Module 3 Commissioning Instrument Transformers

SUBSTATION COMMISSIONING COURSE of 58

SUBSTATION COMMISSIONINGCOURSE

MODULE THREE

COMMISSIONING INSTRUMENT TRANSFORMERS

Written by:Raymond Lee, Technical trainerCopyright ©2010

Electrical Industry Training Centre of Alberta4234 – 93 StreetEdmonton, Alberta, Canada

Phone: (780) 462-5729Fax: (780) 437-0248

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TABLE OF CONTENT

Headings Page

1. Instrument transformer......................................................................................61.1 Polarity and Terminal Markings..................................................................61.2 Nameplate Data..............................................................................................71.3: Symbols..........................................................................................................91.4 Instrument Transformer Standards...........................................................10

Canadian standards........................................................................................10US Standards......................................................................................................10

2. Current Transformer........................................................................................112.1 How Does a CT Work..................................................................................12

2.1.1 Secondary Winding-Induced Voltages................................................132.1.2Operation With Open Circuit Secondary............................................13

2.2 Types.............................................................................................................14Torroidal..........................................................................................................14Wound Primary..............................................................................................14Bar Type..........................................................................................................15Split Core.........................................................................................................15

2.3 Polarity..........................................................................................................152.4 Ratings...........................................................................................................15

2.4.1 Ratio........................................................................................................162.4.2 Ratio Adjustment (for low voltage applications)................................16

2.5 Standard Burden..........................................................................................182.6 Accuracy Classes for Metering...................................................................18

Basis for Accuracy Class................................................................................182.6.1 Standard Accuracy classes....................................................................192.6.2 Accuracy Rating for Metering..............................................................192.6.3 Type M (Metering) Accuracy Class.....................................................20

2.7 Accuracy Rating for Relaying.....................................................................21Class C.............................................................................................................21Class T.............................................................................................................21Class X.............................................................................................................212.7.1 Secondary Terminal Voltage................................................................222.7.2 Saturation Curve...................................................................................22

Class C saturation curve.............................................................................24Class T Overcurrent Ratio Curve.............................................................26

3. Voltage Transformer.........................................................................................263.1 Groups...........................................................................................................27

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TABLE OF CONTENT

Headings Page

Group 1............................................................................................................27Group 2............................................................................................................27Group 3............................................................................................................27Group 4............................................................................................................27Group 5............................................................................................................27

3.2.Overvoltage Requirements..........................................................................273.3 Accuracy Rating...........................................................................................283.4 Standard Burden..........................................................................................293.5 Thermal Burden Rating..............................................................................29

4. Electrical Testing of Instrument Transformers..............................................30Safety Consideration..........................................................................................304.1 CT Testing....................................................................................................30

4.1.1 Insulation Resistance Test.....................................................................304.1.2 Insulation Resistance Test Procedure..................................................324.1.4 Dielectric Withstand Test.....................................................................324.1.5 Dielectric Withstand Test Procedure...................................................344.1.6 Secondary Wiring Insulation Resistance Test....................................344.1.7 Secondary Circuit Insulation Resistance Test Procedure..................354.1.8 Secondary Circuit Burden Tests..........................................................364.1.9 Secondary Burden Test Procedure......................................................374.1.10 Saturation Test.....................................................................................384.1.11 Saturation Test Procedure..................................................................394.1.12 Ratio and Polarity Test.......................................................................404.1.13 Ratio and Polarity Test Procedure.....................................................404.1.14 Ratio and Polarity Test Procedure (Alternative)..............................414.1.15 Ratio and Polarity Test Procedure (Alternative)..........................42

4.2 VT Testing....................................................................................................434.2.1 Insulation Resistance Test.....................................................................434.2.2 Insulation Resistance Test Procedure..................................................434.2.3 Dielectric Withstand Test.....................................................................444.2.4 Dielectric Withstand Test Procedure...................................................454.2.5 Ratio and Polarity Test.........................................................................454.2.6 Ratio and Polarity Test Procedure.......................................................464.2.7 Ratio and Polarity Test Procedure (Alternative)................................464.2.8 Secondary Circuit Burden Test............................................................474.2.9 Secondary Circuit Burden Test Procedure.........................................47

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TABLE OF CONTENT

Headings Page

5. NETA Acceptance Testing Procedures........................................................495.1 Visual and Mechanical Inspection..............................................................495.2 Electrical Tests.............................................................................................50

5.2.1 Current Transformers..........................................................................505.2.2 Voltage Transformers...........................................................................51

5.3. Test Values Analysis................................................................................515.3.1 Visual and Mechanical.........................................................................51

5.3.2. CT Electrical Test.................................................................................525.3.3 VT Electrical Test..................................................................................52

6. Test Set Operational Instruction Manual.......................................................547. Instrument Transformer Test Forms..............................................................558. References and Suggested Reading..................................................................58

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This module will introduce the NETA acceptance testing procedures for the instrument transformers comprising of mechanical and visual inspections, electrical tests and test data analysis. The instruments transformers are the current transformers (CT) and the voltage transformers (VT), which are used for metering and protective relay applications.

An understanding on the theory of operations, functions, types, industry ratings and typical applications will be useful when performing acceptance testing. The discussion will be limited to the medium voltage application where potential transformers are directly connected to the primary system without the use of capacitive voltage divider and where current transformers are either individually mounted within a switchboard assembly or an integral part of the medium voltage apparatus.

By the end of this module the participants will have the basic skills to perform acceptance testing on instrument transformers, conduct visual and mechanical inspections, insulation resistance tests, dielectric withstand test, turns ratio tests, polarity tests, CT saturation tests, CT circuit burden tests, CT secondary winding resistance test and completing the inspection / test forms and conducting an assessment of the test data.

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1. Instrument transformerInstrument transformer is a general classification which includes the current transformers (CT) and the voltage transformers (VT). These devices are used to isolate the primary quantities and convert it into usable secondary values through transformation, which protect personnel and end devices from high voltages and currents. Instrument transformers are designed specifically for use with a broad category of electrical instruments such as protective relays, switchboard instruments and metering devices.

Standard North American secondary ratings operating at 60 Hz power frequency are:

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

Instrument transformers provide voltage and current quantities from which all other measurement values are derived for use in protection, metering and control system applications. Some of these are:

Frequency Power factor Active and reactive power (watts and vars) Energy

1.1 Polarity and Terminal MarkingsPolarity marking is a designation of the relative instantaneous direction of currents in its leads. Primary and secondary leads are said to have the same polarity at any instant the current enters the primary and leaves the secondary lead in the same direction as though the leads formed a continuous circuit. It is denoted by a white dot on the secondary terminals and a white dot or marking on the primary connection points.

Current flowing into the polarity mark of the primary winding is said to be relatively in phase with current flowing out of the polarity mark on the secondary terminal.

H1 is used to distinguish the leads or terminals connected to the primary windings for a 2 winding transformer. Odd numbers are used to denote polarity for mutiple winding transformer. X (also Y and Z, etc as per table 1 is used for multiple windings) is used to distinguish the leads connected to the secondary windings with the numeral 1 to denote polarity in phase with the primary leads or terminals.

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H1 and X1 are representative of the polarity marks.

Table 1: Instrument Transformer Polarity and Terminal MarkingsOne winding Tapped winding

Single primary winding H1 - H2Multiple primary winding H3 - H4

H5 - H6Single secondary winding X1 - X2 X1, X2, X3, etcMultiple secondary winding Y1 - Y2 Y1, Y2, Y3, etc

Z1 - Z2 Z1, Z2, Z3, etcU1 - U2 U1, U2, U3, etcW1 - W2 W1, W2, W3, etcV1 - V2 V1, V2, V3, etc

1.2 Nameplate DataNameplate data show the minimum requirement for equipment information.

Current transformer nameplate data shall include the following: Manufacturer’s name or trademark Manufacture’s type Manufacture’s serial number Rated Primary and secondary current Nominal system voltage (NSV) or maximum system voltage (MSV). None

are required for bushing CT’s Ratio or ratios Basic impulse insulation level (BIL kV). None for bushing CT’s. Rated frequency (Hz) Continuous thermal current rating factor (RF) Accuracy rating

1. Metering accuracy class at specified burdens and as a minimum at the burden for .3 accuracy class.

2. Relaying accuracy rating for transformers intended for relaying applications.

Voltage transformer nameplate data shall include the following: Manufacturer’s name or trademark Manufacture’s type Manufacture’s serial number, numerals only Rated voltage

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Ratio or ratios Basic impulse insulation level (BIL kV). Rated frequency (Hz) Thermal burden rating or rating at ambient temperature(s), in Volt-amperes

in degrees Celcius Accuracy rating: the maximum standard burden at the lowest accuracy rating

as a minimum.

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1.3: SymbolsTable 2: Instrument Transformer Nameplate SymbolsVoltage Transformer Current Transformer

:(colon)

Ratio expression to show ratio between primary and secondary voltages or between primary and tertiary voltages

o 14400:120Vo Ratio 120:1

Ratio between primary and secondary ampereso 600:5A

x(multiplication sign)

Voltage ratings or ratios for a primary or secondary winding having two or more coils for series or parallel connection

o 2400 x 4800Vo Ratio 20 x 40:1

Current ratings or ratios with a primary or secondary winding having two or more coils for series or parallel connections

o 100 x 200:5A

//(double slant line)

(Not used) Ampere rating of separate secondary winding each having an independent core

o 100:5//5A&(ampersand)

Voltage ratings or ratios of separate secondary windings on one core

o 14400:120 & 72Vo Ratio 120 & 200:1

Ampere rating of separate primary windings on one core

o 100 & 600:5Ao 5 & 5 & 5:5Ao 100 & 100:5A

/(single slant line)

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

o 8400/1200/1400:120Vo Ratio 70/100/120:1

Different primary current ratings obtained by taps in the secondary winding

o 300/400/600:5A

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1.4 Instrument Transformer StandardsThe major North American standards that govern manufacture and testing of instrument transformers are CSA and IEEE. Many of the standards deals with the design and factory testing requirements that are required for establishing accuracy related to revenue metering.

Canadian standards CAN/CSA-C60044-x-07; Instrument Transformers –

Part 1: Current Transformers Part 2: Inductive Voltage Transformers Part 3: Combined Transformers Part 5: Capacitive Voltage Transformers Part 6: Requirements for protective current transformers for transient

performance Part 7: Electronic Voltage TransformerPart 8: Electronic Current Transformer

All of the above standards are an adoption of IEC 60044, “x” in the standard ID number is used to denote a numeral corresponding to the respective standards.

US Standards IEEE C57.13-2008 Standard Requirements for Instrument Transformers IEEE C57.13.6 High Accuracy Instrument transformers (Draft) ANSI/IEEE C57.13.1-1981 Guide for Field Testing of relaying Current

Transformers

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2. Current TransformerA CT is a device designed to isolate and transform the primary current quantity into a proportional secondary current quantity. The CT has a primary winding or primary conductor, a magnetic core and a secondary winding.

Figure 1: Bushing, Window of Bar-Type CT with Uniformly Distributed windings.

Figure 2. Primary Wound CT Without Uniformly Distributed windings (simplistic representation).

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2.1 How Does a CT WorkWhen AC current flows in the primary conductor or primary winding it produces a changing magnetic field in the magnetic core which in turn induces a voltage in the secondary winding. If the secondary winding was short circuited, short circuit currents would flow in the secondary winding. The amount of energy flowing in the secondary circuit is equal to the amount of energy flowing in the primary winding for an ideal current transformer based on the circuit’s magneto motive force (MMF) or ampere-turns.

Example 1:If an ideal CT has a ratio of 1 turn (T) primary and 500 turns secondary, how much current will be generated if the secondary winding if it was short circuited, when 1000 A is flowing in the primary winding ?

Ipri x Npri = Isec x Nsec

Where: Ipri = Current flowing in primary winding (Amps)Isec i = Current generated in secondry winding (Amps)Npri = Number of turns in primary winding (Turns)Nsec = Number of turns in secondary winding (Turns)

Isec = (Ipri x Npri) / Nsec = 1000A x 1T / 500T = 2 Amp

There would be 2 Amps generated in the secondary winding.

NOTE: While the above example shows 100% of the MMF is being utilized to convert the primary current into secondary current, in reality a portion of the primary AT primary energy is used to magnetized the core to perform the transformation function. Therefore the actual current will be less than 2 Amps.

The difference between the nameplate ratio and the actual primary to secondary current ratio is known as the ratio error.

All CTs have ratio errors.

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2.1.1 Secondary Winding-Induced Voltages

There is an old saying:

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

If the secondary burden is removed while the primary current is flowing, all of the primary MMF will be used to magnetize the core. The field flux density will increase by one order of magnitude since there is no counter MMF generated by the secondary current. The large increase in magnetic field will induce high crest voltages until the core saturates. During the changing AC waveform the core will be unsaturated twice per cycle, during the zero crossing which is when high peak voltages are produced. Voltage induced is in the order of 2 to 3 thousand volts and can be higher. The limiting factors affecting the voltage peaks magnitude is the core saturation point govern primarily by the cross-sectional area of the core. The larger the core cross-sectional area, the higher the voltage produced.

High crest voltages can break down the low voltage secondary winding insulation system, resulting in persistent arcing which can propagate into more serious faults. If the fault point moves along the secondary conductor path into the CT winding or creating other short circuits in the control wiring, a DC battery bank fault could occur. A primary to secondary fault in the CT device will also be very destructive possibly resulting into a primary phase to ground type fault.

2.1.2Operation With Open Circuit SecondaryCT conforming to IEEE C57.13-2008 is capable of operating under emergency conditions for 1 minute with rated primary current times the rating factor with the secondary circuit open if the open-circuit voltage does not exceed 3500V crest.

When the open circuit voltage exceeds 3500V peak, spark gaps or varistors are provided as part of the voltage limiting devices. Voltage limiting devices are able to withstand open circuit situation for 1 minute without damage to the secondary winding but such devices may have to be replaced after an open circuit condition.

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2.2 TypesFour general types of design and construction for current transformers are:

Torroidal Wound primary Bar type Split Core

They all have a magnetic core and windings.

TorroidalThe torroidal or donut type CT is the most common type of CT. It has no internal primary winding. The primary winding is the conductor in which the current is to be monitored and is placed through the window of the CT. The core is a band of continuous magnetic grade steel concentrically wound upon itself, producing an efficient core design which has no breaks or gaps in the magnetic path and operating with minimal audible noise.

Torroidal CTs are commonly available at 600V voltage class but can be insulated up to 5.0 kV voltage class. It is possible to use a 600V class torroidal type CT in higher voltage class circuit if the primary conductor is fully insulated or the CT is mounted on insulated bushing for the circuit voltage class.

Torroidal CTs are available for measuring currents from 50 to 5000amps, with windows opening size from 1" to 8" diameter.

Wound PrimaryWound Primary CT has an internal primary winding that is wound for more than 1 full turn on the core. It has no window for a primary conductor to pass through and may be designed using the torroidal type core for best efficiency.

The wound primary CT’s are inserted in series with the conductor which is to be monitored. Connections of the conductors are made to the bus bars which serves as a connection point for the internal primary winding and the conductor. These type of CTs are commonly mistaken for the bar type CT’s.

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Figure 3. Cross sectional view of wound primary CT

Bar TypeBar Type CTs is a toroidal type CT with a continuous bus bar as an integral part which is permanently inserted through the torroid or window. The bus bar serves as the primary conductor. Bar type CT’s are inserted in series with the conductor which is to be monitored and insulated for its voltage class. Connection of the conductor is made to the bus bar which serves as the internal primary winding.

Split CoreSplit core CTs have one end removable so that the load conductor or bus bardoes not have to be disconnected to install the CT. They are available for measuring currents from 100 to 5000 amps, with windows in varying sizes from 1" by 2" to 13" x 30", they can be rectangular or circular in shape.

2.3 PolarityAll CTs are substractive polarity. Polarity refers to the instantaneous direction of the primary current flow with respect to the secondary current flow.

Polarity is determined by the way the leads are brought out of the transformer case. On subtractive polarity units, the primary H1 lead and secondary X1 lead will be on the same side.

2.4 RatingsThe rating of a CT shall include:

Basic Impluse Level (BIL full wave voltage test) Nominal or maximum system voltage Frequency (Hz) Rated primary and secondary currents Accuracy classes at standard burden

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Continuous thermal current rating factor (based on 30ºC ambient) Short-time mechanical and short time thermal current rating (available from

the data sheets and may not be on the nameplate)

2.4.1 RatioThe CT ratios are expressed as a ratio of the primary rated current to the rated secondary current. For a 600:5A CT, the ratio will be 120:1.

CT ratio is defined as the number of turns in the secondary winding as compared to the number of turns in the primary winding. For a 600:5A CT, the secondary winding may have 120 turns when the primary winding has only 1 turn. The ratio will be 120:1.

2.4.2 Ratio Adjustment (for low voltage applications)While the nameplate ratio is fixed, the operating ratio can be modified under certain physical conditions even with tapped secondary CTs.

Adjusting the CT ratio for some applications are possible when the primary conductor size are small and large window CT are available. The ratio relationship between the primary and secondary turns can be expressed as:

KA = (KN ± NSA) / NP

Where: KA = Actual ratioKN = Nameplate ratioNSA = Number of adjusted secondary turns added or subtracted

(To Add: loop X1 lead through H1. To Subtract: loop X1 opposite H1)

NP = Number of primary turns

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Figure 4. Adjusted Ratio CT connection:3 Primary Loops + 2 Secondary Loops added

Example: An application requires a 20:5 CT ratio, but only a 50:5 CT is available. Using the above formula it can be determined that by using 3 primary loops and adding 2 secondary loops that a 20:5 actual ratio will be obtained.

KA = (KN ± NSA) / NP = (10 + 2) / 3 = 4Actual CT Ratio = 20:5

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2.5 Standard BurdenThe standard burden for CT with 5A rated secondary is a specified impedance comprising of resistance and inductance and the rated frequency.

Table 3: Standard Burden for CT with 5A secondary @ 60 HzBurdens Burden

IDR

(Ω)L

(mH)Z

(Ω)aVA

@ 5APF

Metering B-0.1 0.09 0.116 0.1 2.5 .9B-0.2 0.18 0.232 0.2 5.0 .9B-0.5 0.45 0.580 0.5 12.5 .9B-0.9 0.81 1.040 0.9 22.5 .9B-1.8 1.62 2.080 1.8 45.0 .9

Relaying B-1.0 0.50 2.300 1.0 25.0 .5B-2.0 1.00 4.600 2.0 50.0 .5B-4.0 2.00 9.200 4.0 100.0 .5B-8.0 4.00 18.400 8.0 200.0 .5

a The impedance tolerance is +5% and -0%

2.6 Accuracy Classes for MeteringAccuracy is defined as the allowable percentage error for instrument transformers operating at specified conditions.

Basis for Accuracy ClassAccuracy classes for revenue metering are based on the requirements that the transformer correction factor (TCF) of the voltage transformer or the current transformer be within specified limits when the (lagging) power factor of the metered load has any value between .6 to unity power factor under specified operating conditions:

For CT:Operating between 10% to 100% of rated current or operating at its rating factor (RF) at the specified burden.

For VT: Operating between 90% to 110% of the rated voltage at the specified burden power factor for any burden in voltamperes (VA) from zero to the specified burden.

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2.6.1 Standard Accuracy classes

Table 4: Standard Accuracy Class for Metering Service and Limits of Transformer Correction Factor (TCF) Between .6 to 1.0 Lagging PF of Metered Load

MeteringAccuracy

Class (%)

VTs CTs90-100% Vrated 100% Irateda 10% Irated or as

notedMin. Max. Min. Max. Min. Max

.15 .9985 1.0015 .9985 1.0015 .9985b 1.0015b

.15S N/A N/A .9985 1.0015 .997c 1.003c

.3 .997 1.003 .997 1.003 .994 1.006

.6 .994 1.006 .994 1.006 .988 1.0121.2 .998 1.012 .988 1.012 .976 1.024

a For CTs, the 100% rated current limit applies to the current corresponding to the continuous thermal current rating factorb The minimum sustained current shall not be less than 5% for .15 accuracy class (proposed amendments CSA and new IEEE standard)c The minimum sustained current shall not be less than 1% for .15S accuracy class (proposed amendments CSA and new IEEE standard)i. Continuous current rating factor is the number by which the rated primary current of a CT is multiplied to obtain the maximum primary current that can be carried continuously without exceeding the limiting temperature rise from 30°C above average ambient air temperature. The RF of tapped-secondary or multi-ration CT applies to the highest ratio unless otherwise stated.

2.6.2 Accuracy Rating for MeteringA CT for metering is given an accuracy rating for each standard burden for which it is rated. The accuracy class can be stated for the maximum burden implying that all other lower burden will have the same accuracy within that class.

Example:A CT has an accuracy rating of 0.3 B-1.8.This would imply that this CT also has a rating of 0.3 B-0.1, B-0.2, B-0.5, B-0.9 and B-1.8.

A CT with an accuracy class at a specified burden is not guaranteed for other burden.

Example:A CT has an accuracy rating of 0.3 @ B-0.2The accuracy is only guaranteed for burden below .2 ohms.

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2.6.3 Type M (Metering) Accuracy ClassMetering CTs have accuracy classes of .3%, .6% and 1.2%. Accuracy class is the same as saying that the CT can have a maximum ratio error of .3%, .6% and 1.2%.

CSA, ANSI/IEEE specifies the accuracy classes for metering CT is for a connected primary load between .6 lagging and unity PF.

The available Maximum Ratio Error Classes are: + 0.3%, + 0.6% and + 1.2% for a maximum specified secondary burden of .1Ω, .2 Ω, .5 Ω, .9 Ω and 1.8 Ω.

At 5A rated secondary current the VA rating of the CT will be 2.5 VA, 5.0VA, 12.5VA, 22.5VA and 45.0VA for the specified burden;VA = I2 * Z

A nameplate designation is 0.3 B 0.2 is interpreted as having .3% maximum error into a burden of .2 Ω operating between .6 to 1.0 lagging power factor at 60 hz.

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2.7 Accuracy Rating for RelayingCT designed for relaying purposes is given the following classification for its accuracy rating:

Table 5: Limits of Ratio ErrorClass 5A

Rated Current100A

(20 X rated current)C and T 3% 10%

X 1% User defined

The relay accuracy class applies only to the full winding, unless otherwise specified.

Class CClass C cover CT’s in which the leakage flux in the core does not have an appreciable effect on the ratio(s) with standard burden applied, so that the ratio can be calculated by standard formula.

Class C CTs are torroidal in design.

Class TClass T cover CT’s in which the leakage flux in the core does have an appreciable effects of the ratio(s) with standard burden applied such that it is not practical to calculate the ratio by standard formula and must be tested.

Class T CTs are primary wound in design.

Class XClass X covers CT’s in which the accuracy is user defined and identified by:

Minimum knee point voltage, Vk Maximum exciting current at knee point voltage, Ik The maximum allowable secondary winding DC resistance corrected to

75°C, Rs.

Class X CTs are special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependent on CT accuracy.

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2.7.1 Secondary Terminal VoltageRelaying CTs, class C and T are capable of producing secondary voltage at 20X rated current into the standard burden. The voltage is a product of the current times the impedance.

Table 6: CT Secondary voltage at 20X rated current (100A)

Standard Burden (Ω)

Terminal Voltage (V)

B-0.1 10B-0.2 20B-0.3 50B-1.0 100B-2.0 200B-4.0 400B-8.0 800

Secondary voltage is the saturation voltage level ranging from 10 to 800V. To specify a CT under the new standard one must select either a C or T type CTs and then specify the burden.

Note: If the CT secondary winding is rated at other that 5A, the appropriate burden should be calculated.

Example:What is the rated burden for a relay class C-100 with a 1A rated current ?

Burden rating = 100V / (1A x 20) = 5Ω

2.7.2 Saturation CurveMeasuring CT’s, type M are accurate up to 120% of rated current. Core construction is made with nickel iron alloy ore with low exciting current and knee point at low flux density. Low saturation level protect instruments during fault conditions when primary current can exceed more that 120% of rated current during fault conditions.

Protection CTs, type P accuracy is not as importance as type M. Protection CTs can operate up to many times the rated current (≈20x) and produce an accurate

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representation of the primary current in the secondary circuits. Core construction is made with grain oriented silicon steel with high saturation flux density capability.

Figure 5. Excitation curves for Metering CT vs Protective CT

In protection applications, secondary burden becomes an important consideration to prevent early CT saturation and ensure proper operation of protective devices. When high primary fault current level are generated the CT with the higher saturation voltage rating are better able to produce the correct secondary current without distortion in its waveform. This is accomplished by placing more material which increases the cross sectional area of the core.

The knee point voltage is a point in the excitation curve at which the CT is approaching saturation. Vk can be identified at the starting point where a 10% change in excitation voltage produces a 50% change in excitation current.

Vk ≈ ΔVe of 10% → Δ Ie of 50%

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The secondary induce voltage formula for a CT is identical to the power transformer formula and known as the transformer universal equation when the flux is purely sinusoidal. The relationship for the windings for its RMS voltage is given by:

ERMS = 2 π f N a Bpeak / √2

ERMS ≈ 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 curveTypical Class C manufacturer’s excitation curves are plotted on a log-log coordinate paper with square decades. Induced secondary voltage is plotted on the Y-axis and corresponding excitation current is plotted on the X-axis.

Voltage start at 1% of secondary terminal voltage rating Voltage ends at 5x rated secondary current Knee of curve at 45º tangent to abscissa identifies the knee point voltage, Vk

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Figure 6. Type C class excitation curve for multi-ratio CT with non-gapped cores

Note: The knee point voltage is the point of maximum permeability of the steel and is not the saturation point. It is location close to the saturation point.

Figure 7. Type C Class Excitation curve with Saturation Point

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Class T Overcurrent Ratio CurveClass T overcurrent ratio curves are plotted on linear rectangular coordinate paper. Secondary current is plotted on the Y-axis and corresponding multiple of primary current is plotted on the X-axis.

Secondary current plotted from 1 to 22 times rated primary current For all the standard burdens up to the burden which causes a ratio correction

of 50%

Figure 8. Class T Overcurrent Ratio Curve (typical).

3. Voltage TransformerA VT is a device designed to isolate and transform the primary voltage quantity into a proportional secondary voltage quantity. The VT has a primary winding, a magnetic core and a secondary winding. Dual secondary windings are very common.

The main attribute of a VT is its accuracy rating. The voltage transformation is constant and linear within specified burden.

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3.1 GroupsVTs are grouped according to their applications. There are five groups.

Group 1Used for application with 100% of rated primary voltage across the primary winding when connected line-to-line or line-to-ground. Rated voltages range from 120V/208V Y to 14.4kV/24.94kV Y.

Group 2Used primarily for phase-to-phase application but can be used for phase-to-neutral providing the thermal burden capability are reduced. Rated voltages range from 120V/120V Y to 69kV/69kV Y.

Group 3Used for phase-to-ground connection only and have two secondary windings. Rated voltages range from14.4kV/24.94kV Grd Y to 431.25kV/750kV Grd Y.

Group 4Used for phase-to-ground connections in grounded or ungrounded wye system . They are capable to be operated at 110% rated voltage continuously within their thermal burden rating.

Rated voltages range:Group 4A from 2.4kV/4.16kV Grd Y to 8.4kV/14.4kVGroup 4B from 4.16kV/4.16kV Grd Y to 14.4kV/14.kV Grd Y.

Group 5Used for phase-to-ground connection only for indoor used on grounded system only. Rated voltages range from 72kV/12.47kV Grd Y to 20.125kV/34.5kV Grd Y.

3.2.Overvoltage RequirementsAll VTs are capable of operating at 110% of rated voltage continuously provided the thermal rating is not exceeded. Conditions such as winding temperature rise during the overvoltage may also apply; details are not included in this write up.

The emergency rating is defined at one minute of operation which should allow protective equipment to operate. Other overvoltage rating requirements are shown in table 7.

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Table 7: VT Emergency RatingGroup

IDNo of

Bushing(s)Voltage

Type Additional Emergency

Requirements1 2 L-G 125% for 8 hours2 2 L-L 110% for 1 min3 1 L-G 92 kV to 161 kV

@ 173% for 1 min138 kV to 230 kV @ 140% for 1 min

4A 1 L-G 125% for 1 min4B L-G 110% for 1 min5 1 L-G 140% for 1 min

3.3 Accuracy RatingA VT is given an accuracy rating for each standard (VA) burden for which it is rated. The accuracy class can be stated for the maximum VA implying that all other lower VA will have the same accuracy within that class.

Example:A VT has an accuracy rating of 0.3 Z.This would imply that this VT also has a rating of 0.3 classat W, X, M, Y and Z.

Note: from the standard burden table implies Z to be a specified 200VA (burden), therefore a VT loaded with less that 200 VA (burden as in W, X, M and Y) will also have an accuracy of .3%.

A VT with an accuracy class at a specified burden is not guaranteed for any other burden unless specifically stated.

Example:A CT has an accuracy rating of 0.3 @ Y.The accuracy is only guaranteed for burden at Y.

The burden on any two secondary terminals affects the accuracy on all other terminals. The accuracy rating is the total burden on the transformer. The accuracy class is applicable with the burden divided between the secondary outputs in any portion.

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3.4 Standard BurdenStandard Burden for VT are for rating purpose only.

Table 8: Standard Burden of VTs For Rating PurposeDesignation VA Power Factor Z on 120V

basis (Ω)Z on 69.3V basis (Ω)

W 12.5 0.10 1152 384X 25.0 0.70 576 192M 35.0 0.20 411 137Y 75.0 0.85 192 64Z 200.0 0.85 72 24

ZZ 400.0 0.85 36 12

3.5 Thermal Burden RatingThe thermal burden rating of a VT is the maximum VA that it can carry at rated secondary voltage without exceeding its design temperature rise. If no VA rating is specified, the maximum VA is the same as the maximum standard burden for its accuracy. Refer to table 8.

Each winding is given a VA rating. If only one VA rating is specified, then the total VA is applicable to any distribution of secondary VA, including the use of taps.

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4. Electrical Testing of Instrument Transformers

Safety ConsiderationMany of the testing procedures involve high voltage and testing should be done by qualified experienced personnel familiar with the hazards associated with the specific tests.

Safety working practices and guidelines are covered in module two.

Refer to module two for safety work practice when testing with high voltages

Refer to IEEE Standard 510 – 1983, Recommended Practice for Safety in High Voltage & High Power Testing

4.1 CT TestingThe testing procedure described is for the typical case where the secondary turns are more numerous than the primary turns. Excitation, ratio and polarity tests are performed by the application of AC voltage to the secondary winding with the primary open circuited and the step-down transformation measured. In the unusual case where the primary turns are more numerous than the secondary turns, the primary and secondary connections should be interchanged to effect a step-down transformation.

4.1.1 Insulation Resistance TestThe insulation resistance test is a DC voltage test conducted at 100% of the rated AC insulation phase-to-ground crest level. The DC equivalent is at 1.414 of the AC RMS rated insulation value.

Note: The above value is higher than the recommended NETA insulation resistance test level. In the opinion of this writer, the higher value is a more practical level since the insulation will be stressed at the operating value.

The result of the test serves as a preliminary assessment of the primary insulation system to determine if it is should be subjected to the High-potential (Hipot) test.

The test is applicable for bar and the primary wound type CTs and conducted by energizing the primary winding with the secondary winding grounded. Bushing or window type CTs are excluded from this test.

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Figure 9. CT Insulation Resistance Test Connection

There is little consensus on a standardized accepted insulation resistance value. The following table is duplicated from the NETA standard.

Table 9. Transformer Insulation Resistance Acceptance TestingWinding Rating

Type(V)

Minimum Test Voltage(DCV)

Recommended Minimum Insulation Resistance in Megohms

Liquid Filled Dry

0 – 600 1000 100 500601 – 5000 2500 1000 5000

> 5000 5000 5000 25000Note: Since Insulation resistance depends on insulation rating (kV) and windng capacity (kVA), values obtained should be compared to manufacturer’s published data.

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4.1.2 Insulation Resistance Test Procedure1. Isolate the primary winding for phase-A CT and disconnect the secondary

wiring.

2. Connect the insulation resistance tester HV lead to the H1 terminal.

3. Ground and connect the LV lead to the secondary winding.

4. Apply the test voltage for the test duration.

5. Record resistance value

6. Repeat step1 through 5 for Phase-B and Phase-C CTs.

7. Reconnect all disconnected wiring.

4.1.4 Dielectric Withstand TestThe AC hipot testing of instrument transformers is conducted at the rated power frequency, at 75% of the factory test voltage level.

The AC test voltage shall have a crest equal to 1.414 times the RMS value specified in Table 1. The wave shape shall be essentially sinusoidal. The frequency shall be within 20% of the rated power frequency. The test voltage is to be increased gradually from zero at a rate no greater than 1000 V per second to reach the required test value and shall be held there for 1 minute.

The test is applicable for bar and the primary wound type CTs and conducted by energizing the primary winding with the secondary winding grounded.

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Table 10. CT Dielectric Withstand Test Voltage Levels

Nominal system voltage

(kV)

BIL rating

(kV)

Dielectric testvoltage(kV)1

.6 10 31.2 3. 7.52.4 45 11.255.0 60 14.258.7 75 19.515 95 & 110 25.525 125 3025 150 37.5

34.5 200 25.546 250 52.5†

1. In the absence of an AC Hipot test set, the dielectric may be substituted for the DC Hipot at 1.4.1 the AC RMS value as shown in column 3. the DC Hipot test is not a recommended test† DC hipot is not recommended for instrument transformer with BIL above 200 kV.

Figure 10. CT Dieclectric Withstand Test Connection

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4.1.5 Dielectric Withstand Test Procedure1. Isolate the primary winding for phase-A CT and disconnect the secondary

wiring.

2. Connect the AC Hipot tester HV lead to the H1 terminal.

3. Ground and connect the LV lead to the secondary terminal.


5. Record leakage current value.



4.1.6 Secondary Wiring Insulation Resistance TestThe secondary insulation resistance test is a 1 minute test at 1000 Vdc, used to determine the integrity of secondary wiring insulation level. The test is conducted with the CT secondary single point grounding removed and the CT isolated from its secondary burden

Consult the manufacturer’s manual for electronic instruments and relays that are connected in the secondary circuits if it can be subjected to high voltage levels. Some electronic devices may have overvoltage suppression circuit connected on its CT input terminals which may be damaged when subjected to high voltage.

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Figure 11. CT Secondary Wiring Insulation Resistance Test Connection

4.1.7 Secondary Circuit Insulation Resistance Test Procedure1. Disconnect Phase-A, Phase-B, Phase-C and neutral CT secondary wiring.

2. Disconnect and isolate the CT secondary single point ground wiring.

Review the applicable schematic or assembly diagrams for the location of the grounding point.

4. Confirm that the CT secondary circuit is isolated from ground with a low voltage insulation tester.

5. Apply 1000Vdc for 1 minute duration on any CT secondary wiring.

6. Record resistance value.


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4.1.8 Secondary Circuit Burden TestsThe secondary burden tests are performed in 2 parts.

Part one measures the per phase secondary circuit burden (RL + ZB).One amp AC current is injected into each phase-to-neutral wiring with the CT winding isolated.

Voltage, current and phase angle is measured and the circuit burden is calculated for each phase.

The measured burden comprises of: Leads resistances (RL) Device burden (ZB)

Figure 12. CT Secondary Burden Test

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Part two measures the secondary winding DC resistance (RS).

Figure 13 Secondary Winding Resistance Test

The total secondary burden is the vector sum of the two measurements.

Total burden (ZT) calculation:

ZT = RS + (RL + ZB)

Where ZT = Total burden (Ω)RS = DC Winding resistance (Ω)RL = Lead resistance (Ω)ZB = Device impedance (Ω)

The burden is used to verify if the CTs has the voltage capability to power the connected burden as determined from its nameplate rating.4.1.9 Secondary Burden Test ProcedurePart One:1. Isolate the CT secondary phase wiring at the CT terminal blocks for phase A,B,

C and neutral.

2. Inject 1A current into phase A wiring.

3. Measure and record current, voltage and phase angle values.

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4. Calculate the phase impedance value.

5. Repeat step 2 to 4 for phase B and C.

Part Two:6. Measure and record the secondary winding resistance for Phase-A. using a four-

wire low resistance ohm meter.

7. Repeat step 6 for Phase-B and Phase-C.

Part Three:8. Calculate and record the per phase burden values.

9. Confirm that per phase burden is within the burden accuracy rating for the CTs.

For metering type CTs, the total burden value shall be less that the nameplate burden rating

For type C and T protection CTs, the total burden (ZT) multiplied by 100Amps shall be less that the nameplate saturation voltage rating

10.Reconnect all disconnected wiring.

4.1.10 Saturation TestThe saturation test is performed by exciting the secondary winding with the primary winding open-circuited. The excitation voltages and currents are recorded as the test is being performed at various levels.

The saturation test is used to identify the knee point voltage where the CT is close to the saturation point. The knee point voltage is identified as the starting point when a 10% change in excitation voltage produces a 50% change in excitation current. Beyond the knee point voltage a small change in excitation voltage will produce a large change in excitation current.

The CT should be taken into the saturation mode; up to 5A maximum excitation current value is permissible. The voltage should then be reduced to zero. This process will remove any residual magnetism.

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The measured data can be plotted on a log log scale to show the location of the knee point voltage relative to the linear and saturated portion of the curve.

The power supply requirements can be large for CT with an 800 V saturation rating. At 5 amps, the power requirements will be about 4 kVA.

Figure 14. Saturation Voltage Test Connection

4.1.11 Saturation Test Procedure1. Connect the test set across the full secondary winding of phase-A, with the

primary winding open-circuited. Ensure that the voltage supply has a maximum scale that is above the rated saturation voltage level by 25%.

2. Raise the voltage slowly at fixed increments to the rated saturation voltage level. Record both voltage and current values.

3. Observe where a small change in voltage produces a large change in and current. This will occurs when the applied voltage gets closer to the rated saturation voltage level.

4. Identify the starting voltage level where a 10% increase in excitation voltage results in a 50% increase in excitation current. Record the starting voltage level (or the knee-point voltage).

5. Increase the voltage until the winding is saturated up to 2.5A. Do not exceed the rated 5 amps current rating of the secondary winding.

6. Slowly reduce the voltage to zero.

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7. Repeat steps 1 -6 for phase B and C secondary windings.


4.1.12 Ratio and Polarity TestThe ratio and polarity tests are performed to verify the nameplate ratio and the polarity markings. The two tests is accomplished using one test connection.

The phase angles are compared between the applied voltage and the induced voltage to determine polarity. Like polarity will show a 0º electrical displacement while unlike polarity will show a 180 º electrical displacement.

Figure 15. Ratio and Polarity Test Connection

4.1.13 Ratio and Polarity Test Procedure1. Ensure that the CT has been demagnetized.

2. Connect the test as per the connection diagram

3. Measure and record the test values

4. Confirm that the measured ratio matches the nameplate ratio

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5. Confirm that H1 and X1 is marked correctly as indicated by the phase value.


4.1.14 Ratio and Polarity Test Procedure (Alternative)An alternative connections using individual equipment requires a variable AC source, phase angle meter and voltmeters. Voltage is applied at 1V per turn on the secondary winding and the induce voltage and phase angle is measured on the primary winding.

The applied voltage is connected to the X1-X2 terminals to affect a step-down transformation action measured on the H1-H2 terminals

Figure 16. Ratio and Polarity Test Connection

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4.1.15 Ratio and Polarity Test Procedure (Alternative)1. Ensure that the CT has been demagnetized.

2. Connect a variable power supply across the full secondary winding of phase-A, with the primary winding open-circuited. The power supply polarity terminal is to be connected to the X1 terminal.

3. Connect a voltmeter to measure the applied voltage across the secondary winding X1-X2.

4. Connect a voltmeter to measure the induced voltage across the primary winding H1-H2.

5. Connect a phase angle meter (PAM) to compare the phase relationship between the voltages at X1-X2 and H1-H2.

The polarity terminal for channel 1 is to be connected to X1.

The polarity terminal for channel 2 to be connected to H1

6 Apply 1 volt per turn to the secondary winding. Record all values.

7. Confirm that the phase relationship between H1 and X1 is around 0° electrical.

8. Calculate the ratio.

9. Confirm that the calculated ratio matches the nameplate ratio

10.Confirm that H1 and X1 is marked correctly as indicated by the phase value.

11.Reconnect all disconnected wiring.

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4.2 VT Testing

4.2.1 Insulation Resistance TestThe insulation resistance test is a 1 minute DC voltage test conducted at 100% of the rated AC insulation phase-to-ground crest level. The DC equivalent is at 1.414 of the AC RMS rated insulation value.

Note: The above value is higher than the recommended NETA insulation resistance test level. In the opinion of this writer, the higher value is a more practical level since the insulation will be stressed at the operating value.

The result of the test serves as a preliminary assessment of the primary insulation system to determine if it is should be subjected to the High-potential (Hipot) test.

The test is performed on winding to winding and each winding to ground.

Figure 17. VT Insulation Resistance Test Connections

4.2.2 Insulation Resistance Test Procedure1. Isolate the primary winding for phase-A VT and disconnect the secondary

wiring.

2. Connect the insulation resistance tester HV lead to the H1 terminal.

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5. Record resistance value



4.2.3 Dielectric Withstand TestThe dielectric withstand test is a 1 minute 60 Hz, AC high voltage stress test conducted at 75% of the factory test voltage levels.

The test is performed on the primary winding with the secondary winding grounded.

Refer to Table 9 for test voltage levels

Figure 18. Dielectric Withstand Test Connection

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4.2.4 Dielectric Withstand Test Procedure1. Isolate the primary winding for phase-A VT and disconnect the secondary

wiring.

2. Connect the AC Hipot tester HV lead to the H1 terminal.


4. Apply the test voltage for the test duration on the primary winding.

5. Record the leakage current.

6. Repeat for phase B and C.


4.2.5 Ratio and Polarity TestThe ratio and polarity tests are performed to verify the nameplate ratio and the polarity markings. The two tests can be accomplished using one test connection.

The test is performed on all tap positions.

The connections are similar to the CT ratio and polarity test with the exception that the voltage is applied to the primary winding to affect the step-down transformation.

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Figure 19: VT ratio and Polarity Test Connection

4.2.6 Ratio and Polarity Test Procedure1. Isolate the primary winding for phase-A VT and disconnect the secondary

wiring.

2. Connect the test set as per the connection diagram

3. Measure and record the test values

4. Confirm that the measured ratio matches the nameplate ratio

5. Confirm that H1 and X1 is marked correctly as indicated by the phase value.


4.2.7 Ratio and Polarity Test Procedure (Alternative)An alternative connections using individual equipment requires a variable AC source, phase angle meter and voltmeters. Voltage is applied at 1V per turn on the primary winding and the induce voltage and phase angle is measured on the secondary winding.

The test connection to the H1-H2 terminal is to affect a step-down transformation.

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The procedure will not be duplicated here and the reader is suggested to review the CT ratio and polarity test procedure.

4.2.8 Secondary Circuit Burden TestThe secondary burden test measure the apparent power supplied to the secondary load at the VT terminals. The sum of all the product of the current and the voltage for all windings should be within the thermal burden rating of the transformer.

Figure 20. Secondary Burden Measurement Connection

4.2.9 Secondary Circuit Burden Test Procedure1. Isolate the secondary wiring from the secondary winding.

2. Connect a suitable power supply to provide rated secondary voltage to the secondary wiring.

3. Measure and record total current, voltage and phase angle for all secondary circuits.

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4. Calculate the total apparent power.

5. Ensure that the total apparent power is within the thermal burden rating.

6. reconnect all disconnected wiring.

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5. NETA Acceptance Testing ProceduresEn situ commissioning tests should be performed on newly installed equipment to ensure that the equipment will meet the requirements as detailed in the design specifications. The NETA standards should be used in the absence of manufacture’s instructions for acceptance testing procedures.

Note: Always consult the manufacturer’s manual for information on the required tests to be performed and the test levels to be applied

5.1 Visual and Mechanical Inspection

1. Compare equipment nameplate data with drawings and specifications.

The following minimum information shall be given on the nameplate: Manufacturer’s name or trademark Manufacture’s type Manufacture’s serial number Rated primary voltage for VTs; Rated Primary and secondary current for CTs Nominal system voltage or maximum system voltage for CTs Ratio or ratios Basic impulse insulation level (BIL kV). Rated continuous thermal current rating factor (RF) for CTs Thermal burden rating or rating at ambient temperature(s), in Volt-amperes

in degrees Celcius for VTs Rated Frequency (Hz) Accuracy rating

2. Inspect physical and mechanical condition.

3. Verify correct connection of transformers with system requirements.

4. Verify that adequate clearances exist between primary and secondary circuit wiring.

5. Verify the unit is clean.

6. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 2 of Module 2.

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7. Verify that all required grounding and shorting connections provide contact.

8. Verify correct operation of transformer withdrawal mechanism and grounding operation.

9. Verify correct primary and secondary fuse sizes for voltage transformers.

10.Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.

5.2 Electrical Tests

5.2.1 Current Transformers

1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter for bar type CT.

Caution:DO NOT put DC current through the primary winding of the CT.Perform resistance measurement at the bolted connections only.

2. Perform insulation-resistance test of each current transformer and its secondary wiring with respect to ground at 1000 volts dc for one minute.

For units with solid-state components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.

3. Perform a polarity test of each current transformer.

4. Perform a ratio-verification test.

5. Perform an excitation test on transformers used for relaying.

6. Measure current circuit burdens at transformer terminals.

7. When applicable, perform insulation-resistance tests on the primary winding with the secondary grounded. Test voltages shall be at the calculated rated crest voltage.

8. Perform dielectric withstand tests on the primary winding with the secondary grounded for Bar and primary wound units. Test voltages shall be in accordance with Table 10.

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9. Verify that current transformer secondary circuits are grounded and have only one grounding point. Confirm grounding point as marked on the drawings.

5.2.2 Voltage Transformers1. Perform resistance measurements through bolted connections with a low-

resistance ohmmeter, if applicable.

2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute at the calculated rated crest voltage. For units with solid-state components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.

3. Perform a polarity test on each transformer.

4. Perform a turns-ratio test on all tap positions.

5. Measure voltage circuit burdens at transformer terminals.

6. Perform a dielectric withstand test on the primary windings with the secondary windings connected to ground. The dielectric voltage shall be in accordance with Table 10. The test voltage shall be applied for one minute.

7. Perform power-factor or dissipation-factor tests in accordance with test equipment manufacturer's published data.

Note: Item 7 is a test requirement which have been left within the listing of recommended tests to be performed as per NETA standard. It is in the opinion of this writer that the power factor or dissipation test is a maintenance test and not an acceptance test. Maintenance testing is describe in the maintenance testing procedure course and has been excluded from this write-up.

8. Verify that voltage transformer secondary circuits are grounded and have only one grounding point as marked on the drawings.

5.3. Test Values Analysis

5.3.1 Visual and Mechanical1. Compare Bolted connection resistance values to values of similar

connections. Investigate values which deviate from those of similar bolted connections by more than 50% of the lowest value.

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2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s data, use Table 2 in Module 2.

5.3.2. CT Electrical Test1. Compare bolted connection resistance values to values of similar

connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.

2. Polarity results shall agree with transformer markings.

3. Ratio errors shall be within nameplate accuracy class

4. Excitation results shall match the manufacturer excitation curve.

5. Measured burdens shall be within the nameplate rating.

6. Insulation-resistance values of instrument transformers shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 9.

7. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the primary winding is considered to have passed the test.

8. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.

9. Test results shall indicate that the secondary circuits are grounded at one point only.

5.3.3 VT Electrical Test1. Compare bolted connection resistance values to values of similar

connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.

2. Insulation-resistance values of instrument transformers shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 9.

3. Polarity results shall agree with transformer markings.

4. Ratio errors shall be in accordance with nameplate data.

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5. Measured burdens shall be within the nameplate rating.

6. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the primary windings are considered to have passed the test.

7. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.

8. Test results shall indicate that the secondary circuits are grounded at only one point only

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6. Test Set Operational Instruction Manual

CT Tester

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7. Instrument Transformer Test Forms

Current Transformer Test Form

Voltage Transformer Test Form

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8. References and Suggested Reading

IEEE C57.13-2008Standard Requirements for Instrument TransformersCopyright © 2008 by the Institute of Electrical and Electronic Engineers Inc.3 Park Avenue, New York, NY 100-16-5997, USAPDF: ISBN 978-0-7381-5410-7Print: ISBN 978-0-7381-5411-4

ANSI/IEEE C57.13.1-1981An American National Standard IEEE Guide for Field Testing of Relaying Current TransformersCopyright © 1981 by the Institute of electrical and electronics Engineers, Inc

ANSI/NETA ATS 2009American National StandardStandard for Acceptance Testing Specifications for Electrical Power Equipment and SystemsCopyright © 209 by International Elelctrical testing Association3050 Old Centre Avenue, Suite 102, Portage, MI 49024,USA

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Module 3 Commissioning Instrument Transformers

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Transcript of Module 3 Commissioning Instrument Transformers