Module 3 Commissioning Instrument Transformers

04/09/23 Module Three 1

SUBSTATION COMMISSIONING COURSE

MODULE THREE

INSTRUMENT TRANSFORMERTESTING PROCEDURES

INTRODUCTION

• The electrical commissioning and acceptance testing of electric power systems is essential for the energization of any electrical system for the first time.

INTRODUCTION

Module Three• Discuss theory and ratings• Safety issues• Test equipment operation &

hands on testing

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

INSTRUMENT TRANSFORMERS

• A general classification

• Current transformers (CT)• Voltage transformers (VT)

• Isolate primary quantities and 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 / reactive power, energy, power factor, impedance

Polarity markings

• A designation of the relative instantaneous direction of currents

• Current flowing into the polarity mark in the primary is relatively in phase with current flowing out the polarity mark in the secondary

• As if the primary and secondary are connected straight through

• Manufacturer’s name or trademark

• Manufacturer’s type• Serial number• Rated Voltage, maximum

voltage or nominal system voltage

• Rated frequency

• Basic impulse insulation level (N/A for bushing CT’s)

• Continuous rating factor for CT (1.0 or more)

• Thermal burden rating for VT (VA)

• Accuracy rating

Nameplate Data

Ratio expression between winding groups

Example: VT with one primary and one secondary winding

14,400 : 120 VRatio 120 : 1

Ratio between primary and secondary amperes

Example: CT with one primary and one secondary winding

100 : 5 ARatio 20 : 1

Nameplate Symbols (: colon)

Voltage rating or ratiosbetween winding groups

Example: VT with two windings for series or parallel connection

2,400 x 4800 VRatio 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)

(Not used for VT) Ampere rating of separate secondary winding with independent core

Example: CT with two secondary windings and two cores.

100 : 5 // 5A

Nameplate Symbols (// double slant lines)

Voltage rating or ratios of separate winding on one core.

Example: VT with one primary and two secondary winding

14,400 V: 120 & 72 VRatio 120 & 200:1

Ampere rating of separate primary winding on one core.

Example 1: CT with two primary windings designed to be used individually.

100 & 600 : 5A

Nameplate Symbols (& ampersand)

Example 2: Totalizing CT with four primary windings designed to be used simultaneously.

5 & 5 & 5 & 5 : 5A

Example 3: CT for three-wire single phase.

100 & 100 : 5A

Two or more primary or secondary ratings obtained by taps in the secondary winding

Example 1: VT with secondary tap for more primary voltage rating

12,000/14,400 :120 VRatio 100/120 : 1

Different primary current ratings obtained by taps in the secondary winding

Example: CT with taps in the secondary for additional ratios

100 : 5 // 5A

Nameplate Symbols (/ single slant line)

Example 2: VT with secondary tap for additional secondary voltage ratings

14,400 : 120/72 VRatio 120/200:1

Nameplate Symbols (/ single slant line)

North American Standards• CSA

– Originated in 2007– An adoption of IEC 60044 part 1 to part 7

• ANSI/IEEE– IEEE C57.13-2008 Standard Requirements for

Instrument Transformers – ANSI/IEEE C57.13.1-1981 Guide for Field Testing of

Relaying Current Transformers

• 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

What is a CT ?

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

• The CT has:– Primary winding or primary conductor– Magnetic core– Secondary winding

Distributed uniform winding with negligible Leakage Flux

Non-uniform distributedwinding with some Leakage Flux

How does a CT work ?

• Primary current flows generates magnetic field around the 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

Ipri x Npri = Isec x Nsec

Ipri = Current flowing in primary winding

Isec = Current generated in secondary winding

Npri = Number of turns in primary winding

Nsec = Number of turns in secondary winding

What is the secondary current ?

CT ratio: 5000:5A; 1000:1Iprimary = 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 %

“Never open the secondary of a CT when primary current is flowing because dangerous high voltages will appear”.

IEEE C57.13 allow open circuit crest voltages up to 3500 V peak without

voltage limiting devices.

CT Types

• Torroidal • Wound Primary• Bar Type• Split Core

Tororoidal

• Low flux leakage• Typical of bushing and

window CTs• Primary conductor pass

through center• 600V insulation can be

used in MV application with insulated primary conductors

CT Types

• Torroidal (donut)• Wound Primary• Bar Type• Split Core

Wound Primary• Has an internal primary

winding– Wound more than 1

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

with the conductors• Bar provides primary

connection points, often mistaken for bar type

CT Types

Bar Type

• Has a continuous bar for the primary winding

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

with the conductors• Primary insulation part of

CT construction

CT Types

Split Core

• Core is separated into two parts

• For use where primary conductor cannot be easily removed

CT Ratio

• Ratio or primary current to secondary current

– Expressed as current quantities• 1000 : 5 A

– Expressed as ratio numbers• 200 : 1

Adjusting CT Ratio (low V)• For some applications• KA = (KN ± NSA) / NP

KA = Actual ratio

KN = Nameplate ratio

NSA = Number of adjusted secondary turns adjusted

To Add: loop X1 lead through H1.

To Subtract: loop X1 opposite H1)

NP = Number of primary turns

CT Ratio Adjustment

• 20:5 CT needed• 50:5 CT available

• KA = (KN ± NSA) / NP = (10 + 2) / 3 = 4

or 4:1

• Actual CT Ratio = 20:5

CT Standard Burden @ 5A, @ 60 Hz

Basis for Accuracy class for Metering

• CTs and VTs correction factor is within specified limits for specified load conditions

• .6 lagging to unity PF

• CT operating between 10% to 100% rated current

• VT operating between 90% to 100% rated voltage

IT Standard Accuracy Class for Metering @ 60 Hz

CT Accuracy Rating for Metering

• Accuracy for a specified burdenExample: 0.3 B – 1.8.3% accuracy for all other burden up to 1.8Ω

Example: 0.3 B @ 0.2.3% accuracy for burden of .2 and 3Ω

CT Accuracy Rating for Relaying @ 60 Hz

Accuracy classification apply to full winding unless specified

Relaying CT Class

• C (Calculated)• T• X

Class C

• Low leakage flux CTs• Torroidal type• 3% error @ 5A• 10% error @ 100A

Relaying CT Class

• C• T (Tested)• X

Class T

• High leakage flux CTs• Primary Wound type• % error obtained from

testing• Tested to 100A or > until

10% error is establish at specified burdens

• Test curve plotted for Ipr Vs Isec, the overcurrent curve

Relaying CT Class

• C• T• X (Defined)

Class X

• User defined along with expected accuracy rating for protection application– Minimum knee point

voltage; Vk– Maximum excitation

current Ik at Vk– Maximum secondary

winding resistance at 75°C, Rs

Secondary Terminal Voltage

• Saturation voltage level at 20X rated current

• 100 A

• Vsat = 100 A x Burden

Saturation Curve

• B-H CurveExcitation Current Vs Excitation voltage

• Knee Point voltage, Vk

Δ 10% Ve → Δ 50% Ie

Transformer Formula

• Erms = 2 π f N a Bpeak / √2 ≈ 4.44 f N a B

Where: f = the supply frequencyN = number of turnsa = cross sectional area of the coreB = peak magnetic flux density

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

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 causes 50% ratio correction

What is a VT ?

• Isolate and transform the primary voltage into proportional secondary voltage

• Construction– Primary winding– Magnetic core– Secondary winding

5 Groups of VT

• All capable of operating at 110% for emergency condition

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

temperature rise

VT Accuracy Rating

• Accuracy rating for each standard burden– For maximum burden– For specified burden

Example: 0.3 Z; 0.3 Y, 0.6 Z and 1.2 ZZ; 0.3 Z @ Y

• Accuracy for two 2ndary or tapped 2ndary windings– Burden is the total for the VT– Accuracy class apply with the burden divided between

the 2ndaries outputs in any manner

Thermal Burden Rating

• The maximum volt-ampere (VA) it can carry within temperature design limits– Each winding can have its own rating– One specified rating is for combined distribution

between the 2ndaries, including taps

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 High Voltage & High Power Testing

CT Testing

• Primary Insulation Resistance

• Dielectric Withstand• Secondary Insulation

Resistance• Secondary Burden• Saturation• Ratio and Polarity

CT Testing

Primary Ins. Resistance Test

• For Bar and Primary wound CT’s

• DC voltage test at the rated crest voltage on the primary winding

• Secondary grounded• 1 minute test

CT 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

CT Testing

Dielectric Withstand Test • For Bar and Primary wound

CT’s• AC voltage test at the 75%

factory test voltage• Secondary grounded• 1 minute test

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

CT Testing

2ndary Ins. Resistance Test

• Test for secondary wiring• Secondary wiring isolated

and ungrounded• 1000 VDC• 1 minute test

CT Secondary Circuit Insulation Test

CT Testing

2ndary Burden Test

• Conducted in 2 parts• Part 1, 1A current injection

– Measure circuit burden• Part 2, four-wire resistance

measurement– Measure secondary

winding resistance• Burden = part 1 + part 2

Part 1: CT Secondary Circuit Burden Measurement

Part 2: CT Secondary Winding Resistance Measurement

Total burden calculation

ZT = RCT + (RL + ZB)

Where ZT = Total burden (Ω)

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

CT Testing

Saturation Test

• Secondary winding excitation test

• Primary open circuited• Step voltage increase,

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

CT Saturation test

CT Testing

Ratio and Polarity test

• Secondary winding excited at 1V per turn

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

calculated• Phase difference ≈ 0 means

polarity correct

CT Ratio and Polarity Test

CT Testing

Ratio and Polarity test (ALT)

• Use individual meters, phase angle and voltmeter

• Secondary winding excited at 1V per turn

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

calculated• Phase difference ≈ 0 means

polarity correct

CT Ratio and Polarity Test (alternate connections)

VT Testing

• Insulation Resistance• Dielectric Withstand• Ratio and Polarity

VT Testing

Insulation Resistance Test

• DC voltage test at the rated crest voltage

• 3 tests– HV to Gnd– HV to LV– LV to Gnd

VT Insulation Resistance Tests

VT Testing

Dielectric Withstand Test

• AC voltage test at the 75% factory test voltage

• Secondary grounded• 1 minute test

VT Dielectric Withstand Tests

VT Testing

Ratio and Polarity test

• Primary winding excited at 1V per turns ratio

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

calculated• Phase difference ≈ 0° means

polarity correct• Compare ratio with

nameplate

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

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

TEST FORMS

Current Transformer Test Form

Voltage Transformer Test Form

Module 3 Commissioning Instrument Transformers

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