Excitation Systems: The Basics

Today we speak with our resident application specialist Bryan Snyder about the basics of excitation systems for Generator Sets.  Feedback?  Write to us at Powerbytes@cat.com

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Transcript

Lou:  Good Day and welcome everyone to Power Bytes!  I am your host Lou Signorelli and Power Bytes is your destination Podcast for power generation conversations.  As always please know how much we appreciate you, our listeners.  We hope you find our topics both helpful and interesting.  We would appreciate feedback and suggestions for new topics. You can send us an email at powerbytes@cat.com, visit us at Cat Electric Power on Facebook or LinkedIn.  If you enjoy your time with us today, please leave us a 5-star rating wherever you listen to your favorite podcast; It really does help.
 
Lou:  Today's topic is an overview of excitation systems.  Cat has published a white paper on this topic  which can be found on Cat.com.  Simply search the term "Excitation Selections" to get a copy of the white paper.

Lou:   I'm joined today by  Bryan Snyder,  the author of the white paper and for those returning listeners, you may recall he spoke with us about over-current protection in an earlier podcast.   Bryan has 19 years of experience with Caterpillar and a career of focusing on integrated systems and gensets.  Welcome Bryan and thanks for joining us today.

Lou: Bryan I read your paper and was amazed at how much there is to consider when deciding on the right excitation system.  Where should our listeners start when thinking about excitation systems.

Bryan: So as a basic overview it is good to understand that without an excitation system you won't have a generator.  The basics of power generation is to pass a magnetic field across a wire to generate a current in the wire.  The excitation system along with the voltage regulator is what creates the magnetic field in the rotor.  

Lou) So from the paper I understand there are two kinds of excitation systems: Direct and Indirect.  Can you take us through them one at a time?

Bryan: So the direct excitation system is also called a brushed system because it uses a direct connection through brushes and slip rings to pass a DC current from the voltage regulator into the rotor.  These systems can provide great response capability, but are also high maintenance and can considered less reliable.

Lou) ok, so that leaves Indirect systems?

Bryan: So the Indirect system doesn't use the brushes, and for this reason it is also called brushless.  The indirect system uses an exciter, which is basically a small generator also attached to the rotor to generate the necessary current. The exciter receives DC power from the voltage regulator to generate a magnetic field in the exciters stator.  This allow the exciters rotor to generate AC current which is rectified into a DC current to generate the magnetic field in the rotor.  Most modern generators use Brushless or Indirect excitation due to the improved reliability and lower maintenance.  

Lou:  So, in these indirect systems, where does the voltage regulator receive power from to generate the magnetic field in the exciter?

Bryan: There are three primary ways to provide power to the voltage regulator and this is actually very important because it will determine the performance of entire system when large loads are added or when there is a fault in the system.  

The three main sources for power are Self-excited (or Shunt), Permanent Magnet, and Internally Excited (or AREP).  The Self-excited systems use the output of the generator to directly power the voltage regulator.  This means any disturbance on the output can impact the generators ability to produce power and a short (or fault) will cause the field to collapse rapidly providing minimal fault current.  

A Permanent Magnet system is a separately sourced power by having a Permanent Magnet Generator attached to the rotor which generates its own power for the voltage regulator.  This allows it to be isolated from load disturbances and provide improved response and fault current.  

The final system is an Internally Excited system.  This system is also a separately sourced power system by have isolated auxiliary windings included within the generator stator to power the voltage regulator.  The Internally Excited system provides the same response qualities of the Permanent Magnet system but typically at a lower cost. 

Our white paper goes into greater details on each of these systems and provides some guidance for customers that are trying to choose between them.  

Lou) Can you help our customers understand how to select between these different technologies? 

Typically in applications where you want to limit the available fault current or you are looking for a low cost standby system, a shut system will meet most customer needs.  Whereas in an application which requires large fault current, greater transient response, or motor starting capabilities, you will want to consider a separately sourced system like the PMG or the Internally Excited.  From my experience the Internally Excited system provides the same response and capabilities as a PMG system.  Where available this would be a good product which may be able to help reduce the overall cost for an application. 

Lou: There you have it folks.  Thank you Bryan for taking us through that today and thank you to you, our listeners.  If you would like a copy of the whitepaper we have been reviewing, go to cat.com and type "excitation" into the search bar.  If you'd like to reach us here at Power Bytes, please write us at powerbytes@cat.com or visit Cat Electric Power on Facebook or Linkedin. Till next time, thanks for listening to Power Bytes and have a great day!