Capacitors installation

Yet another reason for harmonic disorder in a power circuit is the occurrence of series or parallel resonance or a combination of both. Such a situation may occur at certiiin frequencies generated by the harmonic generating sources in the system. The capacitors installed in the system magnify the same. 

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. 

If Xl is the 6% series reactance of the value of the shunt capacitors installed, then... 

When many small motors are operating simultaneously it is economical to install larger capacitors in several sections than to have many small capacitors installed at each motor. 

Pr = l,000(tan31.8 - tan 25.8) = 1,000(0.6197 - 0.4843) = 135.4kV A. So, we need a capacitor installation equivalent to about 135 kV A per megawatt. The reactive power decreases from 62 to 48% of useful power, and the transmission load is 5.6% lower (1 — 0.85/0.90). Improvements in power factor become more expensive as we go on especially above 90%, the capacitance required for a given amount of change increases with the power factor itself. The next 5% increase, to 95%, requires about 156kV AMW", an increase of about 15%. The next increment would have to be more than twice this to reach 100%, an unstable situation in any case. 

Note to paragraph (a) See 1926.961 and 1926.962 for requirements pertaining to the deenergizing and grounding of capacitor installations. 

The efficiency of an induction furnace installation is determined by the ratio of the load usehil power, P, to the input power P, drawn from the utihty. Losses that must be considered include those in the power converter (transformer, capacitors, frequency converter, etc), transmission lines, cod electrical losses, and thermal loss from the furnace. Figure 1 illustrates the relationships for an induction furnace operating at a constant load temperature with variable input power. Thermal losses are constant, cod losses are a constant percentage of the cod input power, and the usehd out power varies linearly once the fixed losses are satisfied. 

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. 

Power capacitors are installed to compensate for the reactive power, particularly that which the machine draws from the grid. The capacitors must be suitable for a minimum voltage of + 13% (generally V, + 15%). 

On the supply side of the interrupter, an equivalent busbar, inductance and lumped capacitance with a provision to connect p.f. correction capacitors, if required, to represent a replica of the actual installation. 

By installing p.f. improvement capacitors to compensate the no-load magnetizing current of the circuit being switched (Section 23.1 3). These may be installed in the same switching circuit to switch together with the inductive load,... 

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

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

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... 

This is a highly recommended method of capacitor switching for installations that are large and require very fine monitoring and correction of p.f. with the smallest number of banks. The entire reactive requirement is arranged in only a few steps yet a small correction up to the smallest capacitor unit is possible. The relay is sequenced so that through its binary counter the required switching is achieved in small steps, with just four or six sets of capacitor units or banks. The operation of the entire sequence can be illustrated as follows ... 

Shunt power capacitors of non self-healing types up to and including 1000 V General performance, testing and rating. Safety requirements. Guide for installation and operation 13585-1/1994 BS EN 60931-1/1996... 

Figure 24.3 The installation of HT capacitor banks (Courtesy Khatau Junker Ltd)...

The power factor can be improved with the use of shunt capacitors at the load points or at the receiving end, as discussed above. It is not practical to have a near-fixed loading for all hours of the day. Moreover, there may also be seasonal loads which may upset the parameters considered while installing the capacitor banks. In such conditions the system may therefore have to be underutilized or run under a high risk of instability during... 

Reliability of shunt or series power capacitors is of utmost importance for the security of the system on which they are installed. Their failure may disturb the system or result in a system outage. 

Coupling capacitors and capacitor dividers Safety in electioheal installations Common specifications for high voltage switchgear and controlgear standards... 

Figure 26.7 Installation of 11 kV capacitor banks and their protective equipment...

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

I Overheating shortens the life of a capacitor. Adequate ventilation and cooling facilities, through convection and radiation, must be made available at the place of installation. When housed inside a cubicle, as in a capacitor control panel and in a tier fonnation. sufficient space must be provided between each unit. Adequate... 

Concentration of capacitor banks at one point in a system may cause amplification of harmonics, selfexcitation of machines, overvoltages due to switching and unsatisfactory working of audiofrequency remote-control apparatus. To overcome this, a concentration of capacitor banks at one point must be avoided as far as possible. These may be installed at different locations in the system. 

Although the manufacture of PCB-filled capacitor units has been discontinued, for older installations some may still be used. For the benefit of their users we give below the precautions that they must take when handling these units ... 

---