KVAr rating

Consider Figure 23.29(a), where, the p.f. of a power circuit is to be improved from cos 0, to cos 02- If kVAri is the reactive component of power at p.f. cos 0, which is to be improved to kVAr2, at p.f. co.s 0i, through the reactive power coinpensation, then the reactive component of power compensated or kVAr rating of the required capacitor banks... 

Figure 23.29(a) Determining the kVAr rating of a shunt capacitor... 

For smaller units, they are u.sed to limit only the inrush currents during a parallel operation and may be rated for 0.2% of the capacitors kVAr rating. They must be connected on the neutral side of a star-connected capacitor bank. Also refer to the notes below step 4 of Example 23.4. 

The practice adopted by manufacturers, for all voltages and kVAr ratings in the niakiitg of the shell, partiettlarly for its size, material and thickness, assume almost the same I" versus l eharacteristics, as provided by the NEMA curves (Figure 2fr.1). 

These are employed for the measurement of power circuit currents through an ammeter, kW, kWh or KVAr and power factor meter, or similar instruments requiring a current measurement. They must have a specified accuracy class as in lEC 60044-1 and the secondary current substantially proportional to the primary within a working range of about 5-120% of its primary rated current. They... 

The rating of a capacitor unit will thus vary in a square proportion of the effective harmonic voltage and in a direct proportion to the harmonic frequency. This rise in kVAr, however, will not contribute to improvement of the system p.f. but only of the overloading of the capacitors themselves. 

When a capacitor circuit is compensated through a series reaetor. either to suppress the system harmonics or to limit the switching inrush currents (Section 23.11) or both, it will require suitable adjustment in its voltage and capacitive ratings, fhe series reactor will dampen the switching currents but consume an inductively reactive power and offset an equivalent amount of capacitive kVAr. and require compensation. The following example w ill elucidate this. 

To determine the basic parameters of a 6% series reactor and its capacitive compensation, consider Example 23.6 with 3000 kVAr banks (1000 kVAr per phase) rated for 33.4 kV ... 

The capacitors rating must be enhanced by the rating of fhe reactor to obtain the same levei of effective kVAr. 

The rating of the capacitor banks should be chosen for = 3000 + 180 = 3180 kVAr. 

Site of Continuous rating Maximum siz.e of three-phase capacitors in kVAr at different svitching currents /, ... 

Thus, for a single capacitor switching of a 60 kVAr bank, a rating of 125 A will be required for the switch, fuse and the contactor etc. 

If we are able to provide an inductance of this value with each capacitor bank of 60 kVAr the problem of excessive inrush transient current can be overcome and the component ratings as chosen above will be sufficient to switch a parallel circuit. 

If 3000 kVAr capacitor banks are required for a system of 33 kV + 7.5%, -5% and the capacitors are rated for 34/v 3 kV then the output of the capacitors at nominal voltage will reduce to... 

Motor Capacitor rating in kVAr at a motor r.p.rn. Motor Capacitor rating in KVAr a a motor r.p.rn. ... 

For low-voltage applications these elements may be made up to I kVAr or so each and designed up to 440 V, depending upon the system requirement, while for high-voltage applications, they may be designed up to 2-3 kV, each rated for 20-100 kVAr. These values are only indicative and may vary from one manufacturer to another depending upon the dielectric used, the process adopted and the site requirements. 

Rating of each capacitor unit kVAr Line voltage kV... 

For a capacitor bank of 500 kVAr in 10 units of 50 kVAr each, for an automatic power factor correction the rating of components would be... 

Power-factor can be rated at unity, leading, or even lagging. The synchronous motor can supply corrective kvar to counteract lagging power factor caused by induction motors or other inductive loads. 

Typically, electrical utilities charge a penalty for power factors below 0.95. The method of calculating the penalty depends on the utility. In some cases, the formula is simple, but in other cases the formula for the power factor penalty can be much more complex. Let s assume that one utility charges a rate of 0.200/kVAR-hr for all the reactive energy used if the power factor falls below 0.95. No kVar-hr charges are levied if the power factor is above 0.95. 

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