Transformer

TRANSFORMER

BY pradeeprajr93@gmail.com

What is it ?

▪ It transforms the VOLTAGE level without changing FREQUENCY.

V v

▪ Rated in – MVA (POWER TRANSFORMERS)

– KVA (DISTRIBUTION TRANSFORMERS)

int const f = 50;

Static Device

Let’s Explore it

▪ Various parts – Core – Winding

▪ HV winding ▪ LV Winding

– HV and LV Bushings – Conservator Tank – Breather – Buchholz Relay – Cooling Fins and Fans

▪ Tests involved

▪ Recent trends

CORE OF A TRANSFORMER

CONSTRUCTION

Laminated core is made of SILICON STEEL.(prevent eddy current loss) It has low hysteresis area and high permeability. Core is also used to give mechanical support to the windings. Proper distribution of flux

Types involved

▪ In case of Shell type transformer Core surrounds the coil.

▪ In case of core type transformer coil surrounds the core.

Transformer Winding

HV windings – It carries very low current

– So it’s thickness is less compared to

LV winding

– We can fully wound the entire coil

continuously (DRUM Type coil).

But during fault entire coil has to be

replaced.

– We can section/segment the coil into two to four sections for ease of winding and replacement.

– DPC (Double Paper Coating) is used as insulation

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Section 8

Sectioned coil

Transformer Winding

LV Windings – As we know LV winding has to carry

more current than HV, it’s thickness is large.

– Based on number of turns in LV winding and primary secondary Voltage/Current, we can calculate the primary turns.

𝑁1

𝑁2 =

𝐸1

𝐸2 =

𝐼2

𝐼1

Transformer Winding

▪ In general, LV windings are wounded around the core first, then HV windings.

▪ So less amount of insulation is enough.

▪ Every windings in LV have two terminals (Inner and Outer)

▪ In this case, all inner terminals of three phases(R,Y,B) are shorted and welded together to form neutral winding

▪ The remaining three outer terminals forms the phases R,Y,B.

Transformer Bushings

▪ The HT windings are mostly Δ (delta) connected and LT windings are Υ (star) connected.

▪ The terminals are taken out and connected to bushings.

Arrangement of Bushings

Primary

R Y B

Secondary

E N R Y B

Earth* Neutral

* Newly introduced especially in Star rated Energy Efficient transformers

CONSERVATOR TANK

▪ Reservoir for Transformer oil situated at top.

▪ It should be half-filled, because to provide space for oil during expansion while heating and to supply oil when oil level reduced in transformer during cooling.

Breather

▪ As we know that level of oil rise and fall during heating and cooling.

▪ Cooling takes air in and heating let the air out.

▪ This aspirated air contain some moisture to breather is filled with SILICA GEL (desiccant air drier) else oil will be deteriorated.

Buchholz Relay (Only in power Transformers)

▪ On a slow accumulation of gas, due 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.

Cooling fins and fans

▪ Due to convection hot oil rise up and flow through cooling tubes and fins. The oil is cooled by either natural air or forced air using Fans.

Cooling tubes

Tap Changing

▪ It allows a variable number of turns to be selected in discrete steps.

▪ A transformer with a variable turns ratio is produced, enabling stepped voltage regulation of the output.

▪ The tap selection may be made via an automatic or manual tap changer mechanism.

Types of TAP-CHANGING

▪ OLTC (On Load Tap Changer) – Used in large Power Transformer where

power interruption is unaccepted.

▪ NLTC (No Load Tap Changer) – Due to short circuit problems, the power

is shut down during tap changing operation

Power Transformer Protection

▪ OTA - Oil Temperature Alarm (if exceeds 85°C)

▪ WTA- Winding Temperature Alarm (if exceeds 90°C)

▪ WTT- Winding Temperature Trip

▪ BA – Buchholz Alarm

▪ BT – Buchholz Trip

▪ MOGA- Minimum Oil Gauge Measurement

▪ PRV – Pressure Release Valve

▪ OSTC – OLTC Surge Relay Trip

▪ DRT- Differential Relay Trip

Transformer Tests

▪ Short Circuit Test

▪ Open Circuit Test

▪ Induced Voltage Test

▪ Back-to-Back Test

▪ Megger test – Ground-Ground (zero resistance)

– Line-Ground (infinite resistance)

– Line-Line (infinite resistance)

▪ Heated in HOT AIR CHAMBER at 200°C around 6hours

Transformer Oil Tests

▪ Medium used for cooling and insulation is hydro carbonated Transformer oil.

▪ Tests involved are – BDV (Break Down Voltage test)

– Acidity Test

– Interfacial Tension Test

Recycling of Transformer Oil

▪ It has – Three chambers

– 3 x 3 kW heater

– Propulsion System

– Belt driven Clay

Process in Recycling chamber

▪ Chamber 1 & 2 – Impure oil is circulated inside chamber at 60°C – Then heated clay at 110°C is mixed and kept circulated for

three hours. – Leave 16hours for settling of clay with carbon and impurities

(REST PERIOD) – Pure oil is let out and further filtered using bladder

▪ Chamber 3 – It takes in the pure oil from bladder and mixes with chemicals

and CO2

– This improves the quality of transformer oil and immune to corrosion problems in transformer.

▪ Finally the output is filtered in cartridge and filled in Drums for transportation.

Energy Efficient Transformers

▪ In a typical power distribution grid, electric transformer power loss typically contributes about 40-50% of the total transmission and distribution loss.

▪ Energy efficient transformers are therefore an important means to reduce transmission and distribution loss.

▪ With the improvement of electrical steel (silicon steel) properties, to achieve higher efficiency

▪ E.g.: Amorphous Metal Transformers

▪ These transformers are star-rated. ✭✭✭

Features of Energy Efficient Transformers

▪ Addition Earth bushing to ground fault current instantly, high safety and better efficiency.

▪ Guaranteed maximum temperature rise in Oil/Winding is 35/40°C.

▪ Guaranteed Total loss @ 100% loading is 3320watts.

▪ Guaranteed Total loss @ 50% loading is 1050watts.

▪ These losses are double the amount in case of ordinary distribution transformers.

Some specifications for your REFERENCE*

▪ Type of cooling involved – ONAN (Oil Natural Air Natural)

▪ Weight of Core and Windings – around 775Kg

▪ Weight of Oil – around 418Kg (480 litres)

▪ Total weight – around 1433 Kg

* The above mentioned values vary based on the capacity of transformer, it is 250KVA Energy Efficient distribution transformer

And finally, it’s my turn to