Is PCS7 capable of executing the load shedding program technically? What are the requirements (hardware and software) if I want to implement the load shedding program by PCS7 at a small power generation plant (30 Mw)

 

Mehdi,

Just about any PLC that can be configured to use appropriate inputs and outputs and interface with the other control systems in a plant is capable of executing a load-shedding program.

Having said that, I've seen some very expensive PLCs improperly configured, and in addition poorly programmed, and so were incapable of executing a load-shedding scheme that didn't cause unintended frequency fluctuations and tripping.

The key for any load-shedding system is to understand what needs to be done and how to integrate the load-shedding system into the plant operation, and that requires a good knowledge of the plant and the other control systems in use for the prime mover(s) and switchgear. Don't over-complicate the load-shedding system, and don't try to make it do things it can't, understanding the other equipment the load-shedding system has to interface with, and the types of interfaces required for the load-shedding system to accomplish its task.

Probably most importantly prior to the selection of a PLC as the basis for a load-shedding scheme there needs to be a good definition of the conditions that would initiate a load-shedding condition and the intended outcome(s) of any load-shedding action.

And, there needs to be a good plan for testing the load-shedding system, as well as a plan for being able to make any modifications which might be necessary and to test (re-test) the modifications to ensure the desired load-shedding operation is obtained.

What happens during implementation and testing of most load-shedding systems is that there is a huge miss on the configuration and programming of the load-shedding system, and a huge misunderstanding of how the system should operate by the plant personnel. All of these things lead to lots of confusion and finger-pointing during testing and implementation of the load-shedding scheme.

And often the PLC chosen for the basis of the load-shedding scheme gets improperly blamed for the inability of the system to shed load and keep the plant running.

So, the answer is a qualified yes.

 

I would like to know more about response time of PCS7 modules regarding the load shedding program timing.

I see that SPPA-T2000(TXP)is applied instead of PCS7. Why?

 

Make sure that you have access to the entire system...hardware and software. We made the mistake of hiring a national climate/environment control company "specialists" to install a load shedding system and it was far from a good experience. The unreliable hardware was all proprietary, the software was not accessible or understandable to us, and half of the functions that the original system had, were never used. After several costly go-arounds with them, we finally scrapped the whole system.

We then installed a small PLC (same brand as the rest of the plant) and we now have a much simpler system that is serviceable and can be easily modified if need be.

Be aware that just because the company might be national or international in scope does not ensure that you will have what you need at the end of the day. The mistakes were entirely ours for allowing ourselves to be led down that original path.

 

I can't express how important this advice is for _any_ crucial system, not just load shedding, that you need to be able to support. If you don't stress this up front, it's almost certain you'll wish you had someday. Many more systems are replaced because they are unsupportable than because they can no longer do the task. It can easily be the difference between success and failure of a project as Gerald points out. I would buy a "pig in a poke" only if there is no alternative. A supportable system is worth a lot more than an unsupportable one.

Regards
cww

 

For a project, I need to sudy more about load shedding.What are the standards about load shedding published by IEEE? other useful guidelines and standards?

 

I do not know if there are any standards for load shedding but i am uploading a sample load shedding scheme in one of the indian refineries.

http://www.2shared.com/document/90XVid7X/Sample_Load_Shedding_System.html

Most of the industrial facilities i have seen ( and i mostly see refineries) have only frequency based load shedding scheme it is simple and effective, but the document i am uploading here combines many different load shedding paradigms rolled into one. This scheme though complicated is working well. read through it and post any clarifications if needed. i will summarize the scheme below.

The refinery had a complex electrical distribution system with 7 generators and 11 main load buses and almost 56 nodes. they had the following three load shedding types implemented.

1. contingency based load shedding - load is shed if the total load exceeds the total generation capacity in the particular island.

2. Slow load shedding - this it to protect the transformers from overload , if the overload persists then the load is shed to reduce the load to acceptable levels

3. frequency based load shedding - there are 4 different frequency points configured with time delay , when each of the frequency low points is reached the designated load is shed.

the main load shedding algorithm does the following. under all conditions , all the three load shedding types are online.

a. scans the breaker status and does the network determination in the system. this divides the total network into different groups. a load list with priority is compiled for each of the groups.

b. checks if the particular group is connected to the gird or not . depending on it 5 different cases emerge.

b.1 CONTINGENCY BASED LOGIC FOR GENERATOR TRIPPING
b.2 CONTINGENCY BASED LOGIC FOR TIE / TRANSFORMER FEEDER TRIPPING
b.3 SLOW LOAD SHEDDING FOR TRANSFORMER
b.4 UNDERFERQUENCY LOAD SHEDDING
B.5 MANUAL LOAD SHEDDING

each has a separate algorithm which is explained in the paper.

contingency forms the primary load shedding scheme. it is often seen that in complex distribution schemes , loss of a generator will cause a severe voltage and frequency in the associated/connected buses. the contingency load shedding eliminates this. by immediately tripping the excessive load mitigates the voltage and frequency dip.

the frequency based load shedding forms the backup , which comes into play if the contingency load shed is for some reason not sufficient and the frequency begins to drop.

the slow load shed also acts as a overload protection for transformers in case of a contingency scenario , the idea being that it is better to lose a few loads than a whole transformer.

hope this is of some help to your project. tell us what you have done or decided to do with the load shedding after you finish your study/research .

 

Mehdi... IEEE Std 100 defines "Load Shedding" and a number of IEEE Std’s discuss it, but none provide a step-by-step procedure.

Power providers and Protective Relay vendors have their own methodology using Protective Relays, in conjunction with PLCs, applied to system disturbances such as Overload, Sudden Load Loss, and Load Rejection. Still, none provide "How-To" guidelines.

In closing, if you are interested in schemes/circuits covering Load Reacceleration following Power Restoration, I can give you some useful direction!

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

> For a project, I need to study more about load shedding. What are the standards about load shedding published by IEEE? other useful guidelines and standards? <

I would just keep it simple with a basic PLC. As long as you have good resolution on the load, for instance a capability to isolate good sized segments of load that doesn't affect other equipment this is a fairly simple task. Often as a previous poster suggested you could implement this with your existing protection relays.

What kind of generation is it?

Is it prime power or standby system?

50 or 60Hz?

Is the load distribution on motor operated breakers or switches?

Also as another poster mentioned, the real trick is putting the load back on getting it off is the easy part!

www.controlpowerconcepts.com

 

Regarding power plant application specially load shedding program, what are the major differences between PCS7 and SPPA-T2000 (TELEPERM XP)?

 

Hi,
Tried unsuccessfully to access the scheme that you had posted earlier in the shared site. Is it possible to email me the same or refer another location where I may be able to access it.

Thanks in Advance

<b>Moderator's note: the forum software automatically places a space after 70 characters in a URL. The link Rana11 is referring to, http://www.2shared.com/document/90XVid7X/Sample_Load_Shedding_System.html works if you take out the space</b>

 

Very Interesting topic is going on. I tried to down load the load shedding scheme through web site as mentioned but i am unable to down load the same can you help.

Rangacharya

 

hi,

thanks for good information. can i have your paper? this file has been deleted from 2shared.

best regards