03 sep674 ret670 differential protection

1MRG019260

Transformer protection RET670Restricted earth fault protection

Substation Automation Products

Chapter 04

© ABB GroupSeptember 10, 2015 | Slide 2

Contents

Application

Operating characteristics

Logical Diagram

In and output signals

Settings

Monitored data

Chapter 04


Faults resulting in large fault currents may cause severe damages to windings and the core

Could also result in high gas pressure damaging the tank

The restricted earth fault protection is fast and sensitive and can detect faults in the winding: in impedance earthed systems

close to the star point in solid earthed systems

The restricted earth fault protection is often used as main protection for all winding faults involving earth

Application restricted earth fault protection

Chapter 04


Low impedance type

Can be used to protect directly or low impedance earthed windings

Zero sequence differential protection An additional directional comparison feature is

included

No need for magnitude or phase angle correction

Internally summation if multi breaker arrangement

Fundamental frequency components

OLTC does not influence the function

Insensitive to inrush and overexcitation

Only danger is eventual CT saturation 2nd harmonic restraint included

Restricted earth fault protectionApplication

Chapter 04


Restricted earth fault protection Application

1. TransformerA. Solidly earthed winding with or without OLTC

B. Earthed though Z-0 earthing transformer

2. Autotransformer with or without OLTC

3. ReactorA. Solidly earthed

1A 1B

2 3A

Chapter 04


Restricted earth fault protection Application

Single circuit breaker or

Multiple circuit breaker applications summation inside the function

Number of instances Customized: 0 - 3

Pre-configured (A10, A30, B30, A40, B40, A25): Included: A10: 1; x30/y40: 2; A25: 0

Optional: y40: 1

Chapter 04


Calculated or measured residual current on terminal side, 3I0 (Ires)

The differential current, Idiff, is the vector sum:

where

3I0_Wx is the summation of CT1 and CT2 for winding x

3I0_W2 is used in autotransformer applications

Stabilizing current, Ibias The highest current among all

current inputs

125

Operating characteristicsRestricted earth fault protection

Chapter 04


Two fixed slopes defines as:

1st: 70% ; 2nd: 100%

End of slope 1: 125% of IBase

Setting: Minimum operation, Idmin In percent of IBase: 4.0 - 100.0

IBase

Rated current of the winding or CT rating

For autotransformers the highest winding rated current i.e. low voltage side

125

Operating characteristicsRestricted earth fault protection

Chapter 04


The bias current stabilizes the protection

Higher currents harder conditions higher probability that Idiff > Idmin due to: CT saturation

Ibias is defined as the maximum current of: 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) = INwhere the CTFactor = Irated_CT / Irated_W

default setting: 1.0 (1.0-10.0)

Operating characteristicsRestricted earth fault protection, Ibias

Chapter 04


The CTFactor : gives a possibility to adjust the sensitivity

does not influence curr(x-z)

Single breaker application Normally the CT ratio matches the winding and the

CTFactor = 1.0

Multiple breaker application Could be necessary to modify CTFactor

IBase = Irated_W and

Irated_CT > or >> Irated_W

curr(a and/or b) might stabilize to much due to transfer currents (fault or load currents)

Example: CTFactor = 1000/500 = 2.0

CT1 1000 A

500 A

CT2 1000 A

curr(a) = MAX [I3PW1CT1] / CTFactorPri1

curr(x) = MAX [I3PW1CT1 + I3PW1CT2]

curr(b) = MAX [I3PW1CT2] / CTFactorPri2

Example:

Operating characteristicsRestricted earth fault protection, Ibias

Chapter 04


The restricted earth fault protection may be very sensitive No room for any false unbalance currents

External faults may cause CT saturation false residual currents

false tripping

The stabilizing characteristics helps tostabilize to some extent

Objective: Positively distinguish between internal and

external earth faults

Enhancing the security for heavy external faults

IN , 3I0, IDiff

Operating characteristicsDirectional criterion

Chapter 04


Same direction in the neutral for both external and internal faults Current in the neutral serves as directional reference

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

Operating characteristicsDirectional criterion

Chapter 04


Internal fault The residual currents are approximately in phase

Not measuring the same primary current, IN and 3I0

Setting: Relay operate angle (ROA) 60 – 90o

Operating characteristics Directional criterion

Chapter 04


The directional check is executed if: within the operate area of the REF-

characteristic

3I0 (terminal side) is at least 3% of IBase

The trip is issued if: both 3I0 an IN are within the operating region

If the check is not executed (small currents) then: the check is not a condition for trip

Operating characteristics Directional criterion

Chapter 04


Prevent unwanted operation on reactor energizing due to terminal side CT saturation cased by

high DC component

The calculation adaptive (input: 3I0, IN, IBias and timing)

No setting: 2nd harmonic ratio: fixed to 60%

IN 3I0

Cur

rent

[pu]

Cycles

Shunt reactor switching150 Mvar 220 kV 50 Hz

Operating characteristics Second harmonic blocking

Chapter 04


Directionalcheck

TRIP

START

&

≥1

IN > 50% of Idmin&

3I0 < 3% of IBase

3I0 > 3% of Ibase

3I0

IN

Simplified logical diagramRestricted earth fault protection

Chapter 04


Residual current - W1 - CT1

Block input

Neutral point current

- W1 - CT2

Binary input signal

Analog input signals (SMAI group signal output)

- W2 - CT1- W2 - CT2

In- and output signals Restricted earth fault protection

Chapter 04


TripStartDirectionality: High Internal faultSecond harmonic blockingResidual current W1+W2 (3I0)

Binary output signals

Analog output signals HMI and Disturbance recorder

Neutral current (IN) Bias currentDifferential currentAngle from zero sequence feature Second harmonic ratio

In- and output signals Restricted earth fault protection

Chapter 04


Base valuesOperation On/Off

Pick up levelFactors influencing bias

Restricted earth fault protection settingsOverview

Chapter 04


Tests/Functions status/Differential Protection/LowImpREF(PDIF,87N) TRIP

START

DIROK – Internal fault

BLK2H – Block due to 2nd harmonic

IRES – Magnitude of residual current (3I0, A)

IN – Magnitude of neutral current (A)

IBIAS – Magnitude of bias current (A)

IDIFF – Magnitude of differential current (A)

ANGLE – Direction angle from zero sequence feature (deg)

I2RATIO – Second harmonic ratio

Monitored dataRestricted earth fault protection

03 sep674 ret670 differential protection

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