Lightning Arresters

The purpose of a lightning arrester installed on primary lines is twofold: first, to provide a point in the circuit at which the lightning impulse can pass to earth without injuring line insulators, transformers, or other connected equipment; and second, to prevent any follow-up power current from flowing to ground. Lightning arresters must be installed on the primary side of all substations, distribution centers, distribution transformers, and capacitor banks.

Ground Wire

A ground wire on an overhead line may be a galvanized steel, copper-covered steel, or aluminum-covered steel cable. It is strung along the transmission line but is mounted above the other conductors. The ground wire is not a part of any electrical circuit, but is, instead, connected to earth ground at frequent intervals (usually every fifth pole) on pole lines. The ground or earth potential is brought above the transmission lines, therefore reducing the stress placed on the lines and insulators caused by lightning. The effectiveness of a ground wire depends on low ground resistance. Good ground connections should be made at frequent intervals along the line;

CAPACITOR BANKS

Capacitors are an important part of the distribution system. They provide a convenient and practical means of improving the power factor by neutralizing the effect of lagging power factor loads. This action reduces the line current and line losses and improves the line voltage regulation. Capacitors can be installed in relatively small banks and placed in the circuit near the source of the reactive load or on the primary feeders. Be sure to use high-voltage capacitors when you place capacitors between the transformer and the high-voltage primary lines. Typical capacitor bank connections are shown in figure 5-18.

The questions you should now be asking yourself are, How much capacity do I add to the circuit? and How do I go about computing the required capacity with the information I now have available to me? Perhaps the following illustration will help.

Figure 5-18.\Typical capacitor bank connections.

Assume you have a 100-kilowatt supply. You have determined by various measurements that it is operating at 70-percent power factor (PF). You would like to improve the PF to 85 percent. How much capacity should you add? Refer to figure 5-19 and the following calculations.

Given:

cos A = .70 = present PF

cos B = .85 = desired PF

From the cosines given above and trigonometric tables, you can determine the following angles and tangents:

Angle C (fig. 5-19) is the amount of leading capacitive reactance you need to add to the circuit to reduce the lagging reactive component. Attempts to improve the PF above 90 percent by means of capacitors are seldom economical.

Listed below are a few rules to help you supervise the installation of capacitors.

Use lightning arresters of the line type on the capacitor banks of the sizes normally used.

Use primary cutouts in the majority of installations to connect and disconnect the capacitor bank. A liberal margin between normal current ratings and fuse rating is necessary to avoid unnecessary operation on current transients.

Install capacitors at load centers to improve lagging power factor.

Install capacitors at the end of the line for voltage improvement.

WARNING

A disconnected capacitor retains its electrical charge for some time and may have full line voltage across its terminals. When capacitors are removed from service for any purpose, consider them to be at full voltage until the terminals have been short-circuited and grounded. DO NOT SHORT-CIRCUIT TERMINALS UNTIL CAPACITORS HAVE BEEN DE-ENERGIZED FOR AT LEAST 5 MINUTES.