Grounding Transformer: Solution To The Ungrounded System?

Introduction:

What does one do when the neutral point is not available? As the ungrounded system problems became more apparent to industry, they recognized that ground their delta-connected systems was to their advantage. Some took the approach of purposely grounding one phase.

Neutral points are an excellent way to ground an ungrounded delta system because this reduces line-to-line overvoltages and eliminates the multiple faults mentioned previously. However, a great solution is that you can derive a neutral point through grounding transformers. Let’s discuss the topic from the basics.

The concept of grounding:

Grounding is a safety measure that prevents an electric shock by connecting the ground wire of an electrical system to the earth. The ground wire can be connected to the earth by digging a hole and placing an electrode on the earth. This is one of the simplest ways of providing a path for current to flow to the ground. 

Why Grounding is necessary?

Grounding is a safety feature that helps protect people and equipment from electrical shock and equipment damage. It is essential for the proper functioning of electrical equipment and the electrical system. 

Without proper grounding, anyone can get an electrical shock or equipment may get damaged. For example, improper grounding can: cause equipment to malfunction increase wear and tear on equipment reduce the life of the equipment. Incorrectly grounded systems are often not detected until there is a problem.

What is a grounding transformer:

A grounding transformer is a device that is used to provide a ground path to either an ungrounded Wye or a Delta-connected system. Typically delta configuration doesn’t have any neutral wire, so with the help of a grounding transformer phase-to-neutral load can be used in a delta-connected three-phase system.

Grounding transformer can be both zigzag winding and wye-delta winding configuration.

Grounding Transformer
Courtesy: © Olsun Electrics Corporation

How does a grounding transformer work?

A grounding transformer is used to provide an electrical ground in a three-phase electrical system, where there is no neutral wire. 

Let’s say we have a three-phase electrical system with two phases connected to a building and the third phase connected to the ground. This system might work for transient overvoltage criterion but the system will be left with continuous L-L overvoltage conditions.

If you connect a delta-connected grounding transformer between the two phases of the system, it will provide a path for the current to flow to the ground. This way, if any of the phases become ungrounded, the current will be able to find its way back to the ground via the delta-connected grounding transformer. 

Benefits of Grounding Transformer:

It provides a low resistance path for the current to return to the ground in case of an ungrounded phase. Delta-connected systems often malfunction when there is no neutral wire. By using a delta-connected grounding transformer, we can provide a neutral point for the electrical equipment. This way, if any of the phases become ungrounded, the electrical equipment will not be damaged as much. It extends the life of the electrical system and equipment. 

The grounding transformer provides a source for zero-sequence current and stabilizes the system neutral. [1]

When a single line-to-ground fault occurs on an ungrounded or isolated system, due to the lack of return path, the fault current cannot flow. However, the system will not operate correctly because the other two un-faulted lines will not have been put at zero potential, so they will raise their voltage by the square root of three, overstressing the transformer insulation and other associated components on the system by 173%. By using a delta-connected grounding transformer, we can protect our electrical system from happening this incident.

Grounding transformers help to prevent overloading the electrical system and also limit the magnitude of transient overvoltages during a restriking ground fault.

Large multi-turbine wind farms require grounding transformers. They provide a ground path in the event the turbine strings become isolated from the system ground.

Types of Grounding Transformer:

Wye-Delta grounding transformers:

The “standard” or most common type of grounding transformer. It is basically a 2-winding wye-delta transformer. It is more expensive than a zig-zag grounded transformer but provides better performance.  Below provide (a) is the conceptual diagram of Wye- delta grounding transformer.

Zig-zag grounding  Transformer:

A “zig-zag” transformer geometry is very useful to limit the third harmonic currents circulation and can be used at a lower cost than a two-winding delta connected transformer. It also provides better grounding than a 2-winding wye-delta transformer. The “standard” or most common type of grounding transformer. It is basically a 2-winding wye-delta transformer. It is more expensive than a zig-zag grounded transformer but provides better performance.  Below provide (b) is the conceptual diagram of Zig-zag grounding transformer.

Rating of the Grounding Transformer:

A grounding transformer is commonly rated as short time such as 10 s as it only carries short-circuit ground current until the protection system clears the fault and disconnected the circuit. In this way, costs can be reduced.

Like a power transformer, a three-phase grounding transformer also rated as kVA, is equal to the rated line-to-neutral voltage in kilovolts multiplied by the rated neutral current that the transformer is intended to carry during fault conditions for a specified time.

Typically they are designed to carry the rated current for a limited time e.g. 10 s or 1 min. As a result they a small in size.

The rated voltage of a grounding transformer is the line-to-line voltage for which the unit is designed.

As per IEEE Std 32-1972 a grounding transformer will have a continuous current rating of 3% for 10 s and 7% for 1 min. [2]

Please ref. to here for overall idea of how to order a Grounding Transformer.

Conclusion:

In summary, the choice of grounding transformer is dependent on the type of load, and system design requirements. The concepts of grounding, isolation, and protection are important to understand in order to ensure safe and reliable grounding transformer installations. 

Reference:

  1. Edson R. Detjen, and Kanu R. Shah, “Grounding Transformer Applications and Associated Protection Schemes”, IEEE Transactions on Industry applications, Vol. 28, N0.4, July / August 1992
  2. M. Shen, L. Ingratta and G. Roberts, “Grounding transformer application, modeling, and simulation,” 2008 IEEE Power and Energy Society General Meeting – Conversion and Delivery of Electrical Energy in the 21st Century, 2008, pp. 1-8, doi: 10.1109/PES.2008.4595995.

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