09 differential protection

8/7/2019 09 Differential Protection

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Differential Protection


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/ Copyright Siemens Australia & NZ 2007. All rights reserved. Siemens. Innovation for generations.

Measuring Principle

Basis is the first Kirchhoffs law e.g. Transformer

The currents to a nodeare positive defined.

Internal fault(nfeed from two sides) External fault

I1 = I1,FI2 = I2,FI= II1,F + I2,FI

trip

I1 = IFI2 = -IFI= IIF - IFI = 0

no trip

I= II1 + I2I

In the case of load:I1 = ILI2 = -IL

I= 0}

1 2

I

I=0I

I1

I2

IL

2

1


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Peculiarity of the Transformer Differential Protection

1. Vector group (e. g. Yd5) 2. Different CTs,tap changer, magnetising current

vector group adaptation restraint function (stabilising) is necessary

3. Dynamic currents

I= f (Irestr.)

Irestr. = |I1| + |I2|

inrush current overflux (overexcitation)

CT saturation duringexternal faults

blocking via harmonics

saturation detector

trip region

-

I2

I1

150

current transformer

tap changer,

CT adaptation(will be eliminated)

magnetising current

I

I

ITr, IRestr.


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Differential Currents with Harmonics

energising

Y y

D y

T2

T1

t = 0

t = 0

i2

i1

i1

even,

2nd harm.

i

i

i

20 40 60 80 100ms t

iDiff = i1

iDiff = i1

iDiff = i2

20 40 60 80 100ms t

20 40 60 80ms t

energising

paralleling(energising transformer T1)

even

2ndharm.

evenand

odd

2ndharm.


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Differential Currents with Harmonics

i1

i1

i1 i2 ~ 0

i2

i2

20 40 60 80ms t

20 40 60 80ms t

20 40 60 80ms t

i

i

i

iDiff = i1 - i2

iDiff = i1 - i2

iDiff = i1

odd

3rd and5thharm.

evenandodd

evenandodd

Over-excitationUTr > UN

External short circuit withsaturation of the CTs at thelow-voltage side

internal short circuit with

saturation of the CTs at thehigh-voltage side


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Differential Protection for Generators and MotorsBasic principle can be nearly direct used.

1. Stabilising characteristic

2. Transients sensitive settings

generators: external short circuit with largedc time constants

motors: start-up currents

transient transfer features of a CT

are important (dc component)

Insensitive settings at

matching transformersin the secondary circuit

different primary CTs

different burden

Trip area I

I

IRestr.

Error currents

via CT

identically current transformers

sensitive setting is possible


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Devices with Differential Protection Functions

7UM62 and7UT6xx -Family

7UT613 for protection objects with threeends

7UM621 and 7UM622for protection objects with two ends(machines)

7UT612 for protection objects with twoends

7UT633 for protection objects with threeends

7UT635 for protection objects up to fiveends


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Applications

7UM 62

7UT612

7UT613

7UT6

Trans-former

Two winding transformer2 or 3phases

1 1/2 circuit breaker applicationwith two winding transformer

Short lines2 ends

7UT613

7UM 62

7UT612

7UT613

Generator/Motor longitudinal ortransversal differential protection

GS3~

Three winding transformer2 or 3phases

Short lines3 ends


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Connection Example

Side 2(Winding 2)

Side 3(Winding 3)

Side1(Winding1)

7UT613; 633

7UM 62

7UT 612 )*

)* direct zero sequence currentalso possible

direct connection to the main CTs

no matching transformers / no

matching connections

numerical vector group adaptationwithout zero sequence currentcorrection depending of the type ofearthing of the winding.

increased sensitivity by 33% bymeasuring of the zero sequencecurrent (7UT6) for single-polefaults.

1A/5A main CTs adaptation in therelay

permissible ratio CT nominal currentto transformer nominal current up to1 : 8


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Functional Diagram of the Differential Protection

i1P

i2P

i3P

i1A*

i2A*

i3A*

iDIFF = i1A* + i2A* + i3A*basic wavefiltering IDiff

iStab = | i*1A | +| i*2A | + | i*3A |rectified mean value

IStab fast tripping withevaluation of

IDiff and iDiff

iDIFF>> Stage

tripping by IDIFF>>

Blockingbyharmonics

tripping by IDIFF>

&

trippinglogic TRIP-

command

TRIP L1

TRIP L2

TRIP L3>1

IDiff

IDiff>

IStabmeasured valuepreprocessingwinding 1

measured valuepreprocessingwinding 2

measured valuepreprocessingwinding 3

tripping characteristicand saturat. detection

harmonics analysiscrossblock

blocking by2nd harmonicsand 3rd or 4th.or 5th harmonics

i1P, i2P, i3P sampling values from winding 1, 2, 3i*1A, i*2A, i*3A values after vector group and CT matchingIDiff basic wave contents in the differential currentiDiff differential currentIStab rectified mean value of the stabilising currentiStab stabilising current


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Measuring Pre-processing, Example for CT Matching (Part 1)

IP1 = 500A(load current)

IP2 = 1833A(load current)

1000/1A 2000/1A

UN1 = 110kV UN2 = 30kV

SN = 100MVA

Side 1 Side 2

7UM 627UT6

IDiff = ?IRestr. = ?

IN, Trafo = 525A IN, Trafo = 1924A

IS1 = 0,5A IS2 = - 0,92A

measuredsecondary currents


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Measuring Pre-processing, Example for CT Matching

(Part 2)

1. Calculation of the transformer nominal current INTrafo =

2. Correction factor kW =

3. Correction nominal current I= kW IS

4. Calculation of the differential and stabilising current Idiff= |I1' + I2' | Istab = |I1' | + |I2' |

Calculation example:

SN = 100MVA; UN1 = 110kV; UN2 = 30kV; IN1CT = 1000A; IN2CT = 2000A

Correction factors: kW1 = 1,9; kW2 = 1,04 Idiff = 0A

Load conditions: Ip1 = 500A; Ip2 = 1833A IStab= 1,9A

Secondary currents: Is1 = 0,5A; Is2 = 0,92A Idiff = 0 IN Trafo

Matched currents: I1' = 0,95A; I2' = 0,95A Istab = 1,9IN Trafo

S

3

N

N U

NTrafo

NCT

I

I


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Measuring Preprocessing: Vector Group Adaptation)

L1

L2

L3

Ip IsYN d5

IE

Protection

IpL1

IpL3 IpL2IsL1

IsL2

IsL3

I

I

I

I

I

I

I

I

I

*

*

*

1 0 0

0 1 0

0 0 1

+1

3

pL1

pL2

pL3

pL1

pL2

pL3

E

E

E

=

I

I

I

I

I

I

*

*

*

1

3

2 -1 -1

-1 2 -1

-1 - 1 2

pL1

pL2

pL3

pL1

pL2

pL3

=

1-10

01-1

101-

3

1

*

*

*

sL3

sL2

sL1

sL3

sL2

sL1

=

I

I

I

I

I

I

1-10

01-1

101-

3

1

*

*

*

sL3

sL2

sL1

sL3

sL2

sL1

=

I

I

I

I

I

I

standard setting

increasing of thesensitivity via theconnection of IE-CT(only at 7UT6 possible)

zero sequence elimination

additional earth current measurement (I0-correction)


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Example Calculation: Vector Group Correction

Situation: Single Phase Fault, No Load

Source

Ynd1F1 F2

ISC ISC/3

ISC/3

ISC

F1 F2

IL1 = -ISC IL1 = 0IL2 = 0 IL2 = 0

IL3 = 0 IL3 = 0

IE = ISC IE = ISC

Il 1 = ISC /3Il 2 = - ISC /3Il 3 = 0

L1

L2

L3


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I Zero Elimination

I*L1

I*L2I*L3

= 1/3

2 -1 -1

-1 2 -1-1 -1 2

- ISC

00

F1 F2

0

00

I*l 1

I*l 2I*l 3

= 1/3

1 -1 0

0 1 -1-1 0 1

ISC /3

- ISC /30

I*L1 = -2/3 ISC 0

I*L2 = 1/3 ISC 0I*L3 = 1/3 ISC 0

I*l 1

= 1/3 ISC

+ 1/3 ISC

= 2/3 ISC

I*l 2 = 0 - 1/3 ISC = -1/3 ISCI*l 3 = -1/3 ISC + 0 = -1/3 ISC

IDIFF1 = I*L1 + I*l 1 = 0 2/3 ISC

IDIFF2 = I*L2 + I*l 2 = 0 1/3 ISCIDIFF3 = I*L3 + I*l 3 = 0 1/3 ISC

We see in all three phases adifferential current; this mustbe considered during asingle phase testUse only the trip signalfrom the tested phase!


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I Zero Correction

I*L1I*L2I*L3

=

1 0 0

0 1 0

0 0 1

- ISC0

0

F1 F2

0

0

0

I*l 1I*l 2I*l 3

13

1 -1 0

0 1 -1

-1 0 1

I*L1 = -2/3ISC 1/3 ISC

I*L2 = 1/3 ISC 1/3 ISC

I*L3 = 1/3 ISC 1/3 ISC

IDIFF1 =I*L1 + I*l 1 = 0 ISCIDIFF2 =I*L2 + I*l 2 = 0 0IDIFF3 =I*L3 + I*l 3 = 0 0

1/3 ISC1/3 ISC1/3 ISC

+ =

ISC

/3- ISC /30

I*l = 1/3 ISC + 1/3 ISC = 2/3 ISCI*l 2 = 0 - 1/3 ISC = -1/3 ISC

I*l 3 = -1/3 ISC + 0 = -1/3 ISC

We see only in the faultyphase the currents


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Tripping Characteristic

flexible adaptation to various transformers, e.g. with tap changer or different main CTs

high stability against external faults with CT saturation

fast tripping for solid short-circuits within one period


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Recommendation for voltage setting:

U = 2U U

U + U= U (1 - cN, New

max min

max min

N

2)

c: steps of tap changer (p.u.)

Problem: The tap changer modifies the transformer ratio

additional error in the differential current

Example: tap changer c = 16%

res

resdiff

0,16=c

cc

I,I

II

diff 0870

2

=m

Relay Settings

Influence of the Tap Changer

If IN.Tr. is flowing the additional Idiff is approx. 17,5% of the transformercurrent. With slope 1 = 0,25 there is the pick-up threshold at 50%. Thesecurity margin is for steady state conditions high enough. Consideringtransient conditions (CT-influence) a small increasing of slope 1 (to 0,3) isrecommended.


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Pick up of Differential Protection

For triggering of internal tasks, events and fault records the differential protectionfunction needs a pickup information. This pickup becomes active, if thedifferential current or the restraint current is over an internal threshold (dottedline). Each external large current leads to a pickup.

Pickup doesnt always means internal failure!


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IDiff / IRestr.- Areas for Short Circuit / Normal Operation

1

2 4

NTr

diff

I

I

NTr

Restr.

I

I

limiting curve

internalshort circuit/Inrush

external short

circuit withCT saturation

external shortcircuit witha high current

externalshort circuit (low current)

normal operation(nominal current)


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Rush Stabilisation

Recognise inrush condition by evaluating the ratio 2nd harmonic I2HAR to basicwave IDiff.

Time limit for cross-block. Reliable reaction to the inrush condition with cross-block.Trip of a short circuit after the set time delay.

Recognise over-excitation by evaluating the ratio 3rd or 5th harmonic to basic wave

filter window1 cycle L1-block

L2-block

L3-block

Cross-block = No (phase separate blocking)

Cross-block = Yes (blocking of all phases)

Idiff, L2 > trip blocking

O R 1

t

t1P 2P 3P

iRUSH = iDiff

15 % setting value

block

no block

I2HARIdiff

Inrush currentin one phase

L1-block

L2-block

L3-block



IDiff > trip blocking for a limited time

&

&

&


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Example of an Inrush CurrentA unit transformer (IN = 396 A) was switched on from the high voltage side


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Stabilising at Motor StartingTypical for motor starting is the starting current and the superimposed dc componentwith a large time constant. The current transformers (ct) transfer different this dccomponent. The result is a differential current and the risk of an over-function is given.

Detection of motor starting:Increases the pick-up values for a restricted time

Criterion:

Supervision ofrestraint current

Istab > I-Restr. Startup(until 2 I/InO)

than theStart-Factor (max. 2)is active for a restrictedtime

T Start Max(Duration of dynamicalincreasing of pickup)


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Setting RecommendationsPower System Data 1The setting of this parameters are important, because they are necessary for thescaling and direction definition of the measurands. At the protection objecttransformer the setting for star point Solid Earthed leads to a zero sequenceelimination. Isolated leads to a direct current comparison(without zero sequenceconsideration). This setting is only allowed at a really free star point (no over voltagearrester, no Peterson coil).At the protection object generator always the direct current comparisonmethodis active (no zero sequence elimination).

Setting values:Under normal conditions the factory settings correspond with the practical experiences.Its not necessary to change these parameters.At transformers with tap changer the inclination of SLOPE 1 should be increased. Thesetting for the inrush detection can be final select during the primary test. If we are onthe limits (low setting value) an activation of CROSSBL. 2. Harm. is recommended.If are current transformer operates on their limit the pickup value Idiff > and die slope 1should be increased.The Idiff >> - stage must be set over the maximum inrush current.

At generators and motors SLOPE 1 can be reduced (to 0,15), if the currenttransformers are identical. At generators the Idiff>>-stage must be set over thetransient fault current (3 to 7 IN,G).

Additional at generators its also recommended an activation of increasing the pickupthreshold during starting or at external faults (Start-Factor > 1).


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Earth Current Differential Protection in the 7UM62The earth current differential protection (restricted earth fault protection - REF) offersa higher sensitivity at single phase faults (approx. 5 %) against the conventionalprotection. Its used at generators with low ohmic star point or at earthed Wyeconnected transformer windings .

protection

object

protection

object

Connection 1 Connection 2At connection 1 the zerosequence current is calculatedfrom the phase currents and

direct measured on the starpoint(transformer application)

At connection 2 the zero

sequence current is calculatedonly from the phase currents.(generator application, wheremore than one generator feedsinto the busbar)


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Measuring Principle of Earth Current Differential Protection in

the 7UM62

iL1S1

iL2S1

iL3S1

iL1S2

iL2S2

iL3S2

iee2

protection object:generator

1

3I023I01

I0Diff

I0Stab

I/InO

I/InO

1

1

I-EDS>

Stabilizing rangeRange not possible

Tripping range0201Diff0 I3I3I +=

0201Stab0 I3I3I +=

L3S1L2S1L1S101 III3I ++=

L3S2L2S2L1S202 III3I ++=

EE202 I3I =or


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Earth Current Differential Protection in the 7UM62

Stabilizing against Overfunction

90

| | = 0180

65115

I.

II.

III.IV.

2104 EDS || I MIN>

Direction of zero sequence current:

I. internal fault

III. external fault

IV. direction not consideredII. measuring repetition

( Imax (max. 2.5 IN), than blocking

Main problem are external faults:Transient conditions with large time constants; Short current circuitswith current transformer saturation

Release at zero sequence voltage:A release of zero sequence current measurement is be caused by a measured zero

sequence voltage (can be switched off).


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Restricted Earth Fault Protection (REF) in the 7UT6

i1i2

i3

i'0

i"0 = i1 + i2 + i3

i0" + i0' = iF

iF

Trip

0

Trip = I0' - k SStab < 0 Trip = I0' Trip - tripping quantity

Stab 0 Stab - I0 angle-dependentstabilization

IEDF- pick-up valve

Stab = I I '0 - I " 0| - | I ' 0 + I " 0|

restrictedearth faultprotection7UT6

fault currentagainst earth

stabilizationarea

tripping area

calculation of the basic wave and the complex vectors of I0' and I0"

insensitive against DC components and CT saturation

evaluation of the modulus and angle between I0' and I0"

sensitive fault detection starting with 5% transformer nominal current


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Commissioning with Browser- Support

Currents on the highand low voltage side

Tripping characteristicwith actualoperating points


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Benefits for the Customer

Protection relay with flexible adaptation to the transformer/generator/motoror short line. Programming of the device data in the relay.

Reduced amount of wiring by direct connection to the main CTs.

No matching transformers and therefore no wiring errors.

Zero sequence current can be measured (in the 7UT6).Sensitivity for single-pole faults in the transformer increased by 33% .

Flexible adaptation of the tripping characteristic to various main CTs,

tapped transformers.

Exact discrimination between the short circuit condition and the inrushcondition by on-line analysis of the harmonics. Fast tripping forhigh-current faults. Saturation detector for external faults.

Thermal monitoring of two transformer windings. Back-up DMTL/IDMTL with reverse blocking for one winding.

Sensitive short circuit protection for faults winding against earth.

Manifold commissioning aids.

09 differential protection

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