GROUNDING METHODS OF MEDIUM-VOLTAGE DISTRIBUTION NETWORKS

Proper system grounding will means safety. Its goals are to minimize voltage and thermal stresses on equipment, provide personnel safety, reduce communications system interference, and give assistance in rapid detection and elimination of ground faults.

With the exception of voltage stress, operating a system as ungrounded, high-impedance grounded, or resonant grounded restricts ground fault current magnitudes and achieves most of the goals listed above. The drawback of these grounding methods is that they also create fault detection (protection) sensitivity problems.

We can create a system grounding that reduces voltage stress at the cost of large fault current magnitudes. However, in such a system the faulted circuit must be de-energized immediately to avoid thermal stress, communications channel interference, and human safety hazards. The disadvantage of this system is that service must be interrupted even for temporary faults.

Ungrounded or Isolated Neutral 


Characteristics:
-Neutral has no intentional connection to ground

-System is connected to ground through the line-to-ground capacitances
-Systems can remain operational during sustained, low-magnitude
-Fault detection is not selective
-Elements respond to the quadrature component of the zero-sequence current with respect to the zero-sequence voltage

Effective or Solid Grounding 
Characteristics:

-(X0 / X1) ≤ 3 and (R0 / X1) ≤ 1, where X0 and R0 are the zerosequence reactance and resistance, and X1 is the positive-sequence reactance of the power system
-Load unbalance and ground fault currents divide between the neutral conductor and earth
-Detecting high-resistance ground faults on these systems is difficult because the protective relay measures
the high-resistance ground fault current combined with the unbalance current
-Solid grounding reduces the risk of overvoltages during ground faults since these faults do not shift the system neutral

Low-Impedance Grounding 
Characteristics:
-Grounded through a low-impedance resistor or reactor with the objective of limiting the ground fault current -This method is equivalent to solid grounding in many other ways, including ground fault protection methods
-Industrial plant engineers also use low-impedance grounding in their plant and distribution circuits.

High-Impedance Grounding 
Characteristics
-Grounded through a high-impedance resistor or reactor with an impedance equal to or slightly less than the total system capacitive reactance to ground. 
-High-impedance grounding method limits ground fault current to 25 A or less
-Grounding resistor may be connected in the neutral of a power or grounding transformer, generator or
generator-grounding bus, or across a broken delta connection of distribution transformers


Resonant Grounding 

Characteristics:
-System is grounded through a high-impedance reactor
-Resonant grounding a system can reduce the ground fault current to about 3 to 10 percent of that for an ungrounded system
-Detecting high-impedance faults in compensated distribution circuits requires a device with a
very sensitive residual current input
-Residual current compensation methods inject a current through the reactor to the system during the fault, reducing the fault current almost to zero


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