POWER QUALITY ENGINEERING

POWER QUALITY ENGINEERINGUNIT-4HARMONICSHarmonics is defined as a component of periodic wave(signal).Whose frequency is integer multiple ofnfundamental frequency.Harmonics are classified inti two different types:1.current harmonics 2. Voltage harmonicsVoltage harmonics depends on grid or source.Current harmonics depends on network impedance and load dependent.

HARMONIC DISTORTIONTotal Harmonic Distortion of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.Electronic ballasts reduce THD in two ways.1. The electronic ballast has a lower THD percentage than the magnetic ballast. 2.The biggest reduction comes from the fact that electronic ballasts reduce the total load.

VOLTAGE VERSUS CURRENT DISTORTION:-

The word harmonics is often used by itself without further qualification. Generally, it could mean one of the following three things:1. The harmonic voltages are too great (the voltage too distorted) for the control to properly determine firing angles.2. The harmonic currents are too great for the capacity of some device in the power supply system such as a transformer, and the machine must be operated at a lower than rated power.3. The harmonic voltages are too great because the harmonic currents produced by the device are too great for the given system condition.

Fig 1. Harmonic currents flowing through the impedance result in harmonic voltages at load.As Fig. 1 shows, voltage distortion is the result of distorted currents passing through the linear, series impedance of the power delivery system, although, assuming that the source bus is ultimately a pure sinusoid, there is a nonlinear load that draws a distorted current. The harmonic currents passing through the impedance of the system cause a voltage drop for each harmonic. Recognition of this fact is the basis for the division of responsibilities for harmonic control that are found in standards such as IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems:1. The control over the amount of harmonic current injected into the system takes place at the end-use application.2. Assuming the harmonic current injection is within reasonable limits, the control over the voltage distortion is exercised by the entity having control over the system impedance, which is often the utility.HARMONICS VERSUS TRANSIENTS:-

Harmonic distortion is blamed for many power quality disturbances that are actually transients. A measurement of the event may show a distorted waveform with obvious high-frequency components.Although transient disturbances contain high-frequency components, transients and harmonics are distinctly different phenomena and are analyzed differently.Transient waveforms exhibit the high frequencies only briefly after there has been an abrupt change in the power system. The frequencies are not necessarily harmonics; they are the natural frequencies of the system at the time of the switching operation.Transients are usually dissipated within a few cycles. Transients are associated with changes in the system such as switching of a capacitor bank. Harmonics are associated with the continuing operation of a load.

HARMONIC INDICES

The two most commonly used indices for measuring the harmonic content of a waveform are the total harmonic distortion and the total demand distortion. Both are measures of the effective value of a waveform and may be applied to either voltage or current.Total harmonic distortion:-The THD is a measure of the effective value of the harmonic components of a distorted waveform. That is, it is the potential heating value of the harmonics relative to the fundamental. This index can be calculated for either voltage or current.

where Mh is the rms value of harmonic component h of the quantity M.The rms value of a distorted waveform is the square root of the sum of the squares as shown in Eqs. (5.3) and (5.4). The THD is related to the rms value of the waveform as follows:

HARMONIC SOURCES FROM COMMERCIAL LOADS

Commercial facilities such as office complexes, department stores, hospitals, and Internet data centers are dominated with high-efficiency fluorescent lighting with electronic ballasts, adjustable-speed drives for the heating, ventilation, and air conditioning (HVAC) loads, elevator drives, and sensitive electronic equipment supplied by single-phase switch-mode power supplies.Commercial loads are characterized by a large number of small harmonic-producing loads. Depending on the diversity of the different load types, these small harmonic currents may add in phase or cancel each other.The voltage distortion levels depend on both the circuit impedances and the overall harmonic current distortion.HARMONIC SOURCES FROM INDUSTRIAL LOADSModern industrial facilities are characterized by the widespread application of nonlinear loads.Nonlinear industrial loads can generally be grouped into three categories:1. Three-phase power converters,2. Arcing devices, and3.Saturable devices.

LOCATING HARMONIC SOURCES

On radial utility distribution feeders and industrial plant power systems, the main tendency is for the harmonic currents to flow from the harmonic-producing load to the power system source.This is illustrated in Fig. 13. This general tendency of harmonic current flows can be used to locate sources of harmonics.

Transformer magnetizing current and harmonic spectrum. SYSTEM RESPONSE CHARACTERISTICSIn power systems, the response of the system is equally as important as the sources of harmonics. In fact, power systems are quite tolerant of the currents injected by harmonic-producing loads unless there is some adverse interaction with the impedance of the system. Identifying the sources is only half the job of harmonic analysis.There are three primary variables affecting the system response characteristics:1.The system impedance, 2.The presence of a capacitor bank, and 3.The amount of resistive loads in the system.

INTERHARMONICSAccording to the Fourier theory, a periodic waveform can be expressed as a sum of pure sine waves of different amplitudes where the frequency of each sinusoid is an integer multiple of the fundamental frequency of the periodic waveform. A frequency that is an integer multiple of the fundamental frequency is called a harmonic frequency.One primary source of interharmonics is the widespread use of electronic power converter loads capable of producing current distortion over a whole range of frequencies, i.e., characteristic and noncharacteristic frequencies.

Typical characteristic harmonics in the ac-side line currents are 11th, 13th, 23rd, 25th,, with some non characteristic harmonics such as the 5th and 7th also possibly present.2fo fs, 4fo fswhere fo and fs are the furnace operating frequency and the fundamental of the ac main frequency, respectively.

14DEVICES FOR CONTROLLING HARMONIC DISTORTIONThere are a number of devices available to control harmonic distortion.They can be as simple as a capacitor bank or a line reactor, or as complex as an active filter. The following material first discusses the effectiveness of a simple inline reactor, or choke, in mitigating harmonic distortion.Two general classes of harmonic filters, 1. passive filters,2. active filters.In-line reactors or chokesA simple, but often successful, method to control harmonic distortion generated by adjustable-speed drives involves a relatively small reactor, or choke, inserted at the line input side of the drive. This is particularly effective for PWM-type drives.It is also important to note that there are other advantages of the choke in ASD applications.

Fig Three phase line choke for ASD applications. Zigzag transformersZigzag transformers are often applied in commercial facilities to control zero-sequence harmonic components. A zigzag transformer acts like a filter to the zero-sequence current by offering a low-impedance path to neutral. The two most important problems in commercial facilities are overloaded neutral conductors and transformer heating.Both of these problems can be solved with proper zigzag transformer placement.Passive filtersPassive filters are inductance, capacitance, and resistance elements configured and tuned to control harmonics. They are commonly used and are relatively inexpensive compared with other means for eliminating harmonic distortion.They are employed either to shunt the harmonic currents off the line or to block their flow between parts of the system by tuning the elements to create a resonance at a selected frequency.

several types of common filter arrangements shown below.

Active filtersActive filters are relatively new types of devices for eliminating harmonics. They are based on sophisticated power electronics and are much more expensive than passive filters.However, they have the distinct advantage that they do not resonate with the system.Active filters can work independently of the system impedance characteristics.Thus, they can be used in very difficult circumstances where passive filters cannot operate successfully because of parallel resonance problems. Active filters can typically be programmed to correct for the power factor as well as harmonics.