If you need to place a current transformer (CT) in the neutral of a transformer, the first step is to distribute the neutral connection of the transformer. This can be configured in the Grounding/Neutral Conductor tab within the Basic Data section of the transformer element dialog.

There are two main options for the N-Connection setting. The first option, "N-Connection", distributes the neutral to the same busbar where the transformer’s phase terminals are connected. It is important to ensure that this busbar supports a phase configuration that includes a neutral, such as ABC-N. This is defined in the Phase Technology parameter of the busbar.

The second option, "Separate N-connection", allows the neutral to be connected to a different busbar than the one used for the phase connections. This is typically a dedicated neutral busbar configured with a phase technology of N. To simplify configuration, it is recommended to leave the Cubicle selection empty at first. After creating and placing the neutral busbar in the model, you can right-click on the transformer and choose "Connect...". When the transformer is connected to the new neutral busbar, the cubicle selection will automatically be populated with the correct information.

Regardless of the chosen method, once the neutral is distributed to a busbar, it must be explicitly connected to earth. This is done using additional network elements that represent the type of earthing required, such as solid, resistive, or reactive grounding. These grounding elements should be configured as single-phase devices. To assist in this process, the "Figure" button on the right-hand side of the branch element dialog can be helpful.

Once the neutral circuit is correctly set up, a current transformer can be inserted into the model in the same way as any other CT. Right-click near the appropriate cubicle in the neutral circuit and select Devices → Current Transformer → New.... The CT should be configured as a single-phase device, and the phase should be set to N.

In Example 1, a CT is placed in the neutral of autotransformer T3. The first option for neutral distribution was used. The transformer is earthed through a series reactor, which is configured as a single-phase element and connected to the neutral terminal of busbar St_6. The CT is located in Cubicle Cub_8 of the same busbar.

In Example 2, a CT is placed in the 20 kV neutral of a two-winding transformer T3. The second option for neutral distribution was used in this case. A dedicated busbar, named "Neutral Busbar", is used. The transformer is again grounded through a series reactor configured as a single-phase element. The CT is placed in Cubicle Cub_2 of the Neutral Busbar.

Example 3 demonstrates an application of Restricted Earth Fault (REF) protection. This protection method works by monitoring both the neutral current at the star point of a transformer and the three-phase current supplying the transformer. A CT is installed at the transformer’s star point to detect zero-sequence (neutral) currents. Three-phase CTs on the supply side allow the calculation of the zero-sequence current entering the transformer. If there is a difference between the measured neutral current and the calculated incoming zero-sequence current, it indicates a earth fault within the protected zone of the transformer. This allows for sensitive differential protection focused specifically on internal transformer earth faults.

The Diagram Layout Tool was used to add the relaying elements to the single-line diagram in the examples.