In the world of Electrical Engineering and Electrical Contracting, we run into problems with businesses experiencing poor power supply. This has been an ongoing problem, but we are seeing the problems with poor power supply affecting machinery and sensitive equipment. A lot of these problems can point back to harmonics.
Harmonics have been around for a long time. Power systems were affected by harmonics when the first AC generator was created which dates back to 1832. So how does this term that seems to be so harmless have such a large impact impact in our business and industrial sector? Lets take a look at what harmonics are and discuss some mitigation techniques.
What are Harmonics?
A pure sinusoidal voltage is a theoretical quantity that could be generated only under ideal conditions like (an ideal AC generator, finely distributed windings and uniform magnetic field). However, in real-time applications, it is impossible to create ideal conditions. Hence, the voltage waveform would be distorted and the voltage-time relationship would deviate from the pure sine function. This distortion in voltage waveform is negligible and appears in the form of a periodic function. In other words, it creates harmonics. Harmonics are created as an integral multiple of fundamental power line frequency.
Loads can be classified broadly into two types:
- Linear loads – here output current is directly proportional to input voltage and there is no distortion. Example : Incandescent lamp
- Non-linear loads – here current is not directly proportional to input voltage and results in distortion. Example : SMPS
When a non-linear load is applied to the circuit, it could result in the superimposition of waveforms. These multiple frequencies are also termed as harmonics of the fundamental frequency.
These harmonics are different from the transient distortions that occur in power units like spikes. Harmonics repeat after cycle and are steady-state distortions. Harmonics can seriously affect the efficiency and quality of operation in both industrial and commercial applications.
Voltage harmonics – Current harmonics result in voltage harmonics. Voltage harmonics created by current harmonics is in direct proportion to source impedance of voltage source.
Impact of harmonics on industrial equipment and commercial equipment
Harmonics related problems are here to stay and could result in a galore of detrimental consequences in both commercial and industrial scenarios. In case of resonance, probability of such detrimental effects are higher. Resonance condition is broadly classified into two: parallel resonance and series resonance.
Parallel resonance – Inductor and capacitor circuit elements are connected in parallel. If the natural frequency of this parallel combination is equivalent to harmonic frequency, heavy current can flow through the circuit and result in transformer overheating.
Series resonance – Inductor and capacitor circuit elements are connected in series. If the natural frequency of this series combination is equivalent to harmonic frequency, it would result in voltage distortion.
Overloading Neutral Conductors
Three individual phase conductors and a neutral conductor is included in a three-phase system. If individual phase conductors carry the same current, phase currents would be cancelled (given that the load is balanced). Hence, the size of neutral conductor can be decreased. However, a very high third-harmonic current is often found in SMPS (used in computers). If there are too much of PCs in buildings, very high current can flow through neutral wires. This current can go higher than the maximum acceptable limits and result in a potential fire hazard. Harmonics could also result in interference or failures in computers and telephones.
Harmonic-producing loads in transformers can result in eddy current loss. This loss is directly proportional to the second power of product of harmonic current and frequency. This results in overheating of transformer and affects the insulation materials as well. This paves the way for transformer failure.
Multiple resonant frequencies would be produced in circuits which contain capacitance and inductance elements. If harmonic frequency created by nonlinear loads, tally with anyone of the resonant frequencies, it could result in harmonic resonance. This could result in distortion of voltage and current waveforms. This results in nuisance tripping in circuit breakers which paves the way for production losses.
Affect life span and affects quality of operation – Harmonics can damage the equipment and reduce the life span. Harmonics can overheat the wires and build up stress on equipment and associated cables.
Increased power consumption – Harmonics result in increased power consumption which paves the way for higher electricity bills.
Driver and power supplies – Harmonics could result in breakdown of commutation circuits present in AC/DC drives.
Harmonic distortion could also result in poor power factor. Some of the other problems caused by harmonics are metering inaccuracies, generator breakdowns etc.
Harmonic mitigation – Ways to Reduce Harmonics
There are different methods to reduce the effect of harmonics in power systems and each of them corresponds to different levels of efficiency.
Multiple tuned series LC circuits are used to remove the harmonic current from the system. They also help in correcting power factor. There are 3 different types of harmonic filters:
- Passive filters
- Active filters
- Hybrid filters
Passive filters are usually used in
- applications that need power factor correction
- circuits which contain group of non-linear loads with power more than 500kVA .
- Current/voltage distortion has to be decreased.
An LC filter is provided in parallel to the non-linear load which generates harmonics. If there is high harmonic current in the circuit, it could be decreased by connecting several LC filters in parallel to the load.
Active filters are usually used in following cases:
- Current distortion has to be decreased.
- Circuits which contain group of non-linear loads with power less than 500kVA .
Here, filters are installed either series on parallel with the load. Here, current produced by the filter circuit compensates the harmonic current produced by the non-linear load.
Hybrid filters are usually used in following cases:
- Current or voltage distortion has to be decreased.
- Circuits which contain group of non-linear loads with power more than 500kVA .
- Power-factor correction is required
- Harmonic emissions has to be regulated precisely.
Hybrid filter comprises of both passive and active filters. Hence, this type of filter is able to combine the benefits of both these types of filters.
Increase the length of neutral wire
In most of the cases, neutral wire is designed to have similar capacity as power wiring. In buildings that consist of plenty of PCs, it is better to design the wiring in such a way that the neutral wire is bigger than the phase wire by almost 200%. However, this method does not offer any protection to the transformer and it safeguards only the building wiring.
Separate neutral conductors
Separate neutral conductors can be installed for each phase conductor. This helps to improve the efficiency of branch circuit to deal with harmonic loads and to get rid of harmonic currents.
A standard transformer is unable to handle harmonic currents and would be overheated. This could pave the way for early breakdown. K- rated transformers can cope with the heat produced by harmonics. For a standard transformer, K-factor is one. Transformers with higher value of K-factor can deal with more heat energy.
Power system design
While designing the circuit, special care should be given to reduce the non-linear load to 30% of total transformer capacity.
Harmonic trap filter
This type of filters are used in circuits that have high non-linear ratio. Filters are designed in such a way to remove particular harmonic like 3rd, 4th etc. They also offer power factor correction.