Page 124 - Practical Power System and Protective Relays Commissioning

Chapter 11

Voltage Transformers

11.1 INTRODUCTION
This instrument is used to transfer the primary voltage to a secondary voltage
proportional with the primary voltage. The secondary voltage value is
suitable for measuring and protection devices.
Voltage transformers (VTs) are used in:
Metering;
Overvoltage protection, for example, no-load line;
Under voltage protection, for example, overloads line;
Discharging capacitor banks (wound).

11.2 PRINCIPLE OF OPERATION OF ELECTROMAGNETIC
VOLTAGE TRANSFORMERS
The electromagnetic VT is connected across the points at which the voltage
is to be measured, and is therefore much like low-power transformers with
secondary winding operating close to an open circuit. For such a no-load
transformer the voltage transformation is in proportion to the primary (N p )
and the secondary (N s ) turns:

V p N p
5
V s N s
An inductive VT is ideally a transformer under no-load conditions where
the load current is zero and the voltage drop is only caused by the magnetizing
current and is thus negligible. In practice, the winding voltage drops are small,
and the rated flux density in the core is designed to be well below the satura-
tion density. Therefore, the exciting current is low and exciting impedance
constant with a variation of applied voltage over the operating range including
some degree of overvoltage. The parameters that define VT performance are
voltage ratio error and phase displacement error. The ratio error is defined as:
Kn Vs 2 Vp
Ratio Error% 5 3 100
Vp

Practical Power System and Protective Relays Commissioning.
DOI: https://doi.org/10.1016/B978-0-12-816858-5.00011-3 123
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