differential protection principle differential overcurrent

36th Hands‐On Relay School 3/13/2019

Transmission Track 1

Differential Protection Principle

Differential Overcurrent Protection



Unequal CT Saturation

Problem of Unequal CT Performance

• CT saturation causes error differential current for external faults

• Resulting high element pickup limits sensitivity



Compares IOP with IRT:

Percentage Differential Protection

1 2OPI I I 1 2RTI k I I

1 2RTI k I I 1 2Max ,RTI I I

Operation: IOP > IPU and IOP > SLP IRT

Percentage Differential Single-Slope Characteristic



Percentage Differential Dual-Slope Characteristic

Adaptive Percentage Differential Characteristic



Challenges of TransformerDifferential Protection

• CT saturation

• Current magnitude mismatch

• Current phase shift across the transformer

• Zero-sequence current sources

• Magnetizing inrush currents

• High excitation current on overexcitation

Transformer Connections



Common Two-Winding Transformer Connections

DABY or Dy1 Transformer Connection

2 1 2 13 / 30a b aI I N N I N N



YDAC or Yd1 Transformer Connection

1 2 1 2 303

A CA

I II N N N N

Current Scaling and Phase-Shift Compensation



Traditional Compensation

Traditional Compensation Is Not Perfect

Desired CT ratio relationship:

• Not always possible with standard CT ratios

• Electromechanical relays have taps

• Compensation is not perfect

2

1 1 2

1 1N

N CTR CTR

1 12 1 1

2 2

3N V

CTR CTR CTRN V



Mismatch Calculation for Electromechanical Relays

1 1

2 2

1 1

2 2

| || |

Mismatch •100

| |min ,

| |

I TAPI TAP

MR

I TAPMR

I TAP

Compensation With Digital Relays

Digital relays carry out current scaling and phase-shift compensation internally

– Exact current scaling

– Phase-shift compensation for all transformer connections

– Wye CT connection is allowed



Digital relays can fully compensate for the current amplitude differences

Current Scaling With Digital Relays

1,000

3 MAX n

nn n

S CTAP

V CTR

Digital Relays Allow Connection of CTs in Wye



Current Scaling and Phase-Shift Compensation

DAB Connection Compensation

1 2 1 1 0 21

2 2 0 1 1 23

3 2 1 0 1 2

I W C IAW T

I W C IBW T

I W C ICW T



Wye Connection Compensation

1 1 1 0 0 1

2 1 0 1 0 1

3 1 0 0 1 1

I W C IAW T

I W C IBW T

I W C ICW T

Zero-Sequence Current Removal



Delta compensation removes zero-

sequence current

Zero-Sequence Current for an External Fault

Restricted Earth Fault (REF) Protection



Transformer Currents for Internal Ground Faults

REF Protection for a Two-Winding Transformer

*Re •X YT I I



Overexcitation Protection

Overexcitation Protection

• Overexcitation occurs when V/f exceeds 1.05 (full load) or 1.1 (no load)

• Causes heating, high exciting current, vibration, and noise

• Volts/hertz (24) protection should trip the transformer

• NOM

NOM

fV

f V

differential protection principle differential overcurrent

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