A . Definition

Inductive, electrical users need more power than what they convert to real power. Induction motors for example convert only 80 to 90% of the delivered power in active energy. The power that remains (reactive power = apparent power = inductive power) is used to create a magnetic field in the motor. The relation between the real power and the reactive power is expressed in power factor (cosine phi).

Real and apparent power

The electric power that is needed to feed the inductive load consists of two elements. The first (active) is in phase with the delivered tension. The second (reactive) is not in phase with the delivered tension. This reactive power is also called "inductive". This lag of the current is meanly caused by the spools in induction motors. The inductive load needs a current that is the sum of the in and out phase.

The out-phase current causes :

• an increasing current consumption
• chance of heating in the supply cables
• chance of heating in the supply transformer
• voltage drop

It is necessary that these cables and transformers are overdimensioned to anticipate this over-current, what is very expensive.

The power factor is defined as the ratio of the real power flowing to the load to the apparent power, and is a number between 0 and 1.
Real power is the capacity of the circuit for performing work in a particular time, expressed in kW.

Apparent power is the product of the current and voltage of a circuit, expressed in kVA.

Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power.

In an electric power system, a load with low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.

Power factor = real power   (kW)

                        reactive power (kVA)

The highest power factor is 1 : that means that 100% of the delivered power is converted in useful energy.

B. Capacitor bank

More information on Wikipedia.  

Power is composed by two parts : active and reactive power. Reactive power consists also of two elements : inductive (induction motor, transformers,...) and capacitive (capacitor banks) power. Capacitive power is as well as inductive power not in phase with the delivered tensions and is vectorly 90° in advance with the tension. When there is as much inductive as capacitive power, these two will cancel each other.

Remark :

A high tension transformer has an inductive consumption, when he is not loaded (measurable at the high tension side). This consumption is also called the no load operation. This loss  can have a major impact on the power factor (power factor is calculated monthly). In order to compensate this inductive loss, a fixed capacitor is often positioned at the low voltage side of the transformer. A fixed capacitor can also be built into an automatic capacitor bank for the uniformity of the installation. You can find more information  and calculation methods in the section fixed capacitors.

•1.      How to compensate ?

a.  Individual compensation

On each inductive user in the network you can place an appropriate capacitor, who are in phase. This capacitor has a fixed power. This type of compensation is interesting for users with a steady consumption, but is quite expensive.

b.  Central compensation

In an electrical installation with unstable charges, a central compensation is desirable. An automatic capacitor bank with adjustable power is here the best  solution.

The integrated regulator in an automatic capacitor bank can measure different parameters, by which the compensation can be adjustable to the electrical installation. The wanted power factor can be set.

For the choice of the power of an automatic capacitor bank, we refer to the table .

•2.      Why a compensation?

a.    Extra charge on reactive power

The difference between the active and apparent power, force the electricity company to over dimension the distribution networks to deliver a power with a bad power factor.

With an extra charge on reactive power,  the clients are forced to invest in capacitor banks to eliminate this reactive power and work with a good power factor.

b.    Increase of the power of the system

The thermal power of generators, transformers and cables limits the kVA that can be delivered by the system. By reducing the demand of kVAr, by placing capacitor banks, there will be more power in the system.

c.     Improvement of the tension

A demand of reactive power (a bad power factor) increase the chance of voltage drops in transformers, cables and other parts of the network, with a lower working voltage as a result. The improvement of the tension is directly proportional with the improvement of the power factor.

d.     Reducing the losses

The higher the current, the higher the losses, who are attended with. This is not the mean reason to choose for a capacitor bank, but is an extra advantage.

 C. Central compensation

The most obvious solution  for the improvement of COS PHI is a central correction. This is also the cheapest solution. The regulator (digital varmetric relais) in the capacitor bank measures the current (TI) and consumption. This measurement of the current  occurs on the LT side of the transfo or on the main protection of the installation. A measurement of the tension over phase L2 an L3 is also necessary. This measurement is also provided in the capacitor bank and serve also as charge for the circuit (through a built-in transfo 400/230 V)

1. Installation guidelines 

2. Types and capacity

Comar Benelux© has different types of capacitor banks in function of the operating voltage and the harmonic distorsion. Before making a choice, it is preferable to demand us to make a measurement.

Harmonics are mains failures caused by non linear loads (pc, direct current motors, rectifiers, thyristors,...).

These currents appear at frequencies multiple of 50Hz (150Hz, 250Hz, 350Hz, ...). Often we speak of the third  (3 x 50Hz = 150Hz), fifth (5 x 50Hz = 250Hz), ... harmonic. These currents influence and disturb the final shape of the sine.

Due to these harmonics, the current source has to provide more energy than necessary, extra heating is produced and interferences occur in electronic equipment.

To understand better these problems, it is necessary to make a difference between the causes. 

The third harmonic (150 Hz) occurs often in the tertiary sector. The harmonic distortion is due to electronical equipment such as computers, printers, television screens, electronical ballasts in lighting, ...

The fifth, seventh and eleventh harmonic (250, 350, 550 Hz) occur more in industrial applications (drives, DC applications)

To choose the right size of the filter, it is necessary to take some measurements on the installation in order to select the right reactive power of the fundamental component and of the other harmonics.

It is so possible to estimate the value of the parameter of the harmonic current distortion : THD% (total harmonic distortion).

THD is a number that represent the harmonic pollution and it is mathematically given by the next expression :

THD% = sum of the square of all harmonic capacities

                       square of the fundamental value

THD 20% is tolerated. When 35% is overcrossed, it is suitable to look for a solution for the problem.

You can find more information in the rubric Harmonic filters.