19

CHAPTER 2

TESTING OF STATOR WINDING

2.1 INTRODUCTION

The rotating machine consists of three main components in stator

such as the copper conductors, the stator core and the insulation. The copper

is a conduit for the stator winding current. In a generator, the stator output

current is induced to flow in the copper conductors as a reaction to the

rotating magnetic field from the rotor. In a motor, a current is introduced into

the stator, creating a rotating magnetic field that forces the rotor to move. The

copper conductors must have a cross section large enough to carry all the

current required without overheating (Say 1976).

Three basic types of stator winding structures are employed over

the range from 1kW to more than 1000 MW:

i. Random -wound stators

ii. Form- wound stators using multiturn coils

iii. Form- wound stators using Roebel bars

2.1.1 Random-Wound Stators

Random-wound stators consist of round, insulated copper

conductors (magnet wire or winding wire) that are wound continuously (by

hand or by winding machine) through the slots in the stator core to form a

20

coil. Most of the turns in the coils can be easily seen. Each turn (loop) of the

magnet wire could, in principle, be placed randomly against any other turn of

magnet wire in the coil, independent of the voltage level of the turn, thus the

term random. Since a turn that is connected to the phase terminal can be

adjacent to a turn that is operating at low voltage (i.e., at the neutral point),

random -wound stators usually operate at voltages less than 1000 V. This

effectively limits random-wound stators to machines less than several

hundred kilo-watts (kW) or Horse Power (HP).

2.1.2 Form-Wound Stators-Coil Type

Form-wound stators are usually intended for machines operating at

1000 V and above. Such windings are made from insulated coils that have

been preformed prior to insertion in the slots in the stator core. The preformed

coil consists of a continuous loop of magnet wire shaped into a coil

(sometimes referred to as a diamond shape), with additional insulation applied

over the coil loops. Usually, each coil can have from 2 to 12 turns, and several

coils are connected in series to create the proper number of poles and turns

between the phase terminal and ground (or neutral). By minimizing the

voltage between adjacent turns, thinner insulation can be used to separate the

turns. For example, in a 4160 volt stator winding (2400 Volt line-to-ground),

the winding may have 10 coils connected in series, with each coil consisting

of 10 turns, yielding 100 turns between the phase terminal and neutral. The

maximum voltage between the adjacent turns is 24 V.

2.1.3 Form-Wound Stators - Roebel Bar Type

In large generators, the more the power output, the larger and

mechanically stiffer each coil usually is. In stators larger than about 50 MW,

the form-wound coil is large enough that there are difficulties in inserting

both legs of the coil in the narrow slots in the stator core without risking

21

mechanical damage to the coil during the insertion process. Thus, most large

generators today are not made from multi-turn coils, but rather from half-turn

coils, often referred to as Roebel bars. With a Roebel bar construction, only

one half of a coil is inserted into the slot at a time, which is considerably

easier than inserting two sides of a coil in two slots simultaneously.

2.2 STATOR WINDING INSULATION SYSTEM

The Stator winding insulation system contains several different

components and features, which together ensure that electrical shorts do not

occur, that the heat from the conductor I2R losses are transmitted to a heat

sink and that the conductors do not vibrate in spite of the magnetic forces.

The basic stator insulation system components are listed below (Stone et al

2004)

i. Strand (or sub conductor) insulation

ii. Turn insulation

iii. Groundwall (or ground or earth) insulation

Figure 2.1 shows the cross section of form-wound coils in a stator

slot, and identifies the above components. In addition to the main insulation

components, the insulation system sometimes has high-voltage stress-relief

coatings and end-winding support components (Allison 2000). The Stress-

Grading systems have to regulate the potential distribution on the surface of

the coil overhangs at the slot to prevent the surface discharge, which acting

over a longer period of time could destroy the insulation. They have to retain

this ability not only during the testing of windings when the voltage

considerably exceeds the rated voltage, but also during the whole operating

life of the machines. So the task is to suppress any discharges which may

occur at the maximum voltage applied to the winding, both within the slot and

at the ends.

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Iron

Slot wedge

Top packing

Semi conductiveCoating

Turn insulation

Strand insulation

Bottom packing

Midstick packing

Groundwall Insulation

Figure 2.1 Form-Wound Multiturn Coils

Normally inside the slot, semi-conductive, low resistance black

carbon/graphite corona shield materials are used; because conductive

materials have a uniform resistance which is not voltage dependent. They are

always black in appearance and are applied only on the straight portion of the

coil/bar. At the coil ends, semi-conductive high resistance gray silicon-

carbide materials are used for stress grading system.

Stress grading materials are non-linear i.e. their resistivity is

changing depending on the applied electrical stress, even though it may seem

uniform over part of the range of applied voltage. They are normally gray in

color and are only applied at the coil ends. The characteristics of conductive

and stress-grading materials are shown in Figure 2.2.

23

Stress grading tape at ends (non-linear)

Current (in mA)

Voltage (V)

Conductive tape in Slot 0.12

0.10

0.08

0.06

0.04

0.02

0 500 1000 1500 2000 2500 3000

Figure 2.2 Material Characteristic Curves

2.2.1 Strand Insulation

Many form-wound machines employ separate strand and turn

insulation. The following mainly addresses the strand insulation in form-

wound coils and bars. There are both electrical and mechanical reasons for

stranding a conductor in a form-wound coil bar. From a mechanical point of

view, a conductor that is big enough to carry the current needed in the coil or

bar for a large machine will have a relatively large cross sectional area. That

is, a large conductor cross section is needed to achieve the desired capacity.

Such a large conductor is difficult to bend and form into the required coil/bar

shape. A conductor formed from smaller strands (also called sub-conductors)

is easier to bend into the required shape than one large conductor.

From an electrical point of view there are reasons to make strands

and insulate them from one another. A copper conductor has a large enough

cross-sectional area, where the current will tend to flow on the periphery of

the conductor. This is known as the skin effect. The skin effect gives rise to a

24

skin depth through which most of the current flows. The skin depth of copper

is 8.5 mm at 60 Hertz (Hz). If the conductor has a cross section such that the

thickness is greater than 8.5 mm, there is a tendency for the current not to

flow through the centre of the conductor, which implies that the current is not

making use of all the available cross section. This is reflected as an effective

AC resistance that is higher than the DC resistance.

The higher AC resistance gives rise to a larger I2R loss than if the

same cross-section had been made from the strands that are insulated from

one another to prevent the skin effect from occurring by making the required

cross section from the strands that are insulated from one another, all the

copper cross section is used for current flow, the skin effect is negated, and

the losses are reduced. The electrical reason for stranding requires the strands

to be insulated from one another. The voltage across the strands is less than a

few tens of volts; therefore, the strand insulation can be very thin. The strand

insulation is subject to damage during the coil manufacturing process, so it

must have good mechanical properties. Since the strand insulation is

immediately adjacent to the copper conductors that are carrying the main

stator current, which produces the I2R loss, the strand insulation is exposed to

highest temperatures in the stator.

2.2.2 Turn Insulation

The purpose of the turn insulation in form wound stators is to

prevent the shorts between the turns in a coil. A turn short occurs whereas the

shorted turn will appear as the secondary winding of an autotransformer. For

example, the winding has 100 turns between the phase terminal and neutral

and when the dead short appears across one turn then 100 times the normal

current will flow in the shorted turn. This follows from the transformer law as

given in equation (2.1).

25

p p s sn I n I (2.1)

where n refers to the number of turns in the primary or secondary, and I refers

to the current in the primary or secondary. Consequently, a huge circulating

current will flow in the faulted turn, rapidly overheating it. This high current

will be followed quickly by a ground fault due to melted copper burning

through any groundwall insulation. Clearly, effective turn insulation is needed

for long stator winding life.

The power frequency voltage across the adjacent turns in a form-

wound multiturn coil is well defined. Essentially, one can take the number of

turns between the phase terminal and neutral and divide it into the phase-

ground voltage to get the voltage across each turn. For example, if a motor is

rated 4160Vrms (phase-phase) and the phase-ground voltage is 2400 V, this

will result in about a 24 Vrms across each turn, if there are 100 turns between

phase end and neutral. This occurs because coil manufacturers take

considerable trouble to ensure that the inductance of the coil is the same, and

that the inductance of each turn within a coil is the same. Since the inductive

reactance (XL) in ohms is given in equation (2.2).

2LX fL (2.2)

where f is the frequency of the AC voltage and L is the coil or turn

inductance, the turns appear as impedances in a voltage divider, where the

coil series impedances are equal. In general, the voltage across each turn will

be between 10 Vac (small form-wound motors) to 250 Vac (for large generator

multiturn coils). The turn insulation in form-wound coils can be exposed to

very high transient voltages associated with motor starts, Inverter Fed Drives

(IFDs) operation, or lightning strikes (Gupta 1987). Such transient voltages

may age or puncture the turn insulation.

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2.2.3 Groundwall Insulation

Groundwall insulation is the component that separates the copper

conductors from the grounded stator core. Groundwall insulation failure

usually triggers a ground fault relay, taking the motor or generator off-line.

Thus the stator groundwall insulation is critical to the proper operation of a

motor or generator. For a long service life, the groundwall must meet the

rigors of the electrical, thermal and mechanical stresses.

The groundwall insulation in form-wound multi-turn coils or bars

connected to the phase end of the winding will have the full rated phase to

ground voltage across it. A stator rated at 13.8 kV (phase-to-phase) will have

a maximum of 8 kV (13.8/ 3 ) between the copper conductors and the

grounded stator core. This high voltage requires a substantial groundwall

insulation thickness. The high groundwall voltage only occurs in the

coils/bars connected to the phase terminals. The coils/bars connected to the

neutral have essentially no voltage across the groundwall during the normal

operation.

The groundwall insulation in indirectly cooled form-wound

machines is the main path for transmitting the heat from the copper

conductors to the stator core. Thus the groundwall insulation should have as

low a thermal resistance as possible, to prevent high temperatures in the

copper. To achieve a low thermal resistance it requires the groundwall

materials to have a high thermal conductivity as far as possible and for the

groundwall to be free of voids. Such air voids block the flow of heat, in the

same way that two layers of glass separated by a small air space inhibits the

flow of heat through a window. Therefore the insulation must be able to

operate at high temperatures and be manufactured in such a way as to

minimize the formation of air pockets within the groundwall.

27

There are large magnetic forces acting on the copper conductors.

These magnetic forces are primarily the result of the two magnetic fields from

the current flowing in the top and bottom coils/bars in each slot. These fields

interact, exerting a force that makes the individual copper conductors as well

as the entire coil or bar vibrate up and down in the slot.

The groundwall insulation must also help to prevent the copper

conductors from vibrating in response to the magnetic forces. The groundwall

were full of air pockets, the copper conductors might be free to vibrate. This

would cause the conductors to bang against the remaining groundwall

insulations, as well as allowing the copper strands and turns to vibrate against

one another, leading to insulation abrasion (Stone and Maughan 2008). An

incompressible insulating mass exists between the copper and the coil surface,

and then the conductors cannot move.

2.2.3.1 Groundwall Partial Discharge Suppression

In form-wound bars and coils rated greater than about 4 kV, Partial

Discharges (PDs) can occur within the groundwall insulation or between the

surface of the coil or bar and the stator coil. These Partial Discharges, which

are sometimes called coronas, are created by the high voltage stress that

occurs in the groundwall. An air pocket exists in the groundwall; the high

electric stress will break down the air, causing a spark. This spark will

degrade the insulation and, repeated discharges will eventually erode a hole

through the groundwall to prevent stator winding failure. In addition, a partial

discharge suppression system is needed to prevent PD in any air gaps between

the surface of the coils and bars and the core.

Electric breakdown strength is also a property of an insulating

material. Electric breakdown is not governed by voltage alone. Also it

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4 mm Groundwall insulation

0.5 mm internal air-filled void

Copper

Stator core

depends on the electric field. Electric stress, E in parallel plate geometry is

given by the equation (2.3).

V kVEd mm

(2.3)

where V is the voltage across the metal plates in kV and d is the distance

between the plates in mm. If the voltage is gradually increased across the

metal plates, there will be a voltage at which electric break down occurs, i.e.,

at which a spark will cross between the plates.

The presence of air pockets within the groundwall can lead to the

electric break down of the air pockets, a process called a Partial Discharge

(Kuffel et al 1998). To understand this process, consider the groundwall cross

section in Figure 2.3. For electric breakdown to occur in the air pocket there

must be a high electric stress across it.

Figure 2.3 Coils with Air Packets Next to the Turn Insulation

29

Va Ca

Cin

Core

V0

Copper

Using a simple capacitive voltage divider circuit as shown in

Figure 2.4, one can calculate the voltage across the air pocket. The

capacitance of the air pocket, to a first approximation, can be calculated,

assuming it is a parallel plate capacitor, i.e.,

Figure 2.4 An Electrical Equivalent Circuit of a Coil with Air Packets

Next to the Turn Insulation

a

ACad

(2.4)

where is the permittivity of the insulating material, A is the cross sectional

area of the void, and da is the thickness of the void. The permittivity is often

represented as shown in equation (2.5).

0r (2.5)

where r is the relative dielectric constant and 0 is the permittivity of free

space, equal to 8.85×10-12 F/m. the dielectric constant for air is 1.0. For most

stator winding insulation materials, the dielectric constant is about 4.

30

Thus, assuming unity cross-sectional area, the capacitance of the air

pocket in Figure 2.4 can be calculated. The air pocket is in series with another

capacitor, which represents the capacitance (Cin) of the solid insulating

material. Using equation (2.4), assuming a dielectric constant of 4 and the

thickness of the insulation is 4mm, the insulation capacitance can be

calculated to a first approximation. Using simple circuit theory, the voltage

across the air pocket can be calculated by the equation (2.6).

0ina

a in

C VVC C

(2.6)

where V0 is applied AC voltage (8 kV rms if the coil is at the phase terminal)

and Cin is the solid insulating material capacitance. Using the above

equations, the dimensions in Figure 2.3, recognizing that A and 0 will cancel

out, and assuming the dielectric constants are 1 and 4 for air and the

insulation, respectively, one can calculate that the voltage across the air

pocket is 33% of the applied voltage. For a V0 of 8 kV rms (rated phase-

ground voltage for a phase-end coil in a 13.8 kV stator), the voltage across the

air pocket is about 2.6 kV. From Equation 2.3, this implies that the electric

stress within the air pocket is 5.2 kV/mm. This far exceeds the 3 kV/mm

electric strength of the air, and thus electric breakdown will occur within the

air. The resulting spark is called a partial discharge. The discharge is referred

to as partial since the spark is only in the air pocket or void (Bartnikas 1980).

2.3 OFF-LINE TEST

The off-line tests describe the main tests that are commercially

available for assessing the condition of the insulation of stator windings as

shown in Figure 2.5. All the tests require the machine to be removed from

service, at least for a short time. Tremendous advancements in testing

technology have been made, due to availability of better electronic

31

equipments like 4½ Digit Micro Ohm Meter, Insulation Tester, Surge

Comparison Tester, Tan Delta Test Kit, AC Hi - Pot Tester, DC Hi - Pot

Tester, Inter Turn Tester (High Voltage High Frequency test kit), Recurrent

Surge Oscilloscope (RSO), Digital Volt and Clamp Meter, Infrared

Temperature Scanner, Partial Discharge Detector, High Voltage Probe and

computers with sophisticated data analysis software like MATLAB 7.0.4.

Figure 2.5 Stator Winding of 11 kV Machine

In this chapter the purpose of each test is described, together with

the types of machines and/or windings it is used for. The procedure of the test

is also described where it is not obvious. In addition, each test is being

compared with other similar tests (Stefan Grubic et al 2008). Practical

information is given on how to apply the test, including the state the winding

must be in to do the test and the normal time required to conduct the test.

Finally, a practical guide is given on interpreting the results. This

interpretation will reflect the experience of test users. The condition of the

stator winding is critical for the overall motor healthiness. To ensure the

32

flawless operation of a motor system, various off-line tests can be performed.

These tests allow the user to assess the condition of the motor under test.

Off-line methods are normally more direct and accurate. However, most of

these tests can only be applied to motors and generators that are disconnected

from services. This is one of the main drawbacks compared to the online-

monitoring methods. The off-line tests are summarized in Table 2.1.

Table 2.1 Different Methods of Tests in the Stator Insulation of

Electrical Drives

S.No. Method Standards Insulation Tested and

Diagnostic Value

1. Insulation Resistance (IR) / Megohm

IEEE 43.NEMA MG 1

Find contaminations and defects in phase-to-ground insulation

2. Polarization Index(PI)

IEEE 43 Find contaminations and defects in phase-to-ground insulation

3. DC High Potential Test (DC Hi Pot)

IEEE 95,IEC 34.1, NEMA MG 1

Find contaminations and defects in phase-to-ground insulation

4. AC High Potential Test (AC Hi Pot)

IEC 60034NEMA MG 1

Find contaminations and defects in phase-to-ground insulation

5. Surge Comparison Test

IEEE 522 NEMA MG 1

Detects deterioration of the turn-to-turn insulation

6. Offline Partial Discharge(PD) Test

IEEE 1434 Detects deterioration of the phase-to-ground and turn-to-turn insulation

7. Dissipation-Factor IEEE 286 IEC 60894

Detects deterioration of the phase-to-ground and phase-to-phase insulation

33

Common methods used to test phase-to-ground insulation are the

Insulation Resistance (IR) Test, the Polarization Index Test, the DC and AC

High Potential Test, and the Dissipation Factor Test. Since these tests can be

conducted on a frequent basis without using additional equipment, ground-

wall insulation problems can be diagnosed at an early stage.

2.4 IR, PI AND LEAKAGE CURRENT

The most widely used diagnostic test for windings of rotating

machines is IR/PI test and this test successfully locates pollution and

contamination problems in windings. In older insulation systems, the test can

also detect thermal deterioration. Insulation Resistance (IR) and Polarization

Index (PI) tests have been in use for more than 70 years. Both tests are

performed with the same instrument, and are usually done at the same time

(IEEE 43-2000).

The IR test measures the resistance of the electrical insulation

between the copper conductors and the core of the stator. Ideally, this

resistance should be infinite. In practice, the IR is not infinitely high. Usually,

lower the insulation resistance, it is more likely that there is a problem with

the insulation.

The PI test is a variation of the IR test. PI is the ratio of the IR

measured after voltage has been applied for 10 minutes (R10), to the IR

measured after just one minute (R1), i.e. PI is defined as given in

equation (2.7).

10

1

RPIR

(2.7)

A low PI indicates that a winding may be contaminated or wet. The

IEEE 43-2000 guide gives an extensive discussion of the theory of the IR/PI

tests as applied to rotating machines.

34

In the test, a relatively high DC voltage is applied between the

winding copper and the stator. The current flowing in the circuit is then mea-

sured. The insulation resistance at time t is given in equation (2.8).

tt

VRI

(2.8)

where V is the applied DC voltage and It is the total current measured after

t minutes, as the current is not usually constant. There are at least four

currents that may flow, when a DC voltage is applied to the winding (Rux

1977). They include:

2.4.1 Capacitive Charging Current

This Current de

where C is the capacitance and R is the internal resistance of the voltage

source, typically a few hundred kilo ohm. The Capacitance of a form-wound,

stator coil may be about 10 nF between the copper and the core, and that of

large hydro generator may be about 1 F. This current may decay to zero in

less than 10 seconds.

2.4.2 Conduction Current

This current can flow if the insulation has cracks, cuts, or pinholes

and has absorbed moisture, and some contamination is present. This current is

constant with time. With modern insulation, like epoxy-mica or film

insulation, this current is usually zero. If this current is significant, it indicates

that the winding insulation has a problem.

35

2.4.3 Surface Leakage Current

This is a constant DC current that flows over the surface of the

insulation caused by oil or moisture mixed with dust, dirt, fly ash, chemicals

on the surface of the windings. If this current is large, it is likely that

contamination-induced deterioration (electrical tracking) may occur.

2.4.4 Absorption Current

The absorption current is due to reorientation of certain types of

polar molecules which many insulating materials contain. Polar molecules

have an internal electric field due to the distribution of electrons within the

molecule. Water molecules are polar. When an electric field is applied across

water, the H2O molecules all align. The energy required to align the

molecules comes from the current in the DC voltage supply.

Once the molecules are all aligned, the current stops. This current is

the polarization current. There are many polar molecules in asphalt, mica,

polyester and epoxy. Experience shows that after a DC electric field is applied

to such materials, the absorption current is first relatively high, and decays to

zero after about 10 minutes. In all practical respects, the absorption current

behaves like an RC circuit with a long time constant. It is merely a property of

the insulation materials.

The total current It is the sum of all these current components.

Unfortunately, we cannot directly measure any of these components of

currents. The Polarization Index (PI) was developed to make interpretation

less sensitive to temperature. If we assume that Rl0 and R1 were measured

with the winding at the same temperature, which is usually very reasonable to

assume, then the temperature correction factor will be the same for both R1

and R10, and will be ratioed out (Lamarre and David 2006).

36

The PI effectively allows us to use the absorption current as a

yardstick to see if the leakage and conduction currents are excessive. If the PI

value is about one, the leakage and conduction currents are high, so that the

electrical tracking occurs in the insulation. If the above currents are low when

compared with the absorption current after 1 minute and then the PI is greater

than 2, the insulation is not being affected.

The IR and PI can be measured with a high-voltage DC supply and

a sensitive ammeter of nano ampere range. The DC supply must have a well-

regulated voltage; otherwise a capacitive charging current will flow

(= C dV/dt).

There are several special-purpose megohmeters available

commercially. Modern instruments can apply voltages up to and exceeding

10 kV DC, and measure resistances higher than 100 G .The IR/PI test will

depend strongly on humidity, if the results are poor it may be necessary to

heat the winding for hours or days to dry off the moisture that has condensed

on it. After each IR and PI test, the winding should be grounded for at least

four times as long as the voltage was applied, i.e., 40 minutes. Premature

removal of the ground will cause a high voltage to reappear, due to the time it

takes for the molecules to again become random in orientation, and for the

space charge to dissipate. Thus, a shock hazard exists.

2.4.5 Test Results and Inference

A 11 kV, 6 MW generator stator winding, having class F insulation

was tested. Results of the IR/PI tests and DC leakage current measurements

obtained on the three phases of the stator winding are presented in Table 2.2.

37

Table 2.2 Measurement of IR, PI and Leakage Current of 6 MW

Generator

Phases IR (M PI DC leakage current ( A)

R 7860 2.63 1.0

Y 9750 4.92 0.9

B 9470 4.59 1.0

Referring to the Table 2.2 the IR and PI values lie in the normal

range expected for a class F insulation machine. For clean and dry class F

insulation, the PI is higher than the minimum permissible value of 2. The DC

leakage current values are also quite low. These results indicate that the stator

winding is clean and dry.

2.5 CAPACITANCE TEST

Measurement of the winding capacitance can indicate thermal

deterioration or saturation of the insulation by moisture. This test is most

useful on form-wound motor stator, or very large direct-water-cooled

generator stators. Capacitance measurements are also made during

manufacturing.

If a form-wound stator deteriorates due to long-term overheating,

the groundwall insulation layers delaminate. The result is that the groundwall

now contains some gas, usually air. The dielectric constant of air is one,

whereas the dielectric constant of epoxy-mica is about 4. As the percentage of

gas within the ground-wall increases as a result of thermal deterioration, the

average dielectric constant decreases. The coil in a slot may be approximated

by a parallel plate capacitor. Its capacitance will decrease, as the dielectric

constant decreases. The dielectric constant of water is 80 and it increases the

38

capacitance. The end-winding of a stator is polluted with conductive

contaminant. This effectively increases the surface area A, of the capacitor

plate and winding contamination will increase the capacitance. If the

capacitance is unchanged over the years, then little deterioration will occur. If

the capacitance is found decreasing, then the winding is likely to have a

thermal deterioration, and when the capacitance is increasing, then the

winding has absorbed moisture.

The capacitance tests are generally performed with commercial

capacitance bridges having a precision better than 0.1 percent. Since the

amount of gas or moisture within the groundwall is usually a small percentage

of the normal insulation, the change in capacitance over the years is also very

small, even for very significant deterioration.

The capacitance of the entire phase or winding can be measured in

a global measurement. This version of the test determines the overall

insulation condition. In addition, for the specific problem of stator winding

water leaks, the local capacitance of a portion of a stator bar is to be

measured.

For the capacitance measurement the key for interpretation is the

trend. A significant amount of thermal deterioration will result in only one

percent drop in capacitance over the years. If the winding has been soaked

with water there will be a 5 percent increase in capacitance. If the entire

winding is affected, then the capacitance test is more likely to detect it.

Coil manufacturers often use the capacitance test to monitor the

impregnation and curing process. Impregnating resins have a very high

dielectric constant when in the liquid state. As they cure, the dielectric

constant asymptotically reaches it low value of about 4. When a coil is first

impregnated with the liquid resin, its capacitance increases as the resin

39

replaces the air between the mica-paper tape layers. It reaches a high steady-

state value when complete impregnation is achieved. The capacitance starts to

decrease as it cures. With experience, manufacturers can define the optimum

cure time by monitoring the initial increase and then decrease of the

capacitance.

2.5.1 Capacitance Tip-Up Test

The capacitance tip-up test is a variation of the capacitance

measurement on complete windings (Emery 2002). Thermal deterioration,

load cycling, and poor impregnation methods can result in air pockets within

the ground wall insulation. The electrical stress within the voids exceeding

3 kV per mm, a partial discharge will occur within these voids. The ionized

gas has sufficiently high conductivity and increases the capacitance. There are

thousands of voids undergoing PD and then there will be a noticeable increase in the capacitance.

By measuring the capacitance at high voltage and subtracting from

this the capacitance at low voltage, the result is the increase in capacitance

due to PD activity (Wolmarans and Geldenhuys 1991). By taking the

difference, one can estimate the capacitance of the voids. The larger the void

capacitance, the more deterioration within the groundwall and presumably,

the closer the winding is to failure. The HV Schering bridge with the accuracy

better than 0.1 percent is most commonly used. The low-voltage capacitance

(Clv) is first measured at about 20 percent of the rated voltage and the high

voltage capacitance is measured with 58 percent of rated voltage.

The capacitance tip-up is given in equation (2.9).

hv lv

lv

C CCC

(2.9)

It is usually expressed in percent rather than in Farads ( C < 1%).

40

2.5.2 Test Results and Inference

A 11 kV, 6 MW generator stator winding having class F insulation

was tested. The capacitance tip-up test results are shown in the Table 2.3.

Table 2.3 Measurement of Capacitance Tip-Up for 6MW Machine

Phases C (%)

R 0.25

Y 0.26

B 0.27

Referring to the Table 2.3, the capacitance tip-up value lies in the

normal acceptable range.

2.6 DISSIPATION FACTOR TEST

Dissipation factor (Tan ) provides an indication of the dielectric

losses within the insulation. Due to thermal deterioration and moisture

absorption, the dielectric loss increases. This test is relevant for stator

windings only and is usually applied only to form-wound stators. Dielectric

loss is a property of any insulating material and its chemical composition. An

increase in dielectric loss in a winding over the years may indicate insulation

aging due to overheating or radiation. Similarly, if a winding has been soaked

with water, the dielectric loss will increase. This occurs since the H2O

molecules are polar. The Dissipation Factor (DF) is measured with a balanced

bridge-type instrument, which can easily achieve 0.01 percent accuracy.

Typical DFs are about 0.5 percent for modern epoxy and polyester

impregnated insulations. The DF can be 3 to 5 percent for asphaltic mica

windings. If DF is measured every few years regularly, and if it remains

41

constant over time, then it indicates that there is no thermal aging or gross

contamination of the winding. If it increases over time, then it indicates that

insulation overheating is occurring or the winding is becoming more

contaminated by moisture or partly conductive contaminants (IEC std 60894-

1987).

If the Dissipation Factor (DF) has increased by 1 percent or more

from the initial value, deterioration is significant. The loss angle

representation is shown in the Figure 2.6. If the capacitance and DF are

measured at the same time, and if the Capacitance (C) decreases with the

increase in DF, it strongly indicates general thermal deterioration. If both C

and DF increase over time, then it indicates that the winding is contaminated

or has absorbed moisture.

Figure 2.6 Loss Angle Representation Graph

The dissipation factor test is used as a process monitor for the

impregnation process (Sang et al 2005). As the groundwall is impregnated,

the DF will increase, since liquid resin has a higher DF than the air. As the

I

Ic

Ir

V

I

- Loss angle

- Phase angle

42

coil cures, the DF will decrease to its steady final level, since the DF of liquid

resin is higher than that of the DF of cured epoxy or polyester.

2.6.1 Dissipation Factor Tip-Up Test

The dissipation factor tip-up is a complement of capacitance tip-up,

relevant to form-wound stator coils rated at 2300 volts and above. It is an

indirect way of determining partial discharges occurring in a high-voltage

stator winding. PD is a symptom of many high-voltage winding insulation

deterioration mechanisms. The tip-up test can indicate many failure processes.

The tip-up test is widely used by stator coil and bar manufacturers, as a

quality control test to ensure proper impregnation by epoxy and polyester

during coil manufacture (Emery 2002).

At low voltages, the DF is not dependent on voltage. As the AC

voltage is increased across the insulation in a form-wound coil, and if voids

are present within the groundwall, then at some voltage, partial discharges

will occur, producing heat, light, and sound, which consume energy from the

power supply. In a delaminated coil, as the voltage increases and PD starts to

occur, the DF will increase above the normal level due to dielectric loss, since

the PD constitutes an additional loss component in the insulation. The greater

the increase in DF, more energy is consumed by the partial discharge.

In the tip-up test, the DF is measured at about 20 percent of the

rated line-to-ground voltage of the stator and at the rated line-to-ground

voltage (Wolmarans and Geldenhuys1991). The tip-up is calculated as given

in equation (2.10).

hv lvTip Up DF DF (2.10)

43

The higher the tip-up, the greater is the energy consumed by PD. The

DF may be recorded at several voltage levels and the tip-up as a function of

voltage may be plotted as shown in the Figure 2.7. The voltage at which PD

starts is sometimes measurable. The DF is measured in percentage, and hence

the tip-up is also represented in percentage. The tip-up on windings rated

greater than 6 kV is usually significant.

Figure 2.7

In tests on machines, it is important to test a few coils as possible,

at a time, since this will increase sensitivity. Each phase should be tested

separately while the other two phases are grounded. The winding should be

partitioned into parallels or coil groups to gain maximum sensitivity. At low

voltage, the silicon carbide is essentially a very high resistance coating, and

no current flows through it. Thus, there is no power loss in the coating. At

rated voltage viz. 6 kV or above, the silicon carbide coating will have a

relatively low resistance. Capacitive charging currents flow through the

insulation and then through the coating. The charging currents flowing

through the resistance of the coating produce an I2R loss in the coating

(IEEE std 286-2000).

Voltage in kV

Sound

Deteriorated

44

2.6.2 Test Results and Inference

Tables

normal acceptable range. These results indicate that the dielectric losses and

void contents in the stator insulation are quite low.

Table 2.4 Dissipation Factor Measurement on 11 kV Machine

Phase Grounded Terminals

Applied Voltage in

kV

Leakage Current in

mA

Capacitancein nF

Dissipation Factor

R Y and B

4.40 135.7 99.73 2.458

6.60 202.9 99.85 2.586

8.80 269.5 99.49 2.923

11.0 339.5 100.3 3.315

Y B and R

4.40 137.8 101.0 2.421

6.60 205.8 100.9 2.549

8.80 275.5 101.3 2.856

11.0 341.1 100.5 3.372

B R and Y

4.40 136.1 99.97 2.462

6.60 203.6 100.1 2.592

8.80 272.1 100.2 2.898

11.0 340.7 100.4 3.343

Table 2.5 Tip-Up Test for 11 kV, 6MW Generator

Phase -up (%)

R 0.273

Y 0.291

B 0.282

45

2.7 PARTIAL DISCHARGE TEST

Form-wound stator winding, rated at 2300 V and above, at rated

line to ground voltage, the pulse currents resulting from PD, is measured.

Thus, any failure process that creates PD as a symptom can be detected with

this method. When a partial discharge pulse occurs, there is a very fast flow

of electrons constituting i = dq/dt from one side of the gas filled void to the

other side. The pulse has a very short duration, typically a few nanoseconds.

Also there will be a flow of the heavy positive ions in the opposite direction,

moving slowly with large transition time. The magnitude of the current pulse

due to ions is negligible.

Any device, sensitive to high frequencies can detect the PD pulse

currents. In the off-line PD test, the most common means of detecting the PD

currents is to use a high-voltage capacitor connected to the stator terminal.

Typical capacitances are 80 pF to 1000 pF. The capacitor offers very high

impedance to the high AC voltage with power frequency and very low

impedance to the high-frequency PD pulse currents (IEEE std 1434, 2000).

This can be displayed on an oscilloscope, frequency spectrum

analyzer, or other display devices having band width in MHz range. PD pulse

is proportional to the size of the void in which the PD occurred. Higher the

magnitude of the PD pulse, the larger is the defect. The advantage of the PD

test is that one concentrates on the larger pulses and ignores the smaller

pulses.

Unlike capacitance or power factor tip-up tests, which are a

measure of the total PD activity, the PD test enables the measurement of the

biggest defects. Since failure is likely to originate at the biggest defects and

not at the smaller defects, the PD test can indicate the condition of the

46

winding at its most deteriorated portion. The measurement of the PD pulses is

shown in Figure 2.8.

Figure 2.8 Measurement of PD Pulse Current

The off-line PD test requires a power supply to energize the

winding to at least rated phase-to-ground voltage. For large generator stators,

a conventional or resonant transformer rated at 20 to 40 kVA may be needed.

The PD test is performed at the machine terminals, energizing one phase at a

time, grounding the other two phases. In the off-line PD test, the applied

voltage is raised gradually while monitoring the PD pulses on an oscilloscope

screen. The voltage at which the PD is first detected is called the Discharge

Inception Voltage (DIV). The voltage then is raised to normal line-to-ground

operating voltage. The winding should remain energized for the soak time of

10 to 15 minutes at this voltage, and then the PD is recorded. Now the voltage

is then gradually lowered. The voltage at which the PD is not detectable is

called the Discharge Extinction Voltage (DEV), usually lower than the DIV is

measured. It is desirable to have the DIV and DEV as high as possible

(Campbell and Stone 2000).

47

For machines rated at 6 kV or more, the maximum test voltage is

normally the rated line-to-ground voltage. A test at this voltage will usually

detect deterioration, years before an in-service failure is likely. For machines

rated at 2300-4100 V, a test at rated voltage may not produce significant PD,

even in severely deteriorated stator insulation. This is because there may be

insufficient electric stress within the defects to achieve the 3 kV/mm needed

in atmospheric air to cause PD.

2.7.1 Test Results and Inference

Unlike the tip-up test, which produces a single number representing

the total PD activity, direct measurement of the PD produces several results.

The key measure is the peak PD magnitude Qm, i.e., the magnitude of the

highest PD pulse. This can be measured in several units like pico Coulombs

(pC), millivolts (mV), milliamps (mA) and decibel (dB). The detected PD

magnitude of a PD pulse within the winding but measured at the stator

terminals depends on the size of the defect, the capacitance of the winding,

the inductance between the PD site and the PD detector (Stone 1998).

The off-line PD test is a comparison test (Zhu et al 2005). One can

determine which phase has the highest Qm i.e. the greatest deterioration. One

can also compare several similar machines to see which has the highest PD or

the lowest DIV or DEV. Finally, one can compare the PD from the same

stator over time. If the PD doubles every 6 months, then the rate of

deterioration is increasing (IEEE 1434-2000). Direct measurement of the PD

indicates how widespread the PD is. As many as 10,000 PD pulses may occur

per second in a stator winding. It seems that a single defect only produces at

most one or two PD pulses per half AC cycle.

Thus, if only a few hundred PD pulses are occurring per second,

then there are only a few PD sites in the winding and the deterioration is

48

localized. If there are 10,000 PD pulses per second, then there are thousands

of PD sites and the deterioration is widespread. The pulse count rate can be

easily measured with a pulse magnitude analyzer, which is incorporated into

most modern commercial PD analyzers (Kurtz et al 1984). If there is

dominant deterioration in a winding, the PD test can sometimes give the

approximate location of the deterioration within the groundwall. Both positive

and negative PD pulses are created. If the positive PD pulses are larger than

negative PD pulses, then the PD is occurring on the surface of the coil. If the

negative PD is predominant, then the PD is occurring at the copper. If there is

no polarity predominance, then the PD may be between the groundwall

insulation layers.

Table 2.6 PD Test Results of 6 MW Generator

Phases PD inceptionvoltage (kV)

PD magnitude at 5 kV (pC)

R 3.78 600

Y 3.83 600

B 3.83 600

Referring to the Table 2.6 the PD magnitude lies in the normal

acceptable range.

2.8 SURGE COMPARISON TEST

None of the tests discussed above directly measure the integrity of

the turn insulation in form-wound or random-wound stator windings. The

stator voltage surge test directly measures the integrity of the turn insulation

by applying a relatively high voltage surge between the turns. This test is a

49

hipot test for the turn insulation. The insulation, may fail requiring a repair,

coil replacement, or rewind.

Voltage surges occur from Inverter Fed Drive Motors (IFDs) and

faults in the power system. These fast risetime surges result in a non uniform

voltage distribution across the turns in the stator winding. If the rise time is

short and, the surge voltage is high, the turn insulation becomes weak and

punctures. This test is analogous to the AC and DC hipot tests. The surge test

is a destructive test. If the turn insulation fails, then the assumption is that the

stator would fail in service due to transients (Stefan et al 2008).

If the winding does not puncture, then the turn insulation will

survive any likely surge over the next few years. The main difficulty with the

surge test is determining when the turn insulation puncture has occurred.

A turn-to-turn puncture in a winding does not cause a huge increase in current

from the power supply. In fact, if there are 50 turns between the phase

terminal and neutral, the failure of one turn will only slightly reduce the

inductive impedance of the winding, since the impedance of only one turn has

been eliminated. Thus the other 49 turns can continue to impede current flow,

and the circuit breaker does not trip.

In the surge test, turn failure is detected by means of the change in

resonant frequency caused by shorting out one turn. The inductor is the

inductance of one phase of a stator winding, or two phases in series. A high-

voltage capacitor within the surge tester is charged from a high-voltage DC

supply via the winding inductance. Once the capacitor is charged to the

desired voltage, the switch is closed. The energy stored in the capacitor then

oscillates back and forth with the winding inductance. If there is no turn fault,

there will be a fixed frequency of oscillation. If a turn fault occurs together

with weak turn insulation, the inductance of the winding will decrease and the

resonant frequency will increase. The increase in frequency being small, it is

50

difficult to detect. Modern surge testers digitally capture the resonant

waveform at low voltage, where the turn insulation is still intact. The surge

voltage is gradually increased by raising the voltage and triggering the switch

after the capacitor has charged up (once per second or once per 50 or 60 Hz

cycle). If a change in the waveform is noted above a certain voltage, then turn

insulation puncture has occurred.

It is easy to detect turn insulation failure on individual coils, since

the shorting of one turn will have a much larger impact on the total inductance

of a coil, thus drastically changing the waveform. Machine manufacturers and

rewind companies use individual coil surge testing to check the quality of the

turn insulation (IEEE std 522- 992).

Such testing is best done after the coils are wound, wedged, and

braced, since by then they have been exposed to all the mechanical handling

and stresses associated with the winding process. As a quality check, the

surge test is done prior to inserting coils into the slot. Ground faults are easily

detected by a surge test, since the waveform collapses. Surge testing is also

useful to identify wrong connections in the winding. As an acceptance test,

the surge is recommended to have a risetime of 100 ns and a maximum

magnitude of 3.5 per unit, where l per unit is the peak line-to-ground rated

voltage. For a maintenance test performed after the winding has been service,

the surge should have the same risetime, but reach only 2.6 per unit. Voltages

higher than these maximum should not be applied to the stator winding,

otherwise there is a significant risk that good turn insulation will fail

unnecessarily.

National Electrical Manufacturing Association (NEMA) requires

that Inverter Fed Drive Motors should withstand 100 ns risetime surges at

3.7 per unit. The longer the surge risetime, the lower is the voltage applied

across the turn insulation. Thus, longer rise-time surges are not as severe as

51

short risetime surges. The surge voltage is gradually increased to the

maximum recommended test voltage. If the waveform changes on the

oscilloscope, then the turn insulation has likely been punctured. If the winding

is form wound, the failed coil will have to be located and isolated. The coils

must be separated from one another and tested separately until the faulted coil

is found. If a turn puncture has occurred, the punctured turn insulation will

break down again, allowing power-frequency currents to flow, rapidly leading

to groundwall failure.

2.8.1 Interpretation

The surge test and the partial discharge test are combined, and then

it may be possible to detect significant voids between the turns, before actual

puncture occurs. This requires a special PD detector, since conventional PD

detectors will be damaged by the high-voltage surges. Although the surge test

stated above to be the only test that directly determines the condition of the

turn insulation, (Gupta et al 1987) the IR/PI, capacitance, and dissipation

factor tests discussed above will also indicate the condition of the turn

insulation.

The Capacitance, Tan delta and Partial Discharge measurements are

adequate for testing winding insulation to ground but not the insulation

between turns. Surge testing is accurate methods of identifying inter turn

faults. Surge voltage is applied on a winding consisting of a number of coils

which in turn consist of many turns. A ringing pattern is seen on the Cathode

Ray Tube (CRT). The fast raising pulse spreads along the coil and creates a

voltage gradient along the turns. Since the three phases are wound identically,

comparison of all the phases will show the same single pattern as shown in

Figure 2.9(a). In case of faults in any one of the phases, the wave pattern gets

separated indicating a fault as shown in Figure 2.9(b).

52

(a) (b)

Figure 2.9 Surge Comparison Test

2.9 AC HIGH POTENTIAL TEST

In AC High Potential (Hipot) test, power-frequency (50 or 60 Hz)

voltage is used. The AC voltage dropped across each component in the

groundwall or in the end-winding depends on the capacitance (dielectric

constant) of each component (Gupta et al 1995, 2001).

For modern windings, the AC hipot test yields an electric stress

distribution that is the same as that which occurs during normal operation.

Consequently, the AC hipot test is more likely to find defects that could result

in an in-service stator failure if a phase-to-ground fault occurs in the power

system, causing an overvoltage in the unfaulted phases. Figure 2.10 shows

AC Hipot test set up. NEMA MG1 and IEC 60034 define the AC acceptance

hipot level as (2E + 1) kV, where E is the rated rms phase-to-phase voltage of

the stator. IEEE 56 recommends the AC maintenance hipot be 1.25 to 1.5 E.

The AC hipot for a 4.0 kV motor that has been in service, is about 6 kV rms,

applied between the copper conductor and the stator core. A winding usually

either passes the test or it fails because of a puncture. A one minute AC hipot

test at 1.5 E is equivalent to about 235 hours or 10 days of operation at normal

53

operating voltage. Therefore, the life is not significantly reduced by a hipot

test if the expected life is about 30 years.

Figure 2.10 AC High Voltage Test on Form Wound Coil

2.9.1 Test Results and Inference

An 11 kV generator stator winding having class F insulation was

tested. The Hipot test results are shown in the Table 2.7.

Table 2.7 Measurements of Hipot Test Parameters

PhaseGrounded

PhaseApplied Voltage

in kVLeakage current

in mA

R Y and B 23 1181

Y B and R 23 1185

B R and Y 23 1177

54

Referring to the Table 2.7 the stator winding passes the AC hipot

test. If the winding fails, as determined by the power supply circuit breaker

tripping or an observed insulation puncture, then repairing of coil or

replacement of winding is required.

2.10 CONCLUSION

Offline testing is the main approach used for insulation testing of

the stator winding and the stator insulation problems prove to be drastic as far

as failure mechanisms are concerned.

One of the restrictions in using online PD monitors for the

insulation monitoring is the capital cost in the deployment of the dedicated

sensors and data processing hardware. In addition, as there are symptoms

which are the causes of insulation failure other than corona, relying on PD

monitoring alone may not be fruitful for reliable assessments of the overall

insulation condition.

So tests like, Insulation resistance test, Polarization Index test,

Leakage current test, Capacitance and Capacitance tip-up test, Dissipation

factor and Dissipation factor tip-up test, Hipot test and Surge comparison test

are conducted for the precise determination of the insulation performance.