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Dynamic insulation in transmission-line design
Dynamic insulation in transmission-line design A Rodriquez and R de la Rosa Instituto de Investigaciones El~ctricas, Cuernavaca, Mor., Mdxico
C Y Wu and T C Cheng University of Southern California, Los Angeles, CA, USA
H W Dommel University of British Columbia, Vancouver, BC, Canada
The evolution o f the dynamic insulation concept from the integration o f two existing technologies is presented. Separate solutions proposed in the recent literature for the reduction o f voltage stresses due to nonuniform electric fields and to transient overvoltages in EHV and UHV transmission line insulation are discussed and then integrated into one. The resulting combination, i.e. dynamic insulation, has the potential o f provMing a superior alternative for the abatement o f these two problems and others. Dynamic" insulation is a new concept which provides a systems approach to the electrical and mechanical design o f transmission-line insulation. Because of i t s multifunction capability, dynamic insulation offers various technical and economical advantages, including the mitigation o f the effects o f contamination on insulation and also the control o f fault initiation overvoltages as indicated by transmissionline simulations. Further theoretical studies and experimental work applying known technology is suggested to verify the ideas proposed. Keywords: electric" power transmission lines, dynamic insulation reliability
I. Introduction There has been much effort to improve most of today's power system components with the objectives of improving reliability and lowering costs. For example, modern highspeed breakers and microprocessor-based electronic relays have made it possible to obtain significant savings in the construction of more reliable transmission lines. However, there has been a definite lapse in insulation technology. As insulation levels in EHV and UHV lines are reduced by improvements to terminal equipment, a point has been reached where there is no real benefit in making further refinements to these devices. The limiting factor can be traced in many cases to design deficiencies of c o n -
Received: 5 July 1983, Revised: 6 March 1984
Vol 6 No 4 October 1984
ventional line insulation. In the field, this is indicated by the fact that the principal causes of transmission system outages in the United States, Canada and Mdxico are overvoltages and pollution. The common s y m p t o m of both of these problems is line-insulation failure. In the following, two solutions which have been proposed for two important line-insulation problems are reviewed 1'2. The problems are insulation voltage stresses due to nonuniform electric fields and to transient surges; the solutions are the application o f nonlinear or active insulators and zinc-oxide arresters, respectively. The two solutions are then combined into a new solution called dynamic insulation, which has the capability to solve both of these problems and others. Digital simulation results of AC and DC transmission line performance are presented, and the advantages and limitations of the dynamic insulation concept are discussed.
I1. Active insulators I1.1 The nonuniform voltage distribution problem The conceptual design of the standard porcelain insulator unit, in widespread use in today's modern transmission systems, is basically the same as that of the first insulators installed around the turn of the century. Although time has proven the porcelain insulator to be adequate on HV lines, in particular with respect to its durability, as early as 1914 one of the main drawbacks arising from its nature as a completely passive device was foreseen 3. At that time, it was noted that the relation between the total applied voltage on a string of standard insulators and the voltage across each individual unit along the string was n o t linear and that the marginal effectiveness of each additional insulator unit decreased rapidly. Since then, transmission system voltages have increased almost an order of magnitude, thus increasing the severity of this problem. This is illustrated in Figure 1 by the
0142-0615/84/040203-09 $03.00 © 1984 Butterworth & Co (Publishers) Ltd
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11.2 Embodiment of the active insulator
Nonlinear analytical models have been developed u~ sinlulate the effects of the nonlinear impedances embedded in the dielectric material ~. These models form the design basis of an insulation assembly which can actively respond in a favourable manner and in various ways to changes in its enviromnent. Advantage is taken of the increasing I L l curve, characteristic of the nonlinear impedance materials used. ('urrent flow through the insulators can be controlled by judiciously selecting the operating poin! on tile curve, thus grading the field and regulating power dissipation.
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For example, with active insulators using barium titanate capacitors, when an insulator or a section of an insulation assembly is stressed due to a nonuniform field or 1o contamination, the increase in voltage will instantaneously increase tile capacitance (Figure 2). The relative change in impedance will automatically result in a uniform field along the entire assembly.
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20 30 Insulator position
40
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Figure 1. Calculated voltage distribution along a gO unit insulator string showing that the first 10% of the units on the line side sustain 60% of the total applied voltage
voltage distribution calculated for a UHV insulator string. In this example the first 5 units on the line side must withstand almost 60% of the total applied voltage. Very long insulator strings must be used because the flashover strength of the string is not directly proportional to its length. The result is substantial increases in tower and right-of-way costs, among others. A nonuniform voltage distribution in addition to decreasing the flashover voltage of the string has other detrimental effects. Radio interference and audible noise increase due to high field stresses on the insulator units and hardware at the ends of the string. Contamination deposition is also influenced by the configuration of the electric field. The string will be contaminated in a nonuniform manner, further decreasing the string's flashover strength 4. Hot spots may originate at those places along the string which are highly stressed and/or have localized partial arcing caused by contamination s . The nonuniform voltage distribution is caused by stray capacitances which electrically load the string in a nonuniform manner. A relatively successful attempt was made as early as 1932 to capacitively grade a string by using oversized insulator hardware to increase the intrinsic capacitance of the units 6. More recently, capacitors were placed inside the hollowed-out caps of suspension insulators with electrical connections between the cap and the pin ~. With the advent o f new dielectric materials and nonlinear impedances for high voltage and high power applications, prototype graded post-type insulators were built 8' 9. Capacitive and resistive grading using bulk-graded polymer concrete posttype insulators has also been investigated in the laboratory.
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I2R and dielectric losses will be beneficial under contamination conditions because moisture condensation on the surface contaminants will be reduced, preventing the fommtion of electrolytes. These losses will be lower than the losses present with semiconductor-glazed insulators because they will occur only when and where needed. Transient behaviour studies indicate that if a material with a high dielectric constant is used, a uniform field can be obtained even for highly contaminated insulator strings 1°. This has important practical consequences, especially for DC line insulation where the field is uniform in the steady state, but during high frequency transients it will become as nonuniform as in the AC case. Also, under DC the contamination problem is much more severe. Nonlinear embedded resistors will grade and provide thermal heating in tile same way as the nonlinear capacitors described previously. Both resistive and capacitive grading can be achieved with zinc oxide which has a highly non-
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Figure 2. A voltage increase will automatically increase the capacitance of barium titanate, instantaneously increasing the current and shunting out all stray capacitance, thus grading the insulator assembly
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any overvoltage magnitude and duration, the two solutions will converge. For example, an active insulator designed to withstand high power surges will be functionally equivalent to a surge suppressor; and vice versa, a surge suppressor designed with a certain leakage distance and operating at a specified point on the nonlinear impedance curve to provide the desired voltage grading, thermal dissipation, mechanical support, etc. will function as an active insulator.
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Figure 3. The nonlinear impedance of zinc oxide causes an increase in current flow when an overvoltage occurs, immediately relieving the stress across the insulation; a = ZnO; b = SiC
linear resistance and also a high dielectric constant (Figure 3). In addition, zinc oxide blocks can support power surges of magnitudes typical o f large transmission systems. This protection function is described in the following. Figure 3 shows a V - I curve for zinc oxide given by I = ( V / K ) c~, where c~ and K depend on the material properties. Zinc oxide lightning arresters have been applied in power systems for several years with good results n-13. It is in this context, as lightning arresters, that surge suppressors at every tower have been proposed to eliminate the overhead shielding wire of a transmission line 2. This reference offers an excellent analysis of the lightning performance of such a line and of the thermal requirements o f the arresters. It is shown that the shielding wire can in theory be eliminated with this scheme. This idea was applied to a 138 kV link of the AEP system in an effort to reduce the number of power (ailures caused by lightning ~4. At the distribution voltage level the 'Thortec' system of pole-top insulation ~s and the 'Darverter q6 have been used to ensure a good phase to ground path and successful operation of unshielded line. However, this application by itself would not justify the added expense of a special single-purpose device at every tower. To be an attractive alternative, both technically and economically, such a device must perform several problemsolving functions for the user. This requirement can be satisfied by extending and fusing the ideas of active insulators and of surge suppressors at every tower.
III. Dynamic insulation Figure 4 outlines the evolution of the dynamic insulation concept from the solution of two separate problems in voltage stress reduction and control on transmission-line insulation. The two problems were discussed in Section lI together with their respective solutions. if the two particular solutions are generalized such that they apply to any dielectric electrode configuration and to
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The resulting dynamic insulation with its enhanced technical characteristics has the potential to solve other problems in addition to the original two. A properly designed dynamic insulator, constructed with the new dielectric and nonlinear impedance materials available today, should be capable of performing many functions which might include the following features: Electrical functions o Electrical field grading and shaping o Thermal heating for contamination abatement o Overvoltage control (switching, lightning, faults, resonance, etc.) o RI and audible noise reduction Mechanical functions o Directed fibre-reinforcement for greater strength/kg o Line drop-proof design o Integral construction of dielectric structure, nonlinear components, hardware and other accessories o Aesthetic form and colours o Shatterproof (internal explosion resistant and antigunshot) o Compact installations As the dynamic insulation concept is based on the solution of various problems integrated into one, it is best to use also a systems approach in its evaluation. At this conceptual stage of development, only a prelhninary assessment of its benefits and disadvantages can be made. However, a quantitative indication of how dynamic insulation would perform some of its functions can be obtained through digital simulations. One of the important electrical functions of dynamic insulation, that of overvottage control, is investigated here. The results of six cases in which overvoltages are generated in practical transmission lines are presented in Section IV.
Voltage stress problems (i) and Q
Nonuniform (~ field stress on insulation
Tronsient (~) overvoltoges
t Proposed solutions
Field graded (~11 insulation J
insulation with (~ Implementation imbedded nonlinear impedance
Synergistic result
(
Surge suppressors
I
Zinc oxide arresters
°1 01
Dynamic 1,2, others ) insulation
Figure 4. Evolution of the dynamic insulation concept which provides a solution for the t w o original problems and many others
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IV. AC and DC transmission-line simulations The Electromagnetic Transients Program ~7 was used to study the effects of dynamic insulation on transmission lines under switching, faults and lightning. Table 1 summarizes the six cases simulated. In each case a comparison was made of the voltage profiles along the entire length of a Line equipped with conventional passive insulation and then with dynamic insulation.
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The AC Line is energized at 400 kV and is 203 km in length, between Manzanillo and Atequiza. It is operated by M~xico's Federal Power Commission (CFE). The physical characteristics and the modelling details of this line and the DC line of case 5 appear in the Appendix. The DC line was assumed to operate at -+400 kV and carry a current of 900 A.
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IV.1 Case 1 In case 1, as in the other cases, many three-phase simulations were made in an effort to select the highest overvoltages under those particular conditions. Figure 5 shows that the maximum overvoltage is determined to a large extent by the parameter eeof the zinc oxide. The high (x values are technologically feasible today, but further research is needed in this area to obtain the design flexibility and thermal stability of the material required for the multiple functions of high-voltage dynamic insulation.
Figure 5. Maximum voltage profiles along a 203 km, 400 kV line for various values of the nonlinear coefficient (~ of zinc oxide, with dynamic insulation every 29 km. during energization with an SLGF at the receiving end (case 1 );
(a) N = 0 ; ( b ) ( x =
11, N = 8; (c) (~= 21, N = 8 ; ( d ) ( ~ = 3 1 ,
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Table 1. Summary of transmission-line simulations Case
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Figures Comments 20
i Energize with 1-@ to ground at receiving end
5 6
60% overvoltage reduction with dynamic insulation Dynamic insulation is needed every 29 km only
7 2 Energize open line
8 9
Reenergize with 1-@ to ground at receiving end Fault initiation overvoltage, 1-0 to ground
l0 ll 12 13 14
DC system with 1 pole to ground
15
Overvoltage reduction from 2.7 to 1.3 p.u. Faster recovery to steady state with dynamic insulation Overvoltage reduction from 4.8 to 1.5 p.u. Trapped charge drained off by dynamic insulation Dynamic insulation every 25 km for flat voltage profile Further overvoltage reduction is required in this case An ee of 41 reduces max. overvoltages to 1.2 p.u. c~has a strong effect on the line voltage profile
16 Direct lightning strike to centre phase
17 18 19
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Overvoltage reduction from 1.87 to 1.35 p.u. Dynamic insulation dampens lightning surges Over 80% of the lightning current can be drained by the dynamic insulation at the point of strike
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Figure 6. Maximum voltage profiles for varying numbers of towers (N) w i t h dynamic insulation (o~ = 21 ) during energization with an SLGF at the receiving end (case 1);
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(1,2,3,4,5,6,7,8)
Figure 6 clearly shows that it is not necessary to install dynamic insulation at every tower for overvoltage control purposes. For this case, dynamic insulation with an ~ of 2 l is needed only at 6 towers (or every 34 km) to limit overvoltages to 1.4 p.u. and to obtain a voltage profile with a 0.1 p.u. variation along the length of the line during the transient. If the number of towers with dynamic insulation is increased from 6 to 8, the voltage profile flattens, but the maximum overvoltage is not significantly reduced. These two findings have important practical implications. 111 Figure 7 the voltage waveforms for the line with and without dynamic insulation are compared. The overvoltage is reduced from 3.0 to 1.4 p.u, with dynamic insulation. There is also less distortion, indicating a faster recovery to the steady-state condition.
Electrical Power & Energy Systems
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Approximately 90% of all transmission-line faults are single line-to-ground faults. These faults induce transient overvoltages on the unfaulted phases or on adjacent circuits. The magnitude of the overvoltages is of the order of 1.7 p.u. If the insulation of these lines fails, a double line-to-ground fault or even a multiple contingency can develop is. Such fault initiation overvoltages also reduce the benefit of single-pole switching. Currently there are no economic methods to reduce or to control this type of overvoltage x9'2°. It is then unjustifiable to reduce other types of surges to levels below those due to fault initiation. Hence, the limiting factors in reducing line insulation are overvoltages initiated by single line-to-ground faults and by power frequency stresses.
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Figure 7. Switching overvoltage reduction ( - - - ) with dynamic insulation ((x = 21 ) at towers every 29 km during energization with an SLGF at the receiving end (case 1)
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Figure 8. Maximum voltage profiles for varying numbers of towers (N) with dynamic insulation (e = 21) during energization of the open line (case 2); (a) N = 0; (b) N = 2 (1, 8); (c) N = 4 (1, 3, 6, 8); (d) N = 6 (1, 2, 3, 6, 7, 8); (e) N = 8 ( 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 )
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Figure 9. Switching overvoltage reduction ( - - - ) with dynamic insulation (e = 21) at every 29 km during energization of an open line (case 2)
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IV.2 Case 2 This case is not as severe as case 1 and the overvoltage is reduced from 2.7 to 1.3 p.u. as shown in Figure 8. With dynamic insulation, the power frequency time period is maintained during the transient (Figure 9). IV.3 Case 3 An extremely high overvoltage of 4.8 p.u. is generated on the unfaulted phase under this condition, but is limited to 1.5 p.u. with dynamic insulation (Figure 10). The threephase reclosing time was deliberately shortened below the actual time lapse to reduce line discharge of the trapped charge. In spite of this, Figure 11 shows that dynamic insulation ahnost instantly starts to drain off the trapped charge, which helps to explain the large reduction in overvoltage.
Vol 6 No 4 October 1984
50
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1 2
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Figure 10. Maximum voltage profiles with varying number of towers (N) with dynamic insulation (e -- 21) during reenergization with an SLGF at the receiving end (case 3); (a) N = 0 ; ( b ) N = 2 ( 1 , 8 ) ; ( c ) N = 4 ( 1 , 3 , 6 , 8 ) ; ( d ) N = 6 (1,2,3,6,7,8);(e)N=8(1,2,3,4,5,6,7,8)
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IV.5 Case 5 In this case, a single line-to-ground fault was simulaled ~,l the midp~int (poinl 5, Figure 15) of the -+400 kV HVD(' line described in lhe Appendix. Figure 15 shows the cltecl of increasing the parameter oe. A _" ~/e" ~ overvollage reduction is seen as oeis increased from 21 to 41. These voltage profiles and those of Figure 1(~ indicate the strong ini]uence ot Q. in determining the voltage distribution along the DC line. which was also evident for the AC line.
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The waveshapes of the overvoltages calculated at the midpoint of the line on the unfaulted pole are shown in Figure 17. Tile overvoltage is reduced from 1.87 to t.35 p.u. with
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Figure 11. Switching overvoltage reduction ( - - - ) with dynamic insulation (c~ = 21) at towers every 29 km during reenergization with an S L G F at the receiving end (case 3)
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Figure 13. Fault initiation overvoltage reduction with dynamic insulation (e = 21) at towers every 25 km during an SLGF at the midpoint of the line (case 4)
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Figure 12. Maximum voltage profile for varying numbers of towers (N) with dynamic insulation ((~ = 21) during fault initiation overvoltage caused by an S L G F at the midpoint of the line (case 4); (a) N = 0; (b) N = 2 (1, 8); (c) N = 4 (1, 3, 6, 8); (d) N = 6 (1, 2, 3, 6, 7, 8); (e) N = 8 (1,2,3,4,5,6,7,8)
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In case 4 an overvottage on the unfaulted lines caused by a single line-to-ground fault at the mid-point of the line can be reduced to 1.4 p.u. with dynamic insulation having an o~of 21 (Figures 12 and 13). However, other sinmlations show that with an oe of 41 the maximum overvoltage could be limited to only 1.2 p.u. as shown in Figure 14. The application of dynamic insulation in controlling this type of surge should also reduce phase-to-phase surges, which will facilitate the design of compact lines and substations.
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Figure 14. Maximum voltage profiles for various values of (x with dynamic insulation every 2 9 k m during an SLGF at the midpoint of the line (case 4); (a) N = 0; (b) ~ = 11, N = 8; (c) e = 21, N = 8; (d) ~ = 31, N = 8; (e) ~ = 41, N=8
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The 20 km section was divided into 1 km spans. Dynamic insulation was placed at the midpoint and at the two ends of the section or every 10 kin. Figure 18 shows the maxiinure voltage profiles along the section when a 10 kA stroke occurs at point 10 and a is varied from 11 to 41. The voltage profile due to a 30 kA, 5/50 ,us stroke is also shown for comparison. Calculations made with no dynamic insulation resulted in overvoltages of 6.8 p.u. and 20.5 p.u. for lightning currents of 10 kA and 30 kA, respectively.
15
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Instantaneous power was also calculated for various values ofoe. These calculations indicate a 33% decrease in instant0
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4
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6
7
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Figure 15. Maximum voltage profiles for an H V D C l i n e with 11 towers with dynamic insulation and varying values of the zinc oxide parameter (x during a single line-to-ground fault at point 5 (case 5); (a) N = 0; (b) a = 11, N = 11; (c) a = 2 1 , N = 11;(d) a = 3 1 , N = 11;(e) e = 4 1 , N = 11 .
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Figure 16. Maximum voltage profiles with varying number of towers (N) with dynamic insulation and varying zinc oxide parameter ~ during a single line-to-ground fault at point 5 of an HVDC line (case 5); (a) N = 0; (b) a = 21, N = 1 1 (1,2,3, 4,5,6, 7,8,9,10,11); (c)~=41, N = 6 (1,3,5,7,9,11)
/
1.87
The lightning current was assumed to be a 5/50 ,us wave having a 10 kA magnitude and striking directly the conductor at the midpoint of the 20 km section. The flashover voltage of the line was chosen at 5.59 p.u. (1 800 kV), and it was simulated with voltage dependent switches at the strike point and at the two adjacent towers on either side.
Vol 6 No 4 October 1984
I I
0
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I 5
I li 4 5 Time, ms
I 6
l 8
Figure 17. Voltage waveshapes on the unfaulted pole at the fault location during a single line-to-ground fault at the midpoint of an HVDC line (case 5); - conventional insulation; - - - dynamic insulation (I = 21; . . . . . . dynamic insulation ~ = 41
dynamic insulation with an a = 41 at 11 towers or every 135 km. This fast acting overvoltage reduction should also benefit AC/DC systems by complementing the slower acting DC breakers. IV.6 Case6 To analyse the effects of lightning, a 20 km section of the 400 kV AC line was modelled. The line was assumed to be infinite in length to avoid reflections. It was represented by a single conductor with a characteristic impedance of 446 ohm, a travel time of 3.9 ,us/kin and a per unit length resistance of 0.086 ohm/kin.
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Figure 18. Maximum voltage profiles a l o n g a 2 0 k m section of a 400 kV AC line varying the parameter ~ of zinc oxide, during a direct lightning strike at point 10, with dynamic insulation every 10 km (case 6); (a) (x= 11, / - - 30 kA; (b) e = 1 1 , / = 1 0 k A ; ( c ) e = 2 1 , / = 10kA; (d) a = 3 1 , / = 10kA;(e) c~=41,/- 10kA
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could have dynamic insulation with smaller c~ valuc~ I{~ field grading or for reducing the effects of contamination, or performing other functions. Overvoltages initiated by single line-to-ground faults and power frequency stresses are presently the limiting factors in reducing line insulation. Case 4 and the discussion concerning the active insulator indicate that dynamic insulation is a promising alternative in overconring these limitations.
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Dylmmic insulation also appears to have potential applications in DC and AC/DC systems. In particular, dynamic insulation should be beneficial in reducing contamination problems, it could complement DC breaker operation and improve surge protection in gas insulated substations.
,c'o Time, ms
Figure 19. Lightning current waveshapes on the struck conductor for various values of the nonlinear parameter a of zinc oxide (case 6); (a) ~ = 41; (b) ~ = 31; (c) c~= 21; (d)(~ = 11
aneous power as a is increased from 11 to 41. This is because, although higher current flows through the dynamic insulation with the higher oe values (Figure 19), the voltage reduction is greater than the current increase. The energy handling capability of the zinc oxide material appears to be the major problem in designing practical insulation devices. Investigation of this is currently being carried out at various laboratories.
V. Conclusions
The multifunction capability of dynamic insulation offers the design engineer the option of using a true systems approach in specifying insulation. Electrical, mechanical, environmental, aesthetic and operational considerations can all be integrated into the design process through dynamic insulation, because it can actively respond in a favourable manner to all of these factors. The multi-purpose characteristic of dynamic insulation makes it a technically and economically attractive alternative in transmission-line design. For example, dynamic insulation has the potential to distribute voltage stresses uniformly along an insulator assembly, abate contamination, reduce corona and RI noise and control voltage surges: and at the salne time function as a mechanically strong and aesthetically pleasing compact line structure. The benefits derived from these features could offset any additional costs of dynamic insulation. Tire manufacturing control of the values of tire nonlinear paraineter a of the zinc oxide material is a key factor in the application of dynamic insulation. The cases studied show that only a small number of towers need to be equipped with dynamic insulation to linrit overvoltages to values of 1.0 p.u., provided that the a value is high. This requirement could present thermal stability problems in the zinc oxide and possibly higher costs. However, all other towers
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Dynamic insulation dampens lightning overvoltages along the entire length of the line. Calculations indicate that the high a values necessary for the desired performance of dynamic insulation do not increase power dissipation during transients: but, on the contrary, as 0e increases, instantaneous power decreases. This study provides only a general frame of reference for future more detailed studies and experimental work on dynamic insulation design and its economics. Further work on a practical prototype device is needed for laboratory testing, in particular thermal dissipation and material stability measurements for field grading, controlled heating and surge suppression with high a values. These results can then be applied on a large scale and long-term field evaluation program to assess quantitatively the full range of functions on dynainic insulation as an integral part of the power system.
V I . References 1 Rodrlguez, A Analysis of voltage stress grading in power system insulator assemblies PhD Thesis, University of Southern California, USA (April 1981) Los, E J 'Transmission line lightning performance with surge suppressors at towers' IEEE Trans. Power Appar. & Syst. Vol PAS 98 (January/February 1979) Brenneman, J L and Crothers, H M 'Distributing potential over a string of insulators' Electrical World Vol 64 (December 1914) p 1095 Wu, C T, Cheng, T C and Rodriguez, A 'A study on the use of internal grading to improve the performance of i nsu lators' IEEE Trans. Electr. Insulation Vol E1-t6 No 3 (June 198t) pp 250-257 Bohne, E W and Weiner, G £ 'Contamination of EHV insulation, Part I, An analytical study' Proc. IEEE New Orleans, USA (July 1966) pp 466-481 6 Schwaiger, A Theory of dielectrics translated by R W Sorensen, John Wiley, NY, USA (1932) General Electric Co. Three-phase UH V A C transmission research EPRI EL-823, Project 68-2, Final Report (July 1978)
Electrical Power & Energy Systems
Perry, E R and Frey, A M 'Epoxy post type graded insulator' Proc. PES Summer Meeting Portland, USA, Paper No 31 CP 67-454 (July 1967)
Brealey, R H, Juneau, P W and Zlupko, J E 'Fabrication design and electrical evaluation of bulk-graded filled polymer post insulators' IEEE Int. Symp. Electr. Insulation Philadelphia, USA (June 1982)
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10 Rodrlguez, A, Wu, C Y and Cheng, T C 'Theoretical analysis of a high voltage active insulator assembly' Electrical Engineering Dept., University of Southern California, USA, Internal Report (March 1981)
40 rn
11 Bronikowski, R J and Du Pont, J P 'Development and testing of MOVE arrester elements' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (June 1982)
//////,/////~/////?///~I/ ///////~///
12 Niebuhr, W D 'Application of metal-oxide-varistor surge arresters on distribution systems' IEEE Trans. PowerAppar. & Syst. Vol PAS-101 (June 1982) 13 Sweetana, A et al. 'Design, development and testing of 1200 kV and 500 kV gapless surge arresters' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (July 1982)
i Figure 20. Physical arrangement of the 400 kV tower used in the AC transmission line model
14 IEEE 'Supplying demand efficiently' Spectrum Vol 20 (January 1983) p 60 15 Ohio Brass Co. 'Step by step analysis of Thortec lightning protection' OB High Tension News Vol 37 (June 1968) p 204 16 Darveniza, M 'A lightning portective device for overhead lines using the arc quenching properties of wood' Electrical Eng. Trans. Australia Vol EEl 5 (October 1979) pp 88-94 17 Dommel, H W 'Digital computer solution of electromagnetic transients in single and multiphase networks' IEEE Trans. Power Appar. & Syst. Vol PAS-88 (1969) p 388 18 Kimbark, E W and Legate, A C 'Fault surge versus switching surge. A study of transient overvoltages caused by line-to-ground faults' IEEE Trans. Power Appar. & Syst. Vol PAS-87 (September 1968) 19 Clerici, A et al. 'Overvoltages due to fault initiation and fault clearing and their influence on the design of UHV lines' Proc. CIGRE Conf. Paper 33-17 (1974) 20
Rocamora, R G et al. 'Switching surges: Part IV. Control and reduction on AC transmission lines' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (August 1982)
To include the dynamic insulation nonlinear elements, the line was segmented into seven sections of 29 km in length, connected in cascade and the corresponding parameters were calculated accordingly. The e and K constants of the nonlinear elements were calculated such that the leakage current passing through the nonlinear insulators was 0.8 mA at rated voltage. The time step, At, used in the four AC cases was equal to 50/is, with a total simulation time of 30 ms. For the DC case, the 1 350 km Pacific [ntertie line was used in the simulation21, and again the line was segmented into ten sections in order to include 11 towers with dynamic insulation. In this case the time step and the simulation time were 100 its and 9 ms respectively. In the lightning stroke case, the same AC line model was used but only a 20 km section was considered in the simulation. A flashover voltage (FOV) of 5.5 p.u. was assumed for the line and this was simulated as a voltage dependent switch, which closes as soon as the voltage attains the FOV level. The lightning stroke model was a 5/50/is waveshape with l 0 kA crest value striking the midpoint of the line (tower 11). The simulation time was 50/3s and the time step was 0.1/is. The line was equipped with dynamic insulation every l 0 kin.
Appendix The electrical parameters of the 400 kV AC transmission line studied were calculated according to the physical arrangement of the tower shown in Figure 20, considering two Bluejay ACSR subcontractors per bundle.
Vol 6 No 4 October 1984
21
Sasaki, S and Matsubara, H 'Fault surges on HVDC transmission lines in both bipolar operation and monopolar metallic return operation modes and comparison with field test results' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (July 1982) pp 2221-2228
211
C Y Wu and T C Cheng University of Southern California, Los Angeles, CA, USA
H W Dommel University of British Columbia, Vancouver, BC, Canada
The evolution o f the dynamic insulation concept from the integration o f two existing technologies is presented. Separate solutions proposed in the recent literature for the reduction o f voltage stresses due to nonuniform electric fields and to transient overvoltages in EHV and UHV transmission line insulation are discussed and then integrated into one. The resulting combination, i.e. dynamic insulation, has the potential o f provMing a superior alternative for the abatement o f these two problems and others. Dynamic" insulation is a new concept which provides a systems approach to the electrical and mechanical design o f transmission-line insulation. Because of i t s multifunction capability, dynamic insulation offers various technical and economical advantages, including the mitigation o f the effects o f contamination on insulation and also the control o f fault initiation overvoltages as indicated by transmissionline simulations. Further theoretical studies and experimental work applying known technology is suggested to verify the ideas proposed. Keywords: electric" power transmission lines, dynamic insulation reliability
I. Introduction There has been much effort to improve most of today's power system components with the objectives of improving reliability and lowering costs. For example, modern highspeed breakers and microprocessor-based electronic relays have made it possible to obtain significant savings in the construction of more reliable transmission lines. However, there has been a definite lapse in insulation technology. As insulation levels in EHV and UHV lines are reduced by improvements to terminal equipment, a point has been reached where there is no real benefit in making further refinements to these devices. The limiting factor can be traced in many cases to design deficiencies of c o n -
Received: 5 July 1983, Revised: 6 March 1984
Vol 6 No 4 October 1984
ventional line insulation. In the field, this is indicated by the fact that the principal causes of transmission system outages in the United States, Canada and Mdxico are overvoltages and pollution. The common s y m p t o m of both of these problems is line-insulation failure. In the following, two solutions which have been proposed for two important line-insulation problems are reviewed 1'2. The problems are insulation voltage stresses due to nonuniform electric fields and to transient surges; the solutions are the application o f nonlinear or active insulators and zinc-oxide arresters, respectively. The two solutions are then combined into a new solution called dynamic insulation, which has the capability to solve both of these problems and others. Digital simulation results of AC and DC transmission line performance are presented, and the advantages and limitations of the dynamic insulation concept are discussed.
I1. Active insulators I1.1 The nonuniform voltage distribution problem The conceptual design of the standard porcelain insulator unit, in widespread use in today's modern transmission systems, is basically the same as that of the first insulators installed around the turn of the century. Although time has proven the porcelain insulator to be adequate on HV lines, in particular with respect to its durability, as early as 1914 one of the main drawbacks arising from its nature as a completely passive device was foreseen 3. At that time, it was noted that the relation between the total applied voltage on a string of standard insulators and the voltage across each individual unit along the string was n o t linear and that the marginal effectiveness of each additional insulator unit decreased rapidly. Since then, transmission system voltages have increased almost an order of magnitude, thus increasing the severity of this problem. This is illustrated in Figure 1 by the
0142-0615/84/040203-09 $03.00 © 1984 Butterworth & Co (Publishers) Ltd
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11.2 Embodiment of the active insulator
Nonlinear analytical models have been developed u~ sinlulate the effects of the nonlinear impedances embedded in the dielectric material ~. These models form the design basis of an insulation assembly which can actively respond in a favourable manner and in various ways to changes in its enviromnent. Advantage is taken of the increasing I L l curve, characteristic of the nonlinear impedance materials used. ('urrent flow through the insulators can be controlled by judiciously selecting the operating poin! on tile curve, thus grading the field and regulating power dissipation.
:a "5 12
For example, with active insulators using barium titanate capacitors, when an insulator or a section of an insulation assembly is stressed due to a nonuniform field or 1o contamination, the increase in voltage will instantaneously increase tile capacitance (Figure 2). The relative change in impedance will automatically result in a uniform field along the entire assembly.
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20 30 Insulator position
40
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Figure 1. Calculated voltage distribution along a gO unit insulator string showing that the first 10% of the units on the line side sustain 60% of the total applied voltage
voltage distribution calculated for a UHV insulator string. In this example the first 5 units on the line side must withstand almost 60% of the total applied voltage. Very long insulator strings must be used because the flashover strength of the string is not directly proportional to its length. The result is substantial increases in tower and right-of-way costs, among others. A nonuniform voltage distribution in addition to decreasing the flashover voltage of the string has other detrimental effects. Radio interference and audible noise increase due to high field stresses on the insulator units and hardware at the ends of the string. Contamination deposition is also influenced by the configuration of the electric field. The string will be contaminated in a nonuniform manner, further decreasing the string's flashover strength 4. Hot spots may originate at those places along the string which are highly stressed and/or have localized partial arcing caused by contamination s . The nonuniform voltage distribution is caused by stray capacitances which electrically load the string in a nonuniform manner. A relatively successful attempt was made as early as 1932 to capacitively grade a string by using oversized insulator hardware to increase the intrinsic capacitance of the units 6. More recently, capacitors were placed inside the hollowed-out caps of suspension insulators with electrical connections between the cap and the pin ~. With the advent o f new dielectric materials and nonlinear impedances for high voltage and high power applications, prototype graded post-type insulators were built 8' 9. Capacitive and resistive grading using bulk-graded polymer concrete posttype insulators has also been investigated in the laboratory.
204
I2R and dielectric losses will be beneficial under contamination conditions because moisture condensation on the surface contaminants will be reduced, preventing the fommtion of electrolytes. These losses will be lower than the losses present with semiconductor-glazed insulators because they will occur only when and where needed. Transient behaviour studies indicate that if a material with a high dielectric constant is used, a uniform field can be obtained even for highly contaminated insulator strings 1°. This has important practical consequences, especially for DC line insulation where the field is uniform in the steady state, but during high frequency transients it will become as nonuniform as in the AC case. Also, under DC the contamination problem is much more severe. Nonlinear embedded resistors will grade and provide thermal heating in tile same way as the nonlinear capacitors described previously. Both resistive and capacitive grading can be achieved with zinc oxide which has a highly non-
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Electrical P o w e r & Energy Systems
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any overvoltage magnitude and duration, the two solutions will converge. For example, an active insulator designed to withstand high power surges will be functionally equivalent to a surge suppressor; and vice versa, a surge suppressor designed with a certain leakage distance and operating at a specified point on the nonlinear impedance curve to provide the desired voltage grading, thermal dissipation, mechanical support, etc. will function as an active insulator.
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linear resistance and also a high dielectric constant (Figure 3). In addition, zinc oxide blocks can support power surges of magnitudes typical o f large transmission systems. This protection function is described in the following. Figure 3 shows a V - I curve for zinc oxide given by I = ( V / K ) c~, where c~ and K depend on the material properties. Zinc oxide lightning arresters have been applied in power systems for several years with good results n-13. It is in this context, as lightning arresters, that surge suppressors at every tower have been proposed to eliminate the overhead shielding wire of a transmission line 2. This reference offers an excellent analysis of the lightning performance of such a line and of the thermal requirements o f the arresters. It is shown that the shielding wire can in theory be eliminated with this scheme. This idea was applied to a 138 kV link of the AEP system in an effort to reduce the number of power (ailures caused by lightning ~4. At the distribution voltage level the 'Thortec' system of pole-top insulation ~s and the 'Darverter q6 have been used to ensure a good phase to ground path and successful operation of unshielded line. However, this application by itself would not justify the added expense of a special single-purpose device at every tower. To be an attractive alternative, both technically and economically, such a device must perform several problemsolving functions for the user. This requirement can be satisfied by extending and fusing the ideas of active insulators and of surge suppressors at every tower.
III. Dynamic insulation Figure 4 outlines the evolution of the dynamic insulation concept from the solution of two separate problems in voltage stress reduction and control on transmission-line insulation. The two problems were discussed in Section lI together with their respective solutions. if the two particular solutions are generalized such that they apply to any dielectric electrode configuration and to
Vol 6 No 4 October 1984
The resulting dynamic insulation with its enhanced technical characteristics has the potential to solve other problems in addition to the original two. A properly designed dynamic insulator, constructed with the new dielectric and nonlinear impedance materials available today, should be capable of performing many functions which might include the following features: Electrical functions o Electrical field grading and shaping o Thermal heating for contamination abatement o Overvoltage control (switching, lightning, faults, resonance, etc.) o RI and audible noise reduction Mechanical functions o Directed fibre-reinforcement for greater strength/kg o Line drop-proof design o Integral construction of dielectric structure, nonlinear components, hardware and other accessories o Aesthetic form and colours o Shatterproof (internal explosion resistant and antigunshot) o Compact installations As the dynamic insulation concept is based on the solution of various problems integrated into one, it is best to use also a systems approach in its evaluation. At this conceptual stage of development, only a prelhninary assessment of its benefits and disadvantages can be made. However, a quantitative indication of how dynamic insulation would perform some of its functions can be obtained through digital simulations. One of the important electrical functions of dynamic insulation, that of overvottage control, is investigated here. The results of six cases in which overvoltages are generated in practical transmission lines are presented in Section IV.
Voltage stress problems (i) and Q
Nonuniform (~ field stress on insulation
Tronsient (~) overvoltoges
t Proposed solutions
Field graded (~11 insulation J
insulation with (~ Implementation imbedded nonlinear impedance
Synergistic result
(
Surge suppressors
I
Zinc oxide arresters
°1 01
Dynamic 1,2, others ) insulation
Figure 4. Evolution of the dynamic insulation concept which provides a solution for the t w o original problems and many others
205
IV. AC and DC transmission-line simulations The Electromagnetic Transients Program ~7 was used to study the effects of dynamic insulation on transmission lines under switching, faults and lightning. Table 1 summarizes the six cases simulated. In each case a comparison was made of the voltage profiles along the entire length of a Line equipped with conventional passive insulation and then with dynamic insulation.
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The AC Line is energized at 400 kV and is 203 km in length, between Manzanillo and Atequiza. It is operated by M~xico's Federal Power Commission (CFE). The physical characteristics and the modelling details of this line and the DC line of case 5 appear in the Appendix. The DC line was assumed to operate at -+400 kV and carry a current of 900 A.
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IV.1 Case 1 In case 1, as in the other cases, many three-phase simulations were made in an effort to select the highest overvoltages under those particular conditions. Figure 5 shows that the maximum overvoltage is determined to a large extent by the parameter eeof the zinc oxide. The high (x values are technologically feasible today, but further research is needed in this area to obtain the design flexibility and thermal stability of the material required for the multiple functions of high-voltage dynamic insulation.
Figure 5. Maximum voltage profiles along a 203 km, 400 kV line for various values of the nonlinear coefficient (~ of zinc oxide, with dynamic insulation every 29 km. during energization with an SLGF at the receiving end (case 1 );
(a) N = 0 ; ( b ) ( x =
11, N = 8; (c) (~= 21, N = 8 ; ( d ) ( ~ = 3 1 ,
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Table 1. Summary of transmission-line simulations Case
2.5
Figures Comments 20
i Energize with 1-@ to ground at receiving end
5 6
60% overvoltage reduction with dynamic insulation Dynamic insulation is needed every 29 km only
7 2 Energize open line
8 9
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l0 ll 12 13 14
DC system with 1 pole to ground
15
Overvoltage reduction from 2.7 to 1.3 p.u. Faster recovery to steady state with dynamic insulation Overvoltage reduction from 4.8 to 1.5 p.u. Trapped charge drained off by dynamic insulation Dynamic insulation every 25 km for flat voltage profile Further overvoltage reduction is required in this case An ee of 41 reduces max. overvoltages to 1.2 p.u. c~has a strong effect on the line voltage profile
16 Direct lightning strike to centre phase
17 18 19
206
Overvoltage reduction from 1.87 to 1.35 p.u. Dynamic insulation dampens lightning surges Over 80% of the lightning current can be drained by the dynamic insulation at the point of strike
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(1,2,3,4,5,6,7,8)
Figure 6 clearly shows that it is not necessary to install dynamic insulation at every tower for overvoltage control purposes. For this case, dynamic insulation with an ~ of 2 l is needed only at 6 towers (or every 34 km) to limit overvoltages to 1.4 p.u. and to obtain a voltage profile with a 0.1 p.u. variation along the length of the line during the transient. If the number of towers with dynamic insulation is increased from 6 to 8, the voltage profile flattens, but the maximum overvoltage is not significantly reduced. These two findings have important practical implications. 111 Figure 7 the voltage waveforms for the line with and without dynamic insulation are compared. The overvoltage is reduced from 3.0 to 1.4 p.u, with dynamic insulation. There is also less distortion, indicating a faster recovery to the steady-state condition.
Electrical Power & Energy Systems
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Approximately 90% of all transmission-line faults are single line-to-ground faults. These faults induce transient overvoltages on the unfaulted phases or on adjacent circuits. The magnitude of the overvoltages is of the order of 1.7 p.u. If the insulation of these lines fails, a double line-to-ground fault or even a multiple contingency can develop is. Such fault initiation overvoltages also reduce the benefit of single-pole switching. Currently there are no economic methods to reduce or to control this type of overvoltage x9'2°. It is then unjustifiable to reduce other types of surges to levels below those due to fault initiation. Hence, the limiting factors in reducing line insulation are overvoltages initiated by single line-to-ground faults and by power frequency stresses.
16 18 20 22 24 26 28 150 :52 Time, ms
Figure 7. Switching overvoltage reduction ( - - - ) with dynamic insulation ((x = 21 ) at towers every 29 km during energization with an SLGF at the receiving end (case 1)
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Figure 8. Maximum voltage profiles for varying numbers of towers (N) with dynamic insulation (e = 21) during energization of the open line (case 2); (a) N = 0; (b) N = 2 (1, 8); (c) N = 4 (1, 3, 6, 8); (d) N = 6 (1, 2, 3, 6, 7, 8); (e) N = 8 ( 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 )
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IV.2 Case 2 This case is not as severe as case 1 and the overvoltage is reduced from 2.7 to 1.3 p.u. as shown in Figure 8. With dynamic insulation, the power frequency time period is maintained during the transient (Figure 9). IV.3 Case 3 An extremely high overvoltage of 4.8 p.u. is generated on the unfaulted phase under this condition, but is limited to 1.5 p.u. with dynamic insulation (Figure 10). The threephase reclosing time was deliberately shortened below the actual time lapse to reduce line discharge of the trapped charge. In spite of this, Figure 11 shows that dynamic insulation ahnost instantly starts to drain off the trapped charge, which helps to explain the large reduction in overvoltage.
Vol 6 No 4 October 1984
50
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Figure 10. Maximum voltage profiles with varying number of towers (N) with dynamic insulation (e -- 21) during reenergization with an SLGF at the receiving end (case 3); (a) N = 0 ; ( b ) N = 2 ( 1 , 8 ) ; ( c ) N = 4 ( 1 , 3 , 6 , 8 ) ; ( d ) N = 6 (1,2,3,6,7,8);(e)N=8(1,2,3,4,5,6,7,8)
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IV.5 Case 5 In this case, a single line-to-ground fault was simulaled ~,l the midp~int (poinl 5, Figure 15) of the -+400 kV HVD(' line described in lhe Appendix. Figure 15 shows the cltecl of increasing the parameter oe. A _" ~/e" ~ overvollage reduction is seen as oeis increased from 21 to 41. These voltage profiles and those of Figure 1(~ indicate the strong ini]uence ot Q. in determining the voltage distribution along the DC line. which was also evident for the AC line.
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Figure 11. Switching overvoltage reduction ( - - - ) with dynamic insulation (c~ = 21) at towers every 29 km during reenergization with an S L G F at the receiving end (case 3)
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Figure 12. Maximum voltage profile for varying numbers of towers (N) with dynamic insulation ((~ = 21) during fault initiation overvoltage caused by an S L G F at the midpoint of the line (case 4); (a) N = 0; (b) N = 2 (1, 8); (c) N = 4 (1, 3, 6, 8); (d) N = 6 (1, 2, 3, 6, 7, 8); (e) N = 8 (1,2,3,4,5,6,7,8)
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In case 4 an overvottage on the unfaulted lines caused by a single line-to-ground fault at the mid-point of the line can be reduced to 1.4 p.u. with dynamic insulation having an o~of 21 (Figures 12 and 13). However, other sinmlations show that with an oe of 41 the maximum overvoltage could be limited to only 1.2 p.u. as shown in Figure 14. The application of dynamic insulation in controlling this type of surge should also reduce phase-to-phase surges, which will facilitate the design of compact lines and substations.
208
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Figure 14. Maximum voltage profiles for various values of (x with dynamic insulation every 2 9 k m during an SLGF at the midpoint of the line (case 4); (a) N = 0; (b) ~ = 11, N = 8; (c) e = 21, N = 8; (d) ~ = 31, N = 8; (e) ~ = 41, N=8
Electrical Power & Energy Systems
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The 20 km section was divided into 1 km spans. Dynamic insulation was placed at the midpoint and at the two ends of the section or every 10 kin. Figure 18 shows the maxiinure voltage profiles along the section when a 10 kA stroke occurs at point 10 and a is varied from 11 to 41. The voltage profile due to a 30 kA, 5/50 ,us stroke is also shown for comparison. Calculations made with no dynamic insulation resulted in overvoltages of 6.8 p.u. and 20.5 p.u. for lightning currents of 10 kA and 30 kA, respectively.
15
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4
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6
7
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Figure 15. Maximum voltage profiles for an H V D C l i n e with 11 towers with dynamic insulation and varying values of the zinc oxide parameter (x during a single line-to-ground fault at point 5 (case 5); (a) N = 0; (b) a = 11, N = 11; (c) a = 2 1 , N = 11;(d) a = 3 1 , N = 11;(e) e = 4 1 , N = 11 .
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Figure 16. Maximum voltage profiles with varying number of towers (N) with dynamic insulation and varying zinc oxide parameter ~ during a single line-to-ground fault at point 5 of an HVDC line (case 5); (a) N = 0; (b) a = 21, N = 1 1 (1,2,3, 4,5,6, 7,8,9,10,11); (c)~=41, N = 6 (1,3,5,7,9,11)
/
1.87
The lightning current was assumed to be a 5/50 ,us wave having a 10 kA magnitude and striking directly the conductor at the midpoint of the 20 km section. The flashover voltage of the line was chosen at 5.59 p.u. (1 800 kV), and it was simulated with voltage dependent switches at the strike point and at the two adjacent towers on either side.
Vol 6 No 4 October 1984
I I
0
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I li 4 5 Time, ms
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l 8
Figure 17. Voltage waveshapes on the unfaulted pole at the fault location during a single line-to-ground fault at the midpoint of an HVDC line (case 5); - conventional insulation; - - - dynamic insulation (I = 21; . . . . . . dynamic insulation ~ = 41
dynamic insulation with an a = 41 at 11 towers or every 135 km. This fast acting overvoltage reduction should also benefit AC/DC systems by complementing the slower acting DC breakers. IV.6 Case6 To analyse the effects of lightning, a 20 km section of the 400 kV AC line was modelled. The line was assumed to be infinite in length to avoid reflections. It was represented by a single conductor with a characteristic impedance of 446 ohm, a travel time of 3.9 ,us/kin and a per unit length resistance of 0.086 ohm/kin.
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Figure 18. Maximum voltage profiles a l o n g a 2 0 k m section of a 400 kV AC line varying the parameter ~ of zinc oxide, during a direct lightning strike at point 10, with dynamic insulation every 10 km (case 6); (a) (x= 11, / - - 30 kA; (b) e = 1 1 , / = 1 0 k A ; ( c ) e = 2 1 , / = 10kA; (d) a = 3 1 , / = 10kA;(e) c~=41,/- 10kA
209
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could have dynamic insulation with smaller c~ valuc~ I{~ field grading or for reducing the effects of contamination, or performing other functions. Overvoltages initiated by single line-to-ground faults and power frequency stresses are presently the limiting factors in reducing line insulation. Case 4 and the discussion concerning the active insulator indicate that dynamic insulation is a promising alternative in overconring these limitations.
/ °olo
Dylmmic insulation also appears to have potential applications in DC and AC/DC systems. In particular, dynamic insulation should be beneficial in reducing contamination problems, it could complement DC breaker operation and improve surge protection in gas insulated substations.
,c'o Time, ms
Figure 19. Lightning current waveshapes on the struck conductor for various values of the nonlinear parameter a of zinc oxide (case 6); (a) ~ = 41; (b) ~ = 31; (c) c~= 21; (d)(~ = 11
aneous power as a is increased from 11 to 41. This is because, although higher current flows through the dynamic insulation with the higher oe values (Figure 19), the voltage reduction is greater than the current increase. The energy handling capability of the zinc oxide material appears to be the major problem in designing practical insulation devices. Investigation of this is currently being carried out at various laboratories.
V. Conclusions
The multifunction capability of dynamic insulation offers the design engineer the option of using a true systems approach in specifying insulation. Electrical, mechanical, environmental, aesthetic and operational considerations can all be integrated into the design process through dynamic insulation, because it can actively respond in a favourable manner to all of these factors. The multi-purpose characteristic of dynamic insulation makes it a technically and economically attractive alternative in transmission-line design. For example, dynamic insulation has the potential to distribute voltage stresses uniformly along an insulator assembly, abate contamination, reduce corona and RI noise and control voltage surges: and at the salne time function as a mechanically strong and aesthetically pleasing compact line structure. The benefits derived from these features could offset any additional costs of dynamic insulation. Tire manufacturing control of the values of tire nonlinear paraineter a of the zinc oxide material is a key factor in the application of dynamic insulation. The cases studied show that only a small number of towers need to be equipped with dynamic insulation to linrit overvoltages to values of 1.0 p.u., provided that the a value is high. This requirement could present thermal stability problems in the zinc oxide and possibly higher costs. However, all other towers
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Dynamic insulation dampens lightning overvoltages along the entire length of the line. Calculations indicate that the high a values necessary for the desired performance of dynamic insulation do not increase power dissipation during transients: but, on the contrary, as 0e increases, instantaneous power decreases. This study provides only a general frame of reference for future more detailed studies and experimental work on dynamic insulation design and its economics. Further work on a practical prototype device is needed for laboratory testing, in particular thermal dissipation and material stability measurements for field grading, controlled heating and surge suppression with high a values. These results can then be applied on a large scale and long-term field evaluation program to assess quantitatively the full range of functions on dynainic insulation as an integral part of the power system.
V I . References 1 Rodrlguez, A Analysis of voltage stress grading in power system insulator assemblies PhD Thesis, University of Southern California, USA (April 1981) Los, E J 'Transmission line lightning performance with surge suppressors at towers' IEEE Trans. Power Appar. & Syst. Vol PAS 98 (January/February 1979) Brenneman, J L and Crothers, H M 'Distributing potential over a string of insulators' Electrical World Vol 64 (December 1914) p 1095 Wu, C T, Cheng, T C and Rodriguez, A 'A study on the use of internal grading to improve the performance of i nsu lators' IEEE Trans. Electr. Insulation Vol E1-t6 No 3 (June 198t) pp 250-257 Bohne, E W and Weiner, G £ 'Contamination of EHV insulation, Part I, An analytical study' Proc. IEEE New Orleans, USA (July 1966) pp 466-481 6 Schwaiger, A Theory of dielectrics translated by R W Sorensen, John Wiley, NY, USA (1932) General Electric Co. Three-phase UH V A C transmission research EPRI EL-823, Project 68-2, Final Report (July 1978)
Electrical Power & Energy Systems
Perry, E R and Frey, A M 'Epoxy post type graded insulator' Proc. PES Summer Meeting Portland, USA, Paper No 31 CP 67-454 (July 1967)
Brealey, R H, Juneau, P W and Zlupko, J E 'Fabrication design and electrical evaluation of bulk-graded filled polymer post insulators' IEEE Int. Symp. Electr. Insulation Philadelphia, USA (June 1982)
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10 Rodrlguez, A, Wu, C Y and Cheng, T C 'Theoretical analysis of a high voltage active insulator assembly' Electrical Engineering Dept., University of Southern California, USA, Internal Report (March 1981)
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11 Bronikowski, R J and Du Pont, J P 'Development and testing of MOVE arrester elements' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (June 1982)
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12 Niebuhr, W D 'Application of metal-oxide-varistor surge arresters on distribution systems' IEEE Trans. PowerAppar. & Syst. Vol PAS-101 (June 1982) 13 Sweetana, A et al. 'Design, development and testing of 1200 kV and 500 kV gapless surge arresters' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (July 1982)
i Figure 20. Physical arrangement of the 400 kV tower used in the AC transmission line model
14 IEEE 'Supplying demand efficiently' Spectrum Vol 20 (January 1983) p 60 15 Ohio Brass Co. 'Step by step analysis of Thortec lightning protection' OB High Tension News Vol 37 (June 1968) p 204 16 Darveniza, M 'A lightning portective device for overhead lines using the arc quenching properties of wood' Electrical Eng. Trans. Australia Vol EEl 5 (October 1979) pp 88-94 17 Dommel, H W 'Digital computer solution of electromagnetic transients in single and multiphase networks' IEEE Trans. Power Appar. & Syst. Vol PAS-88 (1969) p 388 18 Kimbark, E W and Legate, A C 'Fault surge versus switching surge. A study of transient overvoltages caused by line-to-ground faults' IEEE Trans. Power Appar. & Syst. Vol PAS-87 (September 1968) 19 Clerici, A et al. 'Overvoltages due to fault initiation and fault clearing and their influence on the design of UHV lines' Proc. CIGRE Conf. Paper 33-17 (1974) 20
Rocamora, R G et al. 'Switching surges: Part IV. Control and reduction on AC transmission lines' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (August 1982)
To include the dynamic insulation nonlinear elements, the line was segmented into seven sections of 29 km in length, connected in cascade and the corresponding parameters were calculated accordingly. The e and K constants of the nonlinear elements were calculated such that the leakage current passing through the nonlinear insulators was 0.8 mA at rated voltage. The time step, At, used in the four AC cases was equal to 50/is, with a total simulation time of 30 ms. For the DC case, the 1 350 km Pacific [ntertie line was used in the simulation21, and again the line was segmented into ten sections in order to include 11 towers with dynamic insulation. In this case the time step and the simulation time were 100 its and 9 ms respectively. In the lightning stroke case, the same AC line model was used but only a 20 km section was considered in the simulation. A flashover voltage (FOV) of 5.5 p.u. was assumed for the line and this was simulated as a voltage dependent switch, which closes as soon as the voltage attains the FOV level. The lightning stroke model was a 5/50/is waveshape with l 0 kA crest value striking the midpoint of the line (tower 11). The simulation time was 50/3s and the time step was 0.1/is. The line was equipped with dynamic insulation every l 0 kin.
Appendix The electrical parameters of the 400 kV AC transmission line studied were calculated according to the physical arrangement of the tower shown in Figure 20, considering two Bluejay ACSR subcontractors per bundle.
Vol 6 No 4 October 1984
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Sasaki, S and Matsubara, H 'Fault surges on HVDC transmission lines in both bipolar operation and monopolar metallic return operation modes and comparison with field test results' IEEE Trans. Power Appar. & Syst. Vol PAS-101 (July 1982) pp 2221-2228
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