Power Transformer Load Loss Measurement

Gert RietveldErnest HoutzagerMilos AcanskiDennis HoogenboomEnrico MohnsHenrik BaduraIlija Pecelj

ELPOW workshop, 31 Aug 2016

• Power transformers - losses

• Why loss measurements?

• Transformer loss measurement systems

• ELPOW: TLMS calibration systems

• Summary

Outline

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Power transformers are used toscale voltages in the grid

High Voltage low losses

Power Transformers

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Loss mechanisms:1. No load losses (Zb = ), caused by iron core, continuous2. Load losses (Zb = 0 ), copper loss, depends on current3. Stray losses depend on design

Power transformer losses

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Why loss measurements (1)

• Cost of losses in power transformers are comparable to the productcost TCO (total cost ownership)

• Environmental impact is significant

Cost of (no-)load loss equals product costs

Consequence: regulations - EcoDesign Directive:“improve environmental performance of energy-

related products through better design”

Why loss measurements (2)

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Saving potential in use phase through more efficient designsestimated as 16 TWh/year ( 17 % of present losses)3.7 Mt of CO2 emissions (50 % of total Danish electricity consumption)

Consequence: more requirements in standardsIEC 60076-19 (uncertainty calculation), and upcoming IEC 60076-20:

Why loss measurements (3)

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Tier 1: 1 July 2015Tier 2: 1 July 2021

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0 20 40 60 80 100 120

Min

imum

PEI

[%]

Rated Power [MVA]

Liquid immersed - Tier 1Liquid immersed - Tier 2Dry-Type - Tier 1Dry-Type - Tier 2

Consequence: customers (utilities) put fines on losses in excess ofguaranteed maximum losses

Example calculation for a large transformer (100 MVA)• Guaranteed maximum losses: Pgar = 500 kW (0.5 % loss)• Fine for excess losses: 10.000 €/kW

Why loss measurements (4)

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Reliable loss measurement with low uncertaintygives confidence and less discussions

3 % measurement uncertainty (150 ppm) corresponds to500 kW · 3 % · 10.000 €/kW = 150.000 €

(manufacturer and customer have to decide who pays for uncertainty)

Prob

abili

ty

Loss

Uncertainty matters! (compliance)

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Actual loss

Ecodesign limit

Test resultfails to detectcompliance

Accurate test

Inaccurate test

Ecodesign allows no tolerance: if measured loss is above the limit,the transformer is not in conformity

High accuracy =low risk of incorrectdecisions

TLMS typical measurement range: 0 – 100 kV, 0 – 2 kAUncertainty: 3 % - 5 % (IEEE C57.12.00, IEC 60076-19, Ecodesign)

Traceable to international measurement standards

Transformer Loss Measurement

Power supply P = V · I · cos

Power: P = U · I · cos with 90

P = U · I · cos (90 - ) = U · I · with = (90 - ) 0

Measurement challenge

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phase accuracy, not amplitude accuracy required!!

U and I are large numbers 1 % uncertainty is very easyis a very small number:for PF=0.01, = 0.57 1 % uncertainty is a big challenge!

(6 m , 0.34 min, 100 µrad)

TLMS consists of high-quality components

1 % losses

Only a calibrated TLMS system, traceable to national standards, givesproven, reliable quality in load loss measurements

TLMS calibrations

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Two approaches in TLMS calibrations• Component calibration (< 0.5 %; at PF=0.01 0.2 min, 3 m )

Easier to perform, larger overall system uncertainty• System calibration (< 3 %; at PF=0.01 1 min, 17 m )

Difficult to perform, low uncertainty, all effects included

Increased measurement challenge: TUR = 3 – 10

Reference measurement accurate to < 0.15 min (50 µrad, 3 m )

Aim: system calibration of TLMS up to 100 kV and 2 kA, with anuncertainty of better than 50 µW/VA

ELPOW aim for TLMS calibration

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Two approaches:

• Simultaneous generation of voltage V and current I

• Phase lock current I to externally applied voltage V

• Realise feedback loop

LMS for testing power trafos

Generating part Measuring part

• “voltage and current up to 150 kV and at least 2 kA…”• “uncertainties better than 50 ppm…”

Generation: phantom power

30 kVA0 V - 250 V

10 kVAR1: 0 A – 50 AR2: 0 A – 25 A

To do:• Stability transconductance amplifier must be optimized• Assessment of the phantom power source

2 channel source• 16 bit DACs• 10 VPK• phase lock• external synchronization

or 15 Hz … 60 Hz intern DACor 50 Hz line

Measuring system

Voltage TransformerClass 0.02

Current TransformerClass 0.002

Power ComparatorClass 0.01

U, I, , P, Q, S

I = 2 kA

U = 150 kV

LabView Software

To do overview:1. Optimization of the software

• timing to read the comparator• automatic correction of the transformer error

2. Uncertainty calculation3. Assessment of the system

Challenge: lock I to Vwithin 0.3 m (5 µrad)

TLMS system calibration

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V I

DSP is a key element2nd reference CT + RD22 watt meter as check

Actual VSL implementation

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Control (DSP)

Power amplifier (G)

Transformer /current generator

Power reference (RD22 watt meter)

CTs(3-stage compensated)

VT (CC-based capacitive divider)

Generation:• Rohrer wideband amplifier (20 A, 150 V) + step-up transformer

Measurement:• VT: HV capacitor (100 pF, 100 kV) + CC-based LV divider

– Uncertainty: 15 – 25 ppm (phase / magnitude)• CT: 3-stage wideband; errors < 1 ppm (uncertainty 5 ppm)

– Wideband 5 A shunt for conversion to voltage• Power: two 24-bit ADCs (NI)Expected overall phase uncertainty: < 25 ppm (25 µrad)

Verification: RD22 power meter (< 10 ppm) + CT + VT

Components of VSL system

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The control loop has 2 main parts:

First part: high speed. Second part: high accuracy ( feedback)(NRC analogue system needs accurate 90 reference signal)

Results:• Extensive testing with different voltage signals; different phase

shifting blocks studied• Low noise: 4 – 5 µrad; agreement with power meter < 10 µrad

Control loop

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ADC

90°

V [n] DAC

I

Q

I [n-m]

Phaseshifting

Z-n

• Much more noise than in lab (30 µrad vs 5 µrad)• Agreement VSL LL and RD22 reference: within 15 ppm• Agreement with NRC better than 25 ppm

First on-site trial

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Power transformer losses are significant– Economically: TCO and fines– Environment: CO2 emissions

Requirements from EU regulations,IEC / IEEE on both the actual lossesand measurement accuracy

Conclusion

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Transformer Loss Measurement Systems need calibration– Proven, validated accuracy < 3 % @ PF = 0.01– ELPOW: System calibration < 0.5 % @ PF = 0.01

– simultaneous generation– current phase locking

(promising first results < 0.3 %!)