Overfluxing protection is a type of protection used for rotating electrical machines like motors and generators. Here are some key points about overfluxing protection:
- It monitors the level of magnetic flux in the machine’s magnetic core or iron core. Too much flux can saturate the core and cause overheating.
- The flux level is indirectly monitored using sensors like voltage or current transformers connected to the machine windings. An increase in flux will cause a proportional increase in winding voltages or currents.
- Overfluxing is caused by operating the machine at excessive excitation levels, usually due to incorrect voltage being applied to the field windings of a generator or problems with the excitation system of a motor.
- It can also occur due to reduced load levels on the machine, allowing excessive excitation for the load conditions.
- Overflux relays use voltage, current or power inputs to calculate the flux level and compare it to adjustable thresholds. They will trip if the flux exceeds normal operating levels.
- This protects the machine’s core from overheating damage due to core saturation from excessive magnetic flux densities.
- The protection acts as a backup to maintain reliable operation if the machine’s regular voltage/current control or excitation system fails.
- Common technology uses are electromagnetic and static relays for generator applications and induction motor differential relays.
- To prevent over fluxing, transformers have several protective elements:
- Voltage regulators and tap changers – These automatically adjust transformer input voltage to prevent over-voltage conditions.
- Overcurrent relays – Detect excessive magnetizing current drawn by the transformer under over fluxing conditions and trip the transformer.
- Temperature sensors – Monitor transformer windings and oil temperature rise. Overheating under over fluxing will trip the transformer.
- Buchholz relays – Detect gas accumulation and oil surge in transformer tank during internal faults triggered by over fluxing.
- Magnetic flux sensors – Directly detect magnetic flux density in the core and trip the transformer if it exceeds the safe limit.
- The transformer design also provides inherent protection against over fluxing. Laminated cores minimize eddy current losses and overheating. Interleaved windings limit voltage per turn.
- Proper transformer sizing, impedance selection, system protections coordination, and monitoring are also critical to prevent over fluxing conditions.