Using the TESLA 4000 DFR to Detect
Transmission Line Open Conductors
Introduction
Open conductors can pose serious problems to the power system. They can cause system instability due to
reduced power transfer capability, overload in the healthy phases, and higher voltages on ungrounded systems.
This application note provides a solution to detect open conductor events in the case where the conductors
have not touched the ground. The TESLA 4000 Digital Fault Recorder functions and Boolean logic are used in
order to create an open conductor detection solution.
Symmetrical Components
The open conductor solution will use the I2 or negative sequence current elements as part of the open
conductor logic detection. Therefore, it is important for the user to understand symmetrical component concepts
and know how to calculate a negative sequence current component. The formulas and phasors below show the
basic symmetrical component equations for positive, negative and zero sequences (for voltage and current).
Figure 1. Positive, Negative and Zero Sequence Voltages
Figure 2. Positive, Negative and Zero Sequence Currents
Open Conductor Fault Calculations
The theory behind open conductor calculations is shown below. The reader is encouraged to study each
calculation in order to understand the math necessary to calculate the sequence components from a real system
open conductor. The calculations below can be found in reference 1. Even though understanding the theory is
important, one need not perform the calculations all the time. The sequence components are readily available in
TESLA Control Panel Software.
$ 0
%$ *$
$(
$ 0 % 0$(/
# 0
)' )*
(#
$$(&'(
0*/
&#
$$($&+
0+/
&#&'(
Figure 3. Open Conductor Power System
1. Convert all Z to the system base:
a. Generator:
&#
,#/ b. Transformer @ bus G
c.
Line
The per unit values for the line remain the same since they are based on the system MVA base
values.
° ° d. Transformer @ bus H
The per unit values for this transformer remain the same since they are based on the system MVA
base.
e. Load
The load is 30MVA which is the same as the system base MVA. Therefore, the per unit impedance
is 1.p.u at 25 deg.
2. Draw the sequence network for the positive, negative and zero sequence impedances.
Figure 4. Positive, Negative and Zero Sequence Impedances
Figure 5. Positive, Negative and Zero Open Conductor Sequence Impedances
3. Perform the calculations: It is important to point out that with motors, the negative sequence impedance
is less than the positive sequence impedance so Z1load and Z2load are not equal. Therefore:
Z2 is in parallel with Z0. Thus,
° °
Total Impedance is:
°
The equivalent voltage:
°
From these, we can calculate:
= 0
Detecting Open Conductors with the TESLA 4000 DFR Functions and
Boolean Logic
The open detection configuration is done by using the following:
1. Phase Currents: All three sets of currents are need as inputs. A high level trigger of 0.3A secondary in
every phase is used in order to detect that the line is closed and energized and that current is flowing
thought the conductors. A low level trigger of 0.1A secondary is used in order to detect the open
conductor. If the open conductor does not touch the ground or other equipment such as an insulator, the
current through that conductor will be very close to zero. Therefore, the low level trigger will let us know
that the line has been opened or an open conductor has occurred. The low level trigger is supervised by
the negative sequence and breaker status as shown below.
2. Phase Voltages: A set of three phase voltages is used only for reference and metering purposes. The
voltages are not used as part of the open conductor detection logic.
3. Negative Sequence: In the event of an open conductor in one of the phases, we know from the
calculations above that the negative sequence value will increase. For this example, we have assumed
that the line is normally loaded to 50% and chosen a trigger value of 0.2A secondary. It is
recommended that the negative sequence value be chosen based on the load of the line. This is due to
the fact that at low load conditions, the negative sequence value has to be very sensitive to detect an
unbalance.
4. Breaker Status: In the occurrence of an open conductor in one phase, and assuming that the
conductor has not touched any grounded elements, the breaker and line would remain closed. As a
result, a 52a contact will also be closed. We can use the breaker status to supervise the open conductor
detection logic. On the other hand, if the open conductor has been grounded, the protective relays will
operate on either impedance or overcurrent ground elements. In this case, the open conductor logic is
not needed since the open conductor fault will be cleared by the relays and breaker. In addition,
because the ground relays will detect the grounded open conductor, the zero sequence value is not
used as part of the open conductor.
5. Open Conductor Logic: For this application, we will use two different types of logic configuration. The
first type will be able to identify an open conductor in any phase, but it won’t tell us the open phase. The
second type of logic will be able to identify an open conductor for each individual phase. In other words,
we will know which phase has the open conductor.
Configuration of Settings
1. Configuring Analog Inputs: Configure the voltage and current inputs as shown below in Figures 6
and 7. Even though the voltage information is not needed in the open conductor logic, the voltages are
used for phasor reference.
Figure 6. Analog Voltages
Figure 7. Analog Currents
2. Breaker Status: Configure the breaker status as seen below in Figure 8.
Figure 8. Breaker Status
3. Negative Sequence: Configure the I2 negative sequence component as seen below in Figure 9.
Figure 9. Negative Sequence Settings
4. Open Conductor Logic Option 1. Configure the general open conductor logic as shown in Figure 10.
This logic will tell us that one of the phases is open, but it won’t tell us which one. A 2000ms time delay
is introduced to give time for relays to clear the open grounded conductor. If the conductor is opened
and has not touched any grounded elements, then the logic will assert after 2000ms or 120 seconds.
Contact 5 is used to send the alarm to the SCADA master which will have to do a soft latch for the
momentary contact closure.
Figure 10. Open Conductor Logic- All Phases
5. Open Conductor Option 2. The second option will certainly let us know about an open conductor
situation and it will also tell us the open phase. Configure each phase as seen in Figures 11 to 13.
Noticed that the logic also is set to close a dry contact 6 for A phase when the logic is activated. This
contact can be wired to an RTU to let operators know of the event. A soft latch will need to be done by
the SCADA master to latch the alarm since the contact closure will be momentary and then go back to
open state. A 2000ms time delay is also used for this logic application.
Figure 11. Open Conductor Logic-A Phase
Figure 12. Open Conductor Logic-B Phase
Figure 13. Open Conductor Logic-C Phase
Testing
The following figures demonstrate the open detection logic for A, B and C phases with open conductors.
Figure 14. A Phase Open
Figure 15. B Phase Open
Figure 16. C Phase Open
s and product information contained in this document are subject to change without notice.
stencies between documents, the version at www.erlphase.com will be considered correct. (D03032R01)
Conclusions
This application note has described a simple method to detect open conductors by using TESLA Control Panel
functions and Boolean logic. The user can use this method on transmission lines or distributions systems.
References
rd
1. Protective Relaying Principles and Applications, J. Lewis Blackburn and Thomas Dumin, 3 Edition, 2007.
s and product information contained in this document are subject to change without notice.
stencies between documents, the version at www.erlphase.com will be considered correct. (D00___R0_)