Wednesday, December 23, 2015

Making the simulator faster

I was almost ready releasing the next version of the circuit simulator. The only drawback with this version has been that it is a bit slow. Reasons for this are the repeated loop manipulations that are being performed - restricting the stiff loops, computing the loop currents every time there is a switch turning on or off etc.

The past couple of days I tried a nodal analysis approach in comparison with the previous loop analysis approach. I didn't find much improvement in the speed of simulation. However, the matrices are much more unstable and the simulation time step needs to be much smaller. There probably are improvements I could have made to reduce this instability, but I would rather go with the loop analysis method.

To speed up the simulations with loop analysis, the only method I can think of is to maintain a logbook of all the changes taking place in the circuit and how this impacts the loops. This is under the assumption that there will be a finite number of changes possible and therefore a lookup table can be used to get rid of all the loop manipulations. So the simulation will be slow in the beginning but will speed up later.

Anyway, I am taking a break for the next four days to hang out with friends who are beginning to wonder if I am still alive. Seasons greetings to those who celebrate Christmas and for those who don't (and that includes me) get drunk and be merry anyway. See you before new year when I hope I will release my next version.

Saturday, December 19, 2015

Diode state determination

Diodes and switches are high value resistances when off and low value resistances when on. The way of detecting when the turn on is to measure the voltage across them at every simulation cycle and change their status when the forward voltage exceeds 1 Volt. For a device in off state this might mean a current of 0.5 microAmps.

When running the simulation of a three-phase inverter, I found that the diodes were trying to turn on at every simulation instant even though the switch across them was conduction. On the contrary, the diodes that were across switches that were on and conducting was significantly higher and therefore were turning on.

To describe this look at this switch/diode combination when the switch is on.

The switch is a part of a loop with a current i1. The diode is off and forms a loop with the switch. By mere observation, it is clear that because of its association with i1, i2 will be large enough for the diode to turn on. There is absolutely nothing wrong with the loop, but the result is blatantly wrong.

This leads me to think about why do I need to use loop analysis to determine the current though stiff elements? The loop i2 could be written in several different ways. If it is written with respect to the dc bus voltage, it would result in a reverse current that would keep the diode off.

The same goes for any stiff element. By making an element stiff, it ensures that it will not disrupt the currents in the circuit. So the effect of the diode can be removed by making it a high resistance. But if the current through the diode matters and in this case it does, loop analysis is not an accurate way to determine the current through the diode.

Since nodal analysis is available and is being used to determine freewheeling, why not use it to determine the current through all stiff elements after the loop analysis ODE is solved.

Here is the code:

Excessive loops

I started working on the case of a three-phase inverter feeding an inductor-capacitor filter with the objective for designing a controller to regulate the voltage across the capacitor. In a nutshell simulate a UPS.

I ran into a number of problems which will be separate posts. I am still not sure if these problems have been completely solved.

The first problem. On simulating in open loop i.e giving balanced sine wave modulating signals I expected smooth balanced current and voltage waveforms. Instead all were badly distorted. The reason was that the circuit has 48 branches and 32 nodes but the number of loops calculated was 20. This isn't so bad because the loop finder works in random ways and could end up with extra loops. The assumption was that these extra loops would be linear combinations of the other loops and would be eliminated by loop manipulations. However, on looking at the system matrices in the differential equations I found 18 loops. So one loop was extra and solving this loop messed up the simulation.

So why did this loop linger? The loop manipulations are done only for stiff loops. This is to restrict stiff branches to the minimum number of loops. But if there are excess loops that are nonstiff to begin with, they may never be eliminated. So, to make sure that the loops are exact, on forming the system loops for the first time, row operations are performed to convert them to an upper triangular form. This will ensure that loops that are linear combinations are eliminated at the beginning of the simulation.

Here's the code:

Wednesday, December 9, 2015

Releasing version 1.0.1

I am releasing version 1.0.1 following a bug I discovered when I tried to simulate four-wire systems with a neutral wire. The problem lies with the solver and the sequence in which it solves equations. The solution has been to ensure that differential equations get solved before static equations rather than vice versa.


Friday, December 4, 2015

Three phase grid connected inverter

I posted the next case study - a three phase inverter connected to a three phase grid in current control mode. Check out:

I have not included the complete control code and will not do so from now on as even though I am releasing Python Power Electronics open source, the control code may be protected legally by someone else. If you would like to write your own control code, check out the tutorials in the documentation section of the website.

Thursday, December 3, 2015

PPE Website

I haven't posted for the past week because I have been launching the Python Power Electronics Website. Check it out:

It probably will keep changing for a while until I finalize it. Suggestions are of course very welcome and can be sent to

I'll continue this blog but the case studies that I was posting will be posted on the website. This will be more of an informal discussion from now onwards.