Neutral Grounding of Industrial Power Systems

Neutral Grounding Resistors | Source: Chase Power AU

 

Neutral grounding of industrial power systems has always been a controversial topic. Historically, systems with ungrounded neutral were dominant because of the service continuity with a ground fault on the system. This resulted in high system availability because there was no need to trip after the first ground-fault inception. However, as industrial power systems became more complex, transient overvoltage during a ground fault became more severe, making ungrounded-neutral systems less attractive.

Different Methods of Neutral Grounding

The figure above shows different methods of neutral grounding: 

1. Ungrounded 
2. Solidly Grounded
3. Resistance Grounded
4. Reactance grounded.

In general, any neutral-grounding method requires a connection of the system’s neutral to ground at one or more points. According to the nature of the grounding connection, those methods can be divided into two categories: solid grounding and impedance grounding. Depending on the impedance type, impedance grounding can be further divided into resistance grounding and reactance grounding.

Related Article: Design Guide: Resistance Grounding in Electrical System

There are two main reasons for neutral grounding:

1. Controlling the voltage-to-ground level (within predictable limits) to limit the voltage stress on the phase conductors’ insulation. Moreover, voltage control reduces the shock hazard to personnel who may come in contact with live conductors. 
2. Providing a low-impedance path to the ground-fault current. This enables the ground-fault relays to detect any the unwanted connection between the phase conductor(s) and ground and to initiate a trip signal to clear the fault.

Solidly Grounded Systems

Connecting the system’s neutral directly to the ground with no intentional impedance (i.e., zero impedance) has the effect of shorting out the system’s charging capacitances. The flow of high ground-fault currents has a few adverse consequences. Severe burn damage at the point of the fault is always associated with such high currents.

Solidly Grounded System

Resistance-Grounded Systems

Grounding neutral via resistance is a common practice in North America, especially for industrial power systems. The equivalent circuit of a resistance-grounded neutral system. Resistance grounding may either be low or high. The distinction is based on the magnitude of the ground-fault current permitted to flow. 

Resistance Grounded System

Both low- and high-resistance grounding is designed to limit the transient overvoltages to 250% of the system’s voltage. Because of the elevation of the neutral voltage with respect to ground, resistance-grounded systems must be protected using surge arresters rated for ungrounded systems.

Reactance-Grounded Systems

In this case, the system’s neutral is connected to ground via a reactor. Similar to resistance-grounded systems, the magnitude of the ground fault current permitted to flow in the reactor is used to describe the level of grounding. Reactance grounding comes in two types—low and high—where both are referred to as untuned-reactance grounding. There is also an important special case of the high-reactance grounding, i.e., a ground-fault neutralizer that can also be referred to as a tuned-reactance grounding or Peterson coil.

Reactance Grounded System

For any given system, there are four factors that should be evaluated before selecting a grounding method: 

1. The sensitivity and selectivity of the ground-fault protection system 
2. The permissible magnitude of the ground-fault current 
3. The required degree of the surge-voltage protection 
4. The limitation of system transient overvoltages.

 

With the availability of various neutral-grounding methods, it is important to understand the operational charracteristics of each. The design and selection of the most appropriate grounding method is always a compromise between competing objectives. Therefore, performance requirements of the power system should be carefully evaluated to choose the most appropriate method.

Characteristics of Different Grounding Methods

Reference: 

Title: A Design Guide to Neutral Grounding of Industrial Power Systems

Authors: Nehad El-Sherif and Sheldon P. Kennedy

Publisher: IEEE Industry Application Magazine, 2019

Document Type: PDF | Download