What is the Function of an Earthing or Grounding Transformer?

Generally, an Earthing or Grounding Transformer is not a three phase transformer, but a single-phase transformer with the primary (HV end) rated voltage equal to the system phase-to-neutral voltage and the secondary (LV end) rated voltage either 240V or 11oV.

To avoid any switching transient in the Transformer primary of the transmission system, or a big noise may damage the Transformer. To avoid such spurious signal grounding is provided in the primary. The grounding also affects third harmonics from switching transient.

The reason why a grounding or earthing transformer is needed in large or heavy-duty power transformers is that a grounding of a transformer allows the power grid to have a common point of reference for the voltages; otherwise, the voltage may vary from place to place. Earthing or Grounding The transformer also allows for safety for both transformer and people to use power.

What is the Use of Earthing Transformer?

Generally grounding transformer or an earthing transformer is used to provide a ground path to either an ungrounded way or a delta-connected system.

  1. Earthing Transformer allows the connection of phase-to-neutral loads when desired.
  1. Earthing the transformer provides a relatively low-impedance path to the ground maintaining the system neutral or ground potential;
  1. During re-striking ground fault Earthing Transformer limits the magnitude of transient overvoltages;
  1. During line-to-ground faults Earthing Transformer supplies a source of ground fault current;

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"Transformer Capacity and Rating"

Electrical point of view ground and earth are same. 

We can assume that earth is not a super conductor but has a zero resistance. So it can act as a big collecting basin for unwanted voltages and currents what is the function of earthing. 

Typically Earth line is connected locally to ground and neutral line goes back to transformer. Neutral provides close circuit path for current. While Earthing provide safety and protection for use.

In star connection there is only one neutral line, which is consider as a reference voltage that is may be not zero.

Why Earthing or Grounding Transformer is Required for Electrical Energy Transmission Grid?


Understanding the requirement of Earthing Transformer or Grounding Transformer in the large power transmission grid, we would like to investigate the following case study-
Why Earthing or Grounding The transformer is required, of course for economic reasons; let us see:  for example, we are dealing with an 11kV system, whose neutral we want to ground through a resistance.

Let consider the desired ground-fault current is 10A. Now, if we want to connect a resistor in the path between the system neutral and earth, the calculated value of the resistor would be (6350/10 = 635 Ohms) and rated voltage would be 6350V (11,000/1.732=6350). Easily we can observe that practically 6350 V rated a 635 Ohms resistor would be very costly that not economical.

But, if we choose for a single-phase 6350V/240V (input voltage 6350 Volt and output 240 Volt) neutral grounding transformer, whose 6350V winding is connected at neutral to the ground path. In this case, we can connect a simple 0.9 Ohms resistor across the 240V secondary of this neutral grounding transformer.

Here 0.9 Ohms resistor connected at 240V side will reflect as 700 ohms [Square (6350/240) = 700]. The 0.9 Ohms resistor connecting at grounding transformer secondary 240 volts we will get reflected 630 ohms (700x0.9= 630). So, the advantage is that this resistor needs to be insulated only for 240V. Reduced the insulation level of resistor means cheaper the cost, the result is an economic system.

A grounding transformer or earthing transformer is one kind of auxiliary transformer that is used in three-phase electric power transmission systems to provide a ground or earth path. In short, we can say Earthing or Grounding Transformers are part of an earthing system of the electrical transmission network.

Download the Earthing Transformer PDF copy of Earthing Transformer design or Grounding Transformer design guide:



Types of Earthing Transformer

The following two types of earthing transformers are generally used grounding transformer is used to provide a low impedance path to the neutral and also limits the transient overvoltage to the system when the system neutral is not available:

  1. delta-star grounding transformer;
  2. zig-zag grounding transformer.

Types of Earthing Transformer: delta-star grounding transformer and zig-zag grounding transformer


  1. NGR is important to a trnasformer earthing. NGR(Neutral Grounding Resistor) is nothing but a high resistance grounding which in context of electrical power system means the grounding of the system
    neutral through a resistance which limits ground-fault current to a value equal to , or slightly greater than the capacitive charging current of that system.

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