[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

• Input frequency or frequency range
• Input voltage range
• Input watts or volt-amperes
• Input current
• Output voltage
• Output current
• Output watts or volt-amperes
• Maximum and/or minimum ambient temperature
• Schematic diagram or connection information
• Maximum working voltage
• Resonant-capacitor information
2.8.6 New Technology Advancements
Because CVTs are primarily constructed with three major components — magnetic core, magnet wire,
and a capacitor — a consensus exists among a number of leading CVT manufacturers that, while nothing
revolutionary is expected in the near-future, their transformers will continue to be enhanced in many
ways. Whatever CVT innovations do occur will essentially be in improving CVT assembly procedures
and techniques. In general, advances in CVT design are focused on reducing CVT sizes and audible noise
levels and increasing efficiencies at all load conditions.
It is worth mentioning that the CVT user market seems to be moving toward controlled CVTs because
of their very precise output-voltage regulation, their ability to easily adjust the output voltage to exactly
the desired reference required, and their extraordinary immunity to becoming unstable in certain loading
applications. In a number of field situations, controlled 3 CVTS can be customer-adjusted for the specific
application.
2.8.7 Addendum
The following is a tutorial description of the difference between a ferroresonant circuit and a linear
circuit. The main differences between a ferroresonant circuit and a linear resonant circuit are, for a given
• Resonance occurs when the inductance is in saturation.
• As the value of inductance in saturation is not known precisely, a wide range of capacitances can
potentially lead to ferroresonance at a given frequency.
• The frequency of the voltage and current waves may be different from that of the sinusoidal voltage
source.
• Initial conditions (initial charge on capacitors, remaining flux in the core of the transformers,
switching instant) determine which steady-state response will result.
A study of the free oscillations of the circuit in Figure 2.8.21a illustrates this specific behavior. Losses
are assumed negligible, and the simplified magnetization curve (i) of the iron-core coil is that represented
in Figure 2.8.21b. Despite these simplifying assumptions, the corresponding waveforms (see Figure
2.8.21c) are typical of a periodic ferroresonance.
FIGURE 2.8.21
C
a − Schematic Diagram
v
R
K
i
b − Simplified
Characteristic φ(i)
φ
φ max L
φ s
sal
L
i
max
−φ sal
C − Voltage V, Current i and Flux φ as a
Function of Time
v
V 0 V 2
t 0 t 1 t 2 t 3
−V 1
i
I max
φ
φ sal φ max
Originally, voltage at the capacitance terminals is assumed equal to V 0 . At the instant t 0 switch K closes,
a current i is created, and oscillates at the pulsation
1
1
LC
(2.8.5)
The flux in the coil and voltage V at the capacitor terminals are then expressed as:
3
The principles of operation for the controlled CVT are in that the transformer’s output winding is on the same
leg of the magnetic core as the resonant winding, and the resonant capacitor acts to maintain this core section at a
high level of saturation, resulting in a fairly constant voltage. To provide a precise constant voltage, it is necessary to
control this level of core saturation. This is frequently accomplished by shunting the resonant circuit with a solidstate
switching device in series with an inductor.
= (V 0 / 1 sin( 1 t) (2.8.6)
V = V 0 sin( 1 t) (2.8.7)
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC