Download Calibration of high-frequency wattmeters used for standby power

Calibration of high-frequency wattmeters
used for standby power testing to IEC
62301
August 2011
I. Budovsky and D. Georgakopoulos
National Measurement Institute, Australia
Low Frequency Electrical Standards
Overview
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IEC 62301 definition of standby power
Motivation for this work
Calibration waveforms
System description
Traceability
Experimental results
Summary
Standby power definition according
to IEC 62301
“the lowest power consumption mode which
cannot be switched off (influenced) by the user
and that may persist for an indefinite time when
an appliance is connected to the main electricity
supply and used in accordance with the
manufacturer’s instructions.”
Motivation
• Electrical appliances operating in standby mode
consume small amount of electrical power
• The current in standby mode can take the form of
pulses or spikes
• Required measurement uncertainty by IEC 62301
< 2%, measured power > 0.5 W
< 0.01 W, measured power < 0.5 W
• The IEC standard specifies current waveforms with
CF up to 10
I max
CF 
I RMS
CF=1.41 for a sinewave
CF=1.00 for a square wave
… Motivation
• Because of the high CF specified by the IEC
standard, power meters for standby power
cannot be calibrated with sinusoidal waveforms
• Hence there is a need to develop techniques to
traceably verify power meters for the specific
conditions described in IEC 62301
Calibration waveforms
• Voltage
– sinewave with maximum distortion of 2% (up
to and including the 13th harmonic)
–crest factor from 1.34 to 1.49
– use of a sinewave (THD <0.02%) for
calibration is good enough
• Current
– specification only for CF (up to 10)
– no distortion specified
– recommends the instrument must measure up
to at least the 50th harmonic
Current waveforms
I sinc
1
Amplitude
I pulse
0.5
0
0
0.2
0.4
0.6
0.8
-0.5
-1
1
Harmonic magnitude
1.5
1.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
Pulse
Sinc
0
20
40
60
Harmonic number
-1.5
Normalised time (s)
Same CF and fundamental magnitude
But different
• harmonic content
• peak values and
• rms values when the harmonics are included
80
100
Selecting current waveform
We are looking for a current waveform to test
the current channel of a power meter which:
• satisfies the IEC requirements or
recommendations
– CF requirement
– contain pulses or spikes
– high bandwidth
• is physically realisable
In our calibration system we use a pulsed current
Advantages of using pulsed current
• A pulse is a wideband signal and is a more
challenging test for the measurement system
I-V
converter
Amplifier
LPF
S/H
ADC
• Sampling frequency > 2  maximum signal frequency
• The channel frequency response known for the
signal frequencies
Magnitude (% of fundamental)
• A crest factor of 10 is more easily achieved with
a pulsed
current waveform
while maximizing the
H(j)
LPF, Fs>2×f
max
measured power
• Satisfies the 50th harmonic recommendation of
the IEC standard
f
f
F
x
• Simple relation
offmax
the fCF
with
the
power
s
120
100
80
60
40
20
0
-20
1
Fs -fx
10
100
Harmonic number
1000
10000
Measured power as a function of CF
i(t)
1.5
t1  T 
4
Im
tp
t2  T 
4
2
tp
t3  3T 
4
2
t p  t 2  t1  t 4  t3
tp
t 4  3T 
4
2
tp
1
U (t)
I
2
0.5
0
0
t1
t2
t3
t4
T
t
1
2
3
4
5
6
7
-0.5
-1
CF
-Im
T/tp
-1.5
tp: pulse width
T: pulse period
Zero power factor
Harmonics
Im
T
CF 

I rms
2t p
5
50
10
200
20
800
  

P  Vm I m sin 
2 

 2(CF ) 
2
In 
2
 n
I
sin

2 m
CF
 2

 n 
sinc


2 

 2CF 
P  V1I1 cos 
Harmonic voltage
45˚90˚
˚
0
• The power for the selected waveforms is
concentrated only on the fundamental
• The calibration system must be characterized
at power frequencies
77
73
69
65
61
57
53
49
45
41
37
33
29
25
21
17
9
13
1
I 0°
I
V
5
V
Magnitude of voltage and current
harmonics
Calibration waveforms (… continued)
System description
High frequency Thermal Power Comparator (TPC)
Measures the difference between the unknown
ac power (ACV and ACI)
and thesystem
known dc (DS1 and DS2)
Calibration
Dual
Channel
Voltage
Source
Voltage
Amplifier
Power Generation
VD
Power Measurement
ACV
UUT
TCA
i(t)
Rs
TCA minimum
ACI
Distortion,
high
bandwidth
TPC
DCS1
DCV
DCS2
DCI
10 MHz
TPC: multi-junction thermal converters, no aliasing
t
Thermal Power Comparator
VD
S 1 Uu
1
S3
S5 I
U~
Uu 2
S4
S 6 III
Ui1
RS
IV
Controlled Switches
Uio
Differential
Amplifiers
Uc 1
S7
Output
Amplifier

Ui2
Uuo
TC1
Y
S2
I~
II

Uc 2
Temperature Compensation
Sum-andDifference
TC2
Traceability
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Power measurement
Thermal Power Comparator
Magnitude and
Current shunts
Phase errors
Voltage divider
Voltage measurement error (dc only)
Frequency (negligible)
Each of these components has been characterized and
is traceable to the NMIA standards of voltage, current,
resistance, frequency and electrical power
Uncertainty budget
Component
TPC AC-DC difference
Shunt
Resistive voltage divider
DVM1
DVM2
Type A
Source adjustment
Distribution
Ui
(W/VA)
ki
ci
ui
(W/VA)
ciui
(W/VA)
i
Normal
Normal
Normal
Normal
Normal
Normal
Normal
67
10
20
8
8
50
10
2
2
2
1
1
2
2
1
1
1
1
1
1
1
33.6
5.0
10.0
8.0
8.0
25.0
5.0
33.6
5.0
10.0
8.0
8.0
25.0
5.0
30
30
30
30
31
19
30
Combined standard uncertainty, uc (W/VA)
Effective degrees of freedom, ueff
Coverage factor, k
Expanded uncertainty U = kuc
(W/VA)
45.1
65
2.00
90.1
Typical uncertainties of commercial
wattmeters compared to IEC 62301
requirements
Current Range Current Applied
(Apk)
(Apk)
5
3
1
0.6
Current
Crest Factor
3
3
3
5
10
3
3
3
5
10
20
Phase Angle Nominal Power Uncertainty
(degrees)
0
+ 60
- 60
0
0
0
+ 60
- 60
0
0
0
(W)
112.564
56.282
56.282
40.703
10.182
22.513
11.256
11.256
8.141
2.036
0.509
(W)
± 0.06
± 0.03
± 0.04
± 0.03
± 0.06
± 0.012
± 0.006
± 0.006
± 0.005
± 0.012
± 0.006
Uncertainty required
by IEC62301
(W)
± 2.3
± 1.1
± 1.1
± 0.81
± 0.20
± 0.45
± 0.23
± 0.23
± 0.16
± 0.04
± 0.01
Summary
• A traceable measurement system for calibration of
wattmeters used in standby electrical power testing has
been developed
• The measurement system is based on a thermal power
comparator, precision current shunts, inductive and
resistive voltage dividers and precision amplifiers
• Each of these components has been evaluated in a
traceable way
• The expanded uncertainty of the system is better than
100 W/VA for a crest factor of 3
• The system has been tested for current waveforms with
crest factors up to 20
National Measurement Institute
Bradfield Road
West Lindfield NSW 2070
Australia
Phone: + 61 2 8467 3600
Email: [email protected]