What is a grounding transformer and why do we need a grounding transformer

What is grounding

Grounding is done in an electrical system so that equipment can be protected from hazardous voltages. There will be a conductor to provide a low impedance path to the earth, during normal conditions this conductor won’t carry current.

What is a neutral and does the neutral wire carry current

A neutral is a reference point in an electrical system where the conductors are not carrying current underbalanced condition.

What is a grounding transformer and what is the purpose of neutral grounding transformer

A transformer is a device that does the transfer of AC voltage from one level to another, it can also increase or decrease the voltage as per requirement. An earthing transformer is a transformer which provides a neutral point for grounding purpose. So the grounding transformer will pass the ground current, in case of any earth fault and it won’t carry any useful load. These transformers can be used to create a ground path for delta connected or ungrounded star or wye connected systems. The system can be maintained at ground or neutral with the help of a ground transformer. If a single line to ground fault occurs in an ungrounded system, then there is no return path for the fault current. So there won’t be any current flow in the faulty line and this will cause an increase in voltage on the other two lines. This will damage the component, by using a grounding transformer this can be prevented.


Grounding transformers are single winding transformers with zigzag configuration and in certain cases these are combined with special star-delta configuration. The star connection of the primary is connected to the supply system and the delta connected secondary remains unloaded. In the event of a fault, the load on the secondary is a short time for a grounding transformer. So the grounding transformer’s power rating is smaller than a power transformer. These transformers are not supposed to carry a load, but in any case, if one of the lines becomes grounded it supplies the current to the load. Due to this the size and cost of a grounding transformer are less when compared to a power transformer.

Where are grounding transformers used

Grounding transformers are used in wind farms. They utilize grounding transformers for fault protection on underground lines. During the ground fault in a collector cable, the circuit breaker in the substation will open, and it will isolate the turbine string. Turbines won’t be able to detect the fault all the time and generators continue to energize the cable. The transformer placed on the turbine string will provide the ground path. Grounding transformers are mostly used in electrical transmission, utilities, and industrial systems.

Why do we need a grounding transformer

Grounding transformers are mostly used to provide a ground path to an ungrounded “Y” or a delta connected system. Grounding transformers has other functions like

  • They can maintain the system neutral at or near ground potential, by providing low impedance path to the ground
  • They can limit the magnitude of transient over-voltages when a re-striking ground fault occurs
  • It can also provide a source of ground-fault current, during line to ground faults
  • It also allows the connection of phase to neutral loads when required

Grounding transformer is used to provide a physical neutral for a power transformer with a delta connected downside. They can also be equipped with a low voltage auxiliary winding to be used as a local power supply to the substation.

How do you create the ground or what if no neutral exists

In a delta system, there won’t be a neutral, so a grounding transformer is installed in a zigzag or wye-delta, from all three phases to create an artificial neutral for grounding purposes.

What are the types of grounding transformers

Grounding transformers are constructed in two types zigzag and wye connected

What is a zigzag grounding transformer

This design can be a transformer with just one winding, and they are zigzag connected. The zero sequence impedance of such a winding is quite low, but it can be increased if the purpose is to limit the current through the transformer in case of an earth-fault in the system. The zigzag connection has two windings on each leg, each leg of the zigzag connection is connected to a winding from another which is out of phase, and the resulting zigzag connection is phase-shifted with respect to the incoming three-phase source.

Delta connection

An alternative connection to zigzag is the delta connection. This type of winding can compensate for the zero sequence magnetic field and due to this it is restricted to a leakage field between the star and the delta winding and makes the zero-sequence impedance of the transformer relatively small.

How to protect grounding transformers

During low currents system earth, fault protection relay won’t be effective. In order to prevent damage to the earthing transformer by such current, an oil thermometer with alarm trip contacts can be used.

What are the applications of earthing or grounding transformer

These transformers are used in utility distribution networks and in some power electronic converters in order to provide a neutral point in a three-wire system. In order to achieve a vector phase shift, a combination of the wye, delta, and zigzag can be used.