Whenever we want to measure a voltage of a very high voltage transmission lines then we require a transformer with a very high voltage insulation to measure the HV transmission line. But with very high insulation, the cost of the transformer become so much expensive. So here comes out the use of Capacitor Voltage Transformer.
Actually CVT is a type of voltage transformer.
A CVT is used to step down the HV into small measurable LV which is used for the measurement, protection and control of HV system. The measurement or calibration of high voltage from the transmission should have to be accurate.
Design of Capacitor Voltage Transformer:
A Capacitor Voltage Transformer is shown as:
Basically, the CVT consists of 3 main components:
The transmission line connection is divided across these capacitors. We can see in the figure there are 2 capacitors. Capacitor 1 can have a stack of small capacitors which act as voltage or potential divider.They are connected in series.
We can also see that the device has 4 terminals. HV transmission line terminal, ground terminal and 2 secondary terminals.
Working of Capacitor Voltage Transformer:
Note that Capacitor 2 has a higher capacitance than the Capacitor 1. So by the mathematical expression:
Hence impedance Z is lower in the capacitor 2, so voltage drop should also be less. As we know that capacitor 1 < capacitor 2, the voltage drop should also be higher in the capacitor 1 according to above equation. Due to this high voltage drop, less insulation is required for capacitor voltage transformer. This makes the CVT much more economical than others like a wound voltage transformer.
Before discussing the inductor, lets discuss the transformer part in circuit.
An auxiliary LV transformer is used to isolate the circuit and for further stepping down the voltage. Output of voltage divider acts as an input to this transformer. After stepping down we can use this step down voltage for:
- Metering equipment
- Protection Relays etc.
As we can see there is a resistive load in the circuit at the secondary side, the inductor is used as there might be a phase shift due to the use of capacitor. Hence to compensate for this an inductor is used. So we can say that inductor is used to tune load to the frequency of the line.
Usually the wound type voltage divider is used upto 132 kV. Above 132 kV, CVT’s are used.
Hope that the function of capacitor voltage transformer is cleared now.