『TRA-Jiro』is a web application tool, developed by CRIEPI, that can calculate the inrush current and the resulting voltage drop that results from that current when a transformer is charged . This tool can be used by anyone.

When a Transformer is charged (by connecting to the power grid), currents up to several dozens above the trasnformer rated current can flow. The current that flows at that time is called inrush current, and it is a phenomenon that has been known for a long time.

The inrush current can depend from several factors, such as the state of the transfomer before it was charged, how big the grid voltage is at the time of the charge, the phase etc, and can sometimes lead to a large flow of current, and sometimes not. When a large current flows, an instantaneous voltage decay occur in the grid around the transformer (transient voltage drop).

In order to perform an accurate calculation, a power system simulator (several types of simulators with specific features exist. In Japan, the application software 『eXpandable Transient Analysis Program - XTAP』, developed by CRIEPI is largely used) is used. This web application can, however under certain assumptions, also calculate with similar accuracy the magnitude of the inrush current as well as the transient voltage drop when a transformer charges.

This web application represents transformer and power systems as electric circuits, solves its differential equation (in relation to the voltage and current) , and by deriving the equations for the current and voltage, can calculate the inrush current and voltage drop with great accuracy and speed. In the derivation of the circuit, the following assumptions are made. if the calculation conditions cannot satisfy these assumptions, the calculation might not be correclty performed.

• The power system and transformer are 3-phase balanced.
• The circuit breaker that charges the transformer are introcuded simultaneously on 3 phases

While transformers have several wiring and winding counts configurations, this web application only supports transformers that are frequently used in power systems. The types of transformers that can be supported is expected to increase.

Please refer to the following reference for more details regarding the calculation principle