- Introduction of types of basic energy parameters
We all are aware that electrical system comprises of three basic parameters that contribute to total energy consumption.
1) Active Energy (kWh)
2) Apparent Energy (kVAh)
3) Reactive Energy (kVArh)
As shown in above figure (a),
> Active energy (kWh) represents the useful part of the energy consumed.
> Reactive energy (kVArh) represents the losses caused to due to the inductive and capacitive elements of the electrical system. It also represents the energy required to setup magnetic fields between armature and rotor of a motor, between primary & secondary winding of transformer and so on.
> Apparent energy (kVAh) measures the combined effect of active and reactive energy.
kVA= √[(kW)^2+(kVAr)^2 ]
Above formula tells us how active power and reactive power are combined to give us apparent power.
- Types of power factor and what do they represent
Now the power factor which is a measure of power quality of the system depends on the level of current and voltage phase shift and also the percentage of voltage & current THD present in the system.
True P.F. = Distortion P.F. X Displacement P.F.
Above expression tells us about three different types of power factors, which are indices giving us an idea about the effect of current & voltage phase shifts and the effect of harmonics present in the system.
1) Distortion power factor :
This index tells us about the effect of harmonics in the system on power quality. Harmonics in the system increases because of the presence of VFDs, inverters, converters, in short such devices that alter original sine wave of supply signal to get desired output which is than fed to operating machines. If power quality of the system is deteriorated, which represents higher percentage of THD in the system, copper & iron losses in the transformer, heating of cables, nuisance tripping of switchgears, in short, efficiency of the plant decreases, i.e. the plant will use more energy for same amount of work as compared to when the harmonic levels are in the safe zone.
2) Displacement power factor :
This index tells about the effect of phase shift between current and voltage. This phase shift occurs because of capacitive and inductive elements in the electrical system. In case displacement power factor is degraded to either leading or lagging side of unity, it indicates excess of capacitors for deviating to leading side or excess of inductor for deviating towards lagging side.
3) True power factor :
True power factor indicates the overall health of electrical system, it comprises of DPF and PFd and gives us an idea about overall health of system and effect of harmonics and phase shift between current and voltage on power quality. True power factor is expressed using following formula
- Minimizing the difference between kWh & kVAh.
To minimize the difference between kWh & kVAh means to control the demand of reactive energy (kVArh) from the grid or supply side. Transformer & motors in every electrical system need reactive power to set up magnetic fields for their internal working, i.e., transformer needs to set up magnetic field between two windings which is called mutual inductance, the working principle of transformer. Motors need to set up magnetic fields between rotor and stator for rotation of rotor. Thus, reactive power is necessary for the system, all we can do in this is to minimize the demand of reactive power from the grid and supply the necessary reactive power from inside the electrical system. The scheme which we use depends on the type of dominant load, i.e., inductive load or capacitive load. For inductive load which includes, transformer, motors, lighting load that works on inductive principle we need to install a capacitive bank in the system to compensate the lagging phase shift between current & voltage. Thus, the APFC bank ,i.e., Automatic Power Factor Compensation. APFC consists of basic two types of switching methods, contactorised switching and thyristorised switching.
The difference between contactorised & thyristorised APFC system is the switching time, contactor needs at least 60 second for switching ON & OFF capacitor steps where as thyristorised switching takes micro seconds to switch the bank. The need of thyristorised or contactorised bank depends on the type of load profile, i.e., fast varying load and slow varying load. For slow varying, as the name suggests, the load increases or decreases slowly, in this case we need our APFC bank to switch in or out capacitor banks according to slow varying load, which means we have more than a minute to switch the capacitor steps in the bank. For fast varying load, we need to switch in or out capacitor steps within seconds of load variation.
Now the question at hand is why is there a need to properly select between contactorised & thyristorised APFC scheme. As discussed above, we need to fulfill the need of reactive power by the load to minimize the difference between active and apparent energy readings.
Above discussed method maintains the displacement power. In case the electrical system consists of non-linear loads like VFDs and electronic devices that alter sinusoidal input current waveforms according to the need of specific loads to work. In this case we need to control the harmonics produced in the system. Note that reactive power is in demand where as the harmonics are produced inside the system and we need to avoid supplying harmonic currents and voltages to the grid. A higher level of harmonics in the system results in heating of cables and devices, nuisance tripping of switchgears and lower efficiency of the over all system. In order to control harmonics produced in the electrical system, Active harmonic filter or specially designed passive filter are the solutions.
So the first step is to control the displacement power factor which controls the phase shift between current and voltage and then calculate the rating of active/passive harmonic filter that controls the harmonic percentage in the electrical system.
Summary:
Step I :
Maintain displacement power factor using thyristorised or contactorised APFC bank.
Step II :
After maintaining displacement power factor, calculate rating of active/passive harmonic filter to keep harmonics within limits.
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