Influence of DC & Phase Shift On Power Measurement of WHM - - PowerPoint PPT Presentation

Presentation in (Korea Electric Power Corporation)

Influence of DC & Phase Shift On Power Measurement of WHM

Consumption of domestic application

Current load profjles for :

Hair dryer

(power regulator using rectifying diode)

The current wave form has a 50Hz in-phase contribution (cos φ=1), a large DC-component is present also (IDC ≠0)

Ch1 0V M2.00ms Ch2 Ch2 ∫ 500mV 100mV Ch1 0V M2.00ms Ch2 Ch2 ∫ 500mV 100mV

Vacuum cleaner and mixer

(motor controller using phase cutting inverters)

50Hz component in current wave form has strong phase shift (cos φ=0.7)

mains voltage mains voltage current current

[ T ] [ T ]

Single Core CTs ‒ The DC Problem (1)

The core can’t get rid of it’s fmux!

some seconds for a good transformer DC-tolerance is required for residential energy meters according to IEC 62053-21 and-23: Operation with completely half rectifjed current, limited accuracy

t B lprim t

1 1 2 2 ΔB Iprim R ~ ~ ω·AFe·N2

sec

* ΔB = B0 (1-e r )

t

R L τ =

Single Core CTs ‒ The DC Problem (1)

Half wave rectifjed primary current Flux density B in the transformer core Secondary current

• f the transformer

Primary current

0.1 0.2 0.3 0.4 0.5 sec

Meter performance with domestic appliances

Realistic meter test (parallel running loads of domestic appliances)

Reference meter Meter under test A/B

N L

Meter A (linear core) Meter B (combined core)

I,φ

V

PF QS E P

I

Results for meter A and B Composition of drawn current 50Hz-current 7.4 A cos φ 0.85 i DC-current 2 A

Clear deviation from 2% target of IEC standard!!!

+4.5%

• 0.1%

kWh

Test procedure according to IEC standards

Iref = = = Imax 60Arms 42.42Arms 2 2 Itest = = = Imax 60Arms 30Arms 2 2 Iref I = = 60A0P 2 Ibalance = 30Arms I = 2I

test= 2·30Arms

= 60A0P

Test Watthour Meter Balancing Impedance Standard Watthour Meter

Rb Rb

+I

• I

Combined and single core current transformers

Strip-wound core solutios on the market: Single core type Combined core type Does it work?

One core material needs to fulfjll requirements Two different cores assembled in a core box :

Demands for various, highly advanced core materials (Linearity, DC capability, temperature dependency...) Competitors attempt to exploit the advantages of Good opportunities for VAC amorphous and nano-crystalline cores with tailored hysteresis loops high-permeability Low error in true AC operation low-permeability cores DC capability

Low cost product from e.g. China

Properties of combined cores

Performance characteristics “Combined Core” :

Part1

1.5

• 1.5

0.5

• 0.5

1

• 1
• 5

5

• 4

4

• 3

3

• 2

2

• 1

1

Part2 B [T] H [A/cm] Hysteresis loop of combined core (part 1, 2 and combined) Extremely non-linear hysteresis loop, i.e. permeability. CT performance should be dependent on :

• signal level
• wave form (esp. DC contributions)

cores provide for comparison : VAC cores provide highly linear loops

1.5

• 1.5

0.5

• 0.5

1

• 1
• 4

4

• 3

3

• 2

2

• 1

1

B [T] H [A/cm]

Meter performance with domestic appliances

Generalized tests and worst case scenarios :

Test conditions : Rectifjed current with variable “phase shift” (Δt)

Meter with linear core CT: IEC req. fulfjlled well (within ± 0.5%) A possible rip-off !!! Meter with combined core CT: Up to +20% (!) error in meter reading ~ ~ Δt

20.00 0.5 0.5 15.00

• 15.00
• 20.000

5 10 15 20 25 10.00

• 10.00

5.00

• 5.00

0.00 Meter A (linear Current Transformer) half rectifjed current in A half rectifjed current in A Meter B (Current Transformer with combined cores)

A true measurement

0.5 1 0.5 20.00 0.5 0.5 15.00

• 15.00
• 20.000

5 10 15 20 25 10.00

• 10.00

5.00

• 5.00

0.00 0.5 1 0.5

Summary and conclusions

Present status :

IEC tests do not cover “real life“ current mix

Meter insuffjciencies due to combined cores are not revealed Substantial meter mismeasurements can occur with combined cores.

Publications on this issue Proposal for norm amendment well received by German IEC council International IEC council will discuss and probably revise norm accordingly (Oct 2007)

Frank Herrmann in Energie Wirtschaft, p. 33, vol. 16, 2007 (German) Frank Herrmann in Metering International, p. 121, issue 3, 2007 (Eng)

Thank you