Reducing Insulation Level of Electrical Equipment by Effective Grounding
Under normal situations, the voltage on the insulation of a power apparatus such as a power transformer is the phase voltage. Considering the neutral point is insulated when the single-phase grounding fault occurs, the voltage on the insulation of the power apparatus is the line voltage before the breaker operates, which is √3 times as high as the phase voltage. The effectively grounded neutral point reduces the voltage on the insulation of the power transformer and the insulation level of the power apparatus is thus reduced, so the purpose of reducing the size of insulation and cost-cutting of the equipment is achieved.
Table of Contents
Ensuring the Safe Operation of the Power System.
The tower footing resistance of transmission line towers must be lower than a certain value to reduce the difference in potential between the tower’s top and the phase conductor. The value of less than 50% of the insulator’s impulse flash-over voltage can guarantee the operational safety of transmission lines. When the resistance of the earthing is too large, it can possibly cause the potential of the tower top, high enough to trigger a flashover of the insulator string and risk a power outage.
Additionally, the lightning protection systems in substations, such as lightning rods, shielding wires, and surge arresters, must be earthed to the grounding devices for swift discharge of the high lightning energy to the earth.
Ensuring Personal Safety.
The protective grounding is intended to make all electrical equipment’s enclosure grounded. When the insulation damage or the aging of equipment’s insulations makes the enclosures live/charged, effective grounding can ensure the safety of any person who comes in contact with the live shell of the electrical equipment.
Moreover, the grounding devices at substations can be made to ensure that the personal touch voltage and step voltage meet the desired safety requirements by reducing the earthing resistance and taking voltage equalization measures. The touch voltage is the potential difference between one hand and one foot of the person, when he/she contacts the equipment shell or metal enclosure under power system failure, while the step voltage is the difference in potential between two feet of the personnel.
Eliminating Electrostatic Accidents.
Static electricity is known to cause an explosion and fire, and oil storage tanks and natural gas pipelines are particularly susceptible to the explosion caused by an unprecedented electrostatic discharge. Further, static electricity can interfere with the normal workings of solid electronic devices and IEDs. Through grounding, these static charges generated and collected by friction and other factors can be released to the earth to prevent such accidents and damage caused by static interference.
Detecting Ground Faults.
For ensuring personal and property safety, the leakage breakers and various other fault leakage protection devices are used in low voltage circuits. When a ground fault occurs at one point in the circuit, there is a very large ground fault current which brings the protection device into operation.
To meet this condition, the neutral point on the secondary side of the step-down transformer must therefore be grounded. In contrast, for a neutral point grounded circuit, if the shell of an electrical equipment is not grounded or when the electrical equipment enclosure is charged due to insulation damage or other factors, the current thus generated in the circuit by the distributed capacitors can never trigger the protection device used in the circuit, so the equipment’s shell must be grounded, as shown
The current I is:
I = V / Ro + RE
where V is the phase voltage of the circuit, RO and RE are the grounding resistance of the neutral point and the electrical equipment respectively.
Equipotential Bonding.
It is a kind of connection to ensure that externally exposed conductive shells of devices have equal potential. The electrical equipment inside a building can achieve equipotential bonding through the grounding of the equipment enclosure through the main ground bus, as shown in Figure 1.5. The purpose of this equipotential bonding is to prevent dangerous differences in potential between different devices, and to avoid forming a loop, because the loop formed by grounding connection is very much vulnerable to external electromagnetic fields as the loop current interferes with the normal operation of equipment.
Reducing Electromagnetic Interference.
External electromagnetic interference may cause electronic devices or IEDs to malfunction or may interfere with the signal transmitted by cable. This can be much reduced or eliminated by grounding of the shielding shell of the electrical equipment and the cable’s shielding sheath. Further, in order to prevent the high-frequency energy, generated by electronic devices from interfering with other devices, the electronic devices should be grounded.
Grounding of electromagnetic interferences has different types, such as grounding of shielding layers, shielding rooms, grounding of cable’s shielding sheaths, grounding of transformer electrostatic shields, grounding of the protection devices for precision instrumentations, etc. The power line filters available at the entrances of electric or electronic devices should therefore also be grounded. In short, the grounding against electromagnetic interference provides a channel for energy to be dissipated to Earth.
Function Grounding.
Some equipment has to be grounded functionally, for example, cathodic protection that uses electrochemistry to prevent metal corrosion. To make the corrosion current flow to the earth, the cathodic protection system has to be grounded.
Additionally, a reference point with a stable potential is adopted in practice to ensure the regular operation of computers and other electrical equipment, which can only be achieved by grounding.
Work Grounding.
When maintenance personnel work on transmission lines under power outages, the energy stored in the transmission line and other equipment should therefore be discharged by grounding, to prevent any hazards to the maintenance personnel from the induced current through the transmission line. Further, any fatal harm to personnel caused by another’s false operation can be prevented by the use of effective grounding.
