Electromagnetic Compatibility and Power Quality Problems at Low Frequency for Loads from an Urban

Transportation System and in a Power Substation Petre-Marian Nicolae #1, Ileana-Diana Nicolae*2, Marian-Ştefan Nicolae #3

#1 / 3 Electrical Eng., Energetics and Aeronautics Dept., University of Craiova, *2 Computer Science Dept., University of Craiova,

Decebal Blv., no. 107, Craiova, Romania 1 pnicolae@elth.ucv.ro, 3 nmarianstefan@yahoo.com, 2nicolae_ileana@software.ucv.ro

Abstract— This paper deals with energy/power quality issues,

correlated with Electromagnetic Compatibility (EMC) and electromagnetic interferences (EMI) which appear at urban transportation systems. One presents, briefly, some aspects regarding powers definitions for harmonic and/or unsymmetrical regimes from three-phase circuits, applicable to loads supplied from the a.c. line through low voltage power transformers and uncontrolled three-phase rectifiers. It is analysed the case of a driving system with d.c. motor and chopper, supplied from the d.c. line, which induces a significant non-sinusoidal regime in the a.c. network. The cases in which the load is not compensated, respectively compensated through an active filter are also analyzed through tests on stand. Aspects regarding powers and power flows for both cases are also discussed. Furthermore, the obtained data are compared using the quality standards IEEE 519/1992 and 1459/2000. One discusses the obeying of quality standards and some aspects, which are not approached in the specialty literature, concerning the corresponding EMC and EMI issues. Problems related to the tested driving system connection to the supplying system from the power substation for an urban transportation system are briefly presented. The waveforms of currents through the active filter are depicted. Some conclusions on approaching EMC and EMI at low frequencies, with an extension over 2 kHz and the relation with the IEC 61000-4-30 standard are also presented. It is underlined the necessity of treating energy/power quality issues in correlation with issues regarding EMC and EMI at low frequency for this type of loads.

I. INTRODUCTION At loads level (either single or three-phase type), owing to

power electronics intensively used in electric driving systems, many new specific problems appear and are not yet approached in the specialty literature. Power flows from superior harmonics can have reverse circulation direction as compared to the power flow from the fundamental harmonic (useful power for the load). Standards establish only limitations concerning harmonic currents (e.g. IEEE 519/1992 Standard or 1459/2010 Standard) [1], [18]. At the loads level the technical issues are not entirely solved. Sometimes, even the problem formulation is wrong! This happens because aiming to solve issues related to energy/power quality for high loads, the corresponding EMC issues are neglected or improperly handled. Additional powers unused by load can form important sources of EMI along with other equipment,

even for low frequencies (under 2 kHz). Even if one performs load compensation from the distorting

regime point of view by means of active or hybrid filters, in order to get at consumer the power/energy quality improvements, the problem is not correctly approached. This happens because the load compensation actually protects the network against the unwanted harmonics generated by load, and the compensation is considered in terms of harmonic currents limitation, with and without filter’s presence. In fact, under these circumstances, a non-sinusoidal load pollutes with harmonic currents the network, and actually causes (at least) the distortion of voltages across the load supply terminals.

The filters which act in these situations have to produce separately powers which correspond to the additional harmonics (not to the fundamental one!). The effect would be that the currents through filter waveform are strongly distorted compared to currents for the fundamental harmonic. In turn, these filters generate significant interference energy, not approached in the specialty literature! Filters cannot be used for its diminishing; this should affect quite the reason why these filters have been made. Additional studies should be developed in order to limit the EMI effects, resulted from assembling filters near high loads. Only filter equipment screenings cannot solve the EMC related issues in these situations.

In all cases, regardless if we are speaking about high power sources or significant loads, it is clear that we must correctly quantify the loads that are useful or useless to the power system and the elements from its componence, because additional unused powers/energies will turn into RF energy, which will interfere with the RF energy from nearby equipment [2]. As this energy is significant, the effects can also be significant from the EMC point of view [3], [4].

II. THEORETICAL CONSIDERATIONS ABOUT POWERS Specialty literature provides different definitions for powers

in three-phase systems because it is difficult to define power categories for non-sinusoidal and/or non-symmetrical regimes. The efforts have been focused in the direction of defining powers based on Fourier decomposition for the non-sinusoidal waveforms of voltages and currents, followed by the definition of some power spectral components. There are still debates at theoretical level regarding the correct definition for various

978-1-4799-0409-9/13/$31.00 ©2013 IEEE 242

power categories. Recent efforts were made to define same power categories, when other transforms (e.g. wavelet) are used.

Below are presented some concepts regarding these powers.

A. Powers Definition Contained in the IEEE Standard 1459 for Unbalanced and Nonlinear Systems

IEEE 1459-2000 standard provides definitions for powers in non-symmetrical and nonlinear regimes. Based on them improvements were provided in a series of previous works [1], [5]. For example, in order to make these theories coherent, in [6] the definitions of Ve and Ie are modified as follows:

3/222cbae VVVV ++=

(1)

3/222cbae IIII ++=

(2)

With (1) and (2) a new expression of the effective apparent power (Se) is defined:

2 2 2 2 2 2 2( ) ( )e a b c a b cS V V V I I I= + + + + (3)

which yields a new expression of the fundamental effective apparent power:

2 2 2 2 2 2 21 1 1 1 1 1 1( ) ( )e a b c a b cS V V V I I I= + + + + (4)

1eS can be expressed by means of the fundamental symmetrical components in the form:

( ) ( ) ( ) ( ) ( ) ( )2 2 2 2 2 22 0 01 1 1 1 1 1 1eS V V V I I I+ − + −⎡ ⎤ ⎡ ⎤= + + + +⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦

(5)

With the proposed expressions for eV , eI and eS , the definitions of the new unbalanced power and the new non-fundamental effective apparent power are:

( )22 21 1 1U eS S S += − (6)

and: ( ) ( ) ( )2 2 2 2 2 2 2 2 2

1 1 19 . . .eN e e e eH eH e eH eHS S S V I V I V I⎡ ⎤= − = + +⎣ ⎦ (7)

( 2 2 21e e eHV V V= + and 2 2 2

1e e eHI I I= + ). In this theory the power factor and total harmonic distortion

are merit factors of the electrical systems. IEEE Standard 1459-2000 defines the effective power factor ( ePF ), the

fundamental positive-sequence power factor ( 1PF + ), and the

equivalent total harmonic distortions ( eVTHD and eITHD ). The total power factor measures the relationship between

the active power under ideal operation conditions and Se:

eT SPPF /1+= (8)

B. A Theory that Uses the Powers’ Definition Based on the Real and Imaginary Powers Akagi, a.o., suggested the definition of a new variable [7],

called “instantaneous imaginary power, q(t)” or pi(t) that is not influenced by the zero-sequence components:

( ) ( ) ( ) ( ) ( ) ( )iq t p t v t i t v t i tβ α α β= = − (9) Based on these definitions, the (re)active powers,

considered as averaged quantities along a period of the real and imaginary instantaneous powers will be obtained as [8]:

- Active power:

( )

3 cosk k kk

p U I ϕ⎡ ⎤

= ⎢ ⎥⎣ ⎦∑ (10)

- Reactive power:

⎥⎦

⎤⎢⎣

⎡ −= ∑∑+=+= 2313

sinsin3mk

kkkmk

kkk IUIUq ϕϕ (11)

Starting from this conception, the most recently publications present a series of variants that use transformations of co-ordinates toward co-ordinates for electric quantities, all starting from the p-q theory. However all these formulations make use of definitions for powers that should provide compensation solutions for the distorting effects and do not provide solutions for their measurement and quantification in quantities that should provide solutions for consumers’ charging.

Moreover, the theory of instantaneous real and imaginary powers cannot approach the power factor, as this theory is unable to define an apparent power (of any nature). Therefore the full load compensation cannot be considered within this theory, only the diminishing of current and/or voltage harmonics being realistic.

C. Powers Definition using Budeanu’s Theory The definitions issued by the Romanian Academician

Constantin Budeanu in 1927 for reactive and fictitious powers were used when obtaining the generalization of the formulas used for powers calculation in the single phase case [9].

Within this theory the powers are considered as bi-linear forms and the active and reactive powers for the case of three-phase systems with four wires are expressed as follows (for a three phase systems with non-symmetrical and nonlinear load) [9]:

- for the active power:

1 2 3P P P P= + + (12)

(where 1 2 3, ,P P P means the active powers of phases); - for the reactive power:

1 2 3Q Q Q Q= + + (13)

(where 1 2 3, ,Q Q Q means the reactive powers of phases). Within this theory the distorting and apparent powers (both

for the fundamental harmonic and at global level – for all harmonics) are defined. When apparent powers are defined, a power factor for a three-phase load can be defined [10].

Due to the still existent controversy at the theoretical level, the IEC 61000-4-30 Standard [19], even if addressing Power quality measurement methods and presenting in detail the measurements and testing techniques for voltages and currents, still does not make clear specifications related to various power categories measurements and power factor improvements for three-phase systems, or for high power energy supplies and for connected loads in power systems.

243

Providing that the Discrete Wavelet Transform is used to yield decompositions of currents and voltages waveforms in “approximations” and “details”, one can define (non)active, reactive, apparent powers, power factors a.o. for the distorted waveforms submitted to decompositions [11]. These definitions are similar to those based on Fourier decompositions for voltages and currents, which make use of FFT and subsequent definitions of certain spectral power components [12 ].

The common point of these theories consists in the active power definition and determination for any practical case.

III. THE ANALYSIS ON TEST STAND OF A LOAD FROM A URBAN TRANSPORTATION SYSTEM WITHOUT AND WITH A

COMPENSATION SYSTEM Usually the effects of filters (mainly the active ones) are

evaluated relative to the network to which the load – “subject of compensation” is connected. The filters’ main role is the diminishing of harmonic pollution for the low or medium voltage network. The network protection against the harmonic currents introduced in the a.c. network should also be accompanied by the power factor improving with respect to the fundamental harmonic (to solve the power quality problems).

Usually the filter efficiency is evaluated only in the terms of harmonic pollution diminishing (mainly for currents), often neglecting other measures, meant to get full load compensation. Such additional techniques should consider the diminishing of both voltage/current harmonics (therefore diminishing the powers along superior harmonics) and respectively the diminishing of reactive power consumption along the fundamental harmonic relative to the load’s supplying source in the connection point.

Actually, the filters should create locally powers along the high order harmonics and should create reactive power, capacitive or reactive (depending on the load specificity) along the fundamental harmonic.

But does the passive, active or hybrid filtration provide a solution to all technical issues which might occur at such consumers? Is if enough, for example, an approach in which standards mention only that through active filtration one should get the harmonic currents diminishing up to an accepted level for practice? One should consider that, after filtration, current and voltage harmonics from a consumer supplying, even if they obey quality standards (e.g. IEEE 519/1992), are not set to zero [18]. This means that, even if the weight of the remaining harmonics is not so harmful, the harmonics related powers are still relatively significant for the EMI’s created by means of the conductors connected to the loads submitted to the filtering process [13]. One must mention that quality standards provides a limit for harmonic levels used in Fourier decomposition (the European standard for energy quality at consumers limits this number to 40, which corresponds to 2 kHz, whereas the IEEE 519/1992 standard limits it to 50, corresponding to 3 kHz). This shows the standards limitations, as they approach only issues of energy/power quality without any correlation between them and EMI issues for the same frequency ranges or for any broader

ones. Therefore the issues related to low frequencies operation (that is the interval from d.c. to 150 kHz for the EMC area) are disconsidered. Moreover, the switching issues at static power convertors can generate EMI issues for frequencies higher than those from the energy/power quality standards [14].

Another technical issue to be addressed refers to the EMI for active filters which used to diminish the harmonics from a consumer of low or medium voltage. Through design, these filters have to provide signals only for superior harmonics (strongly distorted compared to the fundamental harmonic), and the currents from filters injected to consumers might create significant EMI issues. At active filters, the EMC related issue, consists in the fact that quite these distorted currents are useful to compensation! This classifies as unusable the strategies addressing EMC that are used at a common inverter (where the filter is usually an inverter – voltage source or current source). In active filtering, the filtration of signals needed for compensation will be missing. other strategies for the EMI reducing being required.

To provide a detailed picture of the above, below is presented the case of a d.c. motor, supplied through a chopper from a d.c. source that is supplied through a rectifier with diodes and a network transformer from the LV network at a test stand.

The experiments are related to a complex system, designed for the compensation of harmonics and power factor (for the fundamental harmonic) from a system used to supply an electric urban transportation system (tram, trolleybus). Initial experiments were performed in a transformation substation in order to accurately determine the currents and voltages that supply the d.c. line used by trams. In that context we have also determined the powers for various loads of the supplying line, with different harmonics (according to EN 50160 standard) [15].

A. Behavior of a traction motor for urban vehicles, supplied from a chopper without compensation

The schematic used for a traction motor for urban vehicles, supplied from a chopper without active compensation on the test stand in order to evaluate the steady-state and transient regimes and the behavior of the d.c. motor of 150 kW, supplied from a step-down chopper is depicted by Fig. 1. In this schema a transformer of 220V-380V a.c. supplies a three-phase rectifier with diodes. whose output voltage is approximately 750 V d.c. The rectifier’s output is used to supply a step-down chopper (Fig. 2), whose output voltage is approximately 600 V d.c., being used to supply the d.c. motor with series excitation. The motor is coupled on the same shaft with an asynchronous generator (when regenerative braking are accomplished).

Fig. 1. Electric schema for tests without active filter

244

Fig. 2. Chopper – front view.

Fig. 3. Phase voltage that supply the three-phase rectifier with diodes (up) and currents absorbed by rectifier (down).

The waveforms of the supplying phase voltages and of the three-phase currents absorbed by the d.c. motor through chopper were recorded with a performing data acquisition system, specially designed for this application, placed as in Fig. 1. The currents absorbed by motor were recorded in the secondary winding of the raising voltage transformer from the test stand (Fig. 3).

Using an original decomposition algorithm, relying on Fast Fourier Transform (FFT), we performed a detailed harmonic decomposition of the waveforms of phase voltages and currents.

Below are provided some power quality parameters for the case when the load was not compensated (the harmonic distortions of the currents absorbed by the load are considered):

- The phase voltages RMS values: V1-rms = 379.52 V; V2-rms = 379.32V; V3-rms = 378.06 V - The total harmonic distortions of the phase voltages: VTHD1 = 1.39 %; VTHD2 = 1.5 % ; VTHD3 = 1.61 % - The phase currents RMS values: I1-rms = 79.09 A; I2-rms = 80.18 A; I3-rms = 78.39 A. -The RMS values for fundamental harmonic: I(1)

1-rms = 69.87 A; I(1)2--rms = 70.79 A; I(1)

3-rms = 69.67 A - The total harmonic distortions of the phase currents: ITHD1 = 46.85 %; ITHD2 = 46.95 %; ITHD3 = 45.83 %. Considering the previous definitions, one could determine,

based on the above mentioned harmonic decomposition the total (re)active and distorting powers (considering Budeanu’s theory) – Fig. 4. The apparent power and the power factor for the coupling

Fig. 4. Spectral components of the (re)active powers, absorbed from the supplying source, before compensation

point of the load (consisting in the three-phase rectifier with diodes, that supplies the d.c. motor through a step-down chopper) were evaluated as well, resulting the following values:

S = 90.07 kVA; P = 70.052 kW; Q = 10.771 kVAr ; D = 41.787 kVAd

Power factor: PF = 0.877 (with a capacitive character). The experimental data reveal that a part of the absorbed

powers are additional powers, that affect the operation of the assembly load (chopper+motor)-supplying network. As the determinations were performed in the secondary winding of the network’s transformer, it is obvious that a part from the flows of powers (active, reactive, distorting) affect the operation of the load’s supplying network. Considering the quality of energy/power, it is obvious that measures must be taken in order to reduce mainly the current harmonics in this case and to improve the power factor along the fundamental harmonic. The reversed power flows result into electromagnetic interferences between load and the supplying network. The visible harmonic distortion of the currents absorbed by load also affects the EMC. When the same quantities are determined within the substation used to supply the supplying line of the d.c. network, similar values (30%...40%) are obtained for the currents’ harmonic distortions. In these cases, in order to limit the transmision of disturbances through conduction, line filters and filters for the intermediate circuit are used. Despite these measures, owing to the broadening of the frequency range, many EMI related issued were noticed [15]. They are correlated with the limits imposed to the solving of energy/power quality issues. The conclusion is that one should use some filters for the superior harmonics corresponding to the connecting circuits. Should filtering solve the compatibility issues too?

245

B. Behaviour of a traction motor for urban vehicles, supplied from a chopper with an active compensation The schema with an active filter conected in the load

vecinity is depicted by Fig. 5. Below are provided some power quality parameters for the

case when the load was compensated by an active filter: - The phase voltages RMS values: V1-rms = 377.98 V; V2-rms = 377.21V; V3-rms = 376.35 V - The total harmonic distortions of the phase voltages: VTHD1 = 2.02 %; VTHD2 = 1.73 % ; VTHD3 = 2.01 % - The phase currents RMS values: I1-rms = 70.06 A; I2-rms = 71.04 A; I3-rms = 69.61 A - The total harmonic distortions of the phase currents: ITHD1 = 6.24 %; ITHD2 = 6.47 %; ITHD3 = 6.63 %. Considering the previous definitions, one could determine,

based on the above mentioned harmonic decomposition the total (re)active and distorting powers (considering Budeanu’s theory). The apparent power and the power factor for the coupling point of the load (consisting in the three-phase rectifier with diodes, that supplies the d.c. motor through a step-down chopper) were evaluated as well, resulting the following values:

S = 79.48 kVA; P = 79.024 kW; Q = 6.696 kVAr ; D = 5.224 kVAd

Power factor: PF = 0.994 (with an inductive character). When compared with the standard IEEE 519/1992 or with the

European norm EN 50160, the currents’ harmonic distortions are reduced through active filtering when one intends to compensate the higher harmonic currents such as to obey the specification ITHD<8% and respectively to improve the power factor along the fundamental harmonic (over the neutral value of 0.92). So, the prescriptions of quality standard are obeyed, but what can we say about the EMC and EMI issues?

IV. THE ANALYSIS FROM THE EMC POINT OF VIEW The performed analysis revealed that in both situations

Fig. 5. Test schema with active filter of type shunt

Fig. 6. 1-st phase currents without filter (red) and with filter (blue) - up and their difference (green) - down

(with or without compensation) there are additional energies/ powers required by load. This might stand for an EMI-related energy for other equipment from neighborhood. In both situations the RMS values of the phase currents for the fundamental harmonic are almost identical, even though the total active power from the case with compensation is smaller. It is the effect of some additional powers with significant values that occur when the compensator is missing. In the same situation one can notice inverse flows of active powers along the superior harmonics, whose senses are from the load toward the network. They result into some energy of interference with the network through electric conduction. The active filter’s connection does not reduce this energy, but prevent this additional energy from penetrating the supplying network. Practically the active filter provides a flow of power/energy between itself and the load such as this energy does not need to penetrate the network along the superior harmonics. The extraction from the network of the powers/energies required for load supplying along the fundamental harmonic is not affected. Therefore, when one designs the cables used to connect the active filters to the network and load it is considered that theoretically there are no radiations emitted through the supplying cables [16] – according to the immunity standard for industrial environments IEC 61000-6-2 and to the standard for emissions for industrial environments class A, IEC 61000-6-4, and consequently no special shielding of the power cables should be imposed.

Fig. 6 depicts the difference between the currents without filtering and respectively with filtering for the first phase.

246

Similar waveforms were raised to the other two phases. The current harmonic for the fundamental frequency will not be found at the harmonic decomposition.

V. CONCLUSION The currents yielded by filter are significantly distorted and

they flow through the cables connecting the filter and the load. Therefore they must be as short as possible, such as to avoid issues related to electromagnetic interferences with neighboring equipment through conducted emissions.

A correct analysis should be performed and added to the PQ and EMC standards, with specific paragraphs related to the generated or consumed powers.

Usually during the analysis of PQ and energy efficiency improvement one approaches the quality of harmonic currents, disregarding the actual powers/energies. The analyzed example proved that, whichever the powers definition system is used, an additional power(s) should always exist (in the analyzed case – distorting power). This behaves like an EMI source for conducted emissions at low frequency – in any circumstances (with or without compensation).

The limits (40 or 50) imposed to the greatest harmonic order to be considered does not offer the possibility to extend the analysis for EMI issues at low frequency.

Not only the harmonic decompositions of voltages and currents must be done, but also a determination of powers per each consumer should be done, to deal with EMC issues.

For the analyzed case, the currents yielded by filter will penetrate the supplying cables toward loads, making impossible their filtering. Therefore other designing methods should be considered such as to address the EMC issues.

The supplying source is not a harmonic source by itself, its only role being to supply the load. The harmonics’ source is the complex load (composed by the rectifier with diodes+ chopper+d.c. motor). The analysis of the distinct contribution of the rectifier and chopper to the harmonics’ generation was beyond the scope of this paper. The currents absorbed from source (Fig. 1) contain significant harmonic components for many harmonic orders (5, 7, 11, 13, 17, 19, 23, 25,…), which are also reflected by the spectral components of the (re)active powers from Fig. 4. The active filter significantly reduces the harmonic components of currents and powers. After compensation, both low and high harmonics are diminished with similar ratios.

A positioning of filter in the supplying substation for the d.c. line before the rectifier results into a compensation over the energy absorbed from the MV/LV substation. The cables connecting the filter and rectifier must be shielded.

The analysis of currents and especially of the powers absorbed from the a.c. network proves the filter’s efficiency in terms of active compensation: the active power absorbed in the presence of active filtering is higher than that when no filtering is performed, for the same currents absorbed from network along the fundamental harmonic. Another advantage of active filtering consists in the improvement of the power factor relative to the supplying network. Despite all these, the active filtering yields a part of the currents absorbed by load (along

superior harmonics), this meaning that additional (generally undesired by the load) powers should be absorbed by it. The corresponding additional energy will affect the d.c. motor, through conduction (owing to EMI currents). Even if the operating frequencies are low, the undesired currents will reduce the motor’s lifetime owing to additional heating. In this case the EMI is not reduced but those who design such driving systems do not take corresponding protective measures!

Therefore new EMC related standards addressing the above mentioned problems should be conceived.

REFERENCES

[1] A.E. Emanuel, “Summary of IEEE standard 1459: definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced or unbalanced conditions”, IEEE Trans. on Ind. Appl., vol. 40, no. 3, pp. 869-876, May/June 2004

[2] R.B. Timens, F.J.K. Buesink, V. Ćuk, J.F.G. Cobben, W.L. Kling, F.B.J. Leferink, “High Harmonic Distortion in a New Building due to a Multitude of Electronic Equipment”, in Proc. of IEEE ISEMC, 2011, pp. 393-398

[3] R. B. Timens, F. J. K. Buesink, F. B. J. Leferink, “Voltage Quality in Urban and Rural Areas”, in Proc. of IEEE ISEMC, 2012, pp.755-759

[4] S. K. Rönnberg, M. Wahlberg, M.H.J. Bollen, “Harmonic Emission before and after Changing to LED lamps – Field Measurements for an urban area”, in Proc. of IEEE ICHQP 2012, pp. 1-6

[5] S. Pajié and A. E. Emanuel, “Modern Apparent Power Definitions: Theoretical Vesus Practical Approach-The General Case”, IEEE Trans. on Power Delivery, vol. 21, no. 4, October 2006, pp 1787- 1792.

[6] [6] S. Orts-Grau, N. Muñoz-Galeano, J.C. Alfonso-Gil, F.J. Gimeno-Sales, and S. Segui-Chilet, “Discussion on Useless Active and Reactive Powers Contained in the IEEE Standard 1459”, IEEE Trans.on Power Delivery, vol. 26, no. 2, April 2011, pp. 640-649.

[7] H. Akagi, S. Ogasawara, and H. Kim, „The theory of instantaneous powers in three-phase four-wire systems: a comprehensive approach”, in Proceedings of IEEE Ind. Appl. Conf., 1999, vol. 1, pp. 431- 439

[8] [8] P.M. Nicolae, „Instantaneous real and imaginary powers at three-phase networks with balanced loads that function under distorting regime”, Revue Roumaine des Sciences Techniques (RRST), Serie Electrotechnique et Energetique, no. 3, 1995, pp. 311-319

[9] I.S. Antoniu, and M. Gafencu, „L’expresion des puissance dans une systeme triphase desequilibre et deformant en fonction des composantes symetrique”, in Revue Roumaine des Sciences Techniques (RRST), Serie Electrotechnique et Energetique, Tom 22, no.1, 1977, pp. 3-10

[10] A. Tugulea, “Criteria for the definitions of the electric power quality and its measurement systems”, in ETEP, vol. 6, no.5, 1996, pp.357-363

[11] W.G. Morsi, M.E. El-Hawary, “Wavelet Packet Transform-Based Power Quality Indices for Balanced ad Unbalanced Three-Phase Systems under Stationary and Non-stationary Operating Conditions”, IEEE Trans. on Power Delivery, vol. 24, no. 4, pp. 2300-2310, 2009

[12] I.D. Nicolae, P.M. Nicolae, “Using Discrete Wavelet Transform to Evaluate Power Quality at Highly Distorted Three-Phase Systems”, in Proc. of IEEE EPQU, 2011, pp.1-6

[13] S. Ahn, J. Pak, T. Song, et.al., “Low Frequency Electromagnetic Field Reduction Techniques for the On-Line Electric Vehicle (OLEV)”, in Proc. of IEEE ISEMC, 2010, pp. 625-630

[14] P. Drabek, “EMC Problems of Power Electronic Converters”, in Proc. of ISEMC, 2011, pp. 734-739

[15] P.M. Nicolae, I.D. Nicolae, I.G. Sîrbu, “Solution for the Reduction of Electromagnetic Influences from an Electric Driving System”, in Proc. of IEEE ISEMC, 2009, pp.58-63

[16] J. Luszcz, “Modeling of Common Mode Currents Induced by Motor Cable in Converter Fed AC Motor Drives”, in Proc. of IEEE ISEMC, 2011, pp. 459-464

[17] *** IEC 61000-4-30: Electromagnetic Compatibility(EMC) – Part 4-30: “Testing and measurement techniques–Power quality measurement methods”

[18] *** Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, IEEE Std. 519-1992

247