03 SEP674 RET670 Differential Protection

Substation Automation Products

Transformer protection RET670 Differential protection

1MRG019259

Chapter 03

Contents

© ABB Group September 10, 2015 | Slide 2



Application



Factors that affect a transformer protection



Differential protection function 

Differential and bias currents



Zero sequence elimination



Operating characteristics



Blocking criteria



Internal – external fault discriminator



On-load tap-changer compensation



Switch onto fault



Supervision – Open CT and differential current



In and output signals



Settings



Monitored data

Chapter 03

Application Transformer differential protection

© ABB Group September 10, 2015 | Slide 3



The majority of all transformer failures are caused by winding faults



The differential protection is 

the main protection in case of winding failure



a unit protection limited by used CT’s



fast, sensitive and secure 

2nd harmonic and waveform block restraint to avoid unwanted tripping for inrush



5th harmonic to avoid unwanted tripping for overexcitation

Chapter 03

Application Differential protection (TxWPDIF, 87T) 



Phase differential protection function 

single or multi-breaker arrangement,



protects one or two objects,



with or without tap-changer position monitor,



with or without loaded delta winding,

Power transformer, 

© ABB Group September 10, 2015 | Slide 4

Two or three windings



Auto-transformer,



Shunt reactor,



Generator transformer block,



Phase shifting transformer

Chapter 03

Application Differential protection (T2WPDIF, 87T) 

RET670

Two-winding differential protection function with 

Up to 4 three-phase CT inputs



Up to 2 instances



One on-load tap-changer

. . .

RET670

© ABB Group September 10, 2015 | Slide 5

RET670

Chapter 03

Application Differential protection (T3WPDIF, 87T) 

RET670

Three-winding differential protection function with 

Up to 6 three-phase CT inputs



Up to 2 instances



Up to 2 on-load tap-changers

. . .

RET670 RET670

© ABB Group September 10, 2015 | Slide 6

Chapter 03

Application Differential protection (TxWPDIF, 87T) 

Low impedance type



Low requirements on CT’s



No need for interposing transformers magnitude, phase angle correction and zero sequence is implemented in the IED



© ABB Group September 10, 2015 | Slide 7



Using the fundamental frequency components



Features to detect 

turn-to-turn faults



open CT



Tap-changer position monitoring for increased sensitivity



Differential current supervision



Switch on to fault feature

Chapter 03

Common unbalances – Overview Factors that affect a transformer protection 

Unbalances to be handled due to other factors than faults in the transformer 

Currents that flow on only one side of the power transformer 



Magnetizing currents that flow on only the power source side 

Normal magnetizing currents



Inrush magnetizing currents



Overexcitation magnetizing currents

Currents that cannot be transformed to the other windings 



© ABB Group September 10, 2015 | Slide 8

Zero sequence currents

Error in the power transformer turns ratio due to OLTC

Chapter 03

Common unbalances – Overview Factors that affect a transformer protection 

Unbalances to be handled due to other factors than faults in the transformer 





Inequality of the instrument current transformers 

Different ratings of current transformers



Different types of current transformers

Different relative loads on instrument transformers 

Different relative currents on CT primaries



Different relative burdens on CT secondaries

Different DC time constants of the fault currents 

© ABB Group September 10, 2015 | Slide 9

Different time of occurrence, and degree, of CT saturation

Chapter 03

Inrush current Factors that affect a transformer protection

© ABB Group September 10, 2015 | Slide 10



Source impedance



Size of the transformer



Location of energized winding 

Inner 10 to 20 times of Irated



Outer 5 to 10 times of Irated



Connection of windings



Point of wave when the switch closes



Magnetic properties of the core



Remanence of the core



Pre-insertion resistors or switch-synch

Chapter 03

Energizing inrush Factors that affect a transformer protection

IL1 IL2 IL3





Switch-on 

High current peak and long DC time constant



Risk for CT saturation



Phase currents may differ considerably



Different harmonic levels in different phases

2nd harmonic

© ABB Group September 10, 2015 | Slide 11

Typical magnetizing inrush current waveform 60 MVA, 140/40 kV, YNd

Chapter 03

Sympathetic inrush Factors that affect a transformer protection

T2





T1

Energizing transformer T2 

Inrush at T1 can appear and long DC time constant



Maximum after some delay



Oscillations between T1 and T21MRG004868

2nd harmonic

© ABB Group September 10, 2015 | Slide 12

T1 inrush current waveform

Chapter 03

Recovery inrush and overexcitation Factors that affect a transformer protection 

ILx

© ABB Group September 10, 2015 | Slide 13

Clearing of external fault near the transformer 

The voltage at the terminals recovers to it’s normal levels



Inrush currents



2nd harmonic



Overexcitation results from 

Overvoltage and/or



Low frequency



Could be harmful and an overexcitation protection should be considered



5th harmonic

Chapter 03

Current transformer saturation Factors that affect a transformer protection ILw1x ILw2x Idiffx 2nd harmonic



False differential current during external faults due to CT saturation



DC offset will produce phase shift between primary and secondary current



Current includes harmonics that may disturb the protection operation

© ABB Group September 10, 2015 | Slide 14

Chapter 03

Current transformer saturation Factors that affect a transformer protection



Delay for heavy internal faults as a consequence of harmonic distortion of the fault currents



Due to initial heavy saturation, the harmonic restrain can delay/prevent immediate operation of the restrained (percentage) differential protection

© ABB Group September 10, 2015 | Slide 15

Chapter 03

Zero sequence current transformation Factors that affect a transformer protection 

Three phase or two phase faults 



Delta-Wye Winding Effects on Primary Current Ref. ANSI/IEEE C37.91, IEEE Guide for Protective Relay Applications to Power Transformers

© ABB Group September 10, 2015 | Slide 16

Positive and negative sequence components are transformed between the windings (incl. phase shift)

Single phase fault 

Zero sequence currents is not correctly transformed



False differential currents needs to be eliminated

Chapter 03

References Differential protection (TxWPDIF, 87T) 

Internal adaptation to power transformer vector group, turns ratio and CT ratio



Magnitude reference

Yd





All currents (differential, bias) are transferred to the reference side and expressed in primary Amperes



Phase reference

Dy

Yy

Dd



© ABB Group September 10, 2015 | Slide 17

First winding (usually HV)



first star-connected winding HV  MV  LV



if no star winding, first delta-connected winding HV  MV  LV

All current currents are phase shifted with respect to the reference side

Chapter 03

Differential currents Differential protection (TxWPDIF, 87T) 

Phase segregated



Fundamental frequency differential currents is calculated as: or



© ABB Group September 10, 2015 | Slide 18

Where the coefficients A, B and C depend on 

winding connection type,



transformer vector group and



zero sequence current elimination set on or off

Chapter 03

A, B and C matrices Differential protection (TxWPDIF, 87T) Type and vector group

Zero sequence reduction

YN

0o

yn

0o

d1

-30o

Set Off

Set On

Matrix for reference winding Not applicable

yn2 -60o

d5 -150o

… Not applicable

…

…

…

Not applicable

d11 +30o

© ABB Group September 10, 2015 | Slide 19

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 



© ABB Group September 10, 2015 | Slide 20

Higher currents might lead to unbalance currents other than faults 

CT saturation due to high through fault currents



High load and tap-changer in end-position etc



It’s necessary to stabilize the differential protection

The bias current is defined as 

the highest fundamental frequency current amongst all phase currents (high, medium and low voltage side)



and common for all three operate-restrain characteristics



referred to magnitude reference side (W1)



and specific for single breaker application:

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 

Bias currents in multi-breaker arrangements



Through-fault stability (external fault) 

Ideal CT’s, external current summation  stable

+A

+B

Idiff

-A -B To Diff Prot +A -A -B → -B IDIFF = +B -B = 0 IBIAS = MAX(-B; +B)

© ABB Group September 10, 2015 | Slide 21

Ibias

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 

Through-fault stability (external fault) 

Real CT’s, external current summation  loss of information and risk for unwanted trip due to false differential current and insufficient stabilization

+A

+B

Idiff

-A -B To Diff Prot +A -A -B ±error → -B ±error IDIFF = +B -B ±error = ±error IBIAS = MAX(-B ±error; +B)

© ABB Group September 10, 2015 | Slide 22

Ibias

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 

Through-fault stability (external fault) 

Real CT’s, internal current summation  higher security

+A

+B

Idiff

-A -B To RET 670 +A -A -B ±error TxWPDIF

Ibias Σ

© ABB Group September 10, 2015 | Slide 23

to IBIAS calculation

IDIFF = = +A -A -B ±error +B = ±error

to IDIFF & IBIAS calculation

IBIAS = MAX(-B ±error; +B; +A; -A -B ±error)

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 

The bias current for multi-breaker application is defined as

CT1



the highest fundamental frequency current amongst all phase currents from CT1, CT2 and the sum.



If CT rating >> rated current of the winding  a risk of unwanted desensitizing (over-biasing) of the differential protection.



There is a possibility to scale the bias contribution from CT1 and CT2 (parameter: CTxRatingWy) to prevent over-biasing.

Irated Wx

CT2 to IBIAS calculation

Σ © ABB Group September 10, 2015 | Slide 24

to IDIFF & IBIAS calculation

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 

The bias current for multi-breaker application is calculated



and the maximum value is selected (1 out of 27 for a 3 winding multibreaker arrangement).

© ABB Group September 10, 2015 | Slide 25

Chapter 03

Bias currents Differential protection (TxWPDIF, 87T) 

Multi-breaker application – example 

When 1pu primary current flows through the T-side CT, this should contribute a 1pu component to the bias calculation

Irated_CT1: 1000 A Irated_W1: 500 A CT1 

W1

Without action the bias contribution from CT1 is 1000 A (2pu)



By applying scaling factor:

CT1RatingW1: 1000 

The bias contribution from the CT will be

I∙(RatedCurrentWy / CTxRatingWy) = 1000 (500/1000) = 500 A (1pu)

© ABB Group September 10, 2015 | Slide 26

Chapter 03

Zero sequence elimination Differential protection (TxWPDIF, 87T) 

Elimination of zero sequence currents is necessary to avoid unwanted trips for external earth faults when



zero sequence currents

1.

2.

© ABB Group September 10, 2015 | Slide 27

1.

cannot flow to the other side of the transformer

2.

can only flow on one side of the transformer



Removing the zero sequence currents decreases the sensitivity for internal faults but



the bias currents are treated in the same way  this compensates to some degree



Setting per winding: 

On/Off

Chapter 03

Operating characteristic – Unrestrained Differential protection (TxWPDIF, 87T) 

Unrestrained, non-stabilized, instantaneous limit



Independent of bias current



No doubt that the fault is internal



No blocking criteria



Setting: IdUnre

IdUnre





© ABB Group September 10, 2015 | Slide 28

1.0 - 50.0 times IBase

IBase (RatedCurrentW1): rated current of winding 1 (A)

Chapter 03

Operating characteristic – Operate-Restrain Differential protection (TxWPDIF, 87T) 

Operate unconditionally

Start: 

IdMin

Section 3

Section 2

Section 1

Operate conditionally

Restrain





Section 1 

IdMin, independent of bias currents



Normal currents



Natural differential currents due to off-nominal on-load-tap changer position

Section 2 

© ABB Group September 10, 2015 | Slide 29

Differential and bias currents are above the operate-restrain characteristic

Minor slope to cope with false differential currents due to high load

Chapter 03

Operating characteristic – Operate-Restrain Differential protection (TxWPDIF, 87T) 

Section 3 

Operate unconditionally

Higher tolerance to substantial CT saturation at high trough fault currents

IdMin

© ABB Group September 10, 2015 | Slide 30



The slope:



Settings:

Section 3

Section 2

Section 1

Operate conditionally



IdMin 

Restrain

0.05 – 0.60 times IBase



Endsection1 and 2



SlopeSection2 and 3



IBase (RatedCurrentW1): rated current of winding 1 (A)

Chapter 03

Operating characteristics – Blocking criteria Differential protection (TxWPDIF, 87T) IDL3 IDL2



Phase segregated feature



Waveform restrain or



Harmonic restrain

IDL1



IBias

© ABB Group September 10, 2015 | Slide 31

Second or fifth harmonic blocking



All three work in parallel and can block the differential protection function phase-wise



Instantaneous differential currents are calculated and used in the analysis



The same equations are used as for the fundamental differential calculations (IDL1-L3)

Chapter 03

Operating characteristics – Waveform restrain Differential protection (TxWPDIF, 87T)

IL1

Typical magnetizing inrush current waveform

IL2 IL3

60 MVA, 140/40 kV, YNd



Based on pattern recognition algorithm



Looks for intervals with low rate-of-change in the instantaneous differential currents



Typical to power transformer and shunt reactor inrush currents



No settings



Bundled and phase segregated output signals (BLKWAVLx)

© ABB Group September 10, 2015 | Slide 32

Chapter 03

Operating characteristics – Harmonic restrain Differential protection (TxWPDIF, 87T) 

1. Inrush IL1 IL2 IL3

IN

1.

inrush currents or

2.

currents caused by overexcitation (high voltage/low frequency)



Overexcitation could be harmful but this phenomenon should be handled by the overexcitation protection



The analysis is performed after operaterestrain-characteristic start



The ratio I2/I1 and I5/I1 is calculated per phase



Bundled and phase segregated output signals (BLK2HLx and BLK5HLx)

2. Overexcitation

ILx

© ABB Group September 10, 2015 | Slide 33

Required to prevent unwanted tripping due to magnetizing (false differential currents)

Chapter 03

Operating characteristics – Cross-blocking Differential protection (TxWPDIF, 87T) IDL2/3

IDL1 Phase L1 blocked

cross-block Not affected

Ibias

Ibias



Inrush currents in separate phases of a power transformer may differ considerably and



each phase will most likely have different harmonic levels and waveform



If a blocking condition is active in one phase (harmonic or waveform)

© ABB Group September 10, 2015 | Slide 34



This condition can block the differential protection in other two phases



But only if the differential protection in that phase have started

Chapter 03

Blocking criteria – Summary Differential protection (TxWPDIF, 87T) 

Waveform – inrush 







© ABB Group September 10, 2015 | Slide 35

Settings: None

Second harmonic – inrush, CT saturation 

Settings: Settable level in % of fundamental frequency



Default setting: 15.0%

Fifth harmonic – overexcitation 

Settings; settable level in % of fundamental frequency



Default setting: 25.0%

Cross-blocking between phase 

Settings: On/Off



Default setting: On

Chapter 03

Internal – external fault discriminator - Overview Differential protection (TxWPDIF, 87T) 

The level of negative sequence currents are normally very low in a healthy system



Increasing levels is a fault indicator



Positive and negative sequence quantities follow the same laws 

Transformed in the same way by a power transformer



The same way to compensate for vector group and turns ratio



Possible to compare the currents on HV and LV (MV) side

RET670



© ABB Group September 10, 2015 | Slide 36

The point of fault is a fictitious source of the negative sequence current

Chapter 03

Internal – external fault discriminator - Overview Differential protection (TxWPDIF, 87T) 

The fault position can be determined by comparing the flow directions on all sides 



Theoretically a symmetrical fault will not cause negative sequence currents but 

due to the DC component the component will be present



The protected transformer must be loaded, if not – it’s not possible to determine direction



Objective

RET670

© ABB Group September 10, 2015 | Slide 37

Three direction comparisons for a three winding transformer



Use negative sequence currents



Create a sensitive internal/external fault discriminator

Chapter 03

External fault Internal – external fault discriminator

RET670



© ABB Group September 10, 2015 | Slide 38

External fault 

The negative sequence currents are 180o out of phase (IED reference direction: towards the object)



Magnitude are close to identical

Chapter 03

Internal fault Internal – external fault discriminator

RET670



Internal fault 

© ABB Group September 10, 2015 | Slide 39

The residual currents are approximately in phase

Chapter 03

Operating characteristics Internal – external fault discriminator 



The directional comparison is 

performed if the magnitude of both phasors are > IMinNegSeq



not performed during transformer energization

To guarantee good sensitivity: 



© ABB Group September 10, 2015 | Slide 40

IMinNegSeq must not be set too high

Settings: 

Relay operate angle (NegSeqROA) 30 – 90o



Minimum current (IminNegSeq) 0.02 – 0.20

Chapter 03

CT saturation Internal – external fault discriminator 4



CT saturation might cause phase angles ≠ ≈180o or ≈0o



Approximately 5 ms time to saturation of the main CT is sufficient

3

2 5

© ABB Group September 10, 2015 | Slide 41

1

The figure shows a test 1.

7 ms: Internal fault declared

2.

12 ms: Trip command issued

3.

Definitely an internal fault

4.

Excursion due to CT saturation

5.

External fault region

Chapter 03

Internal – external fault discriminator – Features Differential protection (TxWPDIF, 87T) 

Discriminates between internal and external faults with high dependability



Detects even minor faults with high sensitivity and high speed



The feature is used in two sub functions: 1. Unrestrained negative sequence differential protection (NSUNR)

Turn-to-turn fault at the end of the common winding (Courtesy of Croatian Utility – HEP)

© ABB Group September 10, 2015 | Slide 42

2. Sensitive negative sequence protection (NSSENS)

Chapter 03

Traditional differential protection Simplified logical diagram



© ABB Group September 10, 2015 | Slide 43

1MRG004867

Chapter 03

Unrestrained NS differential protection Simplified logical diagram



Dependent on the traditional differential protection



A restrained start is required and the internal / external fault discriminator categorizes the fault as internal  Trip



The blocking supervision is bypassed if the fault is declared: Internal

© ABB Group September 10, 2015 | Slide 44

Chapter 03

Sensitive NS turn-to-turn fault protection Simplified logical diagram Blocking condition: 2nd harmonic, 5th harmonic, waveblock and crossblock

Negative sequence internal/external fault discriminator

Internal I- start & internal

& &

>1 Bias 150%

© ABB Group September 10, 2015 | Slide 45

Chapter 03

On-line compensation for OLTC – General Differential protection (TxWPDIF, 87T)

© ABB Group September 10, 2015 | Slide 46



Without OLTC information, fixed ratio is used when calculating the differential currents



As turns ratio changes off nominal (due to OLTC movement), the currents will change and false differential currents will appear



TxWPDIF has a feature to monitor the tap-position and dynamically compensate the equation above



i.e. if the tap-changer is placed on W1, the no-load voltage Ur_W1 will be treated as a function of the actual tap position



OLTC must be considered when selecting IdMin



Typically IdMin-setting is 0.30 - 0.40 but if position information is available the settings could be halved

Chapter 03

On-line compensation for OLTC Differential protection (TxWPDIF, 87T) 







YLTC (TCL or TCM) is the interface between the OLTC and TxWPDIF 

T2WPDIF can be compensated for one OLTC



T3WPDIF can be compensated for one or two OLTC

Inputs: Tap position information 

mA input signal (requires MIM-input HW-module),



binary input signals (TCLYLTC, max 32 steps) or



coded binary, BCD or Gray coded

Outputs: to be used by TxWPDIF 

Actual tap position (TCPOS – TAPOLTCx, Integer)



OLTC alarm (POSERRAL – OLTCxAL)

Whenever OLTCxAL = 1 (true)  

© ABB Group September 10, 2015 | Slide 47

Actual IdMin will be increased depending of the OLTC settings

Chapter 03

Switch on to fault feature Differential protection (TxWPDIF, 87T) Typical magnetizing inrush current waveform (red) and voltage (blue)



Built-in SOTF feature - only active when energizing the transformer



Ensures faster tripping when energizing a faulted transformer (severe fault)



Based on the fact: different shape of the current when energizing a healthy transformer compared to switch on to a fault



Instantaneous increase in current 



© ABB Group September 10, 2015 | Slide 48

The waveform block will reset and temporary disable the blocking signals  Fast operation of the differential protection

When SOTF disabled: Waveform the other blocking features are completely independent from each other

Chapter 03

Supervision – Differential current alarm Differential protection (TxWPDIF, 87T) 

The differential current level is continuously monitored



When all three phase differential currents are above:



IdMin IDiffAlarm

© ABB Group September 10, 2015 | Slide 49





IDiffAlarm (10% of rated current)



and disabled at high currents (>110% of rated current i.e. overload, external faults, inrush conditions)

Chapter 03

Supervision – Open CT detection Differential protection (TxWPDIF, 87T) 



When open CT detected 

All differential functions, except the unrestrained function, will be blocked immediately



The unrestrained function will be blocked after tOCTUnrstDelay (< 6000 s)



An alarm will be given after tOCTAlarmDelay (< 10 s)

Reset 



© ABB Group September 10, 2015 | Slide 51

A detected open CT condition will be reset automatically once the condition has disappeared 

Symmetrical bias current within 10 – 110% for at least 1 minute and



this must be fulfilled at least for tOCTResetDelay (< 10 s) to prevent maloperation at reconnection

It is not possible to externally reset the open CT detection

Chapter 03

In- and output signals Differential protection (TxWPDIF, 87T)

Phase currents W1 CT1 W1 CT2 Analog input signals Phase currents W2 CT1 (SMAI group signal output) W2 CT2 Phase currents W3 CT1 W3 CT2 Tap changer #1 position OLTC input signal #2 position (YLTC function outputs) Tap changer #1 alarm #2 alarm Block of function Binary input signal Block of restrained trip Block of unrestrained trip Block of unrestrained negative seq differential Block of negative seq differential protection

© ABB Group September 10, 2015 | Slide 52

Chapter 03

In- and output signals Differential protection (TxWPDIF, 87T)

Binary output signals General trip Restrained differential trip Unrestrained differential trip Unrestrained negative sequence differential trip Sensitive negative sequence differential protection trip General start Start signal from phase Lx Second harmonic blocking

Fifth harmonic blocking

Waveform blocking

© ABB Group September 10, 2015 | Slide 53

Chapter 03

In- and output signals Differential protection (TxWPDIF, 87T)

Sustained differential current alarm Open CT detected Open CT, delayed signal Differential current, samples, phase Lx

Differential current, RMS-value, phase Lx Bias current Negative sequence differential current

© ABB Group September 10, 2015 | Slide 54

Binary output signals Analog output signals HMI and Disturbance recorder

Chapter 03

Differential protection settings Overview Two-winding transformer 1st

instance

General settings Rated values for winding 1 and 2 (3) Winding 1 and 2 (3) connection type Phase angle between W1 and W2 (W1 and W3) Zero sequence subtraction on/off per winding Multi-breaker and bias correction W1 and W2 (W3)

On-Load tap-changer: Not used, W1, W2 (W3) Tap-changer data

© ABB Group September 10, 2015 | Slide 55

Chapter 03

Differential protection settings – OLTC Overview General settings On-load tap-changer 1st instance

Conversion data

mA, BCD, BIN, Gray, Single OLTC Ctrl settings

Setting group 1

© ABB Group September 10, 2015 | Slide 56

Chapter 03

Differential protection settings Group N settings Specific settings Operation On/Off Switch onto fault function Operation characteristic

Blocking

Negative sequence differential function

Open CT Alarm settings

© ABB Group September 10, 2015 | Slide 57

Chapter 03

Monitored data Differential protection 

© ABB Group September 10, 2015 | Slide 58

Tests/Functions status/Differential protection/ TransformerDiffxWind(PDIF,87T) 

TRIP – General trip



TRIPRES – Restrained trip



TRIPUNRE – Unrestrained trip



TRNSUNR – Unrestrained NS trip



TRNSSENS – Sensitive NS trip



START – General start signal



STL1-3 – Start signal from phase Lx



BLK2H, L1-3 – Second harmonic blocking



BLK5H, L1-3 – Fifth harmonic blocking



BLKWAV, L1-3 – Waveform blocking



IDALARM – Sustained differential currents alarm



OPENCT – Open CT detected



OPENCTAL – Open CT delayed alarm

Chapter 03

Monitored data Differential protection 

© ABB Group September 10, 2015 | Slide 59

Tests/Functions status status/Differential protection/ TransformerDiffxWind(PDIF,87T) 

IDL1MAG – Magnitude of differential current, phase L1 (A)



IDL2MAG – Phase L2 (A)

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IDL3MAG – Phase L3 (A)

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IBIAS – Magnitude of bias current (A, Common for all phases)

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IDNSMAG – Magnitude of negative sequence differential current (A)

Chapter 03

© ABB SA-TGroup Training September 10, 2015 | Slide 60