Capacitive Voltage Transformers are large-scale measuring transducers. They are intended for operation in AC electric systems of frequency 50 Hz.
The main distinction feature of these transformers is that secondary voltage is proportionally to the primary voltage. Thus, a voltage of electricity transmission lines is transformed with a definite transformation ratio into a secondary one. A deviation during transformation (error) is determined by accuracy class of secondary windings.
Two-step voltage reduction lies in the basis of CVT operation principle. Capacitive voltage divider is used as the first step. The second step is a transformer of electromagnetic unit (hereafter – EMU).
For maximum measurement accuracy securing a combined paper-film dielectric is used in a voltage divider design, impregnated with a synthetic dielectric fluid. This combination permits to keep a required capacitance value in all ranges of working temperatures from + 40°С up to -60°С.
Elements made from paper-film dielectric are assembled into packets and placed into a porcelain or composite insulator filled with impregnating fluid. Impregnating fluid is ecologically safe and biodegradable. Metal bellows from stainless steel are installed on dividers for compensation of thermal expansion of dielectric and also for prevention of insulators explosion caused by emergency pressure growth.
The main purpose of CVT is a commercial electric energy accounting as well as a transmission of measurement information signal to indicators, protection and control devices. With the help of these transformers it is also possible to organize a high frequency communication at the frequencies 30 – 1000 kHz, for that there is an output on the back panel of transformer for connection of communication equipment.
Electromagnetic unit is an intermediate transformer placed in a hermetic metal tank fulfilled with transformer oil. Electromagnetic unit tank is made from steel with anticorrosive coating. Secondary windings outputs are placed in a special box on the front panel of tank, at that the secondary windings outputs are intended for commercial accounting. There is a possibility of sealing.
In case of need it is possible to produce a transformer with different power values of secondary windings.
Transformer can be used at temperatures from +40 up to -60 °С, at heights up to 1 000 m from the sea level. The standard creepage distance of transformer corresponds to a high degree of pollution III - 2,5 cm/kV. In case of need it is possible to produce the transformer for installation in regions with very high degree of pollution IV—3,1 cm/kV.
Types and basic characteristics are represented in the table:
|Type||Urated primary, V||Urated secondary, V||Frequency, Hz||Accuracy class – power, VA||Divider capacitance, nF||Weight, kg|
* It is possible to produce secondary windings with different power values as per customer’s approval.
Overall dimensions (Figure 1)
| Н max
|h**, mm||L**, mm||В, mm||b, mm||Figure|
CVT ultimate power is 1 000 VA.
** Depending on output availability for connection to high-frequency equipment, it is possible to increase overall dimensions of electromagnetic unit and oil weight by 10%.
Each transformer is equipped with a high-voltage terminal made of aluminum to connect a hardware clamp with four holes (figure 2). In agreement with the customer, it is possible to produce high-voltage output with four or six holes, as well as the choice of output material: aluminum or copper (figure 2 and 3).
Output for connection of high-frequency communication equipment, cable inputs, ground output
The terminals of the secondary windings with the possibility of sealing
The grounding switch to measure capacitance and tangent of dielectric loss angle of capacitor units
Copper tinned terminal for hardware clamp connection
Aluminum electrostatic shield for reducing corona discharge at a voltage of 500 kV and more
HANDLING AND INSTALLATION OF CVT
Transport packing of CVT-500 UHL1
Installation of CVT-220 UHL1, Ul’ken village, substation 500 kV "YUKGRES", "UKCP", Ltd.
The rise of the transformer is carried out by soft slings with the help of eyelets on the EMU tank with strapping at the top of the divider to avoid its tipping.
Voltage transformers of CVT type have been tested in accredited testing laboratories, including:
– Test center of high electrical branch of JSC "STC FGC UES" – SibNIIE (Novosibirsk);
- Test center of high-voltage electric equipment of FSUE "All-Russian electrotechnical Institute of V. I. Lenin" (Moscow);
- PJSC "Ukrainian research design and technological Institute of transformer construction "PJSC" VIT " (Zaporozhye).
- Test center of the electrotechnical equipment and consumer goods of UKCP Ltd. (Ust-Kamenogorsk).