Power Transformers

A transformer is a static electrical device used in electric power systems to transfer power between circuits through the use of electromagnetic induction. Transformers are devices that transfer energy from one circuit to another by means of a common magnetic field.

When an alternating current flows in a conductor, a magnetic field exists around the conductor. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor, then a voltage is induced into the second conductor. The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based.

The transformer is based on two principles:

• An electric current can produce a magnetic field. 

• A changing magnetic field within a coil induces a voltage across the ends of the coil (electromagnetic induction).

The flux lines link the winding itself and the flux is varying so they will be self-induced EMF across the winding, this phenomenon is due to Faraday's laws of electromagnetic induction. This induced EMF or voltage is same in magnitude and opposite in polarity of supply voltage supplied alternating voltage causes alternating current in the winding which produces continually varying flux inside and outside the winding this continually varying flux produces induced EMF across the winding so we can say that the supply voltage is caused an induced voltage the winding is an effect of this cause hence according to Lenz’s law this induced voltage will be in opposite polarity of supply voltage since according to Lenz's law effect always opposes cause.

This self-induced voltage across the winding does not depend upon the number of turns in the winding but depends on the supply voltage but the voltage induced per turn depends on the number of turns in the winding this is nothing but induced EMF divided by the number of turns in the winding.

Supposing one separate winding is brought nearer to the first winding then this second winding gets linked with a portion of varying flux of first winding due to this varying flux linkage there will also be an induced EMF across it, this induced EMF would be quite small as because the flux linkages is small hence rate of change of flux linkage is also small and according to Faraday's laws induced EMF across a coil is directly proportional to the rate of change of flux linkage.

If now we connect a closed circuit across the second winding, we will get a very tiny current through the circuit provided the second winding is placed much nearer to the first, so we have seen that some portion of the input power is transformed to output through the second winding this is because some portion of generated flux of first winding is linked with second.

Now if we want to transform maximum electric power from first winding to second winding we have to link maximum flux of first winding to second winding this is done by placing a low reluctant magnetic core in between these windings Steel is a well-known low reluctant magnetic material so we'd normally use steel for making low reluctant magnetic core in the transformer as soon as we place a steel core in between these windings nearly the entire flux which was surrounding the first winding will be concentrated inside the core and linked with the second winding as nearly the same flux links with second winding now the rate of change of flux with respect to time is equal in both windings

Since as Faraday’s laws of electromagnetic induction, induced EMF across a conductor is directly proportional to the rate of change of flux linkage. The voltage induced per turn in both windings will be the same, actually here we consider that there is no voltage drop between supply terminals and the first winding this is an ideal case, for theoretical purposes we will consider that condition as this first winding is connected with supply it is referred to as primary winding now if this primary winding has N1 number of turns and supply voltage or adduced voltage across primary winding is N1 that voltage per turn in the primary winding is V1 by N1 so far we have understood that exactly this V1 by N1 voltage will appear across each turn of second winding so if this second winding has n2 number of turns then total voltage across the second winding is N2 into V1 by N1 and let us say this is V2 if now any closed circuit is connected across the second winding it will provide voltage V to across the circuit and due to the voltage there will be current flowing through the circuit normally in a transformer the second winding is connected with load circuit this winding is referred to as secondary winding if the number of turns of secondary winding is not equal to that or primary winding that is if N2 is not equal to N1 then secondary voltage of the transformer is different from primary voltage now if N2 is greater than N1 the secondary voltage will be more than primary voltage on the other hand if N2 is less than N1 the secondary voltage is less that the primary voltage former is called a step-up transformer and the latter is called a step-down transformer.

In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformers or 3 winding transformers.

Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements:

• It reduces the unbalancing in the primary due to unbalancing in three-phase load.

• It redistributes the flow of fault current.

• Sometimes it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.

• As the tertiary winding is connected in delta formation in three-winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

The following figure is simply showing the Oil Filled Transformer components.

The Transformer Core is made up of thin slices of Highly permeable material where the thickness is in the range of 0.25 to 0.5 mm. and there are two types of the Transformer Core

When an electrical transformer is loaded, the current starts flowing through its windings. Due to this flowing of electric current, heat is produced in the windings, this heat ultimately rises the temperature of transformer oil. We know that the rating of any electrical equipment depends upon its allowable temperature rise limit. Hence, if the temperature rises of the transformer insulating oil is controlled, the capacity or rating of transformer can be extended up to significant range. The radiator of transformer accelerates the cooling rate of transformer. Thus, it plays a vital role in increasing loading capacity of an electrical transformer.

Oil immersed power transformer is generally provided with detachable pressed sheet radiator with isolating valves. But in case of small size distributing transformer, the radiators are generally integrated parts of transformer body and projected from the main tank. The working principle of radiator is very simple. It just increases the surface area for dissipating heat of the oil. In case of electrical power transformer, due to the transport limitation, this units are sent separately and assembled at site with transformer main body. At the time of dispatching, the flings of radiator are blanketed by gasket and blanketing plates. The radiator valves on the main tank are also blanketed by gasket and blanketing plates.

There are three types of Transformer Radiator: -

• Tube Type

• Panel Type

• Cooler Type “will always have forced Oil (Pumps)”

Power Transformer Cooling Types:

a. Insulation Liquid “Oil”

• Provide dielectric strength/medium

• Provide a cooling medium

• Protects the paper

• Use as a diagnostic tool to monitor the condition of the paper “Paper + H2O + O2 + Heat =Oil sludge & degraded paper”

b. Cooling System Designation

Examples

ONAN = Oil Natural Air Natural

ONAF = Oil Natural Air Forced

ONAN/ODAF (Oil Directed)

ONAN/ODAF/ODAF (Oil Directed Two Stages of Pumps)

ONAN/ONAF/ONAF (Two Stages of Fans)

OFAF or ODAF (One Stage of Forced Oil and Air)

ONAN/ONAF (Two Stages of Fans One Stage of Pumps)

This is a cylindrical tank mounted on supporting structure on the roof the transformer main tank. The main function of conservator tank of transformer is to provide adequate space for expansion of oil inside the transformer and also it acts as a reservoir for transformer insulating oil.

When volume of transformer insulating oil increases due to load and ambient temperature, the vacant space above the oil level inside the conservator is partially occupied by the expanded oil. Consequently, corresponding quantity of air of that space is pushed away through breather. On other hand, when load of transformer decreases, the transformer is switched off and when the ambient temperature decreases, the oil inside the transformer contracts. This causes outside air to enter in the conservator tank of transformer through silica gel breather.

Transformer protection devices respond reliable in case of occurring failures and help to determine or prevent serious damages. Depending on the transformer type and application, different protection devices are commonly used. The devices are equipped with switches to either give an alarm signal or, in case of a serious failure, to switch off the transformer.

Buchholz relays have been applied on oil-filled power and distribution transformers and Reactors. The relay is connected to the oil piping between the conservator and oil tank of a transformer. The piping between the main tank and conservator is arranged so that any gas evolved in the main tank tends to flow upward toward the conservator and gas detector relay.

Oil is decomposed in different Hydrocarbon Gases, CO2 “Carbon Dioxide and CO “Carbon Monoxide”.

Depending on the model, the relay has multiple methods to detect a failing transformer. On a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float switch in the relay is used to initiate an alarm signal. Depending on design, a second float may also serve to detect slow oil leaks.

If an electrical arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage. Buchholz relays have a test port to allow the accumulated gas to be withdrawn for testing.

Flammable gas found in the relay indicates some internal fault such as overheating or arcing, whereas air found in the relay may only indicate low oil level or a leakage.

Oil surge relay (OSR) is a single float Buchholz relay fixed to OLTC and its conservator tank. The purpose of OSR is similar to the Buchholz relay and acts whenever sudden oil rush towards OLTC conservator tank happens due to flashover in OLTC tank. Generally, Buchholz relay connected to the main tank contains two floats i.e. upper and lower, electrically connected to alarm and trip circuits. Apart from low oil level condition, upper float acts when gasses accumulate due to minor internal faults in the Transformer and lower float acts when sudden rush in oil towards to conservator tank is sensed due to major internal faults in the Transformer. In the OSR connected to OLTC, only single float is provided which acts due to oil rush caused from OLTC internal fault. If the upper float, which acts due to accumulation of gasses, is also provided in the OSR, many times gasses that are normally generated in the frequent operation of OLTC trips the Transformer without any internal fault. Hence only single float is provided in the OSR connected to OLTC to avoid unwanted interruption of supply to the Transformer and ensures safety tripping when any internal fault occurs or low oil level condition in the OLTC tank

Sudden and violent short circuits inside the oil-cooled transformer’s metal tanks instantly generate an enormous amount of gas, which causes a substantial increase in internal pressure. If the pressure cannot be externally discharged, there is a danger that the transformer could explode and potentially cause harm and damage.

• In case of a sudden and uncontrolled increase in pressure inside the transformer, the pressure relief device allows the insulating fluid to be discharged in milliseconds.

• In case tripping occurs, the directed oil drain prevents the uncontrollable escape of transformer oil.

The winding is the component with the highest temperature within the transformer and it is the component which is subjected to the fastest temperature increases as the load increases.

Thermal Consideration: -

• High ambient temperature

• Cooling system failure

• Overload

• Slow clearing of faults

• Abnormal system conditions (low frequency, high voltage, harmonics)

The purpose of WTI is to indicate the winding temperature of HV and LV winding of the Transformer and operates the alarm, trip and cooler control contacts.

As can be seen from the figure, Black needle shows the current winding temperature while the Red needle shows the highest winding temperature reached for a particular day. This Red needle needs to be reset manually.

Also, for setting winding temperature for Alarm and Trip, two knobs are provided. The Green knob shows the setting of winding temperature for Alarm and Red knob shows the Trip temperature setting.

Operation Principles

Winding temperature thermometers work the same as the top oil thermometer, except that the bulb is in a separate thermometer well near the top of the tank. A wire-type heater coil is either inserted into or wrapped around the thermometer well which surrounds the temperature sensitive bulb.

• In some transformers a current transformer (CT) is around one of the three winding leads and provides current directly to the heater coil in proportion to winding current.

• In other transformers, the CT supplies current to an autotransformer that supplies current to the heater coil. The heater warms the bulb, and the dial indicates a temperature, but it is not the true hottest-spot temperature.

This device is used to indicate the position of transformer insulating oil level in conservator of transformer. This is a mechanical device. Magnetic oil level indicator of transformer consists of mainly three parts-

a.  One float

b.  Bevel gear arrangement

c.   An indicating dial

The dial of this device has scale from empty to full. It has some intermediate divisions such as 1/4, 1/1, 3/4. The prescribed oil level at either 30oC or 35oC ambient temperature may also be indicated on the dial. A mercury switch and bevel gear are fixed with pointer. When pointer rotates, the alignment of mercury switch also changes according to the angle of rotation of the pointer.

One ball type or drum type float is attached with a sufficiently long float arm. One unit of bevel gear is fitted on the other side of the float arm. Other unit of the bevel gear is magnetically coupled with pointer and mercury switch arrangement. The bevel gear arrangement is positioned inside the conservator tank of transformer and dial, pointer and mercury switch are positioned outside the conservator tank.

There are two main types of transformer oil used in transformers:

• Paraffin based transformer oil

• Naphtha based transformer oil

Naphtha oil is more easily oxidized than paraffin oil. But the product of oxidation – i.e. sludge – in the naphtha oil is more soluble than the sludge from the paraffin oil. Thus sludge of naphtha-based oil is not precipitated in the bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system.

Although Paraffin oil has a lower oxidation rate than Naphtha oil, the oxidation product (sludge) is insoluble and precipitated at the bottom of the tank. This sludge acts as an obstruction to the transformer cooling system.

Transformers can be faced to variety of faults as following:

• • The most common type of fault being the winding to core faults because of weakening of insulation. Phase faults inside the transformers are rare. However, such faults may occur on terminals, which fall within the transformer protection zone.

  • Power transformers are generally provided with on-line tap changing (OLTC) gear, this is another major area of occurrence of fault.

  • All large transformers are oil immersed type. There is a possibility of oil leakage.

  • Transformers experience large inrush currents that are rich in harmonic content at the time of switching if they happen to be unloaded.

  • A transformer may develop inter turn faults giving rise to local hot spots within the winding.

  • Transformers may suffer from over fluxing due to under frequency operation at rated voltage. Over fluxing may also be caused when the transformer is subjected to over voltage at the rated frequency.

In case of sustained overload conditions, the transformer should not be allowed to operate for long duration.