RELIABILITY OF INDUSTRIAL PLANT POWER SYSTEM -
PART I
by Doods A. Amora, PEE
WHAT IS RELIABILITY?
This article is an assimilation of the interwoven factors & issues that contribute to the reliability of power systems in industrial plants. As Reliability is a broad scope, understanding what it is, comes first.
IEEE defines RELIABILITY as "the ability of a system or component to perform its required functions under stated conditions for a specified period of time." Likewise, from the Wikipedia, Reliability may be defined in several ways, as follows:
* The capacity of a device or system to perform as designed;
* The resistance to faults or failures of a device or system;
* The ability of a device or system to perform a required function under stated conditions for a specified period of time;
* The probability that a functional unit will perform its required function for a specified interval under stated conditions.
* The ability of something to "fail well" (to fail without catastrophic consequences and is restorable in a reasonable period of time).
RELIABILITY IN EVERYDAY LIFE
In the Philippines, electricity is usually taken for granted. As many observers say, ‘electrical power is somewhat like the air people breathe’. Electricity seems to be just ‘in there,’ meeting every man’s need constantly, somewhat eternally. As it is always expected that light would come on every time a switch is flipped, fact is, humanity doesn’t really think about it until it is lost. It is only during a power failure, when one enters into a dark room and instinctively hits the useless switch, realizing how important power is in daily life. But, a reliable & continuous presence of electricity is more than just comfort or convenience. It's a necessity. Take power out and industries will grind to a halt - the nation’s economy, as well. Without it today, life gets clumsy and gawky.
In an industrial plant scenario for instance: It’s only a matter of pushing a button and a 6,000 kW motor kicks up to life - just like that! And nobody seems to be thinking about it. That in a sense is reliability..! But if somebody is worrying about what could result if a button is activated (e.g., huge voltage dips or source trip-outs), then that’s another story…
Now, if nobody seems to be worrying about it - then that is good reliability..! Thus, in practical terms, power system reliability is simply: “There is power at sufficient capacity when needed, at any given time, all the time..!”
For an industrial plant, maintaining a high level of reliability requires continuing purposive watch. Of course, a plant relies on a dependable interconnected network of generation (by NPC or PNOC or IPP’s), transmission (TRANSCO & other Power Distributors), and the industry’s own Distribution Systems to power up various processes of an industrial plant whose appetite for power may be small or big time. The questions thus, are:
1) What is the industrial plant’s desired condition in so far as operational continuity is concerned?
2) How long can the industrial plant endure a forced shutdown?
The type of process and the behavior of manufacturing operations of the plant dictate the continuity of service requirements of the power system. Some plants can tolerate interruptions while others require the highest degree of continuity. Where adequacy & continuity of service is of prime importance, these plants deserve a much higher degree of sophistication in their own distribution systems than others.
THE RELIABILITY ‘NINES’
So the question is – “What levels of reliability can a manufacturing plant live with?” First of all, we need to be oriented with the ‘Reliability Nines’ as a technical lingo.
The telephone network has always been identified as a good example of a highly reliable system. But then, bad weather conditions among other factors threaten to derail its high reliability for obvious reasons. As experts say, “if the telephone system was out of service for a total of nine (9) hours over an entire year, then it was available for 525,060 minutes out of a possible 525,600 minutes. Its reliability is therefore: 525,060 divided by 525,600 minutes = .999”. Global experts label it as “three-nines” – and is a good reliability by most standards.
Note that the ‘Reliability Nines’ are new measurements for service dependability, consistency and trust-worthiness packaged in official terms as reliability. Depending on the nature of the business, the desired ‘nines’ in reliability depends on how essential the service is and what are the downtime benchmarks with other industries of similar nature. For telephone systems for instance, a nine-hour downtime per year may be excellent, but not acceptable for life-support systems in hospitals.
To compare with other types of services, the following new global standards may give some insights & discoveries as follows:
1) Homes: Three 9’s (99.9%), 9 hours downtime per year
2) Factories/Manufacturing Plants: Four 9’s (99.99%), 59 min/year
3) Hospitals, Airports: Five 9’s (99.999%), 5 minutes per year
4) Banks: Six 9’s (99.9999), 32 seconds per year
5) E-Commerce/ On-Line Markets: Nine 9’s, 30 milliseconds per year
As can be seen, reliability performance has got to do with the quality of services. In its everyday sense, quality of service means "consistency" and "repeatability". Reliability is when the service, whatever it is, is available or unavailable depending on one's perspective. A perfectly reliable system therefore is said to have a reliability of 1.0000, or a hundred percent reliable.
RELIABILITY IN THE MANUFACTURING PLANT
Note that with today’s technology, the ‘three-nines’ for telephone systems is no longer what the industry is looking at. The standard now often mentioned for traditional telephone service is the stiffer “five-nines”. This had become a motivational goal for new competing players, and a bragging right for those who have achieved it.
What about the factory? A factory or manufacturing plant supposedly belongs to ‘four-nines’. The expressed term ‘four-nines’ refer to the figure 99.99%. It's not just how frequent pieces of equipment in a power system burst into flames that solely counts. It's how much of the time the manufacturing plant is available for production. Availability already imbeds how often it breaks down and how fast it gets back into service. In addition, how long a system is out of service due to routine maintenance.
If a reliability of ‘two nines’ is acceptable to a manufacturing plant, this means that it could afford an average 87.6 hours of downtime annually (3 days, 15 hours and 40 minutes). To increase this reliability to four-nines, it means redundant systems where maintenance can be performed without necessarily shutting down production, while not loading the transformers, switchgears or cables heavily to their thresholds, and to make the system resistant to faults & failures and if it should fail, it should “fail well”.
“Failing Well” is what experts refer to in systems that sturdy enough to resist faults. And faults here mean that there shall be no disturbance to other systems that are unfaulted. The system therefore should be designed to isolate faults selectively with least disturbance to other parts of the system and should have the features for maximum reliability consistent with plant requirements. “Failing Well” also means that no damage in catastrophic proportions must result out of these faults. In Europe it is referred to as “fault control”.
To be continued…
DOODS (September, 2008)
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