safe protection of distribution transformers

SIBA GmbH & Co. KG 16.06.2005

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Safe protection of distribution transformers by means of switch-fuse combinations

IEC 60271 Part 105: Continuation of an approved strategy

By Heinz Ulrich Haas and Dirk Wilhelm

With regard to the fuse protection of distribution transformers it is rather difficult to give information with a substancial news value. After all, this concerns transformers, which, at least in part, have been installed for more than 30 years. The corresponding high-voltage fuses are used to limit possible fault currents for just as many years. And in only a few cases it was necessary to replace the fuses, e.g. if, following a short circuit, there had been a switch-off. The fuses are often part of switch-fuse combinations and these switches also have satisfactorily done what they were expected to do under fault conditions. So then, what is there to be reported?

Now the publication of IEC 60271 Part 105 has caused some degree of uncertainty since, with the use of switch-fuse combinations, approved fuse protection strategies are suddenly called into question. On closer examination, however, it is to be found that in most cases the concern is unnecessary because previously published specifications regarding the protection of transformers remain valid. The present publication is intended to assist the expert in reviewing his fuse protection strategies in order to confirm their correctness or to persuade him to consider using a more appropriate fuse-link.

Criteria for the transformer protection

At first, we would like to expand on the criteria for the protection of the distribution transformer that are fundamental for a correct fuse protection. A prerequisite for this is to know the nominal rating of the trans-former as well as the maximum permissible overload, operating voltage, transformer impedance and inrush current. Subsequently, and as illustrated in Figure 1, the dependences applicable to the selection of a suitable high-voltage fuse-link (HH fuse) are listed:

In: Rated current of the transformer

Inmax: Maximum permissible overload of the transformer

Iinrush: Maximum inrush current of the transformer

ISC: Sustained short-circuit current on the secondary relative to the primary side

HH-ZSK: Time/current characteristic of the HH fuse

NH-ZSK: Time/current characteristic of the NH fuse on the secondary relative to the primary side

HV-K: Characteristic of the protective device on the supply side

Figure 1 Dependences applying to the transformer protection

The HH fuse is required to carry the nominal transformer current as well as its maximum permissible overload. Given this, neither the values specified for the temperature rise nor the maximum permissi-ble power consumption of the switchgear (if used) shall be exceeded.

Depending on its type and size, the inrush current of the transformer is between 6 and 20 times its rated current over a duration of 0,1 s. The HH fuse has to withstand this inrush peak.

10 4

103

102

10 1

10 0

10-1

10-2

101 10 2 10 3

Tim

e

s

CurrentA

InIn

ma x

NH-ZSK

HH

-ZSK

HV-K

Inrush

ISC


2

When short-circuiting the three poles on the secondary, the sustained short-circuit current ISC on the primary side is derived from the transformer impedance uk [%] of the transformer. ISC has to be inter-rupted by the HH fuse-link within a specified time interval so as to prevent the transformer from burst-ing.

In addition to that, the fuse protection strategy in its entirety has to take into account the discrimination of the HH fuse-link with regard to primary or secondary protective devices. On the secondary side the low-voltage fuses (NH fuses) have to be considered, whereas on the supply side it is protective de-vices such as primary fuses, relays etc. that have to be taken into account.

Standards and recommendations

The provisions for the determination of a suitable fuse protection of mains transformers focus primarily on the International standard IEC 60787 and its German translation VDE 0670 Part 402. When using High-Voltage Fuse-Links in air- or gas-insulated switch-fuse combinations, the specifications of IEC 61271-105 or VDE 0671 Part 105, respectively, have to be considered. [25]

In addition to the specifications of the IEC publication, VDE 0670 Part 402 contains recommendations for high-voltage fuses to be used when transformer protection fuses of operating class gTr according to VDE 0636 Part 2011 are used on the low-voltage side (see Table 1). Furthermore, the recommendations of this standard are widely accepted by the users, even if line protection fuses of operating class gG according to VDE 0636 Part 201 are used on the secondary side instead of the transformer protection fuses of operating class gTr, or when no busbar protection is provided either. [67]

Table 1 Recommended fuse protection according to VDE 0670 Part 402

Nominal rating of the transformer in kVA

100 125 160 200 250 315 400 500 630 800 1 000

u2 = 4 % u2 = 5 %

Maximum permissible duration of short circuit 2 s

Nominal voltage

kV

Rated currents of the High-Voltage Fuse-Links in A

6/7,2 20 & 25 25 & 31,5 31,5 & 40 40 & 60 50 & 63 63 & 80 80 & 100 100 & 125 125 & 160 160 160 & 200

10/12 16 16 20 & 25 25 & 31,5 31,5 & 40 40 & 50 50 & 63 63 & 80 80 & 100 100 & 125 125 & 160

20/24 10 10 16 16 16 & 25 25 25 & 31,5 31,5 & 40 40 & 50 63 63 & 80

30/36 6,3 10 10 16 16 & 20 20 & 25 25 25 & 31,5 31,5 & 40 40 & 50 40 & 50

For further considerations it is important to distinguish between the following three cases of application since this leads to a wider range for assigning the rated current of the fuse to the respective transformer. The three possible configurations are shown in Figure 2.

Figure 2 Assignment of fuses to the transformer


3

10 4

10 3

s

10 2

10 1

100

10-1

10-2

101 102 103 1045 5 5

Inrush=12 x In

Ik-transf with uk = 4 %

630kVAgTr

gG400A

HH

-80AH

H- 100A

In-tr

ansf

1,4x

In-tr

ansf

Mel

ting

time

CurrentA

In order to give an example, the approach is described in the following for a mains transformer with 630 kVA at 10 kV. The inrush current is set to be 12 times the nominal transformer current und the transformer imped-ance uk to be 4 % over a maximum duration of 2 s.

For this transformer, the fuses relevant to be considered according to VDE 0670 Part 402 have been pre-sented in Case A in Figure 2. It is possible to use high-voltage fuses with rated currents of 80 A and 100 A (see Table 1). On the secondary side transformer protection fuses gTr with 630 kVA (909 A) are used. For the protection of the outgoing cables the use of low-voltage fuses of operating class gG with maximum 400 A is permitted. As can be seen from the comparison of the relevant characteristics given in Figure 3, full discrimi-nation as achieved. In case of a fault current, only the fuse-link upstream to the respective branch operates.

The protection on the busbar according to B in Figure 2 is taken over by low-voltage fuses of operating class gG. The rated current is selected to be 800 A, which is the highest value below the nominal transformer current. Due to the quicker reaction of these fuses it is possible to use HH fuses with rated currents of 80 A to 125 A. Thus, it can be seen that also in this case all of the three fuse assemblies concerned show a discrimi-native behaviour towards each other.

In accordance with C in Figure 2 there is no busbar protection provided on the low-voltage side. If the distance between the characteristics of the high-voltage fuses and the inrush point of the transformer is sufficient, it is now even possible to use HH fuses with rated currents of 63 A to 125 A.

Figure 3 Discriminative behaviour of the fuses (Case A)


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Criteria for selecting the suitable fuse

Now that the rated current range of the possible HH fuses is defined, the suitable fuse type has to be se-lected. The criteria to be considered are the rated voltage, the operating class, the highest and lowest break-ing currents as well as the type of striker used.

The rated voltage UN of the HH fuse-link must always be greater than or equal to the maximum oper-ating voltage Utrafo of the transformer. For the transformer used in the example a rated voltage of 12 kV would have to be applied.

The rated breaking capacity I1 of the fuse-link must be sufficiently high. A typical value for fuses within the rated voltage range considered is 63 kA.

For the protection of transformers usually HH fuse-links of the backup type are used. These fuse-links offer a protection ranging from their respective minimum breaking current Imin up to the aforemen-tioned rated breaking capacity I1. HH fuse-links made by SIBA are manufactured to have an Imin as low as 3.2 to 4 times the rated fuse current.

Especially when using switchgear with a three-pole tripping mechanism it is highly recommended to use a temperature-limiting striker. High temperatures that are likely to increase the loads acting on the switchgear or the fuse are detected by the striker and cause him to respond. This in turn leads the switchgear to trigger and to the interruption of the fault current. Up to a rated current of 160 A, the HH fuses made by SIBA are by default equipped with such a striker system. [8]

Incorporating the switch-fuse combination into the protection strategy

With regard to a sufficient protection in transformer circuits IEC 60271 Part 105 contains, in addition to the criteria mentioned above, requirements for the combined action of switches and fuses. In this standard the switching task is specified for the switch or the fuse depending on the fault current. With this, the interruption is carried out at a transfer current to be determined from the fuse characteristics and the technical data of the switch.

The fuses are required to interrupt all fault currents greater than the transfer current. The short-circuit current ISC derived from the transformer impedance shall also be interrupted by the HH fuses.

Currents of less than the transfer current of the fuses are interrupted by a combined action of the switch and fuses. At the same time the striker of the fuse responding first actuates the three-pole trip-ping mechanism of the switchgear. Finally, the mains disconnection is carried out by the switch.

For the purpose of testing these dependencies, the switch manufacturer provides the rated transfer current of the switch Itransfer and the opening time T0 taken by the switch in response to the striker operation. Also to be used are the time/current characteristics of the preferential fuses obtained from the fuse manufacturer. In order to continue the example, we began with, a rated transfer current of the switch of 1100 A and an opening time of 45 ms.

The determination of the transfer current is the most extensive part in establishing the suitability of fuse-links. At SIBA, this current is derived by means of the method Mathematical determination of T indicated in IEC 60271 Part 105, Annex B.2. Since the slopes of the characteristics of the HH fuses made by SIBA prove to be constant within the time interval of the usual switch opening times, it is possible to considerably shorten this otherwise rather extensive iterative procedure. The essential steps of calculating the transfer current are represented in Figure 4.


5

Current

Tim

e

II

Averagecurve

Minimumcurve

3.T

5. 4.

m

TTransfer

1. Determining the slope of the characteristic in the range of the switch opening time T0

2. Calculating the pre-arcing time of the fuse responding first Tm with: Tm = T0 0.87/((1 + 0.13) 1)

3. Plotting Tm against the characteristic

4. Reading out IT

5. Establishing ITransfer from the lower ZSK of the fusewith: ITransfer = IT 0.935

Figure 4 Calculation scheme to establish the transfer current

For the example calculation a rated fuse current of 80 A was chosen, which could be used in all three cases presented in figure 2. From the calculation the following values are obtained:

Transfer current of the fuse = 850 A

Short-circuit current ISC at uK = 4% = 910 A

The next step is to compare the transfer current obtained for the fuse to the rated transfer current of the switch:

Transfer current of the fuse rated transfer current of the switch

850 A 1,100 A? Compliance with IN = 80 A

By means of a second comparison it is to be checked whether the transfer current of the fuse is less than the short-circuit current derived from the transformer impedance:

Transfer current of the fuse < short-circuit current ISC (at uK = 4 %)

850 A < 910 A? Compliance with IN = 80 A

Thus, the fuse-link with a rated current of 80 A would comply with the requirements for transformer protection with regard to the relevant state of standardization.

SSK fuses: How to bridge the gap between the Standards

However, in a few cases it will be found on detailed calculation that the preferential fuse is not suited to meet the switch requirements. For instance, a higher transformer impedance uK reduces the short-circuit current; in our example it would only be 606 A for a uK of 6 %. Lower values of the rated transfer current or a shorter opening time would also lead to non-compliance with the switch requirements. It is necessary to use a fuse-link with a quick-acting characteristic to obtain a transfer current below the calculated short-circuit current.

The characteristic of such a fuse-link would be that of the fuse with the next lower rated current. However, to use this fuse would be disadvantageous since it would cause the power dissipation to increase and the temperature to rise. Moreover, the discrimination to the downstream protective device would be questionable.

At SIBA fuse-links have been developed that show at the same rated current and with similarly low values of power dissipation a considerably quicker reaction. These fusesdesignated as SSK fusesare intended to be used when the calculation lined out above has led to a result showing that the switch requirements are not complied with. Within the rated current range of 63 A-SSK to 160 A-SSK these fuses allow for a switch-fuse combination to be used in many cases where the standard fuses prove to be too slow-acting. [9]


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Modifying the example set out above a fuse of 80 A-SSK is used with uK = 6 %:

Transfer current of the fuse < short-circuit current (at uK = 6 %)

850 A < 606 A? Non-compliance with IN = 80 A

Transfer current of the fuse < short-circuit current (at uK = 6 %)

590 A < 606 A? Compliance with IN = 80 A-SSK

In terms of the minimum and maximum breaking currents SSK fuses are comparable with the standard HH fuses. While having the same dimensions their power dissipation is usually even lower. The fuses are also equipped with a temperature-limiting striker and therefore fulfil all the requirements applying to modern transformer protection.

These fuses have attracted international attention from the switch manufacturers. Fuses of type SSK were taken for type-tests of the combination and are included in the lists of recommended products.

Summary

As far as the use of switch-fuse combinations according to IEC 60271 Part 105 is concerned, it has, at least in the theoretical approach, become more difficult to establish whether a high-voltage fuse is suited to protect the distribution transformers. Whereas, in most cases, the use of fuses that are proven to be suitable for many years can be continued, it may in a few instances be necessary to introduce a change towards a lower rated current or to another type of fuse in order to meet the specifications of this standard in full.

However, before replacing a fuse, the switchgear manufacturer or the fuse manufacturer, respectively, should be consulted. Taking the practical aspects into account it is then necessary to consider as to how far the technical data of the system allow for the specified values to be enhanced and thereby to decide whether it is possible to continue the use of the previously employed fuses.


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Sources

[1] www.SIBA.de

[2] IEC 60787:1983-01 Application guide for the selection of fuse-links of high-voltage fuses for transformer circuit application

[3] DIN VDE 0670-402 (VDE 0670 Teil 402):1988-05 Wechselstromschaltgerte fr Spannungen ber 1 kV Auswahl von strombegrenzenden Sicherungseinstzen fr Transformatorstromkreise

[4] IEC 61272-105:2002 High-voltage switchgear and controlgear Part 105: Alternating current switch-fuse combinations

[5] DIN EN 62271-105 (VDE 0671 Teil 105):2003-12 Hochspannungs-Schaltgerte und -Schaltanlagen Part 105: Hochspannungs-Lastschalter-Sicherungs-Kombinationen

[6] DIN VDE 0636-2011 (VDE 0636 Teil 2011):1999-05 Niederspannungssicherungen (NS-System) Part 2-1: Zustzliche Anforderungen an Sicherungen zum Gebrauch durch Elektrofachkrfte bzw. elekt-rotechnisch unterwiesene Personen National Amendment 1: Schutz von elektrischen Sonderanlagen

[7] DIN VDE 0636-201 (VDE 0636 Teil 201):2004-10 Niederspannungssicherungen (NS-System) Part 2-1: Zustzliche Anforderungen an Sicherungen zum Gebrauch durch Elektrofachkrfte bzw. elekt-rotechnisch unterwiesene Personen Clauses I to VI: Beispiele von genormten Sicherungstypen

[8] Haas, Heinz Ulrich: Thermal system protection of switchgear through high-voltage fuse-links with inte-grated temperature limiter under consideration of IEC 420:1990. Proceedings of the 5th International Conference on Electrical Fuses and their Application (ICEFA), 25.27.09.1995, Technical University Ilmenau

[9] Lffler, Ralf and Haas, Heinz Ulrich: Hochspannungssicherungen fr Schalter-Sicherungs-Kombina-tionen. Etz, Elektrotechnik und Automation, VDE-Verlag 78/2001

Dipl.-Ing. Heinz Ulrich Haas (50) is head of the research and development department of SIBA GmbH & Co. KG in Luenen, Germany.

Dipl.-Ing. Dipl.-Wirt.-Ing. Dirk Wilhelm (34) is technical project manager at SIBA GmbH & Co. KG in Luenen, Germany.

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