New Paradigms for Power Conditioning

The term "standard" has always been misleading when applied to electrical system design for buildings, given the uniqueness of each project and facility. Designing systems to accommodate well-behaved linear loads is a thing of the past, and today's standard design approach must include working through a myriad of questions that simply didn't exist before now.

12/01/2001


The term "standard" has always been misleading when applied to electrical system design for buildings, given the uniqueness of each project and facility. Designing systems to accommodate well-behaved linear loads is a thing of the past, and today's standard design approach must include working through a myriad of questions that simply didn't exist before now.

Power quality has become increasingly important, because sensitive electronic equipment typically cannot tolerate direct exposure to the power system. Ironically, it is often this same equipment that generates "dirty" power to begin with.

Power-quality problems originate both outside—from the power distribution grid—and from within a facility. Lightning-induced spikes and surges, power outages and capacitor switching transients come from the grid. Typical in-house problems can include switching spikes, surges and sags caused by equipment cycling and harmonic current. When determining whether to add power conditioning to a system, several basic issues must be considered:

  • Cost of creating a cleaner power supply.

  • Value and use of connected equipment, and the consequences if the equipment fails.

  • Frequency of power problems at the site.

  • Level of power quality needed: Is it for the entire facility, or only for certain key areas?

  • Types of equipment within the facility that contribute to poor power quality, and the nature of power disturbance.

Power conditioning that is placed as close to the source of the disturbance as possible creates the most benefits. Lightning protection located at the building service, harmonic filters at connections to each nonlinear load and surge protection located at key points in the system will minimize system losses and equipment damage.

But in reality, this is rarely the practice. While unwanted impulses from the utility are usually filtered at a building's incoming service, filtering out surges and harmonics within a facility is rarely done at the source, due to the large number of loads that are potential surge and harmonic generators. In most situations the cost is prohibitive.

Instead, various levels of protection are installed throughout the system, which generally reduce , but do not eliminate , the harmful effects of poor power quality.

Fighting the Surge

In determining surge protection needs, one must consider the equipment's ability to tolerate voltage spikes and surges. For example, surge protection for cooking equipment may be much different from telephone and computer equipment.

Once it has been determined that surge protection is needed, the next step is to ensure that the protection equipment has a UL 1449 listing. Not only does this listing mean that equipment will fail in a safe manner—without causing a fire—but it also provides performance criteria.

Perhaps the most important indication of a surge protector's effectiveness is its let-through voltage , which is the maximum voltage the unit will pass on to the connected load. Ratings for clamping voltage , the amount of current a surge suppressor will allow to pass through, can be misleading, as they may only disclose the voltage level at which the protector begins to operate. This rating has little meaning if the let-through voltage is unacceptably high.

Grounding systems are also an important consideration in selecting surge protection. Because surge protectors generally shunt overvoltages to ground, the systems must be properly designed, installed and maintained. Current that is diverted by surge protectors to the ground system can easily create other problems, including differential ground potential and tripping of ground-fault interrupter (GFI) devices that compare ground current to neutral current.

In one case, personnel at a recently completed laboratory were experiencing problems with GFI outlets tripping regularly; they wanted these "defective" outlets replaced. It soon became clear, however, that the problem was surge protectors plugged into the GFI outlets to protect connected computers. Whenever a surge protector shunted more than five milliamps to ground, it caused the GFI to trip.

Proper installation of surge protection is critical to optimal performance. Any surge or spike can cause a very rapid rise in voltage—one that must be seen and then "clamped" before damage to the protected load occurs. For parallel-connected surge protection, lead length and power-system location are essential to minimizing peak voltage levels.

Electricity propagates through conductors at a fixed speed. An increase in conductive path length—wiring or bus—means a longer time for surges to reach a connected device. Even the best-performing surge protection will not protect a load that is upstream of its protection. The impedance of the connecting leads—in particular that due to the inductance—plays a major role in system performance (see Figure 1 on page 18).

Fighting Harmonics

Harmonic distortion of current and voltage waveforms within facilities is a growing problem. Computers, faxes, modems, photocopiers, variable frequency drives and lighting systems all generate some level of harmonic distortion—and feed it back on the power distribution system.

Good design practice includes modeling any questionable areas within a facility to determine anticipated levels of harmonic current. Only after this model has been completed can the designer properly choose and size equipment to deal with harmonics. Electrical system software packages that can perform harmonic modeling are available.

There are many ways of dealing with harmonic currents. A low-tech method is to upsize (derate) feeders, transformer, motors and generators to add to system capacity that is being consumed by harmonic current. This technique does not remove harmonics from the system; it simply reduces problems, such as equipment overheating, that stem from their presence.

Zero-sequence harmonics have a tendency to accumulate on the neutral conductor. On a three-phase Y-connected system, it is theoretically possible to have 1.73 times the phase current on the neutral. So, it has become common to "oversize" (which is really a misnomer) the neutral conductor in systems with high levels of harmonic current.

The National Electrical Code recognizes that in cases where harmonics are present, neutrals can carry as much or more current than the individual phase conductors. Neutrals can be increased to conduct more current without exceeding insulation temperature ratings. But increasing size or quantity of neutral conductors increases the cost of the electrical system, so it is only done when necessary.

Third harmonics , often the greatest in magnitude, are "trapped" in transformer delta windings as eddy currents and dissipate as heat. The excessive heat generated in systems with high harmonic content can damage transformers.

As the problem of harmonic content in electrical systems increased, the electrical industry began to produce transformers with the ability to handle high harmonic current. K rating has become a standard means of identifying transformers that are specifically designed to handle various levels of harmonic current. The higher a transformer's K rating, the higher the level of harmonic current it is designed to handle.

K-rated transformers rely on parallel windings to reduce skin effect and on reduced core resistance to minimize I2R heating. Changing a facility's standard transformers to lower-impedance K-rated transformers normally require that a new short circuit study be performed to make sure withstand ratings of existing equipment have not been exceeded.

Special transformers that create a phase shift in the harmonic current as it passes through the transformer are available. The real advantage of phase-shifting is that harmonic currents from one electrical system branch can be used to cancel harmonics from another branch. Harmonic current is cancelled, rather than dissipated as heat so that system capacity is regained.

For example, a phase-shift transformer could be installed to feed one floor of a building, with a standard transformer feeding another level. With proper design, a cancellation of the harmonic current will occur at the bus upstream of the two transformers.

A New World The need for an increasingly higher quality of power and the simultaneous increase in the loads that contribute to poor power quality have resulted in a host of new products and techniques to remedy the situation. While not as straightforward as in years past, system design can be performed to improve power quality well beyond the needs of even the most intolerant equipment.

Standard design practice must now consider both the quality of power that a load requires and the amount of disruption the load creates on the system.

Table 1. K Rating Tabulation

Harmonic Number

Sequence

Switch Mode Power Supply

PWM VFD

1

Pos

1.000

0.955

3

Zero

0.8

5

Neg

0.6

0.235

7

Pos

0.4

0.135

9

Zero

0.16

11

Neg

0.025

0.08

13

Pos

0.065

0.07

15

Zero

0.08

17

Neg

0.055

19

Pos

0.045

K factor

12.70

6.50


From Pure Power, Winter 2001.



What Is K Rating?

K rating is a convenient method of identifying the quantity of harmonics present. K rating is calculated using the formula shown in Figure 2 on page 18.

The higher the corresponding K rating, the more harmonics are in the system. Using the spectra for typical switch-mode power supplies and pulse-width-modulated variable frequency drives ("Non-linear Loads"), the K rating shown in Table 1 is obtained.

About Non-Linear Loads

Nonlinear loads exhibit predictable harmonic spectra resulting from the technology used to supply power to the load. Computers, faxes, modems, copiers, variable frequency drives, printers, lighting systems and welders are all examples of loads that produce harmonic currents in significant levels.

See Figure 3 on page 18 for more information. Levels shown are typical; exact levels of current at each harmonic are generally available from the equipment manufacturer.



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