View
234
Download
0
Category
Preview:
Citation preview
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
• Current magnitude mismatch • Phase shift across transformer• Zero-sequence sources• Energization inrush• Overexcitation• Unequal CT performance
Challenges to TransformerDifferential Protection
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
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
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
• Element instantaneously trips on magnitude of differential current No percentage restraint
No harmonic block
• CT saturation includes harmonics
Unrestrained Element
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
• 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
Prevention Basics –Control Fan Banks
Ambient Temperature
Top-Oil Temperature
Temperature Data
Cooling Stage Control
Thermal Model
Recommended