SEIDEL and KLIMA: CRITERIA FOR ELECTRICAL HARVESTING 



capable of delivering up to 1,000 V to an 

 electrode array. However, at this voltage, the 

 array shape has to be changed to produce a much 

 higher load resistance to maintain the current at 

 a value which is within the 120-kVA rating of 

 the system and the current and voltage carrying 

 capability of various components in the unit. For 

 instance, at 1,000 V the electrode array has to 

 have a total resistance of 0.3 ohm. 



Mid- Water Trawling Mode 



The pulse generator was also designed for appli- 

 cation to electrical trawling. This use of the sys- 

 tem requires a significantly different configura- 

 tion than in netless harvesting. Since the pulse 

 generator components are far too large to con- 

 sider underwater mounting of the system on a 

 trawl, it was necessary to design the unit for 

 operation through a long power cable. The cable 

 transmits the pulsed power from the vessel to the 

 trawl. A cable length of 2,200 feet (670.1 m) was 

 chosen to allow trawling to depths of 100 fm 

 (fathoms) (182.9 m) with a cable to depth ratio 

 of at least 3:1. The length of the cable is impor- 

 tant because as it gets longer its direct current 

 resistance increases and therefore either the cable 

 losses become greater or the size of the conductors 

 has to be increased to prevent excessive losses. 

 Since a large power loss is not acceptable, con- 

 ductor size and the resulting cable diameter 

 eventually become too large and are limiting 

 factors in the total length and therefore the power 

 then can be transmitted down the cable. 



The operation of a pulse generator into a long 

 cable requires careful design in order to work. 

 First, the impedences of the pulse generator, 

 cable, and electrode array have to be properly 

 matched through step-up and step-down trans- 

 formers to accomplish transmission of the pulse 

 down the cable. Unless impedences are properly 

 matched, the pulse will become very distorted or 

 can be totally lost in the cable. Another serious 

 limiting factor in the operation of a pulse 

 generator through a long cable for trawling is the 

 underwater transformers which match the power 

 supply cable to the electrode array. The delivery of 

 significant levels of power, such as 120 kVA, 

 through a single transformer would require a 

 transformer that is quite large and would weigh 

 several hundred pounds to handle the pulse 

 current into an array with a load resistance of 

 0.05 ohm. 



Our first intended application of the pulse 

 generator in a trawling mode was with a mid- 

 water trawl. The standard mid- water trawl being 

 used at the Pascagoula Laboratory was a net that 

 opened approximately 9 x 9 m under water. In 

 actual field measurements, it has been found that 

 the net generally opens between 7.5 and 9.0 m in 

 height. Therefore we required the pulse generator 

 to accomplish effective electrical trawling on a 

 vertical opening of 7.5 to 9.0 m and a horizontal 

 opening of approximately 9.0 m. In the mid- water 

 trawl application we expect the electricity to 

 provide a combination of fright, leading, and some 

 tetanus to aid in harvesting of fish. Past experi- 

 ments at the Pascagoula Laboratory demon- 

 strated that fish generally accumulate in the 

 mouth and forward body of the trawl. Therefore 

 an electrical field applied periodically should force 

 the fish back into the cod end. 



Because of component ratings, loading of under- 

 water transformers, and design restrictions, a 

 power of 80 kVA was chosen as the maximum 

 which could be supplied to our electrode array in 

 a mid-water trawling mode. Since the application 

 of 80 kVA through a single transformer is difficult 

 under water, we chose four electrode pairs and 

 four underwater transformers matched to each 

 electrode pair to cover the 9.0 x 9.0 m net. It was 

 found that a reasonable electrode size could be 

 used which would provide a load resistance of 0.2 

 ohm for each pair and deliver 20 kVA from each 

 transformer. This meant that by connecting the 

 electrode pairs in parallel, each transformer 

 would carry one fourth the current which would be 

 required of a single transformer at the same total 

 output. In addition, the four parallel electrode 

 pairs would represent a total load resistance of 

 0.05 ohm which could easily be matched to the 

 other impedences of the system. 



Within the impedence matching requirements 

 of the pulse generator, cable, and electrode 

 array, and using the maximum output voltage of 

 2,500 V that the unit is capable of supplying in 

 this mode, 450 V can be supplied to each elec- 

 trode pair through the matching transformers. 

 The surface area of each electrode pair must be 

 adjusted to provide a resistance value of 0.2 ohm. 

 Therefore, the pulse current of this condition is: 



V 450 



/ = — = = 2,250 A. 



R 0.2 



667 



