Vivek Jha_DC and AC Test

7/29/2019 Vivek Jha_DC and AC Test

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1

DC &

AC

Voltage

Testing

of

Electrical Equipments

Vivek Jha



Presentation outline

y Introduction

y DC testing of Insulation :

y DC Testing Methods

y DC Testing Applications

y AC Testing :

y Power Factor and Dissipation Factor Test

y AC Testing Applications

y Conclusion

2



Introduction



Need of

Maintenance

&

Testing

y To lengthen the mean time between failures (MTBF) of

the electrical equipment.

y Reduced cost of repairs

yReduced downtime of equipment

y Improved safety of personnel and property.


http://slidepdf.com/reader/full/vivek-jhadc-and-ac-test 5/9422 November 2010 5

ROLE OF

MAINTENANCE

MANAGEMENT

COST OF MAINTENANCE PLANT AVAILABILITY

MAINTENANCE MANAGEMENT

TO BALANCE BETWEEN



Different approaches to maintenance

y Run to failure or Reactive maintenance:

y Inspect and service as necessary

y Time Based Scheduled preventive maintenance

y Condition Based Preventive Maintenance

y Reliability centered maintenance (RCM)

22 November 2010 6


http://slidepdf.com/reader/full/vivek-jhadc-and-ac-test 7/9422 November 2010 7



Rules of maintenancey Keep it dry

y Keep it cool

y Keep it clean

y Keep it tight

22 November 2010 8



Causes of Insulation Degradation and Failure Modes

of Electrical Equipment

y Mechanical stress

y Thermal stress , hot spots

y Environmental (moisture, chemicals, dirt, and oils)

y Electrical stresses (corona, surges, and partial discharges)

9



TEAM Stresses

10

• Temperature

• Electrical

• Ambient

• Mechanical



Increasing AgeNew Old

Insulation Strength

Insulation Stress

Insulation Spare Margin

Insulation Strength

no TEAM

transients

Poorer design &

manufacture- faster rate

of decline

Good design, manufacture

& O&M: significant margin





Types of tests

y Factory Tests

y Acceptance tests

y Routine maintenance tests

y Special maintenance tests



Insulation as

a Capacitor

14



Insulation as

a Capacitor

15

The dielectric constant of an insulator is an indication of how much dielectric

flux the insulation will allow through it. Under identical conditions insulation

with a higher dielectric constant will pass more dielectric flux through it than another insulation having a lower dielectric constant



Insulation as

a Capacitor

16

Perfect

Insulator



Insulation as

a Capacitor

17

Practical

Insulator



Insulation Test

: AC

or

DC?

18



Insulation Test

: AC

or

DC?

19



Insulation Test

: AC

or

DC?

20



Insulation Test

: AC

or

DC?

21



DC testing of Insulation

22



Introduction : DC

tests

Information provided by DC Tests will be basis for :

y Decision making about corrective maintenance / replacement

needed

y Decision making about energizing the new equipment

y Recording trend of gradual deterioration over time

23



DC voltage

Application

to

Insulation

When DC voltage is applied to insulation, current drawn by insulation can be analyzed 3 components:

y Capacitance charging current

y Dielectric absorption current

y Leakage current

24



DC voltage

Application

to

Insulation

…



Capacitance charging

current

• It is initial charging current when voltage is applied • It is a function of time, and decreases as time of application increases

• Test readings should not be taken until this charging current decreases

to quite low value

Charging current

ic= --- e-t/RCER

• E = Voltage in KV

• R = Resistance in MΩ

• C= capacitance in μF

• t = Time in sec.



Dielectric Absorption

current

Absorption current

ia= K ECt-n

• E = Voltage in KV

• K = proportionality constant

• C= capacitance in μF• t = Time in sec.

• n = constant

• Initially high (not as high as capacitance charging current)• Decreases with time (at a slower rate than capacitance charging current)

• charging current when voltage is applied

• It is a function of time, and decreases as time of application increases

• Test readings should not be taken until this absorption current decreases to

quite low value



Leakage current

• It is the current which flows through volume of

insulation• this is the current that is used to evaluate the

status of insulation system.

• charging current when voltage is applied

• Test readings should be taken only after

stabilization of this leakage current.



Advantages of

DC

Voltage

Testing

y Preferred for equipments having high charging current,

such as cables

y DC voltage stress is much less damaging as compared to

AC voltages

y Time of voltage application is not as critical as AC voltage

y Historical data can be compiled accurately for

comparisony Size and weight of DC test equipment is significantly

reduced as compared to AC test equipment

29



Disadvantages of

DC

Voltage

Testing

y Stress distribution for electrical equipments is different

under AC voltage

y Defects, untraceable with DC, can sometimes cause

failure under AC

y DC Test results are affected by temperature and

humidity.

y

Residual charge after DC test should be carefully discharged

30



DC

Testing

Methods



DC Testing

Methods

Two types of tests are done with DC voltage application:

• Insulation resistance testing (IR)

• High potential testing (Hi‐Pot)

32



DC Testing Methods

Insulation Resistance Testing

• Test voltage application : 100 – 15000 v

• Instrument used: Megohmmeter

(hand/motor/electronic)

• All readings to be corrected to standard

temp, by

correction

factor

table

• MΩ value is inversely proportional to

volume of insulation under test

• IR value by themselves do not indicate

weakness or strength, they can indicate trouble if downward trend continues

further



DC Testing Methods

Insulation Resistance

Testing

Four common IR test methods:

y Short Time Readings or Spot checking

y Dielectric Absorption Ratio test

y

Polarization Index Test y Step Voltage Readings

34



Short‐Time

Readings

y This test simply measures the insulation resistance value for a short

duration of time, through a spot reading. y The reading only allows a rough check of the insulation condition.

y Readings can be used for comparison of this value with previous values . A

continued downward trend is indicative of insulation deterioration ahead.

y For interpreting the results, the values used for comparison should all be

normalized to 20°C.

35

DC Testing Methods



Time–Resistance Readings:

Dielectric Absorption

Ratio

test

y A good insulation system shows a continued increase in its resistance value over

the period of time. On the other hand, an insulation system that is contaminated

with moisture, dirt, etc will show a low resistance value. y In good insulation, the effects of absorption current decreases as time increases. In

bad insulation, the absorption effect is shown by persisting high leakage current.

y The time‐resistance method is independent of temperature and equipment size.

The ratio of time‐resistance readings can be used to indicate the condition of theinsulation system.

y The ratio of a 60 s reading to a 30 s reading is called the DAR (Dielectric Absorption

Ratio)

Resistance reading at 60 sResistance reading at 30 s:

DAR

36



DC Testing Methods

PI (Polarization

Index)

Test

y The PI test is a specialized application of the dielectric

absorption test. This test is used for dry insulation.

37

PI =insulation resistance at 10 min

insulation resistance at 1 min



DC Testing Methods

Step Voltage

Test

38

VOLTAGE IN KV

L E A K

A G E C U R R E N T i n

μ A

Breakdown of

insulation

system



DC Testing Methods

High potential

testing

(Hi

‐Pot)

y DC voltage applied across the insulation at or above DC

equivalent of power frequency AC voltage

y Test can be applied as ‘gradual’ test or step‐voltage test

y In gradual test, max voltage is applied slowly, gradually

and max voltage is held for a period. Leakage current

readings are recorded.

y

In step

voltage

method,

max

voltage

is applied

in steps

and readings taken at each step.

39



DC Testing Applications



DC Testing

Applications:

Transformers

Test procedure

y Do not disconnect ground connection to tank

y Disconnect all HV,LV, neutral connections, cooling

system, meters, LA and other LV control system

y HV/LV jumpers should not touch metal or grounded parts

41





Test connections for 3 ph transformer

HV to LV & Earth

H1 • • X1

H2

• •X2

H3 • • X3

• X0

Guard Line Earth

Megohmmeter Transformer




HV to Earth, LV Guarded

H1 • • X1

H2

• •X2

H3 • • X3

• X0

Guard Line Earth





LV to Earth, HV Guarded

H1 • • X1

H2

• •X2

H3 • • X3

• X0

Guard Line Earth




46

Guard Line Earth

Megohmmeter



47

Guard Line Earth

Megohmmeter



48

Guard Line Earth

Megohmmeter



Interpretation of DC test readings



Interpretation

of

DC

test

readings

Dc test results can serve as guide to decide one of the

following actions:y Put the equipment into service till next scheduled

inspection

y Put the equipment into service now, but plan to repair/replace as soon as possible

y Put the equipment out of service

50



Interpretation

of

DC

test

readingsHow to determine whether insulation is good or bad:

y Manufacturer’s information

y Comparison with values obtained during installation

y Comparison with values from previous routine tests

y Comparison with values of similar equipment

y

Rule of thumb:

y Min 1 MΩ / rated KV + 1 MΩ

y Never < 1 MΩ in any case

51



Interpretation

of

DC

test

readings

More rules of thumb:

y For transformers, Min IR = (CE)/ √ kVAIR = wdg to Earth MΩ, E= voltage rating, kVA = rated kVA

C = constant

y For cables, Min IR = K log10 (D/d)IR = cond.to Earth MΩ per 100 ft of cable

D= outside dia of cond insulation

k = constant

for

insulating

material,

d=

dia of

conductor

value of C

Trafo type value of C at 20 deg C

oil filled 1.5

Dry type 30

52

Min value of K

Impreg paper 2640

polyethylene 2000

Synthetic rubber 2000

XLPE 20000



Acceptance

criteria

for

judging

insulation

53

Nominal voltage class Typical system voltage Min acceptable IR value at

20 0 C (MΩ)

600 v 120/240/440 v 1.5

7.2 kv 6.9 kv 8.2

15

kv 11

kv /

13‐

8

kv 14.8

33 kv 33 kv 35

72 kv 69 kv 70

242 kv 220

kv 231

550 kv 500 kv 501

Resistances above do not necessarily indicate sound insulation condition, but only that the

equipment may be energized without significant risk of failure



Interpretation

of

DC

test

readings

y A clean, dry insulation in excellent condition should have

resistance several times the min. valuey IR value has little significance on one time, absolute value

basis. Long term trend indicates progressive deterioration

y To allow meaningful trending, influence of temp,

humidity should be minimized

54



55

AC Voltage Testing of

Electrical Equipments:

PF

Test

Vivek Jha



Introduction

y Power factor (PF) and dissipation factor (DF) tests are

conducted in the field for acceptance and field maintenance testing of insulation of electrical equipments.

y Purpose of AC Tests:

yIdentify if the equipment has been installed correctly

y Determine need for corrective maint/ repair

y Track the gradual deterioration of the equipment

y

Why AC

tests:

Gives

best

info

about

equipment

condition

(near true operating conditions)

56



Introduction

y Various types of AC voltage tests:

y Power Factor (PF) and Dissipation Factor (DF) Test

y AC High Potential Test

y 0.1 Hz Voltage Test

Power Factor (PF) and Dissipation Factor (DF) Test is the most

widely used test. It is considered non destructive test because

test voltages normally do not exceed designed voltage of

equipment.

AC Hi‐pot is considered destructive test and should not be

repeated frequently

57



58



Power

Factor

and

Dissipation

Factor

Test

y Used since early 1900s

y Based on Schering Bridge

y The test uses AC and pursues to know loss angle of the tested equipment, to know the condition of insulation.

y The test provides information on overall condition of insulation in the form of a ratio (PF or DF) which is independent of the volume of insulation being tested.

y Provide assessment of insulation under normal frequency working condition, which are not time dependent like DC tests

y The test does not overstress the insulation

y Provide handy tool for comparison with similar equipments

y It is important to take note of transformer temperature and environmental moisture (surface leakage).

y This test is sometimes referred as ‘Doble Test ’ because of historical significance of Doble test kit.

59

Power Factor and Dissipation Factor Test :



Principle

of

PF/DF

TestPF and DF

y The PF of insulation is defined as the ratio of watt loss

to total charging volt‐amperes, or the cosine of the angle θ between total current vector (IT)and the

impressed voltage vector. It is a measure of the energy

component (resistive component) of the charging

current. y The DF is defined as the ratio of the watt loss to

charging amperes, or the tangent of the angle δ

between the total current vector and the capacitive

current vector. The angle δ is the complementary angle of the PF angle θ.

60





Exampley insulation of PF = 1.0%.

y PF is approximately equal to DF when PF and DF <10.0%, that is

y Cos θ=tanδ=cotan θ

y cos(89.43)=tan(0.57)=0.01(1%)

62



Factors

that

influence

PF

measurementy Changes in insulation quality result in measurable changes in some of the

basic electrical characteristics of the insulation, such as capacitance,

dielectric loss,

or PF.

Therefore,

by

measuring

these

electricalcharacteristics over time, changes in the integrity of the insulation can be

assessed, The main factors influencing PF measurement are:

y Temperature

y Humidityy Surface leakage

y It is necessary to normalize the results to a common base temperature

y Equipment should be retested if ambient temp is too high or too low

y PF tests should not be performed for detection of presence of moisture in

the insulation when the temperatures are much below freezing,

63



PF

Test

Equipment

64

Cs = standard reference capacitor

Cx = insulation under test

A voltage is applied to both CS and CX.

The ratio arms, Ns and Nx are adjusted to

balance capacitive current, and the variable

resistor Rs is adjusted to balance resistive

current.

The null indicator is used to determine when the bridge circuit is balanced.

The values of Ns and Nx are used to determine capacitance and the value of RS correlates to power ( dissipation) factor of the test insulation.

Safety features, self diagnostic and calibration check facility is provided in the

kit



General

procedure

for

the

Operation

of

the

PF

Test

Set1. Assemble the test set in accordance with the operating instruction manual.

2. Connect ground lead from the test set to a station ground.

Caution: PF tests are performed only on de‐energized and isolated apparatus. Verify the

equipment is cleared before attempting to connect leads.

3. Prepare the specimen for testing. This may include removing external connections, shorting

winding terminals, etc.

4. Connect test leads: first to the test set, then to the apparatus to be tested following the

instructions in the operating manual.

5. Check operation of safety and ground interlocks if supplied on the test set.

6. Select the proper test configuration for the insulation to be measured.

7. Initiate voltage output from the test set. Raise output voltage to the desired level.

8. Continue operation of the test set to obtain test readings, following the specific instructions in

the operating manual. Balance the bridge for capacitance and PF.9. Reduce voltage to zero, or lowest setting and cut off voltage output.

10. Record all values as provided by the test set: test voltage, current, watts‐loss, capacitance,

and PF.

65



Basic

Test

Connections

(Test

Modes)

for

PF

Testing

y

Grounded‐Specimen

Test

Mode

(GST)

y GST Mode with Guard (GST‐G)

y Ungrounded‐Specimen Test Mode (UST)

66



Basic

Test

Connections

(Test

Modes)

for

PF

TestingGrounded‐Specimen Test Mode (GST)

67



Basic

Test

Connections

(Test

Modes)

for

PF

TestingGST Mode with Guard (GST‐G)

68



Basic

Test

Connections

(Test

Modes)

for

PF

TestingUngrounded‐Specimen Test Mode (UST)

69

Safety Precautions with PF Testing




y All PF tests are performed with the apparatus to be tested completely de‐

energized and isolated from the power system. In addition, the apparatus

housing or tank must be properly grounded. There is no substitute for a

visual check

to ensure that the apparatus terminals are isolated from the

power source

y Safety grounds should be applied to all apparatus terminals before doing any

work

on

them,

and

before

connecting

and

disconnecting

the

PF

test

leadsy operator should have an unobstructed view of the apparatus under test and

of various personnel assisting in the tests.

y proper clearance should be maintained between the test set and apparatus;

it should be recognized that damaged or defective apparatus may fail during test.

70




Safety Precautions with PF Testing …..

y Test sets should have ground‐relay that prevents test voltage being applied

until :

1. A heavy‐duty safety (station ground) has been applied to the ground receptacle of

the test set.

2. The test case has been grounded through the power supply cord.

3. The voltage control is at the fully counterclockwise zero voltage position

y

Do not connect test leads to the apparatus terminals unless the leads are already connected to the PF test set.

y Before making the first test, both safety switch operation should be checked

y Never short circuit safety switch

y The heavy‐duty test set ground is the last lead to be removed from the test

set.

71



Applications of PF Test





y PF testing is normally used for acceptance testing, preventing

maintenance, and post maintenance insulation assessment, and for condition trending.

y The test voltage used for PF testing should be sufficient to

detect any

latent

weaknesses

in the

insulation,

but

since

the

test is intended to be non‐destructive, the voltage should not

exceed normal line‐to‐neutral or line‐to‐ground operating

voltage of the apparatus under test.

73

Applications of PF Test : Transformers




74





75












pp

80

Calculation of results :

Test 1 minus test 2

1. Subtract charging current of test 2 from test 1

2. Subtract watt loss of test 2 from test 1

3. Then calculate the CHL and PF, that is [CH+CHL]−[CH]=CHL

Test 3 minus

test

4

1. Subtract charging current of test 4 from test 3

2. Subtract watt loss of test 4 from test 3

3. Then calculate the CHL and PF, that is [CL+CHL]−[CL]=CHL

y The calculated value of CHL from the above calculation should be same. If it

is not then there is either an error in the test results or the calculations


Bushings



g

81

Applications of PF Test: Bushings



pp g

82


Rotating Machinery: PF measurement of Phase to



g y

ground insulation.

83




Rotating Machinery:

PF

measurement

of

Interphase

(End‐turn) insulation.

84



Evaluation of PF and DF Test Results



y Four categories:

1 Good Insulation : condition is good and suitable for continued service

2 Deteriorated Insulation : condition is satisfactory for service but should be

checked within six months to see if the condition has further degraded

3 Marginal Insulation : condition is not satisfactory for service—immediate

investigation of the degraded conditions should be begun and if this is not possible then it should be begun as soon as possible

4 Bad Insulation: Remove from service and recondition to restore insulation

to good condition, if not possible, then replace The

86

Evaluation of PF and DF Test Results



y References for evaluating test results:

y Manufacturer’s recommendations

y year‐to‐year test results

y Results of similar equipments

y Whenever the test results are questionable or marginal, it is generally recommended to perform tests on more frequent basis

y A gradual and consistent increase in PF may be due to contamination,

deterioration, or normal aging, where as a sudden increase in the PF is a

cause of immediate concern even when the absolute PF value is not

considered excessive

87

Typical of maximum allowable PF values *



88

Equipment

Typical max allowable PF

values at 20 Deg C

Oil‐Filled Power and Distribution Transformers

0.5%

may be up to 1% for old

transformers

Dry‐Type Power and Distribution Transformers

up to 2%

may be up to 5% for old

transformers

Transformer Oil 0.05%

Bushings 0.50%Rotating Machines 1%

Cable insulation:

Paper 0.50%

XLPE 0.05 ‐ 1.0 %

Ethylene/propylene rubber 0.5 ‐ 1.0 %Rubber (older type) 3 to 5 %

Varnished cambric 4 to 8 %

These are general guidelines. Please Refer Manufacturer’s recommendations and equipment history*

AC High Potential Test



AC Hi‐pot is used as acceptance test (go/no go type)

Thumb rule: Acceptance :75 % of factory test voltage, as certified by

manufacturer

Maintenance : 60 % of factory test voltage, as certified by

manufacturer

89



Conclusion

Conclusion



For optimum benefit :

yEssential to record all test data in proper formats for future reference

y Test Equipment

yProper selection

y Regular maintenance and

y Regular calibration

y Trained persons with knowledge, experience and cool mind

91

Testing Information



The information on testing can be obtained from several different sources

such as

y Power equipment manufacturer’s manuals (BHEL etc),

y Test equipment manufacturer’s manuals such as y AVO, International: http://www.avointl.com

y Doble Engineering: http://www.doble.com

y AEMC: http://www.aemc.com

y

industry standards

such

as

y Indian Standards (IS)

y Institute of Electrical and Electronic Engineers (IEEE),

y American National Standard Institute (ANSI),

y National Electrical Manufacturers Association (NEMA),

y National Fire Protection Association (NFPA)

y International Electrical Testing Association (NETA),

y Insulated Cable Engineering Association (ICEA),

y And of course our best friend, INTERNET 92

http://www.aemc.com/

http://www.aemc.com/



93

THANK YOU

Temp Correction factors

Rotating Equipment Transformer

Deg C Class B oil filled Temp

correction

factor

of

2

for



g

0 0.4 0.25

5 0.5 0.36

10 0.63 0.5

15 0.81 0.74

20 1 1

25 1.25 1.4

30 1.58 1.98

35 2 2.8

40 2.5 3.95

45 3.15 5.6

50 3.98 7.85

55 5 11.2

60 6.3 15.85

65 7.9 22.4

70 10 31.75

75 12.6 44.7

Temp correction factor of 2 for

•Approx every 10 deg increase

in Temp for oil filled

Transformers

•Approx every 15 deg increase

in Temp for Rotating

equipments having class B insulation

Reproduced from NETA MTS – 1997 Table 10.14

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Vivek Jha_DC and AC Test