Download Report

Winding
Resistance
Testing
ndb Technologies
WRT-10
Winding resistance measurements in transformers / generators &
large motors are of fundamental importance for the following
purposes:
• Manufacturers will use the test to determine “copper losses”
which are calculated using the formula I2*R
• At the time of commissioning, a winding resistance test can be
used to detect loose connections, typically at the connections to
bushings, and abnormalities with the tap-changer as a result of
poor workmanship or shipping damage.
• Calculation of winding temperature at the end of a temperature
test cycle.
• The voltage across an inductor is proportional to the
time rate of change of the current through it.
• The DC current source must be extremely stable. Refer
to formula for DC voltage across a transformer
below: v = I * R + (L di/dt) where:
•
•
•
•
•
vdc = voltage across transformer winding
I = DC current through transformer winding
R = resistance of the transformer winding
L = inductance of the transformer winding
di/dt = changing value of current (ripple)
• Selecting the Proper Test Current Range
• Transformer manufacturers typically recommend
that the current output selected should not
exceed 10% of the rated winding current. This
could cause erroneous readings due to heating
of the winding. Test current ranges are typically
from 0.01% to maximum of 10 % of rated
winding current.
Stabilised resistance measurements
• On large transformers with high inductance windings, it could take
a few minutes for readings to stabilize. The time required for
readings to stabilize will vary based on the rating of the
transformer, the winding configuration, the instruments output
current selected.
• Readings on a Star/Wye-configured transformer should typically
stabilize in 10 to 30 seconds
• For large transformers with delta configuration, magnetization
and getting stable readings can take significantly longer time,
sometimes as long as 30-60 minutes.
Stabilised resistance measurements - B
By connecting the primary and secondary windings in series
(using a Jumper cable), the speed of saturation and stability of the
readings is increased because there are more amp-turns
contributing to the flux in the core. The resistance measurement
of both windings is made at the same time on channels A and B
of the WRT-10.
Delta Windings
If possible, always inject test current to HV & LV simultaneously.
Correct measurements values can be obtained approximately 7
times faster when injecting in both HV & LV in comparison to
measuring LV only
WRT-10 Reading Stability indicator
Indicates if the voltage reading (and displayed resistance reading)
has stabilized.
WRT-10 Reading Stability indicator
- Gray when the WRT-10 is idle, when it is dumping
current, or when measuring has begun but is not yet
stabilized.
- Yellow when approaching stability (1% or less
variation in the value for ten consecutive readings).
- Green once the nominal current value is reached and
the resistance measurement is almost the same each
time (within a 0.25% tolerance for ten readings in a row).
Winding resistance measurements
• How much heat is being generated by the higher
resistance of a transformer winding? This can be
calculated (I²R) using the rated full load current.
Is this heat sufficient to generate fault gases and
possibly result in catastrophic failure? This will depend
on the rate at which heat is being generated and
dissipated. Consider the mass of the connector or
contact involved, the size of the conductor, and its
location with respect to the flow of the cooling medium
and the general efficiency of the transformer design
Diagnostics
Windings resistance measurements can reveal a
great deal of information.
- Faulted winding (open winding or shorted turn)
- Integrity of numerous welded and mechanical
connections
- Poor Joints
- RA switch, LTC (Diverter switch, Tap Selector switch)
damage
Transformer preventive maintenance
- winding resistance measurements (vibrations - a)
Slight changes of length exhibited by a
ferromagnetic object when magnetized, result in the familiar
“hum” heard around large power transformers, it’s the
sound of the iron core expanding and contracting at 120
Hz (twice the system frequency, which is 60 Hz in the
Canada & the USA), one cycle of core contraction and
expansion for every peak of the magnetic flux waveform.
The “hum” also includes the noises created by mechanical
forces (Magneto-Motive Forces - mmf) between primary and
secondary windings.
Transformer preventive maintenance
- winding resistance measurements (vibrations - b)
• When the secondary winding is “loaded” (current
supplied to a load), the winding generates an mmf
(magneto-motive force), which becomes counteracted by
a “reflected” mmf in the primary winding to prevent core
flux levels from changing. These opposing mmf's
generated between primary and secondary windings
as a result of secondary (load) current produce a
repulsive, physical force between the windings
which will tend to make them vibrate. These vibrations
can affect the integrity of different joints and connections
in power transformers
Tap Changers maintenance
• A transformer tap is a connection point along a transformer winding
that allows a certain number of turns to be selected. By this means,
a transformer with a variable turns ratio is produced, enabling
voltage regulation of the output. The tap selection is made via a tap
changer mechanism.
• Tap changers can be On-load (LTC) or Off-load (RA switch)
• In power distribution networks, transformers commonly include an
off-load tap changer on the primary winding to accommodate system
variations within a narrow band around the nominal rating. The tap
changer will often be set just once, at the time of installation,
although it may be changed later during a scheduled outage in order
to accommodate a long-term change in the system voltage profile.
LTC
About 96% of all power transformers today
above 10MVA incorporate on load tap changers
as a means of voltage regulation
For many power transformer applications, a
supply interruption during a tap change is
unacceptable, and the transformer is often fitted
with a more expensive and complex on-load tapchanging mechanism. On-load tap changers
may be generally classified as either mechanical,
electronically assisted, or fully electronic. typical
LTC are step switches, reversing switch &
diverter switches.
These mechanisims have many different joints
and connections that can be tested with winding
resistance measurement.
Reversing switches
• In most transformers the tap-windings are connected to
the main winding through a reversing switch. The
reversing switch allows the tap-windings to be either
additive or subtractive (boost and buck) to the main
winding. This effectively reduces the number of required
taps by half. In other words, a tap changer that has for
example; 16 raise taps and 16 lower taps, would have 33
different ratios, but only 16 tap selector switches (plus
the neutral position).
Reversing Switch-B
• The diagram below illustrates the complete tap changer circuit.
Note the switches “R” (raise) and “L” (lower) in the dotted circle.
• N1 is the main winding. N2 is the tap winding. The diagram on the
right is just a simplified version of that on the left.
+
R
IL
L
L
I2
+
E2
2
+
N
L
Preventive
autoxfmr
IL
IS
+
S
VL
VL
V
S
SL
1
-
I
-
E
1
-
+
VS
IS
N1
+
-
S
SL
LTC – Open circuit tap change
• LTC’s transfer load currents and are designed for
make-before-break, they are NOT designed to
interrupt load currents. An open circuit would likely
result in catastrophic failure. On installation and after
maintenance it is certainly advised to verify operating
integrity by checking for open circuits.
• If an open circuit is detected while changing taps the
WRT-10 will automatically stop and go into its
discharge cycle indicated by the discharge indicator.
ndb Report Manager Software
V-graph analysis tool for Tap-changers
• This graphical representation of the Lower to Riser tap winding
resistance is an easy and fast tool for Tap-changer analysis. We
are looking for an ideal smooth & symetrical V shape graph.
ndb Report Manager Software
V-Graph 3 phase overlaping
( possible Reversing switch problem on Phase B )
ndb Report Manager Software
V-Graph, Before and After analysis
Example of V-Graph information
• High Resistance in Diverter Switch
• In the example below, it can be seen that all of the “even
numbered” taps (2, 4, 6, 8…) are higher than the norm.
(Norm being the imaginary V-shaped line drawn through
the data points). Note that all the odd taps are
connected through side “B” of the diverter switch and all
the even taps are connected through side “A” of the
diverter switch. The pattern of results below would
indicate that the contact resistance of the “A” side of the
diverter switch is high.
High resistance of diverter switch - V-graph
Winding Resistance
8.0000
Resistance in milliohms
7.5000
7.0000
6.5000
6.0000
5.5000
5.0000
-16
-12
-8
-4
0
4
Tap Changer Position
8
12
16
• Please note that when graphed, there will normally be some
deviation from the norm between the odd and even values. This
does not necessarily indicate a problem with the diverter switch.
This may be due to physical installation conditions. For example the
internal cables connected to one side of the diverter switch may be
longer than those connected to the opposite side. In fact if the
diverter switch is damaged, the results will be inconsistent. In other
words, some of the odd (or even) taps will have elevated resistance
and other will appear normal. Or, if a given tap is tested more than
once, after operating the tap changer, the result for the first test will
be different from the second test and so on.
• Sound judgment and the ability to compare readings to factory
values, historic values, similar transformers or other phases will help
in drawing the correct conclusion.
• High Resistance in Reversing Switch
• In the example below, when the reversing switch is in the “raise”
position all the resistance values are high than when the reversing
switch is in the “lower” position. In this case, a high resistance on
the “raise” side of the reversing switch is suspected.
• Refer back to figure 2. The reversing switch only operates as the
tap changer changes from operating in the additive connection to a
subtractive connection. For example if the tap changer has been in
the subtractive position (taps -1 to -16) and is then required to
operate in the additive mode, the reversing switch will reverse the
connections of the tap winding as the mechanism moves through
neutral and makes its connection to the first raise tap (+1).
High Resistance in reversing switch - V-Graph
Winding Resistance
8.0000
Resistance in milliohms
7.5000
7.0000
6.5000
6.0000
5.5000
5.0000
-16
-12
-8
-4
0
4
Tap Changer Position
8
12
16
• High Resistance in Tap Selector Switch
• In the example below, it can be seen that tap +5 and -5 both have
higher than normal resistance.
• In this example both these taps use the same tap selection, but with
the reversing switch in different positions, it is likely that the cause of
this inconsistency is a high resistance in position five of the tap
selector switch.
• Note that the entire circuit to tap 5 is suspect, not just the tap switch
itself. This would include any connections between the taps switch
and the winding.
• The reversing switch can not be to blame because in every case the
resistance of the odd tap is similar to its even counter part.
High Resistance in Tap Selector Switch - V-Graph
Winding Resistance
8.0000
Resistance in milliohms
7.5000
7.0000
6.5000
6.0000
5.5000
5.0000
-16
-12
-8
-4
0
4
Tap Changer Position
8
12
16
Induction / Noise
The WRT-10 has Sigma-delta A/D
converters and notch filters on input
channels to eliminate effects of substation
noise.
Safety - Induction in Substation
transformer located in substations in close proximity to
energized conductors are subject electrostatic charges
induced onto floating windings. This hazard can be
eliminated by grounding all windings. However, to
perform a winding resistance test only one terminal of
any winding can be tied to ground. It’s important that the
windings are grounded prior to connecting the current
and potential test leads, and when disconnecting leads
the ground is the last to be removed.
WRT-10 Color Screen
• Large, Bright, Color LCD screen, 120 mm x 90 mm (4.75" x 3.5"),
easy viewing on a sunny day.
AUTO T/C Mode screen
WRT-10 Full QWERTY Keyboard
Easy data entry
WRT-10 Tap changer control
On-Board Printer
Communication interface
What is the Transformer Temperature Rise
Test or “Heat Run Test”?
• This test determines the average winding temperature rise of the
transformer under rated load (generally performed by Transformer
manufacturers)
• Thermometers are installed to measure ambient temperatures, oil
temperatures, etc... Readings on these are taken before the test is
started to obtain base temperatures on which to determine the rise at
rated load, These are also read periodically during the test, and
continued until temperatures do not vary more than a few degrees over
a period of hours.
• If the temperature rises is above the transformers winding heat
Class tolerance, overheating of the insulation takes place and that
leads to an accelerated ageing of the insulation and, in excessive
cases, could damage the transformer.
What does the Winding Resistance Measurements
have to do with the « Heat Run Test »?
• The average winding temperature is determined by the equation:
T = R/R0 (Tk + T0 ) – Tk
(note: IEEE: C57.12.90)
• Where:
T is the temperature (°C) corresponding to hot resistance R,
T0 is the temperature (°C) at which cold resistance R0 was measured
R0 is the cold resistance (Ohm)
R is the hot resistance (Ohm)
Tk is 234,5 °C for copper (resp. 225,0 for aluminium)
The WRT-10 is equiped with a timed interval resistance
measurement logging feature for heat run testing
•
•
•
•
IEEE C57.12.90 requirements for Hot Resistance measurements.
The time from instant of shutdown shall be recorded for each
resistance measurements.
At least one resistance measurement shall be taken on all terminal pairs
within 4 min after shutdown. Resistance-time measurements shall be
made on all windings.
The resistance-time data shall be corrected to the instant of shutdown
using a resistance-time cooling curve determined by plotting data on
suitable coordinate paper, or by using a curve fitting program.
The resistance-time data obtained on one phase of a winding shall be
used to determine the correction to shutdown for the other phases of the
same winding, provided the first measurement on each of the other phases
has been taken within 4 min after shutdown.
WRT-10 Advantages
• High accuracy 0.25%
• 0.01A, 0.1A, 1A & 10A (30V) Current regulated
output source for fast stabilization and saturation
• V-Graph analysis for fast tap-changer diagnostics (
PC software )
• Reading stabilization indicator, ( we look for no
more than 0.25% deviation for 10 measurements )
to indicate stabilization is completed.
• Load Tap Changer channel, ( we can control
change the taps of a transformer from the WRT-10 )
• Auto-Tap Changer mode ( automatically performs
measurement , data log at tap change sequence )
• Duty Cycle: the maximum tested 4.5 hours
continuous at full 10A output.
WRT-10 Advantages…
• RS-232C & USB communications
• Passive Circuit discharge indicator and .. LED for applications
where while the instrument is charging a transformer a power
outage occurs , the user can see that the instrument is still
discharging the transformer – SAFETY !
• In the event that the current circuit is opened, the WRT-10 has a
fast Crowbar discharge circuit (voltage channel)
• 50 feet / 16 meters of testing cables + 1 jumper cable for dual
channel performance measurements ( included)
• Special Sigma / Delta input measurement filter, looks out
substation noise and induction influence of measurements
• Large Memory, 100Files of 120 Measurements each
• Break before Make feature for On-Load Tap-Changer
analysis, the instrument automatically discharges the
transformer when inductive kick back is detected showing that
there is an open circuit between 2 taps of the LTC!!
WRT-10 Advantages
…
• Heat Run testing mode, where the resistance data can be logged for
cool down curve information. The instrument can record the time
and interval of the measurements automatically ( 5 seconds to 100
seconds intervals )
• Transformer information can be entered directly on the instrument
• FULL keyboard, for easy data entry.
• Beep sound level adjustment ( off, low- med – high )
• FULL Color display, easy to see even in bright daylight !
• Housing includes a storage compartment for cables, or accessories
• Housing has wheel and a retractable handle
• Lightweight instrument, 25 LB or 11.4 KG
• Resistance , temperature correction feature ( Aluminum , Cooper
and custom )
• On board Printer
Visit us at
www.ndbtech.com