59270197 Module 3 Commissioning Instrument Transformers

7/27/2019 59270197 Module 3 Commissioning Instrument Transformers

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10/5/2013 Module Three 1

SUBSTATIONCOMMISSIONING COURSE

MODULE THREE

INSTRUMENT TRANSFORMER

TESTING PROCEDURES


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INTRODUCTION

The electrical commissioning andacceptance testing of electric

power systems is essential for theenergization of any electrical

system for the first time.


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INTRODUCTION

Module Three

Discuss theory and ratings

Safety issues Test equipment operation &

hands on testing


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INTRODUCTION

Skills learned in MODULE THREE:

Acceptance testing for CTs and VTs

Operate CT tester Apply safe work practices Analyze test values Complete test forms


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INSTRUMENT TRANSFORMERS

A general classification

Current transformers (CT) Voltage transformers (VT)

Isolate primary quantitiesand convert into useable

secondary values via

transformation

Standard secondary rating:

5A for CT 120V for VT < 24 kV 115V for VT > 24 kV

Measurement values from

CTs and VTs:

Voltage, current,

frequency, real / reactivepower, energy, power

factor, impedance


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Polarity markings

A designation of the relative instantaneous direction ofcurrents

Current flowing into the polarity mark in the primary isrelatively in phase with current flowing out the polarity

mark in the secondary

As if the primary and secondary are connected straightthrough


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Manufacturers name ortrademark

Manufacturers type Serial number Rated Voltage, maximum

voltage or nominalsystem voltage

Rated frequency

Basic impulse insulationlevel (N/A for bushing

CTs) Continuous rating factorfor CT (1.0 or more)

Thermal burden rating forVT (VA)

Accuracy rating

Nameplate Data


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Ratio expression betweenwinding groups

Example: VT with oneprimary and onesecondary winding

14,400 : 120 VRatio 120 : 1

Ratio between primary

and secondary amperes

Example: CT with one

primary and one

secondary winding

100 : 5 A

Ratio 20 : 1

Nameplate Symbols (: colon)


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Voltage rating or ratios

between winding groups

Example: VT with two

windings for series or

parallel connection

2,400 x 4800 V

Ratio 20 x 40 : 1

Current rating with two or

more coils for series of

parallel connection

Example: CT with two

primary windings for

series or parallel conn.

100 x 200 : 5 A

Nameplate Symbols (x multiplication sign)


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(Not used for VT) Ampere rating ofseparate secondary

winding with independentcore

Example: CT with twosecondary windings and

two cores.

100 : 5// 5A

Nameplate Symbols (// double slant lines)


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Voltage rating or ratios ofseparate winding on one

core.

Example: VT with oneprimary and twosecondary winding

14,400 V: 120 & 72 V

Ratio 120 & 200:1

Ampere rating ofseparate primary winding

on one core.

Example 1: CT with twoprimary windingsdesigned to be used

individually.

100 & 600 : 5A

Nameplate Symbols (& ampersand)


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Example 2: Totalizing CT

with four primary

windings designed to beused simultaneously.

5 & 5 & 5 & 5 : 5A



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Example 3: CT for three-

wire single phase.

100 & 100 : 5A



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Two or more primary orsecondary ratingsobtained by taps in the

secondary winding

Example 1: VT withsecondary tap for moreprimary voltage rating

12,000/14,400 :120 V

Ratio 100/120 : 1

Different primary currentratings obtained by tapsin the secondary winding

Example: CT with taps inthe secondary foradditional ratios

100 : 5// 5A

Nameplate Symbols (/ single slant line)


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Example 2: VT with

secondary tap for

additional secondaryvoltage ratings

14,400 : 120/72 V

Ratio 120/200:1

Nameplate Symbols (/ single slant line)


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CSA Standards CAN/CSA-C60044-1 to -8 (part 1 to part 8), 2007

Part 1: Current Transformers Part 2: Inductive Voltage transformers

Part 3: Combined Transformers Part 4: N/A Part 5: Capacitive Voltage transformers Part 6: Requirements for Protective Current

Transformers for Transient Performance Part 7: Electronic Voltage Transformers Part 8: Electronic Current Transformers


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What is a CT ?

A device that isolates and transforms the primary currentquantity into a proportional secondary current quantity

The CT has: Primary winding or primary conductor Magnetic core

Secondary winding


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Distributed uniformwinding with negligible

Leakage Flux

Non-uniform distributedwinding with some

Leakage Flux


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How does a CT work ?

Primary current flows generates magnetic field aroundthe conductor

Magnetic field flows in the magnetic core Magnetic field induces a voltage in the secondary

winding

Burden connected to secondary winding completes the

circuit and allows current to flow


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Ipri x Npri = Isec x Nsec

Ipri = Current flowing in primary winding

Isec = Current generated in secondary windingNpri = Number of turns in primary winding

Nsec = Number of turns in secondary winding


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What is the secondary current ?

CT ratio: 5000:5A; 1000:1

Iprimary = 1000A

Isec = 1000A / (ratio) = 1000A /1000

= 1A

In reality, there will slightly less than 1A flowing.

The difference is the known as the Ratio Error in %


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Never open the secondary of a CT when

pr imary cu rrent is f low ing because

dangerous high voltages will appear.

IEEE C57.13 allow open c ircu it crest

vo l tages up to 3500 V peak w ithou t

vo l tage lim it ing devices.


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CT Types

Torroidal Wound Primary

Bar Type Split Core

Tororoidal

Low flux leakage Typical of bushing and

window CTs Primary conductor passthrough center

600V insulation can beused in MV application

with insulated primaryconductors


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CT Types

Torroidal (donut) Wound Primary

Bar Type Split Core

Wound Primary

Has an internal primarywinding

Wound more than 1

full turn on the core Core can be torroidal Must be inserted in series

with the conductors

Bar provides primaryconnection points, oftenmistaken for bar type


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CT Types


Bar Type Split Core

Bar Type

Has a continuous bar forthe primary winding

Only 1 turn primary Core can be torroidal Must be inserted in series

with the conductors

Primary insulation part ofCT construction


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CT Types


Bar Type Split Core

Split Core

Core is separated intotwo parts

For use where primaryconductor cannot be

easily removed


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CT Ratio

Ratio or primary current to secondarycurrent

Expressed as current quantities 1000 : 5 A

Expressed as ratio numbers 200 : 1


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CT Standard Burden @ 5A, @ 60 Hz


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Basis for Accuracy class for Metering

CTs and VTs correction factor is within specified limitsfor specified load conditions

.6 lagging to unity PF

CT operating between 10% to 100% rated current

VT operating between 90% to 100% rated voltage


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IT Standard Accuracy Class for Metering @ 60 Hz


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CT Accuracy Rating for Metering

Accuracy for a specified burdenExample: 0.3 B 1.8

.3% accuracy for all other burden up to 1.8

Example: 0.3 B @ 0.2

.3% accuracy for burden of .2 and 3


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CT Accuracy Rating for Relaying @ 60 Hz

Accuracy classification apply to full

winding unless specified


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Relaying CT Class

C (Calculated) T

X

Class C

Low leakage flux CTs Torroidal type

3% error @ 5A 10% error @ 100A


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Relaying CT Class

C T (Tested)

X

Class T

High leakage flux CTs Primary Wound type

% error obtained fromtesting

Tested to 100A or > until10% error is establish at

specified burdens Test curve plotted for IprVs Isec, the overcurrent

curve


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Relaying CT Class

C T

X (Defined)

Class X

User defined along withexpected accuracy rating

for protection application Minimum knee point

voltage; Vk

Maximum excitation

current Ik at Vk Maximum secondarywinding resistance at

75C, Rs


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Secondary Terminal Voltage

Saturation voltage level at20X rated current

100 A

Vsat = 100 A x Burden


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Saturation Curve

B-H CurveExcitation Current Vs

Excitation voltage

Knee Point voltage, Vk

10% Ve 50% Ie


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Transformer Formula

Erms = 2 f N a Bpeak / 2 4.44 f N a B

Where:

f = the supply frequency

N = number of turns

a = cross sectional area of the coreB = peak magnetic flux density


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Class C Saturation Curve

Plotted on log-log scale Induced voltage Y-axis

Excitation current X-axis Voltage begins at 1% Voltage ends at 5X Irated Vk identified where curve

is at 45 tangent axis


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Class T O/C Ratio Curve

Plotted of linear scale Secondary current Y-axis

Primary current X-axis 1 to 22 x rated I-primary

for all standard burdens

Curve identifies the

burden which causes50% ratio correction


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5 Groups of VT

All capable of operating at 110% for emergencycondition

Emergency for 1 minute Some have special emergency rating conditions All must operate within thermal limits and or specified

temperature rise


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Thermal Burden Rating

The maximum volt-ampere (VA) it can carry withintemperature design limits

Each winding can have its own rating One specified rating is for combined distribution

between the 2ndaries, including taps

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

Safety Consideration

Many testing involves using high voltages Should be done by trained qualified personnel

Safe work practice and procedures required

Review Module 2 IEEE Standard 510 1983,

Recommended Practice for Safety in HighVoltage & High Power Testing

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing

Primary InsulationResistance

Dielectric Withstand Secondary Insulation

Resistance

Secondary Burden

Saturation Ratio and Polarity

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing



Resistance

Secondary Burden


Primary Ins. Resistance Test

For Bar and Primary woundCTs

DC voltage test at the ratedcrest voltage on the primary

winding

Secondary grounded

1 minute test

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERSCT Insulation Resistance: Bar Type and Primary Wound

DC Voltage test

Isolate Primary

Isolate & Gnd Secondary Connect HV lead to H1/H2 Connect LV to Gnd /

2ndary

Test for 1 minute Record test value

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing



Resistance

Secondary Burden


Dielectric Withstand Test

For Bar and Primary woundCTs

AC voltage test at the 75%factory test voltage

Secondary grounded 1 minute test

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Dielectric Withstand: Bar Type and Primary Wound

AC Voltage test

Isolate & Gnd Secondary

Connect HV lead to H1/H2 Connect LV to Gnd /

2ndary

Test for 1 minute

Record test value


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TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Secondary Circuit Insulation Test

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing


Dielectric Withstand Secondary InsulationResistance

Secondary Burden


2ndary Burden Test

Conducted in 2 parts Part 1, 1A current injection

Measure circuit burden Part 2, four-wire resistancemeasurement

Measure secondary

winding resistance Burden = part 1 + part 2

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

Part 1: CT Secondary Circuit Burden Measurement


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TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

Total burden calculation

ZT = RCT + (RL + ZB)

WhereZT = Total burden ()

RCT = DC Winding resistance ()RL = Lead resistance ()ZB = Device impedance ()

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing



Secondary Burden Saturation Ratio and Polarity

Saturation Test

Secondary winding excitationtest

Primary open circuited Step voltage increase,current proportional

Test for knee-point voltage Rapid rise is I-excitation 10% V = 50% I

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Saturation test

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing




Ratio and Polarity test

Secondary winding excited at1V per turn

Primary open circuited H1-H2 compared to X1-X2 Phase measured, ratio

calculated

Phase difference 0 meanspolarity correct

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Ratio and Polarity Test

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Testing




Ratio and Polarity test (ALT)

Use individual meters, phaseangle and voltmeter

Secondary winding excited at1V per turn

Primary open circuited H1-H2 compared to X1-X2 Phase measured, ratio

calculated


TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

CT Ratio and Polarity Test (alternate connections)


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TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

VT Testing

Insulation Resistance Dielectric Withstand

Ratio and Polarity

Insulation Resistance Test

DC voltage test at the ratedcrest voltage

3 tests HV to Gnd HV to LV LV to Gnd


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TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

VT Testing


Ratio and Polarity

Dielectric Withstand Test

AC voltage test at the 75%factory test voltage

Secondary grounded 1 minute test

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

VT Dielectric Withstand Tests

TESTING INSTRUMENT


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TESTING INSTRUMENT

TRANSFORMERS

VT Testing


Ratio and Polarity

Ratio and Polarity test

Primary winding excited at1V per turns ratio

Secondary open circuited X1-X2 compared to H1-H2 Phase measured, ratio

calculated


Compare ratio withnameplate


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NETA STANDARD

5. NETA Acceptance Testing Procedure

5.1 Visual and Mechanical Inspection

5.2 Electrical Test 5.2.1 Current Transformer

5.2.2 Voltage Transformer

5.3 Test Value Analysis

5.3.1 Visual and Mechanical

5.3.2 CT Electrical Test

5.3.3 VT Electrical Test


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TEST SET MANUAL

Module 3: CT Tester

Module 2:

15 KV Insulation Resistance Test set AC High Potential Test set

DC High Potential Test set

Digital Low Resistance Test set

S O S


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TEST FORMS

Current Transformer Test Form

Voltage Transformer Test Form


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59270197 Module 3 Commissioning Instrument Transformers

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