Voltage Total Harmonic Distortion Analysis Through: Fill & Download for Free

A bridge rectifier circuit presents a non-linear load to the mains supply. This means that the wave shape of the line current drawn by the rectifier does not have a sinusoidal waveform, to follow the supply voltage waveform. The current waveform can often take on a quasi-square wave shape. It is this distorted current waveform that is the root cause of subsequent voltage harmonic problems.Three-phase Bridge Rectifier CharacteristicsAs an example. check out the three-phase bridge rectifier circuit in Figure 1(a). This is typical of that used in high power UPS, motor VFDs, electrochemical rectifiers and induction furnaces.Figure 1(a): Three-phase Bridge Rectifier CircuitFilter inductor(s) are used to improve input current waveforms, which are shown in Figure 1(b). It can be seen that these are distinctly square wave in shape. Other answers have correctly said that this waveform can be analysed as a series of superimposed sine waves, each at a multiple of the frequency of the supply voltage. These multiples are called harmonic currents. This process is called Fourier Analysis.Figure 1(b): Three Phase Bridge Rectifier Circuit Voltage & Current WaveformsVoltage DistortionNormally, current distortions produce voltage distortions. However, when there is a stiff sinusoidal voltage source (when there is a low impedance path from the power source, which has sufficient capacity so that loads placed upon it will not affect the voltage), one need not be concerned about current distortions producing voltage distortions. The further away from the non-linear load that we sample the voltage waveform, the less the voltage distortion will be. This effect is shown in Figure 2.Figure 2: The Effect of System Impedances on Voltage DistortionAs nonlinear currents flow through a facility's electrical system and the distribution-transmission lines, additional voltage distortions are produced due to the impedance associated with the electrical network. Thus, as electrical power is generated, distributed, and utilized, voltage and current waveform distortions are produced.There are prescribed limits on the maximum harmonic distortion that an electrical installation may cause at the point of common coupling (P.C.C.) which is the point on the electricity network that you share with your neighbour. Under IEEE 519-1992 these limits are:total harmonic distortion (T.H.D.); 5%;no individual harmonic to exceed 3%.

Consider the following signal.It represents an ideal voltage (or current) that has sinusoidal waveform with a desired frequency [1]. This is the signal at which all alternating current sources (Generators at power stations) and loads (Motors, etc) are designed and developed during the last century. The frequency is usually 50Hz or 60Hz depending upon geographic locations and is called fundamental frequency in further discussions below.The system operates without any trouble as long as the signals (voltage / current) are at fundamental frequency. The motors operate as expected, and generators generate as expected and the power gets transferred (from electricity to any other form) as expected.In recent years, the field of power electronics have picked up on the trends due to numerous advantages (Better control of motors, Energy Savings by LEDs, etc). These power electronic devices operate independently compared to supplied signals. As a result, many times they tend to change the signal waveforms. Let us consider the innocent looking diode rectifier circuit for example [3]:The current waveform of a basic CFL or LED lamp, that is supplied through a bridge rectifier circuit with a capacitor at the output looks something like below[4]:Not exactly sinusoidal, huh? Now you understand the problem.These distorted signals contain waveforms different than fundamental sinusoidal signals.Mathematically, they can be easily represented by addition of a fundamental sinusoid signal with frequency multiples called Harmonics. I won’t explain the math, as it is better explained on allaboutciruits.com at [1] and [2].In short, using some math – the fourier series – it is possible to represent the above distorted signals as a function of fundamental signal. On the same lines, the following waveforms represent the %presence of the harmonics in the above signal [4].This math and analysis is used by various control algorithms to improvise on the waveforms. The technique is called Power Factor Correction. But, I’ll limit the discussion here for briefness and get back to the original question.THD (Total Harmonic Distortion) is a standard factor that represents rms value of this distortion with respect to the fundamental frequency. For example, THD factor of voltage signals, is computed as below [5–7].ORSimilarly, THD in current is represented as,The %THD in signal gives a measure of the distortion of the signal. A high THD is a representation of poorly engineered product (All cheap phone adapters and LED lamps invariably have high THDs). Often, the suffix mentions the signal - The THD in voltage is written as THDv (or THDu) and for current signal it is written as THDi.For above discussed circuit, THDi is > 99% (very poor).Systems with THD < 5% are considered good. The ones with THD < 3% are better.Less THD → Better signals → Better PF → Better Power conversions → Less losses.References:[1] Square Wave Signals[2] Harmonics in Polyphase Power Systems[3] Power Supply, Full-Wave Bridge Rectifier[4] https://ntmm.org/~nt/elecpow/le_lamps/[5] Total harmonic distortion[6]http://www.aptsources.com/resources/pdf/Total%20Harmonic%20Distortion.pdf[7] Essential Basics of Total Harmonic Distortion (THD) | EEP

Harmonic voltages and currents in a Power System are a result of non-linear electric loads.In a normal AC power system, the current varies sinusoidally at a specific frequency, usually 50 or 60 Hertz. When a linear electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage (though usually not in phase with the voltage).Current harmonics are caused by non-linear loads. When a non-linear load, such as a rectifier/inverter, is connected to the system, it draws a current that is not necessarily sinusoidal. The current waveform can become quite complex, depending on the type of load and its interaction with other components of the system. Regardless of how complex the current waveform becomes, as described through Fourier Series analysis, it is possible to decompose it into a series of simple sinusoids, which start at the power system fundamental frequency and occur at integer multiples of the fundamental frequency.Further examples of non-linear loads include common office equipment such as computers and printers, Fluorescent lighting, battery chargers, electronic ballasts, variable frequency drives, and switching mode power supplies.Total Harmonic Distortion, or THD is a common measurement of the level of harmonic distortion present in power systems. THD is defined as the ratio of total harmonics to the value at fundamental frequency.where Vn is the RMS voltage of nth harmonic and n = 1 is the fundamental frequency.EFFECTS:One of the major effects of power system harmonics is to increase the current in the system. This is particularly the case for the third harmonic, which causes a sharp increase in the zero sequence current, and therefore increases the current in the neutral conductor.Electric motors experience losses due to hysteresis and losses due to eddy currents set up in the iron core of the motor. These are proportional to the frequency of the current. Since the harmonics are at higher frequencies, they produce higher core losses in a motor than the power frequency would. This results in increased heating of the motor core, which (if excessive) can shorten the life of the motor. The 5th harmonic causes a CEMF (counter electromotive force) in large motors which acts in the opposite direction of rotation. The CEMF is not large enough to counteract the rotation, however it does play a small role in the resulting rotating speed of the motor.Courtesy: WikiPedia