Harmonics and its types based on harmonic numbers.pptx

harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the fundamental frequency of a periodic signal. The fundamental frequency is also called the 1st harmonic; the other harmonics are known as higher harmonics.

1. HARMONICS

2. Harmonic Distortion Harmonic distortion is caused by nonlinear devices in the power system. A nonlinear device is one in which the current is not proportional to the applied voltage. Figure illustrates this concept by the case of a sinusoidal voltage applied to a simple nonlinear resistor in which the voltage and current vary according to the curve shown. While the applied voltage is perfectly sinusoidal, the resulting current is distorted. Increasing the voltage by a few percent may cause the current to double and take on a different waveshape . This is the source of most harmonic distortion in a power system.

3. For the periodic non-sinusoidal waveform shown in Figure, the simplified Fourier expression states:

4. Adding the fundamental and third harmonic frequency waveforms

5. Harmonic Number Harmonic number ( h ) refers to the individual frequency elements that comprise a composite waveform. For example, h = 5 refers to the fifth harmonic component with a frequency equal to five times the fundamental frequency. If the fundamental frequency is 50 Hz, then the fifth harmonic frequency is 5 × 50, or 250 Hz. The harmonic number 6 is a component with a frequency of 300 Hz.

6. ODD AND EVEN ORDER HARMONICS As their names imply, odd harmonics have odd numbers (e.g., 3, 5, 7, 9, 11), and even harmonics have even numbers (e.g., 2, 4, 6, 8, 10). Harmonic number 1 is assigned to the fundamental frequency component of the periodic wave. Harmonic number 0 represents the constant or DC component of the waveform. The DC component is the net difference between the positive and negative halves of one complete waveform cycle.

7. Harmonic Phase R otation and Phase A ngle R elationship

8. So far we have treated harmonics as stand-alone entities working to produce waveform distortion in AC voltages and currents. This approach is valid if we are looking at single-phase voltages or currents; however, in a three-phase power system, the harmonics of one phase have a rotational and phase angle relationship with the harmonics of the other phases. In a balanced three-phase electrical system, the voltages and currents have a positional relationship as shown in Figure . The three voltages are 120° apart and so are the three currents. The normal phase rotation or sequence is a–b–c, which is counterclockwise and designated as the positive-phase sequence in this book. For harmonic analyses, these relationships are still applicable, but the fundamental components of voltages and currents are used as reference. All other harmonics use the fundamental frequency as the reference. The fundamental frequencies have a positive-phase sequence. The angle between the fundamental voltage and the fundamental current is the displacement power factor angle

9. The following relationships are true for the fundamental frequency current components in a three-phase power system The negative displacement angles indicate that the fundamental phasors i b1 and i c1 trail the i a1 phasor by the indicated angle.

10. Voltage versus Current Distortion 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. This results in voltage harmonics appearing at the load bus. The amount of voltage distortion depends on the impedance and the current. Assuming the load bus distortion stays within reasonable limits (e.g., less than 5 percent), the amount of harmonic current produced by the load is generally constant.

11. 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. These frequencies have no relation to the system fundamental frequency.

12. Harmonics, by definition, occur in the steady state and are integer multiples of the fundamental frequency. The waveform distortion that produces the harmonics is present continually, or at least for several seconds. 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.

13. Harmonic Indices The two most commonly used indices for measuring the harmonic content of a waveform are the total harmonic distortion (THD) and the total demand distortion (TDD) . Both are measures of the effective value of a waveform and may be applied to either voltage or current.

14. 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:

15. Total demand distortion The total demand distortion is defined as the square root of the sum of the squares of the RMS value of the currents from 2nd to the highest harmonic (say 25 th maximum in power system) divided by the peak demand load current and is expressed as a percent.

16. Causes of Voltage and Current H armonics

17. A pure sinusoidal waveform with zero harmonic distortion is a hypothetical quantity and not a practical one. The voltage waveform, even at the point of generation, contains a small amount of distortion due to nonuniformity in the excitation magnetic field and discrete spatial distribution of coils around the generator stator slots. The distortion at the point of generation is usually very low, typically less than 1.0%. The generated voltage is transmitted many hundreds of miles,transformed to several levels, and ultimately distributed to the power user. The user equipment generates currents that are rich in harmonic frequency components, especially in large commercial or industrial installations. As harmonic currents travel to the power source, the current distortion results in additional voltage distortion due to impedance voltages associated with the various power distribution equipment, such as transmission and distribution lines, transformers, cables, buses, and so on. Figure illustrates how current distortion is transformed into voltage distortion. Not all voltage distortion, however, is due to the flow of distorted current through the power system impedance.

18. For instance, static uninterruptible power source (UPS) systems can generate appreciable voltage distortion due to the nature of their operation. Normal AC voltage is converted to DC and then reconverted to AC in the inverter section of the UPS. Unless waveform shaping circuitry is provided, the voltage waveforms generated in UPS units tend to be distorted. As nonlinear loads are propagated into the power system, voltage distortions are introduced which become greater moving from the source to the load because of the circuit impedances. Current distortions for the most part are caused by loads. Even loads that are linear will generate nonlinear currents if the supply voltage waveform is significantly distorted. When several power users share a common power line, the voltage distortion produced by harmonic current injection of one user can affect the other users. This is why standards are being issued that will limit the amount of harmonic currents that individual power users can feed into the source. The major causes of current distortion are nonlinear loads due to adjustable speed drives, fluorescent lighting, rectifier banks, computer and data -processing loads, arc furnaces, and so on. One can easily visualize an environment where a wide spectrum of harmonic frequencies are generated and transmitted to other loads or other power users, thereby producing undesirable results throughout the system.

19. Individual and Total Harmonic D istortion Individual harmonic distortion (IHD) is the ratio between the root mean square (RMS) value of the individual harmonic and the RMS value of the fundamental

20. Harmonic Signatures Many of the loads installed in present-day power systems are harmonic current generators. Combined with the impedance of the electrical system, the loads also produce harmonic voltages. The nonlinear loads may therefore be viewed as both harmonic current generators and harmonic voltage generators. Prior to the 1970s, speed control of AC motors was primarily achieved using belts and pulleys. Now, adjustable speed drives (ASDs) perform speed control functions very efficiently. ASDs are generators of large harmonic currents. Fluorescent lights use less electrical energy for the same light output as incandescent lighting but produce substantial harmonic currents in the process. The explosion of personal computer use has resulted in harmonic current proliferation in commercial buildings. This section is devoted to describing, in no particular order, a few of the more common nonlinear loads that surround us in our everyday life.

21. 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.

22. 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.

23. Switched mode power supply. A distinctive characteristic of switch-mode power supplies is a very high third-harmonic content in the current

24. SMPS current spectrum

25. Fluorescent lighting Fluorescent lights are discharge lamps; thus they require a ballast to provide a high initial voltage to initiate the discharge for the electric current to flow between two electrodes in the fluorescent tube. Once the discharge is established, the voltage decreases as the arc current increases. It is essentially a short circuit between the two electrodes, and the ballast has to quickly reduce the current to a level to maintain the specified lumen output. Thus, a ballast is also a current-limiting device in lighting applications.

26. Fluorescent lamp current spectrum

27. PERSONAL COMPUTER AND MONITOR

28. Adjustable-speed drives for HVAC and elevators Common applications of adjustable-speed drives (ASDs) in commercial loads can be found in elevator motors and in pumps and fans in HVAC systems. An ASD consists of an electronic power converter that converts ac voltage and frequency into variable voltage and frequency. The variable voltage and frequency allows the ASD to control motor speed

29. Current Spectrum

30. Harmonic Sources from Industrial Loads Three-phase power converters DC Drives AC Drives Arcing devices

31. Three-phase power converters Three-phase electronic power converters differ from single-phase converters mainly because they do not generate third-harmonic currents. This is a great advantage because the third-harmonic current is the largest component of harmonics.

32. DC Drives Most dc drives use the six-pulse rectifier, Large drives may employ a 12-pulse rectifier. This reduces thyristor current. duties and reduces some of the larger ac current harmonics. The two largest harmonic currents for the six-pulse drive are the fifth and seventh. They are also the most troublesome in terms of system response. A 12-pulse rectifier in this application can be expected to eliminate about 90 percent of the fifth and seventh harmonics, depending on system imbalances. The disadvantages of the 12-pulse drive are that there is more cost in electronics and another transformer is generally required.

33. Six-pulse dc ASD

34. Current spectrum for PWM-type ASD.

35. AC Drives In ac drives, the rectifier output is inverted to produce a variable-frequency ac voltage for the motor. Inverters are classified as voltage source inverters (VSIs) or current source inverters (CSIs). A VSI requires a constant dc (i.e., low-ripple) voltage input to the inverter stage. This is achieved with a capacitor or LC filter in the dc link. The CSI requires a constant current input; hence, a series inductor is placed in the dc link.

36. AC drives generally use standard squirrel cage induction motors. These motors are rugged, relatively low in cost, and require little maintenance. Synchronous motors are used where precise speed control is critical.

37. A popular ac drive configuration uses a VSI employing PWM techniques to synthesize an ac waveform as a train of variable-width dc pulses

38. Arcing devices This category includes arc furnaces, arc welders, and discharge-type lighting (fluorescent, sodium vapor, mercury vapor) with magnetic (rather than electronic) ballasts. As shown in Fig. the arc is basically a voltage clamp in series with a reactance that limits current to a reasonable value.

39. 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.

40. System Response Characteristics In power systems, the response of the system is equally as important as the sources of harmonics. Identifying the sources is only half the job of harmonic analysis. The response of the power system at each harmonic frequency determines the true impact of the nonlinear load on harmonic voltage distortion.

41. There are three primary variables affecting the system response characteristics, i.e., The system impedance, The presence of a capacitor bank, and The amount of resistive loads in the system

42. System Impedance At the fundamental frequency, power systems are primarily inductive, and the equivalent impedance is sometimes called simply the short-circuit reactance. Capacitive effects are frequently neglected on utility distribution systems and industrial power systems. One of the most frequently used quantities in the analysis of harmonics on power systems is the short-circuit impedance to the point on a network at which a capacitor is located.

43. Capacitor impedance Shunt capacitors, either at the customer location for power factor correction or on the distribution system for voltage control, dramatically alter the system impedance variation with frequency. Capacitors do not create harmonics, but severe harmonic distortion can sometimes be attributed to their presence.

44. While the reactance of inductive components increases proportionately to frequency, capacitive reactance XC decreases proportionately:

45. Parallel resonance All circuits containing both capacitances and inductances have one or more natural frequencies. When one of those frequencies lines up with a frequency that is being produced on the power system, a resonance may develop in which the voltage and current at that frequency continue to persist at very high values. This is the root of most problems with harmonic distortion on power systems.

46. Parallel resonance occurs when the reactance of XC and the distribution system cancel each other out. The frequency at which this phenomenon occurs is called the parallel resonant frequency. It can be expressed as follows:

47. Principles of controlling harmonics Harmonic distortion is present to some degree on all power systems. Fundamentally, one needs to control harmonics only when they become a problem. There are three common causes of harmonic problems: 1. The source of harmonic currents is too great. 2. The path in which the currents flow is too long (electrically), resulting in either high voltage distortion or telephone interference. 3. The response of the system magnifies one or more harmonics to a greater degree than can be tolerated

48. When a problem occurs, the basic options for controlling harmonics are: 1. Reduce the harmonic currents produced by the load. 2. Add filters to either siphon the harmonic currents off the system, block the currents from entering the system, or supply the harmonic currents locally. 3. Modify the frequency response of the system by filters, inductors, or capacitors.

49. Reducing harmonic currents in loads There is often little that can be done with existing load equipment to significantly reduce the amount of harmonic current it is producing unless it is being misoperated. While an overexcited transformer can be brought back into normal operation by lowering the applied voltage to the correct range, arcing devices and most electronic power converters are locked into their designed characteristics. PWM drives that charge the dc bus capacitor directly from the line without any intentional impedance are one exception to this. Adding a line reactor or transformer in series will significantly reduce harmonics, as well as provide transient protection benefits.

50. Transformer connections can be employed to reduce harmonic currents in three-phase systems. Phase-shifting half of the 6-pulse power converters in a plant load by 30° can approximate the benefits of 12- pulse loads by dramatically reducing the fifth and seventh harmonics. Delta-connected transformers can block the flow of zero-sequence harmonics (typically triplens ) from the line. Zigzag and grounding transformers can shunt the triplens off the line. Purchasing specifications can go a long way toward preventing harmonic problems by penalizing bids from vendors with high harmonic content. This is particularly important for such loads as high-efficiency lighting.

51. Filtering The shunt filter works by short-circuiting harmonic currents as close to the source of distortion as practical. This keeps the currents out of the supply system. This is the most common type of filtering applied because of economics and because it also tends to correct the load power factor as well as remove the harmonic current. Another approach is to apply a series filter that blocks the harmonic currents. This is a parallel-tuned circuit that offers a high impedance to the harmonic current. It is not often used because it is difficult to insulate and the load voltage is very distorted. One common application is in the neutral of a grounded- wye capacitor to block the flow of triplen harmonics while still retaining a good ground at fundamental frequency.

52. Triplen harmonics Triplen harmonics are the odd multiples of the third harmonic (h 3, 9, 15, 21,…). They deserve special consideration because the system response is often considerably different for triplens than for the rest of the harmonics.

53. Harmonic Phase Sequences