Technical Comparison of Harmonic Mitigation

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Techniques for Industrial Electrical Power Systems. S. F. Mekhamer, A. Y. ... between the various practical harmonics mitigation techniques in the industrial electrical ... Hence, this paper is considered as a helpful guide for design engineers ...
Proceedings of the 15th International Middle East Power Systems Conference (MEPCON’12), Alexandria University, Egypt, December 23-25, 2012, Paper ID 214.

Technical Comparison of Harmonic Mitigation Techniques for Industrial Electrical Power Systems S. F. Mekhamer, A. Y. Abdelaziz

Sherif M. Ismael

Electrical Power and Machines Department Faculty of Engineering Ain Shams University, Cairo, Egypt

Electrical Engineering Department Engineering for the Petroleum and Process Industries - ENPPI Cairo, Egypt

[email protected] Abstract - Power system harmonics cause many problems like equipment failures, malfunctions and plant shutdowns. Accordingly, mitigation of these harmonics is considered an important target especially for the industrial applications where any short downtime period may lead to great economic losses. There are at least ten different mitigation techniques to choose from, each with specific technical advantages and disadvantages. Comparative studies for the harmonic mitigation techniques in industrial electrical systems are rarely found in the literature even though they are strongly needed. This paper, almost for the first time, provides comprehensive technical comparisons between the various practical harmonics mitigation techniques in the industrial electrical systems. This has been carried out with the aid of our research and experience in the industrial field. Hence, this paper is considered as a helpful guide for design engineers, consultants and customers of the industrial sector. Index Terms - Harmonics, Distortion, Mitigation, Filter, Variable frequency drives (VFD).

I. INTRODUCTION Due to the dramatic increase in the applications and usage of nonlinear loads in the industrial applications (mainly, the variable frequency drives VFD's), the power system harmonics problems arise and represent a big obstacle against the wide application of the VFD's although they enhance system efficiency and provide great energy saving. The power system harmonics have many harmful effects including: - Overheating of generators, motors, transformers, and power cables that lead to early equipment failures - Failure of capacitor banks - Nuisance tripping to protection relays and circuit breakers - Interference to communication systems and sensitive electronic devices Accordingly, mitigation of the power system harmonics represents a great importance in the industrial electrical systems to increase system reliability, enhance operation economics and avoid unwanted equipment failure and process downtimes [1-2]. When an engineering consultant or an industrial plant owner decides to mitigate the harmonics within his plant, he will find out that there are at least ten harmonic mitigation techniques, each having advantages, thus the selection of the optimal mitigating technique will be a difficult task. Brief technical comparisons for some harmonic mitigation techniques are presented in [3], but authors did not

focus on the industrial sector as well as they did not take into account the various available mitigation techniques. Similarly, Ref. [4] introduces various harmonic mitigation product features but it is focusing only on the merits of the author's products without addressing the demerits as well. An extensive literature review over the past thirty years makes us ensure that almost there is no article compare and summarize the various available harmonic mitigation techniques especially for the industrial sector. Our paper goals can be summarized as follows: 1- To extract and highlight some design precautions that can lead to mitigation of power system harmonics 2- To investigate the various harmonic mitigation techniques with the pros and cons of each technique 3- To provide comprehensive detailed comparative studies between these various harmonic mitigation techniques to enable design engineers, consultants and plant owners of selecting the optimal harmonic mitigating technique for their plant II. HARMONIC MITIGATION TECHNIQUES First, harmonic mitigation design precautions (during the design stage of the project) are discussed followed by harmonic mitigation techniques after project completion or solutions for an existing facility. 1. Harmonic mitigation design precautions (during the design stage of the project): 1.1 Segregation of harmonic producing loads from sensitive loads Harmonic producing loads may be separated from sensitive loads so that sensitive loads are not influenced by loads with high harmonics as shown in Fig. 1. All heavy loads (in the order of several MVA) should have their own dedicated transformers, such as the practical case of the large drives or arc furnaces in a steel mill. These transformers should be suitably designed to avoid overheating. If the secondary buses are connected together via tie breaker, precautions should be taken to ensure that the harmonic source and sensitive loads are not simultaneously energized during this event [5].

1

1.4 Optimum selection of the generator pitch factor The magnetic circuit of an AC generator produces voltage harmonics that can be minimized through the arrangement of the stator windings. A particular harmonic can be eliminated from the generated voltage wave by choosing a pitch factor that eliminates this particular harmonic [6]. The magnitude of the pitch factor, KP, for any particular harmonic, n, is equal to: (1)

KP = Sin (n p / 2)

Fig. 1 Segregation of harmonic producing loads from sensitive loads

1.2 Using star/ delta transformers for trapping triplen (3rd, 9th…etc) harmonics Transformer connections can be utilized to reduce harmonics in a three-phase system by using delta connected transformers to block the third order (triplen) harmonics, the delta transformer connection provides a zero sequence trap for the triplen harmonics. This is accomplished by the fact that the third and multiples of third harmonic currents have the same magnitude and direction in all three legs of the delta connected transformer, accordingly there is no resulting change in the zero sequence flux, (dΦo/dt), thus the current in the line side of the transformer contains no third harmonics. Instead, these third harmonic currents circulate in the transformer primary windings causing an additional temperature rise. Fig. 2 explains the usage of the transformer delta connection to trap the triplen harmonics.

Fig. 2 Usage of delta transformer connection for trapping triplen harmonics

1.3 Optimum usage of the VFD and soft starters The variable frequency drive has great advantages such as speed control and energy saving. In addition, the VFD limits the motor starting current to the nominal operating full load current. Some design engineers use the VFD as a soft starter only, thus giving the benefit of reducing the motor inrush currents but at the same time injects a great amount of unwanted harmonics into the electrical network. In addition, the large cost of the VFD (the VFD cost is approximately 5 times the cost of the soft starter) limits its usage to the applications that need speed control or energy saving requirements.

Where, P: Winding pitch, the pitch of two conductors is normally defined to be at 180 electrical degrees apart n: Harmonic order With proper planning during the design phase of the project, certain harmonics can be eliminated from the generation units. Table 1 shows the effect of varying the generator pitch factor on the harmonic voltage content output from the generator. Table 1 EFFECT OF PITCH FACTOR VARIATION ON VOLTAGE HARMONIC MAGNITUDES Pitch factor

Fundamental

3rd

5th

7th

9th

2/3

0.87

0.00

0.86

0.87

0.87

4/6

0.95

0.58

0.00

0.59

0.95

5/6

0.96

0.71

0.26

0.26

0.97

6/7

0.97

0.78

0.44

0.00

0.78

By adjusting the winding pitch at any angle other than the default setting (180 electrical degrees), the magnitude of the fundamental and all harmonic components will be impacted. From Table 1, it can be concluded that the optimum pitch factor is (5/6) because it results in reducing the 5th and 7th harmonics, in addition, as clearly described in section 1.2, using delta connected transformer would filter the 3rd and 9th harmonics, thus finally a relatively good reduction in harmonics arising from the generation units is achieved. 2. Harmonic mitigation techniques after project completion or solutions for an existing facility: 2.1 Harmonic mitigation by using AC line reactors (AC chokes) The AC line reactor is the simplest and cheapest mean of mitigating harmonics. It is connected in series with an individual nonlinear load such as a VFD as shown in Fig. 3.

Fig. 3 AC line reactor used to mitigate harmonics created by a VFD

2

The impedance rating of the line reactor indicates the per unit impedance relative to its rated full load current. This effective impedance is proportionally reduced with the reduction of the actual load current [3]. The percent impedance, relative to a given load, is the voltage drop across that impedance caused by the fundamental load current flowing though this impedance. This can be explained by the following equation: Reactor impedance = [ If . Xf .

3 ] / V L-L

the IEEE 519-1992 guidelines where up to 15% to 40% of system loads are VFDs, depending on the stiffness of the utility network, the amount of linear loads, and the value of choke inductance [3, 4].

(2)

Where, If: Fundamental load current Xf: Reactance at fundamental frequency VL-L: Line to line voltage (RMS) Table 2 shows the effect of AC line reactor variation on the current harmonic distortion. TABLE 2 EFFECT OF LINE REACTOR VARIATION ON THE DRIVE CURRENT HARMONIC DISTORTION Line reactor rating

Drive total current harmonic distortion (THDI)

1%

80 %

3%

35 - 45 %

5%

30 - 35 %

Using AC line reactor for harmonics mitigation has the following advantages: • Cheapest solution for harmonic mitigation • Can provide moderate reduction in voltage and current harmonics • Available in various values of percent impedance • Provides increased input protection for VFD and its semiconductors from line voltage transients • Represents a damping for the short circuit faults • Simple and easy in installation On the contrary, using AC line reactor for harmonics mitigation has the following disadvantages: • Additional voltage drop across the reactor terminals is provided, line reactors greater than 5% are not recommended due to the excessive voltage drops • May require separate mounting or larger VFD enclosure • May not reduce harmonic levels to below IEEE519-1992 guidelines [7] • Fixed inductance value (Uncontrollable) • Produce large heat during operation 2.2 Harmonic mitigation by using DC reactors (DC choke) The DC reactor is simply a series inductance (reactor) installed on the DC link of the VFD as shown in Fig. 4. In many ways, the DC choke effect on the harmonic mitigation is comparable to an equivalent AC-side line reactor. The DC choke provides a greater harmonic reduction primarily of the 5th and 7th harmonics. If DC chokes (or AC line reactors) are applied to all the VFDs within a plant, it is possible to meet

Fig.4 AC Line rector and DC reactor applications in the VFDs

Using the DC choke for harmonics mitigation has the following advantages: • Packaged integrally within the VFD • Can provide moderate reduction in voltage and current harmonics • Provide less voltage drop than an equivalent line reactor • Cheap Using the DC choke for harmonics mitigation has the following disadvantages: • Provide less line voltage protection than AC line reactor • May not reduce harmonic levels to below IEEE 519-1992 guidelines • DC choke impedance is typically fixed by design (not field selectable) • DC choke cannot be installed at site because it must be installed by the VFD manufacturer only. 2.3 K-factor transformers and drive isolation transformers Underwriters’ laboratories (UL) and many transformer manufacturers established a rating method called the K-factor, for dry-type transformers, to evaluate their suitability for operation in a harmonic polluted environment. This factor is specifically defined for transformers that feed variable frequency drives [3, 8]. The K-factor is defined by the IEEE Standard C57.110.1998 [8] as follows: The K-factor is a rating optionally applied to a dry type transformer indicating its suitability for use with loads that draw non-sinusoidal currents. The K-factor can be calculated by the following equation:

k 



2  h  h 1

 I   h  IL

  

2

(3)

Where, h: Harmonic order Ih: Harmonic current at order (h) IL: Line current (or fundamental current) The K-factor indicates the transformer capability to supply various nonlinear loads without exceeding the rated temperature rise limits of the transformer. K-factor rated 3

transformers offer no means to reduce the magnitudes of harmonic current except that they act as line reactance as described in section 2.1, but the K-factor method allows the engineer to choose a dry type transformer that can withstand the harmonic distortion without damage or loss of performance. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. Drive isolation transformers are similar to the Kfactor transformers in that they offer line impedance similar to a line reactor and reduce the amount of harmonic current that is allowed to flow to the load but they do not reduce the harmonics generated from the drive itself. Generally, these transformers are 1:1 ratio transformers and are also used in combinations of connections to create the 12 pulse, 18 pulse and 24 pulse VFD configurations. K-factor transformers have the following advantages: • Can provide moderate reduction in voltage and current harmonics by adding line reactance to the VFD • Provides increased input protection for VFD and its semiconductors from line transients • Can be used in combinations to create special phase shifts for harmonic cancellation • Enhance performance reliability and reduce transformer damage due to excessive temperature rise compared with the standard transformers

From Table 3, the following points can be extracted:

K-factor transformers have the following disadvantages: • K-factor transformers by themselves are a method for “living with” harmonics but will not significantly reduce the harmonics over the less expensive reactor solution • Must be sized at full load ratings to match each drive or group of drives • May not reduce harmonic levels to below IEEE 519- 1992 guidelines • More expensive than standard transformers 2.4 Multi pulse drive configurations (6 pulse, 12 pulse, 18 pulse, and 24 pulse) Transformers can have several impacts on the application of power electronics. First, a transformer can help to isolate the power electronic loads from the power system with its impedance. Next, if the transformer has a winding connection other than a wye-wye or delta-delta, there will be an inherent phase shift that can be beneficial in harmonics cancellation. In fact, this phase shift is the key in minimizing the generated harmonic currents in the 12, 18, 24 and higher pulse power electronic equipment. The relation between the harmonics present in the system and the number pulses is as follows: hP= P.n ± 1

The six pulse drive is the simplest and least expensive drive. The input supply current to a six pulse VFD is approximately a square wave. The 12 pulse VFD is formed by connecting two 6 pulse rectifiers in parallel to feed a common DC bus. The input to these rectifiers is provided with one three-winding transformer. This transformer has double secondaries that are in 30o phase shift. The benefit of this arrangement is that in the supply side, some of the harmonics are in opposite phase and thus cancel each other. Theoretically, this transformer arrangement yields to eliminating the 5th and 7th harmonics. The 18 pulse VFD uses three-phase three winding transformer that make a nine-phase system and the equivalent of 20o phase shift which eliminate the 5th, 7th, 11th and 13th harmonics. The 24 pulse VFD has two 12-pulse rectifiers in parallel with two three winding transformers, thus having 15o phase shift. The benefit of this connection is that, theoretically, most of the low frequency harmonics are eliminated but the major drawback is the high cost. An extensive literature review over the past thirty years leads to the fact that there are no single article compare and summarize the various VFD configurations. Table 3 provides this novel helpful comparative study between the various VFD configurations.

(4)

Where, h: Harmonic order present in the VFD of the (P) pulse number P: Number of pulses of a VFD (typical values are: 6, 12, 18, 24…etc) n: Integer = 1,2,3,….

1- The 6 pulse VFD is the cheapest VFD but it creates the largest amount of harmonics. Accordingly, it could be used in the following cases: a. Where small number and low ratings drives are present in the system b. Where there are no strict harmonic limits imposed by the utility companies c. Where minimum initial cost is required by the plant owner 2- The 24 pulse VFD is the most expensive drive but it produces the least harmonic distortion so it could be used in the following cases: a. In the case of a high power single drive or large multi-drive installation (larger than 1 MW), a 24pulse system may be the most economical solution with lowest harmonic distortion b. Where there are strict harmonic limits imposed by the utility companies The conclusion is that the selection of the optimal VFD configuration is a technical and economical compromise which mainly depends on the drive rating, project budget and customer and utility harmonic limits. Using multi-Pulse VFD has the following advantages: • Harmonics are eliminated from its source (not propagating inside the electrical network), thus avoiding excessive energy losses within system cabling and avoid low power factor problems • Excellent harmonic control for larger drives (>100 kW) • Ensures IEEE 519-1992 limits compliance 4

TABLE 3 A COMPLETE TECHNICAL COMPARISON BETWEEN THE VARIOUS VFD CONFIGURATIONS Drive configuration

6 Pulse

12 Pulse

h6 = 6n ± 1

Harmonics created formula th

th

th

th

h 12 = 12n ± 1 th

th

th

th

th

18 Pulse

24 Pulse

h 18 = 18n ± 1

h 24 = 24n ± 1

Harmonics created orders

5 , 7 , 11 , 13 , 17 , 19th …etc

11 , 13 , 23 , 25 , 35th, 27th…etc

17th, 19th, 35th , 37th …etc

23th, 25th , 47th , 49th …etc

Least harmonic order created

5th

11th

17th

23th

Input transformer

Not required

Required

Required

Required

Not applicable

Primary: Star Secondary: double windings (Star / Delta)

Primary: Delta Secondary: Triple windings (Delta connected but phase shifted)

Four Input transformers (each Star/ Delta) but phase shifted from each other

Not applicable

30 °

20 ° or 15 °

15 ° or 7.5 °

Current THD (%)

25 – 40 %

9 – 11 %

3–4%

1-2 %

Meet IEEE 519 limits

No

Maybe

Yes

Yes Available and economic for large power rating drives, (> 800 kW)

Input transformer connection* (*Connections may vary according to manufacturer) Phase shift achieved* (*Phase shift may vary according to manufacturer)

Available ratings in the market (KW)

All (KW) ratings are available

All (KW) ratings are available

Available and economic for large power rating drives, (> 300 kW)

Overall dimensions (Relative to 6 pulse VFD)

100 %

150 % to 200 %

300 %

350 %

Installation options

Stand-alone installation or inside a standard switchgear

Stand-alone installation or inside a standard switchgear

Stand-alone installation

Stand-alone installation

Cooling

Air cooled

Air cooled

Air cooled or water cooled

Air cooled or water cooled

Complexity

Simple

Moderate

Complicated

Very complicated

Cost (Relative to 6 pulse VFD)

100 %

200 %

250 %

300 %

Using multi-Pulse VFD has the following disadvantages: • The higher the number of pulses the higher the drive cost and complexity • 18 Pulse and 24 Pulse VFD's are considered as economical solutions only for large power rating drives (>300 kW) • Special motor's insulation may be required when the motor is fed from a high pulse VFD system • Most of VFD’s are suitable to be installed in an indoor and clean environment only • Special cooling requirements may be required for larger VFD ratings 2.5 Passive tuned harmonic filters One of the most common and effective methods for control of harmonics in the industry is the use of passive filtering techniques that simply make use of combinations of inductances, capacitances and resistances to form a trap for specific harmonic orders. Passive filters can be classified, according to their connections, into single-tuned, damped and

high pass filters. Passive filters are designed (tuned) to provide a low impedance path to harmonic currents at a specific frequency called the tuning frequency (fr) or as bandpass devices that can filter harmonics over a certain frequency bandwidth [2, 3,9] thus preventing the flow of these harmonic currents into the electrical system. Single tuned passive filter circuit diagram, reactance response and impedance versus frequency characteristics are shown in Fig. 5.

Fig. 5 (a) Single tuned shunt filter circuit diagram, (b) Single tuned shunt filter inductive and capacitive reactances Vs. frequency response, (c)Single tuned shunt filter impedance Vs. frequency response

5

A shunt filter is said to be tuned to the frequency that makes its inductive and capacitive reactances equal. Passive filters are constructed by using capacitors and inductors in carefully selected combinations. For a simple single tuned filter, at the tuning frequency, the inductive reactance equals the capacitive reactance as follow: XL = XC

(at the tuning frequency, fr)

2..fr.L = 1/ (2..fr.C)

(5)



• •

(6) •

Solving (6) for (fr) leads to: (7) Where, XL: Inductive reactance XC: Capacitive reactance L: Inductance of the filter C: Capacitance of the filter fr: Tuning frequency of the filter A properly designed passive filter can remove 50-80% of the target harmonic current. In order to design a passive filter, some time-consuming calculations, computer models and simulations are often required. Large rating filters are divided into stages and are often switched in steps using electromechanical contactors controlled by a microprocessor controller that operates the stages based on the load power factor. If the harmonic producing loads are varying rapidly, as in the polystyrene and ethylene petrochemical plants, the passive filters inherent slow response time (up to 30 seconds per stage) can cause short-term overload conditions that may cause the capacitors in the passive filter to deteriorate prematurely. It should be noted that as the capacitors decline (losing microfarads due to overloading and aging), the filter tuning frequency shifts upwards in frequency and this is the reason of tuning the passive filter at 10% below the required tuning frequency. Recently, many manufacturers of low voltage passive filters have replaced the electromechanical contactors with solid-state silicon controlled rectifier (SCR) switching devices that have the advantage of improving the speed of response of the passive filter to cope with the fast load changes. There are other types of passive filters such as: second order damped filters, third order damped filters, and CType filters [10]. Passive filters have the following advantages: • Cheap • Simple circuitry • Easy maintenance • Provides power factor correction • A single filter can compensate for multiple drives Passive filters have the following disadvantages: • Harmonic analysis studies using computer aided softwares like electrical transient analyzer program (ETAP) are required for identifying the harmonics present

• •

in the system and their relevant levels, then the passive filter can be sized accordingly Man-hour and man-power are required by engineering consultants to perform the required plant harmonic analysis studies May not reduce harmonic levels to below IEEE 519-1992 guidelines Separate mounting and protective device (breaker/fuse) are required Care is needed in filter's components sizing to avoid overloading During light loading conditions, the passive filter may lead to leading power factors Each single tuned filter eliminate single harmonic order only

2.6 Active harmonic filters (AHF) Active harmonic filter (AHF) is an intelligent and interactive filter. The basic principle of the active filter is that it generates a current equal and opposite in polarity (180° phase shifted) to the harmonic current drawn by the nonlinear load and injects it to the point of common coupling (PCC). Accordingly, the active filter forces the source current to be pure sinusoidal as clearly shown in Fig. 6. The characteristic of harmonic compensation is strongly dependent on the filtering algorithm used for extraction and calculation of the harmonic load current [2, 9, 11]. The current waveform generated from the active harmonic filter is produced by using a voltage source or current source inverter. The voltage source inverter is more commonly used in the active harmonic filters' market. The desired current waveform generated by the filter is strictly dependent on controlling the switching of the inverter switches that are usually insulated gate bipolar transistors (IGBT).

Fig. 6 Priniciple of operation of the shunt active harmonic filter

As shown in Fig. 7 and Fig. 8, there are two types of active filters: • Shunt active harmonic filter • Series active harmonic filter

6

• Can be designed and allocated into main distribution switchgear to provide global harmonic compensation for several VFDs • Extendable solution, many AHF units can be connected in parallel in order to achieve higher power requirements. Most active harmonic filters disadvantages’ are:

Fig. 7 Shunt active harmonic filter

• Typically the AHF is considered the most expensive mitigation solution due to its intelligent performance and sophisticated control • Series unit must be sized to carry the full load current and insulated for full line voltage, and accordingly their cost are even much higher the shunt filters. • Difficult maintenance and needs for specialized personnel Table 4 introduce a comprehensive comparison between the passive and active harmonic filters TABLE 4 COMPARISON BETWEEN THE PASSIVE AND ACTIVE HARMONIC FILTERS Passive harmonic filters Harmonic analysis studies required Target harmonics Meet IEEE 519 limits

Fig. 8 Series active harmonic filter

Series AHF’s are not commonly used in the market because they are designed to carry the full load current and are insulated for full line voltage, hence their cost are much higher than shunt filters. The active harmonic filter main components are summarized as follows: a. Power section, which contain an input filter, inverter and energy storage components (capacitors) b. Control section, which contain a harmonic current extraction circuit, inverter control circuit and capacitor voltage monitoring circuit Active harmonic filters have many advantages including the following: • Extensive harmonic analysis studies are not required because the active filter mitigate all the harmonic present in the system automatically • Save man-hour and man-power required by engineering consultants to perform the plant harmonic analysis studies • Avoids series and parallel resonance problems • Guarantees compliance with IEEE 519-1992 limits • Shunt AHF cannot be overloaded even if future harmonic loads are added • Provides harmonic cancellation from the 2ndto 50th harmonic components • Shunt AHF provides easy installation with no major system rework or interruption. • Provides reactive power compensation which improves the system power factor

Possibility of overloading

Yes Only single harmonic order (typically 5th) Sometimes, if properly sized Yes, due to slow response and/or variation of harmonic loads

Active harmonic filters No From 2ndto 50th Yes No. AHF current is limited to maximum RMS current rating of the filter Yes. Addition of parallel units is possible. Only low voltage AHF is available, higher voltage levels can be achieved by using step up transformer

Field expandable

Sometimes

Available voltages

Low voltage and medium voltage passive filters are available

Response to load changes

Typically 30 seconds delay with electromechanical contactor based systems

Installation options

Indoor or Outdoor

Indoor only

Mainly defined by

Target harmonic order and the amount of reactive power injected

RMS current of the filter

Yes

No

Easy Cheap

Difficult Most expensive solution

Probability of series or parallel resonance Maintenance Cost

Very fast response (less than 1 cycle)

From Table 4, we conclude that the AHF has many advantages over the passive filter, but the major drawback of the AHF is its great cost that may reach four times the cost of the passive filter. 2.7 Hybrid harmonic filters In order to extend the application range of the active filters, and to improve the performance of passive filters, another approach consists of combining both technologies within the same device called (Hybrid) filter [12]. The available hybrid filter configurations are parallel/ series type 7

hybrid filter and series/ parallel type hybrid filter. Both hybrid filter configurations are shown in Fig. 9 and Fig. 10 respectively.

• Filtering a wide frequency range (elimination of harmonics numbered 2nd to 50th) • Compensation of reactive power and improving power factor Hybrid harmonic filter has many disadvantages including the following: • Care should be given to ensure that the passive portion of the hybrid filter will not be overloaded • Disadvantages of the passive filter and active filter described above in sections (2.5, 2.6) are valid for hybrid filter • Expensive, difficult maintenance solution

Fig. 9 Parallel / Series type hybrid filter

Each harmonic mitigation technique has many advantages and disadvantages as described earlier in this paper. Furthermore, Table 5 provides a novel helpful comparative study between most of the harmonic mitigation solutions. III. CONCLUSION

Fig. 10 Series / Parallel type hybrid filter

Parallel/ Series hybrid filter main components can be summarized as: a. One (or more) bank(s) of resonant passive filters (Fi), parallel-connected across the nonlinear load(s). b. An active harmonic filter, made up of: ‐ A magnetic coupler (Tr), the primary of which is inserted in series with the passive filter(s), ‐ Active harmonic filter (AHF) connected to the secondary of the magnetic coupler. The hybrid filter is composed of a combination of passive and active filters. The passive filter portion is tuned to the dominant harmonic frequency in the system and is supplying the required reactive power for power factor correction requirements. The active filter portion is dedicated for removing all other harmonic orders. The active filter must be programmed to ignore the harmonic frequency to which the passive filter is tuned to avoid any negative interaction between the two elements in the hybrid configuration. With the reduction of size of the active filter portion and the supply of reactive energy from the passive portion, the hybrid technology offers an optimum economical solution for the most demanding high power installations. It combines the advantages of both passive and active filters technologies. Hybrid harmonic filter has many advantages including the following: • Combine the advantages of passive and active filtering solutions

Mitigating the power system harmonics represents a great importance in the industrial applications in order to increase system reliability, enhance operation economics and avoid unwanted equipment failure and process downtimes. There are at least ten harmonic mitigation techniques, each having many pros and cons. Previous articles addressed limited specific harmonic mitigation techniques in details. This paper provides a comprehensive technical comparison between most various harmonics mitigation techniques highlighting the pros and cons of each technique to enable the design engineers, consultants and plant owners of selecting the optimum harmonic mitigating technique for their plant. REFERENCES [1] Mohamed Zaki El-Sadek, Power System Harmonics, 2nd edition, 2007, Mukhtar Press, Egypt. [2] Mohamed Zaki El-Sadek, Power System Harmonic Filters, 1st edition, 2007, Mukhtar Press, Egypt [3] Daniel J. Carnovale, Thomas J. Dionise, Thomas M. Blooming, “Price and Performance Considerations for Harmonic Solutions”, unpublished. [4] Schneider Electric, Harmonics Solution Handbook, 1st edition, June 2009 [5] IEEE, Recommended practice for industrial and commercial power systems, ANSI/ IEEE 399-1997, Chapter 10, PP. 265-312 [6] John P. Nelson, “A Better Understanding of Harmonics Distortions in the Petrochemical industry”, IEEE Petroleum and Chemical Industry Conference, 2002, PP. 237-250 [7] IEEE, Recommended Practice and Requirements for Harmonics Control in Electrical Power Systems, ANSI/ IEEE 519-1992 [8] IEEE, Recommended Practice for Establishing Transformer Capability when Supplying Non-sinusoidal Load Currents. ANSI/IEEE C57.1101986. [9] Francisco C. De La Rosa, Harmonics and Power Systems, 1st edition, 2006, CRC press [10] Jos Arrillaga, Neville R. Watson, Power System Harmonics, 2nd edition, 2007, John Wiley & Sons Press [11] Schneider Electric Cahier technique no. 183, “Active harmonic conditioners and unity power factor rectifiers”, 1st edition, June 1999 [12] Schneider Electric Cahier technique no. 152, “Harmonic disturbances in networks, and their treatment”, 1st edition, Dec. 1999

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TABLE 5 A COMPLETE TECHNICAL COMPARISON BETWEEN MOST OF THE HARMONIC MITIGATION SOLUTIONS Harmonic mitigation solution

AC line reactor

DC reactor

K-factor transformer

Multi-pulse VFD

Passive filter

Active filter

Hybrid filter

Current THD (%)