Doble Power Factor Document

Together We Power

The World

Basic Instruction

Notes

Doble Engineering Company

85 Walnut Street

Watertown, MA 02472

Tel (617) 926-4900

Fax (617) 926-0528

Table of Contents

1 Doble

Services

2

M-Series Safety Features & Practices

3

Power Factor Basic Theory

4

Transformer Overall Power Factor

5

Bushing Power Factor

6

Transformer Excitation Current

7

Transformer Turns Ratio

8

Insulating Fluids Power Factor Tests

9

Surge Arrestor Tests

10 Circuit

Breaker

tests

11

Grounded-Tank SF6 Circuit Breaker Tests Oil

12 Potential

Transformers

Doble Corporate Headquarters

Telephone: (617) 926-4900

Fax: (617) 926-0528

www.doble.com

Doble Service & Equipment Agreement

Mike Horning, Principal Engineer Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

Overview of Services Included with Lease

Doble Service & Equipment Agreement

¾ 24 Hour Technical Support ¾ Perpetual Warrantee ¾ Customized Training

¾ Doble Client Committees & Conferences ¾ Doble E-mail Forums

¾ Doble Knowledgebase ¾ Doble Laboratories

3 of 9

Doble Service & Equipment Agreement

Technical Support

¾ Assigned Client Service Engineers (normal

business hours)

¾ On-call Client Service Engineer (after hours,

weekends, & holidays)

¾ Assistance with test procedures, data evaluation

(written reports on request), and troubleshooting

Doble Service & Equipment Agreement

Perpetual Warrantee

¾ Client Service Engineers will assist with the

troubleshooting and diagnosis of problems with Doble test equipment.

¾ Replacement of worn, damaged, or malfunctioning

equipment.

¾ There are no additional costs for replaced equipment

(unless client is negligent).

¾ Client is responsible for shipping costs. ¾ Overnight shipping is available.

5 of 9

Customized Training

¾ Five (5) days of training per contract year. ¾ Training is tailored to clients needs. ¾ Client is responsible for travel expenses.

Doble Service & Equipment Agreement

Doble Client Committees & Conferences

manufacturer (or affiliate) may participate in the Doble

Client Committees. Committee meetings are held twice annually (spring & fall).

¾ Any Doble client may attend the annual Doble Client

Conference (spring).

¾ Committees and Conference Sessions fall into 8

categories: (1) Transformers, (2) Bushings, Insulators, and Instrument Transformers, (3) Circuit Breakers, (4) Arrestors, Capacitors & Cables, (5) Rotating Machinery, (6) Insulating Materials, (7) Protective Apparatus, and (8) Asset Maintenance Management.

7 of 9

Doble Service & Equipment Agreement

Doble E-mail Forums

¾ Maintenance Engineers – an open form where clients may

converse electronically about anything related to the power industry … system operations, safety procedures,

maintenance/testing practices, equipment issues related to specific equipment, urgent equipment needs, etc. (open to

non-manufacturing clients only).

¾ DTA Users - an e-mail forum for users of the DTA software. ¾ SFRA Users – an e-mail forum for users of the Doble’s SFRA

test equipment and software.

¾ TRX Users - an e-mail forum for users of the TRX software. ¾ PROTEST Users - an e-mail forum for users of the PROTEST

software.

Doble Service & Equipment Agreement

Doble Knowledgebase

¾ The Doble Knowledgebase is an electronic system

that may be accessed through the Doble website (www.doble.com).

¾ The Doble Knowledgebase contains a large collection

of information … Doble Conference papers, manuals and guides, frequently asked questions (from Maintenance Engineers e-mail forum), manufacture service

9 of 9

Doble Laboratories

¾ Doble’s HV laboratory and oil/material laboratories

services are available to Doble clients at an additional cost.

¾ Doble has three oil/material laboratories: (1)

Watertown, Massachusetts, (2) Indianapolis, Indiana, and (3) Kent, Washington.

¾ Contract includes $500.00 worth of laboratory

1 Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

M-Series Safety Features & Practices

Michael Horning, Principal Engineer Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

M-Series Safety Features

Ground Relay. During normal operation, there are two grounds connected to the M4000; the #6 AWG ground lead and the ground provided by the 120V power supply.

If the resistance between these two grounds exceeds 50-100Ω, then the ground relay will not pick up, thus preventing the operation of the test set.

The purpose of the ground relay is to protect against hazards associated with differences in ground potential.

3

An Acceptable Method

5

Improper method!

Safety Switches.

Two safety switches are provided.

Both must be depressed in order for test voltage to be applied.

If either of these switches is released during test, then the test voltage will be immediately removed.

The short safety switch is used by the “Operator”, and the long (extension) safety switch is used by the “Safety

7

Safety Beeper . (M4000 only)

For the first few seconds after a test is initiated, the safety beeper will sound. This provides an audible signal that a test has been initiated.

Safety Strobe. (M4000 only)

Whenever voltage is being applied, the safety strobe will flash.

This strobe has a magnetic base for convenient mounting. It should be positioned in a location that will alert all personnel in the area whenever a test is in progress.

9

Prepare the Specimen for Testing Conduct crew meetings, de-energize, ground, isolate,

safeguard, etc., using your company’s established safe work procedures, and in compliance with applicable safety

regulations (OSHA, NFPA, NESC etc).

Grounding the Test Equipment

The #6 ground lead should always be the first test lead connected and the last test lead removed. This ensures that the test set chassis is safely grounded, and it removes touch potential hazards.

11

voltages on ungrounded equipment. The following

procedures will minimize the chance of exposure to static or induced voltages while applying and removing test leads:

Before applying test lead connections, a ground should be applied to the specimen connection point. For Energized, UST, or Guarded circuits the ground should be removed after the test lead is connected and before initiating the test.*

* Note: Proper protective equipment and live line tools must be utilized while applying and removing grounds.

• When applying connections, all test leads should be

connected to the M4000 first and then to the specimen connection point.

• Before removing test lead connections, a ground should be

applied to the specimen connection point.*

• When removing connections, all test leads should be

removed from the specimen connection point first and then from the M4000.

* Note: Proper protective equipment and live line tools must be utilized while applying and removing grounds.

13

Click picture

Safety During Tests

Good Communication. A uniform system of communication between the operator and the safety lookout (and all other affected personnel) should be established in order to eliminate confusion during testing.

The following is an example of common communication:

1. Operator - “Ready?”

2. Safety Lookout – Responds “Ready” if the connections are made and

the work area is safe, or “No” if not ready and safe.

3. Operator – “Going hot.”

4. Safety Lookout – Echoes “Going hot” to acknowledge the operator.

5. Test is initiated … completes.

6. Operator – “All Clear.” Operator extends the operators safety switch at

15

Safety Lookout. The Safety Lookout should position himself in an area where he can observe all terminals and access points to the apparatus under test.

Safety Switches. The Safety Switches can be released at any time to terminate a test. This may be necessary if unauthorized personnel enter the area or if some other undesirable situation develops.

Safety During Tests (continued)

Testing with Personnel on the Specimen. Testing with personnel on the specimen is

strongly discouraged

Handling the HV Cable. Handling the HV cable during test, even when wearing insulated gloves, is

strongly

discouraged

17

Strongly Discouraged!

The Ladder Was NOT Tied-Up Either

- Do not hold high-voltage cable during a test.

19

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Power Factor Basic Theory

Mike Horning, Principal Engineer Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

Capacitors, Resistors, & Inductors

0 90 180 270 360 IR ↓ ↓ ↓ IL IC E ↓ 0 ¼ 1/2 3/4 1 Cycle

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C = Aε/4πd

I

= E(2πf)C

ε

Question: Is an insulation system like a capacitor?

Capacitors (continued)

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A “Real” Capacitor is “Imperfect”

As the insulation becomes contaminated or deteriorates… (1) the resistance (R) goes down,

(2) the resistive current (I_R) goes up, (3) and the dielectric losses (watts) go up.

Capacitors (continued)

In a perfect capacitor, no current flows through the capacitor. Rather, the current ICflows back-and-forth from plate-to-plate through the source. A real capacitor is imperfect, and a small amount of current flows through. This current (I_R) generates dielectric losses [watts]. P [watts] = I_R2_R

Power Factor

Power Factor = cos(θ) %PF = 100cos(θ) %PF = 100(I_R/I_T) = 100(W/VA)

%PF = 100(I_RE/I_TE) =100(P/ I_TE) Assuming E=10,000 volts, and converting I_Tfrom

7 of 20 Power Factor is affected by both contamination (watts) and capacitance (mA).

IC = 100 m A IR= 10 mA E WLOSS= 100 C = 26,500 pF PF = 9.95% IT≅ IC IT= 100 .5 m A Case 2 Contamination IC = 100 m A IR= 1 mA I=T 100 .005 mA E WLOSS= 10 C = 26,500 pF PF = 1.00% IT≅ IC Case 1 Starting Condition IC = 80 m A IR= 1 mA E I=T 80. 006 mA WLOSS= 10 C = 21,200 pF PF = 1.25% IT≅ IC Case 3 Change in A, d, or ε

Except for extreme cases, contamination has only a small effect on the measured current IT.

A significant change in ITis usually related to a change in capacitance; IT≅ IC= EωC.

Power Factor vs. Specimen Size

Specimen #1, 5 MVA Transformer 0.5 20 Specimen #2, 10 MVA Transformer 0.5 10 If specimen #1 and #2 are made with the same insulation, and the insulation

is in the same condition, then the power factors will be the same.

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Importance of Testing the Smallest Subsystem

P.F.=0.5% 0.3% 1.1% 0.2% 0.4% LV Circuit HV Circuit Buswork Bushings Case 2 •Four (4) subsystems of non-equal quality. •Each subsystem may have a different power factor. •The total system power factor is a measure of the average quality/condition of all insulation included in the test.

Total System Test

0.5%

0.5% 0.5%

0.5%

Case 1 •Four (4) subsystems of equal quality.

•Each subsystem has equal power factor and they are equal to the total system power factor (power factor is independent of size).

•Each subsystem may have a higher meggar reading than the total system.

Subsystem Tests

It is important to test the smallest subsystem possible (economically feasible) in order to evaluate the quality of each individual subsystem. Otherwise, bad insulation could be disguised by good insulation (and vice-versa).

Power Factor vs. DC Resistance Testing

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Kirchoff’s Current Law – All current leaving must return. Therefore, by KCL all current leaving the test set through the HV Cable must return to it … either through the LV Test Leads (red or blue) or the Ground Lead.

mA &W Meter

M4100

Guard

Meter = Measured Guard = Not Measured

Low Voltage (LV) Test Leads Ground Test Lead High Voltage (HV)

We can choose to measure the RED LEAD, the BLUE LEAD, the GROUND LEAD, or ANY COMBINATION (any two, or all three) by specifying the correct TEST MODE.

Internal to the M4100, test leads that are connected to the METER will be measured, and test leads that are connected to GUARD will not be measured.

The TEST MODE is specified in the DTAF software. It is an instruction that tells the M4100 which test leads to connect to the meter and which leads to connect to guard circuit.

Test Modes

GST= Grounded Specimen Test

Measures anything connected to ground Measures grounded insulation.

UST- Ungrounded Specimen Test

DOES NOT measure anything connected to ground (ground is guarded)

Measures ungrounded insulation

(GST-) Ground or Guard- Describes the connection of the LV leads … either connected to the ground point (measured) or the guard point (not measured). (UST-) Measure or Ground- Describes the connection of the LV leads … either connected to the meter (measured) or the ground point (not measured).

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Test Modes - Ground Lead Only

mA &W Meter B C A M4100 Guard IA TEST MODE #1 GST DTAF “GND” Measures I_A

Test Modes – Ground and One LV lead

mA &W Meter B C A M4100 Guard IA IB TEST MODE #1 GST Ground Red DTAF “GND-R” Measures I_A+ I_B mA &W Meter B C A M4100 Guard I_A IB TEST MODE #2 GST Guard Red DTAF “GAR-R” Measures IA B C M4100 IB TEST MODE #3

15 of 20 mA &W Meter B C A M4100 Guard

Test Mode DTAF Abbreviation Measures

#1 GST Ground Red, Ground Blue GND-RB IA+ IB+ IC

#2 GST Guard Red, Guard Blue GAR-RB I_A

#3 GST Guard Red, Ground Blue GAR-R I_A+ I_C #4 GST Ground Red, Guard Blue GAR-B I_A+ I_B #5 UST Measure Red, Measure Blue UST-RB I_B+ I_C #6 UST Measure Red, Ground Blue UST-R IB

#7 UST Ground Red, Measure Blue UST-B IC

Power Factor vs. Voltage, Tip-Up

Stator Winding Motor Insulation

Voids 2kV L-G E %PF %PF @ 2kV %PF @ L-G

In dry type insulation systems (i.e. generators, dry-type transformers) there may be gas pockets or voids in the insulation.

As the voltage stress is increased, tracking may begin to occur across the voids. This results in a higher watts loss and Power Factor values.

Tip-Up = %PF@V_L-G - %PF@2KV

When possible, it is also suggested to test at 110% or 125% of the line-to-ground rating. This may give an indication of what the future might bring.

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Electrostatic Interference

H-V Test Cable

GND Lead

Test Set Step Up Transformer CA Guard Point IA CE IE 60 Hz Lines IA+IE H-V Test Cable GND Lead

Test Set Step Up Transformer CA Guard Point IA CE IE 60 Hz Lines IA-IE

Interference current, IE, follows the path of least impedance to ground.

The Line Sync Reversal method reverses the polarity of the test set applied voltage resulting in a reversed current, IA. The effects of interference are eliminated by calculating the average of the currents measured in the forward and reverse polarity tests.

[IFOR+ IREV]/2 = [(IA+ IE) + (IA– IE)]/2 = (2IA)/2 = IA

Note: If IE> IA, then the above equation is incorrect unless the polarity of the current is recorded. Therefore, when taking watts readings, it is important to check and record the polarity.

The Line Freqency Modulation method conducts test at 57 Hz and 63 Hz and averages the results. By testing and measuring “off frequency” the effects of the 60 Hz interference are eliminated.

Forward Polarity Test Reverse Polarity Test

Power Factor vs. Dissipation Factor

Power Factor = cos(θ) = IR/IT

%PF = 100cos(θ) = 100(I_R/I_T)

Dissipation Factor = tan(Δ) = I_R/I_C %DF = 100 tan(Δ) = 100(IR/IC)

For values less than 10%, %PF

≅ %DF

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Whenever possible, it is desirable to have the apparatus temperature above freezing before conducting insulation tests.

If a specimen that is contaminated with water is tested below freezing (apparatus temperature), the effects of water contamination may be much less noticeable. The resulting watts loss and power factors may not be representative of the condition of the equipment when tested above freezing (i.e. testing the same specimen above freezing may yield significantly higher power factors).

Alternative to testing below freezing: (1) Choose another day and/or time to test;

(2) Test transformers immediately after removing from service before the oil temperature falls below freezing; (3) Construct a hasty shelter and apply heat with radiant or forced air heaters.

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Transformer Power Factor Tests

Mike Horning, Principal Engineer Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

Test Voltages

Liquid-Filled Transformers - Full Oil Level

Transformer Power Factor

Rating, V_L-L (KV) Test Voltage (KV) 12 and Above 10

5.04 to 9.72 5

2.4 to 4.8 2

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Test Voltages

Liquid-Filled Transformers – Drained or Reduced Fluid Level

Rating, V_L-L (KV) Test Voltage (KV) 161 and Above 10

115 to 138 5

34 to 69 2

12 to 25 1

Below 12 0.5

Rating, VL-L (KV) Test Voltage (KV)

12 and Above 1

Below 12 0.5

Delta and Ungrounded/Ungraded Wye Windings

Grounded/Graded Wye Windings and Single Phase with Grounded Neutral

SAFETY

SEE

NEXT

PAGE!

SAFETY

Liquid-Filled Transformers – Drained or Reduced Fluid Level

Transformer Power Factor

In the presence of oxygen, oil vapors and combustible gases can be ignited by an energy source such as an electrical arc or spark. Do not apply test voltage before determining - by direct

measurement - that the gas space and insulating liquid contain safe combustible gas levels. Purging with dry nitrogen is recommended to reduce the oxygen level in the gas to less than 2%.

Never apply test voltage to a transformer whose windings are under vacuum.

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Transformer Power Factor

Test Voltages Dry-Type Transformers

Rating, V_L-L (KV) Test Voltage (KV) 14.4 and Above 2 and 10

12 to 14.4 2, V_L-G, and 10 5.04 to 8.72 2 and 5

2.4 to 4.8 2

Below 2.4 1

Rating, VL-L (KV) Test Voltage (KV)

2.4 and Above 2

Below 2.4 1

Delta and Ungrounded/Ungraded Wye Windings

Grounded/Graded Wye Windings and Single Phase with Grounded Neutral

Transformer Power Factor

Load Tap Changers

If the transformer contains a LTC, then it should be moved to any non-neutral tap position for/during overall power factor testing.

Certain LTC schemes contain non-linear resistor elements (surge protection) that may cause abnormal test results (high or negative power factors) if tested in the neutral tap position.

H1

H2 X2

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Physical Representation of a Three-Phase Two-Winding Transformer One of Three Phases Shown HV Winding LV Winding Core - Grounded Tank - Grounded CL CHL CH

Short the Bushings for Each Winding

Transformer Power Factor

H1 H2 H3

X0 X1 X2 X3

If the windings are not shorted, an inductance is introduced into the current reading. Instead of measuring I_T, you will measure I_T’. This will cause the calculated power factor to be higher than the true value, and the calculated capacitance will be lower than the true value.

The neutral bushing, X0, must be ungrounded. Isolate the neutral bushing from any grounding resistors or reactors.

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Two-Winding Transformer – Dielectric Model

Transformer Power Factor

TANK & CORE

C_H C_L C_HL x₁ x2 x3 x0 H2 H₃ H₁

Two-Winding Transformer Test Circuits

Transformer Power Factor

11 of 21

Two-Winding Transformer Test Table

Test No. Mode Energize Ground Guard UST Measure

1 GST High Low - - CH+ CHL

2 GST High - Low - CH

3 UST High - - Low CHL

4 CHL

5 GST Low High - - CL+CHL

6 GST Low - High - CL

7 UST Low - - High CHL

8 CHL

Test 1 minus Test 2 (W, mA)

Test 5 minus Test 6 (W, mA)

Liquid-Filled Transformers – Temperature Correction Factors

Transformer Power Factor

13 of 21

Transformer Power Factor

Liquid-Filled Transformers – Temperature Correction Factors Required Data for DTAF Software

1. Manufacturer

2. KV Rating (Left Box, Primary KV)

3. KVA Rating (Left Box, Base KVA)

4. Coolant Type

5. Year of Manufacture

7. Apparatus Temperature

8. Ambient Temperature

6. Tank Type

Transformer Power Factor

Three-Winding Transformer Test Table

Test No. Mode Energize Ground Guard UST Measure

1 GST High Low Tert - CH+ CHL

8 CLT

9 GST Tert High Low - CT+ CHT

10 GST Tert - High&Low - CT

2 GST High - Low&Tert - CH

3 UST High Tert - Low CHL

4 CHL

5 GST Low Tert High - CL+CLT

6 GST Low - Tert&High - CL

7 UST Low High - Tert CLT

Test 1 minus Test 2 (W, mA)

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Autotransformer – Shorted Bushings

Y1 Y2 Y3 Tertiary Delta H1 H0X0 X1 X2 X3 H3 H2 Autotransformer

Transformer Power Factor

Autotransformer Test Tables

Test No. Mode Energize Ground Guard UST Measure

1 GST High&Low Tert - - CH+ CHT

8 CTH

2 GST High&Low - Tert - CH

3 UST High&Low - - Tert CHT

4 CHT

5 GST Tert High&Low - - CT+CTH

6 GST Tert - High&Low - CT

7 UST Tert - - High&Low CTH

Test 1 minus Test 2 (W, mA)

Test 5 minus Test 6 (W, mA)

Test No. Mode Energize Ground Guard UST Measure

1 GST High&Low - - - CH

Autotransformer Without Tertiary or With Buried Tertiary (i.e. no tertiary bushings).

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Transformer Power Factor

Step-Voltage Regulator

A step-voltage regulator is an autotransformer with a load tap changing switch.

S L

SL

1:1 Autotransformer

±10% Load Tap Changer Test No. Mode Energize* Measure

1 GST S-L-SL CH

Rating (KV) Test Voltage (KV) 12.47 and Above 10 <12.47 and >4.16 5

4.16 and Below 2

Test Voltages

Test Table

* Short S, L, and SL bushings

Transformer Power Factor Tests – Evaluation of Results

Transformer Power Factor

General

•Whenever possible, compare to prior tests.

•Significant changes in W, mA, %PF, or capacitance should be investigated.

Liquid-Filled Transformers

•New(er) – Expect power factors of 0.5% or less. •Service Aged – Expect power factors of 1.0% or less.

•Investigate if one overall insulation power factor is significantly higher than the others (i.e. CHhigher than CLand CHL).

•Free-breathing designs tend to have higher power factors due to higher moisture content in the oil and cellulose insulation.

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Transformer Power Factor Tests – Evaluation of Results

Voltage Regulators

•The expected power factors for voltage regulators vary according to the Manufacturer and Model.

•On larger units, the LTC switch may be housed in a separate compartment (i.e. it does not share the same oil volume as the transformer). For these units, the expected power factors should be similar to those stated for liquid-filled transformers.

Ventilated Dry-Type Transformers

•Expect C_H ≤ 3%, C_HL≤ 2%, C_L≤ 4%, and Tip-up ≤ 0.5% Epoxy-Encapsulated Dry-Type Transformers

•Expect CH ≤ 3%, CHL≤ 1%, CL≤ 2%, and Tip-up ≅ 0% Dry-Type Transformers

•The power factor results for dry-type transformers are often very sensitive to humidity. Hence, cold units may need to be dried in order to obtain acceptable power factors.

Power Factors for Service Aged Transformers

Transformer Power Factor

30Y 32Y 34Y 36Y 38Y 40Y

0.50 0.25 0.75 1.00 1.25 1.50 1.75 42Y Po w e r Factor [% ]

Transformer Age Current

Test

Future Test

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CH Power Factors of Liquid-Filled Transformers

Transformer Power Factor

20 120 240 180 80 60 25 20 15 0 50 100 150 200 250 300 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 N u m b er o f T ran sf o rm e rs

Bushing Power Factor & Capacitance Tests

Mike Horning, Principal Engineer Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

Typical Capacitance-Graded Bushing

Sight-Glass Liquid or Compound Filler Insulating Weathershed

Main Insulating Core Tap Insulation Tap Electrode Mounting Flange Ground Sleeve Tapped Layer Lower Insulator

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Graded Bushing - Core Construction

Core Wind C2 Plate Foil or Paint Common Construction Semi-Conducting Paper Herringbone Pattern C2 Plate Core Wind Distributed Capacitance

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C2 Plate

ABB O + C, 27 kV

7 of 57

Graded Bushing – Electrical Characteristics

Voltage stress is evenly distributed across the insulation. As capacitive layers (i.e.. CA, CB, etc.) are shorted out the

overall capacitance, C₁, increases. CA CB CC CD CJ CE NTE R CONDUCTOR CK CE CF CG CH CI V1 = V2 = V3 = V4 = V5 = V6 = V7 = V8 = V9 = V10

Line-to-Ground System Voltage Main Insulation C1 CA = CB = CC = CD = CE = CF = CG = CH = CI = CJ CK Tap Insulation C2 Center Conductor Tap Electrode Grounded Layer or Flange

Adding Capacitors in Series

C₁ C₂ C₃ C_N C_T C_T2> C_T1 Shorting out a capacitor results in an increase in C_N 1 C_T 1 C₁ 1 C₂ 1 C₃ 1 = + + + … + 2 2 2 C_T1

Case 1: 3 Capacitors in Series

2 3 C_T1 1 2 1 2 1 2 1 = + + = 2 2 C_T2

Case 2: Shorted Capacitor

2 C_T2 1 2 1 2 1 = + = 2

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Test Tap Potential Tap

Test Tap Potential Tap

Test Voltage applied to Tap Electrode, Test Voltage applied to Tap Electrode , Recommended Test Voltage = 500V Max Recommended Test Voltage = 2000V Max Exception: Ohio Brass Type L = 250V Max

Bushing Rated ≤ 69KV Bushing Rated > 69KV

C2 ≅ C1 [capacitance] C2 ≅ C1 x 10 …or… C2 >> C1 [capacitance] In Service:Tap Grounded In Service:Tap Grounded,

Used as a Potential Source, OR Floating

C2 Plate is Outermost Foil C2 Plate is Next Inner Foil Outer Foil Permanently Grounded Tap Connected to C2 Plate by Friction Tap Connected to C2 Plate by Solder,

Clamp, or Other “Solid” Connection Tap Cover 1 ½” or Smaller Tap Cover 2 ½” or Larger

Tap Well Dry Tap Well Dry or Oil/Grease Filled

Test Taps vs. Potential Taps – “Typical” Differences

11 of 57

Doble Recommended Test Voltages

For Voltage Applied to Bushing Center Conductor

Test Voltages Applied to Center Conductor

Bushing Rating (kV) Recommended Test Voltage (kV)

>8.7 10

8.7 8

5 5

4.3 4

1.2 1

NOTE: The test voltages recommended for the bushing C1 UST test are applicable to spare bushings and for bushings installed in apparatus. For bushings in apparatus there may be unusual circumstances whereby the voltage rating of a bushing is greater than the voltage rating of the apparatus terminal to which it is connected; For example, the neutral terminal of a transformer winding. In such cases, though rare, the normal test voltage for the bushing C1 UST tests may have to be reduced to that which can be applied for the overall tests on the apparatus itself.

Bushing Test Connections

Short the bushings of each winding. Short the bushings of each winding.

Ground windings not under test. Ground windings not under test.

DO NOT FORGET to replace the tap cover after test! DO NOT FORGET to replace the tap cover after test! Ground Lead

LV Test Lead

Only remove the tap cover from the bushing under test. Only remove the tap cover from the bushing under test.

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Incorrect

(Unshorted)

Phase A

0.80% = D

Phase B

1.05% = I

Phase C

0.76% = D

Correct

(Shorted)

0.39% = G

0.40% = G

0.39% = G

For apparatus containing windings, when the bushings are not shorted there may be a difference in potential at each bushing (due to winding inductance). If so, there may be a capacitive cross-coupling between phases which can result an increase in watts and power factor.

Bushing C1 Test, Routine Method - Connections HV Cable

Guard LV Test Lead

Apparatus Ground Ground Lead

mA & W

Test Mode: UST

C1 Test Includes •Core insulation between center conductor and tapped layer. C1 Test Includes •Core insulation between center conductor and tapped layer. C1 %PF is temperature corrected to 20°C using the average of the apparatus and ambient temperature. C1 %PF is temperature corrected to 20°C using the average of the apparatus and ambient temperature.

Connection to Parent Apparatus

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Bushing C1 Test, Routine Method

Gro

und

Lead

LV Test

Lead

Test Tap Center Co

nduc to r mA & W ICG HV Cable Guard C2 C1 IC1

Test Mode: UST

CG

CG = Capacitance from center conductor circuit to ground. Includes parent apparatus insulation and upper and lower insulators of bushings. CG = Capacitance from center conductor circuit to ground. Includes parent apparatus insulation and upper and lower insulators of bushings.

Typical C1 Test Data

Description Current (mA) Watts %PF

Typical Good 1.08 0.03 .28 Bushing Same Bushing, 1.09 0.06 .55 Contaminated Same Bushing, 1.19 0.04 .34 Shorted Condenser layers

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DTAF Software – Required Fields

Bushing Temperature Correction Factor

1. Manufacturer 2. Type

3. Ambient Temperature (Probe) 4. Apparatus Temperature

Transformer – Top Oil Temperature

Oil Circuit Breaker – Ambient (with discretion)

Bushing C1 Tests – Abnormal Results

¾ Clean and dry upper (and lower) porcelain and retest using Routine C1 method.

¾ Perform Inverted C1 test. This test is less sensitive to the effects of surface contamination (see slides 17-18).

¾ Repeat test using a guard-collar to guard surface leakage on upper weathershed (see slide 19).

¾ Perform Hot-Collar tests using both the GST-Ground and UST ¾ Check connections. Ensure that all intentional connections have

good metal-to-metal contact (no paint or oxidation). Verify that there are no unintentional grounds on test leads. Avoid using insulated wire for phase-to-phase shorts. Inspect test leads for damage. Verify proper test mode (UST).

21 of 57

Test Kv mA Watts Measure %

Power Factor

10 1.313 -.007 -.053

Bushing C1 Tests, Inverted Method - Connections

LV Test Lead

HV Cable

Ground Lead

mA & W Guard

Test Mode: UST

Apparatus Ground Inverted Method •Useful for investigating negative power factors obtained using the C1 Routine Method.

•A common cause of

negative C1 power factors is surface contamination on the upper or lower insulators of the bushings. Inverted Method •Useful for investigating negative power factors obtained using the C1 Routine Method.

•A common cause of

negative C1 power factors is surface contamination on the upper or lower insulators of the bushings.

0.5 or 2 kV Test

23 of 57

Bushing C1 Test, Inverted Method

LV Test Lead

Ground Lead

Test Tap Center Co

nduc to r mA & W IC2 Guard HV Cable C2 C1 IC1 CG

Using the C1 Inverted Method, the center conductor is effectively grounded via connection to the LV Test Lead. Hence, there is no voltage stress across CG. Using the C1 Inverted Method, the center conductor is effectively grounded via connection to the LV Test Lead. Hence, there is no voltage stress across CG.

Test Mode: UST

0.5 or 2 kV Test

Bushing C1 Test with Guard-Collar

HV Cable

Guard LV Test Lead

Apparatus Ground Ground Lead

mA & W

Test Mode: UST

Guard-Collar

•May eliminate the

effects of surface contamination from the test result.

•Use one guard

collar positioned near the bottom skirt.

•Or, use multiple

collars located at various locations on the upper weathershed. Guard-Collar

•May eliminate the

effects of surface contamination from the test result.

•Use one guard

collar positioned near the bottom skirt.

•Or, use multiple

collars located at various locations on the upper weathershed.

25 of 57 C2 Test Includes

•Tap insulator •Core insulation

between tapped layer and bushing ground sleeve •Portion of liquid or compound filler •Portion of weathershed near ground sleeve C2 Test Includes •Tap insulator •Core insulation between tapped layer and bushing ground sleeve •Portion of liquid or compound filler •Portion of weathershed near ground sleeve C2 %PF is not temperature corrected. C2 %PF is not temperature corrected. LV Test Lead HV Cable Ground Lead mA & W Guard

Test Mode: GST-Guard

Apparatus Ground

0.5 or 2 kV Test

Bushing C2 Test

Test Mode: GST-Guard

27 of 57

Why Perform C2 Tests

?????

•Internal Flashover Around the Main Core is

a Real and Serious Threat

to all Sealed

Capacitance Graded Bushings

•The C2 Power Factor Test has Been

Shown, in Some Cases, to be a More

Apparent Indicator of Internal Fluid

Contamination Than the C1 Test

29 of 57

GE Type U 230kV Bushing:

C1

C2

Date

%PF Cap

%PF

Cap

1/6/82

.31

508.8

.28

4134

5/1/96

.58

510.2

2.26

4138

1/30/97

Failed in service

Example:

McGraw-Edison Type PA 23kV Bushings

Bushing #

C1(%PF)

C2(%PF)

X1 0.46

0.50

X2

0.60

2.78

X3

0.45

0.50

• X2 was removed from service and found

to have highly contaminated fluid with low

dielectric-breakdown strength

31 of 57

Bushing C2 Tests – Abnormal Results

C2 Troubleshooting & Investigations

¾ Clean and dry tap insulator and retest.

¾ Add an additional ground to the bushing flange and retest. Poorly grounded bushing flanges can cause both high and low/negative C2 test results. If a poorly grounded flange is discovered, then corrective actions should be taken to ensure proper grounding before returning to service.

¾ Check connections. Ensure that all intentional connections have good metal-to-metal contact (no paint or oxidation). Verify that there are no unintentional grounds on test leads. Avoid using insulated wire for phase-to-phase shorts. Inspect test leads for damage. Verify proper test mode (GST-Guard).

33 of 57

35 of 57

Spare Bushing Tests

Comments on Spare Bushing Tests

¾ Do not test in wooden crate or on wooden stand.

¾ Support bushing on a grounded metal stand if possible.

¾ Web slings may be used for tests.

™ Cleanliness of sling may affect test results. ™ Sling should be kept clear of energized points.

¾ Connect ground lead directly to bushing flange.

¾ Ground bushing flange to substation/building ground.

¾ Clean upper and lower surfaces before testing.

Bushing “Overall” Test

Test Includes •Main C1 Core Insulation •Upper Insulating Weathershed •Sight-Glass •Lower Insulator •Portion of Liquid or Compound Filler Test Includes •Main C1 Core Insulation •Upper Insulating Weathershed •Sight-Glass •Lower Insulator •Portion of Liquid or Compound Filler %PF is temperature corrected to 20°C using the ambient %PF is temperature corrected to 20°C using the ambient Guard

Bushing and

Test Mode: GST-Ground

Ground Lead

HV Cable

37 of 57 Note: For most bushing types, the C2 insulation will be shorted out (as shown) via the tap cover. Note: For most bushing types, the C2 insulation will be shorted out (as shown) via the tap cover.

Ground Lead

Test Tap Center Co

nduc to r mA & W IC1+ICG HV Cable Guard C2 C1

Test Mode:

GST-Ground

39 of 57

Capacitance

Bushing C1, C2, and Overall – Evaluation of Results

¾Suggested limits:

±5% of Nameplate Capacitance = Investigate

¾Each shorted capacitance layer will cause an increase in C1 capacitance of 5% to 15%.

¾If the tap electrode becomes disconnected from the C2 plate there may be a dramatic decrease in C2 capacitance. This may also cause a change in the C1 capacitance.

¾Oil or compound leaks may cause a decrease in capacitance.

¾Differences in factory and field test procedures and/or test conditions may result in differences in capacitances.

C2 Capacitance – Factory vs. Field Test “Conditions”

C2 capacitance varies depending on the length of the outer condenser layer and the distance to the grounded test tank wall.

C2 = C2A + C2B [pF]

Flange

Test Tank Outer

C2A

C2B

C1 Bushing

Center Conductor

41 of 57

Haefley Type COTA Bushing

Factory C1 Test Field C1 Test

Because the test tap is buried, the factory C1 test cannot be reproduced in the field. Using the nameplate capacitances, the Doble C1 capacitance may be calculated:

C1_DOBLE= (C1_NPx C2_NP) / (C2_NP– C1_NP) [pF]

C1 C2 Potential Tap

Test Tap (burried)

Center Conductor Flange C1 C2 Potential Tap Test Tap Center Conductor Flange C0

Bushing C1, C2, and Overall – Evaluation of Results

Power Factor

¾Most modern oil-filled condenser type bushings have C1 power factors of approximately 0.5% or less, and values exceeding 1.0% are questionable. Specific limits for various manufactures and types are given on the following slides.

¾Bushings that exhibit a history of continued increase in power factor should be investigated and considered for removal from service.

¾Power factors that are significantly lower than nameplate or prior tests may be the result of extreme contamination patterns and/or tracking conditions, and these results should be investigated.

¾A common cause of high C2 power factors is moisture or contamination on the tap insulator. Cleaning and drying of the tap insulator will frequently correct the problem.

43 of 57

Bushing C1 & Overall Power Factor Limits

ASEA

Type (* All Types) Description Typical %PF Questionable %PF

GOA 250 0.5% 0.7% GOA OTHER 0.45% 0.65% GOB 0.5% 0.7% GOBK 0.5% 0.7% GOC 0.4% 0.6% GOE <800 kV 0.45% 0.65% GOE 800kV 0.4% 0.6% GOEK 0.4% 0.6% GOEL 0.4% 0.6% GOF 0.45% 0.65% GOFL 0.4% 0.6% GOG 0.45% 0.65% GOH 0.25% 0.45% GOM 0.45% 0.65%

* Up to 3% deviation from nameplate capacitance is considered acceptable. * Remove from service when the difference between the nameplate and measured C1 power factors exceeds 75%.

* This information per ABB Components bulletin #2750 515E-56 dated 1990.

ASEA BROWN BOVERI (ABB)

Type Typical %PF Questionable %PF

O+C * 0.5% Double Nameplate

T * 0.5% Double Nameplate

* Per ABB instruction leaflet 44-666E dated 7/1/1990, contact the manufacturer if the C1 capacitance increases over 110% of the initial installed value.

BUSHING COMPANY (REYROLLE LIMITED)

Type Typical %PF Questionable %PF

OTA * 0.35% 0.6%

45 of 57

FEDERAL PACIFIC and PENNSYLVANIA TRANSFORMER

Type DESCRIPTION Typical %PF Questionable %PF

P Paper-oil condenser, sealed 0.5% 1.0% PA Paper-oil condenser, sealed 0.5% 1.0% PB Paper-oil condenser, sealed 0.5% 1.0%

HAEFELEY TRENCH

Type Description Typical %PF Questionable %PF

COTA * Under 1,400 kV BIL 0.30% Double Nameplate COTA * 1,400 kV BIL and above 0.35% Double Nameplate * C1 capacitances 10% over nameplate or 5% over first installed measurement are questionable.

* C2 capacitance may vary by 20% per Heafely fax dated 4/5/1994.

GENERAL ELECTRIC and LOCKE INSULATORS, INC.

Type Description Typical %PF Questionable %PF

A * Through Porcelain 3.0% 5.0%

A ** High Current 1.0% 2.0%

B * Flexible Cable, Compound Filled 5.0% 12.0%

D Oil Filled Upper Portion, Sealed 1.0% 2.0%

F Oil Filled, Sealed 0.7% 1.5%

L Oil Filled Upper Portion, Sealed 1.5% 3.0%

LC Oil Filled Upper Portion, Sealed 1.5% 3.0%

OF Oil Filled Expansion Chamber 0.8% 2.0%

S * Forms C & CG, Rigid Core, Compound Filled 1.5% 6.0%

U Oil-Filled, Sealed 0.5% 1.0%

T Oil-Filled, Sealed 0.5% 1.0%

* Type S, Form F, DF & EF (flexible cable) redesigned as Types B, BD, and BE, respectively. Type S, no Form letter (through porcelain) redesigned as Type A. ** Modern high-current oil-filled solid-porcelain design.

47 of 57

LAPP INSULATORS (INTERPACE CORP.)

Type Description Typical %PF Questionable %PF

PA Paper-Oil Condenser, Sealed 0.5% 1.5% POC Paper-Oil Condenser, Sealed 0.5% 1.5% ERC Epoxy-Resin Condenser Core 0.8% 1.5% PRC * Paper-Resin Condenser Core 0.8% 1.5%

* For older PRC bushings (Serial #s of 00-189100 and lower) the C2 power factor ranged from 4-15% due to high-loss potting compound injected during manufacturing process.

* For newer PRC bushings (Serial #s of 00-189100 and higher), the C2 power factor is normally similar to the C1 value. Newer PRC bushings may have C2 capacitances ranging from 2000-5000 pF (accept the first test capacitance as benchmark, and then compare future test to the benchmark value).

Bushing C1 & Overall Power Factor Limits

LAPP PRC Bushings – Old design (top) and New Design (Bottom)

49 of 57

OHIO BRASS

Type/Description Typical %PF Questionable %PF

Class GK - Type C, 15 to 196 kV 0.4% 1.0% Oil-Impregnated Paper Condenser Core

Class LK - Type A, 23 to 69 kV 0.4% 1.0% Resin-Paper Condenser Core, Oil-filled

ODOF, Class G, and Class L Oil-Filled,

Prior to 1926 and after 1938 1.0-5.0% Initial %PF plus 22% Between 1926 and 1938 2.0-4.0% Initial %PF plus 16%

PASSONI & VILLA

Type Typical %PF Questionable %PF

PNO * 0.4% 0.7%

PAO * 0.4% 0.7%

* This information from Passoni & Villa bulletin #1005 dated 1992.

WESTINGHOUSE

Type / Description Typical %PF Questionable %PF

S *, OS, and FS 0.8% 2.0%

On OCB & Inst. Tx., 69kV & below 1.5% 3.0% (except Types S, OS, and FS)

On OCB & Inst. Tx., 92kV to 138kV 1.5% 3.0% (except Types O, O-Al, OC, and O Plus)

On Power & Dist. Tx. of all ratings, 1.0% 2.0% and OCB & Inst. Tx., 161kV to 288kV

(except Types O, O-Al, OC, and O Plus)

O and O-AL, 92kV to 288kV 0.3% 1.0% O Plus 0.3% 1.0% D Transformer Bushings 1.5% 3.0% (Semi-condenser type) RJ 1.0% 2.0% (Solid Porcelain)

* For Type S bushings the C2 power factor may be as high as 12%.

51 of 57 Test Includes •Portion of Insulating Weathershed •Sight-Glass •Core Insulation in Vicinity of Collar •Liquid or Compound Filler in Vicinity of Collar

•Surface leakage from

Collar to LV test lead & from collar to bushing flange Test Includes •Portion of Insulating Weathershed •Sight-Glass •Core Insulation in Vicinity of Collar •Liquid or Compound Filler in Vicinity of Collar

•Surface leakage from

Collar to LV test lead

& from collar to bushing flange

Hot-Collar Test, GST-Ground Mode

Guard

LV Test Lead

Apparatus Ground Test Mode: GST-Ground

Ground Lead

HV Cable

mA & W

Hot-Collar Test, UST Mode

Test Includes •Portion of Insulating Weathershed •Sight-Glass •Core Insulation in Vicinity of Collar. •Liquid or Compound Filler in Vicinity of Collar. •Surface leakage

from Collar to LV test lead. Test Includes •Portion of Insulating Weathershed •Sight-Glass •Core Insulation in Vicinity of Collar. •Liquid or Compound Filler in Vicinity of Collar. •Surface leakage

from Collar to LV test lead.

Guard

LV Test Lead

Test Mode: UST Ground Lead

HV Cable

53 of 57

When to Perform Hot Collar Tests

¾Bushings not equipped with taps, and overall tests cannot be performed (i.e. cannot be isolated from apparatus).

¾Investigative test when overall, C1, or C2 tests indicate a possible problem.

¾Compound filled bushings with or without taps.

¾Verify oil level in bushings without site glasses or level gauges, or if level gauge is suspect.

Comments on Hot-Collar Tests

Hot-Collar Tests

¾Hot-Collar tests should be conducted at 10KV.

¾Hot-Collar tests can be performed in either the GST-Ground or UST Modes.

™The UST Mode is more susceptible to electrostatic interference.

™The GST-Ground Mode reads additional surface leakage between the collar and the grounded bushing flange.

¾Hot-Collar tests evaluate the weathershed, sight glass, and the core insulation/liquid/compound between the collar and the center conductor.

55 of 57

Comments on Hot-Collar Tests (continued)

Hot-Collar Tests

¾Tests may be performed under various skirts depending on the bushing KV and purpose of test.

™Routine - single Hot-Collar test under top skirt on small bushings rated 15KV and below.

™Routine - several single Hot-Collar tests or one multicollar test on bushings rated above 15KV.

™Additional skirts may be tested for purposes of benchmarking or as part of an investigation.

Hot Collar Tests - Evaluation of Results

1. Good bushings should have losses less than 0.1 watts.

2. Similar bushings should have similar currents and watts when tested

under the same skirt.

3. For historical comparisons, it is important to use the same width of hot

collar band as used in prior test (and same test mode).

Example

Current [uA] Watts

1. Typical Good Bushing / New 90 0.020

2. Same / Similar Bushing Contaminated 95 0.310

57 of 57

H1 H2 H3

T1 = 0.095 mA T1 = 0.097 mA T1 = 0.074 mA

T2 = 0.098 mA T2 = 0.099 mA T2 = 0.080 mA

T3 = 0.101 mA T3 = 0.103 mA T3 = 0.102 mA

Conclusion: The oil level in the H3 bushing is somewhere between the locations used for tests hot-collar tests 2 and 3.

Hot Collar Tests – Abnormal Results

Hot Collar Troubleshooting & Investigations

¾ Clean and dry upper porcelain and retest.

¾ Add an additional ground to the bushing flange and retest. Poorly grounded bushing flanges could cause high hot collar test results. If a poorly grounded flange is discovered, then corrective actions should be taken to ensure proper grounding before returning to service.

¾ Check connections. Ensure that all intentional connections have good metal-to-metal contact (no paint or oxidation). Verify that there are no unintentional grounds on test leads. Avoid using insulated wire for phase-to-phase shorts. Inspect test leads for damage. Verify proper test mode (GST-Ground).

¾ Repeat test in UST Mode. A significant reduction in watt during the UST test generally indicates that there is surface contamination between the collar and the bushings flange.

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Transformer Excitation Current

Mike Horning, Principal Engineer Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

What is Excitation Current?

Transformer Excitation Current

∼

E

I

1:1

E

Excitation current is the current that flows when the winding of the transformer is energized under no-load conditions. It supplies the energy necessary to create the magnetic flux Φ

3 of 15

Excitation Current Tests Can Identify Problems in the Iron Core, Windings , and Tap Changers

Core

•Interlaminar insulation damage

•Abnormal core grounds •Dislocated joints

Windings

•Turn-to-turn shorts •Turn-to-ground shorts (grounded windings) •High resistance turn-to turn or turn-to-ground shorts Tap Changers •Mechanical failures •Auxiliary transformer problems Test Voltages

Transformer Excitation Current

•Maximum test voltage must be limited to the rated line-to-ground voltage. •Test voltage may be limited by the test set current maximums (M4000 maximum 300mA at 10,000V).

•The relationship between excitation current and applied voltage is non-linear. Therefore, for historical comparisons it is necessary to use the same voltage for each test on a given transformer.

5 of 15

Test Procedure – Single Phase Transformer

Transformer Excitation Current

Test No. Mode Energize HV Lead

UST LV Lead

Float * Ground Measure 1 UST H1 H2 X1,X2 Y1,Y2 * * I1-2 2 UST H2 H1 X1,X2 Y1,Y2 I2-1 *Normally grounded terminals of X and/or Y windings must be grounded.

*Ground one leg of any very-low voltage (ie. 120V) secondaries.

Transformer Excitation Current

Test Procedure – Wye Primary (Routine Method)

Test No. Mode Energize HV Lead

UST LV Lead

Float * Ground Measure 1 UST H1 H0 X1,X2,X3 Y1,Y2,Y3 * * * I1-0 2 UST H2 H0 X1,X2,X3 Y1,Y2,Y3 I2-0 3 UST H3 H0 X1,X2,X3 Y1,Y2,Y3 I3-0

7 of 15

Test Procedure – Delta Primary (Routine Method)

Test No. Mode Energize HV Lead

UST LV Lead

Float * Ground Measure 1 UST H1 H2 X1,X2,X3 Y1,Y2,Y3 H3, * H1, * H2, * I1-2 2 UST H2 H3 X1,X2,X3 Y1,Y2,Y3 I2-3 3 UST H3 H1 X1,X2,X3 Y1,Y2,Y3 I3-1

*Normally grounded terminals of X and/or Y windings must be grounded. *Ground one leg of any very-low voltage (ie. 120V) secondaries. *For Y connected secondaries, the neutral bushings must be grounded.

“Traditional” Phase Excitation Current Patterns for Various Core, Winding, and Test Configurations

Transformer Excitation Current

Two Similar (H) and One Lower (L)

•3 leg core form, U primary, routine method.

•3 leg core form, Y primary, neutral available, routine method. •Shell form, with U secondary or tertiary, routine method.

Three Similar

•5 leg core form, any.

•Shell form, without U secondary or tertiary (includes open U).

Two Similar (L) and One Higher (H)

•3 leg core form, U primary, alternate test method.

•3 leg core form, Y primary, neutral unavailable, alternate test method. •Shell form, with U secondary or tertiary, alternate method.

Three Dissimilar

9 of 15

Transformer Design with “Traditional” H-L-H Patterns

Transformer Excitation Current

I_L-1 I_L-2 I_L-3 I_T-1 I_T-2 I_T-3 I_C-1 I_C-2 I_C-3

•During the Excitation Current Test, we measure the total currents IT.

•What would happen to the total current I_Tif the magnitude of the capacitive current ICwas larger than the

magnitude of the inductive current IL?

•For the traditional H-L-H pattern all total currents ITare inductive.

“Newer” Transformer Designs with “Non-Traditional” Patterns

Transformer Excitation Current

I I_L-2 I IC-1 I_C-2 IC-3 I_T-1 I_T-2 IT-3

•Depending on the magnitude of the inductive and capacitive components, the total current measured may be inductive or capacitive (or zero) in each phase.

•Assuming equal magnitudes for the capacitive components, the resulting phase pattern could be HLH, 3 similar, or LHL.

•If the magnitudes of the capacitive components are not equal in each phase, then the pattern could be any combination of H, M, and L

11 of 15

Recommended Tap Positions for Tests

Load Tap Changers

New - 1L, N, 1R, 2R, … , 15R, 16R (18 tests/phase) Routine - 1L, N, 1R, 16R (4 tests/phase)

De-Energized Tap Changers

New - A, B, C, D, E (5 tests/phase) Routine - As found (1 test/phase)

Preventive-Autotransformer LTC Scheme

Transformer Excitation Current

HV Windings U → LV Windings Y → Reversing Switches → Tap Windings → Taps → Preventive Autotransformer → HV Bushings → LV Bushings →

13 of 15

Load Tap Changer Excitation Current Patterns

Transformer Excitation Current

LV LTC, Preventive Autotransform er N _{1R 2R 3R 4R 5R 6R 7R 8R 9R} 10R 11R 12R 13R 14R 15R 16R LTC Tap Positon E xc ita ti o n C u rr en t

LV LTC, Preventive Auto & Series Transform er

N _{1R 2R 3R 4R 5R 6R 7R 8R 9R} 10R 11R 12R 13R 14R 15R 16R LTC Tap Position E xc ita ti o n C u rr en t HV LTC, Preventive Autotransform er N _{1R 2R 3R 4R 5R 6R 7R 8R 9R} 10R 11R 12R 13R 14R 15R 16R LTC Tap Position E xc ita ti o n C u rr en t HV LTC, Resistive Bridge N _{1R 2R 3R 4R 5R 6R 7R 8R 9R} 10R 11R 12R 13R 14R 15R 16R LTC Tap Positon E xc ita ti o n C u rr en t

Transformer Excitation Current

Load Tap Changer Excitation Current Patterns (Continued)

Six examples of LTC transformer excitation current patterns have been given … other patterns do exist.

The most common (numerous) LTC schemes are represented LV LTC, Preventive Auto & Com pensating Winding

N _{1R 2R 3R 4R 5R 6R 7R 8R 9R} 10R 11R 12R 13R 14R 15R 16R LTC Tap Positon E xc ita ti o n C u rr en t

LV LTC, Preventive Auto & Com pensating Winding

N _{1R 2R 3R 4R 5R 6R 7R 8R 9R} 10R 11R 12R 13R 14R 15R 16R LTC Tap Position E xc ita ti o n C u rr en t