Oct
26

Mitigating harmonics in electrical systems

Nicholas Rich, PE, LEED AP, Interface Engineering, Seattle
3/13/2014

Although devices using power electronics can produce distortion in electrical distribution systems, it’s up to the engineer to apply effective solutions to mitigate them.

Learning objectives

  • Understand current and voltage harmonics in electrical systems, and their negative effects on the facility electrical system.
  • Know how electronic power equipment such as VFDs creates harmonics.
  • Understand characteristic and noncharacteristic harmonics.
  • Understand IEEE 519 guidelines for the reduction of electrical harmonics.
  • Learn design techniques for mitigating harmonics with recommended applications.

Harmonics and detrimental effects

In North America, alternating current (ac) electrical power is generated and distributed in the form of a sinusoidal voltage waveform with a fundamental frequency of 60 cycles/sec, or 60 Hz. In the context of electrical power distribution, harmonics are voltage and current waveforms superimposed on the fundamental, with frequencies that are multiples of the fundamental. These higher frequencies distort the intended ideal sinusoid into a periodic, but very different shaped waveform.

Many modern power electronic devices have harmonic correction integrated into the equipment, such as 12- and 18-pulse VFDs and active front-end VFDs. However, many nonlinear electronic loads, such as 6-pulse VFDs, are still in operation. These nonlinear loads generate significant magnitudes of fifth-order and seventh-order harmonics in the input current, resulting in a distorted current waveform (see Figure 1).

The characteristics of the harmonic currents produced by a rectifier depend on the number of pulses, and are determined by the following equation:

h = kp ±1

Where:

  • h is the harmonic number, an integral multiple of the fundamental
  • k is any positive integer
  • p is the pulse number of the rectifier


RTEmagicC CSEPP1403 FHARMONIC Fig 1 Time Domain Freq Domain Harmonic Graphing 6 pulse.jpg

Figure 1: Transformers are available in a variety of sizes and distribution voltages, and can be installed indoors or outdoors. All images courtesy: TLC Engineering for ArchitectureThus, the waveform of a typical 6-pulse VFD rectifier includes harmonics of the 5th, 7th, 11th, 13th, etc., orders, with amplitude decreasing in inverse proportion to the order number, as a rule of thumb. In a 3-phase circuit, harmonics divisible by 3 are canceled in each phase. And because the conversion equipment’s current pulses are symmetrical in each half wave, the even order harmonics are canceled. While of concern, harmonic currents drawn by nonlinear loads result in true systemic problems when the voltage drop they cause over electrical sources and conductors results in harmonics in the voltage delivered to potentially all of the building electrical system loads—even those not related to the nonlinear loads. These resulting harmonics in the building voltage can have several detrimental effects on connected electrical equipment, such as conductors, transformers, motors, and other VFDs.

Conductors: Conductors can overheat and experience energy losses due to the skin effect, where higher frequency currents are forced to travel through a smaller cross-sectional area of the conductor, bunched toward the surface of the conductor.

Transformers: Transformers can experience increased eddy current and hysteresis losses due to higher frequency currents circulating in the transformer core.

Motors: Motors can experience higher iron and eddy current losses. Mechanical oscillations induced by current harmonics into the motor shaft can cause premature failure and increased audible noise during operation.

Other VFDs and electronic power supplies: Distortion to the increasing voltage waveform in other VFDs and electronic (switch mode) power supplies can cause failure of commutation circuits in dc drives and ac drives with silicon controlled rectifiers (SCRs).

Establishing mitigation criteria

The critical question is: When do harmonics in electrical systems become a significant enough problem that they must be mitigated? Operational problems from electrical harmonics tend to manifest themselves when two conditions are met:

  • Generally, facilities with the fraction of nonlinear loads to total electrical capacity that exceeds 15%.
  • A finite power source at the service or within the facility power distribution system with relatively high source impedance, resulting in greater voltage distortion resulting from the harmonic current flow.

IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Power Systems, was written in part by the IEEE Power Engineering Society to help define the limits on what harmonics will appear in the voltage the utility supplies to its customers, and the limits on current harmonics that facility loads inject into the utility. Following this standard for power systems of 69 kV and below, the harmonic voltage distortion at the facility’s electrical service connection point, or point of common coupling (PCC), is limited to 5.0% total harmonic distortion with each individual harmonic limited to 3%.

In this standard, the highest constraint is for facilities with the ratio of maximum short-circuit current (ISC) to maximum demand load current (IL) of less than 20, with the following limits placed on the individual harmonic order: (Ref. Table 10.3, IEEE Std. 519)

  • For odd harmonics below the 11th order: 4.0%
  • For odd harmonics of the 11th to the 17th order: 2.0%
  • For odd harmonics of the 17th to the 23rd order: 1.5%
  • For odd harmonics of the 23rd to the 35th order: 0.6%
  • For odd harmonics of higher order: 0.3%
  • For even harmonics, the limit is 25% of the next higher odd harmonic.
  • The total demand distortion (TDD) is 5.0%.

There are various harmonic mitigation methods available to address harmonics in the distribution system. They are all valid solutions depending on circumstances, each with their own benefits and detriments. The primary solutions are harmonic mitigating transformers; active harmonic filters; and line reactors, dc bus chokes, and passive filters.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar

P3 strives to bring you quality relevant industry related news.
See the origial article at: https://www.plantengineering.com/single-article/mitigating-harmonics-in-electrical-systems/195ae63d6e6e0d6d87421de194bd2c88.html

 

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Oct
19

How To Check For Harmonics In Electrical Power Systems

How To Check For Harmonics In Electrical Power Systems

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Harmonics are electric voltages and currents on an electric power system that can cause power quality problems. Because equipment and machinery can malfunction or fail in the presence of high harmonic voltage and/or current levels, harmonic distortion has become a growing concern for facility managers, users of automation equipment, and engineers. While the presence of harmonics won't make it impossible for a factory or office to operate, the degree of impact depends on how much the power system can withstand and how susceptible the equipment is to harmonic distortion.

What Causes Harmonics?

Harmonics are created by electronic equipment with nonlinear loads drawing in current in abrupt short pulses. The short pulses cause distorted current waveforms, which in turn cause harmonic currents to flow back into other parts of the power system. Harmonics are especially prevalent when there are many personal computers, laser printers, fax machines, copiers, or medical test equipment, fluorescent lighting, uninterruptible power supplies (UPSs), and variable speed drives all on the same electrical system.

Harmonics degrade the level of power quality and its efficiency, particularly in a commercial building or industrial facility. In general, most buildings can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern. If the nonlinear loads exceed 15%, some non-apparent negative consequences can result.
Common Problems Caused by Harmonics

Overloading Neutral Conductors

The three-phase system consists of three individual phase conductors and a neutral conductor. If all the phase conductors carry the same current, the phase currents tend to cancel one another out provided there is a balanced load. This balanced load makes it possible to reduce the size of the neutral conductor. Unfortunately, switched mode power supplies used in computers have a very high third-harmonic current. While harmonic currents cancel out on the neutral wire, the third harmonic current is additive in the neutral. In buildings with a large number of installed personal computers, the neutral wire can carry much higher currents than the wire was designed to accommodate, creating a potential fire hazard.

Overheating Transformers and Increased Associated Losses

For transformers feeding harmonic-producing loads, the eddy current loss in the windings is the most dominant loss component in the transformer. This eddy current loss increases proportionate to the square of the product's harmonic current and its corresponding frequency. The total transformer loss to a fully loaded transformer supplying to a nonlinear load is twice as high as for an equivalent linear load. This causes excessive transformer heating and degrades the insulation materials in the transformer, which eventually leads to transformer failure.

Nuisance Tripping of Circuit Breakers

All circuits containing capacitance and inductance have one or more resonant frequencies. When any of the resonant frequencies correspond to the harmonic frequency produced by nonlinear loads, harmonic resonance can occur. Voltage and current during resonant frequency can be highly distorted. This distortion can cause nuisance tripping in an electrical power system, which can ultimately result in production losses.

How to Diagnose and Fix Harmonics

A harmonics analyzer is the most effective instrument for performing detailed analysis of power quality to determine the wave shapes of voltage and current on respective frequency spectrums. A harmonic analyzer is also useful in instances where the lack of obvious symptoms prevent you from determining if harmonics are a cause for concern.

A harmonics analyzer is used to provide a detailed analysis of the suspect source. Using this data, the harmonic ratio function calculates a value from 0% to 100% to indicate the deviation of non-sinusoidal and sinusoidal waveform. This value indicates the presence of harmonics.

With built-in harmonic ratio function, the Agilent U1242 Series handheld DMM helps technicians and engineers quickly verify the presence of harmonics in AC signals. This information can be used to prevent or reduce equipment downtime and repair costs.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar

P3 strives to bring you quality relevant industry related news.
See the origial article at: http://www.grainger.com/content/safety-electrical-power-system-harmonics

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Oct
12

Principles for Controlling Harmonics

principles for controlling harmonics

Figure 1 - Variation of the voltage THD over a 1-week period

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. Harmonic distortion is present to some degree on all power systems.

Fundamentally, one needs to control harmonics only when they become a problem. Harmonic distortion is not a new phenomenon on power systems.

Concern over distortion has ebbed and flowed a number of times during the history of ac electric power systems.

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.


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.

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.

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.


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.

Active filters work by electronically supplying the harmonic component of the current into a nonlinear load.

Modifying the system frequency response

There are a number of methods to modify adverse system responses to harmonics:

  1. Add a shunt filter. Not only does this shunt a troublesome harmonic current off the system, but it completely changes the system response, most often, but not always, for the better.
  2. Add a reactor to detune the system. Harmful resonances generally occur between the system inductance and shunt power factor correction capacitors. The reactor must be added between the capacitor and the supply system source. One method is to simply put a reactor in series with the capacitor to move the system resonance without actually tuning the capacitor to create a filter. Another is to add reactance in the line.
  3. Change the capacitor size. This is often one of the least expensive options for both utilities and industrial customers.
  4. Move a capacitor to a point on the system with a different short-circuit impedance or higher losses. This is also an option for utilities when a new bank causes telephone interference—moving the bank to another branch of the feeder may very well resolve the problem. This is frequently not an option for industrial users because the capacitor cannot be moved far enough to make a difference.
  5. Remove the capacitor and simply accept the higher losses, lower voltage, and power factor penalty. If technically feasible, this is occasionally the best economic choice.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar

P3 strives to bring you quality relevant industry related news.
See the origial article at: http://electrical-engineering-portal.com/principles-for-controlling-harmonics

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Oct
05

State-Of-The-Art Solutions for Controlling Harmonics

Introduction:
Harmonics are the multiples of fundamental frequencies. They are generated due to non-linear loads. Non-linear loads, by definition are the equipments which draw non-sinusoidal current even from a sinusoidal voltage source. The examples of non-linear loads are Rectifiers, Induction furnaces, UPS Systems, Variable frequency drives and so on . . .

The adverse effects of harmonics in industrial plant are well known:

Effects of harmonics:

The harmonics adversely affect almost all the components of any industrial plant :

  • Power factor improvement capacitors draw excessively high current if voltage is contaminated with harmonics.
  • The magnetic equipments like motors, generators and transformers are abnormally heated up due to harmonics. This is due to increased copper loss, hysterisis loss and eddy current loss.
  • Fuses, Circuit breakers, Protective relays malfunction due to harmonic currents.
  • Neutral cables get over heated due to addition of zero-phase sequence triplen harmonic current.
  • Due to the adverse effects of the harmonics, harmonics needs to be controlled.
  • There are two philosophies of harmonic control.
  • To eliminate or reduce the harmonics by taking care in the equipment design. This is essentially in the “Green Power Technology”.
  • Elimination or reduction of harmonics which are already generated by the non-linear equipment which was not designed to take care of harmonics.

In industries both these philosophies are prevalent for harmonics control.

Prevention of harmonics by design

1) Multipulse converter
3-phase rectifier consisting of 6-diodes having 6-pulse design is shown in Fig – 1. This is a basic building block of variable frequency drives, UPS systems, battery chargers and so on . . . This rectifier has typically 62% current distortion (THD).

1420808863 ER1412 Technology Power SB Mahajani 01

Please refer Fig – 2 showing the input current waveform. In order to reduce this current distortion by design multi-pulse converters are commonly used.

1420808878 ER1412 Technology Power SB Mahajani 02

Please refer to Fig – 3 showing schematic of 12-pulse rectifier. It consists of 12 diodes instead of 6 diodes. The two 6-pulse converters are connected to two secondaries of input transformer. One secondary is star connected and other secondary is delta connected to give 30º phase shift.

1420808890 ER1412 Technology Power SB Mahajani 03

The current distortion is reduced from 62% to about 8% in this configuration. The input current waveform of 12-pulse rectifier is shown in Fig – 4. This technology is further extended to 18-pulse or 24-pulse converters to further reduce the current distortion.

1420808899 ER1412 Technology Power SB Mahajani 04

This technique is used in high power rectifiers. For example in HVDC transmission (High Voltage Direct Current transmission) multiple converters are used. However, multi-pulse converters have disadvantage of using more number of devices leading to relatively poor efficiency. They also required intricate transformer design and balancing required for current sharing by multiple converters. Hence the state-of-art technology in converters is PWM converter.

2) PWM converter
The schematic of PWM converter is shown in Fig – 5. It uses 6-IGBTs in place of 6-diodes. PWM converter has following advantages.

  • It can work at unity power factor and it can also be made to operate at leading power factor to compensate for the poor power factor created by other lagging power factor loads.
  • It can work in both ways i.e. it can transfer the power from mains input to output as well as it can feed back power from regenerative loads to mains. Thus it can lead to energy conservation in case of some applications like Centrifuge.
  • It can stabilize DC link output voltage against fluctuations in mains input voltage.
  • All the above techniques of harmonics control are the examples of harmonics controlled by design. However, these techniques can not take care of existing harmonics in the plant. The equipments which controlled the existing level of harmonics are given in the following session.

1) Passive harmonic filter:
Passive harmonic filter consists of inductor and capacitor in series. This combination is tuned to the harmonics to be eliminated. The schematic of passive harmonic filter is shown in Fig – 6. The passive harmonic filter is simple and economical. It is very effective for applications where the load configuration is fixed and supply frequency is relatively constant. However, these filters have the following limitations.

These filters can be over loaded due to harmonic inrush current coming from some other load which can damage the filter.

  • This filter becomes less effective if the supply frequency varies.
  • If the load configuration changes this filter can not effectively filter the harmonics.
  • The inductor and capacitor used in the filter can resonate with power factor improvement capacitor used in the plant at some harmonic frequency.

To overcome these limitations Active Harmonic Filter is invented.

2) Active harmonic filter:

The schematic of active harmonic filter is shown in Fig – 7.

1420808912 ER1412 Technology Power SB Mahajani 05

Active harmonic filter has a current sensor connected in series with a non-linear load which is to be compensated to reduce the harmonics. The harmonic components of current in the non-linear load is sensed and equal and opposite current is generated by active harmonic filter. The current of active harmonic filter cancels the harmonic current of the non-linear load. As a result the source current is pure sinusoidal which does not contain harmonics.

Advantages of active harmonic filter

  • It reduces the harmonic current distortion by eliminating the harmonics.
  • The harmonics can be selectively eliminated by configuring the active harmonic filter in user programmable manner.
  • Like passive harmonic filter this filter does not resonate with any components of industrial plant.
  • This filter is dynamic by design and can adapt to changes in load configuration.
  • This filter can compensate for lagging power factor and it can also take care of 3-phase current balancing.
  • Thus active harmonic filter is a state of art solution for harmonic mitigation.

Conclusion :
Use of more and more non-linear loads is becoming common in today’s industrial plants. Therefore, harmonic elimination has become the necessity of the day. Depending upon the equipment used in the industrial plant different harmonic elimination techniques can be adopted. Whenever any new equipment is to be designed it should be designed to take care of harmonics by using multiple converter or PWM converter whereas, if the existing harmonics are to be taken care of either passive filter or active filter can be used.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar

P3 strives to bring you quality relevant industry related news.
See the origial article at: http://www.engrreview.com/Articles/State-of-the-art-solutions-for-controlling-harmonics/937650/110250/45150

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Sep
28

"Harmonic Distortion": The Invisible Electricity Thief You've Been Paying For

"Harmonic Distortion": The Invisible Electricity Thief You've Been Paying For

New Solutions Help Optimize Power Usage and Eliminate Unnecessary Power Costs

Did you know that 10 to 30 percent of your energy costs might have been wasted because of bad power quality?

Harmonic currents, which are generated by non-linear electronic loads in personal computers, laser printers, photocopiers, fax machines, battery chargers, UPS devices, switch-mode power supplies (SMPS), and variable speed motors and drives, are a frequent cause of power quality problems. In fact, power quality problems can have a significant impact on electrical distribution systems and the facilities they feed. All computer systems, for example, use SMPS that convert utility AC (alternating current) voltage to regulated low-voltage DC (direct current) for internal electronics. Because these non-linear power supplies draw current in high-amplitude short pulses, significant distortion in the electrical current and voltage wave can occur. This is known as "harmonic distortion." In addition, this distortion can travel back to the power source and affect the other equipment that is connected to the same source. Most power systems can accommodate a certain level of harmonic current, but will experience problems when harmonic currents become a significant component of the overall load. As higher frequency harmonic currents flow through a power system, they can reduce system efficiency; cause apparatus to overheat as well as cause misfires in variable speed drives and torque pulsations in motors; and increase power costs, among other things.

Learn more about Harmonics in this month's PQU Harmonics seminars: http://www.p3-inc.com/power-quality-university/seminar-info/harmonics-seminar

P3 strives to bring you quality relevant industry related news.

 

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