Overview
For industry energy consumer, lagging reactive power generated from inductive loads is one of the major causes of power and financial losses i.e. poor power factor, incorporating power factor correction devices in the electricity network helps in generating leading reactive power to compensate lagging reactive power, This techniques helps consumer to improve power factor (cos phi) close to unity.
The necessary leading power can be produced by LV capacitor connected in parallel to the supply network close to the lagging power source (like induction motors, MCC panels etc)
The capacitors connected can be fixed type for given fixed lagging pf of the system at a point in power system or variable in steps for a changing connected load.
What is Harmonic and Power Factor?
In past decades the use of power non-linear loads like VFD,SMPS, UPS etc has increased tremendously. These devices distort the pure sinusoidal waveform of power supply. These distortions can be called as harmonics. When a capacitor is used for power factor correction,it might create a resonating circuit with the feeding transformer.The resonance frequency is generally from 250Hz to350Hz, that means 5th to 7th harmonics.
This resonance is undesired condition and it might lead to Overloading of capacitors-reduce the life of capacitor Overloading of transformer, cables and other switchgear elements in the circuit- reduces life of all components Voltage distortion,increased power losses,nuisance tripping of protection equipment.
This resonance can be avoided by putting a detuned reactor in series with the capacitor. The reactor shall be such that the tuning frequency with capacitor shall be less than the dominant harmonics. This combination of power factor correction capacitor and detuned reactors behaves inductively to frequencies above tuning frequency. Thus provide high impedance path to harmonics present in the system.
Detuning factor can be defined by following formula:
Where
XL= Inductive reactance
XC=Capacitive reactancep= detuning factor in percentage
Tuning frequency of LC filtercan be calculated by below formula:
Where
ft = tuning frequency
fs = supply frequency
p= detuning factor in percentage
This combination of detuned LC filter will act capacitive for frequency below ft and inductive for frequency above ft, Thus for base frequency of 50 or 60Hz this detune filter will act as capacitive and improves the power factor. This LC detuned filter is selected such that the tuning frequency is much less than the dominant harmonic frequency. Thus harmonics always seeshigher impedance and the condition of resonance with feeding transformer is avoided.For example if the dominant harmonics is 5th harmonic and base frequency is 50Hz, a 7% detuned reactor shall be selected. The tuning frequency of this filter will be 189Hz, the tuning frequency in this case is lesser then 250Hz, the harmonic frequency.
Hence there will not be a situation of resonance between the feeding transformer and capacitor.
Why power factor correction and harmonic filtering is matter?
Different types of PF correction should be adopted depending on the performance requirements and complexity of control:Fixed, by connection of a fixed-value capacitor bank Automatic, by connection of a different number of steps, allowing adjustment of the reactive energy to the required value Hybrid, for PF correction of highly fluctuating loads.
1-Fixed compensation
This arrangement uses one or more capacitors to provide a constant level of compensation.Control may be:
Manual: by circuit-breaker or load-break switch
Semi-automatic: by Thyristor Switch Controller
Direct connection to an appliance and switched with it.
These capacitors are installed:
For no-load compensation of Distribution tranformers.At the terminals of inductive loads (mainly motors)At busbars supplying numerous small motors and inductive appliances for which individual compensation would be too costly. In cases where the load factor is reasonably constant.
2-Automatic compensation
This kind of PF correction provides automatic control and adapts the quantity of reactive power to the variations of the installation in order to maintain the targeted cos p.
Control is usually provided by an electronic device(Power Factor Controller)which monitors the actual power factor and orders the connection or disconnection of capacitors in order to obtain the targeted power factor. The reactive energy is thus controlled by steps.In addition, the Power Factor Controller provides information on the electricity network characteristics (voltage amplitude and distortion, power factor,actual active and reactive power...) and equipment status. Alarm signals are transmitted in case of malfunction.
Connection is usually provided by contactors. For compensation of highly fluctuating loads use of active filters or Static Var Generators(SVG)are recommended, Contact Sinavar Power for SVG electronic compensation solutions.
3-Hybrid compensation
This kind of compensation is required when fluctuating loads are present, and voltage fluctuations have to be prevented. The principle of dynamic compensation is to associate a fixed capacitor bank and a Static Var Generator (SVG) ,providing either leading or lagging reactive Currents.The result is continuously varying fast compensation, perfectly suitable for loads such as lifts, crushers, spot welding, etc.
The equipment is installed at points in an installation where the active-power and/or reactive-power variations are relatively large, for example: on the busbars of a main distribution switchboard on the terminals of a heavily-loaded feeder cable.
Where the kvar rating of the capacitors is less than or equal to 15% of the power supply transformer rating, a fixed value of compensation is appropriate.Above the 15%level,it is advisable to install an automatically-controlled capacitor bank, to avoid over voltages.
