OWTS M28.ppt

Contents
1. Measurement connection
2. StartMask
3. Calibration
4. Measurement
5. Evaluation “Quick Overview”

One of the most important parts of the PD measurement is ensuring a
good connection to the test-object.
A faulty connection to the test-object could cause the following problems:
 increased Background noise;
 PD‘s at the near termination;
 no PD‘s;
 „not loading“ the test-object.
Measurement connection

Earthing requirements:
 when having a corroded earthing or sheath, remove corrosion (on the place where
the connectors will be connected)
 otherwise increased Background noise
 when possible always connect the sheath of the extension-cable directly on the
sheath of the test object
 Otherwise increased damping of the signals
 always connect the earthing of the test-van itself on the same point as the sheath of
the extension cable
 because of loop, increased Background noise
Earthing
Copper corosion

With the connection of the extension cable it should be observed that the
unshielded part of the cable does not touch or is close to earthed
components.
Earthed components are:
- Floor
- Switchgear
- Cabledrum
- Sheath of the extension cable
A minimum clearance of 5 till 10cm is strictly
necessary.
(between unshielded part and earthed component)
Connecting extension cable

But like this:
Connecting extension cable

During the measurement the following requirements apply.
 Current-transformers (CT‘s) can be left connected;
 Voltage-transformers (VT‘s) must be disconnected;
 Surge-arresters ideally have to be disconnected. They can be left connected,
however the chance exists that the cable at voltages >= U0 cannot be loaded
anymore since the leakage current through the surge-arrester is to high;
 Other two phases have to be earthed during the measurement.
What kind of field-distribution we have got in joints???
Connection during measurement

Needs to be applied for „short“ cables to decrease the frequency:
 e.g. XLPE cables < 300m
 at Error message: “Frequency too high, please use additional capacitor!”
Important, earth properly!!! -> otherwise PD can be created
Test
Object
Additional capacitor

Important for the measurement!
 Data entry of U0 (for 20kV cables enter 11.6kV as
U0)
 Data entry of the cable-length
Joint-entry; acc. to position from near
termination.
Load loading an existing Startmask (e.g.
created by the Excel-Macro);
Clone (Copying) copying an existing
Startmask (creating new folder with
current date and time);
OK accepting the standard definition;
New creating a new Startmask;
Hint: With three one-phase cables first
enter the data as a single three-phase
cable afterwards change it to three one-
phase cables.
Generating Startmask

Goal of the calibration is to:
 measure according to the international norm IEC 60270;
 determine the propagation velocity v of the cable, which is required for the
localization.
Additionally joints can be recognized in this modus.
However attention has to be paid!!!!!
Faulty calibrations can lead to:
 Extremely high background noise;
 A scattered PD-Mapping (PD‘s not on straight line) because of difference in v ‘s.
Calibration

 Connect between core and sheath;
 Twist connection cables as far as possible;
 Always keep the frequency on 100Hz.
SebaKMT recommends to calibrate the Calibrator every two years
and to replace the batteries every year (no matter what the battery
indication shows).
Calibration
Connection
100Hz
500pC

 Always start calibration with the highest value (100nC).
Note: The only exception is cables shorter than 300m, then start with 500pC.
 SebaKMT recommends to calibrate the entire range from 100nC to 200
(100)pC. This to avoid time-consuming re-calibration.
 Especially 100 und 200pC can sometimes not be calibrated because of
high background noise activity. „What doesn‘t go, does not go…“
 When the automatic calibration takes to long it is better to stop the
automatic calibration and continue manually. Best practice is to use the
PgUp and PgDn keys instead of the logarithmic slide-bar on the left hand
side!!!
Calibration
The calibration

One calibration for all
phases is sufficient.
At three singe-phase
cables change Phase
to „All“.
Calibration
The calibration

Press arrow in upper
right-hand corner.
 move yellow marker
to the maximum of
the reflection of the
joint and the distance
of the joint can be
read.
Calibration
Joint recognition

Check VHF
For cables shorter
then 100m the
following settings
are required:
1)Press setup and
check if VHF is
turned on;
(is automatically
turned on for cables shorter
than 500m)
2)Adjust slide-bar to
short cable;
3)Then continue
with the calibration. 1
2
Calibration
Calibration of short cable lengths

For long cables or cables with a high attenuation it could happen that the
endreflection is not visible anymore. How to set the second marker then?
Approach in such cases:
a) Set 100nC, stop automatic calibration and
manually increase the amplification until an
end-reflection becomes visible.
b) Move second marker to the correct position
and adjust amplification level back to 80%
level.
c) Notice the v and position the marker for
further calibrations based on this v.
When at a) no end-reflection could have been
recognized, the second marker has to be set,
based on a v of 160m/µsec.
Of course always measure from both ends!!!
Calibration
Calibration of long cable lengths

Cable-data:
Voltage: 20kV
Type: NA2XS(f)2Y
Year: 2006
Length: 3500m
Joints: 6 pieces
All_1000.cal
N:...All_1000.cal Monday, May 26, 2008 8:16 PM
Time (us)
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
Range
[%]
100
90
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
One side will do.
Calibration
Example attenuation of the cable

Cable-data:
Voltage: 10kV
Type: NAKBA
Year: 1970
Length: 570m
Joints: 3 pieces
All_1000.cal
N:...All_1000.cal 09/04/2010 08:35
Time (us)
12
11
10
9
8
7
6
5
4
3
2
1
0
Range
[%]
100
90
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Measurement from both sides
recommended!!
Calibration

Cable-data:
Voltage: 11,6kV
Type: NA2XS2Y
Year: 1993
Length: 2370m
Joints: - pieces
All_1000.cal
N:...All_1000.cal Sunday, January 18, 2009 13:33
Time (us)
50
48
46
44
42
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
Range
[%]
100
90
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Calibration

The calibration provides the user from valuable information. Based on the
calibration it e.g. becomes clear if a measurement has to be performed from
both sides yes or no.
This decision for measuring from both ends is depending on the attenuation
of the cable.
The attenuation is influenced by several factors:
 Cable-type (Isolation)
 Cable condition (new, aged)
 Voltage class
 Manufacturer
 Number of joints
Generally we recommend to measure cables from both sides if they are
longer then 2km. However as the following examples will show that this will
not always be correct!!!! Better is to always make the decision based on the
calibration itself.
Calibration
What can we learn from the calibration?

Settings:
Display modus: 16 AC-Cycles
Phase: L1
PD-Range: lowest one
Voltage entry: U0
Measurement
Measurment approach

0kV
0,5U0
- Determine PDIV
- Save PDIV
- Increase voltage with 0,1U0
and every time save data
1U0
1,3U0
1,5U0
Save
PD existing No PD
- Determine PDIV
- Save PDIV
- Determine and save PDIV
- Determine and save PDEV
PD existing No PD
Save
PD existing No PD
Measurement
Measurment approach

Summary Data Collection Procedure
PDIV Less Than Uo Condition
Test Voltage Function Key Freq
0Uo F7 1
↑Uo →PDIV F8 1
1.0Uo F7 5
1.3Uo
F7 5
F9 1
PDIV More Than Uo Condition
Test Voltage Function Key Freq
0Uo F7 1
↑Uo →PDIV
F8 1
F7 5
1.3Uo
F7 5
F9 1

Practical tips:
 Always select the proper PD-range, e.g. do not save PD‘s of 200pC
height with a PD-range of 5nC.
Or the other way around save PD‘s which are higher then the selected
PD-range!!!
 Save as PDIV if from 2 of the 3 measurements PD is present;
 Only save PDEV when PDIV ≥ U0;
 Also for the PDEV perform 3 measurements and take the average value;
 In total three data-sets have to be saved if the cable is PD-free (0kV, U0
and1.3U0).
Measurement
Practical tips

 Determining the extinction voltage is then possible if PD‘s occur over
more then two AC cycles.
 PDEV must not necessarily be saved at 1.3U0, if PD‘s occur over more
then two AC cycles directly determine the PDEV and save it.
 Of major importance is to select the correct PD-range (500pC). Only in
this case PD‘s may clip. Goal is to determine the PDEV, not to save data
for the TDR analysis!!!
Selecting faulty PD-range will in most cases result in a PDEV higher then the PDIV, which is not possible!!!
Measurement
Determination of PDEV

0
0
V0
t → s
t → s
kV
↑
V
i↑
PDIV
PDEV
0
0
V0
t → s
t → s
kV
↑
V
i↑
PDEV
PDIV
PDIV – Partial Discharge Inception Voltage; the voltage where the first discharges
appear (incept).
PDEV – Partial Discharge Extinction Voltage; the voltage where the discharges
disappear (extinct). In practice 10 to 35% below inception voltage.
PDEV below U0.
Dangerous
situation!! PD
remains after
overvoltage.
PDEV above U0.
PD only occurs
during
overvoltage
Measurement
Importance of PDEV

Dielectric losses are the losses that occur in the insulation material.
An increase in tanδ as function of the operating voltage like in the picture
below, means aged insulation or local weak-spots.
Values measured with the OWTS are given in percentages, according to the
value of the dielectric losses the following distinctions can be made:
Tan delta Condition of the cable
0.1 < tan δ < 0.9% dry
0.9% < tan δ < 2% moisture
tan δ > 2% wet
Measurement
Dielectric loss summary

The purpose of the Quick overview is to give a first estimation of the
condition of the cable.
Based on this overview direct measures can be scheduled e.g. replacement
of PD-affected joints.
However because it is an automatically compiled PD-mapping, a manual
evaluation is still required/ advised!!!
Evaluation „Quick Overview“

Step 1: Select data

Correctly set
Bandwidth!!!
Select Quick
Overview and
press Accept
Step 2: Start quick overview

 Check PD-concentrations
by right-clicking on PD spot
with the Mouse and select
„View PD-Pulse“
 With help from the
„Threshold“ function PD-
concentrations can be
displayed more clearly
 Based on these results
direct measures can be
scheduled e.g. replacement
of PD affected joints
 However a manual
evaluation is always
required.
Step 3: Check quick overview result

Before
adaptation
Step 3: Example threshold

After adaptation
Threshold-level of
510pC
Step 3: Example threshold

The Cluster Strength is a new feature (only available as a Beta Version
and only functions when having OWTS Explorer V4.0) which looks to PD
concentrations.
If there is a weak spot, there will always be several PD pulses.
The software skips the cable parts where the PD concentration looking to
the cable length is 2 or less.
If two or less the PD will not be shown in the PD Mapping.
This new feature significantly increases the usability of the quick
overview function.
Note: scattered PD in paper-mass will also not be shown in a PD
mapping
New: Cluster strength (Beta Version only)

“Threshold” 0 pC and “Cluster strength” 0

“Threshold” 0 pC and “Cluster strength” 40 (default)

Manual evaluation
L1
L2
L3
PD mapping for
3phase20100720_man.3PF
Location (m)
2.000
1.900
1.800
1.700
1.600
1.500
1.400
1.300
1.200
1.100
1.000
900
800
700
600
500
400
300
200
100
0
PD
(pC)
1.200
1.100
1.000
900
800
700
600
500
400
300
200
100
0

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