Differential protection device of a power transformer

Details Differential protection device of a power transformer Differential protection device of a power transformer

1996-06-28

1998-07-21

Gaffin, Jeffrey A.

Electricity: electrical systems and devices

Safety and protection of systems and devices

Transformer protection

361 93, 361115, H02H 500

Patent

active

057842337

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a differential protection device of a power transformer comprising first measuring means for measuring the current flowing in a primary winding of the transformer, second measuring means for measuring the current flowing in a secondary winding of the transformer, interrupting means connected to the primary winding of the transformer to interrupt the electrical power supply of the transformer and a processing circuit connected to the first and second measuring means, supplying a tripping signal to the interrupting means and/or information signals to annunciation or monitoring means, the processing circuit comprising sampling means connected to the first and second measuring means to sample signals representative of the currents measured in the primary and secondary windings.
Known differential protection devices analyze the currents of the primary and secondary windings of transformer. The object of differential protection is to detect an internal fault between any two turns of the transformer windings or a fault between a turn and ground of the transformer. The faults are detected by measuring a differential current representative of the current difference between the current flowing in the primary windings and the current flowing in the secondary windings, taking account of the transformation ratio and of the coupling. A through-current, also called passing current, representative of a mean value between the currents flowing in the primary and secondary windings, more or less the transformation ratio and of the coupling, is also used.
The differential protection used in tripping relays causes breaking of an electrical power supply connected to the primary winding of the transformer. Generally a tripping order is produced if tripping conditions are verified, for example when the differential current is greater than a preset threshold or when it exceeds a fraction of the through-current. The differential and through-currents are pre-processed so as to keep only the fundamental component of the signals, at the frequency of the current supplied by the electrical power supply of the transformer.
The tripping conditions alone are not sufficient to ensure correct operation of differential protection devices. On power-up, notably, a remanent flux in a magnetic circuit of the transformer can cause saturation of the magnetic circuit according to the closing phase of the supply voltage and supply very high transient differential current measurements. In continuous operation at high voltage levels the magnetic circuit of the transformer can also saturate and make the differential current measurement false. In certain devices, primary and secondary current measuring means comprise a magnetic circuit which saturate at high current levels and supply wrong measurements.
Conventional electronic relays comprising analog or digital circuits enable certain transient or continuous operating states representing events other than faults to be identified, but do however generate a differential current due for example to saturation of the magnetic circuit of the transformer or of the measurement sensors, in order to prevent spurious trips. However taking all the operating cases fully into account would lead to very complex and costly circuits or algorithms, and/or very high response times, for an efficient differential protection of the transformers.
The object of the invention is to achieve a fast and inexpensive transformer differential protection device able to differentiate between a large number of operating situations and tripping conditions.
This object is achieved by the fact that the processing circuit comprises pre-processing means connected to the sampling means and a neuron network comprising inputs connected to the pre-processing means and at least one output to supply the tripping and/or information signals, the pre-processing means supplying to the neuron network signals representative of the fundamental component, of the second harmonic and of the fifth harmonic of a differential current a

REFERENCES:
patent: 5014153 (1991-05-01), Wilkerson
patent: 5629870 (1997-05-01), Farag et al.
IEEE Transactions on Power Delivery, vol. 9, No. 1, Jan. 1994, pp. 434-441, "Training an Artificial Neural Network to Discriminate Between Magnetizing Inrush and Internal Faults."
IEEE Transactions on Power Delivery, vol. 5, No. 3, Jul. 1990, pp. 1299-1308, "A Kalman Filter Based Digital Percentage Differential and Ground Fault Relay for a 3-Phase Power Transformer."
TENCON '93 IEEE Region 10 Conference on Computer, Communication, Control and Power Engineering, vol. 2, Oct. 1993, pp. 754-757, "Detection of Magnetizing Inrush Current Using Artificial Neural Network."

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