HT systems

The leakage current is measured as in lEC 60298. This method is generally applicable to an HT system by applying the full rated voltage between the insulating surface, say, between a phase and the ground. The leakage current thus measured should not exceed 0.5 mA. 

The values thus shown in Table 14.7 take cognisance of the recommendations of I EC 60298 to maintain the leakage current at less than 0.5 mA for all HT systems. 

Rated primary voltage The normal practice for an HT system is to provide a separate LT feeder for the auxiliary supplies. The primary voltage will be the normal system voltage, V, when the transformer is connected line to line or V,/ l3 when connected line to neutral. 

In most applications, a bar primary CT is generally used and a normal CT may be suitable. But for too small ratings, where the use of a wound primary CT is imperative, short-circuit effects must be considered, except the CTs for an LT system, where the fault level for such small ratings may be very low and may not matter (Section 13.4.1(5)). For applications on an HT system, where a wound primary CT is imperative, choice of a CT from standard wound primary CTs may still be possible, meeting the minimum requirements of class of accuracy, VA burden and short-time rating. lEC 60044-1 indicates for measuring and protection CTs the maximum short-time factors (STF) that can be obtained economically for a normal wound primary CT where... 

ACBs were the first to be produced commercially. They are simple to operate and cause no fire hazards. But at atmospheric pressure, they possess a low dielectric strength and are therefore normally manufactured only in low voltages. Air has less contamination and therefore these breakers require negligible maintenance, compared to oil. They require no contact cleaning. Since there is a limit to producing these breakers for HT systems, their normal application is for LT systems alone, where they... 

From this equaiion one can determine the required value of neutral circuit impedance for a particular level of ground fault current. The external impedance will be Z, less the ground impedance. In HT systems one c in also delermine the likely value of a ground inductor coil to achieve a near-resonance condition, to eliminate the arcing grounds, on the one hand, and facilitate a strike-free extinction of an arc hy the interrupting device, on the other. 

For a three-phase three-wire system (generally HT systems)... 

This is a mailer of system design and is different for LT and HT systems, as discussed above. The main criterion... 

The odd harmonics (non-multiples of 3), generated by hexaphase rectifiers, are more severe than any other harmonic disorders present in the system, except LT systems, where third harmonics too are pronounced. An LT system and the devices connected on it must take cognisance of these effects and suitable measures be taken, either to diminish such effects or to select higher ratings of the devices to operate on such systems without overloading. HT systems must take particular cognizance of odd harmonics. 

In an HT system, either the star is not grounded or it is a delta-connected system and hence the third harmonic is mostly absent, while the content of the. second harmonic nuiy be too small to be of any significance. For this purpose, where harmonic analysis is not possible, or for a new installation where the content of harmonies is not known, it is common practice to use a series reactor of 6% of the reactive value of the capacitors installed. This will suppress most of the harmonics by making the circuit inductive, up to almost the fourth harmonic, as derived subsequently. Where, however, second harmonics are significant, the circuit may be tuned for just below the second harmonic. To arrive at a more accurate choice of filters, it is better to conduct a harmonic analysis of the system through a harmonic analyser and ascertain the actual harmonic quantities and their magnitudes present in the system, and provide a correct series or parallel filter-circuits for each harmonic. 

In HT systems, where a series reactor is already being used, to suppress the system harmonics this would also serve to limit the switching inrush currents and no separate reactor would be necessary,... 

For HT systems, refer to Table 19.1, for the selection of interrupters for capacilor duly. The breaker should be chosen restrike free, as far as possible. 

The following are suggestions for the suceessful operation of the capacitor units installed on an LT or an HT system ... 

These values are considered for an altitude of up to 2000 m for LT and 1000 m for HT systems. For higher altitudes to achieve the same level of dielectric strength, the values of clearances and creepage distances, as given in Tables 28.4 and 28.5, may be increased by at least 1 % for every 100 m rise in altitude. 

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