a DIY approach to a robust, accurate, low-cost wind turbine controller
Most people who have small battery charging wind electric systems should be familiar with dump or diversion loads. A diversion load is basically a resistive device (and its associated diversion load controller) that is designed and programmed to allow proportional current (PWM) to flow from the battery bank to a heating element based on battery voltage and how fast it rises above a certain threshold. A diversion load is wired in parallel with the charging source across the battery and is designed to “divert” incoming charge current away from a fully charged battery bank, and onto a properly sized load to use up the excess energy that may be available at the time. It can be used with wind electric generators, and even solar PV to help maintain battery charge in renewable energy systems. Diversion loads can use excess incoming energy to heat water, or simply to heat air and burn off the excess charge current. Diversion loads have their pros and cons but historically must be used specifically with a wind electric system as to avoid rotor unloading and free-wheel conditions when “100% SOC” charge conditions are met (in reality, for FLA banks, its more like 97% SOC, when battery internal resistance starts to rise quickly and charge current should be decreasing during absorb).
A diversion load is a sub-system in and of itself. It is often thought to be part of the wind electric system, however it must be designed to accommodate charging current from ALL sources, not just the wind turbine. When utilized in off-grid systems that tend to grow in capacity over time -most often with additional PV generating capacity, upgrading or resizing of the diversion load system if often overlooked. In hybrid systems, it seemed to me that a more forgiving approach would be to simply shut the turbine down when it was not needed and allow the PV charge controllers do what they normally do, which is to go mostly open circuit when power is not needed. Again, you normally cannot do this with a wind turbine. Also, if a DC coupled backup generator is utilized, things get even more complex.
A More Simple Solution
In hybrid off grid systems (wind/PV) it can work well to simply shut the turbine down when not needed. In this instance, the wind turbine would be used primarily as a bulk charging source and during gusty conditions, would be turned off when the battery becomes mostly full or voltage rises above the batteries programmed Absorb voltage. This then allows the PV controllers to remain online and finish the absorption stage. If they get fully charged and enter the Float stage, the turbine will remain off until a load is turned on and causes the battery voltage to dip below another programmed set-point (similar to RE-Bulk voltage). Using the turbine in this manner allows you to use its energy when needed, and keep the rotor from turning when its not needed. Yes, some minor energy production is ultimately lost from the turbine when it is not running, but this approach prevents a lot of premature leading edge wear on the blades which would have happened anyway had it been running full tilt into a dump load all the time, especially during the most inefficient period of the charge cycle.
Over the last few years, I’ve build a handful of custom turbine controllers for customers. The purpose of these controllers, was to monitor turbine voltage at the input to the rectifier, and quickly apply the “brakes” if the voltage was to cross over a pre-programmed threshold. This of course was to protect the input of either, an MPPT charge controller, or an MPPT grid-tie inverter from over voltage in the event of a wind gust or in the case of sudden grid loss which would result in a loss of load on the rotor. Lots of folks (myself included) have been doing this for years with the axial flux turbines, utilizing a contactor to short out the wild-AC from the alternator if the inverter or charge controllers auxiliary relay set point happened to be crossed. Aside from a few of my controllers being extremely experimental (what really isn’t experimental!?) I started to use more solid state devices in my designs (such as SCR’s, GTO Thyristors, IGBT’s, Triacs) and got away from using contactors, which tend to be noisy and consume more energy. The majority of these controllers operated on 24V and 48V nominal systems, with a few running on higher voltage DC systems, one at 550VDC, which worked well. Although not the most eloquent approach, most of these controllers are all built around a type of circuit called a “Crowbar”, which if you are familiar with high quality linear power supplies, you know what that is. I chose to utilize a Crowbar circuit because its construction and functions are relatively simple. I felt that was important when using it with an Axial Flux turbine, a turbine that is designed around being simple to construct, -the controller should also be relatively simple to construct. There is no doubt that one could make it a lot more complicated if they wanted to, but it does not need to be that way to work well. This control concept can easily be applied to grid-tie wind systems as well.
A Crowbar circuit is a type of circuit that is basically the equivalent of taking a big screw driver or wrench and holding it across the output of the power supply (which would be extremely detrimental and dangerous under most circumstances!!). While this may seem silly on the surface, in a linear power supply, it actually pulls the output voltage to zero, draws an extremely large amount of current, and ultimately (and rather quickly) blows the mains 120v supply fuse, effectively shutting down the power supply and preventing damage (from over voltage) to the presumably expensive devices the linear supply was powering. It is ultimately an over-voltage protection circuit, usually using a zener diode triggering the gate of an SCR which is basically wired across the power supply output. Traditionally, this approach works ok for power supplies, but has a few key flaws when used in a wind turbine overvoltage application. Once the crowbar is triggered, its a one-shot deal. The SCR conducts and the power supply is then presented with a short circuit, and the mains input fuse blows. The fuse would then need to be replaced in order for the power supply to work again, granted no damage was done to the other devices it was powering.
In a wind turbine over voltage protection application, this approach would not work for two reasons-
1) When we want to protect a renewable energy system from turbine over-voltage, we can use a crowbar circuit, but we never want the fuse to actually blow. If the circuit happened to go open (such as if a fuse or breaker tripped) that would mean that we lose the load on the turbine and can therefore have another over voltage condition after the rotor speeds up again, since we have no dynamic brake/clamping current. So we have to make sure that if we use a Crowbar, that it remains shorted once triggered, in order to keep the turbine dynamically braked.
2) The second reason why the traditional crowbar circuit will not work, is primarily because once the fuse or breaker trips, someone needs to be able to reset it in order to regain turbine energy production when battery voltage falls below the programmed threshold. So basically, the reset needs to happen automatically.
The way I get around these two issues is to use a circuit that I call an Active Crowbar.
You can probably guess what an Active Crowbar does…right?
Active Crowbar circuits have the ability to utilize a few voltage/frequency measurement functions to actively control when to apply (or not apply) a short circuit. This effectively brings the turbine rotor to a crawl until the proper requirements are met for it to open the circuit and allow the turbine rotor to begin spinning again and producing energy. There are no fuses to blow, and no resetting required for this to take place. Its fully automatic. However, I have built in some other safety features that will require a reset if it happens to “latch-out” upon sensing a phase imbalance or an over-speed (unloaded condition). Yes it does that too!! Its a pretty neat device.
Compared to other turbines that use radial PM alternators, home-built Axial Flux alternators tend to use more magnetic material in their magnet rotors which, while making them great low wind performers, also seems to allow them to take take a bit more of abuse from short circuits. What I mean by that is that because of the large amount of magnet density, they exhibit quicker rotor stop times when they are dynamically braked. Voltage tends to drop faster during a braking event, and that means the inrush current from the short circuit is quickly dissipated in the alternator winding and the feed line. Ive yet to even need a braking resistor with an axial flux turbine and have omitted it in lieu of less expensive IGBT modules used to apply the short circuit. Most Axial Flux turbines will even stop the rotor with one phase shorted (if you happen to lose one phase).
Up until this point, if you needed a device like this, you would need to purchase a Midnite Solar Clipper, which works in a similar manner to the Active Crowbar (with a three phase Triac on the AC side), but needs a Midnite Classic controller to work. This cost can get to be around $2000 just for the equipment and it seems to me that it is sort of out of reach for the average homebrew turbine builder, although Midnite makes some really great products that work really well.
I just figured I would share my alternative approach. The Active Crowbar can be built for around $500 using quality off-the-shelf components. Included in that price are the components for the interface board which can either be built using the schematics I provided here, or possibly purchased from me in the future if there is enough demand. The Interface board is composed mainly of a bunch of precision voltage dividers and allows the PLC to read the proper voltages and therefore control the turbine via programmable voltage settings. I chose a Siemens LOGO! controller because its relatively inexpensive, expandable and has lots of input filtering and opto-isolation. Its designed to be installed in an industrial environment and you would be hard-pressed to duplicate these features and reliability if for instance, you chose to use an Arduino (but its certainly doable).
Here is the circuit that I came up with for the interface board:
This is the circuit I came up with to measure independent phase voltage and battery voltage. Future version will support alternator frequency or anemometer frequency measurement.
Here is the programming that I wrote for the Siemens LOGO! controller using Siemens LOGO! Soft Comfort demo version. It is possible to convert this block programming to Ladder Logic and enter it via button-pushing through the front panel, but a lot easier to use the LOGO! software.
I know… It looks a little confusing. Basically, if you took the file that I have provided below (in parts list) and loaded it into a LOGO! (7 or 8) via the SD card slot, you will have programming that will work. Voltage settings and parameters will then be accessed through the front panel, parameters tab in the LOGO! controller.
Parts list for the controller:
Siemens LOGO! PLC/Smart Relay: LOGO!8 24 CE – 6ED1052-1CC01-0BA8 (or equivalent) you can read more about LOGO! here
DC/DC Converter (to power PLC, input 18-72VDC): Traco Power TCL 024-124 DC (or equivalent)
Three phase full wave bridge rectifier: 200A, 1600V MDS200A
IGBT Module: 600A, 1200V CM600HA-24H (data sheet here)
LED indicators (one green, one red) 24V. I used something similar to this, but its not critical what you use.
Reset Switch (i used a din rail mount switch, any single pole, momentary will do)
Interface board parts list (BOM): Ill add this soon
Interface Board Schematic: here
My current code I wrote for this controller. Download here
Siemens Soft Comfort Programming Software (DEMO) can be found here
Some images of the TAC controller