ENERGE CAPACITORS

  
 AN ISO 9001:2000 COMPANY

The Principles of Power Factor Correction:

Under normal operating conditions 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 reative 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 cosφ. The closer cosφ is to unity, the less reactive power is drawn from the supply.

If cosφ = 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 cosφ = 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 standard results in higher expenses both for the supply distribution companies and the consumer.

Another reason for higher expenses are losses 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 our above example that the cosφ = 1 the power dissipated would amount to about 10 kW, then a power factor of 0.6 would result in a 180% increase in the overall dissipation i.e. 28 kW.

• 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 penalize consumers for poor power factor.

Conclusion :

• A reduction in the overall cost of electricity can be achieved by improving the power factor to more economic level

• The supply will be able to support additional load which may be of 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

Reactive Power Compensation

Power factor

when a load is connected to a line, the load absorbs a current that depends on its electrical characteristics. The product of this current by the applied voltage is called apparent power.

The apparent power is composed by the active power load can supply (as mechanical power or heat) and the reactive power which is needed to generate the magnetic field necessary for the operation of certain types of loads. The relation between the active power and the apparent power is defined as power factor or (in pure sinusoidal systems) cos φ:

                   Cos φ  =    Active power /  Apparent Power

Power factor value varies from 0 to 1.

Table 1 shows the approximated power factor values for some common loads.

Table 1 :

Reaction power compensation

Although the reactive power required by inductive loads is not  used in profitable work, it has to be generated, transported and distributed through the electrical network. This fact forces to oversize transformers, generators and lines, as well as losses and line voltage drops. For this reason, most of the electrical authorities penalize reactive power consumption with additional charges.

Electrical capacitors installed near the inductive loads produce the reactive power required by these inductive loads. Their connection to the electrical network is called compensation or improvement of the power factor (cos φ). It is the most economical, easy, and safe system to supply the required reactive power. Power factor improvement gives the following advantages to the user :

• Removal of additional charges in the electricity supply invoice

• Decrease of active power losses in the cables

• More power available at the secondary of transformers

• Reduction of voltage drops (increase of voltage at the end of the lines)

The saving in the electricity bill obtained by the elimination of the additional charges for the reactive power, allows a fast payback of the installation of the capacitors, usually ranging between one and one and a half year.

Compensation systems

Individual compensation

This type of compensation is applied to motors, transformers, and in general to loads with a long operating time.

Capacitors are connected directly in parallel to the terminals of the loads

ADVANTAGES

• Elimination of additional charges in the electricity bill

• savings from the omission of devices for the connection and disconnection of the capacitors

• Reduction to a minimum the current flowing in the lines, allowing the use of smaller size cables and switchgear. In existing installations additional power is released

DISADVANTAGES

Expensive system, as when the loads do not  work full time, part of the capacitors are kept out of operation

An indication of the necessary compensation of motors and transformers is table 2.

To avoid the danger of self-excitation it is necessary to limit the power of the capacitor to 90% of the no load reactive power of the motor.

Qc = 0.9 X lо X UΝ√3

where:

Qc = capacitor power (var)

lo = No load current of the motor (A)

UN = voltage between phases (V)

Centralized compensation

When there is a large and spread number of inductive loads in the installation, the individual compensation can become uneconomical.

In these cases the centralized compensation by means of an automatic capacitor bank with automatic regulation offers the most simple and economical solution.

Total power is subdivided in a number of capacitors steps that can be connected independently. A reactive power controller measures continuously the needs of the installation and connects or disconnects the capacitors to reach a prefixed cosφ.

Table 2

ADVANTAGES

• Elimination of additional charges in the electrical energy invoice

• Total capacitor power smaller than the one needed in individual compensation

• Reduced installed costs

MIXED COMPENSATION

Usually applied in the case of having an installation with its own distribution transformer and with the power meter in the H.V. side.

Reactive power consumed by the transformer when connected to the mains is compensated by permanently connecting a capacitor to the secondary of the transformer.

This type of compensation can also be applied when the installed has a very important load, as for example, a motor of very high power, by using the individual compensation for the motor and the centralized compensation for the rest of the installation.

RECOMMENDATION FOR

CABLE FUSES AND SWITCH GEAR

Recommended ratings of accessories for uses with 400/440 V.A.C 50 cycles three phase delta connected Energe Power Factor Improvement Capacitors.

How to carry out general testing of Capacitors:

Capacitors can be tested by applying following formula for computing its KVAr rating at rated voltage

(KVAr)2 = (KVAr)1 * ((v2)^2 / (v1)^2 )

Wherein:

(KVAr)2 =  KVAr at measured voltage = √3 V1I1

(KVAR)1 = KVAr at rated voltage

V1 =  Rated Voltage

V2 = Test voltage applied

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