GROUNDING METHODS OF MEDIUM-VOLTAGE DISTRIBUTION NETWORKS

Ungrounded or Isolated Neutral
In an isolated neutral system, the neutral has no intentional connection to ground: the system is connected to ground through the line-to-ground capacitances. Single line-to-ground faults shift the system-neutral voltage but leave the phase-to-phase voltage triangle intact.

For these systems, two major ground fault current magnitude-limiting factors are the zero sequence line-to-ground capacitance and fault resistance. Because the voltage triangle is relatively undisturbed, these systems can remain operational during sustained, low-magnitude faults.

Self-extinction of ground faults in overhead-ungrounded lines is possible for low values of ground fault current. At higher magnitudes of fault current, faults are less likely to self extinguish at the fault current natural zero-crossing because of the high transient recovery voltage.

Effective or Solid Grounding
Effective, or solid, grounding is popular in the United States. To be classified as solidly grounded, the system must have (X0 / X1) ≤ 3 and (R0 / X1) ≤ 1, where X0 and R0 are the zero sequence reactance and resistance, and X1 is the positive-sequence reactance of the power system. In practice, solidly grounded systems have all power system neutrals connected to earth (or ground) without any intentional impedance between the neutral and earth.

There are two different practical implementations of solid grounding in medium-voltage distribution systems: uni grounded and multigrounded. In uni grounded systems there may only be three wires with all loads connected phase-to-phase , or there may be four wires with an isolated neutral and all loads connected phase-to-neutral.

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. Ground faults on these systems may produce high-magnitude currents that require tripping the
entire circuit and interrupting load to many customers.

Low-Impedance Grounding
In this type of grounding the system is grounded through a low-impedance resistor or reactor with the objective of limiting the ground fault current. By limiting the ground fault current magnitudes to tens or hundreds of amperes, you reduce equipment thermal stress, which allows you to purchase less expensive switchgear. This method is equivalent to solid grounding in many other ways, including ground fault protection methods.

Many of the distributed networks in France are low-resistance grounded. In rural distribution networks the ground fault current is limited to 150–300 A primary, and in the urban networks, which have higher capacitive currents, the resistor is selected to limit the ground fault current to a maximum of 1000 A. Industrial plant engineers also use low-impedance grounding in their plant and distribution circuits.

High-Impedance Grounding
In this method the system is grounded through a high-impedance resistor or reactor with an impedance equal to or slightly less than the total system capacitive reactance to ground. The high-impedance grounding method limits ground fault current to 25 A or less. High-resistance grounding limits transient over voltages to safe values during ground faults. The 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.

As with isolated neutral systems, ground faults on these systems shift the system neutral voltage without modifying the phase-to-phase voltage triangle. Again, this grounding method permits the utility to continue operating the system during sustained ground faults.

Non selective ground fault detection is possible by sensing system zero-sequence voltage magnitude and comparing it with an over voltage threshold, or by measuring all three phase-to ground voltages and comparing each voltage magnitude against an under voltage threshold. To find the faulted feeder, you must use sensitive zero-sequence directional elements or disconnect feeders to determine when the zero-sequence voltage drops to a normal level.

Resonant Grounding
In this method of grounding, the system is grounded through a high-impedance reactor, ideally tuned to the overall system phase-to-ground capacitance. The variable impedance reactor is called a Petersen coil after its inventor, who introduced the concept in 1917. It is also known as an arc-suppression coil or ground-fault neutralizer. The coil is typically connected to the neutral of the distribution transformer or a zigzag grounding transformer.

Systems with this type of grounding are often referred to as resonant-grounded or compensated systems. When the system capacitance is matched by the inductance of the coil, the system is fully compensated, or at 100 percent tuning. If the reactor inductance does not match the system capacitance, the system is off tuned. It can be over- or under compensated, depending on the relationship between inductance and capacitance.

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