Principle of Tan Delta Test

Principle Principl e of Tan Delta Test Test A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity. In pure capacitor, capacitor, the capacitive electric current leads the applied voltage voltage by  by 0!. In practice, the insulator cannot be made 100% pure. Also due to the ageing of insulator the impurities li"e dirt and moisture enter into it. These impurities provide the conductive path to the current. #onse$uently, lea"age electric current flowing from line earth earth through insulator has also also resistive component.

ence, it is needless to say that, for good insulator, this resistive component of lea"age electric current is $uite low. In other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive component. &or good insulator this ratio would be $uite low. This ratio is commonly "nown as tan' or tan delta. (ometimes it is also referred as dissipation factor.

In the vector diagram above, the system voltage is drawn along )*a)is. #onductive electric current  current   i.e. resistive component of lea"age current, I +  will  will also be along )*a)is. As the capacitive component of lea"age electric current I# leads system voltage voltage by  by 0!, it will be drawn along y*a)is.  ow, total lea"age electric current I-Ic / I+  ma"es an angle ' say with y*a)is.  ow, from the diagram above, above, it is cleared, the ratio, I+  to  to I# is nothing but tan' or tan delta.

NB: This δ angle is known as loss angle.

Method of Tan Delta Testing The cable, winding, current transformer , potential transformer , transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. A very low fre$uency test voltage is applied across the e$uipment whose insulation to be tested. &irst the normal voltage is applied. If  the value of tan delta appears good enough, the applied voltage is raised to 1. to 2 times of normal voltage, of the e$uipment. The tan delta controller unit ta"es measurement of tan delta values. A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results. 3uring test it is essential to apply test voltage at very low fre$uency.

Reason of applying Very Low re!"ency If fre$uency of applied voltage is high, then capacitive reactance of the insulator becomes low, hence capacitive component of electric current is high. The resistive component is nearly fi)ed, it depends upon applied voltage and conductivity of the insulator. At high fre$uency as capacitive current, is large, hence, the amplitude of vector sum of capacitive and resistive components of electric current becomes large too. Therefore, re$uired apparent power for tan delta test would become high enough which is not practical. (o to "eep the power re$uirement for this dissipation factor test, very low fre$uency test voltage is re$uired. The fre$uency range for tan delta test is generally from 0.1 to 0.01 z depending upon size and nature of insulation. There is another reason for which it is essential to "eep the input fre$uency of the test as low as possible. As we "now,

That means, dissipation factor tan' ∝ 1 4 f. ence, at low fre$uency, the tan delta number is high, the measurement becomes easier.

#ow to predict the Res"lt of Tan Delta Testing There are two ways to predict the condition of an insulation system during tan delta or dissipation factor test. &irst, one is, comparing the results of previous tests to determine, the deterioration of the condition of insulation due ageing effect. The second one is, determining the condition of insulation from the value of tan', directly. o re$uirement of comparing previous results of tan delta test. If the insulation is perfect, the loss factor will be appro)imately same for all range of test voltages. 5ut if the insulation is not sufficient, the value of tan delta increases in the higher range of test voltage.

&rom the graph, it is clear that the tan6delta number nonlinearly increases with increasing test very low* fre$uency voltage. The increasing tan6delta, means, high resistive electric current component, in the insulation. These results can be compared with the results of previously tested insulators, to ta"e the proper  decision whether the e$uipment would be replaced or not.

Insulation Resistance Test and Polarization Index Test

Both Insulation Resistance Test (IR Value Test) and Polarization Index Test  (PI Value Test) are conducted on HV machine to determine service condition of the insulation. In HV machines and windin are li!el" to #e a$ected #" moisture and contamination. IP test is conducted s%eciall" to determine the dr"ness and cleanliness of windin insulation. In insulation resistance test& a hih ' voltae is a%%lied across& conductor and round more s%ecicall". The voltae is a%%lied across the insulator. 'ue to this a%%lied hih ' voltae there will #e a current throuh the electrical insulator. Bi dividin the a%%lied voltae #" this current we et the actual resistive value of the insulator.

*a" the a%%lied hih voltae is V and corres%ondin current throuh the insulator is I. Hence as %er +hm,s law the value of insulation resistance is

 This test is enerall" done #" means of meer. -eer ives reuired direct (') voltae across the insulator an it also shows the resistive value of insulator directl" in -ohm rane.The meer are enerall" of /00 V& 1./ 2V an / 2V. /00 V meer are used for insulation test u%to 3.3 2V rated insulation. 4or hih voltae transformer& other HV eui%ment and machines& 1./ or / 2V meer are used. 5s all insulators are dielectric in nature the" have alwa"s a ca%acitive %ro%ert". 'ue to that& durin a%%lication of voltae across the electrical insulator& initiall" there will #e a charin current. But after some time when the insulator is totall" chared& the ca%acitive chanin current #ecomes zero and then onl" resistive conductive current %resents in the insulator. That is wh" it is alwa"s recommended to do insulation resistance test at least for 3 minute as it is %roved that charin current totall" #ecomes zero after 3 minute. +nl" measurin insulation resistance #" meer for 3 minute does not alwa"s ive relia#le result. 5s the resistive value of an electrical insulator also varies with tem%erature.  This di6cult" is %artiall" solved #" introducin %olarit" index test or in short PI value test. The %hiloso%h" #ehind PI test is discussed #elow. 5ctuall" when a voltae is a%%lied across an insulator there will #e a lea!ae current from line to round. 5lthouh this lea!ae current is ver" small is in millam%ere or sometimes in micro am%ere rane& #ut it has mainl" four com%onents.

3. a%acitive com%onent.

1. Resistive or conductive com%onent.

7. *urface lea!ae com%onent.

8. Polarization com%onent.

9et us discuss one #" one.

 a%acitive om%onent

:hen a ' voltae is a%%lied across in insulator& #ecause of its dielectric nature there will #e an initial hih charin current throuh the insulator from line to round. 5lthouh this current deca"s ex%onentiall" and #ecomes zero. ;enerall" this current exists for initial 30 seconds of the test. But it ta!es nearl" ver" insulator should have this com%onent of electric current. *ince in %ractice ever" material in this universe %ersists some sensitive nature.  The resistive or conductive com%onent of insulator lea!ae current  remains constant throuhout the test.

*urface 9ea!ae om%onent

'ue to dust& moisture and other contaminants on the surface of the insulator& there is one small com%onent of lea!ae current throuh the outer surface of the insulator.

Polarization om%onent

>ver" insulator is h"rosco%ic in nature. *ome contaminant molecules and mainl" moisture in the insulator are ver" %olar. :hen an electric eld is a%%lied across the insulator the %olar molecules alin themselves alon the direction of electric eld. The ener" reuired for this alinment of %olar molecules& comes from voltae source in form of electric current. This current is called %olarization current. It continues until all the %olar molecules allied themselves alon the direction of electric eld.

It ta!es around 30 minutes to alin the %olar molecules alon electric eld and that is wh" if we ta!e meer result for 30 minutes& there would #e no e$ect of %olarizin in meer result. *o when we ta!e meer value of an insulator for 3 minute& the results re=ects& the IR value which is free from e$ect of ca%acitive com%onent of lea!ae current. 5ain when we ta!e meer value of an insulator for 30 minutes& the meer result shows the 31 value& free from a$ects of #oth ca%acitive com%onent and %olarization com%onent of lea!ae current. Polarization index is the ratio of meer value ta!en for 30 minutes to the meer value ta!en for 3 minute.  The sinicance of %olarization index test. 9et I is the total initial current durin %olarization index test or PI test. I is the ca%acitive current. IR is resistive or conductive current. I* is surface lea!ae current. IP is %olarization current of the insulator.

Value of insulation resistance test or IR value test& i.e. value meer readin ?ust after 3 minute of the test& is@

-eer value of 30 minute test& is

 Therefore& result of %olarization index test& is

4rom the a#ove euation it is clear that& if the value of (I R A I*)  IP& the PI of insulator a%%roaches to 3. 5nd lare I R or I* or #oth indicate unhealthiness of the insulation.  Th value of IP #ecomes hih if (IR A I*) is ver" small com%ared to IP. This euation indicates that hih %olarization index of an insulator im%lies healthiness of insulator. 4or ood insulator resistive lea!ae current I9 is ver" tin". It is alwa"s desired to have %olarization index of an electrical insulator more than 1. It is hazardous to have %olarization index less than 3./.

TAN-DELTA TEST SETS

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Why to measure the CAPACITANCE & TAN-DELTA of the insulating material? Electrical properties of the insulating system change due to age and continuous electrical street. The principal c the unexpected breakdown of the high voltage equipment is the insulation failure. As compared to the magnetic, condu insulating materials which form the basics of any electrical equipment, the insulating material is more prone to service thermal stress, electrical stress, mechanicals stress, environment stress etc. y measuring the electrical properties such as capacitance and Tan!"elta regularly on periodical basis, it is pos ensure the operational unexpected breakdown. "issipation factor #Tan!"elta$ is one of the most powerful off!line nond diagnostic tool to monitor the condition of solid insulation of various high voltage equipment. %apacitance and Tan!"elta values obtained on new insulation are treated as benchmark readings. Then by me comparing the periodical readings of the capacitance and Tan!"elta of the insulating material with the benchmark readi can know the rate of deterioration of the health of the insulation.

Knowing the rate of eterioration! we "an #e a#le to redict the future unexpected breakdown of the insulation of '( equipment. lan the maintenance schedule. )epair the insulation before actual flashover, saving high cost of replacement of material which will reduce the inven delay in procurement at the last minute.  After repair, quality of insulation can be checked before returning the equipment to service.

When to "he"$ "a%a"itan"e & Tan-Delta "uring manufacturing process preferably at each stage. *n service & will be depend on rate of change of Tan!"elta (s (oltage (s Time #+onth-ear$. requency of testing depends on history of past failures on same machine. requency of testing depends on environmental conditions. +ore humidity, temperature, pollution would require frequency measurement of Tan!"elta.

Ca%a"itan"e & Tan-Delta Test ystem '(oel TD-)* The instrument after some minor calculation directly gives the capacitance and dissipation factor #Tan!"elta$ of the ins material when used with the required power source. This is a battery operated instrument and very useful to monitor th health of the insulating material as per */. This instrument is specially shielded with high permeability metal sheets to a effect of external interference.

+eatures uilt!in protection 0 *nstrument is provided with built in high voltage protection device which protects the instrument operator against failure of test ob1ect or standard capacitor. 2ull "etector 0 uilt!in battery operated null detector is provided which is most suitable for quick balancing of the inst gives high sensitivity and accuracy. *%3 0 *nterference compensation #unit$ is useful only in the case of heavy induction area so the to get the correct rea

Te"hni"al s%e"ifi"ation , Ca%a"itan"e ange , 4  to 4.4+" in three or four ranges depend upon capacitor and can be extended further # external current transformer$. esolution

, C. multi%lying fa"tor 5.54 5.4 4 45

esolution 5.4  4  45  455 

A""ura"y

, 6 5.47 of reading.

Tan-Delta ange

, 5.5554 to 44.4 in three ranges.

esolution

, Tan-Delta fa"tor 5.4 4 45

A""ura"y

, 6 47 of reading 6 4 to ; x 45 !9.

esolution 4 x 45 !8 4 x 45 !9 4 x 45 !:

/igh 0oltage Power our"e '(oel /-)1* The instrument is used to get the high voltage 5 to 4;