Using Fuses to Provide Fault Protection of Windfarm Transformers

Transcription

1 Using Fuses to Provide Fault Protection of Windfarm Transformers Dan Gardner R&D Manager, Hi-Tech Fuses, Thomas & Betts John G. Leach Consultant, Hi-Tech Fuses Abstract A rising concern among utilities that use wind power generation is how to adequately protect the 24.9kV-34.5kV delta-connected step-up transformers that are commonly used from internal faults. This paper examines this very important issue and discusses considerations that must be addressed when providing fault protection for transformers of this type. A solution is then proposed using L-L rated expulsion fuses and matchmelt coordinated L-N rated oil-submersible currentlimiting backup fuses that will provide fault protection for transformers as large as 2600KVA at 34.5kV. 1 INTRODUCTION Most utilities prefer that the 24.94kV-34.5kV deltaconnected step-up transformers that are typically used to connect wind power generators to the system be protected from eventful failure, and be quickly disconnected from the system, in the case of an internal fault. However, finding protection that is suitable for use on the high-voltage (HV) side of these transformers can be problematic as the protective device(s) must be capable of interrupting the full line-to-line (L-L) voltage that results from the delta connection. Fuses are commonly used to provide such fault protection due to their low cost, convenience, and proven reliability. However, standard fuse coordination techniques used to select fusing for pad-mounted distribution transformers often cannot be used for these applications due to the unavailability of suitable 38kV current-limiting (CL) fuses. Alternative coordination techniques must therefore be employed. This paper describes a method of coordination being used to fuse the HV side of these transformers. The method uses L-L rated under oil weak-link cartridge expulsion fuses and matchmelt coordinated line-to-neutral (L-N) rated oilsubmersible CL backup fuses. It begins by explaining how each type of fuse works. Coordination between them is then covered, and some recent research that enables 34.5kV transformers to be protected with existing fuses is explained. Finally, application cautions are discussed. very short time (less than about 5 ms). The two types are called current-limiting and non-currentlimiting. The most common type of non-currentlimiting fuse is the expulsion fuse, and only that type will be covered here. 2.2 Expulsion fuses There are many types of expulsion fuse, but all, after melting, arc until at least the next natural current zero. Expulsion fuses have relatively short elements that produce a short arc upon melting (compared to current-limiting fuses). They introduce very little resistance into the fault circuit during arcing, so if they melt before the first peak of a high fault current, this peak is not significantly affected (that is they limit the duration of a fault current, but not its magnitude). The arc is extinguished at a natural current zero because gas, generated by the arc and its surrounding medium, is blown out of the tube, removing ionized material (i.e. an expulsion action, hence the name). If the voltage withstand of the resulting gap is sufficient to withstand the circuit transient recovery voltage (TRV), the arc does not re-ignite and the current ceases. Figure 1 shows an expulsion fuse melting in the first loop of a relatively high current, and interrupting at the first natural current zero. If instead the arc is re-established, a further loop of arcing ensues, and the fuse tries to extinguish the current at the next natural current zero. Fuse Current Element Melting Fuse Voltage System voltage Current interruption TRV 2 FUSE TYPES 2.1 General All conventional fuses, after melting, continue to carry the current in the form of an arc for some period of time. Fuses can be divided into two basic types depending on how, during the arcing, they influence the current that causes them to melt in a Figure 1: Expulsion fuse interrupting in one loop of current. The current shown in figure 1 is asymmetrical. This occurs when the point of initiation of a fault does not occur close to the peak of the voltage waveform (where a natural current zero occurs in a predominately inductive circuit), and the resulting sinusoidal fault current begins with a mean value

2 offset from the zero current axis. The higher the circuit X/R, the more dc offset occurs, and the higher the first major current peak will be, for the same rms symmetrical fault current. Larger first current peaks produce more I 2 t in the first loop. I 2 t (strictly i 2 dt) is proportional to energy when multiplied by resistance it gives the energy generated in that resistance by the passage of current having that value of I 2 t. The concept of I 2 t is useful in fuse coordination and as a measure of the protective ability of fuses (particularly currentlimiting fuses). I 2 t is also a measure of the burning effect on, and energy dissipation in, an expulsion fuse s tube. If it is too high, the tube could burst or be damaged, and the expulsion fuse will not be able to interrupt. Expulsion fuses therefore tend to have a well-defined rated maximum interrupting current ( rated max I/C ), and higher currents and X/R values make interruption more difficult. They are also very sensitive to circuit TRV, so fuse rated max I/C tends to decrease with increasing voltage. Under oil expulsion fuses typically used in windfarm transformers use a 70mm to 250mm long element suspended in a tube, made of, or lined with, a material that will evolve gas under the action of an arc. The tube is open to, and filled with, the transformer oil. They are relatively simple in construction, and therefore low cost, but tend to have relatively low rated max I/C values, and limited X/R capability. At higher voltages, they are normally not suitable for use alone, unless the available fault current is quite low. They do, however, have very good low current interrupting performance. In the case of low currents (causing melting in many cycles) they may require several loops of arcing to build up sufficient arc gap and expulsion action to interrupt such currents. This is illustrated in Figure 2, which shows a fuse taking two and a quarter loops of arcing to interrupt the fault current. measure of overload protection for the transformer. This is only possible with fuses having a relatively low current rating, as the low melting point elements tend to be larger in diameter than the equivalent high temperature element. Expulsion fuses typically employed in windfarm transformers use elements made from high melting point materials and provide little or no overload protection. However, this is true of many fuses used in transformers since the primary function of an expulsion fuse is to isolate a faulted transformer from the system (i.e. protect the system). Any protection of the transformer from overloading is primarily the responsibility of protection on the low voltage (LV) side of the transformer. This is true of conventional transformers (fed from the HV side) and windfarm transformers (fed from the LV side). A popular type of transformer expulsion fuse is the so-called bayonet type. This fuse uses a replaceable cartridge in an assembly that can be removed from the transformer through a sealed housing, mounted in the sidewall of the transformer. While they are very convenient and have replaceable links available with a variety of melting time-current curve shapes, they are limited to a maximum rated voltage of 23kV. This makes them unsuitable for most windfarm applications. For larger transformers, cartridge expulsion fuses are therefore often used. These are fixed inside the transformer, making replacement harder (a handhole is required) but they tend to have a higher rated max. I/C. They are available with voltages up to 34.5kV, where their max I/C is 1200A. Such a cartridge fuse is shown in figure 3. System Voltage Fuse Voltage Fuse Current Figure 3: Cartridge type expulsion fuse rated 34.5kV. Element Melting Figure 2: Expulsion fuse interrupting a current that requires more than one loop of arcing. In general, expulsion fuses have elements made from either a low or high melting point material (e.g. tin or copper). When low melting point materials are used, the fuse is capable of providing some Current-limiting fuses The other main type of fuse is the current-limiting (CL) fuse. CL fuses usually consist of long elements wound helically around a former or core (to make the fuse compact), enclosed in a body that is filled with quartz sand. Figure 4 shows a cutaway view of such a fuse.

3 Figure 4: A cut-away view of an oil-submersible current-limiting fuse, commonly mounted inside a transformer (the quartz sand has been removed for clarity). The element, typically made from one or more ribbons of high melting point material (silver or copper), has multiple areas of reduced crosssection. At a high current, these restrictions melt virtually simultaneously, rapidly introducing a significant resistance into the circuit. This causes the current to quickly reduce to zero, and prevents the fault current from having the opportunity to reach its normal peak value (hence the currentlimiting name). This action is shown in figure 5. Fuse Current Element Melting Fuse Voltage Available fault current Arc voltage Figure 5: A current-limiting fuse, interrupting a high fault current. Because of the rapid current reduction, the voltage across the fuse during arcing (arc voltage) exceeds the system voltage (the difference being made up by the circuit s inductive di/dt). The element and restriction design is therefore optimized in a CL fuse to control this voltage to an acceptable limit. It is very important to note that no fuse, expulsion or current-limiting, should be exposed to a recovery voltage that exceeds its rated maximum voltage (test voltage), or failure to interrupt can result (often in a spectacular fashion). As described, a CL fuse is extremely good at interrupting high currents. Standard testing [1] includes tests at the fuse s Rated Maximum Interrupting Current (commonly termed max I/C ), and also referred to as I 1, or Test Duty 1. CL fuses are also tested at a lower current (I 2, Test Duty 2, or critical current ). This current depends on the design and current rating of the fuse, and approximates the maximum arc energy (absorbed energy) for that fuse. These two tests ensure that the tested design can interrupt any current that causes a current-limiting action to occur. CL fuse rated maximum I/C values tend to be very high, normally 50,000A symmetrical, unless test station 3 limitations require a lower value. Because a CL fuse introduces a high resistance into the circuit upon arcing, current and voltage are pulled into phase, so circuit X/R and TRV have much less influence than for expulsion fuses. While testing standards require X/R values to be at least 10 for distribution class fuses, higher values (often 25 or more) are therefore often used when testing at high currents for convenience or due to test station limitations. While current-limiting fuses are very good at interrupting high currents, they are not always as good at interrupting lower currents. This has led to the introduction of three categories of CL fuse, based on their ability to interrupt low currents: backup, general purpose, and full-range. General purpose and full-range fuses are designed to be able to interrupt low currents as well as high currents. Full-range fuses are quite often used to protect pad-mounted transformers. However they are not normally recommended for windfarm applications for reasons discussed in section 5. A fuse that uses a simple punched strip element in quartz sand, as shown in figure 4, is normally classed as a backup fuse. If the current is so low that only one notch melts, the current cannot be interrupted at any significant voltage. Such a fuse therefore has a minimum current that will result in enough series notches melting together to produce current interruption. This current is termed the fuse s rated minimum interrupting current (commonly minimum I/C ). A backup fuse interrupting such a current is shown in figure 6. Fuse Voltage Fuse Current Element Melting Figure 6: Current-limiting fuse interrupting lower current, requiring more than one loop of arcing. The waveforms are similar to an expulsion fuse interrupting low current except that, when multiple parallel elements are used, current switching between the elements occurs, and the voltage across the fuse during arcing tends to be higher, reducing the severity of the inherent TRV. Backup fuses are tested at this current, termed I 3, or Test Duty 3, and so can interrupt any current between their minimum and maximum I/C. A backup fuse must always be used in conjunction with some other device that will operate at a current below its

4 minimum I/C to prevent the backup fuse from melting at a current it cannot interrupt. Failure to protect the backup fuse from melting at low currents (currents below its minimum I/C) could result in a failure of the backup fuse and possibly cause an eventful failure of an otherwise undamaged transformer. The most common protection system for windfarm applications (and indeed pad-mount transformers in general) is the so-called two fuse approach where the best features of expulsion and CL fuss are obtained by pairing the two fuses. The CL fuse protects the expulsion fuse for currents above its max I/C while the expulsion fuse protects the backup fuse at currents below its minimum I/C. 6 x I r at 1s (cold load pick-up) 3 x I r at 10s (cold load pickup) The specified points must lie on, or to the left of the minimum melt TCC ( safety margins have been built in). Obviously, for a windfarm transformer, the concept of cold-load pickup has no meaning, and instead anticipated surges, from variations in the wind speed for example, must be accommodated TRANSFORMER PROTECTION As has been stated in 2.2, the primary function of the HV side fusing is to remove a faulted device from the power system (protect the system). However, if the available fault current is high, it is also important to remove a faulted transformer in a way that minimizes the chance of an eventful transformer failure. Current-limiting fuses not only allow transformer disconnection at high fault currents (currents that an expulsion fuse cannot interrupt) but they also do so in a way that significantly limits the fault energy that flows into the transformer from the power system during the fault. This minimizes the chance of an eventful transformer failure. In the case of two fuse coordination, normally the expulsion fuse is chosen first, taking into account transformer inrush and anticipated overloads and surges, and then a suitably coordinated backup fuse is selected to complete the protection. 4 FUSE SELECTION 4.1 Expulsion fuse current rating We will first examine the selection of an expulsion fuse. In general, the smallest fuse that will meet the transformer s requirements is selected, in order to minimize the risk of eventful failure (in general the smaller the expulsion fuse, the smaller the coordinated current-limiting fuse). However, it is important that the selected fuse is large enough to avoid nuisance operations due to damaging surge currents. Rules to achieve fuse coordination with inrush, etc. are well established, and figure 7 shows the usual inrush and cold-load pickup points for a conventional transformer application, together with a minimum melt TCC for an expulsion fuse. The specified points, where I r is the transformer rated current, are: 25 x I r at 0.01s (transformer inrush) 12 x I r at 0.1s (transformer inrush) Time - Seconds Inrush/cold load pick-up points Expulsion Fuse Minimum Melting TCC ,000 Current in Amperes rms Symmetrical Figure 7: Expulsion fuse transformer inrush coordination. Experience has shown, however, that if the usual cold-load pick-up points are used for choosing the expulsion fuse, nuisance operation is minimized. To achieve coordination with a low voltage side circuit breaker (for overload protection) then sufficient margin should be left between the breaker curve (adjusted for turns ratio) and the HV fuse. A frequently used rule for this is to shift the HV fuse s minimum melting TCC to 75% of time and compare this with the total clearing TCC of the low voltage device. The total clearing TCC must lie to the left of the shifted minimum melt TCC to achieve coordination. When the expulsion fuse uses a low melting point material, its TCC curve shifts significantly to the left with increasing oil temperature (this is how it can provide long-time overload protection). This shift would have to be

5 taken into account when comparison with an external LV device is considered. Fortunately, with the high melting point expulsion fuse elements normally used for windfarm applications, TCC shift is minor (generally less than 5%), similar to the shift in an under oil backup CL fuse. 4.2 Expulsion fuse voltage rating For conventional transformers, the selection of the expulsion and current-limiting fuse voltage ratings is influenced by design of the transformer (delta or wye), the nature of the connected load, the method of coordination to be used, operating experience and practices, and the ratings of the available expulsion and current-limiting fuses. In the case of certain grounded-wye/grounded-wye transformers, line to neutral rated expulsion fuses are used. However, since most windfarm applications use transformers having a delta connected winding, a line-to-line rated expulsion fuse is necessary [2]. 4.3 Expulsion fuse/backup CL fuse coordination General Having selected a suitable expulsion fuse, the next step in creating a two-fuse protection scheme is to appropriately coordinate a backup CL fuse. The total clearing TCC of the expulsion fuse and the minimum melting TCC of the backup fuse are used for this. Note Most of the difference between minimum melt and total clearing TCC curves, at times of more than 1s, is due to manufacturing tolerances. Standards require that the maximum melting curve not be more than 20% greater (in terms of current) than the minimum. Arcing is added to the maximum melting TCC to produce the total clearing TCC, but arcing time is only significant at very short melting times. If an average clearing TCC is published for a fuse, this must be shifted 10% to the right to give an approximation of its total clearing TCC. Several different coordination criteria must be met to ensure a successful two fuse scheme Mutual protection The first criterion concerns the arrangement whereby each fuse protects the other in its area of non-operation. Figure 8 shows the TCC curves for an expulsion fuse crossing that of the backup fuse. The curve for the backup fuse is shown dashed for currents below the fuse s minimum I/C to indicate that although the fuse will melt at such currents, it cannot interrupt them. It must therefore be protected from melting in this region. Coordination exists if the total clearing TCC of the expulsion fuse crosses the minimum melting TCC of the backup fuse at a current equal to or greater than the backup fuse s rated minimum interrupting current. To protect the expulsion fuse from attempting to interrupt at a current higher than its rated maximum interrupting current, the crossing of the two curves 5 should also occur at a current below the expulsion fuse s max I/C Time in Seconds Overload Coordination (25% Margin) Bolted Secondary Fault Coordination (25% Margin) Total Clearing CL Backup Fuse Minimum Melting Minimum I/C of CL Backup Fuse Crossover Maximum I/C of Current in Amperes Figure 8: Coordination of expulsion fuse and backup fuse. In the case of larger sized transformers, this latter constraint may not be possible. However, provided that the minimum I/C of the backup fuse is lower than the expulsion fuse s maximum I/C, fusing may still be possible. At currents above the max I/C of the expulsion fuse, the expulsion fuse will arc until the backup fuse melts and interrupts. In the case of an internal cartridge fuse, the additional arcing will increase the severity of the fault inside the transformer (which has failed anyway - see later) and so may be acceptable Bolted secondary fault coordination The purpose of the backup fuse is to provide transformer protection in the case of high current faults, with an emphasis on significantly reducing the risk of an eventful failure. Because the backup fuse needs only to operate in the event of an internal transformer failure, the backup fuse does not have to be field replaceable. It is therefore important that a fault external to the transformer (the worst case being a bolted LV fault) causes the expulsion fuse to operate without melting, or more importantly damaging, the backup fuse. This is achieved by leaving a suitable margin between the expulsion fuse total clearing TCC and the backup fuse minimum melt TCC, at a current equal to the bolted LV fault current. A commonly used margin (that appears in the IEEE Fuse Guide C37.48 [3]) is to ensure that at a time equal to the expulsion fuse bolted secondary fault current clearing time, the backup fuse minimum melting current is at least 125% of the bolted secondary fault current. This is shown in figure 8. The 25% margin prevents

6 element damage, and also takes care of the situation where the transformer has taps. The bolted secondary fault current should be calculated from the transformer nominal kva and voltage, and its minimum impedance. It may be noted that this is equivalent to shifting the backup fuse s minimum melting TCC by 20%, in terms of current (reducing the current by 20%), creating a no damage curve for the fuse Overload/low level fault The safety margin discussed in also applies for any current lower than the bolted secondary fault current. Clearly any current at which the expulsion fuse should operate must not damage the backup fuse. Otherwise, after the expulsion fuse has been replaced, the damaged backup fuse could melt at a current that it cannot interrupt, and subsequently fail. Therefore the 25% margin should also be used at the top of the expulsion fuse curve, and anywhere in between, should the expulsion fuse curve have a knee in it (this occurs when dual element links, having two different elements in series, are used). This is also shown in Figure 8. An additional consideration is that the rated continuous current of the backup fuse should not be exceeded by any overload that can persist for a significant time. Fuses have a relatively short thermal time constant, and respond quite rapidly to currents. However, oil-submersible backup fuses frequently can carry more than their name plate rating in 85ºC oil and, fortunately, most applications use a backup fuse that has a significantly higher current rating than the transformer rated current, due to the coordination criteria discussed Voltage rating of the backup fuse As with expulsion fuses, there are some transformer configurations and load arrangements that allow for the use of line-to-neutral rated backup fuses when using the basic coordination method described in 4.3.2, 4.3.3, and (which is often termed time-current-curve (TCC) crossover coordination ). However, in the case of transformers that have a delta-connected winding, a line-to-line rated backup fuse is necessary when only TCC coordination is used to select the fusing. This presents a problem for 34.5kV windfarm transformers having a delta connection, as there are no suitable backup fuses rated at 34.5kV. This is the reason that additional coordination criteria are needed, and why appropriate testing of certain existing fuses was done to show how fusing can be achieved Matched Melt coordination There is a second method, or subset, of TCC crossover coordination, termed matched melt coordination. It requires all of the criteria already discussed and adds one more. This additional 6 criterion ensures that the expulsion fuse always melts open, even if it is the backup CL fuse that is doing the interruption. At very short melting times, the I 2 t to melt a fuse element tends towards a fixed value (minimum melt I 2 t). Clearly, if the minimum melt I 2 t of the backup CL fuse is more than the maximum melt I 2 t of the expulsion fuse (using minimum and maximum manufacturing tolerances) the CL fuse could not melt without melting the expulsion fuse also. To use this as the criterion would be rather restrictive, however, and excellent results have been achieved by having the minimum melt I 2 t of the backup fuse at least half of the maximum melt I 2 t of the expulsion fuse. This is because, at most practical melting times, the CL fuse does have a higher melt I 2 t than its minimum, and because a fuse cannot interrupt current without some arcing, which will add additional I 2 t. This is illustrated in Figure 9 below. 2 x minimum melt I 2 t of the backup fuse must be greater than or equal to the maximum melt I 2 t of the expulsion fuse Current Melting I 2 t Melting Arcing I 2 t Figure 9: Illustration of matched melt coordination criteria. Matched melt coordination is extensively used to coordinate external backup fuses and cutouts (often used to protect pole-type transformers). In this case the primary reason for the matched melt coordination is to ensure that the cutout always drops open, but it does also allow L-N rated backup fuses to be used in most cases. The drop-open action gives visual indication of the fuse operation. There have also been occasions, before windfarm applications, when matched melt coordination was used with the two-fuse protection of deltaconnected pad-mounted distribution transformers. Delta connected transformers produce L-L voltages across primary fuses for a variety of faults. [2] It is therefore normal to use L-L rated fuses. However if one uses the two fuse approach, and if the expulsion fuse is rated lineto-line, it was often possible to coordinate the expulsion fuse and the current-limiting fuse using matched melt coordination such that a currentlimiting fuse having a voltage rating corresponding to the system L-N voltage could be used. In practice, this was, and is, seldom done where L-L rated backup CL fuses are available, but often becomes the only way that higher voltage transformers can be fused.

7 Matched melt coordination ensures sufficient I 2 t will be let through by the current-limiting fuse(s) to melt open the expulsion fuse(s). If the fault is a line-to-neutral fault, the line-to-neutral rated current-limiting and the line-to-line rated expulsion fuse combination obviously should have no difficulty in clearing, as both fuses have voltage ratings equal to or greater than the voltage that they would have to clear against. Even if the fault is transient (e.g. a flashover) and a L-L recovery voltage appears across the operated fuses, the L- L rated expulsion fuse could block the voltage if the backup fuse proved to be inadequate for the task. In the case of line-to-line faults, the situation becomes more complicated. If the magnitude of the fault current is less than the interrupting rating of the expulsion fuses, the two expulsion fuses that would "see" the fault (one from each phase) should have no difficulty in clearing it. If the fault current exceeds the rated maximum I/C of the expulsion fuse and doesn't involve ground, two current-limiting fuses in series with two expulsion fuses will be attempting to interrupt the fault current. If the two current-limiting fuses share the interrupting duty, they will be able to clear despite the fact that they must do so against a voltage that exceeds each's individual rated voltage. In addition, the fact that the line-to-line rated expulsion fuses will have melted open means that the system's line-to-line voltage will not be impressed across the line-to-neutral rated currentlimiting fuses after the clearing has occurred. Testing and many years of experience support this proposition. Tests have shown that two series connected current-limiting fuses share voltage well when they both melt in the first loop of current (i.e. when they are operating in their current-limiting mode or at currents a little less). Thus, it has long been assumed that as long as the rated max I/C of the expulsion fuse is at, or above, the point at which the current-limiting fuses go into their current-limiting mode, there should be no question as to whether the two current-limiting fuses share the interrupting duty. An illustration showing how the combination of the series connected expulsion and CL backup fuses interrupt L-L faults, when the CL fuses are in their current-limiting mode at the maximum I/C of the expulsion fuse, can be seen in Figure 10. Using this criterion, fusing windfarm transformers as large as 1250kVA at 34.5kV delta was possible. However, fusing larger kva units, required CL backup fuses that were not in their current-limiting mode at the max I/C of the expulsion fuse (1200A). In this case at currents just above this max I/C, the backup CL fuses would take several cycles to melt, and there was no guarantee that the two fuses would share the interrupting duty. In fact test data [4] confirmed 7 the predictions that two backup current-limiting fuses, of a particular design, would not share the interrupting duty for melt times longer than that of a few loops of current. This would imply that, unless the current corresponding to the maximum interrupting current of the expulsion fuse is high enough to cause the current-limiting fuses to melt within one or two loops of current, L-L rated backup fuses should be used. However, the same testing referred to previously also showed that, at currents below those causing a single backup fuse to operate in its current-limiting mode, these designs of fuse were capable of interrupting L-L faults even though they were only rated L-N. This was shown to be true from currents causing melting in a few loops down to currents well below the maximum interrupting current of the expulsion fuse. This is because the rated maximum voltage of a backup fuse is established under the most severe conditions, i.e. I 2 and I 1 and the light duty conditions below I 2 are easier for the fuse Time in Seconds Total Clearing CL Backup Fuse Minimum Melting Interrupted by Interrupted by Two Series CL Fuse CL Fuses Enter Current-limiting Mode Max I/C of Current in Amperes Figure 10: Interruption of L-L faults when CL fuses are in their current-limiting mode at the max I/C of the expulsion fuse. Figure 11 shows a single 23kV fuse interrupting 1150A at 37kV, melting after several cycles of current, while Figure 12 shows a single 23kV fuse interrupting 3090A at 40kV (melting towards the end of the first loop of current). At this higher current, when two fuses were tested in series, both melted in the first loop and shared the interruption duty (figure 13).

8 large as 2750kVA to be fused using L-L expulsion fuses and L-N rated backup fuses. An illustration showing how the combination of the series connected expulsion and CL backup fuses interrupt L-L faults, when the CL fuses are not in their current-limiting mode at the maximum I/C of the expulsion fuse, can be seen in Figure Figure 11: Single 23kV fuse interrupting 1150A at 37kV. 100 Interrupted by Interrupted by a Single CL Fuse Interrupted by Two Series CL Fuses 10 Time in Seconds 1 CL Fuses Enter Currentlimiting Mode 0.1 Total Clearing CL Backup Fuse Minimum Melting Max I/C of Expulsion Fuse Figure 12: Single 23kV fuse interrupting 3090A at 40kV Current in Amperes Figure 14: Interruption of L-L faults when CL fuses are not in their current-limiting mode at the max I/C of the expulsion fuse. Figure 13: Series 23kV fuses interrupting 3090A at 40kV. Such testing has been performed on certain types of 23kV (nominal) rated backup fuses, and has shown them to be capable of interrupting in a 37kV circuit from a current well below the maximum interrupting rating of 34.5kV expulsion fuses (1200A) up to a current at which a single fuse can interrupt alone or two fuses in series will melt in the same loop and share the duty. This enables 34.5kV delta connected transformers as 8 5 APPLICATION CONSIDERATIONS As is the case with all applications in which fuses rated less than full line-to-line voltage are used, several assumptions need to be valid. Also windfarm transformers require some additional cautionary comments. The overriding assumption is that a single CL fuse will not be called upon to interrupt line-to-line voltage except in the case of a tested backup CL fuse discussed in Even here, there is a restriction that a single fuse must not be called upon to interrupt line-to-line voltage in its current-limiting mode (that is at severe duty conditions when the current is higher than that causing melting in one to two loops of current). Conditions that could cause this to occur are quite rare, but are discussed in fuse standards. Three conditions where this could occur are: 1) If a three phase primary fault that does not involve ground can occur 2) If a system has an isolated neutral or is resonant grounded and does not have protection that will operate when a single ground fault occurs and (or before such protection can operate) a second phase fails to ground with one fault upstream of the fuses and the other downstream

9 3) If a system is such that a neutral shift can occur that would produce a higher than normal voltage across the fuse during a high current L-N fault. Fuse Disconnecting Switches, and Accessories (ANSI). For a windfarm transformer it is also important that, in the event of a fault that causes operation of the HV fuses, the generator be isolated from the low voltage side of the transformer as quickly as possible. There are two main reasons for this. The first is that if the transformer windings are intact and able to step up the LV, and if the generator and system voltage are not held in phase, up to double voltage could be imposed across the operated fuses. For this reason, the two-fuse approach using matched melt coordination is particularly helpful, especially when cartridge expulsion fuses are used as they produce a large, under oil, isolation gap. This is one reason why full-range fuses are not usually recommended for this type of application. The second reason is that it must also be recognized that an internal transformer fault is being fed from both the HV and LV sides of the transformer. The HV fusing, as described in this paper isolates the transformer from the power system. However a transformer having a high voltage, or indeed low voltage, failure will continue to receive fault current and arc energy (albeit at reduced values) until the LV protection operates. N. R. Schultz, R. H. Hopkinson, and J. H. Easley Single phase switching and fusing in threeth Power Distribution phase circuits, 27 Conference, University of Texas at Austin, October, 1974 [3] IEEE Guide for IEEE C , Application, Operation, and Maintenance of High-Voltage Fuses, Distribution Enclosed Single-Pole Air Switches, Fuse Disconnecting Switches, and Accessories (ANSI). [4] D. L. Gardner, "Analysis of the operation of series-connected high-voltage current-limiting fuses under multi-cycle melting conditions," MS. dissertation, Dept. Electrical. Eng., Univ. North Carolina, Charlotte, 2003 [5] W. W. Olive and A. C. Westrom Low impedance fault energy withstand evaluation of pad mounted distribution transformers, 1976 Underground Transmission and Distribution Conference. TM BIOGRAPHIES Dan Gardner is the R&D Manager for the Hi-Tech Fuses product line for Thomas & Betts Corporation. He joined Hi-Tech Fuses, Inc. in 1999, and has held the positions of both Design Engineer and Sr. Design Engineer before becoming the R&D Manager, upon the sale to T&B. Dan is a Professional Engineer (P.E.) certified with the North Carolina Board of Examiners for Engineers and Surveyors and is an active member of the IEEE. He holds a Bachelor of Science degree in Electrical Engineering from the Pennsylvania State University and a Masters of Science in Electrical Engineering from the University of North Carolina Charlotte, where he focused on power systems. 6 CONCLUSIONS Using the two-fuse approach with matched melt coordination, it is now possible to provide protection on the HV side of delta-connected windfarm transformers as large as 2750KVA at 34.5kV, providing of course, that the backup fuses have been properly tested. Although attempts have been made to assess the energy withstand of various types of transformer [5] in order to determine 2 appropriate I t limits for the use of CL fuses in particular sizes of transformer, this has proved difficult as the transformer arc energy is 2 proportional to I t times arc resistance, which depends on the length of the internal fault arc. However, what has been found is that current2 limiting fuses tend to let through an I t having an order of magnitude less than that necessary to cause tank failure, so the use of any appropriately sized fuse seems to provide good transformer protection under nearly all practical circumstances. John Leach was born in Birmingham, England, on October 8, He graduated from the University of Aston, Birmingham, with a B.Sc degree in Electrical Engineering in While working for Brush Fusegear (now Cooper-Bussmann), he graduated from Nottingham University in 1972 with a Ph. D in electrical engineering (having worked on the numerical analysis of fuse pre-arcing behavior). In 1978 he moved to the USA to work for General Electric, and then Hi-Tech Fuses, Inc., a company that he co-founded in When Thomas and Betts purchased the assets of Hi-Tech Fuses in 2006, he became a consultant to the product line. He has written numerous technical papers, and been awarded eight US patents on fuse technology. Leach has been active in IEEE and IEC Fuse standards for over twenty-eight years. He is the chair of the IEEE High Voltage Fuses Subcommittee and its Revision of Fuse Standards Working Group. He is the US technical advisor to IEC TC 32 (Fuses) and SC32A (High-Voltage Fuses). REFERENCES [1] [2] TM IEEE Std C , IEEE Standard Design Tests for High-Voltage Fuses, Distribution Enclosed Single-Pole Air Switches, 9