DCDC control loop design in Simulink

[lia_lia]

Hello!
I have very unusual configuration of DC-DC chopper. I want to make the Simulink model for it.
The idea is as follows: the motor operates in both motoring and generating modes. In generating mode, excess voltage appears on the DC bus. I would like to utilize this excess energy to charge a supercapacitor, which is connected in parallel to the DC link.

I have several questions regarding this configuration:

1. How should the control system be designed for this configuration?
2. What are the necessary equations for calculating the regulators?
3. Is it possible to fully discharge the supercapacitor, or should its voltage always be maintained above a certain threshold?
   
   

 

[Easy peasy]

you essentially have 3 buck converters off the main input - this is usually done to reduce the power in each buck to manageable levels

e.g. 10kW required - use 3 x 3.4kW bucks, 120 degrees phase shifted for lower Iin ripple


p.s. they can also boost if run in the reverse direction.

 

[FvM]

Post #1 design idea makes only sense if DC bus is powered from simple rectifier or unidirectional PFC rather than bidirectional PFC. This is the way how most motor inverters in industry are still operated.

Generative mode is only temporary, e.g. when spinning-down a load with large moment of inertia and typically using a braking chopper and resistors to dissipate energy.

I agree with Easy peasy that a bidirectional PFC stage (active front end) is the appropriate way to handle the problem. Super capacitor storage has been implemented in special cases like hybrid vehicles. But we can take the scenario nevertheless as interesting exercise problem.

 

[lia_lia]

Post #1 design idea makes only sense if DC bus is powered from simple rectifier or unidirectional PFC rather than bidirectional PFC. This is the way how most motor inverters in industry are still operated.

Generative mode is only temporary, e.g. when spinning-down a load with large moment of inertia and typically using a braking chopper and resistors to dissipate energy.

I agree with Easy peasy that a bidirectional PFC stage (active front end) is the appropriate way to handle the problem. Super capacitor storage has been implemented in special cases like hybrid vehicles. But we can take the scenario nevertheless as interesting exercise problem.

Hi,
In my system, the DC bus is supplied by a simple six-pulse diode rectifier (as shown in the attached schematic). I want to connect a bidirectional DC-DC converter between the DC bus and an LVS (supercapacitor).
My goal:

• During regeneration, send excess energy from the DC bus to the LVS.
• When DC bus voltage drops, draw energy from the LVS to support it.

Question:
Should I use a dual-loop control system for the DC-DC converter - with an outer voltage loop (to keep DC bus voltage stable) and an inner current loop (to control power flow)?
Is this a standard approach?
Thanks in advance!

 

[FvM]

Should I use a dual-loop control system for the DC-DC converter - with an outer voltage loop (to keep DC bus voltage stable) and an inner current loop (to control power flow)?
Is this a standard approach?

Basically yes. Problem is however likely that you don't have defined setpoint for voltage controller because DC-Bus voltage depends on grid voltage and load current. A solution could be to have a setpoint clearly above rectified grid voltage, supercapacitor charging starts if a certain overvoltage is reached, similar to brake chopper operation, and discharging starts immediately if Bus voltage falls below the threshold.

 

[mtwieg]

I don't understand the point of putting the supercap directly in parallel with the motor like that. Even if the motor does "generate" some energy into the capacitor, you won't have any control over how the energy regenerated in this way is actually used. It will basically make it as if the rotor has extremely high intertia, making it difficult to change its speed. Is that what you want?

 

[FvM]

I don't understand the point of putting the supercap directly in parallel with the motor like that.

I don't recognize direct parallel connection. Supercap chopper is connected to DC bus, similar to a brake resistor chopper. It doesn't affect motor speed in the first place, except for a case where you stop motor deceleration because supercap is fully charged.

 

[mtwieg]

I don't recognize direct parallel connection. Supercap chopper is connected to DC bus, similar to a brake resistor chopper. It doesn't affect motor speed in the first place, except for a case where you stop motor deceleration because supercap is fully charged.

Ok I think I misinterpreted their description. Referring to the first schematic, I assumed "CL" is the supercap, and the battery symbol is the motor. Didn't notice that this is just a block in the system described in the second figure. In that case, things make a lot more sense. Actually quite similar to something I've been working on...

Question:
Should I use a dual-loop control system for the DC-DC converter - with an outer voltage loop (to keep DC bus voltage stable) and an inner current loop (to control power flow)?

Sure you could make use of nested loops, but the real question is what top-level control law(s). In particular, what do you want to do with energy stored in the supercaps: spend it immediately once the motor stops regenerating, or wait until a particularly heavy load is present, reducing the peak power seen by your rectifier?

 

[lia_lia]

Ok I think I misinterpreted their description. Referring to the first schematic, I assumed "CL" is the supercap, and the battery symbol is the motor. Didn't notice that this is just a block in the system described in the second figure. In that case, things make a lot more sense. Actually quite similar to something I've been working on...


Sure you could make use of nested loops, but the real question is what top-level control law(s). In particular, what do you want to do with energy stored in the supercaps: spend it immediately once the motor stops regenerating, or wait until a particularly heavy load is present, reducing the peak power seen by your rectifier?

Good question, in my case i need wait until a particularly heavy load is present, reducing the peak power seen by your rectifier