Causes of Over Fluxing in Transformer
As per present day transformer design practice, the peak rated value of the flux density is kept about 1.7 to 1.8 Tesla, while the saturation flux density of CRGD steel sheet of core of transformer is of the order of 1.9 to 2 Tesla which corresponds to about 1.1 times the rated value. If during operation, an electrical power transformer is subjected to carry rather swallow more than above mentioned flux density as per its design limitations, the transformer is said to have faced over fluxing problem and consequent bad effects towards its operation and life. Depending upon the design and saturation flux densities and the thermal time constants of the heated component parts, a transformer has some over excitation capacity. I.S. specification for electrical power transformer does not stipulate the short time permissible over excitation, though in a round about way it does indicate that the maximum over fluxing in transformer shall not exceed 110%.
The flux density in a transformer can be expressed by
The magnetic flux density is, therefore, proportional to the quotient of voltage and frequency (V/f). Over fluxing can, therefore, occur either due to increase in voltage or decrease in-frequency of both. The probability of over fluxing is relatively high in step-up transformers in Power stations compared to step down transformers in Sub-Stations, where voltage and frequency usually remain constant. However, under very abnormal system condition, over-fluxing trouble can arise in step-down substation transformers as well. the over fluxing relays are so adjusted, that for every transient over flux condition the transformer does not trip for maintaining the power system stability. But till it should trip as soon as the duration and severity of the condition crosses the specified safe limit.
There are are several causes of transformer over fluxing, but some common causes are given below for information
- 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 to the surrounding volume. When the flux density in the increases beyond saturation point, a substantial amount of flux is diverted to steel structural parts and into the air. At saturation flux density the core steel will over heat.
Structural steel parts which are nu-laminated and are not designed to carry magnetic flux will heat rapidly. Flux flowing in unplanned air paths may link conducing loops in the windings, loads, tank base at the bottom of the core and structural parts and the resulting circulating currents in these loops can cause dangerous temperature increase. Under conditions of excessive over fluxing the heating of the inner portion of the windings may be sufficiently extreme as the exciting current is rich in harmonies. It is obvious that the levels of loss which occur in the winding at high excitation cannot be tolerated for long if the damage is to be a voided.
Physical evidences of damage due to over fluxing will very with the degree of over excitation, the time applied and the particular design of transformer. The Table given below summarizes 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.|
It may be seen that metallic support structures for core and coil, windings, lead conductors, core lamination, tank etc. may attain sufficient temperature with the evolution of combustible gas in each case due to over fluxing of transformer and the same gas may be collected in Buchholz Relay with consequent Alarm/Trip depending upon the quantity of gas collected which again depends upon the duration of time the transformer is subjected to over fluxing.
Due to over fluxing in transformer its core becomes saturated as such induced voltage in the primary circuit becomes more or less constant. If the supply voltage to the primary is increased to abnormal high value, there must be high magnetising current in the primary circuit. Under such magnetic state of condition of transformer core linear relations between primary and secondary quantities (viz. for voltage and currents) are lost. So there may not be sufficient and appropriate reflection of this high primary magnetising current to secondary circuit as such mismatching of primary currents and secondary currents is likely to occur, causing differential relay to operate as we do not have over fluxing protection for sub-stn. transformers.
Stipulated Withstand-Duration of Over Fluxing in Transformers
Over fluxing in transformer has sufficient harmful effect towards its life which has been explained. As over fluxing protection is not generally provided in step-down transformers of Sub-Station, there must be a stipulated time which can be allowed matching with the transformer design to withstand such over fluxing without causing appreciable damage to the transformer and other protections shall be sensitive enough to trip the transformer well within such stipulated time, if cause of over fluxing is not removed by this time.
It is already mentioned that the flux density ‘B’ in transformer core is proportional to 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 higher v/f causes higher core loss and core heating. The capability of a transformer to withstand higher v/f values i.e. over fluxing effect, is limited to a few minutes as furnished below in the Table.
|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 it may be seen that when over fluxing due to system hazards reaches such that the factor F attains a values 1.4, the transformer shall be tripped out of service instantaneously otherwise there may be a permanent damage.
Protection Against Over fluxing (v/f – Protection) in Transformer
The condition arising out of over-fluxing does not call for high speed tripping. Instantaneous operation is undesirable as this would cause tripping on momentary system disturbances which can be borne safely but the normal condition must be restored or the transformer must be isolated within one or two minutes at the most.
Flux density is proportional to V/f and it is necessary to detect a ratio of V/f exceeding unity, V and f being expressed in per unit value of rated quantities. In a typical scheme designed for over fluxing protection, the system voltage as measured by the voltages transformer is applied to a resistance to product a proportionate current; this current on being passed through a capacitor, produces a voltage drop which is proportional to the functioning in question i.e. V/f and hence to flux in the power transformer. This is accompanied 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 is initiated after a fixed time delay, the other after an additional time delay which is adjustable. The over fluxing protection operates when the ratio of the terminal voltage to frequency exceeds a predetermined setting 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 output from the first auxiliary element, which operates after fixed time delay available between 20 to 120 secs. second output relay operates and performs the tripping function.
It is already pointed out that high V/f occur in Generator Transformers and Unit-Auxiliary Transformers if full exaltation is applied to generator before full synchronous speed is reached. V/f relay is provided in the automatic voltage regulator of generator. This relay blocks and prevents increasing excitation current before full frequency is reached.
When applying V/f relay to step down transformer it is preferable to connect it to the secondary (L.V. said of the transformer so that change in tap position on the H.V. is automatically taken care of Further the relay should initiate an Alarm and the corrective operation be done/got done by the operator. On extreme eventuality the transformer controlling breaker may be allowed to trip.