Main keywords for this article are Surge arrester selection. See ieee c62.22. Surge arrester. Arrester selection. Procedure for selection of arresters. Considerations for Class Arrester.
Surge Arrester selection
- Suggested procedure for selection of arresters. The general procedure for selection of arresters should be as follows:
a. Select an arrester and determine its protective characteristics
b. Determine (or select) insulation with stand
c. Evaluate the insulation coordination - Other sequences may be equally acceptable. The key step is insulation coordination evaluation. Withstand voltages may be selected to match the characteristics of certain arresters or arresters may be matched to available insulation. See IEEE C62.22 Table 2 for Factors for Estimating Withstand Voltages in Mineral Oil Immersed Equipment . Also see IEEE C62.22 for more information regarding the arrestor selection procedure.
- Arrester class selection considerations. The selection of an appropriate arrester involves consideration of MCOV protective characteristics (lightning and switching impulse), durability (temporary over voltage TOV and switching surge), service conditions and pressure-relief requirements. Durability and protective level primarily determine the class of arrester selected. See protective levels summarized in IEEE 141 Tables 5 and 6 .
- The following arrester classes are listed in order of decreasing cost and overall protection and durability:
a. Station class arresters are designed for heavy-duty applications. They have the widest range of ratings, the lowest protective characteristics, and the highest durability. See Table I for station class arrester characteristics.
b. Intermediate class arresters are designed for moderate duty and system voltages of 169 kV and below. See Table I for intermediate class arrester characteristics. c. Distribution class arresters are used to protect lower voltage transformers and lines where the system-imposed duty is minimal, and there is a need for an economical design. - Specific TOV duty. An Surge Arrester selection shall be capable of withstanding the maximum anticipated TOV duty. TOV requirements shall take into account both magnitudes and duration of temporary over voltages, the combinations of which shall be equal to or less than the capability of the arrester as shown by the TOV capability curves published by the manufacturers. The following conditions are sources of TOV and can affect arrester operation:
a. Line-to-ground fault, particularly on an ungrounded or resistance grounded system b. Loss of neutral ground on a normally grounded system
c. Sudden loss of load or generator over speed, or both
d. Resonance effects and induction from parallel circuits - The most common source of TOV and the most common basis of TOV determination is the voltage rise on unfaulted phases during a line to ground fault. In the case of ungrounded systems, this shift is virtually complete; that is, the unfaulted (sound) phase arrester(s) will be subjected to 100 percent of the line-to-line operating voltage. However, a solidly grounded system (depending upon degree) provides considerable restraint in voltage pattern shift and usually permits a considerable reduction in arrester rating requirement. See IEEE 141.
- When detailed system studies or detailed calculations about Surge Arrester selection are unavailable, over voltages due to line to ground faults should be addressed. Single line-toground faults are the most common type of system disturbance, the magnitudes of these over voltages are related to system grounding and can be estimated by the coefficient of grounding (COG). COG can be calculated as follows. The equations are applicable for Z1=Z2, but do not include fault resistance.
e. See IEEE C62.22 Annex ‘A’ for additional COG calculation information
- The class of Surge Arrester selection may be influenced by the importance of the station or equipment to be protected. For example, station-class arresters should be used in large substations. Intermediate-class arresters may be used in smaller substations and on sub-transmission lines and cable terminal poles at 161 kV and below. Distribution-class arresters might be used in small
distribution substations to protect distribution voltage buses. - There is relatively little difference in the protective levels of the normal-duty and heavy-duty arresters at typical industrial plant discharge voltages. The heavy-duty arrester has substantially higher discharge current capability (high current, short duration; low current, long duration; and duty cycle classifying current) than the normal-duty arrester. The heavy-duty arrester is applicable in exposed severe lightning area distribution circuits.
- For additional information concerning Surge Arrester selection and application, see IEEE C62.22.
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Other Considerations for Station Class Arresters
- Terminal Connections. Arrester lead wire length shall be kept as short as possible. The size of the arrester lead wire is primarily determined by mechanical and corona free considerations. Arrester lead wires and ground conductors for various voltages, shall be of minimum sizes as mentioned in Table III.
- Mechanical Loading. The arrester shall not be used as a bus support device. The maximum permissible mechanical loading applied to a metallic-top shall be 40 percent of manufacturer’s rated cantilever strength. If manufacturer’s rated cantilever strength is not available, the maximum side force applied to a metallic-top arrester shall be 667 N. The line connection on a porcelain-top arrester (ideally suited for confined spaces, for example cubicle mounting, where clearance between live parts are critical) shall be made in a way that no excessive mechanical stress is placed on the arrester.
- Grounding. The lead from the ground terminal of the arrester shall be connected by the shortest possible route to the insulated bushing of the discharge counter and leakage current indicator (if provided) and thereon directly to a dedicated ground rod, which shall also be connected to the station grid conductor in the substation. The protected transformer tank shall also be connected to the same grounding system as the arrester with as low ground resistance as possible, one ohm or less. Any difference in ground potential between the protected apparatus ground and the arrester ground shall add to the voltage impressed across the apparatus insulation. If the distances stated are not maintained, there is a risk that the value of the discharge voltage of the arrester may differ. Also, the arrester may fail due to unbalanced stresses on it from nearby conducting or grounded objects. Surge Arrester selection.
- Distance to Surrounding Objects. To prevent outside objects from affecting the voltage grading, no conducting parts shall be situated in the area included by a hemisphere drawn with arrester line terminal as center and radius in millimeters equivalent to eleven times the arrester rated voltage in kV. Also, no object shall be situated within a cylindrical space with the same radius and coaxial to arrester.
- Discharge Counters and Leakage Current Indicators. All arresters for system voltages of 69 kV and above shall have a discharge counter and a continuous ac leakage/internal current indicator. As contamination increases on the porcelain, the leakage current increases. The level of current registered serves as a guide to insulation cleaning frequency, and abnormally high reading of counter over a given period of time would indicate the need for inspection and checking of arrester.
a. Due to the high contamination level and humid atmospheric conditions in the Eastern Province, an ammeter with a scale range of 0-50 mA Peak/√2 (nonlinear scale) shall be provided to monitor and measure the leakage current across the external porcelain insulator, in addition to the internal current flowing through the arrester. The surge counter shall be nonresettable and capable of registering up to 5 discharges per second on a five digit cyclometer dial.
b. The discharge counter and the leakage current indicator shall be connected between the arrester ground terminal and the station ground, and shall be installed in a position that allows readings to be taken easily. The lead between the ground terminal and of an arrester and the insulated bushing of an arrester, and the insulated bushing of the counter, shall have a withstand strength to ground of not less than 5 kV crest. It shall not be more than 3 m in length, to avoid sparkover to ground. The arrester shall be insulated from ground with 5 kV rated standoff insulators, so the
leakage current will pass through the discharge counter and leakage current indicator. - Pressure Relief Device. Arrester pressure relief device is intended to vent internal arc gases, and prevent violent porcelain shattering when an internal fault occurs. An arrester that has vented shall be replaced immediately. The arrester class to be selected for a given application shall have a pressure relief capability greater than the maximum short circuit current available at the intended arrester location.
- Shielding. As the provision of an arrester is meant to protect the transformers by limiting over voltage in the form of traveling waves entering the substation over connecting lines, the arresters shall be protected from direct lightning strokes. A basic principle of arrester application (Surge Arrester selection) is the provision of substation shielding to protect the equipment against direct lightning strokes.
