Understanding Harmonics

Transcription

1 Understanding Harmonics Terry Gaiser Sensus What Are Harmonics? 1

2 » What is Power Quality?» Power quality is the degree to which both the utilization and delivery of electric power affects the performance of electrical equipment.» Ontario Hydro Power Quality Committee (Gibson, B. (1996). What is power quality? Electricity Today. today.com/et/june96/quality.htm)» Who is Responsible for Power Quality?» Customer (according to most utilities)» Utility (according to most customers)» Electrical power problems cost U.S. industry $40 $150 billion each year, according to some estimates. Most problems originate outside our plant and are therefore beyond our control for example, outages, voltage interruptions, voltage sag, voltage reductions, and blackouts.» Department of Energy 2

3 » What are Harmonics?» Harmonic frequencies in power systems are a primary cause of power quality problems.» The presence of harmonics in power systems means that current and voltage waveforms are distorted and deviate from sinusoidal waveforms.» What are Harmonics?» Harmonics describe disturbances which repeat every cycle for a significant number of cycles.» Harmonic voltages and currents in an electric power system are a result of non linear electric loads. 3

4 » What are Harmonics?» Harmonics in power systems result in increased heating in equipment and conductors.» Harmonics stress the distribution system and potentially damage equipment. They may disrupt the normal operation of equipment and increase operating costs.» What are Harmonics?» Non linear electric loads are increasingly more abundant in all industrial, commercial, and residential installations and their percentage of the total load is growing steadily. 4

5 » What are Harmonics?» A load is said to be non linear when the current it draws does not have the same waveform as the supply voltage. The flow of harmonic currents through system impedances in turn creates voltage harmonics, which distort the supply voltage.» Harmonics can result in metering errors and disputes.» Harmonics: Distorted Waveforms 5

6 » What are Harmonics?» The mathematical definition of a harmonic is a component frequency of the signal that is an integer multiple of the fundamental frequency.» What are Harmonics?» Harmonics are AC voltages and currents with frequencies that are integer multiples of the fundamental frequency (60 Hz). Fundamental frequency (60 Hz) 2nd order harmonics (120 Hz) 3rd order harmonics (180 Hz) 4th order harmonics (240 Hz) 5th order harmonics (300 Hz) and so on. Normally, only odd order harmonics (3rd, 5th, 7th, 9th) occur on a 3 phase power system 6

7 » What are Harmonics? Background 7

8 » Background» Harmonics are one of the most important issues effecting power quality. Even though the issue of harmonics seems relatively new, the issue has been around for a long time.» In 1893, only eight years after first AC power plant was built, engineers conducted a harmonic analysis to identify and solve a motor heating problem.» A paper written by E.J. Houston and A.E. Kennely in 1894, was one of the first documents in which the word harmonic was used.» Background» Up to the 1960 s, most of the electric loads were linear loads in other words, the current flowing through the appliances had a sinusoidal sine wave.» These electric loads included induction motors, domestic lighting, stoves and other household appliances. 8

9 » Power System Loads: Linear» Linear: Responds in a straight line, predictable fashion» Linear Load: Current and voltage behave similar to each other: change proportionately» Linear load (Incandescent lamp) 9

10 » Background» With the introduction of semi conductor technology, harmonics have become one of the most important issues in managing power quality in distribution systems.» Modern electronic appliances, variable frequency drives and UPS systems are a major source of harmonics being introduced into the distribution system.» Without proper protection, these harmonics can damage customer equipment and parts of the distribution system.» Background» UPS (uninterruptible power supply) systems convert the incoming AC supply to DC in order to charge batteries in the event of a power outage.» The DC component has a very high frequency signal and interferes with the AC power supplies.» Most of the distortion in the modern distribution system is customer generated. 10

11 » Background» Today, electric distribution systems cover large geographic areas and a wide variety of loads are connected to the distribution system.» During the planning and management of power systems, an accurate understanding of the customer loads that are connected to the distribution system is very important.» Thus, proactive preventative measures can be taken to reduce the potential for power quality issues.» Power System Loads: Non linear» Non linear: Responds in an unpredictable, distorted, or suddenly changing response» Non linear Load: Current and voltage behave differently they do not change proportional to each other. 11

12 » Non linear loads: Examples» Non linear load: Dimmer 12

13 » Non linear load: Laser Printer» Non linear load: Hair Dryer, low 13

14 » Non linear load: Adjustable Speed Drives Causes of Harmonics 14

15 » Causes of Harmonics» Harmonics have existed from the earliest days of the industry and were (and still are) caused by the non linear magnetizing impedances of transformers, reactors, fluorescent lamp ballasts, etc» In addition, power electronic devices have become abundant today because of their capabilities for precise process control and energy savings benefits.» However, they also bring drawbacks to distribution systems: harmonics.» Causes of Harmonics» What causes harmonics? If harmonics aren't generated intentionally, where do they come from?» Harmonics are the byproduct of the continually expanding number of power electronic devices 15

16 » Causes of Harmonics» Some sources of harmonics include non linear loads like battery chargers, variable frequency drives, electronic fluorescent lighting ballasts, switching mode power supplies, rectifiers, inverters, adjustablespeed motor drives, welders, arc furnaces, voltage controllers, and frequency converters,» Another source of harmonics are iron core devices like transformers. Effects of Harmonics 16

17 » Effects of Harmonics» It is easy to see how the power quality of a system might be compromised by the presence of harmonics.» Modern plant equipment and the associated controlling power electronics are more reliant than ever on good power quality for optimum operation.» Deviations from ideal power quality is acceptable. But even a small amount of interference can have a serious effect on the power system.» Effects of Harmonics» Some of the main risks linked to harmonics:» Overload of distribution networks due to the increase of rms currents.» Overload of neutral conductors neutral current can exceed the phase currents. 17

18 » Effects of Harmonics» Overload, vibration and premature aging of generators, transformers and motors as well as increased transformer noise.» Nuisance tripping of circuit breakers» Distortion of the supply voltage that can disturb sensitive loads» Effects of Harmonics» Disturbance in communication networks and telephone lines.» Malfunction of UPS systems and generator systems.» Computer malfunctions» Additionally, the life span of many devices are reduced by elevated operating temperatures. 18

19 » Effects of Harmonics» Harmonic currents can cause problems for both the distribution system and the customer s installation.» The effects and the solutions are very different and need to be addressed separately.» The measures that are appropriate for controlling the effects of harmonics within the installation may not necessarily reduce the distortion caused on the supply and vice versa.» Effects of Harmonics» Metering issues:» Mechanical meters will not register harmonics well 80% error at higher harmonics.» Solid state meters can measure harmonics well depending on the sampling rate and VAR calculation used.» CT s and PT s can only pass a limited range of harmonics. 19

20 » Effects of Harmonics» Harmonics and Metering Accuracy:» Primarily affect the calculation of VA, VAR, and Power Factor.» No ANSI standard for these calculations at this time.» Different manufacturers use different methods and definitions most allow the user to make several choices for each. Economic Impact of Harmonics 20

21 » Economic Impact of Harmonics» Harmonics can damage customer equipment and parts of the distribution system.» Harmonics can cause premature aging of equipment more frequent replacement costs.» Economic Impact of Harmonics» Overload on the distribution network means higher equipment rating, increased subscribed power level for the industrial customer, and increased power losses.» Unexpected current distortion can lead to nuisance tripping and production interruptions for the customer. 21

22 Measurement Devices» Measurement Devices» Measurement devices can provide instantaneous and average harmonic information.» Instantaneous values are used for analysis of the disturbances linked to harmonics.» Average values are used for Power Quality assessment. 22

23 » Measurement Devices» Most recent measurement devices are designed referring to IEC standard : General Guide on Harmonics and Inter harmonics Measurements and Instrumentation.» For practical considerations, this standard distinguishes between harmonics, inter harmonics and other components above the harmonic frequency range, up to 9 khz.» This standard defines the measurement instrumentation intended for testing individual items of equipment.» Measurement Devices» Portable instruments Traditional observation and measurement methods include: Oscilloscope Digital power quality analyzer Calculate the values of the harmonic indicators (power factor, crest factor, individual harmonic amplitude, THD) 23

24 » Measurement Devices» Installing a digital Power Quality recorder in a cabinet» Measurement Devices» Fixed instruments Panel instrumentation can provide continuous information of an electrical installation. Data can be accessed through dedicated power monitoring devices or through the digital trip units of circuit breakers. 24

25 » Measurement Devices» Example of Power and Energy meter» Measurement Devices» Example of electronic trip units of circuit breakers providing harmonic related information 25

26 » Harmonics: Spectrum Analysis» Transformer Harmonic Spectrum 26

27 Reducing the Effect of Harmonics» Reducing the Effects of Harmonics» Because of the adverse effect of harmonics on power system components, the IEEE developed standard to define recommended practices for harmonic control.» This standard also stipulates the maximum allowable harmonic distortion allowed in the voltage and current waveforms on various types of systems. 27

28 » Reducing the Effects of Harmonics» There are two approaches available for mitigating the effects of excessive heating due to harmonics. A combination of the two approaches is often implemented.» One method is to reduce the magnitude of the harmonic waveforms, usually by filtering.» Reducing the Effects of Harmonics» The other method is to use system components that can more effectively manage the effects of harmonics, such as finely stranded conductors, oversized neutral conductors, and k factor transformers.» The most significant harmonic orders in three phase distribution networks are the odd orders (3, 5, 7, 9, 11, 13...) 28

29 » Reducing the Effects of Harmonics» Triplen harmonics (order multiple of 3) are present only in three phase, four wire systems, when single phase loads are connected between phase and neutral.» Utilities are mainly focusing on low harmonic orders (5, 7, 11, and 13).» Reducing the Effects of Harmonics» Generally speaking, harmonic conditioning of the lowest orders (up to 13) is sufficient. More comprehensive conditioning takes into account harmonic orders up to 25.» Harmonic amplitudes normally decrease as the frequency increases. Sufficiently accurate measurements are obtained by measuring harmonics up to order

30 » Reducing the Effects of Harmonics» However, current manufacturers are providing equipment that can measure up to the 128th harmonic.» There is no one size fits all solution.» Reducing the Effects of Harmonics» Utilize over sized neutral conductors» Utilize separate neutral conductors» Oversizing transformers and generators» K Rated transformers 30

31 » Reducing the Effects of Harmonics» Isolation transformers» Special transformers» Filtering passive and active» Special metering Summary 31

32 » Summary» Harmonics will undoubtedly continue to become more of a concern as more equipment that produces harmonics are added to electrical systems.» But if adequately considered during the initial design of the system, the detrimental effects of harmonics can be managed.» Utilities and their customers have to cooperate to identify power quality issues and to take steps to manage effects of harmonics. 32