Protecting Power Quality
Power-quality issues have been around for many years, but in today's power-distribution systems, sensitive electronic equipment makes the subject more critical than ever before. The traditional sources of voltage dips, sags, surges, spikes and other longer-term interruptions are still around, but today...
Power-quality issues have been around for many years, but in today's power-distribution systems, sensitive electronic equipment makes the subject more critical than ever before. The traditional sources of voltage dips, sags, surges, spikes and other longer-term interruptions are still around, but today's facilities have all types of new power-quality issues. Harmonics, silicon-controlled-rectifier switching transients and vacuum contact-wave chopping are all new challenges to overcome in maintaining the power quality needed for the high-speed, sensitive electronics commonly seen in offices, schools and industrial complexes.
In this new environment, it is important for electrical engineers to know what the forces are that make power quality such a pressing issue, how to find out what is causing power-quality problems and what can be done to solve them.
In the past, electric utilities were vertically integrated and contained every aspect of generation, transmission and distribution in a single company. Power reliability and quality were of direct interest to them, and each time one of these issues arose, there was an immediate response by one of their discipline experts to mitigate the source of the problem.
Deregulation of the electrical supply has created another attitude toward these issues in a number of electrical utilities. Different companies may be responsible for generation, transmission and distribution, with none of these companies really assigned to address reliability or power quality. With the abolition of the guaranteed return-on-investment of the regulated environment and the emphasis on quarterly profits, the forces driving preventive maintenance and optimum reliability are now gone. For a number of utilities, it is becoming increasingly common to address emergency maintenance only after a failure, without any preventive maintenance functions. The reduction in maintenance and qualified personnel has created an industry where power quality and reliability will continue to decline. Coupled with the overall decrease in generation spinning reserves, monitoring power quality will be increasingly important.
Traditionally, power-monitoring strategies utilize a reactive approach where a problem is observed and then power-monitoring equipment is installed. The results are then analyzed and a course of action determined.
Today, mission-critical processes require something more. Continuous monitoring allows earlier detection of utility faults, equipment deterioration and load changes-before they escalate into a hard failure. State-of-the-art monitoring equipment uses digital signal processing, high-speed communications interfaces and sophisticated software packages to provide real-time analysis of system data to automatically identify power-disturbance sources. This equipment can eliminate all post-processing requirements by identifying capacitor switching, voltage sags and distances to upstream or downstream radial faults. By enabling an automatic notification device, a remote alarm-monitoring station or maintenance individual may be alerted to a problem by pager, e-mail or hardwired connection. This way, both the character and location of the problem is known before the site-maintenance personnel reach the site-reducing downtime to the lowest levels.
In addition, power-quality monitoring may be combined with load-flow and demand monitoring to provide an even more comprehensive continuum of the mission-critical systems' status. The collected data may be viewed in real time from any location by utilizing transmission-control protocol/Internet protocol (TCP/IP), where each meter has a unique IP address and anyone with an Internet connection may access each meter point individually.
The accepted standard for monitoring is Institute of Electrical and Electronics Engineers' (IEEE) 1159-1995, "Recommended Practice for Monitoring Electric Power Quality." (See "Standard IEEE Terminology," above, for some of the common terms used in the industry to describe the various types of outages. The included Table lists the ranges for the standard types of disturbances.)
Power conditioning today
The selection of power conditioners for particular applications is completely dependent on the type and magnitude of power disturbance at the location. While there are many types of power conditioners, the four common units in use today are the noise-isolation transformer, the line conditioner, the voltage stabilizer and the harmonic filter.
For minimal power-quality problems, the noise-isolation transformer performs very nicely on the high-frequency "noise" that is routinely seen on distribution systems. Typically, an isolation transformer provides 120 dB of high-frequency common mode-ground-to-neutral or ground-to-line-noise rejection. To achieve this high level of protection, the primary and secondary windings of the transformer are virtually open circuits to high frequencies, and the electrostatic shielding between the primary and secondary winding provides additional isolation. While the traverse mode (line-to-neutral) noise rejection is only 30 dB, there is still some protection there as well.
Two downsides in using isolation transformers are that they do not provide long-term voltage stabilization and do a very poor job of rejecting harmonic voltages. The longer-term voltage variations-for both above and below nominal-are not mitigated because the isolation transformer is just that, a transformer. Any voltage variations on the primary of the transformer are reflected on the secondary as variations in the same direction; that is, a 5-percent sag on the primary will result in a 5-percent sag on the secondary.
Likewise, the harmonic voltages are passed through the transformer with little attenuation. The maximum harmonic commonly seen is the 13th (780 Hz). Since the windings of a 60-Hz transformer attenuates 400 Hz of power by only a nominal amount, the effect on the common harmonic voltages are no more than double that attenuation, which is insignificant compared to what would be required to fully mitigate the harmonics and prevent interferences.
Line conditioners provide an increased level of power conditioning for sensitive circuits. The devices start with an isolation transformer and add active circuitry to improve voltage-sag protection, traverse-mode noise rejection and voltage regulation.
Because there are variations from one manufacturer to another and multiple offerings from many of the manufacturers, a unit may be selected to provide the desired level of power conditioning for most installations.
Stabilizing significant disturbances
For installations where there are significant voltage swings, voltage stabilizers provide a 1-percent output-voltage regulation with input-voltage fluctuations of plus or minus 15 percent. Having only nominal common and traverse-mode noise rejection, the voltage stabilizers may require additional line conditioning to provide adequate noise filtering if there are significant high-frequency disturbances in the system. Some voltage stabilizers can provide wave shaping that will reduce the total harmonic distortion (THD) in the system.
In installations where power-quality problems are created by heavy concentrations of switching power supplies, variable-frequency drives, uninterruptible power supplies or other loads with high harmonic contents, none of the previous power conditioners are adequate for the task. Significant harmonic content requires some type of harmonic filtering to mitigate the harmonic voltages and currents. The harmonic filters seen in the past were passive filters having a combination of inductors, capacitors and resistors arranged in Pi, T or L configurations to block or shunt harmful harmonic voltages from the critical load. These filters are designed to resonate at one particular frequency and provide a high in-line impedance to the selected frequency, a low impedance to ground for the selected frequency or both.
The passive filters work reasonably well for individual harmonics but do not work so well for broader-spectrum cases where there are high levels of objectionable harmonics at several frequencies. For these installations, there are active harmonic filters that sample the incoming line, amplify and invert the signal sample, and reinject the mirror image of the offending harmonics onto the line. Since this is done in real time, variations in the magnitude and frequency of the voltage are accommodated automatically and the output has a THD below 5 percent.
There are also fixed-frequency, active filters that mitigate selected harmonic currents and can accommodate varying current magnitudes. These filters are designed to reduce the triplen harmonics (third, ninth, 15th) which are otherwise difficult to mitigate. Because most passive filters do not work well for these triplen harmonics, zig-zag transformers may be utilized to cancel these harmonics without having to provide a resonant solution.
Super, code-compliant grounding
Grounding in a mission-critical application seems to always produce strong feelings by both the electronic systems designer and the power-distribution engineer. The electronics designer asks for a superb, totally isolated ground for his signal reference. The power engineer wants the system to meet the National Electrical Code (NEC) requirements of having all grounds in a facility connected to a single point.
Oftentimes, each party is convinced that his own viewpoint is the only one that matters and that the opposing view is just something to try to circumvent. In reality, a properly designed grounding electrode system and suitably connected bonding and grounding electrode conductors can give the electronics designer an excellent ground point, give the power engineer a properly bonded grounding system-and improve power quality.
Beginning with the electronics designer's criteria, an excellent signal-reference ground may be achieved by using one of the chemically enhanced grounds that draw moisture from the air to improve the earth-to-electrode interface. This grounding electrode would be connected to the mission-critical isolated ground bus in the distribution utilizing an identified, insulated grounding conductor sized as large as is practical. To meet the NEC criteria, Article 250 requires a connection of the service ground bus to the metallic cold-water piping system, a supplementary made electrode such as a driven ground rod, the building structural steel and the service-grounded conductor (neutral). In addition, other metallic piping systems that may accidentally become energized and the neutrals of all separately derived systems should be bonded to the common ground point. Connections for all of these system-grounding electrode conductors may be green insulated or bare copper conductors sized in accordance with Table 250-66 of the NEC, based on the equivalent service-entrance conductor size.
One important issue on the connection between the ground and the neutral at the service is that this connection must be the only point where they are attached together. In addition to the NEC requiring that there be only a single neutral-to-ground connection, multiple neutral-to-ground connections permit the passage of neutral currents onto the ground conductor. The varying neutral currents cause an elevation of the voltage level of the ground near the utilization devices. When these devices use the ground as a signal reference, the elevated state of the ground creates a false and varying reference for the signal-reference ground, causing device malfunctions.
Finally, a common connection must be made between the grounding electrode conductor system and the isolated signal-reference ground to meet Article 250-50. This connection should be made utilizing an identified #4 copper conductor from a point conveniently near the chemically enhanced ground electrode to the point at which the grounding electrode is bonded to the grounding electrode conductor. With the low impedance of the Article 250 grounding-electrode conductor, the lower impedance of the signal-reference grounding conductor and the relatively high impedance of the #4 bonding conductor, the result is the overall reduction of power disturbances in the critical system (see Figure above).
Correct diagnosis is key
For every installation, the engineer must analyze the system voltage and current with one or more metering instruments selected to measure the kinds of power-quality problems that his experience anticipates. After every piece of data is collected, the engineer must analyze them to determine whether the suspected problem is really present. If it is unclear, modifications to the meter's data-selection settings-or the use of another meter-can collect the pertinent data. Once the relevant data is in hand, the selection and sizing process can begin to determine the appropriate power conditioner for mitigating the power-quality problems.
As a design engineer, one must make sure never to select a power conditioner without knowing exactly what kind of power-quality problems are present. Otherwise, the engineer may find that he has fixed something that was not broken, leaving the client with an uncorrected problem.