Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-12-31
2004-10-26
Nguyen, Vincent Q. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S547000, C324S726000
Reexamination Certificate
active
06809525
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to transformers. More particularly, the invention relates to a method and a system for estimating the conductor losses that occur in transformers during operation thereof.
BACKGROUND OF THE INVENTION
Transformers are alternating current (“ac”) devices that transfer energy from one ac circuit to another ac circuit.
FIG. 1
depicts a single-phase transformer
100
. The transformer
100
comprises a laminated iron core
101
, a high-voltage (“HV”) winding
102
, and a low-voltage (“LV”) winding
104
. The HV and LV windings
102
,
104
arc each wound around the core
101
. The HV winding
102
can be electrically coupled to an external ac power source (not shown), and the LV winding
104
can be electrically coupled to an external load (not shown) during normal (in-service) operation of the transformer
100
.
Energizing the power source causes an alternating current to flow within the HV winding
102
. The alternating current induces an alternating magnetic flux within the core
101
. The core
101
conducts the magnetic flux to the LV winding
104
. The magnetic flux induces a voltage across the LV winding
104
. The voltage across each of the HV and LV windings
102
,
104
is proportional to the number of turns in the respective HV and LV windings
102
,
104
. The resulting current in each of the HV and LV windings
102
,
104
is inversely proportional to the number of turns in the respective HV and LV windings
102
,
104
.
Various losses occur in the transformer
100
during operation thereof. These losses are typically classified as “core” losses and “conductor” (copper) losses. Core losses result from the alternating magnetic flux within the core
101
. More particularly, the alternating magnetic flux causes eddy currents and hysteresis within the core
101
, which decrease the amount of energy transferred from the HV winding
102
to the LV winding
104
. Conductor losses result from the resistance of the HV and LV windings
102
,
104
to the flow of current therein (losses of this type are commonly referred to as “IR
2
” losses). These losses represent energy losses that occur during the transformation of power by the transformer
100
, and can contribute substantially to the operating cost of the transformer
100
.
In a competitive sales environment, the decision of a potential customer whether to purchase a transformer such as the transformer
100
is often based on the concept of total owning cost. In other words, the purchaser typically seeks the lowest combination (sum) of initial purchase price and projected operating cost over the life of the transformer. The purchaser is usually willing to pay more for a transformer having a relatively low estimated operating cost. Conversely, a transformer with a relatively high estimated operating cost will usually sell for a lower price. Hence, transformer manufacturers are subject to an economic penalty for transformers having relatively high operating costs.
Most purchasers of transformers such as the transformer
100
expect to receive a certified report from the manufacturer documenting the operating cost of the transformer, reflected in current monetary terms. Hence, transformer losses are usually measured by the manufacturer prior to shipping the transformer to the purchaser.
Conductor losses are typically measured using a so-called “short-circuit” test. The short-circuit test is performed by placing a “shorting bar”
50
(or other suitable electrical conductor) across one of the windings (typically the LV winding
104
) of the transformer
100
(see FIG.
1
). In practice, the shorting bar is electrically coupled to a first and a second bushing
106
,
107
of the transformer
100
. The first and second bushings
106
,
107
are normally used to electrically couple the external load to the LV winding
104
during normal operation of the transformer
100
.
A suitable ac power source
51
, wattmeter
52
, and ammeter
54
are electrically coupled to one of the windings
102
,
104
(typically the HV winding
102
) of the transformer
100
as shown in FIG.
1
. In practice, the wattmeter
52
is electrically coupled to a third and a fourth bushing
108
,
109
of the transformer
100
. The third and fourth bushings
108
,
109
are normally used to electrically couple the external power source to the HV winding
102
during normal operation of the transformer
100
.
Energizing the power source
51
causes an alternating current to flow through the HV winding
102
. Preferably, the voltage produced by the power source
51
is adjusted so that the alternating current flowing through the HV winding
102
(as measured by the ammeter
54
) is approximately equal to the rated current for the HV winding
102
. The alternating current within the HV winding
102
induces an alternating magnetic flux in the core
101
. The core
101
conducts the magnetic flux to the LV winding
104
. The magnetic flux induces a voltage across the LV winding
104
.
The LV winding
104
is short-circuited by the shorting bar
50
. The induced voltage across the LV winding
104
therefore causes a current to flow through the LV winding
104
.
The wattmeter
52
measures the power delivered to the HV winding
102
. The power delivered to the HV winding
102
is approximately equal to the conductor losses of the transformer
100
(including the conductor losses associated with the LV winding
104
) and, in addition, the power losses associated with the shorting bar
50
.
The power losses associated with the shorting bar
50
include the losses caused by the resistance of the shorting bar
50
, i.e., the IR
2
losses of the shorting bar
50
. The losses associated with the shorting bar
50
also include the losses caused by the contact resistance between the shorting bar
50
and the first and second bushings
106
,
107
.
The power losses associated with the shorting bar
50
are typically included in the value of the conductor losses used to estimate the operating cost of the transformer
100
. Thus, the transformer manufacturer is subject to an economic penalty, in the form of a lower purchase price, due to the inclusion of the power losses associated with the shorting bar
50
with the estimated conductor losses.
SUMMARY OF THE INVENTION
A preferred method for estimating conductor losses in a transformer having a first and a second winding comprises energizing the first winding while the second winding is short-circuited by an electrical conductor so that power is supplied to the first winding and a portion of the power is dissipated due to a resistance associated with the electrical conductor. A preferred method also comprises measuring the power supplied to the first winding, calculating the portion of the power dissipated due to the resistance associated with the electrical conductor, and subtracting the portion of the power dissipated due to the resistance associated with the electrical conductor from the power supplied to the first winding.
A preferred method for estimating conductor losses in a transformer comprises supplying power to a first winding of the transformer while a second winding of the transformer is short-circuited by an electrical conductor, and measuring the power supplied to the first winding. A preferred method also comprises calculating power dissipated by the electrical conductor in response to supplying power to the first winding, and subtracting the power dissipated by the electrical conductor from the power supplied to the first winding.
Another preferred method for estimating conductor losses in a transformer comprises electrically coupling an electrical conductor to a first and a second end of a first winding of the transformer, and energizing a second winding of the transformer. A preferred method also comprises measuring power delivered to the second winding, calculating power dissipated by resistance associated with the electrical conductor in response to energization of the second winding, and subtracting the power dissipated by the resistance associated wit
ABB Technology AG
Nguyen Vincent Q.
Woodcock & Washburn LLP
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