With the advent of power electronics in 1902, control of electricity became much easier. Since then, several devices including valves, transistors, SCRs, Mosfets, IGBTs etc. took part in the success story. All these developments have been contributing to the idea of green buildings and reduction of carbon foot print over the years.
With advantages come certain draw backs as well. Harmonics in electric circuits is one of the main disadvantages which reduced the glamour of power electronics and gave hard times to Electrical Contractors during the testing and commissioning of building projects. In theoretical terms, Harmonics are unwanted higher frequency waves developed in the power systems due to non linear loads. These waves distort the original wave form and hence the quality of power supply. The effects of harmonics can be seen in lighting projects in hotels in the form of flickers, high neutral currents etc.
Let’s try to understand what are Harmonics in real life terms.
First of all, what are linear and nonlinear loads? In a linear load, when voltage is applied, current wave form follows the voltage wave form. There may be a phase shift due to inductive or capacitive loads. But the wave forms will be similar. Application of voltage in a nonlinear load may not result in a current wave form similar to the voltage wave form. The reason is that, in nonlinear loads, the current flow is controlled by something else restricting the natural flow.
Typical example of a nonlinear load is a Variable frequency drive (VFD). A VFD is used for the control of the speed of motors which can reduce starting currents, avoid wastage of electrical energy and help control the connected system efficiently. There are two main sections in a VFD. Rectifier and inverter. Rectifier converts AC to DC. Inverter section converts DC to AC at desired frequency which is often decided by the demand of the load.
When looked from the power supply end, the current flow through the rectifier is not similar to the voltage. The current drawn through the inverter is controlled by the inverter’s input. Inverter needs power in the form of pulses in order to create a fly wheel effect. Even if a voltage is applied to inverter input, the current drawn will be restricted to pulses. These pulses are very short duration high amplitude pulses. The pulse carries whole energy required for the load. If there is no VFD the current flow occurs in 10ms corresponding to a half wave of voltage. With VFD the entire load flow occurs in a fraction of the above mentioned time. Obviously, the sudden flow in pulse form creates a dip in the voltage wave form.
In a mathematical point of view, a dip in the voltage waveform can only be explained with the help of super imposing additional higher frequency waves to original wave form. The shape of the vector sum of such additional super imposed higher frequency waves will be same as the shape and size of the dip.
A dip in the voltage wave form or a group pf higher frequency waves or Harmonics, creates chaos in the power system. This leads to poor quality of power supply resulting in flickering of phase dimmer controlled lamps etc.
In order to avoid harmonics affecting your power distribution system, several methods can be adopted.
In design stage the electrical consultant or electrical contractor can distribute the nonlinear loads such as VFDs by feeding from various transformers avoiding the impact on a single transformer. This will improve the ratio of linear to nonlinear load and thus reduce the effect of harmonics on electrical systems in the project.
A detuned capacitor bank will smoothen the irregularities caused by unwanted higher frequency waves.
If harmonics are still out of control, a passive filter or active filter can be used. Passive filters smoothen the dips in wave forms by filtering sharp changes. It acts like a shock absorber. Disadvantages are power loss and low efficiency.
Active harmonic filters feed the wave form with waveforms complimentary to the harmonics to nullify the effect. Now a days, active filters have become intelligent and they inject only required frequencies at that instant. Modern VFDs employ Active Front End technology employing IGBTs as rectifiers thus solving the problem at the origin itself.
Harmonic study needs to conducted after energizing the building as well. Corrective actions can be taken based on actual harmonic content.
Dubai Electricity and water authority has made it compulsory for the Electrical consultant or Electrical contractor to ensure that the harmonics in electrical distribution systems are within the limits. This is applicable to new and old buildings. DEWA may conduct a visit in existing projects if they notice excessive harmonics in the power network. It is recommended to harmonic studies in the new and existing projects and take corrective actions.
In Existing hotels, malls, residential buildings, if the clients employ electrical and mechanical contractor to add AC systems, solar PV systems, LED lighting, UPS etc. it is advised to conduct harmonic study after the completion of works and add necessary filters if required.
Livisis is a Dubai based company specializing in Electrical Contracting, HVAC contracting, MEP contracting, industrial automation, harmonic studies, power system peak load analysis, testing and commissioning etc. Livisis can conduct a survey, evaluate the power system issues and rectify them with minimum cost.