Long Distance Power Transmission for Hydro and PV Systems Paul Cunningham c.1992 Paul Cunningham Many times the best site to generate power must be overlooked because of the distances involved. You may have located your house in the shade of trees because of their summer cooling value and the aesthetics of being in the midst of the woods. Likewise the brook with hydroelectric potential may be too distant from the chosen house site. This article explains how power can be generated at a high voltage for easy transmission and then converted to battery voltage. Todd 'DC to DC Switching' Battery Chargers For PV long distance power transmission is made easy by the Todd line of battery chargers. These were are also marketed by Heliotrope General (with higher quality control) and are described in HP#17. They are 'switching' DC to DC converters with efficiencies around 90%. They receive power at 90 to 130 volts and produce a regulated 12 VDC nominal output. Inside the converter the input is rectified, so the input can be ac or DC of either polarity. The Todds are compact, inexpensive, light in weight, and run quietly. Where transformer based chargers 'brown out' when given alternating current from a genset with low peak to peak voltages, these switchers continue to convert power efficiently. They are available in sizes of 30 to 75 Amperes. To use them in a PV system where long distance power transmission is needed, wire panels in series strings of eight (ten to twelve in cloudy areas) to produce 3 Amperes at around 120 VDC. Transmitting the power at this voltage allows efficient transmission for long distances without monster cable diameters. The 120 VDC is fed into the Todd charger, which in turn feeds high amperage power at a lower voltage to the nearby batteries. When the battery voltage rises to a certain threshold (13.8, 14.8, and 16.5 VDC are available) the charger reduces its output to the batteries... Not only do you get efficient long distance power transmission, but you get a built in PV regulator! What a deal! If you've got a 24 Volt system, it is possible to use two Todd chargers with the outputs in series. When the PVs aren't producing, simply plug the Todd into the old gasoline generator. Figure 1 PV long distance power transmission system Induction Generators (For Hydro Folks) Hydro systems can make use of the same high voltage long distance power transmission scheme, by using high voltage generators attached to their turbines. An inexpensive and efficient alternative to a permanent magnet generator or alternators is a three phase induction motor as a generator. Induction generation was introduced to Home Power readers in HP#3 (now out of print), and time has proven it reliable. Modifications are easy, connect capacitors across each of the motor's power lines. You'll need 10µF to 60µF of capacitance between each line. Use "ac motor run capacitors" (not DC electrolytics!). Varying the size of the capacitors provides a method of tuning the system so that it operates at the maximum power point. Output voltage can be in the range of 100 to 400 volts ac, but you'll want less than 130 volts if you're feeding it into a Todd charger. To change the three phase ac current to DC, use the rectifier shown in Figure 2. Before Todd chargers, the conversion of high to low voltages in hydro systems was done with standard 60 Hertz transformers. This required assembling an expensive custom circuit for each job. The efficiency was poor under normal conditions and wretched in low head sites where the induction generator frequency drops substantially below 60Hz. Using the Todd chargers it has been found that the efficiency of the generator and charger together is undiminished at even 30Hz. As the battery voltage climbs above the preset threshold and the Todd begins to regulate current, it does so by switching off the input power. Running unloaded, the hydro generator's voltage rises to levels which may damage the generator or the charger. For this reason, a separate shunt type regulator (e.g. Trace C-30 or Enermaxer) is needed. It must operate at a voltage lower than that of the charger set point. Figure 2. Hydro long distance power transmission system I've specified and sold equipment for some 20 hydro systems following the induction generator/ Todd charger scheme suggested here. They've operated reliably and with high efficiency. Alternative Energy Engineering has installed several PV systems of the type described here, and they report good results. In fact they suggest such a system has advantages even where long distance transmission is not an issue, because it eliminates the need for a PV regulator, and because wire runs to the array can be small gauge (a major consideration if using conduit). Access: Author: Paul Cunningham, Energy Systems and Design, POB 1557, Sussex, N.B. Canada EOE1P0 ¥ 506-433-3151 PV systems: Wes Edwards, Alternative Energy Engineering, POB 339HP, Redway, CA 95560 ¥ 1-800-777-6609 Induction motors and ac motor run capacitors: Leeson Induction motors, 230/460 V, three phase. Order from Paul Cunningham (above), or Grainger Inc, 5959 West Howard St., Chicago IL 60648 ¥ 1-800-323-0620. Also consult local yellow pages.