POWER SYSTEM RELIABILITY IN INDUSTRIES - PART II

RELIABILITY OF INDUSTRIAL PLANT POWER SYSTEM -
PART II

by Doods A. Amora, PEE

THE PILLARS OF RELIABILITY

Are “RELIABILITY NINES” achievable…?

Reliability could partially mean the best equipment or systems that are the easiest to repair or maintain. That’s maintainability.

But on top of these, redundancy is still needed. Highly reliable systems often include multiple power supplies, UPS’s (stationary or rotary), backup diesel generators (for longer power outages) and extras of whatever else is likely to fail. Troublesome equipment & apparatuses that break down a lot and take a long time to get back online are going to spoil reliability. So, the chosen system depends on the duration of outage the plant can tolerate. Diesel generator sets require about 5–10 seconds to start, come up to rated speed, develop rated voltage, and begin to powering up loads. Where even momentary outages are unacceptable, UPS or flywheels (rotary UPS’s) are now common.

Note that the concept of ‘Reliability Nines’ can be achieved through:


1) Good Design of the System
2) Effective Maintenance Program
3) Error-Free Operation

While it is true that reliability is fundamentally influenced by the sturdiness of equipment & apparatuses in the system, trouble-free operation and effective maintenance starts at the drawing board when the design of a system is conceptualized. The design of an electrical system is to provide continuous operation under all foreseeable circumstances, including utility outages and equipment breakdown. When considering the implications of reliability, all three pillars of system reliability: design, operations, and maintenance, must be inputted in the design concept.

Experts in Reliability say, “There is no maintenance program that can improve the reliability of a poorly designed system. Additionally, whatever maintenance program developed by a plant is determined by the design of the system and the goals of the organization. One goal for reasonable levels of reliability given the nature of the technology is a good selection of equipment or system that provides a Mean Time Between Failure (MTBF) that is as long as possible. It is desirable to have a few relatively long but planned service interruptions rather than lots and lots of short ones that are unexpected. Maintenance also aims to provide a Mean Time To Resolution (MTTR) that is as short as possible, so that when a failure does occur service can get back quickly”. Again, this is maintainability.


Reliability practitioners further say, “The telephone system is a good example of reliability improvement over time. When telephones first became widely available in the early twentieth century, their reliability was poor by today's standards, with outages, dropped calls, line noise and crosstalk quite common. As time passed, technology improved to the point where five nines of reliability are now common. It did, however, take nearly eighty years to reach that standard of reliability”.

For sure, reliability comes at a cost - and it doesn’t come in cheap. For electric systems of any manufacturing plant for that matter; operational continuity frequently is synonymous to 'how fast the restoration of electric service' is. But swift restoration of service can not be achieved when there are no alternate paths of power flow provided in the system.


REDUNDANCY IN THE ‘N + n’ SYSTEMS

Hereunder is to introduce the terms, [N + 1], [N + 2], [N + 3]… as reliability through good system design:

1) A system with one redundant path is termed an N+1 design.

2) N + 1 would allow for one of the paths to be de-energized for maintenance while the other is still energized, allowing maintenance without system shutdown.

3) If the system is designed with a normal path and two alternate paths (N+2 design), one path could be down for maintenance, a failure could occur in a second path, and ideally, the third path would supply power to the load without interruption.

Thus new reliability jargon has given rise to the novel terms as: N+1, N+2, N+3 or N+n which speak for the degree of redundancy. How then does the Power System of an industrial plant fare with the ‘N + n’ principle?

Note that in a system that has been operating for 20 years, the more honest-to-goodness maintenance is needed to sustain continuous operation. But decent maintenance (other than wiping, air-blowing or cleaning the externals of the equipment & apparatuses) can not be done if there is no degree of redundancy in the system. Chances are, maintenance time would only be a few hours usually allocated during scheduled plant-wide annual shutdown. In this case, maintenance becomes superficial and hasty as production group would be scratching their backs when schedule to re-start operation has come.

So then, maintenance can’t be effective if the plant itself is not designed to be ‘maintainable’. The power system configuration must be maintenance-friendly such that maintaining major equipment does not mean shutting down the plant. If maintenance requires shutting down the plant, so then the plant is "not maintainable”. If continuous round-the-clock operation of all or some identifiable parts of the process is required, then system configuration must have redundant feeders or separate supplies to these components to support maintenance at other portions of the system. The power system must also be flexible in events of failures of major equipment such that the plant can still operate partially in a considerable production capacity.

But then, redundant power supplies in some instances do not always improve reliability. If two redundant feeders supply power to an industrial facility but originate at the same utility substation and are carried on the same set of power poles, reliability will be lower than if they originate at separate substations and travel to the site on different sets of power poles. The problem with redundant feeders carried on the same set of poles is that a single-point failure (e.g., a weather-related event, pole fire, or traffic accident) could cause simultaneous outages on both sources.

RATIONALE OF THE RELIABILITY STUDY

The importance of a Reliability Audit can not be over-emphasized. Its value speaks for itself the moment power is out during peak production days or during the visit of the company president. An industrial plant therefore continues to face the challenge of improving its power system availability of existing facilities in a very competitive global market. These challenges are aggravated by the condition that many plant facilities may have been in operation for more than 20 years with constant exposure to corrosive materials, fumes and a hostile environment which contribute to the gradual deterioration of equipment integrity. As the plant grows older, poor system availability may mean loss of competitiveness.

Experience has shown that capital investments to extend the life of facilities can be expensive if these are done when reacting to unplanned outages. Industry’s best practices are aimed to maximize reliability, and minimize unplanned production losses by using structured systems that implement pro-active reliability programs.

Note that if the industrial plant is already 20 years old, a sort of a Reliability Assessment should be sought for. Of course, reliability assessment should have been wanting during the drawing board conceptualization but an assessment today would not place the effort for naught. After 20 years, this kind of assessment is even more meaningful as a lurking system fault increases with age. This study therefore is used to identify improvement opportunities and manage to sustain system reliability in a cost effective manner.

The Reliability Assessment for an industrial plant may have the following objectives:


1) To identify “system vulnerabilities” or “gaps” that includes the following:

a) Loading Behavior or Load Profile of the Primary & Intermediate Substations that could have resulted to over-stresses and poor voltage regulations,

b) Review on the Flexibility of the System with regards to power supply and feeders, its capacity or capability in supporting the loads in alternate paths,

2) To identify major deviations from normal industry practices, including but not limited to:

a) Voltage-Dip levels on each major feeder or intermediate substations during starting of large motors and under other operating conditions,

b) Review on the Power Factor condition of each substation or feeder,


3) To identify whether the power system is resistant to faults & failures to include but not limited to the ability of a device or system to perform a required function under fault conditions and the ability of the system to "fail well". This includes the following:

a) Review & Confirmation of Actual Fault Duties against Interrupting or Momentary Ratings of the Existing Sets-Up,

b) Engineering Calculations Leading to Coordination and/or Discrimination of Protective Devices within the entire system,

c) Actual Re-setting, Testing & Simulations on these Protective relays whether they are performing as expected,

d) Evaluate actual condition of the equipment & apparatuses within the system if they could still last longer than expected.

4) Recommend measures to close identified gaps & vulnerabilities, including application of technology and/or system modifications that will facilitate improvement in reliability, as follows:

a) Capacity Review on all electrical equipment whether additional capacities are necessary to maintain reliable service under various operating conditions.



To be continued...

DOODS (September, 2008)