Page 487 - Industrial Power Engineering and Applications Handbook

Instrument and control transformers: application and selection 15/461

Transformers with HT fuse

Figure 15.3(a) Typical indoor epoxy resin cast instrument voltage transformers up to 11 kV (Courtesy: Kappa Electricals)

Table 15.6 Recommended limits of voltage and phase
displacement errors, applicable for all types of protection VTs
(electromagnetic, capacitor and residual VTs)
Class of accuracy % voltage (ratio) Phase displacement (6)
error? fminutes

3P 3.0 120
6P 6.0 240
As per IEC-60044-2
aThese errors are valid only when the-voltage is between 5% to
‘rated voltage factor x loo%’, burden 25-100% of the rated burden,
and p.f., 0.8 lagging.
t
a At voltages lower than 5%, the limits of error may increase. They
9 become up to twice the specified errors at about 2% of the rated
voltage, the limits of VA burden and p.f. remaining the same.
Note
(b) 11 kV three-phase indoor The choice of class 3P or 6P will depend upon the application and
(a) 33 kV single-phase outdoor the protection scheme of the system. The following may be considered
Figure 15.3(b) Typical HT instrument voltage transformers as a rule of thumb when making this choice.
a residual or zero phase sequence voltage across the (i) Class 3P
open terminals at the secondary. This represents the This class may be selected for protective devices that operate
residual or the zero phase sequence voltage, whatever on the basis of phase relationship between the voltage and the
current phasors, such as in a directional overcurrent protection,
may exist in the main supply system. This voltage will reverse power or directional distance protection.
be zero when the main primary system is balanced and (ii) Class 6P
healthy. This class may be selected for protective devices where their
operation does not depend upon the phase relationship between
Important parameters the voltage and the current phasors, such as for an overvoltage,
1 Residual voltage The residual voltage appearing overcurrent or an undervoltage protection. For instance, a
residual VT should have this accuracy class.
across the secondary windings will be three times the (iii) When a residual VT is employed for capacitor discharges it
zero sequence voltage, if it is found in the primary requires no accuracy class.