FIELD OF THE INVENTION

The present invention relates to high voltage engineering/electrical engineering, more particularly relates to area of pollution flashover studies on porcelain/ceramic insulator strings to improve the pollution flashover strength of the insulators.

BACKGROUND OF THE INVENTION AND PRIOR ARTS

Currently transmission of bulk power at high voltages over very long distances has become very essential and this task has been mostly performed by overhead transmission lines. The dual task of mechanically supporting and electrically isolating the live phase conductors from the support tower is performed by string insulators. The electrical stress distribution along the insulators governs the possible flashover whether in clean condition W or under polluted conditions, which is quite detrimental to the system.

On way to minimize flashover in high-voltage electrical insulator is by arranging discrete conducting regions and by placing sufficient number. For example, U.S.3,963,858 (the 858' patent) describes a high voltage electrical insulator adapted to prevent flashover.

The electrical insulator composed of one orimore electrical insulating skirts or sheds or shells at least some of which skirts or sheds or shells have a plurality of discrete conductive regions at one surface thereof, the discrete conductive regions being appropriately arranged and sufficient in number to intercept an arc in the event of incipient flashover from proceeding radially past the discrete conductive regions, thereby to prevent said flashover. i Some of the remedial measures currently followed to improve the pollution flashover strength are; increasing creepage length of the insulator, going in for special designs, applying semi-conducting glaze/ silicon grease/ RTV silicon rubber coatings etc, pre-monsoon maintenance, additional insulators, hotline or live-line washing etc. The improvements achieved are not commensurate with expectations and the problem persists.

OBJECTS OF THE INVENTION

The principal object of the invention is to develop field reduction electrodes to improve pollution flashover strength for a disc insulator.

Still another object of the invention is to provide for a method to improve pollution flashover strength in an insulator of a transmission line.

Yet another object of the invention is to;provide for a method of assembling field reduction electrodes with an insulator to improve flashover strength in a transmission line and/or distribution systems

STATEMENT OF THE INVENTION

Accordingly, the present invention provides for an insulator comprising field reduction electrodes to improve pollution flashover strength in a transmission line and/or 'J distribution systems, also provides for a method to improve pollution flashover strength in an insulator of a transmission line and/or distribution systems, said method comprising act of reducing electric field intensity at pin region for improving flashover strength of the insulator in the transmission line and/or1 the distribution systems by installing field reduction electrodes in the insulators, and also provides for a method of assembling field reduction electrodes with an insulator to improve flashover strength in a transmission line and/or distribution systems, said method comprising acts of; joining outer electrode onto inner electrode to form an unified field reduction electrode; and installing the unified field reduction electrode with the insulator to improve flashover strength in a transmission line.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

FIGURES la-lh shows field reduction electrodes in different views designed for normal working conditions,

FIGURES 2a-2h shows field reduction electrodes in different views designed for anti-fog working conditions,

FIGURE 3 shows a three-disc string with field reduction electrode mounted at the pin,

FIGURE 4 shows equipotential plot for single disc,

FIGURE 5 shows equipotential plot for 14 disc insulator string,

FIGURE 6 shows surface potential profile for single disc without electrode,

FIGURE 7 shows surface potential profile for single disc with electrode,

FIGURE 8 shows surface field for single disc without electrode,

FIGURE 9 shows surface field for single disc with electrode,

FIGURE 10 shows surface field for three discs without electrode,

FIGURE 11 shows surface field for three discs with electrode,

FIGURE 12 shows surface field for 14-disc string without electrode,

FIGURE 13 shows surface field for 14-disc string with electrode,

FIGURE 14 shows bulk stress distribution for single disc,

FIGURE 15 shows bulk stress distribution for 6-disc string,

FIGURE 16 shows scintillation activity on 6-disc insulator string without field reduction electrodes,

FIGURE 17 shows scintillation activity on 6-disc string with field reduction electrodes,

FIGURE 18 shows developed surface (porcelain-air interface), and

FIGURE 19 shows normalized surface resistance.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is in relation to an insulator comprising field reduction electrodes to improve pollution flashover strength in a transmission line and/or distribution systems.

In still another embodiment of the present invention said insulator is preferably a disc insulator or insulator strings.

In yet another embodiment of the present invention the field reduction electrodes are installed into the surface at pin region of the insulator.

In yet another embodiment of the present invention the field reduction electrode comprises an outer electrode preferably a disc shaped electrode having an opening at centre and an inner electrode comprising plurality of semicircular arc shaped elements.

In yet another embodiment of the present invention the inner electrodes are joined to the outer electrode at the opening using screws.

In yet another embodiment of the present invention the outer electrode has outer diameter ranging from about 100mm to about 120mm and height ranging from 20 mm-23 mm.

In yet another embodiment of the present invention said field reduction electrodes are optionally configured to anti-fog insulators having outer diameter of the outer electrode of about 104 mm and height of about 20 mm.:

In yet another embodiment of the present invention the inner electrode has outer diameter ranging from 48 mm to 50 mm and height ranging from 5mm to 8mm.

The present invention is in relation to a method to improve pollution flashover strength in an insulator of a transmission line and/or distribution systems, said method comprising act of reducing electric field intensity at pin region for improving flashover strength of the insulator in the transmission line and/or the distribution systems by installing field reduction electrodes in the insulators.

[n yet another embodiment of the present invention said method improves pollution flashover strength by about 15% for 6 insulator string and about 20% for single disc.

[n yet another embodiment of the present invention said method reduces electric field of about 40%.

The present invention is in relation to a method of assembling field reduction electrodes with an insulator to improve flashover strength in a transmission line and/or distribution systems, said method comprising acts of; joining outer electrode onto inner electrode to form an unified field reduction electrode; and installing the unified field reduction electrode with the insulator to improve flashover strength in a transmission line.

In yet another embodiment of the present invention said joining of the outer electrode with the inner electrode is made using screws.

In yet another embodiment of the present (invention said installing of field reduction electrodes are optionally retrofitted to existing insulators of the transmission line.

The performance of insulators used in overhead transmission lines, overhead distribution lines, and in outdoor substations is one of the critical factors which govern the reliability of power delivery systems. The deposition of the air-borne particulates and vapors on the surface cause a reduction in the performance of outdoor insulators. It is impractical in most situations to prevent the formation of; such a layer and therefore insulators are designed so as to ensure adequate flashover strength even under such pollution depositions. This aspect becomes more and more critical at higher transmission voltages. It has often been stated that the upper limit of open-air transmission voltage is set by the pollution performance of insulator strings.

In spite of knowing this phenomenon for the past several decades, a solution has remained still elusive. It is the single largest cause of transmission/distribution line outages, next to lightning. A major significance of the problem is that it repeatedly occurs even at working voltages. Because of these factors, a great deal of effort has already gone into to understand, as well as, model the same. Despite best efforts, success has only been nominal in improving the performance of transmission lines under polluted conditions.

The failure at any single point brings down the entire system. Following example amply demonstrates the consequence of such failure. Recent reports [2, 3] on grid disturbance in India indicate the loss of five thousand million rupees and 97% of interconnected generation on 2n January 2001. Similar disturbances of lesser magnitudes were also observed during the period of December 2002 & 2005, Feb & Dec 2006, Jan/Feb 2007 & March 2008. One of the major causes identified was the pollution induced flashovers. These events have amply portrayed that the performance of overhead transmission line string insulators and that in outdoor substations is a critical factor which governs the reliability of power delivery systems.

The present work is basically aims to seek simple alternative solutions for the problem. An interrogative study on the problem has shown that the maximum electric field occurring on the surface near the pin and cap, especially the former, is the accelerating source for pollution induced flashover. However, a reliable electric field distribution data on commonly used disc insulators are rather Scarce to find. In fact, the field concentration

near the pin can lead to early formation of dry band and scintillation. Considering this, it is aimed to seek possible minimization of the maximum field occurring at the surface near the pin region. This is expected to yield enhanced pollution flashover strength and in addition, improved wet flashover strength, as well as, reduction in audible and radio noise levels.

For obtaining the prevailing fields, suitable electric field computation tool is required. As the present problem under investigation is of open geometry type and having multiple dielectrics, it is impractical to employ analytical methods as the geometry does not fit into orthogonal curvilinear coordinates. Hence boundary based method namely Surface Charge Simulation Method (SCSM), which is well suited for the current work, is employed. Currently 2D axi-symmetric computer codes have been developed. Using the same, the contours of field reduction element/electrodes have been designed for the pin region.

Performance of string insulators used in;overhead power transmission lines is very critical and it is dictated by the electric field distribution prevailing under different operating contingencies. However, a reliable electric field distribution data on commonly used disc insulators are rather scarce to find.

Considering this, potential and electric field profile for commonly used porcelain insulators are investigated. To improve the flashover performance strength of the insulators during normal and polluted conditions novel field reduction element/ electrodes are developed.

Both theoretical and experimental investigations are carried out for ascertaining the improvement in the performance. Preliminary experiments carried out are in agreement with the theoretical deductions, which confirm the improvement in the performance of string insulators fitted with our field reduction electrode.

Phenomena underlying pollution induced flashover

The basic phenomena leading to flashover of an insulator are;

(i) A layer of pollution (uniform/non-uniform) forms on the surface of the insulator over a period of time. Almost always, there is no problem when the pollutant layer is dry.

(ii) When the pollutant layer becomes wet, as by a light drizzle or fog (heavy rains are likely to wash away the pollutant layer thus mitigating the problem), it starts conducting. The current flow causes Joule ^ heating and hence drying of the wet layer resulting in formation of dry bands. Non uniform pollution deposition would result in non-uniform current densities and this would aggravate the situation by promoting dry-band formation at locations of higher current densities. These dry bands being relatively high resistance have higher electric field intensities. If the field intensity is adequate, there will be local arcing across the dry bands, commonly known as scintillations.

Formation of the dry bands is obviously the consequence of vaporization of water which implies steam formation. Our experiments have shown that presence of steam plays a role in lowering the breakdown voltage across rib tips of insulators and for a lesser extent across insulating surfaces. Higher the rate of formation of steam, lower is the field required for breakdown and more vigorous the scintillations. Conversely,. lower the rate of drying (i.e. steam formation), higher the required field for initiation of scintillation.

(iii)One of two things happen further: ;

(a) The dry bands may develop so much that the voltage is inadequate to maintain the scintillations. Therefore, the scintillations stop and thus no flashover occur.

(b) The scintillations continue and progress vigorously and cause a flashover.

Principles and techniques for improving pollution performance
From the above, it is concluded that, other things being equal;

(i) formation of dry bands are delayed & slowed down by reducing current density at critical places. The reduced current density also reduces the rate of steam formation thus reducing the frequency and intensity of scintillations. In the limiting case, scintillations would not occur.

(ii) The severity of scintillations is reduced by reducing the electric field intensity at the critical place/s; this in turn reduces the risk of scintillations developing into a flashover.

The present invention describes a method for improving the flashover strength of disc insulators/insulator strings significantly under polluted conditions. The method is adopted either at the time of manufacture of the insulators or for retro-fitting to the existing insulators in service.

The detail dimensions of the two types of fielcl reduction electrodes are shown in figures la-lh and Figures 2a-2h:

Electrode-1: Developed for Normal type of Ceramic Insulator

Dimensions: OD: 118 mm, height: 23mm, width of inner electrode: 50mm height of inner
electrode: 8mm

Electrode-2: Developed for Anti-fog type of Ceramic insulators

Dimensions: OD: 104 mm, height: 20mm, width of inner electrode: 50mm height of inner
electrode: 5mm

Surface Charge Simulation Method (SCSM)

In an electrostatic field, the applied excitation induces real charges on the conductor surfaces and apparent (polarisation) charges on dielectric interfaces (for linear media). The resulting field distribution is equivalent to that produced by surface charge distributions on the conductor boundaries and fictitious surface charge distributions at the dielectric interfaces with dielectrics replaced by vacuum. The SCSM attempts to simulate these real and fictitious charges by piecewise-defined surface charge distributions. In other words, the SCSM involves discretisation of conductor surfaces and dielectric interfaces. As a consequence, the solution Satisfies the governing differential equation exactly, but satisfies the boundary conditions only approximately. The present work employs segments with a linearly varying charge distribution for the discretisation and Galerkin's method for deriving the SCSM equations.

The potential and field due to an arbitrary axi-symmetric strip is necessary for the formulation.

T is the value of / till that range potential is computed analytically and rest is computed numerically. If T covers whole ■ range, then potential is computed analytically.

For the singularity at other portion of the segment, original segment is suitably divided near the singularity and each is handled separately.

The above mathematical steps are fully coded in C along with many error checking routines. The resulting matrices are inverted in'Matlab and obtained charge densities are stored. Two more C programs are run to obtain the data for the equipotential and interface potential distribution. The typical run times involved for the execution of the programs are about 2 hours for single disc and 6 hours for 15 discs (corresponding to a 220kV string).

Simulation

Type N porcelain insulator (i.e. 160 kN normal type) is chosen. The results for other types of insulators are on similar lines.

For a better understanding both bulk and surface stress along the air interface was studied for a single disc. The potential and bulk stress distribution is presented in Figures 4 and 14. Figure 15 presents bulk distribution for 6-disc string respectively. The maximum bulk stress appears at the top corner of the pin lying in the cement. The potential and tangential surface field distribution along the air insulator is shown in Figures 6 and 8. The maximum field on the surface occurs at the pin along the cement-air interface. The maximum field is very high at the pin region in comparison to the average field along the surface of the insulator. There is clearly field intensification by a factor of 6. This region has been identified as the most probable source of starting the initial scintillation/partial arcs.

After getting a clear picture on field distribution in a single disc, now analysis is extended to string of insulators with 3, 6, 9 & 14 disc strings, which correspond to 33, 66,132 and 220 kV class respectively. Figure 5, presents the field (equipotential) distribution at the operating voltage of 220kV class. The corresponding surface field plot along the insulator-air interface for 3-string is shown in Figures 10 and 11 and for 14-disc string is shown in Figures 12 and 13.

The laboratory tests shown that considerable voltage drop was seen on the line end disc. But for the pin of the first disc and cap of the last disc, all other cap and pins form floating conductors dictating a capacitive voltage distribution. Consequently the capacitance (arising between cap and pin) of the first disc needs to carry most of the chain current which imposes a large voltage drop across it. Field plot, which is an image of the potential plot, shows much larger surface field at the first disc as compared to the single disc. The simulations carried out for other type of discs showed a similar trend in potential and field distribution.

The basic goal of the invention is to reduce the maximum surface stress occurring at the pin region. Based on the field plot obtained above, few contours for the field control electrode were arrived at and based on the pertinent study on the modified field distribution a suitable profile for the electrode was chosen. The insulators with field reduction element are shown in Figure 3.

The simulation results for the selected control electrode are carried-out using single disc firstly. The surface potential and field profiles are presented in Figures 7 and 9. By comparison with the results for normal single disc, it is inferred that the maximum surface field is now reduced by about 40%^ a significant reduction. The insertion of the electrode causes a sacrifice on the creepage length by only 2-3 % (as per the practice, cement region is not considered in specifying the creepage length).

Figures 11 and 13 presents the surface field profile for 3-disc & 14-disc string with all insulators fitted with field reduction electrode. By comparison with the string of normal discs, an overall reduction by > 40% is seen in the maximum surface field.

Similar field concentration is expected to prevail during polluted conditions, during which the surface conduction field dominates over the dielectric field. Consequently, the surface profile of the insulator assumes prime importance. Figure 18 shows cross sectional width along the surface, which implicitly indicates the local surface resistance. Normalised surface resistance (as shown in figure 19) indicates a large concentration of resistance at the pin region. The ratio of rriaximum to average surface stress under polluted conditions is a quantitative indicator for the pollution performance. For the surface conduction field, this normalised surface resistance is directly proportional to the resulting electric field and therefore used for depicting the field.

Experimentation

A preliminary, set of experimentation was carried out on type N insulator disc/strings with and without field reduction elements. Flashover strength under dry, wet and polluted conditions is evaluated up to 132 kV class string. Photographs presented in Figure 16 and 17 shows the scintillation activity at 29 kV (RMS) in a 6-disc insulator string without and with field reduction element/electrode. Table 1 presents the results for pollution flashover strength. The improvement is appreciable for the investment made (which is less than 2-3% of the cost of the insulator) on field reduction electrodes.

The invention has been described in connection with its preferred embodiments. However, it is not limited thereto. Changes, variations and modifications to the basic design may be made without departing from' the inventive concepts in this invention. In addition, these changes, variations and modifications would be obvious to those skilled in the art having the benefit of the foregoing teachings. All such changes, variations and modifications are intended to be within the scope of this invention. The technology of the instant Application explained with the examples should not be construed to limit the scope of the invention.

REFERENCES

[1] Ravi S Gorur, Edward Cherney and Jeffrey Burnham, "Outdoor Insulators", Phoenix, Arizona, USA, 1999.

[2] Rebati Dass, "Grid disturbance in India on 2nd January 2001", No: 196, pp 6-15, Electra, June 2001,

[3] CEA Enquiry Committee report of Grid incident of Northern region, 2007

[4] CIGRE Task force 33-04-01, "Polluted Insulators: A review of current knowledge", 2000

[5] IEC-507-1991-04 "Artificial Pollution tests on high voltage Insulators used on ac systems".1991

[6] Eric H. Allen and Peter L. Levin, "Two dimensional and Axi-symmetric Boundary value problems in Electrostatics", Computational Fields Laboratory, Dept. of Electrical and Computer Engineering, Worcester Polytechnic Institute, Worcester, MA-USA -1993.

[7] D Beatovic et al, "A Galerkin formulation of the Boundary Element Method for two dimensional and axi-symmetric problems in Electrostatics", IEEE Trans on Electric Insulation, Vol.27, No.l, Feb 1992, pp.135-1.43.

[8] Udaya Kumar and Vasu Mogaveera, "Studies on Voltage distribution in ZnO Surge Arrester", IEE Proceedings, Generation, Transmission & Distribution, Vol. 149, No. 4, July 2002

[9] S Chakravorti and H Steinbigler, "Capacitive resistive field calculation on H V bushing using the boundary element method", IEEE Trans. Dielectrics & Electrical Insulation, Vol. 5, No.2, pp 237-244, April 1998.

[10] O W Andersen, "Finite element solution of complex electric fields", IEEE Trans, on PAS, Vol. 96, No.4, pp 1156-1160, 1977.

[11] H El Kishky and R S Gorur, "Electric potential and field computation along ac HV insulators", IEEE Trans, on DEIS, Vol.1, No.6, pp 982-990, Dec 1994.

[12] Chakravorti S. and Steinbigler H.: 'Boundary Element studies on insulator shape and electric field around HV insulators with or without pollution', IEEE Trans. DEIS, 2000, Vol.7 (2), pp. 169-176.

[13] Zafer Aydogmus and Mehmet Cebeci, "A New flashover dynamic model of polluted H V Insulators", IEEE Trans, DEIS, Vol.11, No.4, Aug 2004, pp.577-584.

[14] IEC Report Publication - 815 - 1986, *'Guide for the selection of insulators in respect of polluted conditions", 1986.

[15] Subba Reddy B and Udayakumar, i "Potential and Electric Field Profiles for Transmission line Insulators", Fourth IASTED Asian Conference on Power and Energy Systems (Asia-PES 2008), paper No: 606-115 held at Langkawi, Malaysia, April 2 - 4, 2008

[16] Muhsin Tunay Gencoglu and Mehmet Cebeci, "The Pollution flashover on high voltage insulators", Electric Power Systems Research, Volume 78, Issue 11, Pages 1914-1921, November 2008.

WE CLAIM:

1. An insulator comprising field reduction electrodes to improve pollution flashover strength in a transmission line and/or distribution systems.

2. The insulator as claimed in claim 1, wherein said insulator is preferably a disc insulator or insulator strings.

3. The insulator as claimed in claim 1, wherein the field reduction electrodes are installed into the surface at pin region of the insulator.

4. The insulators as claimed in claim 1, wherein the field reduction electrode comprises an outer electrode preferably a disc shaped electrode having an opening at centre and an inner electrode comprising plurality of semicircular arc shaped elements.

5. The insulator as claimed in claim 4, wherein the inner electrodes are joined to the outer electrode at the opening using screws.

6. The insulator as claimed in claim 4, wherein the outer electrode has outer diameter ranging from about 100mm to about 120mm and height ranging from 20 mm-23 mm.

7. The insulator as claimed in claims 1 and 6, wherein said field reduction electrodes are optionally configured to anti-fog insulators having outer diameter of the outer electrode of about 104 mm and height of about 20 mm.

8. The insulator as claimed in claim 4, wherein the inner electrode has outer diameter ranging from 48 mm to 50 mm and height ranging from 5mm to 8mm.

9. A method to improve pollution flashover strength in an insulator of a transmission line and/or distribution systems, said method comprising act of reducing electric field intensity at pin region for improving flashover strength of the insulator in the transmission line and/or the distribution systems by installing field reduction electrodes in the insulators.

10. The method as claimed in claim 9, wherein said method improves pollution flashover strength by about 15% for 6 insulator string and about 20% for single disc.

11. The method as claimed in claim 9, wherein said method reduces electric field of about 40%.

12. A method of assembling field reduction electrodes with an insulator to improve flashover strength in a transmission line, and/or distribution systems said method comprising acts of;

a. joining outer electrode onto inner electrode to form an unified field reduction
electrode; and

b. installing the unified field reduction electrode with the insulator to improve
flashover strength in a transmission line.

13. The method as claimed in claim 12, wherein said joining of the outer electrode with the inner electrode is made using screws.

14. The method as claimed in claim 12, wherein said installing of field reduction electrodes are optionally retrofitted to existing insulators of the transmission line.

15. An insulator comprising field reduction electrodes, a method to improve pollution flashover strength and a method of assembling is substantially as herein above