What is the Rated Voltage Factor in voltage transformers, and how is it determined?
This is the ratio of the maximum operating voltage to the rated primary voltage of the PT. This factor is to be multiplied with the primary rated voltage to determine the maximum operating voltage of the PT at which the PT can operate without deterioration of its performance and accuracy. The voltage factor of a PT or CVT connected between line and earth in an effectively earthed system is 1.2.
What factors determine the choice between a PT and a CVT in high-voltage applications?
The transient behavior of a Capacitive Voltage Transformer (CVT) is slightly inferior to that of a Magnetic Voltage Transformer (PT). When a short circuit occurs at a very close point in the network to the CVT, the primary voltage suddenly drops. Due to the capacitance of the CVT and the inductance of damping inductance (Z), compensating inductance (D), and inductive transformers, an oscillatory circuit will be produced. This oscillation lasts for quite a few cycles. The protection relay connected to the CVT must have some time delay to overcome the oscillation zone, otherwise, malfunction of the protection relay may occur. Because of that, a time delay of 10 to 30 milliseconds has to be introduced in the relays.
When very fast relay operation is required, a normal PT is used.
What are the essential components of a Capacitive Voltage Transformer?
The figure below shows a line diagram of a coupling capacitor type voltage transformer or CVT.
The capacitors C₁ and C₂ are made of oil-impregnated paper and aluminum foil. A tap is taken in between to connect the magnetic voltage transformer (TR) across the capacitor and earth. This point is fixed in consideration of the system voltage between line and earth. It is a usual practice to design the magnetic transformer for a standard primary voltage of 5, 10, 15, or 20 kV, depending on the requirement of burden and accuracy. There are different special auxiliary circuit elements connected in CVT circuit. These are (i) Compensating inductance coil (D), (ii) Damping impedance (Z), (iii) Resistor (R), and (iv) Spark gap (F).
The compensating inductance coil in series with the primary of the induction transformer is used to compensate the capacitive reactance on the capacitive voltage divider. The damping impedance (Z) across the secondary circuit is used to avoid ferro-resonance. The resistor and spark gap provide necessary protection arrangements.
What is the main purpose of compensating inductance coil in a CVT?
The most important advantage of the compensating coil in a Capacitor Voltage Transformer (CVT) is its ability to correct the phase angle of the output voltage. A CVT uses a capacitive divider, which introduces phase shifts of divided voltage due to capacitive reactance. The compensating coil provides inductive reactance, to compensate the capacitive reactance. Therefore, the error in the output voltage remains under limit.
What is the purpose of damping impedance across the secondary circuit of a CVT?
A CVT has inductive circuits as well as capacitive circuits. Due to interaction of these capacitive and inductive circuits and due to ferro – resonance in inductive circuit during a faulty or transient over voltage condition, there may be a non linear oscillation on the output voltage. The damping impedance (Z) across the secondary circuit is used to damp this oscillation.