transformer protection open lecture

7/27/2019 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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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' = Ia' / 2IB' = Ib' / 2IC' = Ic' / 2





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Compensation (2)

Change in

CT

Ratio





There must be an easier way..


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Transformer Basics

100MVAIN

100MVAOUT


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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


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


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


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.)


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



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Assume 1:1 transformer

a

b

c

AB

C


US Standard Yd Example:H1 (a) leads X1 (A) by 30Currents on X bushings are delta quantities


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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



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Typical Transformer Inrush Waveform

2nd and 4thHarmonicsDuringInrush



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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



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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)



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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



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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



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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



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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



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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



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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



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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



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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



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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)



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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


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



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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 Differential


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0

90

180

270

IG

-3I O


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



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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



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Xside Over Current Elements: Negative sequence over

current used to protect against unbalanced loads & open conductors

Easy to coordinate46




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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




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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


Overexcitation Curve


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This is typically how the apparatus manufacturer specs it



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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

transformer protection open lecture

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