Energy and Power Engineering, 2013, 5, 1259-1265
doi:10.4236/epe.2013.54B239 Published Online July 2013 (
Pr otection Scheme Half Wavelength Tr ansmission Trunk
Using Conventiona l Relay
Renzo. G. Fabián1, Elson C. Gomes1, Maria C. Tavares1, Carlos A. Floriano2
1School of Electrical and Computing Engineering, University of Campinas, Campinas, Brazil
2ELETRONORTE -Brasília, Brazil
Received April, 2013
This paper presents the main results on the use of conventional protection hardware to protect a Half Wavelength
Transmission Line, or shortly AC-Link, for possible occurrence of three-phase and single-phase fault during energiza-
tion maneuver. Lines of these dimensions do not exist today, but in countries of continental size they could be an inter-
esting alternative to HVDC transmission, since the AC-Link has less dependence on Power Electronics technology. The
studies on performance and relay settings were implemented using the RTDS real-time simulator. It is important to note
that the AC-Link protection for the energization maneuver can be performed with existing hardware on conventional
Keywords: Protection; Half Wavelength Transmission; AC-Link, RTDS; Digital Relays
1. Introduction
Countries with continental extensions such as Brazil,
China and Russia are faced with the need to transport
large blocks of energy over long distances, namely above
2000 km. The use of transmission lines with a little more
than a half wavelength is an important alternative that
must be properly considered.
Currently, these long transmission trunks are made by
high-voltage direct current transmission lines (HVDC),
but an alternating current (AC) alternative with some
particular characteristics might be the most economical
one, having much less dependence on the Power Elec-
tronics Technology.
In the 1960s, the first studies were done showing that
the AC line has an interesting behavior in terms of volt-
age, current and stability of the system when it has elec-
trical length a little more than half the length of the elec-
tromagnetic wave [1-3]. Nowadays new researches have
been studying this alternative, as [4-12].
In a 60 Hz power system the length of the transmission
trunk is approximately 2600 km [4, 6]. The half wave-
length lines exhibit behavior very similar to short lines in
steady state and are more robust during transients. This is
an alternative to the HVDC line, because the AC-Link
may present a cost of implementation up to 20% lower [6,
As there is not a half-wavelength transmission line in
the world, Brazilian Electrical Energy Agency (ANEEL)
proposed to carry out an energization test connecting in
series sections of existing lines in the Brazilian intercon-
nected system [8-11]. This test will allow the analysis of
the behavior of the AC-Link and the comparison with the
studies made so far.
The energization test needs a protection system, and it
is necessary to analyze whether the current existing re-
lays could protect against possible faults along the line.
Otherwise, a more efficient protection scheme should be
The results of the study on the protection for three-
phase and single-phase faults using the relays available
in substations are presented.
The studies were performed using the RTDS real-time
simulator together with the same relays available on-site.
The analysis to identify the relays settings are presented
in the following sections, as well as the results of the
protection system proposed and the main conclusions.
2. Analyzed System Description
The used transmission system is based on the lines of
500 kV that can form an AC-Link with 2600 km long.
These lines are the interconnections North-South I and
North-South II that are parallel with a distance of 60 m
between towers, and part of the interconnection North
East-South East, as seen in Figure 1 [8,9]. The AC-Link
will be energized at shot through the breaker of Serra da
Mesa I substation.
Copyright © 2013 SciRes. EPE
The lines that will compose the AC-Link Test are of
the same voltage level, but they are not equal. Tables 1
and 2 present the series and shunt parameters per unit
length in sequence components. The lines were supposed
ideally transposed lines and the parameters were calcu-
lated for 60 Hz.
Traditional soil representation was applied during the
tests, specifically the soil resistively was considered con-
stant with frequency over the entire length of the
AC-Link trunk with value of 4000 .m due to high soil
resistively in these regions [13].
North -South 1
256 km255 km173 km330 km
500 kV500 kV500 kV500 kV500 kV
500 kV500 kV500 kV500 kV500 kV
Serra da Mesa 1
Gurupi 1
Miracema 1
Colinas 1
North -South2
256 km255 km173 km330 km
Serra da Mesa 2
Gurupi 2
Miracema 2
Colinas 2
NorthEast -SouthEast
251.3 km321.3 km
500 kV500 kV500 kV
Serra da Mesa
Rio das Eguas
Bom Jesus da Lapa
Switching breaker
Breaker always close
Breaker always open
Assumed trunk connection for the test
Figure 1. Single-phase diagram of AC-Link Test - 500 kV.
Table 1. Positive/negative sequences longitudinal and
transversal parameters calculated at 60 Hz.
Line Unitary
North-South I 0.01589 0.70700 0.01612
North-South II 0.01602 0.71089 0.01634
East-South East 0.01602 0.72403 0.01603
Table 2. Zero sequence longitudinal and transversal pa-
rameters calculated at 60 Hz - North-South II.
Line Unitary Re-
North-South I 0.37138 4.11662 0.00725
North-South II 0.34822 3.74452 0.00946
East-South East 0.34821 3.75767 0.00934
2.1. Protection System
In the protection study for the AC-Link energization
maneuver, it was initially observed the behavior of the
variables: voltage, current and impedance for three-phase
and single-phase faults along the transmission line. Ini-
tially, it was studied the existent distance protection and
later other additional functions of the relay SEL 321-1.
The faults were represented as impedance at the site of
defect of 20 .
The system simulated in the RTDS is shown in Figure
1 and there were monitored the voltages in the buses and
the currents in the circuit-breakers (CB), as well as the
magnitudes measured by instruments transformers (CT
and PT). The CTs ratio was 3000:1 and the PTs ratio was
4500:1. Formerly only the relay from the substation Serra
da Mesa 1, SEL-321-1, was used.
3. Three-phase Faults Protection
3.1. Using Conventional Distance Protection
In this section the distance protection was analyzed to see
if it would be suitable for the energization manouver test
of the AC-Link. To calculate the impedances, a compo-
nent was programmed for RTDS in the Standard C lan-
guage using the CBuilder tool of RSCAD. The compo-
nent was programmed according to [14].
In certain locations of the line, faults reach quasi reso-
nance conditions, either of positive (for three-phase
faults) or zero (for single-phase faults) sequence, rising
the voltages and currents along the AC-Link. In the case
of grounded and isolated three-phase faults, for faults
occurring in the region between 65% and 90% from the
AC-Link measured from the sending terminal, the posi-
tive sequence quasi-resonance condition becomes critical,
with rapid increases in voltage at the terminals of the
breaker and high currents. This critical region depends on
the terminal systems interaction with the AC-Link. In the
present case the AC-Link is supposed isolated, connected
to sending end substation (SE).
It is important to emphasize that during three-phase
faults at critical location the voltage along the line will
grow to a limit when insulation level will be reached and
flashover will occur along the line. In [12] it was pro-
posed the use of a reduced isolation distance (RID) posi-
tioned at approximately 40% of the total length of the
line from the SE to quickly remove the AC-Link Test
from the quasi-resonance condition. This RID mitigation
method consists in adjusting the number of insulators in
order to provoke the flashover for overvoltages higher
than 1.6 p.u. In that location the overvoltages will not be
higher than 1.2 p.u. during energization transient and
therefore RID will not operate during the energization
maneuver without defect. However, if a severe condition
appears RID will actuate immediately, removing the
Copyright © 2013 SciRes. EPE
R. G. FABIÁN ET AL. 1261
Link from the quasi-resonance condition.
Figure 2 shows the impedances calculated from the
voltages and currents of the PTs and CTs from Serra da
Mesa 1, for faults applied along the line (at intervals of
10% of the length of each line section). Each point on the
graph indicates the place of fault applied. In this graph, it
was supposed that the insulator string of the Imperatriz
substation had a normal length, i.e. they were not re-
duced. The squares are the impedances at the energiza-
tion steady state of the AC-Link without the occurrence
of fault.
As the maximum impedance that the relay SEL 321-1
can identify is 320 , it can be verified that the relay
could only protect the section of Serra da Mesa 1 (SM1 -
km 0) up to the area near the Imperatriz (IMP - km 1040)
substation. However, the reverse protection zones can be
used to protect the section from Bom Jesus da Lapa (BJL
- km 2601) up to the area near Miracema 2 (MI2 - km
1517). The no fault steady state impedances should be
dealt properly (squares), as these points are close to the
impedances for faults that occur near line center.
The distance protection cannot cover the section be-
tween Imperatriz and Colinas 2 (central region) and that
it is necessary to block the distance protection trip for
faults near Colinas 2.
The RID in a tower near the Imperatriz substation will
modify the distance protection performance for faults
near the remote end. RID starts working for faults that
affect the Link from the Gurupi 2 (GU2 - km 1772) sub-
station toward the remote terminal. The impedance
changes drastically for fault between Colinas 2 and Gu-
rupi 2, specifically the upstream and downstream of the
first location where the RID acts.
Figure 2. Impedances seen at Serra da Mesa 1, without RID
in Imperatriz.
3.2. Additional Protection Functions
From the above, the existing distance protection can pro-
tect the line in its initial and final sections through the
forward and reverse zone. It can indeed be used two for-
ward zones and two reverse zones, with Zone 1 forward
with instant action, Zone 2 forward with delayed action,
Zone 3 reverse with instant action and Zone 4 reverse
with delayed action. This makes it possible to identify
the fault site, as the zone would indicate the section of
the line where the fault occurred. However, the distance
protection of the relay SEL 321-1 would not be capable
of protecting the entire line during the energization test in
the event of three phase faults in the central region of the
Alternatives were analyzed to obtain a more efficient
protection scheme. As the faults are three-phase faults,
the components of zero and negative sequence will only
appear during fault transient and they are of low magni-
In Figure 3 the voltages and currents in the primary
and secondary sides of the PTs and CTs are presented for
the occurrence of three-phase to ground fault along the
entire line. The maximum voltage for energization is
567.05 kV and voltage at operation without fault is
286.76 kVrms. It can be observed that for faults in SM 1
voltage is reduced to close to zero, then, as the fault
moves away from the generator terminal, voltage at SM1
increases, which is the expected behavior for a conven-
tional line. However, for faults near the middle of the
Figure 3. Voltages and currents at sending end for tree-
phase faults along the line. a) Voltage in primary side of PT.
b) Voltage in secondary side of PT. c) Current in primary
side of CT. d) Current in secondary side of CT.
Copyright © 2013 SciRes. EPE
line, the voltage at SM1 exceeds the voltage of the steady
state without fault, which is the characteristic behavior of
he AC-Link. As the fault location moves toward the open
terminal, the voltages at SM1 reach maximum for faults
near Serra da Mesa 2 (SM2) and then decrease as the
fault moves to Bom Jesus da Lapa (BJL).
The current on the primary side of the CT reaches
1765 A (peak) on energization and has a current of
285 Arms in the operation without fault. It can be veri-
fied that for faults near SM1, the current measured is
quite high when compared to the current value without
fault. As the fault moves toward the open terminal the
current decreases, but is still higher than the current from
the operation without fault. This would be an expected
behavior in conventional lines; however, for faults lo-
cated near the middle of the line, the current in SM1 be-
comes lower than the current from the operation without
fault. This is a characteristic of the AC-Link.
After reaching its lowest value, the current begins to
increase as the fault moves to the remote terminal, peak-
ing near SM2 and finally begins to decrease as the fault
continues to move to the remote end. The high currents
measured for faults near Serra da Mesa 2 are similar to
those obtained for terminal faults, a characteristic behav-
ior of the AC-Link that is different from a line with con-
ventional length.
Analyzing the voltages and currents, it can be ob-
served situations characteristics of an AC-Link, mainly
in the region corresponding to faults near the middle of
the line. It can be seen that it is possible to use Under-
voltage, Overvoltage and Overcurrent devices to identify
three-phase faults along the AC-Link, but there are still
problems of selectivity for faults that occur near the mid-
dle of the line. Figure 4 shows the voltages in all buses
for three-phase fault at Colinas 2.
In Figure 4 it can be verified that voltages hardly suf-
fer variation in the substations at the upstream of the
fault location. The currents at the fault location do not
suffer variation. This explains the difficulty in identify-
ing these faults.
3.3. Additional Settings for the Relay SEL 321-1
With the above considerations, it was analyzed the pos-
sibility of additional settings in order to complement the
distance protection trying to protect the entire line for
three-phase faults.
Initially, it was used the distance protection (21) of the
relay as it is more selective. It was necessary to consider
the operation impedance without fault to block the dis-
tance protection using the Load Encroachment.
Some additional devices were used, as described be-
Undervoltage Device (27L): was set at 0.8 p.u.,
timed in 5 cycles. The 27L device was adjusted with the
loss of potential device (LOP), and the final logical was:
27L*! LOP. The performance of the relay would improve
significantly if it were used the state of the breaker to-
gether with the 27L device, in place of the LOP device;
Overvoltage Device (59N): was set at 1.2 p.u.,
timed in 12 cycles.
Overcurrent Device (51P): pick-up value of 0.5;
family of curve: U3; and time dial: 2.
Figure 4. Voltages in all buses for fault at Colinas 2.
Copyright © 2013 SciRes. EPE
R. G. FABIÁN ET AL. 1263
3.4. Tests Performed and Results
To verify the protection performance in the electrical
system, the simulations were performed with RTDS, ge-
nerating signals of voltages and currents that were ampli-
fied and finally injected into the relay SEL 321-1. The
relay trip was injected back into RTDS.
The three-phase-to-ground faults have been applied
along the entire line, initially every 20% of the length of
each section, to allow observing the performance of each
protective device. Finally all the devices were tested si-
multaneously for faults every 10% of the length of each
The distance protection operates for faults that occur
in the first 948 km and in the last 828.8 km of AC-Link.
The RID starts to operate from kilometer 1772, which
corresponds to the Gurupi 2 substation. The distance re-
lay operates at 1721 km from Serra da Mesa 1, and after
that the distance protection does not operate for a 308 km
section due to the operation of RID, returning to operate
in the last 572.6 km from the AC-Link. The central sec-
tion of 480.5 km is not protected.
Finally a relay located in Serra da Mesa 2 was consid-
ered. For this relay, the Undervoltage device (27L) is
used. It can be observed that it suitably covers the central
region and it does not operate for faults at the beginning
of the AC-link because the Undervoltage device (27L)
was adjusted to act together with the negative (!) of the
loss of potential device (LOP), and the Trip logic is:
The relay performance can be improved using the state
of the breaker instead of the LOP device in the trip logic
for Undervoltage.
4. Single-phase Faults Protection
The phase current and voltage characteristics were ob-
served at the sending end considering the application of
single-phase faults every 20 km along the line. Beside
the peak values, the faults were applied for 700 ms in
order to observe its steady state, as presented in Figure
Faults in central region of the line provide voltages
and currents values similar or even lower than the ones
seen with the system in steady state. This characteristic
does not allow to distinguish a single-phase fault from a
no load case. Moreover, the behavior of voltages and
currents are not monotonic, which increase the problem
of identification to the fault location.
There were obtained current and voltage curves of se-
quence components observed also in the sending end,
searching for a situation in which the single-phase fault is
clearly observed, Figure 6. These curves permit the es-
tablishment of limits for each voltages and currents se-
quence observed at sending end, together with the case
for no load condition.
4.1. Adjustments of Digital Relay
AC-Link the distance protection functions are not suffi-
cient to protect the AC-Link. Other relay functions have
to be used to provide an efficient protection in the case of
a single-phase fault. In Figure 7 some functions and the
protected areas obtained with RTDS simulations are
Faults in the central region of the line behave as high
impedance, producing little variation in the voltages and
currents seen as phases or sequences components at line
SE, as shown in Figure 8. Function 59N (zero sequence
overvoltage) properly identified central region faults.
Figure 5. Peak values of current and voltages in phase
components at sending end for single-phase faults applied
along the AC-Link.
Figure 6. Zero se que nc e component phase to ground voltage
of at SE - peak values (V0P) and sustained (V0S) for sin-
gle-phase faults along the AC-Link.
Figure 7. Single-phase fault protection.
Copyright © 2013 SciRes. EPE
Figure 8. Phase-to-ground current and voltages in Serra da
Mesa I during a single-phase fault in Colinas II substation
(km 1344).
Figure 9. Phase-to-ground current and voltages in Serra da
Mesa I during a single-phase fault in Colinas II substation
(km 1344) and relay operation.
Figure 9 shows the waveforms of the phase-to-ground
voltages and currents in Serra da Mesa I substation with
a single-phase faults occurrence at the middle of the
AC-Link. The relay rapidly responds and sends the trip
signal. Thus, the overvoltages at SE are low, even in case
of a fault in the most severe location, near the far end of
the AC-Link.
5. Conclusions
In this paper, there were presented the necessary settings
so that the existent conventional relay (SEL 321-1) at
SM1 relay SEL 321-1 can quickly identify the occur-
rence of three-phase and single-phase faults along the
AC-Link Test during an energization maneuver.
The main conclusions for three-phase faults are:
The conventional transmission lines’ protection
philosophy is not the most efficient way to protect the
AC-Link in the energization test. It is necessary to in-
clude the under and overvoltage protection, and also the
overcurrent protection, supposing that only the relay
from Serra da Mesa 1 may be used for the test;
The entire AC-Link could not be properly protected
with only the relay from SM1, leaving a central section
of 480.5 km without protection;
If the relay from Gurupi 2, or another one located in
a substation between Gurupi 2 and the remote terminal of
the Link were used, it would be possible to protect the
entire Link during the test with conventional relays.
It is worth noting that the occurrence of three-phase
faults in the central region does not damage or reduce
life-time of assets located in the generation terminal.
Regarding single-phase fault, the main protection used
is based on instantaneous zero sequence voltage (59N)
that does not identify the site of the fault but quickly
protects the system. Three other adjustments were made,
both instantaneous and timed to allow greater operating
It should be noted that the adjustments were made for
the no load energization test of the AC-Link. The protec-
tion of a line with a little more than half-wavelength un-
der operation will be presented in a future paper.
The AC-Link is a reliable and technically robust solu-
tion for very long transmission trunks. Due its constant
terminal voltage for all load profile it should also be
carefully analyzed for intermittent power transfer as the
ones produced by green energy sources, as large solar
and wind power plants.
It can be stated that the AC-Link energization test can
be implemented in the analyzed system without impact-
ing the equipment involved. There will be no damage to
the devices involved and their life-time will not be re-
duced due to the stresses imposed by the experiment.
6. Acknowledgements
This work was supported CNPq, CAPES and FAPESP in
Brazil. The results presented in this paper are partial re-
sults from the project supported by ELETROBRAS/
as part of ANEEL (Brazilian Electrical Energy Regula-
tory Agency) strategic project 2008 (proc. 4500072477).
The project was coordinated by UNICAMP with the par-
ticipation of UFBA and UEFS.
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