development of high and extra-high voltage compact...

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DEVELOPMENT OF HIGH AND EXTRA-HIGH VOLTAGE COMPACT OVERHEAD POWER LINES

Waldemar Skomudek / Opole University of Technology

1. INTRODUCTION

The results of research, analysis and discussions carried out by the author of this paper in objectifying the threat to insulation systems caused by surges show that these specific electrical loads created during various types of transient states in power systems have a significant impact on making decisions on the method of op-eration and equipment in medium, high and extra-high voltage power networks [2, 3, 4 and 8]. The importance of surges justifies the need for their testing and analysis, which should be continued and developed with the progress of measuring and calculating capabilities, as well as in the field of new design solutions, in particular those affecting the reliability and quality of electricity supply.

However, a possible alternative approach to the principles of insulation coordination and protection against surges in medium, high and extra-high voltage networks presented in publications [1, 8 and 9] indicates directions of modification, based mainly on:

• increasing the role of natural reduction of particular surge types• optimization of surge insulation strength against surges (reducing the oversize of insulation systems)• rational approach to the insulation protection factor, including the use of modern equipment for limit-

ing surges• unification of provisions contained in the standards relating to the issues of insulation coordination and

protection against surges.An in-depth analysis of the proposed courses of modifications takes into account also the economic aspect

of the investment process in the power sector.

2. ASSESSMENT OF INVESTMENT NEEDS

The national power sector, as well as the companies belonging to this sector in other European Union countries, is undergoing profound changes. Since the electricity markets, dominated by competition, demopoli-sation and privatisation, were established in the 1990s, they have undergone substantial changes. The economy of this sector is becoming important.

The analysis of macroeconomic statistics characterizing the electricity market in relation to economic growth determined by GDP1 shows that electricity consumption in Poland has been increasing over the last few years; it was temporarily slowed down only by the crisis at the end of 2009. Currently, the national electricity consumption is at the level equal to 2008. However, the observed consumption of electricity in the first half of 2010 is over 4.7% higher than at the same period last year. Although the last two years brought a significant re-

Development of High and Extra-high Voltage Compact Overhead Power Lines

Abstract

A comprehensive analysis of the principles of insu-lation coordination indicates the possibility of modifying the existing procedure. Taking action in this direction wo-uld lead to measurable economic results, which, in light of current investment needs in power transmission and di-stribution network infrastructure, the financial capabilities

of entities that manage this infrastructure and the existing legal and environmental barriers for their implementation (particularly in the case of line investments) would be a determining factor.

1 Gross domestic product – the aggregate value of final goods and services produced within the particular country in a given time unit (e.g. one year).

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duction in fuel and electricity consumption in the Polish power sector, confirming the risks arising from the finan-cial crisis, the aforementioned increase should be taken as a clear signal of economic recovery in the country.

Based on the current and projected trends in GDP growth, the demand for gross electricity 2and simulta-neous reduction of energy consumption in the economy, it will reach about 200 TWh in 2030, while in 2010 the national consumption of electricity amounted to almost 155 TWh3. However, given the current and future rates of electricity demand in network capacity analysis, we arrive at alarming conclusions, as the depreciation of both transmission and distribution power networks reaches 40-65% (fig. 1).

Fig. 1. Age structure of national overhead power lines

As a result, the rules of a competitive market based on balanced supply and demand for electricity reveals a significant gap, also in overhead power lines. It is estimated that to close the gap, an increase in the length oftransmission lines by about 2000 km and distribution lines by about 2800 km is needed in between 2011 and 2020. In addition, the age structure of existing overhead lines indicates the great need for modernization, which is estimated for about 9000 km4.

The above scale of investment needs is motivation for seeking and implementing solutions that will allow avoiding interruptions in energy supply, and thus will increase energy security. Thus, each investment task will trigger the need for an in-depth economic analysis, which will optimize the spending of funds.

3. LEGAL AND ENVIRONMENTAL RESTRICTIONS

Regulation of many legal acts, ranging from energy law, through construction law, public procurement law, spatial planning and development law to environmental protection law, plays an important role in imple-mentation of electric power investments, especially line investments. Environmental restrictions dictated by localization conditions are particularly important. They are based on a complicated procedure of spatial plan-ning, including:

• introduction of investment to the local development plans• obtaining the-so called right of way, i.e. the right to use the area for construction of lines or stations,

along with granting the decision on immediate enforceability• proceedings associated with environmental impact assessment of the planned project• obtaining social approval for the investment implementation.

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2 Electricity obtained from the energy conversion process in the form of electrical current measured at the generator terminals.3 Data according to McKinsey & Company, Global Insight, ARE.4 The principle of cautious estimate was applied.


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The experience of the national and international power sector shows that most problems are caused by the proceedings related to environmental impact assessment of the project. Many of the problems that need to be solved in the course of investment are social in nature and are caused directly by the fact that the investment is implemented in a specific natural environment. The relationship between investor, local administration andlocal communities are governed by the law that imposes the obligation to implement the investment in such a way as to ensure public participation in environmental decision-making. In such a case, the concept of environ-ment has both a natural and socio-environmental meaning.

The specificity of line investments lies in the fact that the line route usually runs through much diversifiedareas, including in terms of ownership. Therefore, solutions that maximally limit interference in the environ-ment and private property (e.g. small-sized stations, compact lines built on steel poles with narrow shafts, steel poles or concrete poles) are preferred in line construction. Hence, the selection of the proper route and type of line becomes an issue of crucial importance.

The electromagnetic field that accompanies the operation of every power line and station is equally im-portant. This field may be a nuisance to the environment. Particularly its impact on living organisms requiresthe use of solutions that reduce the negative impact to the lowest level possible when using reasonable techni-cal means and expenditures. The maximum value and distribution of electric and magnetic field in the vicinityof overhead lines are mainly affected by the type and value of rated voltage, the distance of phase conductors from the ground, the gaps between the wires of particular phases, the geometric arrangement of wires and their height.

The need to create a protection (technology) belt for the line, the width of which depends on the rated voltage and type of line is also a significant nuisance to the environment5. Therefore, the goal is to ensure that overhead lines are constructed using poles that are hardly noticeable and perfectly integrated into the land-scape, and are additionally located in forest areas or woodlands as lines with very tall towers that allow running cables over tree-tops.

4. IMPLEMENTATION OF GROUNDS FOR MODIFICATION OF INSULATION COORDINATION

The behaviour of overhead lines during lightning discharges depends on many factors, of which the most important are:

• density of lightning discharges• resistance of insulation (insulation system)• resistance of pole earthing• protection by lightning arresters• cable system• height of cable lines.Analysis of principles and rules applied in determining the levels of surge amplitudes and strength of

insulation (insulation system) described in the monograph [8] showed that the distance between phase cables and earthed pole structure may be optimized. Risk assessment for breakdown, expressed in threat to insulation, indicates also the possibility of optimizing the levels of protection and insulation. In both cases, optimization may bring technical and economic benefits.

To determine a measurable efficiency of implementation of research, analysis and evaluation results inpower lines, an attempt was made to assess the benefits that will result in:

• minimizing the reserves of impulse insulation strength• coordination of distances between insulation on the pole• reduction of the protection belt width in the line route• natural reduction of lightning surges by reducing resistance and inductance of earthed poles.

5 The minimum distance from the trees (vertically) is 1.5 m + Del; according to PN-EN 50341-1 Del is the minimum distance in the air required to avoid a complete discharge between the phase conductor and earth potential objects during overvoltages with mild or steep front; for the highest voltage of devices Um = 123 kV the distance.


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4.1. Optimization of impulse insulation strengthIn order to implement the economic projects of medium, high and extra-high voltage overhead power

lines, the reserve of impulse insulation strength should be optimized. In this process, crucial importance is at-tributed to impulse voltages that have an impact on the selection of insulation distances of both external and internal lines and on the width of protection belt in the line route. Example values of minimum distances in the air for various values of standard withstand lightning impulse voltage (Uw) shown in fig. 2 were determinedcarefully, taking into consideration the operation experience and economy, and ensure a particular level of in-sulation.

Therefore, assuming in further considerations minimum distances for overhead lines with a rated voltage of 110 kV for the extreme values of voltage Uw (fig. 2 - values on grey background), we may reduce the weight ofthe supporting structure by shortening the length of cross arms and insulators, which has an additional influenceon reducing cable sag or pole height in adjusting the length of the insulator (insulator chain). Correction of the external and internal distances within the lines also has an influence on the width of the protection belt.

To estimate the achievable level of economic benefits resulting from the coordination of insulation dis-tances, calculations were made using the following data as basic assumptions:

• unit length of the line is 100 km• the share of straight-line (P) and power (M) poles in the line is 70% and 30%, respectively• power poles (M) are indicated as M3 (or ON 150)• shortening of cross arms does not change the cost of other line elements• assumed construction costs as the percentage of the cost of materials (100% of the cost of materials)• the cost of line does not include the expenses on acquiring the rights to land• unit prices of the 110 kV line elements are assumed as of 1 October 2009.

Fig. 2. Presentation of the minimum insulation distance in the air for overhead lines with a rated voltage of 110 kV; values on the grey back-ground show the differences in the minimum distances for extreme values of the rated withstand lightning impulse voltage

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The obtained calculation results are presented in fig. 3; the final iteration assumes that steel cross arms will be replaced with insulators, e.g. according to the suggestions included in fig. 4

Fig. 3. Effect of reduction of the pole cross arms per the level of unit cost of line segment

Analysis of the calculation results showed that reduction of the cross arm length that does not exceed 40 cm may reduce the line construction costs by 0.3-0.5%. However, the replacement of traditional cross arms on overhead line poles with insulator systems leads to further reduction of the line construction costs by 1.3-2.5%. Implementation of the above actions involves the possibility of reducing the surface area of protection belt along the line route by about 6%. It also leads to reducing the influence of the maximum values of electric field by about 19% and magnetic field by about 28% (assuming the maximum acceptable reduction of the length of pole cross arms in terms of impact strength).

Therefore, it can be concluded that restriction of insulation distances (while maintaining the required levels of impulse insulation strength) leads to a measurable economic effect, while maximization of this effect takes into account the investment needs for overhead power lines existing in the country. The type of lines used (single-track or multi-track) is also of significance to the level of obtained benefits, as is the selected system for installing cables (steel cross arms with reduced length or insulator systems replacing steel cross arms).

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Fig. 4. Selected examples of insulator systems used to hang the cables of high and extra-high voltage overhead lines: a) traditional system with possibility of reducing the length of cross arms, b) triangular system with vertical installation of insulators, c) triangular system with horizontal installation of insulators

a) b) c)


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4.2. Natural reduction of lightning surgesOne way to protect overhead power lines against surges is to prevent the surges generated in the line

from causing damage to the line insulation. The results of surge propagation analysis in overhead lines de-scribed in publications [5, 6 and 7] show that modifications in this area are possible, and one of the directionsfor their practical use is to increase the role of natural reduction of lightning surges in overhead power lines. This can be achieved by changing the value of resistance and inductance of earthed supporting structures. To demonstrate the correctness of the above action, the phenomenon of direct lightning striking an overhead line pole was analysed. This incident creates the potential difference between the pole tip and the ground surface, which is the earth voltage of the pole, determined by the relation

uws = ip × Rs (1)

where Rs – resistance of the pole earthing. However, this relation does not include the inductive voltage drop. Assuming the likely parameters of lightning current (about 30-50 kA) and the value of the pole earthing resistance of 20 Ω, we can determine the peak voltage drop in the pole. It is high enough to make the difference between earth fault voltage in the pole and earth fault voltage in the return conductor6 usually exceed the value of withstand lightning impulse surge in the insulator (insulation system). This causes an earth fault. Because in this case the pole has a higher potential than the return conductor, a so-called reverse breakdown in the line insulation occurs. Reverse breakdowns may occur in insulation of one phase or simultaneously in the insulation of two or all phases.

Because the rate of rise of lightning current is high, the pole inductance can not be ignored. In this case, the pole tip voltage is increased by the inductive drop of voltage

dtdi

L p . In such a situation, the pole tip voltage can be determined using the formula

(2)

Using formula (2), it is possible to determine the value of maximum resistance of pole earthing, which causes a reverse breakdown for specific values of lightning current and its rate of rise. After making simpletransformations, the condition takes the following form

(3)

If the pole tip voltage is replaced with the value of withstand lightning impulse surge of the insulator (insulation system) in relation (3) U, the condition can be written using the formula

(4)

Independent calculations for two types of steel poles: lattice towers and solid poles (so-called tubular poles) were made using relations (3) and (4). Three peak values and rates of rise for the lightning discharge current were selected, and unit inductance values for poles in the range of 0.5-1.2 µH/m were assumed for this calculation7; the extreme values relate to solid pole and lattice tower, respectively. The calculations were made assuming that the overhead line is equipped with a lightning arrester that connects in parallel the pole struck by

dtdi

LiRuuu ppsLRws �

p

pws

s Idtdi

LUR

dtdi

LU pw

6 Earth fault voltage of return conductor consists of working voltage and lightning current-induced voltage. The voltage induced by lightning current in working cable in the line is determined by the relation 2.41.7 Steel line pole inductance Ls = (0,5 ÷ 1,2) x l, w μH [ 2]; it can be also determined using the formula for inductance of long coil (with the number of turns z = 1)

lRzLs22��

, where: l - pole length, z - number of turns, R - long coil radius [10].


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lightning with the neighbouring poles. Under this assumption, which additionally took into account the occur-rence of lightning arrester inductance and the poles of adjacent lines, it can be assumed that about 60% of the peak lightning current flows through the pole struck by lightning. Spread of the remaining 40% of the lightning current is symmetrical in both sides of the line. The calculation results are presented in fig. 5.

The obtained results lead to the conclusion that the comparison of two values, i.e. the pole earth fault voltage Uws with the withstand lightning impulse surge in the insulator Uw allows determining the acceptable value of pole earthing resistance Rs, at which the expected values of parameters describing the lightning current (ipmax i dipmax / dt do not cause a breakdown (fig. 5a). Thus, at the specified value of pole earthing resistance (e.g. arising from technical capabilities), it is possible to assess whether there is a threat to insulation due to the oc-currence of reverse breakdowns.

It should also be noted that in the case under consideration the insulators are tensioned in an electric field of the line with the voltage that is the difference between the earth fault voltage in the pole, which consists of voltage in the earth electrode and induction voltage drop in the pole, and the earth fault voltage of the return conductor, consisting of working voltage and lightning current-induced voltage. Therefore, in order to reduce the earth fault voltage in the pole, and thus reduce the influence of field on the insulation, earth electrodes with low impact resistances in supporting structures and/or supporting structures with low inductance (e.g. steel tubular poles) should be used. An example of dependence of the pole earthing voltage on its inductance is shown in fig. 5b. Using appropriately low values of inductance in the line supporting structures, it can be demonstrated that increase of the level of withstand lightning impulse voltage is unnecessary, which leads tooptimization of the margin betweenthe primary insulation level and protection level. Such action reduces the cost of insulation and protection.

Fig. 5. Illustration of impact of the diffe-rence in pole earth voltage and voltage drop in its inductance on the pole earthing resistance (a) and the pole inductance on the earth voltage in the case of a direct lightning striking the pole (b)

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b)

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htni

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impu

lse

volta

ge, i

n kV

Earthing resistance, in Ω

Pole

ear

thin

g vo

ltage

, in

kV

Pole inductance, in μH


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BIBLIOGRAPHYBIBLIOGRAPHY

Therefore, using the supporting structures with a low inductance, we can reduce the likelihood of reverse breakdowns and thus improve the reliability of these elements in the power network.

5. SUMMARY

The assessment of the correctness of ranking of the current strength levels in overhead power lines under consideration, the estimation of the breakdown risk in a self-regenerating line insulation and the evaluation of safety rules applicable in these networks showed that there are reasonable grounds for modifying the rules of insulation coordination and surge protection. In particular, the possibility of lowering the level of protection us-ing the current solutions in devices for surge protection was demonstrated. In practice, the choice of insulation and protection levels should also take into account the expected surge waveforms, the ageing of insulation, the impact of environmental factors and mutual location of protected and protecting devices. Although the actual electrical parameters for lightning surges generally differ (are more mild) from the values assumed for standard waveform (1.2/50 µs), the scope of these changes requires an individual assessment of each tested case, taking into account the requirements for electrical strength and breakdown risk.

It should be stressed that the deliberations based on the results of research, analysis and evaluations are the reason to take further rational actions towards modifying the insulation coordination rules, leading to the achievement of economic effects. The expected economic effects will be invested in both technical solutions and environmental impact. But the common benefit will result from the economy of implementation of power sector investment plans.

The conducted analyses also justify the use of steel solid poles, i.e. the so-called tubular poles as an al-ternative to steel lattice towers in the network construction. Undoubtedly, the structures of tubular poles taking into account the factors optimizing their construction, replacement of the traditional steel cross arms with the composite insulation systems, and improving the reliability of overhead lines are solutions that are more and more often used by operators of transmission and distribution networks in the country and abroad.

1. Flisowski Z., Technika wysokich napięć (ed. 5), WNT, Warszawa 2005.2. Gacek Z., Technika wysokich napięć. Izolacja wysokonapięciowa w elektroenergetyce. Przepięcia i ochrona przed

przepięciami (ed. 3), Draft of the Silesian University of Technology, no. 2137, Gliwice 1999.3. Jakubowski J.L., Podstawy teorii przepięć w układach elektroenergetycznych, PWN, Warszawa 1968.4. Kosztaluk R., Flisowski Z., Koordynacja izolacji polskich sieci wysokich napięć, Przegląd Elektrotechniczny, no. 2/

1998, pp. 41–45.5. Skomudek W., Computer analysis of overvoltage hazard due to lightning discharges in medium voltage overhead

lines with covered conductors, Journal of Electrical Engineering, vol. 55, no. 5-6, Slovakia 2004, pp. 161–164.6. Skomudek W., Assessment of overvoltage hazard for the polymer insulation of medium voltage electricity distribu-

tion cables. CIGRE Gen. Session 2008, rep. B1-201.7. Skomudek W., The Comparative Analysis of Lightning Overvoltages in Distribution Lines on the Ground of Laboratory

Tests and Measurements, Journal of Material Science, 3/2009.8. Skomudek W., Analiza i ocena skutków przepięć w elektroenergetycznych sieciach średniego i wysokiego napięcia,

Oficyna Wydawnicza Politechniki Opolskiej, Opole 2008.9. Skomudek W., Modyfikacja zasad koordynacji izolacji w sieciach wysokiego napięcia w aspekcie ekonomicznym,

Przegląd Elektrotechniczny, no. 11b/2010.10. Rawa H., Elektryczność i magnetyzm w technice, PWN, Warszawa 1994.


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