Grounding Transformers Explained

Grounding transformers are normally constructed with one of the two configurations

Grounding transformers are used to provide a path to ground for unbalanced load current and for fault currents on systems where a suitable ground is otherwise not available. Grounding transformers are normally constructed with one of the two configurations: Zig-Zag (Zn) with or without an auxiliary winding or a Wye (Ynd) with a delta connected secondary. The Zig-Zag connection is preferred as it is more cost effective and physically smaller than the Delta-Wye unit. For added safety, neutral ground resistors (NGR) are often used in conjunction with the grounding transformer to limit neutral ground fault current magnitude. The rated voltage of the NGR should be equal to the line to ground voltage of the grounding transformer. The current rating and duration should match the grounding transformer ratings. It is important to set the current rating high enough to be above the cable charging current and grounding transformer magnetizing current.

Optional Features

• High fire point liquid : FR3, Silicone or Beta Fluid
• Detachable cooling radiators with shut off valves
• Galvanized cooling radiators
• Substation type design
• Tamperproof type design
• Primary, secondary and neutral current transformers

Transformer Grounding

The importance of proper grounding for electrical systems in general and electrical transformers in particular cannot be overemphasized. Under normal conditions, an electrical circuit may continue to operate satisfactorily (that is, deliver power to the utilization equipment) even without proper grounding. It is not until an abnormal condition has occurred and after either someone has been injured, equipment has been damaged, or a fire has been started, that it is realized that improper or faulty grounding was the reason. Electrical systems are grounded to:

• Ensure that, at all times, the noncurrent-carrying metallic parts of an electrical system are kept at zero potential of the earth. This is done to prevent people who come in contact with them from receiving an electric shock
• To limit voltages to ground. Unintentional line surges, lightning strikes, or contact with higher voltage lines may result in voltage being placed on the electrical system that could damage or destroy electrical components and equipment.
• To stabilize the voltage with respect to ground. Grounding electrical systems protects equipment by ensuring that the maximum phase-to-ground voltage is not exceeded.
• To ensure that overcurrent protection devices open under ground-fault conditions. Grounding electrical systems ensures that a low-impedance ground path for fault current is present. The impedance of the ground path is kept low by ensuring that all electrical connections and any other connections or terminations in the ground path are electrically continuous and installed in a manner that ensures very little opposition to the flow of current.
• Article 250 of the NEC covers the general requirements for grounding and bonding of electrical installations. Requirements for the grounding of transformers are divided according to the primary and secondary voltages. As an example, a transformer having a secondary voltage of less than 50 volts is required to have its system grounded if the primary voltage is greater than 150 volts to ground.

Terms used in grounding are similar and may be misinterpreted if care is not taken. Article 100 of the NEC contains important definitions that relate to grounding of electrical systems. These include the terms ground, grounded, effectively grounded, grounded conductor, grounding conductor, equipment grounding conductor, grounding electrode conductor and bonded. Grounded is defined as connection to earth or to some conducting body that serves as earth. Bonded is defined as the permanent joining Df metallic parts to form an electrically conductive path that will ensure electrical continuity and capacity.

In general, an electrical system derived from a transformer is required to have noncurrent-carrying metal parts and equipment grounded the same as any cther part of the complete system. Most transformer installations are considered to be separately derived systems, and their grounding requirements are covered in Section 250.30 of the NEC. According to this section the methods used for establishing a ground system shall be the nearest one of the following:

• The steel frame of the building
• A metallic cold water pipe
• A rod driven into the ground

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