Links Fuses

22Aug/09Off

Links Fuses

Matched-Melt Coordination as used for Selecting windfarm Fuses

What is Matched-Melt coordination?

Matched-melt coordination as defined in IEEE C37.48 is a variation of time-current curve coordination that is used to ensure that the expulsion fuse melts open during any overload or fault condition. As stated in IEEE C37.48,The principal advantage of the matched-melt method is that the expulsion fuse will melt open even if the current limiting fuse does the actual clearing. Because the expulsion fuse opens, the current-limiting fuse is not likely to have the system's voltage impressed across it after it has operated.

The main advantage of this type of coordination is that in most three-phase applications, the voltage rating of the backup current-limiting fuse need only be equal to the system's line-toneutral voltage as long as the voltage rating of the expulsion fuse is equal to the system's line-toline voltage.

What does Matched- Melt coordination have to do with windfarm Transformer Protection?

Windfarm transformers are predominately 34.5kV and are delta-connected (or ungrounded).IEEE C37.48 specifies that fuses used to protect ungrounded systems should have a maximum voltage rating equal to or exceeding the maximum system L-L voltage. The only exception to this rule allowed by the standard is when matched-melt coordination is used. As stated previously, when matched-melt coordination requirements are met, L-N rated backup fuses can often be used.

Until recently, no 38kV oil-submersible backup fuses were available. It was therefore not possible to provide a L-L rated backup fuse for a 34.5kV delta-connected transformer.

Why is it that Matched- Melt coordination Requirements are Strictly Observed in Delta connected Windfarm Application if L-N Rated backup Fuses are to be used?

Due to the nature of a windfarm application, a number of conditions exist that are not explicitly covered in IEEE C37.48 as it focuses on distribution transformer protection. Most importantly, in a windfarm application, both sides of the transformer are "active", meaning that a fault can be fed from either side, the generator on the low voltage side and the collector system on the high voltage side (this as opposed to a distribution transformer application where the load side of the transformer is "passive").

When a fault occurs causing the fusing on the 34.5kV side of the transformer to operate, a brief period of de-synchronization can occur between the phase voltages on the generator side of the transformer and the phase voltages on the collector side of the transformer. The phase voltages can move out of sync until such a time as the generator protection opens, entirely isolating the transformer. During this period of time, the voltage across the open fuses could be on the magnitude of twice system L-L voltage.

There is not sufficient testing or experience to show that an L-N rated backup fuse could withstand what could be on the order of three times its rated voltage alone, even if it did initially interrupt.

What is Required to achieve Matched- Melt coordination with weak links?

In addition to meeting all requirements needed for time-current curve crossover coordination as specified in IEEE C37.48, there are two other requirements that must be met:

1. The first requirement is that one must ensure that the expulsion fuse melts open any time the two fuse combination clears an overload or fault. IEEE C37.48 states that Coordination can be realized as long as the maximum melting I2t of the expulsion fuse does not exceed approximately twice the minimum melting I2t of the current-limiting fuse.

While the minimum melting I2t for backup fuses is readily available in published literature, the maximum melt I2t for expulsion fuses is not so readily available. IEEE C37.48 therefore recommends a method for calculating this value from the minimum melting curves of the expulsion fuses as stated below

One method of calculation involves first determining the current corresponding to the value of time representing the fewest whole numbers of quarter-cycles.

For many published curves this might be the current corresponding to three (3) quarter cycles (0.0125 s). Once the current has been determined from the expulsion fuse's minimum melting curve, it should be increased by an appropriate factor to take into account variations resulting from manufacturing tolerances. In the case of expulsion fuses having silver elements, this factor is 10%. For fuses with elements made from other materials, this factor is normally 20%.

After the current has been corrected to allow for manufacturing tolerances, the maximum melting I 2t of the expulsion fuse can be calculated by first squaring this current and then multiplying that value by the time (expressed in seconds) that was the basis for determining the current.

C37.48 assumes that published curves meet the requirements set forth in IEEE C37.47 which states that When publishing time current curves, the maximum melting current shall not exceed the minimum melting current by more than 20% for any given melting time.

2. The second requirement is one that is specific to applications where match-melt coordination is used to coordinate fuses used in pad-mounted transformers due to the very low maximum interrupting currents of the expulsion fuses in such applications. Specifically, matched-melt coordination as specified in IEEE

C37.48 assumes that the current-limiting fuses being used will be melting in the first loop of fault current at the maximum interrupting current of the expulsion fuse.

About the Author

To know more about Matched-Melt coordination and wind farm transformers. Visit Pacific Crest Transformers website

where can i go to download a fuse box diagram of what fuses go to what?

i need a fuse diagram that is usually in the owners manual but i don't have that. i need to know what fuse operates what accessory/device. this is for a 97 ford ranger w/ 2.3 engine and 2 wheel drive. any links to the information would be greatly appreciated

Here's the page you want to go to at the MotorCraft site;

http://www.motorcraftservice.com/vdirs/SPubs/default.asp?pageid=oghome&gutsid=spubs_serv_all_new&TYPE=OWNERGUIDE&menuIndex1=17

You can't just go direct to the previously posted link.
(I see that now it says 3.4mb, which sounds about right. Previously, it had said it was a 1kb file. Might be because I was trying to open it in a new tab?)

Or try this alternate site;

https://www.fleet.ford.com/maintenance/owners_manuals/default.asp

Ignore the advice about needing to use Internet Explorer. Firefox works fine.

In either case (from those pages) you have to select year, make and model, and then either download or open in your browser then save a copy.