By Antero R. Farias
In order to reduce ice storm damage to overhead lines in the voltage range of 8 kV feeders to 115 kV transmission lines, Manitoba Hydro has developed an Ice Storm Management Program. This program consists of the de-icing of overhead lines, either by AC ice melting -the placement of 3 phase short at a calculated point, or by ice rolling -the mechanical stripping of ice from conductors. Ice storm damage control is also achieved by prevention measures such as underground distribution systems.
Recent experience in Manitoba
The last significant ice storm experienced in Manitoba was in the winter of 1997/98. El Nino likely had the greatest temperature effect last winter on the southern parts of Manitoba, more so than in any other parts of Canada. Average temperatures were several degrees above normal, and the amount of sunshine was well below normal, as many consecutive days were cloudy or foggy, resulting in hoarfrost. As well there were many calm days, unusual for normally windy Prairies. These weather conditions precipitated drizzle fog, rain and hoarfrost for days.
During the period from February 6- 17, 1998 ice storm conditions were experienced over a wide area of the province. In addition to "ice rolling", a total of 83 "ice melt" procedures were performed to melt the ice from 2,628 km of overhead line (7,883 km of conductor).
To put these figures into perspective, Manitoba Hydro has removed ice from an average of 713 km of line per year for the past three years. This is less than one-third of what was removed in those eleven days during 1998. In order to perform these melts, voltages of 8 kV, 12 kV, 25 kV and 33 kV were applied to the lines (66 and 115 kV).
The table below shows the distance of each type of line melted:
Voltage of the Circuit-km of Line Line Melted Melted
33 kV 106 km
66 kV 1,941 km
115 kV 565 km
On February 9, 1998, Manitoba Hydro melted fifteen separate cases in a period of approximately ten hours. To be able to have done this amount of melting during the height of an ice storm, with a large portion of our resources dedicated to ice rolling and restoring service, was only possible because of experience, confidence, pre-planning and teamwork. During 1997/98 Manitoba Hydro melted 3,419 km of line in 110 procedures.
In the 1998/99 ice storm season Manitoba Hydro has melted 714 km of line to date.
AC Ice melting Manitoba Hydro has accumulated experience with melting ice from conductors for over twenty-five years. A literary search of papers and articles reveals that ice melting by slightly heating the ice covered conductors has been theorized and practiced successfully in many parts of the world in the past fifty years.
The theory and the basis of using AC short circuit currents to melt ice is to use the short circuit capacity inherent in the local power system. These sources have included using mobile transformers (66-12/25 kV), substation transformers (66-12 or 25 kV), and terminal station transformers (115 or 230-66 kV). In most cases the power transformer in substations is the most popular source.
Mobile transformers are too cumbersome to move to a desired location in the midst of an ice storm. Roads and highways can become impassible and time delay to set up the mobile unit is valuable time lost.
The concept, in the most basic terms, is to use the impedances in the melting circuit to limit the current (short circuit) to a safe enough level so that no component in the circuit will be overloaded and damaged but high enough to warm the conductors sufficiently in a reasonably short period of time to melt the top section of ice and allow the unmelted ice to drop.
AC Ice Melting Limitations
1. The size of the conductor of the line to be de-iced -the larger the conductor, the higher the current needed.
2. The MVA rating of the bank to be used as the source for the melting current -a larger bank can deliver a higher current.
3. The voltage of the source transformer -the melt voltage is equal to or less than the line voltage.
4. The length of the conductor to be de-iced.
5. The availability of interconnections between the source transformer and the line to be de-iced.
The amount or magnitude of current required is predetermined by studies and short circuit calculations which take into account many factors such as conductor size, ambient temperatures, wind speeds, thermal limits of the other current carrying components, loads, voltages, etc. Once these limits or boundaries are known, the optimum location for the short circuit to be applied to generate the required short circuit current, is selected. The case is then documented and filed in an ice melt manual.
In most cases on Manitoba Hydro's system, the concept relies upon using the short circuit capacity of the substation transformer, de-energizing the subtransmission supply line, re-energizing it at the lower voltage of the substation ( 8 kV, 12 kV, up to 33 kV) and using the short circuit at a pre-calculated and pre-determined distance away to generate the required melting current on the iced circuit. Cases have also been developed for melting subtransmission and transmission lines at their operating voltages (33 kV, 66 kV and 115 kV).
AC Ice Melting -Advantages
AC Ice Melting -Disadvantages
Rolling
Ice can be removed from lines by using a manual roller. This device consists of a 11.43 cm pulley, mounted on the end of a 6 m. laminated 3.8 cm X 3.8 cm wooden handle. A rope is attached to the end of this handle to extend the reach of the device (see diagram). The roller assembly is placed on the conductor by a lineman up the pole or tower -one roller per conductor. The rope at the end of the stick is passed to staff on the ground who pull the roller down and along the conductor to the next tower/pole.
The roller wheel assembly is made of aluminum for lightness as well as for softness to reduce conductor and insulator damage. The wheel of the roller moving over the ice plus the bend in the conductor caused by the pull on the rope causes the ice to fracture and fall off.
Transmission lines are de-energized before rolling starts. Distribution lines up to 12 kV can be rolled while energized with no modification to sticks. 25 kV distribution lines can be rolled while energized by installing an insulated link in the stick handle.
Ice Rolling -Advantages
Ice Rolling -Disadvantages
DC Ice Melting
As an alternative to using conventional 3-phase AC impedance faults to melt ice off transmission lines, direct current heating may also be employed. Using DC has the distinct advantage of eliminating the reactive losses which limit the distance in conventional melting. By adjusting the timing angles of the thyristor control, it is possible to adjust the amount of current down from a theoretical maximum.
A direct current ice melting system may be used on short transmission lines, may reside on a mobile platform for use in many stations, and be used to melt large conductors.
Manitoba Hydro reviewed the pros and cons of DC ice melting in the 1980's. For a number of reasons, mainly economic, the approach was shelved. The time may now be right for considering this concept again.
There are reports that in the former Soviet Union, DC ice melting has been successfully used for a number of years. Apparently a large ice melting complex -120 MVA, 3000 A, 40 kV -has been installed in Kazakhstan for 500 kV lines.
Also the concept of injecting pulse direct current into 3-phase AC lines has been proposed. Due to better communications now with these countries more knowledge should be available regarding these interesting approaches.
DC Ice Melting - Advantages
DC Ice Melting -Disadvantages
DC ice melting can be best applied in situations where ice prone conductors are very large and cannot easily be switched out of the system. If the icing is very localized, DC ice melting is also a good candidate if the lines are short or the capacity does not exist to use more conventional ice melting methods.
Conclusion
The Ice Storm Task Forces which reviewed ice storms in 1984 and 1991 concluded that ice melting was the most effective way of removing ice from overhead transmission and subtransmission lines. It was recommended that ice melting should be expanded wherever possible and that the system should be strengthened in ice prone areas.
Ice melting should not be viewed as the panacea to combat the effects of ice storms but only as one tool among others including improved structure and line designs, system designs that provide more redundancies and emergency sources, standby generators at critical loads, etc.
It is recognized that AC ice melting may not be as effective for other utilities as it is for Manitoba Hydro. Utilities interested in ice melting would have to review their system to determine if ice melting was possible and how effective it would be. Ice rolling is very effective and could be used by most utilities.
Future Research
The effects of the "Ice Storm of the Century" in eastern Canada and the northeastern United States was costly, daunting, and devastating to utilities, their employees, to citizens, to businesses and society.
The power utility industry, manufacturers, research laboratories, academics, educational institutions, and individuals should collaborate on ideas and research to develop improved methods and technologies to combat future ice storms. International alliances, workshops, and conferences are encouraged to promote activities and to make progress. Manitoba Hydro is willing to participate in these activities as there is much to be learned yet and much needs to be done. Melting of ice off skywires and off large transmission conductors remains a serious challenge.
Antero R. Farias is with Manitoba Hydro. This article was originally presented at CEA's Electricity'99.
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