Re: CAT 3412 750F Standby generator Block load and stability issues

3 words:  Power System Study!  (PSS)


Until you have a complete power system study, you cannot do anything but guess. Sure, educated guesses often fix the problem and you might get lucky, but it is only luck and you don't know what will happen over the long time.


Short disclaimer: No, I am not selling power system studies!


Many things can be done. Mikel is right: caps on UPS's can be a problem. I used to be in the business. They are sized for the worst-case senario because they cannot put too much/many harmonics back into the system. Some UPS's can switch on/off the AC caps as needed. Some can be set to "soft start" the converter section to prevent  this exact problem. They have SCR front ends and they can be switched on slowly. Ask your UPS vendor/service dude.


Only 1 genset on site? Often hospitals have 2N or N+1 configurations. What happens on the other genset, if there is one?


I have seen flywheels help, but those systems do take longer to recover. They don't drop as low, but they take longer. That seems a rather expensive fix. I would get the PSS done first.


Good luck



Super Contributor

Re: CAT 3412 750F Standby generator Block load and stability issues

A lttle more information adds a few more wrinkles. If you have a UPS with front end power factor correction capacitors, you may be going into a leading power factor when on your emergency generator, if it goes too far leading you will get voltage instability, if the voltage goes unstable it will cause instability in the frequency. As for the CT scanner, depending on it's power electronics it may have similar issues. You may also find that if you look closely enough it may be causing power quality problems when on utility power, since I see a number of sites where this is a problem.


What probably really needs to happen is that someone who knows what they are looking for should do a proper survey of the engine and generator with your connected loads, with an instrument like a good quality power analyzer, and look not only at the prime mover but also the generator and AVR as well.


Frankly the 3412 PEEC engine generator set is quite capable of driving most any electrical load, and please don't think a modern mining or materials site is not a "complex and rapidly varying load", you obviously haven't been around a large jaw crusher and multiple VFD driven conveying systems. What is most likely the problem is that is may not have been properly adjusted to respond to your site loads, maybe it has an AVR that is not performing well with your loads, or you maybe you have problems you have not correctly identifed the root cause of.


Since this is a pretty old unit, how long has it been on site? Was it purchased used? If it hasn't worked properly since day one why was it accepted and put into service?


Mike L.

New member

Re: CAT 3412 750F Standby generator Block load and stability issues

Thank you both for your replies, you have confirmed what was also suspected - wrong tool for the wrong job - and probably why we were offered this unit at the same price of a 550VA genset.


The units configuration suggests it was designed to drive equipment on a mining site rather than complex and rapidly varying loads.  


Lack of engineering input for the project has once again proven to be unwise.


The 3412/750 is used as a standby generator at our main Hospital, it hasn't work properly from day 1. Although most of the load can deal with the voltage sags and frequency deviations the large UPS's with onboard power factor  

don't like it. If the CT scanner is working things become interesting.


Regards and once again thank you










Super Contributor

Re: CAT 3412 750F Standby generator Block load and stability issues

Frequency stability and block load capability are usually two different issues. Do you have a steady state frequency stability problem? Or do you have a transient response problem? Or do you have both?


On block load capability, a number of factors come into play.  The PEEC 3412 will usually handle a 50% block load with approximate 10% voltage drop and 5-6% frequency drop and recovery in about 3-4 seconds. I have successfully block loaded this vintage engine to 75% of rating with a 15% voltage dip, 7% frequency dip and a 6 second recovery time. This did require assuring the v/Hz and knee frequency settings were correct in the AVR. What AVR do you have? Main issue for anyone dealing with block load response, what is the limits of your connected loads to voltage and frequency variations?


This is a pretty old engine, has it never worked right, or is this a new problem on an existing installation? Also what is the overall mechanical condition of the engine?


A little more info will get you a better response.


Mike L.



Trusted Contributor

Re: CAT 3412 750F Standby generator Block load and stability issues

ADEMII controlled engines would most likely have a better load response compared to their older prodecessors.  Are there multiple load steps or just a single block load that you are applying?  You may be able to manipulate the the settings to improve load responses, but I would suggest you give the genset serial number to your local dealer and get the frequency dip information from TMI if available.  Typically, you may be able to oversize the generator to assist in the dip issues associated with the intial block loading response. 


These are very basic suggestions as we have no information on your actual load application.  Softstarts, governnor adjustments, etc, all can help improve but the initial response at load step initiation may be more than the the generator can actually handle.  What is your actual response time and what do you need for an actual reponse?  Also, the 220KW to 480KW load swings are another thing to look at. SkVA ratings is where I would start of the load and contrast against the nameplate rating of the genset.  


I would contact your local dealer and discuss your application to see what is best to do in your specific case.


Hope this helps,


Nick C.

New member
Accepted Solution

CAT 3412 750F Standby generator Block load and stability issues

[ Edited ]

I have a 3412/750F/PEEC controlled by an Asco switch (PWM). What is the block loading specification for the genset setup and is there anyway of resolving the frequency stability issues? A 200lb flywheel?

Its a mechanically inject TA engine, its been suggested that its response time is to sluggish and needs replacing with an electronically injected engine with the ADEM III control. The engine runs for 10 seconds before the load is applied.

Load varies, anything from 220kw to 480kw