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
13
Negative Power Factor
Doble Corporate Headquarters
Telephone: (617) 926-4900
Fax: (617) 926-0528
www.doble.com
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Doble Service & Equipment Agreement
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Doble Service & Equipment Agreement
Overview of Services Included with Lease
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
2 of 9
1
Doble Service & Equipment Agreement
Technical Support
¾ 24 hour support, 365 days a year
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
3 of 9
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.
4 of 9
2
Doble Service & Equipment Agreement
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.
5 of 9
Doble Service & Equipment Agreement
Doble Client Committees & Conferences
¾ Any Doble client who is not an equipment
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.
6 of 9
3
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 nonmanufacturing 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.
7 of 9
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
advisories, etc.
8 of 9
4
Doble Service & Equipment Agreement
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
services per year … an incentive to try our services.
9 of 9
5
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
M-Series Safety Features & Practices
Michael Horning, Principal Engineer
Doble Engineering Company
1
85 Walnut Street, Watertown, MA 02472
Doble Engineering Company
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.
2
1
•
3
An Acceptable Method
Power Cord
Feeding the
M4000
Ground Jumper
From
Specimen Ground
4
2
Improper method!
5
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
Lookout”.
6
3
Wrong!
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.
8
4
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.
10
5
Static and Induced Voltages
Care must be taken to avoid exposure to static or induced
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.
11
• 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.
12
6
Click picture
13
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
arms length with the button released for all to see.
14
7
Safety During Tests (continued)
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.
15
Safety During Tests (continued)
Testing with Personnel on the Specimen. Testing with
personnel on the specimen is strongly discouraged (i.e.
on top of the transformer under test).
Handling the HV Cable. Handling the HV cable during test,
even when wearing insulated gloves, is strongly
discouraged. If a flashover occurs while testing, transient
voltages higher than 10kV can be developed resulting in a
puncture in the cable’s insulation and a hazard to the
personnel holding the cable.
16
8
Strongly Discouraged!
The Ladder
Was NOT
Tied-Up Either
17
Strongly Discouraged!
- Do not hold high-voltage cable during a test.
Source: 1995 DCCM, Page Sec. 1-2.1, David Train and Lawrence Melia
18
9
Know the equipment under test!
19
Don’t be compared to this crew!
20
10
Knowledge Is Power
SM
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
E
↓
IC
↓
IR
↓
IL
↓
0
90
180
270
0
¼
1/2
3/4
0
1/240
1/120
3/240
360
1 Cycle
1/60 Sec
2 of 20
1
Capacitors
εDielectric = CDielectric/CVacuum
Vacuum
Air
Paper
Oil
Silicone Fluid
Porcelain
Water (20ºC)
C = Aε/4πd
IC = E(2πf)C
ε Vacuum = 1.0
ε Air = 1.000549
ε Paper = 2.0
ε Oil = 2.1
ε Silicone = 2.75
ε Porcelain = 7.0
ε Water = 80
3 of 20
Capacitors (continued)
Question: Is an insulation system like a capacitor?
Answer = YES
4 of 20
2
Capacitors (continued)
A “Real” Capacitor is “Imperfect”
In a perfect capacitor, no current flows through the capacitor. Rather, the
current IC flows 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 (IR) generates dielectric losses [watts]. P [watts] = IR2R
As the insulation becomes contaminated or deteriorates…
(1) the resistance (R) goes down,
(2) the resistive current (IR) goes up,
(3) and the dielectric losses (watts) go up.
5 of 20
Power Factor
Power Factor = cos(θ)
%PF = 100cos(θ)
%PF = 100(IR/IT) = 100(W/VA)
%PF = 100(IRE/ITE) =100(P/ ITE)
Assuming E=10,000 volts, and converting IT from
amperes to milliamperes this equation is simplified to
%PF = 10P/IT = 10x[W]/[mA]
6 of 20
3
Changes in Power Factor
Case 1
Starting Condition
Case 2
Contamination
IR = 10 mA
E
WLOSS = 10
C = 26,500 pF
PF = 1.00%
IT ≅ IC
06 mA
IC = 80 mA
IT =
100.
IT = 8
0.0
A
5m
IC = 100 mA
05 mA
IR = 1 mA
IT = 10
0.0
IC = 100 mA
IR = 1 mA
Case 3
Change in A, d, or ε
E
WLOSS = 10
C = 21,200 pF
PF = 1.25%
IT ≅ IC
E
WLOSS = 100
C = 26,500 pF
PF = 9.95%
IT ≅ IC
Except for extreme cases, contamination has only a small effect on the measured current IT.
A significant change in IT is usually related to a change in capacitance; IT ≅ IC = EωC.
Power Factor is affected by both contamination (watts) and capacitance (mA).
7 of 20
Power Factor vs. Specimen Size
IT2
IC2
IT1
IC1
θ
IR1
IR2
Test
Specimen #1, 5 MVA Transformer
Specimen #2, 10 MVA Transformer
E
%PF
0.5
0.5
MΩ
20
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.
Power Factor measures the quality of the insulation, and it is independent of size.
8 of 20
4
Importance of Testing the Smallest Subsystem
Subsystem Tests
Case 1
0.5%
0.5%
•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).
Total System Test
0.5%
0.5%
•Each subsystem may have a higher meggar reading
than the total system.
P.F.=0.5%
0.3%
LV Circuit
0.2%
Buswork
Case 2
•Four (4) subsystems of non-equal quality.
•Each subsystem may have a different power factor.
1.1%
0.4%
HV Circuit
Bushings
•The total system power factor is a measure of the
average quality/condition of all insulation included in the
test.
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).
9 of 20
Power Factor vs. DC Resistance Testing
For multiple layer insulation systems (i.e. condenser type bushings)
AC tests, such as Power Factor, are much more sensitive
to a single deteriorating layer than DC tests.
10 of 20
5
Test Modes
M4100
High Voltage (HV)
Guard
Low Voltage (LV)
Test Leads
mA &W
Meter
Meter = Measured
Guard = Not Measured
Ground Test Lead
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.
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.
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.
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.
11 of 20
Test Modes
TEST MODE Terminology
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).
12 of 20
6
Test Modes - Ground Lead Only
M4100
B
Guard
mA &W
Meter
C
A
TEST MODE #1
GST
DTAF “GND”
IA
Measures IA
13 of 20
Test Modes – Ground and One LV lead
M4100
IB
Guard
B
C
mA &W
Meter
A
TEST MODE #1
GST Ground Red
DTAF “GND-R”
Measures IA + IB
IA
M4100
IB
Guard
B
C
mA &W
Meter
A
IA
TEST MODE #2
GST Guard Red
DTAF “GAR-R”
Measures IA
M4100
IB
Guard
B
C
mA &W
Meter
A
IA
TEST MODE #3
UST Measure Red
DTAF “UST-R”
Measures IB
14 of 20
7
Test Modes – Ground and Two LV leads
M4100
B
Guard
mA &W
Meter
C
A
DTAF Abbreviation
Measures
#1 GST Ground Red, Ground Blue
Test Mode
GND-RB
I A + IB + IC
#2 GST Guard Red, Guard Blue
GAR-RB
IA
GAR-R
I A + IC
#4 GST Ground Red, Guard Blue
GAR-B
IA + IB
#5 UST Measure Red, Measure Blue
UST-RB
IB + IC
#6 UST Measure Red, Ground Blue
UST-R
IB
#7 UST Ground Red, Measure Blue
UST-B
IC
#3 GST Guard Red, Ground Blue
15 of 20
Power Factor vs. Voltage, Tip-Up
Voids
%PF
%PF @ L-G
%PF @ 2kV
E
Winding
Motor Insulation
2kV
Stator
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@VL-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.
16 of 20
8
Electrostatic Interference
60 Hz Lines
IE
60 Hz Lines
CE
H-V Test Cable
CE
H-V Test Cable
IE
CA
Test Set Step
Up Transformer
CA
Test Set Step
Up Transformer
GND Lead
Guard
Point
GND Lead
Guard
Point
IA
IA
IA-IE
IA+IE
Forward Polarity Test
Reverse Polarity Test
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.
17 of 20
Power Factor vs. Dissipation Factor
Power Factor = cos(θ) = IR/IT
Dissipation Factor = tan(Δ) = IR/IC
%PF = 100cos(θ) = 100(IR/IT)
%DF = 100 tan(Δ) = 100(IR/IC)
For values less than 10%, %PF ≅ %DF
When %PF=10, %DF=10.05
As the values get smaller, they get closer.
18 of 20
9
Testing Below Freezing
Whenever possible, it is desirable to have the apparatus
temperature above freezing before conducting insulation tests.
Ice has a volumetric resistance 144 times larger than that of water.
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.
19 of 20
10
Knowledge Is Power
SM
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
Transformer Power Factor
Test Voltages
Liquid-Filled Transformers - Full Oil Level
Rating, VL-L (KV)
Test Voltage (KV)
12 and Above
10
5.04 to 9.72
5
2.4 to 4.8
2
Below 2.4
1
2 of 21
1
Transformer Power Factor
Test Voltages
Liquid-Filled Transformers – Drained or Reduced Fluid Level
Delta and Ungrounded/Ungraded Wye Windings
Rating, VL-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
SAFETY
SEE
NEXT
PAGE!
Grounded/Graded Wye Windings and Single Phase with Grounded Neutral
Rating, VL-L (KV)
Test Voltage (KV)
12 and Above
1
Below 12
0.5
3 of 21
Transformer Power Factor
SAFETY
Liquid-Filled Transformers – Drained or Reduced Fluid Level
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.
4 of 21
2
Transformer Power Factor
Test Voltages
Dry-Type Transformers
Delta and Ungrounded/Ungraded Wye Windings
Rating, VL-L (KV)
Test Voltage (KV)
14.4 and Above
2 and 10
12 to 14.4
2, VL-G, and 10
5.04 to 8.72
2 and 5
2.4 to 4.8
2
Below 2.4
1
Grounded/Graded Wye Windings and Single Phase with Grounded Neutral
Rating, VL-L (KV)
Test Voltage (KV)
2.4 and Above
2
Below 2.4
1
5 of 21
Transformer Power Factor
Load Tap Changers
H1
X1
H2
X2
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.
6 of 21
3
Transformer Power Factor
Physical Representation of a Three-Phase Two-Winding Transformer
One of Three
Phases Shown
HV Winding
CH
CHL
LV Winding
CL
Core - Grounded
Tank - Grounded
7 of 21
Transformer Power Factor
Short the Bushings for Each Winding
H1
H2
X0
X1
H3
X2
X3
If the windings are not shorted, an inductance is introduced into the current reading.
Instead of measuring IT, you will measure IT’. 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.
Use bare (non-insulated) wire for shorting.
The neutral bushing, X0, must be ungrounded. Isolate the neutral bushing from any
grounding resistors or reactors.
8 of 21
4
Transformer Power Factor
Two-Winding Transformer – Dielectric Model
H2
x2
x1
H1
H3
CHL
x0
x3
CL
CH
TANK & CORE
9 of 21
Transformer Power Factor
Two-Winding Transformer Test Circuits
10 of 21
5
Transformer Power Factor
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
Test 1 minus Test 2 (W, mA)
5
GST
6
GST
7
UST
8
Test 5 minus Test 6 (W, mA)
Low
CHL
High
-
-
CL+CHL
Low
-
High
-
CL
Low
-
-
High
CHL
CHL
11 of 21
Transformer Power Factor
Liquid-Filled Transformers – Temperature Correction Factors
12 of 21
6
Transformer Power Factor
Liquid-Filled Transformers – Temperature Correction Factors
Required Data for DTAF Software
1.
2.
3.
4.
5.
6.
7.
8.
Manufacturer
KV Rating (Left Box, Primary KV)
KVA Rating (Left Box, Base KVA)
Coolant Type
Year of Manufacture
Tank Type
Apparatus Temperature
Ambient Temperature
13 of 21
Transformer Power Factor
Three-Winding Transformer Test Table
Test No.
Mode
Energize
Ground
Guard
UST
Measure
1
GST
High
Low
Tert
-
CH + CHL
2
GST
High
-
Low&Tert
-
CH
3
UST
High
Tert
-
Low
CHL
Tert
High
-
CL+CLT
4
Test 1 minus Test 2 (W, mA)
5
GST
6
GST
Low
-
Tert&High
-
CL
7
UST
Low
High
-
Tert
CLT
8
Test 5 minus Test 6 (W, mA)
9
GST
Tert
High
Low
-
CT + CHT
10
GST
Tert
-
High&Low
-
CT
11
UST
Tert
Low
-
High
CHT
12
Test 9 minus Test 10 (W, mA)
Low
CHL
CLT
CHT
14 of 21
7
Transformer Power Factor
Autotransformer – Shorted Bushings
H1
X1
Y1
Y3
H0X0
X3
X2
Y2
H3
Tertiary Delta
H2
Autotransformer
15 of 21
Transformer Power Factor
Autotransformer Test Tables
Autotransformer Without Tertiary or With Buried Tertiary (i.e. no tertiary bushings).
Test No.
Mode
Energize
Ground
Guard
UST
Measure
1
GST
High&Low
-
-
-
CH
Autotransformer with Tertiary (i.e. tertiary bushings available)
Test No.
Mode
Energize
Ground
Guard
UST
Measure
1
GST
High&Low
Tert
-
-
CH + CHT
2
GST
High&Low
-
Tert
-
CH
3
UST
High&Low
-
-
Tert
CHT
High&Low
-
-
CT+CTH
4
CHT
Test 1 minus Test 2 (W, mA)
5
GST
6
GST
Tert
-
High&Low
-
CT
7
UST
Tert
-
-
High&Low
CTH
8
Tert
Test 5 minus Test 6 (W, mA)
CTH
16 of 21
8
Transformer Power Factor
Step-Voltage Regulator
A step-voltage regulator is an autotransformer with a load tap changing switch.
Test Voltages
L
S
Rating (KV)
Test Voltage (KV)
12.47 and Above
10
<12.47 and >4.16
5
4.16 and Below
2
Test Table
±10% Load Tap Changer
Test No.
Mode
Energize*
Measure
1
GST
S-L-SL
CH
1:1 Autotransformer
SL
* Short S, L, and SL bushings
17 of 21
Transformer Power Factor
Transformer Power Factor Tests – Evaluation of Results
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. CH higher than CL and CHL).
•Free-breathing designs tend to have higher power factors due to higher moisture
content in the oil and cellulose insulation.
•Lower voltage units (15 kV and below) tend to have higher power factors.
18 of 21
9
Transformer Power Factor
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.
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.
Ventilated Dry-Type Transformers
•Expect CH ≤ 3%, CHL ≤ 2%, CL ≤ 4%, and Tip-up ≤ 0.5%
Epoxy-Encapsulated Dry-Type Transformers
•Expect CH ≤ 3%, CHL ≤ 1%, CL ≤ 2%, and Tip-up ≅ 0%
19 of 21
Transformer Power Factor
Power Factors for Service Aged Transformers
Power Factor [%]
1.75
1.50
1.25
1.00
0.75
0.50
0.25
30Y
32Y
34Y
36Y
Transformer Age
38Y
40Y
Current
Test
42Y
Future
Test
20 of 21
10
Transformer Power Factor
Number of Transformers
CH Power Factors of Liquid-Filled Transformers
300
240
250
180
200
150
120
80
100
50
60
25 20 15
20
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1
% Power Factor at 20 Degrees C
21 of 21
11
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
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
Center Conductor
Sight-Glass
Liquid or Compound Filler
Insulating Weathershed
Main Insulating Core
Tap Insulation
Tap Electrode
Mounting Flange
Ground Sleeve
Tapped Layer
Lower Insulator
2 of 57
1
3 of 57
Graded Bushing - Core Construction
Semi-Conducting Paper
Herringbone Pattern
Foil or Paint
Core Wind
Core Wind
C2 Plate
C2 Plate
Common Construction
Distributed Capacitance
GE Type U
4 of 57
2
ABB O + C Construction
C2 Plate
5 of 57
ABB O + C, 27 kV
Conductors
6 of 57
3
Graded Bushing – Electrical Characteristics
CA
Main Insulation C1
CB
CC
Tap Insulation C2
CD
CE
Center CA = CB = CC = CD = CE = CF = CG = CH = CI = CJ
Conductor
CF
CG
CENTER CONDUCTOR
CH
CI
CJ
Grounded
Layer or
Flange
CK
V1 = V2 = V3 = V4 = V5 = V6 = V7 = V8 = V9 = V10
Tap
Electrode
CK
Line-to-Ground System Voltage
Voltage stress is evenly distributed across the insulation.
As capacitive layers (i.e.. CA, CB, etc.) are shorted out the
overall capacitance, C1, increases.
7 of 57
Adding Capacitors in Series
1
C1
C2
C3
CT
CN
1
=
C1
1
+
1
+
C2
1
C3
+…+
CN
CT
Case 1: 3 Capacitors in Series
2
2
2
1
1
=
2
1
+
2
1
+
2
2
CT1 =
3
=
2
Shorting out
a capacitor
results in an
increase in
capacitance.
2
2
CT2 > CT1
CT1
CT1
Case 2: Shorted Capacitor
CT2
1
CT2
1
=
2
1
+
2
2
=
2
CT2 = 1
3
8 of 57
4
Test Taps vs. Potential Taps
Test Tap
Potential Tap
9 of 57
Test Taps vs. Potential Taps – “Typical” Differences
Test Tap
Potential Tap
Test Voltage applied to Tap Electrode,
Recommended Test Voltage = 500V Max
Exception: Ohio Brass Type L = 250V Max
Test Voltage applied to Tap Electrode ,
Recommended Test Voltage = 2000V 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
Note: There are exceptions to most of the above statements.
10 of 57
5
Test Voltages Applied to Center Conductor
Doble Recommended Test Voltages
For Voltage Applied to Bushing Center Conductor
Bushing Rating (kV)
>8.7
8.7
5
4.3
1.2
Recommended Test Voltage (kV)
10
8
5
4
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.
11 of 57
Bushing Test Connections
HV Cable
Short
Shortthe
thebushings
bushings
of
ofeach
eachwinding.
winding.
LV Test Lead
Only
Onlyremove
removethe
the
tap
tapcover
coverfrom
fromthe
the
bushing
bushingunder
undertest.
test.
Ground Lead
Ground
Groundwindings
windings
not
notunder
undertest.
test.
DO
DONOT
NOTFORGET
FORGET
to
toreplace
replacethe
thetap
tap
cover
coverafter
aftertest!
test!
12 of 57
6
Effects of Unshorted Bushings
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.
Incorrect
(Unshorted)
0.80% = D
1.05% = I
0.76% = D
Phase A
Phase B
Phase C
Correct
(Shorted)
0.39% = G
0.40% = G
0.39% = G
13 of 57
Bushing C1 Test, Routine Method - Connections
HV Cable
C1
C1Test
TestIncludes
Includes
•Core
insulation
•Core insulation
between
betweencenter
center
conductor
conductorand
and
tapped
tappedlayer.
layer.
Test Mode: UST
mA & W
Guard
LV Test Lead
Ground Lead
Apparatus
Ground
C1
C1%PF
%PFisistemperature
temperature
corrected
correctedto
to20°C
20°C
using
usingthe
theaverage
averageof
of
the
theapparatus
apparatusand
and
ambient
ambienttemperature.
temperature.
Connection to
Parent Apparatus
14 of 57
7
Bushing C1 Test, Routine Method
HV Cable
C1
Center Conductor
Test Tap
C2
Test Mode: UST
IC1
Ground Lead
LV Test Lead
mA & W
Guard
CG
ICG
CG
CG==Capacitance
Capacitancefrom
fromcenter
center
conductor
conductorcircuit
circuitto
toground.
ground.
Includes
Includesparent
parentapparatus
apparatus
insulation
insulationand
andupper
upperand
andlower
lower
insulators
insulatorsof
ofbushings.
bushings.
15 of 57
Typical C1 Test Data
Description
Current (mA)
Watts
%PF
Typical Good
Bushing
1.08
0.03
.28
Same Bushing,
Contaminated
1.09
0.06
.55
Same Bushing,
Shorted
Condenser
layers
1.19
0.04
.34
16 of 57
8
Bushing Temperature Correction Factor (Page 1 of 2)
17 of 57
Bushing Temperature Correction Factor (Page 2 of 2)
18 of 57
9
Bushing Temperature Correction Factor
DTAF Software – Required Fields
1. Manufacturer
2. Type
3. Ambient Temperature (Probe)
4. Apparatus Temperature
Transformer – Top Oil Temperature
Oil Circuit Breaker – Ambient (with discretion)
19 of 57
Bushing C1 Tests – Abnormal Results
C1 Troubleshooting & Investigations
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).
¾
Clean and dry upper (and lower) porcelain and retest using Routine
C1 method.
¾
Repeat test using a guard-collar to guard surface leakage on upper
weathershed (see slide 19).
¾
Perform Inverted C1 test. This test is less sensitive to the effects of
surface contamination (see slides 17-18).
¾
Perform Hot-Collar tests using both the GST-Ground and UST
Methods (see slides 38-45).
¾
Perform reduced voltage test using Routine C1 method.
20 of 57
10
Resistive Path-to-Ground
Test Kv
mA
10
1.313
Watts
-.007
Measure %
Power Factor
-.053
21 of 57
Bushing C1 Tests, Inverted Method - Connections
LV Test Lead
Inverted
InvertedMethod
Method
•Useful
for
•Useful for
investigating
investigatingnegative
negative
power
powerfactors
factors
obtained
obtainedusing
usingthe
theC1
C1
Routine
RoutineMethod.
Method.
Test Mode: UST
0.5 or 2 kV Test
HV Cable
Ground Lead
mA & W
Guard
Apparatus
Ground
•A
•Acommon
commoncause
causeof
of
negative
negativeC1
C1power
power
factors
factorsisissurface
surface
contamination
contaminationon
onthe
the
upper
upperor
orlower
lower
insulators
insulatorsof
ofthe
the
bushings.
bushings.
22 of 57
11
Bushing C1 Test, Inverted Method
LV Test Lead
Test Mode: UST
0.5 or 2 kV Test
C2
Ground Lead
CG
IC2
mA & W
Guard
IC1
Center Conductor
Test Tap C1
HV Cable
Using
Usingthe
theC1
C1Inverted
InvertedMethod,
Method,
the
thecenter
centerconductor
conductorisis
effectively
effectivelygrounded
groundedvia
via
connection
connectionto
tothe
theLV
LVTest
Test
Lead.
Lead. Hence,
Hence,there
thereisisno
no
voltage
voltagestress
stressacross
acrossCG.
CG.
23 of 57
Bushing C1 Test with Guard-Collar
HV Cable
Guard-Collar
Guard-Collar
•May
•Mayeliminate
eliminatethe
the
effects
effectsof
ofsurface
surface
contamination
contamination
from
fromthe
thetest
test
result.
result.
Test Mode: UST
mA & W
Guard
LV Test Lead
Ground Lead
Apparatus
Ground
•Use
•Useone
oneguard
guard
collar
collarpositioned
positioned
near
nearthe
thebottom
bottom
skirt.
skirt.
•Or,
•Or,use
use multiple
multiple
collars
collarslocated
locatedat
at
various
variouslocations
locations
on
onthe
theupper
upper
weathershed.
weathershed.
Connection to
Parent Apparatus
24 of 57
12
Bushing C2 Test - Connections
LV Test Lead
C2
C2Test
TestIncludes
Includes
•Tap
insulator
•Tap insulator
Test Mode: GST-Guard
0.5 or 2 kV Test
•Core
•Coreinsulation
insulation
between
betweentapped
tapped
layer
layerand
andbushing
bushing
ground
groundsleeve
sleeve
HV Cable
•Portion
•Portionof
ofliquid
liquid
or
orcompound
compoundfiller
filler
Guard
mA & W
•Portion
•Portionof
of
weathershed
weathershednear
near
ground
groundsleeve
sleeve
Ground Lead
C2
C2%PF
%PFisisnot
not
temperature
temperaturecorrected.
corrected.
Apparatus
Ground
25 of 57
Bushing C2 Test
LV Test Lead
C1
C2
Test Mode: GST-Guard
0.5 or 2 kV Test
Center Conductor
Test Tap
HV Cable
CG
mA & W
Guard
Ground Lead
IC2
IC1
26 of 57
13
Why Perform C2 Tests
?????
27 of 57
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
28 of 57
14
C2 TESTS
GE Type U 230kV Bushing:
C1
C2
Date
1/6/82
%PF Cap
.31
508.8
%PF
.28
Cap
4134
5/1/96
.58
2.26
4138
1/30/97
Failed in service
510.2
29 of 57
C2 TESTS
Example:
McGraw-Edison Type PA 23kV Bushings
Bushing #
X1
X2
X3
C1(%PF)
0.46
0.60
0.45
C2(%PF)
0.50
2.78
0.50
• X2 was removed from service and found
to have highly contaminated fluid with low
dielectric-breakdown strength
30 of 57
15
Bushing C2 Tests – Abnormal Results
C2 Troubleshooting & Investigations
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).
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.
31 of 57
Doble Bushing Tap Adapters
32 of 57
16
Westinghouse Type O Bushing Tap Adapter
33 of 57
Westinghouse Type O+ Bushing Tap Adapter
34 of 57
17
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.
35 of 57
Bushing “Overall” Test
HV Cable
Test
TestIncludes
Includes
•Main
•MainC1
C1Core
Core
Insulation
Insulation
Test Mode: GST-Ground
•Upper
•UpperInsulating
Insulating
Weathershed
Weathershed
•Sight-Glass
•Sight-Glass
•Lower
•LowerInsulator
Insulator
•Portion
•Portionof
ofLiquid
Liquidor
or
Compound
CompoundFiller
Filler
mA & W
Guard
Ground Lead
Bushing and
Test Stand
Ground
%PF
%PFisistemperature
temperature
corrected
correctedto
to20°C
20°C
using
the
ambient
using the ambient
temperature.
temperature.
36 of 57
18
Bushing “Overall” Test
HV Cable
C1
IC1
C2
Test Mode:
GST-Ground
CG
mA & W
Guard
Ground Lead
IC1+ICG
Center Conductor
Test Tap
ICG
Note:
Note: For
Formost
mostbushing
bushing
types,
types,the
theC2
C2insulation
insulation
will
willbe
beshorted
shortedout
out(as
(as
shown)
shown)via
viathe
thetap
tapcover.
cover.
37 of 57
DTA Spare Bushing Test Screen
38 of 57
19
Bushing C1, C2, and Overall – Evaluation of Results
Capacitance
¾
Suggested limits:
±5% of Nameplate Capacitance = Investigate
¾
Each shorted capacitance layer will cause an increase in C1
capacitance of 5% to 15%.
¾
If 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.
¾
Oil or compound leaks may cause a decrease in capacitance.
¾
Differences in factory and field test procedures and/or test
conditions may result in differences in capacitances.
39 of 57
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]
C2A
C1
Flange
Bushing
Center
Conductor
C2B
Test Tank
Wall
Outer
Condenser
Layer
40 of 57
20
C1 Capacitance – Factory vs. Field Test “Procedures”
Haefley Type COTA Bushing
Flange
Flange
Potential Tap
Potential Tap
Test Tap (burried)
Test Tap
C0
C2
C2
C1
C1
Center Conductor
Center Conductor
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:
C1DOBLE = (C1NP x C2NP) / (C2NP – C1NP) [pF]
41 of 57
Bushing C1, C2, and Overall – Evaluation of Results
Power Factor
¾
Most 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.
¾
Bushings that exhibit a history of continued increase in power
factor should be investigated and considered for removal from
service.
¾
Power 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.
¾
A 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.
42 of 57
21
Bushing C1 & Overall Power Factor Limits
ASEA
Type (* All Types)
GOA 250
GOA OTHER
GOB
GOBK
GOC
GOE
GOE
GOEK
GOEL
GOF
GOFL
GOG
GOH
GOM
Description
Typical %PF
0.5%
0.45%
0.5%
0.5%
0.4%
0.45%
0.4%
0.4%
0.4%
0.45%
0.4%
0.45%
0.25%
0.45%
<800 kV
800kV
Questionable %PF
0.7%
0.65%
0.7%
0.7%
0.6%
0.65%
0.6%
0.6%
0.6%
0.65%
0.6%
0.65%
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.
43 of 57
Bushing C1 & Overall Power Factor Limits
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
OTA *
0.35%
Questionable %PF
0.6%
* This information received from The Bushing Company by fax dates 9/1/1993.
44 of 57
22
Bushing C1 & Overall Power Factor Limits
FEDERAL PACIFIC and PENNSYLVANIA TRANSFORMER
Type
DESCRIPTION
P
Paper-oil condenser, sealed
Typical %PF
0.5%
Questionable %PF
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.
45 of 57
Bushing C1 & Overall Power Factor Limits
GENERAL ELECTRIC and LOCKE INSULATORS, INC.
Type
A*
A **
B*
D
F
L
LC
OF
S*
U
T
Description
Through Porcelain
High Current
Flexible Cable, Compound Filled
Oil Filled Upper Portion, Sealed
Oil Filled, Sealed
Oil Filled Upper Portion, Sealed
Oil Filled Upper Portion, Sealed
Oil Filled Expansion Chamber
Forms C & CG, Rigid Core, Compound Filled
Oil-Filled, Sealed
Oil-Filled, Sealed
Typical %PF
3.0%
1.0%
5.0%
1.0%
0.7%
1.5%
1.5%
0.8%
1.5%
0.5%
0.5%
Questionable %PF
5.0%
2.0%
12.0%
2.0%
1.5%
3.0%
3.0%
2.0%
6.0%
1.0%
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.
46 of 57
23
Bushing C1 & Overall Power Factor Limits
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).
47 of 57
Bushing C1 & Overall Power Factor Limits
LAPP PRC Bushings – Old design (top) and New Design (Bottom)
48 of 57
24
Bushing C1 & Overall Power Factor Limits
OHIO BRASS
Type/Description
Typical %PF
Questionable %PF
Class GK - Type C, 15 to 196 kV
Oil-Impregnated Paper Condenser Core
0.4%
1.0%
Class LK - Type A, 23 to 69 kV
Resin-Paper Condenser Core, Oil-filled
0.4%
1.0%
ODOF, Class G, and Class L Oil-Filled,
Prior to 1926 and after 1938
Between 1926 and 1938
1.0-5.0%
2.0-4.0%
Initial %PF plus 22%
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.
49 of 57
Bushing C1 & Overall Power Factor Limits
WESTINGHOUSE
Type / Description
Typical %PF
Questionable %PF
S *, OS, and FS
0.8%
2.0%
On OCB & Inst. Tx., 69kV & below
(except Types S, OS, and FS)
1.5%
3.0%
On OCB & Inst. Tx., 92kV to 138kV
(except Types O, O-Al, OC, and O Plus)
1.5%
3.0%
On Power & Dist. Tx. of all ratings,
and OCB & Inst. Tx., 161kV to 288kV
(except Types O, O-Al, OC, and O Plus)
1.0%
2.0%
O and O-AL, 92kV to 288kV
0.3%
1.0%
O Plus
0.3%
1.0%
D Transformer Bushings
(Semi-condenser type)
1.5%
3.0%
RJ
(Solid Porcelain)
1.0%
2.0%
* For Type S bushings the C2 power factor may be as high as 12%.
50 of 57
25
Hot-Collar Test, GST-Ground Mode
LV Test Lead
Test
TestIncludes
Includes
•Portion
of
•Portion ofInsulating
Insulating
Weathershed
Weathershed
HV Cable
•Sight-Glass
•Sight-Glass
•Core
•CoreInsulation
Insulationin
in
Vicinity
Vicinityof
ofCollar
Collar
Test Mode: GST-Ground
Ground
Lead
mA & W
Guard
Apparatus
Ground
•Liquid
•Liquidor
orCompound
Compound
Filler
Fillerin
inVicinity
Vicinityof
of
Collar
Collar
•Surface
•Surfaceleakage
leakagefrom
from
Collar
Collarto
toLV
LVtest
testlead
lead
&&from
fromcollar
collarto
to
bushing
bushingflange
flange
51 of 57
Hot-Collar Test, UST Mode
LV Test Lead
Test
TestIncludes
Includes
•Portion
of
•Portion ofInsulating
Insulating
Weathershed
Weathershed
HV Cable
•Sight-Glass
•Sight-Glass
•Core
•CoreInsulation
Insulationin
in
Vicinity
Vicinityof
ofCollar.
Collar.
Test Mode: UST
Ground Lead
•Liquid
•Liquidor
orCompound
Compound
Filler
Fillerin
inVicinity
Vicinityof
of
Collar.
Collar.
mA & W
Guard
Apparatus
Ground
•Surface
•Surfaceleakage
leakage
from
fromCollar
Collarto
toLV
LVtest
test
lead.
lead.
52 of 57
26
Hot-Collar Tests
When to Perform Hot Collar Tests
¾
Bushings not equipped with taps, and overall tests cannot
be performed (i.e. cannot be isolated from apparatus).
¾
Investigative test when overall, C1, or C2 tests indicate a
possible problem.
¾
Compound filled bushings with or without taps.
¾
Verify oil level in bushings without site glasses or level
gauges, or if level gauge is suspect.
53 of 57
Hot-Collar Tests
Comments on Hot-Collar Tests
¾
Hot-Collar tests should be conducted at 10KV.
¾
Hot-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.
¾
Hot-Collar tests evaluate the weathershed, sight glass, and the
core insulation/liquid/compound between the collar and the
center conductor.
¾
Hot-Collar tests do not test the whole bushing.
54 of 57
27
Hot-Collar Tests
Comments on Hot-Collar Tests (continued)
¾
Tests 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.
55 of 57
Hot-Collar Tests
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]
90
Watts
0.020
1.
Typical Good Bushing / New
2.
Same / Similar Bushing Contaminated
95
0.310
3.
Same Bushing with Lower-than-Normal
Liquid or Compound Level.
70
0.020
56 of 57
28
Hot-Collar Tests – Oil or Compound Level
H1
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
H2
H3
Conclusion: The oil level in the H3 bushing is somewhere
between the locations used for tests hot-collar tests 2 and 3.
57 of 57
Hot Collar Tests – Abnormal Results
Hot Collar Troubleshooting & Investigations
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
Clean and dry upper porcelain and retest.
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.
¾
Perform tests under additional skirts.
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.
58 of 57
29
Knowledge Is Power
SM
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
Transformer Excitation Current
What is Excitation Current?
1:1
Iex
E1
Φ
+
+
∼
E2
−
-
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
in the iron core.
2 of 15
1
Transformer Excitation Current
Excitation Current Tests Can Identify Problems in the Iron
Core, Windings , and Tap Changers
Core
Windings
•Interlaminar insulation
damage
•Turn-to-turn shorts
•Turn-to-ground shorts
(grounded windings)
•Abnormal core grounds
•Dislocated joints
•High resistance turn-to turn
or turn-to-ground shorts
Tap Changers
•Mechanical failures
•Auxiliary transformer
problems
3 of 15
Transformer Excitation Current
Test Voltages
•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.
4 of 15
2
Transformer Excitation Current
Test Procedure – Single Phase Transformer
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.
5 of 15
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
*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.
6 of 15
3
Transformer Excitation Current
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, *
I1-2
2
UST
H2
H3
X1,X2,X3
Y1,Y2,Y3
H1, *
I2-3
3
UST
H3
H1
X1,X2,X3
Y1,Y2,Y3
H2, *
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.
7 of 15
Transformer Excitation Current
“Traditional” Phase Excitation Current Patterns
for Various Core, Winding, and Test Configurations
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
•This pattern “may” indicate a problem.
8 of 15
4
Transformer Excitation Current
Transformer Design with “Traditional” H-L-H Patterns
IT-1
IT-2
IT-3
•During the Excitation Current Test, we
measure the total currents IT.
•For the traditional H-L-H pattern all
total currents IT are inductive.
IL-2
IL-1
•What would happen to the total current
IT if the magnitude of the capacitive
current IC was larger than the
magnitude of the inductive current IL?
IC-2
IL-3
IC-1
IC-3
9 of 15
Transformer Excitation Current
“Newer” Transformer Designs with “Non-Traditional” Patterns
IT-2
IT-1
IC-2
IT-3
IC-1
IC-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
(M=medium).
IL-2
IL-1
IL-3
10 of 15
5
Transformer Excitation Current
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)
11 of 15
Transformer Excitation Current
Preventive-Autotransformer LTC Scheme
HV Bushings →
HV Windings U →
LV Windings Y →
Reversing Switches →
Tap Windings →
Taps →
Preventive
Autotransformer →
LV Bushings →
12 of 15
6
Transformer Excitation Current
Load Tap Changer Excitation Current Patterns
LTC Tap Positon
16R
15R
14R
13R
12R
11R
9R
10R
8R
7R
6R
5R
4R
3R
2R
1R
N
16R
15R
14R
13R
12R
11R
9R
10R
8R
7R
6R
5R
4R
3R
2R
1R
N
Excitation Current
LV LTC, Pre ventive Auto & Series Transform er
Excitation Current
LV LTC, Preve ntive Autotransform er
LTC Tap Position
LTC Tap Position
16R
15R
14R
13R
12R
11R
10R
9R
8R
7R
6R
5R
4R
3R
2R
1R
N
16R
15R
14R
13R
12R
11R
10R
9R
8R
7R
6R
5R
4R
3R
2R
1R
N
Excitation Current
HV LTC, Resistive Bridge
Excitation Current
HV LTC, Preve ntive Autotransform er
LTC Tap Positon
13 of 15
Transformer Excitation Current
Load Tap Changer Excitation Current Patterns (Continued)
LTC Tap Positon
LTC Tap Position
Six examples of LTC transformer excitation current patterns
have been given … other patterns do exist.
The most common (numerous) LTC schemes are represented
by the patterns shown in the first 4 examples (prior slide).
14 of 15
7
16R
15R
14R
13R
12R
11R
10R
9R
8R
7R
6R
5R
4R
3R
2R
1R
N
16R
15R
14R
13R
12R
11R
10R
9R
8R
7R
6R
5R
4R
3R
2R
1R
N
Excitation Current
LV LTC, Preve ntive Auto & Com pensating Winding
Excitation Current
LV LTC, Preve ntive Auto & Com pensating Winding
Transformer Excitation Current
Evaluation of Results
1.
Look for expected phase-to-phase patterns (ie. HLH or 3 Similar).
A.
Readings are considered similar if:
(1) They are between 0 and 50 mA and they are with 10%.
(2) They are greater than 50 mA and they are within 5%.
B.
For 3 dissimilar readings:
(1) Core, winding, or tap changer problems could exist.
(2) Residual magnetism could be present. De-magnetize and re-test.
(3) Some newer designs do not conform to historical patterns.
2.
For LTC transformers, look for expected Tap-to-Tap patterns.
3.
Compare to historical tests. Readings should be within 10% of the original test
if test is performed at the same test voltage.
15 of 15
Position
Test
DETC LTC
kV
April 13, 2007 Test
4
N
10
H3 – H1
mA
Watts
Connections
H1 – H2
mA
Watts
H2 – H3
mA
Watts
Case 6: Residual Magnetism
10.593 54.168 6.208 30.944 10.726 53.926
Winding resistance test performed numerous times after this test.
May 18, 2007 Test
4
N
10
18.121 86.645 7.941 39.423 17.515 84.910
Demagnetized H3-H1 winding. Initial interval was 4 min. Reversed
polarity after 4, 3.5, 3, 2.5, 2, 1.5 and 1 min., and after 50, 40, 30, 20 and 10
sec.
4
N
10
11.025 58.477 6.622 34.792 11.841 62.075
16 of 15
8
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Transformer Turns Ratio – Doble Method
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Transformer Turns Ratio
Comments on Doble Transformer Turns Ratio
Uses for TTR
•Verify turns ratio.
•Identify turn-to-turn or turn-to-ground shorts.
•Detect open circuits.
Benefits of Doble Method vs. Conventional TTR
•Doble method uses voltages as high at 10KV, while most conventional TTRs use
less than 100V. Incipient failures are more likely to be found using higher voltages.
•Doble capacitor is small, light weight, and inexpensive.
•Test results are automatically recorded in DTA software (M4000 users).
2 of 8
1
Transformer Turns Ratio
Doble TTR – A Two Step Process
CTRUE
n
UST
UST
L-V Lead
L-V Lead
I&W Meter
I&W Meter
STEP 2
STEP 1
I1=V1ωC
I2=V2ωC
Measured capacitance C is identified at C1
Transformer ration n = V1/V2
So V2= V1 /n and I2=V1ωC/n
Measured capacitance C/n is identified as C2
Ratio C1/C2 = C/(C/n) = n
Also, ratio I1/I2 = n
3 of 8
Transformer Turns Ratio
Step 2, Wye-Wye Transformer
Test
Energize
Ground
Capacitor
Ground
1
H1
H0
X1
X0
2
H2
H0
X2
X0
3
H3
H0
X3
X0
4 of 8
2
Transformer Turns Ratio
Step 2, Wye-Delta Transformer
Test
Energize
Ground
Capacitor
Ground
1
H1
H0
X1
X2
2
H2
H0
X2
X3
3
H3
H0
X3
X1
5 of 8
Transformer Turns Ratio
Step 2, Delta-Wye Transformer
Test
Energize
Ground
Capacitor
Ground
1
H1
H3
X1
X0
2
H2
H1
X2
X0
3
H3
H2
X3
X0
6 of 8
3
Transformer Turns Ratio
Comments on Test Procedures
Test Voltages
•Test voltages should be limited to the rated line-to-ground voltage of the winding.
•Test voltage may be limited by the test set current maximums (M4000 maximum
300mA at 10,000V).
Tap Positions
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)
7 of 8
Transformer Turns Ratio
Evaluation of Results
1.
Compare to nameplate.
Per ANSI, the tested values must be within 0.5% of nameplate.
2.
Compare to similar units (“sisters”).
3.
For three-phase units, compare phases.
4.
Compare to prior tests.
8 of 8
4
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Insulating Fluids Test
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Insulating Fluids
Doble “Liquid-Insulation Power-Factor Test Cell”
Carrying Case
Outer Electrode
Inner Electrode
2 of 8
1
Insulating Fluids
Test Connections for Doble Liquid Insulation Test Cell
Guard Ring
Test
TestVoltages
Voltages
10
10kV
kVFluid
FluidFilled
Filled
55kV
kVEmpty/Dry
Empty/Dry
Guard Ring
Oil Level
LV Lead
Outer Electrode
Inner Electrode
Bottom Half of
Carrying Case
3 of 8
Insulating Fluids
Temperature Correction Factors
4 of 8
2
Insulating Fluids
Comments on Collecting Samples
For gas blanketed apparatus, it is important to verify that the gas head is at
positive pressure before attempting to collect a sample from the drain valve. If a
vacuum is present (i.e. negative pressure) then air and contaminants may be drawn
into the apparatus when the drain valve is opened.
For low volume systems, such as CTs and PTs, care must be taken to avoid
reducing the oil level to dangerously low levels.
A “good” sample is one that is representative of the bulk liquid insulation. To
collect a good sample, the following procedures should be taken:
¾ Clean the drain valve inside and out. The outside should be cleaned with a rag
to remove any debris that may fall into the sample container. The inside may be
cleaned by opening the drain valve and flushing with at least 1 to 2 quarts of oil.
¾ Use of the sample cock is discouraged by many reputable oil laboratories
because these valves are often difficult to flush out. In order to properly draw a
sample from the sample cock, first flush the drain valve with 1 to 2 quarts of oil,
cap the drain valve, flush the sample cock, and then collect a sample.
5 of 8
Insulating Fluids
Interpretation of Results
New:
%PF ≤ 0.05%
Investigate:
%PF ≥ 0.5%
Problems:
%PF ≥ 1.0%
Note: While power factors for in-service fluids may be “good” up to 0.5%PF, some
consideration should be given to the type of equipment that the sample was taken
from.
Free-breathing apparatus may have higher moisture content due to the interaction
of the oil and air, and thus may have higher oil power factors.
Arcing apparatus such as circuit breakers, load tap changers, and voltage
regulators produce arcing by-products such as carbon and metals particals during
normal operation. These by-products are contaminants that cause the the oil
power factor to be higher.
Well-maintained, sealed, non-arcing apparatus usually have oil power factors well
below 0.5%.
6 of 8
3
Insulating Fluids
Dry Oil Cell Test M4000 / M2H
Purpose
Verify integrity of the test set, cables, and accessories.
Procedure
1. Make sure oil cell is empty.
2. Test at 5KV (*).
3. Test without oil as shown in prior diagram.
4. UST Mode, should read 0.400 mA, 0.002 W, 106 pF.
5. GST-Ground Mode, should read 0.420 mA, 0.002 W, 111 pF.
Note: DO NOT test a dry oil cell at 10KV.
Flashover may occur at/above 7.0KV!
7 of 8
Insulating Fluids
Dry Oil Cell Test MEU
Purpose
Verify integrity of the test set, cables, and accessories.
Procedure
1. Make sure oil cell is empty.
2. Test at 2.5KV.
3. Test without oil as shown in prior diagram.
4. UST Mode, should read 250 mVA, 0.2 mW, 106 pF.
5. GST-Ground Mode, should read 263 mVA, 0.2 mW, 111 pF.
8 of 8
4
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Surge Arrestor Tests
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Surge Arrestors
Surge Arrestor Volt-Ampere Relationship
Volts
System BIL kV
Protective Margin
Arrestor kV
Conductor
Arrestor MCOV
System Operating kV
Insulator
Doble Test kV
Amperes
2 of 7
1
Surge Arrestors
Recommended Test Voltages
Arrestor Type
Silicon Carbide
MCOV KV
N/A
Metal Oxide
2.2 and 2.55
3.7 to 10.6
12.7 and higher
Arrestor Rating [KV]
3.0
4.5
6.0
7.0
9.0 and 10.0
12.0 and higher
2.7 and 3.0
4.5 to 12.0
15.0 and higher
Test Voltage [KV]
2.5
4.0
5.0
7.0
7.5
10.0
2.0
2.5
10.0
Surge arrestors have nonlinear volt-ampere relationships. Hence, it is important to test surge
arrestors at prescribed voltages in order to make meaningful comparisons.
If the surge arrestor is equipped with a leakage current detector or a discharge counter, it must
be shorted out during the test. Short the bottom flange of the arrestor stack to directly to ground.
3 of 7
Surge Arrestors
Test Connections & Test Modes
Single Arrestor
H-V Test Cable
A
Measure A, GST-Ground Mode
Test Set Step
Up Transformer
Guard
Point
GND Lead
I&W Meter
4 of 7
2
Surge Arrestors
Test Connections & Test Modes
Two-Stack Arrestor
A
H-V Test Cable
Measure A, UST Mode
B
Measure B, GST-Guard Mode
Test Set Step
Up Transformer
Guard
Point
GND Lead
L-V Lead
I&W Meter
5 of 7
Surge Arrestors
Test Connections & Test Modes
Three-Stack Arrestor
A
Measure A, GST-Guard Mode
H-V Test Cable
Measure B, UST Mode
B
Test Set Step
Up Transformer
Measure C, Reverse HV/LV Test leads,
GST-Guard Mode
L-V Lead
C
Guard
Point
GND Lead
I&W Meter
6 of 7
3
Surge Arrestors
Interpretation of Results
1.
Measure and evaluate watts-loss [W].
2.
Power factor is not calculated or considered.
3.
Compare to tabulated data for the given manufacturer, catalog
number, KV rating, and test voltage.
4.
When tabulated data is not available, compare to watt-loss values
obtained from tests of similar units under similar conditions. This is
usually possible since similar arrestors are normally installed at the
same location.
7 of 7
4
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Oil Circuit Breaker Tests
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Typical OCB Construction
2 of 14
1
OCB Insulation Tests
Overall Tests
• Perform two (2) open breaker tests and one (1) closed breaker
test on each phase.
• For each test evaluate %PF and capacitance.
• %PF temperature corrected to 20ºC based on bushing type and
ambient temperature.
• Calculate and evaluate tank-loss index (TLI) for each phase.
Bushing Tests
• C1, C2, and hot collar tests.
Oil Tests
• For each tank.
3 of 14
OCB Test Voltages & Procedure
Breaker Voltage Ratings
15 KV and Above
7.2 and 7.5 KV
5 KV and Below
Test Voltages (KV)
10
5
2
4 of 14
2
OCB Open vs. Closed Breaker Tests
Open Breaker Tests
Closed Breaker Tests
LIFT ROD GUIDE
LOWER LIFT ROD
UPPER LIFT ROD
TANK LINER
OIL
INTERRUPTORS
BUSHINGS
LIFT ROD GUIDE
LOWER LIFT ROD
UPPER LIFT ROD
TANK LINER
OIL
INTERRUPTORS
BUSHINGS
Oil
5 of 14
Tank Loss Index (TLI)
TLI = (Closed-Breaker Watts) – (Sum of Two Open Breaker Watts)
Tank Loss Index (TLI)
Below
-0.20 W
-0.20 W to
-0.10 W
-0.10 W to
+0.05 W
+0.05 W to
+0.10 W
Above
+0.10 W
Investigate
immediately
Retest or
place on
more
frequent test
schedule
Normal for
most
breaker
types.
Retest or
place on
more
frequent test
schedule
Investigate
immediately
Interruptor, upper lift rod,
lift rod guide
Oil, tank liner, lower lift rod,
high contact resistance,
interruptor support
insulation
6 of 14
3
OCB Investigative Tests
Wood and Other Insulating Members - “Three Electrode Method”
Test 6” section at 10KV, UST Mode
Test individual 3” sections using guard circuit.
Material
Wood Members
Plastic Coated
Desired Loss (W)
W ≤ 0.2 for a 3” section
W ≤ 0.15 for a 3” section
7 of 14
OCB Investigative Tests
Interruptors - “Foil Method”
Desired results 10% ≤ PF ≤ 30%
For PF > 50% drying is recommended.
Overly wet interruptors may swell resulting in binding and/or overstressing of clamping bolts.
Overly dry interruptors may loose their mechanical strength or develop looseness of their plate structure.
8 of 14
4
OCB Test Analysis – Example 1
9 of 14
OCB Test Analysis – Example 2
10 of 14
5
OCB Test Analysis – Example 3
11 of 14
OCB Test Analysis – Example 4
12 of 14
6
OCB Test Analysis – Example 5
13 of 14
OCB Test Analysis – Example 6
14 of 14
7
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Grounded Tank SF6 Circuit Breaker Tests
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Grounded Tank SF6 Circuit Breaker
Power Factor Test Procedure
• Bushings (Hot-Collar)
• Open-Breaker Tests (6 Tests)
• Open-Breaker UST Tests (3 Tests)
• Closed-Breaker Tests (3 Tests)
for some multi-contact breakers
• Diagnostic (questionable results)
2 of 10
1
Grounded Tank SF6 Circuit Breaker
Recommended Test Voltages
Breakers Rated Above 15KV
Overall tests are performed at 10KV
Breakers Rated 15KV and Below
Initial Tests
1. Below corona inception, 2KV or less.
2. Rated operating (system) line-to-ground voltage, VL-G.
3. 10% to 25% above rated operating (system) line-to-ground voltage, VL-G.
Routine Tests
10% to 25% above rated operating (system) line-to-ground voltage, VL-G.
Use same voltage as first test.
3 of 10
Grounded Tank SF6 Circuit Breaker
Test Procedure
Test No.
Breaker Position
Test Mode
Bushing Energized
Bushing Floating *
1
Open
GST-Ground
1
2
-
2
2
1
-
3
3
4
-
4
4
3
-
5
5
6
-
6
6
5
-
1
-
2
8
3
-
4
9
5
-
6
1&2
-
-
11 **
3&4
-
-
12 **
5&6
-
-
7
10 **
UST
Closed
GST-Ground
Bushing UST
* Bushings of phases not under test should be floating
** Test 10-12 are supplementary tests for breakers containing internal support insulators (ie. those that are
isolated when the breaker is open). In general, this applies to some circuit breakers with more than one break
per phase. Some of these designs include:
Brown Boveri / Gould / ITE, Types GA/GB
High Voltage Breakers, SF6 Puffers
Westinghouse, Type SFV (two interrupters/phase)
4 of 10
2
Grounded Tank SF6 Circuit Breaker
Open Breaker GST-Ground Tests
HV Lead
Included In Test Results:
• Bushing
• Any Support Insulation
or Operating Rod on
Same Phase & Side
Ground Lead
5 of 10
Grounded Tank SF6 Circuit Breaker
Open Breaker UST Tests
HV Lead
LV Lead
Included In Test Results:
• Contact Assembly
• Gas
• Any Grading Capacitor(s)
Ground Lead
6 of 10
3
Grounded Tank SF6 Circuit Breaker
Closed Breaker GST-Ground Tests
HV Lead
Necessary to test
operating rod and
support insulation
Ground Lead
Included In Test Results:
• Bushings (Both)
• “All” Support Insulation and
Operating Rod on Same Phase
7 of 10
Grounded Tank SF6 Circuit Breaker
Evaluation of Results
• If current is greater than 300 μA evaluate % PF.
• No temperature correction.
• Compare to previous tests, similar breakers, and Doble’s tabulated
data for similar breakers (TDRB).
• Compare results:
¾ Tests 1, 3, and 5
¾ Tests 2, 4, and 6
¾ Tests 7, 8, and 9
¾ Tests 10, 11, and 12
8 of 10
4
Grounded Tank SF6 Circuit Breaker
Evaluation of Results (continued)
• Tests 1 through 6
¾ Dominated by bushing.
¾ Also includes operating rod and any support insulation.
¾ % PF generally 1.0 % or less.
• Tests 10, 11, and 12
¾ Dominated by bushings (tests both bushings).
¾ Also includes “all” operating rod and any support insulation
(including those located between the breaks).
¾ % PF generally 1.0 % or less.
9 of 10
Grounded Tank SF6 Circuit Breaker
Evaluation of Results (continued)
• Tests 7, 8, and 9 (contact assembly without a grading capacitor)
¾ Condition of contact assembly and SF6 gas.
¾ Very low current, evaluate losses.
¾ Losses generally 0.010 Watts or less.
• Tests 7, 8, and 9 (contact assembly with grading capacitor(s))
¾ Dominated by grading capacitors.
¾ Condition of contact assembly and SF6 gas.
10 of 10
5
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Potential Transformer Tests
Mike Horning-Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Common PT, Line-to-Line & Line-to-Ground
H1
H2
H1
CH2
CH1
CHY
CH1
CHX
X1
X2
X3
CHX
X1
X2
X3
Y1
Y2
Y3
H0
Y1
Y2
Y3
CHY
CH0
CH = CH1 + CH2
CH = CH1 + CH0
2 of 12
1
Recommended Test Voltages
Line-to-Line PTs (Liquid Filled)
PT Voltage Rating [kV]
Test Voltage [kV]
15 kV and Above
10.0
7.2 to 8.7
5.0
4.2 to 5.0
2.5
2.4
2.0
3 of 12
Recommended Test Voltages
Line-to-Ground PTs (Liquid Filled and Dry-Type)
Test Description
Any Test with
H0 Energized
Any Test with
H0 Not Energized
Test Voltage
The maximum test voltage must be limited to the
rated line-to-ground voltage of the H0 bushing
(usually 5 kV or less), the rated line-to-ground
voltage of the PT, or 10kV, whichever is lower.
This statement applies to all tests except H1 CrossCheck Test #2 and H1 Bushing Test #8 (see Routine
and Supplemental Procedures on following pages).
The maximum test voltage should be limited to the
rated line to ground voltage of the PT or 10 kV,
whichever is lower.
This statement applies to H1 Cross-Check Test 2 and
H1 Bushing Test 8 (see Routine and Supplemental
Procedures on following pages).
4 of 12
2
Recommended Test Voltages
Dry-Type PTs (Line-to-Line and Line-to-Ground)
Test Description
Test Voltage
a. 2 kV
b. Line-to-ground operating voltage.**
Overall *
c. 10% to 25% above line-to-ground operating voltage.
a. 2 kV
Cross-Check *
b. Line-to-ground operating voltage.**
Exciting Current *
a. Line-to-ground operating voltage.
* For line-to-ground applications, The maximum test voltage must be limited
to the rated line-to-ground voltage of the H0 bushing (usually 5 kV or less),
the rated line-to-ground voltage of the PT, or 10kV, whichever is lower.
** Calculate and analyze Power Factor Tip-Up by subtracting the 2 kV test
result from the operating line-to-ground test result.
5 of 12
Test Procedures
Routine Tests, Single-Phase PT
Test
No.
Test
Mode
Energize
Ground
Guard
UST
Test Description
1*
GST
H1 & H0 (H2)
X1 & Y1
-
-
Overall (CH+CHX+CHY)
2
GST
H1
X1 & Y1
H0 (H2)
-
H1 Cross-Check
3*
GST
H0 (H2)
X1 & Y1
H1
-
H0 (H2) Cross-Check
4**
UST
H1
X1 & Y1
-
H0 (H2)
Excitation H1 to H0 (H2)
5*
UST
H0 (H2)
X1 & Y1
-
H1
Excitation H0 (H2) to H1
* Maximum test voltages for Tests #1, 3, and 5 must be limited to (1) the rated
line-to-ground voltage of the H0 bushing, (2) the recommended test voltage
based upon the kV rating of the PT, or (3) 10 kV, whichever is lower.
** For purposes of comparison, Test #4 should be conducted at the same
voltage as Test #5.
6 of 12
3
Test Procedures
Supplemental Tests, Single Phase PT
Test
No.
Test
Mode
Energize
Ground
Guard
UST
Test Description
6*
UST
H1 & H0 (H2)
Y1
-
X1**
CHX
7*
UST
H1 & H0 (H2)
X1
-
Y1**
CHY
8
GST
H1
-
H0 (H2), X1, & Y1
-
CH1
9*
GST
H0 (H2)
-
H1, X1, & Y1
-
CH0 (CH2)
* Maximum test voltages for Tests #6, 7, and 9 must be limited to (1) the rated
line-to-ground voltage of the H0 bushing, (2) the recommended test voltage
based upon the kV rating of the PT, or (3) 10 kV, whichever is lower.
** For Test #6 the X circuit must be ungrounded. For Test #7 the Y circuit must
be ungrounded.
7 of 12
Test Procedures
Routine Tests, 3-Phase PT
Test
No.
Test
Mode
Energize
Ground
Guard
UST
Test Description
1
GST
H1, H2, H3, & H0
X1 & Y1
-
-
Overall
2
GST
H1
X1 & Y1
H0, H2, & H3
-
H1 Cross-Check
3
GST
H2
X1 & Y1
H0, H1, & H3
-
H2 Cross-Check
4
GST
H3
X1 & Y1
H0, H1, & H1
-
H3 Cross-Check
5
GST
H0
X1 & Y1
H1, H2, & H3
-
H0 Cross-Check
6
UST
H1
X1 & Y1
H2 & H3
H0
Excitation H1 to H0
7
UST
H2
X1 & Y1
H1 & H3
H0
Excitation H2 to H0
8
UST
H3
X1 & Y1
H1 & H2
H0
Excitation H3 to H0
* Maximum test voltages for Tests #1 and 5 must be limited to (1) the rated line-to-ground
voltage of the H0 bushing, (2) the recommended test voltage based upon the kV rating of
the PT, or (3) 10 kV, whichever is lower.
** For purposes of comparison, Test #6, 7, and 8 should be conducted at the same voltage.
8 of 12
4
Temperature Correction Factors
9 of 12
Line-to-Ground PT, Internally Grounded H0
H1
CH1
CHX
X1
X2
X3
CH = CH1 + CH0
CHY
CH0
Y1
Y2
Y3
H0
10 of 12
5
Test Procedures
Routine Tests, Line-to-Ground PT with Internally Grounded H0
Test No.
Test Mode
Energize
Ground
Guard
UST
Test Description
1
UST
H1
H0
-
X1 & Y1
Line End of CHX+CHY*
2
GST
H1
H0
X1 & Y1
-
Excitation H1 to H0
and CH1
Supplemental Tests, Line-to-Ground PT with Internally Grounded H0
Test No.
Test Mode
Energize
Ground
Guard
UST
Test Description
1
UST
H1
H0 & Y1
-
X1
Line End of CHX*
2
UST
H1
H0 & X1
-
Y1
Line End of CHY*
* Because H0 cannot be ungrounded, the test voltage is graded across the H winding.
Therefore the insulation at the line end of the winding is stressed while the insulation
at the ground end is not. This is only a partial test of the insulation system CHX and
CHY.
11 of 12
Potential Transformers
Evaluation of Results
1. Overall power factor test results (test 1) should be compared to prior tests, similar units,
tabulated data (Doble Test Data Reference Book), and manufacturer recommendations.
2. For most single-phase PTs, the cross-check test power factor results (tests 2 and 3)
compare closely to the overall test results. However, in some units it is normal for one
cross-check power factor to be higher than the overall.
3. The cross check provides useful supplementary data, particularly when the overall test
results are questionable. For example, in a single-phase PT if the overall power factor is
higher than expected, then the cross-check could help differentiate between a general
condition (overall and both cross-checks elevated) or a problem localized in one bushing
or a end of a winding (overall on one cross-check elevated).
4. The sum of the two cross-check tests’ current and watts should be approximately equal
to the overall current and watts respectively. Failure of these results to agree could
indicate winding problems (open circuits), poor connections at the bushings, or some
other voltage sensitive problem (i.e. if the cross check tests are conducted at two
different test voltages).
5. Tip-Up analysis may be performed on dry-type PTs.
6. For single-phase PTs the excitation current tests (tests 4 and 5) should provide similar
results if performed at the same voltage. For 3-phase PTs, a excitation current pattern of
two similar values and one lower value is expected.
12 of 12
6
Knowledge Is Power
SM
Apparatus Maintenance and Power Management
for Energy Delivery
Negative Power Factor
Mike Horning, Principal Engineer
Doble Engineering Company
Doble Engineering Company
85 Walnut Street, Watertown, MA 02472
Negative Power Factor Theory
HV Cable
ET
IT
Z1
EG
Test Mode: UST
IT’
IG
Z2
RG ≅ ZG
mA & W
Guard
LV Test Lead
Apparatus
Ground
Ground Lead
2 of 21
1
Negative Power Factor Theory
Case 1 – Normal Conditions
IT
Z1
ET
Case 2 – Low Resistance Path to Guard
Z1
EG
Z2
EG
IG
IT ’
Z2
mA,W
ETZ2
EG =
Z1 + Z2
IT’ = EG / Z2 =
IT ’
ZG
mA,W
EG =
IT
ET
ET
Z1 + Z2
ETZ2||ZG
Z1 + Z2||ZG
IT’ = EG / Z2 =
= IT
=
ETZ2
Z1Z2 + Z1 + Z2
ZG
ET
Z1Z2 + Z1 + Z2
ZG
≠ IT
3 of 21
Negative Power Factor Theory
Assuming Z1 and Z2 are primarily capacitive, and ZG is primarily resistive
Z1 = 1/jωC1 = -j/ωC1 = -jXC1
Z2 = -jXC2
ZG = RG
EG =
IT’ =
ETZ2
Z1Z2 + Z1 + Z2
ZG
ET
Z1Z2 + Z1 + Z2
ZG
=
=
-jETXC1
– XC1XC2 – j(XC1 + XC2)
RG
ET
– XC1XC2 – j(XC1 + XC2)
RG
EG = EG α°
0° < α < 90°
IT’ = IT’
δ°
90° < δ < 180°
4 of 21
2
Negative Power Factor Theory
-IG
IT
IT’
EG
IT
IG
α° ET
EG
ET
0° < α < 90°
1. Applied voltage ET produces total
current IT through RC network
between HV hook and
ground/guard.
2. Current IT produces voltage EG
across the RC network between the
leakage origination point and
guard. EG is phase shifted α° due
to the leakage resistance RG.
3. Leakage point voltage EG produces
leakage current IG. Because the
leakage current is predominantly
resistive, IG and EG are shown in phase
with each other.
4. The measured current IT’ is equal to the
total current IT minus the leakage
current IG.
5 of 21
Negative Power Factor Theory
IT’
IC’
δ°
IR’
ET
90° < δ < 180°
5. The measured current IT’ has resistive IR’ and capacitive IC’
components as shown.
6. The resistive current IR’ has a negative value. Hence, the measured
watts value and calculated power factor are also negative.
w = IR’ x ET / 1000
%PF = w x 10 / IT’ [currents in mA]
6 of 21
3
Negative Power Factor Theory
Effects of Origination Location and Resistance
IT’ =
100%
75%
IT ‘
100%
XC1
25%
75%
XC
50%
ET
– XC1XC2 – j(XC1 + XC2)
RG
XC2
Maximum
XC2 / 4RG
50%
25%
RG
0%
0%
• If RG >> XC2 / 4 then no phase shifting
mA,W
• If RG is small then phase shifting everywhere
7 of 21
Negative Power Factor - Laboratory Test
Effects of Origination Location and Resistance
RG = 5 MΩ
RG
mA,W
RG = 11.5 MΩ
11
Node
mA
W
%PF
Cap
mA
W
%PF
Cap
10
1
3.66
0.02
0.05
971
3.66
0.02
0.07
972
9
2
3.65
-1.77
-4.84
968
3.63
-0.31
-0.87
962
8
3
3.64
-3.11
-8.54
962
3.60
-0.62
-1.72
955
7
4
3.63
-4.07
-11.18
958
3.60
-0.87
-2.41
953
6
5
3.63
-4.62
-12.72
954
3.60
-1.04
-2.90
954
5
6
1.88
-15.88
-84.47
265
3.58
-1.23
-3.44
948
4
7
3.62
-4.61
-12.71
953
3.58
-1.32
-3.67
950
3
8
3.63
-4.06
-11.18
957
3.60
-1.27
-3.53
953
2
9
3.65
-3.12
-8.55
964
3.63
-1.03
-2.83
961
1
10
3.66
-1.77
-4.83
970
3.65
-0.77
-2.12
968
11
3.67
0.02
0.05
972
3.67
0.03
0.07
974
Ten Doble TTR capacitors in series, approximately 10,000 pF each
8 of 21
4
Negative Power Factor - Slung Specimen
Grading Capacitor for Alsthom Circuit Breaker, Dirty/Wet Sling
C1
Sling
C2
RG
N.P. Capacitance 1200 pF
Test / Mode
KV
Test Mode: UST
mA
W
%PF
Cap [pF]
Slung / UST
10
4.5
-0.014
-0.03
1193
Mounted / UST
10
4.505
0.038
0.08
1194
Note: The slung specimen could also be a bushing, arrestor, stand-off insulator, etc.
9 of 21
Negative Power Factor - Bushing
ABB Type O+C Bushing, Surface Contamination
C1
C1-1
C1-2
RG
Test Mode: UST
Test / Mode
Test KV
mA
W
%PF
C1 / UST
10
1.313
-0.007
-0.053
10 of 21
5
Negative Power Factor - Bushing
Surface Contamination – C1 Test, Routine vs. Inverted Method
Routine Method
C1-1
Inverted Method
Weakest Coupling
Strongest Coupling
Strongest Coupling
Weakest Coupling
C1-2
C1-2
C1-1
RG
RG
11 of 21
Negative Power Factor - Bushing
LAPP Type POC-A 34.5kV Bushing, Internal Tracking
C1
C1-1
C1-2
RG
Test Mode: UST
Bonding tape loose shunting outer surface
of bushing core to ground flange.
Test / Mode
Test KV
mA
C1 / UST
10
0.551
C2 / Guard
0.5
W
%PF
-0.18
-3.3
Test set tripped off
12 of 21
6
Negative Power Factor - Transformer
Epoxy Encapsulated Transformer with Surface Contamination
Surface
Contamination
HV Winding
LV Winding
CORE
Air
Gap
CHL
CH-Air
CAir
RHG
CL-Air
RLG
Test Mode: UST
Ground Potential
13 of 21
Negative Power Factor - Transformer
Epoxy Encapsulated Transformer
National, 3-phase, 2250 kVA, 34/0.48 kV, 1984
#
Insul.
kV
mA
Watts
%PF
Cap [pF]
1
CH + CHL
10
4.772
0.092
2
CH
10
1.522
0.120
0.79
403.5
3
CHL (UST)
10
3.252
-0.020
-0.06
862.5
4
CHL
3.250
-0.028
-0.09
861.5
5
CL + CHL
.5
15.230
3.059
6
CL
.5
11.990
3.051
2.54
3179
7
CHL (UST)
.5
3.254
0
0
863.2
8
CHL
3.240
0.008
0.02
861.0
1265
4040
14 of 21
7
Negative Power Factor - Transformer
Transformer with Ground Shield, Interwinding Test
HV
Winding
Ground
Shield
LV
Winding
CORE
CHL
CH-GS
CL-GS
RGS
Test Mode: UST
Note: If RGS=0, then mA ≅ 0 and Watts ≅ 0
15 of 21
Negative Power Factor - Transformer
Transformer with Ground Shield
McGraw Edison, 1250 kVA, 20.9/2.3 kV, 1984
#
Insul.
kV
mA
Watts
%PF
Cap [pF]
1
CH + CHL
10
13.040
0.477
2
CH
10
7.854
0.752
0.88
2083
3
CHL (UST)
10
5.189
-0.280
-0.59
1376
4
CHL
5.186
-0.275
-0.58
1377
5
CL + CHL
10
16.85
2.024
6
CL
10
11.67
2.322
1.83
3095
7
CHL (UST)
10
5.186
-0.290
-0.61
1375
8
CHL
5.180
-0.298
-0.63
1373
3460
4470
16 of 21
8
Negative Power Factor - Transformer
Three-Winding Transformer, Interwinding Tests
HV
Winding
LV
Winding
TV
Winding
CORE
CHT
H
L
T
CHL
CLT
RB
Test Mode:
UST Meas R, Gnd B
Note: If RB=0, then mA ≅ 0 and Watts ≅ 0
17 of 21
Negative Power Factor - Transformer
Transformer with Poor Grounding, Interwinding Tests
CHL
LV Winding
HV Winding
ICHL
CHL
CH
CH
CL
EG
CL
ICL
RG
Poor Core Ground
RG
Poor Tank Ground
Poor Test Lead Ground
ICHL + ICL
wCHL + wCL
Note(1): wCL may be negative. Therefore the measured sum wCHL+wCL
may also be negative. This would result in a negative power factor.
Note(2): If RG=0, then EG = 0. Therefore ICL ≅ 0 and wCL ≅ 0.
9
18 of 21
Negative Power Factor - Transformer
Transformer with Poor Core Ground (High Resistance)
Trafo-Union, 58 MVA, 230/20.72 kV
#
Insul.
kV
mA
Watts
%PF
1
CH + CHL
10
92
3.0
2
CH
10
29
3.7
0.95
3
CHL (UST)
10
63
-0.7
-0.11
6
CL
10
108
19.5
1.8
19 of 21
Negative Power Factor - Transformer
Transformer with Poor Tank Ground
General Electric, 7.5 MVA, 13.8/4.16 kV, D-Y
#
Insul.
kV
mA
Watts
%PF
Cap [pF]
1
CH + CHL
10
57.050
1.844
2
CH
10
0.913
-0.46
-5.0
241.9
3
CHL (UST)
10
1.669
-0.76
-4.5
442.1
4
CHL
56.137
2.3
0.41
14892
5
CL + CHL
5
42.100
1.4
6
CL
5
0.399
-0.1
-2.5
105.9
7
CHL (UST)
5
1.542
-0.7
-4.5
408.5
8
CHL
41.701
1.59
0.38
11061
15134
11167
20 of 21
10
Negative Power Factor - Stator
CAB
Coils
CA
CAB
Insulation Outside Slots
CA
EG
ICAB
CB
ICB
RG
CB
Insulation Inside Slots
RG
ICAB + ICB
Core
A Phase
B Phase
wCAB + wCB
C Phase
Note(1): wCL may be negative. Therefore the measured sum wCHL+wCL
may also be negative. This would result in a negative power factor.
Note(2): If RG=0, then EG = 0. Therefore ICB ≅ 0 and wCB ≅ 0.
11
21 of 21