06 Sep-603b Ref Ret 670

RET 670 Transformer Protection Restricted Earthfault

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ABB Power Technologies AB, 2007

Substation Automation and Protection Training

2008-05-23


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RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

Up to three instances available

Low impedance type

Can be used on all directly or low impedance earthed windings

Up to 4 three-phase CT inputs REF1REFPDIF_87N

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I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

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Able to handle multiple circuit breaker applications as well as autotransformers in 1½ CB applications


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Functionality

Zero sequence differential protection with a percentage biased operate/restrain characteristic, plus an additional zero sequence current directional comparison criterion

Requires no current magnitude or phase correction, even when OLTC present

Insensitive to inrush and overexcitation currents

Only danger is eventual CT saturation 2nd harmonic restraint (reactor energizing)

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Functionality

Three-section operate/restrain characteristic Idiff

Settings: operate

Idmin

1st slope 70%

IdMin

2nd slope 100%

restrain

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Ibias

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Functionality

Operate/restrain characteristic

Differential current – vector sum of the fundamental frequency neutral current and the fundamental frequency residual current (calculated from the three line-side fundamental frequency phase currents) Idiff = IN + 3I0

I3PW1 = I3PW1CT1 + I1PW1CT2


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For multiple circuit breaker applications, the two contributing residual currents are internally summed to produce the overall line-side residual current

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Functionality

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The bias current stabilizes the restricted earthfault protection

It is a measure of how high the currents are (i.e. a measure of how difficult the conditions are under which the CTs are operating)

The higher the currents, the more difficult the conditions, and the higher the probability that the calculated differential current has a false component, primarily due to CT saturation

The bias current is calculated as the highest fundamental frequency current amongst all the input currents (three phases / neutral) to the restricted earthfault protection

Included in the bias current calculation is an enhanced feature that ensures security when external faults are disconnected


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Functionality


Single circuit breaker applications – ordinary power trfrs


curr(x) = MAX [I3PW1CT1]

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curr(z) = IN i.e. Ibias = MAX [curr(x); curr(z)]

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Functionality


Multiple circuit breaker applications – ordinary power trfrs


curr(a) = MAX [I3PW1CT1] / CTFactorPri1

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curr(b) = MAX [I3PW1CT2] / CTFactorPri2 curr(x) = MAX [I3PW1CT1 + I3PW1CT2] curr(z) = IN i.e. Ibias = MAX [curr(a); curr(b); curr(x); curr(z)] 2008-05-23


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Functionality


Multiple circuit breaker applications

CTFactor = Irated pri_CT / Irated_W

The CTFactor is generally chosen so that when 1pu primary current flows through the CT, this would contribute a 1pu component to the bias calculation – e.g. Irated pri_CT = 1000A Irated_W = 500A contribution to bias calculation = 1000 / (1000/500) = 500A

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Ibias (pu) = Ibias (pri A) / IBase, where IBase = Irated_W

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Without CTFactor, a 1pu CT primary current would contribute a 1000A component to the bias calculation (i.e. 2pu)



Functionality Operate/restrain characteristic

Multiple circuit breaker applications

Applying the CTFactor gives a gain in the sensitivity for internal faults (by reducing the contribution to the bias from the T-side CTs), but still ensures adequate biasing for through fault conditions

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Functionality


Single circuit breaker applications – autotransformers


curr(x) = MAX [I3PW1CT1] curr(y) = MAX [I3PW2CT1]

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curr(z) = IN i.e. Ibias = MAX [curr(x); curr(y); curr(z)]

Note: set IBase = highest winding rated current, normally Irated_MV

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Functionality


Multiple circuit breaker applications – autotransformers

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Functionality


Multiple circuit breaker autotransformer applications curr(a) = MAX [I3PW1CT1] / CTFactorPri1 curr(b) = MAX [I3PW1CT2] / CTFactorPri2 curr(c) = MAX [I3PW2CT1] / CTFactorSec1 curr(d) = MAX [I3PW2CT2] / CTFactorSec2 curr(x) = MAX [I3PW1CT1 + I3PW1CT2] curr(y) = MAX [I3PW2CT1 + I3PW2CT2] curr(z) = IN

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i.e. Ibias = MAX [curr(a);…….curr(z)]

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Directional criterion – external earth fault zone of protection

s o A (L1) u r B (L2) c C (L3) e Uzs

zs voltage is max. at the earth fault

I zs1

I a0=

a (L1)

I zs1

I b0=

b (L2)

I zs1

c= 0

c (L3)

I

3Io=3I zs1

IN = -3Izs1

IN

Current in the neutral (IN) serves as a directional reference because it has the same direction for both internal and external faults.

For external fault ROA

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3I0 IN

Reference is neutral current

ROA

Restrain for external fault

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Currents 3Io and IN are theoretically 180o out of phase for any external earth fault.

MTA

Operate for internal fault


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Directional criterion – internal earth fault power system contribution to fault current

s o u r c e

zone of protection

A (L1) B (L2) C (L3)

Izs2

Izs1

I

a= 0

a (L1)

Izs2

Izs1

I

b= 0

b (L2)

Izs2

Izs1

I

c= 0

c (L2)

Ifault

Uz s

3Io = -3Izs2

IN

IN = - 3Izs1 Return path for 3Izs2

Return path for 3Izs1

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ROA

For internal fault 3I0

IN ROA

Restrain for external fault 2008-05-23 16


Current in the neutral (IN) serves as a directional reference because it has the same direction for both internal and external faults.

MTA

Reference is neutral current

Operate for internal fault

currents 3Io and IN are approximately in phase for an internal earth fault.



Directional criterion

The directional criterion is applied in order to positively distinguish between internal and external earth faults – this check is an additional criterion, with the purpose of enhancing the security against maloperations for heavy external earth faults, and during the disconnection of such faults by other protections

For an external earth fault the residual current 3I0 and the neutral current IN are theoretically equal in magnitude and 180 degrees out of phase

For an internal earth fault

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the residual current 3I0 and the neutral current IN are approximately in phase

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The current in the neutral (IN) serves as the directional reference because it has the same direction for all earth faults, both internal and external – current polarized


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Directional criterion

a start has been registered from the operate-restrain characteristic

the magnitude of the line-side residual current (3I0) is at least 3% of IBase

If a directional check is not executed (due to too small currents), then the internal directional requirement is cancelled as a condition for trip

If a directional check is executed, then a trip output is only allowed if the currents 3I0 and IN are both within the operating (internal) region

Relay Characteristic Angle (RCA)


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A directional check is only executed if

Relay Operate Angle (ROA)

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Constant (fixed) 0° (determined by where the line-side residual current (3I0) should lie with respect to the directional reference (neutral current) for an internal earth fault) Settable 60° to 90°




2nd harmonic analysis / blocking

Required primarily to prevent unwanted operation on reactor energizing due to possible line-side CT saturation caused by a high dc component of long duration

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The current through the neutral CT does not have the same dccomponent, or the same amplitude, and consequently the risk of saturation in this CT is much less

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The differential current that appears as a result of the saturation may be so high that it reaches the operate area of the characteristic

Calculation of 2nd harmonic content in the neutral current is made when certain criteria are fulfilled

If the ratio between 2nd harmonic and fundamental exceeds 60%, the function is blocked


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Overview of restricted earthfault protection function 3I0 < 3% of Ibase 3I0 > 3% of Ibase

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Directional check

IN > 50% of Idmin 2008-05-23 20


≥1

&

TRIP

&

START



Function block

‘Analog’ quantity inputs

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‘Analog’ quantity outputs

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RET 670 – Restricted Earthfault Protection - Settings

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RET 670 – Restricted Earthfault Protection - Settings Settings Settings group N1 Differential protection LowImpREF(PDIF,87N)

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Off; On

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Restricted earthfault protection (PDIF, 87N) Service values

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Restricted earthfault protection (PDIF, 87N) Service values

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06 Sep-603b Ref Ret 670

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