TRANSFORMER

TRANSFORMER

Transformer: Transformer is a static device, which is used for step up or step down the voltage without change in frequency, it works according to Faraday law of electromagnetic induction.
Types of transformers

According to construction
1. Oil type.    2.Dry type

According to phase
1. Single phase.  2. Three phase

According to type
1.Shell type   2. Core type 3. Berry type
(1&2 used at distribution side)
(3 is used at generating stations)

According to voltage
1. Step down (used at distribution side)
2. Step up (used at generating station)

Parts of the transformer
Winding
Core
Body
Oil
On load tap changer
Winding temperature meter
Oil temperature meter
Terminal
Breather
Buchholz relay
Conservator
Cooling system

Winding: it is copper wire with insulation, transformer consists two types of winding

1. Primary 2. Secondary Winding.

Oil: it is mineral oil with high viscosity its BDV (break down voltage) is between 40 kv to 50 kv in oil testing the gap produces 2.5mm/minute.

OLTC(On load tap changer): it is used to change the tap position of the transformer on load condition depends on output voltage, it is located at the primary side of the transformer because cable size depends on the current at secondary side high current and at primary side low current is available that's OLTC is located at primary side.

Buchholz relay: This relay is located between the transformer & conservator. This relay is activated whenever abnormal pressure is produced in the transformer tank & then it will trip the main incoming source of the transformer & gives the alarm. The abnormal pressure is produced due to moisture or the burning of winding/loose contacts etc.

Breather: it is connected to the conservator tank, it consists silicone, it is used to absorbs the moisture from the in the transformer tank, under normal condition it is in sky blue color if moisture is observed it turns in to pink color.

Temperature Sensor: Temperature raises due to heavy load, unbalance load, loose contact, loss of property of the transformer oil, low level of the oil, frequent tap change, poor maintaining of power factor etc.

Conservator: it is located at the top of the transformer, the oil level in transformer can raise and falls due to temperature this oil can enters in to the conservator chamber, if temperature increases the oil in the transformer expands, if temperature decrease the oil in the transformer also decrease, the conservator consists oil above the half of the tank.

Types of cooling for the transformer:

ONAN (oil natural air natural)
ONAF (oil natural air forced)
Water cooling system

Connection of the transformer:
1. Delta to Delta Connection is used in low voltage transformer
2. Star to Star Connection is used in high voltage transformer
3. Delta to Star Connection is used in step down transformer

Delta Connection:

I_L=√3 I_ph         V_L= V_ph

Star Connection:

V_L=√3 V_PH      I_L=I_PH

HT means high tension
LT means low tension

Daily observations in transformer:
1. Check the temperature of oil
2. Check the temperature of Winding
3. Check the colour of the breath
4. Check the working & position of tap changer
5. Check the oil level in the conservator

Earthing for transformer: There are totally five earth pits are required for transformer
Two earth pits for body earthing
Two earth pits for neutral earthing
One earth pit for lighting arrestor

Protective devices of transformer:
Winding temperature relay
Oil temperature relay
Buchholz relay
Breather
Pressure release valve
Short circuit relay
Under voltage relay
Over voltage relay
Earth Fault relay

RTCC (Remote tap Control Changer): It is used to maintain the constant output voltage and change the tap position of the transformer. It senses the output voltage and gives the command through AVR (Automatic voltage relay) to the OLTC to change the tap position of the transformer.
AVR operating voltage is 110 volts

Types faults occurred in transformer:

Ac motor failure
Surge relay trip
Buchholz relay alarm
Buchholz relay trip
Winding temperature high alarm
Winding temperature high trip
Low oil level

Types of losses in transformer: There are two types of losses in transformer

1. Copper losses
2. Iron losses

Iron losses are again classified in to two types
Eddy Current losses
Hysteresis losses

Current Transformer (CT): It is a protective device which is used to measure the high current and it is a step-down transformer with ratio depend on load.

Potential Transformer (PT): It is a protective device which is used to measure the high voltage and it is a step down transformer with ratio depend on load.

ACCP (Automatic Capacitor Control Panel): it consists capacitor bank which are used to improve the power factor of the load. it works with the help of the APFC (Automatic Power Factor Control) Relay. It is connected in parallel to the load.