Is there anyone having experience in electric arc furnace transformers ? As we know that metal is melted with help of electric arc which is created by short circuiting. How the overcurrent or instantaneous overcurrent setting is decided as we have short ckt.many times. Also as the transformers are very big (bigger than 100MVA) so have high inrush currents.
overcurrent settings for transformer protection
- Thread starter panthripu
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panthripu,
Scan this stuff which may provide some ideas.
I personally would start with the transformer manufacturer who should have information regarding how to protect their transformer.
http://www.brosz.net/newsletters/electrical_digest_11.pdf
http://steellibrary.com/BookContents/PR-351-085 1 View Abstract.pdf
http://www.pacw.org/no-cache/issue/..._electric_arc_furnace_power_transformers.html
http://www.scribd.com/doc/6767205/Transformer-Protection
Scan this stuff which may provide some ideas.
I personally would start with the transformer manufacturer who should have information regarding how to protect their transformer.
http://www.brosz.net/newsletters/electrical_digest_11.pdf
http://steellibrary.com/BookContents/PR-351-085 1 View Abstract.pdf
http://www.pacw.org/no-cache/issue/..._electric_arc_furnace_power_transformers.html
http://www.scribd.com/doc/6767205/Transformer-Protection
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
The following is from the GE Book, "Art and Science of Protective Relaying" Chapter 11 - Transformer Protection
ELECTRIC ARC-FURNACE TRANSFORMERS
Electric arc-furnace power transformers are not protected with percentage-differential relays because of the complications that would be introduced by the very frequent tap changing on the power transformer. Every time a furnace-transformer tap was changed, the low-voltage CT ratio or a tap on the relay would have to be changed. Also, the connections of the furnace-transformer primary windings are usually changed from delta to wye and back again, which would require changing the CT connections.
Protection against short circuits inside the power transformer should be provided by inverse-time phase (and ground if required) overcurrent relays operating from the current on the high-voltage side of the power transformer. The phase relays should have torque-control coils and should be adjusted to pick up at currents only slightly in excess of the transformer's rated full-load current; they should have time delay only long enough to prevent operation on transformer magnetizing-current inrush. High-speed overcurrent relays on the low-voltage side of the transformer, adjusted to pick up at current slightly above rated full load but slightly below the current that will pick up the high-voltage phase relays, should be arranged to control the operation of the high-voltage phase relays through their torque-control coils so as to permit the high-voltage relays to operate only when the low-voltage relays do not operate. In this way, the high-voltage relays may normally be sensitive and fast so as to provide as good protection to the transformer as it is possible to provide with overcurrent relays, while at the same time avoiding undesired operation on external faults, the most common of which are short circuits in the furnace.
For primary protection against short circuits between the back-up breaker and the power transformer, and for back-up protection against faults in the transformer or beyond it, inverse-time phase (and ground if required) overcurrent relays should be provided. These relays should obtain their current from the source side of the back-up breaker. This so called back-up breaker is the breaker that is provided to interrupt short-circuit currents in the transformer or on the high-voltage side, and it may serve several transformers.
Both of the foregoing groups of relays should trip the back-up breaker.
ELECTRIC ARC-FURNACE TRANSFORMERS
Electric arc-furnace power transformers are not protected with percentage-differential relays because of the complications that would be introduced by the very frequent tap changing on the power transformer. Every time a furnace-transformer tap was changed, the low-voltage CT ratio or a tap on the relay would have to be changed. Also, the connections of the furnace-transformer primary windings are usually changed from delta to wye and back again, which would require changing the CT connections.
Protection against short circuits inside the power transformer should be provided by inverse-time phase (and ground if required) overcurrent relays operating from the current on the high-voltage side of the power transformer. The phase relays should have torque-control coils and should be adjusted to pick up at currents only slightly in excess of the transformer's rated full-load current; they should have time delay only long enough to prevent operation on transformer magnetizing-current inrush. High-speed overcurrent relays on the low-voltage side of the transformer, adjusted to pick up at current slightly above rated full load but slightly below the current that will pick up the high-voltage phase relays, should be arranged to control the operation of the high-voltage phase relays through their torque-control coils so as to permit the high-voltage relays to operate only when the low-voltage relays do not operate. In this way, the high-voltage relays may normally be sensitive and fast so as to provide as good protection to the transformer as it is possible to provide with overcurrent relays, while at the same time avoiding undesired operation on external faults, the most common of which are short circuits in the furnace.
For primary protection against short circuits between the back-up breaker and the power transformer, and for back-up protection against faults in the transformer or beyond it, inverse-time phase (and ground if required) overcurrent relays should be provided. These relays should obtain their current from the source side of the back-up breaker. This so called back-up breaker is the breaker that is provided to interrupt short-circuit currents in the transformer or on the high-voltage side, and it may serve several transformers.
Both of the foregoing groups of relays should trip the back-up breaker.
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