applied to the primary winding as well as the larger capacitance
that was used for the primary winding application. Using
(2) with the magnetizing inductance of 230 mH seen on the
primary winding and the device capacitance of 22 mF used
for the application yields a resonance frequency of 2.2 Hz,
or a quarter resonance period of approximately 110 ms, very
close to the quarter period measured in Fig. 8, for the primary
winding application.
Ultimately, as long as the proper amount of volt-seconds is
supplied to the transformer, there is great flexibility in the initial
voltage level of the prefluxing device capacitor. In applying
the device to a 230-kV winding of a substation transformer, the
capacitor’s voltage can be a fraction of the size, allowing the
capacitor to be charged from the substation ac or dc supply.
Assume that a single-phase 100-MVA transformer with a primary
winding rated for 132 kV has a magnetizing inductance of
9 H. If the capacitor is charged using a 125-V dc supply, then (4)
solves for a needed capacitance of roughly 1.75 F. This amount
of capacitance is readily achievable using high Farad capacitors.
The prefluxing capacitor would store approximately 13.8 kJ of
energy. Using a 100-W power supply, it would take approximately
2.5 min to charge the prefluxing capacitor.
C. Approximating
A key piece of information needed to produce a prefluxing
design is the transformer’s magnetizing inductance. While manufactures
include this information when new transformers are
purchased, the data for older units may no longer be available,
and is of little consequence since the magnetizing branch is
ignored in most power system calculations. Conveniently, the
transformer’s magnetizing inductance can be approximated for
the purposes of sizing the prefluxing device.
Newer transformers use highly efficient designs which can
operate with a magnetizing current of 1% or less of the rated
current. However, there are many older transformers still in use
today whose magnetizing current is significantly higher, between
2%–4% of the rated current [8]. If the magnetizing current
of a given transformer is estimated at 5% of the rated current,
it will result in an estimated magnetizing inductance that is
smaller than that which actually exists within the transformer.
This estimation will result in a slightly oversized prefluxing design
that would then be solved using the actual .
The following equation gives the magnetizing inductance of
a transformer assuming a magnetizing current is equal to 5% of
the transformer’s rated current:
(5)
where is the rated power of the transformer.
Once the magnetizing inductance is approximated with (5),
the value is then used in (4) to complete the design of the prefluxing
device.
The approximate design technique was tested on the laboratory
transformer with satisfactory results. With the voltage and
power rating of the primary winding, the approximated magnetizing
inductance is 167 mH, a reduced value from the measured
(230 mH), as expected with the approximation. Using the same
applied to the primary winding as well as the larger capacitance
that was used for the primary winding application. Using
(2) with the magnetizing inductance of 230 mH seen on the
primary winding and the device capacitance of 22 mF used
for the application yields a resonance frequency of 2.2 Hz,
or a quarter resonance period of approximately 110 ms, very
close to the quarter period measured in Fig. 8, for the primary
winding application.
Ultimately, as long as the proper amount of volt-seconds is
supplied to the transformer, there is great flexibility in the initial
voltage level of the prefluxing device capacitor. In applying
the device to a 230-kV winding of a substation transformer, the
capacitor’s voltage can be a fraction of the size, allowing the
capacitor to be charged from the substation ac or dc supply.
Assume that a single-phase 100-MVA transformer with a primary
winding rated for 132 kV has a magnetizing inductance of
9 H. If the capacitor is charged using a 125-V dc supply, then (4)
solves for a needed capacitance of roughly 1.75 F. This amount
of capacitance is readily achievable using high Farad capacitors.
The prefluxing capacitor would store approximately 13.8 kJ of
energy. Using a 100-W power supply, it would take approximately
2.5 min to charge the prefluxing capacitor.
C. Approximating
A key piece of information needed to produce a prefluxing
design is the transformer’s magnetizing inductance. While manufactures
include this information when new transformers are
purchased, the data for older units may no longer be available,
and is of little consequence since the magnetizing branch is
ignored in most power system calculations. Conveniently, the
transformer’s magnetizing inductance can be approximated for
the purposes of sizing the prefluxing device.
Newer transformers use highly efficient designs which can
operate with a magnetizing current of 1% or less of the rated
current. However, there are many older transformers still in use
today whose magnetizing current is significantly higher, between
2%–4% of the rated current [8]. If the magnetizing current
of a given transformer is estimated at 5% of the rated current,
it will result in an estimated magnetizing inductance that is
smaller than that which actually exists within the transformer.
This estimation will result in a slightly oversized prefluxing design
that would then be solved using the actual .
The following equation gives the magnetizing inductance of
a transformer assuming a magnetizing current is equal to 5% of
the transformer’s rated current:
(5)
where is the rated power of the transformer.
Once the magnetizing inductance is approximated with (5),
the value is then used in (4) to complete the design of the prefluxing
device.
The approximate design technique was tested on the laboratory
transformer with satisfactory results. With the voltage and
power rating of the primary winding, the approximated magnetizing
inductance is 167 mH, a reduced value from the measured
(230 mH), as expected with the approximation. Using the same
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