Interior Design and Home Accessories

by Wendy Tanner

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 systems 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 systems line-to neutral voltage as long as the voltage rating of the expulsion fuse is equal to the systems 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 fuses 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.

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by theloushe

Article by Allen Shaw

Blown art glass is quickly becoming one of the fastest growing hobbies in North America?and it is about time. Glassblowing has been around since 27 BC in Syria, though the first evidence of manmade glass products occurs in Mesopotamia in the late 3rd century BC. But the advancement of actual “blowing” glass using a tube transformed the materials usefulness. The new technique quickly spread throughout the Roman world. Harvey Littleton, a ceramics professor, and Dominick Labino, a chemist and engineer, are credited with starting the most recent “studio glass movement” in 1962. The two held workshops at the Toledo Museum of Art. This is where the current method of melting glass in a furnace for use in blown glass art was originated. Thus, Littleton and Labino are credited with making molten glass available to artists in private studios. The actual process of preparing the glass for blowing is very involved though. The glass is melted in furnaces using the sand, limestone, soda, potash and other compounds. The actual transformation of raw materials into glass takes place well above 2000 degrees Fahrenheit. After the glass has melted, the artist uses a blowpipe to shape the glass. The blowpipe is about five feet long and is used for blowing a parison of molten glass. Molds are used to impress decorative patterns. There are two types of modern glassblowing but offhand glassblowing is the type most people picture in their mind when they think about this kind of art. The artist gathers a glob of fused glass at the end of a hollow tube called a blowpipe or blowing iron. The molten glass is then fashioned into its final form by various techniques of blowing and shaping with hands, tolls and molds. The second kind of glassblowing is lampworking. Lampworking is the softening of a glass tube by heating it in the flame of a torch. Next, the softened glass is manipulated into its final form by blowing and shaping with hands and tools. Any number of things can be created using either technique; sculpted animals, ashtrays, vases, aquarium pieces, beads, paper weights, perfume bottles?the list goes on and on. Moreover, practically every major part of the world at one time or another in its history has been known, in some part, for its glass art. However, Mexican glass art is the most popular. Mexico is the land of the master craftsman. Known throughout history as being expert potters, weavers and wood carvers, Mexican artists have really made their mark as glassblowers. Beginning in 1542 in Puebla, these artists produced glass items in a variety of shapes with little more than a long pipe and a glob of melted glass. Experienced Mexican glass blowers will even add effects as small bubbles, blobs of color or pebbles to their finished to pieces to make them stand out. Authentic Mexican glass is easily identified by a ponti, or a place at the bottom of the finished product that indicates it was mouth blown. Another feature of Mexican glass is its individual nature. No two pieces are ever exactly alike in size, shape or design, which simply adds to the unique nature of each piece. But the most unique detail about this glassblowing is how little it’s changed since its inception. For the most part, glassblowers are still producing beautiful works of art the same way they did back in 27 BC?melted glass and a metal tube. This is one instance where my grandpa was wrong. In the case of glassblowing, “They are still making them like they used to.”

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