Power Transformer.pdf

Power Transformer

By Dr. Tarek Saad Abdel-Salam

1

YOU WILL LEARN

An understanding of the fundamental theory and principles of the operation of power transformers

An insight into the identification and application of transformers‘ types An understanding of the power transformers components and their construction

2

YOU WILL LEARN

Knowledge of power transformer protection An understanding of power transformers oil and oil tests and interpretation of results Knowledge of the most effective power transformer electrical tests Skills in how to manage power transformer breakdowns to ensure a minimum disruption 3

WHO SHOULD ATTEND

Power System Engineers Electrical Engineers Consulting Engineers Project Engineers Power System Technicians Electrical Contractors

4

WHO SHOULD ATTEND

Electrical Technicians Tradesman Electricians Electrical Inspectors Utility Engineers

5

Transformer Theory Transformers are used extensively for AC power transmissions and for various control and indication circuits. Knowledge of the basic theory of how these components operate is necessary to und erstand the role transformers play in today’s nuclear facilities 6

Mutual Induction If flux lines from the expanding and contracting magnetic field of one coil cut the windings of another nearby coil, a voltage will be induced in that coil. The inducing of an EMF in a coil by magnetic flux lines generated in anoth er coil is called mutual induction. The amount of electromotive force (EMF) that is induced depends on the relative positions of the two coils. 7

Turns Ratio Each winding of a transformer contains a certain number of turns of wire. The turns ratio is defined as the ratio of turns of wire in the primary winding to the number of turns of wire in the secondary winding TurnsRatio =

Np Ns 8

Impedance Ratio Maximum power is transferred from one circuit to another through a transformer when the impedances are equal, or matched. A transformer winding constructed with a definite turns ratio can perform an impedance matching function. The turns ratio will establish the proper relationship between the primary and secondary winding impedances. 2

Zp ⎛ Np ⎞ ⎜⎜ ⎟⎟ = Zs ⎝ Ns ⎠

9

Impedance Ratio

10

Efficiency

Efficiency of a transformer is the ratio of the power output to the power input

Power Efficiency = Power

Output Ps = x100 Input Pp 11

Theory of Operation

A transformer works on the principle that energy can be transferred by magnetic induction from one set of coils to another set by means of a varying magnetic flux. The magnetic flux is produced by an AC source. The coil of a transformer that is energized from an AC source is called the primary winding (coil), and the coil that delivers this AC to the load is called the secondary winding (coil)

12

Theory of Operation

13

Voltage Ratio

The voltage of the windings in a transformer is directly proportional to the number of turns on the coils

Vp Vs

=

Np Ns 14

Current Ratio

The current is inversely windings

in the windings of proportional to the

a transformer voltage in the

Vs I p = Vp I s 15

Three-Phase Transformer Connections

Delta Connection

Wye Connection

16

Combinations of Delta and Wye Transformer Connections

17

Voltage and Current Ratings of Transformers TABLE 1: Voltage and Current Ratings of Transformers Transformer Connection (Primary to Secondary)

Primary Line

Secondary Phase

Line

Phase

Volt.

Current

Volt.

Current

Volt. *

Current

Volt.

Current

∆-∆

V

I

V

I/ 3

V/a

aI

V/a

aI/ 3

Y-Y

V

I

V/ 3

I

V/a

aI

V / 3a

aI

Y-∆

V

I

V/ 3

I

V / 3a

3aI V / 3a

aI

∆-Y

V

I

V

I/ 3

3V/ a aI/ 3

V/a

aI/ 3 18

Transformer Losses and Efficiency

Losses: z

Copper loss is power lost in the primary and secondary windings of a transformer due to the ohmic resistance of the windings

Copper

Loss = I R p + I Rs 2 P

2 S

19

Transformer Losses and Efficiency

Core losses are caused by two factors: hysteresis and eddy current losses

20

Efficiency Output Efficiency = Input

Power Ps = x100 Power Pp

Vs I s pf Efficiency = Vs I s pf + Copper Loss + Core

Loss

x100

21

Transformer Operation Under No-Load

If the secondary of a transformer is left open-circuited, primary current is very low and is called the no-load current

22

Coil Polarity

The phase of that voltage depends on the direction of the windings around the core

23

Transformer Theory Summary

¾ The induction of an EMF in a coil by magnetic flux lines generated in another coil is called mutual induction. ¾ The turns ratio is defined as the ratio of turns of wire in the primary winding to the number of turns of wire in the secondary winding. ¾ The ratio between the primary and secondary impedances is referred to as the impedance ratio. ¾ Efficiency of a transformer is the ratio of the power output to the power input. ¾ In a delta connection, all three phases are connected in series to form a closed loop. ¾ In a wye connection, three common ends of each phase are connected together at a common terminal, and the other three ends are connected to a three-phase line. In a ∆connected transformer: VL=Vφ IL= 3 Iφ ¾ ¾ In a Y connected transformer: VL= 3 Vφ

IL=Iφ 24

Transformer Equivalent Circuit

25

Transformer Approximated Equivalent Circuit

26

Voltage Regulation

ε=

I s Req cos θ ± I s X eq sin θ Vp

x100

27

Transformer Phasor Diagram

28

Power Transformer


1

TRANSFORMER TYPES • Transformers can be constructed so that they are designed to perform a specific function. A basic understanding of the various types of transformers is necessary to understand the role transformers play in today’s facilities

2

TRANSFORMER TYPES • • • • • •

a. Power b. Control c. Auto d. Isolation e. Instrument current f. Instrument potential

3

Power Transformer • Power transformers are generally used in electrical power d stribution and transmission systems. This class of transformer has the highest power, or volt-ampere ratings, and the highest continuous voltage rating. The power rating is normally determined by the type of cooling methods the transformer may use. Some commonly-used methods of cooling are by using oil or some other heat-conducting material. Ampere rating is increased in a distribution transformer by increasing the size of the primary and secondary windings; voltage ratings are increased by increasing the voltage rating of the insulation used in making the transformer.

4

Control Transformer • Control transformers are generally used in electronic circuits that require const ant voltage or constant current with a low power or volt-amp rating. • Various filtering devices, such as capacitors, are used to minimize the v ariations in the output. This results in a more constant voltage or current. 5

Auto Transformer • The auto transformer is generally used in low power applications where a variable voltage is required. The auto transformer is a special type of power transformer. It consists of only one winding. By tapping or connecting at certain points along the winding, different voltages can be obtained

6

Auto Transformer

7

Isolation Transformer • Isolation transformers are normally low pow er transformers used to isolate noise from or to ground electronic circuits. • Since a transformer cannot pass DC voltage from primary to secondary, any DC voltage (such as noise) cannot be passed, and the transformer acts to isolate this noise.

8

Instrument Potential Transformer • The instrument potential transformer (PT) steps down voltage of a circuit to a low value that can be effectively and safely used for operation of instruments such as voltmeters, watt meters, and relays used for various protective purposes.

9

Instrument Current Transformer • The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer. This is done by constructing the secondary coil consisting of many turns of wire, around the primary coil, which contains only a few turns of wire. In this manner, measurements of high values of current can be obtained 10

Summary • Power transformers are generally used in power distribution and transmission systems. • Control transformers are generally used in circuits that require constant voltage or constant current with a low power or voltamp rating.

11

Summary • Auto transformers are generally used in low power applications where a variable voltage is required. • Isolation transformers are normally low power transformers used to isolate noise from or to ground electronic circuits. • Instrument potential and instrument current transformers are used for operation of instruments such as ammeters, voltmeters, watt meters, and relays used for various protective purposes. 12

Types of power transformer • (A) Oil Immersed Power Transformers • (B) Dry type Power Transformers • (C) Special Power Transformers

13

Oil Immersed Power Transformers • Double Wounded, Three Phase Power Transformers with On Load Tap Changer rated 12 ► 63 MVA • Double Wounded, Three Phase Power Transformers with On Load Voltage Control • rated 2.5 ► 10 MVA 14

Oil Immersed Power Transformers • Electrical Network Interconnecting Transformers and Autotransformers • Three Phase Power Transformers and Autotransformers having 3 Windings, with On Load Voltage Control • rated 25 ► 200 MVA 15

Oil Immersed Power Transformers • Three Phase Power Transformers and Autotransformers without Voltage Control or with No Load Voltage Control • rated 67 ► 160 MVA • Three Phase Power Transformers for Drilling Plants 16

Oil Immersed Power Transformers • Three Phase Power Transformers for the Internal Services of the Nuclear Power Plant • Three Phase Power Transformers for the Internal Services of the Power Plant

17

Oil Immersed Power Transformers • Three Phase Power Transformers with Fixed Ratio rated 170 ► 440 MVA • Three Phase Power Transformers with No Load Tap Changing rated 1000 ► 1600 kVA, in special construction 18

Oil Immersed Power Transformers • Transformers used to supply the Static Power Converter

19

Double Wounded, Three Phase Power Transformers with On Load Tap Changer

• Conditions of operation • The transformers are meant to operate in power transport networks at the ambient temperatures of max. + 40°C and min. - 35°C. • The altitude of the installation site is max. 1000 m above the sea level. • At the customer's request, the transformers may be manufactured to operate in tropical environment with ambient temp. 45-55°C and protected by encapsulation for polluted areas. 20


• Construction features • The magnetic circuit is made of carlyte insulated, grain oriented, cold rolled silicous steel sheet with low specific losses • The windings are made of paper insulated copper wire. • They are helical type or flat coils on the LV side and continuous in the flat coils on the HV side. • The tank is bell or cover type, made of welded steel plate, stiffened with ribs. 21

Double Wounded, Three Phase Power Transformers with On Load Tap Changer Rating (MVA)

Voltage (kV)

Uk (%)

Po Pk (kW) (kW)

Connection group

Wdg. Material

Overall Weight (kg)

Overall Dimensions (LxWxH)(mm)

12

33/11

8

10.5

65

Dyn5

Cu

33400

5340x3825x3890

15/20

132/15/3.3

9/19/8

21

100

Ynyn0d

Cu

56700

6415x4150x4430

16

110/22

11

18

94

YND11

Al

34800

5980x3084x4278

25

110/22

17

24

139

YNd11

Al

50200

6480x3715x4755

25

154/6.3

11

25

110

YNyn0

Cu

62800

7634x4730x4820

25

110/22

11

21

143

YNd11

Al

42400

3970x3215x4175

25

116/6.3

14

25

143

YNd11

Cu

40300

5866x3890x4476

25

154/34.5

10

24

115

YNyn0

Cu

62500

7505x4630x5635

25

110/22

11

21

143

Ynd11

Al

29650

5490x3774x4660

17.5/35

66/11

15.5

18.5 36/145

YNd11

Cu

54000

6150x4450x4435

40

110/22

1212

31

165

YNd11

Cu

59250

6305x4066x4520

40

110/6.6

18.5

34

220

YNd11

Al

62300

7380x3525x5045

50

150/15.75

21

42

190

Dyn1

Cu

110500

7375x5620x6220

63

110/22

12

55

265

YNd11

Al

80800

7070x5165x6170

150/10.5

12

55

300

Ynd11

Al

88150

7715x4105x6010

110/6.3/6.3 1.114-13

65

110

Dyn1yn1

Cu

121500

8250x3725x6470

63 6.3/31.5/31.5 63

110/38.5

13

45

260

YNd11

Cu

76000

6515x4350x5210

16

110/6.6

11

18

94

YNd11

Al

34800

5900x3084x4278

15/20

66/11

10

16

90

Dyn11

Cu

33000

4550x4630x4519

15/20

66/6.6

10

16

90

Dyn11

Cu

33000

4550x4630x4510

25

110/6.3

11

21

143

YNd11

Al

43030

5940x3772x4660

25

66/11/6.3

11

22

140

YNyn0d1

Cu

55800

6770x4230x4600

25/30

150/15/6.6

10

28

160

YNyn0d1

Cu

76000

7850x4650x5900

30/50

115/22

16.66

26

200

YNd11

Cu

79000

7000x5000x5330

40/50 161.25/6.6-6,6

10

33

159

YNy2y2

Cu

103000

7350x4600x6100

>19

25

161

YNyn0

Cu

105000

7875x5060x5900

40/50

150/21

22


1. HV bushings

10. Jacking supports

2. HV neutral bushing

11. Oil draining and filtering valve

3. LV bushings

12. Control cabinet

4. Oil conservator

13. Wheels

5. On load tap changer

14. Fans

6. Motor drive for OLTC

15. Winding temperature indicator

7. Pressure relief device

16. Oil temperature indicator

8. Dehydrating breather

17. Buchholz relay

9. Radiator

23

Oil Immersed Power Transformers • Double Wounded, Three Phase Power Transformers with On Load Voltage Control rated 2.5 ► 10 MVA

24

Double Wounded, Three Phase Power Transformers with On Load Voltage Control

• Conditions of operation • The transformers are destined to operate in electric power carrying networks at the ambient temperatures of max. + 40º C and min. - 40º C. The altitude of the installation site is max 1000 m above the sea level. At the user's request, transformers being able to operate at tropical temperatures can be manufactured. 25


• Construction features • The magnetic circuit is made of carlyte insulated, grain oriented, cold rolled silicous steel sheet with low specific losses. • The transformers windings are made of paper insulated copper wire and their type is: helical or continuous in flat coils on the HV side and continuous in flat coils on the LV side. • The tank is bell or cover type, made of steel plate, welded construction, stiffened by ribs. 26


Rating Voltage Uk Po Pk Overall Weight Overall Dimensions Connection group Bob. (MVA) (kV) (%) (kW) (kW) (kg) (LxWxH)(mm) 2.55

66/11.5

9

4.1

21

Dyn11

Cu

22325

4800x2800x4265

5

66/11.5

9

7.25

28.8

Dyn11

Cu

27200

4780x2970x4470

6.3 132/34.5

8

8.6

32

YnZn11

Cu

33300

5600x2875x4765

7.5

33/1

7.5

6.3

41

Ynd11

Cu

22900

5115x3810x3410

10

110/22

11

16

68

Ynd11

Al

32390

5505x3120x3470

10/13 132/11.5

12

10

73

Dyn1

Cu

34900

5410x5010x4600

27


1. HV bushings

10. Jacking supports

2. HV neutral bushing

11. Oil draining and filtering valve

3. LV bushings

12. Control cabinet

4. Oil conservator

13. Wheels

5. On load tap changer

14. Fans

6. Motor drive for OLTC

15. Winding temperature indicator

7. Pressure relief device

16. Oil temperature indicator

8. Dehydrating breather

17. Buchholz relay

9. Radiator

28

Dry type Power Transformers • Dry type Power Transformers rated 10kVA ► 1600 kVA • Dry type Power Transformers for Rectifier Plants and for Converters rated 430; 550; 630; 720; 900 kVA

29

Dry type Power Transformers rated 10kVA ► 1600 kVA • Conditions of operation • The transformers are meant to supply the inner auxiliaries of the power stations within the underground train stations and residential districts. The transformers are rated to operate in suitably ventilated closed rooms and are supplied from electrical grids free from the atmospheric surge voltages. • The ambient temperatures are: • max. ambient temperature: +40° C • max. air daily average temperature +30° C • min. air temperature -15° C • max. altitude: 1000 m above the sea level • Protection degree: IP00 or IP20 30

Dry type Power Transformers rated 10kVA ► 1600 kVA

• Construction features • The magnetic circuit is made of carlyte insulated, grain oriented, cold rolled silicous steel sheet • The windings are manufactured of glass-fiber and enamel insulated cooper wire, impregnated in electro-insulating varnish. • The winding types used are: cylindrical or helical on the LV side and layered or continuous, in flat coils on the HV side. • The voltage control on the HV side is achieved at a terminal plate, by means of ringlets (bars). 31


• Construction features • The HV and LV connections are manufactured of glass fiber strip insulated copper buses. • The outlets are taken out on the transformer small side or top, on the HV side through bushings and on the LV one through buses. Function of the customer's request and with the manufacturer's agreement, the outlets may be located otherwise too: by performing the required changes of the overall dimensions. • The protection housing is made of steel sheet placed on a metal frame. At the top it is provided with louvers enabling air free flow. The transformer may be lifted by means of lugs provided on its structure.

32

Dry type Power Transformers rated 10kVA ► 1600 kVA Rating (kVA)

Voltage (kV)

Uk Po Pk Overall Weight Overall Dimensions Connection group (%) (kW) (kW) (kg) (LxWxH)(mm)

10 0.38/0.048

3

0.11

0.25

Dy11

140

700x300x440

25 0.38/0.052

4

0.17

0.4

Yyn0

240

850x355x530

40 0.38/0.122

4

0.24

0.68

Yy0

300

805x350x570

60

0.5/0.23

4

0.4

1

Yy0

450

950x510x780

100

10/0.4

3.5

0.86

1.05

Yy0

1045

1925x840x1060

160

6/0.47

8

0.55

2.8

Yyn0

746

1240x440x960

200

6.3/0.4

4

0.95

2.5

Yyn0

1085

1280x620x1258

250

20/0.4

6

1.3

3.2

Dyn5

1670

1640x676x1500

315

6/0.47

4

1.4

2.4

Yyn0

2125

2340x780x1725

400

6.3/0.4

6

1.5

3.8

Dyn5

1885

2200x1000x1350

630

6.3/0.66

6

1.75

6

Dy05

2600

2155x1030x1670

630

20/0.4

6

2.3

7.2

Dyn5

2485

1840x800x1620

750

13.8/0.4

8

1.9

6.5

Dyn11

3120

1750x850x1880

800

6/0.427

6

2.1

7

Dd6

2846

1630x790x1643

900

6/0.66

6.5

2.2

6.8

Dy11

3300

2150x800x1355

1000

6/0.4

6

2.5

8.5

Dyn5

3370

2200x1050x1840

1000

10/0.4

6

2.7

8.5

Dyn5

3640

2300x1100x1950

1600

6.3/0.4

6

3

11

Dyn-5

5655

2660x1050x2360

33


1. LV buses

7. Grounding terminal

2. HV bushing

8. Undercarriage

3. Lifting lug

9. Temperature micro relay

4. Name plate

10. Shutter

5. Warning name plate

11. Lifting lugs for housing

6. Alarms name plate

12. Removable cover 34

Dry type Power Transformers for Rectifier Plants and for Converters

• Conditions of operation • Temperate environment in properly ventilated closed room: • the max. ambient temperature +40°C; the min. ambient temperature -15°C; • the max. altitude 1000m. • IP00 or IP22 protection degree (with housing for protection). 35

Dry type Power Transformers for Rectifier Plants and for Converters • Construction features • The magnetic circuit is of carlyte insulated, grain oriented, cold rolled silicous steel sheet with low specific losses. • The windings are made of copper wire, insulated with enamel and glass fibber impregnated in electroinsulating varnish. • The practical winding types are: cylindrical on the LV side and continuous on the HV side. • The insulation of the winding is made of glass fibber fabric in layers. • The control on the high voltage range is made at a terminal box. 36

Dry type Power Transformers for Rectifier Plants and for Converters Construction features The connections are made of copper buses, insulated with a glass-fibber strip. • The terminals are brought out of the shorter housing walls through bushings for high voltage side, and through copper buses for the low voltage side. • The protection housing is made of steel sheet, fixed on a metallic structure. • At the upper part, it is provided with louvers and holes for air exchange and at the lower side, the transformer has a skid and earthing pad. • The transformer lifting is made by means of lifting eyes provided on the structures. 37


Rating Voltage Uk Po Pk Connection group (kVA) (kV) (%) (kW) (kW)

Overall Weight (kg)


30

60/0.38

6.5

1.4

4

Dy11

1936

1960x800x1150

430

10/0.42

6.5

1.5

4

Dy11

2060

2270x1000x2000

550

6/0.44

6.5

1.5

5

Dy11

2135

1960x760x1270

630

6/0.44

5.5

1.75

5.5

Dy11

2490

1590x840x1490

720

6/0.36

6.5

2.05

6

Dd0

2950

2100x1000x1625

720

10/0.42

6.5

1.95

6

Dy11

2925

2280x1010x1565

900

6/0.825

6.5

2.2

6.6

Dy11

3360

2220x750x1480

900

6/0.43

6.5

2.2

6.6

Dy11

3490

2220x750x1480

38


1. LV buses

6. Lifting lugs

2. HV bushing


3. HV protection box

8. Undercarriage


9. Removable cover

5. Housing

10. Name plat 39

Special Power Transformers • Fire Damp Transformers • rated 100 ► 500 kVA • Transformers Supplying the Induction Electrical Furnaces • rated 250 kVA ► 5000 kVA

40

Fire Damp Transformers

• Conditions of operation • in explosive atmosphere, in the coal mines and other industrial departments • max. ambient temperature +40° C; min. ambient temperature -15° C • air relative humidity 95 ± 3%; max. altitude 1000m • protection degree of the housing and terminal boxes IP-54 41

Fire Damp Transformers • Construction parameters • The transformer is made of: • The transformer proper, with its components: the magnetic circuit, which is made of carlyte insulated, cold rolled siliceous steel sheet ; the windings, which are made of glass-fibber and enamel insulated copper wire, impregnated in electro-insulating varnish. The winding types used are: cylindrical on the LV side and layer type on the HV side. The winding insulation is of glass-fiber fabric in layers. Voltage control on the HV side is performed by changing the connections to the bushings.

42

Fire Damp Transformers • •

Construction parameters The LV compartment affords achieving electrical diagram of the LV wiring within the explosionproof housing and has the following components: –

–

USOL automatic breaker provided with maximal and overload protection, with remote making and breaking tapping device, manually driven at remote making and breaking operations; insulation resistance permanent and warning control device, disconnecting the secondary circuit when the insulation resistance decreases below a certain value; measuring instruments : ammeter, voltmeter, current transformer; terminal box with removable bushings affording connection of the supply outlets of the users, signalling-control; 43

Fire Damp Transformers •

Construction parameters

•

The HV side duty is to energize and de-energize the transformer unit. Components and instruments: load special separating switch for making and breaking the no load currents in the dry type transformer; it is manually operated, mechanically and electrically interlocked by means of the LV circuit breaker; device to visually check voltage availability at the load separating switch inlet terminals before penetrating into the HV compartment; mini-circuit breaker which automatically opens the dry type transformer supply circuit, before penetrating into the HV compartment; terminal box with removable bushings affording to connect the supply and control signaling incoming wires. 44


• •

Construction parameters The fire dump housing is made of steel sheet, having different thickness for the 3 chambers. The LV and HV compartment housings are provided with doors ensuring safety, as per STAS 6877/2. The housing of transformer proper is provided with tubes for heat exchange/dissipation. Outside, the housing is provided with: 4 wheels-undercarriage with 375 mm diameter, having the gauge required by the customer, limit facility on the length, lifting lugs. 45


Rating Voltage Uk Po Pk Connection group (kVA) (kV) (%) (kW) (kW)

Overall Weight (kg)


250

6/0.4

3.5

1.3

2.3

Yy0

3925

2970x930x1595

400

6/0.4

4

1.5

3.1

Yy0

4376

2970x930x1595

400

6/0.69

4

1.5

3.2

Yy0

4250

2970x930x1600

500

6/0.69

3

2.75

2.6

Yy0

4857

3125x930x1325

46


. LV buses


2. HV bushing

8. Undercarriage

3. Lifting lug

9. Temperature microrelay

4. Name plate

10. Shutter

5. Warning name plate


6. Alarms name plate

12. Removable cover

47

Transformers Supplying the Induction Electrical Furnaces

• Conditions of operation Furnace capacity

t

0.5

1.1

2

3

6.3

12

20

Transformer rating

KVA

250

400

630

1000

1600

2800

5000

6000

6000

6000

6000

6000

6000

6000

10000

10000

10000

10000

10000

10000

10000

20000

20000

20000

20000

20000

20000

20000

550-125

550-125

550-125

550-125

550-125

1000-222

2500-500

Steps number

10

10

10

10

10

10

10

Connection group vectors

Dyn5

Dyn5

Dyn5

Dyn5

Dyn5

Dyn5

Dyn5

800

900

1750

2500

3000

4500

6500

850

1100

1950

6800

7600

8000

12000

21000

26000

38000

Primary voltage Secondary voltage

V

V

No load losses

W

Copper losses

W

No load current

%

2.1

1.9

1.8

1.6

1.6

1.4

1.2

Impedance voltage

%

6

6

6

6

7

6

7

Overall weight

Kg

2580

2750

3650

5200

6250

9350

15500

2600

2900

4200

5300

9450

17000

930

960

1250

1600

2850

4100

1000

1410

1700

Oil Weight

7200

8500

1850

5000

48

Transformers Supplying the Induction Electrical Furnaces

1. Oil conservator

9. Name plate

2. Oil filling plug

10. Dial type thermometer

3. Oil level indicator

11. Oil draining valve

4. Buchholz relay

12. Undercarriage

5. Tap changer

13. Earthing terminal

6. Silicagel breather

14. HV bushing

7. Untanking part lifting lug

15. LV bushing

8. Lifting lug 49

Power Transformer

By Dr. Tarek Saad Abdel-Salam 1

PROTECTION OF TRANSFORMERS

Low-voltage circuit breakers can be used for the transformer protection required by NEC Section 450-3. Refer to this section for the specific primary and secondary protection requirements for transformers over 600 V nominal and equal to or less than 600 V nominal. The protection discussed in this section of the NEC is intended to protect the transformer only. Protection of the primary and secondary conductors may be obtained by proper selection of cables.

2

Application considerations

Considerations for the application of low-voltage circuit breakers for transformer protection include the following: a) Will they clear the system for short circuits within the transformer? b) Will they prevent the transformer from becoming overloaded beyond its ability? c) Will they protect the transformer from damage during a through-fault condition on the load side? 3

Application considerations

d) Do they have adequate interrupting ratings for faults at their load-side terminals? e) Will they handle the transformer inrush current without nuisance tripping? f) Can they tolerate the current transients during inrush and during other operating conditions? g) Do they provide conductor protection? h) Is ground-fault protection provided (if required)? 4

Transformer with a primary rated over 600 V

When the transformer primary is over 600 V and the secondary is 600 V or less, low-voltage circuit breakers might be used as the secondary transformer protection. The rating of this secondary protection must not exceed 125% of the transformer rated secondary current, or the next higher standard rating or setting for unsupervised transformer applications per NEC Section 450-3(a)(1); NEC Section 450-3(a)(2) allows 250% of the transformer rated secondary current for “supervised” installations. 5

Transformer primary and secondary rated 600 V or below

Primary protection only

The overload ratings or settings determined by the following paragraph do not necessarily provide conductor protection. For example, NEC Section 2403(i) states that transformer secondary conductors (other than two-wire) are not considered to be protected by the primary over current protection. Before making the final selection of the circuitbreaker rating, conductor protection must be verified

6


Primary protection only

NEC Section 450-3(b)(1) states that if only primary protection is to be used for a transformer of 600 V or less, that protection shall be an individual over current device on the primary side, rated or set at not more than 125% of the rated primary current of the transformer as shown in Figure 4-20. If the primary current rating of the transformer is less than 9 A, the exceptions allow the over current device to be rated up to, but no more than, 167% of the transformer primary current rating. If the primary current rating of the transformer is less than 2 A, the exceptions allow the over current device to be rated up to, but no more than, 300% of the transformer primary current rating.

7


Transformers with secondary protection

When the transformer has secondary protection, an individual over current device is not required on the primary side if: a) The over current device on the secondary side is rated or set at not more than 125% of the transformer secondary rating, and b) The primary feeder over current device is rated or set at not more than 250% of the transformer primary current rating 8

PROTECTION OF TRANSFORMERS

Other considerations for protecting transformers Selecting the current ratings is only part of the job of protecting the transformer. Transformer damage curves, current inrush data, overload capabilities, and information on transient tolerances can be obtained from the manufacturers of the transformers and IEEE standards. Refer to the IEEE C57 Collection and IEEE Std 242-1986. This type of information will help the designer determine the proper trip unit settings.

9

Need for protection

Transformer failure may result in loss of service. However, prompt fault clearing, in addition to minimizing the damage and cost of repairs, usually minimizes system disturbance, the magnitude of the service outage, and the duration of the outage. Prompt fault clearing usually prevents catastrophic damage. Proper protection is, therefore, important for transformers of all sizes, even though they are among the simplest and most reliable components in the plant’s electrical system. 10

Need for protection

11

Causes of Failure

a)Winding breakdown, the most frequent cause of transformer failure. Reasons for this type of failure include insulation deterioration or defects in manufacturing, overheating, mechanical stress, vibration, and voltage surges b)Terminal boards and no-load tap changers. Failures are attributed to improper assembly, damage during transportation, excessive vibration, or inadequate design. 12

Causes of Failure

c)Bushing failures.Causes include vandalism, contamination, aging, cracking, and animals. d)Load-tap-changer failures.Causes include mechanism malfunction, contact problems, insulating liquid contamination, vibration, improper assembly, and excessive stresses within the unit. Load-tap-changing units are normally applied on utility systems rather than on industrial systems. 13

Causes of Failure

e)Miscellaneous failures.Causes include core insulation breakdown, bushing current transformer (CT) failure, liquid leakage due to poor welds or tank damage, shipping damage, and foreign materials left within the tank.

14

Causes of Failure

Failure of other equipment within the transformer protective device’s zone of protection could cause the loss of the transformer to the system. This type of failure includes any equipment (e.g., cables, bus ducts, switches, instrument transformers, surge arresters, neutral grounding devices) between the next upstream protective device and the next downstream device. 15

Objectives in transformer protection

Protection is achieved by the proper combination of system design, physical layout, and protective devices as required to: a) Economically meet the requirements of the application, ) Protect the electrical system from the effects of transformer failure, c) Protect the transformer from disturbances occurring on the electrical system to which it is connected,

16

Objectives in transformer protection

d) Protect the transformer as much as possible from incipient malfunction within the transformer itself, and e) Protect the transformer from physical conditions in the environment that may affect reliable performance.

17

Protection of different Types of transformers

Under the broad category of transformers, two types are widely used in industrial and commercial power systems: liquid and dry. Liquid transformers are constructed to have the essential element, the core and coils of the transformer, contained in the liquid-filled enclosure. This liquid serves both as an insulating medium and as a heat-transfer medium. The dry transformers are constructed to have the core and coils surrounded by an atmosphere, which may be the surrounding air, free to circulate from the outside to the inside of the transformer enclosure. The dry coils may be conventional (with exposed, insulated conductors) or encapsulated (with the coils completely vacuum-cast in an epoxy resin). 18

Preservation systems

Dry preservation systems

Liquid preservation systems

19


Dry preservation systems are used to ensure an adequate supply of clean ventilating air at an acceptable ambient temperature. Contamination of the insulating ducts within the transformer can lead to reduced insulation strength and severe overheating. The protection method most commonly used in commercial applications consists of a temperature-indicating device with probes installed in the transformer winding ducts and contacts to signal dangerously high temperature by visual and audible alarm. Figure 11-1 illustrates this feature.

20


The following types of dry systems are commonly used: — Open ventilated — Filtered ventilated — Totally enclosed, non-ventilated — Sealed air- or gas-filled

21


22


Liquid preservation systems are used to preserve the amount of liquid and to prevent its contamination by the surrounding atmosphere that may introduce moisture and oxygen leading to reduced insulation strength and to sludge formation in cooling ducts. The importance of maintaining the purity of insulating oil becomes increasingly critical at higher voltages because of increased electrical stress on the insulating oil.

23


The sealed tank system is now used almost to the total exclusion of other types in industrial and commercial applications. The following types of systems are commonly used:

— Sealed tank — Positive-pressure inert gas — Gas-oil seal — Conservator tank

24

Sealed tank

The sealed-tank design is most commonly used and is standard on most substation transformers. As the name implies, the transformer tank is sealed to isolate it from the outside atmosphere. A gas space equal to about one-tenth of the liquid volume is maintained above the liquid to allow for thermal expansion. This space may be purged of air and filled with nitrogen. A pressure-vacuum gauge and bleeder device may be furnished on the tank to allow the internal pressure or vacuum to be monitored and any excessive static pressure buildup to be relieved to avoid damage to the enclosure and operation of the pressure-relief device. This system is the simplest and most maintenance-free of all of the preservation systems. 25

Positive-pressure inert gas

The positive-pressure inert gas design shown in Figure is similar to the sealed-tank design with the addition of a gas (usually nitrogen) pressurizing the assembly. This assembly provides a slight positive pressure in the gas supply line to prevent air from entering the transformer during operating mode or temperature changes. Transformers with primary windings rated 69 kV and above and rated 7500 kVA and above typically are equipped with this device.

26

Gas-oil seal

The gas-oil seal design incorporates a captive gas space that isolates a second auxiliary oil tank from the main transformer oil, as shown in Figure. The auxiliary oil tank is open to the atmosphere and provides room for thermal expansion of the main transformer oil volume.

27

Conservator tank

The conservator tank design shown in Figure does not have a gas space above the oil in the main tank. It includes a second oil tank above the main tank cover with a gas space adequate to absorb the thermal expansion of the main tank oil volume. The second tank is connected to the main tank by an oil-filled tube or pipe. 28

Protective devices for liquid preservation systems

Liquid-level gauge Pressure-vacuum gauge Pressure-vacuum bleeder valve Pressure-relief device

29

Liquid-level gauge

30

Pressure-vacuum gauge Pressure-vacuum bleeder valve

31

Pressure-relief device

32

Mechanical detection of faults

Two methods of detecting transformer faults exist other than by electric measurements: a) Accumulation of gases due to slow decomposition of the transformer insulation or oil. These relays can also detect heating due to high-resistance joints or due to high eddy currents between laminations. b) Increases in tank oil or gas pressures caused by internal transformer faults. 33

Gas-accumulator relay

A gas-accumulator relay, commonly known as the Buchholz relay, is applicable only to transformers equipped with conservator tanks and with no gas space inside the transformer tank.

34

Gas-detector relay

The gas-detector relay shown in Figure is a special device used to detect and indicate an accumulation of gas from a transformer with a conservator tank, either conventional or sealed.

35

Static pressure relay

The static pressure relay can be used on all types of oil-immersed transformers. They are mounted on the tank wall under oil and respond to the static or total pressure.

36

Sudden pressure relays

Sudden pressure relays are normally used to initiate isolation of the transformer from the electrical system and to limit damage to the unit when the transformer internal pressure abruptly rises. The abrupt pressure rise is due to the vaporization of the insulating liquid by an internal fault, such as internal shorted turns, ground faults, or winding-to-winding faults. 37

Sudden oil-pressure relay

The sudden oil-pressure relay is applicable to all oil-immersed transformers and is mounted on the transformer tank wall below the minimum liquid level.

38

Sudden oil-pressure relay

39

Sudden gas-pressure relay

The sudden gas-pressure relay is applicable to all gas-cushioned oil-immersed transformers and is mounted in the region of the gas space. It consists of a pressure-actuated switch, housed in a hermetically sealed case and isolated from the transformer gas space except for a pressureequalizing orifice

40

Sudden gas/oil-pressure relay

A more recent design of the relays is the sudden gas/oil-pressure relay, which utilizes two chambers, two control bellows, and a single sensing bellows. All three bellows have a common interconnecting silicone-oil passage with an orifice, and an ambient-temperaturecompensating assembly is inserted at the entrance to one of the two control bellows.

41

Dissolved fault-gases detection device

The dissolved faultgases detection device can be used for continuous monitoring of hydrogen

42

Thermal detection of abnormalities

Causes of transformer overheating — High ambient temperature — Failure of cooling system — External fault not cleared promptly — Overload — Abnormal system conditions, such as low frequency, high voltage, non-sinusoidal load current, or phase-voltage unbalance.

43

Undesirable results of overheating

The consequences of overheating include the following:

— Overheating shortens the life of the transformer insulation in proportion to the duration of the high temperature and in proportion to the degree of the high temperature. — Severe over temperature may result in an immediate insulation failure. — Severe over temperature may cause the transformer coolant to heat above its flash temperature and result in fire. 44

Liquid temperature indicator (top oil)

The liquid temperature indicator measures the temperature of the insulating liquid at the top of the transformer. Because the hottest liquid is less dense and rises to the top of the tank, the temperature of the liquid at the top partially reflects the temperature of the transformer windings and is related to the loading of the transformer.

45

Thermal relays

Thermal relays are used to give a more direct indication of winding temperatures of either liquid or dry transformers. A CT is mounted on one of the three phases of the transformer bushing. It supplies current to the thermometer bulb heater coil, which contributes the proper heat to closely simulate the transformer hotspot temperature

46

Hot-spot temperature thermometers

Hot-spot temperature equipment is similar to the thermal relay equipment on a transformer because it indicates the hottest-spot temperature of the transformer. While the thermal relay works with fluid expansion and a bourdon gauge, the hot-spot temperature equipment works electrically using a Wheatstone bridge method. In other words, it measures the resistance of a resistance temperature detector (RTD) that is responsive to transformer temperature changes and increases with higher temperature.

47

Forced-air cooling

Another means of protecting against overloads is to increase the transformer’s capacity by auxiliary cooling. Forced-air-cooling equipment is used to increase the capacity of a transformer by 15% to 33% of base rating, depending upon transformer size and design.

48

Fuses or over current relays

Other forms of transformer protection, such as fuses or over current relays, provide some degree of thermal protection to the transformer.

49

Over excitation protection

Over-excitation may be of concern on direct-connected generator unit transformers. Excessive excitation current leads directly to overheating of core and un-laminated metal parts of a transformer. Such overheating in turn causes damage to adjacent insulation and leads to ultimate failure.

50

Nonlinear loads

Nonlinear electrical loads may cause severe overheating even when the transformer is operating below rated capacity. This overheating may cause failure of both the winding and the neutral conductor. Electronic equipment such as computers, printers, uninterruptible power supply (UPS) systems, variable-speed motor drives, and other rectified systems are nonlinear loads. Arc furnace and rectifier transformers also provide power to nonlinear loads. 51

Nonlinear loads

The nonlinear load causes transformer overheating in three ways: —Hysteresis — Eddy currents —Skin effect

52

Nonlinear loads

Overheating of neutral conductors from nonlinear loads is due to the following: —Zero-sequence and odd-order harmonics —Skin effect

53

Nonlinear loads

Failures of transformers due to nonlinear loads can be prevented by de-rating the transformer.

54

Protecting the transformer from electrical disturbances

Transformer failures arising from abusive operating conditions are caused by — Continuous overloading — Short circuits — Ground faults — Transient over voltages

55

Overload protection

An overload causes a rise in the temperature of the various transformer components. If the final temperature is above the design temperature limit, deterioration of the insulation system occurs and causes a reduction in the useful life of the transformer Protection against overloads consists of both load limitation and overload detection. Loading on the transformers may be limited by designing a system where the transformer capacity is greater than the total connected load when a diversity in load usage is assumed 56

Over current relays

Transformer overload protection may be provided by relays. These relays are applied in conjunction with CTs and a circuit breaker or circuit switcher, sized for the maximum continuous and interrupting duty requirements of the application

57

Short-circuit current protection

In addition to thermal damage from prolonged overloads, transformers are also adversely affected by internal or external short-circuit conditions, which can result in internal electromagnetic forces, temperature rise, and arc-energy release. Protection of the transformer for both internal and external faults should be as rapid as possible to keep damage to a minimum. This protection, however, may be reduced by selective-coordination system design and operating procedure limitations.

58

Time-current Characteristics (Tccs)

59

Different types of protection

Over current relay protection

Over current relays may be used to clear the transformer from the faulted bus or line before the transformer is damaged. On some small transformers, over current relays may also protect for internal transformer faults. On larger transformers, over current relays may be used to provide backup for differential or pressure relays. 60


Time over current relays

Over current relays applied on the primary side of a transformer provide protection for transformer faults in the winding, and provide backup protection for the transformer for secondary-side faults.

61


Instantaneous over current relays

Phase instantaneous over current relays provide short-circuit protection to the transformers in addition to overload protection. When used on the primary side, they usually coordinate with secondary protective devices. Fast clearing of severe internal faults can be obtained

62


Tertiary winding over current relays

The tertiary winding of an autotransformer, or three-winding transformer, is usually of much smaller kilo volt ampere rating than the main windings. Therefore, fuses or over current relays set to protect the main windings offer almost no protection to such tertiaries. During external system ground faults, these tertiary windings may carry very heavy currents. 63


Phase differential relays

Differential relaying compares the sum of currents entering the protected zone to the sum of currents leaving the protected zone; these sums should be equal. If more than a certain amount or percentage of current enters than leaves the protected zone, a fault is indicated in the protected zone; and the relay operates to isolate the faulted zone. 64


65


66


67

Ground differential relays

Protection of the transformer by percentage differential relays improves the overall effectiveness in detecting phase-to-phase internal faults. However, line-to-ground faults in a wye winding may not be detected if the transformer is low-resistance-grounded where ground fault current is limited to a low value below the differential relay pickup level.

68


69


70


71

Protection against over voltages

Transient over voltages produced by lightning, switching surges, switching of power factor correction capacitors, and other system disturbances can cause transformer failures. High voltage disturbances can be generated by certain types of loads and from the incoming line. A common misconception is that underground services are isolated from these disturbances. 72


Surge arresters

Ordinarily, if the liquid-insulated transformer is supplied by enclosed conductors from the secondaries of transformers with adequate primary surge protection, additional protection may not be required, depending on the system design. However, if the transformer primary or secondary is connected to conductors that are exposed to lightning, the installation of surge arresters is necessary

73


Surge capacitors

Additional protection in the form of surge capacitors located as closely as possible to the transformer terminals may also be appropriate for all types of transformers

74

Ferroresonance

Ferroresonance is a phenomenon resulting in the development of a higher than normal voltage in the windings of a transformer. These over-voltages may result in surge arrester operation, damage to the transformer, and electrical shock hazard.

75

Ferroresonance

The following conditions combine to produce ferroresonance: a) No load on the transformer b) An open circuit on one of the primary terminals of the transformer and, at the same time, an energized terminal. In the case of three-phase transformers, either one or two of the three primary terminals may be disconnected. c) The location of the point of disconnection if it is not close to the transformer d) A voltage potential between the disconnected terminal conductor and ground 76

Ferroresonance

77

Protection from the environment

Undesirable conditions include: a) Average ambient temperatures above 30 °C when the transformer is loaded at rated kilo voltampere or more b) Corrosive agents, abrasive particulate matter, and surface contaminants derived from the surrounding atmosphere c) Conditions that can lead to moisture penetration or to condensation on windings and other internal electrical components d) Submersion in water or mud e) Obstruction to proper ventilation of liquid transformer radiators or, in the case of dry transformers, ventilating openings f) Exposure to damage from collision by vehicles g) Excessive vibration h) Exposure to vandalism 78

Conclusion

Protection of today’s larger and more expensive transformers can be achieved by the proper selection and application of protective devices. Published application guides covering transformers are readily available, for example, ANSI C37.91-2000. The system design engineer should rely heavily on sound engineering judgment to achieve an adequate protection system. 79

Power Transformer


1

Definitions • acceptable conditions:

• The conditions in which an item is able to perform its required function and/or meet the relevant specification.

2

Definitions • electrical station:

• building, rooms, or designated space that houses the electrical equipment in an installation.

3

Definitions • electrical equipment:

• A general term for the equipment (e.g., materials, fittings, devices, appliances, fixtures, apparatus, machines) used as a part of, or in connection with, an electric installation. 4

Definitions • emergency action:

• Action that should be taken immediately to avoid serious consequence.

5

Definitions • examination:

• An inspection with the addition of partial dismantling as required, supplemented by means such as measurements and nondestructive tests or high-potential tests, in order to arrive at a reliable conclusion about the condition of an item.

6

Definitions • failure (or breakdown):

• The termination of the ability of an item to perform its required function.

7

Definitions • inspection:

• Maintenance action comprising a careful scrutiny of an item carried out without significant dismantling and using all the senses as required to detect anything that causes the item to fail to meet an acceptable condition. 8

Definitions • item:

• Any part of equipment, including a composite, which can be individually considered.

9

Definitions • maintenance:

• A combination of any actions carried out to retain an item in, or restore it to, an acceptable condition.

10

Definitions • electrical preventive maintenance:

• A system of planned inspection, testing, cleaning, drying, monitoring, adjusting, corrective modification, and minor repair of electric equipment to minimize or forestall future equipment operating problems or failures. 11

Definitions • nonroutine maintenance:

• Unplanned maintenance that is not the result of a breakdown.

12

Definitions • post-fault maintenance:

• Maintenance necessary on switchgear after a specified number of fault clearance operations.

13

Definitions • preventive maintenance:

• Maintenance carried out with the objective of preventing breakdown. It may include routine or nonroutine maintenance.

14

Definitions • repair or corrective maintenance:

• Maintenance necessary to restore to an acceptable condition an item that has ceased to meet an acceptable condition.

15

Definitions • routine maintenance:

• Maintenance organized and carried out in accordance with a predetermined policy or plan to prevent breakdown or reduce the likelihood that an item will fail to meet an acceptable condition. 16

Definitions • operational check:

• An action carried out to determine whether an item functions correctly.

17

Definitions • test:

• A measurement carried out to determine the condition of an item.

18

Definitions • diagnostic testing:

• A technique involving the establishing of comparative data for monitoring and checking the condition of equipment.

19

Definitions • overhaul:

• Maintenance of an item including examination and replacement or rebuilding as required.

20

Definitions • major overhaul:

• An overhaul that includes major dismantling and/or replacement of items to complete the maintenance.

21

Definitions • minor overhaul or servicing:

• An overhaul that is limited to lubrication and/or replacement of consumables.

22

MAINTENANCE TESTING OVERVIEW • Introduction – Maintenance testing is an important procedure that is used to detect deficiencies in electrical equipment before the equipment fails in service

23

MAINTENANCE TESTING OVERVIEW • Introduction – Maintenance testing needs may vary with each of these categories. Keep in mind that the purpose of maintenance testing is to determine if the equipment will continue to properly perform its function. In many cases, the testing consists of simulating different operating conditions and evaluating how the equipment responds. 24

MAINTENANCE TESTING OVERVIEW • Insulation tests – One characteristic that all types of electrical equipment have in common is the use of some form of insulation. At its most basic level, all electrical equipment have some part or parts that conduct electricity and other parts that do not. A bare overhead distribution line is held up by insulators and also utilizes the air around it for insulation. Transformer windings have insulation around each turn of the conductors and, in some cases, use oil, in addition, to raise the insulation value between the conductor and the grounded components. 25

MAINTENANCE TESTING OVERVIEW • Insulation tests – The primary factor that determines the level of insulation that is required is the operating voltage. Other factors, such as current and frequency, also play a part; however, they are secondary to voltage. Therefore, the first consideration in testing insulation is whether it can support the required voltage without breakdown. This is accomplished by measuring the leakage current that flows through the insulation medium when a voltage is applied.

26

MAINTENANCE TESTING OVERVIEW • Insulation tests – There are almost as many types of insulation as there are different applications. There are several things, however, that they all have in common. Moisture and contamination decrease an insulators ability to withstand voltage and increase the amount of leakage current that will flow. The insulation will also deteriorate with age. Overheating causes deterioration to be greatly accelerated. A common rule of thumb is that the life expectancy of the insulation is cut in half for every 10 °C above its rating that the equipment operates. 27

MAINTENANCE TESTING OVERVIEW • Insulation tests – Another characteristic that some types of insulation have is that, as the voltage rises, the insulation will maintain its integrity until it reaches the point of failure. Then, near the point of failure, the insulating capability drops very rapidly, often accompanied by an arc or puncture in the insulation. 28

MAINTENANCE TESTING OVERVIEW • DC tests – The most common method of testing insulation integrity is to apply a dc voltage and measure the leakage current. The insulation resistance is then determined by dividing the voltage by the current. There are many commercially available test instruments that have specific voltage outputs and provide the readings directly in ohms. This is referred to as insulation resistance testing, and frequently as Megger testing or Meggering. 29

MAINTENANCE TESTING OVERVIEW • DC tests – For low-voltage equipment, common test voltages are 100 V, 250 V, 500 V, and 1000 V. Test instruments are also available that have test voltages of 2500 V, 5000 V, and 10 000 V for use on medium-voltage equipment. – For most testing beyond 10 000 V, the test equipment no longer has a fixed output voltage. This is considered high-potential testing, commonly called hipot testing; and the voltage is continuously adjustable so that it can be ramped up slowly. – The leakage current is usually measured directly when hi-pot testing is being performed, since the voltage is no longer fixed and getting a direct readout in ohms would be more difficult. 30

MAINTENANCE TESTING OVERVIEW • Insulation resistance tests – Insulation resistance tests are typically performed on motors, circuit breakers, transformers, low-voltage (unshielded) cables, switchboards, and panel boards to determine if degradation due to aging, environmental, or other factors has affected the integrity of the insulation. This test is normally conducted for 1 min, and the insulation resistance value is then recorded. 31

MAINTENANCE TESTING OVERVIEW • High-potential testing – High-potential testing, as its name implies, utilizes higher levels of voltage in performing the tests. It is generally utilized on medium-voltage (1000V-69 000 V) and on high-voltage (above 69 000 V) equipment – The leakage current is usually measured. In some cases, such as in cable hi-potting, the value of leakage current is significant and can be used analytically. In other applications, such as switchgear hi-potting, it is a pass/fail type of test, in which sustaining the voltage level for the appropriate time (usually 1 min) is considered passing. 32

MAINTENANCE TESTING OVERVIEW • High-potential testing

Circuit breaker high-potential testing 33

MAINTENANCE TESTING OVERVIEW • AC tests – The most common ac insulation test is ac hi-pot testing at 60 Hz. – The capacitance of the insulation is a large factor in ac testing. With dc insulation testing, the capacitance of the insulation charges up over time and the residual leakage current is an indication of the resistance of the insulation. – This is not true of ac insulation testing. Since the voltage is changing at 60 Hz, the leakage current may be predominantly the capacitive charging current of the insulation under test. – AC hi-pot testing is usually a pass/fail type test in which passing means that the insulation was capable of holding the required test voltage, usually for 1 min. 34

MAINTENANCE TESTING OVERVIEW • Power-factor testing dielectric loss angle (DLA) testing – Power-factor testing is a special type of ac insulation testing. In power-factor testing, the phase relationship between the applied test voltage and the resulting leakage current is determined.

35

MAINTENANCE TESTING OVERVIEW • Power-factor testing dielectric loss angle (DLA) testing – A special test set is used that supplies voltage and current at 60 Hz . – The volt amperes and watts are measured with the test set and the power factor is determined.

36

MAINTENANCE TESTING OVERVIEW

• Insulation power-factor testing 37

MAINTENANCE TESTING OVERVIEW • Transformer turns ratio (TTR) testing – The voltage across the primary of a transformer is directly proportional to the voltage across the secondary, multiplied by the ratio of primary winding turns to secondary winding turns. – In order to ensure that the transformer was wound properly when it was new, and to help locate subsequent turn-to-turn faults in the winding, it is common practice to perform a TTR test.

38

MAINTENANCE TESTING OVERVIEW • Infrared scanning – Infrared scanning is a method that is utilized to locate high-resistance connections (hot spots) by using a camera that turns infrared radiation into a visible image – This test is performed with the equipment in service carrying normal load current, which is a major advantage because it does not interrupt normal production. Exposure to energized equipment, of course, carries the possibility of exposure to electrical hazards. 39

MAINTENANCE TESTING OVERVIEW • Infrared scanning

Infrared inspection 40

MAINTENANCE TESTING OVERVIEW • Oil testing – Many medium- and high-voltage transformers and circuit breakers utilize different types of oils for insulation. – Chemical testing of the oil has proven to be a very dependable method of locating existing or potential problems – This can be tested on-site by measuring the voltage at which dielectric breakdown occurs with a special test set that is designed for this purpose.

41

MAINTENANCE TESTING OVERVIEW • Grounding tests – Power system grounding is for the purpose of minimizing the electrical hazard that the power distribution system poses to people. – Signal reference grounding refers to the use of ground as a reference for electronic controls and communication. In many instances, electronic equipment uses its own metal frame or case as the signal reference. – Lightning protection is intended primarily to dissipate the energy from a lightning strike in a manner that is safe for personnel and that causes the least amount of equipment damage 42

MAINTENANCE TESTING OVERVIEW • Grounding electrode test – The NEC [B53] uses the term grounding electrode for the electrical conductor or ground rod that is buried in the earth. The test utilized to determine the resistance between a grounding electrode and the earth is called a fall-of-potential test

43

MAINTENANCE TESTING OVERVIEW

Ground resistance testing

44

MAINTENANCE TESTING OVERVIEW • Two-point resistance tests – Two-point resistance tests are used to measure the resistance of the equipment grounding conductor and its bonding of the electrical equipment

45

MAINTENANCE TESTING OVERVIEW • Soil resistivity test – Soil resistivity is a measure of resistance per unit length of a uniform cross-section of earth, usually expressed in ohmcentimeters – It is performed with a four-terminal test set that uses four equally spaced electrodes, driven into the ground

46

MAINTENANCE TESTING OVERVIEW • Functional testing – Functional testing consists of simulating various normal and abnormal conditions, and monitoring the system performance for proper operation – This can be as simple as opening and closing a circuit breaker electrically

47

MAINTENANCE TESTING OVERVIEW • Testing procedures and specifications – There are many sources of information on testing and maintenance of electrical equipment.

48

MAINTENANCE TESTING OVERVIEW • Sources of electrical equipment testing and maintenance information – a) American National Standards Institute (ANSI); – b) American Society for Testing and Materials (ASTM); – c) Association of Edison Illuminating Companies (AEIC); – d) Institute of Electrical and Electronics Engineers (IEEE);

49

MAINTENANCE TESTING OVERVIEW Sources of electrical equipment testing and maintenance information – e) Insulated Cable Engineers Association (ICEA); – f) InterNational Electrical Testing Association (NETA); – g) National Electrical Manufacturers Association (NEMA); – h) National Fire Protection Association (NFPA); – i) Occupational Safety and Health Administration (OSHA). 50

NFPA 70B-1998, Electrical Equipment Maintenance [B54]

• The development of NFPA 70B began in 1968 with the Board of Directors of the National Fire Protection Association who established a committee to develop suitable texts relating to preventive maintenance of electrical systems and equipment used in industrial-type applications with the view of reducing loss of life and property 51

NFPA 70B-1998, Electrical Equipment Maintenance [B54]

• The purpose is to correlate generally applicable procedures for preventive maintenance that have broad application to the more common classes of industrial electric systems and equipment without duplicating or superseding instructions that manufacturers normally provide.

52

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