- Over Fluxing Definition: Over fluxing in a transformer occurs when the magnetic flux density exceeds the designed limit, leading to potential damage.
- Causes of Over Fluxing: Over fluxing can be caused by overvoltage, low-frequency power generation, lightly loaded transmission lines, and inadequate shunt compensation.
- Effects of Over Fluxing: Excessive flux causes overheating and damage to various transformer components, potentially producing combustible gases.
- Overfluxing Protection: Overfluxing protection involves detecting the V/f ratio and tripping the transformer if the ratio exceeds safe limits.
- Protection Mechanisms: A typical protection scheme uses a combination of voltage transformers, resistors, capacitors, and Zener diodes to monitor and respond to over fluxing conditions.
Causes of Over Fluxing in Transformer
Modern transformer designs keep the peak flux density around 1.7 to 1.8 Tesla, while the core’s saturation flux density is about 1.9 to 2 Tesla. Over fluxing happens when an electrical power transformer runs at a flux density above those design limits. That condition can harm operation and life. Transformers have some over-excitation capacity from their design and thermal limits. Indian Standard (I.S.) texts do not set a short-time over-excitation limit, but they suggest over fluxing should not exceed 110%.
The flux density in a transformer can be expressed by
The magnetic flux density in a transformer is proportional to the voltage-to-frequency ratio (V/f). Over fluxing can happen if the voltage increases or the frequency decreases. This issue is more common in step-up transformers at power stations than in step down transformers at Sub-Stations, where voltage and frequency are usually stable. In rare cases, step-down transformers can also face over fluxing. Over fluxing relays are set so the transformer does not trip on brief flux changes, but they will trip if the condition lasts beyond safe limits.
There are several causes of transformer over fluxing. Common ones are listed below.
- Over voltage causes due to sudden load rejection
- Low frequency generation of power
- Transmission line is lightly loaded
- Proper shunt compensation in transmission system is not provided. etc.
Effect of Over Fluxing in Transformers
The flux in a transformer, under normal conditions, is confined to the core of transformer because of its high permeability compared with the surrounding volume. When the flux density in the core increases beyond saturation, a substantial amount of flux is diverted to steel structural parts and into the air. At saturation flux density the core steel will overheat.
Structural steel parts which are un-laminated and are not designed to carry magnetic flux will heat rapidly. Flux flowing in unplanned air paths may link conducting loops in the windings, leads, tank base and structural parts. The resulting circulating currents in these loops can cause a dangerous temperature rise. Under excessive over fluxing the heating of the inner portion of the windings may be extreme, as the exciting current is rich in harmonics. The winding loss at high excitation cannot be tolerated for long if damage is to be avoided.
Physical evidence of damage due to over fluxing will vary with the degree of over excitation, the time applied and the particular design of transformer. The table below summarises such physical damage and probable consequences.
| SL | Component involved | Physical evidences | Consequences |
| 1 | Metallic support and surfaces structure for core and coils | Discoloration or metallic parts and adjacent insulation.Possible carbonized material in oil. Evolution of combustible gas. | Contamination of a oil and surfaces of insulation. Mechanical weakening of insulation Loosing of structure. Mechanical structure |
| 2 | Windings | Discoloration winding insulation evolution of gas. | Electrical and mechanical weakling of winding insulation |
| 3 | Lead conductors. | Discoloration of conductor insulation or support, evolution of gas. | Electrical and mechanical weakening of insulation, Mechanical Weakening of support. |
| 4 | Core lamination. | Discoloration of insulating material in contact with core. Discoloration and carbonization of organic/lamination insulation Evaluation of gas. | Electrical weakening of major insulation (winding to core) increased interlaminar eddy loss. |
| 5 | Tank | Blistering of paints | Contamination of oil if paint inside tank is blistered. |
Metallic support structures for the core and coils, windings, lead conductors, core laminations and the tank may reach a high enough temperature that combustible gas forms. That gas may collect in the Buchholz Relay and give an alarm or trip, depending on how much gas is collected and how long the transformer stays over-fluxed.
Due to over fluxing in transformer the core saturates, so the induced voltage in the primary circuit becomes more or less constant. If the primary supply voltage is then raised to an abnormal value, magnetising current in the primary becomes high. In that saturated state the usual linear relation between primary and secondary voltage and current is lost. The high primary magnetising current may not appear correctly on the secondary, so primary and secondary currents can mismatch and a differential relay may operate. Substation transformers often have no separate over-fluxing protection.
Stipulated Withstand-Duration of Over Fluxing in Transformers
Over fluxing in transformer can shorten life, as already explained. As over-fluxing protection is not generally fitted on step-down transformers of a substation, there must be a stated time the design can withstand without much damage. Other protection should trip the transformer within that time if the cause is not removed.
Flux density B in the transformer core is proportional to the V/f ratio. Power transformers are designed to withstand (Vn/fn x 1.1) continuously, where Vn is the normal highest r.m.s. voltage and fn is the standard frequency. Core design is such that a higher V/f causes higher core loss and core heating. The ability of a transformer to withstand higher V/f (over fluxing) is limited to a few minutes, as in the typical table below.
| F = (V/f)/(Vn/fn) | 1.1 | 1.2 | 1.25 | 1.3 | 1.4 |
| Duration of with stand limit (minutes) | continuous | 2 | 1 | 0.5 | 0 |
From the table above, if over fluxing from a system hazard pushes factor F to 1.4, the transformer should be tripped at once, or lasting damage can follow. That table is a typical design example, not a universal IEC limit.
Protection Against Over fluxing (v/f – Protection) in Transformer
Over-fluxing does not need high-speed tripping. Instantaneous operation is undesirable because it would trip on momentary system disturbances that the machine can bear. Normal V/f must be restored, or the transformer must be isolated within one or two minutes at most.
Flux density is proportional to V/f, so the scheme must detect a V/f ratio above unity, with V and f in per-unit of rated quantities. In a typical over-fluxing scheme, the system voltage as measured by the voltages transformer is applied to a resistance to produce a proportionate current. That current, passed through a capacitor, produces a voltage drop proportional to V/f and hence to flux in the power transformer. The measured drop is compared with a fixed reference DC voltage obtained across a Zener diode. When the peak AC signal exceeds the DC reference it triggers a transistor circuit which operates two electro-mechanical auxiliary elements. One starts after a fixed time delay, the other after an extra adjustable delay. The protection operates when the terminal voltage-to-frequency ratio exceeds a set value and resets when the ratio falls below 95 to 98% of the operating ratio. By adjustment of a potentiometer, the setting is calibrated from 1 to 1.25 times the ratio of rated volts to rated frequency. The first auxiliary element operates after a fixed delay of 20 to 120 seconds. The second output relay then trips.
High V/f occurs on generator transformers and unit auxiliary transformers if full excitation is applied before the generator reaches synchronous speed. A V/f relay in the automatic voltage regulator blocks a rise in excitation current before full frequency is reached.
When applying a V/f relay to a step down transformer it is preferable to connect it to the secondary (LV side) of the transformer so that a change in HV tap position is taken into account. The relay should give an alarm so the operator can correct the condition. Only in an extreme case should the transformer breaker be allowed to trip.
