I have few questions on Governor control:

1. Whether the isochronous and droop speed control are default features of all governors, both electronic and electro mechanical?

2. I presume the automatic switching over from one mode to another is effected by the utility grid circuit breaker status during islanding and grid parallel operation of the generator. However, when multiple generators are in parallel operation,which generator is selected for isochronous operation, when separated from grid?

3.Is it a must a load sharing system is incorporated for multiple generator operation for smooth running on islanding & grid parallel?

3.When an external load sharing system in place, whether all the generators will be operated in isochronous load sharing or droop speed control load sharing by the controllers?

4.Finally,how much effective the external load sharing system will be for steam turbine generators?

Please bear with me for too many questions.
Thanks.

 

Shanks,

It's not the number of questions, it's the vagueness of the scenarios.

1. Every electronic or electro-mechanical or electro-hydraulic or mechanical-hydraulic governor I've ever worked on had both types of speed control modes by default. I'm sure there are some (probably for small prime movers and some hydro turbines) that don't have Isochronous speed control mode, but they are the exception rather than the rule.

2. Presumption, while better than assumption, can still be a dangerous thing. One would think for plants/locales with the ability to operate independently of a large grid ("island mode") that the transfer of the generator-set from Droop to Isochronous would be automatic and directly tied to the utility tie breaker status. But the reality is that quite frequently the transfer is left up to a human which can mean wide variations in frequency until the responsible human initiates the transfer. And, if the governor is not properly tuned for the prime mover's operating characteristics then there is a risk of tripping--which is usually why the transfer is not automatic to begin with.

A control system, or even a group of control systems (governors) can be configured (programmed) to do whatever is desired on some event. There are lead-lag scenarios, least recently used scenarios (where counters or timers choose the least recently used gen-set to be the Isochronous unit--the thought process being it should be the more reliable of the group), and any number of other unique (and one-off) schemes for multiple units which need to be able to power an island load. There are Power Management Systems which (allegedly) can control multiple machines ine Isochronous mode (or Droop mode) which have to occasionally operate in island mode with some very "creative" schemes (many of which never work properly). If you can dream it up, it's been sold and tried.

3. One. For automatic, non-human operator intervention? It's pretty difficult without a power management system, especially if the load varies rapidly and is not very stable. But, there are some very "sophisticated" power (load) management systems which don't work well at all.

3. Two. It depends on the "experience" (whims in most cases) of the programmer(s).

4. Steam turbine-driven generators are limited in their ability to respond to load changes because while the valve controlling the steam can be opened/closed quickly the ability of the steam generator (boiler) to maintain pressure/flow can lag far behind the load demand (which drives the control valve operation). This isn't a problem for "small" load changes (say, 50 MW) on a 600 MW steam turbine, but can be a real problem for smaller steam turbines (say, 20 MW) trying to respond to a sudden 12 MW change in load. The ability of the "boiler" to increase--or decrease--steam production is not as fast as fuel flow can be varied to a gas turbine or Diesel engine. Boilers are inherently slower than some other types of prime movers. But it all really depends on the nature of the load--how much it varies and how fast it varies.

Since we're talking about switching from Droop to Isochronous, the change in load when making the transition is also important. Is the load in Droop mode for three gen-sets 33 MW, but when the utility tie breaker suddenly opens does the load drop to 31 MW, or 11 MW? There are a lot of factors, including the type(s) of prime movers as well as the load changes (both from grid operation to island operation, as well as during island operation).

Most load-sharing systems are made using PACs (Programmable Action Controllers--the new politically correct name for PLCs, Programmable Logic Controllers), and there are lots of very bright programmers for many of these generic controllers. But, it really takes someone with power generation experience and knowledge to be able to make these systems work well. There are some load-sharing systems, such as from Woodward Governor Co., which work better than others--if the person configuring and programming them know and have experience with power generation.

It's really an experience thing, because a lot of stuff taught in universities and written in textbooks and reference books is pretty much ivory tower stuff--all theory and little or no practical knowledge or experience. "Load sharing" is a great example of this--it's a term frequently used but never fully explained. Especially in discussions about Droop. And most written descriptions of Droop say that speed changes with load--which is only true for a single gen-set powering an isolated load while in Droop mode--but they never say that. When a machine operating in Droop is synchronized with other units on a well-regulated grid the speed NEVER changes with load. But, that's never properly described, either in most texts or reference books.

Hope this helps!

 

Thanks CSA. It is of great help.

It boils down to one has to consider various factors like, the type and behavior of the plant loads, fluctuating or steady, the response of the governing system and the type of turbine (steam, gas, diesel) before going for load sharing system, hoping for a smooth running of a captive power plant. That calls for an in depth study of the plant operation, power plant experience and installation & commissioning support of PMS system supplier, in close coordination with plant people. We should not invite more trouble by installing a load sharing system with out a thorough study. This is especially true in the case of steam turbine generators. We have to keep it simple.

 

> ....That calls for an in depth study of the plant operation, power
> plant experience and installation & commissioning support of PMS
> system supplier, in close coordination with plant people.

This is one of the most important things that people fail to do when installing new controls or upgrading controls: they fail to ask for references from the suppliers bidding on the work and even if given references they fail to contact the references and ask for their impression of the commissioning and after-installation support. Just about any programmable control system can be programmed to act as a PMS (Power Management System). BUT, the people programming it need to know what's supposed to happen and when, and in some cases, how. I have seen PLCs used for turbine control systems that had some really brilliant programmers (as in more than one or two programmers) and creative sequencing/logic--but it didn't do what needed to be done. Because while the programmers were extremely well-versed in how to program the controller and wrote some amazing code/logic/sequencing, it just didn't do what it needed to do. And, you could tell that multiple programmers had worked on the project--there was widely varying differences in signal naming, comments, and over-writes of other bits of code/programming.

I talk with people regularly who say they wish they'd known about the issues with a company they'd chosen to supply or upgrade their controls. Because, now that they have the system and they've talked with other people/sites with systems from the same supplier they have learned they wouldn't have chosen that supplier. Either the commissioning was late and fraught with issues and there are unresolved issues, or they can't get the supplier back to help with on-going issues after the commissioning. (In some cases, they won't have the supplier back, and in some cases the supplier won't return--so there are sometimes two sides to every story/situation.)

But, if you or your company is considering adding something like a PMS to existing generator-sets at your plant, it would behoove you and your company to do the research you have mentioned but to also get references and contact those references before you decide on a supplier. It's not easy or fun to call people and ask them what their experience with a supplier was like, but it's better to find out before you are saddled with a system that isn't working and the supplier doesn't seem capable of making it work, or had serious problems getting it to work.

And, lastly, the power system study you mentioned is a critical part of the whole project--maybe the second most critical. Understanding the nature of the captive load, the types of prime movers, and the overall power system and interconnection is crucial to the control system supplier. You might find it's easier to commission the power system study as part of the project supply, or to elicit bids from control system suppliers to perform the power system study, as the power system study might be more "meaningful" to the control system supplier than something prepared by someone else who really doesn't understand why the study was commissioned and couldn't really answer problems the control system supplier might have. This probably limits the choice of suppliers, but it could be a benefit that would be very important.

Best of luck with your project! There are lots of captive loads with island power generating systems, some work better than others, and some don't really work very well at all. And, it's important to also realize that if the load changes (the plant expands, or contracts) that the power system study and coordination will also likely change. So, the PMS might need modification/changes, too. It's only a part of the whole system.

 

CSA,

What you have written is very true and thought provoking. It is very important to check with PMS suppliers past references that too of a similar plant of yours before taking the plunge.

Whenever any existing plant is going for an expansion to increase production, additional steam turbine generators and boilers are added. And we are asked for advice whether to go for Load sharing (PMS) controls or not. And the plant is steel plant with wildly fluctuating loads arc furnaces, rolling mills, induction furnaces etc. Sometimes it is sugar plant with its own characteristic loads. The situation is tricky.