Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Reexamination Certificate
2000-11-13
2003-08-19
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C340S646000
Reexamination Certificate
active
06609079
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for screening state variables, in particular temperatures, in an oil-cooled transformer.
2. Discussion of Background Information
Transformers are electrotechnical devices which have today already reached a very high level of technical and technological sophistication. Nevertheless, further developments are still possible and necessary, but involve a correspondingly high level of expenditure and risk, including from a commercial viewpoint.
In the currently evolving situation with the free exchange of energy over large distances, the question concerning the extent to which a transformer can be operated with overload without a significant loss in terms of service life is gaining increasingly in importance.
To be regarded as the current state of the art in this respect are the documents U.S. Pat. No. 4,654,804 and FR-A-2 526 599.
The first-mentioned document discloses a method for monitoring and analyzing transformer parameters, such as for example temperatures, winding currents, oil pressure, etc. All the parameters to be monitored must in this case be picked up by use of sensors and they are then compared with predetermined limit values. For this purpose, limit values which do not take into consideration external conditions surrounding the transformer, such as for example the ambient temperature, must be fixed in advance. Since the analysis of all the transformer parameters is based here on a simple comparison, this method also cannot be used for a simulation of specific situations.
The second document relates, inter alia, to an air-cooled transformer, that is to say to a thermopneumatic system and not to a thermohydraulic system, as is present in an oil-cooled transformer.
SUMMARY OF THE INVENTION
An aspect of the invention is consequently to provide a method which supplies information on important temperatures, for example the hotspot and hot-oil temperatures, and can additionally be used for forecasting, simulation and analysis.
This aspect is achieved by input variables, in particular the voltages at the transformer terminals, the currents in the windings and the ambient temperature, being measured, and the status of cooling units, which are fans, pumps etc., and possibly the position of a switch, such as for example a stepping switch or the like being established, by these input variables and the status of the cooling units and the switch position being fed to a thermohydraulic model and by state variables being calculated in the thermohydraulic model with auxiliary variables, which are for example losses in the transformer, parameters for the heat transfer, flow resistances and the flow rates in the oil circuit, and a hydraulic network of the oil circuit, which has branches and nodes, these state variables preferably being the average temperatures and the hotspot temperatures in the loss-producing parts of the transformer and the average oil temperatures in branches and in nodes of the hydraulic network of the oil circuit, and by the auxiliary variables being adapted appropriately to the current value of the state variables when there is a change in the input variables and/or the status of the cooling units and/or of the switch position, and the change over time of the state variables subsequently being calculated and new state variables being calculated with these changes. With this method, it is consequently possible for the first time to ascertain the operating temperatures and also the critical temperatures as well as their changes in parts of the transformer without temperature sensors, whereby the optimum operation of the transformer is ensured. With the thermohydraulic model of the transformer cooled with oil, the behavior of the temperatures in the core, the windings and in the oil is ascertained both in the case of steady-state processes and in the case of transient processes. The suffix “hydraulic” indicates that the hydraulic behavior of the oil is also described by the model.
According to an aspect of the present invention, a method is provided wherein the rate of change of the state variables and the state variables are calculated with differential equations. This aspect is advantageous, since it is the way leading most directly to the aim of ascertaining the rate of change of the state variables.
According to another aspect of the present invention, the measured voltages are used to ascertain the relationship between these voltages and the magnetic flux in the individual parts of the core of the transformer by means of an assignment matrix, and wherein the idling losses are subsequently ascertained in dependence on this magnetic flux by a characteristic curve defined by parameters. The aforementioned aspect makes it possible to calculate on a digital computer the idling losses in the individual parts of the core directly from the terminal voltages.
In yet another aspect of the invention, the transformer-internal winding branches are defined, linked with the measured currents, preferably the terminal currents, by the use of a current assignment matrix. All the currents are represented as a vector with two components because of the necessity to take the phase position into consideration. The amount of the current with which the ohmic losses are calculated via the appropriate resistance is ascertained for each internal winding branch. The distribution of the magnetic leakage flux is ascertained from the distribution of the currents between the individual windings with matrices which are defined both for axial components and for radial components. From the matrices, the eddy-current losses in the windings and the inactive parts of the transformer are ascertained. Also, the relationship between the currents in winding branches and the magnetic leakage flux relevant for the loss-producing branches is ascertained by a matrix, and the eddy-current losses are calculated with factors which are ascertained from the winding geometry. According to the implementational variant discussed above, all the increases in temperature of parts in the transformer caused by the measure currents are ascertained.
In another aspect of the present invention, the matrices and the relevant assignment matrix and ohmic winding resistances are dependent on the switch position. According to a further aspect of the present invention, an initializing operation is carried out with every change, of the switch position, the voltage—magnetic flux assignment matrix, the currents—internal winding currents matrix and the ohmic winding resistances being checked and changed if appropriate. The aspects above take into consideration the influence of the switch position on the distribution of the main magnetic flux the distribution of the internal currents and also on the values of the ohmic resistances of individual winding branches.
In another aspect of the present invention, the status of the cooling units is included in at least one of the parameters of the heat transfer between the oil and the ambience, and the corresponding relationship between the fans and the cooling branches is expressed in an assignment matrix. According to this aspect, the influence of the operating state of the fans on the cooling is described.
According to a further aspect of the present invention, the average temperatures in loss-producing parts of the transformer are calculated with the differential equation:
ⅆ
ⅆ
t
tqm
=
V
tot
-
wim
cqg
wherein tqm is the average temperature of a loss-producing part of the transformer, V
tot
is the total losses of the loss-producing part of the transformer, cqg is the thermal capacity of the loss-producing part of the transformer, and wim is the power loss dissipated from the loss-producing part of the transformer to the oil flowing past. This configuration is advantageous, since with this differential equation the average temperatures in loss-producing parts are ascertained in a simple way.
In another aspect of the present invention, the power loss wim dissipated to the oil flowing past is c
Barlow John
Dougherty Anthony T.
Greenblum & Bernstein P.L.C.
Va Tech Elin Transformatoren GmbH
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