Difference Between Power System Reliability & Stability

[GRAEE]

Have read some references regarding this subject but it seems that there areas in the power system that these two will go hand in hand.

Can somebody provide me a clear definition of the two and how it differs from one point to another?

 

[waross]

Reliability is on-and-off.
Stability is up-and-down.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[magoo2]

Stability has to do mainly with the generation and transmission system and being able to keep the generators running and in synch with each other as the load changes.

Reliability has to do more with the load end of the business and keeping all of the customers connected. The unreliability (and exposure to interruptions) of the system is mostly at the distribution end where trees, lightning, equipment failures, etc. can interrupt service to some of the customers.

 

[rbulsara]

Stability is essential for survival of the equipment at any given moment. Reliability is essential for survival of business as magoo2 stated.

You cannot have a reliable system without stability but the converse can be true. You can have a very stable system, but it may not have any backup, in case it fails then the system is stable but not reliable to maintain availability of power.

Stability is the function of system's or individual machine's operating characteristics. Reliability is the function of system configuration.




Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Hi Rafiq;
I may have misunderstood you, but I have been in rural areas where power failures were rare but the voltage went from "TOO-HIGH" to "TOO-LOW" once or twice a day. I did not consider that to be stable. Maybe a misunderstanding of terms.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Bill:
Yes, that fits the unstable bill. I do not see the misunderstanding.

Rafiq Bulsara

http://www.srengineersct.com

 

[magoo2]

There's voltage stability as well as system stability which Bill mentions. Voltage stability deals more with a system being able to maintain adequate voltage control. I interpreted Graee's post as system stability and not voltage stability.

 

[waross]

You cannot have a reliable system without stability but the converse can be true.

I don't agree with this statement but I may be wrong.
We have different understandings of "Reliable".

I remember being in a community in the Yukon territory. There was a mine about 250 miles out from the generating station. We were about 100 miles out on the same line. The truck hauling the voltage regulator to the mine went off the road and the voltage regulator was wrecked before it reached the jobsite. Replacement took over one year. The voltage at the mine was regulated at the head end at the generating plant. Our voltage was low when the load was low and rose as the head end voltage was raised to accommodate load increases. When the mine was at high load, our voltage was too high. We had a local diesel backup generator. It was old, too small, and overloaded. When we were on the generator the voltage was about 5 or 90 Volts and the frequency was low also.
We always had some sort of power but it was very unstable.
I see the system as reliable but unstable.

I am presently at my Southern place in Central America. Our voltage and frequency are acceptable. I would call them stable. However we have a power outage at least every Sunday and failures during the week are common.
Stable but not reliable.
I understand that you consider the instability to imply unreliability.
Fair enough. Both of our viewpoints have been presented. Both have merit.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Bill:

For example, a system cab be unstable because of a mismtached generator or bad gen plant or temporary bad batch of fuel.

If the system had a back up to or alternate source to fall on by taking that bad unit off line but continue the service. That is a reliable system as for as the customer goes and business goes. It may be a unstable system in particular situation, that need be fixed.

On the other hand you can have a very stable system with a single generator, but if that fails for any reason and there is no back-up, that is an unreliable system design, even if the system appears stable at a given time.

Hope that helps clarify my thinking.


Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

Reliability is driven by the number of failures. Parameters like MTBF(mean time before failure), FITS (failure in billion hours), MTTR(mean time to repair), Availability, and Downtime are the usual measures if reliability.

On the other hand, Stability is a criterion for the operation of a system. Variations from allowable values are considered "instability".

Stability is usually compromised by "unreliable" system elements. One could say these two are synonymous but a system could be composed of low-reliability elements that can be made "stable" through redundant and over-reaching zones of protection, innovative configurations,etc and ample reserves management.

For example "An old but reliable horse" means the horse is always usable when needed but the same horse could be slow when you want it fast or doesn't run true and sways side to side(unstable).

 

[rbulsara]

Bill:
Re-reading your post..I see more agreement than disagreement!

To me, unstable would also be unacceptable. Or a unstable system would not be able to stay on line. What you described in your mine example, is not necessarily unstable (to me), it was stable enough to stay on and also acceptable to the mine loads. There were variations in V and f but once adjusted it stayed "there" , high or low. Not unstable enough to fall out of sync or off line.

Stability is a necessary but not sufficient condition for reliability.

I think, the horse is dead now, so I will leave it alone!





Rafiq Bulsara

http://www.srengineersct.com

 

[GRAEE]

Thank you guys...

Based on my own interpretation of your comments, generally, power system reliability talks about how the power network performs to deliver electric power to the loads while the power system stability deals with the equilibrium of the system, how the system regain its operating state after being subjected to disturbances.

 

[stevenal]

Language evolves, sometimes by decree. I offer the NERC definition:

Definition of “Adequate Level of Reliability”
The Bulk-Power System (“System”) will achieve an adequate level of reliability when it possesses following characteristics:
1. The System is controlled to stay within acceptable limits during normal conditions;
2. The System performs acceptably after credible Contingencies;
3. The System limits the impact and scope of instability and cascading outages when they occur;
4. The System’s Facilities are protected from unacceptable damage by operating them within Facility Ratings;
5. The System’s integrity can be restored promptly if it is lost; and
6. The System has the ability to supply the aggregate electric power and energy requirements of the electricity consumers at all times, taking into account scheduled and reasonably expected unscheduled outages of system components

Note that reliability includes stability.

What we formerly called "reliability" at the customer level, we now call "level of service", since most customer events are not Bulk Power System events.

 

[mgtrp]

Hi Bill,

I would love to hear any stories you have about the Yukon electrical system - I work for the utility that is probably providing to generation to whichever community it is that you're talking about!

 

[waross]

I was hanging out in Carmacks. When Anvil mine was being built they lost the voltage regulator bond for Faro. With Whitehorse trying to regulate the voltage for Faro and Anvil you can guess what our voltages were like in Carmacks partway out on the line. Carmacks had a diesel standby. When on diesel the voltage and frequency were both low. WE would drink coffee at the Carmacks Hotel and watch the circular fluorescent light fixtures cycle off and on. The balasts would overheat and the thermal protection would trip them off. After cooling down they would come back on. There would be about 1/3 of the fixtures out at any one time.
Is Roxie's trading post still in operation? It was just across the bridge heading north.
I spent a short time filling in for a friend as bartender at the Hotel beer parlor. I have a lot of outrageous memories of that episode.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mgtrp]

Thanks Bill - sounds like interesting times! Carmacks has a couple of petrol stations/general stores by the hotel and a small shop just over the bridge... no trading post that I'm aware of, but then again I tend just to pass straight through.