The SSN algorithm is able to detect Impulse Events during a simulation. By Impulse Event, we mean the instantaneous opening or closing of a switch (most often a diode) following the open or closing of another switch in the system.
This happens, for example, in a buck converter in which the free-wheeling diode turn-on instantaneously when the forced switch (IGBT or MOSFET) opens.
In a real-time simulation, this type of event is difficult to simulate accurately: switch natural conduction conditions are usually evaluated at the beginning of a time step--so if a forced switch changes state, its effect is only detected on the next time step.
In ARTEMiS and ARTEMiS-SSN algorithm, we use the fact that the state of a system cannot change instantaneously when a switch changes of conduction state. We can, therefore, re-evaluate the switch voltage after any forced switching by simply re-evaluating the outputs of state equations.
In the ARTEMiS-SSN algorithm, some caution is to be taken for the Impulse Event Detection to work correctly because this Impulse Event verification is made on a group basis.
The above figure depicts a 3-level Neutral clamped inverter drive system in SimPowerSystems and SSN. Each arm is composed of 4 IGBT/Diode pairs plus 2 clamping diodes.
The real-time simulation of this model is challenging because it is composed of 30 coupled switches.
In the solution above, since SPS has a switch model for the IGBT/Diode pairs, the internal number of switches reduce to 18, which make real-time simulation impractical because of the high number of matrix permutations to compute (2^18).
The solution in SSN is to put each arm in a separate group of 10 switches (6 internal SPS switches, considering the IGBT/Diode pairs as one device).
The resulting model has 5 groups and 6 nodes. The inverter was separated at the arm level to obtain 6 SPS switches per group, which can be precomputed and run in real-time after.
Note: The NIB blocks ARE the nodes in SSN.
A key aspect of the separation with Impulse Events: the NIB that connects to the output of the inverters (A connector) are of I-type to provide the group with an image of the continuity variable during impulse events, that are the transformer input currents.
This so-called continuity variable is the model variable that will not force instantaneous switching in the inverter.
In the above example model, suppose that all IGBTs are suddenly turned off. The transformer input current will drop but without discontinuities. This current will force some anti-parallel diode in the inverter to turn on momentarily.
Such methods were actually used by ABB to simulate such 3-level NPC converters. See References for details.
Impulse Events in SSN
This last point is important to understand. It is caused by the fact that the SSN algorithm does not make multiple iterations of the equation to verify Impulse Events like instantaneous diode turn-on effects. It only re-evaluates the Outputs of a group for natural switch threshold crossing each time a forced switch is activated. This can be done on the basis that the states of a system cannot change instantaneously on a switching action.
In general, a switched device using diodes as a free-wheeling diode (for example) will have a branch that forces the continuity of the current at switching time. This element must either be grouped with the switching elements (best case) or the NIB must have an I-type interface to give an image of this element in the groups for the SSN Impulse Event Detection to work. In the example of the 3-level NCP inverter, this element is the transformer primary leakage inductance.