Systematic Evaluation for Harmonic Distortion Limits from IEEE 519

Transcription

1 Systematic Evaluation for Harmonic Distortion Limits from IEEE 519 J. Barreiro, M. Hernandez and G. Ramos Department of Electrical and Electronics Engineering Universidad de los Andes Bogotá D. C. Colombia Abstract Every day industry growths and with it, non-linear load components growth as well. This causes sever implications on the power quality. Some standards establish rules and limits in order to guarantee adequate conditions for equipment, and to supply appropriate power to final users. Automatic mechanisms should be explored to contribute with this process of power quality improvement, helping with design and necessary corrections related to harmonic distortion. Index Terms Power quality, harmonic distortion, software algorithm. I. INTRODUCTION The amount of non-linear loads has increased considerably since there are many devices useful (for instance adjust speed drives or converter ac-dc, ac-ac, dc-ac and dc-dc) in industry which generate harmonics; however despite the fact this aspect affects electrical systems in several ways, it is necessary to find solutions different from stop using this kind of equipment. That is why the study and analysis of harmonic limits is significant in electric power systems [1]. As mentioned above, there are many devices which generate harmonics in industry and whose applications are necessary and irreplaceable, therefore, it is imperative to study and analyze harmonics limits to mitigate them without stopping using this equipment. Some common harmonic sources in industry are converters, arc furnaces, switched mode power supplies, etc. In order to analyze harmonic contents, equations (1) and (2) are used to determine harmonics generated h by a converter respect to the number of pulses q and the harmonic I h and fundamental I 1 magnitude. h = kq ± 1 where k = 0, 1, 2, 3,... (1) I h = I 1 h IEEE 519 [2] presents tables of current distortion limits whose values depend on the voltage at point of common coupling (PCC) and additionally, each table establish some ranges for the maximum short-circuit current at PCC I sc and maximum demand load current for the fundamental frequency The authors are with the Universidad de los Andes, Bogotá D.C., Colombia. ( j.barreiro135@uniandes.edu.co; me.hernandez47@uniandes.edu.co; gramos@uniandes.edu.co). (2) component at PCC I L. These tables will be shown in section IV as part of the algorithm. This paper proposes an automatic way to analyze and study harmonic limit according IEEE 519 through a program developed in LabView, in section II explains the importance to have an automatic way to evaluate harmonic limits in electrical applications is presented; section III illustrates the algorithm developed and its advantages in comparison with other alternative analysis are shown; section IV presents possible solutions to fulfill harmonics limits by adding another load and establish a restriction for loads in case the limits are not exceeded; section V shows an application example simulated in ATPdraw, gotten from the IEEE 519, is analyzed with the algorithm and conclusions about possible loads are made; finally, in section VI and the last one, further work related to the algorithm presented in this paper is discussed and general conclusions are made. II. IMPORTANCE OF DOING AUTOMATIC ANALYSIS The analysis of harmonic limits based on IEEE 519 is an analysis oriented to design power systems with nonlinear loads, considering static critic conditions in steady-state event when these conditions are or could be dynamic [3]. This paper proposes to have an alternative way to analyze harmonic limits in an automatic way in order to accomplish evaluate a system in short time, what would allow analyze dynamic power systems online. Currently, solutions to mitigate harmonic distortion limits are established based on a critical or steady-state condition which is not an optimal solution when conditions change [4]; for instance, when a user is disconnected during a period of time or when another load is added at the PCC, the case of study changes dynamically and the analysis should be done over again. However, some variables could be considered constant since they belong to the PCC as own parameters like voltage at PCC, maximum shortcircuit current at PCC, maximum demand load current at PCC and pulse number [5] [6]; these parameters could maintain their value for some conditions (for which this paper focuses) event if a load is added. In section VII, applications using the algorithm presented in this paper are discussed /13/$ IEEE

2 Maximum Demand Load Maximum short-circuit current Voltage at PCC Pulse Number Wave of Current at PCC Figure 2. Current distortion limits for 120 < v pcc Harmonic Limits Tables Information of the System Harmonic Spectrum Recalculate Limits yes Pulse Number greater than 6 Figure 3. Current distortion limits for < v pcc no Limits According the Case Figure 4. Current distortion limits for < v pcc Evaluate Limits Calculate Possible Loads / Conclusions Figure 1. Flow-chart algorithm evaluation harmonic limits III. ALGORITHM The figure 1 illustrates the general scheme of the algorithm to calculate the limits of the IEEE 519 according to the parameters of the electric power system. This section explains each part of the algorithm and shows tables calculated to make conclusions afterwards. There are some initial data that describes the characteristics of the system; these data are maximum demand load I L, mximum short-circuit current I sc, voltaje at PCC v pcc, pulse number q and wave of current at PCC. It is relevant to clarify that the mentioned wave can be gotten from different sources and it is exactly what makes this application useful for diverse cases of study and applications; wave of current is obtained from a file extension.mat what guarantees the possibility to get this information from simulation in ATPdraw as shown in section VI, from simulation in Simulink, and by using a CompactRIO for an application in real-time as proposed in section VII. Broadly, this analysis can be carried out from a file with this format. Tables of current distortion limits in IEEE Standard 519 are stored; the corresponding table is selected based on voltage at PCC given by the user and the limits within the table are selected based on the relation Isc/I L. Figure 2 shows the table of current distortion limits in IEEE Std 519 obtained from the algorithm storage for a voltage at PCC 120v < v pcc 69000v, Figure 3 for 69000v < v pcc v and figure 4 for v < v pcc. After having selected the correct row of the relation fraci sc I L from the corrected table, the limits are recalculated if the pulse number is greater than 6 with a factor of q 6 for characteristic harmonics and with a factor of 0.25 for the non-characteristic harmonics [7]. Once limits are identified, the actual values for the harmonic spectrum at PCC are determined [8], and harmonics which do not fulfill the limits can be detected for their analysis and calculation for possible solutions or limits for additional loads as explained in section V. IV. POSSIBLE CASES IN THE ANALYSIS Once the limits for harmonic distortion are analyzed, the developed algorithm evaluates two main cases. The first case is when limits are exceeded for any or various harmonics, and the second case is when the limits are not exceeded [9]. In any case, it is possible to fulfill or continue fulfilling limits by adding another load at PCC [10]. The algorithm presented in this paper only considers two kind of loads, a linear load what just affects the fundamental component and a six pulse load what affects its characteristic harmonic which can be calculated with (1); additionally this load can be added after a wye/wye transformer or a delta/wye transformer that imply a shift and consequently an annulation of some harmonic components. Figure 5 shows the two main cases that could occur and the critical harmonic; the critical harmonic is the critical to fulfill limits, for example, in case there are various harmonics components exceeding limits (suppose in figure 5 that limits are represented by color red and actual values are represented by color black), it is necessary to reduce in a proportion that guaranties that all of them would be under limits. For a case where there are limits over the limits, it is possible to calculate the differential between actual value and

3 Figure 5. Differential between actual values and limits. limit denoted by I h and the critical harmonic component is the corresponding with the maximum I h because if this harmonic is reduced, all of the others unfulfilling harmonics will be satisfied reduced under limits as well. For the second case, when all of the harmonic components are under limits (suppose in figure 5 that limits are represented by color black and actual values are represented by color red), there is a I h for each harmonic and a critical harmonic corresponding with the minimum I h, it means, the harmonic which is the nearer to the limit. Then, if the closer harmonic to the limit does not exceed the limit, all others do not exceed it either. Case 1: Limits are exceeded Six pulse load from a transformer delta/wye with shift 30: When there is a shift at the transformer of 30, there is a shift for some harmonics what produces reduction of their value at PCC since they are subtracted with the actual corresponding harmonics; the shift for each harmonic is calculated from (3). shift h = 30 (h ± sq(h)) where sq is sequense of h (3) It is important to point out that if any harmonic that does not suffer a shift with (3), could not be reduce with this strategy and it is necessary to check a linear load to fulfill harmonic distortion limits. In order to determine the possible load that reduces harmonics which are exceeding limits, it is necessary to determine the critical harmonic as explained above excluding harmonics h = 11, 13, 23, 25 and 35 because they are added instead of subtracted, this is obtained by (4) and considering that the critical fundamental component depends not only on the maximum critical harmonic but also on the harmonic component like in (5). I Critical h = max (I h I h Limit ) (4) Once the critical harmonic is identified, the critical fundamental component I C1 for that with (5). I C1 = h c I Critical h (5) After that, with this value for a fundamental component the magnitudes for six pulse characteristic harmonics are calculated and also the I RMS by (6). ( I RMS = 35 ) 2 I c1 3 h where h for 6 pulse (6) 2 h=1 Finally a load, that decreases harmonics that suffer shift in (3), is determined by (7). Load decrease = v pcc I RMS (7) The same procedure must be done with harmonics which do not suffer shift, and then, a load that increases harmonics h = 11, 13, 23, 25 and 35 is determined Load increase. And the conditions for the possible non-linear load are shown in (8). In case that this relation between loads does not make sense the algorithm gives a message saying that it is not possible to correct harmonics for this case. Lineal load: Load decrease < Load < Load increase (8) When a lineal load is added at the PCC, this load only affects the fundamental component which is the reference to calculate the magnitude of other harmonics, and this produces always a decreasing in the percentage of harmonic values. However, there is a maximum possible load depending on the short-circuit current and voltage at PCC that is calculated by (9). And the current load at PCC for the analysis is determined by (10). Load max = v pcc I sc (9) Load act = v pcc I L (10) In the same way, when limits are exceeded, the critical harmonic has to be detected and after having identified the percentage corresponding this critical harmonic, the I c1 can be calculated by (11). I c1 = h[%] c I L I h [%] L (11) limit After that, the limits for the lineal load is calculated by (12) and (13), what results in (14). Load lin = v pcc I c1 (12) Load maxlin = Load max Load act (13) Load lin < Load < Load maxlin (14) Case 2: Limits are exceeded. Six pulse load from a transformer wye/wye: For this case, the calculation of the load has fewer restrictions since there are not problems with harmonic limits but it

4 A 13.8kV 25 MVA 115kV A K 12.5 MW 50 MVA K 12.5 MW Figure 6. Application example IEEE 519 is necessary to establish a limit for a non-lineal load added at PCC. Critical harmonic is calculated as explained in the beginning of the section by (15). I Critical h = min (I h I h Limit ) (15) Once critical harmonic is identified, the procedure to determine the load that set the value just at the harmonic limit is obtained in the same way as explained above. This is (5) and I RMS is gotten from (6) but including all harmonics components. Finally the load calculated is (16) and (17) Load increase = v pcc I RMS (16) Figure 7. Current wave at PCC. ATPdraw and LabView Load < Load increase (17) Lineal load: For this case, there is not a lower limit for the lineal load, because there is not a value exceeding harmonic limit that should be compensated, however, upper limit is still the same one established by the short-circuit current and maximum load current as in the first case. This load is (18). Load < Load maxlin (18) V. EXAMPLE APPLICATION FROM IEEE 519 The analysis of harmonic limits for the first application example of the IEEE 519 (Figure6) is made in this section by using the algorithm presented. Current wave is obtained by a simulation made in ATPdraw and opened in the program to get results. Current wave in ATP and in LabView is shown in figure 7. Once current wave is loaded at the LabView program, the application gets the spectrum Figure 8, calculates limits and points out harmonics that do not fulfill limits. For the first case proposed in the application example of the IEEE 519 v pcc = , I sc = and I L = 250 and results from the program are shown in figure 9. For second case v pcc = , I sc = and I L = 251, 4285 and results from the program are shown in figure 10. And for the third case v pcc = , I sc = and I L = 25, 1 and results from the program are shown in figure 11. Figure 8. Results from LabView program for case 1. Figure 9. Results from LabView program for case 1.

5 For this case, it is important to clarify the necessity of evaluating the error as quickly as possible, because this fact determines the sample time for the control system and at this point the importance of the application shown in this paper is highlighted. Figure 10. Results from LabView program for case 1. Figure 11. Results from LabView program for case 1. VI. FURTHER WORK As mentioned in the introduction of this paper, devices which are harmonic sources are necessary in industry and could not be replaced; therefore, it is imperative to find alternative solutions. Additionally, power systems are dynamic, what implies different values for harmonic distortion in time, and of course, different dynamic solutions. The figure 12 shows a general scheme for a dynamic application with feedback to control harmonic distortion; this scheme measure current wave at PCC and dynamically can adjust different parameters on the system, for instance, can control an adaptive filter for a specific harmonic or change loads to guarantee the fulfilling of limits IEEE 519. There is a CompactRIO in charge of doing this control by measuring the current wave at PCC and calculates the harmonic spectrum to identify an error between the measurement and the limits (This is a classic control scheme where there is a feedback measurement and a reference which is a desirable value). After having detected the error, in case the harmonics are exceeded (since there could be an error that does not exceed limits, and should not be considered), it is necessary to determine a control action, that could be changing filter parameters, modifying an actual load or simply indicate through an alarm that limits are being exceeded to evaluate design problems. Figure 12. General Application that Implies the Algorithm Proposed VII. CONCLUSIONS It is important to highlight the necessity that knowledge about efficient harmonic values and the angle for each component has, among different methods to analyze power in current and non-sine voltage systems. In this way, the sum of vector components can be made and calculate the fundamental and harmonic powers in the system. Considering the instrument capabilities is fundamental in the implementation of power calculation methodologies, since there are alternatives which require calculating the Fourier transformation so current and voltage waves are decompound (with the benefit to supply a graphic tool to make analysis), and alternatives based on wave discrete treatment whose results in power are precise without demanding significant computational effort. It is posible to verify pulse conversion by calculating the displacement harmonic in a group of transformer connections, and based on the knowledge of characteristic displacement factor for each coil. The vector harmonic component sum can be generated easily for the automatic calculation under balanced conditions; however, sequence decomposition, to analyze unbalanced multi-pulse systems, is necessary. Changing load in balanced systems with multiple nodes implies the necessity of hierarchical calculations of the influence on the network. Moreover, it is necessary to average the load in two nodes which contribute current at the same PCC to obtain the value of THD gotten from the total current; to make the mentioned average, information about the harmonic displacements is required and to consider the vector sum correctly. REFERENCES [1] R. Chu and J. Burns, Impact of cycloconverter harmonics, Industry Applications, IEEE Transactions on, vol. 25, no. 3, pp , may/jun [2] Ieee recommended practices and requirements for harmonic control in electrical power systems, IEEE Std , [3] S. Halpin, Overview of revisions to ieee standard , in Power Engineering Society Summer Meeting, 2002 IEEE, vol. 2, july 2002, pp vol.2. [4] J. Mazumdar and R. Harley, Determining ieee 519 compliance of a customer in a power system, in Power Electronics Specialists Conference, PESC IEEE, june 2007, pp [5] M. McGranaghan, Overview of the guide for applying harmonic limits on power systems-ieee p519a, in Harmonics and Quality of Power Proceedings, Proceedings. 8th International Conference On, vol. 1, oct 1998, pp vol.1. [6] A. Zobaa, Cost-effective applications of power factor correction for nonlinear loads, Power Delivery, IEEE Transactions on, vol. 20, no. 1, pp , jan [7] E. Gunther, Interharmonics in power systems, in Power Engineering Society Summer Meeting, 2001, vol. 2, 2001, pp vol.2. [8] J. Mazumdar, R. Harley, and F. Lambert, System and method for determining harmonic contributions from non-linear loads, in Industry Applications Conference, Fourtieth IAS Annual Meeting. Conference Record of the 2005, vol. 4, oct. 2005, pp Vol. 4.

6 [9] P. Ribeiro, Common misapplications of the ieee 519 harmonic standard: Voltage or current limits, in Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE, july 2008, pp [10] N. Kandev and S. Chenard, Method for determining customer contribution to harmonic variations in a large power network, in Harmonics and Quality of Power (ICHQP), th International Conference on, sept. 2010, pp. 1 7.