RET 670

A B B P ow er T ec hn ol og ie s A B , 20 07 RET 670 Transformer Protection Restricted Earthfault Substation Automation and

P ow er T ec hn ol og ie s A B , 20 07

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

RET 670 – Restricted Earthfault Protection

REF1-I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2 REFPDIF_87N

A B B P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

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

RET 670 – Restricted Earthfault Protection

I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2 I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

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

A B B P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

Functionality

Three-section operate/restrain characteristic

Settings:

IdMin

Dependent on the magnitude of the bias current, the corresponding zone (section) of

P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

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)

For multiple circuit breaker applications, the two contributing residual currents are internally summed to produce the

overall line-side residual current Idiff = IN + 3I0

I3PW1 = I3PW1CT1 + I3PW1CT2 I3PI3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

A B B P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

Functionality

Operate/restrain characteristic

Single circuit breaker applications – ordinary power trfrs

curr(x) = MAX [I3PW1CT1] curr(z) = IN

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

I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Functionality

Operate/restrain characteristic

Multiple circuit breaker applications-CTFactor

A factor to allow a sensitive function also at multi-breaker

arrangement where the rating in the bay is much higher than the rated current of the transformer winding.

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

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

A B B P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Functionality

Operate/restrain characteristic

Multiple circuit breaker applications – ordinary power trfrs

curr(a) = MAX [I3PW1CT1] / CTFactorPri1

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)]

I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

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

A B B P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

Functionality

Operate/restrain characteristic

Single circuit breaker applications – autotransformers

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

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

Note: set IBase = highest winding rated current, normally Irated_MV I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Functionality

Operate/restrain characteristic

Multiple circuit breaker applications – autotransformers

RET 670 – Restricted Earthfault Protection

I3P I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

A B B P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

Functionality

Operate/restrain characteristic

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

i.e. Ibias = MAX [curr(a);…….curr(z)]

I3P

I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Directional criterion – external earth fault

RET 670 – Restricted Earthfault Protection

A (L1) B (L2) Ia =₀ Ia =₀ Ib =₀ Ib =₀ Ic =₀ Ic =₀ Uzs Uzs 3Io=3Izs1

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

180o _{out of phase for any external}

earth fault.

Currents 3Io and I_N are theoretically zone of protection

I_N = -3I-3Izs1

zs voltage is max. at the earth fault zs voltage is max.

at the earth fault C (L3) a (L1) b (L2) c (L3) s o u r c e s o u r c e I_N Izs1 Izs1 Izs1 ROA MTA For external fault

Restrain for external fault Reference is neutral current ROA IN Operate for internal fault 3I₀

Relay Operate Angle (ROA)

A B B P ow er T ec hn ol og ie s A B , 20

07 currents 3Io and IN are approximately

in phase for an internal earth fault. A (L1) B (L2) C (L3) a (L1) b (L2) c (L2) Ia = 0 Ia = 0 Ib =₀ Ib =₀ Ic =₀ Ic =₀ Uz s

3Io = -3Izs2 _{reference because it has}serves as a directional

the same direction for both internal and external faults. Current in the neutral (IN)

serves as a directional reference because it has the same direction for both internal and external faults. zone of protection I_N = - 3Izs1 Ifault power system contribution to fault current power system contribution to fault current s o u r c e s o u r c e

Return path for 3Izs2 Return path for 3Izs1

I_N Izs2 Izs2 Izs2 Izs1 Izs1 Izs1 ROA MTA For internal fault

Restrain for external fault Reference is neutral current ROA IN Operate for internal fault 3I₀

Restricted earthfault protection (PDIF, 87N)

Directional criterion – internal earth fault

RET 670 – Restricted Earthfault Protection

Relay Operate Angle (ROA)

Settable 60° to 90

The residual current 3I0and the

neutral current INare theoretically

equal in magnitude and 180 degrees

P ow er T ec hn ol og ie s A B , 20 07

RET 670 – Restricted Earthfault Protection

Restricted earthfault protection (PDIF, 87N)

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.

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

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

If the ratio between 2nd _{harmonic and fundamental exceeds 60%, the}

A B B P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Overview of restricted earthfault protection function

RET 670 – Restricted Earthfault Protection

Directional check TRIP START & ≥1 I_N > 50% of Idmin & 3I0 < 3% of Ibase 3I0 > 3% of Ibase

A directional check is only executed if The magnitude of the line-side residual

current (3I0) is at least 3% of IBase

start has been registered from the operate-restrain characteristic

P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Function block

RET 670 – Restricted Earthfault Protection

‘Analog’ quantity inputs

A B B P ow er T ec hn ol og ie s A B , 20 07

P ow er T ec hn ol og ie s A B , 20 07 Settings Settings group N1 Differential protection LowImpREF(PDIF,87N) Off; On

A B B P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Service values

P ow er T ec hn ol og ie s A B , 20 07

Restricted earthfault protection (PDIF, 87N)

Service values

A B B P ow er T ec hn ol og ie s A B , 20 07