Power Factor Correction

Improving the PF can maximize current-carrying capacity, improve voltage to equipment, reduce power losses, and lower electric bills. The simplest way to improve power factor is to add PF correction capacitors to the electrical system. PF correction capacitors act as reactive current generators. They help offset the non-working power used by inductive loads, thereby improving the power factor. The interaction between PF capacitors and specialized equipment, such as variable speed drives, requires a well-designed system.

The selection of the Power Factor Correction equipment can follow a 4-step process:

1.     Calculation of the requested reactive energy

Calculation of Reactive Energy Based on the Application

Power Factor Correction for Transformer no-load compensation

The transformer works on the principle of Mutual Induction. The transformer will consume reactive power for magnetizing purpose. Following equivalent circuit of transformer provides the details of reactive power demand inside the transformer

kVA rating of Transformer : kVA rating of Transformer

kVA rating of Transformer: 2% of kVA rating

Power Factor Correction where Load and present Power Factor is Known

The objective is to determine the requested reactive power QC(kvar) to be installed, in order to improve the power factor cosφ and reduce the apparent power S.

For φ’ < φ, we’ll get: cosφ’ > cosφ and tanφ’ < tanφ.

This is illustrated on the diagram in the left.

 

QC can be determined from the formula:

 QC = P. (tanφ - tanφ‘), which is deduced from the diagram

QC : power of the capacitor bank, in kvar

P : active power, in kW

tanφ: tangent of the phase angle - before compensation,

tanφ‘: tangent of the phase angle - after compensation

The parameters φ and tanφ can be obtained from the billing data, or from direct measurement in the installation.

2.     Selection of the compensation mode:

The location of low-voltage capacitors in an installation constitutes the mode of compensation, which may be global (one location for the entire installation), by sectors (section-by-section), at load level, or some combination of the latter two. In principle, the ideal compensation is applied at a point of consumption and at the level required at any instant.

In practice, technical and economic factors govern the choice.

The place for connection of capacitor banks in the electrical network is determined by:

- Global objective (avoid penalties on reactive energy, relieve of transformer or cables, avoid voltage drops and sags),

- Operating mode (stable or fluctuating loads),

- Foreseeable influence of capacitors on the network characteristics,

- Installation cost.

Global compensation

The capacitor bank is connected at the head of the installation to be compensated in order to provide reactive energy for the whole installation. This configuration is convenient for stable and continuous load factor.

Compensation by sectors

The capacitor bank is connected at the head of the feeders supplying one particular sector to be compensated. This configuration is convenient for a wide installation, with work shop shaving different load factors.

Compensation of individual loads

The capacitor bank is connected right at the inductive load terminals (especially large motors). This configuration is well adapted when the load power is significant compared to the subscribed power. This is the technical ideal configuration, as the reactive energy is produced exactly where it is needed, and adjusted to the demand

3.     Selection of the compensation type

Different types of compensation shall be adopted depending on the performance requirements and complexity of control:

- Fixed, by connection of a fixed-value capacitor bank,

- Automatic, by connection of different number of steps, allowing

the adjustment of the reactive energy to the requested value,

- Dynamic, for compensation of highly fluctuating loads.

Fixed compensation

This arrangement uses one or more capacitor(s) to provide a constant level of compensation. Control may be:

- Manual: by circuit-breaker or load-break switch,

- Semi-automatic: by contactor,

- Direct connection to an appliance and switched with it.

These capacitors are applied:

- At the terminals of inductive loads (mainly motors),

- At bus bars supplying numerous small motors and inductive appliances for which individual compensation would be too costly,

- In cases where the load factor is reasonably constant.

Automatic compensation

This kind of compensation provides automatic control and adapts the quantity of reactive power to the variations of the installation in order to maintain the targeted cos φ. The equipment is applied at points in an installation where the active-power and/or reactive power variations are relatively large, for example:

- At the busbars of a main distribution switch-board,

- At the terminals of a heavily-loaded feeder cable.

Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate. Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors.

Control is usually provided by contactors. For compensation of highly fluctuating loads, fast and highly repetitive connection of capacitors is necessary, and static switches must be used.

Dynamic compensation

This kind of compensation is requested when fluctuating loads are present, and voltage fluctuations should be avoided. The principle of dynamic compensation is to associate a fixed capacitor bank and an electronic var compensator, providing either leading or lagging reactive currents.

The result is a continuously varying and fast compensation, perfectly suitable for loads such as lifts, crushers, spot welding

4- Conditions and harmonics

We will explain this point in the next topics

Reference:  "Guide for the Design and Production of LV Power Factor Correction Cubicles" By Schneider Electric