Electrical Grounding Using Effective, Solid, and Low-impedance Methods

Effective grounding has the best attributes for transient overvoltages control, easy relaying, and cost, among others. However, this method produces the highest magnitudes of ground-fault currents with potentially harmful effects. The low-impedance method reduces the ground-fault current levels to safe values, keeping some of the advantages of effective grounding. This arrangement requires the insertion of an impedance (reactor or resistor) into the power system neutral.

 

 

Previous articles highlighted the purposes of grounding system neutrals: limit overvoltages on the sound phases, control the ground-fault current to reduce damages, increase safety and allow protective devices to detect and clear the fault. A compromise is necessary, as these properties may be conflicting. That is where effective grounding comes in.

 

Solid vs. Effective Grounding

The best method to control overvoltages is effective, or solid, grounding.

This method supplies phase-to-neutral connected loads without the risk of finding dangerous neutral-to-ground voltages when a ground-fault arises. Additionally, straightforward ground relaying schemes isolate the defective portions of the network.

However, an effectively grounded system also has the highest values of ground-fault currents, which may range from zero to three times the three-phase short-circuit current. 

Before going further, it is essential to clarify the difference between solid and effective grounding.

 

What is Solid Grounding?

Solid grounding means that there is no impedance placed intentionally between the neutral and ground of power transformers, grounding transformers, or generators. But it is not a zero-impedance neutral circuit because the electrical machines and system attributes impose a reactance in the zero-sequence circuit. The magnitude of the ground-fault current depends on the power system constants and configuration, any fault resistance, and location of the fault.

 

What is Effective Grounding?

The term “solid” is limited and evolved into the superior concept of effective grounding, which considers the electrical constants of the network as they are seen at fault.

A meaningful way to determine the degree of grounding of a power system is by the ratio of the ground-fault current to the three-phase fault current. The higher the ratio, the higher the grounding, e.g., 25%, 60%, 100%.

The ANSI/IEEE standards state that a system, or a portion of it, is effectively grounded when the ratio of zero-sequence reactance to positive-sequence reactance is not greater than three (Xₒ/X1 ≤ 3), and the ratio of zero-sequence resistance to positive-sequence reactance is not greater than one (Rₒ/X1 ≤ 1)  for any condition of operation and any amount of connected generator capacity. This means that the same power system may be effectively grounded in one portion but not in other parts depending on the ratios of network constants seen at fault.

 

Low-reactance Grounding

Low-reactance grounding is done by connecting the neutrals to the ground through a reactor, making it different from solid.

The degree of grounding depends on the network constants ratios, mentioned above, and not on the impedance inserted in the neutral. When the impedance is a reactor, the power system will not be reactance grounded if the ratios of the constants show that it is effectively grounded.

Based on network constant ratios, the criteria to qualify as reactance grounded is: 

Xₒ/X1 > 3

as seen at fault, but less than the value necessary for resonant grounding. 

Inserting a low-reactance such that 

Xₒ/X1 ≤ 3

at fault, is not reactance grounding.

When employing a solidly grounded grounding transformer, its reactance may be such that

Xₒ/X1 > 3

and the system is deemed reactance grounded.

It is not proper to endanger a generator winding with fault currents higher than the three-phase current at the terminals. In generators, the ground-fault currents are higher than the three-phase fault currents because the internal impedance seen by the ground-fault is less than the impedance to three-phase faults. The high current produces excessive heating and mechanical forces.

A suitable current limiting impedance, like a low-reactance reactor, should be installed in the neutral to avoid generator damage.

In transmission and distribution systems, without directly connected rotating machines, the neutrals of the transformers are usually effectively grounded and reactors are not typical.

The phase-to-ground fault current should range from 25% to 100% of the three-phase fault current. Less than 25% may cause damaging transient overvoltages. Choose the value of reactance needed to limit the ground-fault current to the preferred amount.

The ratio is 

• Xₒ/X1 = 10 when 25%

• Xₒ/X1 = 1 when 100%

• Xₒ/X1 = 3 when 60%

Limiting the ground-fault current to 60% of the three-phase fault current is the borderline between effective grounding and reactance grounding.

When installing a 100% reactor in a generator neutral, the system is not reactance grounded but effective grounded, by definition, and the maximum fault-current contribution of this generator to a line-to-ground fault anywhere in the system outside of the generator will be its three-phase fault current.

 

Low-resistance Grounding

In the USA, low-resistance grounding is the most popular method utilized to limit ground-fault current. The value of resistance is much lower than the high-resistance method and ranges from 5% to 20% of the three-phase fault current. Some applications limit the ground current to around 50A to 600A.

A typical resistor of 400A will allow enough current flow to operate the protective relays for fast fault clearing. These resistors are also time-rated. A standard figure is 10s because, like in effective grounding, the branch will shut down after the first ground-fault.

Pros and Cons of Effective and Low-impedance Grounding

The effective grounding method does not produce excessive transient overvoltages, fault tracing is straightforward, the protective relays selectively segregate the faulty zone, the required insulation level is small, and the cost is the lowest. However, the ground-fault current magnitudes can fluctuate substantially from very small to higher than the three-phase value, with detrimental effects on the power system.

The low-impedance method limits the ground-fault current to a figure that does not damage generators, power transformers, or other devices in the power system and suits the protective scheme needed to clear the fault selectively. Faults are removed immediately and the method provides higher safety, but the cost of the array is higher than in effective grounding.

There are some particularities for low-reactance and low-resistance grounding:

• Low-reactance cannot adequately limit the ground-fault current to less than 25% of the three-phase fault current because damaging transient overvoltages may occur.

• Low-resistance can limit the ground-fault current to values lower than low-reactance and have the same risks of damaging overvoltages, protecting equipment, and threatening people's safety.

 

Areas of Application

The NEC requires most low-voltage power systems to be solidly grounded. Part II of Article 250 titled System Grounding lists the dos and don’ts for less than and more than 1kV.

In general, the recommendation for effective grounding is for low-voltage (≤ 1kV) and medium-voltage (> 15kV) systems in industrial and commercial applications. In voltages above 15kV, economic considerations dictate the use of effective grounding, which can result in reduced insulation levels and no need for grounding equipment.

Utilities' transmission and distribution systems are mainly overhead lines, which do not have the potential dangers of high magnitude ground currents as when employing insulated cables. Therefore, the recommendation is effective grounding. 

Transmission and distribution lines are very long, making the zero-sequence impedance predominant, and reducing the magnitude of line-to-ground fault current to a value lower than the three-phase fault current. Limiting the current at the neutral in distribution systems complicates the detection of distant faults, posing a threat to people and property.

Effective grounding may provide enough fault current to melt the fuses in transformers with primary protection only on secondary ground faults.

Low-resistance grounding is not advised in low-voltage ( ≤ 1kV) systems because the fault current may not be enough to operate the circuit breakers and fuses that protect phase-to-phase and phase-to-ground faults.

Low-resistance is the preferred method in medium-voltage power systems (1000 < V ≤ 15000) for industrial and commercial applications, as well as in utility generating stations. The main reason for this is to protect rotating machines by reducing the damage at fault. The frequently short lengths found in this type of distribution system produce high ground-fault currents.

When achieving low fault current values, the improved transient overvoltages performance of low-resistance grounding favors this method in generator applications, and low-resistance has replaced low-reactance grounding in most cases.

Low-reactance grounding is used in generators to reduce the ground-fault currents to magnitudes equal to or lower than the three-phase fault current, but not lower than 25% of the latter to deter transient overvoltages.

Autotransformers have a low zero-sequence reactance that helps to generate large ground-fault currents. A reactor inserted in the neutral reduces the current to the three-phase value or less.

 

Hybrid High-resistance Grounding (HHRG)

As mentioned in a previous article, grounding medium-voltage generators through low-resistance provides adequate ground-fault current to stabilize the neutral shift and allows the correct operation of the ground-fault protection scheme. But when the fault is inside the generator, low-resistance grounding cannot prevent damage caused by the ground-fault current.

An IEEE/IAS Working Group has proposed the hybrid high-resistance grounding (HHRG) method. The objective behind HHRG is to minimize the damage to generators when subjected to internal ground-faults. With the HHRG method, the conventional system is low-resistance grounded, reacting rightly for external ground faults, and switching to high-resistance grounding (HRG) in the event of an internal generator ground-fault.

For more information, see "Switching Transient Analysis and Specifications for Practical Hybrid High-resistance Grounded Generator Applications," by the IEEE/IAS Working Group, Presented at the 2009 IEEE IAS Pulp & Paper Industry Conference in Birmingham, AL.

 

A Review of Effective and Low-impedance Grounding

In a solidly grounded power system, the connection to the ground of the generator, transformer, or grounding transformer neutral does not include an intentionally inserted impedance. But the neutral link is not zero-impedance because of the impedances in the zero-sequence circuit. The ratio ground-fault current/three-phase fault current commonly describes the degree of grounding. The ANSI/IEEE standards use the power system constants to define that a system, or a portion of it, is effectively grounded when Xₒ/X1 ≤ 3 and Rₒ/X1 ≤ 1.

Effectively grounded networks generate the highest ground-fault current magnitude. This could range from a low value to thousands of amperes, posing a risk for people and equipment.

The low-impedance grounding method is mainly used to protect generators by limiting the level of the ground-fault current to a value less than or equal to the three-phase fault current. The impedance can be a reactor or a resistor.

The lower limit of ground-fault current in low-reactance grounding is 25% of the three-phase fault current. Lower values may cause destructive transient overvoltages.

Low-resistance grounding may limit the ground-fault current to lower values than low-reactance grounding with less risk of creating damaging overvoltages. This higher current limitation capability is the main reason to prefer low-resistance grounding to low-reactance grounding.

 

Read more about how to calculate effective and low-impedance grounding for power systems.