Problems with POWER

The concept of load and source compatibility is not new. In fact, the need to provide power with steady voltage and frequency has been recognized since the inception of the electric utility industry. However, the definition of "steady" has changed over the years, with increasingly sophisticated electronic equipment that is much more susceptible to departures from steady conditions.

03/01/2001


The concept of load and source compatibility is not new. In fact, the need to provide power with steady voltage and frequency has been recognized since the inception of the electric utility industry. However, the definition of "steady" has changed over the years, with increasingly sophisticated electronic equipment that is much more susceptible to departures from steady conditions.

While the advent of electronic power conversion has been widely applauded by users, the drawbacks-from the power-quality point-of-view-have generally been ignored. The very advantages of solid-state devices-which make possible modern switching power supplies, electronic ballast, inverter-rectifiers, high-frequency induction heating and adjustable-speed drives-turn these power converters into generators of harmonic currents and additional sources of line-voltage drops.

In addition to the disturbances generated by the normal operation of familiar power-delivery and load equipment, the disturbances resulting from the new electronic loads must be taken into consideration.

A Dynamic Revolution

Semiconductor devices that constitute the heart of modern power electronics have been undergoing dynamic evolution in recent years. Never before in the history of power-semiconductor devices have we seen the emergence of so many exotic devices in such a short span of time. A power semiconductor device is indeed the most complex-and delicate-element in a converter.

Building power-system designers and installers need to understand these devices thoroughly for efficient, reliable and cost-effective specification of converters. The availability of high-power, high-frequency devices at economical prices permits mass application of variable-speed motor drives, uninterruptible power-supply (UPS) systems and active power-line conditioners in building systems.

It is important to understand what pure power looks like. Perfectly clean power would have a perfect sinusoidal voltage of constant amplitude and frequency. Voltage amplitude would be adequate for the application, the voltage source would have no impedance and the frequency would be 60 Hz (or 50 Hz in some foreign countries). In other words, the wave shape would be totally free of harmonics, noise and transients.

Of course, such a perfect source of power does not exist-not even in the laboratory. The term "power quality" is often used today to describe an aspect of the electricity supply. A power-quality problem is any power occurrence that leads to failure or misoperation of electronic equipment. Such shortfalls in power quality can be very expensive in terms of building systems shutdowns and equipment malfunctions.

Power Impurities

The kinds of system events that cause power-quality problems warrant closer examination in order to appreciate the remedies. Power events that affect power quality include:

Voltage variations. Voltage swells, which are brief increases in system voltage, can upset system electric/electronic controls and electric motor drives, including common adjustable-speed drives, which trip because of their built-in protective circuitry. Swells may also stress delicate computer components to the point of premature failure. Overvoltages have a less immediate effect than swells but may shorten the life of building power-system equipment and motors. Undervoltages are sometimes caused by the deliberate reduction of voltage by the utility in order to lessen the load during periods of peak demand, called brownouts.

Transients. These are voltage disturbances of even shorter duration than sags and swells. They fall into two basic classes: impulsive, attributable in many cases to lightning and building heavy-load switching, and oscillatory, usually caused by utility capacitor-bank switching. Utility capacitor banks are customarily switched into service early in the morning in anticipation of a higher power demand. With the exception of those caused by lightning, almost all transients are generated as the result of interaction between stored electrical energy in building power-system inductance and capacitance.

When the current is interrupted at peak current flow, the building system inductive load (L) is left with considerable stored energy. When the flux collapses, this energy interacts with system capacitance (C), causing an LC circuit oscillation with a theoretical peak voltage of perhaps ten times the normal peak voltage. This type of interruption is known as current chopping and is a form of current suppression. Current chopping refers to a vertical cut in the current wave and is caused by circuit breakers and switches interrupting light inductive loads such as the excitation current of an unloaded transformer. Current chopping is also common with high-speed static switches and circuit breakers.

Current-limiting fuses, thyristors or silicon-controlled rectifier switches used in building power-system configurations also generate large spikes or transients. Transients can cause computer data errors because of the dv/dt (rapid change of voltage in short time) coupling through the stray and interwinding capacitance of the power supply.

Powerfail. The powerfail, as defined by the Information Technology Industry Council (ITIC, formerly CBEMA) curve, which is included in IEEE standard 1100-1999, is the total removal of the input voltage for 20 milliseconds or longer. Powerfails can cause the floating heads of old building-system computer disk drives to "crash down" on the disk, causing memory loss, unscheduled shutdown or equipment damage. Modern disk drives have heads that automatically retract upon loss of power, but the designer cannot assume that this is the case in an existing facility.

Harmonic Distortion. Harmonic currents are a result of building-system and office-automation equipment that require currents other than a sinusoid. The amount of harmonic voltage distortion occurring on any building power-distribution system will depend on the impedance vs. frequency characteristic seen by the equipment and by the magnitude of the currents generated by nonlinear loads. The distortion factor can refer to either voltage or current.

Total harmonic distortion (THD) is calculated by breaking a periodic wave into its sinusoidal components and then doing a quantitative analysis of its parts. Buildings with high harmonic-current problems will see increased heating in power distribution components as a result of iron and copper losses. The heating of power-distribution components can lead to premature failures and give rise to high audible noise emission. In induction motors, harmonic problems can cause the phenomena known as cogging (the refusal to start smoothly) or crawling (very high slip).

A system resonance condition is the most important factor affecting system harmonic levels. Parallel resonance is a high impedance to the flow of harmonic current, while series resonance is a low impedance to the flow of harmonic current. In building systems where capacitor banks are used for voltage control or power factor improvement, the way in which capacitors are connected can cause resonance conditions-both series and parallel-that can magnify harmonic current levels.

Parallel resonance occurs when system current oscillates between the energy storage in the system inductance and that stored in the capacitance. The frequency at which parallel resonance occurs can be estimated by the following simple equation:

Hresonance =

short circuit MVA/capacitor bank size in MVA=

X c / X L

H is the harmonic order, and X C and X L are capacitive and inductive reactances of the building power system at the fundamental frequency.

Series resonance occurs as a result of the series combination of building power-system capacitor banks and line or transformer inductances. Series resonance presents a low impedance path to harmonic currents and tends to draw in, or "trap," any harmonic current to which it is tuned.

Building cables involved in system resonances may be subjected to voltage stress and corona which can lead to dielectric (insulation) failure. Cables which are subjected to ordinary levels of harmonic current are prone to parasitic heating. The flow of nonsinusoidal current in a conductor will cause additional heating over and above that expected for the RMS value of the waveform.

Harmonic currents flowing through the resistance of the building power system represent heat as:

P h = I2 harmonic x R harmonic

R harmonic for a given building power system can vary with applied harmonics because of such issues as skin effect, proximity effect, stray currents and eddy currents. These vary as a function of frequency as well as conductor size and spacing. As a result, the effective alternating-current resistance (RAC), is raised higher, increasing the I2R AC loss.

Solutions

The most common power-quality protective device-used to shield equipment from power transients and surges-is the transient-voltage surge suppressor. One disadvantage of system transient suppressors is that they pump transients to ground, shifting line power-quality problems to potential building-ground problems. This can be avoided by separating surge suppressors and lightning arrestors from the building critical equipment ground with an isolation transformer. The shielded isolation transformer is often used in tandem with voltage regulators, transient suppressors and other devices and power conditioners, because it offers excellent power dirt rejection and a clean ground.

Where a high level of power quality and total isolation from sags, dips, surges and transients is required, motor generator (MG) sets and UPS systems are used. Since MG sets and UPS units actually reconstruct the power sinewave by converting the system alternating-current (AC) input voltage to another form of energy and then regenerating an AC voltage, these units are not pure power conditioners. The UPS often comes with input and output filters to prevent damage to its solid-state components and to limit harmonic feed to its source of power. Harmonics are a necessary evil of modern solid-state electronics-but one that can be dealt with using the technology currently available.

From Pure Power, Spring 2001





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