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DETERMINE CURRENT TRANSFORMER SUITABILITY USING EMTP MODELS
1

DETERMINE CURRENT TRANSFORMER SUITABILITY
USING EMTP MODELS
Ralph Folkers
Schweitzer Engineering Laboratories, Inc.
Pullman, WA USA
A
BSTRACT

Current transformer (CT) and relay modeling are practical tools to evaluate protection equipment
performance. This paper demonstrates the use of a set of software tools - Electromagnetic
Transients Program (EMTP), The Output Processor (TOP), and Mathcad - to model transient
events in the power system, as well as relay response to those events. The paper provides step-
by-step instructions for using these tools to better understand and protect power systems.
Specifically, in this paper we:
1. Model CTs using EMTP to visualize transient events.
2. Transfer EMTP output into Mathcad to examine CT accuracy, burden effects, saturation,
and subsidence.
3. Model digital relays in Mathcad to show the effects of CT saturation on overcurrent,
distance, and directional element operation, making relay response to transient events
easier to understand.
I
NTRODUCTION

Older existing or spare equipment is often used in new construction or retrofit projects. Changing
system conditions can cause existing and spare equipment to operate outside of its intended
rating. To effectively evaluate equipment suitability, you must have the tools to determine power
transformer or circuit breaker CT performance in a protection scheme.
The Alternative Transients Program (ATP) version of EMTP is an inexpensive, powerful tool for
evaluating CT performance. This paper briefly describes ATP software, provides instructions for
constructing a CT model using ATP, and presents a method of modeling relay response by using
the CT model as input to digital relay models in Mathcad. The paper uses the CT and relay
models to demonstrate:
Secondary burden and connection effects on Ratio Correction Factor (RCF) and Phase
Angle Connection Factor (PACF) to answer the question When can a relay-accuracy
class CT be used for metering?
The effects of X/R, CT class, and burden on CT saturation and recovery times
Saturated secondary current reduction and its effects on overcurrent, inverse time-
overcurrent, and breaker failure element pickup
The effect of CT subsidence current on breaker failure element dropout time
Saturated secondary current and its effects on distance and directional element
performance. 2
Examples in this paper show methods of analysis rather than illustrating the performance of
particular CTs or relays. Appendices A through E provide ATPDraw circuits and detailed
Mathcad calculations used in these examples.
S
OFTWARE

The choice of power system transient analysis software is a matter of suitability, cost, and
individual preference. Cost can range from $0 to $15,000. We chose the four programs used in
the following work for their power, availability, and reasonable price.
ATP
The ATP version of EMTP is the basic software tool for electric system transient modeling.
Different computer operating systems use different versions of the program. Version ATPMING
works very well with MS Windows 95 and 98.
ATP is free to licensed users who meet the requirements of the ATP users group. Most utilities,
consultants, and manufacturers easily meet these requirements. Licensing information is
available on the World Wide Web at http://www.ee.mtu.edu/atp/index.html. Once licensed,
simply download the program from a password-protected site on the World Wide Web.
ATPDraw
ATPDraw is a graphical, mouse-driven preprocessor to ATP on the MS Windows platform and
uses a standard Windows layout. Users build a picture of an electric circuit by selecting
components from menus and using dialog boxes to enter component values and ATP parameters.
ATPDraw then creates the ATP input file and runs ATP.
Basic ATP model development is much easier in this environment, particularly for new users.
You can download ATPDraw for Windows free of charge from the ftp server ftp.ee.mtu.edu
(user: anonymous; password: your e-mail address). The Bonneville Power Administration,
USA, and SINTEF Energy Research, Norway, own the proprietary rights.
TOP
TOP, written and supported by Electrotek Concepts, Inc., is a graphical postprocessor for
transient data. TOP will graph ATP output files (*.pl4) and allow users to save the data in
different formats, including COMTRADE and comma separated variable (CSV) text files. This
program is the bridge between ATP and Mathcad.
You can download TOP free of charge from the Electrotek website at http://www.electrotek.com/.
Mathcad 7 Professional
Mathcad worksheets process the CT transient data generated by ATP. The Mathcad desktop
interface uses mathematical equations similar to those seen in textbooks. Concepts are easy to
see and understand, although the same results can be achieved in other programs such as
MATLAB . Mathcad 7 Professional is available from Mathsoft, Inc. 3
C
ONSTRUCTING A
CT M
ODEL
U
SING
ATP
This section demonstrates CT modeling using the ATP Saturable Transformer Component, shown
in Figure 1.
L P
R P
R S
L S
IDEAL
N1 : N2
BUS1-1
BUS2-1
BUS1-2
BUS2-2
SATURA
RMAG
Low Voltage
Winding 1
High Voltage
Winding 2

Figure 1: ATP Saturable Transformer Component
To model the CT, use the CT accuracy class, ratio, secondary winding resistance, and excitation
curve. Some manufacturers provide the ratio and phase angle correction curves which are useful
while testing the model.
Accuracy class C indicates the CT relay accuracy can be calculated adequately [9]. This paper
considers only C-class CTs.
Using the step-by-step instructions in Appendix A: Develop a 1200/5 CT Model, create the CT
model in ATP as follows:
1. Model the CT secondary on Winding 1 of the saturable transformer component (Figure 1).
2. On Winding 2, set resistor RS equal to zero. Set inductor LS, which must have a value
greater than zero, equal to 10E-6.
3. Set LP equal to zero, since a C-class CT secondary leakage reactance is very small.
4. Set resistor RP equal to the CT secondary winding resistance. Add separate circuit
components to model lead resistance and burden resistance.
5. Set magnetizing resistance, RMAG, to infinity, since RMAG is very large. Enter a 0 in
the ATP model for infinite RMAG.
6. Select seven to ten excitation-current versus voltage points from the CT excitation curve,
to include saturation in the model.
7. Convert these current versus voltage points to current versus flux points using the ATP
supporting routine SATURA.
8. Create the CT model in ATPDraw (Figure 2).
9. Test the model by recreating the CT excitation curve using the ATPDraw circuit shown
in Figure 2. 4

Figure 2: CT Excitation Test Circuit in ATPDraw
Figure 3 shows the results of three excitation curve tests using three, four, and nine points to
model saturation. The nine-point model gives the best results of the three.

Figure 3: Comparison of CT Models with Different Numbers of Excitation Points
Always test the CT model. Mistakes appear as ratio errors and irregularities in the excitation
curve. Use the model only after it has been tested.
In ATP, saturation is a piecewise linear model that can be unstable in certain conditions. Picking
too many points on the excitation curve or selecting a time step that is too large can cause high
frequency oscillations in the output.
Appendix A: Develop a 1200/5 CT Model describes the development of a 1200/5, C800 CT
model using nine points from the excitation curve. 5
S
ECONDARY
B
URDEN AND
C
ONNECTION
E
FFECTS ON
RCF
AND
PACF
Increasing CT burden increases induced secondary voltage and exciting current, causing ratio and
phase angle errors in a CT. Since C-class CT accuracy can be calculated accurately, use ATP to
examine the effects of secondary burden at different primary current levels. Appendix B:
Calculate CT Accuracy describes the ATPDraw circuit and Mathcad calculations in the
following example:
This example uses a 1200/5, class C800, CT model in the ATPDraw circuit in Figure 4. The six
sources turn on and off in sequence to apply 5%, 10%, 20%, 60%, 100%, and 150% rated current.
Each source is on for three cycles.

Figure 4: Accuracy Test Circuit in ATPDraw
The CT secondary resistor in Figure 4 is a standard burden, B-1.8 (1.62 + j0.784). This burden is
equivalent to 1,800 feet of No. 10 AWG.
Figure 5 shows the ATP output of primary and secondary current in the graphical postprocessor,
TOP. The secondary quantities appear very small because of the plot vertical scale. Notice the
six increasing levels of primary current.

Figure 5: Primary and Secondary Current in TOP

Save the TOP active window containing the CT primary and secondary currents shown in Figure
5 as a CSV text file using the File Save As menu item. Read the CSV text file into Mathcad