SURGE PROTECTION OF SUBSTATION BASIC INFORMATION AND TUTORIALS



What Are The Means To protect The Substation From Surges?

A substation should be designed to include safeguards against the hazards of abnormally high voltage surges that can appear across the insulation of electrical equipment in the station. The most severe overvoltages are caused by lightning strokes and by switching surges.

The main methods to prevent these overvoltages from causing insulation failures include:
1. Use of surge arresters
2. Equipment neutral grounding
3. Proper selection of equipment impulse insulation level
4. Proper selection and coordination of equipment basic insulation levels
5. Careful study of switching-surge levels that can appear in the substation

The main device used to prevent dangerous overvoltages, flashovers, and serious damage to equipment is the surge arrester. The surge arrester conducts high surge currents, such as can be caused by a lightning stroke, harmlessly to ground and thus prevents excessive overvoltages from appearing across equipment insulation.

The important consideration in applying surge arresters and in selecting equipment insulation levels depends greatly on the method of grounding used. Systems are considered to be effectively grounded when the coefficient of grounding does not exceed 80%. Similarly, systems are noneffectively grounded or ungrounded when the coefficient of grounding exceeds 80%.

A value not exceeding 80% is obtained approximately when, for all system conditions, the ratio of zero sequence reactance to positive sequence reactance (X0/X1) is positive and less than 3 and the ratio of zero sequence resistance to positive sequence reactance (R0/X1) is positive and less than 1.

What this says in effect is that if neutrals are grounded solidly everywhere and if a ground occurs on one of the conductors, then the voltage that can appear on the healthy phases cannot exceed 80% of normal phase-to-phase voltage.

Thus, the coefficient of grounding is defined as the ratio of maximum sustained line-to-ground voltage during faults to the maximum operating line-to-line voltage. On many HV and EHV systems, the coefficient of grounding may be as low as 70%.

Surge-arrester ratings are normally selected on the basis of the coefficient of grounding; thus, for effectively grounded systems, the 80% arrester is selected when using the conventional gap-type arrester. When using the gapless metal oxide arrester, a lower-value arrester may be selected based on the maximum continuous operating voltage (MCOV) equal to the maximum normal line-to-neutral voltage.

For example, a 115-kV system (maximum operating voltage equals 121 kV) can use a 97-kV conventional arrester, that is, 80% of 121 kV, when operating on a solidly grounded system, and can use a gapless-type metal oxide arrester rated 70 kV. It should be noted that other factors, such as resonant conditions and system switching, could increase the value of the coefficient of grounding and thus should be studied in each individual system.

The impulse insulation level of a piece of equipment is a measure of its ability to withstand impulse voltage. It is the crest value, in kilovolts, of the wave of impulse voltage that the equipment must withstand. However, at EHV, the switching-surge insulation level may be lower than the corresponding impulse level, and thus the switching-surge level becomes the dominant factor in establishing insulation levels.

Basically, the coordination of insulation in a substation means the use of no higher-rated arrester than required to withstand the 60-Hz voltage and the choice of equipment insulation levels that can be protected by the arrester.

Careful study of switching-surge levels that can occur at the substation as determined, for example, by transient network analyzer studies also can be used to determine and coordinate proper impulse insulation and switching-surge strength required in a substation electrical equipment.

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