Rate of change of frequency for Grid islanding

[RAgrawal]

We are having a large industrial load with about 500 MW internal generation. Our plant normally operates in parallel with Grid. In recent past, there had been severe frequency fluctuation in the Grid, which resulted into tripping of some of our internal Generators because of delayed islanding (at present islanding is given on pure Under frequency or pure Over frequency, dir O/C, Dir E/F, reverse power). I want to install df / dt protection for islanding.

For this i need to know how to arrive at setting of df /dt. I will appreciate, if you can provide guidelines / standards / technical papers about the same.

 

[slavag]

Hi Ragrawal.
From my point of view, good information about islanding ( not only about df/dt, also about voltage jump), you can found on the Woodward/SEG site:
for example:

http://doc.seg-pp.com/doku.pdf/htl/mrg3/mrg3_gb.pdf

.
From our expirience, setting of df/dt will start from 1Hz/sec, less is very sensetive, you can take in account that system colapse is usually a very low value of df/dt.
Second, what do you do if you have balance of load, that mean "0" supply from grid to your plant.
It's very complicateded issue and only df/dt for my pinion don't help.
Best Regards.
Slava
Will add additional documentation, please search also on the Forum

 

[RRaghunath]

Agarwal,

Islanding is an issue to be discussed and agreed with the grid authority. I am afraid the generator cannot unilaterally decide. It helps to check the agreement that your company had with the grid authority/regulator in the area.
Generally it is a requirement for the generator to remain connected to the grid when there is disturbance and support the grid in terms of MVAR or MW as the case may be. This is the case for all power companies except those with very small units.

Are you sure your generator AVR and Turbine Governor are functioning as they should be! Many a time, re-tuning of these would take care of tripping issue.

 

[RAgrawal]

Thanks a lot Mr Raghun. I also agree that we should not unnecessarily island our self during disturbance in the Grid. But we are not primarily generating company. We are industry having our captive Generators and we want to save our critical load along with captive generators from tripping by islanding our selves by anticipating problem in Grid. There is not agreement with our supplier that we shall remain connected with the Grid. The only agreement is that we have to pay minimum guarented energy charges. As far as functioning of AVR and governors are concerned, yes we have checked the functioning and most of them are functioning properly.

 

[RRaghunath]

Another aspect is - Is the generator power factor allowed to be on lagging side! This is important as this gives stability to the machines during grid faults. I have seen many companies think that it is waste of money to keep the generators on lagging power factor (higher power consumption in the excitation system).
Is the AVR kept in Auto mode always (the chances are high for the machine to trip if the AVR is in manual mode (for what so ever reason).

With regard to islanding protection, I still think you should see the agreement made by your company with the grid authority and review the requirements stipulated. The generators are not allowed to be synchronised with the grid unless certain requirements are met.

 

[RAgrawal]

Our AVRs are always in the Auto mode and we keep excitation in lagging mode. I am very much sure about the agreement with Unitlity and i can island myself based on our requirement.