TRANSFORMER DESIGN

2010BIA3526G

Praveen Sharma

INTRODUCTION

• Practical transformer design requires knowledge of electrical principles, materials, and economics.

• For a transformer using a sine or square wave, one needs to know the incoming line voltage, the operating frequency, the secondary voltage(s), the secondary current(s), the permissible temperature rise, the target efficiency etc.

• Specifications for transformer design varies with transformer application, tolerance level and cost limitations.

• Data in next slide gives an idea of technical parameter’s specified for designing a 500KVA , 3 Phase transformer

TECHNICAL SPECIFICATIONFOR A 500 KVA, 60HZ, K-FACTOR, THREE PHASE DRY TRANSFORMER

Input voltage 3 x 480V, delta

Output voltage 3 x 208/120V, star

Output power 500kVA, K-Factor=20, continuous operating mode

Frequency 60Hz

Ambient temperature 40°C, in a cabinet

Temperature rise Max. 120°K, insulation class H

°Short-circuit voltage 4-5%

Steel&core M6, not annealed, strips for alternated stacking (90°)

INITIAL CALCULATIONS

• The designer first starts with the primary voltage and frequency

• After that power of each secondary winding is calculated by multiplying the voltage by the current of each coil. These are added together to get the total power the transformer must provide to the load.

• The transformer losses are estimated and added to this sum to give a total power the primary coil must supply.

• The losses arise from :Winding resistance : Eddy currents Hysteresis losses

LOSSES IN TRANSFORMER

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.

Stray losses : 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.

Cooling system : The power used to operate the cooling system

POWER APPLICATIONS

• Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.

• Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.

• Transformer Size / Insulation Level:Power transformer : big in size as compare to distribution transformer,

high insulation levelDistribution transformer : small size, easy in installation and low

insulation

POWER APPLICATIONS

• Iron Losses and Copper Losses : Power Transformers :

1. used in Transmission network so they do not directly connect to the consumers, so load fluctuations are very less.

2.  These are loaded fully during 24 hr’s a day, so Cu losses & Fe losses takes place throughout day. Distribution Transformer :

1. directly connected to the consumer so load fluctuations are very high.

2. not loaded fully at all time so iron losses takes place 24hr a day and cu losses takes place based on load cycle.

POWER APPLICATIONS

Power transformer generally operated at full load. Hence, it is designed such that copper losses are minimal. However, a distribution transformer is always online and operated at loads less than full load for most of time. Hence, it is designed such that core losses are minimal.

• Maximum Efficiency : Distribution transformer : designed for maximum efficiency at 60% to

70% load as normally doesn’t operate at full load all the time. Its load depends on distribution demand.

Power transformer :designed for maximum efficiency at 100% load as it always runs at 100% load being near to generating station.

In A Nutshell

CORE

•  Core material should have a high permeability and a high flux density

• A steel core's magnetic hysteresis means that it retains a static magnetic field when power is removed.

• Certain types have gaps inserted in the magnetic path to prevent magnetic saturation. These gaps may be used to limit the current on a short-circuit.

• Distribution transformers can achieve low off-load losses by using cores made with amorphous (non-crystalline) steel having very large permeability .

• Various type of cores are used : Steel cores, Solid cores, Air cores , Toroidal core etc. depending upon the frequency of operation

TOROIDAL CORE

• Ring-shaped core, which is made from a long strip of silicon steel

• Strip construction ensures that all the grain boundaries are pointing in the optimum direction, making the transformer more efficient by reducing the core's reluctance.

• The ring shape eliminates the air gaps in core

• The primary and secondary coils are wound concentrically to cover the entire surface of the core.

• This minimizes the length of wire needed, and also provides screening to prevent the core's magnetic field from generating electromagnetic interference.

CORE STACK

• Total amount of the steel laminations needed to produce the correct core area for the power in watts, or volt amperes that the transformer is required to handle

• Core can be stacked in two ways :

Interleaved fashion : each lamination is staggered opposite to the other (turned 180 degrees opposite the other)provides for the least amount of air gap in the core, and the highest

efficiency.

Butt Stacked fashion : all the E type lams are stacked on one side, and all the I type lams are stacked on the other. This however creates an air gap thus increasing the losses.

CORE STACK However, when a DC current is superimposed on an AC current as in an audio transformer or filter choke, the air gap can stop the core from saturating.

WINDINGS

• Primary and secondary conductors are coils of conducting wire because each turn of the coil contributes to the magnetic field, creating a higher magnetic flux density than would a single conductor.

• Larger power transformers are wounded with wire, copper or aluminum rectangular conductors, or strip conductors for very heavy currents.

• High frequency transformers operating in the tens to hundreds of kilohertz uses windings made of Litz wire, to minimize the skin effect losses in the conductors.

• Windings on both primary and secondary of a power transformer may have external connections (called taps) to intermediate points on the winding to allow adjustment of the voltage ratio

WINDINGS

• Thickness of winding also plays a major part in designing process.

• The voltage that each winding sees determine the wires insulation thickness.

• Once this voltage is known, the diameter of the selected insulated wire can be used

•  By knowing the wire diameter, the number of turns per layer is calculated

• Number of layers are calculated using the window height and winding margins.

• The windows are the openings on either side of the core. The window area = window width * height

WINDINGS

•  The thickness of the insulation paper for the layers of each winding is adjusted in accordance to the voltage between the coils.

• This thickness is added to the total coil thickness by multiplying the paper thickness by the number of layers

• Finally the bobbin thickness is added.

•  Bobbin is a permanent container for the wire, acting to form the shape of the coil.

• This total thickness is compared to the window dimensions for a fit

• The design should not exceed 80-85% of available opening to allow for manufacturing tolerances

PLASTIC BOBBIN .

THROUGH FAULT - SHORT CIRCUIT WITHSTAND CONSIDERATIONS

• The windings are subject to both radial and axial forces related to the current and flux interactions.

• Radial forces in the inner winding (normally the LV winding) are in compression while the outer winding (normally the HV winding) forces are in tension

• Design of the windings and bracing must consider the magnitude of these forces and provide adequate strength to withstand them without significant mechanical deformation which could result in a dielectric failure.

AXIAL FORCES

• Flux fields are dependent of the balance of the ampere turn distribution of the HV and LV windings.

• When the ampere turns of the HV and LV windings are equal and balanced, the only forces are radial.

• If the HV and LV windings are not aligned axially or one winding is physically shorter than the other, ampere turn balance is significantly affected and axial forces are magnified.

CORE FORM AND SHELL FORM TRANSFORMERS

•  When windings surround the core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form.

• Shell form design are more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils

• Core form design tends to be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications

INSULATION

• The conductor material must have insulation to ensure the current travels around the core, and not through a turn-to-turn short-circuit.

• In power transformers, the voltage difference between parts of the primary and secondary windings can be quite large. 

• So insulation is inserted between layers of windings to prevent arcing.

• Transformer may also be immersed in transformer oil that provides further insulation. 

• To ensure that the insulating capability of the transformer oil does not deteriorate, the transformer casing is completely sealed against moisture ingress.

VARNISHING

•  The main purpose of the varnish, besides increasing the electrical insulation, is to keep any form of moisture from effecting the coil

• Varnish dip is done in a vacuum chamber.

• Accomplished by preheating the conductor coils and then, when heated, dipping them in varnish at an elevated temperature.

• The coils are then baked to cure the varnish.

COOLANT

• To remove the waste heat produced by losses.

• The windings of high-power or high-voltage transformers are immersed in transformer oil - a highly-refined mineral oil that is stable at high temperatures

• Earlier, polychlorinated biphenyl (PCB) was used as it has very high ignition temperature and is highly stable.

• Now a day, nontoxic, stable silicone-based oils or fluorinated hydrocarbons are used.

•  To improve cooling of large power transformers, the oil-filled tank may have external radiators  through which the oil circulates by natural convection

COOLANT

Oil transformer with air convection cooled heat exchangers

LIQUID-IMMERSED CONSTRUCTION TRANSFORMER

Cutaway view of liquid-immersed construction transformer.

The conservator (reservoir) at top provides liquid-to-atmosphere isolation as coolant level and temperature changes. 

BUSHINGS

• An electrical bushing is an insulated device that allows the safe passage of electrical energy through an earth field.

• Larger transformers are provided with high-voltage insulated bushings made of polymers or porcelain

• A basic porcelain bushing is a hollow porcelain shape that fits through a hole in a wall or metal case, allowing a conductor to pass through its center, and connect at both ends to other equipment..

• Outside surfaces have a series of skirts to increase the leakage path distance to the grounded metal case

• The inside of the porcelain bushing is often filled with oil to provide additional insulation

PERCENT IMPEDANCE VOLTAGE

• The percent impedance is the percent voltage required to circulate rated current flow through one transformer winding when another winding is short-circuited at the rated voltage tap at rated frequency.

•  This drop in voltage is due to the winding resistance and leakage reactance.

• The percentage impedance of the transformer is calculated as

Z % = (Impedance Voltage/Rated Voltage)*100

• So for a two winding transformer with a 5% impedance, it would require 5% input voltage applied on the high voltage winding to draw 100% rated current on the secondary winding when the secondary winding is short-circuited.

IMPEDANCE LEVELS

Based kVA Minimum Impedance, %

0 – 150 Manufacturer’s standard

151 – 300 4

301 – 600 5

601 – 2,500 6

2,501 – 5,000 6.5

5,001 – 7,500 7.5

7,501 – 10,000 8.5

Above 10,000 9.5

CONNECTION SYMBOL

• The primary and secondary windings of a transformer can be connected in different configuration to meet practically any requirement.

• In the case of three phase transformer windings, three forms of connection are possible: "star" (wye), "delta" (mesh) and "interconnected-star" (zig-zag).

• The combinations of the three windings may be with the primary delta-connected and the secondary star-connected, or star-delta, star-star or delta-delta, depending on the transformers use.

• The standard method for marking three phase transformer windings is to label the three primary windings with capital (upper case) letters A, B and C, used to represent the three-phases of RED, YELLOW and BLUE. The secondary windings are labelled with small (lower case) letters a, b and c

CONNECTION SYMBOL

• Symbols are generally used on a three phase transformer to indicate the type or types of connections used.

Connection Primary Winding Secondary WindingDelta D dStar Y yInterconnected Z z