Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-02-19
2004-03-30
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C340S646000, C361S037000
Reexamination Certificate
active
06714022
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to apparatus and methods for cooling power transformers.
Electric utilities use large power transformers to distribute power (voltage and current) along and within their distribution territory. These “power” transformers handle large amounts of power (e.g. 10 million volt-amperes—10 MVA) and are normally made to have very low winding resistance (Rw). However, at elevated load currents (I), the power dissipation (I
2
Rw losses) in the transformer winding translate into the generation of large amounts of heat which in turn causes the temperature of the power transformer to rise. Accordingly, the temperature of these power transformers increases as a function of the load (power drawn from the transformer) and ambient temperature.
The power drawn through a transformer may increase significantly due to a fault on a distribution line or some other overload condition. In addition, a large increase in the power drawn through a power transformer may occur due to certain operating procedures as illustrated with reference to
FIGS. 1 and 2
.
FIG. 1
shows a block diagram of a substation
10
used to distribute power from a primary source
12
to various loads connected to the substation. Input power from source
12
is coupled via circuit breaker CB
1
to a transformer T
1
and is distributed via a closed switch SW
1
to a station bus section line
14
from which power is then distributed via circuit breakers CB
4
and CB
5
to feeder lines F
1
A and F
1
B to which loads L
1
and L
2
are, respectively, connected. Likewise, input power coupled via circuit breaker CB
2
to a transformer T
2
is distributed via a closed switch SW
2
to a station bus section
16
from which power is then distributed via circuit breakers CB
6
and CB
7
to feeder lines F
2
A and F
2
B to which loads L
3
and L
4
are, respectively, connected.
From time to time the load from one transformer (e.g., T
2
) is switched to another transformer (e.g., T
1
) in accordance with some standard operating procedure, e.g., whenever it is necessary to service power lines or equipment inside and/or outside the substation. By way of example, this is illustrated with reference to
FIG. 2
when switch SW
2
is opened and bus tie breaker CB
3
is closed. Then, all the currents for loads L
1
through L
4
are drawn from T
1
. Because of the increased loading on the transformer (e.g., T
1
), the temperature of the transformer will increase with time and may rise above the ambient temperature by a significant amount. Insofar as T
1
is concerned this load condition would represent a “high” load condition.
Conventional cooling systems rely on sensing the temperature of the power transformer and/or other points representative of the actual transformer temperature. If and when the temperature being sensed rises above a predetermined level, a cooling system is activated; where, for example, the cooling system may include banks of fans blowing air over the transformer or pumps causing cooling oil to be circulated about the transformer windings. However, it should be noted that the power transformers are physically massive devices which have large thermal time constants (e.g., one-half hour). Thus, by the time the free air maximum rating temperature of the transformer is sensed and the cooling system is activated, the temperature of the winding will continue to rise and may exceed the “rating” temperature of the transformer. The temperature of the transformer and its windings may thus continue to rise above critical values giving rise to “service life” problems, as discussed below.
It is important to maintain the temperature of a power transformer at, or below, certain specified temperature ratings because the service life of the transformer is reduced when these specified temperature ratings are exceeded. By way of example, at elevated temperatures the winding insulation begins to breakdown Also, the circulating oil may break down and/or volatile gases may be produced creating potentially hazardous conditions. To ensure that the temperature rating of the transformer is not exceeded a variety of cooling systems (e.g., forced air or circulating oil) may be used, as already noted, to ensure that the winding temperature of the transformer stays below its specified ratings.
As noted above, known methods for controlling the temperature of a power transformer includes sensing the temperature of the transformer and/or making direct temperature measurements of selected points associated with the transformer and then turning on fans for blowing air onto the transformers or causing cooling oil to be circulated. This is not satisfactory because of the potentially large thermal overshoots.
It should also be appreciated that operating the cooling system on a continuous basis is expensive and increases the wear and tear on the cooling equipment. Therefore, it is undesirable to operate the cooling system continuously if such operation is not needed. On the other hand, as just noted, the delay in energizing the cooling system causes the temperature to overshoot which in turn reduces the life of the transformer.
SUMMARY OF THE INVENTION
The problems present in the prior art are mitigated using apparatus and methods embodying the invention. In accordance with the invention the load current drawn from a power transformer is sensed (by sensing the current in the primary or secondary of the transformer) to determine if and when the current exceeds a predetermined threshold. The length of time the load current exceeds the threshold is also sensed. By monitoring the excess current flow and the length of time for which it flows, it is possible to anticipate a rise in the temperature of the transformer and in its winding and to initiate cooling before the transformer and its windings have reached a critical temperature. Thus, systems embodying the invention include means for sensing the current drawn from and/or by a transformer, determining when the current exceeds a predetermined value and timing means for sensing the length of time for which the excess current flows. The timing means are needed, in part, to differentiate between a transitory overload condition and a static, continuous, high load condition.
Applicant's invention thus bypasses the long thermal time constant and enables an appropriate cooling response to be initiated at an early point of a heat cycle which can prevent the transformer temperature from rising significantly above its rated value, thereby extending the useful life of the transformer.
Thus, in systems embodying the invention the turn-on of the cooling system is made a function of the electrical power dissipation which causes heat dissipation which in turn causes a rise in the temperature of the transformer. This is in sharp contrast to the prior art schemes where the temperature of various surfaces or items is sensed to determine when the cooling system is to be turned on.
REFERENCES:
patent: 4360849 (1982-11-01), Harris et al.
patent: 4623265 (1986-11-01), Poyser
patent: 4654806 (1987-03-01), Poyser et al.
patent: 4754405 (1988-06-01), Foster
patent: 4775245 (1988-10-01), Hagerman et al.
patent: 6424266 (2002-07-01), Weekes et al.
Dole Timothy J.
Le N.
Schanzer Henry I.
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