Introduction

The goal of this document is to explain the procedure that allows to find the snubbers parameter values. An eHS model containing machines may eventually behave improperly depending on the topology and its parameters. In this case, it may be necessary to tune the snubbers which are connected in parallel to the current sources in order to stabilize its behavior.

Context

Electrical machines are modeled in eHS by a current-source representation with time lag : At simulation time , all variables are known at ; thus, they can be used to calculate the current flow of the machine at time and inject it as a current source at time .

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Electrical machine current-source representation

However, under this principle, any sudden change in voltage causes a current response only in the next time step. Thus, for the previous time step, the machine looks like an open circuit and spurious spikes may appear in the machine terminal voltage [1]. In order to increase the stability of such representation, RC snubbers are inserted in parallel to the current sources. Their impedance are usually elevated, so the current passing through them is negligible if compared with current injected by the sources.

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Stator circuit

A good initial guess for snubber parameters is and (that is: the snubber impedance is 1000 times larger than the typical impedance of the system). However, if the simulation presents a diverging behavior, it may be necessary to change those values.

Procedure

The following procedure may be used to find the snubber parameters that could make the circuit more stable.

1. Isolate the machine by inserting a very high-impedance in their terminals

The first step is to isolate the machine from the rest of the circuit by inserting a high impedance in the machine terminals, for instance resistances with .

The goal is to isolate the effects of the external circuit from the machine, leaving just the iteration between the current source and the snubbers.

2. Reduce the snubber impedances

Now that the current source is directly connected to the snubbers, their impedance is reduced by decreasing the resistance or increasing the capacitance. The simulation is executed and the results are analyzed. If the model is still unstable, the impedance is reduced again until the results converge.

3. If the results are convergent, removing the input impedance inserted in step 1

Once the results are stable, the input impedance may be removed. The machine will continue to present proper behavior.

Drawback

To reduce the snubbers impedance may eventually increase the current passing through them, and so the dissipated power. There is a trade off to be respected between the model stability and its accuracy, so the snubbers impedance cannot be set indefinitely small. Also, depending on the model, there could be other sources of instability that could interfere with the results and should be fixed before reducing the snubber impedance any further.

Conclusion

The procedure described in this section may be very useful to tune the snubber parameters when the machine presents a unstable behavior. Usually, snubbers with smaller impedances make the model more stable, but they could interfere with the results by draining more current, so a careful search for these values have to be performed in order to have both stable and accurate results.

Reference

[1] A. M. Gole, R. W. Menzies, H. M. Turanli and D. A. Woodford, "Improved Interfacing of Electrical Machine Models to Electromagnetic Transients Programs," in IEEE Transactions on Power Apparatus and Systems, vol. PAS-103, no. 9, pp. 2446-2451, Sept. 1984, doi: 10.1109/TPAS.1984.318398.