Calibration of high-frequency wattmeters used for standby power testing to IEC 62301 August 2011 I. Budovsky and D. Georgakopoulos National Measurement.

Calibration of high-frequency wattmeters used for standby power testing to IEC

62301

August 2011I. Budovsky and D. Georgakopoulos

National Measurement Institute, Australia Low Frequency Electrical Standards

Overview

• 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

RMSI

ICF max 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

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.2 0.4 0.6 0.8 1 1.2

Normalised time (s)

Am

plit

ude

I sinc

I pulse

Same CF and fundamental magnitudeBut different • harmonic content• peak values and• rms values when the harmonics are included

00.020.040.060.08

0.10.120.140.160.18

0 20 40 60 80 100

Harmonic number

Har

mon

ic m

agni

tude Pulse

Sinc

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

• A crest factor of 10 is more easily achieved with a pulsed current waveform while maximizing the measured power

• Satisfies the 50th harmonic recommendation of the IEC standard

• Simple relation of the CF with the power -20

0

20

40

60

80

100

120

1 10 100 1000 10000

Harmonic number

Mag

nit

ud

e (%

of

fun

dam

enta

l)

Fs -fx

LPF, Fs>2×fmax

fx Fs ff

H(j)

fmax

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7

U (t)

I

Measured power as a function of CF

t1 t2 t3 t4 T t

i(t)

Im

-Im

3412

43

21

243

243

2424

ttttt

tTttTt

tTttTt

p

pp

pp

t1 t2 t3 t4 T t

i(t)

Im

-Im

3412

43

21

243

243

2424

ttttt

tTttTt

tTttTt

p

pp

pp

prms

m

t

T

I

ICF

2

2)(2sin

2

CFIVP mm

Zero power factor

tp: pulse widthT: pulse period

22 2sinc

2sin

2

CF

nnI

CFI mn

Harmonics

CF T/tp

5 50

10 200

20 800

Calibration waveforms (… continued)V

I 0°

cos11IVP • The power for the selected waveforms is concentrated only on the fundamental

• The calibration system must be characterized at power frequencies

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77

Harmonic voltage

Mag

nit

ud

e o

f vo

ltag

e an

d c

urr

ent

har

mo

nic

s

IV

0˚ 90˚45˚

Calibration system

Power Generation Power Measurement

System descriptionHigh frequency Thermal Power Comparator (TPC)Measures the difference between the unknown ac power (ACV and ACI) and the known dc (DS1 and DS2)

TPC: multi-junction thermal converters, no aliasing

UUT

Rs

VD

TPC

DCS1

DCS2

ACV

ACI

DCV

DCI

Dual Channel VoltageSource

VoltageAmplifier

TCA

10 MHz

i(t)

t

TCA minimum Distortion, highbandwidth

Thermal Power Comparator

Controlled Switches Amplifiers

U~

I~

Uu

Ui

Temperature Compensation

VD

R

TC1

TC2

OutputAmplifier

S1

S2

S3

S4

S5

S6

S7

Uu1

Uu2

Ui1

Ui2

I

II

III

IV

Uc1

Uc 2o

o

S

Differential

Sum-and-Difference

Y

Traceability

• Thermal Power Comparator• Current shunts• 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

Magnitude andPhase errors

Power measurement

Uncertainty budgetComponent Distribution

Ui

(W/VA) ki ci

ui

(W/VA) ciui

(W/VA) i

TPC AC-DC difference Normal 67 2 1 33.6 33.6 30

Shunt Normal 10 2 1 5.0 5.0 30

Resistive voltage divider Normal 20 2 1 10.0 10.0 30

DVM1 Normal 8 1 1 8.0 8.0 30

DVM2 Normal 8 1 1 8.0 8.0 31

Type A Normal 50 2 1 25.0 25.0 19

Source adjustment Normal 10 2 1 5.0 5.0 30

Combined standard uncertainty, uc (W/VA) 45.1

Effective degrees of freedom, ueff 65 Coverage factor, k 2.00

Expanded uncertainty U = kuc (W/VA) 90.1

Typical uncertainties of commercial wattmeters compared to IEC 62301

requirementsCurrent Range Current Applied Current Phase Angle Nominal Power Uncertainty

Uncertainty requiredby IEC62301

(Apk) (Apk) Crest Factor (degrees) (W) (W) (W)

3 0 112.564 ± 0.06 ± 2.3 3 + 60 56.282 ± 0.03 ± 1.1 3 - 60 56.282 ± 0.04 ± 1.1 5 0 40.703 ± 0.03 ± 0.81 10 0 10.182 ± 0.06 ± 0.20 3 0 22.513 ± 0.012 ± 0.45 3 + 60 11.256 ± 0.006 ± 0.23 3 - 60 11.256 ± 0.006 ± 0.23 5 0 8.141 ± 0.005 ± 0.16 10 0 2.036 ± 0.012 ± 0.04 20 0 0.509 ± 0.006 ± 0.01

5 3

1 0.6

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 InstituteBradfield RoadWest Lindfield NSW 2070Australia

Phone: + 61 2 8467 3600

Email: [email protected]