AC Power Systems
Introduction
Every electronic installation requires a steady supply of clean power to function properly. Recent advances in technology have made the question of alternating current (ac) power quality even more important, as microcomputers are integrated into a wide variety of electronic products.
When the subject of power quality is discussed, the mistaken assumption is often made that the topic only has to do with computers. At one time this may have been true, because data processing (DP) centers were among the first significant loads that did not always operate reliably on the raw power received from the serving electrical utility. With the widespread implementation of control by microprocessor-based sin- gle-board computers (or single-chip computers), however, there is a host of equipment that now operates at voltage levels and clock speeds similar to that of the desktop or mainframe computer. Equipment as diverse as electronic instrumentation, cash registers, scanners, motor drives, and television sets all depend upon onboard computers to give them instructions. Thus, the quality of the power this equipment receives is as important as that supplied to a data processing center. The broader category, which covers all such equip- ment, including computers, is perhaps best described as sensitive electronic equipment.
The heart of the problem that seems to have suddenly appeared is that although the upper limit of cir- cuit speed of modern digital devices is continuously being raised, the logic voltages have simultaneously been reduced. Such a relationship is not accidental. As more transistors and other devices are packed together onto the same surface area, the spacing between them is necessarily reduced. This reduced distance between components tends to lower the time the circuit requires to perform its designed function. A reduc- tion in the operating voltage level is a necessary — and from the standpoint of overall performance, particu- larly heat dissipation, desirable — by-product of the shrinking integrated circuit (IC) architectures.
The ac power line into a facility is, of course, the lifeblood of any operation. It is also, however, a fre- quent source of equipment malfunctions and component failures. The utility company ac feed contains not only the 60 Hz power needed to run the facility, but also a variety of voltage sags, surges, and transients. These abnormalities cause different problems for different types of equipment.
Defining Terms
To explain the ac power-distribution system, and how to protect sensitive loads from damage resulting from disturbances, it is necessary first to define key terms:
• active filter. A switching power processor connected between the line and a nonlinear load, with the purpose of reducing the harmonic currents generated by the load.
• alternator. An ac generator.
• boost rectifier. An unfiltered rectifier with a voltage-boosting direct current (dc)/dc converter between it and the load that shapes the line current to maintain low distortion.
• circular mil. The unit of measurement for current-carrying conductors. One mil is equal to 0.001 in.
(0.025 mm). One circular mil is equal to a circle with a diameter of 0.001 in. The area of a circle with a 1- in. diameter is 1,000,000 circular mils.
• common-mode noise. Unwanted signals in the form of voltages appearing between the local ground reference and each of the power conductors, including neutral and the equipment ground.
• cone of protection (lightning). The space enclosed by a cone formed with its apex at the highest point of a lightning rod or protecting tower, the diameter of the base of the cone having a definite relationship to the height of the rod or tower. When overhead ground wires are used, the space protected is referred to as a protected zone.
• cosmic rays. Charged particles (ions) emitted by all radiating bodies in space.
• Coulomb. A unit of electric charge. The coulomb is the quantity of electric charge that passes the cross section of a conductor when the current is maintained constant at 1 A.
• counter-electromotive force. The effective electromotive force within a system that opposes the passage of current in a specified direction.
• counterpoise. A conductor or system of conductors arranged (typically) below the surface of the earth and connected to the footings of a tower or pole to provide grounding for the structure.
• demand meter. A measuring device used to monitor the power demand of a system; it compares the peak power of the system with the average power.
• dielectric (ideal). An insulating material in which all of the energy required to establish an electric field in the dielectric is recoverable when the field or impressed voltage is removed. A perfect dielectric has zero conductivity, and all absorption phenomena are absent. A complete vacuum is the only known perfect dielectric.
• eddy currents. The currents that are induced in the body of a conducting mass by the time variations of magnetic flux.
• efficiency (electric equipment). Output power divided by input power, expressed as a percentage.
• electromagnetic compatibility (EMC). The ability of a device, piece of equipment, or system to func- tion satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances.
• generator. A machine that converts mechanical power into electrical power. (In this book, the terms alternator and generator will be used interchangeably.)
• grid stability. The capacity of a power distribution grid to supply the loads at any node with stable voltages; its opposite is grid instability, manifested by irregular behavior of the grid voltages at some nodes.
• ground loop. Sections of conductors shared by two different electronic or electric circuits, usually referring to circuit return paths.
• horsepower. The basic unit of mechanical power. One horsepower (hp) equals 550 foot-pounds per second or 746 watts.
• HVAC. Abbreviation for heating, ventilation, and air-conditioning system.
• hysteresis loss (magnetic, power, and distribution transformer). The energy loss in magnetic material that results from an alternating magnetic field as the elementary magnets within the material seek to align themselves with the reversing field.
• impedance. A linear operator expressing the relationship between voltage and current. The inverse of impedance is admittance.
• induced voltage. A voltage produced around a closed path or circuit by a time rate of change in a mag- netic flux linking that path when there is no relative motion between the path or circuit and the mag- netic flux.
• joule. A unit of energy equal to 1 watt-second.
• life safety system. System designed to protect life and property, such as emergency lighting, fire alarms, smoke exhaust and ventilating fans, and site security.
• lightning flash. An electrostatic atmospheric discharge. The typical duration of a lightning flash is approximately 0.5 scc. A single flash is made up of various discharge components, usually including three or four high-current pulses called strokes.
• metal-oxide varistor. A solid-state voltage-clamping device used for transient-suppression applica- tions.
• normal-mode noise. Unwanted signals in the form of voltages appearing in line-to-line and line-to- neutral signals.
• permeability. A general term used to express relationships between magnetic induction and magnetiz- ing force. These relationships are either (1) absolute permeability, which is the quotient of a change in
magnetic induction divided by the corresponding change in magnetizing force, or (2) specific (rela- tive) permeability, which is the ratio of absolute permeability to the magnetic constant.
• point of common coupling (PCC). The point at which the utility and the consumer’s power systems are connected (usually where the energy meter is located).
• power factor (PF). The ratio of total watts to the total root-mean-square (rms) volt-amperes in a given circuit. Power factor = W/VA.
• power quality. The degree to which the utility voltage approaches the ideal case of a stable, uninter- rupted, zero-distortion, and disturbance-free source.
• radio frequency interference. Noise resulting from the interception of transmitted radio frequency energy.
• reactance. The imaginary part of impedance.
• reactive power. The quantity of unused power that is developed by reactive components (inductive or capacitive) in an ac circuit or system.
• safe operating area. A semiconductor device parameter, usually provided in chart form, that outlines the maximum permissible limits of operation.
• saturation (in a transformer). The maximum intrinsic value of induction possible in a material.
• self-inductance. The property of an electric circuit whereby a change of current induces an electromo- tive force in that circuit.
• single-phasing. A fault condition in which one of the three legs in a three-phase power system becomes disconnected, usually because of an open fuse or fault condition.
• solar wind. Charged particles from the sun that continuously bombard the surface of the earth.
• switching power supply. Any type of ac/ac, ac/dc, dc/ac, or dc/dc power converter using periodically operated switching elements. Energy-storage devices (capacitors and inductors) are usually included in such supplies.
• transient disturbance. A voltage pulse of high energy and short duration impressed upon the ac wave- form. The overvoltage pulse may be 1 to 100 times the normal ac potential (or more in some cases) and may last up to 15 ms. Rise times typically measure in the nanosecond range.
• uninterruptible power system (UPS). An ac power-supply system that is used for computers and other sensitive loads to (1) protect the load from power interruptions, and (2) protect the load from tran- sient disturbances.
• VAR compensator. A switching power processor, operating at the line frequency, with the purpose of reducing the reactive power being produced by a piece of load equipment.
• voltage regulation. The deviation from a nominal voltage, expressed as a percentage of the nominal voltage.
Power Electronics
Power electronics is a multidisciplinary technology that encompasses power semiconductor devices, con- verter circuits, electrical machines, signal electronics, control theory, microcomputers, very-large-scale integration (VLSI) circuits, and computer-aided design techniques. Power electronics in its present state has been possible as a consequence of a century of technological evolution. In the late 19th and early 20th centuries, the use of rotating machines for power control and conversion was well known [1]. Popular examples are the Ward Leonard speed control of dc motors and the Kramer and Scherbius drives of wound rotor induction motors.
The history of power electronics began with the introduction of the glass bulb mercury arc rectifier in 1900 [2]. Gradually, metal tank rectifiers, grid-controlled rectifiers, ignitions, phanotrons, and thyra- trons were introduced. During World War II, magnetic amplifiers based on saturable core reactors and selenium rectifiers became especially attractive because of their ruggedness, reliability, and radiation- hardened characteristics.
Possibly the greatest revolution in the history of electrical engineering occurred with the invention of the transistor by Bardeen, Brattain, and Shockley at the Bell Telephone Laboratories in 1948. In 1956,
the same laboratory invented the PNPN triggering transistor, which later came to be known as the thyristor or silicon controlled rectifier (SCR). In 1958, the General Electric Company introduced the first commercial thyristor, marking the beginning of the modern era of power electronics. Many different types of power semiconductor devices have been introduced since that time, further pushing the limits of operating power and efficiency, and long-term reliability.
It is interesting to note that in modern power electronics systems, there are essentially two types of semiconductor elements: the power semiconductors, which can be regarded as the muscle of the equip- ment, and the microelectronic control chips, which make up the brain. Both are digital in nature, except that one manipulates power up to gigawatt levels and the other deals with milliwatts or microwatts. Today’s power electronics systems integrate both of these end-of-the-spectrum devices, providing large size and cost advantages, and intelligent operation.