TRANSFORMERS

TRANSFORMER

Transformer is used to convert electrical energy of higher voltage (usually 11-22-33kV) to a lower voltage (250 or 433V) with frequency identical before and after the transformation. Its main application is mainly within suburban areas, public supply authorities and industrial customers. With given secondary voltage, distribution transformer is usually the last in the chain of electrical energy supply to households and industrial enterprises.

INTRODUCTION
A transformer has no internal moving parts, and it transfers energy from one circuit to another by electromagnetic induction. External cooling may include heat exchangers, radiators, fans, and oil pumps. Radiators and fans are evident in figure. The large horizontal tank at the top is a conservator. Transformers are typically used because a change in voltage is needed. Power transformers are defined as transformers rated 500 kVA and larger. Larger transformers are oil-filled for insulation and cooling. Transformers smaller than 500 kVA are generally called distribution transformers.

Radiator : Any of numerous devices, units, or surfaces that emit heat, mainly by radiation, to objects in the space in which they are installed.

All electrical devices using coils (in this case, transformers) are constant wattage devices. This means voltage multiplied by current must remain constant; therefore, when voltage is “stepped-up,” the current is “stepped-down” (and vice versa). Transformers transfer electrical energy between circuits completely insulated from each other. This makes it possible to use very high (stepped-up) voltages for transmission lines, resulting in a lower (stepped-down) current. Higher voltage and lower current reduce the required size and cost of transmission lines and reduce transmission losses as well. Transformers have made possible economic delivery of electric power over long distances. The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.

Basic principle

A simple transformer consists of two electrical conductors called the primary winding and secondary winding, and a steel core that magnetically links them together These two windings can be considered as a pair of mutually coupled coils. At the instant a transformer primary is energized with AC, a flow of electrons (current) begins. During the instant of switch closing, buildup of current and magnetic field occurs. As current begins the positive portion of the sine wave, lines of magnetic force (flux) develop outward from the coil and continue to expand until the current is at its positive peak. The magnetic field is also at its positive peak. The current sine wave then begins to decrease, crosses zero, and goes negative until it reaches its negative peak. The magnetic flux switches direction and also reaches its peak in the opposite direction.

NOTE: Direct current (DC) is not transformed, as DC does not vary its magnetic fields. A transformer usually consists of two insulated windings on a common iron (steel) core:
With an AC power circuit, the current changes (alternates) continually 60 times per second, which is standard in the United States. Other countries may use other frequencies. In Europe, 50 cycles per second is common. Strength of a magnetic field depends on the amount of current and number of turns in the winding. When current is reduced, the magnetic field shrinks. When the current is switched off, the magnetic field collapses

Transformer Voltage and Current If the secondary coil has twice as many turns as the primary, it will be cut twice as many times by the flux, and twice the applied primary voltage will be induced in the secondary. The total induced voltage in each winding is proportional to the number of turns in that winding. If E1 is the primary voltage and I1 the primary current, E2 the secondary voltage and I2 the secondary current, N1 the primary turns and N2 the secondary turns, then:

E1/E2 = N1/N2 = I2/I1

Note that the current is inversely proportional to both voltage and number of turns. This means that if voltage is stepped up, the current must be stepped down and vice versa. The number of turns remains constant unless there is a tap changer. The power output or input of a transformer equals volts times amperes (E x I). If the small amount of transformer loss is disregarded, input equals output or:

E1 x I1 = E2 x I2
Transformers are adapted to numerous engineering applications and may be classified in many ways:

§ By power level (from fraction of a watt to many megawatts),
§ By application (power supply, impedance matching, circuit isolation),
§ By frequency range (power, audio, RF)
§ By voltage class (a few volts to about 750 kilovolts)
§ By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.)
§ By purpose (rectifier, arc furnace, amplifier output, etc.).
§ By ratio of the number of turns in the coils

Step Down Transformers
If there are fewer turns in the secondary winding than inthe primary winding, the secondary voltage will be lower than the primary.

Step Up TransformersIf there are fewer turns in the primary winding than in the secondary winding, the secondary voltage will be higher than the secondary circuit. Isolating
When the primary winding and the secondary winding havethe same amount of turns there is no change voltage, the ratio is 1/1 unity. Variable
The primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.
Note: The primary winding is the winding which receives the energy; it is not always the high-voltage winding.

CONSTRUCTION
There are 3 main parts in the distribution transformer:

1) Coils/winding
where incoming alternating current (through primary winding) generates magnetic flux, which in turn induces a voltage in the secondary coil.

2) Magnetic corematerial allowing transfer of magnetic field generated by primary winding to secondary winding by the principle of electromagnetic induction. A transformers core and windings are called its Active Parts. This is because these two are responsible for transformer s operation.

3) Tank
serving as a mechanical package to protect active parts, as a holding vessel for transformer oil used for cooling and insulation.

Transformer Accessories
Bucholz relay
Breather
Pressure relief device etc

(Breather pipe A pipe that opens into a container for ventilation, as in a crankcase or oil tank. Also known as crankcase breather.)

A Buchholz relay, also called a gas relay or a sudden pressure relay, is a safety device mounted on some oil-filled power transformers, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment

Losses
An ideal transformer would have no losses, and would therefore be 100% efficient. In practice energy is dissipated due both to the resistance of the windings (known as copper loss), and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient, and large power transformers (around 100 MVA and larger) may attain an efficiency as high as 99.75%. Small transformers such as a plug-in "power brick" used to power small consumer electronics may be less than 85% efficient.

The losses arise from:
1) Winding resistance
Current flowing through the windings causes resistive heating of the conductors.
2) Eddy currents
Induced currents circulate in the core and cause its resistive heating.
3) Stray losses
Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects such as the transformer's support structure, and be converted to heat. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also from magnetostriction vibration of the core.
4) Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. The level of hysteresis is affected by the core material.
5) Mechanical losses
The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.
6) Magnetostriction
The flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores.

The magnetic circuit and windings are the principal sources of losses and resulting temperature rise in various parts of a transformer. Core loss, copper loss in windings (I2R loss), stray loss in windings and stray loss due to leakage/high current field are mainly responsible for heat generation within the transformer.

§ Cooling system
Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper and iron losses. The power used to operate the cooling system is typically considered part of the losses of the transformer.

Modes of Heat Transfer
The heat transfer mechanism in a transformer takes place by three modes, viz. conduction, convection and radiation. In the oil cooled transformers, convection plays the most important role and conduction the least important.

Conduction
Almost all the types of transformers are either oil or gas filled, and heat flows from the core and windings into the cooling medium. From the core, heat can flow directly, but from the winding it flows through the insulation provided on the winding conductor. In large transformers, at least one side of insulated conductors is exposed to the cooling medium, and the heat flows through a small thickness of the conductor insulation. But in small transformers the heat may have to flow through several layers of copper and insulation before reaching the cooling medium.

Radiation
Any body, at a raised temperature compared to its surroundings, radiates heat energy in the form of waves. The heat dissipation from a transformer tank occurs by means of both radiation and natural convection. The cooling of radiators also occurs by radiation, but it is far less as compared to that by convection.

Convection
The oil, being a liquid, has one important mechanical property that its volume changes with temperature and pressure .The change of volume with temperature provides the essential convective or thermosiphon cooling. The change of volume with pressure affects the amount of transferred vibrations from the core to tank.

The heat dissipation from the core and windings occurs mainly due to convection. When a heated surface is immersed in a fluid, heat flows from the surface to the cooling medium. Due to increase in the fluid temperature, its density (or specific gravity) reduces. The fluid (oil) in oil-cooled transformers, rises upwards and transfers its heat to outside ambient through tank and radiators. The rising oil is replaced by the colder oil from the bottom, and thus the continuous oil circulation occurs.

Common Cooling Arrangements ONAN/OA cooling (Oil Natural and Air Natural)
In small rating transformers, the tank surface area may be able to dissipate heat directly to the atmosphere; while the bigger rating transformers usually require much larger dissipating surface in the form of radiators/tubes mounted directly on the tank or mounted on a separate structure. If the number of radiators is small, they are preferably mounted directly on the tank so that it results in smaller overall dimensions.

Oil is kept in circulation by the gravitational buoyancy in the closed-loop cooling system as shown in figure. The heat developed in active parts is passed on to the surrounding oil through the surface transfer (convection) mechanism. The oil temperature increases and its specific gravity drops, due to which it flows upwards and then into the coolers. The oil heat gets dissipated along the colder surfaces of the coolers which increases its specific gravity, and it flows downwards and enters the transformer tank from the inlet at the bottom level.
Since the heat dissipation from the oil to atmospheric air is by natural means (the circulation mechanism for oil is the natural thermosiphon flow in the cooling
equipment and windings), the cooling is termed as ONAN (Oil Natural and Air Natural) or OA type of cooling. Since the heat dissipation from the oil to atmospheric air is by natural means (the circulation mechanism for oil is the natural thermosiphon flow in the cooling equipment and windings), the cooling is termed as ONAN (Oil Natural and Air Natural) or OA type of cooling.

ONAF/FA cooling (Oil Natural and Air Forced)
If fans are used to blow air on to the cooling surfaces of the radiators, the heat transfer coefficient is significantly increased. For a given set of ambient air temperature and oil temperature, a compact arrangement is possible since less number of radiators is required to cool the oil. This type of cooling is termed as ONAF (Oil Natural and Air Forced) or FA type of cooling.

OTHER TRANSFORMER TYPES
Current Transformer(CT)
Current transformers are used in electric metering for large load situations to reduce the current level presented to the metering circuit in order to make it more manageable and safe. A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.

Potential Transformer

A Potential Transformer is a special type of transformer that allows meters to take readings from electrical service connections with higher voltage (potential) than the meter is normally capable of handling without at potential transformer. Potential transformers are used with voltmeters, wattmeters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and undervoltage and overvoltage trip coils of circuit breakers. One potential transformer can be used for a number of instruments if the total current required by the instruments connected to the secondary winding does not exceed the transformer rating.

Autotransformers
It is possible to obtain transformer action by means of a single coil, provided that there is a “tap connection” somewhere along the winding. Transformers having only one winding are called autotransformers, shown schematically in figure . An autotransformer has the usual magnetic core but only one winding, which is common to both the primary and secondary circuits. The primary is always the portion of the winding connected to the AC power source. This transformer may be used to step voltage up or down. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding (as shown), the secondary voltage is “stepped-down.”

Step-up and step-down transformers
A transformer designed to increase voltage from primary to secondary is called a stepup
transformer. A transformer designed to reduce voltage from primary to secondary is
called a step-down transformer.

Power Transformer
Power transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus.
The construction of a transformer depends upon the application. Transformers intended for indoor use are primarily of the dry type but can also be liquid immersed. For outdoor use, transformers are usually liquid immersed.

AccessoriesThere are many different accessories used to monitor and protect power transformers, some of which are considered standard features, and others of which are used based on miscellaneous requirements. A few of the basic accessories are briefly discussed here.

Liquid-Level Indicator
A liquid-level indicator is a standard feature on liquid-filled transformer tanks, since the liquid medium is critical for cooling and insulation. This indicator is typically a round-faced gauge on the side of the tank, with a float and float arm that moves a dial pointer as the liquid level changes.

Pressure-Relief Devices
Pressure-relief devices are mounted on transformer tanks to relieve excess internal pressures
that might build up during operating conditions. These devices are intended to avoid damage to the tank. On larger transformers, several pressure-relief devices may be required due to the large quantities of oil.

Liquid-Temperature IndicatorLiquid-temperature indicators measure the temperature of the internal liquid at a point near the top of the liquid using a probe inserted in a well and mounted through the side of the transformer tank.

Sudden-Pressure Relay
A sudden- (or rapid-) pressure relay is intended to indicate a quick increase in internal pressure that can occur when there is an internal fault. These relays can be mounted on the top or side of the transformer, or they can operate in liquid or gas space.

Desiccant (Dehydrating) Breathers
Desiccant breathers use a material such as silica gel to allow air to enter and exit the tank, removing moisture as the air passes through. Most tanks are somewhat free breathing, and such a device, if properly maintained, allows a degree of control over the quality of air entering the transformer

‘‘Buchholz’’ Relay
On power transformers using a conservator liquid-preservation system, a ‘‘Buchholz’’ relay can be installed in the piping between the main transformer tank and the conservator. The purpose of the Buchholz relay is to detect faults that may occur in the transformer. One mode of operation is based on the generation of gases in the transformer during certain minor internal faults. Gases accumulate in the relay, displacing the liquid in the relay, until a specified volume is collected, at which time a float actuates a contact or switch. Another mode of operation involves sudden increases in pressure in the main transformer tank, a sign of a major fault in the transformer. Such an increase in pressure forces the liquid to surge through the piping between the main tank and the conservator, through the ‘‘Buchholz’’ relay, which actuates another contact or switch.