module 3 commissioning instrument transformers
TRANSCRIPT
04/09/23 Module Three 1
SUBSTATION COMMISSIONING COURSE
MODULE THREE
INSTRUMENT TRANSFORMERTESTING PROCEDURES
04/09/23 Module Three 2
INTRODUCTION
• The electrical commissioning and acceptance testing of electric power systems is essential for the energization of any electrical system for the first time.
04/09/23 Module Three 3
INTRODUCTION
Module Three• Discuss theory and ratings• Safety issues• Test equipment operation &
hands on testing
04/09/23 Module Three 4
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
04/09/23 Module Three 5
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
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
• 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
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INSTRUMENT TRANSFORMERS
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)
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INSTRUMENT TRANSFORMERS
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)
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INSTRUMENT TRANSFORMERS
(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)
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INSTRUMENT TRANSFORMERS
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)
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INSTRUMENT TRANSFORMERS
Example 2: Totalizing CT with four primary windings designed to be used simultaneously.
5 & 5 & 5 & 5 : 5A
Nameplate Symbols (& ampersand)
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INSTRUMENT TRANSFORMERS
Example 3: CT for three-wire single phase.
100 & 100 : 5A
Nameplate Symbols (& ampersand)
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INSTRUMENT TRANSFORMERS
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)
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INSTRUMENT TRANSFORMERS
Example 2: VT with secondary tap for additional secondary voltage ratings
14,400 : 120/72 VRatio 120/200:1
Nameplate Symbols (/ single slant line)
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT 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
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INSTRUMENT 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
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INSTRUMENT TRANSFORMERS
Distributed uniform winding with negligible Leakage Flux
Non-uniform distributedwinding with some Leakage Flux
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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 %
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INSTRUMENT TRANSFORMERS
“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.
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
CT Types
• Torroidal (donut)• Wound Primary• Bar Type• Split Core
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
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INSTRUMENT TRANSFORMERS
CT Types
• Torroidal (donut)• Wound Primary• Bar Type• Split Core
Split Core
• Core is separated into two parts
• For use where primary conductor cannot be easily removed
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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
CT Ratio
• Ratio or primary current to secondary current
– Expressed as current quantities• 1000 : 5 A
– Expressed as ratio numbers• 200 : 1
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
CT Standard Burden @ 5A, @ 60 Hz
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
IT Standard Accuracy Class for Metering @ 60 Hz
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
CT Accuracy Rating for Relaying @ 60 Hz
Accuracy classification apply to full winding unless specified
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INSTRUMENT TRANSFORMERS
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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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
Secondary Terminal Voltage
• Saturation voltage level at 20X rated current
• 100 A
• Vsat = 100 A x Burden
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INSTRUMENT TRANSFORMERS
Saturation Curve
• B-H CurveExcitation Current Vs Excitation voltage
• Knee Point voltage, Vk
Δ 10% Ve → Δ 50% Ie
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
What is a VT ?
• Isolate and transform the primary voltage into proportional secondary voltage
• Construction– Primary winding– Magnetic core– Secondary winding
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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INSTRUMENT TRANSFORMERS
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
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INSTRUMENT TRANSFORMERS
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
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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 High Voltage & High Power Testing
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
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
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TESTING INSTRUMENT TRANSFORMERS
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
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
Dielectric Withstand Test • For Bar and Primary wound
CT’s• AC voltage test at the 75%
factory test voltage• Secondary grounded• 1 minute test
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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 TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
2ndary Ins. Resistance Test
• Test for secondary wiring• Secondary wiring isolated
and ungrounded• 1000 VDC• 1 minute test
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TESTING INSTRUMENT TRANSFORMERS
CT Secondary Circuit Insulation Test
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
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
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TESTING INSTRUMENT TRANSFORMERS
Part 1: CT Secondary Circuit Burden Measurement
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TESTING INSTRUMENT TRANSFORMERS
Part 2: CT Secondary Winding Resistance Measurement
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TESTING INSTRUMENT TRANSFORMERS
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)
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
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
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TESTING INSTRUMENT TRANSFORMERS
CT Saturation test
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
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
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TESTING INSTRUMENT TRANSFORMERS
CT Ratio and Polarity Test
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TESTING INSTRUMENT TRANSFORMERS
CT Testing
• Primary Insulation Resistance
• Dielectric Withstand• Secondary Insulation
Resistance• Secondary Burden• Saturation• Ratio and Polarity
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
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TESTING INSTRUMENT TRANSFORMERS
CT Ratio and Polarity Test (alternate connections)
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TESTING INSTRUMENT TRANSFORMERS
VT Testing
• Insulation Resistance• Dielectric Withstand• Ratio and Polarity
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TESTING INSTRUMENT TRANSFORMERS
VT Testing
• Insulation Resistance• Dielectric Withstand• Ratio and Polarity
Insulation Resistance Test
• DC voltage test at the rated crest voltage
• 3 tests– HV to Gnd– HV to LV– LV to Gnd
04/09/23 Module Three 72
TESTING INSTRUMENT TRANSFORMERS
VT Insulation Resistance Tests
04/09/23 Module Three 73
TESTING INSTRUMENT TRANSFORMERS
VT Testing
• Insulation Resistance• Dielectric Withstand• Ratio and Polarity
Dielectric Withstand Test
• AC voltage test at the 75% factory test voltage
• Secondary grounded• 1 minute test
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TESTING INSTRUMENT TRANSFORMERS
VT Dielectric Withstand Tests
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TESTING INSTRUMENT TRANSFORMERS
VT Testing
• Insulation Resistance• Dielectric Withstand• Ratio and Polarity
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
04/09/23 Module Three 76
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
04/09/23 Module Three 78
TEST FORMS
Current Transformer Test Form
Voltage Transformer Test Form
04/09/23 Module Three 79
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