Transformer Protection - IEEE · Transformer Cooling System • Contact inputs indicate active cooling system status • Thermal model selects constants for three cooling systems
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• Maximum asymmetrical fault current for X/R = 12: IFmax = 20 / 0.25 • (12 + 1) = 6.15 pu = 12.3 kA
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IF ZB(1 + X/R) = 20
IF ZB(1 + X/R) = 50
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Common CT Connections
Wye Delta
Ia
Ib
Ic
Ia
Ib
Ic
Ias Ibs Ics IresIas – IcsIcs – Ibs
Ibs – Ias
Effective Burden Depends on CT Connections and Fault Types
CT Connection
Effective Burden Impedance (ZB) for Different Types of Faults
Three Phase or Phase to Phase
Phase to Ground
Wye ZLEADS + ZDEVICE 2 ZLEADS + ZDEVICE
Delta 3 (ZLEADS + ZDEVICE) 2 (ZLEADS + ZDEVICE)
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Select CT That Will Not Saturate
• Know maximum available symmetrical fault current (use VS ≤ VSTD and IFZB ≤ 20 to verify no saturation)
• Determine X/R ratio and worst-case asymmetrical fault (use IFZB (X/R + 1) ≤ 20 to determine CT will not saturation under asymmetrical fault conditions)
Determine Maximum Emergency Rating of Transformer
• Calculate full load rating (FLA) of transformer
• Ensure CTR matches FLA as closely as possible
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DABY or DY1 Transformer Connection
YDAC or YD1 Transformer Connection
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Traditional Compensation a b 2 1I – I N / N
b c 2 1I – I N / N
c a 2 1I – I N / N
aI
bI
cI
2
1 1 2
N 1 1
N CTR CTR
b c 2I – I / CTR
c a 2I – I / CTR
a c 2I – I / CTR
c a 2 1 1I – I N / N / CTR
b c 2 1 1I – I N / N / CTR
a b 2 1 1I – I N / N / CTR
Compensation With Digital RelaysCurrent Scaling and Phase-Shift
Compensation Are Internal
• Exact current scaling
• Phase-shift compensation for all transformer connections
• Allowed wye-CT connection
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Current Scaling With Digital Relays
Digital relays can fully compensate for current amplitude differences
Digital Relays Allow Connection of CTs in Wye
Winding 2Winding 1
X2
X3
X1a
b
cH3
H2
H1A
B
C
ICW1
IBW1
IAW1
ICW2
IBW2
IAW2
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Current Scaling and Phase-Shift Compensation
1
1
TAP 2
1
TAP1
1
TAP
Zero-Sequence Current for an External Fault
Delta compensation removes
zero-sequence current
87
Zero Sequence
Negative Sequence
Positive Sequence
S1ZT1Z
S2ZT2Z
T0ZS0Z
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Zero-Sequence Current RemovalTraditional Relays
Auxiliary CTs connected aszero-sequence trap
87
Zero-Sequence Current RemovalDigital Relay
1
1
TAP 2
1
TAP
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Differential Current Caused by Magnetizing Inrush, Overexcitation,
and CT Saturation
Magnetizing Inrush Current Obtained From Transformer Testing
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Inrush Current Harmonic Content
Harmonic-Based Methods in a Relay With Three Differential Elements
• Independent harmonic restraint
• Independent harmonic blocking
• Common harmonic blocking
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Harmonic-Based Method Comparison
FeatureIndependent Even-Harmonic Restraint
Common Even-Harmonic Blocking
Security for external faults
High High
Security for inrush High High
Dependability High High
Speed for internal faults Lower Higher
Speed for internal faults during energization
Higher Lower
Slope characteristicAdaptive
(harmonic dependent)Fixed
(harmonic independent)
Combined Harmonic Blocking and Restraint for Optimal Protection
• Faults during inrush conditions
• Faults during normal conditions
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Harmonic Restraint Mode
• Operation conditions
♦ IOP > IPU
♦ IOP > SLP IRT + K2I2 + K4I4
• Blocking condition (K5I5 > IOP)
Application Considerations
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Selection of Characteristic Settings
• Minimum pickup: constant differential current
• Slope 1: proportional differential current
• Slope 2: CT saturation
Constant and Proportional Differential Currents
• Constant
♦ Exciting current (1 to 4% of rated current)
♦ Unmonitored load in protection zone
• Proportional
♦ Tap mismatch: 0% in digital relays
♦ Tap changers: NLTC ±5%; LTC ±10%
♦ Linear CT errors: ≤3%
♦ Relay errors: ±5%
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DO NOT DELETE
Combined Transformer
Bus and Feeder
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Results of Repeated Faults and Mechanical Stresses
Transformer Overcurrent and Mechanical Protection
• Apply overcurrent protection for through-fault damage to transformers
• Review IEEE thermal model
• Understand how sudden pressure relays provide sensitive protection for turn-to-turn faults and how to apply them
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Overcurrent Protection
• Possible primary protection for small transformers
• Backup of primary protection (87 and 63)
• Backup protection for faults in adjacent protection zones (trip transformer before it is damaged)
Transformer Damage Curves
• Infrequent fault incident curve (fewer than 5 faults in life of transformer)
• Use infrequent fault curve
♦ For faults in zones that are cleared by high-speed protection
♦ For systems without overhead lines
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Category IV
Above 10,000 KVA –single phase
Above 30,000 KVA –three phase
Source: IEEE Std. C57.12.00-2010, IEEE Standard for Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
IEEE Standard C57.91-2011 Guide for Loading Mineral
Oil-Immersed Transformers
• Top-oil temperature
• Hottest-spot temperature
• Loss of life
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Transformer Cooling System
• Contact inputs indicate active cooling system status
• Thermal model selects constants for three cooling systems