Capacitors banks

Power Supplies and Controls. Induction heating furnace loads rarely can be connected directiy to the user s electric power distribution system. If the load is to operate at the supply frequency, a transformer is used to provide the proper load voltage as weU as isolation from the supply system. Adjustment of the load voltage can be achieved by means of a tapped transformer or by use of a solid-state switch. The low power factor of an induction load can be corrected by installing a capacitor bank in the primary or secondary circuit. 

Note If a power capacitor is connected after the relay, say across the motor terminals, to improve the p.f. of the machine, then only the corrected value of the current musi be used. For example, if a capacitor bank of 130 kVAr is used for an individual correction of the machine then... 

AC6b AC23 High capacitive switching such as capacitor banks... 

The size of the capacitor banks, their voltage and the impedance of the capacitor circuit. 

To detect an unbalance in a three-phase normally balanced capacitor bank (Section 26.1.1(8)). 

To discharge a charged capacitor bank over a very short period, particularly when a fully charged capacitor is interrupted. See also Section 25.7. 

They also pose limitations for installations that connect a number of cables and capacitor banks, as these also require a higher energy handling capability. 

Interrupting a charged capacitor bank, when the current will lead the impressed voltage by nearly 90°. 

Suitable for all applications.. Switching of capacitor banks, however, may j)ose problem because of high TRVs. These may cause a restrike of the arc plasma and give rise to switching surges... 

Summarizing the above, the harmonic quantities when present in a system on which are connected a few capacitor banks affect the capacitors as follows ... 

It is common practice to leave the star-connected capacitor banks ungrounded when used in the system or use delta-connected banks to prevent the flow of third harmonic currents into the power system through the grounded neutral. 

Refer to Table 23.1, which shows the average cumulative effect of all the harmonics that may be present in a power system. If we can provide a series reactor of 6% of the total kVAr of the capacitor banks connected on the system, most of the harmonics present in the. system can be suppressed. With this reactance, the system would be tuned to below the fifth harmonic (at 204 Hz) for a 50 Hz system as derived below. 

Use of a reactor will enhance the voltage across the capacitor banks and must be considered in the design of the capacitor units. Refer to Figures 23.16(a) and (b) illustrating this. If... 

V( = voltage across the capacitor banks = Ic Ul = voltage across the series reactor = /[ X ... 

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

Consider the same LT system of 415 V, as of Example 23.8, three-phase 50 Hz, and having a fault level of 36 MVA and employing capacitor banks of 360 kVAr. 

Here we have considered the fault level of the switching circuit to be same as that of the entire system, as they are the total capacitor banks connected in the system and switched together as a single unit. 

We have considered the switching of capacitor banks that are installed on one power circuit and mounted in... 

Z.2 = circuit inductance between the banks in henry (ignoring the negligible selfinductance of the capacitor banks)... 

Parallel operation of capacitor bank C2 with an already switched capacitor C,... 

Figure 23.22 Equivalent circuit for n number of capacitor bank already switched on the circuit and another (C ) being switched...

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. 

For obtaining a self-inductance of 42.93 pH consider a coil of 15 cm mean diameter (r= 7.5 cm) made of the same cable that is connecting each capacitor bank through the switching device. 

By providing 12 turns of 150 mm mean diameter of the 50 mm flexible copper cable connecting each 60 kVAr capacitor bank a self-inductance of roughly 42.93 x 10 H can be introduced into each switching circuit, which will limit the switching inrush current to almost the permissible value of the making current (/, ) of the switching device. 

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... 

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... 

Diversity When a tiumber of motor loads are connected on a common bus. normally not all the motors will be operating at a time. A capacitor bank near the MCC w oidd permit the use of lesser total kVAr than if the capaeitors were located separately at each individual load. 

Figure 23.34 suggests possible locations where the capacitor banks can be installed for individual or group controls, depending upon cost and simplicity. Location 1 will be suited for individual loads and is effective when there are not many load points. For group controls, locatioti... 

Thus capacitor banks of 360 kVAr should be adequate for this system and can be arranged in six steps of 60 kVAr each. 

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