Behaviour of grounding loop with bentonite during a ground fault at an overhead line tower Z.R.Radakovic and M.B.Kostic Abstract: The results of a single-phase short-circuit experiment at a 35kV overhead line are presented. During the experiment, the grounding loops, backfilled with bentonite and waste drilling mud, were exposed to real ground fault currents. As a consequence of the increased thermal stresses of bentonite and waste drilling mud, only short time increases of the resistance of the grounding loops were registered. It is shown that, independently of the manner of the neutral point grounding in 35 kV and 1lOkV networks, as a backfill material, bentonite retains its positive characteristics even after being exposed to real ground fault currents. 1 Introduction Requests for low grounding resistances arise particularly in the case of transmission line tower footings. They assure not only the proper operation of high-speed protective relaying, but also a considerable reduction in the transmis- sion line outages caused by backflashes during lightning strikes [l]. Since it is often impossible (or too expensive) to reach the desired grounding resistance by adding more grid conductors, an alternative solution was found by using conductive backfills, i.e. in modifying the soil surrounding the grounding electrodes [2, 31. During the past few decades, bentonite has become the most popular backfill material. It has been shown previ- ously [2] that bentonite possesses practically all required positive properties: very low and stable resistivity; low cost; ability to hydrate and swell (absorbing and chemically retaining water in its structure); ability to adhere to any surface; and absence of corrosive activities to zinc and iron. Owing to these precious properties, the use of bentonite results in a significant reduction in the grounding resist- ance, especially during drought periods [4]. In addition, a multiple reduction in the maximum touch voltage has been recorded. Waste drilling mud, a major pollution problem worldwide, has also been presented [4] as an acceptable bacMill material. A comprehensive technical-economic analysis [5] resulted in very simple recommendations regarding the performance of the grounding system of 1 lOkV transmission line tower footings containing ben- tonite suspension or powder. Field tests also confirmed that bentonite belongs to the group of inactive corrosive materi- als [4, 51. An important aspect of the use of bentonite is its behav- iour after increased thermal stresses caused by ground fault currents (high-current discharges) [6]. The results of an investigation on the model of a grounding rod (length = 0 IEE, 2001 IEE Proceedings online no. 20010407 DO1 10.1049/ipgtd:20010407 Paper received 26th September 2000 The authors are with the Department of Electrical Enginering, University of Belgrade, Bulevar Revolucije 73, 1 1000 Belgrade, Yugoslavia lm, diameter = 9Omm), have been presented in [7]. The most important conclusion was that bentonite did not change its resistivity under conditions when the tempera- ture at the joint conductor-bentonite surface was up to 80°C. The experiments were performed on the model and did not take into account the real electrical contact resist- ance and cooling conditions. It is thus not possible to pre- dict bentonite behaviour when exposed to a high-current discharge. This is why 35 kV overhead line grounding sys- tems (with bentonite and waste drilling mud as backfill materials) were subjected to real ground fault currents. In this paper we present relevant results of the test. 2 currents Bentonite stresses when exposed to ground fault All elements in a short-circuit path are exposed to mechan- ical and thermal stresses. Mechanical stresses are dependent on a maximum instantaneous (peak) current, whereas ther- mal stresses are dependent on a thermal equivalent short time current (ZJ. The calculation procedures for peak and thermal equivalent short-time currents are given in IEC Standards [8, 91. Both mechanical and thermal bentonite stresses depend on these currents per grounding system length values (see Section 4), and thermal stresses also depend on bentonite resistivity. 3 Grounding systems under test The experimental site was selected near a 110kV/35kV sub- station, named Belgrade 17. Using Wenner's four-pin method [lo] and earth resistivity measurement interpreta- tion techniques [Ill, the site can be described by a two- layer soil. The upper layer is thin (H = 0.3m) and consists of an organic soil (p, = 15Qm), and the resistivity of the lower layer is p2 = 130Qm. Two equal grounding loops were installed at a depth of 0.5m at this site. Their dimensions (2.5m x 2.5m) are simi- lar to those existing in real 35kV and 1 lOkV overhead line tower grounding systems. The distance between the loops was 25m. They were made of the usual zinc-protected steel strips with a rectangular cross-section (30mm x 4"). The first one was backfilled with bentonite suspension and the second with waste drilling mud; about 10001 of backfill 275 IEE Proc.-Gener. Transin. Distrib.. Vol. 148, No. 4, July 2001
Transcript
Behaviour of grounding loop with bentonite during a ground fault at an overhead line tower
Z.R.Radakovic and M.B.Kostic
Abstract: The results of a single-phase short-circuit experiment at a 35kV overhead line are presented. During the experiment, the grounding loops, backfilled with bentonite and waste drilling mud, were exposed to real ground fault currents. As a consequence of the increased thermal stresses of bentonite and waste drilling mud, only short time increases of the resistance of the grounding loops were registered. It is shown that, independently of the manner of the neutral point grounding in 35 kV and 1 lOkV networks, as a backfill material, bentonite retains its positive characteristics even after being exposed to real ground fault currents.
1 Introduction
Requests for low grounding resistances arise particularly in the case of transmission line tower footings. They assure not only the proper operation of high-speed protective relaying, but also a considerable reduction in the transmis- sion line outages caused by backflashes during lightning strikes [l]. Since it is often impossible (or too expensive) to reach the desired grounding resistance by adding more grid conductors, an alternative solution was found by using conductive backfills, i.e. in modifying the soil surrounding the grounding electrodes [2, 31.
During the past few decades, bentonite has become the most popular backfill material. It has been shown previ- ously [2] that bentonite possesses practically all required positive properties: very low and stable resistivity; low cost; ability to hydrate and swell (absorbing and chemically retaining water in its structure); ability to adhere to any surface; and absence of corrosive activities to zinc and iron. Owing to these precious properties, the use of bentonite results in a significant reduction in the grounding resist- ance, especially during drought periods [4]. In addition, a multiple reduction in the maximum touch voltage has been recorded. Waste drilling mud, a major pollution problem worldwide, has also been presented [4] as an acceptable bacMill material. A comprehensive technical-economic analysis [5] resulted in very simple recommendations regarding the performance of the grounding system of 1 lOkV transmission line tower footings containing ben- tonite suspension or powder. Field tests also confirmed that bentonite belongs to the group of inactive corrosive materi- als [4, 51.
An important aspect of the use of bentonite is its behav- iour after increased thermal stresses caused by ground fault currents (high-current discharges) [6]. The results of an investigation on the model of a grounding rod (length =
0 IEE, 2001 IEE Proceedings online no. 20010407 DO1 10.1049/ipgtd:20010407 Paper received 26th September 2000 The authors are with the Department of Electrical Enginering, University of Belgrade, Bulevar Revolucije 73, 1 1000 Belgrade, Yugoslavia
lm, diameter = 9Omm), have been presented in [7]. The most important conclusion was that bentonite did not change its resistivity under conditions when the tempera- ture at the joint conductor-bentonite surface was up to 80°C. The experiments were performed on the model and did not take into account the real electrical contact resist- ance and cooling conditions. It is thus not possible to pre- dict bentonite behaviour when exposed to a high-current discharge. This is why 35 kV overhead line grounding sys- tems (with bentonite and waste drilling mud as backfill materials) were subjected to real ground fault currents. In this paper we present relevant results of the test.
2 currents
Bentonite stresses when exposed to ground fault
All elements in a short-circuit path are exposed to mechan- ical and thermal stresses. Mechanical stresses are dependent on a maximum instantaneous (peak) current, whereas ther- mal stresses are dependent on a thermal equivalent short time current (ZJ. The calculation procedures for peak and thermal equivalent short-time currents are given in IEC Standards [8, 91. Both mechanical and thermal bentonite stresses depend on these currents per grounding system length values (see Section 4), and thermal stresses also depend on bentonite resistivity.
3 Grounding systems under test
The experimental site was selected near a 110 kV/35 kV sub- station, named Belgrade 17. Using Wenner's four-pin method [lo] and earth resistivity measurement interpreta- tion techniques [Ill, the site can be described by a two- layer soil. The upper layer is thin (H = 0.3m) and consists of an organic soil (p, = 15Qm), and the resistivity of the lower layer is p2 = 130Qm.
Two equal grounding loops were installed at a depth of 0.5m at this site. Their dimensions (2.5m x 2.5m) are simi- lar to those existing in real 35kV and 1 lOkV overhead line tower grounding systems. The distance between the loops was 25m. They were made of the usual zinc-protected steel strips with a rectangular cross-section (30mm x 4"). The first one was backfilled with bentonite suspension and the second with waste drilling mud; about 10001 of backfill
material was poured into the channel of each grounding loop. The rest of each channel was then covered by the excavated soil.
Bentonite suspension was prepared using natural ben- tonite ores from Sipovo deposits (Serb Republic, Bosnia and Herzegovina). The activation was carried out using 4.5% Na2C03. The principal characteristics of the activated bentonite were: cation exchange capacity = 86meqllOOg bentonite number = 30ml Ateberg's plasticity index = 310 resistivity of 20% suspension = 2.552m
4 Power and measuring equipment
The basic elements of power and measuring equipment are presented in Fig. 1, where T = power transformer of rated power 63MVA, voltages llOkVl35 kVIlOkV, short-circuit voltage 11.3%, YyOd5 CB = circuit-breaker of rated current 600A, breaking capacity 12.1 kA, rated voltage 38 kV R,, = water resistor with resistance of 41.6552 at 24°C R,, = substation grounding system with resistance of about 0.0452 Cable 1 = IPZ013 3 x 95mm2, Cu, 35kV, 196m Cable2 = XHP48 1 x 25mm2, Al, 35kV, 120m Cable 3 = PPOO 1 x 70mm2, Al, 35kV, 3m CT = current transformer of rated voltage 35 kV, 2 x 300 AI5 AI5 AI5 A, with secondary winding 30VA, 0.5 Fs 10 used in the test VT = voltage transformer 35kV/lOOV, rated power 18OVA, maximum power lOOOVA R,-- = resistor with resistance of 0.5 Q RVT = resistor with resistance of 56Q R = disconnector of rated current 630A, rated voltage 38 kV Apart from the signal measuring by PC, a parallel measur- ing system was used (digital oscilloscope 2430 Tektronix, AM 503 current Tektronix probe amplifier, current probe Tektronix A6303) in order to increase the certainty of recording the measuring data and to verify their accuracy.
5 Measurement results
The ground fault tests were carried out on both grounding loops. Prior to the short-circuit experiments, using the fall- of-potential method [12], the following values of the
CB n
grounding resistance of the loops with bentonite and waste drilling mud were measured: Rbent = 3.9052 and Rwdm = 4.3852. In addition, the tripping time of the protective relaying was set to the maximal possible value, of T, = 1.5s. The purpose of such a protection setting was to obtain, in the given conditions, the maximal possible ther- mal impulse, i.e. the maximal thermal strain of bentonite (waste drilling mud).
The anticipated effective values of the ground fault cur- rents were approximately equal to the ratio of the phase voltage (35OOOVld3) and the sum of the resistance of the water resistor (R,,,.) and the grounding resistance of the loop (Rbent or Rwdm). They amounted to 443.6A and 439A for the grounding loops with bentonite and waste drilling mud, respectively. These values were determined under the assumptions that the value of the water resistor resistance is equal to that measured in a cold state, and that the ground- ing resistance of each grounding loop is equal to that meas- ured prior to the ground fault experiment.
The ground fault tests were performed twice (after an interval of 60 s) on both grounding loops. Current and volt- age sampling time was 1 ms. Figs. 2 and 3 show the current and voltage changes during the first ground fault tests on the loops with bentonite and waste drilling mud, respec- tively.
Table 1 contains the rms values of the current (Zrm) and the voltage (U,,), related to the time of a test duration (T,>. Note that practically the same values were obtained by the oscilloscope. The values of the ratio (UrJIrm) are also shown, presenting the loops' grounding resistance with high accuracy. The penultimate column contains the values of the grounding resistance increase with respect to those obtained prior to the ground fault tests. Since the thermal energy released in a backfill material amounts to
Eh = RI,,: T,, = RI,," (1) where R is the resistance of the backfill material, and since R - 111 [13] (1 = 10m is the circumference of each loop), the temperature increment of the backfdl material, representing a measure of the thermal stress, is directly proportional to the quantity (IthIo2, whose values are quoted in the last col- umn of Table 1.
Table 1 shows a significant increase in the grounding resistances with respect to the values measured prior to the tests. However, measurements of the grounding resistances (by applying the fall-of-potential method), made only 10 minutes after each of the repeated ground fault tests, showed that their values are close to those obtained before the tests. In the case of the grounding loop with waste
30m . .
, . . . . . . . . .
*i electrode >40m
Fig. 1
216
G r d fmlt test scheme
IEE Proc.-Gener. Transm. Distrib.. Vol. 148, No. 4, July 2001
500
I 250
4 E o 5
-250
-500
0 0.25 0.50 0.75 1.00 1.25 1.50 time, s
a
1 - --___- --
-6 -5 > 0 0.25 0.50 0.75 1.00 1.25 1.50
time, s b
Fig.2 bentonite n Current b Voltage
Current and voltage chunge5 during ground fault test on loop with
500
250 U L
= o : U
-250
-500
0 0.25 0.50 0.75 1.00 1.25 1.50 time, s
a _____ .-
4
3 2
2 1 6 0 3 -1 9 -2
-3 I
-4 I
-6 -5 1 0 0.25 0.50 0.75 1.00 1.25 1.50
time, s b
Fig.3 waste drdling mud n Cuirent b Voltage
IEE Pioc -Genet Ttanmtiz D i w i b , Vol 148, No 4, July 2001
Current and voltage chunges during ground fault test on loop with
drilling mud the difference was unnoticeable, while in the case of the grounding loop with bentonite a value of 4Q was obtained. A value equal to that obtained prior to the test (3.9Q) was measured after the next 25 minutes.
Simultaneously with the first measurement of the resist- ance of the grounding loop with bentonite made after the test, the temperature measurement was started at this grounding system. The thermocouple junctions were placed in the bentonite block at five selected points (Fig. 4). In the 20 minutes period since the beginning of the measurement, slight drops in temperature were recorded at both strip measuring sites; 13~ decreased from 12.3"C to 11.7"C, and 7Y4 from 13.5"C to 12.9"C. However, the remaining meas- ured temperatures retained approximately constant values
= 11.2"C. During the test the air temperature was 6°C. The control measurement of temperatures was made a week after the test, and the following constant values were recorded: a1 = 1O"C, 82 = 9.8"C, a3 = lO"C, a4 = 10.1"C and 19~ = 10°C.
= 10.3"C, 19~ = 10.8"C, and
/ I
I
Fig. 4 I = inner edge of channel horizontal cross-section, 11 = strip, I11 = outer edge of channel horizontal cross-section
Dkposal of thermocouples ut groundkg .rystem with bentonite
6 Discussion
The test results showed satisfactory features of the consid- ered backfill materials (bentonite and waste drilling mud) when they are exposed to thermal strains due to ground fault currents. Since waste drilling mud is not dense, it only soaks the soil around the grounding loop, improv- ing the contact between the grounding strips and the sur- rounding soil, as well as the soil conductive properties. This is why it was assumed that the ground fault current
211
would not have any damaging effect on it. As bentonite suspension forms a monolithic block, the verification of the behaviour of bentonite exposed to the ground fault current was a primary goal.
The ground fault test was made in the network with a neutral point grounded by means of impedance (the water resistor of resistance Itw,.). This is why the ground fault cur- rent was considerably smaller than in the case of a network with a directly grounded neutral point. For example, if the neutral point was directly grounded and the soil resistivity was 175 Qm (the minimal value when bentonite suspension or powder should be applied [5]), the ground fault current would be about 2.5 times higher than that obtained during our experiment. However, as the total real time of reaction of overcurrent protection (including the circuit-breaker opening time) is about 0.2s, even in this case the value of (Ith/02 is less than in the case of our ground fault experi- ment. In addition, if we take into consideration the ground fault current decrease due to the existence of the overhead ground wire, as well as the fact that two successive ground fault tests were carried out for each loop, it is obvious that the value of the quantity (Ith/02 obtained in our test is greater, and in the majority of cases much greater, than the values obtained in practice. A simple analysis shows that it is also representative in the case of 1 lOkV networks.
The increase in the grounding resistance of the loop with bentonite recorded during the ground fault reduces the ground fault current and the protective relaying reliability. However, this negative effect (which is considerably smaller in real bentonite applications, at bigger soil resistivities) is multiply compensated by the decrease in the grounding resistance compared to the conventional grounding systems without backfill materials [4].
Several days after the test, diggings were made, showing that the bentonite suspension was still in the form of a monolithic block, containing no cracks. The peak values of the ground fault current in the 11OkV networks and in directly grounded 35kV networks are higher than those obtained during the test. Since they are considerably less than those characterising lightning strikes, and since the test is scheduled to investigate the behaviour of bentonite exposed to currents whose intensity, waveshape and dura- tion are similar to those existing at atmospheric discharges, this aspect of the bentonite application was not analysed here.
7 Conclusions
To check if positive properties of bentonite and waste drill- ing mud are retained after their exposure to ground fault
currents, real ground fault tests were carried out in a 35 kV network. As a consequence of the increased thermal strain of bentonite and waste drilling mud, only short-term grounding loop resistance increases were registered. Analy- sis has shown that the conclusion of the steadiness of posi- tive properties of bentonite is valid, independent of the type of 35kV and llOkV networks. The scheduled investigation of the behaviour of grounding systems backfilled with ben- tonite during lightning strikes would complete the charac- teristics of bentonite relevant for its practical application.
8 Acknowledgment
The authors would like to thank Mr. Nebojsa S. Radovanovic, Department of Electrical Design, Power Utility of Belgrade, for his kind help during the experimen- tal stage of this research.
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References
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