epe2005-harmonic calculation software for industrial applications with asd

HARMONIC CALCULATION SOFTWARE FOR INDUSTRIAL APPLICATIONS WITH ADJUSTABLE SPEED DRIVES

Lucian Asiminoaei ♦, Steffan Hansen ♣, Frede Blaabjerg ♦,

♦ AALBORG UNIVERSITY, INSTITUTE OF ENERGY

TECHNOLOGY, DK-9220 Aalborg SE, Denmark

Tel.: +45 / 9635.9254 Fax: +45 / 9815.1411

E-Mail: [email protected], [email protected]://www.iet.aau.dk

♣ DANFOSS DRIVES A/S, DK-6300 Graasten, Denmark

Tel.: / +45 7488.4599 Fax: / +45 7465.0434

E-Mail: [email protected] http://drives.danfoss.com/SW/DDsoftwaredownload/en

Keywords Variable speed drive, Harmonics, Industrial application, Simulation, Design, Power Quality

Acknowledgments The authors want to acknowledge the support from Danish Research Agency FUR project 562/06-14-23632 and the help received from Aalborg Fjernvarme, Svendborgvej, Aalborg, Denmark.

Abstract This paper describes the evaluation of a new harmonic calculation software. By using a combination of a pre-stored database and new interpolation techniques the software can very fast provide the harmonic data on real applications. The harmonic results obtained with this software have acceptable precision even with limited input data. The evaluation concludes here that this approach is very practical compared to other advanced harmonic analysis methods. The results are supported by comparisons of calculations and measurements given in an industrial application.

I. Introduction Often many companies are using simulation programs to predict, design or analyze different issues with harmonics. The desired harmonics assessment method is by doing a real measurement that reveal exactly the harmonic sources, their flow [1] and their effects. However, sometimes such alternative might be difficult in practice due to the required time to arrange the experimental setup and due to the cost of the measuring equipments. Even more, for the case of a new system design or prior to certain modifications in an existing system, the choice of doing harmonic measurements may not be possible. Thus, only by using simulation software programs and simulating an existing case it is possible to predict different problems and issues that may arise, as like harmonics. In such cases the harmonic estimation is given by a harmonic analysis software. Usually, by using a graphical interface, one can create a design that reproduces the existing power system and can configure the models of the power components and the initial conditions. Thus, one can obtain very similar results in simulations as it would be obtained from a real measurement [2]-[6]. There are many possibilities for such software products (as it will be given in §III), the choice varying from simple harmonic calculations up to very advanced harmonic simulators [7], [8]. Which of these

Harmonic Calculation Software for Industrial Applications with Adjustable Speed Drives STEFFAN Hansen

EPE 2005 - Dresden ISBN : 90-75815-08-5 P.1

might be useful for a given case, it is sometimes just a matter of knowledge, experience, effort spent in modelling and the required precision for the estimated harmonic data [9]. Among this diversity of harmonic simulators, there is one solution that emerged lately in industry, which is the choice of a dedicated software program. Different companies have developed software programs, which provide harmonic calculations for their manufactured products. These programs turn to be very efficient tools for harmonic estimations in industry, because they are fast and practical. The present work evaluates such dedicated harmonic analysis software. The software calculates the harmonic results for a typical distribution network (Fig. 1) with multiple adjustable speed drives (ASD's) connected at the point of common coupling (PCC). The ASD's can be configured in different ways in respect to the load conditions and the harmonic mitigation solutions (filters, coils, phase multiplication). Other parameters of the system can also be changed (i.e. grid, transformer, cables, linear load).

Other Loads

Transformer ih Cable

Drive 1

PCC

Panel

ih

ih

ih

ih

Grid

Drive 2

Drive 3

Drive n

Cable

Fig. 1. Typical distribution network used for the implementation of the proposed harmonic calculation software.

This paper presents the main steps for implementation of the harmonic calculation software and the algorithms used. The estimation of the harmonic currents is done by using a combination of a pre-stored database and new interpolation techniques, which can provide the harmonic data with acceptable accuracy especially for lower order harmonics. The results are evaluated by using a set of measurements from an industrial case. The topology is also simulated with different other software programs, which allows a comparison of the actual work with other state of the art simulators. The advantages and the drawbacks are discussed for each presented simulators. The results obtained validate the actual implementation of the harmonic software and conclude that it may be used for designing future ASD applications and for analyzing future harmonic mitigation techniques.

II. Software Implementation for the Harmonic Toolbox A. General Description The developed harmonic software [10] uses a pre-established simple topology for the power system as shown in Fig. 1. It includes a transformer, two sets of cables, a linear load and multiple connected drives. The drives are configured as front-end diode rectifier types, either six-pulse rectifiers with different harmonic mitigation solutions (filters, ac-, dc-coils) or multi-pulse rectifiers (12- or 18-pulse). The number of drives and their characteristics may vary (i.e. nominal power, loading, harmonic filtering). Therefore, for simplicity when using the toolbox the selection of the ASD's is done from a pre-defined list of drives. But there is also a possibility to create a custom configured ASD. Once the input parameters are given, the results can be collected either as a short report with only a few main indications like harmonic spectrum of the voltage and current at the transformer, or as a full harmonic report with detailed harmonic indications in all the points.



B. Fundamental Current Calculation A load-flow calculation can do the determination of the voltages and currents in a power system network. The basic schematic is displayed in Fig. 2. Having the active and reactive power (denoted by P1 respective Q1 in Fig. 2) and also the cable impedance (denoted by R1 and X1), the problem is reduced to the equation of voltage drop due to the current flow (1).

2

111112 V

QXPRVVV ⋅+⋅=−=∆ ,

2

12 3V

SI =

where, ∆V is the voltage drop created by real and reactive powers (P1 respective Q1) that flow through an impedance with resistance and reactance of R1 respective X1.

(1)

The algorithm for calculating the fundamental voltages and currents is sequentially repeated from the transformer point down to the drive’s connection point (Panel point in Fig. 1) [10].

C. Building the Harmonic Database The harmonic calculation approach, as presented in [10], [11], splits the initial circuit in Fig. 1, into multiple simple topologies, as shown in Fig. 3. Each system contains an equivalent model of the power system and one single ASD. The ASD is separately analyzed using a circuit simulator to obtain the line-side harmonic currents, and then the harmonic data are minimized using different interpolation functions [10], [11], [12] and stored in databases for off-line estimations later in the developed software.

Cable PCC

V2 V1

P LIN cos( ϕ )

Q SVC

ΣP DRIVES

Power flow P1, Q1

R1

ΣQ DRIVES

X1

I2

ac-impedance Drive 1

ih Grid

ac-impedance Drive 2

ih Grid

ac-impedance Drive n

ih Grid

…

Fig. 2. Load-flow example at PCC for calculation of the fundamental values.

Fig. 3. System diagram used in each individual study approach.

D. Integration of the Harmonic Calculation Software

Once the harmonic currents are known for the individual ASD the superposition principle is used to regroup the original diagram (Fig. 1).

Fig. 4. Graphical interface from MCT 31 Harmonic Calculation Software used for a case study with a Heat Power Station. The picture shows the interface to the short calculation report from the toolbox.



Ultimately, the algorithm, the database achieved and all analytical functions are compiled using Matlab programming into a graphical interface application (Fig. 4) called Danfoss MCT31 Harmonic Calculation Software (hereafter referred to as MCT31) [13]. Thus, for a given case, as it is selected here, a Heat Power Station, the input parameters (e.g. Table I) are used to search inside the embedded database retrieving the best-fitted information and estimating the harmonic currents and voltages. Other indications are also provided in the output report as explained, like fundamental voltages, currents, losses, power factor, displacement angle, etc.

Table I. Input data required for the harmonic calculation software Danfoss MCT31.

Transformer: Snom 1250 kVA ex 6%

Vnom 400 V Frequency 50 Hz

Primary-side Short circuit power 200 MVA Cable: Length 15 m Area 240 mm2 Material Aluminum

Number or pairs 1 ASD's: Snom See Fig. 5 Load 100 %

dc-link inductance Customized ASD's Linear Load: PLoad 220 kW Cos(ϕ) 0.85

Capacitive power factor correction 0 kVAr

III. Harmonic Software Programs This section gives a comparison between the actual harmonic software [13] and other state of the art harmonic calculation programs. There can be different alternatives in selecting the type of software, which could do a harmonic analysis. Thus, the harmonic software products have here been categorized (as an extension of [9]), in:

• Equation-solvers software - like Mathcad, Matlab, Simulink, etc., typically used for solving algebraic calculations expressed as analytic equations, usually given in text file. Thus, the software can be relatively simple to use for implementing an analytical model, which is most probably particular for a specific case or for a common class of applications. Both time-domain and frequency-domain can be implemented depending of the user choice. The simulation speed depends on the size of the iteration step for time domain simulations and the number of the equations to be solved. The main drawback is the drastic increase of the model with many input parameters or for large models.

• General circuit simulators - like Saber, PSpice, Psim, EMTP, etc., used for electrical circuit calculations. The most common usage of such software is to simulate a given electrical circuit in time-domain. Thus, one may do a harmonic simulation for power systems first by building the models of transformers, cables, ASD's with simple available elements as resistor, inductors, capacitors, diodes, etc., and finally having the transient response of the whole system. Once the time-domain signal is available, by using built-in FFT functions the time-domain signal is decomposed in the associated spectrum for a given period. Such software products are very powerful as they include libraries with relatively advanced models that allows the user to obtain very realistic modelling. Thus, the accuracy of the results can be very good if the user knows sufficient input parameters. However, there are drawbacks like possibly a long simulation time or convergence errors for large models and also the need of good practice with the software.



• Professional power circuit simulator - like HarmFlo, EMTDC, DIgSILENT, etc. are especially developed

for simulation of the power system components. This type of software is similar with the previous type, although it is developed for power system electromagnetic phenomena. The models are provided here for larger electrical components as transformers, electrical towers, three-phase loads, transmission cables, etc. The simulations can be selected either in time- or in frequency-domain depending on the required purpose. As they include larger models, the simulations are not as accurate as in circuit simulations, but still enough for the power system phenomena, also including different possibilities for non-ideal conditions. The speed of the simulations is also dependent on the model size and step selection as explained above, but they provide relatively faster harmonic results for larger design compared to the previous simulator types.

• Dedicated simulation software - as Harmac/Siemens, Cymharmo/Cgi-Cyme, TCI/Trans-coil, MCT31/Danfoss, etc., which are developed for harmonic calculations for specific application like transmission, power distribution, adjustable speed drives. Usually the software is restricted to a limited number of applications and is not as flexible as the previously explained types. The user may configure an electrical diagram in different conditions and may have the results for certain input values, the results being satisfactory for some engineering evaluations and comparisons. The simulation speed is relatively fast, the precision is acceptable and there is no need to have a big experience with the program.

The actual comparison done consists in analyzing the accuracy of the harmonic results and the tasks required for simulating a given case. The data are taken from a real application where the harmonics are known from measurements. The actual topology is simulated in 3 different harmonic simulators, which are: MCT31, PSpice and DIgSILENT. The application is a 1 MVA Heat Power Station that uses large ASD's installation as shown in Fig. 5. Here the ASD's are used for controlling gas-burners and water-pumps, hence the total produced hot water delivered to the customers. The application uses large rated ASD’s (in the amount of 730 kVA ASD's power from a total installation of 1250 kVA), and as it is expected there are harmonic currents generated into the grid. The harmonics are measured at the PCC for a loading of 100 % of the total installation and the harmonic data are displayed in Table II. Table III gives the measured voltage predistortion at the PCC.

Table II. Harmonic measurements at the PCC for 100 % loading capacity of the Heat Power Station.

Table III. Measured voltage predistortion at PCC

Harmonic order

Measured harmonic currents at PCC,

Ih [%]

Harmonic order

Measured harmonic voltages at PCC,

Vh [%]

Harmonic order

Voltage predistortion

Vh [%] I5 17.5 V5 6.2 V5 1.2 I7 5.9 V7 3.0 V7 1.5 I11 4.3 V11 3.2 V11 0.6 I13 3.2 V13 2.3 V13 1.1 I17 1.7 V17 2.0 V17 0.8 I19 1.8 V19 1.5 V19 0.1

THDi 19.5 THDv 9.1 THDv 2.4

The one-line diagram of the Heat Power Station is given in Fig. 5, which is designed in each of the selected software programs. Some of the issues encountered in the design and simulation are mentioned next. The simulations done in PSpice are in time-domain. Thus, this software will give results that may easily be interpreted and may create a close correspondence with the measured transient waveforms. After obtaining the results the waveforms are processed with an FFT operator, which gives the harmonic spectrum. There could be some difficulties in building the PSpice design for some of the models like the transformer and the ASD's. As these models do not exist in the default PSpice library, they have to be



0.02 0.025 0.03 0.035 0.04 0.045 0.05-400

-300

-200

-100

0

100

200

300

400

Tran

sfor

mer

, Exp

. No.

18

Time [s]

VAN [V]0.2 x IA [A]

Fig. 5. The diagram of a Heat Power Station with large ASD's installations. This diagram is simulated in PSpice, DIgSILENT and Danfoss-MCT31.

Fig. 6. Measured waveforms for phase current and voltage at PCC for a loading capacity of 100 % of the Heat Power Station.

realized by using basic components as resistors, inductors, capacitors and diodes. The cable models were simulated as simplified LR lumped models. Few attempts were done with distributed parameter models, and the results were better for the higher frequencies, but due to the longer simulation time (almost 5-10 time compared to the lumped parameters), this approach was not used. In DIgSILENT a frequency-domain simulation was selected. The software can also do time-domain simulations, but one limitation is that the rectifier model is ideal; therefore, it is impossible to be configured as the existing ASD's. Thus, the approach was a frequency-domain simulation, where the ASD's are configured as harmonic current sources with values defined for the actual case. This gives an other dilemma, which is how to find the correct values for the harmonic currents for a given case. As it is known the harmonic currents are generated based on the existing voltage, predistortion, impedance and level of non-linearity. Therefore, there is not a unique value, which can be applied for all cases, as like the well-known value of “1/h” for ideal models of ASD's with dc-smoothing inductor [9]. In the actual work the harmonic currents have been established for simplicity reasons based on the knowledge from similar cases. The other power system component models are simpler to be used and configured, as the existing library provides them with sufficient predefined settings. For the developed harmonic toolbox MCT31, the design is relatively simple, as the user has to select the drives from an existing database. One limitation is mentioned here, that MCT31 has a predefined circuit configuration, which makes it difficult to analyze complex power systems. However, for the actual case of the Heat Power Station, MCT31 suits the needs.

After building each design, the input parameters are set according to Table I, Table II and Fig. 5. The background distortion was used only in PSpice and DIgSILENT simulations since these softwares allow the user to configure additional voltage sources to simulate the voltage predistortion. For the MCT31, the predistortion was not implemented, thus, the power supply is an ideal voltage source. The results of the simulations at 100 % loading capacity of the installation are presented in Table IV. As it can be seen the closest results to the measured data are obtained from PSpice. Then follows DIgSILENT and MCT31, with small differences between each other. However, none of the software was capable to give exact data as the measured values in Table II. The explanation may be found in different uncertainties that come from some of the considered input parameters as like the transformer parameters, cable type and lengths, unknown loadings, unbalance currents, etc. Fig. 7 shows the results of the simulations at different loadings of the Heat Power Station. Each plot shows the comparison between the measured and the simulated data for THDi respective THDv.



Table IV. Simulated low-frequency harmonics at the PCC for the Heat Power Station case at 100 % load.

Harm.

Simulated harmonic currents at PCC, Ih [%]

Harm.

Simulated harmonic voltages at PCC, Vh [%]

order PSpice DIgSILENT MCT31 Order PSpice DIgSILENT MCT31I5 20.0 20.0 20.0 V5 5.3 4.9 5.0 I7 6.1 5.5 6.5 V7 2.9 2.5 2.3 I11 5.0 4.9 4.2 V11 2.9 2.5 2.3 I13 3.2 3.1 2.8 V13 2.3 1.7 1.8 I17 1.9 2.0 1.4 V17 1.8 1.8 1.2 I19 1.7 1.7 1.2 V19 1.2 1.5 1.1

THDi 22.0 21.7 21.8 THDv 8.6 7.1 6.7

PSpice gives the closest simulation data compared to the measurements. For DIgSILENT the simulated harmonic currents have large errors compared to the measured currents. This is because the ASD's use the same values in percentage for modelling the current source behavior. Certainly, better simulation results can be obtained in DIgSILENT if one changes accordingly the values of the harmonic currents each time the load of the ASD's changes, but this will lead to a very difficult task as previously explained of determining each time the harmonic currents depending on the existing conditions. For MCT31, the simulated harmonic currents are very similar with the values obtained in PSpice. However, here the biggest errors are for the simulated harmonic voltages. This is because MCT31 did not consider the voltage predistortion in the development. Therefore, with this lack of knowledge the results are different. However, the difference between the MCT31 simulations and the measured data is relatively constant for the whole span of the loading with a value of 2.3 %. The value of the measured predistortion as indicated in Table III is 2.4 %. Thus, for future estimations using MCT31 the simulated harmonic voltages may be corrected by adding the value of the voltage predistortion, if known.

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d) e) f) Fig. 7. Simulated THDi respective THDv at different loads at PCC obtained in a), d) PSpice, b), e) DIgSILENT, c), f) MCT31.



Table V. Comparing different issues for each considered simulators. PSpice DIgSILENT MCT31 Required experience Expert level User level Beginner level Time required for training Month Month/days Hours Time required in design Tens of hours Hours Minutes Time required for simulation Minutes Seconds Seconds Time to create a harmonic report Minutes* Minutes/Seconds Seconds Need to build ASD's models Yes* Maybe* No Need to configure the existing models Yes Yes No Possibility of including non-ideal conditions Yes Yes No Versatility in making changes High High Low Number of required parameters Many Medium Minimum Type of analysis Time Time or Freq. Frequency Type of simulators Electrical circuit Differential eq. Table-based Possible encountered convergence errors Medium Medium Non Precision High Good Acceptable PC resources (processor, hard-disk) High High Low

* this option can be customized, but it was not used in this comparison Table V summarizes a number of aspects that one may encounter when using such simulators. Seen from this perspective it seems that the actual developed harmonic toolbox MCT31 is suitable especially when quick harmonic estimations are needed without requiring many details about the power system. IV. Discussion A. Simulation Objective As it seems from the developed comparison the PSpice program may mainly be used for deep harmonic investigations. The design is flexible and can include many realistic models. Also the modification of a design is relatively easy to be done, for example if the topology of the network is changed or if a new element must be included. The results may be very close to the real life if there are enough informations included in the design. However, these require appreciative time either for constructing the design or for simulation. The objective of using PSpice in harmonic analyses is that it will provide insight information about the ASD as a non-linear load and the mechanism of harmonic generation. Also the interaction between the harmonic currents, harmonic voltages and impedances (also frequency dependent) can be easily simulated. DIgSILENT may be easier in usage since some of the models are simplified and customized for power system applications. However, the harmonic frequency domain analyzes depend on how the ASD's are configured as harmonic current sources, which may introduce significant errors if they are not carefully configured. Therefore, in respect to harmonic analysis the software may be used for cases where there is not so much interest in finding very precise details, especially for harmonic sources. Thus, the objective of a harmonic analysis in DIgSILENT could be investigations at the system level, for example the harmonic-flow in large or complex networks. Also, all associated indexes as like the power losses, voltage-drop on cables, loading of the neutral wire, etc. may be simulated with a relatively good precision. The actual developed toolbox MCT31 is reduced in flexibility since there is a predefined diagram for which the existing industrial application should comply with. Another limitation is that the results are given only for ideal grid conditions. However, as the user is asked to provide a relative reduced number of input parameters, MCT31 proves to be a very practical toolbox for harmonic investigation. The harmonic analysis may be reliable enough for harmonic currents generated from ASD's since the internal database is



obtained from such simulations [10]. Thus, MCT31 may be used for harmonic estimations either at the load level (ASD's) or at the distribution level. As a result, MCT31 may be useful for plant engineer and end-user customers, or wherever a prior estimation is required to evaluate possible performances or initial implementations.

B. Non-ideal Conditions

As it was presented until now one limitation of the developed toolbox is that the toolbox calculates the harmonic results only in ideal conditions, i.e. ideal pure sinusoidal voltage supply, balanced system, linear behavior and no parasitics included for the system components. Further investigations [11] have found this conclusion much more convenient than the integration of the non-ideal situations into the actual harmonic toolbox. As an image, considering only few possibilities, the next list shows that such integration under a software tool is too far from a simple implementation. Thus, in order to improve the actual estimations, the toolbox would have to include: • type of the voltage unbalance (in amplitude and in phase), • value of the voltage unbalance, • phase on which the voltage unbalance occurs, • unbalance of the system components, • spectrum (harmonic order) of the pre-distortion, • amplitude of the each harmonic in this spectrum, • phase angle of the each harmonic, • duration and the repetition rate of other different types of distortions [1], • presence of different resonances in the network, • frequency dependences for the system components, • nonlinear behavior as saturation curves, hysteresis, etc. If one may try to include these non-ideal conditions in the actual harmonic toolbox MCT31, which is basically a table-based approach, it may reach a relatively complicated structure for the toolbox implementation, probably impossible to implement such volume of data. On the other hand these parameters listed above are most of the time hard to be known without any measurements. Thus, even if advanced harmonic simulators (as presented in this work like PSpice or DIgSILENT) allow implementation of the non-ideal conditions, these simulators will not be able to give realistic results (i.e. as like in a real measurement) if the parameters are not known with a relative good precision. Thus, even if they allow very powerful simulations, the results will suffer from the lack of knowledge. Therefore, by using a harmonic toolbox as the one described in this paper it might be reasonable from a practical point of view. Even if the results are obtained in ideal conditions, they are still valid to some extent and can be used in an engineering way to predict the influence of harmonics in the power system. V. Conclusion This paper describes a dedicated harmonic calculation software. The software is applied for harmonic estimation in an industrial case with large ASD installation and compared against two other professional simulators PSpice and DIgSILENT. The comparison shows that the exact knowledge of the existing conditions improves the accuracy of the simulations in PSpice. As for DIgSILENT, since the results are based on the harmonic current model configured for the ASD's, the interpretation is biased by the values used. MCT31 gives fairly good estimations for the harmonic currents and the harmonic voltages may be corrected with the existing value of the voltage predistortion. The accuracy obtained with the proposed harmonic calculation software MCT31 is within acceptable limits, which makes it a very practical tool.



References [1] Std 519-1992 “Recommended Practice and Requirement for Harmonic Control in Electrical Power Systems“,

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Conf. Publ. No. 291, 1988, pp. 153-156. [3] M. Grötzbach, “Line Side Behavior of Uncontrolled Rectifier Bridges with Capacitive dc Smoothing”, Proc.

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Rectifier Line-Side Harmonics”, Proc. of IAS’03, 2003, Vol. 3, pp. 2056-2062. [10] L. Asiminoaei, S. Hansen, F. Blaabjerg, “Harmonic calculation toolbox in industry application for adjustable

speed drive”, Proc. of APEC’04, 2004, Vol. 3, pp. 1628-1634. [11] L. Asiminoaei, S. Hansen, F. Blaabjerg, “Development of calculation toolbox for harmonic estimation on

multi-pulse drive”, Proc. of IAS’04, 2004, Vol. 2, pp. 878-885. [12] L. Asiminoaei, S. Hansen, F. Blaabjerg, “Evaluation of an advanced harmonic filter for Adjustable Speed

Drive using a toolbox approach”, Proc. of NORPIE’04, 2004, http://www.elkraft.ntnu.no/norpie/. [13] Danfoss Drives A/S, “MCT31 Harmonic Calculation Software”,

http://drives.danfoss.com/SW/DDsoftwaredownload/en



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