The finest instrumentation, final control, and control systems won’t deliver the performance you require if the power and grounding are not right. I caught up with Emerson’s Stephan Kerrebijn who highlighted its importance as the basis for a reliable and proper working system. This is particularly important with all the digital communication busses to get solid and reliable communication.
He pointed me to a quick start guide specific to DeltaV systems, Quick Start Guide for DeltaV Power, Grounding, and Surge Suppression. Getting this right the first time before you start up your DeltaV system means that you will have the full capabilities that the technology provides through the lifecycle of the system.
The quick start guide shares these basic premises:
The following principles provide a foundation for system design with respect to mitigating interference issues through power and grounding.
- Power, ground, and surge should always be considered together because they frequently interact. A system where power, ground, and surge suppression work in unison provides the most stable system.
- There is not a “magic hole” that we can dump all of our unwanted interference into. However by establishing a stable ground reference (preferably 1 Ω to 3 Ω) for the control system, voltage events such as those caused by facility faults, dramatic load changes, or lightning that affect one area of the ground system will not adversely cause issues with the control system’s ground reference.
- Noise (interference) always wants to return to its source following the path of least resistance (Ohm’s law)
Five reasons for grounding:
- Safety ground (protective earth)—protects personnel from injury resulting from defective supply feeds.
- High frequency ground—ground systems that improve signal integrity by reducing noise caused by machinery such as variable speed drives, welders, or commutated DC motors.
- Stable DC reference ground—A low impedance ground (1 Ω to 3 Ω between the ground system—triad or plant grid—and earth) maintains the control system at a stable reference.
- Lightning protection—protects property and personnel from lightning strikes.
- Lightning mitigation—protects equipment from induced energy as a result of lightning.
Stephan noted that he’s seen process manufacturers that did not have correct grounding in place and faced several problems with their communication busses like fieldbus. His experience is that most of the ‘strange’ and ‘unexpected’ failures did have a root cause of improper grounding or bad power quality.
From a power quality standpoint, several standards address this:
- IEEE Recommended Practice for Monitoring Electric Power Quality
- IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
- IEC 61000-3-11 Electromagnetic compatibility (EMC) Limitations of voltage changes, voltage fluctuations and flicker in public low voltage supply systems
- IEC 61000-3-12 Electromagnetic compatibility (EMC) Limits for harmonic currents produced by equipment connected to public low voltage systems
Rock solid, clean power is never a given. Uninterruptable Power Supplies (UPS) can address electrical supply issues such as service interruptions, voltage sag, undervoltage, voltage swell, and overvoltage. Other issues in the AC power waveform that UPSs can help mitigate include impulse transients, oscillatory transients, EFI/RMI noise, notching and harmonics.
Appendix C of the guide has thorough checklists to help ensure a well-designed and installed system from a power, grounding, and surge suppression standpoint. It’s well worth the time to work through the checklist to avoid problems down the road.
Stephan pointed out that maintaining and measuring the power quality can greatly improve the availability of the control system. Whether it is maintaining minimum power requirements as devices and I/O cards are added or the long-term health of the electronic components through the quality of the power they consume—power and grounding impacts the reliability and ability to fully use the technology.
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