transformer - s; e

TRANSFORMER Knowledge Sharing

Mahersa ALI

• Basic Theory • Definition Of Transformer • Electrical Parameters

• Types of Transformer • Cooling and Insulation • Surface Treatment • Monitoring & Protection Device • Testing • Installation, Operation &

Maintenance

Content

Basic Theory

Definition, Electrical Parameters,

Confidential Property of Schneider Electric |

Basic Theory

Transformer is… … an electrical equipment which convert electrical energy from one level voltage to another level voltage at a same frequency

Transformation Ratio

1

2

2

1

2

1

II

UU

nn

==

P = U x I x √3

kVA = kV x A x √3

P = rated power

U = rated voltage (at no-load)

I = rated current

Transformer ratio :

n= number of turns

U1, n1

I1 I2

U2, n2

I1

U1

I2

U2 / √

3

3nn

II

UU

2

1

1

2

2

1

×==

Rated voltage: step-up or step-down • Step-down:

• Primary = the higher voltage

• Secondary = the lower voltage

i.e. 20000/400 V

• Step-up: • Primary = the lower voltage (supply)

• Secondary = the higher voltage (operating)

i.e. 400/20000 V

Tapping

Example: 20 000 V ± 2.5 %

U1 n1

= pos 1 20 500 1904

= pos 2 20 000 1858

= pos 3 19 500 1812

tapping range 2.5 %

46 turns by step

To adjust the rated voltage of the transformer to the system.

⇐ plus tapping

⇐ nominal tapping

⇐ minus tapping

Magnetic core


Iron Core

Sometimes referred as Magnetic Core, it is made of laminations of grain oriented silicone steel insulated with mineral oxide. Its performance depends on the grade of the steel, the cutting pattern and the method of assembly

No Load Loss (P0) and Parameters Related to Magnetic Core

No Load Loss (P0) •Composed of:

– Hysteresis losses (reversing of magnetic flux)

– Eddy - current losses (the magnetic core is an electrical conductor)

No load current • is the current required for the core magnetisation

• is often expressed as a percentage of the rated current of the same winding.

Noise/Sound/Acoustic Level • acoustic power level Lw(A)

• acoustic pressure level Lp(A)

Windings

LV winding HV winding ● Conductor material:

● Copper (Cu) ● Aluminum (Al)

● Conductor shape: ● Round wire

● Rectangular wire ● Foil ● Strip

Iron core


Load Loss (PCU)

Load loss

● Is composed of ohmic losses (R x I2), eddy-current losses and stray-load losses

●Resistance increases as conductor temperature increase

●Reference temperature :

●75°C for the oil immersed transformers

●120°C for the cast resin transformers

● Thus, load loss should be corrected according to reference temperature

Vector Group

• 1st letters group : CAPITAL letters

⇒the higher voltage

if the neutral is brought out : YN or ZN

• 2nd letters group : small letters

⇒the lower voltage

if the neutral is brought out : yn or zn

• 3rd letters group : number ⇒clock-hour figure, indicating phase displacement between HV and LV windings

The vector group of a transformer defines the connection of the HV and LV windings


Vector Group - example

Dyn11 HV : Delta LV : Star Phase Angle =11x30°= 330°

Types of Transformer

Oil Immersed Transformer, Dry Type Transformer

Confidential Property of Schneider Electric

Types of Transformers Based on Cooling and Insulation

Based on insulation and cooling medium:

●Oil immersed transformers

● Sealed type transformer ● Hermetically Sealed Totally Oil Filled

● Sealed type with gas (N2) cushion

● Breathing type with conservator

●Dry type transformers ● Cast Resin

● Encapsulated Resin

● Vacuum Pressure Impregnated (VPI)


Types of Transformers Based on Cooling and Insulation

Dry Type Transformer. Use air as cooling medium Oil Immersed Transformer. Use oil as cooling and insulation medium

Standard solution. Up to 36 kV and 4000 kVA.

Oil Immersed Transformer

Suitable for high rating up to 170 kV and 100 MVA. Customized solution. Up to 36 kV and 10 MVA.

Based on method to absorb oil expansion

Hermetically sealed totally oil filled

• Transformers is fully filled with oil.

• Expansion of oil is compensated with deformable fins radiator

Sealed type with gas cushion

• Not fully filled with oil, this transformer has N2 gas cushion on the top of tank to absorb oil expansion.

Breathing Type with Conservator

• Oil expansion is absorbed by conservator tank on top of main tank.


Cooling and Insulation


Temperature Rise

For Oil Immersed Transformer

Part Temperature rise °K

Windings : Temperature class A (measured by resistance)

65, for natural or forced oil circulation, 70, for forced and directed oil circulation

Top oil (measured by thermometer)

60, when the transformer is sealed or equipped with a conservator, 55, when the transformer is not sealed

Part Cooling method Class of temp.

Temp. rise °K

Windings (measurement by resistance)

Air natural or forced A E B F H

60 75 80

100 125 150

For Dry Type Transformer

Cooling

Examples

1st letter 2nd letter 3rd letter 4th letter

Indicating the cooling medium that is in contact with windings Indicating the cooling medium that is in contact with external cooling system

Kind of medium Kind of circulation Kind of medium Kind of circulation

Example :

O N A N

ONAN

AN

Order of symbols

Symbols Kind of cooling medium Symbol

Mineral oil or synthetic insulating fluid with flame point ≤ 300°C Insulating fluid with flame point > 300°C Askarel Gas Water Air Solid insulant

O K L G W A S

Kind of circulation Natural Forced Directed

N F D

Surface Treatment


Corrosivity Categories ISO 12944-2 Standard

EXTERIOR INTERIOR

Atm

ospheric corrosivity category IS

O12944-2

C1 Very Low Corrosivity

- ● Heated buildings with clean atmospheres ● Examples : Offices, shops, schools, hotels

C2 Low Corrosivity

● Atmospheres with very low level of pollution ● Mostly rural areas

● Unheated buildings where condensation may occur ● Examples : Depots, sports halls

C3 Medium Corrosivity

● Urban and industrial atmospheres, moderate sulphur dioxide pollution

● Coastal areas with low salinity

● Productions rooms with high humidity and some air pollution

● Examples : Food-processing plants, laundries, breweries, dairies

C4 High Corrosivity

● Industrial areas and coastal areas with moderate salinity

● Chemical plants, swimming pools, coastal ship-and boatyards

C5Marine Very High Corrosivity

● Coastal and offshore areas with high salinity ● Buildings or areas with almost permanent condensation and with high pollution

C5Industrial Very High Corrosivity

● Industrial areas with high humidity and aggressive atmosphere

● Buildings or areas with almost permanent condensation and with high pollution

Durability Categories ISO 12944-2 Standard

LOW 2 - 5 years

MEDIUM Durability Maintenance : 5 - 15 years

HIGH Durability Maintenance : > 15 years

Atm

ospheric corrosivity category IS

O12944-2

Guarantee Ri 3 – 5 years Ri 2 – 5 years

C3 Medium Corrosivity

> 240 Hours (Neutral Salt Spray Resistance - ISO 7253 standard)

> 480 Hours

C4 High Corrosivity

> 480 Hours > 720 Hours

C5 Marine Very High Corrosivity


C5 Industrial Very High Corrosivity

> 720 Hours > 168 Hours (Chemical Resistance –

ISO 2812-1)


Monitoring & Protection Device


Off circuit tap changer

• To adjust the voltage of the transformer at off circuit (not only at no-load) condition, the transformer has to be disconnected from the network.

• Tap changer is operated by means of a handle on the cover or a hand wheel on top or on sidewall of the transformer.

On Load Tap Changer (OLTC)

• To adjust the voltage automatically, without de-energizing the transformer

1. Integrated Safety Detector

2. Oil Level Indicator

3. Pressure Relief Device

4. Oil Temperature Indicator

5. Winding Temperature Indicator

6. Buchholz Relay

7. Pressure Vacuum Gage

8. Sudden Pressure Relay

Monitoring & Protection Devices


Function:

• Oil level indication

• Oil temperature indication

• Over pressure detection

All functions are provided with contact for alarm and/or trip

Standard monitoring device


Integrated Safety Detector (R.I.S®, DMCR®, DGPT2®) Hermetically Sealed Totally Oil Filled type

Oil Level Indicator For Sealed Type with N2 Gas Cushion Type and Breathing Type

Function : to measure the level reached

by the oil in a transformer conservator

Pressure Relief Device Standard device for all type

Function : A device to relieve large volumes of gas or insulating fluid rapidly when the pressure inside a transformer reaches a pre-determined limit.

Oil Temperature Indicator

For Sealed Type with N2 Gas Cushion Type and Breathing Type with Conservator

Function : to measure the oil temperature inside transformer tank

Winding Temperature Indicator Standard device for high rated power

Function: • to measure the winding temperature by means of a special

bulb surrounded by heating resistance, through which passes a current proportional to the current passing through the transformer winding.

Buchholz Relay for breathing type with conservator

Function : • to trip out the transformer if there is a release of gas arising

from the decomposition of the solid and liquid insulating material due to the action of heat or electric arcing.

Pressure Vacuum Gauge

for sealed type with gas cushion

Function : To check the vacuum pressure inside the tank, applicable for sealed type transformer with N2 gas cushion

Test ing


Transformer Testing

Performed on one unit to validate design. Subject to additional cost. Performed on all units, free of charge.

For distribution transformer, according to IEC 60076

ROUTINE TEST

1. Measurement of Winding Resistance

2. Measurement of Turn Ratio and Vector Group Check

3. Measurement of No Load and No Load Current

4. Measurement of Load Losses and Impedance Voltage

5. Induce Test

6. Applied Test

7. Oil Break Down Voltage Test

TYPE TEST

1. Temperature Rise Test

2. Lightning Impulse Test


Winding Resistance Test

Test Objective: To Measure the resistance in

ohms of each of the windings of the transformer


Turn Ratio Test

Test Objective: To check conformity to specification of the transformer turn ratio and vector group connection Acceptance Criteria: Tolerance : ± 0,5% of theoretical ratio


No Load Losses & Current Measurement

Test Objective: 1. To measure no load losses

and no load current of the transformer

2. To check whether the characteristics conform the applicable standards

Acceptance Criteria: +15% tolerance from declared

value according to IEC 60076


Load Losses & Impedance Measurement

Test Objective: 1. Measurement of the

transformer’s load losses 2. Measurement of impedance

voltage Acceptance Criteria:

•Load losses tolerance: +15% from declared value •Impedance tolerance:± 10% from declared value • All according to IEC 60076


Induce Overvoltage Test

Test Objective: The test the insulation strength

inter turns and inter layer of the windings.

Acceptance Criteria: The transformer is considered to have passed when there is no voltage collapse or variation in current


Applied Voltage Test

Test Objective: To test the insulation strength

between live parts to earth Acceptance Criteria:

The transformer is considered to have passed when there is no voltage collapse in 60 second

HV LV

kV HV side

LV Side

TEST FOR HV SIDE

TEST FOR LV SIDE

HV LV

kV HV side

LV Side

Temperature Rise Test

Test Objective:

1. To determine the temperature rise of the oil (ΔT oil) 2. To determine the temperature rise of the HV and LV windings

3. To confirm the rated power of the transformer Acceptance Criteria:

•According to approved documents and specifications; ex ΔT oil : 60°C, ΔT windings : 65°C

Lightning Impulse Test Test Objective:

•To check the impulse withstand of each winding relative to earth, relative and to other windings and along the tested winding

Acceptance Criteria: •Reduced and full wave should be similar

Function Test of Measuring & Protection Devices

Test Objective: • Check of control and protection devices : relay function, accessories function and

continuity of wiring from devices into wiring box (Regulator Control Panel, Lamp, Power socket, Heater, Oil level gauge, Pressure Relief Device, Oil Thermometer, Winding Thermometer, Buchholz Relay, Junction Relay (RS. 2001, OLTC), Control Panel, Current Transformer (CT), etc.)

Installation, Operation & Maintenance

INSTALLATION Installation should be carried out in compliance with following rules / conditions:

1. All external cables connections are tight and secure

2. All accessories circuits operate properly

3. The tap changer is securely in correct position

4. The neutral and earth connections have been properly installed

5. Fans (if any) has been installed and operates properly

6. Proper clearance is maintained from high voltage bus to terminal equipment

7. All windings are free from un-intended earths

8. There is no dust, dirt or foreign material on core and coils (dry-type transformers)

9. There is no visible moisture on or inside the core and coils (dry-type transformers)

10. All plastic wrappings are removed from the core and coils (dry-type transformers)

11. There are no obstructions in or near the openings for ventilation

12. Commissioning should be carried out prior to energizing

Checklist before operation (part 1)

OPERATION

No.

CHECK ITEMS

DESCRIPTION

SPECIFIED

REMARK 1

Out line drawing

Specification

Compliance with drawing and specification

2

Oil level

Main tank

Level gauge to read correctly for existing level and oil temperature

3

Valve position

1. Valves

Open and close position 2. Radiator valves

3. Other valves 4

Off load tap changer

Manual operation through all tap position

As specified

No.

CHECK ITEMS

DESCRIPTION

SPECIFIED

REMARK

5 Electrical tests

1. Insulation resistance of windings No insulation breakdown

As reference

2. Ratio and polarity Acc. to specification

3. Dielectric strength of oil >30 kV

4. Alarm, tripped, control circuits, and auxiliary circuit 1. Wired properly and comply with wiring 2. Contacts open or close as specified.

6 Earthing Tank and Marshalling kiosk It should be connected to earth

7 Bushing

1. Porcelain Clean Proper continuity and tightness

2. Spark Gap

3. Connected

8 Other Cleans rust Cleans Clean and touch point

Checklist before operation (part 2)

OPERATION

ROUTINE MAINTENANCE No Task Acceptance Criteria Frequency Remarks

1 Check the pressure relief disc No crack Yearly Contact the transformer supplier

2 Check ventilation and ambient temperature inside the transformer room

< 40o C Yearly Ensure adequate ventilation and the compliance with Section 7 - Installation.

3 Check insulating liquid temperature < 100o C Yearly Conduct investigation if the insulating liquid temperature is abnormally high with due respect to the load current and ambient temperature

4 Check for leakage of insulating liquid level No insulating liquid leak Yearly If leakage is found, refer to Appendix 1.1

5 Check insulating liquid level Middle between Max & Min Yearly If level on the RIS insulating liquid level indicator is too low, refer to Appendix 1.2

6 Check the RIS settings Refer to the RIS Catalogue & Instructions 2-yearly Reset the settings if different from that in the RIS

Catalogue & Instruction

7 Check dielectric strength of the insulating liquid

Breakdown voltage test > 30kV/2.5mm 2-yearly For sampling details, please refer to Appendix 1.3

8 Check paintwork No paint peeling or rust 2-yearly The frequency should be increased for hostile environment

9 Arrange outage to check and clean bushings No dust 5-yearly The frequency should be increased to 4 years or

less for hostile environment

Electrical Test: •Winding Resistance •No Load Current •Capacitance •Transmission ratio

Advance (Special Test): •Partial Discharge •Dielectric Response •SFRA

Oil Analysis: •Color •BDV •Water Content •DGA

On Line Monitoring: •Voltage, Current, Temperature •Oil, tap changer, cooling •Bushings

Post Mortem : • Degree of polymerization •Moisture in pressboard/paper

Periodic/Routine Maintenance

Diagnostic Method for Transformer

Questions?


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