Generator Field Resistance Varies with Generator Operating Point

Has anyone modeled an SPS "Synchronous Machine pu Standard" generator field circuit and found that the generator field resistance varies significantly with the changes in the generator's operating point? The field resistance would change with temperature, not system conditions. Or does anyone know how the field resistance is calculated inside the generator block?

The field current calculation is done within the synchronous machine block using a value of field resistance generated within that block. Field resistance is standard data provided by the generator manufacturer, but it is not an input parameter for this generator model.

I am modeling a synchronous generator and a connected power system to study the effects of including a field discharge resistor, arrester, or short circuit in the generator's field circuit.

The field circuit is modeled in physical units (volts and ohms) and is connected to a generator block through a Field Connection block.

The generator I am modeling has a field resistance of 0.088 ohms at operating temperature and requires a 5,860 ADC field at rated load, according to the manufacturer's data. The required generator field voltage at rated load is then 515.6VDC. When the model is run with the excitation voltage fixed to that value, the steady state field current is 9035A and the Mvar output is twice rated.

The field resistance used by the block for this run was determined from the applied field voltage and the reported field current, 515.5V/9035A=0.057 ohm.

I added a 0.031 ohm resistor (0.088-0.057) in series with the F+ terminal of the Field Connection block to establish a 0.088 ohm field circuit. Re-running the model, the steady state field current was now 4920A, 83% of the manufacturer's value, and the Var output was 137Mvar, 28% of the rated Var capability. The voltage at the generator block Vf input was 362V. The field resistance across the generator F +/- terminals was now 362/4920=0.073 ohm, an increase of 28%.

Trial and error determined that a series resistor of 0.018 ohms resulted in a field current of 5875A and a generator reactive output of 430 Mvar. Both are near the manufacturer's values for full load, and verify the model will provide reasonable steady state results if the field current is correct. The apparent internal field resistance of the generator block was 0.07 ohms.

I am concerned that large variations in field resistance will make transient analysis inaccurate.

Answer 1

[Leepakshi]

Hey Donald,

The Synchronous Machine pu Standard block does not use the manufacturer’s physical field resistance directly. Instead:

It computes an apparent field resistance internally based on per-unit machine parameters and operating conditions (including saturation).

Variations you see are due to the pu model and nonlinear magnetization, not temperature.

For physical studies (e.g., discharge resistor, arrester), use the Field Connection block and adjust series resistance to match manufacturer field current and reactive power.

These internal variations do not compromise transient accuracy because the block models flux linkage dynamics rather than physical resistance.

Refer to below link for Synchronous Machine pu Standard Mathworks Documentation:

https://www.mathworks.com/help/sps/powersys/ref/synchronousmachinepustandard.html

Hope it helps!