Reverse Power setting for Co-Generation Main breaker

[rockman7892]

I was looking for some guidance on the pickup and time delay settings for a reverse power relay located on the main breaker of a facility which contains Co-Generation. Incoming Utility voltage is 4.16kV with M-T-M breakers set to 1200A. Generator is a 2MW gas turbine unit connected to the main 4.16kV bus though a feeder breaker. The utility has not provided any guidance and/or requirements on what to set the 32 protection at other than stating that this protection was required.

Are there particular guidelines on setting the 32 function in this application? Should it simply be set as a percentage of the generator rating or a percentage of the loading on the system? Are there specific requirements for the time delay? Does this setting need to coordinate with any other settings on the Main breakers?

Also I have seen in some cases where there is also a 67 function included in the main breaker to prevent the generator from contributing to faults on the utility system? Is this typically required in all Co-gen applications? Is there any guidance for settings this function based on the system parameters?


Thanks for the help!

 

[ScottyUK]

What type of turbine? A single-shaft type will have a substantial load from the compressor during a reverse power event which is easy to detect and initiate a trip. A multi-shaft machine with separate gas generator and power turbine presents a much lower load under reverse power conditions. You'll need some data from the engine manufacturer to help you, but likely around 5% - 10% of rating.

Generally a 32 relay doesn't need to co-ordinate with anything.

 

[rockman7892]

Thanks for your response ScottyUK

Perhaps my post above was slightly mis-leading but I was looking for typical reverse power settings at the plants main 4.16kV breakers at the utility POCC not the generator main breaker. I am looking for guidance on setting the 32 element at the plants main breaker in order to prevent the Co-Gen from pushing power back onto the utility system.

So I'm trying to figure out a typical setting for the 32 element for the main breaker. Since the utility didn't give any guidance I would guess this value would be 0kW to prevent any power backflow however I may be missing something in terms of a minimum setting and delay in order to prevent nuisance tripping.

Thank you.

 

[catserveng]

In most of the cogen applications I work with those settings are defined by the utility in the interconnect agreement. Most utilities around where I work use PG&E's Rule 21 as a standard with some variations.

Normally we see a minimum import requirement, in some cases there is an allowance for inadvertent export, but every site seems to be a little different. In general I see a 5% of rated nameplate as a minimum import requirement, with a trip at some point of above that for a fairly long time delay, then a second trip on a much shorter time delay closer to 0 kw import. Some sites have an inadvertent export that allows as much as 10-15% of rating for a short time, mainly to respond to a large load coming off line and giving the generator set time to respond. I would be surprised if your utility really doesn't have any input on that.

Where are you? Maybe someone in the forum could give a better answer depending on where you are at.

On the 67 settings, again in my area, those settings are usually driven by the utilities protection group to protect their system from you.

You could look up "Rule 21 interconnect" on Google and see if it applies to your utility.

Hope that helps, MikeL.

 

[davidbeach]

What Mike said. There's no particular setting that anybody here can give you that will make any difference whatsoever if your utility doesn't agree.

 

[rockman7892]

Mike

This particular plant is located in S. Florida and thus the utility is Florida Power and Light (FP&L).

I was able to dig up a little more info and see that the addition of this generator is part of a Net Metering Project. To me Net Metering means that the customer can push power out onto the utility when the plant loading is less than generation and thus get credit for this outflow of power. If it is indeed a net metering project wouldn't the reverse power element be obsolete? I'm in the process of trying to get more clarification on the interconnect agreement. I did see in the spec though that it should conform to IEEE1547 and although I am familiar with this standard I'm not intimately familiar with all the requirements in it.

I did not see any requirements by the utility and I'm trying to get some sort of input from them. I did notice in the project description/spec that it stated that the customers facility will have a minimum 500kW buffer between the plant load and maximum generator output power. I'm not exactly sure how this buffer will be maintained but I'm assuming perhaps it will be done with the generator controls?

I've come to learn though further reading that even if there is a buffer like described above there may be instances where large loads such as motors in the plant drop offline and the generator controls cannot react fast enough to ramp the generator down and thus the generator will momentarily push power out to the utility. I guess it is for cases like this that the 32 element is required?

 

[davidbeach]

IEEE-1547 does not, per se, require reverse power relaying at the point of common coupling. Nor does it prohibit any requirement to provide reverse power. The utility should provide the requirements. If they don't explicitly require reverse power then don't implement it. Any decent relay used at the point of common coupling will have reverse power capability if enabled.

Now, on my soap box, the requirement for reverse power relaying seems highly misguided. Under normal system conditions the presence of some amount of "negative load" won't have any particular impact on the system. That's just not the problem with the interconnected generation. Where the problem arises is during fault conditions. If the generator is there and running it will contribute fault current based on the machine parameters. The prefault load does not appreciably affect the fault current; some effect, but not much and certainly not related to how much power is being pushed back on to the utility system. If there's a system fault, the generator will supply lots of reactive current but very little power. A directional overcurrent relay will do far better at detecting that fault current than a reverse power relay will; heck, even a reverse VAR relay will do much better than a reverse power relay.

No, the reverse power relay is a tool of the bean counters who don't want to deal with four quadrant metering. Cheaper to burden the customer with a reverse power relay than to install a four quadrant meter and figure out how to do four quadrant accounting.

 

[catserveng]

Hopefully this helps, at least based on my experience,

On Net Metering, this is how it is described in California,

http://www.cpuc.ca.gov/PUC/energy/DistGen/netmetering.htm

On cogeneration or distributed generation plants in my area, the ability to export power into the grid is usually defined as an "export agreement", some utilities only allow export in certain block sizes, others typically only allow export for systems generating with "renewables, like solar, wind or bio-fuels like landfill or digester gas.

I believe most areas in the USA allow utilities to set actual requirements for interconnection based off of guidelines from their respective regulatory bodies, so I'm guessing that this is where you'd start for info in your area,

http://www.psc.state.fl.us/utilities/electricgas/index.aspx

"I did notice in the project description/spec that it stated that the customers facility will have a minimum 500kW buffer between the plant load and maximum generator output power. I'm not exactly sure how this buffer will be maintained but I'm assuming perhaps it will be done with the generator controls?" I would take a guess at this (only a guess because different utilities define some of these terms differently), but based on your description I would assume this to mean you are being required to maintain a 500 kW minimum import level, and is what I am used to calling "load following". This is where the generator controls use a kW reading across the PCC and adjust the generator load to assure that that utility is always providing at least 500kW.

"I've come to learn though further reading that even if there is a buffer like described above there may be instances where large loads such as motors in the plant drop offline and the generator controls cannot react fast enough to ramp the generator down and thus the generator will momentarily push power out to the utility. I guess it is for cases like this that the 32 element is required?" What you are describing here is what, at least in most of the areas I work in, what gets described as "inadvertent export", in some cases the utility will allow a short time for the generator controls to adjust to the large load drop and get the unit back to the desired minimum import level. Again, the utility gets to define what they will allow.

Regards, MikeL.

 

[rockman7892]

It turns out that this particular application isn't a traditional Net metering arrangement agreement, but rather a peak shaving arrangement where the generator will supplement the utility.

It was decided that the reverse power element in the main breakers relay would not trip the main breaker (in order o avoid nuisance tripping of plant), but rather will feed the feeder breaker in the main 4.16kV swichgear which feeds the generator and generator breaker. This way the generator will be tripped offline instead of the main breaker for reverse power conditions. It was decided to set the reverse power for 250kW for a duration of 5s.

It was also decided that a directional 67 element would not be used at the main breaker in order to detect faults occurring on the utility. The concern was that if the generator has a fixed pf then it could export reactive power and current to the utility which would nuisance trip the main breaker. I believe this can be avoided by coupling the 67 element with the 32 element but it was decided not to pursue this.

Instead of the 67 element it was decided to rely on the generator backup protection 51V and 21 elements to trip the generator breaker for faults occurring on the facility. In addition to the generator backup protection the generator feeder breaker will also trip with its 50/51 elements when current is flowing backwards through the breaker to the utility fault. Will the generator backup protection provide adequate generator breaker clearing for faults located on the utility system? How far into the utility system will the 21 element typically reach?

Lastly this 2MW 4.16kV generator is solidly grounded as is the rest of the 4.16kV system. I thought most MV generators were resistance grounded in order to limit ground fault current and prevent damage from the generator?