Page 15 - Catalogo FTM 2017

9. Limits of voltage error and phase displacement for measuring voltage transformer, the voltage error % phase displacement (minutes or centiradians) at rated frequency shall not exceed the values given in table 5 at any voltage between 80% and 120% of rated voltage and with burdens:
• any value from 0 VA to 100 % of the rated burden, at a cos φ 1 for burden “range I”.
• between 25 % and 100 % of rated burden at cos φ 0,8 lagging for burden “range II”.
10. Protective voltage transformer, is a voltage transformer intended to provide a supply to electrical protective relays.
11. Limits of voltage error and phase displacement for protective voltage transformer, the voltage error % phase displacement (minutes or centiradians) at rated frequency shall not exceed the values given in table 6 at 5 % rated voltage and at rated voltage multiplied by the rated voltage factor, with burdens of:
• any value from 0 VA to 100 % of the rated burden, at a cos φ 1 for burden “range I”.
• between 25 % and 100 % of rated burden at cos φ 0,8 lagging for burden “range II”.
At 2 % of rated voltage, the limits of voltage error and phase displacement will be twice as high as those given in Table 6.
12. Thermal limiting output is the maximum burden withstood by VT (errors excluded)
13. Ferroresonance phenomenon, in three phase networks with an insulated grounding cable that run through induction transformers connected between a phase and the earth, should there for any reason (earth-fault condition on a phase or switching maneuvers) be sustained excess voltage at network frequencies or at subharmonic frequencies, in association with capacitor components (insulated cables with MT shielding, batteries of rephasing capacitors); these may result in effects known as “ferroresonance phenomena”, which constitute the principal cause of malfunction in voltage transformers.
The principle governing this phenomenon is linked to an increase in magnetic induction of the operating TV, due to the excess load, and therefore to a lessening of the magnetization impedance, whose values approach those of the capacitor’s components. When these two physical parameters are equal, the circuit resonates and oscillations are formed when the excess voltage decreases and which keep the nucleus of the TV saturated, resulting in a high magnetization charge in the primary coils.
An increase in the current will thus cause overheating of the conductor wires of the primary coil and a loss of insulation that eventually leads to a short circuit in the coils.
The oscillations of the three phase currents, in a three phase system, clearly refer to the zero component only, meaning that they do not alter the charges of one phase with respect to the others. If these oscillations are not eliminated, they can persist for extended periods.
The most common precautions taken to avoid this phenomenon are:
A) The manufacture of voltage transformers designed to operate at very low magnetic induction, so as to avoid saturation when overcharged. This means, however, that precision characteristics for TVs manufactured must be lowered.
B) The addition of a suitable resistor, to be connected to the secondary transformer coils in an open triangle manner (see drawing A), which acts to quickly cut off oscillations. This resistor may also be connected in parallel to the safety relays. The indicated resistance and power values recommended are 25 Ohm – 500 W or 100 Ohm -200 W.
15. Reference standard, if not specified in different way:
IEC 61869-1 Instrument transformers , part 1 general requirement.
IEC 61869-3 Instrument transformers, part 3 Additional Requirements, for Inductive Voltage Transformers
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