Method of monitoring the contact burnoff in tap changers

Details Method of monitoring the contact burnoff in tap changers Method of monitoring the contact burnoff in tap changers

2001-01-29

2003-02-11

Le, N. (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S421000, C200S0110TC

Reexamination Certificate

active

06518771

ABSTRACT:

FIELD OF THE INVENTION
Our present invention relates to a method of monitoring the contact burnoff of a tap changer and especially the burnoff of contacts which tend to arc in tap changers.
BACKGROUND OF THE INVENTION
Tap changers have been used for a considerable time for the uninterrupted switching between taps of a tap transformer in electrical power distribution and such tapped transformers and their tap changers are utilized in large number throughout the world. The tap changer is utilized to select the transformer winding which is to be effective and has been designed to allow such switching under load. The tap changer for tap selection under load generally comprises switching contacts and resistance contacts. The switching contacts can directly connect the particular tap and section of the transformer winding, with the lines running to the load. The resistance contacts are briefly connected in circuit and bridge resistance into the circuit to allow uninterrupted tap selection under load. In recent years the tap changer could be equipped with thyristors (electronic switch devices) and vacuum switch cells as the switching elements but by far the greatest number of tap changers in use today and in the near future utilize mechanical contacts which are subject to burn off by the switching arc which may be formed.
To minimize the contact burnoff, the switching contacts and the resistance contacts can be composed of arc-resistant copper-tungsten alloys. Nevertheless, upon switchover of the contacts arcs are generated which can melt small quantities of the contact material and cause burnoff and evaporation of some of the contact material. The result is a contact burnoff which is an important criterium in the maintenance and operation of a tap changer. The contacts in the past have been frequently inspected and determinations as to burnoff have been made. The burnoff in the switching and resistance contacts is a significant consideration in the operation of the tap changer. If the contacts burn off at different rates, the switching and overlapping or bridging intervals of the individual switching steps can vary within the sequence in a tap changing operation so that the tap changer if the contacts burn off at different rates, the switching and overlapping or bridging intervals of the individual switching function can become unreliable. In general, the burnoff will have a maximum permissible burnoff difference or burnoff limit. If these values are exceeded the contacts must be replaced by new contacts or the resistance contacts and the switching contacts must be interchanged. When contacts are completely burned off, they must be replaced immediately.
There are numerous processes available for contact burnoff or contact wear monitoring whereby the residual life of a contact or some other similar factor can be reviewed for switching contacts and tap changers or other high voltage switching contacts. These can be considered in different groups.
For example, DE-GM 296 19 365 and EP 0 948 006 provide a purely optical process for determining residual life or burnoff state.
DE-OS 35 15 027 and DE-PS 40 28 721 describe processes in which the arc current between the contacts is determined and is used as a criterium for the burnoff.
DE-PS 195 44 926 describes a process in which the arc voltage is used.
DE-OS 44 27 006 describes a process in which the contact pressure of the switching element is utilized as a criterium of contact burnoff. WO 97/28549 describes a process for monitoring the switch movements, i.e. the timed sequence in tap selection or tap changing.
In WO 96/13732 a process has been described for monitoring the insulation breakdown criterium for a switch contact subject to wear, utilizing an additional signal line.
Japanese open application Hei-4-64206 describes a process utilizing a calculation which is a function of the number of switchovers carried out by a tap changer.
Reference may also be had to DE 195 30 776 C1 which discloses a process for monitoring a tap changer operable under load whereby during the switching under load, the arc which is formed is detected from time to time and by comparison of the times between the individual arcs or by comparison of the durations of the length of the individual arc with respect setpoint values as characteristic values for the tap changer, a determination of the contact burnoff can be obtained. The determination is indirect and when the life of the contacts is exceeded, i.e. the burnoff has progressed beyond a permissible limit, the replacement can be undertaken. A direct detection of burnoff or monitoring is not however possible.
DE-OS 27 27 378 describes a device for monitoring operation of a tap changer in a general way in which the burnoff is determined by a load current measurement utilizing a current converter. In many cases this system is not suitable for certain tap changers.
By and large the processes described above have not found widespread use with tap changers for a variety of reasons. Direct optical and mechanical techniques are not practical because of the location of the contacts to be monitored in the interior of the tap changer, i.e. usually in an oil bath. Processes which require additional measuring conductors to run into the region of the contacts in the tap chamber are also not suitable since the passage of these converters through the tap changer wall reduces the breakdown voltage of the housing and the system. Processes which utilize the arc current, the arc voltage or the number of switching operations have generally been found to be insufficiently reliable.
OBJECTS OF THE INVENTION
It is therefore the principal object of the present invention to provide an improved process or method for monitoring contact burnoff in tap changers which can ensure in a reliable and simple manner a substantially exact measurement of the burnoff of the contact without requiring visual examination or direct measurements at the respective contacts and which can generate an output upon a contact burnoff exceeding a predetermined degree.
Another object of the invention is to provide a burnoff monitoring method which is free from the drawbacks of the prior art system mentioned previously and which does not adversely affect breakdown voltage of the tap changer.
It is also an object of this invention to provide a method of monitoring contact burnoff in the switching contacts and resistance contacts of a tap changer wherein the contacts which tend to arc can be submerged in oil.
SUMMARY OF THE INVENTION
These objects are attained, in accordance with the invention in a method of monitoring contact burnoff in a tap changer for a transformer having a multiplicity of taps, the method comprising the steps of:
(a) storing values of respective nominal tap voltages (U
S
), a limiting value for the permissible contact burnoff for switching contacts and resistance contacts of the tap changer, and tap-changer-specific parameters a, b and k;
(b) detecting a current tap setting of the tap changer;
(c) incrementing an index n with each tap change by
(c
1
) stepping the tap changer to a selected tap,
(c
2
) measuring a respective load current (J
L
) of the selected tap, and
(c
3
) reading out the permanently stored values for the nominal tap voltage (U
S
) of the selected tap;
(d) calculating a switching current (J
SK
) of the respective switching contacts and a switching current (J
WK
) of the respective resistance contacts in accordance with the relationships:
J
SK
=
J
L
ParSec
J
WK
=
U
S
+
J
L
·
R
0
s
res
2
·
R
0
wherein ParSec is a number of parallel sectors, R
0
is a magnitude of a bridging resistance of the tap changer for the selected tap and s
res
is a resulting current splitting;
(e) calculating the respective burnoff rates A
sk
of the respective switching contacts and A
wk
of the respective resistance contacts from the relationships:
A
SK
=
a
·
J
SK
b
A
WK
=
a
·
J
WK
b
(f) summing up the burnoff rates (A
sk
) and (A
wk
) to obtain total volume burnoffs (GA
SK
n
)for the switching contacts and

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