From the Energy Management Discussion Group (http://in.groups.yahoo.com/group/aipnpc/ )

“Is it advisable to install DF/DT relay for load shedding during power failure”- A technical discussion

1.	Ours is a chemical plant of home load 2.7MW. Our capitive power plant capacity and generation both is 2.3MW(92% actuator opening). We are synchronising the DG power (11/22KV) with EB Grid. During any heavy fault in EB side we are facing block out condition (i.e) due to O/Fin powerplant. So we decided to install DF/DT relay for load shedding during power failure. Is this advisable to install this relay . Can anyone give suggestions or any comments. Kandangowri via email
2.	Dear Mr. Kandangawri, Would you pl. elaborate the type of problem as what means 'Blocking'. Does it go Islanding the own Generation ? I wish you all a happy inteaction and great success for ur. objectives. Thanks N L Singh
3.	Hi Regarding incorporating df/dt relay for islanding operation of your TG from the grid is very much recommended way of reliable operation. The df/dt does function very well with the following conditons i.e In export conditon, under frequency (say 47.8 Hz) and rate of change of freq with time. Balaji.M
4.	Blockout means all the breakers (i.e)our EB,TIE,TG breakergot opened.After islanding,within few seconds blockout happens.
5.	Your df/dt relay shall initiate a logic to trip selected load of your Plant so that balanced load comes within your capacity of your generation and that should be immidiate. Then your Plant will survive. Selection of Load Shedding shall be proper i.e. Lighting,Emergency Loads of selected drive,Auxiliaries of your own Generation,Control & Instrumentation Power. Relay also shall be provided with two stage of U/Frequency operation and in extreme condition Aux.of Generation and Lighting & Control. Thing will work like magic and you can forget Black out. The same is experienced by me. Amar Nath Mukherjee
6.	Dear Sir/Madam, While the engine is running in parrelel to grid and whenever there is disturvance on grid, the engine should get Island mode which can be done by installing the vector sum relay as well all Ht, medium breakers in the distribution should be taken as grid failure contact to Island the own generation. Once the engine goes on Island mode with respect to its capacity load has to arranged by load shedding deciding the critical v/s non critical load. When the grid runs in parrelel the frequency is normaly found lower but in Island mode it will take standard frequency and to avoid any problem the own generation can be put on constant power mode. Regards N L Singh
7.	Yes it is better to put dV/dt and dF/dt relay to isolate the generator from grid during sudden change in grid frequency / voltage. Also since your Plant power requirement is 2.7MW and your generation is 2.3MW , it is good to put AUTO LOAD SHEDDING of 0.4-0.5MW load during islanding with import power condition. regards, Ananth
8.	Dear Sir, Calculate the sustainable capacity after islanding & then incorporate islanding scheme. Regards, vijay
9.	DF/Dt protection will be suitable for your requirement but you may think also vector surge protection relay (mains decoupler) which includes the protection of vector surge (andle) in addition of DF/Dt. Furhter vector surge protection relay sensing is fastet than Df/Dt. Prabhu