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Extended Range CT’sMEA Electric Operations Summit
May 2015
Presented by:
Dennis Krause
General Electric
Extended Range CT’s for Transformer-Rated Revenue Metering
600V Current Transformer
Secondary
Winding
Core
No Base
The challenge with CT’s … VARIATION!
50:5
200:5
400:5
300:5
500:5
600:5
800:5
1000:5
1200:5
1500:52000:5
3000:5
4000:5
Low Base
High Base
Wide Base
Primary Bar
No Bar
2” Window
3.06” Window
3.5x4.5” Window
5” Window
30°C
Single Ratio
B0.5
B0.2
B0.1
0.3
0.15
55°CDual Ratio
Multi Ratio
RF 4
RF 3
RF 2
RF 1
RF 1.5
Solution … SIMPLIFY!With Extended Range CT’s
What can
they do?Simplify meter shop operations
& reduce costs for utilities
How do
they do it?A single “one-size-fits-all” CT
can operate accurately over a
wide range of current
Where are
they used?Revenue metering in commercial
and industrial applications
Product Feature Benefit
Features & Benefits
Same mounting construction Easy substitution & retrofitting
Reduces Inventory & Part NumbersReduce inventory holding costs, less risk
of stock out
Simplifies CT Selection Save engineering time
Simplifies Billing MultipliersSignificant reduced risk of meter
programming error or billing error
Equal to or better accuracy, over a wider
range
One extended rnage unit can functional
replace multiple standard CT ratios
Agenda
• Instrument Transformer Fundamentals
• Selection Characteristics: “BRAVER”
• Next Generation of CT
• CT Selection Quiz
Instrument Transformer Selection Characteristics
IT Selection
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
IT Selection
Load on the secondary
side of the transformer
CT
E-0.2
B-0.1
B-0.2
B-0.5
B-0.9
B-1.8
VT
W
X
M
Y
Z
ZZ
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
IT Selection
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
Primary (Input)
24,000 Volts
Secondary (Output)
120 Volts
H1
X2X1
H2
24000:120V or 200:1 75:5
Primary (Input)
75A Volts
Secondary (Output)
5 Amps
H1
X2X1
H2
IT Selection
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
Class Description
0.6 Indicating
0.3 Standard Revenue Metering
0.15 High Accuracy Metering
0.15S Special High Accuracy
(Accubute™)
0.3 B-0.9
IT Selection
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
IT Selection
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
Window Size (CT)
Indoor vs. Outdoor
Insulation Type
1 Bushing or 2 Bushing
Hardware (Fuse, Baseplates)
IT Selection
B – Burden
R – Ratio
A – Accuracy
V – Voltage Class
E – Etc.
R – Rating Factor
RF
1.0
1.33
1.5
2.0
3.0
4.0
Rated current x (RF) =
Maximum continuous operating
current, without:
• Exceeding temperature limits
• Loss of published accuracy
Burden
Graph: Bill Hardy – TEC PowerMetrix
CT Accuracy – Burden – Rating Factor
Less Burden = Higher Accuracy More Load = Higher Acc. (until saturated)
Primary
Current
CT
X1X2
Burden of
Devices ()
Burden of
Leads ()
Secondary current
Total
Burden ZT
What creates Burden?Metering/Indication
Standard Burdens per IEEE C57.13
Burden Impedance
(Ohms)
Volt-amps
(at 5
amps)
Power
Factor
B0.1 0.1 2.5 0.9
B0.2 0.2 5.0 0.9
B0.5 0.5 12.5 0.9
B0.9 0.9 22.5 0.9
B1.8 1.8 45.0 0.9
E0.2 0.2 5.0 1.0
E0.04 0.04 1.0 1.0
Leads Resistance Values
Wire Size
(AWG)
Resistance
Ohms / 1000 FT
@ 20 deg C
10 0.9989
11 1.260
12 1.588
13 2.003
14 2.525
15 3.184
16 4.016
For reference only
CT Burden Calculation
100ft. of #10 = 100* (0.999/1000) = 0.10
Ω
100ft. of #12 = 100* (1.588/1000) = 0.16
Ω
With classic rotating style meter w/25’ #12 leads:
Ammeter = 0.75VA pf 0.9
WHr meter = 0.50VA pf 0.6
Leads = 50’ total * 1.588/1000’ = 0.080 Ω
Convert Meter VA to an impedance:
Z = 0.75 / (5A)2 = 0.03 ohms = .027 + j 0.013
Z = 0.50 / (5A)2 = 0.02 ohms = .012 + j 0.016
Total Meter = 0.039 Ω + j0.029
Total Burden on CT = 0.119 + j0.029 = 0.122 Ω
Minimum requirement of CT = B0.2
Burden Example #1
Burden Impedance
(Ω)
B0.1 0.1
B0.2 0.2
B0.5 0.5
B0.9 0.9
B1.8 1.8
E0.2 0.2
E0.04 0.04
Burden Example #2
Burden Impedance
(Ω)
B0.1 0.1
B0.2 0.2
B0.5 0.5
B0.9 0.9
B1.8 1.8
E0.2 0.2
E0.04 0.04
*or* E0.2
With electronic meters w/25’ #12 leads:
Ammeter = 0.2VA pf 1.0
WHr meter = 0.0125VA pf 1.0
Leads = 50’ * 1.588/1000’ = 0.08 Ω
Convert Meter VA to an impedance:
Z = 0.2 / (5A)2 = 0.008 ohms
Z = 0.0125 / (5A)2 = 0.0005 ohms
Total Meter = 0.008 + 0.0005 = 0.0085 Ω
Total Burden on CT = 0.08 + 0.0085 = 0.0885 Ω
Minimum requirement of CT = B0.1
Electronic meter burdens are very small
Therefore:
Actual field burdens < 0.1 ohm
Burden Power Factor can approach 1.0
CT burden length of connecting leads
Per IEEE C57.13.6
Accuracy
Actual secondary
currentRated secondary
Current (5A)=
Difference in % is known as
the “Accuracy”
of the CT
CT Accuracy
Sample CT Test Card
The Ratio Correction Factor (RCF) has
been defined as the factor by which the
marked ratio must be multiplied in order to
obtain the true ratio.
Example:
Current Transformer ratio is 150:5
The tested RCF is .9981
The true ratio is 150 x .9981 = 149.7:5
Ratio Correction Factor (RCF)
The Phase Angle of an instrument
transformer (PA) is the phase
displacement in minutes between the
primary and secondary values.
Phase Angle (PA)
The Ratio Correction Factor (RCF) and
the Phase Angle (PA) are combined to
determine whether or not the transformer
meets accuracy requirements.
CT Accuracy
What causes
ratio error
and
phase error?
Relative factors that DECREASE CT accuracy:
Operating current <100% of rated current
Operating at higher burden
Design Factors:
Larger Window
Low Ratios (less primary or secondary turns)
Smaller Core
Relative factors that INCREASE CT accuracy:
Operating current near rated current or within rating factor
Operating at low burden
High Ratios (more primary or secondary turns)
Higher grade core steel (higher accuracy class)
CT Accuracy Factors
Medium Voltage Extended Range & High Accuracy CT’sPer IEEE C57.13.6
Class Description
0.6 Indicating Instruments
0.3 Standard Revenue Metering
0.15 High Accuracy Metering
0.15S Special High Accuracy (Accubute™)
-- Extended Range
CT Accuracy Parallelogram per IEEE
0.9940
0.9970
1.0000
1.0030
1.0060
-40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0
RC
F
Phase Angle, minutes
100%
10%
0.3 Metering Accuracy Class
0.9940
0.9970
1.0000
1.0030
1.0060
-40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0
RC
F
Phase Angle, minutes
100%
10%
0.3 Metering Accuracy Class
By their physics, a CT
accuracy must always
be here … unless…Fa
vo
rs
Cu
sto
me
r
Fa
vo
rs
Utilit
y
Compensation
A true 200:5 has 40 Turns
200 Amps / 40 Turns = 5 Amps
5 Amps * 0.997 = 4.985 Amps
A “compensated” 200:5 (or 199.4:5)
200 Amps / 39.88 Turns = 5.015 Amps
5.128 Amps * 0.997 = 5.0 Amps (!!)
0.3
0.6
0.3
0.6
B0.1
B1.8
Fa
vo
rs
Cu
sto
me
r
Fa
vo
rs
Utilit
y
0.9970
0.9985
1.0000
1.0015
1.0030
-16.0 -12.0 -8.0 -4.0 0.0 4.0 8.0 12.0 16.0
RC
F
Phase Angle, minutes
0.15
Accuracy
Parallelogra
m100%
5%0.3 Accuracy
Parallelogra
m
High Accuracy Class
0.9970
0.9985
1.0000
1.0015
1.0030
-16.0 -12.0 -8.0 -4.0 0.0 4.0 8.0 12.0 16.0
RC
F
Phase Angle, minutes
New
0.15s
Accuracy
100%
5%
Special High Accuracy Class
0.30
0.30
0.60
0.60
Rating Factor
Accu
racy C
lass -
%
10%
1.0 RF
IEEE C57.13 Accuracy
0.3 @ BX.X; RF = X.X±0.3% Accuracy
from 100% rated
current up to RF
100%
Standard 0.3 Accuracy Class±0.6% Accuracy
from 10% to 100%
of rated current
No accuracy guaranteed at
current levels less than 10%
Typical 0.3 CT
Performance Curve
0.30
0.30
0.60
0.60
Accu
racy C
lass -
%
5%
1.0
±0.15% Accuracy
from 100% up to
RF
100%
0.15 High Accuracy Class
±0.3% Acc.
from 5% -
100%
0.15
0.15
IEEE C57.13.6 Accuracy
0.15 @ BX.X; RF = X.X
Typical 0.15 CT
Performance Curve
No accuracy guaranteed at
current levels less than 5%
RFRating Factor
0.30
0.30
0.60
0.60
Accu
racy C
lass -
%
100%
1.0
0.15
0.15
0.15S Special High Accuracy(Accubute) IEEE C57.13.6 Accuracy
0.15 @ E0.04, E0.20; 0.15 @ BX.X;
RFX.X
5%
±0.15% Accuracy
from 5% up to RF
Typical CT
Accubute™
Performance Curve
No accuracy guaranteed at
current levels less than 5%
RFRating Factor
Accuracies are stated at a BurdenJKW-5A RCF Curve
0.3 B1.8 *OR* 0.15 B0.9 *OR* 0.15S B0.5
Higher Accuracy means Higher Revenue……Generally.
In a typical installation:
1. The load a CT sees will be below the
Rated Primary Current, for a majority of
the time
2. CT error is almost always negative, and
becomes exponentially more negative as
the current load decreases.
3. Negative error means that the CT under
reports current into the meter, meaning
the power being measured is increasingly
lower than what is actually on the line
4. Therefore, less CT error means more
current being metered, meaning more
revenue recorded
Example, a 40A input load on a 100:5 CT, metered as
0.00 Error: 40A ÷ 100/(5 * 1.0000) * 20 multiplier =
40.00A
0.15 Error: 40A ÷ 100/(5 * 0.9985) * 20 multiplier =
39.94A
0.30 Error: 40A ÷ 100/(5 * 0.9970) * 20 multiplier =
39.88A
0.60 Error: 40A ÷ 100/(5 * 0.9940) * 20 multiplier =
39.76A
Some Considerations with High Acc.
• A more “accurate” description would be “tighter tolerance CT”
• A specially compensated CT could have error favoring the utility and therefore increasing accuracy *decreases* total customer bill
• At B-0.1, many standard CT designs will be extremely accurate, especially within Rating Factor.
• The more extreme the conditions, the more potential benefit from high accuracy class (i.e. high voltage, high burden, high variation in operating voltage)
• IEEE standards for Instrument Transformers are written with consideration for Metering standards (ANSI C12.20)
Reason why 5% of Inom is minimum current
ANSI C12.20 Standard for Electricity Meters Class 0.2 - having ± 0.2% accuracy
1% 3% 5%
9S - C20, 120V
Rating Factor
CT Sizing (Rating Factor)
Example:
200:5 CT with RF4.0
May be operated up to:
4.0 x 200A = 800A Primary Current
Note: Secondary current = 4.0 x 5A = 20A
CT Rating Factor
0.25A or 0.5A to 20A = CL20 Meter Op.
Range
0.30
0.30
0.60
0.60
Rating Factor
Accu
racy C
lass -
%
10%
1.0 RF
100%
CT Rating Factor
For optimal performance with a
standard CT, size to operate CT
within Rating Factor
High Accuracy vs. Rating FactorScenario:
• You use a 200:5 ratio with a RF of 3.0.
• Accuracy performance is ± 0.3% from 200A to 600A, and ± 0.6% from 20A to 200A
Suppose you want to increased accuracy. You may be disappointed to learn the higher accuracy option only has an RF of 1.5, and therefore the maximum guaranteed current is only 300A…
Solution:
• Choose a 400:5 high accuracy guaranteed at ±0.15% from 5% to 150%.
• ± 0.15% accuracy from 20amps to 600amps.
• Despite a lower RF, you have improved accuracy over the entire range you had before!
Furthermore, your biggest improvement in accuracy will come at the 20-200amp range.
20
0.30
0.60
0.15
0.30
0.15
0.60
200 400 600
Primary Current Range (Amps)
ACCUBUTE™
400:5 RF=1.5
Conventional CT
200:5, RF = 3
Accu
racy C
lass -
%
Next Generation of CT’s
Redefining CT Performance
Many manufacturers offer performance beyond the current IEEE definition
Standard Accuracy (IEEE vs. Wide
Range)
High Accuracy (IEEE vs. Extended
Range)
Wide Range: How it worksExtending the test limits
Standard CT Wide Range CT
Standard CT Extended Range
Extended Range: How it worksWider range plus tighter test tolerances
Features & Benefits
System Current 6A 20A 40A 60A 80A 100A 200A 400A 600A 800A 1000A 1200A 1400A 1600A 1800A
200:5 JAK-0C
(Rating Factor 4.0)±0.6% Accuracy ±0.3% Accuracy
400:5 JAK-0C
(Rating Factor 4.0)_ ±0.6% Accuracy ±0.3% Accuracy
1200:5 JAK-0C
(Rating Factor 1.5)±0.6% Accuracy ±0.3% Accuracy
600:5 JAK-0S
(1% to 300%)±0.15% Accuracy
…Plus higher accuracy, which translates to higher revenue
• CT error becomes exponentially more
negative at light loads
• Negative error means that the CT under
reports current into the meter
• Less CT error means an increase in kWh
measured
Sample Business Case
Scenario
• Commercial customer, open M-F, 9am-5pm
• 480/277V system with 200A nominal current
• Load is 100% on weekdays, 10% on weeknights, and 5% on weekends
With these conservative assumptions - less than an 18 month payback!
CT Type Ratio kWh Metered Annual Bill Additional Revenue Price Adder
JAK-0C Standard CT 200:5 217,211 $19,549 --- ---
JAK-0S RevenueSense™ 600:5 217,665 $19,590 +$41$20/unit * 3
CT’s
The RevenueSense CT, with its premium accuracy increases billing revenue by $41 annually versus the
standard CT. The additional cost to use (3) high accuracy CT’s is roughly $60, in this example.
Result & Benefits
Sizing CT’s
Nameplate
Majority of revenue loss in most utility systems come from CT-metered installations
because
Installations are larger customers such that an error will create obviously a
much larger loss
CT installations are more complex and comprise more components resulting
in a higher probability of failures.
Most CT-metered installation errors favor the customer resulting in under-billing of the
customer.CT - Basics of Operation & Testing
Norm Ackermann - SpinLab
Why is the correct selection of CT’s important?
Estimated annual impact at US Utility is $450M
Example #1
Secondary Metering Cabinet, Bar-type CT
Need burden of B0.2
Normal operating current of 400A
Maximum operating current of 800A
Minimum operating current near 0
Choices
• 200:5 rated 0.3 B0.5, RF=4.0
• 400:5 rated 0.3 B0.5, RF=4.0
• 600:5 rated 0.3 B0.5, RF=2.0
Example #2
Secondary Metering Cabinet, Bar-type CT
Need burden of B0.2
Normal operating current of 100A
Maximum operating current of 1200A
Minimum operating current near 0
Choices
• 200:5 rated 0.3 B0.5, RF=4.0
• 400:5 rated 0.3 B0.5, RF=4.0
• 600:5 rated 0.15S B0.2, RF=2.0
Example #3
Primary Metering Cabinet, Wound CT
Need burden of B0.5
Normal operating current of 350A
Maximum operating current of 800A
Choices
• 300:5 rated 0.3 B0.5, RF=1.5
• 400:5 rated 0.3 B0.5, RF=1.5
• 600:5 rated 0.3 B0.5, RF=1.5
• 800:5 rated 0.3 B0.5, RF=1.5
Example #4
Primary Metering Cabinet, Wound CT
Need burden of B0.5
Normal operating current of 350A
Maximum operating current of 800A
Choices
• 600:5 rated 0.3 B0.5, RF=1.5
• 600:5 rated 0.15S B0.5, RF=1.5
Example #5
Secondary Metering Cabinet, Bar-type CT
Need burden of E0.2
Normal operating current of 40A
Max operating current near 200A
Choices
• 200:5 rated 0.3 B0.2, RF=2.0
• 600:5 rated 0.15S B0.2, RF=2.0
Example #6
Secondary Metering Cabinet, Bar-type CT
Need burden of E0.2
Normal operating current of 300A
Maximum operating current of 400A
Minimum operating current near 200A
Choices
• 200:5 rated 0.3 B0.2, RF=2.0
• 600:5 rated 0.15S B0.2, RF=2.0
Should I use lowest ratio possible and
operate within rating factor for extended
range CT’s?
Ratio (RF=3) Standard CT Extended Range
200:5 20A to 600A 2A to 600A
400:5 40A to 1200A 4A to 1200A
600:5 60A to 1800A 6A to 1800A