38 Behaviour DC

7/29/2019 38 Behaviour DC

1/10

BEHAVIOUR OF INSULATION TO DC SUPPLY

P. K. Pattanaik

E & MR Sub-Divn, Orissa Power Transmission Corp. Ltd. ,

Burla-768018, (Orissa)09437209480 (Mob), 06632430514 (o), 06632431232 (R),[email protected]

INTRODUCTION

The electrical conductor in the electrical equipment needs certain insulation to be

covered up for protection of the leakage current flow to the adjacent path or

conductor. Good insulator is regarded as the bad conductor and provides highresistive path for the current flow that restricts the leakage current through it and

theoretically offers infinity resistance and zero leakage current. But practically no

such insulator is possible to offer infinity resistance. However an insulator with

higher value and well above the allowable standard can be considered as a goodinsulator. To interpret ate for the acceptance level of this insulator such used in the

electrical system is needed to be tested to the available methods. The behavior of thesame to the application of DC supply and to the application of AC supply has been

explained in this article with some standard values.

1. CAUSES OF INSULATION DEGRADATION

The different environmental factors like electrical, mechanical and chemical cause the

degradation of the insulation used in the electrical system. These factors are the basic

initiators for the deterioration and considered as (Electrical Stress, Mechanical Stress,

Chemical Effects, Thermal Stress, and Environmental Contamination.) Due to these factors,

during normal operation in the system the insulators start deterioration slowly till to the

permanent aging. However sometimes due to some severest application of the factors or

continual use of the same in adverse condition causes the deterioration of the insulation in a

faster manner than that of the usual practice. This behavior of the insulation is needed to be

monitored to know the status of the same for the use in the system.

1.1 Electrical stressInsulation of any material is designed to a particular application of supply voltage. The over

and under voltage in the system causes the abnormal stresses in the insulation that leads to the

rupture, pitting or crack in the insulation. This stress for the application of Electrical supply

to the insulation is called Electrical Stress

1.2 Mechanical StressDue to abnormal running of the equipment along with electrical stress certain mechanical

stress also results on the insulation that causes deterioration ob the cable. The practical

deformity and displacement of the insulation are caused due to the Mechanical Stress.

1.3 Chemical Effects
mailto:[email protected]:[email protected]


2/10

After continuous use of the insulation, the material composition in the insulator causes

chemical reaction which in turn starts deterioration of the insulation in long process. This

stress due to chemical change is called Chemical Effect.

1.4 Thermal StressRunning of any equipment in abnormal temperature like extreme hot or extreme cold

condition causes the failure of the insulation. Sometimes irregular start and stop of themachine also causes abnormal rise of the temperature and deterioration of the insulation.

1.5 Environmental ContaminationThe surrounding factors like moisture, humidity, dust and foreign particles also cause the

slow damage to the insulation.

So it is a continuous process that from the day of service, the insulation starts to degrade.

In general the insulation in the system is designed to provide good service over many years

under normal operating condition. However abnormal conditions may have a damaging

effect which, if left unchecked, will expedite the rate of degradation and ultimately cause afailure in the insulation.

2. Effect of Insulation to DC supplyApplication of DC voltage is the basic principle of the IR (Insulation Resistance) value

measurement by any tester ( Megger ). The test current through the insulation becomes the

resultant of three components (Capacitive Current, Absorption or Polarization Current,

Leakage or Conduction Current). For the application of DC voltage and the current that flows

in the insulation being calculated by Ohms Law provides the resistance of the insulation

( R= V/I )

2.1 Capacitive Current

Insulation in the electrical system acts as a dielectric medium and possesses somecapacitance to the system. So the application of DC voltage results certain charging current

which becomes initially large and goes to zero exponentially when the insulation under test

becomes fully charged.

2.2 Absorption or Polarization CurrentFor the application of electric field to the insulation, the electrons in the material start

drifting for molecular alignment. The di-pole lining in the polarized form depends upon the

nature of binding material available in the insulation. This polarization current after final

alignment becomes zero and the nature of approach to final zero value depends upon the

polarization index.

2.3 Leakage or Conduction CurrentEvery insulating material also contains conducting ions and according to the degree of

ionization the current flow through the insulation is called Conduction current. This current is

steady through the insulation and is usually represented by a very high value resistance in

parallel to the capacitance of the insulation. This current also includes surface leakage current

that depends upon the contamination of the insulation with the factors like moistures,chemical effects etc..


3/10

Total Current

Current

AbsorptionCurrent

Leakage Current

Capacitive Charging Current

Time in Secs

2.4 Total CurrentNow the effect of all the possible currents through the insulation provides the total

current as the concept of exponential decay value according to the time factor. During the

measurement of insulation resistance for the application of DC voltage to the system, the

value of the initial resistance becomes small and rises according to the kind of insulation

used. For good quality of insulation value rises quickly and attains the final value of charging

at minimum time. The PI (Polarization Index) becomes more.

Insulation resistance is time dependant and in usual practice the behavior of resistance for

the first few seconds becomes capacitive rise till to the capacitive charging of the insulation.

At about 1minute (60 Sec) the current becomes the combination of polarization and leakage

current. By the end of 10 minutes only leakage current remains and contributes the final

constant value of the resistance. Most of insulation takes maximum of 10 minutes to reach the

final value. But some times it also takes hours together to reach the final value. So in general

10 minutes is considered as the standard of testing practice.

Important Note

Every insulation specimen after testing with DC voltage has to be discharged to

decay the stored energy during the charging condition.

3. DIAGNOSTIC PRACTICE OF INSULATION

Different Methods and testing practices are used for the diagnosis of the nature and status

of insulation. The interpretation of the test results are important for arriving to the conclusion

of the kind of insulation. The test practices are listed below.

3.1 Spot Reading Test

3.2 Polarization Index Method


4/10

3.3 Step Voltage Technique

3.4 Dielectric Discharge

4.1 Spot Reading Test

This is one of the usual practices of the insulation resistance measurement for which thetest voltage is applied for a specific time period (usually 1 minute is considered as capacitive

current decays within this period) and reading is taken after that period. The reading thatobtained is called SPOT READING. This value is compared with the standard and also to the

previous value if any to decide the nature and status of the insulation. But the value of

insulation is temperature dependant. So during the time of comparison the reference

temperature has to be taken into the account. As a thumb rule for 10 degree (Celsius) of

temperature rise causes double of the current (Half of the Resistance). For actual comparison

the table mentioned can be referred for.

It is also important to consider the scale used in the instrument for the measurement of

the value. Some times the range of the instrument does not cover the actual value of the

insulation. If the start range becomes higher then after 1 minute, the pointer/needle/scale may

not touch the initial value and lead confusion to the operator. Similarly if the maximum range

is less then the reading may go beyond the limit declaring the value as infinity. But a value as

infinity can not be compared with the previous result.

4.1.1 Points to be considered for testing/ comparison of valuesFollowing points are to be considered for the correct interpretation of the result and for

obtaining the correct value of the insulation resistance.

1. Periodic test should be made in same manner for every testing practicea. Use same test connection

b. Same Test voltage

c. Same Length of time

2. Guard connection should be provided

3. Temperature factor should be considered for relative comparison.

4.1.2 Nature of readings

From the readings obtained during the test practice the status of the insulation can beassessed and accordingly the corrective actions can be taken. The following table can

provide the guide lines for the same.

4.2 Polarization Index MethodThis concept has been originated from the spot reading test. The study of the polarization

is important to know the nature of the insulation for the use in electrical equipment. Thepolarization of the electrons for the correct alignment depends upon the nature of binding

material available in the insulation. It is seen that most of the insulation takes maximum of 10

Sl Nature of Reading Corrective Action1 High Value and maintained in correct way of decrement No cause of concern and insulation

considered as correct.

2 Low Value ( above standard or allowable value ) and

maintained in correct way of decrement

No cause of concern and insulation

considered as correct.

3 High or fair value but sudden decrement Clean, dry out or take corrective action for t

sudden decrement of the result.

4 Low value and sudden decrement Unsafe and corrective action to be investigat

or else discard the insulation.


5/10

minutes to reach the final value of the polarization. So the behavior of the insulation till to the

period of 10 minutes is studied. Now the ratio of the values obtained for 1 minute with 10

minutes reading is considered for the polarization index. Such study is also called Time

versus Resistance study.

Polarization Index = (Insulation resistance obtained at 10 minutes of testing) /

(Insulation resistance obtained at 1 minute of testing)

The ratio such obtained is purely a number and does not depend upon the temperature value

and other climatic conditions. So this method of PI value and its interpretation helps the

operator to decide the status of the insulation and regarded as one of the quite useful

techniques.

NOTES ON P I VALUE

PI value test is particularly useful for large equipments, which yields self contained

evaluation based on relative readings rather than an absolute values. The following chart may

provide useful hints to the operator.

PI INSULATION4 Good

Note(1) Values above 4, indicate excellent equipment for which no action is likely to be

necessarily taken.

(2) However some values of PI (above 5) could be considered critically. Such valueindicates regarding dryness, brittle or cracked insulation and volatile one. This value

is objectionable and investigation is to be done for such kind of insulation. These

insulations are very prone to the fire and causes fire for slightest fire inside.

(3) Sudden rise of PI value greater than 20% without any maintenance of the insulation

is also objectionable, as this also indicates the dried and cracked tendency of

insulation by the time of the operation.

(4) The time factor of 10 minutes of reading is quite OK, for almost of all insulation, but

for large pieces of equipment, the insulation may take an hour or more for fully

change.

SPECIAL NOTE

The concept of PI value and IR value for fluid filled insulation can not be considered as

confirmatory method, because for such insulation, the passage of test current through oil filled

sample creates convection current, than continuously swirl the oil, resulting in a chaotic lack of

bonding and final change of the insulation.


6/10

4.3 STEP VOLTAGE TECHNIQUE

By this method the insulation under test needs to be applied with voltages in successive

manner. According to the range of voltage application, the IR value of the insulation is noteddown for study. At lower test voltages say 500v or 1000v, the defects in the insulation may not

be noticed .But further rise of voltage causes the ionization of insulation within cracks or cavities,resulting the increase in current and decrease in resistance.

A recognized standard practice for step voltage test is to raise the voltage in 5 equal steps

at one minutes increments and to record the final insulation resistance at each level.

Note

1. If a deviation or fall of 25% in resistance measurement is observed over the range ofthe successive voltages, it is an indication of the presence of moisture or other

contamination.

2. No appreciable difference in values of insulation indicates the good quality.

4.4 DIELECTRIC DISCHARGE TEST

This method is quite different to the other methods as described. For dielectric discharge

test, the current that flows during discharge is measured for evaluation.

The insulation is measured for evaluation of the condition of insulation .The insulation is charged

with certain supply voltage, up to 10 to 30 minutes for which the charge stored in the insulation.

Then the stored charge is allowed to discharge through a standard resistance. After approximately

60 seconds of discharge, any remaining current flow is measured. At this time the capacitance

gets discharged and the voltage is collapsed to zero value.

The following formulae are used for evaluation of insulation

Index = The current flowing after one minute (mA)

Test voltage (V) X Capacitance (F)

The following chart may be helpful for the operator to know the condition of insulation.

DD value(mA/VF) Insulation condition

>7 Bad4-7 Poor

2-4 Questionable


7/10

CURVE FOR DIELECTRIC DISCHARGE

Total Current

Absorption CurrentCurrent( Amp)

Capacitive Charging Current

Time in Secs

POINTS TO CONSIDERED DURING THE TEST,

(1) Test leads(a) Quality of test leads most be good and voltage rating more than the applied

voltage

(b) Test leads should not be allowed to touch each other or any other object duringtest.

(c) Sharp points at the test lead connections should be avoided.

(2) Instrument Set up(a) For analog type, the instruments should be leveled properly for correct movement

of pointer over the scale and to avoid parallax error.

(b) Suitable range of scale should be chosen with application of correct voltage

value.

(c) Guard the other points of the equipment to avoid surface leakage current.

NOTES ON GUARD

In the insulation tester, the third terminal that is used, excluding the Main line and Earth

(+ve and ve) is called the Guard Terminal. It is a Shunt circuit that directs surface leakage

current around the measurement function. If parallel leakage paths exist, a guard connection will

eliminate those from the measurement. Lets consider the following example of measurement.


8/10

For the measurement Shown, actual insulation resistance is 1000M

(1) But measured value without Guard terminal

R = 100 X 0.6 = 60 = 596k

(100+0.6) 100.6

(2) For guarding the sample, the value changes and calculated as follows.

Now the effect surface leakage current can be avoided to some extent. The current that flows to

the actual insulation resistance indicate value.

EFFECTS ON INSULATION RESISTANCE VALUE

(1) Temperature: Temperature plays the important role for the IR valuemeasurement. The degree of the orientation of the electrons for insulating

materials differs due to the temperature on it. In general rise in temperature

causes drop of IR value and vice versa. As a thumb rule for every 10 0c increase,

causes halve the resistance and every 10 0c decrease results double the resistance.

The table for convection factor can be referred for IR value at different

temperature.

(2) Effect of Humidity: The humidity (moisture content) has an effect upon theIR, but it can not be quantified properly, because of its invariant absorbent

quality. So, during measurement, care should be taken so that the surrounding

temperature to be more than the dew point.

(3) Ingress Protection: IP stands for Ingress Protection, i.e the degree ofprotection of the instrument to withstand for the ingress of foreign materials. This

rating is designated by two digits with their own significance. For example:

IP54: I P Five Four

5 indicates dust protected along with invasion of wire down to 1 mm.

4 indicates resistance to splashing water any direction.

Line

+ve

100 M

Rl1(300 k) Rl2(300 k)

Guard Ground

(-ve)


9/10

CHARTS FOR INGRESS PROTECTION

Chart 1Protection Against Ingress of solid foreign objects (First Digit)

Number Description0 Non protected

1 Objects 50 mm2 Objects 12.5 mm

3 Objects 2.5 mm4 Objects 1 mm

5 Dust-protected6 Dust-tight

Chart 2Protection Against Ingress of liquid (Second Digit)

Number Description0 Non-protected

1 Water dripping vertically2 Water dripping, enclosure tilted up to 150c

3 Spraying water, up to 600c angle from vertical4 Splashing water, any direction5 Jetting water, any direction6 Powerful jetting water, any direction7 Temporary immersion in water8 Continuous immersion in water

So, the operator has to consider the case of I P rating during measurement.

INFORMATIONS AND DATA

5 INSULATION RESISTANCE5.1 Conversion factor for I.R. value (Ref = 600C)

Temp in 0C M.F. Temp in 0C M.F. Temp in 0C M.F.10 0.05 24 0.12 30 0.165

15 0.067 25 0.125 31 0.17

20 0.088 26 0.13 32 0.18

21 0.091 27 0.135 33 0.195

22 0.097 28 0.14 34 0.21

23 0.11 29 0.15 35 0.22

36 0.235 51 0.58 66 1.4

37 0.25 52 0.64 67 1.5

38 0.265 53 0.67 68 1.7

39 0.29 54 0.69 69 1.75

4 0.30 55 0.74 70 1.85

41 0.31 56 0.78 71 1.95

42 0.34 57 0.84 72 2.1


10/10

43 0.37 58 0.90 73 2.2

44 0.39 59 0.93 74 2.35

45 0.40 60 1.00 75 2.4546 0.45 61 1.05 76 2.6

47 0.48 62 1.11 77 2.75

48 0.49 63 1.12 78 2.9549 0.52 64 1.25 79 3.12

50 0.55 65 1.35 80 3.3

Note : Thumb Rule for every 100C change (reduction) I.R. valuechanges by ratio 2/1.

5.2 INSULATION CONDITION OF TRANSFORMER(AS PER IEEE)

P.I (IR at 600 s/ IRat 60 s

InsulationCondition

P.I (IR at 600 s/ IRat 60 s

InsulationCondition

Less than 1 Dangerous Between

1.25 to2.0

Fair

Between

1 to1.1

Poor Above 2 Good

Between

1.1 to1.25

Questionable

5.3 Insulation Resistance and P.I Value

(a)Minimum insulation resistance is obtained by following formula: R=CE/KVA

Where R = I.R of winding with ground when other windings are grounded in M.

C = Constant = 0.8 for ONAN at 200C.= 16 for dry, compound filled or un tanked oil filled,

KVA = Rated capacity of winding under test.E = 1 voltage (Y) and line voltage for () in volt.

5.4 Minimum I.R. Value

Rated voltage ofwinding

Minimum safe I.R. in M300C 400C 500C 600C

66 KV and above

22 KV and 33 KV

6.6 KV and 11 KVBelow 6.6 KV

600

500

400200

300

250

200100

150

125

10050

75

65

5025

The I.R. value should be taken with all windings earthedexcept the tested winding.

5.5 Minimum P.I. Value

PI1=R15/R0 PI2=R60/R15 PI3 = R600/R602.5 to 3 1.5 to 2 1.2 To 1.5

38 Behaviour DC

Documents

Transcript of 38 Behaviour DC