Impact of Incorrect Gap Spacing
Incorrect gap spacing can lead to an array of problems. These will vary depending on whether the spacing is set below the minimum level or above the maximum level. If the gap spacing is installed below the minimum, the risk of flashover during a temporary overvoltage event increases and could lead to arrester damage or even failure.
If the gap is installed with its separation greater than the maximum recommended level, the insulator could flashover before the gap sparks over. While this will not cause arrester failure, the flashover could result in a momentary or even long-term outage. Fortunately, EGLA gap spacing is quite flexible. For additional insight, see the various system voltages and recommended gap settings shown in Table 2.
Arrester Overload Considerations
In arrester overload situations, the optimum configuration would be to use a disconnector or failure indicator, as shown in Fig 4.
Function & Design of Electrodes
Although the electrodes might seem a simple component in this type of arrester, they fulfill important functions, such as:
1. Creating a durable location on the arrester or phase conductor for any arc due to a surge to land on without damage;
2. Allowing for adjustability of gap spacing, when needed;
3. Offering a surface that is equidistant from the opposite electrode given periodic movement of insulator or arrester;
4. Producing a shape that has consistent and predictable flashover;
5. Generating full CFO to the system upon failure if the electrode and EGLA is also equipped with a disconnector (as in Fig. 4).
Note that every EGLA needs at least two electrodes and that the second electrode can be part of the existing system but not necessarily a separate component of the arrester. The electrodes must also meet additional requirements to provide the functions stated above, including:
1. Mechanical strength;
2. Non-ageing finish;
3. Shape conducive to system voltage and potential swinging of arrester or phase conductor;
4. Conductivity adequate for surges but not power frequency current;
5. Acceptable failure mode;
6. Sufficient withstand to wind loads.
Series Varistor Unit (SVU)
The varistor component of an EGLA, also known as a series varistor unit (SVU), is comprised of non-linear MOV disks and performs a single basic function, namely to limit surge current. In effect, it terminates the surge event as the surge drops in voltage. The SVU is for all practical purposes an NGLA.
Voltage Rating of SVU
The most important consideration when selecting the voltage rating of the SVU is the maximum line-to-ground voltage the system will experience. As demonstrated in Fig. 5, the turn on level of the SVU is set just above the peak line-to-ground voltage of the system. As a general rule, the 1mA Vref of the SVU must equal or exceed peak line-to-ground voltage. The discharge voltage of the EGLA is the sum of the voltage across the gaps, the varistors and the leads. In most cases, this voltage is lower than in the case of a standard arrester so it can easily clamp the voltage well below the CFO of the protected insulator.