by Matt Owen (owencm) on 09-10-2009 12:24 PM
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.
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:
- 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?
- 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.
- 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.
What other lessons have you learned? Post your thoughts below.
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