Electric Transmission of Water Power

by Alton D. Adams

In one case a line that ran near a certain chemical factory was said to be much troubled by the burning of its pins, though the voltage employed was only 440, and the insulators were designed for circuits of 10,000 volts. In rainy weather, when insulators, pins, and cross-arms were washed clear of the chemical deposits, there was no pin burning. Similar trouble has been met with on sections of the 40,000-volt Provo line, in Utah, where dust, mixed with salt, is deposited on the insulators, pins, and cross-arms. On page 708 a 2,000-volt line is mentioned on which fog, dust, and rain caused much burning of pins.

When circuits are operated at voltages of 40,000 to 60,000, no very severe climatic conditions are necessary to develop serious trouble in the wooden pins by leakage currents, even where the transmission lines are supported in insulators of the largest and best types yet developed. Striking examples along this line may be seen in the transmission systems between Colgate and Oakland, Cal., and between Electra and San Francisco. Both of these systems were designed to transmit energy at 60,000 volts, but the actual pressure of operation seems to have been limited to about 40,000 volts during much of their period of service.

Insulators of a single type and size are used on both of these transmission lines, and are among the largest ever put into service on long circuits. Each of these insulators is 11 inches in diameter, and 11¹⁄₄ inches high from the lower edge to the top, the line wire being carried in a central top groove. The wooden pins used on the two lines vary a little in size, so that on the Electra line each pin stands 11¹⁄₂ inches above its cross-arm, while on the Colgate line the corresponding distance is 12 inches. As the insulators are of the same size in each case, the length of the pin between the lower edge of each insulator and the top of the cross-arm is 4 inches on the Colgate line and 3¹⁄₂ inches on the Electra line.

On the latter line a porcelain sleeve, entirely separate from and making no contact with the insulator, covers each pin from the top of its cross-arm to a point above the lower edge of the insulator. On the Colgate line each insulator makes contact with its pin for a length of 2¹⁄₂ inches down from the top of its thread, and on the Electra line the contact of each insulator with its pin runs down 3¹⁄₂ inches below the top of the thread. This leaves 9 inches in the length of the pin between the insulator contact and the top of each cross-arm on the Colgate line, and a corresponding length of pin amounting to 8¹⁄₂ inches on the Electra line. Of this 8¹⁄₂ inches of pin surface, about 6 inches is covered by the porcelain insulating sleeve used on each pin of the Electra line, so that only about 2¹⁄₂ inches of the length of each pin on that line is exposed to the leakage of current from the insulator directly through the air. Both the sizes of pins just mentioned were made of eucalyptus wood, boiled in linseed oil.

Each one of three pins taken from a pole, between North Tomer and Cordelia, on the Colgate line, was badly charred and burned on its side that faced the damp ocean winds. This charring extended all the way down each pin from the point where the insulator made contact with it, a little under the threads, to the top of the cross-arm nine inches below. Two of these pins were located at the opposite ends of a cross-arm, and the third was fixed in the top of the pole. This cross-arm was charred or burnt, as well as the pin, but no defects could be detected in the insulators that the pins supported.

As to these three pins, the most reasonable explanation seems to be that enough current leaked over both the outside and inside surfaces of each insulator and through the air to char the pin and cross-arm. In flowing down each pin, the current was naturally concentrated on the side exposed to the damp winds of the ocean, because the deposit of moisture by these winds lowered the resistance on that side. When these winds were not blowing, and before a pin became charred on one side, its resistance was probably about the same all the way around, and the leakage current, being distributed over the pin, was not sufficient to char it. The damp wind would, of course, lower the surface resistance of each insulator, and this, with the deposit of moisture on the pins and cross-arm, many have made a very material reduction in the total resistance from wire to wire.

The insulators used on these pins each had two petticoats, an upper one, 11 inches in diameter, and a lower one, 6¹⁄₂ inches in diameter, the lower edge of the smaller petticoat being 7¹⁄₂ inches beneath the lower outside edge of the larger petticoat. As the inner surface of the larger petticoat was much nearer to a horizontal plane than the inner surface of the smaller petticoat, moisture would have been more readily retained on it, and the greater part of the surface resistance of the insulator during wet weather must therefore have been on the inside of the smaller petticoat. At its lower edge the smaller petticoat was distant radially about 1³⁄₄ inches from the pin, and the distance between the pin and the inside surface of the smaller petticoat gradually decreased to actual contact at a point 5¹⁄₂ inches above this lower edge.

The path of the current from the line wire to the pin in this case seems to have been first over the entire insulator surface to the lower edge of the smaller petticoat and then partly up over the inner surface of this petticoat and partly from that surface through the air. On each of these three pins the charring was quite as bad just below the thread as it was further down, so that a large part of the leakage current seems to have gone up over the interior surface of the smaller petticoat. The charred portion of these pins extended but little, if at all, into the threads near the tops or into the part of the pin fitting into the cross-arm. The preservation of the part of each pin that entered the cross-arm seems to have been due to the increase of surface and decrease of resistance of the cross-arm in comparison with the pin. Preservation of the threaded part of each pin seems to have been due to its protection from moisture and its high resistance, so that little or no current passed over it.

Another pin taken from the same line as the three just considered was badly burned at a point about 1.75 inches below the threads, but on sawing it completely across at two points below the charred spot the entire section was found to be perfectly sound and free from any sign of burning. The explanation of the condition of this pin is, perhaps, that the resistance of the burned part, owing to its additional protection and dryness, was high compared with that of the lower part of the pin, and thus developed most of the heat on the passage of current. It is not clear, however, why this pin should burn only just below the thread, while other pins of the same kind on the same line were charred all the way down from the thread to the cross-arm.