In Todays era of PowerElectronicsWhere rectifiers & inverter circuits are highly utilized, Harmonics are a necessary concomitant. In order to prevent the ill effects of their presence, We need to understand harmonics well and employ suitable mitigation techniques. This article discusses the basic concepts; related to harmonics like :
Harmonics
Causes of harmonics
Effects of harmonics
Mitigation/ Elimination of harmonics
HARMONICS
“Harmonics are defined as the component of periodic wave whose frequency is integral multiple of the fundamental frequency ! “
Non linear loads such as rectifiers, inverters, VFDs, furnaces etc. create harmonics. These currents consists of a fundamental frequencycomponents rated at 50 Hz & a series of overlapping currents with frequencies that are multiples of the fundamental frequency.
The result is the deformation of the current (and as a consequence, voltage) that has a series of secondary effects.
The diagram given below illustrates how the harmonic waveforms of the higher frequency give a net effect of a distorted sine wave.
Harmonics effect on Fundamental waveform
Energy Meter ELITE 440 by secure, shows Voltage total harmonic distortion |
CAUSES OF HARMONICS
Increased use of modern PowerElectronic; Apparatus produces non-linear current Which loads the network with harmonics.
All devices that are “energy efficient” work on the principle of saving power by means of chopping the sine wave. As soon as the sine wave is chopped i.e. distorted, Harmonics are induced in the system. This means that whenever sinusoidal current passes through a non-linear device, The output wave would get distorted.
Major loads that are responsible for Inducing harmonics in the system are:
Variable frequency derives & UPS
3phase & 1phase rectifiers
Arc/Induction Furnaces
Discharge Lamps
1phase converters
Computers
SMPS
TVs
EFFECTS OF HARMONICS
1. Current harmonics increases the rms current flowing through the circuit and thereby increases the power loss.
2. Current harmonics affect the entire distribution all the way down to the loads. They may cause increased eddy currents and hysteresislosses in motors and transformers; Resulting in overheating.
3.Overloading in neutral conductors.
4. Nuisance tripping of circuit breakers.
5.Over stressing of power factor correction capacitors.
6.Interference with communication etc.
7. They can also lead to overheating and saturation of de-tuned reactors.
8. Voltageharmonics do not originate directly from non-linear loads. The current harmonics flow through the system impedance (source and line impedance) and cause harmonic voltage drops across the impedance. This will distort the supply voltage waveform. Thus voltage harmonics are generated. Hence even if the current harmonics are low, if the source impedance is high – like in case of Diesel Generators – It can lead to higher levels of voltage harmonics. Voltage harmonics affect the entire system irrespective of the type of load. They affect sensitive instrument throughout the facility like those that work on zero voltage crossing as they introduce voltage distortions.
9. Presence of high harmonics can also lead to phenomenon like harmonic amplification by the power factor capacitors Where certain orders of harmonic current shoot up to dangerous level.
MITIGATION OF HARMONICS
There are multiple techniques; Available in the market for harmonics mitigation. But applying these techniques directly on the line without any type of analyzation will not provide appropriate results; Because :
There are multiple reasons and sources of harmonics generation; also there are integral multiple of harmonics.
Further, We need to analyze the complete cycle of the system where; at which time period; at what level of magnitude; harmonics is getting generating.
Next, We are going to discuss some of the mitigation techniques for harmonics.
1. LINE REACTORS
Line reactors are the type of inductive coil also known as ‘choke‘ which is been placed in series before the load. Since coil is connected in series; Whenever changes occurs across the line current like generation of harmonics ; Coil oppose the changes as per according to the ‘Lenz’s law’.
Line Reactor connected before the 2.2KW VFD
2. ACTIVE FILTER
Active filters have having its own power source; Which ; While connected to power line, Produces signal of equal magnitude but in opposite direction to counter the harmonics. Since these filters produces its own power signals that’s why it is known as ‘Active filter‘.
Schematic diagram of active filter shown below:
Schematic diagram of Active Filter
3. PASSIVE FILTER
Passive filter is an arrangement of series combination of Line rector & Capacitor which is been connected parallelly across the load. According to filer’s configuration, It absorbs particular type of harmonics.
Schematic diagram of passive filter is shown below:
Schematic diagram of Passive Filter
Passive Filter | Power to capacitor passes through the reactor in a series
4. HYBRID FILTER
Hybrid filter is the combination of both the filters; ‘Active’ & ‘passive’. Both the filters work simultaneously to eliminate the harmonics.
Schematic diagram of Hybrid filter
5. LOAD GROUPING
Load Grouping consists of separating the loads according the type. Grouping the non linear load away from the sensitive loads.
Schematic diagram shows load grouping ; Divided linear & Non linear loads
6. POSITIONING OF LOADS
By arranging non linear loads near the power source while sensitive loads far away from the source, Harmonics can be minimized.
Schematic diagram shows positioning of non linear load near to the source
7. SEPEARATE ENERGY SOURCES
In this method, Non linear loads can be grouped & energized from different energy source. So that. Sensitive load runs from safe & smooth power source.
Through a schematic diagram, A separate energy sources are shown below :
Separate energy sources for Linear and Non Linear loads
8. TRANSFORMERS WITH SPECIAL CONNECTIONS
By employing the transformers with special connection; Harmonics can be eliminated. Each transformer connection mitigate harmonics of particular type. Such as :
Delta-Wye configuration transformers can eliminate the 3rd Harmonics.
Delta-Delta-Wye configuration transformers can eliminate 5th & 7th Harmonics.
Schematic diagram shows transformers with special winding configuration for harmonics mitigation
9. SUITABLE EARTHING ARRANGEMENT
In TN-C(PEN) System; Earthing & Neutral remains combined which works as a return path for unbalanced current While same PEN conductor is used for earthing. This system is dangerous for the loads which are having high harmonics.
In TN-S system, Earthing & Neutral remains separated. This system is better system for the loads which are having high harmonics.
TN-C & TN-S Earthing System
10. PHASE BALANCING
Load distribution across all the three phases should be balanced so that if harmonics mitigation technique get applied on all the three phases, evenly.
CONCLUSIONS
Harmonics are defined as the component of periodic wave whose frequency is integral multiple of the fundamental frequency.
Non linear loads such as rectifiers, Inverters, VFDs, Furnaces etc. creates harmonics.
Presence of harmonics in the system leads to power loss, overheating, nuisance breaker tripping, damage to electronic circuitry, poor power factor.
There are multiple techniques to mitigate the harmonics like Line reactors, Active filter, Passive filter, Hybrid filter, Separate power sources, load grouping, Load positioning, Special transformers, Phase balancing & Different Earthing systems.
Before applying the mitigation techniques, It is important to identify the non linear loads & Study of harmonics which are getting generating through out the cycle.
Hope, You like the post. If you have any doubt, Suggestion or query please do comments !
