Transcript Wer viel misst, misst Mist?

Metering a lot means a
lot of metering errors?
Uncommon and unexpected metering errors
Stefan Fassbinder
Deutsches Kupferinstitut
Am Bonneshof 5
D-40474 Düsseldorf
Tel.: +49 211 4796-323
Fax: +49 211 4796-310
[email protected]
www.kupferinstitut.de
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The German Copper Institute, DKI, is the
central information and advisory service
dealing with all uses of copper and
copper alloys. We offer our services to:
 Commercial companies
 The skilled trades
 Industry
 R & D institutes
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We can be contacted by:
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 online database, or
 personally
2
1. True root-mean-square or
reprocessed arithmetic mean?
The TRMS value of an AC or mixed AC and DC is the
magnitude which a smooth (pure) DC would need to have to
cause the same thermal effect.
Moving iron metering systems: TRMS displayed
Moving coil metering systems: Arithmetic mean displayed;
with bridge rectifier, the absolute arithmetic mean
Analog metering systems: Insignificant price difference
between AV and TRMS display, but not any longer commonly
in use
Digital metering systems: TRMS display significantly more
expensive!
3
1. True root-mean-square or
reprocessed arithmetic mean? blind
î = 1A
R = 1W
uR = R*i = 1V
pR = uR*i = ûR²/R = 1W
q = 2*10ms*1A = 20mAs
WR = 2*10ms*1W = 20mJ
 UR = 1,00V, IR = 1,00A
blind
î = 2A
R = 1W
uR = R*i = 2V
pR = uR*i = ûR²/R = 4W
q = 2*5ms*2A = 20mAs
WR = 2*5ms*4W = 40mJ
 UR = 1,41V, IR = 1,41A
4
The end of the sine waves
300
300
3,0
3,0
200
200
Load
250
250
150
150
Mains voltage:
Mains frequency:
Mains resistance:
Mains inductance:
Mains impedance:
DC load:
Smoothing capacitance:
100
100
50
50
00
00
i /A 
3,5
3,5
u /V 
350
350
55
10
10
230
50
500
904
575
180
220
V
Hz
mW
µH
mW
mA
µF
2,5
2,5
2,0
2,0
1,5
1,5
1,0
1,0
0,5
0,5
tt // ms
ms 

15
15
0,0
0,0
20
20
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1. True root-mean-square or
reprocessed arithmetic mean?
Following, an “RMS” and a TRMS meter
measure the same current:
6
What actually is THD?
Example shown here: triangular current
profile with an r.m.s. current of 1000 mA
and a pulse duty cycle of 1:7
i 
 Squared values 
6A
Without
5A
Fundamental:
502 mA
251986 mA²
fundamental
4A
3rd harmonic:
-479 mA
229000 mA²
229000 mA²
3A
5th harmonic:
434 mA
188595 mA²
188595 mA²
2A
7th harmonic:
-374 mA
139903 mA²
139903 mA²
1A
9th harmonic:
304 mA
92527 mA²
92527 mA²
t 
11th harmonic:
-232 mA
53671 mA²
53671 mA²
0A
13th harmonic:
163 mA
26600 mA²
-1A0ms
5ms
10ms
15ms 26600 mA²
20ms
15th harmonic:
-104 mA
10780 mA²
10780 mA²
-2A
17th harmonic:
57 mA
3299 mA²
3299 mA²
-3A
Sum of
squares:
996362 mA²
744377 mA²
-4A of this:
Root out

998 mA (eff.)
863 mA
-5A THDr (root mean square) = 863mA/1000mA
= 86%
-6A THDf (fundamental)
= 863mA/502mA
= 172%
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2. Current harmonic metering errors
arising on account of voltage
harmonics
E. g. when
operating
1000
compact
fluorescent
lamps rated
11 W (15 VA)
each on a 15
kVA
transformer
8
Inverse impact of single-phase
rectifier loads on the line-to-neutral
and line-to-line voltages
Load
9
Measurements on a mercury
vapour lamp
Voltage
Current
10
Mains voltage tolerances to be
kept during test
according to EN 61000-3-2:
2.0% deviation from rated value,
0.9% content of 3rd harmonic,
0.4% content of 5th harmonic
0.3% content of 7th harmonic,
0.2% content of 9th harmonic,
0.2% content of even harmonics
0.1% content of 11th to 40th harmonics
during test! And this is virtually necessary, since
the influence is major!
11
Why does the 3rd harmonic always
dominate in the current, while in the
voltage the 5th prevails?
This can only
happen while
all phase
conductors
are feeding
single-phase
rectifier
loads,
otherwise no
cancellation
can ever
take place
12
3. Self-caused additional
voltage harmonics within
the circuit under test
E. g. by using
the wrong
transformer
13
3. Self-caused additional
voltage harmonics within
the circuit under test
Without
transformer
influence
With
transformer
influence
14
4. Compatibility of metering
appliances and accessories
When an incandescent
lamp becomes
inductive, scepticism is
correctly placed!
15
4. Compatibility of metering
appliances and accessories
Useful measuring supplements, but...
16
Cable
Cable 1
Cable 2
Cable 3
Cable 4
Cable 5
Cable 6
Cable 7
Cable 8
Sum
5. Parallel cables
What Hogeschool
West-Vlaanderen,
Kortrijk, found out...
*>47,7°C
L1
330 A
145 A
109 A
109 A
153 A
104 A
115 A
181 A
1246 A
L2
206 A
147 A
166 A
173 A
128 A
119 A
205 A
307 A
1451 A
L3
72 A
141 A
136 A
236 A
135 A
33 A
289 A
170 A
1212 A
*>47,7°C
46,0
46,0
44,0
44,0
42,0
42,0
40,0
38,0
40,0
36,0
38,0
34,0
36,0
32,0
30,0
34,0
28,0
32,0
*<27,3°C
30,0
17
5. Parallel cables
...what a simple home
test revealed...
without with
“conduit” around cable 1
Cable 1: 249.1 A 242.6 A
Cable 2: 240.2 A 240.4 A
Cable 3: 247.0 A 246.2 A
18
5. Parallel cables
...and how a trade journal explained the differences:
• Fabrication tolerances
• Differences in cable lengths on account of different
bending radiuses
• Different temperature rises
19
6. Pitfalls with earth
resistance measurement
6.1 The test earth rods
have a much higher earthing resistance than the object under test!
6.2 Extraneous voltages
may be caused by galvanic effects or operational currents within the
earthing system.
6.3 The current carrying capability
should be measured right along by using a high probe current!
6.4 Z includes R and X
A lightning pulse with its rise time edges of about 100 kV/µs is considered
a high frequency event, encountering the according reactances.
6.5 Non-linearities
On the other hand the strike enhances the conductivity of the earth.
6.6 Do not measure metal instead of earth!
Are you sure there is no metallic connection between the objects under
test?
20
6.1 The supplementary earth rods
have a much higher earthing resistance
than the object under test!
Object under test,
e. g. RE0 = 2W
Test earth rod 1,
e. g. RE1 = 87W
Test earth rod 2,
e. g. RE2 = 93W
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6.2 Extraneous voltages
If there is such a
lot of current
flowing in the
earthing system
because of
inadequate
network
configuration,
major earth
resistance
measurement
errors will be the
consequence
22
6.3 You should measure the current
carrying capability right along
instead of using metering equipment
operating with milliamp
probe currents.
Some surveyors
even recommend
to use a welding
transformer!
23
6.4 The earthing system fulfills
several functions:
• Divert great mains frequency fault currents
• Divert small high frequency leakage
currents
• Divert great high “frequency” lightning
currents
Z  R  2fL 
2
2
24
6.4 The earthing system fulfills
several functions:
Divert great high frequency lightning currents
150A
i 
100Aeff @ 50Hz  44A/ms
100A
50A
t 
0A
0ms
5ms
10ms
15ms
20ms
-50A
-100A
-150A
25
6.4 The earthing system fulfills
several functions:
Divert great high frequency lightning currents
110kA
60kA in 1.2µs  50kA/µs
100kA
80kA
i 
90kA
70kA
60kA
50kA
40kA
30kA
20kA
tR
tS
10kA
t 
0kA
0µs
1µs
2µs
3µs
4µs
5µs
6µs
7µs
8µs
9µs
10µs
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6.5 Non-linearities
• Mains frequency and harmonics [A; Hz]:
Linear relationships, R dominates
• HF leakage currents [mA; MHz]:
Linear relationships, X dominates
• Lightning currents [kA; MHz]:
Non-linear relationships, X dominates
27
6.6 Do not measure “metal earth”!
Make sure the objects under test have no
metallic connection to one another!


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Morals: Who measures a lot may
measure garbage, but who doesn't
measure may miss a lot later on!
Perhaps
you better
procure
adequate
test
equipment
and just
measure!
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3
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30
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