Operating Gas Generator Sets in Island Mode

by Visitor owencm on ‎09-10-2009 12:24 PM

Matt Owen of Caterpillar

In recent years, I've seen a lot of growth in the application of natural gas generator sets for electric power. The drivers of this growth can vary from tighter emission requirements, to green/sustainable financial incentives. Sometimes the reason to choose gas can be as simple as the fact that a pipeline can deliver fuel to a site that a diesel truck can't get to on a reliable basis.

Whatever the reasons, these trends have pushed users to consider applying gas genset packages into applications that have traditionally been limited to diesel packages. Specifically, I'm referring to using gas gensets in "standby" or "island mode" type applications, where the units operate without being parallel to the local utility grid. But as many of us have learned, there are inherent challenges associated with using a gas genset in an "off the grid" type setting. Here are a few of the lessons I've learned from my years of experience in this area:

First off, when it comes to transient response, gas gensets are not diesels.
There are core differences that drive very different performance when it comes to accepting and rejecting loads, particularly when operating off the grid. Generally speaking, a gas engine will have less load pickup (transient) capability than a diesel – both in terms of how large a load step the unit can take and how large the voltage/frequency deviation will be. Second, recovery time after a load step is also longer for a gas engine. Even at the steady state condition (no load change), you will often see less frequency stability on a gas unit when compared to a diesel. This is not all to say that gas packages can't be successfully applied in island mode applications, but it is important to understand the application and manage expectations.

Not all gas packages are the same.
Many markets demand lower emissions, better fuel economy, and higher power density. To meet these goals, many engine platforms had to give up the simpler, more robust control systems and oversized air systems. This however, meant the loss of transient capability when compared to their successors. Generally, as fuel efficiency increases, the capability to accept load decreases. To combat this trend, some of the recent platforms offer new control technology, specifically designed to improve transient capability. The key to successful application is understanding that different platforms have different transient capability and the unit should be matched to your applications needs.

Don't oversimplify.
Generic industry terms like island mode and standby do not define a set of transient requirements. For example, just because a unit needs to operate off the grid, doesn't mean the unit will be subject to large load steps or have tight voltage variation requirements. Another common oversimplification is the blind application of industry specifications. The standby/island mode market has been traditionally limited to diesel packages, so it is no surprise that many industry specifications are written around that product.

Details, details, details.
In the end, the only way to ensure that you will have a successful application is to understand your requirements – details create the big picture. This may seem obvious, but you'd be surprised how many times I get asked to review an island mode application without the load requirement spelled out. There are a lot of different factors that can affect the transient performance of a gas package, and you need to try and understand all of them for a successful gas package application. Here are some of the best questions I've learned to ask:
  1. What are the critical transient characteristics?
    Specifically, what is the allowable voltage and frequency deviation? What is the required recovery time? These values are the nitty gritty of any transient application. It's also good to have an understanding as to what happens when one of these requirements isn't met. Does this represent a delay nuisance – or damage to a motor?

  2. What are the loads?
    How many load steps are needed, what types of loads are being applied, and what is the load profile/strategy? Often times, how you load a genset can have a greater effect on transient performance than the unit itself. It is also useful to understand if there is any flexibility in the loading strategy.

  3. What are the extraneous factors that impact a unit's transient performance?
    Probably the most important factor is your fuel delivery setup. What is the available fuel pressure? How much flow is available? How steady is the fuel pressure? How consistent is the fuel quality? Emissions setting and fuel type can also play a role in transient capability, along with any derate for altitude and ambient temperatures. Most genset spec sheets have published min./max. requirements for these types of parameters. Some island mode specific packages may even recommend specific values for these parameters to optimize transient capability.

So these are just some of my experiences, but I know there are a lot of folks out there on the ground at commissioning, making these units run and pass the various load demo tests.

What other lessons have you learned? Post your thoughts below.

Comments
by Regular Contributor
on ‎09-17-2009 07:53 AM
Running isolated is old hat in the Municipal Power industry. Duel fuel  engines (diesel/NG ) have been doing it for 50+ years! They have been able to easily meet all the difficulties running islanded presents. Cat would sell more engines if the had a true "duel fuel" machine. Concentrating on "spark fired"  ICE is looseing them market share to companies like Wartsila....  We're building a 25MW asset now with another to follow in a few years. As much as I would love to put in  CM model engines from Cat. I think we'll end up with Wartsila....... Doesn't help my Cat stock much!
by New member odelrivero
on ‎09-17-2009 11:09 AM

We have ran Cat G3516's and Deutz 32V18 with a textile factory and a plastics factory and operating them in island mode is getting into hell!!

 

Problem is no fuel pressure, nor load type, since both of these factories had standby diesels with no problem.

 

Problem is the inherit response of the spark engine to sudden load changes.  Diesels can handle this without much fuss, but gas engines symply do not react fast enough.  Either they overrev when load drops and the safety provisions shut it down or they get killed when load enters.

 

We fixed both problems, but it cost a lot.  Do you want to know how we did it?  

by New member marcbmyers
on ‎09-17-2009 12:57 PM

Matt is very correct and this is good information for anyone considering this application. I installed (2) G3520's in Standby (island mode) several years ago prior to the island mode rating being available. In doing so we sized the gas lines for max 30psi in the building then we took control of all downstream distribution breakers 800-amp and larger from the paralleling gear, in this fashion we were able to program the load steps not to exceed the engine(s) transient capabilities.

 

Still running strong!

by Visitor Megawatts
on ‎09-17-2009 01:40 PM

Agree with plantpro, I also would like to see CAT develop a true dual fuel engine. The dealers can do the small ones but it's not the same. I would also like to see in the 18 and 32 range, a gas Genset that will meet NFPA 110 level 1 ten second transfer and block loading requirements. There is a market for 500-1000 kW gas Gensets that can do this. Universities and others do not want to deal with deisel fuel if possible, but for life safety it is the only option.

 

We just completed install of a G3516C that seems to deal with transients better than the B model.

by Regular Contributor
on ‎09-17-2009 10:20 PM
I am waiting for answer odelrivero.
by Regular Contributor
on ‎11-02-2009 09:34 PM

Gents,

There are some fundamentals that are important to note before we get too emotional about CAt developing a true dual fuel Engine... Firstly Wartsila is restricted to the CM range of products only.. The better response of these Engines to block loading is not due to the "Dual Fuel Technology".. The DF engines have fine injection systems to provide around 2% "pilot fuel" only.. This fuel is only for combustion.. When there is a block loading the Diesel fuel injection does not increase..  The extract from the Brochure is below...

Combustion of the lean air-fuel mixture

is initiated by injecting a small amount of

LFO (pilot fuel) into the cylinder. The pilot

fuel is ignited in a conventional diesel

process, providing a high-energy ignition

source for the main charge. To obtain

the best effi ciency and lowest emissions,

every cylinder is individually controlled to

ensure operation at the correct air-fuel

ratio and with the correct amount and

timing of pilot fuel injection.

The main reasons for the better block load responses are 2 fold..

1) The rotating interia of the Engine and generator mass is much higher for Medium speed producs than the G3520's and G3516's.. When we have a block load condition the load inertia is taken from the Genset rotating inertia and this is what causes the speed drop.. In a CM range product the speed drop is not very high and hence it can recover faster..

2) Point of fuel injection.. For DF and CM (GCM) engines the point of fuel injection is at theadmission valve at the intake.. In smaller engines the fuel admission is before the Turbo in a carburation chamber.. Hence the response time to provide the additional fuel required increases..

You will notice also that the block load responses for G3600 and GCM of CAT are better than the G3500's as the gas admission is nearer the cylinder..

Another important difference we need to note between gas and Diesel in terms of block load response is that in a diesel when a block load is applied the additional fuel is injected during the POWER stroke and hence response is immediate.. In a gas product the excess fuel required is provided at the intake stroke (direct admission, CM, 3600 or Warts product) or at the carburation point and hence the delay in response..

Hope this clarifies the concept that a true "Dual Fuel' is not the solution to a block loading.. 

 

Hope this is usefull..

Srivatsa

by
on ‎02-13-2010 07:55 AM

I would add a word on my understanding “reasons of slow response running on natural gas (NG)”  

Let’s compare responses of old carburetor gasoline engines vs. new injection system. The second one has much quicker response.

 

1. In old carb sys after load increase and throttle opens wider it takes a time before air increase speed and than it suck more fuel from carb. So, it’s two step process.

2. In new injection system as soon as throttle opens wider fuel increase immediately. So, no air inertia delays time and only one step process.

 

Same thing happening with diesel engines running on gas vs. diesel.

1. At first, the air speed in intake manifold must increase than it suck more gas. Two steps process. Even turbo add some more delay.

2. In case of diesel fuel, ECM reacts right after load arises.

by Super Contributor
on ‎02-14-2010 07:11 PM

The simple reason diesel engines respond much faster to load transients than gas engines is because fuel introduction on a diesel engine is done directly into the combustion chamber, at the next engine cycle from a load change, if the control system is fast enough, the different fuel rate can be introduced.  On the gas engine, fuel is introduced upstream, on older engines at the carburator mixer, on newer G3500's at the Raptor valve, on medium speed gas engines at the inlet port of the cylinder head.  All points of fuel introduction produce delay before the cylinder sees the desired fuel change. 

 

Gas engines do have a number of issues operating "island mode" when compared to diesels.  Properly designed, applied and integrated systems can operate effectively as prime applications, including maintaining very low emissions with installed after-treatment and heat recovery systems.

 

Transient capability is VERY different between gas and diesel.  Newer design gas engines not only have issues with load acceptance, but in many ratings, have a worse time with load rejection.  Also, gas engines have significant degradation in transient capabilites between "normal" service intervals.  Issues such as valve lash, spark plug condition, and ignition component health, like extenders and transformers, all affect engines ability to respond to load changes and maintain stable operation.  Also, newer engines have very different response characteristics depending on load level, low load pickup on some engines is extremely poor, mid range load pickup can be best if sufficient turbo response is available.  Top end transient response can be erratic as control system limitations, emission control and available turbo response all can fight each other.  So a 25% load transient may be acceptable at 25-50% and 50-75%, but unacceptable at 0-25% and 75-100%.  It is also quite possible the engine will not tolerate a 25% load rejection.

 

Also be aware that the applied protection settings for voltage and frequency deviation may not allow for desired transient operation, and using volts/Hz for improved recovery may affect operation of system loads like VFD's and UPS systems.

 

New gas engines also have a much harder time with running extended periods at "low load", actual load levels that would be defined as low load can vary, but gas engines suffer increased problems with spark plug life, especially multi-torch type spark plugs due to accelerated deposit levels.  Increased cylinder deposits also affect engine combustion, detonation levels, and emissions outputs, so assuring the engines are properly matched to the system load profiles is essential in maintaining stable plant operations.

 

As pointed out above, older engine designs with simpler and more robust controls systems were easier to apply in island applications, newer engines needing to meet reduced fuel consumption, reduced emissions and higher power densities have to give up something, and transient response suffers in these engines.

 

Island mode covers a fairly broad topic, and while I agree that gas engines can be successfully operated in prime, peaking and non-critical applications, I am firmly opposed to using a gas engine as a critical standby unit for life safety.  Current design gas engines have a huge number of shutdowns programmed into their ECM's, a large number of these designed to protect the engine, adding complexity and reducing reliablity.  Gas engines have ignition systems, spark plugs on cylinders open to the atmoshpere corrode, ignition wiring deteriorates, tranformers get internal faults, and ignition sources, such as magnetos or ignition modules can appear to operate correctly at no or low loads during testing and fail when called to operate at higher loads.  Fuel systems components, such as gas regulators and carburators, can stick and bind, diaphragm materials deteriorate, springs fatigue.  Even newer fuel systems components, such as the Raptor valve have relatively high failure rates.  While no engine is 100% reliable, I have seen a very large number of fails to start or failure to operate as expected with gas engines in standby service as compared to a much larger population of diesel engines.  I live in southern California, our most likely natural event is an earthquake, after the Northridge earthquake, natural gas supplies were shutoff, propane trucks and large diesel fuel tankers were limited in their ability to travel on damaged roads, but we were able to put 55 gallon drums in the back of a service truck and find fuel where ever we could, and keep critical customers operating.

 

Compounding the problems with using a gas engine as a critical standby unit are two issues, in my opinion.  First, air quality control districts have greatly reduced the number of available hours a unit can be run for maintenance.  Second, most customers don't want to run their unit under load, while some do install permanent load banks or do regular site load testing, their number is small compared to the total population.  And since most standby systems are low cost installations, good monitoring and trending systems for engine mechanical or electrical parameters are likely not installed.  So these engines don't run enough hours at a high enough load in a year to assure their engine systems are functioning correctly.  So if you're going to apply a gas engine in critical standby service, you have to be aware it has a higher incidence of failure, needs more maintenance, and can have reduced performance in between service intervals than a comperable rated diesel engine.

 

My two cents worth.  Mike L.

 

 

by New member Larry
on ‎02-26-2010 12:04 PM

I’m Larry a pretty normal guy who likes to tinker and build things. I spontaneously bought some property in a pretty isolated part of Central America. Then I had to figure out how to build a generator so I could have electricity. I just came across this blog and fidning a ton of helpful data and ideas here. Just wanted to say thanks for helping me. Larry

by New member bluesea7704
on ‎03-22-2010 06:42 AM

4xG3520C/1.95 MW/50 Hz, I am a user of CAT this model 4 units for Textile application. Can anyone please advice whats the minimum load can I apply on this model? At night the load become less than 50% and I need urgent advice as whether this will be okay for this model or not? At someother sites I heard this models are giving troubl when the load is less than 60% and I just can't affort sudden shutdown of my engines at peak production hour.

by Visitor GARYM1955
on ‎03-31-2010 04:58 AM

I propose that this is a method for improving the spark based gas engine design

Gary Minker 561 969-9245 gminker@gate.net

 

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-b...

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