather patterns contribute to this concern. For example, installed solar power now exceeds 20 GW in the U.S., according to the Solar Energy Industries Association, and most of the growth is utility-scale projects.
Two of the key disturbance types to record and track at the service entrance include voltage sags and momentary interruptions.
Voltage sags remain the most common power quality disturbance on a typical electric utility distribution system. A voltage sag is a brief (often lasting less than a second) and sudden decrease in effective voltage, frequently due to single-line-to-ground (SLG) faults on a nearby overhead distribution system. Voltage sags are brief because electric utility systems are designed to open the affected portion of a circuit and re-energize that section automatically, since most of the SLG faults can be extinguished by this operation.
If the portion of the radial overhead circuit on which the SLG fault occurs happens to serve your facility directly, then your disturbance may be a momentary interruption instead of a sag. This type of disturbance is generally more costly to mitigate inside the facility since the supply voltage drops to zero during the event. With most sags, however, voltage seldom dips as low as zero on all three phases.
In either case, the facility’s power monitoring program should record these high-speed events, and report them regularly to their electric utility. While many events may occur during storms and are an unavoidable product of an overhead distribution system design, the utility may be able to make changes that reduce the frequency and magnitude of these events. The facility should log the time, date, and environmental conditions for each event, in addition to charting the magnitude and duration of the sag or interruption. A "mag/dur chart" can be a valuable tool in resolving troublesome disturbances that can adversely affect plant operation.
Larry Ray is director of consulting services for Schneider Electric. He can be reached at Larry.firstname.lastname@example.org.