Co-Generation with Restrictions on P & Q 1

[Kiribanda]

Hello to all,

This may NOT be a typical application of a Co-Generation (Embedded Generation) system.

Please think about a critical industrial plant which has got its own in-house electricity generation by steam or gas but has NO any excess power to sell to utility like in typical co-generation plants. But due to its critical nature, it wishes to tie-in to utility so that utility will be its stand-by power source ONLY. In that case,

1)Is it possible for this industry ONLY to import P & Q from the utility through its interconnection?
2)Is it possible for this industry to BLOCK any kind of export of P & Q to the utility through its interconnection?
3)What is the most suitable method of blocking any export of P & Q to utility while allowing any import of P & Q from the utility?
4)Has any member got any exposure to such an identical system?
5)What are the associated Standards/ Recommended practices (American & European) ?

Thanks in advance!
Kiribanda :->

 

[dpc]

Power export can be "blocked" by use of reverse power relays. But this generally causes the intertie breaker to be tripped, which may not be a very attractive solution.

Depending on your concern, a simple alarm notifying operators when power or vars are being exported could work. This could be automated via control loops with the unit governors and voltage regulators.

What is your concern regarding incidental export of power? Electrically, this is of very little concern. Commercially, it can be a concern if there is no compensation for the exported power.

Also keep in mind that the utility will generally impose significant "standby" charges for such a connection, even if no power or vars are consumed. There is a price to pay for have utility source instantly available for any emergency.

 

[jlazucena]

You need a relay to prevent the generator from exporting power.
The relay will operate over the speed governor and the AVR to keep the gen from exporting power. See

http://www.woodward.com/ic/pdf/01740.pdf

http://www.woodward.com/IC/pdf/82380.pdf

No, I do not work for these people

 

[pablo02]

In the US, such an application will have the utility require you to install intertie relays (which includes the equivalent to a reverse power) -- which will trip your unit (or the intertie) if generation is determined to be flowing back to the grid

although some of the protection is left to the generating party, some specific interconnection areas will be specified, possibly the winding configuration of the interconnecting transformer, general requirements of utility grade inrterconnection relays, CT & VT requirements, functional protection requirements (81O/U, etc.)settings of some of the interconnection functions, and the speed of the operation

... hopefully, with the new IEEE standard (# 1547)regarding distributed generation, etc. the requirements may be somewhat more uniform than previously required... possible relay functions you'll need are 81O/U, 81R, 27/59, 59I, TT, 51V, 67, 21, 51N, 67N, 59N, 27N, 47, 46, 32, 25 (good application for the new digital multi-function relays)

if in the US, you'll need to contact your local utility for what they require (and they may differ depending upon the size of your generation)...

the purpose is alittle more than dpc implied -- concerns deal with fault protection on the lines and substations, system distubances (including transient overvoltages and system stability), auto-reclosure and synchronizing, and also personnel protection...

 

[SidiropoulosM]

It sounds like you want to be a non-participating generator. You still have to apply for an interconnection and will have to comply with certain provisions depending on your jurisdiction. Here's an excerpt from the California ISO Generator Interconnection Manual that would apply to your situation:

"...Non-Participating Generating Units
A Generating Unit that does not Schedule Energy on the ISO Controlled Grid (i.e. does not export power on to the interconnected grid) and does not participate in the ISO¡¦s markets is not required to be included in a PGA or MSA/ISOME. In order for the ISO to fulfill its responsibilities as Control Area operator and to collect all related necessary charges, however, such a unit, unless subject to the provisions of the agreements and exclusions described in Sections 3.1.1 through 3.1.4 above, is subject to all of the following requirements:
„h Provide the ISO with gross metering;
„h Provide the ISO with gross telemetry;
„h Schedule all Generating Unit outages with the ISO;
„h Respond to ISO operating orders to alleviate System Emergencies and other conditions adversely affecting the reliability of the ISO Controlled Grid; and
„h Comply with applicable PTO interconnection requirements and the standards and agreements of the Western Electricity Coordinating Council (WECC)

The ISO intends to develop a pro forma agreement to set forth the requirements for non-participating Generating Units..."

You can find and download the entire document at:

http://www.caiso.com/pubinfo/recent.html

 

[pablo02]

Charles Mozina of Beckwith Electric and Donald Hornak (Basler Electric) with Joe Chau of Florida Power and Light have written papers regarding IPPs and distributed generation protection -- I don't know if these are available on the internet, but should be available from their repective companies or their distributors:

Charles Mozina: "Interconnect Protection of IPP Generators Using Digital Technology"

Hornak & Chau: "Distributed Generation Interconnections: Protection, Monitoring, and Control Opportunities"

Good information; suggest reading these if possible before conversing with your local utility..

 

[peebee]

Your situation is common for generator installations where it is desired to parallel with the utility. I've heard the device you're you're looking for referred to as a "PIEC" (Process Import Export Controller). There are stand-alone hardware-based PIEC's, although it now appears more common for their funcitonality to be built into a PLC routine.

Generally, the PIEC is adjusted to import a set amount of power from the utility. Say, 50kVA. It then automatically controls the generators to provide any power above 50kVA.

By the way, I would say that "prime generation" or "distributed generation" would be better terms to describe your installation than "co-generation". "Cogen" typically implies that waste heat from the generators is being captured and put to some use.

The recommended practices for paralleling with utilities in the US are driven almost solely by the local utility. In California and some other states, standards have been created for small systems. But most anything over about 50 or 100kVA in generation will require custom engineering solutions and specific utility review and approval.

 

[peebee]

Here's a link to a spec sheet for a PIEC made by Woodward Governor:

http://www.woodward.com/IC/pdf/02013.pdf

 

[LDC]

I think what you need is just a simple sofware which reads the telemeteries of the Tie lines flows (P & Q ) and sends a commands to your controllers ( Co-Generating units , and transformers taps, ...ets) to prevent the export of the flow and return the system to the base case within seconds.

Tripping the tie breakers is not a good idea because once the two systems are connected togather the Tie Line flow could happen any time with different amount and opening breakers will cause instability and could trip the generating units.

On other hand, the program can be use to synchronize the two systems if they seperated.

 

[GerH]

We have been using Grid Exchange Point power control on four CoGen plants (35MW) for about 10 years. After a number of iterations this is the scheme that works best. Using PLC's we count the kWHr pulses for Import and Export to/from Grid over 2 minute intervals, multiply the result by 30 to get average kW and adjust the Power setpoint of the generator to achieve the desired Exchange setpoint. If the change required is small e.g. less than 30kW we make no adjustment. We did have analogue PID control but they were fail to extreme and could leave with a huge import bill. This solution is fail stay-put ( more or less). Also the gas turbines do not adjust overly often. We usually never set for Zero exchange, the tariff tends to penalise for small excursions so it is usually set to always import a little ( about 300kw on 10MW CoGen ) and can vary from Night to Day e.g. we export in Day and import a little at Night. Need any more info please ask. By the way try to avoid Reverse Power and Reverse kVAr trips as they can give a lot of nuisance Islanding.

 

[jbartos]

Suggestion: Visit

http://www.woodward.com/ic/pdf/26086.pdf

etc. for more info

 

[ScottyUK]

Kiribanda,

YOu don't describe your generator's controls, so forgive me if I am off target with my assumptions:

On a programmable control system typical of a modern medium-to-large generator set, it would be normal for the control system to have the means to control both the governor and the AVR to some user-defined setpoint.

If suitable transducers to measure the P and Q flow onto the utility grid are connected to the generator control system, it should be a fairly easy job to regulate the P and Q to any desired level (zero, for example) by some relatively simple programming within the control system.

In essence, I guess you would build a P-I control block using the setpoint (zero) and the process value (utility P or Q flow) and use the output of the P-I controller to modulate the governor or AVR respectively.


Scotty.