Current Limiting Fuse for 13.8kV and 34.5kV
Medium Voltage Systems Distribution
Transformers of CEPALCO
CEPALCO Distribution Planning and
Development Department
A. Research Proposal Flowchart
Figure 1 Flowchart of research proposal
B. Introduction
CEPALCO experienced the upstream tripping of reclosers given fault downstream of DT.
Supposedly, only the DT primary side fuse should trip. The primary side of DT uses expulsion
fuse. This research study explores the use and characteristics of “current-limiting fuse” (CLF) as
applied protection to distribution transformers in the 13.8 kV & 34.5 kV systems.
B.1 Research question
Does the use of Current Limiting Fuse (CLF) in the primary side of Distribution
Transformer (DT) limit the tripping of upstream reclosers in an event of DT-level fault?
Investigating the CLF as
1. Tandem with expulsion fuse,
2. Back-up only, or
3. Standalone DT protection fuse (on high vegetation areas)
B.2 Summary of proposal
1. Advantages of Fuses
a. Non-mechanical faster at higher currents
b. Minimizes voltage dips
c. Non-resettable encourages correction of fault
d. No or few moving parts to degrade
e. Fuse replacement restores protection to 100%
f.
Most economical for high degree of system protection
g. Widely available
2. Additional advantages in the case of Current Limiting Fuses (CLF)
a. Current limitation reduces energy let-through
b. Very high maximum interrupting current
c. Silent operation, no gas emission
d. Compact size and lower comparable cost than other forms of protection
e. Allows smaller cables due to reduced short-circuit stresses
f.
Reduced I2t (energy let-through) equals reduced thermal stress on circuit
components
g. Reduced electro-dynamic forces from peak current-limitation
h. Fuse has maximum clearing I2t for easier coordination to minimize outage
Expulsion fuses interrupt fault current but allow high magnitude of fault current to pass
through which can be detected by reclosers resulting to nuisance tripping upstream. CLF will
limit the high magnitude localized at DT level.
C. Literature Review
C.1 Literature on Topic
Topic: Un-necessary tripping of upstream reclosers in a DT-Level fault scenario
The main characteristic of a current limiting fuse is its ability to limit and interrupt
fault current.
ABB, in a published research describes the behavior of Current Limiting Fuse in
limiting high current faults. When a current limiting fuse operates during a fault, multiple
arcs are produced in a long element confined in quartz sand.1 This introduces a significant
resistance in the circuit, and voltage quickly rises across the fuse. The earlier research of Fuji
Electric also supports these findings in a published research which shows that the larger the
rated current, the smaller is the arc times. The cause of that is, when one element melts and
an arc is produced, the arc is transferred sequentially to the remaining elements for breaking
action.2
According to Won and Hyun in their published study of the factors affecting the
breaking performance of CLF; the melting of the element triggers a voltage rise. The higher
the Vfuse, the faster will current reduce to zero. The instantaneous value of the Vfuse should
be higher than Vsource in order to reverse current increase, as soon as possible after fuse
melting occurs. This is true because the fuse voltage is proportional to the fuse resistance.
1
https://library.e.abb.com/public/8c06477331da4439b3fcfe91b5734b02/7TKK000088_HiTech_Current_Limiting_Fuse_Arc_Voltage_case_study_LR.pdf
2
https://www.fujielectric.com/company/tech_archives/pdf/13-05/FER-13-05-145-1967.pdf
Because of this phenomenon, a current limiting fuse can interrupt faults in less than the
traditional half cycle of other types of interrupters.3 This behavior can be best represented in
an illustration as can be seen in Figure 2.
Figure 2 Small current interruption of
fuse with rated voltage of 3.6KV, rated current of 200A
This would mean that the upstream network will be proofed from high current fault that
can last for a longer duration which the recloser may detect causing un-necessary tripping. The
current-limiting characteristics of the CLF give additional advantage in limiting arc-flash
hazard, environmental and equipment hazard as well.4 This is mainly due to the construction of
a CLF: current responsive elements, surrounded by pure silica or quartz sand with controlled
grain size that acts as a cooling and absorbing agent when the fuse interrupts a fault; all
components are housed in a fiberglass reinforced resin tube.
Figure 3 Pure silica or quartz sand will turn into fulgurite
when exposed to heat and will act as absorbing agent preventing any explosion or arc-flash5
3
https://www.yumpu.com/en/document/read/41259820/short-circuit-tests-on-current-limiting-fuses-fordistribution-system
4
https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solutioncenter/bus-ele-tech-lib-current-limitation.pdf
5
https://www.ee.co.za/article/current-limiting-mv-fuses-for-indoor-distribution-networks.html
I2t (Let-through energy) is a measure of the thermal stress of thermal energy during
short circuit interruption. This would be a concern for expulsion fuses as shown in
Figure 2 but not for CLF as indicated in the results of actual pole DT testing published
by A.C. Westrom.6
Current Limiting Fuse are the only products that work against an internal fault
condition to reduce energy and clear fault well before a natural current zero crossing.
C.2 Challenges and debates on the topic
To resolve confusion on terminologies, let us tabulate and compare IEEE terms
with IEC terms. CEPALCO mostly used IEC terms in their equipment.
Table 1 Comparison between IEEE and IEC
6
IEEE term
IEC term
ambient temperature
surrounding temperature
backup current-limiting fuse
Back-Up fuse
clearing
operating
cutout fuse support
cut-out fuse base
drop out
drop-out
full-range current-limiting fuse
Full-Range fuse
fuse, or fuse and fuse support
fuse
fuse, fuse unit
fuse-link
fuse link
fuse-link
fuse support
fuse-base (fuse-mount)
fuse holder
fuse-carrier
fuse holder and fuse support
fuse-holder
fuse cutout
distribution fuse-cutout
general-purpose current-limiting fuse
General-Purpose fuse
ground
earth
guide
technical report
interrupting (current)
breaking (current)
I2t
Joule integral, I2t
A. C. Westrom, “Distribution Fusing Applications, “presented at the International Conference of Distribution
Fusing,1981
melting
pre-arcing
minimum fusing current
minimum melting current
Peak let-through current
Cut-off current
peak overvoltage
switching voltage
Rated Maximum Application Temperature (RMAT)
Maximum Application Temperature (MAT)
wye connected
Star connected
Note: Excerpted from John G. Leach article, an IEEE expert on fuses.
Figure 4 Current – Limiting fuse versus Expulsion fuse in facing high current faults
Peak let-through current differs for both CLF and EF. For EF, fuse starts melting
when fault occurs (according to its TCC) but not limit its magnitude which has
possibility of the upstream protective device to detect. While for CLF, when the fuse
melts, the current is limited to the magnitude when the fuse started melting and
decaying.
Transient recovery voltage (TRV) is a short transient oscillatory voltage that
appears on an opened circuit after current interruption which is due to damped current
oscillation in the circuit inductance and parallel capacitance. For CLF, TRV is not a
problem meaning less in CLF but significant in EF.
In minimizing let-through current and energy, high arc voltage can be produced
to force current to zero before the available short-circuit current reaches its peak.3
Excessively high voltage may risk causing operation of parallel surge arresters or
damaging other parallel voltage sensitive equipment.
But in the laboratory research done by Fuji Electric on the same concern of high voltages
in breaking short circuit current, they found that Full-Range Current Limiting Fuse is
able to break short circuit current with overvoltage within the limit set by the IEC
recommended standards (IEC 60038).
While all current-limiting fuses excel at high-current operation, their ability to
interrupt lower currents leads to sub-classifications of “backup”, general-purpose” and
“full-range”. This is evident in the operation ranges as illustrated in the image below.
Figure 5 Operation ranges for different CL Fuse types7
General – purpose Current Limiting Fuses interrupts currents from the maximum
interrupting rating, down to the current that causes the fuse element to melt in one hour.
This is approximately 170% – 240% of the fuse rating. However, GPF may not be able to
break current below the value stated by the one-hour criterion.
Back-up CLF can only break rated minimum breaking current which is several factors of
the rated current. It must be used in combination with a load break switch which will
automatically break the current in the overload range.
7
https://www.ewh.ieee.org/soc/pes/switchgear/presentations/tp_files/2019-1_Leach_HV_Fuse_Tutorial.pdf
Full-range CL Fuse on the other hand - can interrupt all currents from the maximum rated
interrupting to the minimum continuous current that causes melting of the fusible
element. This fuse does not have a one-hour limitation. Full range fuses provide
comprehensive coverage and fit within an interchangeable cutout mount.
A challenge for Current Limiting Fuse might arise from its operation under minimum
melting time characteristic curve. A published S&C Powerfuse document claims that in
CLF adjustment has to be made to minimum melting time-current characteristic curves
ranging from 25% in terms of time to 25% in terms of current.8
C.3 Literature on method
Application for CLF:
DISTRIBUTION TRANSFORMER APPLICATION:
CLF, High Voltage side of DT
Figure 6. SLD showing the set-up of Kojovic & Hassler in their simulations involving using EMTP & actual testing9
8
https://www.sandcindia.com/globalassets/sac-electric/documents/sharepoint/documents---alldocuments/descriptive-bulletin-242-30.pdf?dt=637695144852835234
9
Lj. Kojovic, S. Hassler, Member, “Application of current limiting fuses in distribution systems for improved power
quality and protection”, IEEE Transactions on Power Delivery, Vol. 12, No. 2, April 1997
Results of simulations in Figure 6: Fault downstream of fuse located in B
Their findings show that fault current measured in point B is significantly lower in CLF than
in expulsion fuses. Voltage variations are also mitigated well by CLF compared to expulsion
fuses. As shown in Figure 16, the lateral lines are slightly affected by the fault voltages but
the feeder voltage is barely affected.
SECTION APPLICATION:
Figure 7. Kojovic, Hassler, Williams, Baker Comparative analysis of EF and CLF: Field Testing
(circuit 12.47KV/7.2 KV grounded wye Florida State DU)
The analysis of Kojovic and Hassler are documented for location 1 and 4 indicated in the left
image above. Fault are simulated in these sections of the line. The response of CLF and
expulsion fuses are documented from each of these locations referred to as Site #1 and Site
#4.
Site #1 Fault current: 6200 A,,, symmetric,
Expulsion Fuse:
1. Tested for 12K, 40 K, 50K, 65K
Lower rated EF interrupt at first current zero.
2. System was subjected to TRV. TRV is dependent
on system PF and loading.
3. Test was conducted on phase C. No overvoltage
on A and B.
CLF
1. Tested on 8.3KV, 20 A CLF
2. Voltage dip shorter than 2ms, overvoltage
is 1.7 pu
3. Test was conducted on phase C. No
overvoltage on A and B.
Site #4 Fault current: 2000 A symmetric;
Expulsion Fuse:
CLF
The simulations and actual testing show that CLF is able to significantly reduce fault current
and fault duration by as much as a quarter compared to the expulsion fuses.
Question: Up to how much current can CLF interrupt?
Maximum Breaking Capacity - the maximum prospective fault current that the fuse is capable of
breaking
20 kA and 50 kA Reference: Cahier Technique Schneider Electric no. 128 / p.6 Schneider Electric
Reference: Eaton OH Utility Fuses Product presentation