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Power quality: what every electrical contractor needs to know

Power quality (PQ) can be defined as the ability of the electrical distribution system within a facility to allow the resident equipment to function at their optimum levels without unscheduled interruptions. PQ engineering is now recognized as an important discipline.Considering our sales volume, we at Continental Electric have not experienced as many PQ problems as one might ordinarily think. However,

Power quality (PQ) can be defined as the ability of the electrical distribution system within a facility to allow the resident equipment to function at their optimum levels without unscheduled interruptions. PQ engineering is now recognized as an important discipline.

Considering our sales volume, we at Continental Electric have not experienced as many PQ problems as one might ordinarily think. However, we do have the resources to address any PQ problems and to bring them to the attention of our customers. Should the customer choose not to act upon any problem we bring up, at least we're on record as having warned them.

We also let our customers know that we're interested in the electrical conditions at a facility after construction is completed and final payment is made. This attitude brings considerable repeat business, which is generally negotiated work-the kind we all like to do.

Our experience indicates that most PQ problems are generated from within the facility's wiring system: errors in grounding, switching large loads, and stress applied to the electrical power system from nonlinear loads. Other problems originate in the utility power distribution system.

Let's concentrate first on problems that commonly come from the utility network. We should remember that the utility power system is complex; the potential for disruptions (whether natural or man-made) is tremendous, and during normal power flow, the system can generate electrical aberrations. These aberrations, which change the smooth line of a 60 Hz power sine wave, include: impulses and dropout caused by grid switching; impulse and surge caused by power factor correction capacitors, impulse caused by arcing contactors; and sag, undervoltage and outage caused by breaker clearing. A recloser is a circuit breaker that will open on a fault, but will automatically reclose after a preprogrammed time delay, and will continue the process for a specific number of attempts. Obviously, the farther away the load is from such an operation, the less effect it will have at a customer's facility. This is also true in the case of a capacitor bank being switched. Relocating or lengthening the feeders will reduce the effects.

Now, let's look at the PQ concerns within a facility. Distorted voltage waveforms can adversely affect not only computers, but also equipment as diverse as electronic instrumentation, scanners, and motor drives. With single-phase power, the voltage waveform of the individual phases are 180 deg apart. One phase is always at the maximum voltage (+120, for example), while the other phase is at the minimum voltage (-120), and they cross the zero plane at the same time, allowing us to combine the phase conductors with a single neutral for branch wiring. Three-phase power is essentially the same, yet with a 120 deg offset. One phase is at the maximum, one at the minimum, and the third is crossing the zero plane. In each case, a vector analysis of the waves is always zero. Imbalanced loads will put current on a neutral conductor, but in theory, that current should never exceed the rating of the overcurrent protection device for the individual phases.

Harmonics, which are caused primarily by nonlinear loads, can cause a variety of problems to power systems serving computer systems, such as waveform distortion, improper voltage readings and especially overheating in the system neutral conductor.

Switch mode power supplies in computers, UPS equipment, variable-speed drives and electronic fluorescent ballasts are typical nonlinear loads that can produce troublesome harmonics. The most common sine waves that distort a power system are whole number multiples of the fundamental 60 Hz power frequency. The multiples are called harmonics. Harmonics combine with the fundamental to form distorted waveforms, which can be detected with sensitive hand-held testers, such as a true RMS meter or a scope meter. Harmonics (and other aberrations) are also detected with more detailed electronics-based systems.

Earth grounding system A majority of the PQ related problems that we, as a full-service electrical contractor, have seen come from improper grounding techniques. The following diagrams will illustrate both correct and incorrect installations. The National Electrical Code (NEC) requires that one ground system be used and that all points of ground (water mains and supplemental electrodes) must be bonded together, although the local utility may provide a separate ground at their equipment. The neutral bus and the ground bus should only be bonded at the main service entrance, as shown in Fig. 1. Mistakes in a grounding system, called ground loops, are a common cause of PQ problems. They occur whenever the equipment grounding conductor is connected to ground points that are not at the same potential.

As Fig. 2 shows, earth ground No. 1 is at a different potential than earth ground No. 2, so current flows, or circulates, in the equipment grounding conductor, which causes ground noise. This ground noise, in turn, can result in logic system malfunction.

In addition, objectionable current can flow in the equipment grounding conductor because of neutral-to-ground bonds in the panelboards throughout the facility. Fig. 3 shows normal current flow in a properly wired power system that serves computer A and computer B. The neutral carries all of the return current. ln Fig. 4, a neutral-to-ground bond is created in the panelboard-an NEC violation that exists far more commonly than you would expect. In this case, the neutral current splits at point A in the panelboard and allows parallel paths. The equipment grounding conductor (green wire) functions as a secondary neutral, with current flowing along the path ABC.

The current flow from B to C can create a voltage that causes the ground reference for computer B to vary, depending on the amount of current and the resistance of the conductor. This condition can cause Computer B to operate erratically, although no hardware failures are experienced.

The problem compounds when a supplemental ground is added to computer B. >From now on, the neutral return current, which enters the equipment grounding system at point A, now splits at point B and flows in the path (BDEFC) as shown with arrows. Current is now flowing through the chassis of computer B, causing both operation and hardware problems. A data link between the two computers further complicates the problem.

Promoting power quality Only a few aspects of PQ have been covered here. As mentioned above, not many contractors have the time or the inclination to conduct PQ studies because we're already very busy. We prefer to let power quality specialist handle these survey--the International Electrical Testing Association can provide a list of members in any area.

It is useful to let other do a survey because data acquisition from certain types of electrical equipment and systems layouts can be difficult. For example, measurements are usually taken both ahead and behind suspected problem locations. Recognize that ahead refers to the utility side and behind the load side of the system. In addition, the measurement interval should be long enough to capture electrical behavior; intermittent transients complicate this matter.

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