As modern industrial facilities become increasingly dependent on microprocessors, reliable data interchange, and complex process control, there is a growing need for improved power quality. But this is an area that is little understood in the UK, and there is widespread ignorance of Regulation ER G5/4-1, which was updated in November 2005 and attempts to control harmonic distortion.
An example of the difficulties that can occur is in the use of UPS technology. There are two important reasons for using UPS technology: ensuring continuity of ac power; and providing a supply of clean, quality power.
But many UPS designs are not acceptable power quality tools, and it is worth examining the issues to prevent costly system failure at a later date.
The obvious reason for fitting a UPS is to provide a secondary source of power if the primary source is lost. Yet, power outage is an infrequent event and focusing on this aspect diverts attention from some very important basic issues.
When a UPS is installed, power quality issues should be carefully considered. A system that should not fail because of a power outage must not be allowed to fail for other reasons either. By carefully selecting a UPS system, protection can be provided from a complete range of power problems. Additionally, many UPS systems can be combined with other devices to further improve power quality.
Power quality is more than having a clean, noise-free electrical supply. It means controlling harmonics, demand charges, power factor, and power factor penalties. Ultimately, it depends on the requirements of the equipment that is being protected. For industrial computers, some of the quality issues are:
Impulses: These short duration, fast rise time, high energy events may pack significant destructive potential, and can be responsible for catastrophic failure of semiconductor devices. The effect of an impulse is mitigated with a surge diverter.
Common mode voltage: This is any voltage measured with reference to safety earth. Since most modem computer systems use safety earth as their logic and communications reference, clean and quiet safety earths are mandatory. Computer problems associated with common mode voltage include processor lockup, lost or fragmented data, communication errors, or unexplainable "no trouble found" failures. Using an isolation transformer to create a separately derived power source eliminates common mode voltage.
Noise: This results from the distribution and use of electrical power. RFI and EMI are generated by every device that uses electricity. Ironically, many of the disturbances that cause computer malfunction are induced by the computer’s electrical system. Noise filters built from capacitive and inductive elements are used to divert disturbances to electrical system earth.
Voltage regulation: This is more critical for linear power supplies than for systems powered by switch mode supplies, and there are several methods of providing well-regulated voltage. These include tap-switching, or ferro-resonant voltage regulators, and buck/boost autoformers. An on-line UPS provides natural voltage regulation because of its double conversion process.
Outage: These can only be mitigated with a UPS device that provides reserve power from batteries. Battery-powered UPS systems function as an effective bridge until either the utility is restored or another source is brought online.
Frequency changes: This problem is caused by variations from the fundamental operating frequency of the electrical system. Double-conversion UPS systems have an ability to function as a frequency regulator, and a high-quality online UPS can tightly regulate output frequency over a very wide range of input frequencies, unlike single conversion designs.
Single-conversion UPS systems may not incorporate every one of these six elements; but double-conversion typically provides superior protection against normal mode noise and impulses.
Rob Morris is UK country manager of Powervar