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Star and Delta Grounding Systems

There are many methods for grounding system. The most widely known and widely used of these methods is the Solidly-Grounded grounding system (neutral point directly connected to the ground without resistance). In this article, we will examine the Solidly-Grounded grounding system and discuss grounding faults.

 

 

Solidly-Grounded Star (Y) System;

 

The Solidly-Grounded system is the most common system arrangement and one of the most functional. Star (Y) system is one of the most commonly used installations as it supports single phase and phase-neutral loads. Grounding of the Solidly-Grounded Star (Y) system considering the neutral point is shown below.

Points to note on the figure above;

First, the grounded system voltage is fixed by the phase-neutral winding voltage. The grounding of parts of the power system, such as the material structure and the rest of the environment, has major consequences as the rest is at zero potential to a significant extent.

Namely, the line-to-ground insulation level of the equipment needs to be as large as the phase-neutral voltage, which is 57.7% of the phase-to-phase voltage. Also, the system is less sensitive to phase-to-ground voltage transients. In other words, the system is suitable for supplying line-neutral loads.

Since the phase voltage magnitudes are equal, the operation of a single phase load connected between a phase and neutral will be equal on any phase.

This system is very common at usage levels such as 480Y/277V and 208Y/120V, including most of the distribution systems of public utilities.

While the Star-connected grounding system is more than most common grounding systems, it is not the only grounding system configured in a Star (Y) arrangement.

Solidly-Grounded Triangle (∆) System;

The groundability of the triangular (∆) system is shown below.

 

 

Compared to the solidly-grounded star (Y) system, the solidly-grounded delta (∆) system has some disadvantages. The phase-to-ground voltage is not equal and as a consequence the triangular (∆) system is not suitable for single overloads. Since the only grounded conductor is phase B and can be misidentified, there is a risk of shock without proper phase identification.

The triangular (∆) arrangement can be configured in other ways. Although the arrangement shown in the figure may not seem logical at first glance, it can be seen that this system is suitable for 3-phase and single-phase loads as long as the single-phase and 3-phase load cables are kept separate.

This system can be widely used in small-scale services requiring 240 V AC 3 phase and 120/240 V AC single phase.

It should be noted that 173% of the amount required to ground the phase A voltage is required to ground the phase B and phase C voltages.

Grounding Faults;

A common feature of the Solidly-Grounded earthing systems shown here and other existing Solidly-Grounded earthing systems is short-circuiting, which results in larger amounts of short-circuit current to ground.

This is known as a ground fault. As shown below, the voltage across the faulted phase is static. Since the phase and the faulty resistor are small, a large amount of current moves through the faulty phase.

The current and voltage on the other two phases are not affected. The fact that a Solidly-Grounded earthing system can support a large amount of ground fault current is an important characteristic of this type of earthing system and influences the system design. Statistically 90-95% of all short-circuit systems are ground faults. This is why this issue is important.

A ground fault on an earthing system requires a fault eliminator as fast as possible. Compared to other grounding systems, this is the biggest disadvantage of a Solidly-Grounded system.

A Solidly-Grounded system is very effective in reducing the possibility of transient fluctuations in the phase-to-ground voltage. However, in order to achieve this, the system must be effectively grounded. One measure of the effectiveness of the grounding system is the ratio of the available ground fault current to the available 3-phase fault current. For effective earthing systems, this ratio should usually be at least 60%.

Most public systems providing services for commercial and industrial systems are Solidly-Grounded. Since there are separate grounding lightning rods used within a commercial or industrial facility, multiple grounded neutrals are not preferred for power systems in these facilities due to the possibility of circulating ground currents.

Within the jurisdiction of the NEC (National Electrical Code), multiple grounded neutrals are prohibited for commercial or industrial facilities. Instead, single-ended grounding is preferred for this system.

In general, the Solidly-Grounded system is the most preferred. It is required when single phase, phase-neutral loads must be provided. It also has the most stable phase-to-ground voltage characteristics.

However, the equipment required by this system and the large ground fault currents that it can support are a disadvantage and may hinder the reliability of the system.

Source:

► electrical-engineering-portal.com

09/08/2024