1. For diesel generators, at what size capacity is a separate excitation panel or AVR panel required?
2. In parallel operation of diesel generators and steam turbine-driven generators, how is load sharing and reactive power management typically achieved?
- Is a master control panel required for load and reactive power sharing, in addition to the individual PLC control panels for each generator? Why can't load sharing and reactive power management be achieved using only the PLC control panel for each generator?
- If each generator set has a separate PLC control panel for load sharing and reactive power, why can't a single PLC control panel (master control panel) manage this for all units?
- In this context, does "PLC" refer to the load sharing controller provided by the vendor, and "AVR" refer to the automatic voltage regulator also supplied by the vendor?
3. What factors should be considered when paralleling dissimilar generators, such as diesel and steam generators, of different brands and capacities?

 

@Selk,

1. I'm not familiar with smaller diesel-generators (less than approximately 750 kW) but most diesel-generators I've encountered all have separate diesel governors and excitation (AVR) controls. There are so many types of exciters (PMG; brushless; static source; etc.) it would seem to be a difficult project to build a single control system to handle several types of exciters AND the engine controls. Not so difficult with smaller generators. I imagine, also, that keeping high-voltage electronics separate from the governor control system might be a consideration for some diesel-generator packagers. Most diesel manufacturers also probably don't manufacture generators and so only provide the diesel governor. (Diesels can provide power/torque for MANY different types of applications, not just generators.)

2. Are you speaking about parallel operation of generator-sets that are supplying a load (loads) independently of a large electric utility grid/system? Or generator-sets that are paralleled with large utility grid/systems? Load sharing is done using Droop Speed Control in most cases, but in some smaller systems there may be some kind of PMS (Power Management System) that can be used to automatically control load-sharing and even reactive power management.

A master control panel is not required--but well-trained and experience operators would be definitely required in this scenario.

Using the individual governor controls and excitation controls for multiple generator-sets synchronized together would definitely be possible, because load-sharing is accomplished using Droop Speed Control. There are some companies that manufacture and sell reactive load-sharing controls (think Woodward Governor, and Basler Electric; I'm sure there are others, as well).

There really aren't a lot of knowledgeable, experienced PLC programmers around who really and truly understand AC power generation controls and are capable of programming "generic" PLCs to accomplish load-sharing and/or reactive load sharing--and do it well. YES; there are LOTS of excellent PLC programmers who can do a lot of very ingenious and creative programming, but that's not what AC power generation requires. AC power generation requires a firm understanding, and usually some hands-on experience, of AC power system fundamentals and practices. Droop Speed Control, and Isochronous Speed Control, are not things which are well documented in textbooks or reference books and there are even fewer people around the world that can explain these concepts to others so they understand them and can implement them in generic PLC control systems--which were not designed with AC power generation in mind, you can be assured of that.

I have seen and had to deal with some very bad implementations of PLC-based generator-set PMSs that were simply incapable of performing even basic load sharing or reactive power management or even frequency control for small- to mid-size facilities that operated independently of a larger utility electrical grid/system. Most are unsupported by the control system integrators who packaged and commissioned them--because the people doing the programming simply don't understand AC power system concepts and principles and they are being badgered by power plant managers and operations supervisors to "make them work!" when they try to go to site and help with issues. And, the power plant personnel don't always use all the functionality the PMSs were supposed to provide, because they simply don't work or don't work well. (I should also mention that some power plant managers and operations supervisors don't understand basic AC power system concepts and fundamentals very well, themselves, and so cause confusion when trying to work through issues with these PMSs.)

I don't think you will find most generator-set manufacturers refer to the control systems they provide for the prime movers (the governors) as PLCs. They might be programmable like PLCs, but, again, in general PLCs don't really make good prime mover control system (governors). If they did, you would see inexpensive PLCs used on many prime movers as governors.

AVR refers to the excitation control system, operating in generator terminal voltage control mode. There may be a VAr or power factor control mode of the AVR which can be enabled and used to control either parameter by varying generator terminal voltage, but AVR refers to Automatic Voltage Regulator--the generator terminal voltage control mode of exciter control. (Lots of people like to use TLAs (Three-Letter Acronyms) to describe technical things; it makes them sound more knowledgable when they can bandy TLAs around without really understanding what they actually mean.) Some AVRs or excitation control systems also have a "manual" control mode that doesn't try to directly regulate/control generator terminal voltage. Generator terminal voltage control requires special sensing devices which can often be damaged and so if the excitation control system can only control generator terminal voltage then they can't be used when the sensors are damaged or out of service--so manual control mode can be used (by trained operators) to allow the generator-set to continue to provide power to supply load(s). But, some smaller generator-sets don't have this manual control mode as most of these types of generator-sets are not usually directly monitored or controlled by humans and so need some kind of automatic control of generator terminal voltage.

3. This is a very difficult question to answer without understanding the way the generators are being used (synchronized to an electric utility grid/system, or a load (loads) that operate independently of an electric utility grid/system). Just about any generator and its prime mover that uses Droop Speed Control or can be operated in Droop Speed Control mode can be paralleled with just about any other generator-set--large or small, and made by just about any manufacturers--because Droop Speed Control is the universally accepted method of operating multiple generators synchronized together to supply a load/loads. This is the way generators and their prime movers have been synchronized together since the beginning of AC power generation and distribution systems. Droop Speed Control is often described as a "load-sharing" mode of prime mover governor operation--because it is. Without Droop Speed Control a generator synchronized to a grid/system with other generators and their prime movers (operating in Droop Speed Control--because that's the universally accepted method of operating multiple generators together) will behave very erratically--the power output will be very unstable (unless the nature of the load(s) the grid/system is supplying are very stable and constant) and will most likely trip at some point. If I understand your definition of load-sharing, it's primarily accomplished using Droop Speed Control and trained human operators. Yes, it can be automated using some kind of PMS (Power Management System) and there are all kinds of PMSs (see Woodward Governor offerings for several, though they're not usually called PMSs), some work better than others; some work MUCH better than others. Even if a machine is "under the control" of a PMS it will still be operated/paralleled with the prime mover governor operating in Droop Speed Control--it will just receive commands to change load from the PMS instead of a human operator.

(If you come to a good understanding of Droop Speed Control, you will see that multiple generators and their prime movers located hundreds of miles apart can operate smoothly and stably WITHOUT ANY CONTROL SYSTEM WIRES INTERCONNECTING THEM TO SHARE LOAD!!! It's all done by monitoring system frequency (and prime mover-generator speed--because the two are directly related). It really is a pretty ingenious--and simple--system.)

 

Thank you for your detailed and thoughtful response!

1. I'm referring to generators in the range of 1 to 5 MW. The reason for my question is that, in smaller generators, the AVR is typically mounted directly on the generator, while the governor controller is usually located on the local control panel on the generator skid or nearby.

2. I’m referring to the parallel operation of generator sets supplying loads independently of a large utility grid or system, specifically in the context of marine ship operations.

A master control panel is not required--but well-trained and experience operators would be definitely required in this scenario.

So, when is a master control panel really required?

Using the individual governor controls and excitation controls for multiple generator-sets synchronized together would definitely be possible, because load-sharing is accomplished using Droop Speed Control

If individual governor controls and excitation controls can handle the task, what additional value do controllers from manufacturers like Woodward Governor and Basler Electric provide?

Generator terminal voltage control requires special sensing devices which can often be damaged

What are those special sensing devices?

I don't think you will find most generator-set manufacturers refer to the control systems they provide for the prime movers (the governors) as PLCs

May be "governor controller" or "load sharing controller" or Just a "controller"
3.

Yes, it can be automated using some kind of PMS (Power Management System) and there are all kinds of PMSs (see Woodward Governor offerings for several, though they're not usually called PMSs)

What are Woodward governor offerings termed as?

 

2. When the owner/operator of the "power plant" wants to have load control, frequency control and reactive power management automated.

3. The additional controller(s) I'm referring to would basically be PMSs, providing load control, frequency control and reactive power management. Automation that reduces the need for constant human monitoring and adjustment.

High-accuracy PTs (Potential Transformers) which usually have some kind of fuse protection (though not always) but are subject to problems from lightning strikes, over- and under-voltage spikes/dips, and high temperatures, poor cleanliness/maintenance, and vibration.

I think you will find that a diesel engine prime mover controller is most often called a governor, or whatever acronym the manufacturer/packager decides to use to describe the control method/equipment used to control speed and load of the diesel engine. Steam turbines also have governors to control load and speed. So do combustion turbines, and hydro turbines.

Woodward uses their own acronyms; if I remember correctly they call one of them a DSLC (usually pronounced/spoken as "D-slick"). You can go to their website and browse their products--they have a very large array of diesel engine controls--some are simple mechanical governors, others are electronic controllers. They have a large array of diesel generator controls, as well, that provide load-sharing and frequency control and even some power factor management options, as well, if I recall correctly. They also publish a number of "white papers" to describe basic governor functions, some for generator applications. Unfortunately, the last time I tried to download a couple of white papers and an instruction manual they asked for a lot of information (employer; job function; address; work phone number; work email address; etc.), and I was just curious--not trying to apply some controller or learn more about speed control, for example (because I have read Woodward white papers on droop speed control--and it's just the same ivory tower drivel found in most textbooks and reference books; and many of their descriptions of other functions (such as "cascade control" are very brief and nebulous))--but just because I wanted to refer to some sections for responses I was preparing for Control.com. So, I chose not to provide all the required information, and they chose not to grant me access to their materials on their website. Seems fair enough.

Look, automation is a good thing. But all programmable controllers are not meant for all applications. Period. Full stop. And just because they're programmable and there are some extremely experienced programmers who primarily work on some of the more common programmable controllers doesn't mean they are experienced with all applications. And, power generation is one of the applications that requires some pretty unique experience and understanding. When a lot of the written documentation available fails to properly describe the conditions for the control function they are attempting to describe it can lead to some very poor programming. For, example, Woodward describes Droop Speed Control as causing a proportionate decrease in machine speed as the machine is loaded--which is true if it's the ONLY machine supplying power to a load (or loads). BUT that's not often the case, and a single machine supplying a load (or loads) should NOT be operating in Droop Speed Control--it should be operating in Isochronous Speed Control. But that description is rampant in most textbooks and reference materials (and on the World Wide Web). I can tell you--and I'm sure you've experienced--that a synchronous generator DOES NOT decrease speed as its being loaded (because that would result in the frequency of the machine being lower than rated). It just doesn't happen in the real world, on this planet, in this part of the universe.

Tchau!