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PRACTICAL CONSIDERATIONS FOR SINGLE-POLE-TRIP LINE-PROTECTION SCHEMES 1
PRACTICAL CONSIDERATIONS FOR
SINGLE-POLE-TRIP LINE-PROTECTION SCHEMES

Fernando Calero
Schweitzer Engineering Laboratories, Inc.
La Paz, Bolivia
Daqing Hou
Schweitzer Engineering Laboratories, Inc.
Boise, ID
A
BSTRACT

This paper discusses some practical aspects of implementing single-pole tripping schemes in
transmission-line protection. Implementation and techniques will vary in the design of protective
relaying systems that implement single-pole trip (SPT); as well as in the philosophy of the users
applying these devices; however, the intent is to discuss issues that need to be considered for SPT
systems.
We present a quick review of the benefits and implementation of SPT, emphasizing the benefits
for power system stability with a relatively minor incremental cost compared to three-pole trip
(3PT) systems for EHV and many HV transmission systems. We also discuss specific aspects of
implementing single-pole-tripping in line-protective relaying schemes. In addition, we discuss
using a faulted-phase selection algorithm, some reclosing scheme philosophy, and the greater
flexibility that improved communication channels give to SPT systems. We analyze the open pole
period of an SPT system to gain some insight into the behavior of the measured quantities and
protective relaying elements.
I
NTRODUCTION

Single-pole tripping (SPT) systems are being used worldwide to enhance the stability, power
transfer capabilities, reliability, and availability of a transmission system during and after a
ground fault [1][2]. The basic idea is to take advantage of the higher probability of occurrence of
single-line-to-ground faults, SLGF, (AG, BG or CG) with respect to the other seven fault types,
designing the system to correctly differentiate between SLGF and the rest of the occurrences, for
the purpose of tripping a single pole.
The benefit of having a single pole of the transmission line breakers open is that the two ends of
the transmission line remain metallically connected by the other two phases, allowing power
transfer and reducing the possibility that the two ends will lose synchronism.
B
ENEFITS OF
SPT
When a single-line-to-ground fault occurs, the protective system should detect the ground fault
and identify the faulted phase, tripping a single pole of the breaker to clear the primary arc
current. The open-pole period should be long enough to ensure that the secondary arc current
(fault current fed by the energy from the other two phases) is extinguished. The design of the
transmission line should consider whether additional shunt compensation reactors are needed to
extinguish the secondary fault current after the single-phase poles are open [2]. Once the
secondary arc current has been extinguished, the reclosing scheme will take care of bringing the
breaker back to normal. 2
The classical use of the power transfer capability equation demonstrates the benefits obtained
when a single-pole-trip system is applied to the protection of a transmission line [3][4]. The
equation describes the maximum electrical power that can flow across a transmission-line
impedance:

)
sin(
X
Vr
Vs
Pe
T =
(1)
Where: Pe is the real electric power transferred in Watts


Vs, Vr are the sending and receiving equivalent source voltages.


is the angle between the two source voltages
In a steady-state power system, the energy conversion is at equilibrium. All the mechanical power
is converted into electrical power (Pm = Pe), if losses are not considered. The power flow across
the line impedances of a power system should ensure that the mechanical power is transmitted to
the loads. Any lack of transmission capacity implies that the equilibrium is lost and there is more
mechanical power than the lines are able to transmit electrically.
The transfer reactance (X
T
) between the two voltages (Vs, Vr) in Equation 1 is the key for
evaluating the power transfer capability of a system configuration. The system in Figure 1, for
example, illustrates a simplified arrangement of two parallel lines. For the sake of simplicity, the
assumptions are that the source impedances are negligible, the lines are reactances, and the zero-
sequence impedances of the lines are three times the positive-sequence impedances (XL0 = 3XL).
Vs
Vr
XL
XL
Pe
Normal
SLGF
Ph-Ph Fault
Ph-Ph-G Fault
Three Phase Fault
1/2 XL
5/9 XL
2/3 XL
3/4 XL
XL
X
T
90
0
180
2
1
P.U. Power Transfer
PeSLG
Pe2pp
Pe2pg
Pe3ph
Pe
Electrical Angle
P.U
.
Pow
e
r

Transer
Normal
SLGF
Ph-Ph
Ph-Ph-G
3 Phase

Figure 1 Power Transfer Capability of a System During Power System Faults
in the Middle of the Line
Figure 1 illustrates the results of calculating X
T
, the equivalent transfer reactance between the two
sources, for different types of faults in the middle of one of the parallel lines. During normal
power flow, the equivalent transfer reactance is X
T
= ½ XL, the parallel combination of the two 3
line reactances. If a fault occurs in the middle of one of the transmission lines, as shown in the
figure, the equivalent transfer reactance is calculated using electric power system analysis
techniques and the theory of symmetrical components [5][3].
The severity of the power system fault can be evaluated. The value of X
T
illustrates how much
electrical power can be transferred across the system. The SLGF, which fortunately is the most
frequent fault, is in general the least severe for the power system. During normal operation of the
simplified power system shown in Figure 1, 2 p.u. can be transferred across the transmission
system. During a SLGF the system can transfer 9/5 p. u. As expected, for a three-phase fault the
system can only transfer 1 p.u., half of the normal operation power, and it is this fault that greatly
affects the stability of the power system. A three-phase fault severely affects the ability of a
power system to transfer power.
Using the same power system, we calculate the power transfer capability during the operation
with open poles in one of the transmission lines. We use the same circuit analysis and
symmetrical component techniques to find the equivalent transfer reactance (X
T
) of the system.
Figure 2 illustrates that having a single pole open allows the maximum power transfer.
Vs
Vr
XL
XL
Pe
Normal
One Pole Open
Two Poles Open
Three Poles Open
1/2 XL
7/11 XL
5/6 XL
XL
X
T
90
0
180
2
1
P.U. Power Transfer
PeOPH
Pe2OPH
Pe3OPH
Pe
Electrical Angle
P.U
.
Pow
e
r
Transer
Normal
1 Poles Open
2 Poles Open
3 Poles Open

Figure 2 Power Transfer Capability of a System During Open-Pole Periods
The simplified example provides arguments for doing single-pole tripping in a power system. A
SLGF is the least damaging fault to the power system and the most common. Moreover, a single
open pole, as would be expected, allows the power transfer over the two remaining phases.
Power systems are subject to faults; single-pole tripping and reclosing can help reduce their
impact on the power system. During normal operation of the power system, the mechanical power
and the electrical power transmitted are equal, neglecting losses for simplicity. Equilibrium is
disturbed when a fault occurs and there are no means to transfer all the mechanical power
delivered to the generators. As a result, the generators accelerate and the power transfer angle (
)
increases.
If this
increase is not limited, the power system goes into instability, because the generators will
start slipping poles. A ground fault disrupts the equilibrium of the power transfer and
changes. 4
The rate of change is less than that of a multiphase fault, as discussed in earlier paragraphs. An
open-pole condition, by allowing power transfer through the two healthy phases, allows the
change in
, and the rate of change is less than the complete opening of the three poles.
The classic equal area criterion is a relatively simple concept that can be used to describe the
benefits of SPT systems for power system stability. This criterion uses Equation 1, which
recognizes an accelerating energy caused by more mechanical power than electrical power being
transferred. This is represented by the area where Pm > Pe, mechanical power greater than the
electrical power transferred. On the other hand, the decelerating energy is represented by the area
where Pe > Pm. For simplicity, it is assumed that the mechanical power (Pm) remains constant.
When the power-transfer capability of the p