How does using multiple SimPowerSystems Ideal Switch blocks in series with snubber impedances compare with reality in my power network in SimPowerSystems 4.3 (R2006b)?

When using multiple ideal switches in series, the snubber circuit must be used. A high value resistance value is then added to allow simulation to work. I am presently building power distribution networks using the Ideal Switch block as the main building block to create virtual power contactors which are then assembled together to create large network of power contactors. I must use the snubber in order to allow this environment to simulate. I would like to know how does using Ideal Switch blocks in series in such a manner compares with reality. My example model is attached as "Basic_network_topology.mdl".

Answer 1

[MathWorks Support Team]

If you build a real circuit with two switches connected in series as in your example "Basic_network_topology.mdl", the voltage appearing across switches when both switches are opened will be dictated by the stray capacitance of each individual switch and by their capacitances to ground. We can assume that the resistive leakage current is negligible.

For a mechanical switch this stray capacitance can be evaluated based on the

switch geometry or it can be measured.

In your example, if you replace the resistive snubber by capacitive snubbers

by setting

 Rs=0 and Cs=10e-12

(assuming 10 pF stray capacitances), you will still obtain 50 V across each switch. See example model "Basic_network_topologyGS.mdl".

Now, if the capacitances are

 Cs1= 10 pF and Cs2=30 pF

 you get 

 Vsw1= 75 V  and Vsw2 = 25 V.

Naturally, if you use a Voltage sensor with given input impedance, the voltage sharing will be affected. For example, using a voltage sensor with 10-Megohm resistive input impedance to measure Vsw1 and Cs1=Cs2= 10 pF will give

 Vsw1| ~0  V Vsw2=100 V

If the voltage source is an AC source of 60 Hz and 100V peak voltage, you would get Vsw1 = 3.8 V peak and Vsw2 = 99.8 V .

In other words, the voltage sharing is highly dependant on the stray capacitances and voltage sensors input impedances as it would be in the real life.