CLASSIFICATION OF GROUNDING SYSTEMS

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.

2.2. LOW IMPEDANCE GROUNDING

This method is similar to the solidly grounding system in that transient over-voltages are not a problem, see Figure 3. The impedance may be a resistor (Rg) or a reactance (Xg). Low impedance grounding is typically used in medium voltage systems, but not on low-voltage system. The impedance is usually a resistor that is selected to limit ground fault current magnitude, but leave the current high enough to be detected by sensitive relays. 

Resistor (resistance) grounding is normally selected as a method to limit ground fault current in the range of 200 A to 500 A and is rated for approximately 10 seconds. Many medium-voltage industrial power systems has a resistance grounding with typical ground fault currents in the range from 100 A to 1000 A. 

In reactance grounding systems, the available ground fault current must be in the range of 25 @ 65 % of the three phase fault current to prevent serious transient over-voltages (X0 ≤ 10.X1), which is higher than the ground fault levels in a resistor grounded systems. Thus, the resistance grounding scheme is preferred, because it allows more reduction of ground fault currents than reactance grounding without risk of transient over-voltage.

2.3. HIGH IMPEDANCE GROUNDING

There are two types of  high-impedance grounding systems: a) high-resistance grounding; and, b) resonant grounding.

2.3.1. HIGH-RESISTANCE GROUNDING

In this scheme, the neutral point is grounded to earth using a high-impedance resistor with magnitude equal to or slightly less than the total system reactive capacitance to ground (R0 ≤ XC0) in order to limit transient over-voltages to safe values during ground faults. This scheme limits transient over-voltages to less than 250% the peak value of the nominal phase-to-ground voltage. The resistor is sized typically to allow a ground fault current below 15 A, this system is illustrated in Figure 4.

                                                    Figure 4 High-Resistance Grounding

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. 

2.3.2. RESONANT GROUNDING

Here, the system is grounded through a high-impedance reactor, the total system capacitive reactance is canceled by an equal inductive reactance connected between the neutral point and ground, illustrated in Figure 5. The variable reactor or reactance is called Petersen Coil, when this neutral reactor is tuned to the total system capacitance, the ground current fault is ideally zero. The circuit is nothing but a parallel resonant circuit, and the very low ground fault current cause minimum fault damage. The low ground fault current require sensitive fault detection devices. If the coil reactance is greater than the system capacitive reactance, the system is overcompensated, and it if is smaller the system is undercompensated. 

Resonant grounding method is used in medium voltage generators connected to a power system through a dedicated step-up power transformer. The reactor magnitude is adjusted to 1/3 the per-phase capacitive reactance to ground. For ground fault currents, the system is in resonance and the fault current is typically below 1 A. However, generator resonant grounding is much less used that high-resistance grounding.

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).

                                                        Figure 6 Ungrounded System

As we can see in Figure 6b and 6c, a ground fault in phase “A” shift the system neutral voltage but leave the phase-to-phase voltage triangle intact. Hence, after a single ground fault the ungrounded system is able to remain in service. In the normal balanced system (see Figure 6b) V_AN = V_AG, V_BN = V_BG, and V_CN = V_CG, so N and G has the same potential. When a ground fault occurs in phase “A” the voltage between phase faulted (“A”) and Ground becomes to zero Volts, but the voltage between N and G is now equal to the zero-sequence voltage (V0). The phase-to-ground voltage on the un-faulted phases are √3 (1.73) of their nominal values. This has consequences because the system must have a phase-to-phase insulation level and all power equipment and loads.

On the other hand, the ground fault current has a magnitude equal to the capacitive current by connecting one of the phases to ground. For medium voltage (MV) systems, these currents are in a range from 5 to 15 Amperes, depending on system voltage characteristics. Thus, the ground fault current is very low, such that equipment damage is minimal, and it is not necessarily essential that the faulted area be rapidly cleared. Generally, undergrounded systems are not  recommended., because they may be exposed to high and destructive transient overvoltages, they represent potential hazards to equipment and personnel.

SUMMARY

The main goals of system grounding are provide personnel safety, minimize potential transient overvoltage and thermal stress, reduce communications system interferences, and give assistance in rapid detection and elimination of ground faults.

Each type of grounding systems has advantages and disadvantages, and no one method is generally accepted. We can mention some factors that influence the choice, for instances:

- Voltage levels.

- Transient over-voltage possibilities.

- Required continuity of service.

- Methods used on existing systems.

- Cost of equipment.

- Availability of convenient grounding point.

- Safety, including fire and shock hazard.

- Tolerable fault damage levels.

- Etc.

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.

REFERENCES

1. IEEE STD 142-1991. IEEE RECOMMENDED PRACTICE FOR GROUNDING OF INDUSTRIAL AND COMMERCIAL POWER SYSTEMS.

2. IEEE STD 242-2001. IEEE RECOMMENDED PRACTICE FOR PROTECTION AND COORDINATION OF INDUSTRIAL AND COMMERCIAL POWEWR SYSTEMS.

3. NATIONAL ELECTRICAL CODE (NEC –NFPA 70).

4. PROTECTIVE RELAYING PRINCIPLES AND APPLICATIONS - THIRD EDITION. J. LEWIS BLACKBURN, THOMAS J. DOMIN.

5. PROTECTIVE RELAYING THEORY AND APPLICATIONS – SECOND EDITION. WALTER A. ELMORE.

6. SYSTEM GROUNDIG. BILL BROWN. SQUARE D ENGINEERING SERVICES.