1682
IEEE Transactions on Power Delivery, Vol. 6, No. 4, October 1991
COMPACT RIGHT-OF-WAYSWITH MULTI-VOLTAGE TOWERS
R.H. Brierley
Member
A.S. Morched
Senior Member
T.E. Grainger
Non-member
ONTARIO HYDRO
Toronto, Ontario, Canada
ABSTRACT:
With increasing transmissior. requirements, and
increasing public pressure to minimize new right-of-ways,
utilities are increasing the circuit density to maximize the
u s e of e x i s t i n g right-of-ways. Reduced c l e a r a n c e s
between circuits, and the arrangement of various voltagelevel circuits on the same tower have resulted in a serious
increase i n induction effects. Problems with voltage
unbalance, residual load voltage, ferroresonance, breaker
recovery voltage, ground switch duty, a n d line
m a i n t e n a n c e have been identified, a n d solutions
presented.
1.0
INTRODUCTION:
In recent years, the public has become more sensitive to
the proliferation of overhead transmission right-of-ways.
There h a s been a n increasing pressure to provide t h e
required transmission capability by increasing the voltage
level of existing lines, and by adding more circuits onto
existing right-of-ways. One solution to this problem, used
by Ontario Hydro and others, is to restring existing lower
voltage transmission or distribution circuits onto new
towers with new high-voltage circuits. Several
configurations have been s t u d i e d , with u p to t h r e e
circuits, each of different voltage, on t h e same tower.
Typical tower configurations are shown in Figure 1.The
following i s a d e s c r i p t i o n of p r o b l e m s i d e n t i f i e d ,
alternatives considered, and solutions developed for such
circuit arrangements. Although these problems are not
new to power systems, t h e i r severity h a s increased,
frequently beyond t h e tolerable level, a s compared to
those experienced with conventional double circuit towers
or multi-circuit right-of-ways.
2.0
POTENTIAL PROBLEMS:
2.1
Voltage Unbalance:
contain various proportions of positive, negative, and zero
sequence components. The positive sequence components
a r e not likely to c a u s e a problem, with t h e possible
exception of voltage level control. The zero sequence
c o m p o n e n t s a r e e i t h e r blocked by d e l t a connected
transformer primary windings, or shorted by transformer
delta-connected secondary windings. I n t h e latter case,
minor additional transformer heating could be expected
due to the delta winding loading. The negative sequence
components a r e passed through transformers, and can
cause serious overheating problems for rotating loads, or
local generation.
Line unbalance effects have traditionally been solved by
regularly spaced phase conductor transpositions. With
s y s t e m e x p a n s i o n , t h e t r a n s p o s i t i o n cycles w e r e
interrupted by new load or switching stations, until they
no longer served t h e i r purpose. New lines were built
w i t h o u t transpositions; a practice which eventually
resulted i n a n increase i n voltage unbalance causing
difficulty in motor starting, and increased motor heating.
Steps were taken to rebalance the system voltage, on a n
ad-hock basis, by modifying the phase arrangements of
selected circuits.
FIG. 1 Multi-Voltage Towers
The close coupling of extra-high-voltage circuits, with
lower voltage circuits, can result in significant voltages
being induced on the lower voltage circuits and appearing
at the load buses. Depending on the tower configuration,
and on the phase arrangement, the induced voltages can
91 WM 096-8 PWRD A paper recommended and approved by
the IEEE Transmission and Distribution Committee of
the IEEE Power Engineering Society for presentation
at the IEEE/PES 1991 Winter Meeting, New York, New
York, February 3-7, 1991. Manuscript submitted
August 30, 1990; made available for printing
January 3 , 1991.
I n t h e mid-seventies t h e O n t a r i o Hydro bulk power
transmission system consisted, predominantly, of doublec i r c u i t 230 kV tower l i n e s , w i t h only a few 500 kV
circuits. Now, a significant portion of a 500 kV overlay
system exists. Some additions to this overlay a r e being
built on existing right-of-ways. The existing lower voltage
circuits a r e frequently retained, sometimes t o radially
supply local loads a s shown in Figure 2. Under system
contingencies, the 500 kV circuits a r e expected to carry
very high c u r r e n t s . T h e negative sequence voltages
induced i n t h e lower voktage circuits by t h e s e l a r g e
currents can cause a n unacceptable voltage unbalance on
local loads.
0885-8977/91$01.ooO1991 IEEE
1683
Figure 1A shows a multi-voltage tower configuration
which, w i t h t h e 500 kV circuit c a r r y i n g load at i t s
t h e r m a l limit of 2300 amps, produces 5.5970 negative
sequence voltage at radially supplied loads. I n t h i s
configuration, two phases of the higher voltage circuit are
in close proximity to phases of t h e lower voltage circuit,
while t h e third phase is remote. Both capacitively and
inductively induced voltage u n b a l a n c e s c a n r e s u l t .
Figure 1B shows a more acceptable tower configuration.
Induced voltages on t h e lower voltage circuit can be
mostly positive sequence for this configuration. However,
i n a particular system arrangement, with t h e phasing
selected so a s to minimize the electromagnetic field at the
edge of the right-of-way, and with thermal limit loading of
t h e 500 kV circuit, t h i s arrangement would produce a
negative sequence voltage of approximately 1.25%.
stationary, this residual voltage i s approximately t h e
maximum allowable. However, values greater than 20%
have been calculated for multi-voltage tower
configurations. With voltages in this range, large motors
will be tripped by undervoltage relays. Small motors, and
household a p p l i a n c e s will l i k e l y stop, a n d will b e
subjected to locked-rotor currents high enough to cause
damage in a short time.
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FIG. 3 Voltage Unbalance Variation with
500kV Power Flow
DS A DSB DS C
FIG. 2 Bulk Transmissionand Distribution
Circuits on a Combined Right-of-way
The tower arrangement shown i n Figure lC, used with
t h e system of Figure 2, produces a variation i n load
negative sequence voltage unbalance with power transfer
in the 500 kV circuit a s shown in Figure 3. Through most
of the normal operating range, the unbalance is below the
acceptable NEMA Standard MG1-12.45a limit of 1.0%.
However, a system contingency resulting i n increased
power flow i n the 500 kV circuit will result in potential
damage to r o t a t i n g loads fed from t h e lower voltage
circuits. Overheating, and difficulty with motor starting
h a s been experienced with voltage unbalance a s low a s
2.%.
2.2
2.3
Ferroresonance:
If a n u n l o a d e d t r a n s f o r m e r i s de-energized w i t h a
transmission line of sufficient length, coupled to another
t r a n s m i s s i o n l i n e , f e r r o r e s o n a n c e m a y occur [ 11.
Essentially t h e nonlinear magnetizing impedance of the
transformer oscillates with t h e line capacitances. With
sufficient energy transmitted across t h e inter-circuit
capacitance to supply the losses, these oscillations can be
sustained.
Ferroresonant oscillations normally 'lock in' at either 60
Hz or at a subharmonic. A fixed relationship exists
b e t w e e n t h e f r e q u e n c y a n d t h e v o l t a g e of t h e
ferroresonant oscillation. This relationship is due to the
Residual Load Voltage:
Step-down transformers supplying loads a r e frequently
t a p p e d from a circuit without individual switching
capability, for example the 115 kV DS's shown in Figure
4. De-energization of the circuit-transformer combination
may not result in zero load voltage if the circuit is closely
coupled with another circuit. The residual voltage can be
considerable, and will likely be unbalanced a s shown in
Figure 5. The effect is caused, almost exclusively, by the
capacitive coupling between the circuits. Therefore the
residual voltage is dependent on the length of the coupled
section, and the size of the connected load.
Residual voltage resulting from coupling on double circuit
towers, with circuits of the same voltage, is of the order of
5%. Considering the normal range of motor locked rotor
i m p e d a n c e s , a n d t h e poor h e a t d i s s i p a t i o n w h e n
STATION A
m
STATION B
DS
DS
3tz
FIG. 4 Multi-Voltage Right-of-way
1684
switching and fault conditions. Increasing or decreasing
a n y p a r a m e t e r m a y r e s u l t i n t u n i n g or d e t u n i n g t h e
s y s t e m . T h e s i t u a t i o n w i t h f e r r o r e s o n a n c e i s more
c o m p l i c a t e d s i n c e t h e n o n l i n e a r b e h a v i o r of t h e
transformer magnetizing reactance permits i t to t u n e
itself to the applied frequency, or its subharmonics, over a
wide r a n g e of s y s t e m conditions. This increases t h e
likelihood for ferroresonance, and makes it difficult, if not
impossible, to predict ferroresonant conditions from
system parameters. Simulation of the system, in the time
domain, appears a s t h e only reliable tool; but even this
tool suffers from a number of short-comings.
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0.50 0.60 0.70 0.80 0.90
Time ( in Sec.) x
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FIG. 5 Residual Load Voltage on
De-energization
need to drive the flux in t h e transformer core from one
saturation point to t h a t of t h e opposite polarity. As the
voltage decreases, t h e frequency of t h e ferromagnetic
oscillation decreases. Conversely, if a switching surge
were t o s t a r t t h e process at a high enough voltage, t h e
o s c i l l a t i o n s could conceivably be m a i n t a i n e d a t a
harmonic frequency and at a voltage well above normal.
Close coupling to a h i g h e r voltage line increases t h e
p o s s i b i l i t y of s u c h h i g h v o l t a g e h i g h f r e q u e n c y
oscillations.
A situation conducive to ferroresonance may be created
inadvertently by a stuck breaker, where a transformer
a n d a transmission line a r e i n adjacent positions i n a
switch yard. Modern air blast a n d SF6 breakers a r e not
mechanically ganged between poles. Consequently, the
independent pole mechanisms can be expected to stick
with a higher frequency than the three phases together.
S u c h a s i n g l e p h a s e connection b e t w e e n a coupled
transmission line a n d a transformer can result i n
ferroresonz ice.
T h e s i t u a t i o n m a y also r e s u l t from t h e omission of
transformer high-voltage breakers as in the Dual Element
Source Network (DESN) arrangement used by Ontario
Hydro. I n t h i s a r r a n g e m e n t , two circuits, with two
attached transformers, feed a load bus as shown in Figure
4. On occurrence of a fault, the faulted circuit-transformer
combination i s disconnected, leaving t h e load
continuously supplied. This system achieves high
reliability by providing continuous voltage to t h e load
during the clearing of line faults. However, the switching
of t h e circuit-transformer combination without a load,
combined with the close coupling of the switched circuit to
a circuit of higher voltage, increases the probability of
ferroresonance.
Identifying resonant conditions in transmission systems
is I] t straight forward due to the complexity of the interphase a n d inter-circuit capacitances a n d inductances
involved, and their distributed nature. Linear resonant
c o n d i t i o n s , as i n t h e c a s e of l i n e r e a c t o r s , c a n b e
accurately identified by frequency scans under different
Studies with the EMTP program are somewhat limited by
t h e lack of information on t h e transformer s a t u r a t i o n
characteristics a n d t h e accuracy of t h e t r a n s f o r m e r
models. EMTP studies, with d a t a varied over a
reasonable range, have almost invariably shown some
possibility of ferroresonance if the line was coupled to one
of higher voltage. The higher saturation kneepoint of the
’quiet’ transformers now being specified has, on occasion,
appeared to aggravate t h e situation by increasing t h e
voltage at which t h e ferroresonance c a n occur. T h e
calculated oscillation has been at extremely high voltage
i n some instances a s shown i n Figure 6. This type of
oscillation creates a risk of transformer damage, a n d of
breaker restrike.
2.4
Breaker Recovery Voltage:
The duty on existing breakers may be increased by t h e
close coupling of a circuit with higher voltage circuits in
two ways: by increasing the trapped charge voltage, and
by increasing t h e ferroresonant oscillation voltage if a
transformer is attached to the circuit. The trapped charge
voltage on a double circuit tower will have a crest of about
1.5 per unit of normal crest voltage. With close coupling to
a higher voltage circuit, voltages u p t o 2.1 per unit have
been calculated. F i g u r e 7 shows t h e r e s u l t s of s u c h
calculations. The breaker stress is especially high when
t h e coupled voltage i s not i n p h a s e with t h e s y s t e m
voltage on the live side of the breaker.
A maximum recovery voltage (1.76 x maximum voltage
for equipment) i s given i n s t a n d a r d s [21 for AC highvoltage circuit breakers. This recovery voltage refers to
the fast transient peak which, in the first few tenths of a
millisecond, follows fault current interruption. Standards
allow for a n increase in this breaker recovery voltage, by
a factor of 1.17, for interruption of small currents. An
e v e n h i g h e r w i t h s t a n d voltage i s applicable t o t h e
longitudinal insulation with t h e breaker i n t h e open
position, fully deionized. However a value is not given
specifically for the very slow recovery voltages involved in
switching closely coupled circuits.
Provided the standard recovery voltage value, augmented
by the low current allowance, (1.76 x 1.17 of maximum
voltage for the equipment) is not exceeded, t h e breaker
can be assumed adequate. Its operation may, however, be
accompanied by an occasional restrike.
1685
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0.10
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Time ( in Sec.)
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Time ( in Sec.) x 10
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FIG. 6 230kV Transformer Ferroresonance
(a) Terminal Voltage
(b) Breaker Recovery Voltage
2.5
Ground Switch Duty:
When a transmission circuit i s taken out of service for
maintenance, i t is usually grounded at both ends using
permanently installed ground switches. O n multiple
circuit right-of-ways, t h e power flow in t h e live circuits
induces currents in t h e grounded circuit. The second to
l a s t ground switch to open may h a v e difficulty
interrupting this circulating current. I n addition, t h e
inter-circuit capacitance can induce charging current in
the last ground switch to open, and can result i n a high
recovery voltage across it.
G r o u n d s w i t c h e s i n common u s e h a v e no r a t e d
interrupting capability. However, over the years, i t has
been obvious t h a t t h e s e switches h a v e successfully
interrupted appreciable currents and withstood
significant recovery voltages. The close coupling of HV
circuits with EHV bulk transmission circuits on the same
tower h a s created a condition where t h e circulating
c u r r e n t a n d r e c o v e r y v o l t a g e c a p a b i l i t y of t h e
conventional g r o u n d s w i t c h e s m a y be exceeded.
Circulating c u r r e n t s i n excess of 300 a m p s rms, a n d
ground switch recovery voltages in excess of 160 kV crest,
on a 115 kV circuit have been calculated.
0.10
0.20
0.30
0.110
Time ( in Sec.)x lo-'
FIG. 7 De-energizationof an Untapped Coupled Circuit
(a) Line Side Voltage
(b) Breaker Recovery Voltage
2.6
Working Grounds:
I n addition to permanent grounds at station entrances,
working grounds a r e usually applied i n t h e vicinity of
maintenance work on transmission circuits to protect
workers from operating errors and from induction effects.
It i s generally expected t h a t negligible currents will flow
in the working grounds since the induced currents on both
sides are almost identical.
Working grounds at a transposition location carry t h e
difference of t h e induced currents i n t h e two adjacent
sections, which a r e approximately 120 degrees out of
phase. The result can be a ground current a t i m e s t h e
normal circulating current. Similarly, high currents can
be experienced at other discontinuities, such as locations
w h e r e circuits join o r leave the right-of-way. These
c u r r e n t s m a y be too high for t h e continuous c u r r e n t
capability of the grounding equipment,and may cause too
long arcs for the working grounds to be safely removed.
I n some arrangements, t h e lower voltage circuits form
major p a r t s of t h e bulk power transmission system. A
maintenance outage to the EHV circuit can result in the
loading of t h e low voltage circuits to t h e e x t e n t t h a t
unacceptably high circulating currents can be induced in
the closely coupled EHV circuit.
POSSIBLE SOLUTIONS:
3.0
A complicating difficulty in finding proper solutions to the
above problems is t h a t a solution t o one may aggravate
another. The accepted solutions must aim at control of all
problems, at minimum cost. Possible solutions to specific
problems are given below.
3.1
Solutions for Voltage Unbalance
Problems:
3.1.1
Transpositions:
which will not appear at the load due t o delta connected
transformer windings.
3.3.2
Secondary Side Load Switching:
On detection of unacceptably high residual voltage, i t
would be possible to open secondary feeder breakers t o
protect the load. The major risk of such a n arrangement is
t h a t i t leaves t h e unloaded transformer connected t o a
circuit with high coupled voltages, a n d could lead to
ferroresonance.
3.2.3
Primary Side Load Switching:
By transposing the high voltage circuit, voltage unbalance
i n t h e low voltage circuit from line coupling c a n be
eliminated. This can be impractical with short lengths of
coupling, and expensive in any situation. Somewhat less
effective is transposition of the lower voltage circuit so a s
to create three equal sections over the coupled range. This
will result in the conversion of the voltage unbalance into
zero sequence. Zero sequence voltage unbalance may not
reach t h e load because of delta connected transformer
windings. Transpositions can result in unexpectedly high
currents in working grounds during maintenance.
O n d e t e c t i o n of r e s i d u a l v o l t a g e , t h e s t e p - d o w n
transformer and its attached load could be switched by a
load switcher. The circuit will be subsequently r e energized without t h e transformer. I n t h e case of a
s u c c e s s f u l r e - e n e r g i z a t i o n , t h e load will s u f f e r a
somewhat prolonged outage until the load interruptor is
closed by control. If the re-energization is not successful,
t h e line b r e a k e r s m a y e n c o u n t e r t h e high recovery
voltages associated with clearing a coupled, unloaded
circuit.
3.1.2
3.3
Solutions for Ferroresonance Problems:
3.3.1
Damping Resistors:
Tower Config-uration:
Voltage u n b a l a n c e m a y be r e d u c e d by t o w e r
configuration, and phase arrangements. The objective is
to provide equal coupling of each phase of the low voltage
circuit with the corresponding phase of the high voltage
circuit. The coupling will t h e n cause predominantly
positive sequence voltages. Close coupling of all phases of
the low voltage circuit with a single high voltage phase
will result in a predominantly zero sequence coupled
voltage, which may be tolerable with some transformer
configurations. Other tower o r phase arrangements can
result in a strong negative sequence component, which
will pass unmodified through transformers, and damage
rotating loads.
3.1.3
Adding damping resistors, switched on t h e low voltage
bus by a fast device, instigated by the same relaying used
t o t r i p t h e HV circuit, m a y provide a solution t o t h e
problem. The high speed is necessary to reduce the high
voltage levels which may accompany the earlier stages of
the ferroresonant oscillation. The fastest switching device,
on closing, is believed to be a vacuum switch. Breakers
have been used, with some delay being introduced in the
line breaker opening s o t h a t t h e r e s i s t e r s will be i n
service when the breakers clear. Damping resisters, in
two independantly switched banks for reliability, with
ratings up t o 400 kW per phase have been suggested.
Voltage Balancing Devices:
3.3.2
Load Break Switches:
T h e voltage of a m u l t i - p h a s e power s y s t e m m a y be
balanced at a point by use of independent phase voltage
control. This may consist of single phase transformer-type
voltage regulators, a series of small single phase switched
shunt capacitors, o r single phase static var compensators.
Although these devices are standard system components,
their use to balance rather than maintain a voltage level
is less common. The logic to decide on t h e amount and
p h a s e of t h e a d j u s t m e n t m u s t be based on t h e small
differences of the bus voltages. Measurement accuracy is,
therefore, a concern.
In circumstances where the transformer is connected t o a
coupled line through a disconnect switch, a n d
ferroresonance is expected, i t may be economic to replace
the disconnect with a load switcher which can successfully
de-energize a ferroresonating transformer, a s well a s
provide the isolation duty of a disconnect. Although the
currents involved in ferroresonance do not approach fault
currents, t h e recovery voltages may be high, a n d t h e
capability of t h e load s w i t c h e r s h o u l d be carefully
checked.
3.2
Solutions for Residual Load
Voltage Problems:
3.3.3
3.2.1.
Transpositions:
As in the voltage unbalance problem, the residual voltage
may be eliminated by complete transposition of the high
volt,age circuits. Transposition of the low voltage circuit
orily can convert the residual voltage to zero sequence,
I
Ground Switches:
Ferroresonance h a s been stopped, on occasion, by t h e
closing of ground switches. This solution has the handicap
of being very slow. It will not suffice if t h e oscillating
voltage is high enough to threaten equipment insulation.
Further, the ground switch must eventually be removed,
and i t is quite possible t h a t the associated transient will
once more i n i t i a t e t h e oscillation. Separation of t h e
1687
transformer and line prior to opening the ground switch
could create a need for special relaying or
communications.
3.3.4
Cross Tripping of Companion Circuit:
For a stuck breaker condition leading to ferroresonance, a
scheme to cross trip the live circuit is under consideration.
Stuck breaker conditions occur very infrequently. If the
system is designed to tolerate a double circuit fault, the
cross tripping will only marginally increase the frequency
of the simultaneous loss of two circuits.
3.4
3.4.1
Solutions for Breaker Recovery Voltage
Problems:
Selection of Optimum Phase
Arrangement:
By selecting t h e p h a s i n g of t h e low voltage circuit,
relative to t h e high voltage circuit, so t h a t the induced
voltage i s i n p h a s e with t h e low voltage system, t h e
voltage across t h e opening b r e a k e r will be reduced.
However, a phase shift between t h e voltage levels, or
other requirements, such a s a need to minimize t h e
electromagnetic field strength at the edge of the right-ofway, may preclude this approach.
3.4.2
Application of Surge Arresters:
I t may be possible to select metal oxide arresters, with a n
adequate continuous operating voltage, and with a low
enough protective level that the breaker recovery voltage
can be limited. The arrester would be installed, on t h e
line side of t h e breaker, to discharge a portion of t h e
t r a p p e d charge voltage d u r i n g line de-energization.
However, in selecting such a n arrester, margins are much
tighter than normal. Consideration must be given to the
temporary overvoltages that may occur on healthy phases
during the fault. Also, following line de-energization, a n
attached rotating load may maintain both the fault and
the system voltage, increasing the arrester thermal duty.
3.4.3
Replacement of Breakers:
If studies indicate t h a t , in spite of stratagems such as
p h a s e a r r a n g e m e n t selection, t h e s t a n d a r d recovery
voltage value for breakers is exceeded, or if a n occasional
restrike is not tolerable, the breaker must be assumed
inadequate. It must then be tested for capacitor switching
d u t y at appropriately high voltages, or replaced by a
breaker of adequate rating. Consideration should be given
to the use of SF6 breakers, which appear to inherently
have a capacitive switching ability much above t h a t of
other types of breakers.
3.5
Solutions for Ground Switch Duty
Problems:
3.5.1
Determination of Capability
of Conventional Ground Switches:
The i n t e r r u p t i n g capability of conventional ground
switches is usually expressed a s a current limit, and a s a
length of circuit corresponding to conditions at which
successful operation h a s been demonstrated. The
increasingly close coupling, particularly with circuits of
higher voltage rating, h a s made this experience-based
data obsolete. A method outlined i n a 1950 NEE paper by
F.E. Andrews et a1 [3] permits the calculation of arc reach
as a function of current and open circuit voltage. If such
arcs are limited to approximately one half of the available
clearance, a basis for evaluation of existing equipment
can be established.
3.5.2
Development of Higher Capability
Ground Switches:
A ground switch with suitable interrupting capability has
been developed for use by Ontario Hydro [4], consisting of
a n SF6 i n t e r r u p t o r , w i t h a disconnect s w i t c h t y p e
isolator. Other desirable features have been included,
such a s remote operation capability and a visible by-pass
of the interruptor in the closed position.
Ground switches with proven interrupting capability have
also been developed for gas insulated substations [51.
3.6
Solution for Workinv Ground Problems:
The compact right-of-ways presently being planned show
numerous locations and conditions where conventional
w o r k i n g g r o u n d s c a n n o t b e u s e d . I n s t a l l a t i o n of
p e r m a n e n t ground switches, with or without special
interruption capability, in these locations, is the present
practice, b u t m a y n o t be a n economic solution. It i s
desirable t h a t a portable ground switch be developed,
capable of easy application in the field.
4.0
CONCLUSIONS:
The addition of more circuits onto existing right-of-ways,
and particularly the use of multi-voltage towers, is adding
a new dimension to problems resulting from
electromagnetic and electrostatic induction. Although
none of the problems are new, and although solutions are
usually possible, the costs can be significant and must be
anticipated in the early planning stages. Problems with
voltage unbalance, residual voltage, ferroresonance,
breaker recovery voltage, ground switch duty, and line
m a i n t e n a n c e have been identified, a n d solutions
presented. Although implemented solutions depend on the
s y s t e m configuration, t h e following conclusions a r e
generally valid:
1. The use of transpositions to minimize induction effects
i s expensive, a n d r e s u l t s i n severe working ground
problems.
2 . T h e n u m b e r of locations w h e r e coupled c i r c u i t s
converge o r diverge should be minimized.
3. Careful selection of circuit p h a s i n g i s required to
minimize negative sequence unbalance, to reduce breaker
recovery voltage, and to reduce ground switch duty.
4.Use of independent phase switched capacitors, or static
var compensators, may be required to compensate load
1688
voltage unbalance i n systems where optimum phase
arrangement is not enough.
5. Breakers with limited recovery voltage capability may
have to be protected with arresters, or be replaced with
SF6 breakers of higher capability.
6. Special m e a s u r e s , s u c h a s d a m p i n g r e s i s t o r s , or
individual transformer switching capability, m a y be
r e q u i r e d t o r e d u c e i n c i d e n t s of t r a n s f o r m e r
ferroresonance.
7. Special high-voltage switching devices may be required
to disconnect loads u n d e r t h e condition of s u s t a i n e d
under-voltage.
REFERENCES:
[ l ] E . J . D o l a n , D.A. G i l l i e s , E . W . K i m b a r k Ferroresonance i n a Transformer with a n EHV Line
IEEE Transactions (PAS-91) pp. 1273, May/June 1972
[21 ANSI Standard C37.06 - 1979 Preferred Ratings and
Related Required Capabilities for AC High-Voltage
Circuit Breakers
[31 F . E . A n d r e w s , L . R . J a n e s , M.A. A n d e r s o n Interrupting Ability of Horn-Gap Switches AIEE
Transactions 1950, pp. 1016
[4] G. Handfield, L. L a m - I n t e r r u p t e r Type Ground
Switch for 550 kV Parallel Transmission Lines CEA
Spring Meeting, 1989
[5] R. Kugler, H.M. Luehrmann, F. Veuhoff - Switching
T e s t s on GroundinP Switches for G a s I n s u l a t e d
Substations IEEE Transactions (PAS-103) pp. 3569,
Dec. 1984
BIOGRAPHIES:
Russell H. Brierley (M'73) was born
i n 1930 in Hamilton, Ontario. H e
received his electrical engineering
degree from Queens University in
Kingston, Ont., in 1953.
He worked for six years for CGE in
Peterborough, Ont., in the design of
Hydraulic Generators; a n d t h r e e
years for Ontario Hydro's Research Division, field testing
electrical equipment. Since 1963 he has worked in various
positions in Ontario Hydro, System Planning Division. He
h a s been involved i n s y s t e m s t u d i e s u s i n g t h e
Electromagnetic Transients Program, since 1969.
Mr. Brierley i s a Professional Engineer in the Province of
O n t a r i o . H e i s a m e m b e r of t h e C a n a d i a n N a t i o n a l
Committee of IEC TC28 on Insulation Coordination, and a
co-author of CSA Publication on t h e Principles a n d
Practice of Insulation Coordination - C308.
Atef S.Morched (M'77-SM790)was
born i n Cairo, Egypt i n 1942. He
o b t a i n e d a B.Sc. i n E l e c t r i c a l
Engineering from Cairo University
in 1964, a Ph.D. a n d a D.Sc. from
t h e N o r w e g i a n I n s t i t u t e of
Technology i n Trondheim i n 1970
and 1972.
He worked for t h e E g y p t i a n
Electricity Corporation between 1964-1967 and 1972-1974.
H e was a Research Associate w i t h t h e U n i v e r s i t y of
Toronto during 1974-1975. Since 1975 h e h a s been with
O n t a r i o Hydro; initially with t h e S t a t i o n s Design
Department, and subsequently with the System Planning
Division where he currently holds the position of Head of
the Electromagnetic Transients Section.
Dr. Morched is a Professional Engineer in the Province of
Ontario. He has authored and co-authored a number of
technical papers. His paper on Network Equivalents for
Electromagnetic Transient Studies won a 1985 PES Prize
Paper Award .
Tom E . Grainger was born in North
Bay, Ontario in 1958. I n 1980 he
received a diploma i n E l e c t r i c a l
Engineering Technology from t h e
Ryerson Polytechnical Institute, in
Toronto, Ont. He i s presently
completing t h e r e q u i r e m e n t s for
membership i n t h e Association of
Professional Engineers of Ontario.
I n 1980 Mr. Grainger joined the System Planning Division
of O n t a r i o H y d r o , a n d i s c u r r e n t l y w o r k i n g i n t h e
Electromagnetic Transients Section.
1689
DISCUSSION
GEORGE GELA, HVTRC, LENOX, MA: The paper provides a
broad overview o f many questions t h a t could a r i s e when
one t r i e s t o minimize t h e r i g h t - o f - w a y through t h e use
o f compact l i n e designs, o r when one t r i e s t o maximize
t h e u t i l i z a t i o n o f t h e r i g h t - o f - w a y through t h e use o f
m u l t i - v o l tage towers.
The paper concentrates mainly
on t h e o p e r a t i o n a l c h a r a c t e r i s t i c s o f t h e o p t i m i z e d
t r a n s m i s s i o n c o r r i d o r s , and presents as conclusions
some f a i r l y generic statements as t o what might be expected.
The d i s c u s s e r would f i r s t l i k e t o e x p l o r e t h e poss i b i l i t y o f d e r i v i n g some more d e f i n i t e statements o r
g u i d e l i n e s . For example, w i t h t h e "more crowded" c o r r i d o r s and towers (i.e., smaller a i r distances, o r a
m i x t u r e o f voltage l e v e l s ) , would t h e outage r a t e be
higher, and could t h e t r a d i t i o n a l l y low outage r a t e s
be recovered a t a reasonable c o s t o f s w i t c h i n g and
p r o t e c t i v e equipment? Would t h e overvoltage charact e r i s t i c s be a l t e r e d t o t h e p o i n t where t h e v a s t
amount o f experience accumulated t o date would need t o
be r e v i s e d ? With t h i s l a s t question, t h e d i s c u s s e r
would l i k e t o address n o t so much t h e general area o f
electromagnetic t r a n s i e n t s , b u t r a t h e r more s p e c i f i c a l l y t h e t o p i c o f l i v e - l i n e maintenance.
The authors address t h e question o f working grounds,
i . e . , o f grounding temporarily the de-energized l i n e
f o r t h e purpose o f p r o t e c t i n g workers from e l e c t r o c u t i o n due t o a c c i d e n t a l 1 i n e r e - e n e r g i z a t i o n [ A ] .
However, de-energizing a l i n e t o perform maintenance
r e p r e s e n t s a l o s s o f revenue, which may be a s i g n i f i c a n t consideration. Performing t h e work w i t h t h e
l i n e energized, i.e., l i v e - l i n e maintenance, o f course
i s an o p t i o n which avoids t h e problem, b u t t h e s a f e t y
o f t h e worker must take p r i o r i t y . Worker's s a f e t y i s
assumed b a s i c a l l y by r e t a i n i n g proper a i r distances t o
grounded and energized e l e c t r o d e s [ B ] .
These d i s tances must i n c l u d e several l a y e r s o f " s a f e t y f a c t o r s "
o r "adders" t o account f o r v a r i o u s working and engonomic issues, and t o b r i n g t h e p r o b a b i l i t y o f a i r
breakdown t o very, very low values.
I n o t h e r words,
t h e distance needed t o perform l i v e - 1 i n e maintenance
s a f e l y i s always considerably greater than t h a t c o r responding t o t h e 50% breakdown, f o r a given t r a n s i e n t
overvoltage l e v e l .
For compact l i n e s and m u l t i voltage towers, t h e a v a i l a b l e physical distances may
not be s u f f i c i e n t due t o compaction, t o perform l i v e 1 i n e maintenance, unless a d d i t i o n a l remedial steps are
taken.
The r e m e d i a l s t e p s may range f r o m dee n e r g i z i n g t h e l i n e a l t o g e t h e r , through t e m p o r a r i l y
c o n t r o l l i n g t h e overvoltages l o c a l l y a t t h e w o r k s i t e
o r p l a c i n g t h e worker on t h e l i n e using, say, h e l i c o p t e r s , t o developing new work methods i n c l u d i n g automat i o n . The thorough understanding o f overvoltages and
o f t h e i m p l i c a t i o n s o f reduced distances i n optimized
c o r r i d o r s , i s e s s e n t i a l i n making decisions r e l a t e d t o
l i n e maintenance. Above a l l , t h e broad q u e s t i o n o f
l i v e - l i n e maintenance should be included e a r l y i n t h e
d i s c u s s i o n o f compact r i g h t - o f - w a y s , as t h e authors
have attempted i n p a r t , so t h a t t h e apparent advantages are n o t b l u n t e d by t h e need t o de-energize
t h e l i n e due t o i n s u f f i c i e n t distances even f o r such
operations as replacement o f bundle spacers o r i n s t a l l a t i o n o f marker b a l l s . The authors' comments i n t h i s
area a r e appreciated.
REFERENCES
A.
ANSI/IEEE Std 1048-1990, " I E E E Guide f o r Protect i v e Grounding o f Power ines"
B.
ANSI/IEEE S t d 516-1987, " I E E E Guide f o r Maintenance Methods on Energ zed Power-Li nes" .
.
Manuscript received February 20, 1991.
R.H. BRIERLEY, A.S. MORCHED, T.E. GRAINGER
The Authors would like to thank Dr. Gela for his interest
in t h i s paper, a n d for pointing out t h e similarities of
problems associated with t h e topic of t h i s paper, and
Compact Line Design.
The increased coupling effects of the multi-voltage tower
h a v e r e s u l t e d i n w h a t m i g h t be called 'bothersome'
overvoltages. Although a concern for terminal equipment,
they do not appear to threaten t h e line insulation itself.
Consequently, higher outage rates are not expected if the
terminal effects have been properly considered. These
terminal effects do represent a modification in 'experience
based' knowledge which will have to be assimilated.
Live-line maintenance is indeed a n important
consideration in the design of multi-voltage towers. In the
p a r t i c u l a r t o w e r d e s i g n s of F i g u r e 1, c l e a r a n c e s
commensurate w i t h t h e maximum s w i t c h i n g s u r g e
a s s o c i a t e d w i t h n o m i n a l 2 3 0 kV c i r c u i t s a n d t h e
'envelope' required for workers and equipment have been
maintained. The maximum switching surge results from
re-energizing a line with a trapped charge, and since the
trapped charge voltage may be increased by t h e multivoltage tower arrangement, so t h e maximum switching
surge may be increased.
Several of the circuits described are actually insulated for
230 kV, while operating at 115 kV. This was done because
of t h e expected high o u t a g e r a t e of 115 kV insulator
s t r i n g s on a high 500 kV t o w e r , a n d b e c a u s e of a n
anticipated need to reconnect t h e circuits for 230 kV
operation in t h e f u t u r e . As a consequence, live-line
maintenance on these circuits, using 230 kV tools a n d
clearances, should pose no problem. The circuits operating
a t 230 kV have been checked to ascertain that switching
s u r g e s h i g h e r t h a n t h e a s s u m e d maximum a r e n o t
possible. Such switching s u r g e s a r e unlikely in t h e
discussed a r r a n g e m e n t , because t h e presence of the
DESN transformers ensures t h a t the trapped charge will
be dissipated prior to reclosure. I t may be necessary, in
other locations, to restrict switching surges using line
mounted arrestors, r a t h e r t h a n increase standardized
safety clearances.
Manuscript received June 30, 1991.