Transformer Protection - IEEE Entity Web Hosting | A free …€¦ · · 2015-06-11Source: IEEE Std. C37.91-2008, IEEE Guide for Protecting Power Transformers Transformer Protection

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?

Documents

Transformer Protection - IEEE Entity Web Hosting | A free …€¦ · · 2015-06-11Source: IEEE Std. C37.91-2008, IEEE Guide for Protecting Power Transformers Transformer Protection