Earthing of Electrical Devices and Safety

Transcription

1 Earthing of Electrical Devices and Safety JOŽE PIHLER Faculty of Electrical Engineering and Computer Sciences University of Maribor Smetanova 17, 2000 Maribor SLOVENIA Abstract: - This paper presents the importance of electrical devices earthing for safety of qualified personnel responsible for operation and maintenance of electrical devices; humans and animals which incidentally approach electrical devices or individual parts of electrical installations. For designers and users it is important that they are acquainted with theoretical backgrounds and voltages that may appear on earthed structures, possible consequences of these voltages and measures for prevention of dangerous impacts on living creatures. The results of laboratory tests are shown some examples if these measures are not taken into account. Key-Words: - Electrical devices, earthing, touch voltage, step voltage, safety. 1 Introduction Electric power system is one of the most complex systems in the nature. This system has to operate all the time reliably and without interruptions. In addition to this it has to be safe for people responsible for operation and maintenance of electrical devices. Earthing systems play an important role both in operational security and in providing safety to people and other living creatures that incidentally approach power system devices. The earth electrode as the most important part of the earthing system plays an important role in power system design. Earthing can have a significant impact on overvoltages and insulation stress in electrical devices. It also directly determines the magnitudes of short-circuit currents. Overvoltages and potential rise on the earth electrode can also appear due to lightning strike to the overhead earth wire of a power line. A certain share of current flows through the line tower to the earth electrode. A reflective voltage surge or damage can occur on the insulator chain. Between the affected phase and the metal cross arm of the line tower earth fault occurs. The fault current depends on the network capacity. A damage of insulator chain units or a sparkover on their surface is also a possible cause of puncture of the insulator chain. In this case the short circuit current also flows through the line tower and the earth electrode to the ground. The established current field causes potential rise inside the potential funnel due to resistance of the earth electrode. A potential rise can also occur on the transmission tower structure due to increased resistance between the elements of the line tower truss. This resistance can increase due to the poor quality execution of corrosion protection of steel structure (hot-dip galvanisation) or due to the colour application to the joints between the elements of the line tower truss. Each occurrence of overvoltage or potential rise causes increased touch and step voltage. If their values exceed the maximum permitted values, they pose a direct threat to humans and animals. Therefore it is of utmost importance that power system designers are aware of the conditions that may appear in order to prepare adequate measures for mitigation of impacts of potential rise on the earthing systems. The primary purpose of this paper is to present some examples of laboratory tests if these measures are not taken into account. The second section of the paper gives an overview of voltages that can appear on earthing systems. The third section presents the basics of earth electrodes design. The fourth section brings an estimation of possible voltage drops and consequently potential rise on earthed structures of electrical devices. Given are results of measurements of ohmic resistance on steel structures of line towers, as well as some results of practical tests with short circuit currents. 2 Voltage that may appear on earthing systems A deviation of voltage conditions in the network from the designed values can occur due to several different reasons. In most cases short- or long-term ISBN:

2 overvoltages occur due to switching manipulations or due to atmospheric discharges. Another source of overvoltages or voltage deviations from the designed values can be faults in the network. In case of a fault in the network potentials and voltages that are significantly higher than the designed values can occur in various parts of the network. Overvoltage is defined as a voltage between different parts of the network or between individual parts of the network and the ground, which is higher than the nominal voltage prescribed by the standard IEC [1], taking into consideration ±10 % tolerance band. Overvoltages that occur in the network have internal (operational) or external (atmospheric) source. Overvoltages can be different; therefore they are described with certain characteristics: amplitude, waveform or front time, duration, probability of occurrence, etc. According to the IEC [2] standard overvoltages are with regard to the cause of occurrence and overvoltage waveform classified in the following main groups: Low frequency continuous and temporary overvoltages. These are power system frequency overvoltages. Sources of them are earth faults, outages of large consumer units, ferroresonance or any combination of them. Slow-front transient overvoltages. These are overvoltages that appear during switching manipulations of power system elements (e.g. switching on or off of power lines) or in case of interrupting earth fault or short circuit currents. Fast-front and very-fast-front transient overvoltages. These are overvoltages that appear in case of lightning strike to an overhead line. Lightning strike can occur as a direct strike to the overhead earth wire, direct strike to the phase conductor or a strike in the vicinity of the line. The selection and design of the earthing system have an important impact on the overvoltage magnitude and duration. The earthing system has an impact on overvoltage magnitude, operational characteristics of the network, safety of users and operators of power devices, as well as of animals that can be found in the immediate vicinity of any electric power system facilities. The design of earthing systems is an extremely important part of power system design, since it has a significant impact on transient overvoltages, insulation stress and safety. It also directly determines the order of magnitude of fault currents. With regard to the method of earthing we distinguish networks with insulated neutral point, networks with resonant earthing, indirectly earthed networks and effectively earthed networks. In addition to the above stated it is of extreme importance to earth metal structures of individual power system elements, such as line towers, to ensure adequate level of safety. An overvoltage can cause local rise of earth potential, which is the voltage between the earthing system and the remote reference earth. The earthing system is a locally limited system of electrically connected earth electrodes or earthing conductors and equipotential bonding conductors or in the same way operating metal parts, such as line tower legs, metal reinforcements in concrete, metal cable coatings, etc. The reference earth (remote earth) is that part of the earth that is outside the area of influence of the earthing system or the earth electrode, where there is no significant perceivable potential difference due to currents in the earth between any two points. Potential rise of the earthing system can also occur due to the current caused by a lightning strike. Potential rise of the earthing system and of the nearby earth above permitted values can cause injury or death of living creatures, either by direct contacts or indirectly. Important criteria that define hazard of potential rise are touch and step voltage. Touch voltage: A part of earth potential rise that is transferred through the human body between hand and foot (it is supposed that the horizontal difference from the exposed part of the device amounts to 1 m). Step voltage: A part of the earth potential rise that can be bridged by a 1 m long step, which means that current flows through the human body from one foot to another [3]. 3 Designing of earth electrodes Earth electrodes are the most important elements of earthing systems of electric power networks and devices. They represent an extremely important part of operation and protection system against hazardous voltages and currents for people and animals. They need to be designed in such a way that in case of an earth fault the potential of the earth electrode does not rise to the magnitude that would cause excessive touch and step voltage on the earth surface. The level of the voltage rise depends on the earth electrode resistance. This can be to the maximum possible extent reduced by the adequate allocation of earth electrodes. This is therefore the primary protection measure against excessive touch and step voltages. This measure is successfully implemented in earthing of substations where the adequate allocation of earth electrode enables ISBN:

3 management of electrical potential on the earth surface. To a certain extent this can also be implemented in earthing systems of a line tower, although this is not a sufficient condition for protection against excessive touch and step voltages. resistance is immediately next to the rod. The graph of surface potential distribution (in the actual 3D space this is a potential funnel) is given in Fig Soil resistivity The designing of earth electrodes is closely connected to the soil resistivity. Soil resistivity varies with regard to the soil composition in the case of the same humidity. Varying the soil humidity additionally changes the earth resistivity. Earth electrodes that are laid in the foundations of buildings have a good conductance with regard to the surrounding earth, since their resistance is practically independent of the soil humidity, which may be drastically reduced during dry periods. Characteristic properties of soil can significantly change up to the depth of 3 m, while in deeper layers the conditions are much more stable. Soil resistivity also changes during the year. 3.2 Earth electrodes Earth electrode is a conductor or a group of conductors that are in the immediate contact with the earth and ensure electrical connection with it. Very important is voltage drop between the earth electrode and the remote reference point in the earth potential of the earth electrode. A distance of 200 m is usually enough for setting the remote reference point. The current density near the earth electrode is high, but since the cross section of the earth increases rapidly with the distance, the current density at the distance of 200 m practically equals to zero. To facilitate the observation of developments let s assume that the earth resistance is much higher than the resistance of the metal rod and that the current density is constant. The picture is more transparent taking into assumption homogenous soil and constant earth resistivity in horizontal and vertical direction. In practical applications we use various types of earth electrodes: horizontal strip earth electrode vertical rod earth electrode, foundation earth electrode, circular plate type and other earth electrodes. When the current flows through a metal rod to the earth, the current flow lines distribute radially the available cross section increases. The highest Fig. 1. Potential distribution and potential funnel of a circular plate type electrode. It is important to achieve such a potential distribution that touch and step voltages never exceed values that are dangerous for human beings. 3.3 Dimensioning of earthing systems with regard to human safety The standard [3] defines requirements for design, installation and testing of earthing systems with the purpose to ensure their operation in all conditions and limit the values of touch and step voltages at an acceptable level. An earthing system should fulfil the following five requirements: a) ensure mechanical strength and corrosion resistance, b) withstand thermal load with the maximum computed fault currents, c) prevent damage of property and equipment, d) ensure safety of people from voltages occurring on the earthing system in the case of an earth fault, e) ensure adequate degree of reliability of overhead lines. The decisive parameters for designing an earthing system are: magnitude of fault current, duration of fault current, soil properties. People are threatened by the electric current passing through the human body. The publication IEC [4] gives instructions about the effects of current passing through the human body, depending on current magnitude and duration of current passing through the body. In practice it is easier to present this dependence with maximum permitted values of touch voltage. Maximum permitted touch ISBN:

4 voltage (potential difference) is the value that can be applied along the body between hand and foot that, depending on duration, does not cause permanent consequences for the human being. Each earth fault needs to be interrupted either automatically or manually. Therefore the duration of occurrence of touch voltages is not infinite. If the earthing system is fully compliant with the requirements regarding the touch voltage, it can be assumed that there will be no dangerous step voltages [3]. (joint b). These two joints were tested in the laboratory with the 16 current lasting for 1 s. The time diagrams of current and voltage drops are shown in Fig Assessment of possible voltage drop on earthed structures of electrical devices Earthing systems comprise, in addition to the so far discussed earth electrodes and soil resistivity, the actual earth resistance, composed of earthing conductor resistance, earth electrode resistance, contact resistance between the electrode and the surrounding soil and soil resistivity. Earthing conductor resistance is the resistance of connecting cable, overhead conductor or a part of the line tower structure. All these elements represent a conductor through which the fault current or lightning surge current will flow through the earth electrode to earth. Fig. 2. Consequences of current test 16, 1 s. 4.1 Occurrence of induced voltages and voltage drops due to poorly designed or executed earthing Lightning strikes or earth faults can cause large currents in the line tower truss. The current will in both cases cause: magnetic field that will in closed loops cause induced voltages; and voltage drop due to resistance on the joints in the line tower truss. The induced voltage and voltage drop can be higher than the maximum permitted values and may cause injuries to human beings and animals. 4.2 Current load of line tower joints The effect of poor contact on the connecting conductor to the earth electrode was analysed by setting the actual joining elements of the line tower as shown in Fig. 2. We put together two joints, where the first was composed of unpainted and painted element (joint a) with the tightening torque of 130 Nm. The second joint was composed of two painted elements, screwed with the same tightening torque of 130 Nm Fig. 3. Time diagrams of current and voltage drops on the joints. During the tests the screws melted (Fig. 2) and an electric arc appeared [5]. Sparkling metal parts flew several metres away. The joint was completely destroyed and lost all required mechanical properties. Voltage drop on the joints exceeded 100 V. The main reason for this was too large contact resistance of the joint. 4.3 Computation of equalisation currents through the earth wire or earthed phase conductors and estimation of voltage drop on poorly joined parts of the structure The occurrence of equalisation currents in the earth wire or in the earthed phase conductors can be ISBN:

5 caused by: a) short circuit on another circuit of the line that is in operation; or b) atmospheric discharges Short circuit Let s assume a double-circuit overhead line with one circuit being switched off and earthed due to maintenance and another being in operation and energized. In the case of a short circuit fault in the operating circuit we assume that the values of equalisation current will not exceed the values of short circuit currents on the line during operation. The values of these currents through the earthing system can only be lower due to the presence of contact resistances on the connection points. The current magnitude also significantly depends on the method of insulation of the network neutral point. The computed potential difference on the line tower amounts to: ΔU = I st R st = Ω = 4,6 kv, (1) where I st is the maximum short circuit current that can flow to earth and R st is the resistance of metal parts of the line tower, if they are painted. This voltage drop that could appear in the line tower is deadly for people and animals Atmospheric discharges Atmospheric discharges causes surge currents of 8/20 µs waveform that can reach amplitudes in the range from 2 to 270, which is up to now the highest recorded value. The average value of 25 is usually used in practice. Table 1 from the IEC standard [6] gives the frequency of lightning currents through a line tower of the overhead line equipped with an earth wire. The table confirms the values that actually occur in practice. If 20 is assumed as an average value of current flowing through the line tower, as given in Table 1, 80 % of all lightning strikes are encompassed. Table 1. Frequency of current magnitudes through a line tower of the overhead line equipped with an earth wire. Current I st through the tower up to Cumulative frequency of all lightning strikes % 90 % 95 % 98 % 99 % observations are needed to determine the actual distribution of lightning currents in an overhead line route. The equation (1) is used to calculate the potential difference on the line tower for 20 current: ΔU = I st R st = Ω = 5,8 kv. The calculation results shows that the potential rise is high above the permitted values. 5 Conclusion The purpose of the paper was to warn the designers of electric power systems against overvoltages that may appear on earthing systems of the electrical devices. By means of measurements the values of resistance between individual elements of steel structure of line tower was established. On the basis of the predicted possible values of short circuit currents the potential rise was calculated. It was much higher than the permitted values of touch voltage. The test performed with the short circuit current through the steel structure with a high contact resistance lead to damaging and destruction of the structure. References: [1] IEC standard voltages, IEC 60038, 2009/06. [2] Insulation co-ordination - Part 1: Definitions, principles and rules, IEC , [3] Overhead electrical lines exceeding AC 45 kv Part 1: General requirements Common specifications, EN , [4] Effect of current passing through human body Part I: General aspects, IEC TS , [5] Test report of ICEM-TC Laboratory, 12-Mlab- 162, Maribor [6] Overhead electrical lines exceeding AC 45 kv- Part 3-21: National Normative, Aspects (NNA) for Slovenia (based on S IST EN :2002), SIST EN , The values in Table 1 are used in absence of actual distribution of frequencies of lightning currents, obtained by modern systems for their localisation. Several years of ISBN: