Capacitive Voltage Transformers
(72.5 kV and 170 kV)
Our CVTs and here to the requirements of the International quality standards and our quality and environment management system, safety management systems are certified to ISO 9001–2000, ISO 14001 and ISO 18001 respectively.
Design and Construction
Each CVT consists of a coupling capacitor (CC) which acts as a voltage divider and an Electro Magnetic Unit (EMU), which transforms the medium voltage to standard low voltage. Depending on the system voltage the CC can be a single or a multi stack unit. The CC and the EMU are individually hermetically sealed to ensure accurate performance and high reliability.
Coupling Capacitor
The Coupling Capacitor (CC) acts as a voltage divider and converts the system voltage to a medium voltage. The active part of the CC consists of a large number of oil impregnated paper (paper and film) capacitor elements connected in series.
Super calendared capacitor tissue paper and pure aluminum foils are used to make the capacitor elements. The capacitor elements are pressed and held in insulating supports to ensure a stable capacitance even for large temperature variations. The electrical connections between the capacitor elements are designed for a natural frequency much above 600 KHz in order to avoid interference with carrier communication.
The processed capacitor stack is assembled inside a porcelain insulator with corrosion resistant aluminum alloy end fittings. Brown glazed porcelain insulators with shed profile as per IEC 815 are used.
The insulators are cemented to aluminum alloy flanges for improved strength. Oil volume changes due to temperature variations are compensated by a stainless steel bellow installed at the upper end of the CC. The unit is completely filled with degassed insulated oil under vacuum. The bellow is pressurized by inert gas (from the top surface) to maintain a positive oil pressure even at lowest ambient temperatures. The CVT thus has very low PD levels even at low ambient temperatures.
Electromagnetic Unit
The Electromagnetic Unit (EMU) consists of a medium voltage transformer, compensating reactor, damping element and surge protection device. The unit is housed inside a steel tank, which is filled with insulating oil leaving a largely dimensioned air cushion at the top in order to take care of changes in the oil volume due to fluctuations in the ambient temperature. An oil level indicator is mounted on the side wall of the tank.
The CC unit is mounted on the EMU tank and the insulated earth terminal of the CC is also accessible for connecting to power line carrier communication equipment. A surge arrester across this terminal and earth serves as the surge protection device. The NHF terminal must always be connected to earth if the CVT is not connected to carrier equipment.
The secondary terminal box is provided on the EMU tank. The EMU is calibrated and adjusted at factory for all burden and accuracy requirements. No site adjustments or measurements are necessary. The EMU is given adequate surface treatment for corrosion protection for life long service.
Maintenance
The product is self-contained, maintenance free and requires no spares over its entire life span. We recommend regular and periodic checks as per pre-specified schedules (specified in the Instruction Manual supplied with the CVTs).
Optional / Accessories
- Terminal Connector (Aluminum/Bimetallic, NEMA or as per customer specifications)
- Three element Carrier Protection Device Level (comprising Drain Coil, surge Arrester & Earth Switch)
- Cable Glands
