Normal to emergency protective devices co-ordination

[colonelbogart]

Hi Everybody,

We have an existing power distribution from 115kV S/S, 20MVA xformer (115kV/4.16kV), a switchgear with 7SJ602 relays, 4160V feeders, 2MVA 4160V/480V xformers, and LV switchgears with micrologic 6A main CBs.

Based on the existing available fault at 115kV which is 2933 MVA We have already setup and commissioned our protective devices base on these parameters. We then have installed our emergency power system composed of 4- 1750kW, 4160V generators (syschronized). Since the available fault of this new system is very small compared to the normal supply do we need to adjust our existing settings? Probably the minimum fault during emergency is so small that the normal setting will be affected?

Any comments please....

 

[davidbeach]

You should certainly look at the performance of the protective system while running on the generators. My guess is that the 480V system will not be significantly affected since those 2MVA 4160/480V transformers will be a big chunk of impedance in either case.

The 4160V system, on the other hand, will have very different values of fault current. It may well be useful to use different settings groups on the relays, (I don't recognize the model number) if available, to use different settings while in the different modes of operation.

 

[colonelbogart]

Thanks davidbeach,,,

We are looking on that options too. The relay model is 7SJ602 and 7SJ60 (siemens). Any idea on how we are going to set the instataneous of each genset CB? do we need to consider the minimum fault at gensets swithgear main cb?

Thanks

 

[davidbeach]

How to set the relays? Excellent question. The standard protective relaying answer: "It depends." It depends on a lot of things, what is the one-line, what are the fault current magnitudes and directions for various fault conditions, where are the CTs, what other protection exists, how are the generators grounded, how is the rest of the system grounded, what are the local preferences for generator protection, etc., etc.