Hacker News Reader

Comment by jacquesm | Hacker News Reader

I'd be really interested in a teardown of what goes into a modern tri-phase solar inverter.

There's not much to it. There's usually some stage-1 converters at a subnetwork of panels that step the panel voltage up to a common DC bus voltage with all the MPPT and relevant OTP/OVP/UVP/OCP/etc. Then the big DC bus gets combined across all panels and fed into a three phase inverter with IGBTs or sometimes SiC FETs (getting more common), which just looks like a hex bridge across a three phase transformer. There's gate drivers (maybe isolated), amps for current and voltage sensing (maybe isolated), some protection circuits, and a half-assed flyback to run the fans and the control DSP. Some models might host an MCU for data logging and comms out to a control plane.

There's a handful of projects where the size of the solar field is large enough to make it economic to step up from 400V or 800V bus. I've seen many 1000V buses, a few 1200V and 1500V buses. Honestly it's exactly the same circuits, just with higher voltage ratings; all your switching elements are still giant hockey pucks, you're still doing a three phase hex bridge, etc. The half-assed flyback is sometimes replaced with something a little less braindead.

When I was 17 or so I built a simple inverter out of a big 12V transformer and some 3055's (RCA's, not Motorola) which was enough to run most of my gear when it was 'lights out' (or not...). So I understand the basics. But those transformerless sine wave inverters are interesting, they seem to get away with squeezing a 20 KW inverter into a relatively modest package at a ridiculously high efficiency.

On top of that they have to comply with a whole host of safety regulations, so even if the theoretical block diagram is as simple as you've outlined it the actual implementation is likely going to be a lot more complex and interesting.

Any pointers to where I can dig around without opening one up myself would be greatly appreciated, most of the youtube stuff is for very cheap or small gear.

Sure. TPT-PV systems are pretty close to the same concept, with a bigger LCL filter on the output. IMO the tricky part is all the compensation and software design required to minimize ground leakage currents, since that needs real-time analysis of the grid state. I've seen some neat tricks with common-mode current injection off the DC bus using another converter.

Most of the inverter designs offload all the complexity to the software controller in an attempt to keep the power component choice and placement simple. The cool control stuff is mostly available in published IEEE papers, particularly from 2015-2020. It's not open-access, but it's definitely easier to get ahold of IEEE papers than a PV inverter. Once you know what you need to implement, the rest is just software engineering.

Thank you! I will definitely try to read up on this. A similar thing happened to servo drivers somewhere around the early 00's up to that point they were pretty involved hardware wise and then from one day to the next it was super simple hardware and a very beefy controller. It's interesting how when you solve math in software (and some of the math involved is quite complex) the marginal cost is like with every other piece of software: close to $0. But doing the same thing in hardware is expensive and it will decrease your MTBF considerably (and it probably will have other negative effects as well). Servo motors are an interesting case (as are steppers, but for very different reasons), especially when driving large loads and decelerating them again at speed. Those are definitely non-trivial control problems and I can see some parallels with these inverters.

I think the big driver for solar inverters was panel cost reductions. With motor drives it's straightforward even 25 years ago to see the benefits and applications of slapping a DSP and some digitizing sensors onto the system to do crazy control stuff. But PV cost was just way too high for this stuff to have tons of active research until recently. Once panels got commodized, the industry pretty quickly noticed how much virtually free compute is available, and moved to fill the gap. It's certainly been interesting watching commercial PV inverter design start with an extra decade of compute improvements and cost reductions as the industry effectively speedruns the design challenges - even with only a few active research projects before 2015, we've had designs in hand for a decade and no scalable way to deploy them.

The commoditization of computing power, and the continuous decades of improvement in digital hardware performance per watt, has reduced numerous classes of analog problems to an exercise in fast enough bit-twiddling. I think in motor drives the big jump to simple hardware and beefy controller was directly downstream of the creation of usable 32-bit motor controller DSPs, along with software toolchains that made it possible to compile optimized C and C++ libraries for these architectures. Up to early 00's there just wasn't enough real-time computing power available for most of the market to take advantage of it, and what little did exist wasn't directly targeted at motor drives. But it is worth pointing out that the S-curve of digital adoption probably got started as far back as the mid-90s; the only people who could really take advantage of it back then were at the cutting edge with very expensive low-volume projects. I'm sure it felt like an overnight event, but it took a decade for motor drive DSPs and software toolchains to get good enough that most people felt compelled to switch.

ETA: oh gosh I forgot FPGAs happened then too, that probably had a lot more to do with it... Ah well, fun trip down memory lane :)

It's been very interesting watching as the compute gets cheap enough that we can start embedding it into the analog chips. The telecom chips all have DSPs in the ADCs and DACs and digital PLLs in the line cards, the battery management ICs all have microcontrollers for charge management and safety, you can buy radar ASICs for automotive proximity detection, there's gate drivers for SiC FETs in automotive traction inverters that incorporate redundant microcontrollers to do monitoring and fault detection/recovery for ASIL D compliance. So I'd add: the same way that software drives the marginal cost of complex math to near-zero, advances in digital circuitry and ease of incorporation into other analog designs drives the marginal cost of complex application requirements down. It's not quite as stark as software, but it's amazing how much quicker a single complex chip design becomes when you can digitize a subclass of the problems and solve them in real-time at virtually no cost on analog ASICs with built-in CPUs and DSPs. Analog hardware advances are extremely challenging by comparison, and can take years of R&D across a wide array of reliability and performance assessments before they become realized in designs.

Interesting conversation. I recall '5 phase' suddenly being all the fashion because it allowed for better control in hardware at a very high expense and then two seasons later 5 phase had simply disappeared because the increased degree in control meant that the intractable problems of two phase resonance could be solved in software (effectively those drivers made it possible to draw energy out of the motor while speeding it up past those resonance points). The cost for a motor, wiring and controller dropped to a fraction of what it was before. Berger Lahr must have been seriously pissed, finally they had those resonance problems licked and then the software revolution simply overtook them.

Reply on news.ycombinator.com