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.

1. TR0902: Report on matched-melt coordination as
used for selecting windfarm fuses
By
Dan Gardner,
P.E., R&D Manager
Hi-Tech, Thomas & Betts Corporation

2. TR0902: Report on matched-melt coordination as used for selecting windfarm fuses
WHAT IS MATCH-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-to-
neutral voltage as long as the voltage rating of the expulsion fuse is equal to the system’s line-to-
line 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. Matched-
melt coordination has therefore been used with the L-L rated 34.5kV ABB weak link expulsion
fuses to allow the use of 23kV backup fuses.
WHY IS IT CRITICAL THAT MATCHED-MELT COORDINATION REQUIREMENTS ARE
STRICTLY OBSERVED IN DELTA-CONNECTED WINDFARM APPLICATIONS 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.
The long oil gap that results in the open ABB 34.5kV weak link appears to be able to withstand
this voltage until the breaker on the low voltage side of the transformer operates, often sharing
that duty with the open backup fuse. 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.
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3. TR0902: Report on matched-melt coordination as used for selecting windfarm fuses
WHAT IS REQUIRED TO ACHIEVE MATCH-MELT COORDINATION WITH THE ABB 34.5KV
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.
However, while this method works well for many types of expulsion fuses, it cannot be used to
calculate the maximum melt I2t of the ABB 34.5kV weak link fuses that are used in Windfarm
applications. This is because 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.
The ABB weak link curves were published well before the present IEEE standards and therefore
do not meet the above requirement as it did not exist. In some cases, the margins on the
published ABB curves are as large as 60%. They also publish an average clearing curve as
opposed to the total clearing curve required by the present standards. Their average clearing
curve must be shifted by 10% to the right in order to model the total clearing curve needed to
perform time-current curve crossover coordination per C37.48. Using the published ABB
minimum melting curves to calculate their maximum melting I2t will result in values that can be
off by a factor of almost two.
Maximum melting I2t values for the ABB weak links must therefore be obtained through other
means. An example of this for the ABB #9 weak link can be seen in Appendix I.
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
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4. TR0902: Report on matched-melt coordination as used for selecting windfarm fuses
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.
When this is not the case, additional testing on the L-N rated backup fuses is needed to show
that they can interrupt L-L voltage at currents that cause melting in times longer than 1 loop
down to a current equal to the maximum interrupting current of the expulsion fuse.
It is well-known that two L-N rated current-limiting fuses (one from each phase) will melt
simultaneously and share the interruption duty to interrupt a L-L fault when operating in their
current-limiting mode (i.e. melting during the first loop of fault current); hence the assumption
made in the standard as described in the preceding paragraph. However, at longer melting
times, conditions can occur due to slight differences in fuse tolerances or starting temperatures
that will result in one fuse melting slightly ahead of the other.
When a fuse in one phase melts even slightly ahead of the fuse in another phase, the
resistance that it introduces into the circuit will cause the current to drop significantly, and the
second fuse may not melt in time to share in the interruption. It is therefore imperative that any
L-N rated backup fuse that might be matched-melt coordinated with the ABB weak links is
shown to be capable of interrupting L-L voltage by itself (as a single fuse) from 1200A (the
maximum interrupting current of the ABB 34.5kV expulsion fuses) up to a current that is high
enough to ensure that the fuses from each phase on a L-L fault will always melt together and
share the interruption duty, regardless of the slight variations described above.
WHAT BACKUP FUSES ARE AVAILABLE TODAY THAT MEET BOTH OF THE
REQUIREMENTS DISCUSSED ABOVE FOR THE ABB #8 AND #9 WEAK LINKS?
In addition to the windfarm specific testing described in the second requirement above, any
backup fuse that is to be used should have a published melt I2t that is greater than the values
listed in the table below in order to meet match-melt coordination requirements per IEEE
C37.48.
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5. TR0902: Report on matched-melt coordination as used for selecting windfarm fuses
T&B Hi-Tech has performed the testing as described in the second requirement above on fuse
ratings that meet match-melt coordination requirements for both the #8 and #9 weak links. The
Hi-Tech HTDS352100 fuse has been tested and match-melt coordinates with the ABB #8 weak
links. The 2xHTSS252100 fuses have been tested and match-melt coordinates with the ABB #9
weak links. Full test reports that show how the fuses were tested are available. A competitor is
now also claiming to have performed the extra testing required on some of their backup fuse
ratings to allow them to be used on windfarm applications. However, some of the fuses they
claim meet match-melt coordination requirements with the ABB #8 and #9 weak links do not
meet the key requirement needed to achieve match-melt coordination as specified by the IEEE
because the competitor used the published ABB curves to calculate the maximum melt I2t’s of
the ABB links. An example of the recommendation that is being made using the #9 ABB weak
link, and why that recommendation is not suitable and does not indeed meet IEEE specified
requirements for match-melt coordination, is shown in Appendix II. The size of the competitor’s
fuse that would be needed to meet match-melt coordination requirements with the #9 weak link
has not had any of the additional testing performed on it and has therefore not been shown to
be suitable for windfarm applications at this time.
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6. TR0902: Report on matched-melt coordination as used for selecting windfarm fuses
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7. TR0902: Report on matched-melt coordination as used for selecting windfarm fuses
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