A white paper on power problems
The availability of business systems is significantly impacted by AC mains power quality. The degree to which power quality affects business systems depends on many factors, which include:
1. The quality of the electrical power
2. The downtime caused by factors unrelated to power
3. The ability of the business systems to recover from power problems
These factors vary greatly from site to site and from business to business and therefore it is inappropriate to make general statements regarding the impact of power on business process availability. Nevertheless, it is possible to take into account the specific issues of a site and a business and determine the quantitative effect of power problems on business operation
What constitutes a power problem?
AC power is imperfect. All AC power exhibits defects almost continuously including harmonic distortion, sags, swells, RF noise etc. Common citations regarding power quality and the frequency of power problems can be highly misleading because they often include power defects that do not affect information equipment. For a meaningful discussion of the effect of power problems on information equipment, a power problem must be defined as a condition where the AC power does not meet the necessary and sufficient conditions required to provide equipment operation. The generally accepted definition of necessary and sufficient power quality is the magnitude and duration of power problems which are likely to affect information equipment.
The Electric Power Distribution System
For purposes of understanding the distribution of Electrical Power, the system is typically separated into the following four levels:
Distribution Level Equipment
Bulk Power — Power Plants
Area Power — High Voltage lines
Distribution Network — Neighborhood power lines
Utilization Equipment — Building wiring
A failure at any of these levels can lead to a failure of equipment operation at the user site.
Bulk Power is defined as a composite of the Generating Stations and the very high voltage transmission network. Problems with Bulk power affect the largest number of users. These problems are caused by
1. fuel shortages
2. human error
3. plant shutdowns
4. planned conservation
The statistics for Bulk Power availability vary widely. For example, on a small island Bulk Power may be a major contributor to down time. In Western Europe, the USA and Japan, the Bulk Power system is highly fault tolerant and a Bulk Power loss may occur only once every ten years or less.
Area Power is defined as the transformer stations and substations supplying a given area.
Problems with Area power affect large blocks of people such as entire towns or cities. These problems are caused by:
1. equipment failure/wear out
The statistics for Area Power availability vary widely. For example, some countries routinely employ interconnected stations with fail over capability while others employ a single path system. Systems with fail over capability provide a much lower mean time to repair and hence higher availability.
The Distribution Network is the local network of wiring which feeds buildings. This wiring typically follows streets and operates in the range of 5kV to 30kV and includes the transformers at the users site, which convert the power to the final utilization voltage. For many sites distribution is the primary cause of power problems. The distribution network is highly complex and exposed to many factors which can cause a power problem, including:
4. vehicular accidents
7. construction accidents
The statistics for power problems in Distribution Networks are most strongly affected by local weather. In systems where Distribution wiring is underground these affects are reduced dramatically. In some cases, a significant degree of fail over redundancy is designed into the local distribution system, which reduces mean time, to repair and therefore increases availability.
The Utilization System consists of building wiring, circuit breakers, and internal building transformers. Power problems arising in the customer’s Utilization System are mainly independent of the geographic location of the site and are caused by factors that are typically in the control of the customer, including:
2. construction accidents
3. scheduled electrical work
4. electrician errors
5. heavy equipment startup
6. poor wiring connections
The statistics for power problems in Utilization Systems are most strongly affected by the existence of construction or wiring changes in the building, the nature of the business (industrial vs. knowledge workers) and the age of the building and wiring. In situations where the quality of the power supplied by the Utility Company is high, power downtime may be dominated by Utilization System problems within the customer’s own facility.
Power protection devices
Power protection devices have traditionally fit neatly into one of two categories; those that alter, change, or otherwise control the character of electricity and those that provide an alternate or secondary source of power in the event of the failure of the primary
source. Products in the first group include surge protectors, filters, voltage regulator, power conditioners, and others. The amount of protection varies from device to device. The operational requirements of LAN systems along with an emphasis on protecting data, software, and processes have created a significant level of interest in the uninterruptible power supply (UPS) products that comprise the second group. While it is possible for a UPS to also function as a power conditioner, such capabilities cannot automatically be assumed. Indeed, along with the rapid growth in the number of UPS suppliers, the industry has seen the distinction between a UPS and a power conditioner become too poorly defined.
The best place to start is by highlighting several of the most common misconceptions concerning UPS products.
A UPS provides total power conditioning. · For total power conditioning, an on-line UPS (as opposed to a standby design) must be used.· Standby UPS systems are undesirable because they only become active when power is lost.
Much has been said and written in the battle between different UPS technologies. It’s important to recognize that today, most UPSs are used in applications where the system is powered by a switch mode power supply. These power supplies make computer systems very tolerant of both voltage variations and short duration (5-20 msec) power losses.
The fact is that systems powered by switch mode supplies (and that’s most systems today) are perfectly compatible with standby UPS designs. Equally inaccurate is the assumption that because of its inverter design, an on-line UPS provides superior power conditioning to a standby UPS.
It is true that on-line UPS systems provide excellent normal mode protection (between line and neutral). Normal mode protection, however, is only one part of the power-conditioning picture The switch mode supply is a significant improvement in electronic system design for a number of reasons. Not only does it make system more tolerant to voltage variations, but it is also smaller, lighter, more efficient, and quite a bit cheaper to produce.
All these advantages come with a price tag, however. The predecessor to the switch mode supply was the linear supply. It was characterized by a step-down isolation transformer on the input side. Elimination of the transformer in switch mode designs accounts for most of the physical and economic advantages.
However, it also results in a distinct operation disadvantage. That is the loss of common mode (neutral to ground) noise immunity for the system. Modern microprocessor system use electrical ground as a signal reference when making logic transitions and for the proper exchange of data between systems and peripherals.
For reliable operation, ultra-quiet ground reference is a necessity. Common mode disturbances disrupt this clean signal reference. Such disturbances can only be eliminated with an isolation transformer. It is important to recognize that a UPS – any UPS – should include an isolation transformer in it output circuit. Without it, the UPS cannot qualify as a power conditioner because it will not be capable of protecting the attached computer system from common mode noise.
There is a proliferation of UPS systems available in the marketplace that do not contain all the elements necessary to provide complete protection to the sensitive electronic load. This is true for both on-line as well as standby designs. Example abound of both types of UPS designs that fail to incorporate an isolation transformer as the final stage of their construction .