Capacitors Bank

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ACTIVECAP Real Time Capacitor Bank Electromechanical Type Automatic Capacitor Bank

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Introduction

Hong Kong Switchgear Limited is one of the leading manufacturers of Automatic P.F. Correction Capacitor Bank in Hong Kong. The design of the capacitor bank is based on German Technologies and therefore is built to VDE 0560 part 41 and EN 60439 part 1. Module technic, non-reactor protected as option.

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Application

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"HKS" Automatic power factor correction equipment is designed for central compensation of reactive power in three phase supply system, wall or floor mounted.

General Description

The Principles of Power Factor Correction

Under normal operating certain electrical loads (e.g. induction motors, welding equipment, arc furnaces and fluorescent lighting) draw not only active power from the supply, but also inductive reaction power (KVAR). This reactive power is necessary for the equipment to operate correctly but could be interpreted as an undesirable burden on the supply.

The power factor of a load is defined as the ratio of active power to apparent power, i.e. kW : kVA and is referred to as cosø. The closer cosø is to unity, the less reactive power is drawn from the supply.

If cosø = 1 the transmission of 500kW in a 400 V three phase mains requires a current of 722 A. The transmission of the same effective power at a cosø = 0.6 would require a far higher current, namely 1203 A. Accordingly, distribution and transmission equipment as well as feeding transformers have to be dimensioned for this higher load. Further, their useful life may decrease.

* For systems with a low power factor the transmission of electric power in accordance with existing standards results in higher expenses both for the supply distribution companies and the consumer.

Another reason for higher expenses are lasses incurred via heat dissipation in the leads caused by the overall current of the system as well as via the windings of both transformers and generators.

If we assume for out above example that with  cosø= 1 the power dissipated would amount to about 10kW, then a power factor of 0.6 would result in a 180% increase in the overall dissipation i.e. 28kW.

* In general terms, as the power factor of a three phase system decreases, the current rises. The heat dissipation in the system rises proportionately by a factor equivalent to the square of the current rise.

This is the main reason behind why Electricity Supply companies in modern economies demand reduction of the reactive load in their networks via improvement of the power factor. In most cases, special reactive current tariffs penalise consumers for poor power factor.

Conclusion:

  • A reduction in the overall cost of electricity can be achieved by improving the power factor to a more economic level.
  • The supply will be able to support additional load which may be benefit for an expanding company.
  • Reducing the load on distribution network components by power factor improvement will result in an extension of their useful life.

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Methods of Power Factor Correction

Opposing capacitive Reactive power resulting from the connection of a correctly sized capacitor can compensate for the inductive reactive power required by the electrical load. This ensures a reduction in the reactive power drawn from the supply and is called Rower Factor correction.

Most common methods of power factor correction are:

Single or fixed PFC, compensating for the reactive power of individual inductive loads right on the spot and reducing the load in the feeling leads (typical for single, permanently operated loads with constant and/or big power)

Group PFC - connecting on fixed capacitor to a group of simultaneously operated inductive loads (e.g. group of motors, discharge lamps)

Central PFC, typical for large electrical systems with fluctuating load where it is common to connect a number of capacitors to a main power distribution station or substation. The capacitors are controlled by a microprocessor based relay which continuously monitors the reactive power demand on the supply. The relay connects or disconnects the capacitors to compensate for the actual reactive power of the total load and to reduce the overall demand on the supply.

A typical power factor correction system would incorporate a number of capacitor sections determined by the characteristics and the reactive power requirements of the installation under consideration.

Sections of 12.5 kvar, 25 kvar, and 50 kvar are usually employed. larger stages (e.g. 100kvar and above) are achieved by cascading a number of smaller sections. This has the beneficial effect of reducing fluctuations in the mains caused by the inrush currents to the capacitors and minimizes supply disturbances. Where harmonic distortion is of concern, appropriate systems are supplied incorporating detuning reactors.

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Safety of the capacitors

1. Self-healing Dielectric

All dielectric structures used by ELECTRONICON are "self-healing":
In the event of a voltage breakdown the metal layers around the breakdown channel are evaporated by the temperature of the electric arc that forms between the electrodes. They are removed within a few microseconds and pushed apart by the overpressure generated in the centre of the breakdown spot. An insulation area is formed which is reliably resistive and voltage proof for all operating requirements of the capacitor.
The capacitor remains fully functional during and after the breakdown.


2. Protection against Overload and Failure at the End of Useful Service Life: The Overpressure Mechanism ("Break Action")

In the event of overvoltage or ageing at the end of the capacitor's useful service life, an increasing number of self-healing breakdowns may cause rising pressure inside the capacitor. To prevent it from bursting the capacitor is fitted with an obligatory "break action mechanism".
This safety mechanism is based on an attenuated spot at one of the connecting wires inside the capacitor. With rising pressure the case begins to expand, mainly by opening the folded crimp and pushing the lid upwards.
As a result, the prepared connecting wire is separated at the attenuated spot, and the current path is interrupted irreversibly.
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Reactors for Detuned Filter Circuits

A consumer whose load includes a high proportion of variable speed motor drives and/or other harmonic generating loads may require a detuned capacitor system. This would perform the function of power factor improvement whilst preventing any amplification of harmonic currents and voltages caused by resonance between capacitors and inductances in the mains.

Our low-loss reactors are designed for such systems. They are available with adjusted and fixed ratings:

  • The adjusted reactor is designed to create exactly the required output of reactive power, allowing for the internal voltage rise inside the resonating circuit formed together with the capacitor connected in the circuit. Note that exact sizing of the capacitor is necessary.
  • The fixed reactor is matched to a power capacitor with standard rating at mains voltage. .

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