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Transient of capacitor voltage transformer

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					Transient Performance of
Capacitor Voltage
Transformers (CVT)


Introduction                           A CVT system can be represented
                                       by its Thevenin equivalent circuit.
In steady state, the secondary volt-   The analysis of a simplified L-C-R
age of a capacitor voltage trans-      circuit for the case of a sudden
former mirrors the primary voltage.    collapse of the primary voltage
Because of the energy-storage          (assuming a given set of initial
elements used, the transient out-      conditions) produced the following
put, caused by sudden change in        conclusions:
primary voltage, is no longer a rep-
lica of the primary. This distorted    1 If the equivalent capacitance Ce
output may last for a few cycles of      is larger and assuming a fixed
the supply frequency. These errors       value of the burden, the magni-
may be termed as the CVT linear          tude of the residual transient
transient since they would still         voltage becomes smaller. The
occur in the event that all of the       intermediate voltage should also
components of the CVT were line-         be maximized in order to achieve
ar. In addition to this, ferroreson-     a high value of the equivalent
ance may occur resulting in volt-        capacitance.
age measurement error. Ferroreson-
ance is caused by the saturation       2 The effect of the burden is so
of the electromagnetic unit or           profound that it can dominate
inductive burden. Unlike the linear      the CVT behavior. The burden
transient it does not occur with         configuration is insignificant
every change in primary voltage.         since it determines the way in
                                         which energy can be stored and
                                         the path through which it can
Linear Transient Error                   discharge. Pure resistive burden
(Transient Response)                     dissipates and does not store
                                         energy. Stored energy burdens
The linear transient error is mainly     (lagging or leading) create harm-
influenced by the following factors:     ful effect on the transient
                                         response producing oscillations
1 The Thevenin equivalent capaci-        of low frequency nature.
  tance referred to the secondary
  voltage level, Ce.                   The oscillograms shown below
2 The burden value and configura-      illustrate the variation in the
  tion.                                residual voltages as a function of
                                       burden at crest fault initiation. At
3 The point of wave voltage inter-     zero fault initiation, the transient
  ruption.                             response worsens.




E 214.60
3 During the sudden application
  or sudden disappearance of
  primary voltage, the distribution
  of energy stored in the CVT is
  a major factor in understanding
  the effect of the point of wave
  voltage interruption. The tran-
  sient performance in those
  cases is similar with the transi-
  ents in the case of the primary
  short circuit.

The amount of energy accumula-
ted by the capacitor at zero in-
itiation is equivalent to the energy
stored by the series reactor at
crest voltage initiation.

The discharge time constant will
vary depending upon where the
maximum energy is stored. A
typical example of transient res-
ponse ( see below) indicates the
worse case with the maximum
energy on the capacitor at the
time the primary voltage wave is
going through the zero voltage
interruption.

				
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posted:4/5/2012
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