Transformer Protection Open Lecture
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Hands On Relay SchoolTransformer Protection Open Lecture
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Hands On Relay SchoolTransformer
Protection
Open
Lecture
Class Outline Transformer protection overview Review transformer connections Discuss challenges and methods of current
differential Protection
Discuss other protective elements used in transformer protection
Scott CooperEastern Regional Manager
Manta Test
Systems
[email protected](727)415-5843
204 37th Avenue North #281Saint Petersburg, FL 33704
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Transformer Protection Overview Transformer Protection Zones
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Types of ProtectionMechanical Protection
Analysis of Accumulated Gases Looks for arcing byproducts
Sudden Pressure Relays Orifice allows for normal thermal expansion/contraction. Arcing
causing pressure waves in oil or gas space overwhelming the orifice and actuating the relay.
Thermal Caused by overload, over excitation, harmonics and geo magnetically
induced currents Hot spot temperature Top Oil LTC Overheating
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Types of ProtectionRelay Protection
Internal Short Circuit
Phase: 87HS, 87T Ground: 87HS, 87T, 87GD
System Short Circuit Back Up Protection
Phase and Ground Faults Buses: 50, 50N, 51, 51N, 46 Lines: 50, 50N, 51, 51N, 46
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Types of ProtectionRelay Protection
Abnormal Operating Conditions Open Circuits: 46 Overexcitation: 24
Undervoltage: 27 Abnormal Frequency: 81U Breaker Failure: 50BF, 50BFN
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Phase DifferentialOverview
What goes into a unit comes out of a unit
Kirchoffs Law: The sum of the currents entering and leaving a junction is (should be) zero
Straight forward concept, but not that simple in practice with transformers
UNITI
1I
2
I3
I 1 + I 2 + I 3 = 0
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Phase DifferentialOverview
A host of issues presents itself to decrease security and reliability of transformer differential protection
CT ratio caused current mismatch Transformation ratio caused current mismatch (fixed taps) LTC induced current mismatch Delta wye transformation of currents
Vector group and current derivation issues Zero sequence current elimination for external ground faults on wye windings Inrush phenomena and its resultant current mismatch Harmonic content availability during inrush period due to point on wave
switching (especially with newer transformers)
Over excitation
phenomena and
its
resultant
current
mismatch Internal ground fault sensitivity concerns
Switch onto fault concerns CT saturation, remnance and tolerance
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Compensation (2)
Change in
CT
Ratio 1:1, Y-Y
1:1, 3Y4:1, 3Y
IA, IB, IC Ia, Ib, Ic
IA'*4 = Ia'IB' * 4 = Ib'IC' * 4 = Ic'
IA', IB', IC' Ia', Ib', Ic'
Phase DifferentialOverview Transformer Basics
Transformer Tap Calculation Per Unit Concept
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Compensation (3)Transformer Ratio 2:1, Y-Y
1:1, 3Y1:1, 3Y
IA, IB, IC Ia, Ib, Ic
IA' = Ia' / 2IB' = Ib' / 2IC' = Ic' / 2
IA', IB', IC' Ia', Ib', Ic'
Phase DifferentialOverview Transformer Basics
Transformer Tap Calculation Per Unit Concept
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Compensation (2)
Change in
CT
Ratio
IA, IB, IC Ia, Ib, Ic
IA', IB', IC' Ia', Ib', Ic'
Phase DifferentialOverview Transformer Basics
Transformer Tap Calculation Per Unit Concept
There must be an easier way..
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Transformer Tap Calculation Per Unit Concept
Phase DifferentialOverview
Transformer Basics
100MVAIN
100MVAOUT
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Transformer Tap Calculation Per Unit Concept
Phase DifferentialOverview
Transformer Basics
Each measured current is divided by the winding Tap. The
result is a percent of rating. These percent of ratings can becompared directly.
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AB connected delta wye transformer
Phase DifferentialOverview
Transformer Basics
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a
b
c -b
Subtracting Vectors: Subtract from reference phase vector theconnected non-polarity vectorin our example I a-Ib
Can be repeated for B & C, or you can assume 120 and 240displacement from A for B&C respectively
I b I c and I c I a would be the vectors
Phase DifferentialOverview
Transformer Basics
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AC connected delta wye transformer
Ia Ia
Ib Ib
IcIc
Ia
Ib
Ic
Ia-Ic
Ib-Ia
Ic-Ib
Ia
Ia-IcIb
Ic
Ib-Ia
Ic-Ib
Phase DifferentialOverview
Transformer Basics
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Angular Displacement Conventions: ANSI YY, @ 0; Y , Y @ X1 lags H1 by 30
ANSI makes life easy
Euro designations use 30 increments of LAG from the X1 bushing to the H1 bushings
Dy11=X1 lags H1 by 11*30 =330 or, H1 leads X1 by 30
Think of a clock each hour is 30 degrees0
6
39
8
7
10
11 1
2
5
4
Dy1 = X1 lags H1 by 1*30 = 30, orH1 leads X1 by 30 (ANSI std.)
Phase DifferentialOverview
Transformer Basics
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US Standard Dy Example: H1 (A) leads X1 (a) by 30 Currents on H bushings are delta quantities
a
b
c A
B
C
Assume 1:1 transformer
Phase DifferentialOverview Transformer Basics
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Assume 1:1 transformer
a
b
c
AB
C
Phase DifferentialOverview Transformer Basics
US Standard Yd Example:H1 (a) leads X1 (A) by 30Currents on X bushings are delta quantities
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Phase DifferentialOverview
Applied with variable percentage slopes to accommodate CT saturation and CT ratio errors
Applied with inrush and over excitation restraints
Set with at least a 20% pick up to accommodate CT performance
Class C CT; +/ 10% at 20X rated
If unit is LTC, add another +/ 10%
May not be sensitive enough for all faults (low level, ground faults near neutral)
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CT ratios and tap settings are selected to account for:
Transformer ratios If delta or wye connected CTs are
applied
Delta increases ratio by 1.73 Delta CTs must be used to filter zero
sequence current on all wye transformer windings
Dy transformer connections compensated by yd CT connections to make the currents apples to apples.
Phase DifferentialEM
Relay
Application
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Zero sequence elimination: In EM relays with wye connected transformers, delta connected CTs are used to remove the ground current.
Phase DifferentialEM Relay Application
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Settings compensate for the following: Transformer ratio CT ratio
Vector quantities Which vectors are used Where the 1.73 factor (3) is applied
When examining line to line quantities on delta connected transformer windings and CT windings
Zero sequence current filtering for wye windings so the differential quantities do not occur from external ground faults
Phase DifferentialDigital
Relay
Application
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Angular displacement (IEC and SEL) IEC (Euro) practice does not
have a standard like ANSI Most common connection is
Dy11 (low lead high by 30!) Obviously observation of
angular displacement is extremely important when paralleling transformers!
*1
*1
*2
*2
*1 = ANSI std. @ 0
*2 = ANSI std. @ X1 lag H1 by 30 ,or high lead low by 30
Phase DifferentialDigital Relay Application
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Digital Relay Application
All wye CTs shown, most can retrofit legacy delta CT applications
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Benefits of Wye CTs Phase segregated line currents
Individual line current oscillography Currents may be easily used for overcurrent
protection and metering Easier to commission and troubleshoot Zero sequence elimination performed by
calculation
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Zero sequence elimination: In digital relays with wye connected transformers and wye connected CTs, ground current must be removed from the differential calculation.
3I 0 = [I a + I b + I c]I0 = 1/3 *[I a + I b + I c]
Used where filtering is
required, such as wyewinding with wye CTs
Phase DifferentialDigital Relay Application
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Typical Transformer Inrush Waveform
2nd and 4thHarmonicsDuringInrush
Phase DifferentialDigital Relay Application
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Harmonically Restrained Differential Element Inrush Detection and Restraint
Inrush occurs on transformer energizing as the core magnetizes Sympathy inrush occurs from adjacent transformer(s) energizing, fault
removal, allowing the transformer to undergo a low level inrush Characterized by current into one winding of transformer, and not out
of the other winding(s) This causes the differential element to pickup Use inrush restraint to block differential element during inrush period
Phase DifferentialDigital Relay Application
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Inrush Detection and Restraint 2nd harmonic restraint has been employed for years Gap detection has also been employed As transformers are designed to closer tolerances, both 2nd harmonic
and low current gaps in waveform have decreased
If 2
nd
harmonic restraint
level
is
set
too
low,
differential
element
may
be blocked for internal faults with CT saturation (with associated harmonics generated)
Phase DifferentialDigital Relay Application
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Inrush Detection and Restraint 4 th harmonic is also generated during inrush Odd harmonics are not as prevalent as Even harmonics during inrush Odd harmonics more prevalent during CT saturation Use 4 th harmonic and 2nd harmonic together
M3310/M 3311 relays use RMS sum of the 2nd
and 4th
harmonic as inrush restraint Result: Improved security while not sacrificing reliability
Phase DifferentialDigital Relay Application
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0.5
1.0
1.5
2.0
0.5 1.0 1.5 2.0
87T Pick Up
87T Pick Upwith 5th Harmonic Restraint
Slope 1
Slope 2
Slope 2Breakpoint
TRIP
RESTRAIN
Phase DifferentialDigital Relay Application
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87T Pick Up Class C CTs, use 20%
LTC, add 10% Magnetizing losses, add 1% 0.3 to 0.4 pu typically setting
Slope 1 Used for low level currents Typically set for 25%
Slope 2 breakpoint
Typically set at 2X rated current This setting assumes that any current over 2X rated is a
through fault or internal fault, and is used to desensitize the element against unfaithful replication
Phase DifferentialDigital Relay Application
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Slope 2 Typically set at 70%
Inrush Restraint (2nd
and 4th
harmonic) Typically set from 15 20% Employ cross phase averaging blocking for security
Over excitation Restraint (5 th harmonic) Typically set at 30% Raise 87T pick up to 0.60 pu during overexcitation No cross phase averaging needed, as overexcitation is
symmetric on the phases
Phase DifferentialDigital Relay Application
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Unrestrained 87H Pick Up Typically set at 810pu rated current
This value should be above maximum possible inrush current and lower than the CT saturation current
C37.91, section 5.2.3, states 10pu an acceptable value Can use data captured from energizations to fine tune the
setting
Phase DifferentialDigital Relay Application
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CT Issues:
Remnance :
Residual magnetism
that
causes
dc
saturation
of
the
CTs
Saturation : Error signal resulting from too high a primary current combined with a large burden
Tolerance : Class C CTs are rated +/ 10% for currents x20 of nominal Thru faults and internal faults may reach those levels depending on ratio
selected
Phase DifferentialDigital Relay Application
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CT Issues (cont.) Best defense is to use high Class C voltage levels
C400, C800 These have superior characteristics against saturation and relay/wiring burden
Use low burden relays
Digital systems
are
typically
0.020
ohms
Use a variable percentage slope characteristic to desensitize the differential element when challenged by high currents that may cause replication errors
Phase DifferentialDigital Relay Application
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Point on Wave Considerations During Energization As most circuit breakers are ganged three pole, each phase is closed at a
different angle resulting in less harmonics on one phase and more on the others
Low levels of harmonics may not provide inrush restraint for affected phase security risk!
Most modern relays employ some kind of cross phase averaging scheme to compensate for this issue Provides security if any phase has low harmonic content during inrush or overexcitation This can occur depending on the voltage point on wave when the transformer is energized for a
given phase Cross phase averaging uses the average of harmonics on all three phases to determine level
Phase DifferentialDigital Relay Application
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Improved Ground Fault Sensitivity: 87T element is typically set with 20 40% pick up This is to accommodate Class C CT accuracy
during a fault plus the effects of LTCs That leaves 20 40% of the winding not covered for
a ground fault Employ a ground differential element to improve
sensitivity (87GD)
Phase DifferentialDigital Relay Application
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Switch onto Fault (cont): Employ 87HS to protect winding that is first energized 87HS is set above inrush current If fault is near the bushing end of the winding, the current will be higher
than inrush Typically 912 pu thru current
87HS does not employ harmonic restraint Fast tripping on high current faults
Phase DifferentialDigital Relay Application
d ff l
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Use 87GD I A + I B + I C = 3I 0
If fault is internal, opposite polarity
If fault is external, same polarity
IG
I A
IB
IC
Ground DifferentialDigital Relay Application
G d Diff i l
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IG
I A
IB
IC
IG
I A
IB
IC
Internal External
Ground DifferentialDigital Relay Application
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G d Diff ti l
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IG
I A
IB
IC
3I0IG
Residual currentcalculated fromindividual phasecurrents. ParalleledCTs shown toillustrate principle.
0
90
180
270
IG
-3I O
Ground DifferentialDigital Relay Application
Ground Differential
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0
90
180
270
IG
-3I O
Ground DifferentialDigital Relay Application
Other Transformer Protection
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Fuses Small transformers (
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Hside over current elements:
Protection against heavy prolonged through faults Transformer Category by nameplate capacity IEEE Std. C57.109 1985 Curves
Other Transformer ProtectionOver current Elements
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Th h F l
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Through Fault
Category 2
Th h F lt
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Through Fault
Category 3
Through Fault
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Through Fault
Category 4
Other Transformer Protection
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Xside Over Current Elements
Used to protect against un cleared faults downstream
of the transformer May consist of phase
and ground elements
Coordinated with line protection off the bus Failed Breaker
5151G
Over current Elements
Other Transformer Protection
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Xside Over Current Elements: Negative sequence over
current used to protect against unbalanced loads & open conductors
Easy to coordinate46
Over current Elements
Other Transformer Protection
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Overexcitation:
Responds to overfluxing; excessive v/Hz
Continuous operational limits ANSI C37.106 & C57.12
1.05 loaded, 1.10 unloaded
Inverse curves typically available for values over the continuous allowable maximum
Over current Elements
Other Transformer Protection
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Causes: Generating Plants
Excitation system runaway
Sudden loss of load Operational issues (reduced frequency)
Static starts Pumped hydro starting Rotor warming
Transmission Systems Voltage and Reactive Support Control Failures
Capacitor banks ON when they should be OFF Shunt reactors OFF when they should be ON Generator unit transformer connected to long line with
no load (Ferranti effect) Runaway LTCs
Over current Elements
Overexcitation Curve
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Overexcitation Curve
This is typically how the apparatus manufacturer specs it
Overexcitation Curve
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Overexcitation Curve
This is how protection engineers enter the v/Hz curve into a protective device
References:ANSI / IEEEC37 91 Guide for Protective Relay Applications for Power Transformers
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ANSI/ IEEEC37.91, Guide for Protective Relay Applications for Power TransformersANSI/IEEE C57.12, Standard General Requirements for Liquid Immersed Distribution, Power and Regulating TransformersProtective Relaying: Principals and applications, Third Edition By J. Lewis Blackburn and Thomas J. DominDigital Transformer Protection from Power Plants to Distribution Substations, CJ MozinaGeneral Electric Transformer Connections including Autotransformer Connections GET2J, Dec, 1970
87T
50
51 51G
High Side Low Side