Proper planning prevents most power quality problems.
It's usually not until the lights go out or equipment burns up that power quality becomes an issue. Why not prevent such problems from occurring in the first place? That's the idea behind establishing a PQ problem prevention policy. The keys to making it successful include planning, communication, interdepartmental cooperation, and enforcement. Follow these steps to successfully implement such a program in your facility.
First, identify your power quality needs based on your facility's operations and mission. Most facilities can tolerate a moderate level of harmonics, so don't assume you must eliminate harmonics at the expense of resolving other power quality issues. You'll want to prevent unbalanced voltages and currents, service interruptions, spikes, sags, and surges. The degree to which you address each of these issues depends on where your budget and your tolerance for power quality problems intersect. You'll need to identify major aspects of your electrical infrastructure and write a specification for preventing problems in each of them. This specification will include predictive and preventive maintenance. Let's look at some of those areas and key points you should consider as you develop your plan.
Most power quality problems result from violations of accepted grounding standards, such as NEC Art. 250. Thus, your plan must include baseline testing of your grounding system followed by periodic inspections of various types. Visual inspections can reveal corrosion, broken parts, and improper configuration changes. Thermographic scans and meter readings reveal problems at specific connections. In many operations, ground-monitoring systems are critical.
In today's frequently reconfigured facilities, you should pay special attention to neutral sizing. Instead of a mix of linear and nonlinear loads using less than half of each circuit's available outlets, you now have routers, servers, and other difficult loads everywhere — not to mention extension cords and multi-receptacle strips. Changing the lighting from incandescent to electronic fluorescent can make the problem worse. If you are rewiring to accept these new loads, go beyond the requirements of the Code and size the neutral at least as large as the current carrying conductors.
Poor protection coordination is a preventable cause of downtime. Even if things are running smoothly, you should perform a baseline study to ensure the coordination is correct. Consider the interrupt and fault current withstand ratings of your fuses and breakers, as well as their time curves. Starting at the service entrance, work your way toward the load and ensure breakers are progressively more sensitive. You cannot design protection coordination by starting at the load end. Any time you add or relocate protection devices, update the study. Stop the fault as far down in the hierarchy as possible. If a breaker trips “upstream” before one closer to the fault, you have a problem.
Proper load segregation.
Improper load segregation is a leading cause of electrical noise, spikes, and wild waveforms. A facility with frequent power quality problems probably has panels that carry a mix of load types. In one panel, maybe three circuits feed lighting, one feeds a mixer, two feed an office, one feeds convenience receptacles, another feeds a conveyor, and so on. Use separate transformer/panel combinations for separate load types. If you set a transformer and panel just for lights, you have a known load on that transformer. Transformers and panels that supply production equipment should never supply other kinds of loads.
Wire separation and cable routing.
Separate wiring by power level. For example, don't mix control wiring and power wiring. Run motor drive output cables in their own wireways. Rather than trying to correct for the effects of inductive coupling, ensure it doesn't happen. You can do so by limiting installation supervision to people who can document their working knowledge of the NEC, such as master electricians.
The more training your maintenance staff has on power quality — specifically the technical areas identified earlier — the more able you will be to ensure a workable plan. Use a mix of formal training, OJT, trade magazines, and other resources.
The level of power quality you have, and the level you need, will change periodically. Your policy for ensuring adequate power quality must also change. Review some part of it each month, and update accordingly.