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Saturday, 20 September 2014

Extra High Voltage measurement using Capacitor Voltage Transformer

In power sub-stations, and testing laboratories, it is necessary to measure the extra high voltages with accuracy. While measuring these enormous voltages, it is essential to ensure the safety of working personnel and related equipments. 
Capacitor Voltage Transformer (CVT) or Capacitance Coupled Voltage Transformer (CCVT) is typically single-phase devices used for measuring voltages exceeding 100 kV and where the use of Potential Transformers (PT) becomes uneconomical. 

CVT or CCVT convert the transmission class voltages for the purpose of metering and protection or in other words these devices isolate the measuring instruments, meters, relays etc from High Voltage (HV) or Extra High Voltage (EHV) circuit or transmission line and provide a low value replica of the original voltage. CVT also serves as coupling capacitors for coupling high frequency Power Line Carrier (PLC) signals to the transmission lines.

Construction of Capacitor Voltage Transformer:

CVT consists of series connected capacitor elements housed in a hermitically sealed porcelain or silicon rubber shells. These capacitors acts as potential dividers and steps down the voltage to be measured to an intermediate value of about 5 to 12 kV. This voltage is fed to an electromagnetic circuit consisting of an auxiliary transformer which gives the final reduced secondary voltage (say 110 V). 

The dielectric of the capacitor is made up of polypropylene film or paper impregnated with synthetic fluid. Stainless steel bellows are provided in each capacitor section and these bellows allow for the expansion and contraction of the insulating fluid taking place because of the change in the operating temperature.

CVT has at least 4 terminals; one connected to the HV or EHV terminals, second to the ground terminal and two secondary terminals which are connected to the measuring instruments or protective relays as shown in the figure below. 

Many renowned manufacturers including Siemens make an entire range of Instrument Transformers from 72.5 kV to 800 kV class consisting of Current Transformers (CT), Potential Transformers (PT), Capacitor Voltage Transformer (CVT) and Combined CT and PT units.

The main advantages of CVT are:
·         High accuracy because of highly stable capacitance.
·         Maintenance-free performance throughout the life.
·         Robust and reduced size.
·         Reliable ferro-resonance suppression system.

The 1200 kV test station at Bina (M.P.) has installed 1200 kV CVT manufactured by Siemens at its Aurangabad plant.