The 1960s to 1970s saw rapid development of the French power system, with erection of many HV overhead lines and substations located mainly in the countryside. However, as towns expanded since the 1980s, many houses, factories, shopping malls and recreation centres were increasingly sited near substations, which were now in mostly suburban areas. In fact, some towers belonging to RTE, the TSO in France, were in public areas or near residential gardens while others were in parking lots. One safety concern in such situations is rise in ground potential due to flow to ground of power frequency short-circuit current. This defines what can be called a ‘hot zone’ around any tower. When properly designed, pollution flashover of insulator assemblies and of the gap formed by line conductor and tower are both unlikely scenarios. But in the event of overvoltage due to lightning, insulator gaps will certainly spark-over and create faults to ground. A related issue is that certain industrial customers connected to the HV grid, such as automobile or electronic plants, are demanding when it comes to quality of electricity supply. Yet another such critical application are overhead lines that connect power plants to the grid. In these cases, all faults must be avoided as much as possible. This edited contribution to INMR by Frédéric Maciela, EDF R&D Les Renardières Electrical Laboratories and Pierre Moreau, EDF Power System & Transmission Engineering Center explains how installation of line arresters can not only protect insulator assemblies from flashover due to lightning but also improve safety and quality of electricity delivered along overhead lines.
Traditionally, Electricité de France (EDF) has tried to improve safety by lowering grounding impedance of towers as much as possible or by installing shield wires. The latter are highly efficient in reducing power frequency current flowing to tower grounding systems by paralleling them and by offering a return path to the substation. What is more difficult to control, however, is the way any rise in potential of a HV tower grounding system is transmitted to a nearby grounding system of a low voltage installation, which is extremely sensitive to power frequency overvoltage. These days, the French grid belongs to RTE. But EDF still owns many overhead lines that connect its power plants, mainly nuclear and hydraulic, to RTE substations. EDF is also owner of power plants as well as the grid in French overseas territories such as French Guyana, the Caribbean and Indian islands, some of which are exposed to severe lightning activity. Overhead power supply lines are important for network stability, proper operation of power plants. Every minute of an outage is extremely costly.
Selection of EGLA Line Arresters
At the beginning of the 1990s, EDF’s R&D and transmission divisions created a working group to study the best way to avoid flashover of insulators assemblies on 63 kV and 90 kV lines due to lightning overvoltage. The main objective was to reinforce safety of towers located in ‘hot zones’ while bearing in mind that any proposed solution could also prove useful in improving electricity quality. Installing line arresters was deemed the only practical solution since alternatives such as lowering tower footing impedance, increasing withstand of insulators assemblies or installing shield wire if mechanically possible, were all seen as insufficient. For example, a storm cloud is considered to be a generator of tens of kA. Considering that line surge impedances are of some hundreds of ohms or tower foot HF impedances are of tens of ohms, overvoltages of hundreds of kV exceeding withstand of insulator strings are easily created. Specification of a suitable line arrester was drawn up a few years later.
There are two basic technologies for line arresters: non-gapped line arresters (NGLAs) and externally-gapped line arresters (EGLAs) – defined in IEC standard 60099-8. NGLAs are similar to substation arresters, both in design and operation. The main challenge is installation on towers, especially the arrester’s ground connection and its associated disconnector.
EDF decided to employ EGLAs with a polymeric housing for the following reasons:
• more compact design, lighter and easier to install (no need for disconnector of ground connection)
• no permanent power frequency voltage and no ageing of ZnO blocks nor housing
• no energy stress due to switching impulses
• less costly than NGLAs.
But use of EGLAs also comes with certain technical challenges, such as:
1. set-up must guarantee right operation of the arrester, even in case of wind (e.g. dimensional stability of the external gap);
2. insulation coordination must be carefully designed and may need systematic testing in a laboratory on a full sized configuration;
3. verification of operation under polluted conditions is difficult.
EDF Line Arresters Design Considerations
All line arresters at EDF are EGLA type. The series varistor unit (SVU) is made of zinc oxide blocks and the housing is polymeric (EPDM was qualified for this application). The same arresters are used on 63 kV and 90 kV lines. The SVU and the external gap are bolted directly onto an insulator assembly and this whole set-up is prepared on the ground. This offers major benefits:
• installation of the arrester sees replacement of the insulator string, which is common for workers in charge of lines maintenance;
• if displacement of the conductor occurs under high wind, the entire insulator assembly also moves and the length of the arrester gap does not change significantly.
Rated voltage of the SVU is an important parameter since it determines the follow current interruption properties of the arrester when stressed by TOV conditions on the network. The final choice involved a compromise for both applicable voltage levels, i.e. rated voltage selected for the SVU was about 60% of what it should be for a 63 kV substation arrester and 75% of that typical value for a 90 kV substation arrester. Such a decrease in rated voltage of an EGLA is possible since the power frequency follow current, limited to a few amps, occurs only within the half-period during which the impulse creating spark-over of the gap is applied. Energy design of line arresters should also take into account several factors: lightning activity, tower foot impedance, rated voltage of the SVU, presence or not of shield wire and acceptable arrester failure rate. This is therefore a statistical approach. In the case of EDF’s application on 63 and 90 kV lines in France, it was considered that energy design of the line arresters should be at least 200 kJ so as to guarantee an average failure rate lower than 1 per 1000 per year. Calculation was based on EMTP models and lightning data collected from CIGRE.
In the event of arrester failure, failure mode must be completely safe since these are installed in public areas. Here, choice of a polymeric housing helped achieve satisfactory results. For example, short-circuit tests were conducted on the complete set-up, including arrester and insulator assembly. The sample was deemed to have passed the test if no hard pieces of the arrester or pieces of glass fell down, if the insulator assembly maintained its mechanical integrity, if the arrester quickly self-extinguished flames, and if the associated fault indicator was visible by the end of the test. Presence of a fault indicator was regarded as necessary to locate with complete certainty any failed arrester during line inspection.
Finally, insulation coordination is key to proper operation of an arrester. When the SVU is considered to have failed, i.e. to be equivalent to a short circuit, the gap shall withstand switching impulses. When the SVU is in good condition, its gap shall spark over before all others gaps surrounding the arrester (i.e. mainly the insulator gap and the distance between line conductor and tower, even in case of displacement due to wind). EDF’s decision was to perform these tests under the most representative configurations of the real application. For this purpose, a full sized 90 kV tower head was sey-up in the laboratory. Calculation of U50 of the gap can be unpredictable because of complex geometry. This leads to some atypical values of gap factor due as well to presence of the SVU, which acts as a capacitive divider when an impulse is applied. The conclusion of these tests was that it is important to check that the arrester gap is the first gap to spark-over when the EGLA is installed on the tower, regardless of the absolute spark-over voltage of the arrester alone.
EGLA Design for 225 kV & 400 kV lines: Additional Challenges
EDF also studied the possibility of using the 63/90 kV EGLA concept on 225 kV and 400 kV lines. Two key points emerged: insulation coordination and compatibility of the presence of EGLAs with live work operations. Insulation coordination was more challenging because of the relatively high voltage amplitude of switching impulses. One solution was to increase number of insulators on protected strings since this would allow more margin in electrical dimensioning (withstand of switching impulses and spark-over for lightning impulses). Adding one or two additional insulator discs increases string length and particular attention must therefore be paid to height of line conductor with respect to the ground as well as to clearance of line conductors in tower windows. An alternative approach is to decrease switching impulse voltage, thereby avoiding some charges being stocked on conductors, e.g. by using instrument voltage transformers.
The other challenge concerned live line work, critical for operation of many 225 and 400 kV lines. The presence of an EGLA introduces into the working zone a gap for which length must be compatible with live working distances. This gives another parameter for choice of EGLA gap length. At EDF, design of line arresters permits live installation of 225 kV EGLAs due to fixing the SVU with a pivot that allows for an increased gap during the live installation process. Once the insulator and EGLA assembly has been installed, the operator can then move the SVU to its normal operating position using a live line tool.
Optimizing Line Arresters: Number & Location
When planning installation of line arresters, the first task is to determine the main purpose, e.g. mitigating ground potential rise in hot zones or improving electricity quality, or both. Then, the line itself must be characterized: geometry (i.e. number of circuits, arrangement of phase conductors, presence or absence of shield wire), length, tower foot impedances, topography, local lightning activity). In the case of aiming mainly to mitigate rise in potential, the conservative approach is to equip every insulator string on the tower with arresters to avoid any spark-over due to lightning on this particular structure. If the line is equipped with shield wire, it may also be necessary to install arresters on nearby towers to avoid some ground faults occurring too close to the protected tower as well as to benefit from a distribution of the power frequency fault current in multiple grounding systems. If the main goal is to improve electricity quality, this objective must be expressed using a statistical approach. If no outages are to be allowed, every insulator string must be protected with line arresters. But this is costly. Optimization can therefore be considered using specialized software, such as EMTP. For example, below is an example of such a study for a 17 km line located on the Caribbean island of La Guadeloupe.
Field experience for this line (no shield wire, no arresters) showed an outage rate of about 1.33 to 1.5 per year. Calculated value was 1.15, independent of value of tower footing resistance, which showed that the computation model was accurate. The main conclusions of this study were:
1. without shield wire, ground connection resistance has no influence;
2. with shield wire, very low tower footing resistance dramatically decreases outage rate;
3. with line arresters, good results can also be achieved. But installation must be carefully chosen considering ground resistance and presence or absence of shield wire.
Another important point to take into account is line topography. For example, on some islands storms always arrive from the same direction and are blocked by mountains extending over part of the island. Line arresters should ideally be installed in this area. Considerations such as this can lead to optimization in number of line arresters to be installed, achieving a given goal of power outages while also reducing costs.
Efficiency of Line Arresters: Past EDF Investigations
To validate computational models, an investigation was conducted on a 10 km 90 kV double circuit line located in the centre of France. This line is equipped with shield wire and tower footing resistance values are quite poor, with average value of 200 ohms due to the poor soil conductivity in the area. Over a 16-year observation period, the line’s outage rate due to the storms was between 5 and 8 per year. This was seen as unacceptably high for proper operation of the line. 60 EGLAs were then installed on every tower of one circuit, but only on the bottom two phase conductors. The fault indicator of each arrester was replaced by a monitoring box supplied by solar cells, capable of dating and recording maximum current value whenever an arrester gap sparked. Data was stored and could be downloaded from ground level by basic radio modem. The investigation lasted 4 years. During this period, the French Lightning Location System indicated that 207 lightning strokes to ground occurred near the overhead line. Peak values of lightning currents recorded through the arresters were between 9 kA and 35 kA. The conclusion was that line arresters were effective in reducing outage rate and optimization of their number and location was possible to achieve any particular goal. For example, in the case of double circuit lines, arresters can be optimally installed only on one circuit. This reduces but does not eliminate outages on this circuit that essentially becomes the ‘weak circuit’ but cancels outages on the other circuit.
Field Experience: Reliability of Line Arresters & Energy Design
The main stress on EGLAs is provided by lightning currents. Energy design is a key point yet evaluation of the energy associated with lightning strokes is difficult since it depends not only on line characteristics but also on topography and storm activity in the area.
Former IEC energy Class 1 was used for the 63/90 kV EGLA at EDF and Class 2 for 225 and 400 kV EGLAs. After some 18 years of operation of thousands of EGLAs across France, failure rate (e.g. due to energy overstress) was extremely low and less than 1/1000/year. In the case of overseas French territories, such as Caribbean islands, failure rate has been higher. For example, in La Martinique Island, 20 EGLAs installed in one part of a 90 kV overhead line were all replaced after 15 years service. This field experience demonstrated that energy class of line arresters must be carefully chosen. But this parameter can be unpredictable and only actual experience can provide the basic data needed for future designs and new projects.
Based on an intensive program of laboratory tests and computational studies and given almost 20 years of experience involving thousands of samples, installation of line arresters is highly effective in mitigating ground potential rise of towers and improving electricity quality wherever needed. Optimization in number and location of arresters is also possible to achieve any specific goal. Some EMTP models are sufficiently accurate to estimate outage rates of overhead lines equipped with arresters. Selection of the EGLA concept leads to an interesting technical-economic compromise but requires a rigorous specification and qualification procedure, based mainly on laboratory tests with full-scale configurations.