Upgraded Reliability for Non-ESS IGBTs

Over the years, we at SEMIKRON have supported numerous requests for custom power electronic stacks. Recently (in power electronic terms) a wind operator asked us to develop a product that is not only compatible with the existing Non-ESS wind turbine converter but also uses our latest technology to increase reliability. Though it took quite some time and numerous visits to test at a wind farm, we succeeded in verifying the design and are now ramping to production.

Our goal is to manufacture these new Non-ESS IGBTs at a competitive price. No need to send us a core, we’ll scrap it anyway. Our design uses two new cold plates each optimized for the line and rotor side, baseplate-less and solder-free IGBT modules, and a new digital driver to replace the AEBM. Below are some details of the system and how we decided to optimize it.

Focus on longevity

One item worth keeping in mind: these wind turbines are getting older. How would you feel about using a phone from 2008 (note the iPhone was released mid-2007). There have been numerous iterations of the iPhone, yet wind turbines in the field are still using much of the same converter technology. To understand what I’m talking about, let’s take a look at the original Non-ESS Power System.

The system’s ultimate goal is to adjust power flow by turning the IGBT switches on and off. This is done by the controller. However, you should not plug the controller directly into IGBTs since you wouldn’t have enough power to turn them on or off. The IGBTs also handle high voltage, which you want to keep the controls away from. Damaging controls means potential damage to the rest of the system, which can become costly!

In order to keep the controller away from high voltage, the controller sends its signals along with power from an auxiliary supply (AEPS) to another board, called the AEBI. The AEBI groups the signals/power together isolates them from the high voltage and sends the isolated signals to a board right on top of the IGBT modules, the AEBM board. The AEBM has passive, analog components that must be near the IGBTs to drive them. This board then drives the IGBTs on and off in a way to adjust output voltage and current, which gets stepped up and sent to the grid. Turning these IGBT switches on and off thousands of times a second at hundreds of amps and near a thousand volts causes the IGBT modules to heat up, so it is attached to a cold plate, cooled by the coolant.

New technology can be used

What new technologies could we use to optimize this system? Looking at each component, we can make separate improvements to the driver, the IGBT module, and the cold plate.

Cold Plate

We approached this with two different heatsinks, each optimized for single- or double-sided cooling rather than one to handle both versions with average cooling. This improvement was not the initial goal. The original cold plate was considered for reduced cost, but our IGBT module was too tall. So, we went with a great heatsink at a competitive cost.

IGBT Module

Moving upward, we now use our SKiM® IGBT module with no baseplate and no solder. If you’re wondering why we removed the baseplate, it causes the higher thermal impedance to the heatsink and more mechanical strain. Higher thermal impedance causes the IGBTs to run hotter, reducing reliability. The mechanical strain is caused by expansion/contraction of dissimilar materials heating and cooling.

As for the solder, we are using a silver sintering process and pressure mounting the terminals to completely eliminate solder. This eliminates solder fatigue, which causes voids. As these voids grow, thermal impedance grows to cause the IGBTs to heat up and reduces reliability.

Driver Board (Replaces AEBM)

On top of the SKiM® IGBT module sits the driver board. The driver board has been quite literally packed with updates. Using the latest driver technology, we have gone completely digital.

Here’s what we added:

  • Isolation right on the driver to help protect the AEBI from faults against the SKiM® IGBT module. However, this will not protect the AEBI from non-SEMIKRON IGBT effects.
  • An auxiliary power supply to increase the safety margins during catastrophic events, such as a short circuit.
  • Board level DESAT protection to reduce propagation delay in a short circuit.

Installation Features

For the technicians out there, we added a couple more features for you:

  • Hardware kit: This includes hose clamps, necessary screws, and a grounding strap so you don’t have to search for missing hardware during the installation.
  • Easy-access connectors: Instead of struggling with plugging-in the ribbon cables, we made the connections front-facing for easier access.
  • Thicker mounting glastic: Doubling the thickness of the red glastic used for mounting will make it less susceptible to cracking or breaking.