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Illia
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How to connect two power grids in SimPowerSystem through DC_link?

Asked by Illia
on 6 Aug 2015
Latest activity Answered by Gert Kruger on 20 Jun 2019
Have a further task: - 2 power grids with different voltage levels have to connected, currents have to flow from 690V grid into 400V grid - Grids are connected through DC_Link. Sequence is AC690 -Diode rectifier - DC - Inverter - AC400. (picture below form left to right).
Unfortunately the currents are pretty strange and far away from sinusoidal ones'. (Picture below, yellow, blue, pink)
Interesting thing is that without connection of DC_Link both sides work properly. Here the ideal DC_source connected to provide power for inverter. As can be seen the power factor and current forms are ideal...
Which can be a reason of such strange behavior?
Thank you, Illia

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Hi Illia,
Without your model, this is extremely difficult to reproduce.
Additionally, can you please clarify what you mean by "DC_Link" or how you are connecting the grids?
I typically find that examples are helpful for finding problems in models like these, and there are a host of examples specifically for SimPowerSystems here: http://www.mathworks.com/examples/simpower
Hope this helps,
-Adam

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3 Answers

Answer by Sebastian Castro on 11 Aug 2015

As Adam said, it's really difficult to trace the source of the problem without having access to the model. This is fine, though -- ultimately, you should try out the troubleshooting :)
It looks like your model runs fine, so the problem is either in the solver settings (is the model running accurately?) or the design itself (are the RLC parameters + Inverter controller a good design?)
For the solver settings, I see that you are using a discrete simulation. If you switch this over to Continuous, it will likely be slowers -- but do the results look the same? If they do not, then you may need to switch a smaller discrete sample time that can adequately capture enough resolution. For these types of systems, the limiting factor is the PWM frequency. I like to choose a sample time that is at least 50-100 times the PWM frequency.
For your design, is it the passive rectifier + RLC filter values, or is it the active inverter control in your "Regulator" subsystem? I would try the following:
  • Instead of using the AC Link on the left, try use a constant DC Voltage source. Does your inverter create a nice sinusoidal signal if the DC link is a constant voltage source? If not, then the rectifier on the left isn't doing its job.
  • Instead of the voltage regulator on the right, try using a constant PWM generation. That is, use a PWM Generator (2-Level) block to generate a constant duty cycle, frequency, and phases (0, -2pi/3, +2pi/3). Does this open-loop inverter strategy give you a sinusoidal output? If so, then your inverter regulator may need improvements.
Since you say that the model works fine without the DC Link, could it be that those resistors in series with the AC links are eating up most of the power, and there is therefore no more power to overcome the semiconductor forward voltages in the rectifiers/inverters? Not sure...
- Sebastian

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Answer by Illia
on 14 Aug 2015

Thank you for your ansvers,
let's start from the beginning.
  • "It looks like your model runs fine, so the problem is either in the solver settings (is the model running accurately?) or the design itself (are the RLC parameters + Inverter controller a good design?)".
The model runs accurately I have tried with continuous mode and with Sampling time 1e-7s. What is RLC parameters? Do you mean DC capacitor and line inductor? If yes, they are calculated accurate and according to usual theory of VSI.
  • For your design, is it the passive rectifier + RLC filter values, or is it the active inverter control in your "Regulator" subsystem?
I have bouth. From one side I have passive rectifier (it is connected to the grid with higher voltage level) and active rectifier which intended to work mostly in recuperative mode.
  • Instead of using the AC Link on the left, try use a constant DC Voltage source. Does your inverter create a nice sinusoidal signal if the DC link is a constant voltage source? If not, then the rectifier on the left isn't doing its job.
I have tried it and diagrams for recuperative mode are attached to my first question. Current looks almost ideal. Controller reacts on change of the mode from recuperative to rectifier and vice versa very accurate.
  • Instead of the voltage regulator on the right, try using a constant PWM generation. That is, use a PWM Generator (2-Level) block to generate a constant duty cycle, frequency, and phases (0, -2pi/3, +2pi/3). Does this open-loop inverter strategy give you a sinusoidal output? If so, then your inverter regulator may need improvements.
As I said, controller works good with constant DC source.
  • Since you say that the model works fine without the DC Link, could it be that those resistors in series with the AC links are eating up most of the power, and there is therefore no more power to overcome the semiconductor forward voltages in the rectifiers/inverters?
I don't think so. Resistor represents real load which is connected to the DC side. But without those resistors I have the same problems.
Here is a model.

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Answer by Gert Kruger on 20 Jun 2019

Sorry, this answer is four years late:
  1. The 3-phase voltage sources you used to represent the grids are both tied to ground, this causes circulating current between the two grids. Change the voltages source from ground connection to isolated neutral connection.
  2. With the ground reference in step 1 removed, there are no more references in the system, hence add a ground symbol to the DC bus negative.
  3. Move the 3-phase line inductor before the rectifier to a DC filter after the rectifier using the same inductance. When it is before the recitifier it destablizes your DC bus control in the inverter, not entirely sure why, it seems to boost the DC bus voltage when it is before the rectifier.
~~ Cheers !!

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