A capacitor voltage transformer consists of a Capacitor Voltage Divider (CVD) and an inductive Intermediate Voltage Transformer (IVT). The IVT voltage level of ABB’s ca-pacitor voltage transformers is about 22/√3 kV, and the rated voltage of the complete capacitor voltage transformer determines the ratio at the capacitor voltage divider.
Figure 7.0
It is more convenient to make an Inductive voltage transformer for lower voltage levels and let the CVD take care of the high voltage.
The ratio of the capacitive divider is
The ratio of the intermediate voltage transformer is
The total ratio factor is therefore
K1 is normally chosen to give E2 = 22/√3 kV. Thus for different primary voltages, only C1 differs and a standard intermediate transformer can be used for all primary voltages. The intermediate voltage transformer (IVT) also contains reactors for com-pensation of the capacitive voltage regulation.
The capacitor voltage transformer has a double function, one for metering/protec-tion and one for power line communicametering/protec-tions (PLC).
C1 C2 E1
E2 E3
CVT quality depends on formula:
A high performance CVT the Q must be >10.
Figure 7.1
Principle diagram for a capacitor voltage transformer
7.1 External disturbances on capacitor voltage transformers
Pollution
External creepage currents due to pollution on insulators can influence the accuracy of a capacitor voltage transformer. When a porcelain insulator is divided into several parts, there can be different creepage currents in each part of the capacitor voltage divider. This has an effect on voltage dividing in the capacitor and results in a ratio error. The proportion of errors is difficult to estimate, as it is not easy to measure the different creepage currents. High capacitance in the voltage divider makes it less sensitive to pollution.
C1 C2
Stray capacitance
The effect of stray capacitance from equipment erected nearby on accuracy is negligible. If two 420 kV capacitor voltage transformers are erected at a distance of 1.25 m from each other, the ratio error of ABB’s high capacitance CVT due to the other capacitor voltage transformer will be 0.01%. Normally the phase distance is longer than 1.25 m. Therefore, the high capacitance of the voltage divider has a positive effect on the accuracy.
7.2 Mechanical stress on capacitor voltage transformers
As with current transformers the capacitor voltage transformers are also exposed to similar mechanical stresses from forces in the primary terminals, wind and earth-quakes. The load on the primary terminals is normally lower than that on a current transformer. The connection lead weighs less because of the very low current in the voltage transformer, which can be transferred by a thinner wire. A typical requirement on static and dynamic load is 1000 N with a safety factor of 2. The limitation of the stress is the bending moment in the porcelain insulator. A typical value for a standard insu-lator is 25 kNm (Tmax), but it is possible to obtain a higher value.
where
Fmax Maximum horizontal force on the primary terminal (kN) S Safety factor, usually 2
H Height of the capacitor (CVD) (m)
Tmax Maximum bending strength of the porcelain insulator (kNm)
Fmax must be reduced according to the wind load.
The following part describes the calculation of wind load on a capacitor voltage transformer. The additional wind load with line traps placed on top of the capacitor voltage transformer is also of importance.
Wind pressure (W) will be specified for cylindrical surfaces:
where
V Wind speed (m/s) (IEC 34 m/s)
7. Design of capacitor voltage transformers
We assume that the capacitor voltage transformer has a cylindrical shape. The force (F) will take up half the height of the CVD:
where
D1 Medium diameter of the porcelain
The moment (M) will be:
If the capacitor voltage transformer is provided with line traps; this must also be taken into account:
where
Hs Line trap height (m) Ds Line trap diameter (m)
Ds
D1
Hs
H v W
7. Design of capacitor voltage transformers
7.3 Seismic properties of ABB’s capacitor voltage transformers
The requirements on the seismic withstand capability of capacitor voltage trans-formers usually depend on the local seismic conditions. This means that a particular calculation has to be performed in each individual case. Some general rules and also a few examples of requirements ABB has been able to fulfill are given below.
Due to its slender shape, a capacitor voltage transformer is more sensitive to hori-zontal movements than vertical ones. Generally, if a capacitor voltage transformer can withstand the horizontal requirements, the vertical requirements are usually satisfied automatically.
The difficulties in meeting earthquake requirements increase rapidly with the height of the CVD, i.e. the system voltage.
Response spectra
If the requirements, formulated as an acceleration spectrum of a probable earth-quake, have to be met at a certain safety factor, a CPA or CPB will always with-stand 0.3 g horizontal ground acceleration according to IEC spectra with a safety factor of 2.0 up to 550 kV highest system voltage.
For particularly heavy applications a capacitor voltage transformer with stronger porcelain can be designed to withstand 0.5 g horizontal and 0.3 g vertical accelera-tion with a safety factor of 2.0.
Resonance frequency tests
If the requirement says that the CVT shall withstand mechanical testing at its reso-nance frequency, the damping will be the most important factor in the calculation.
Earthquake calculations will normally be performed from case to case depending on the requirements. As these calculations could be complicated to perform, modern cost-saving computer programs have been developed. With help of these programs high precision calculations can be made. However, some tests must still be per-formed to verify these calculations.