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5/29/2015
Schweitzer Engineering Laboratories 1
Copyright © SEL 2015
IEEE SF Power and Energy SocietyMay 29, 2015
Transformer Protection
Ali Kazemi, PERegional Technical Manger
Schweitzer Engineering LaboratoriesIrvine, CA
Sources of Transformer Stresses
• Thermal cycling
• Vibration
• Local heating due to magnetic flux
• Impact due to through-fault current
• Heating due to overload or inadequate cooling
Source: IEEE Std. C37.91-2008, IEEE Guide for Protecting Power Transformers
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Transformer Failure Statistics1983–1988
• Winding failures 37%
• Tap changer failures 22%
• Bushing failures 11%
• Terminal board failures 3%
• Core failures 1%
• Miscellaneous failures 26%
Source: IEEE Std. C37.91-2008, IEEE Guide for Protecting Power Transformers
Transformer Protection
• Differential Protection
• Current Transformer Performance
• Through Fault Protection
• Mechanical Protection
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Design Considerations for Transformer Differential Protection
• CT ratio and CT voltage class selection
• CT connections
• Current phase shifts across transformer
• Inrush detection
• Differential pickup settings
• Zero-sequence currents
• Slope
• High excitation currents
Design Considerations for Transformer Backup Protection
• Overcurrent
• Directional overcurrent
• External faults
• Sudden pressure
• Hot spots
• Loss of coolers
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Differential Protection Overcurrent
50
Protected Equipment
IOP = 0
CT CT
Normal Load
Balanced CT Ratio
Differential Protection
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Common Problems With Differential Protection
• False differential current can occur if CT saturates during through fault
• Some measure of through current can be used to desensitize relay when high currents are present
50
Protected Equipment
IOP ≠ 0
CT CT
External Fault
Differential Current and CT Saturation
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Possible Solution Percent Slope Differential
Protected Equipment
IR
CTR CTR
Relay (87)
IS
IRPISP
Compares:OP S R
S RRT
I I I
I Ik • I k •
2
Dual-Slope Differential Digital Relay
IOP
IR1 IR2 IR3 IRT
Minimum Pickup, IPU
Unrestrained Pickup, IHS
Operating Region
Slope 2
Slope 1 Restraining Region
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Adaptive Slope for Security
Slope 1Internal Fault
IRT
IOP
Slope 2External Fault
Restrain
Operate
87P
Differential Protection Summary
• Overcurrent differential scheme is simple and economical but does not respond well to unequal CT performance
• Percentage differential scheme responds better to CT saturation
• Differential principle provides best protection selectivity and speed
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Current Transformers
Current Transformer (CT) Principle
• CT isolates relay from the HV system
• Drastically reduces current
Ideally: is = ip / Ns
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Core and Secondary Winding Example
The Current Transformer Equivalent Circuit
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Induced Secondary Voltage
• Assuming the CT is not saturated, and magnetic flux density (B) is sinusoidal:
• Induced secondary voltage is approximately:
• Note: If Bmax, Ns, and f are fixed, the only way to obtain larger induced voltages is to make A larger. This implies a larger iron core.
max max2
4.44 2
s s sf
V N AB f N AB
max maxsin sin B B t BA AB t
Excitation Curve for a C400 Multiratio CT
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Core-Balance Current Transformer
Relay
Is
a
b
c
Shield
Ia
Ib
Ic
IN
Relay
Core-Balance CTWhich Photo Shows Correct Installation?
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CT Burden Calculation
IP
IS VS+ –
ZLEADS
ZDEVICE
CT Terminal Voltage
S S B S LEADS DEVICEV I Z I Z Z
ANSI Standard Terminal Voltage Rating
• Defines minimum CT terminal voltage for
♦ 20 times nominal current
♦ Standard burden
♦ <10% ratio error
• Applies to full winding
• Using CT taps reduces accuracy
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C Class Terminal Voltage Rating
VSTD = 20 IS RATED ZB STD
For IS RATED = 5 A secondary
C Class ZB STD (Ω) VSTD (V)
C100 1 100
C200 2 200
C400 4 400
C800 8 800
Avoiding CT Saturation for Asymmetrical Faults2
1.5
1
0.5
0
–0.5
–10 0.02 0.04 0.06 0.08 0.1
Time (s)
v S
R– t
LS F BV 2I Z e – cos t
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Avoiding CT Saturation for Asymmetrical Faults
IF = per-unit fault current
ZB = per-unit burden
Predicting CT Saturation in Asymmetrical Faults
• C400, 2000/5 CT with 1 burden
♦ ZB = 1
♦ ZB STD = 4
• 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
♦ Oil-air (OA)
♦ Forced-air cooled (FA)
♦ Forced oil-air (FOA)
Transformer Thermal Monitoring Optimizes Operation
• Transformer protection
• System operation
• System planning
• Capital investment
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Mechanical Protection
Sudden Pressure Relay (ANSI 63)
• Gas space (sudden pressure)
• Under oil (fault pressure)
Qualitrol®
Rapid Pressure Rise Relay (Under-Oil Type)
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Combine Relays for Best Transformer Protection
• Differential relay is primary protection for most faults in tank and bus work
• Sudden pressure relay is primary protection for turn-to-turn faults and backup 87 for large faults inside tank
• Overcurrent relays are primary protection for through-fault damage and provide backup for faults in tank and bus work
Questions?
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