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.

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

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 1. Protective Relaying Principles and Applications, J. Lewis Blackburn and Thomas Dumin, 3

rd 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_)

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)