Load Shedding Scheme
Load Shedding Scheme
(OP)
With a wide range of load shedding devices ranging for less effective under frequency relays with and without df/dt characteristics designed for quick shedding of loads, there still seems to be a lot of options floating around in terms of PLC based and other intelligent devices. I need to conduct loss of generation based transient analysis studies and evaluate subsequently the various options available to finally recommend selection of the most optimum state of the art load shedding scheme for a industrial plant. Would appreciate advice from friends here who have done extensive work on the subject.






RE: Load Shedding Scheme
RE: Load Shedding Scheme
One of the load shedding method depending upon loss of generation we have adopted is as follows.
The scada system continuously monitors the plant load and the generation. Delta P(mw), a operator settable value is set which is a value of load which system can take in addition to existing load. It is assumed that machines are partly loaded and there still exists some margin in terms of power for them being loaded to base load. Now whenever, there is a loss of generation, load shedding would take place only if there is a deficit of power inspite of loading the machines to (P + Dp), Where P is the plant load and Dp is the cushion available in the system.
Please let us know whether this is helpful.
RE: Load Shedding Scheme
The end result is it allows the frequency to fall further below its setting, causing a worsening of the situation, leading then to the need for much larger shedding of load than required. Moreover the relays are pre-wired to trip certain load circuits and does not have any intelligence to know whether these circuits are really loaded to the extent expected. If not the situation gets further worsened, as frequency will continue to fall.
The situation could be improved by using relays with df/dt features. This also has problems as df/dt is a varying value, being subjected to the power mismatch at the instant of loss of generation. This will depend on governor response, the load on the system etc etc. One could work out a df/dt for a worst case, but will not help much. More over the relay needs to filter out harmonics, that causes some delay in its operation
At best the underfrequency relays can be used as a standby scheme to the scheme suggested by "vishalmehta". The scheme suggested by "vishalmehta" also cannot determine which feeders to trip on an intelligent basis. The best that can be done is to pre arrange a trip of certain feeders, without really knowing the extent of load on these feeders. The scheme required should dynamically rearrange the tripping sequence depending on the actual situation of load on each circuit. This is really true for a plant with varying loads, not fixed loads. Each circuit load varies from time to time.
Is there a more intelligent system, even a system that uses neural network engine, that can be trained i.e modeled to read each circuit load, generation loading at each instant and then understand, calculate, think and execute. This way the tripping can be effected quickly, and only a small requisite load and that too the right relevant circuits can be tripped.
Any ideas on any such system that one has used in their plant or have read about it.
Thanks for the responses.
RE: Load Shedding Scheme
I think that dumping more load than necessary is a small price to pay for saving the system from a total collapse, and bugger the lost profits from the lost production!
Bung
RE: Load Shedding Scheme
There are probably others.
If at all possible, I would use an off-the-shelf stand-alone relay rather than a PLC.
RE: Load Shedding Scheme
For your questation answer is first study the load pattern of plant, second critical load & plant base load (utilites)
requirement third connected genrator load throughoff analysis fourth genator stedy state & transient analysis then you decide which kind of load shedding to be applied.
RE: Load Shedding Scheme
thank
RE: Load Shedding Scheme
I have developed a scheme that seems to be similar to what you were looking for. It controls generating plant (power output, system frequency, voltage etc.) for the electricity system on the island of Guernsey. We have 125MW of installed plant plus a 60MW interconnector to France. The system also provides emergency load shedding in the event of failure of a generator, or trip of the interconnector. It monitors for this using a signal from the Master Trip Relays. On failure of a generator, the system analyses load conditions and spinning reserve, calculating how much load needs to be tripped (if any). It then selects a suitable combination of feeders from a priority matrix, and trips them. The time from reciept of the Master Trip signal to tripping load is less than 100ms, it needs to be of this order of speed to maintain stability of the remaining generators. The system also contains algorithms for under frequency load shedding that is a little more intelligent than simple underfrequency relays.
The system is built using standard PLC equipment, which we programmed ourselves. I wrote the load shedding parts and some of the generation control parts. Since it was first commissioned in 1999, we have made several extensions. The system now comprises dual redundant main processor nodes, including a data link to our main SCADA system, plus seven remote "data collector" nodes. The nodes are linked using a 1Mb data bus, either using fibre optic or HDSL ethernet.
Hope this is of interest.
RE: Load Shedding Scheme
We have a client who will have their own generation on site and will be paralleled with the local utility. They plan to import from the utility a certain amount and generate the rest to meet their campus demand.
Our client needs to be able to shed load when necessary to prevent the power imported from the utility from going above that fixed amount. The utility is quite adamant that this needs to happen within 20 cycles, or else the client can suffer signigicant financial penalties.
We're thinking that maybe we not only need to monitor for discrete generator trip events, but perhaps monitor the import power in real time and react. What if an operator inadvertently lowers the onsite generation, and there is no discrete trip input to react to?
I'm curious about how you (and others) acquire the power information into your PLC and at what update rate. Thank you!
RE: Load Shedding Scheme
Direct load tripping as opposed to underfrequency load shedding may be the preferred solution here. You'll have a logic scheme which will trip pre-specified loads immediately when your generators trip. You can make this as simple or as complicated as you like. The scheme may be armed for certain conditions etc.
RE: Load Shedding Scheme
1 spinning reserve caluculations of generators.
2 frequency based.
We have after much deliberations are sticking to spinning reserve theory. Complete scada programming is required depending upon the generator droop charecterstics.
Mode of islanding.etc
RE: Load Shedding Scheme
http://www.selinc.com/appguide/200016.pd...
http://www.maac-rc.org/reference/b_8.pdf
etc. for more info
RE: Load Shedding Scheme
Beat that DCS/SCADA system
RE: Load Shedding Scheme
Because load shedding is initiated by trip marks from the proection equipment, analogue power update rates are not critical - in fact in our system a degree of damping of power measurements is actually beneficial. We use standard (but quite high accuracy) power transducers, feeding a 4-20mA signal into DC voltage analogue cards on the PLC.
The Brush system mentioned by walk3641 is dedicated to this sort of application, and as such has a number of features that make it ideally suitable. For example, the equipment has specially designed AC input cards for power, voltage and frequency measurement. It also uses interrupt cards for load shedding to give high speed response. We in fact used to use a Prismic system. The reason that we replaced it with our own is that the Brush system is hardcoded by them, and requires the operator to revert back to Brush if they need to change the specification of the system.
In terms of trip response, our system is quite distributed. The majority of the 100ms response time I quoted is actually data transmission around the network. We could fine tune this to improve speed (e.g. send trip marks and load shed commands on a higher polling rate) but this has not been worth the effort since the response is perfectly adequate for us. On a small system, where I/O and processing may be done on a single node, then response becomes a factor only of processor execution time, and could possibly be brought down to something like 10-15ms.
RE: Load Shedding Scheme
One thing that seems to be unique is that our load-shedding objective is mainly to protect the client financially, not necessarily to protect the system physically. (Of course there's lots of relaying, outside my scope, for that). Although, an undefrequency event and the load shedding that might result higher up in the system may be what the utility is ultimately trying to prevent with this financial penalty.
We have used GE Fanuc PLC's throughout this project thus far. We're considering GE's power transducer modules, which connect right to a 90-30 backplane, and claim to re-calculate power nearly every cycle. Anybody out there have any experience with these? Coupled with a speedy CPU, we think we can make the decision in about 20 msec. Another aspect I forgot to mention before is that the client gets their utility power via 5 different feeders, so we'll have measure each and sum those up.
RE: Load Shedding Scheme
http://www.cooperpower.com/Library/TheLi...
http://www.pi.energy.gov/library/EWSLphi...
http://www.rge.com/pdfs/JUN2001HighLight...
http://www.metsoautomation.com/automatio...
http://www.iee-unsj.org/ingles/sofware_e...
for:
SICODIS
Dynamic behavior simulation under constraints, including optimal automatic load shedding.
http://www.hometoys.com/htinews/apr99/re...
http://www.shareholder.com/rok/news/2001...
for: "Rockwell Automation Announces RSPower32TM Version 2.0 Software" that includes the emergency load shedding
etc. for more info
RE: Load Shedding Scheme
RE: Load Shedding Scheme
Our load shedding system is also using GE Fanuc, but 90-70 series. I don't have any experience of power transducer modules, maybe they were not available when we designed our system.
The requirement of your utility seem rather stringent - are they saying that NO power excursion (however small) will be tolerated without financial penalty ? If so, then you'll inevitably have to run generation with some margin of error to allow for short term variations in generation power output that could occur for any number of reasons. The other question is how the utility will measure such short excursions, since this level of time resolution is approaching the realms of high speed data logging rather than utility metering !
RE: Load Shedding Scheme
I am involved in designing an adaptive load shedding system for an industrial plant having 2 Nos. GTG's of 19.2MW each and 1 No. STG of 24 MW and paralled to grid.
The system monitors the inplant generations ,import from the grid and power consumptions at important nodes in the plant.
In case of grid disturbance, df/dt realy in the grid incomer trips.The deficit of power at that point in time is calculated,and plant loads as per priority matrix is tripped.
Tomatge,
Since i am continiously monitoring the generation and consumption of power , the scan rate and accuracy for power measurment matters.Can anybody suggest what could be the criteria??
How do I arrive at the time from reciept of the Master Trip signal to tripping the loads, for my system??
Thanking you in anticipation
Rgds,
KSV
RE: Load Shedding Scheme
I would add that perhaps you have a small delay to allow the load stabilize. As I've often experienced in these situations, the initial "bump" which causes the df/dt to trip may also cause several loads to trip off line naturally reducing loads, so long enough for this change to be measured would likely be prudent. I would also suggest you should have some form of underfrequency or df/dt as backup to this system as a final line of defense for the most critical loads, ie powerhouse itself.
RE: Load Shedding Scheme
-Frequency relays: By power companies. They have sufficient time to shed the load. If your plant has a generator operting in standalone mode then it is possible to use it. This is not a loss of generation load shedding. It is protection for turbine generators which are sensitive to frequency and also a kind of avoidance of overload.
-Undervoltage relays: Trip everything on loss of generation and start again in steps. Use motorised breakers and contactors in load circuits to switch the load sequentially and insteps. Transient stability study is not required. If your generator is operating in parallel with the power company supply and if there is a disturbance/fault in the power company system then the protection at the tie point should operate to isolate the two systems. If the generator capacity is not adequate for the load then the generator will trip . It is impossible to save it by load shedding. Load shedding cannot be fast enough.
RE: Load Shedding Scheme
You're wrong!!
Please read my first post. I'm using a load shedding system to protect a whole island community in the event of failure of the incoming European grid connection. I does (and has) worked sufficiently quickly to allow the generators to keep running.
Provided you can gain access to reliable and fast signalling of a grid failure you can respond with load shedding to keep your own generators running and keep some of your load on supply. The speed at which you do this depends on the characteristics of the generators (speed, inertia etc.) and the power shortfall after the grid supply fails. My advice would be to shed the load as quickly as possible, bearing in mind any reliability considerations. In my case, 100ms is fast enough, but we could improve on this if required, as I said above.
RE: Load Shedding Scheme
RE: Load Shedding Scheme