Page 490 - Industrial Power Engineering and Applications Handbook

15/464 Industrial Power Engineering and Applications Handbook
Short-time duty When this transformer is required which is reasonably low compared to the high system
to discharge a charged capacitor bank it should be voltage. This helps restrict the phase error, on the one
capable of withstanding heavy inrush discharge currents hand, and facilitates an economical intermediate wound
(see below for its application). The following may be transformer T,, on the other, to perform the same duty as
considered when designing a transformer for dischar- a normal wound voltage transformer. The purpose of
ging purposes: line capacitors is thus to step down the high to very high
1 The size of the capacitor banks, their voltage and system voltages to an economically low value. Through
the impedance of the capacitor circuit. the tapping point A is connected a conventional and a
2 Rate of discharge of the trapped charge. less expensive wound-type intermediate or auxiliary
3 The temperature of the primary windings after the voltage transformer T,, rated for the intermediate voltage
discharge. VI in association with a reactor L (Figure 15.6(b)). The
4 The magnitude of electrodynamic forces on the use of the reactor is to almost offset the heavy capacitive
primary windings, which may be developed by the voltage component. If possible, the reactor and the
discharge currents. transformer may be combined in one unit to make it
Applications They may be used to carry out the easier to operate. The secondary of the transformer is
following functions: rated for the required standard voltage, say, 110/ <3 (63.5
1 To detect a ground fault or operate a directional V), to feed the auxiliary devices and components fitted
ground fault relay (Section 21.7.4). in the auxiliary circuit.
2 To operate a neutral displacement relay (Figures The inductive reactance of the combined transformer
26.4 and 26.9). and the reactor is chosen so that it will balance the
3 To detect an unbalance in a three-phase normally capacitive reactance of the line capacitors at the rated
balanced capacitor bank (Section 26.1.1(8)). frequency and thus achieve a near-resonant circuit. Since
4 To discharge a charged capacitor bank over a very a frequency variation may cause a de-tuning of the resonant
short period, particularly when a fully charged circuit, tappings are generally provided on the intermediate
capacitor is interrupted. See also Section 25.7. voltage transformer to facilitate adjustment of the circuit
5 To discharge an interrupted HT circuit before a re- reactance at different frequencies, to achieve a near-
closing. An HT system, say, a transmission line or resonant condition even on other frequencies. There is a
a cable when interrupted, develops high transient voltage drop across both the capacitor units Vc and the
voltages as discussed in Sections 23.5.1 and 20.1. reactor VL. Figure 15.7 illustrates a simple equivalent
Unless these transients are damped to a reasonably circuit for the CVT of Figure 15.6(b) for more clarity.
low level so that they are not able to endanger the These voltage drops, being 180" out of phase, are
connected devices on an automatic reclosing, the detrimental in influencing and adding to the phase error
devices installed in the system may become damaged of the intermediate voltage transformer T,. At higher
due to the resulting switching transients. The normal frequencies, the summation of these voltages (V, + V,)
practice to deal with this is to damp the transients may become very high and cause high phase errors, leading
through an electromagnetic transformer such as this. to erratic behaviour of the instruments, devices and
The transformer, however, may have to be designed components connected on the secondary of the
for such a duty to sustain the electrostatic stresses intermediate voltage transformer. It is therefore, imperative
arising from such transients, the discharge time and that these voltage drops be contained as low as pgssibk,
impedance of the interrupting circuit up to the on the one hand, and must offset each other, i.e. Vc + V,
transformer. = 0, on the other, to remain almost ineffective even at
higher frequency variations, in influencing the phase error
15.4.4 Capacitor voltage transformers (CVTs) of the intermediate VT.
Frequency variations are usually caused on a fault or
This type of voltage transformer is normally meant for a a switching operation (Sections 20.1 and 23.5.1) and
high to an ultra-high voltage system, say, 110 kV and also during the changeover of the tapping of the
above. While a conventional wound-type (electro- intermediate VT or the reactor. When the voltage drops
magnetic) voltage transformer will always be the first Vc and VL are not large enough compared to VI, the
choice, it may become costlier and highly uneconomical content of phase error is contained. An intermediate
at such voltages. voltage of almost 12-24 kV is found to be realistic in
The size and therefore the cost of a conventionally restricting the voltage drops across C and L, to a reasonably
wound voltage transformer will be almost proportional low value compared to V, during normal operation. Further,
to the system voltage for which it is wound. As a cost it is essential to offset the reactances Xc and X, through
consideration, therefore, a more economical alternative a variable reactor to achieve a near-resonant circuit when
is found in a Capacitor Voltage Transformer (CVT) (Figure the CVT is in service. This makes the whole system
15.6(a)). behave like a normally wound VT in terms of its rating
A CVT consists of a capacitor divider unit in which a and class of accuracy for both metering and protection
primary capacitor C, and a secondary capacitor C2 are purposes. The same error limits will apply as for a normal
connected in parallel between the line and the ground VT (Tables 15.5 and 15.6). The output for a given accuracy
(Figure 15.6(b)). A tapping at point A is provided at an is dependent on the range of frequency variation over
intermediate voltage VI, usually around 12 to 24 kV, which the voltage transformer is required to operate.