Method for calculating primary time constant of power grid

A method for calculating a primary time constant of a power grid, comprising:

• establishing, according to actual power grid parameters and a network topology, an electromechanical transient model of the power grid by using a Power System Analysis Software Package (PSASP);
• establishing an electromagnetic transient model of a local power grid for a site where the primary time constant of the power grid is calculated, by using the PSASP according to the actual power grid parameters, and setting a ground short circuit fault at the site;
• calculating automatically a transient short circuit current of the site by using a hybrid simulation of the electromechanical transient model and the electromagnetic transient model of the PSASP;
• and
• filtering out a periodic component in the transient short circuit current to obtain a non-periodic component attenuated with time, calculating an attenuation time constant of the non-periodic component, the attenuation time constant being the primary time constant of the power grid,
• wherein the periodic component in the transient short circuit current is filtered out by:
• sampling a first transient short circuit current at a sampling point of time t;
• sampling a second transient short circuit current at a sampling point of time t+T/2, wherein T is a period of the periodic component;
• superposing the first transient short circuit current and the second transient short circuit current to obtain the non-periodic component attenuated with time.