Current-limiting MV fuses for indoor distribution networks

May 28th, 2019, Published in Articles: Energize, Articles: Vector

In today’s world of advanced electronically controlled protection devices, the use of fusegear in networks may seem archaic and out of place, but there is still a range of applications for this simple protection device, and fuses are used extensively in variety of applications in medium and high voltage industrial and distribution networks. Advanced  features such as current limiting make the fuse a critical component of MV industrial and substation networks.

Fault currents in MV distribution networks, can reach high values, and can result in damage to cables and other items of equipment if the fault is not cleared timeously, or the fault current limited  to a level way below the maximum possible peak current of the circuit. Current limiting fuses (CLF)  are used to provide limitation of fault currents in MV circuits, and are used in combination with switchgear such as circuit breakers and isolators. A current limiting fuse is designed to clear the fault in less than one half cycle.

Damage is not only caused by the peak current, but by the time for which there is a flow of fault current during the clearing period of the fuse. The impact of fault current is measured by the let-through current, and let through energy:

  • Let-through current is the maximum value current allowed to flow in a circuit during clearing of a fault. The term usually refers to the  highest instantaneous value of current reached during the interruption or arcing process. Let through current for a CLF will be much lower than the maximum possible peak fault current.
  • Let-through energy is the heat energy let through by a fuse during its clearing  time.  It is equal to the rms arcing current squared,  multiplied by clearing time ( I2tc) ( see Fig. 1). Let-through energy may also be defined in terms of melting time, (tm) and arcing time(ta). Let through energy describes the thermal and mechanical effects of let through current on circuit elements.

Fig. 1: Operation of a current limiting fuse (Eaton).

Current limiting fuses

A current-limiting fuse is a fuse that, when its fusible element is melted by a current within the fuse’s specified current limiting range, abruptly introduces a high resistance to reduce current magnitude and duration, resulting in subsequent current interruption. For a fuse to be considered “current-limiting”, it must interrupt the circuit within 180 electrical degrees (one-half electrical cycle) after the fault occurs. This is accomplished by producing an arc voltage across the fuse greater than the system voltage. This higher arc voltage forces current to zero before the available short-circuit current reaches its first peak. In a non current-limiting fuse the fault current continues to flow until extinguished by the cycle of the supply system (zero crossing).  Fig. 1 illustrates the operation of a current limiting fuse, compared with a non-current limiting fuse and shows the maximum instantaneous peak current during the first half-cycle after initiation of a fault. Depending on the power factor and when the fault occurs, the maximum possible instantaneous peak current (Ipeak) without current limitation can be as high as 2,3 times the available rms fault current when the fault occurs. As shown, if a current-limiting fuse is used, the maximum instantaneous peak current is limited to a small fraction of the peak current that could possibly flow if the fuse were not used. The current-limiting fuse link melts and clears the circuit. The area under the curve is known as the I2t let-through energy of the fuse. The lower the I2t is, the lower the destructive thermal energy is that passes through the circuit before it is interrupted.

CLF construction

A current limiting fuse generally consists of two fusible parts:

  • A fast blow element which operates rapidly in response to an overload.
  • A high resistance element used to reduce the fault current during the remainder of the fault clearing time.

Operation of the fast element switches the high resistance element into the circuit, thus reducing the fault current. The high resistance element then continues with the arc extinction process. A typical current limiting fuse construction is shown in Fig. 2.

Arc voltages are created in the fuse by the melting of the fuse links. This produces a number of high resistance arcs (gaps) in series, and there exists a voltage drop across each gap. When the total voltage drop exceeds system voltage, the current is interrupted. As this occurs, a transient voltage spike is generated in the system. It is important to make sure that this voltage does not exceed the system’s basic insulation level (BIL). Arc (or transient) voltage will usually not be a problem in a system if the fuse’s maximum design voltage rating does not exceed 140% of the system’s voltage.

Fig. 2: Construction of a typical CLF (Cooper).

CLF parameters

The following parameters are important in the selection of CLF:

  • Rated current (In): The rated current is the maximum current value that the fuse can conduct for an indefinite amount of time without reaching fusion, but generating an energy that can be dissipated by the fuse.
  • Minimum interrupting current ( Im): For values above the rated current, In, the fusion times are very long and they decrease as he current increases In this range (In to Im) , heat dissipation capacity is lower than the heat generated inside, therefore severe thermic stresses that may damage the fuse occur. While the current increases, the fusion times are reduced until a point where the fusion occurs in a relatively short time (milliseconds), before thermic stresses and damages to the fuse occur. This current value is called the minimum interrupting current I3 and corresponds to the lower limit of current ranges that the fuse may satisfactorily interrupt.
  • Rated voltage (Un): The maximum rms AC voltage at which the fuse is designed to operate. The rated voltage of the fuse links must be equal to, or higher than the operating line voltage. By choosing the fuse link rated voltage (Un)  considerably higher than the line voltage, the maximum arc voltage must not exceed the insulation level of the network. Current-limiting fuses produce arc voltages that exceed the system voltage. Care must be taken to make sure that the peak voltages do not exceed the insulation level of the system.

Current-limiting fuse let-through charts

The degree of current-limitation of a given size and type of fuse depends, in general, upon the available short-circuit current that can be delivered by the electrical system. Current-limitation of fuses is best described in the form of a let-through chart that, when applied from a practical point of view, is useful to determine the let-through currents when a fuse opens. Fig. 3 shows a typical let-through chart.

Fuse let-through charts are plotted from actual test data. The test circuit that establishes line A-B corresponds to a short circuit power factor of 15%, that is associated with an X/R ratio of 6,6. The fuse curves represent the cut-off value of the prospective available short-circuit current under the given circuit conditions. Each type or class of fuse has its own family of let-through curves. The let-through data has been generated by actual short-circuit tests of current-limiting fuses. It is important to understand how the curves are generated, and what circuit parameters affect the let-through curve data.

Fig. 3: CLF let-through chart (Eaton-Cooper Bussman).

Typically, there are three circuit parameters that can affect fuse let-through performance for a given available short-circuit current. These are:

  • Short-circuit power factor
  • Short-circuit closing angle
  • Applied voltage

How to use let-through charts

The let-through chart pertaining to a Cooper Bussman 800 A low-peak fuse is illustrated in Fig. 4.

1. Determine the peak  let-through current.

  • Enter the chart on the “prospective short-circuit current” scale at 86 kA and proceed vertically until the 800 A fuse curve is intersected.
  • Follow horizontally until the instantaneous peak let-through current scale is intersected.
  • Read the peak let-through current as 49 kA. (If a fuse had not been used, the peak current would have been 198 kA.)

2. Determine the apparent prospective rms symmetrical let through current.

  • Enter the chart on the “prospective short-circuit current” scale at 86 kA and proceed vertically until the 800 A fuse curve is intersected.
  • Follow horizontally until line A-B is intersected.
  • Proceed vertically down to the prospective short-circuit current.
  • Read the apparent prospective rms symmetrical let-through current 21 kA. (The rms symmetrical let-through current would be 86 kA if there was no fuse in the circuit.)

The fuse will limit the short circuit current to 21 kA, and will protect all devices with a short circuit current rating  of 21 kA or higher in the case of an 86 kA prospective current fault.

Fig. 4: CLF let through curves (Cooper Bussman).

Indoor current limiting fuses

Indoor fuse gear is primarily used in MV networks to protect transformers and capacitors. Indoor fuse gear differs from outdoor in several aspects.  Indoor operation places several restrictions on the use of fuses, and indoor types are designed to minimise the negative effects of operation. The following needs to be taken into account when fuses are installed indoors.

  • Although the operation of fuses is of very short duration, sudden loud noises can be disturbing indoors and could result in accidents. The indoor fuse is designed to reduce noise during operation.
  • Expulsion of by-products, especially hot gases, can cause a hazardous situation, especially in confined spaces.  ID fuses are designed to reduce by-product of operation.
  • Derating for high temperatures. The ambient temperature indoors, especially in confined spaces with no environmental control, can be significantly higher than the outdoor ambient temperature. ID fuses can be derated  to compensate for higher temperatures.

Indoor rated versions of fuses are designed to meet these stringent needs of industrial and utility applications. The noiseless operation and lack of expulsion by-products make them ideal for indoor application in confined spaces.

References

[1] Cooper Busmann: “How To Use Current-Limitation Charts”, www.cooperindustries.com
[2] Littlefuse: “Using current-limiting fuses to increase short circuit current ratings of industrial control panels”, www.littelfuse.com
[3] P+C Fuses: “Medium Voltage current limiting Fuse links”, Manual_PCEF-R12.16674050, www.egpandc.com
[4] Cooper Busmann: “NX indoor current-limiting fuses”, Catalog Data CA132049EN.

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