Copyright © SEL 2016

Applying SEL Differential RelaysIntroduction to Transformer

Differential Protection

• Explain challenges of transformer differential protection • Understand need for tap, phase, and zero-sequence

compensation and how they work• Understand how transformer differential relays are

made secure for inrush and overexcitation

Transformer DifferentialProtection Presentation Objectives

Differential Protection is Easy in Theory

Kirchhoff’s Current Law (KCL):n

kk 1

I 0

1

2 3

• Current magnitude mismatch • Phase shift across transformer• Zero-sequence sources• Energization inrush• Overexcitation• Unequal CT performance

Challenges to TransformerDifferential Protection

Differential Protection Principle

Differential Protection Principle

Current Magnitude and Phase Angle Difference Compensation

ANSI Transformer ConnectionsDABY or Dy1

1 2a b 2 1

b c 2 1

c a 2 1

a

b

c

a

A

ANSI Transformer ConnectionsYDAC or Yd1

Secondary Currents

SECSEC

Result of Uncompensated Phase Shift

Traditional Compensation

CTR1 and CTR2 Ideal Relationship

Not always possible with standard CT ratios

2

1 1 2

11 2

2

12 1

2

N 1 1N CTR CTRN CTR CTRN

kVCTR 3 CTRkV

• EM relays provide limited magnitude compensation• SEL relays provide magnitude and phase-shift

compensation (SEL-387 considers not only ANSI / IEEE standard connections, but all possible connections)

Compensation With Relays

• Current magnitude is transformed (transformer is constant KVA device) kVH • Ihigh-side = kVL • Ilow-side

Ihigh-side ≠ Ilow-side

• Load tap changer dynamically changes transformation ratio

Current Magnitude Mismatch

Tap Compensation

WDG

MVA •1000 • CTAP3 • kV • CTR

where:C = 1 for wye-connected CTsC = for delta-connected CTs3

Wye Connection Compensation

1TAP1

1TAP1

1TAP1

IAW1I1W1CTAP1

IBW1I2W1CTAP1

ICW1I3W1CTAP1

DAB Connection Compensation

1TAP2

1TAP2

1TAP2

1 (IAW2 IBW2)I1W2C •TAP2 3

1 (IBW2 ICW2)I2W2C •TAP2 3

1 (ICW2 IAW2)I3W2C •TAP2 3

DAC Connection Compensation

1TAP2

1TAP2

1TAP2

1 (IAW2 ICW2)I1W2C •TAP2 3

1 (IBW2 IAW2)I2W2C •TAP2 3

1 (ICW2 IBW2)I3W2C •TAP2 3

Compensation for Zero-Sequence Currents

Why Eliminate Zero-Sequence Current?

Removal Via Delta Connection

Restrained Differential Element

CT saturation and CT ratio error

Differential Protection PrincipleUnequal CT Performance

Percentage Differential Protection Principle

1 2

1 2IOP I I

1 2IRT 0.5 • I I

Differential Element Operate and Restraint Quantities

1 2I I

1 2I I2

1TAP1

1TAP2

Percentage Restraint Differential Characteristic

Percentage Restraint Differential Characteristic

Transformer Energization

C-Phase Inrush Current

Inrush Current

Cycles

Cycles1 2 3 4 5 6 7

1 2 3 4 5 6 7Prim

ary

Cur

rent

(A)

0

50

100P

erce

ntag

e of

Fund

amen

tal

0

20

40

60

80

Fundamental Frequency Magnitude

Second-Harmonic Magnitude

Second-Harmonic Percentage

Second-Harmonic Block Threshold

• Harmonic blocking• Harmonic restraint• DC blocking

Discriminating Internal FaultsVersus Inrush Conditions

DC Blocking

MinMax

DC Blocking

+

+

–

+

–

DC Blocking

+ +

–

+

–

Inrush Conditions (Blocking)

Inrush Conditions (Restraint)

Unrestrained Differential Element

• Element instantaneously trips on magnitude of differential current No percentage restraint

No harmonic block

• CT saturation includes harmonics

Unrestrained Element

Transformer Overexcitation

Excitation Current From Testing150 Percent Overvoltage

Prim

ary

Cur

rent

(A)

Cycles

0.5 1 1.5 2 2.5 3–60

–40

–20

0

20

40

60

0

Fifth-Harmonic Content in Excitation Current

Frequency Component

Magnitude (primary A) Percent of Nominal

Fundamental 22.5 52.0

3rd 11.1 26.0

5th 4.9 11.0

7th 1.8 4.0

Excitation Current Harmonics

0

10

20

30

40

50

60

70

80

100 110 120 130 140 150 160

Application Considerations

Paralleling CTs on Restraint Input

Paralleling CTs on Restraint Input

Paralleling Transformers Sympathetic Inrush

• Know that third winding may not be rated at same MVA as main windings

• Set taps on multirestraint relay Work through matching taps in pairs

Be sure to use same MVA base for each tap calculation

Setting Taps for Three-Winding Transformers

Restricted Earth Fault (REF) Protection

Protect Windings Close to Neutral

Protection Basics – Why Use REF Protection?

• Thermal model • Through-fault monitoring

Prevention Before Protection

What Causes Transformer Overheating?

What Does Overheating Do?

Prevention Basics –Control Fan Banks

Ambient Temperature

Top-Oil Temperature

Temperature Data

Cooling Stage Control

Thermal Model

Why Enable Through-Fault Monitoring?

Track Through Faults Transformer Has

Experienced

Thermal Model

Prevention Basics –Enable Through-Fault Monitoring

Questions?