Replacement parts, new blades, and blade-actuated governors. We make replacement parts and have new blades for most all wind generators, pre-REA to present models. Many used parts, too. Lots of used equipment available: wind generators, towers, both synchronous and stand alone inverters, and Aermotor waterpumpers.
Best prices on TRACE inverters and SOVONICS PV's. Information: $1; specify interests.
Lake Michigan Wind & Sun 3971 E Bluebird RD., Forestville, WI 54213 Phone 414-837-2267
Batteries Can Be Dangerous
In the last issue we covered the requirements for overcurrent devices. DC rated circuit breakers meet these requirements and combine the overcurrent function with a switching function They also increase system reliability and in many cases lower cost because of fewer parts count. There is a problem, however, in that most DC rated circuit breakers, switches and fuses have limited ability to interrupt short-circuit currents. The fuses and circuit breakers, while functioning properly to open the normal overloads of 150-1000% of rating, can only break or interrupt short-circuit currents of around 5000 Amps. When subjected to higher short-circuit currents, they may be destroyed-- literally blown apart.
It has been estimated that a single 220 amp-hour, 6 volt golf cart battery can deliver 8000 amps into a terminal to terminal short-circuit for a fraction of a second. In PV systems we frequently use batteries of higher capacity and even parallel them which creates significantly greater short-circuit current capabilities. To some extent, the magnitude of the short-circuit currents is reduced by the series resistances of the wiring, fuse holders, switches and circuit breakers. But, when we use 4/0 cable between the inverter and the battery, these resistances are measured in milliohms and do not limit the short-circuit currents a great deal. Current Limiting Fuses
A current limiting fuse should be used in the positive conductor of the battery bank in all stand-alone systems. These fuses have a special internal construction that helps to extinguish the internal, direct current arc created inside the fuse when a short-circuit occurs. When the current limiting action takes place, the peak let-through current is significantly reduced For example, the DC rated UL listed, RK-5 current limiting fuse made by Littelfuse Company has a DC interrupt rating of 20,000 Amps. In ratings up to about 150 amps, this fuse can reduce the short-circuit let-through currents to less than 5000 amps which can be handled safely by lesser rated fuses and circuit breakers. Beware of using current limiting fuses with only an ac rating--they have not been certified by UL as having the necessary DC rating and may not function properly in DC circuits.
The calculations required to determine how to use current limiting fuses are quite complex and not even the battery manufacturers have a good handle on the short-circuit current capabilities of their batteries. To simplify the design of a safe PV installation, it is suggested that every 700 amp-hours of battery bank capacity (i.e. two Trojan L-16's in parallel) have a disconnect switch and a current limiting fuse in series with the positive conductor. The switch is connected nearer the battery and there should be another switch elsewhere in the circuit to insure that both ends of the fuse are dead when it is serviced. If no battery charger is involved, either external to or in the inverter, then the PV disconnect switch will serve as the extra switch removing the only other source of power in the system. For simple systems without inverters see the diagrams in Code Corner in HP 16.
DC rated, fused disconnects are available from Square D and Siemens I-T-E among others, which combine the fuse holder and a switch. Another option is to separate the components with a fuse holder and switch in different enclosures. In either case, there must be no exposed wiring and the single conductor cables must be in conduit. All metal cabinets and boxes must be well grounded-- a subject for the next issue.
DC rated, UL listed current limiting RK-5 fuses are made by Littelfuse. See Code Corner in HP 16 for the address. DC rated, UL listed fused disconnects are available from Square D and Siemens I-T-E at 3333 State Bridge Rd, Alpharetta, GA 30201. Call 404-751-2000 for regional I-T-E representative who can steer you to the nearest source. Joseph Pollak Corp at 195 Freeport Street, Boston MA 02122, phone 617-282-9550 makes a 180 amp 36 volt heavy duty switch. It is not UL listed, but your local inspector might let you use it. Ask for info on product #51-302. JC Whitney and Northern both carry it.
John Wiles, SWRES, POB 30001/Dept 3SOL, Las Cruces, NM 88003 Ktei
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