Methodology and Technology for Power System Grounding (He/Methodology and Technology for Power System Grounding) || Fundamental Concepts of Grounding

Download Methodology and Technology for Power System Grounding (He/Methodology and Technology for Power System Grounding) || Fundamental Concepts of Grounding

Post on 20-Dec-2016

222 views

Category:

Documents

6 download

Embed Size (px)

TRANSCRIPT

<ul><li><p>1Fundamental Conceptsof Grounding</p><p>1.1 Conduction Mechanism of Soil</p><p>1.1.1 Soil Structure</p><p>Soil is a complex system, consisting of solid, liquid and gas components. The solid phase of normal soil</p><p>usually includes minerals and organic matter; the liquid phase means the water solution and the gas</p><p>phase is the air between the solid particles. The solid phase makes up of the basic structure of the soil,</p><p>the liquid and gas phases fill the voids within the structure, as shown in Figure 1.1. Different from</p><p>normal soil, a new kind of solid material, ice, is present in frozen soil.</p><p>Soil conductivity is strongly determined by water content and water state. According to the distance</p><p>from solid particles and the electrostatic force received from solid particles, the water in soil can be</p><p>classified into the following types [1]:</p><p>Strongly Associated Water. Near the surface of soil particles, the water molecules cram together</p><p>closely and cannot move freely due to the great intensity of the electrostatic field. This type of water</p><p>is called strongly associated water.</p><p>Weakly Associated Water. Being farther from the soil particles, the intensity of the electrostatic field</p><p>has comparatively decreased, so the water molecules are more active and weakly oriented. This type</p><p>of water is still mainly affected by the electrostatic field and is called weakly associated water.</p><p>Capillary Water. As the distance between soil particles and water molecules increases, the water mol-</p><p>ecules become mainly affected by gravity. Although the electrostatic field still plays a role, it does</p><p>not have a primary function. This type is called capillary water.</p><p>Gravity Water. As the distance between soil particles and water molecules continues to increase, the</p><p>effect of the electrostatic field becomes negligible to the water molecules, and the water molecules</p><p>are only controlled by gravity. This type is called gravity water or ordinary liquid water.</p><p>1.1.2 Conduction Mechanism of Soil</p><p>Research has shown that soil conductivity falls with dropping temperature. This can be explained by</p><p>the theory of electrochemistry [2,3], because the electrical conduction in soil is predominantly electro-</p><p>lytic conduction in the solutions of water-bearing rocks and soils. Accordingly, the resistivity of soil or</p><p>rock normally depends on the degree of porosity or fracturing of the material, the type of electrolyte</p><p>Methodology and Technology for Power System Grounding, First Edition. Jinliang He, Rong Zeng and Bo Zhang. 2013 John Wiley &amp; Sons Singapore Pte. Ltd. Published 2013 by John Wiley &amp; Sons Singapore Pte. Ltd.</p></li><li><p>and the temperature. Metallic conduction, electronic semiconduction and solid electrolytic conduction</p><p>can occur but only when specific native metals and minerals are present [28]. Similar to the solid</p><p>medium, frozen soil is obviously distinguished from normal soil.</p><p>Because of the charges and ions attracted onto the surface of soil particles, soil can be considered as</p><p>a polyvalent electrolyte. Soil conductance is the contribution of both charged soil particles (known as</p><p>colloidal particle conductance, mainly decided by the amount of charge on the surface of soil particles)</p><p>and ions in solution (known as ion conductance, mainly decided by the diffusion velocity of ions).</p><p>When ions diffuse into the soil solution, the diffusion velocity is affected by the resistance of the water</p><p>molecules. As the temperature drops, the water becomes more viscous and its diffusion becomes slower</p><p>because the resistance of water molecules increases. In contrast, the ions are affected by the soil elec-</p><p>trostatic resistance. As the temperature lowers, the average kinetic energy of ions decreases and the</p><p>capacity to overcome the soil electrostatic resistance also decreases and the diffusion velocity slows</p><p>up. So, ion conductance decreases and soil resistivity increases as the temperature drops.</p><p>When the soil temperature decreases to 0 8C or even lower, most of the water in the soil is frozengradually and the ice (with high resistivity) fills the voids between the soil particles in the form of grains</p><p>or laminas, so the conductive cross-section of soil reduces. The thickness of the water film coating the</p><p>soil particles is reduced and the activity of the water molecules becomes weak. So, the resistivity of</p><p>frozen soil is significantly higher than that of normal soil. When the soil is chilled to a much lower</p><p>temperature, most of the soil water is frozen and the ion conductance created by ion movements gradu-</p><p>ally disappears. Finally, there would be only colloidal particle conductance created by the charges on the</p><p>surface of soil particles, which is not related to temperature, so a saturation phenomenon appears.</p><p>1.2 Functions of Grounding Devices</p><p>1.2.1 Concept of Grounding</p><p>Grounding is provided to connect some parts of electrical equipment and installations or the neutral</p><p>point of a power system to the earth. This provides dispersing paths for fault currents and lightning</p><p>currents in order to stabilize the potential and to act as a zero potential reference point to ensure the</p><p>safe operation of the power system and electrical equipment and the safety of power system operators</p><p>and other persons. Grounding is achieved by grounding devices (or ground devices) buried in soil. The</p><p>grounding devices of a power system can be divided into a relatively simple one for transmission line</p><p>towers, such as a horizontal grounding electrode (or ground electrode), vertical ground rod, or ring</p><p>grounding electrode, and the other is the grounding grid (or ground grid) for a substation or power plant.</p><p>Figure 1.1 Photo showing the microstructure of soil.</p><p>2 Methodology and Technology for Power System Grounding</p></li><li><p>The grounding device is a single metal conductor or a group of metal conductors buried in soil,</p><p>including horizontally or vertically buried metal conductors, metal components, metal pipes, reinforced</p><p>concrete foundations of structures, metal equipment, or a metal grid in soil. The grounding system</p><p>refers to the whole system, including the grounding device of a substation or power plant, and all metal</p><p>tanks for the power apparatus and electrical equipment, towers, overhead ground wires, neutral points</p><p>of transformers and the metal sheaths of cables connected with the grounding device.</p><p>The basic parameter to indicate the electrical property of a grounding device is grounding resist-</p><p>ance (or ground resistance), which is defined as the ratio of the voltage on the grounding device</p><p>with respect to the zero potential point at infinity and the current injected into earth through the</p><p>grounding device. If the current is a power-frequency alternating current (AC), the grounding</p><p>resistance is called a power-frequency grounding resistance. If the current is an impulse current,</p><p>such as a lightning current, then it is called an impulse grounding impedance, which is a time-</p><p>variant transient resistance. The impulse grounding resistance of a grounding device is usually</p><p>defined as the ratio of the peak value Vm of the voltage developed at the feeding point to the peak</p><p>value Im of the injected impulse current into the grounding device.</p><p>1.2.2 Classification of Grounding</p><p>The grounding devices of AC electrical equipment for a power system can be divided into three catego-</p><p>ries according to their functions: working grounding, lightning protection grounding and protective</p><p>grounding. Further, the instrumentation and control equipment of the substation should also be grounded.</p><p>1.2.2.1 Working Grounding</p><p>Based on whether the neutral point is grounded, an AC power system can be classified into a neutral-</p><p>point effective grounding system or a neutral-point ineffective grounding system (including neutral-point</p><p>ungrounded system, neutral-point resistance grounding system and neutral-point reactance grounding</p><p>system). In order to reduce the operating voltage on the insulation of the power apparatus, the neutral-</p><p>points of power systems of 110 kV and above are solidly grounded. This grounding mode is called a</p><p>working grounding. For the neutral-point effectively grounded operation mode, under normal situa-</p><p>tions, the voltage on the insulation of the power apparatus (such as a power transformer) is the phase</p><p>voltage. If the neutral-point is insulated, when the single-phase grounding fault occurs, the voltage on</p><p>the insulation of the power apparatus is the line voltage before the breaker cuts off the fault, which is3</p><p>ptimes as high as the phase voltage. The neutral-point effectively grounded operation mode can</p><p>effectively reduce the voltage on the insulation of the power apparatus and the insulation level of the</p><p>power apparatus is reduced, so the purpose of reducing the insulation size and lowering the cost of the</p><p>equipment is achieved. For the neutral-point solidly grounded system, the current through the ground-</p><p>ing device is the unbalanced current of the system under normal situations and, when a short circuit</p><p>fault occurs, a short-circuit current of tens of kilo-amperes (kA) will flow through the grounding device,</p><p>and usually the short-circuit current will last about 0.5 s.</p><p>Usually, the neutral point of the double-pole DC transmission system is grounded, which can operate</p><p>under single pole mode by using the earth as the return path. Operating in single pole mode, a current of</p><p>several kA will flow through the grounding electrode over a long period of time and we should pay</p><p>particular attention to the electrochemical corrosion of the grounding electrode.</p><p>For a power distribution system, a step-down transformer is used to connect the high-voltage system</p><p>with the low-voltage system and, according to whether the neutral point of the transformer is grounded,</p><p>the low-voltage distribution system can be classified into a grounded system (either solidly or through</p><p>impedance) or an ungrounded system. Figure 1.2 shows a low-voltage distribution system with neutral</p><p>point grounded. If someone touches the low-voltage conductor, a loop will be formed in which the</p><p>current through the body is related to the contact resistance between the body and earth. If the contact</p><p>resistance is small, a dangerous current will flow through the body and harm it.</p><p>Fundamental Concepts of Grounding 3</p></li><li><p>For water lighting and other power supply lines, it is necessary to add an insulated transformer</p><p>with a secondary side neutral point not grounded. This kind of system is called a neutral point</p><p>ungrounded system. As shown in Figure 1.3, when a person contacts the secondary circuit of the</p><p>neutral point ungrounded system, only a very small current flows through the loop circuit, which is</p><p>formed by the distributed capacitance, and it passes through the body, so it is much safer. One short-</p><p>coming of an ungrounded system is that there is no way to inhibit this abnormal voltage and it will</p><p>cause a hazard on the secondary side, when the system voltage is increased for some special reason,</p><p>such as the mixed contact of the high and low voltage circuits, a lightning impulse, a switching</p><p>voltage and so on. Another shortcoming is that an ageing insulation will possibly break down, thus</p><p>leading to a grounding accident.</p><p>1.2.2.2 Protective Grounding</p><p>When the insulation of electrical equipment fails, its enclosure becomes live and a person will suffer an</p><p>electric shock if he or she contacts its enclosure. In order to guarantee personal safety, the enclosures of</p><p>all electrical equipment should be grounded. This kind of grounding is called protective grounding.</p><p>When the enclosure of electrical equipment is live due to insulation damage, the fault current flowing</p><p>through the protective grounding device should trigger a relay protection device to cut off the faulty</p><p>equipment, and we can also reduce the grounding resistance to make sure that the voltage on the enclo-</p><p>sure is lower than the value of the body safety voltage, so that electric shock accidents caused by the</p><p>live enclosure can be avoided.</p><p>Figure 1.3 Ungrounded low-voltage distribution system.</p><p>Figure 1.2 Solid grounding of a low-voltage distribution system.</p><p>4 Methodology and Technology for Power System Grounding</p></li><li><p>1.2.2.3 Lightning Protection Grounding</p><p>In order to prevent the hazard of lightning to power systems and human beings, lightning rods, shield-</p><p>ing wires, surge arresters and other lightning protection equipment are usually adopted. Such lightning</p><p>protection equipment should all be connected to suitable grounding devices to lead the lightning current</p><p>into the earth. This kind of grounding is called lightning protection grounding. The lightning current</p><p>through the lightning protection grounding device is huge and can reach hundreds of kilo-amperes, but</p><p>it has a very short duration, tens of microseconds in general.</p><p>1.2.2.4 Signal Reference Grounding</p><p>A large number of instrumentation and control devices based on solid electronic devices are widely used</p><p>in modern power systems, but these devices need a signal reference point when in operation. Signal</p><p>reference grounding plays a very important role in making sure that the electronic devices and the com-</p><p>puter control system work regularly. But in the modern power system, it is very difficult to provide a</p><p>pure signal reference ground without interference. So, how to improve the anti-interference ability of the</p><p>signal ground is one of the important issues that should be considered during signal ground design. From</p><p>the functional point of view, the signal reference grounding is a kind of special working grounding.</p><p>1.2.3 Purpose of Grounding</p><p>Reducing Insulation Level of Electrical Equipment. As mentioned earlier, the working grounding</p><p>formed by grounding the power system neutral point can decrease the operating voltage on the power</p><p>apparatus and thereby reduces the insulation level of the power apparatus.</p><p>Ensuring Safe Operation of Power System. The grounding resistance of transmission line towers</p><p>must be lower than a certain value to reduce the potential difference between the transmission tower</p><p>top and the phase conductor. A value of less than 50% of the impulse flashover voltage of the insula-</p><p>tor can guarantee the safe operation of transmission lines. If the grounding resistance is too large, it</p><p>could possibly cause a tower top potential which is high enough to trigger an insulators string flash-</p><p>over and a power outage might happen.</p><p>In addition, as mentioned before, lightning protection systems in substations, such as lightning</p><p>rods, shielding wires and surge arresters, must be grounded to the grounding devices to discharge</p><p>the lightning energy to the earth.</p><p>Ensuring Personal Safety. As mentioned above, the protective grounding is intended to make the</p><p>enclosures of all electrical equipment grounded. When damage or the aging of equipment insulations</p><p>make the enclosures live, it can ensure the safety of any person who contacts the shell of the equip-</p><p>ment. However, the grounding devices of substations can make sure that the personal touch voltage</p><p>and step voltage meet the desired safety requirements by...</p></li></ul>

Recommended

View more >