1. AUTHOR: BSEE Hugo E Reyes H
1. INTRODUCTION
The system grounding is very important because its main goal is to provide a path to the earth in order to minimize potential transient overvoltages from lightning strikes, line surges, or unintentional contact by higher-voltage lines, and helps to determine the system protection requirements. The system grounding arrangement is determined by the grounding of the power sources, and also determines the types of loads the system can accommodate.
The National
Electrical Code (NEC – NFPA 70) article 250.4 (A) (1) defines a grounded
systems as follows: “Electrical System that are grounded shall be connected to earth in a
manner that will limit the voltage imposed by lightning, line surges, or
unintentional contact with higher-voltages lines and that will stabilize the
voltage to earth during normal operation”.
2. 2. TYPES OF SYSTEM
GROUNDING
Grounding are
classified as follow:
·
SOLID OR EFFECTIVE GROUNDING
·
LOW-IMPEDANCE GROUNDING
·
HIGH-IMPEDANCE GROUNDING
·
UNGROUNDED SYSTEMS
2.1. SOLID GROUNDING (OR EFFECTIVE GROUNDING)
Solidly grounded systems are in Wye (Y or Star) connection and have all neutrals connected to ground without any intentional impedance between the neutral and ground (Earth). This type of system ground provides excellent protection against overvoltage and ground fault currents. Other important characteristic of solidly-grounded systems is that ground faults may cause high short-circuit current. Thus, the occurrence of a ground fault needs to be cleared as fast as possible.
In order to be classified as solidly grounded, in these system the X0/X1 ratio must be positive and less than 3.0, and the R0/X1 ratio must be positive and less than 1.0 at all point and under all operating conditions, where R0 and X0 are the zero-sequence resistance and reactance, and X1 is the positive-sequence reactance of the power system.
The Figure 1 shows an effective grounding with a single-point grounding. In this application we have one arrangement with three phase and three wires (3Ph/3W) with every load connected phase to phase. On the other hand, we can get a second arrangement with three phase and four wires (3Ph/4W) with an isolated neutral and every load connected phase-to-neutral. In this second arrangement, the load unbalance current returns through the neutral, but any ground fault returns through the earth to the substation neutral.
The Figure 2 shows an effective grounding with a multiple-point grounding. In this application we have three phase with four wires (3Ph/4W) and phase to neutral loads, the system is grounded at the substation, at every power transformer location along the circuit, and every, 1 000 feet (305 m) or so if there is no power transformer ground. In this type of arrangement, both load unbalance and ground fault currents divide between the neutral conductor and earth. Detecting high-resistance ground faults is difficult because the protective relay measures the high-resistance ground fault combined with the unbalanced current. Most utility companies uses multiple-point grounding, the typical case is to ground the transformer neutral at overhead line poles, creating in this way a multiple-point grounding. Due to a separate grounding conductor is not run with the overhead power line, the resistance of the earth limits the circulating ground currents. Multiple-point grounding in National Electrical Code (NEC) jurisdictions, such as commercials or industrial facilities, are actually not allowed in most cases. Instead, a single-point grounding is preferred.
High-resistance
grounding schemes connect a distribution transformer between neutral point and
ground, with a resistor on the transformer secondary. The transformer primary
is rated for primary phase-to-phase voltage, and a 240 Volts secondary limits the
secondary to a 139 Volts maximum.
This scheme is
specially used in medium voltage, permits the utility to continue operating the
system during the first sustained ground fault condition until a favorable time
for an outage to clear the fault, provided that the cable carrying the fault is
rated 173% of the voltage level. If a second ground fault occurs on another
phase before the first ground fault is cleared, a phase-to-ground-to-phase
fault occurs that is not limited by the neutral grounding resistor. Thus, the
second fault may be an arcing fault, whose magnitude is limited by the
ground-path impedance to a value high enough to cause severe arcing damage, but
too low to activate the overcurrent relays quickly enough to prevent or limit
this damage. For this reason, systems of 13.8 kV and higher generate too much
heat to justify a delay in tripping. To avoid arcing damage, we need to use two relay levels, the first level is used to issue an alarm on first phase-to-ground
fault, and the second level to issue a trip on second fault in time to prevent
arcing damages.
Figure 5 Resonant Grounding System
2.4. UNGROUNDED SYSTEMS
These system has no intentional connection between the neutral and ground. However, the system is connected to ground through the lines-to- ground capacitances as we can see in Figure 6. However, ungrounded systems must still be grounded. Do it per NEC 250.4 (B) (1) through (4).
A summary of various systems grounding characteristics are:
- Ungrounded: this system is not generally recommended but sometimes is used in industrial and utility stations for high-service continuity. It is not recommended in utility transmission, sub-transmission and distribution. The fault currents in ungrounded systems are very low and easy to detect. There are possibilities of transient over-voltages and ferro-resonance phenomena.
-
High-Impedance Grounding: is
recommended in industrial and utility stations in medium voltage due to high-service continuity,
but is not recommended in utility transmission, sub-transmission and
distribution. The fault currents are low in the range of 1 @ 10 A, they are easy
to detect. Transient over-voltages are limited to 250% of the phase-to-neutral
voltage.
-
Low-Impedance Grounding: is
recommended in industrial and utility stations in medium
voltage systems. It is not recommended in utility transmission, sub-transmission
and distribution. The fault currents are limited in the range of 50 @ 600 A,
and they are easy to detect.
-
Effective or Solid Grounding: this
system is recommended in industrial and utility stations, and also is
recommended in utility transmission, sub-transmission and distribution. The
fault currents are in the range from low to high levels, they are easy to
detect.
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