capacitive voltage transformers Characteristic by hamada1331


									             Characterization of the capacitive voltage transformers
                 as coupling paths and measuring devices for
                  the high-voltage transients in a substation

  S. Coatu, D. Rucinschi, M. Costea, A. Marinescu                  V. Branescu
      University “Politehnica” of Bucharest                        Electrica S. A.


In frame of the project “TRANSDIST - Transient disturbances in romanian open-air high-voltage substations”,
the first step was directed to the problem of transient disturbances transmitted through instrument (voltage
and current) transformers to the complex and distributed measuring and protection systems.
As in Fig. 1, the events that can produce such disturbances are switching operations of circuit breakers or
disconnectors, as lightning strokes on or nearby the high-voltage (H.V.) transmission lines connected to bus-
bars of the substation, followed or not by breakdowns of the H.V. insulation or by the work of surge arresters.
By magnetic and electric coupling mechanisms, the fast transients that appear on H.V. terminals of
instrument transformers are transmitted to the auxiliary, measuring and protection circuits, connected to their
secondary terminals and can affect seriously theirs working.
According to the present stage of the project, this contribution deals mainly with two aspects:
a) Laboratory experiments regarding the transmission of lightning and switching impulse voltages through a
     capacitive voltage transformer (C.V.T.);
b) The development of measuring device based on C.V.T.’s there are in substation, for the monitoring of
     H.V. transient overvoltages.


The basic diagram of a 110 kV rated voltage C.V.T. is given in Fig. 2. It consists mainly in a capacitive
voltage divider C1-C2 and a medium-to-low voltage transformer. An auxiliary terminal may be used for
telecommunication between substations, with a high-frequency carrier in the range of tens of kHz, having as
support one conductor of H.V. transmission line. In this case, the switch S is open and the earthing reactor L3
assures the 50 Hz grounding of capacitive voltage divider. Regarding to the secondary terminals 1a and 2a,
the 50 Hz ratio of C.V.T. is k50 = 1100.
The maximum peak value of overvoltages between the H.V. terminal and ground is in range of 270...330 kV,
depending of the protection level assured by the surge arresters of substation.
    a) The transmission of slow front transient overvoltages was investigated for applied impulse voltages
with various rise times, between 15 μs and about 100 μs and peak values, between 130 kV and 260 kV, the
time to half-value being practically constant with a value of about 220 μs. The H.V. was applied between the
H.V. and the ground terminals of C.V.T., the transmitted voltage was recorded at the secondary protection
terminals 2a, connected to a resistor equivalent to the rated load.
Fig. 3 gives a sample from the applied and transmitted overvoltages.
The acquired results pointed out:
      • The ratio between the peak values of applied and transmitted voltages, ks is 1.4 ... 1.5 greater than
     the power-frequency ratio, k50; it takes grater values for smaller rise times of applied voltage;
      • The rise times of transmitted voltages are between 6 and 23 times grater than those of the applied
     voltages, the bigger values corresponding to faster of the applied voltages;
      • The time to half-value of transmitted voltages are with about 30% greater than the applied voltage
      • In case of an applied voltage of maximum 330 kV; peak value, the transmitted overvoltages may
     reach about 220 V, respectively 2.7 times the rated power-frequency peak value.

                Fig. 3. Applied (a) and transmitted (t) slow front transient overvoltage
                             (a): Tr = 117 µs; T0.5 - 0.5 = 2290 µs; U = 132 kV

                            (t) :   Tr = 743 µs;   T0.5 - 0.5 = 2770 µs; U = 89 V

All these data show that the transmission of slow front transient overvoltages proceeds, after a reduction
given by the capacitive divider of C.V.T., mainly by an inductive coupling between the primary and secondary
windings of output transformer. This coupling is weaker for the higher frequencies corresponding to faster
b) The transmission of fast front transient overvoltages is illustrated in Fig. 4.
The transmitted voltage has an entirely different shape that the applied voltage, characterized by a strong
oscillation, with a ground frequency about 12 MHz and a duration about 1.5 μs, equal with the time to crest of
applied impulse voltage, followed by a very long tail, in range of hundred of μs, but with meaningful smaller
The shape of transmitted voltage suggests a capacitive coupling of the high-frequencies during the front of
applied voltage, from the output of capacitive divider, through the inter-turns capacities of medium voltages
choke L0 and the capacity between the primary and secondary windings of output transformer.

The ratio between the peak values of applied and transmitted voltages, kF is about 1.3 greater than the
power-frequency ratio k50. In case of an applied voltage of maximum 330 kV, peak value, the transmitted
overvoltage may reaches about 235 V, respectively about 2.9 times the rated power-frequency peak value.


To characterize, in a statistical approach, the share of H.V. events at the electromagnetic environment of an
open-air substation, a long-term record of transient overvoltages may be useful. The main difficulty, to fulfill a
suitable recording system, regarding the costs and locations, in a substation is that of H.V. impulse divider.
The proposed solution is based on a modification of existing C.V.T.’s consisting in; (Fig. 5):
     • The removal of earthing reactor L3;
     • The addition to C.V.T. of a measuring capacitor, between high-frequency terminals, as a low-voltage
      arm of a capacitive voltage divider, as high-voltage arm acting the C.V.T.’s capacitors C1 and C2.
 This relatively cheap solution doesn’t need an additional place in substation and doesn’t disturb the basic
 functions of C.V.T., regarding the measuring and the protection.

 Fig. 6 and Fig. 7 shows, comparatively, the applied and the measured voltages, for a standard lightning
 impulse 1.2/50 μs, respectively a standard switching impulse 250/2500 μs.
 In case of the standard lightning impulse, deviations of the measured voltage against the applied voltage
     - for the front time             - 7.7%:
     - for the half-value time         - 2.3%.
 The crest value ratio was 550.
 In case of the standard switching impulse, the respective deviations were:
     - for the front time             - 1.9%;
     - for the half-value time         - 3.4%.
 The measuring device preserves the crest value ratio of 550.
 The registered deviations, regarding the time’s parameters are in acceptable tolerances for such
  measurements, /1/.


The planned future works are:
    • Characterization of current instrument transformers as coupling path for high-voltage transients in a
    • Development of frequency-dependent coupling mechanism models for H.V. instrument transformers;
    • Completion of transient overvoltages monitoring system, based on existing C.V.T.’s, with an
   adequate acquisition system;
    • Implementation the overvoltage monitoring system, in a representative substation and starting a
   monitoring program;
    • Tests, with simulated disturbances, on measuring and protection systems of a substation.

/1/ I.E.C. Standard 60-2: High-voltage test techniques. Part 2: measuring systems. 2 edition, 1994.


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