Grounding System

Introduction

In any electrical circuit network, the circuitry that provides a path between the parts of the circuit and the ground, is known as the grounding system. The grounding system is required for reliable operation of any electrical equipment including generator, transformer, power system tower, and other power system installation under fault conditions. In a grounding system, the study of ground resistance is very important in designing any electrical network for residential, commercial and industrial areas. Any electrical equipment needs to be grounded for safe operation. In this case, the enclosure of the equipment is grounded in such a way that the voltage on the equipment maintains the voltage of the ground. This is known as equipment grounding. The grounding is also an important for power generation, transmission and distribution systems. In the transmission systems, each leg of the transmission tower is grounded through a vertically inserted earth electrode. In an electrical substation, all high voltage equipment are grounded with a grounding grid system. In this chapter, objectives, symbols, expression of ground resistance with different types of electrode etc. have been discussed.

Objectives of Grounding System

The objectives of any grounding system are as follows:

Reduce insulation level of power system equipment: Power system equipment like transformer’s neutral must be grounded. This can decrease the operating voltage and insulation level of the equipment.

Ensure personal safety: A good grounding system of a substation can ensure that the touch and step voltages meet the standard voltage levels.

Eliminate electrostatic accidents: Static electric current may create interference of electronic devices and generate ﬁre near any ﬂammable object. A good grounding system can release the static current to the earth which can prevent that type of incident.

Reduce electromagnetic interference: The normal operation of any electronic device is interrupted due to electromagnetic interference. This type of interference can be reduced by the good grounding system.

Reduce cathodic protection current: The voltage is usually induced in the pipeline at the same corridor of the transmission lines which can harm the utility operators. The cathodic protection is used in the pipeline to mitigate high touch potential and reduce the current due to a fault condition. Therefore, the cathodic protection system of the pipeline must be grounded to release the current into the earth.

Detecting ground faults: In the substation, there are many leakage breakers and other fault leakage protection devices used in the low voltage circuits. A high magnitude of the ground fault current is required to bring the protection device into action, if there is an earth fault in the circuit. Therefore, to meet this condition, the neutral point on the secondary side of the step-down transformer must be grounded.

Grounding Symbols and Classiﬁcation

In the grounding system, three symbols are used which are signal, chassis and earth in Figure 1.

Figure 1 Grounding Symbol

The signal or equipotential ground is used for the reference voltage of an electrical circuit for simulation activities. The earth ground is used to release high magnitude of current into the earth or soil. The presence of voltage is observed in any system due to electrostatic voltage. This electrostatic voltage can be mitigated using the chassis ground. The earth and chassis grounds are normally used in the power system networks.

The grounding system basically provides a low resistance path for the fault and lightning currents in order to maintain the safe potential with respect to the zero potential. The single-phase to ground fault is the most common fault in the power system and it accounts for 98 % of all failures. The phase-to-phase and three-phase faults are responsible for 1.5 and 0.5 % of all failures respectively. The grounding system is constructed by burying electrodes into the soil. The electrodes are known as a ‘rod’ when buried vertically and ‘conductor’ when buried horizontally in the soil. The grounding with only rod is used in the transmission tower and distribution electric pole. The grounding grid is formed either the combination of the conductor and the rod or only the conductor. The grid is normally used in the substation and power station grounding system. Initially, grounding system is divided into grounded and ungrounded systems. The grounded system is again sub-divided into neutral grounded and non-neutral grounded systems. The neutral grounded is again divided as solidly grounding, resistance grounding, reactance grounding, voltage transformer grounding and Zig-Zag transformer grounding. The resistance grounding is sub-divided as high resistance grounding, low resistance grounding and hybrid high resistance grounding.

Ungrounded Systems

Any grounded or ungrounded systems mainly depend on the customer demands. The ungrounded electrical systems are used where the customer and the design engineer do not want the overcurrent protection device acting on the ground fault line. However, there are some ungrounded elements such as building steel or iron and, water pipeline, which are intentionally grounded.

In an ungrounded system, there is no internal connection between any line (including the neutral) and ground. The ungrounded system is basically grounded through distributed capacitance. There are some advantages and disadvantages of ungrounded systems. The ground fault current in this system is very low (5 A or less) and provides more reliability during fault conditions. The voltage between the healthy lines and ground is very high. The effect of harmonics in the ungrounded system will die out itself within the system. The outside source interferences are usually neglected as there is no connection with the soil. The ungrounded system is very poor to protect the electrical appliances due to transient voltages.

Sometimes, these transient voltages propagate or elongate to the nearby equipment which can destroy the insulation of those equipment. In this system, it is very difﬁcult to locate the line to ground fault. An ungrounded wye-connection is shown in Figure 2, where a ground fault occurs in line R. Each line has distributed earth capacitance. The capacitance of line R will discharge through the faulty path to the capacitance of lines Y and B. The charge and discharge of capacitance of line R will continue due to the healthy lines Y and B. The currents in the lines Y and B can be written as,