CULTURA Y LENGUA ORIGINARIA FORMATO: Taller

For low-voltage power-factor-correction applications it is usual to install a single capacitor in each phase. Simple recommendations, associated with the protection of these capacitors, are usually provided by fuse manufacturers. These are based on

R

unit

Y B

Figure 7.19 Capacitor bank

service experience and take into account the high transient inrush currents, the possible harmonic content of the currents and the capacitor tolerances. As an example, when fuselinks to IEC 60269-2-1 Section II (BS 88-2) are to be used to protect capacitors of ratings in excess of 25 kVA it is usual to use fuselinks with current ratings of at least 1·5 times the capacitor full-load current.

In large installations at higher voltages, capacitor banks are made up of individual capacitors connected to form a number of separate units. For three-phase applications up to 11 kV (line) and 1 MVAr, the phases may be star- or delta-connected and the units are connected in parallel. For higher-voltage systems the phases are star-connected and the units in each are connected in parallel, as shown in Figure 7.19.

A practice widely adopted in European countries apart from the UK is to fuse each individual capacitor element in the units. The fuses used for this purpose contain simple wire elements with the appropriate low-current rating and breaking capacity.

The alternative practice, which is used in the UK, is to fuse each unit as a whole, although frequently a line fuse for each phase is also included for the smaller banks used at voltages up to 11 kV. A unit fuse should operate if its associated unit becomes faulty, leaving the remainder of the bank in service.

As with other applications, the requirements are that a fuse should operate as quickly as possible in the event of a fault but also be able to carry load current and transient overcurrents. The latter arise in the event of a sudden change in the voltage across a bank, a situation which arises on connection to the supply or in the event of a system fault which affects the network voltages. To prevent operation under this condition, it is usually necessary to use fuses with a current rating considerably higher than the normal capacitor current.

If a unit develops a short circuit, a discharge will occur within it and current will flow into the unit from the supply and from other healthy units, as shown in Figure 7.20. Clearly only the fuselink associated with the faulty unit should operate in these circumstances. Considering the situation with four units per phase connected in series–parallel as shown in Figure 7.21, it will be seen that the discharge current of

i_sc

short circuit

faulty unit i_df

i_dh i_s supply

heaithy unit

Figure 7.20 Faulty capacitor unit Where

i_sc current in short circuit i_s current from supply

i_df discharge current of faulty capacitor i_dh discharge current of healthy capacitor i_sc i_s+ idf + idh

i_sc i_dh+ 2i_ch

short circuit i_df

i_dh

i_ch

i_ch 2i_ch

Figure 7.21 Superimposed current flow due to short circuit of a capacitor unit Where

i_sc short-circuit current

i_ch charging current of healthy units i_dh discharge current of healthy unit i_df discharge current of faulty unit i_sc 2i_ch+ idh+ idf

the faulty capacitor does not flow through its fuselink. Each of the two lower fuselinks carries the current needed to double the instantaneous voltage on its associated unit and the fuselink associated with the healthy upper unit carries the current needed to reduce the charge on its capacitors to zero. These three currents are all equal and should not operate the fuselinks because, as explained above, they must be chosen to withstand similar currents when the capacitors are connected to the supply. The fuselink associated with the faulty unit carries the sum of these three currents, as can be seen from Figure 7.21, and it should operate to give correct discrimination.

After the initial surge and until the fuselink melts, it will carry four times the normal steady-state current.

A factor which must be considered is that the voltage on the healthy upper unit will rise to 133 per cent of its normal value and, of course, its VAR input will rise by 77 per cent.

Actual arrangements using more series- and parallel-connected units can be considered in a similar manner.

The special factors which must be considered when choosing fuses which are to protect capacitors are summarised below:

(a) They must not deteriorate or be damaged by the high and rapidly changing inrush currents which may flow when capacitor banks are energised or when healthy units discharge into a fault. To achieve this it is usually necessary to use fuselinks with a current rating considerably higher than the current they normally carry. In this connection it must be recognised that fuselinks of small current rating are more sensitive to inrush currents of given multiples of the rated values than larger fuselinks. Thus, the ratio of permissible fuse rating to load current decreases with increase of load current. Figure 7.22 gives a guide for the selection, based on ability to withstand inrush currents, of typical 10–300 A high-voltage fuselinks, used in air, for different values of full-load capacity current.

(b) The fuselinks must also be of sufficiently high current rating to withstand not only the continuous maximum load current but also the harmonic content, which can be quite significant because of the lower reactance presented to harmonics by capacitors. In practice there is a maximum permitted harmonic content and this dictates a fuse-current rating of not less than 143 per cent of the nominal full-load current of the circuit.

(c) Where the fuses are to be mounted in enclosures having restricted ventilation and/or where the ambient temperatures may exceed 40^◦C, it may be necessary to derate the fuselinks.

(d) Where an installation comprises banks which are in close proximity and where they are switched separately, allowance must be made for the transient inrush current which may flow between banks when one bank is to be switched in parallel with already energised banks. In practice it has been found that it is sufficient to choose the bank fuselinks by assuming the capacitor current in Figure 7.22 to be 1·6 times the actual value.

500

100

fuselink rating, A

10

.3 10

capacitor full-load current, A

100 300

Figure 7.22 Capacitor fuselink selection curve

One or more of factors (a)–(d) dictates the smallest current rating of fuselinks which may be used in a given application, but consideration must then be given to the following factors to ascertain the degree of protection which will be obtained:

(e) In situations where the unit or line fuselinks are to be used without any associated over-current-protective device, consideration should be given to the minimum breaking capacity of the fuses. This should be low enough to prevent malop-eration in the event of an over-current arising owing to the short circuiting of a capacitor unit.

(f ) Calculations of the type described earlier should be done to determine the surge and steady-state currents which will flow when faults occur within individual units of a capacitor bank to ensure that the appropriate unit fuselink will operate and correct discrimination will be achieved. In practice it may happen that the fuselink will be operated by the high surge current but this cannot always be relied on. Consequently, the rating should be such that the steady-state power-frequency current will cause operation. In this connection it must be recognised that the fuse current will be of low leading power factor if a fault occurs in one unit of a bank which has several units in series or it will be of low lagging power factor in banks containing only parallel-connected units, the supply-system impedance then being responsible for the current limitation in the event of a short circuit. Clearance for this particular fault condition should be sufficiently rapid to limit the energy input to the container of the faulty capacitor to a level below that which could cause the case to rupture.

(g) For capacitor installations with many units in parallel, the energy which may be fed to a faulty unit, by the units in parallel with it, should be calculated.

The maximum energy, which may be stored by, and which must therefore be

dissipated by, the capacitors in parallel with a faulty unit is given by maximum energy = ¹₂C_pV_pk² = CpV²

in which C_pis the total capacitance of the parallel capacitors and V_pkand V are the peak and RMS steady-state voltages across them.

The above equation is often expressed in the form:

maximum energy= 3·18 × (kVAr of parallel units) for 50 Hz or 2·65 × (kVAr of parallel units) for 60 Hz

There is a limit to the discharge energy with which fuselinks can cope, a typical value being 10 kJ.

(h) As stated earlier, line fuses are sometimes included, in addition to unit and/or individual capacitor fuses, to isolate a complete capacitor bank in the event of terminal faults against which the other fuses provide no protection. Differ-ent considerations apply to these fuses and in general there is no need to use fuselinks of as low a current rating as possible because the other fuses will operate for faults within the capacitors and prevent rupture of the containers. It is the normal practice to use line fuses with current ratings at least 2·5 times that of the unit fuses to ensure correct discrimination and, of course, even higher values may be necessary if there are many units in parallel in a bank.

Both expulsion and current-limiting cartridge high-voltage fuselinks are used for capacitor protection, the former being limited to outdoor applications and where, because the banks contain units connected in series, high inductive breaking capac-ity is not needed. Cartridge fuses immersed in oil or air are used for all other applications.