Overcoming Lightning Protection Challenges

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One of the ways I excite graduate students about career opportunities in the electrical power industry is a weekly lecture called ‘In the News …’. This year interest has expanded so rapidly in the course Smart Grid Topics that a second section was needed. A factor-of-four increase in students over three years gives me hope that utilities and manufacturers will soon be able to draw on a growing pool of talented new engineers.
Before describing Smart Grid to students, I take time to explain the components of existing power systems, including historical aspects of electrotechnology. For example, I use a video of an obsolete system control center in Clarkson, Ontario. The operators must place calls to dispatch additional 100 MW increments of hydraulic resources while viewing an LED readout of area control error. Actions must be taken each time the frequency readout drops below 59.99 Hz. Strip charts in the video show rate of change of load, in 1 GW increments. Lecture rooms always go quiet when I state that the cost of generation capacity to satisfy each of these increments remains at about $2 billion. As a change of pace, students enjoy a video, streamed from the Internet, of Canadian comic, Rick Mercer, as he hangs a 25-unit alternating aerodynamic and standard glass disc string on a 60m double circuit 500 kV tower along the Bruce to Milton Line. Such an alternating profile was selected here since it breaks up icing patterns under modest ice accretion and is apparently unique to Ontario.
A recent ‘In the news…’ featured company, Valard, that provided engineering and construction services for the Bruce-Milton Project. Valard recently advertised completion of tower erection on another project, Churchill Falls to Muskrat Falls Power Plants – some 500 km spanned using two, parallel 315 kV lines with 1267 towers. They also achieved similar milestones on a companion line – a ±350 kV DC link extending from Muskrat Falls for 1100 km. This line crosses the ocean using undersea cable and then traverses some of the world’s most challenging terrain in terms of combined wind and ice loading. The Muskrat Falls hydroelectric plant will replace an old oil-fired plant at Holyrood and thus help reduce greenhouse gas emissions and other pollution.

Lightning flash density (flashes/km2/yr) in Canada, averaged from 1999 to 2008.

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Manitoba HVDC Corridors


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Newfoundland HVDC Corridor

The entire region from Churchill Falls through to the Avalon Peninsula makes for an interesting case study in use of overhead groundwires (OHGW) for lightning protection. With typical 75 mm radial ice accumulation, these tend to sag into the phase conductors unless the steel cables are massive or the spans very short. Both increase line construction costs and also reduce reliability since the action of side-winds causes strong overturning moment forces at tower bases. Add to this the fact that resistivity on The Rock, (an affectionate name for our island province of Newfoundland) is on the order of 4000 Ωm and it becomes clear that the grounding systems needed to assure good OHGW performance are also expensive. Based on these considerations, the rationale behind building 230 kV lines in Newfoundland without OHGW lightning protection is strong.
Investments in other lightning protection methods, including redundant lines and single-pole reclosing, have generally proven effective in areas where lightning ground flash density is less than 0.2 flashes/km2/year. A flash density map shows that Newfoundland and Labrador meet this criterion. However, in areas with difficult grounding, in-service performance of single-pole reclosing or redundant parallel lines has sometimes been poor. If lightning strikes an unshielded phase conductor and arcs over the insulation to ground, the surge and resulting Line-to-Ground (LG) fault current will flow roughly equally into the foundation and guy wires. If the potential rise from their combined impedance in parallel is too high, there may be a second impulse flashover to another phase. Once two phases flash over, there is an L-L or a 2L-G fault and single-pole reclosing protection reverts to traditional three-pole reclosing to clear the problem. Lightning impulse potential rise at any tower base or guy wire can also transfer to an adjacent circuit, causing it to flashover as well and reducing the reliability benefits of having several circuits in parallel on the same right-of-way.
The new ±350 kV DC link in Newfoundland has been designed with a single OHGW, which provides good but not perfect lightning protection. This single-OHGW option has also been adopted with long-term success for ±400 kV and ±500 kV DC lines in Manitoba, a province that has more than three times more lighting, based on flash density maps for the two.
Protection and reliability engineers often debate about the degree of protection that should be given to HVDC lines in comparison to AC lines. For example, some insist that long-term damage to expensive semiconductors at converter stations can be expected from repetitive incoming surges (in the same way that ten or more applications of a steep-front wave can puncture a porcelain insulator). Others suggest that use of multiple banks and levels of surge arresters at a station entrance will mitigate this risk by limiting the steepness and magnitude of overvoltages.
Our industry will therefore be watching the performance of the Churchill Falls to Muskrat Falls 315 kV AC lines and the Labrador-Island Link ±350 kV DC lines with great interest. I also congratulate Valard on their successful completion of these difficult yet vital projects.

Dr. William A. Chisholm