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# EMTP | Machines

# Synchronous and Asynchronous Machine

## Saturation Characteristic

The saturation characteristic of a machine is represented by an analytical model describing the excitation current versus the open-circuit voltage in a series of exponential functions. The HYPERSIM model, similar to that of the PSSE software, is defined by the parameters EU, EL, SGL, SGU.

HYPERSIM deduces the SGL and SGU parameters from the D-axis data on the EMTP saturation curve (points in the table) for the voltage Vt = 1.0 and 1.2 pu (EL, EU). HYPERSIM does not represent saturation in the Q axis.

The HYPERSIM algorithm uses eight points (positive dial) at Vt = [0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5] calculated from the analytical curve defined by the SGL and SGU parameters. Note that the characteristic of the analytic model is linear from Vt = 0 to 0.8 pu, beyond which it becomes nonlinear. The following figure illustrates a numerical example.

**EMTP Sample Saturation Curve in HYPERSIM**

The figure shows in red a saturation characteristic curve defined in EMTP by two straight segments. The curve in blue is obtained from the EMTP curve by using eight segments to the right. For more precision, we will define the saturation characteristic by at least three segments. To correspond with HYPERSIM we will define eight segments deduced from the analytical curve defined by the SGL parameters and SGU.

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