Application Note
Dissipation / Power Factor and Capacitance
Measurement on Rotating Machines using the
CPC100/TD1 and CR500
Authors
Fabian Öttl
Stefan Böhler
Martin Anglhuber
Fabian.oettl@omicronenergy.com
Stefan.boehler@omicronenergy.com
Martin.Anglhuber@omicronenergy.com
Date
23.03.2016
Related OMICRON Product
CPC 100, CP TD1, CP CR500
Application Area
Rotating Machines
Version
v1.0
Document ID
ANP_16002_ENU
Abstract
This application note shows how to perform Dissipation Factor (Power Factor) and Capacity Measurements
on rotating machines using the CPC 100 in combination with the CP TD1 and the CP CR500.
© OMICRON
Page 1 of 32
Content
1
Using this document .............................................................................................................................3
1.1 Operator qualifications and safety standards .................................................................................3
1.2 Safety measures .............................................................................................................................3
1.3 Related documents .........................................................................................................................3
2
Dielectric measurement on rotating machines ..................................................................................4
2.1 Dissipation factor theory .................................................................................................................4
2.2 Dissipation factor vs. power factor ..................................................................................................5
3
Preparing the dissipation factor test ...................................................................................................7
3.1 CPC Editor and Test Card overview ...............................................................................................7
3.1.1
4
Measurement Setup ..............................................................................................................................9
4.1 Compensation .............................................................................................................................. 10
4.1.1
4.1.2
4.1.3
4.1.4
4.2
4.3
4.4
5
Choosing the right parameter in the TanDelta-PF test card .............................................................. 8
Choosing the right compensation.................................................................................................... 10
Steps to perform the compensation ................................................................................................ 14
Omicron TD1 High-Voltage Source Test Card ................................................................................ 15
OMICRON Excel compensation tool ............................................................................................... 16
Preparation of the test object ....................................................................................................... 17
Buildup of the test circuit.............................................................................................................. 17
Overview: Dissipation factor measurement step by step ............................................................ 22
Assessment of the measurement ..................................................................................................... 23
5.1 Influence of the end potential grading ......................................................................................... 23
5.1.1
5.2
5.2.1
5.3
Practical example............................................................................................................................ 26
Fingerprint measurement............................................................................................................. 27
Phase comparison .......................................................................................................................... 27
Parameters of the dissipation factor measurement ..................................................................... 28
5.3.1
5.3.2
5.3.3
Use cases ....................................................................................................................................... 28
Capacitance measurement ............................................................................................................. 30
Limitation of the measurement ........................................................................................................ 31
Please use this note only in combination with the related product manual which contains several important safety
instructions. The user is responsible for every application that makes use of an OMICRON product.
OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON.
© OMICRON 2015. All rights reserved. This application note is a publication of OMICRON.
All rights including translation reserved. Reproduction of any kind, for example, photocopying, microfilming, optical character
recognition and/or storage in electronic data processing systems, requires the explicit consent of OMICRON. Reprinting,
wholly or in part, is not permitted.
The product information, specifications, and technical data embodied in this application note represent the technical status
at the time of writing and are subject to change without prior notice.
We have done our best to ensure that the information given in this application note is useful, accurate and entirely reliable.
However, OMICRON does not assume responsibility for any inaccuracies which may be present.
OMICRON translates this application note from the source language English into a number of other languages. Any
translation of this document is done for local requirements, and in the event of a dispute between the English and a nonEnglish version, the English version of this note shall govern.
© OMICRON 2015
Page 2 of 32
1
Using this document
This application guide provides detailed information on how to measure and asses a dissipation (power) factor
measurement on a high-voltage rotating machine using the OMICRON CPC 100 in combination with the CP
TD1 and the CP CR500.
Please refer to national and international safety regulations relevant to working with high voltage (HV). The
regulation EN 50191 ("The Erection and Operation of Electrical Test Equipment") as well as all the applicable
regulations for accident prevention in the country and at the site of operation has to be fulfilled.
1.1
Operator qualifications and safety standards
Working on HV devices is extremely dangerous. Measurements must be carried out only by qualified, skilled
and authorized personnel. Before starting to work, clearly establish the responsibilities. Personnel receiving
training, instructions, directions, or education on the measurement setup must be under constant supervision
of an experienced operator while working with the equipment. The measurement must comply with the relevant
national and international safety standards listed below:

EN 50191 (VDE 0104) "Erection and Operation of Electrical Equipment"

EN 50110-1 (VDE 0105 Part 100) "Operation of Electrical Installations"

IEEE 510 "Recommended Practices for Safety in High-Voltage and High-Power Testing"

LAPG 1710.6 NASA "Electrical Safety"
Moreover, additional relevant laws and internal safety standards have to be followed.
1.2
Safety measures
Before starting a measurement, read the safety rules in the CPC 100, CP TD1 and CP CR500 Reference
Manual and observe the application specific safety instructions in this application note when performing
measurements to protect yourself from high-voltage hazards.
1.3
Related documents
Title
CPC 100 Reference Manual
CP TD1 Reference Manual
CP CR500 Reference Manual
Application Guide: Capacitance and
Dissipation Factor Measurement with
CPC 100 + CP TD1
© OMICRON 2015
Description
Contains information on how to use the CPC 100 test system and
relevant safety instructions.
Contains information on how to use the CP TD1 test system and
relevant safety instructions.
Contains information on how to use the CP CR500 test system
and relevant safety instructions
Contains general information about dissipation factor (power
factor) measurement on different assets.
Page 3 of 32
2
Dielectric measurement on rotating machines
The following document applies to all kind of vacuum impregnated or resin-rich taped single bars, coils
or complete windings.
Dielectric tests of rotating machines can help to identify potential insulation problems before defects
or damage occurs.
The dielectric measurement is performed with high AC voltage at nominal frequency in order to have
a close to realistic behavior of the insulation system. The test is non-destructive for healthy insulation
systems and is performed during a standstill of the machine.
2.1
Dissipation factor theory
In an ideal capacitor without any dielectric losses, the insulation current is exactly 90° leading
according to the applied voltage. For a real insulation with dielectric losses, this angle is less than 90°.
The insulation of a rotating electrical machine can be modelled by a loss-free capacitance with a
parallel ohmic resistance representing the losses in the insulation system (Figure 1). The latter ones
are caused by:
 Surface currents
 Leakage currents
 Partial discharges
 Polarization losses
Figure 1: Parallel equivalent circuit diagram and vector diagram of a rotating machine insulation system.
Applying a voltage to the parallel components causes a current IC to flow through the capacitance as
well as through the resistivity IR. The overall current I therefore has a resistive and a capacitive
component. A causal relation between losses and the resistive part can be assumed, as the higher
the losses are, the higher the resistive current will be. The angle δ = 90° - φ is caused by the resistive
part of the overall current and is proportional to the losses, which leads to the definition of the tan(δ)
expressed in Figure 1 as an indication of the overall condition of the insulation system of an electrical
machine:
tan⁡(δ) =
© OMICRON 2015
1
ω ∗ 𝐶𝑃 ∗ 𝑅𝑃
Page 4 of 32
2.2
Dissipation factor vs. power factor
Another parameter commonly used in North America when speaking of dielectric tests on rotating
machines is the so-called power factor. In this case, the definition is different from the dissipation
factor. Assuming again that the vector diagram in Figure 1 shows power factor (PF) as the quotient of
the resistive part of the current IR to the overall current I:
𝑃𝐹 =
𝑃𝐹 =
𝐼𝑅
= cos 𝜑
𝐼
tan 𝛿
√1 + tan² 𝛿
If the dissipation factor (tanδ) is very small – typically less than 10 %, which can be presumed as given
when measuring healthy electrical machine insulation, the dissipation factor and the power factor differ
in a negligible amount and can be assumed to have the same value. Table 1 gives an idea about this
statement.
Table 1: Comparison between correlating values of dielectric power factor cos(φ) and dielectric loss factor tan(δ) and their
difference (IEC 60034_27_3).
Switching between the two forms can be done in the CPC software very easy, which is explained in
Figure 2.
For reasons of simplicity and to avoid repetition, only the dissipation factor is mentioned in the following
chapter. As the difference between dissipation factor and power factor is minimal. When testing
rotating machines, all of the statements in the following chapter apply to the dissipation factor as well
as to the power factor.
© OMICRON 2015
Page 5 of 32
Figure 2: Switching between the two forms of parameter in a dielectric measurement and overview of the OMICRON rotating
machine template according to the CPC Start page.
© OMICRON 2015
Page 6 of 32
3
Preparing the dissipation factor test
The preparation of the test can be easily done in the office with the CPC start page. OMICRON offers
a template for rotating machine testing, which can be adapted and extended to your individual needs.
Figure 2 shows this template, which is described in this chapter in more detail.
3.1
CPC Editor and Test Card overview
The generator template can be found in the CPC start page (Figure 3).
Figure 3: CPC start page with a path to the generator template
By opening the template file, the CPC editor appears with the test card combination shown in Figure
4. The default template consists of two comment test cards and eight dissipation factor test cards.
The build-up is shown according the test procedure and requires a capacitance measurement and a
check of the resonant circuit after the nameplate information of the generator. The reason for these
two measurements are explained in chapter 4 .
After performing the measurement, the Excel file loader template automatically adopts the
measurement data from the measurement file for a fast and easy report.
Both tools – the test template with the test cards and the excel file loader – can be adapted by the
user according to their needs.
For a detailed description of the CPC start page, please refer to the CPC 100 reference manual.
© OMICRON 2015
Page 7 of 32
Figure 4: View of the generator template in the CPC Editor
3.1.1
Choosing the right parameter in the TanDelta-PF test card
The highest voltage during the test is defined in national or international standards, or it is given by
the internal test procedures according to the operator needs.
OMICRON suggests to measure the dissipation factor in steps of 10 % of the maximum voltage
defined for the machine. The resulting ramp is driven upwards (i.e. from 10 % to 100 % of the defined
maximum voltage) and downwards (from 100 % to i.e. 10 % of the maximum voltage).
The reason for driving the ramp in both directions is a plus on information out of the results (more
details are explained in chapter 5 ).
Steps of 20 % of the test voltage are also common. The only difference to the OMICRON suggestion
is a lower resolution of the partial discharge measurement, which is potentially performed in parallel.
The voltage steps are defined in the Auto Test Point List in the TanDelta-PF test card of the CPC. If
doing the measurement in steps up AND downwards, it is not possible to choose exactly the same
voltage for both ramps as no second auto test point with the same value can be set in the device. In
this case, simply choose a voltage which differs by 1 V (in the example in Figure 5 5999 V instead of
6000 V) from the chosen value.
Figure 5: Left side: Auto Test Points slightly differ in the upwards and downwards ramp; Right side: Number of measurements
for an averaging result
© OMICRON 2015
Page 8 of 32
If the measurement frequency is the same as the default frequency in the device (the default frequency
can be checked by pressing the options button on the device and go to the “device setup” tap), then
the CPC uses a noise suppression algorithm, which performs a measurement on two different
frequencies then on the rated frequency. The result is an interpolation of the two measurements.
As for rotating machines, the signal-to-noise level is much higher, so the noise suppression algorithm
is not needed. Additionally, the compensation (see chapter 4.1 ) is optimized for one frequency.
Therefore, please set the default frequency in the device to a frequency different than the test
frequency.
A workaround could be to test at 50.01 Hz if 50Hz is the rated frequency, or 60.01 Hz with 60Hz as
rated frequency.
With the number of averaging marked in Figure 5, the number of measurements is chosen to
determine a single result.
In the example shown in Figure 5 above six measurements are averaged to one result and are
performed at each voltage step. When a PD measurement is performed simultaneously, the duration
of one voltage step can be varied by using the averaging number. On the CPC display (not in the CPC
editor), the approximate duration of the step is displayed when changing the value.
The measurement of the dissipation factor has to be performed in GSTg-A+B mode in order to guard
the current from the CP CR500 parallel path (see also Figure 16). The same applies to the capacitance
measurement. The resonance frequency check is performed in GST mode in order to measure the
whole measurement circuit.
4
Measurement Setup
To ensure a lightweight system with a minimum need of real power, the OMICRON approach is to use
a parallel resonant system together with the test capacitance. This means that the test object’s large
capacitance (represented by CTest) causes a huge apparent power when applying high voltage, but
this is compensated with parallel resonators (CR 500).
The remaining required power is caused by losses and the rest of the apparent power which is not
ideally compensated. This is supplied by the CPC 100 in combination with the CP TD1.
To get a better idea of the concept please refer to the following chapter on Compensation.
© OMICRON 2015
Page 9 of 32
4.1
Compensation
The picture a) in Figure 6 shows the principle of a parallel resonance system; where the parallel
resonators and the test capacitance are considered as ideal, causing a minimum of resulting test
current Itotal as a vectorial sum of ITest and IComp.
a)
b)
Figure 6: Equivalent circuit diagram of the parallel resonance circuit with vector diagram
Picture a) in Figure 6 represents an ideal parallel resonance circuit, where the theoretical capacity and
inductivity are free of losses. Picture b) in Figure 6 represents the vector diagram with real lossy
components. The compensation of the reactive part is not done in a perfect way, and the total current
Itotal has a real part as well. The real part results from the losses of the test object itself (resulting in the
loss angle δ in the picture), from losses in the cables to the test object and from the losses of the
compensation reactor.
Nevertheless Itotal is usually much smaller than the needed test current ITest at a certain voltage and
can be supplied by the CPC 100 in combination of the CP TD1.
4.1.1
Choosing the right compensation
For a better understanding about which parallel resonators (CR 500) are used for a certain capacitive
load, an example of a real measurement on a 56 MVA machine is presented. The hydro generator to
test has the following data:
 10 kV
 352 nF capacitance phase to ground
 50 Hz
The nominal voltage is considered as test voltage for a single phase measurement.
© OMICRON 2015
Page 10 of 32
Figure 7: Tan(delta) testing without compensation reactors
Without compensation, the source has to provide the full capacitive current ITest, including the whole
active part ITa (losses of the test object) as well as reactive part ITr. This case is displayed in Figure 7,
where the reactive part of the test current – as usual – is much higher than the active part. As
mentioned above, the overall maximum apparent power, which the CP TD1 has to provide, can be
calculated as follws:
𝑆 = 𝑈𝑇𝑒𝑠𝑡 ∗ 𝐼𝑇𝑒𝑠𝑡 = 𝑈 2 ∗ 𝑗𝜔𝐶𝑇𝑒𝑠𝑡 = 11,1𝑘𝑉𝐴
By establishing a parallel resonant circuit, the capacitive reactive part ITr can be compensated by the
CP CR 500. Considering the ideal circuit in Figure 6, the system can be assumed in resonance if the
following condition is fulfilled:
1
=2∗𝜋∗𝑓∗𝐿
2∗𝜋∗𝑓∗𝐶
f
C
L
test frequency
capacitance of test object
compensator reactance
The calculation of the compensation is then:
𝐿=
f
C
L
1
4 ∗ 𝜋2 ∗ 𝑓2 ∗ 𝐶
test frequency
capacitance of test object
compensator reactance
In our example, a parallel reactor of LComp = 28,78 H would be required for a perfect compensation
displayed in Figure 8. The resulting current ITotal is the remaining sum of the active part from the lossy
inductance as well as from the lossy capacitance.
© OMICRON 2015
Page 11 of 32
Figure 8: Parallel resonant circuit with vector diagram
As OMICRON offers 80 H and 40 H reactors, the best compensation can be achieved by connecting
one 40 H reactor and an 80 H reactor in parallel, which results in a reactance of 26,6 H.
Figure 9: Compensation with OMICRON reactors
The current ITotal is now slightly higher than in Figure 8 and will be in the range of 100 mA depending
to the losses in the test object and the measurement circuit in general. Nevertheless, a perfect
compensation is not required as the CP TD1 can provide the required current of 100 mA without
problems.
The loss angle δ is not influenced as the current through the compensation coils is guarded.
© OMICRON 2015
Page 12 of 32
In general, an inversely proportional relation between the test capacitance and the compensating
reactance is given (i.e. a larger capacitance requires a smaller compensating inductance – please see
the formulas above). Smaller values for the compensating inductance can be achieved by connecting
multiple CP CR500 reactors in parallel.
The following diagram shows the types and quantities of CR 500 that are required to compensate a
certain capacitance. It also shows the current ITotal, which is not compensated and has to be supplied
by the CPC 100 and the CP TD1. The test voltage is assumed as 12 kV, the maximum output current
of the CP TD1 is 300 mA.
Figure 10: Example of different compensation connections by a given test capacitance in nF
Without any compensation, the CPC 100 in combination with the CP TD1 is able to supply an 80 nF
capacitive load. If the test object has a capacitance of e.g. 120nF, a compensating reactor of 80 H
needs to be connected in parallel to reach the 12 kV.
Compensating reactors have to be used to ensure the compensation of capacitances larger than 80 nF
and also for testing on nominal frequency. A combined 40 H / 80 H can be used to compensate for
capacities up to about 450 nF. For larger capacitances, further 40 H reactors have to be connected in
parallel.
Due to the insulation level of the LV output of the CP CR 500, connecting in serial
of the coils is not permitted, and will cause a damage of the device!
Choosing the best measurement configuration, respectively the best compensation, is described in
the following chapter 4.1.2
© OMICRON 2015
Page 13 of 32
4.1.2
Steps to perform the compensation
There are two possible ways to calculate the best compensation -- either you can do it with the
CPC100 or on your computer by using the OMICRON calculation tool. Both alternatives require the
capacitance of the test object against ground. If this value is not given by a former measurement or a
factory acceptance test, you have to measure the capacitance on site using the following setup without
compensation:
Figure 11: Measurement setup for capacitance measurement
The OMICRON test template for rotating machines in the CPC Start page provides the C-MEAS test
card for measuring the capacitance of the test object.
Measurement of the
capacitance on the test
object
After calculating and
connecting the necessary
compensation reactors, the
resonance frequency is
measured to ensure that
the compensation has been
done correctly.
Performing a tan-delta test
for each phase (in this case
additional test-cards are in
the template for measuring
the dissipation factor over a
frequency range)
Figure 12: OMICRON rotating machines test card overview
© OMICRON 2015
Page 14 of 32
Please note:
The resonance frequency test card is a tool for double-checking if the compensation is performed
correctly. The measurements, which identify the dissipation factor of the single phases, can be
performed at rated frequency.
After getting the value for compensation, the compensation calculation can be done either:
 Manually (please refer to the chapter 4.1.1 )
 By using the OMICRON high-voltage test card
 By using the OMICRON excel compensation tool
4.1.3
Omicron TD1 High-Voltage Source Test Card
Using the TD1 High-Voltage Source Test Card is one possibility to measure the test object
capacitance, to calculate the correct compensation and to measure the resonant frequency is.
The TD1 High-Voltage Source Test Card is designed for using the CP TD1 as a high-voltage source,
but is not able to measure the dissipation factor. However, the feature to measure the capacitance,
calculate the compensation and to measure the resulting resonant frequency is available.
Figure 13: High-Voltage Source Test Card; Detail
Following the description of the points in Figure 13:
1. Insert a test voltage: The test voltage should be lower than the rated voltage of the machine.
Please keep in mind that no compensation is done when measuring the capacitance, therefore
a voltage level between 500 V and 2000 V is suggested.
2. Check the capacitance of test object and calculate the compensation: The calculation of
the compensation is done automatically. After the definition of the available CP CR500 coils,
the device displays the best measurement setup.
3. Second step: Build up the test setup. By pressing the options button, the screen in Figure 14
will appear. In the TD1 High-Voltage Source Test Card, it is possible to enter the available
amount and type of CP CR500 to calculate the best suitable compensation in the test setup.
Connect them according to the suggestion of the test card.
4. Double check if the compensation is performed correctly by measuring the resonance
frequency. The corresponding wiring diagram is displayed by pressing the “Wiring” button.
Please note: Even if the resonance frequency is not equal to the rated frequency, the
measurement can be performed at nominal frequency, as the CPC 100 in combination with
the TD1 is able to supply the missing reactive power.
© OMICRON 2015
Page 15 of 32
Available amount
and type of CR500
Suggested amount
and type of CR500 to
use and calculated
resonant frequency
Figure 14: View of the high-voltage test card by pressing the options button
4.1.4
OMICRON Excel compensation tool
The calculation tool for the calculation of the necessary compensation for a dedicated test object can
be downloaded in the customer area of the OMICRON homepage. This tool calculates the required
current for all possible combinations of all coils currently available for the test.
The current is the total needed current from the TD1 and includes:
 The remaining current from the compensation if the resonant frequency is not the test
frequency
 The active part of the current as result of the dissipation factor (estimated value, based on
OMICRON experience)
 The active part of the current from the losses of the CR500 coils, as an average value over a
certain test sample amount.
Figure 15: Input Form - All yellow cells are required input values provided by the use
© OMICRON 2015
Page 16 of 32
Following is a short explanation of the different input values:

Test frequency: Frequency at which the test is performed (usually rated frequency).

Highest test voltage: The highest voltage reached during the test.

Test capacitance: Capacitance of the test object including additional capacitances (e.g.
coupling capacitors).

Actual inductance required for perfect compensation: Theoretical value of inductance
required for compensation exactly to the test frequency (test frequency = resonant frequency).

Available coils: Enter the amount of available coils for the measurement (Please note: the
amount of CR500 ≠ amount of coils, 1 x CR500 = 2 coils).

Coils for best compensation: Best combination of coils (with lowest current) for
compensation at the entered test frequency.

Actual inductance (with suggested coils): Total inductance as a result of the suggested
coils.
After you fill in the input form, click on the button “Optimize compensation” and all the calculations will
be executed. The output is similar to the one in the high-voltage source test card explained in chapter
4.1.3 .
4.2
Preparation of the test object
On rotating machines, usually phase-to-ground of the stator insulation is measured. As a comparison
of the different phases is very helpful for the analysis, it is important to separate the phases from each
other (e.g. by opening the star point). Any other electrical components, which are directly connected
to the phases like cables etc., should also be disconnected, as they would otherwise influence the
measurement. If this is not possible, it has to be considered that these components will be included in
the measurement as well.
4.3
Buildup of the test circuit
Before Starting
Make sure that the CPC 100, the CP TD1 and the CP CR500 are properly grounded
and the generator is disconnected from the line. Always touch the leads and
terminals with a grounding rod first.
For your own safety:
Be sure to ALWAYS follow the following five safety rules:
1. Disconnect mains
2. Prevent reconnection
3. Test for absence of harmful voltages
4. Ground and short circuit
5. Cover or close off nearby live parts
© OMICRON 2015
Page 17 of 32
The buildup of the test circuit is displayed in Figure 16, where some components are marked with
numbers. The following list gives an overview about the different components:
1) Grounding cable
2) CPC 100 booster cable
3) Serial cable from CPC 100 to CP TD1
4) Safety connection to the CP CR500
5) Guard connection to the CP CR500
6) High-voltage cable from CR500 to the test object
7) CP TD1 high-voltage cable
Figure 16: Setup of the test circuit with numbered items
Following is a description about the items listed above:
1) Grounding Cable:
Before proceeding with the test setup or switching on the CPC 100, please ensure that all
devices are grounded properly.
2) CPC 100 booster cable:
The power connection from the CPC 100 to the voltage booster CP TD1
3) Serial cable from CPC 100 to CP TD1
Ensures the communication between the CPC 100 and the CP TD1
4) Safety connection to the CP CR500
The safety connection to the CP CR500 implements the resonator in the safety system of the
CPC 100. If there is an issue with overcurrent (i.e. due to improper test setup), the CP CR500
will stop the test automatically.
The connection is done with the special accessory cable displayed in Figure 17 on the left
side, where the serial plug replaces the CPC 100 safety dongle, and the special plug
terminares the SAFETY A input (Figure 17, right side). The safety dongle also terminates the
SAFETY B plug.
© OMICRON 2015
Page 18 of 32
By using more than one CP CR500, the connection between the different CP CR500 units is
performed with the special accessory cable. The SAFETY B input of the last CP CR500 is
terminated by the safety dongle.
Figure 17: Connection cable for implementing the CP CR500 into the CPC 100 safety concept
5) Guard connection to the CP CR500:
The guard connection to the CP CR500 ensures that the resonator itself is not influencing the
measurement results. The guard connection is performed with the CP TD1 measurement
cable connecting special plug into the banana socket (See Figure 17 on the left side).
6) High-voltage cable from CR500 to the test object:
The connection from the CP CR 500 to the test object is performed with the red cables in
Figure 18 by using the clamp which provides enough sockets for 4 coils and the CD TD1 high
voltage cable.
Figure 18: Connection cables from the CP CR500 to the test object and clamp for connection to the test object.
The CP CR500 cables are non-shielded cables and therefore they can be the starting point of
partial discharge activities, when touching ground potential. Especially when performing a
tan(delta) measurement and a partial discharge (PD) measurement in parallel, this factor
influeces the PD measurement. The picture in Figure 19 on the right side shows a PD-free
test setup for a parallel measurement of the tan(delta) and PD activity of the machine. Figure
19
© OMICRON 2015
Page 19 of 32
1) High-voltage supply for combined dissipation
factor and partial discharge measurement
2) Connections to the terminals are kept at a
safe distance with Nomex paper from any
ground potential
3) Plastic material tube to ensure that the cables
are not touching the floor
Figure 19: Measurement set-up at a generator site for a combined dissipation factor and partial discharge
measurement (note that the red cables are not touching ground)
The picture on the left side in Figure 19 shows a fully-connected CP CR500 with the special
plug of the safety cable, the measurement cable from the CP TD1 connected to the banana
socket of the cable (blue banana plug) and the safety dongle terminating the safety B plug. In
this case, both coils of the CP CR500 are used. The housing is grounded via the grounding
cable and is connected at the grounding screw.
7) Blocking Impedance BLI1:
A passive filter device when measuring tan(delta) and partial discharge in parallel, which
allows the partial discharge measurement at the same point where the injection is taking place.
The BLI1 blocks the high frequency disturbances coming from the source.
8) CP TD1 high voltage cable
The connection to the CP TD1 is a shielded cable where the shield current is guarded
internally.
Important for not influencing the measurement results is the section to the right of the
connection to the test object, which has a stress coating in order to ensure a smooth potential
grading from the high voltage to the cable shield.
Under high voltage, this section should not touch any grounded parts and respectively should
have a proper distance to any grounded part according to the high voltage output.
An example is shown in Figure 20, where the first section of the cable from the blocking
impedance BLI 1 is not touching any ground component.
© OMICRON 2015
Page 20 of 32
Figure 20: High-voltage connection to the BLI 1 for a combined dissipation factor and PD measurement with coupling
capacitor and the MPD system
© OMICRON 2015
Page 21 of 32
4.4
Overview: Dissipation factor measurement step by step
•Prepare the test in the office
Preparation
•Measure the capacitance if the value is not given by another source
•Due to the symmetric design, all three phases have similar values
Capacitance
of DUT
•Connect the required amount of CP CR500 coils to establish the parallel
resonance circuit
Compensation
Perform the
Test
•Double check of the right compensation by measuring the resonant
frequency
•Perform the dissipation factor mesaurement
•Analyze the test results by exporting the data to the Excel file loader
Analyzing of
the results
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5
Assessment of the measurement
Rotating machines are small serial products or non-serial products with wide spread material
characteristics. An overall limit regarding a dissipation factor does not exist, even in consultation with
the manufacturer. Nevertheless, the dissipation factor measurement gives a reliable statement
regarding the overall state of the rotating machine by comparing fingerprint measurements using the
same conditions. These are explained in the following chapters.
5.1
Influence of the end potential grading
The End Potential Grading (EPG) is a semi conductive area in the end winding of every medium
voltage machine. It can be found under various names in the literature: semi conductive grading, end
winding stress grading, or the mentioned EPG are only a few names, which all indicate the same area,
as shown in Figure 21.
Figure 21: Position of the End Potential Grading (EPG) at the single bar
The semi conductive paint or tape causes a defined reduction of the high-voltage potential from the
soldering contacts to the grounded laminated core. Even if the insulation is applied at the end winding
area, this area is on high-voltage potential due to the capacitive coupling. The slot portion by definition
has ground potential, therefore a boundary surface with high potential gradients is created at the end
winding area.
Without the EPG, high electrical field gradients would appear at the laminated core, causing high
discharges as displayed in Figure 23.
Figure 22: Simulated field strength without EPG and with EPG
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Figure 22 shows the theoretical simulation of the EPG area, which smoothens the electric potential at
the boundary surface from high-voltage to ground.
Figure 23: Different test probes without EPG (top); EPG on the left side (middle); and with EPG (bottom); (Weidner, 2008)
Silicon carbide (SiC) is the primary component and it is responsible for the behavior of the end potential
grading area during the dissipation factor test . It is well known in the abrasives industry and is applied
as a filler in a varnish or a tape component at the bar. The electrical behavior can be assumed as
micro-varistor. Its conduction mechanism is displayed in Figure 24.
Figure 24: Schematic illustration of the influence of different conduction mechanisms of a Schottky barrier
After reaching the breakdown voltage VBD, the Schottky barrier acts as a conductor. For the dissipation
factor measurement this results in a voltage-dependent conducting behavior in the endwinding zone.
It acts as a bypass to the ground wall insulation. Depending on the material properties, the avalanche
point is reached sooner or later, and makes the dissipation factor measurement a fingerprint
measurement as stated above. This leads to the equivalent circuit diagram shown in Figure 25.
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Figure 25: Equivalent circuit diagram of the end potential grading
Another influence of the EPG behavior can be seen in the capacitance measurement. As with higher
voltage, the SiC-layer becomes more and more conductive. Depending on the voltage, it “extends”
the OCP (outher corona protection) layer. Therefore, a rise in capacity value is normal during a
measurement starting from 0,1 Un up to 1,0 Un (Un is the highest test voltage). Also PD-activity (partial
discharge activity) influences the capacitance value by changing the dielectric properties of the voids
in the insulation. The effect of capacitance increase can be observed in every machine, no matter if
they were in operation over years (most likely high PD-activity) or new machines (most likely low PDactivity).
Figure 26: Current paths during a tan(delta) measurement – EPG currents are mentioned as “Surface Currents”
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5.1.1
Practical example
To point out the influence of the end potential grading, one limb of a 6kV coil was modified by
scratching an isolating gap in the OCP. This allows a measurement with the possibility of guarding the
surface currents and measure the pure dielectric parameters of the ground wall insulation.
The principle is explained in Figure 27, where the simulated slot section is representing the measuring
electrode and the guard electrodes were used as Input A or B in the CP TD1.
Figure 27: Modified limb of the coil according to IEEE 286/2000, 2001
The results are represented in Figure 28, where on the left side the measurement setup without guard
electrodes and on the right side the measurement with guard electrodes are displayed.
Figure 28: Results of the different measurement setups
For the measurement without guarding rings, a much higher increase of the tan(delta) can be
observed, as the EPG is relatively conductive and responsible for an almost linear increase. The initial
value for both diagrams is almost equal, since the EPG is not that conductive at 600V.
On the right side of Figure 28, the tan(delta) value is almost equal until 3kV and starts increasing
slowly up to 2% compared to 3.75% in the measurement setup without guard rings.
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5.2
Fingerprint measurement
As the dissipation factor measurement is a fingerprint measurement and heavily influenced by the end
winding surface currents (as described above), the condition of the end winding area should be the
same for a reproducible measurement. Therefore it is suggested to clean this part of the winding to
ensure a clean and dry surface and to give reproducible conditions for every measurement without
any additional current paths.
IMPORTANT:
Prior to cleaning the end winding area, please look for any traces of PD (e.g. white powder) and
document it for the parallel dissipation factor and PD-measurement (Figure 29).
The traces provide an indication of the PD activity in the end winding and also the possible failure
mechanism.
Figure 29: Partial discharge traces in the end winding area (“white powder”)
The comparison of different measurements over time under the same circumstances (i.e. end winding
surface, temperature, humidity) offers the best possibility to assess the condition of the winding .
5.2.1
Phase comparison
If there is no fingerprint measurement available, a phase comparison can be a proper equivalent
solution for a reliable statement regarding the phase condition. Due to the symmetric buildup of
generator stator windings, the results of different phases vary only slightly.
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Figure 30: Example of a phase comparison
The measurement represented in Figure 30, shows an example of a phase comparison of a healthy
winding. A comparison between the phases shows a similar increase of tan(delta) between each steps
and the absolute value of the dissipation factor, as well as the hysteresis of the ramp up and
downwards. A trend analysis with a measurement performed four years earlier showed that the values
have shown consistent behavior over the interval.
5.3
Parameters of the dissipation factor measurement
Four parameters are essential for analyzing the results of a dissipation factor measurement:




Dissipation factor at low voltages (first voltage step) as a fingerprint value
Delta tan(delta) in each interval to identify the highest tan(delta) increase
Characteristic tan(delta) tip-up between different pre-defined voltage steps (e.g. 0.6Un and
0.2Un)
Shape of the curve of the tan(delta) ramp
By using the OMICRON approach with the decreasing ramp, a fifth parameter, the PD hysteresis, can
be included in the analysis. This gives a statement about the PD activity in the winding (keywords:
inception and extinction voltage). A higher tan(delta) hysteresis is an indicator of higher PD-activity in
the ground wall insulation. Generally an assessment can be done by comparing the absolute tan(delta)
values or using the graph of dissipation factor over voltage.
5.3.1
Use cases
The following examples of different measurements should give an idea of the diversity of possible
dissipation factor graphs. The measurements are from different high-voltage machines, representing
different stages of deterioration. A PD-measurement was performed in parallel.
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Figure 31: Tan(delta) measurement example (1)
The example in Figure 31 shows a relatively constant dissipation factor value at lower voltages. At
4 kV, the EPG becomes more conductive. Approximately at the same voltage, PD activity also starts.
This was confirmed by the parallel PD measurement. The rise of tan(delta) is approximately 2 %
starting from 1 % at 1 kV up to 3 % at 12 kV.
Figure 32: tan(delta) measurement example (2)
The example in Figure 32 shows a generator from 1968 with low PD activity (no PD hysteresis until
6kV) and a relatively non-conductive EPG up to 4kV. In general the tan(delta) value is quite low for an
entire winding measurement and increases from 0.6 % up to 0.85 %.
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Figure 33: tan(delta) measurement example (3)
Figure 33 shows results from a brand new generator. The machine shows no PD activity but has a
very conductive EPG. Therefore, the tan(delta) values are rising up to close to 4 %. Another parameter
increasing the dissipation factor is the fact that the machine is a very slow hydro generator. This means
a high-end winding surface area in comparison with the slot portion.
5.3.2
Capacitance measurement
The capacitance measurement is done in parallel to the tan(delta) measurement and acts as a
fingerprint measurement, as in the case with the dissipation factor.
As mentioned above in the EPG description, the capacitance will increase due to the behavior of the
semi conductive area. Figure 34 shows two different capacitance measurements with a maximum
increment of ~3% and ~4%.
a)
b)
Figure 34: Capacitance measurement of two different generators with an increment of the capacitance value of ~3% and ~4%
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Figure 34 a) is the capacitance measurement from the dissipation factor measurement in Figure 33
and Figure 34 b) is correlating with the measurement in Figure 32.
5.3.3
Limitation of the measurement
The measured dissipation factor is an integrative value over the whole winding. Therefore, the value
does not represent the most deteriorated part of the winding, as a large dissipation factor value can
be due to voids distributed all over the winding or a few single heavily aged bars or coils.
The latter case is potentially more severe regarding the longevity of the winding. In such cases, where
high dissipation factor values or high tip-up factor values can be observed, further investigations like
visual inspections or PD measurements should be executed.
© OMICRON 2015
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