Ground Fault Protection-Restricted versus Unrestricted Methods

Ground fault protection for a power system network provides in two ways. They are Restricted Ground fault and Unrestricted Ground Fault Protection methods.

Let’s see briefly what Ground fault protection is,

There is a difference between normal unbalance and ground fault currents. The Unbalanced currents due to Single Phasing or Unequal loading on three phases, will flow only through the neutral path whereas, in the case of ground faults the fault current flows through the grounding path.

Ground fault protection of any system depends upon the number of wires used either 3 wires or 4 wires and the type of grounding either solidly grounded or impedance grounded.

Restricted Ground Fault Protection:

This protection is used when we need to protect the circuit within a particular part or confined area. The outside faults are not detected by Restricted ground fault protection devices.

Generally, this is an equipment protection scheme suitable for generators, transformers, and all similar circuits that require individual protection. This scheme is used to isolate the equipment in case of faults outside the restricted zone.

For transformer, Restricted Earth Fault Protection Scheme is explained in detail here.

Transformer Restricted Earth Fault Protection Scheme

Unrestricted Ground Fault Protection:

Combined Ground fault and Phase fault protection scheme is widely employed. Unrestricted Ground Fault Protection scheme is mostly used for a system where numbers of load points are more such as distribution network. For this type of network, common protection is provided for ground faults and phase faults at the incoming feeder. In the case of MV switchgear, each feeder is provided with a common Phase and Ground fault protection.

In this case, a restricted earth fault protection is not required, as the system needed ground protection rather than individual equipment ground protection. This scheme is usually employed for LV and MV systems; it is neither possible nor recommended to employ restricted ground fault protection for each individual feeder.

Unrestricted ground fault protection-Connecting Methods:

This type of protection is employed for three-phase three-wire or four-wire systems as shown in the below figures.

There are three possible connections available in this scheme. The type of scheme is based on economical factors and the importance of the protection level needed.

Protection for Three-phase Three Wire Systems:

1.Only Ground fault protection

In this scheme, only a single ground fault relay is used for all three phases as shown in the below figure.

2. Three over current and one Ground fault combined protection

In this scheme, three overcurrent relays for each phase and a single ground fault relay is used for all three phases as shown in the below figure.

3. Two over current and one Ground fault combined protection

In this scheme, two overcurrent relays connected in two phases, and a single ground fault relay is used for all three phases as shown in the below figure.

Note: In Numerical relays, a single relay provides all these protections. It has R, Y, B phase elements, and a ground element.

Protection for Three-phase Four Wire Systems:

There are three possible connections available in this scheme also. The type of scheme is based on economical factors and the importance of the protection level needed.

These schemes also same as above except it consists of additional Neutral wire. The connection diagrams are given below.

1. Only Ground fault protection

2. Three over current and one Ground fault combined protection

3. Two over current and one Ground fault combined protection