Surge arresters levels

All electrical equipment are designed for a specific BIL, as indicated in Tables 11.6, 13.2, 14.1, and 32.1(A) for motors, switchgears and bus systems respectively, and Tables 13.2 and 13.3 for the main power system (line clearances and insulators). If the actual severity of a prospective surge, i.e. its amplitude and/or rise time or both, is expected to be higher than these levels (higher amplitude and lower rise time) the same must be damped to a safe level, with the use of surge arresters, surge capacitors or both as discussed later. 

Select the nearest voltage rating of a station class surge arrester (see also Section 18.8) from the manufacturer s catalogue, (Table 18.8) as 6 kV which has the following protective levels ... 

The current limiting gaps, as noted above, in series with the non-linear resi.stors make it possible to adjust the protection level of the surge arrester for different values of discharge currents. They also help to maintain... 

Table 18.3 Establishing the protection level of a surge arrester...

Transient condition as in Table 18.1 Protection level of a gapped surge arrester Protection level of a gapless surge arrester ... 

The protective levels of the surge arresters, al different system voltages are furnished by the manufacturers in their product catalogues. Tables 18.9 and 18.11 furnish typical data for a few established manufacturers. 

TOV is considered only to select the MCOV and the rated voltage, V, of the surge arrester. This is a reference parameter to define the operating characteristics of an arrester. It plays no part in deciding the protective level of the arrester, which is solely dependent on the transient conditions of the system, as discussed later. V, is used to make the right choice of an arrester and its energy absorption capability to ensure that it does not fail under the system s prospective traiisieut conditions. 

The protective level as determined above is true only when the surge arrester is mounted directly on the protected equipment (Figure 18.21), But this is seldom possible, us there is usually a gup between the surge arrester and the equipment, due to arrester height, connecting leads... 

V Z./C is the surge impedance, Z, L and C the circuit constants of the interrupting circuit, as discussed in (Section 17.6.4). C represents the dielectric capacitance between the parting contacts of the interrupter. must be prevented, as far as practicable, from reaching dangerous levels with the use of surge arresters. 

Surge arresters Electrical characteristics of a ZnO surge arrester Basic insulation level (BIL) ... 

Surge arresters. The use of modern surge arresters allows a reduction in the basic impulse-insulation levels of much transmission system equipment. The primary function of early arresters was to protect the system insulation from the effects of lightning. Modern arresters not only dissipate lightning-caused transients, but may also control many other system transients caused by switching or faults. 

The control of the primary coolant inventory is shown in Figures C-62 and C-63. The coolant liquid level in the pressurizer must be maintained essentially constant under various phases of reactor operation to assure surge capacity and pressure control capability Liquid level changes in the pressurizer will operate the primary loop Injection flow control valves Indirectly controlling the liquid level in the pressurizer. Surges in level above the normal control ranges will result in operation of the spill system and will initiate a reactor power setback. Drops in the pressurizer level below the normal control range may lead to serious consequences if not arrested Efforts have been made to avoid excessively low levels or to minimize their consequences If the level drops below the normal control point an alarm sounds If, in spite of corrective actions taken after the alarm, the level continues to drop, a power setback trip will occur. The purpose of the setback is to reduce power before the pressurizer is depleted and losa of pressure control is threatened. In case of a still further drop in level, a reactor scram trip will be incurred automatically. 

With this particular arrester, the motor has to be specially designed for the higher level of FOW impulse voltage withstand. But the manufacturer can always modify the protective characteristics of the arrester, depending upon the system s requirements. The matter may therefore be referred to the manufacturer for recommendations. The arrester may be fitted with a 0.25, uF surge capacitor in parallel, to reduce the steepness of the FOW (for the arrester of example 17.5, it is 14/0.2 kV/jUS) to a safer value. 

The proteetion level of an arrester, is a function of the magnitude of arrester discharge current (/ ), and the time to peak of the surge (/ ). and is influenced by the following. 

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