Download X Switching Overvoltage Mitigation Measures

X Switching Overvoltage Mitigation Measures
X.1 Introduction
Switching overvoltages are the primary dimensioning parameter for air-clearances on
EHV and UHV systems. The reason for this is that, while clearances for lightning
impulse withstand increase linearly with gap distance, those for switching impulse
withstand tend to saturate with increasing gap distances (Fig. 1). Clearly, therefore,
there is great merit to mitigate switching overvoltage levels.
Fig. 1 Lightning and switching impulse withstand versus
gap distance for rod-rod gap
The historical background to this subject is interesting. The first EHV system was the
400 kV system developed in Sweden in the mid 1950s. No account was taken of
switching overvoltages because their influence remained to be identified and
obviously no mitigation was considered. The development of the 400 kV system in
Italy in the 1960s was the first real recognition of switching overvoltages. A very large
number of switching impulse tests were performed and resulted in the now
well-known U-curves and the notion of gap factors [1, 2]. The explanation for the
U-curve was provided by research work at EdF [3]. Switching impulse breakdown
involves both streamer and leader development (lightning impulse breakdown
involves streamers only) with the latter being the main driver. The minimum value of
the U-curves thus represents optimal leader development and occurs at front-times in
the range of 100 to 400 µs leading to a selection of a front-time of 250 µs for
standardization purposes. Detailed descriptions of electrical discharges and
breakdown in air can be found in [4, 5].
A number of switching overvoltage mitigation measures have been proposed or used
starting in the 1960s:





Fast insertion of shunt reactors [6].
Closing resistors [6, 7, 8, 9].
Staggered pole closing [10, 17, 18, 20].
Line terminal arresters [10, 11, 12, 13, 14].
Controlled closing [15, 6, 8, 16, 17, 18, 19, 20, 21].
Each of these measures will be discussed in the following sections.
X.2 Fast Insertion of Shunt Reactors
The primary purpose of shunt reactors is to provide compensation of transmission
line capacitance and have secondary value, if connected, of providing some
reduction in switching overvoltage levels. No utility would consider adding shunt
reactors specifically for the latter purpose. The intent, therefore, with this proposal is
to switch in disconnected reactors preferably prior to energizing or re-energizing the
line [6]. Compared to other more measures discussed below, this measure is not of
significant interest.
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X.3 Closing Resistors
Once the significance of switching overvoltages were introduced, closing resistors
were applied on EHV airblast circuit breakers starting in the 1960s [7, 8, 9]. The
principle behind the use of closing resistors is as follows. Assume a voltage UD
across the open circuit breaker at the instant of closing. Closing effectively produces
a travelling wave of magnitude UD which divides between the resistor R and the line
surge impedance Z. The incident wave at the open end of the line is thus:
UoL 
UD Z
RZ
and after voltage doubling, the line end voltage becomes
UeL 
2UD Z
 UTC
RZ
where UTC is the initial trapped charge on the line.
Taking, for example, UD  2 (1 pu positive source voltage and 1 pu negative trapped
voltage)
UeL 
4Z
1
RZ
If R  0, then UeL  3 pu.
Clearly, the higher the value of R, the lower the value of UeL. However, the resistor is
inserted only momentarily and bypassing generates a travelling wave given by:
UR 
URDR
RZ
where URD is the voltage across the resistor at the instant of bypassing. At the line
end with voltage doubling, the voltage is:
URe L 
2URDR
 Upf
RZ
where Upf is the frequency voltage at the instant of travelling wave arrival. An upper
limit on the value of R is therefore required. The value of R is usually optimized
through computer studies and is typically in the range 250 to 600 ohms dependent on
the application.
Generally, the aim with closing resistors is to limit the voltage at the receiving end of
the line to 2 pu. However, closing resistors do little for the voltage profile along the
line and higher overvoltages than 2 pu at points where incident and reflected waves
cumulatively coincide.
X.4 Staggered Pole Closing
Staggered pole closing is more a complimentary switching overvoltage mitigation
measure than a fundamental measure [10, 17, 18, 20]. The principle is quite simply
closing the individual poles one-half cycle apart in the expectation that transients in
2
the closed phase will have greatly attenuated before the next poles close. The effect
is to reduce the coupling contribution in any one phase from the other two phases.
This measure is inexpensive, easily implemented and reliable.
X.5 Line Surge Arresters
As an alterative to the use of closing resistors, metal oxide surge arresters were
applied on line terminals starting in the late 1980s [10, 11, 12, 13, 14]. This has since
become common practice at EHV and even at HV but there for lightning protection
reasons.
The effect of the line surge arresters is shown in Fig. 2 for a typical EHV application
such as described in [14]. With arresters at the line ends only, the overvoltages at
those points only will be limited to the protective level of the arresters. However, the
voltage profile along the line shows much higher overvoltage levels and suggests a
need to even out the profile. This is achieved by adding arresters at intermediate
points along the line as also shown in Fig. 2.
The surge arresters applied for this purpose are special in the sense that their
characteristics are derived from EMTP system studies. A balance has to be achieved
between the protective level and the energy absorbed which work in opposite
directions, i.e. decreasing the protective level increases the energy absorbed and
vice versa. In the ultimate, further reduction in the overvoltage level requires a more
proactive mitigation measure.
Fig. 2 Effects of line surge arresters on switching overvoltage levels
X.6 Controlled Closing
Controlled closing of circuit breakers to limit switching overvoltages was suggested
as early as 1966 [15]; was viewed as less than practical 1969 [8]; was proposed as
part of a combined closing resistor and controlled closing solution in 1970 [6]; was
actually tested on a power system in 1976 [16]; but finally did not become a reality
until the 1990s [17, 18, 19, 20, 21]. The reason for this delay is discussed below.
When an unloaded transmission line is switched out, the remaining voltage on the
line will be a DC voltage if the line is not compensated or an oscillatory voltage if the
line is compensated, the frequency being dependent on the degree of compensation.
The intent of controlled switching then is to close the circuit breaker when the
difference between the source power frequency voltage and the line voltage is at a
minimum. For the uncompensated line case, the minimum will occur once every
cycle when the polarities of the source and line voltages are the same. For the
compensated line case, the differential voltage minimums are frequency dependent
as shown in Fig. 3. The 45 Hz case corresponds to a high degree compensation
(80%) with shunt reactors at both line ends and the voltage minimums are clearly
evident. At 33 Hz –low degree of compensation (40%) with shunt reactor at one end
only – the minimums are less evident by comparison.
3
Traces from actual field measurements by BC Hydro are shown in Figs. 4 and 5.
Fig. 5 shows that the line side oscillation is more complex than that shown in Fig. 4.
In reality, three oscillation modes are involved in the 40% compensation case and the
minimums become even less distinct.
2.5
60 Hz
Difference 60 vs 33 Hz
2
Difference 60 vs 45 Hz
1.5
Voltage (pu)
1
0.5
0
0
20
40
60
80
100
120
140
-0.5
-1
-1.5
-2
-2.5
Time (ms)
Fig. 3 Differential voltage across circuit breaker
Fig. 4 500 kV line 80% compensated
4
160
180
Fig. 5 500 kV line 40% compensated
It is clear that this mitigation measure requires a dynamic controller which can
analyze the differential voltage across the circuit breaker, locate the minimums,
predict the future minimums and close the breaker accordingly when the close signal
is applied, all within 0.5 s or perhaps less. The sophistication of the control device
obviously goes far beyond the zero crossing detectors and sequence timers available
when this measure was first proposed. As with controlled capacitor bank switching,
the rate of decay of dielectric strength (RDDS) of the circuit breaker is a factor in the
application of this measure.
This measure combined with line end and mid-line 372 kV rated arresters has been
successfully in-service at 500 kV on the BC Hydro system since 1995 [20, 21]. The
intent was to limit switching overvoltages to 1.7 pu anywhere along the line.
X.7 Comparison of Various Measures
Fig. 7 shows the cumulative frequency distribution for the overvoltages at the open
end of a transposed 330 km long 500 kV line following high-speed reclosing for the
various overvoltage limitation measures. The arresters considered were rated at
100.0
No control m easures
Cumulative frequency (%)
90.0
Staggered pole closing
80.0
70.0
Closing resistors
60.0
Line term inal arresters
50.0
40.0
Line term inal plus m id-line
arresters
30.0
Controlled closing plus line
term inal arresters
20.0
Closing resistor 400 ohm s
plus line end arresters
10.0
Closing resistor 800 ohm s
plus line end arresters
0.0
1.0
1.5
2.0
2.5
3.0
Overvoltage level (pu)
Fig. 6 Cumulative frequency overvoltage distributions for various
overvoltage limitation options
372 kV and about an equivalent IEC Line Class 7. Clearly, staggered pole closing
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has only incremental value and cannot be considered as stand-alone option. The
value lies in the options of closing resistors, line terminal (meaning arresters at both
line terminals) and mid-line arresters and controlled closing combined with line
terminal arresters.
The effect of the various options on the voltage profile along the line is of greater
interest and this is shown in Fig. 7 for a 330 km long 500 kV line.
The two options that provide the most significant line overvoltage profile limitation are
closing resistors or controlled closing both combined with line terminal arresters. At
800 kV and below, closing resistors are now rarely used and either an arrester only
option or the controlled closing option is adopted. At the UHV levels of 1000 kV and
above, the situation is different as discussed below.
3.5
3
No control measures
Overvoltgae (pu)
Closing resistors (400 ohm)only
Closing resistors (400 ohm) plus
line terminal arresters
Line terminal arresters plus SPC
2.5
Line terminal and mid-line
arresters plus SPC
Controlled closing and line
terminal arresters
Closing resistors (800 ohm) only
2
Closing resistor (800 ohm) plus
line end arresters
1.5
1
0
1
2
3
4
5
6
7
8
9
10
11
12
Line Section No.
Fig. 7 Overvoltage profile along 330 km long transposed 500 kV line for various
overvoltage limitation options
Circuit breakers applied at 1000 kV and 1200 kV will incorporate opening resistors in
order to meet transient recovery voltage (TRV) requirements during fault clearing.
The resistors, though probably rated resistance-wise for opening, can double as
closing resistors. However, as shown in Figs. 6 and 7, the lower the resistance value
the greater the overvoltage limitation value. The choice between the closing resistor
versus closing controller option is a decision for the user and a comparison is given
in Table 1.
Table 1
Comparison of closing resistor and closing controller overvoltage
limitation options
Attribute
Proven technology
Closing resistor
Yes, in use since 1960
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Closing controller
Yes, in use since early
1990s
Attribute
Complexity
Closing resistor
High; multiple moving
mechanical parts
Location
At line potential
Maintainability
Low; requires circuit breaker
outage
Yes; parts only rather than
complete module
Limited if at all
Provision of spares
Future improvement
potential
Closing controller
Low; no moving parts, circuit
boards and associated
software only
At ground level in control
room
High; does not require circuit
breaker outage
Yes; complete module or
circuit boards
Yes, component and
software advances
The above discussion and study results for a transposed 500 kV transmission line
illustrate the combined contribution of line end arresters and closing resistors or a
controller to switching overvoltage limitation. The results can certainly be viewed as
indicative of performance but, to determine absolute design parameters, the user
needs to conduct studies focused on the actual application details and
considerations. Accurate representation of arrester and controller characteristics,
resistor insertion and line configuration is essential in order to achieve a valid design
basis for lines meeting dependability requirements.
References
1.
G. Carrara, "Investigation on impulse sparkover characteristics of long rod/rod
and rod/plane air gaps". Cigre Report No. 328, 1964.
2.
L. Paris and R. Cortina, "Switching and lightning impulse characteristics of large
air gaps and long insulator strings". IEEE Transactions on Power Apparatus
and Systems No. 87, 1968.
3.
J.N. Ross et al, "Positive discharges in long air gap at Les Renardieres."
Electra No. 53. 1977.
4.
N.L. Allen, "Mechanism of air breakdown." Chapter 1, Advances in High
Voltage Engineering (Book), IEE Press 2004.
5.
V. Cooray, "Mechanism of electrical discharges." Chapter 3, The Lightning
Flash (Book), IEE Press 2003.
6.
H.B. Thoren, "Reduction of Switching Overvoltages in EHV and UHV Systems."
IEEE Trans. PAS-90, 1971.
7.
C.L. Wagner and J.W. Bankoske, "Evaluation of Surge Suppression Resistors
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8.
P.A. Baltensperger and P. Djurdjevic, "Damping of Switching Overvoltages in
EHV Networks – New Economic Aspects and Solutions." IEEE Trans. PAS-88,
1969.
9.
R.G. Colclasser, C.L. Wagner and E.P. Donohue, "Multistep Resistor Control of
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10.
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11.
J.R. Ribeiro and M.E. McCallum, "An Application of Metal Oxide Surge
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12.
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13.
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14.
Y.I. Musa, A.J.F. Keri, J.A. Halladay, A.S. Jagtiani, J.D. Manderville,
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to Control Switching Transient Overvoltages." IEEE Trans. PD, Vol. 17, 2002.
15.
E. Maury, "Synchronous closing of 500 and 765 kV circuit breakers: a means of
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16.
H.E. Konkel, A.C. Legate and H.C. Ramberg, "Limiting Switching Surge
Overvoltages with Conventional Power Circuit Breakers". IEEE Trans. PAS-96,
1977.
17.
B. Avent and J. Sawada, "BC Hydro's Experience with Controlled Circuit
Breaker Closing on a 500 kV Line." Canadian Electrical Association,
Engineering and Operating Division Meeting, March 1995.
18.
K. Froehlich, C. Hoelzl, A.C. Carvalho and W. Hofbauer, "Transmission Line
Controlled Switching." Canadian Electrical Association, Engineering and
Operating Division Meeting, March 1995.
19.
A.H. Khan, D.S. Johnson, J.H. Brunke and D.L. Goldsworthy, "Synchronous
Closing Application in Utility Transmission Systems." Cigre Report No. 13-306,
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20.
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21.
B.L. Avent, D.F. Peelo and J. Sawada, "Application of 500 kV Circuit Breakers
on Transmission Line with MOV Protected Series Capacitor Bank." Cigre
Report No. 13-107, 2002.
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