Experimental Study of Electrical Breakdown

Voltage of a Glass Insulator Strings with

Different Numbers of Broken Units
L. R. P. Leite(1), J. A. Yanaguizawa(1), A. H. Shinohara(1), E. G. Costa(2), G. J. V. Xavier(3) and D.A. Maciel (3)
(1) UFPE - Federal University of Pernambuco, Dept. of Mechanical Engineering, 50740-530, Recife-PE, Brazil.
(2) UFCG - Federal University of Campina Grande,
Dept. of Electrical Engineering, 58109-970, Campina Grande-PB, Brazil.
(3) CHESF – Companhia Hidrelétrica do São Francisco, 50761-901, Recife-PE, Brazil

Abstract-- This work presents results of a comparative study The insulation of tempered glass has excellent dielectric
conducted on breakdown voltage measurements in a glass properties. In insulators type cap and pin, the pin and cap are
insulators string containing several units with broken glass made of forged steel and are fixed by cement. The
part. String with 16 insulator units were tested simulating a
arrangement allows the insulation withstand mechanical
transmission line of 230 kV. Two arrangements have been
adopted: (i) seven good insulators and nine broken units and, loads exceed 240 kN, depending on the type [7]. In an
(ii) six good insulators and ten broken units. The electrical electrical point of view, a glass insulator for 230 kV lines
breakdown voltage measurements have been carried out in dry behaves as an RC circuit [4], exhibiting 80 kV of flashover
and without pollution, in five different configurations. The voltage (12 kV when broken) in dry conditions and under no
experimental works have been conducted in industrial pollution [7].
frequency and with switching impulse with positive and
In the events of the dielectric being destroyed, perhaps
negative polarities. As a result, depend on the distribution and
localization of the good insulators in the chain, the breakdown due to rifhe fire, resulting in the skirt being completely
voltage values of the chain with six good insulators was higher broken away, the mechanical security of the broken
than the chain with seven good insulators. A string with six insulator is assured by the design of metal fittings [13]. The
good insulators in configuration #4 showed higher value of the partial and total breaking of the glass insulator strings by
electrical breakdown voltage than a string with seven good vandalism is responsible for a significant number of non-
insulators in configurations #1 and #2.
programmed disconnections by the companies of the
electrical field, reaching the percentage of 75% [5]. The
I. INTRODUCTION
damages also can cause the programmed disconnection
The electric insulators are used in transmission lines to when the number of good insulators in the string is less than
support the cables and isolating them of the ground. the minimum number established by the company
Consequently, the insulators, individually or grouped in a regulations. Therefore, the amount of remaining insulators
chain, must present enough mechanical resistance to in the string does not offer minimum security to the
maintain the weight other insulators, effect wind and maintenance and intervention in energized lines to substitute
temperature, and forces of the short circuit. In addition, the the damaged units.
insulators string reduces the mechanical vibrations For example, according to the CHESF, a Brazilian
transmitted to the tower and facilitates their maintenance [1- electrical company, procedures for electricians of hot line
3]. are allowed to work in an energized 230 kV line only when,
The insulators in a transmission line are subject to at least, seven good insulators remain in the string,
vulnerable environment, such as mechanical loads, high independently of their position, however it is essential
electric field, vandalism and the action of climate [1-3, 6]. considering atmospheric the good conditions to work. The
Some types of insulators commonly used are: tempered minimum number of good glass insulator units in a string
glass, porcelain and polymeric (or composite). Specifically, for live work was reported by [8].
non-composites insulators possess a relatively long useful The new rules and most rigorous regulations imposed by
life and can be used for more than 40 years. the Brazilian government to the electric field intend to
The essential components of the suspension insulator are improve the quality and continuous supply of energy to
the cap, pin, glass dielectric and cement. The upper part of customers.
the pin is tapered having an angle closely matching he slope
of the lower part of the inside of the cap. As such, when the In this context and with the aim to minimize the non-
insulator is loaded in mechanical tension, the tapered portion programmed disconnections of energy supply caused by
of the pin transmits force through the cement layers to the vandalism on the transmission lines, which remain
wall of the cap, which provides the restraining force. The energized after suffering the attack from vandals, the present
tension force can be resolved into a compressive force work describes a comparative experimental study of
across the dielectric. The design of the glass suspension electrical breakdown voltage using two strings of glass
insulator takes advantage of the fact thar the mechanical insulators with different number of broken units. The
strength of glass is greater in compressive than in tension. comparison was carried out in terms of distribution of good
[11] insulators along the string, as shown in Figures 1 and 2.

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 II. MATERIALS AND METHODS
To reproduce the most probable real conditions in the
field, ten string configurations have been mounted for 230
kV transmission lines, which have a concrete tower without
ring equalizer and composed by sixteen insulators. All the
insulators tested were tempered glass and shell-ball type,
with 254 mm of diameter and produced by a world-wide
supplier. Figures 1 and 2 show the configurations tested
with six good insulators and ten broken units, while Figure 3
and 4 present the configurations with seven broken
insulators and nine good units. All the insulators were
randomly selected for this study. Figure 5 shows the photos
of a typical glass insulator in its good and broken conditions.
Configuration 1 presented all the good insulators placed
together in the beginning of the string (next to the ground
terminal); Configuration 2 had all the good insulators placed
together in the end of the string (next to the phase terminal);
Configuration 3 presented all the good insulators placed
together in the middle of the string; Configuration 4 was
formed by a quasi uniform distribution of the good
insulators along of the string; Configuration 5 had the
broken insulators disposed in the middle of the string.
The measurements of electric breakdown voltage were
conducted at the IEE-USP Electro technical and Energy Fig. 1. Schemes of the tested configurations with six good insulators and
Institute of the University of São Paulo, Brazil, using ten broken units.
industrial frequency (60 Hz) and switching impulse (positive
and negative polarities), in dry conditions and under no
pollution.
For switching impulse tests, that simulate a severe
operation condition in a transmission line, the Haefely
impulse generator of 15 training periods was used, having 3
MV of maximum voltage. For industrial frequency tests, the
Toshiba and General Electric cascade transformers with 4
training periods were used.
The humidity, temperature and atmospheric pressure had
been determined around 70%, 20°C and 703 mmHg,
respectively. The electrical breakdown values were
corrected to the standardized atmospheric conditions
according to the NBR 6936-1992 Brazilian Technical
Standard [9, 12]. A number between 50 and 320 breakdown
voltages measurements were carried out in industrial
frequency and switching impulse, respectively.
Fig. 2. Tested configurations with six good insulators and ten broken units
III. RESULTS AND DISCUSSION

A. Strings with six good and ten broken insulators B. Strings with seven good and nine broken insulators
The five configurations of six good and ten broken The five combinations of seven good and nine broken
insulators in a string (Figures 1 and 2) showed different insulators in a string (Figure 3 and 4) also presented
electrical breakdown voltage values, as shown in Figure 6. different breakdown voltage values, as shown in Figure 7.
For configuration #5, the electrical breakdown voltage Here, also, for configuration #5, the electrical breakdown
measurement was carried only out in the industrial voltage was carried only out in the industrial frequency.
frequency. As a result, configuration #4 presented the Again, the Configuration #4 presented the best performance
highest values of breakdown voltage in industrial frequency during breakdown measurements, reaching 505 kV in the
(459 kV) and of switching impulse (663 kV and 728 kV). industrial frequency and in switching impulse, the electrical
On the other hand, Configuration #1 appeared as the lowest breakdown voltages were 661 kV and 894 kV, respectively.
value (396 kV for industrial frequency; 474 kV and 704 kV The Configuration #1 exhibited the worst performance (449
for switching impulse). Based on the observations, the kV in industrial frequency; 584 kV and 741 kV in switching
highest value in configuration #4 can be attributed to the impulse).
longest path to flashover.

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 C. Comparison and discussion of electrical breakdown
voltages between strings with six and seven good insulators
The experimental results evidenced that the strings
constituted by spaced and intercalated good insulators
(Configuration #4) work in an equal distribution of the
electric load along the string. According to the Paschen’s
Law (formulated in 1889), the electrical breakdown voltage
is a function of pressure and gap distance. Configuration #4
seems to be performed in a way to have the largest gap
between the insulators in relation to the other configurations,
causing a longer conduction way to the electric discharge.

800
734 728
704 696
700 663
600

Breakdown Voltage (kV)

600
519
474
500 459 446
423 412
396
400

300

200

100

0
1 2 3 4 5
Configurations

Industrial frequency Switching impulse (positive polaritie) Switching impulse (negative polaritie)
Fig. 3. Scheme of the tested configurations with seven good insulators and Fig. 6 Breakdown voltages in a string with six good and ten broken
nine broken units. insulators, using five configurations.

1000
894
900

800 766
741 732
Breakdown Voltage (kV)

700 661
623
584 588
600
478 505 496
500 448 455
400

300

200

100

0
1 2 3 4 5
Configurations

Industrial frequency Switching impulse (positive polaritie) Switching impulse (negative polaritie)

Fig. 7. Breakdown voltages in a string with seven good and nine broken
insulators, using five configurations.

Differently from literature [10], the breakdown voltage of

Configuration #2 was exceeded by the one of Configuration
Fig. 4. Pictures of tested configurations with seven good insulators and nine #4, evidencing that the proximity of the good insulators to
broken units. the conducting cables did not improve the electrical
performance of the string. Figure 8 shows the values of the
breakdown voltage obtained for all the configurations tested
in industrial frequency. An important observation can be
extracted from the comparison between the strings with six
and seven good insulators, in terms of the variation in the
electrical breakdown values as a function of the number of
good units: strings with seven good insulators had equal or
greater (or slightly inferior) breakdown voltage than the
strings containing six good insulators. This indicates that
Fig. 5. Photos of typical glass insulators for 230 kV transmission lines: there is a divergence between the established conditions by
good (left) and broken (right) units.
the electric companies to the minimum safety maintenance
in hot lines and the real working conditions, which means a
more detailed study is needed to elucidate the involved
mechanisms in energized lines with shorter number of good

293
insulators and the dependence of their position along of the ACKNOWLEDGMENT
chain. Nowadays, it is established by the CHESF procedures The authors thank the Electro technical and Energy
that seven good insulators is the minimum number of good Institute of the University of São Paulo for the breakdown
units for safety intervention by the electricians. The results voltage experiments. We also thank the continuous support
presented in this work also corroborated the need of of Eng. Osveraldo Vilar F. Lima, head of the Department of
developing a protection device for insulators to minimize Transmission Lines Maintenance of CHESF. We also thank
the damages caused by the vandalism in transmission lines. Miss Maria Paula Moura Pini for critical reading of English.
Considering the best electrical performance of configuration
#4, such protectors could be placed in the string in a manner REFERENCES
to avoid the breaking of glass insulators placed at the
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insulators along the chain. Configuration #4, presenting the
good insulators uniformly distributed along the string, had
the highest values of breakdown voltage. From the present
study, the values of the electrical breakdown voltage of the
string with six good insulators and in configuration #4,
presented higher value of the electrical breakdown voltage
than a string with seven good units in configurations #1 and
#2.
The development of a device to protect the strategic good
glass insulators in a string from vandalism becomes a goal
to be enhanced by the energy suppliers, and also an
interesting method to prevent the damages caused by
vandals in transmission lines. Because of the promising
results regarding the possibility of working with less number
of remaining good insulators in a string, compared to the
current amount adopted by the CHESF, further studies are
needed on the minimum number of insulators to guarantee a
safe intervention in hot lines, including the strings for 500
kV or higher voltage of transmission lines.

294