Comparison of Neutral Grounding Methods
Neutral Grounding Methods
System grounding can generally be examined in three main categories.
Insulated Neutral System
The neutral is not directly grounded. In reality, the electrical system is grounded through the system capacity for grounding.
Ground fault causes leakage current due to excess current in the cable carrying capacity and the voltage of the healthy phases does not rise up to the line voltage. Thus, the system works with the existing ground fault, improving system continuity and supply.
isolated neutral system
“It is very difficult to detect the location of the leak. The main sensing components are a voltmeter. This method is generally used for AG networks.”
Direct Grounding
The neutral of power transformers or generator is connected directly to ground.
Leakage current = Three-phase symmetrical short circuit current and can rise up to 20 to 30 times the nominal current. The overvoltage in the solid phase does not exceed the earth voltage.
Direct grounding scheme
“There is no leakage current limitation when the system is directly grounded.”
Grounding Impedance
The purpose of this method is to limit leakage current for greater safety. There are three types of ground impedance:
► Resistance,
► Reactance,
► Arc suppression coil (petersen coil).
1) Direnç Üzerinden Topraklama
The neutral is connected to ground through a resistor. Leakage current is limited to the selected value:
If=V/R
R = Value of resistance (W)
V = Mains voltage (kV)
A system properly grounded by the resistor is not subject to harmful effects from transient overvoltages.
Grounding scheme through resistor
Reasons for limiting the current with resistance;
► To reduce the effects of burning and melting on faulty electrical devices,
► To reduce mechanical stress on the circuits and devices passing leakage currents,
► To reduce electric shocks, hazards caused by leakage currents for personnel are eliminated.
“There are two classes based on the level of leakage current in the soil where current is allowed; high resistance value and low resistance value (there are no accepted standards for the level of earth leakage current that defines these two classes)”
There is a clear difference in practice;
► High resistance value typically used for ground leakage current levels of 10 A or less.
► Low resistance value typically used for ground leakage current levels above 10 A and up to 3000 A.
Both classes are designed to limit earth leakage current and protect the system from transient overvoltages (to a safe level). However, the high resistance method does not require immediate elimination of an earth leakage. Because the leakage current is limited to a very low level, the protective draft associated with a high resistance value is usually detection and alarm.
The low resistance method has the advantage of immediate and selective purification of the grounded circuit. However, the minimum leakage current must be large enough to positively actuate the movable contact of the applied earth leakage current relay.
2) Grounding via Reactance
The neutral is connected to ground through reactance. Ground fault is a function of neutral reactance; The level of leakage current is often used as a criterion to define the degree of grounding.
Grounding scheme via reactance
In this method, to prevent transient overvoltage, the earth leakage current should be at least 60% of the three-phase leakage current. This is considerably higher than the level of leakage current desired in the system using resistors. Therefore, reactance grounding cannot generally be considered as an alternative to a system using resistors.
“This system is used when the system transformer is not available (Delta connected system).”
3) Grounding via Arc Suppression Coil
The grounding reactor connected between the ground and neutral of a system ensures that under fault conditions, with a relatively high value of specially selected reactance, the reactive current in the ground balances the capacitance current flowing through the lines to the ground. Thus, the current flowing to ground at the fault is limited to almost zero.
Grounding scheme via arc extinguishing coil
“In case of a ground fault, the insulator may extinguish itself if it flashes like a flash in the air. This grounding method occurs primarily in 110 kV systems, largely on overhead power lines or distribution lines.”
Systems of such structures are rarely used in industrial or commercial power systems.
Tablo: Nötr Topraklama Yöntemleri
X0: Zero Row Reactance of the System
X1: Positive Equation reactance of the system
R0: Phase sequence resistance per phase
XC0: Reactance capacitive for grounding the system, distributed per phase
V: Work voltage