Even after 30 years-plus of power-quality monitoring, no national design code standard exists for minimizing the impact of power quality phenomena; there isn’t an equivalent of the National Electrical Code (NEC) anywhere.
Article 90.1 clearly states that the purpose of the NEC “is the practical safeguarding of persons and property from hazards arising from the use of electricity” and “contains provisions that are considered necessary for safety.” It also states, “Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use” (emphasis added). Yet, in the scope in Article 90.2, the “Code covers the installation of electrical conductors, equipment, and raceways; signaling and communications conductors, equipment, and raceways; and optical fiber cables and raceways” and installations of conductors and equipment that connect to the supply of electricity, which are the elements where we measure, monitor and mitigate power quality phenomena. Could the Code serve as the power quality design standard?
Article 100 defines nonlinear loads. The fine print note (FPN) states, “Electronic equipment, electronic/electric--discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads,” but the FPN isn’t directly tied to the concept of harmonics. While derating factors exist for conditions such as temperature in Table 310.16 and number of conductors in a cable or raceway [Table 310.15(B)(2)(a)], there’s no mention of derating of transformers, motors or the grounded (neutral) conductor based on the harmonic current levels beyond the FPN in Article 210.4(A), which states, “A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral-conductor currents.”
With regard to unbalanced voltages (or imbalance), the neutral point FPN states, “At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential,” but in reality, it often isn’t 0 volts (V), with respect to ground, nor is any limit placed on how high it should be to prevent problems. Unbalanced conditions are not uncommon, due to unequal loads, impedances or even the supply voltage itself. Also, the neutral to ground voltage usually requires further investigation if it exceeds 1V on a 120/240 circuit.
Sags are addressed somewhat in FPN No. 4 of Article 210.19(A)(1), which covers the minimum ampacity and size of branch conductors on branch circuits not more than 600V. It states, “Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation.”
Similarly, FPN No. 2 on Article 215.2(A)(3) for voltage drop on feeder conductors states that the feeder conductors must be sized to prevent a similar drop. But that just addresses the wire size, which has a voltage drop based on the current and all of the impedances from the connections, switches, transformers and wire length. It’s a good start, but reality has shown that many sags with drops greater than 10 percent occur within facilities that are designed and built to Code.
Interruptions that are caused by faults originating in a facility will most likely be the result of the breakers operating. Since these almost always require manual re-energizing, the interruption duration on a nonbackup power system will be measured in minutes or hours. It is addressed somewhat in Article 110.10, Circuit Impedance and Other Characteristics, where the “overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit.”
Perhaps the most significant and often misunderstood issue is that, where a very low impedance path to ground is important for safety in tripping breakers and ground-fault circuit interrupters, it is actually a low impedance at high frequencies that would limit transients from disturbances coupled into the wiring. Grounding wouldn’t solve problems resulting from switching power factor capacitors. Article 250.56, Resistance of Rod, Pipe, and Plate Electrodes states, “A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7).” This has led to claims about the need to keep driving more ground rods to lower the impedance.
The equipotential ground plane—-partially mentioned in Article 100 as an “area where wire mesh or other conductive elements are embedded in or placed under concrete or other conductive surface, are bonded together and to all metal structures and fixed nonelectrical equipment that may become energized, and are connected to the electrical grounding system”—is one of the best methods for minimizing many potential power quality problems.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.