TCC Curve

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Multiple curves in one relay make it easy to stock a spare or replace a relay.

Fundamentally, the relay curve must fit inside the "no damage" withstand curve of all the parts it protects, so that no parts are damaged as a consequence of a failure of another part.

Different plant and makes have different withstand curves.

Roughly, inverse time suits most items which are damaged by overheating. An oil filled transformer,for example, would usually take longer to overheat than a dry one of the same rating.

Discrete time would mostly be applied where the fault current varies little with position/distance from source - "close" items would have longer trip times.

Even if many items in a system have similar withstand, there is requirement to only trip the minimum of plant to clear a fault, common or more important plant must not be tripped as a result of a fault at a minor plant which ought to be cleared first by its own relay. This requires succeeding relay settings which never overlap - hence the need for many current settings and steepness of operating curve.

If you want all the details, you need to look at relay makers application literature and textbooks.

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Simply put: The different shapes of the curves are to ease your ability to perform a fuse coordination.

In more detail:

In a coordination, the melt curves of a fuse will go between different TCC for the relay, depending on what you want to accomplish. For example, a "blow" fuse would be faster than any of the relay curves, because one would want the "blow" fuse to blow before any blink on a recloser. A "save" fuse" would be slower than the recloser's fast response, but not as fast as the recloser's slow response. It allows one blink of the recloser to attempt to clear the fault, but then "saves" the rest of the system upstream from that fuse by letting the fuse blow/melt. The recloser then will no longer see the fault and not operate on the slow curves - unless the fault is upstream of the fuse, in which case it will continue to operate/blink to lockout. Different amounts of operations are standard in different locations.

Fuse curves are usually based on the type of fuse. There are 'fast' ones, and 'slow' ones. Reclosing relays usually have a variety of curves available.

Transformers typically have just one damage curve, which has been calculated and extrapolated based on measurement averages.

The way I did it when I coordinated was to start with the transformer, and work downstream. On the HV side of the transformer I added a fuse appropriately sized between the transformer and the transmission, to protect transmission from my distribution transformer.

Then, on the LV side, I sometimes (hopefully) got to start with some sort of a recloser. Downstream from that recloser, I added fuses for different zones.

The important this is also; it is necessary to know what is being protected against. So a max fault current must be calculated in some way - either by system modelling, or by hand. If done by a model, I reccomend a quick by hand sanity check, to ensure the model's accuracy.

Hopefully that answers your questions, along with the other replies.

thank u for the replies

Thank you, vandie,for the acknowledgement. It is good to know the replies have helped, too often (no feedback from person putting original question) it is not known if the "original post" read the reply or not.

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