An Intelligent Control Strategy for Power Factor Compensation on Distorted Low Voltage Power Systems

Transcription

1 1562 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 3, SEPTEMBER 2012 An Intelligent Control Strategy for Power Factor Compensation on Distorted Low Voltage Power Systems Shunfu Lin, Member, IEEE, Diogo Salles, StudentMember,IEEE, Walmir Freitas, Member, IEEE, and Wilsun Xu, Fellow, IEEE Abstract Due to the proliferation of harmonic producing-loads, harmonic resonance has become a major hurdle for performing power factor compensation in commercial power systems, such as office towers and shopping complexes. This paper presents an intelligent power factor compensation controller that can perform power factor correction without exciting harmonic resonance under varying demand conditions. Practical and robust control algorithms are proposed for the purpose of easy implementation in a micro-controller. In addition, the controller relies on common low cost sensing devices and does not require additional measurements. As a result, the proposed controller can be constructed as aretrofitting device to replace existing power factor correction controllers with little effort. Analysis of representative case studies is conducted to illustrate how the proposed controller performs. Index Terms Capacitor switching, commercial buildings, harmonic resonance, power factor control. I. INTRODUCTION E LECTRICAL energy efficiency is of prime importance to commercial facilities, such as office towers and shopping complexes. The application of power factor (PF) compensation has long been accepted as a necessary step to improve the efficiency of these low voltage electrical installations [1] [3]. This is usually achieved by installing capacitors downstream to the supply transformer at the entrance point of the facility. Such capacitor units are switched in and out of circuits as the demand for VAR compensation of the building load fluctuates. If applied properly and controlled, capacitors can improve the performance of distribution circuits since, by providing the reactive current locally, less power needs to be provided by the distribution network resulting in lower losses, improved line voltage, Manuscript received January 28, 2012; revised April 24, 2012; accepted May 13, Date of publication July 13, 2012; date of current version August 20, This work was supported by the Natural Resources Canada through the Technology, Innovation Program as part of the Climate Action Plan for Canada and by FAPESP, Brazil. Paper no. TSG S. Lin is with the Department of Electric Power and Automation Engineering, Shanghai University of Electric Power, Shanghai, China, ( shunfulin@shiep.edu.cn). D. Salles and W. Freitas are with the Department of Electrical Energy Systems, University of Campinas, , Campinas, Brazil ( dsalles@ieee.org, walmir@ieee.org). W. Xu is with the Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada ( wxu@ualberta. ca). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TSG and, ultimately, reduced facility billing charges (utility penalties) [1] [5]. However, facility operators have reported frequent failures or trips of these PF correction capacitor banks. One important reason is the proliferation of harmonic-producing loads; solid-state power conversion devices are prime examples. The problem is that capacitors might aggravate the existing harmonic distortion since, although these devices do not generate harmonics, they provide a network path for possible local or general parallel resonance conditions, which contributes to a significant amplification of harmonic currents produced by the facility loads. In cases of resonance, this current may be very large and may damage the capacitors. Therefore, the consideration of PF compensation capacitor installation should also include harmonic resonance analysis at the design stage [1] [3], [5], [6]. According to [5], possible solutions to avoid harmonic problems include the following: 1) ungrounding grounded-wye capacitors; 2) changing capacitor bank sizes and locations; 3) adding a reactor to an existing bank; 4) adding a filter capacitor; and 5) controlling the capacitor switching scheme to avoid resonance. The objective of this paper covers the fifth approach and consists of developing a practical and robust control algorithm for the capacitor bank switching scheme that is capable of achieving both power factor correction and resonance avoidance requirements. Previous works proposed optimization algorithms for this purpose; however, they are time-consuming and it is not guaranteed they converge to the optimal solution under time-varying load and system impedance conditions [7], [8]. More recent works have focused on the installation of passive and active harmonic filters [9] [11]. This approach can be complex (e.g., stresses in the filters need to be considered, the harmonic spectrum of the nonlinear loads need to be determined, etc.) and be costly (e.g., installation and maintenance costs) to the facility s owner. Another common solution is to add reactors in series with existing capacitor banks [12]. However, the system parameters vary dynamically with the power system configurations and loads. Therefore, the harmonic resonance might occur even if a combination of capacitors connected in series with reactors has been installed. Reference [13] proposed replacing the reactors with power electronics inverters. The concept of the controller proposed in this paper is simpler; whenever resonance conditions exist, the capacitor bank should be changed in size to shift the resonant point to /$ IEEE

2 LIN et al.: AN INTELLIGENT CONTROL STRATEGY FOR POWER FACTOR COMPENSATION 1563 Fig. 1. Typical power factor compensation arrangement for commercial power systems. another frequency [6]. However, one important issue must be solved; how to determine if the resonance condition exists under varying demand conditions? It is not safe and intelligent to decide the suitable number of switched capacitors units assuming constant system impedance since it varies for different operating conditions. In order to solve this issue, the proposed controller uses pre- and postdisturbance (due to the capacitor switching) steady-state waveforms of the voltages and currents at the interface point to estimate the harmonic system impedance [14] [16]. It must be highlighted that the proposed controller relies on common low cost sensing devices and does not require additional measurements. As a result, the controller can be constructed as a retrofitting device to replace existing power factor correction controllers with little effort. This paper is organized as follows. Section II discusses the conventional control approach for power factor compensation. Section III describes the characteristics associated to the harmonic resonance problem. Section IV presents the development of the proposed capacitor controller. Section V presents case studies to evaluate the proposed approach. Section VI summarizes the main findings of this paper II. COMMON REACTIVE POWER COMPENSATION SCHEME As mentioned before, the traditional approach for power factor compensation in commercial facilities consists of placing capacitor banks in parallel with the load at the entrance point of the facility, offsetting the inductive loading (lagging power factor) of the equipments. Fig. 1 shows a common power factor compensation arrangement used in commercial power systems. As one can observe, the scheme typically consists of one or more breaker switched capacitor units along with an intelligent control unit, current (CT) and voltage (VT) transformers, which are connected at the low side of the supply transformer. These banks often include three to nine capacitor units connected in three-phase grounded-wye, ungrounded-wye, or delta configurations [5]. In practice, commercial installations employ switched capacitor banks, instead of fixed banks, in which the capacitors units are switched on and off automatically to compensate for changing load conditions (minimum condition up to peak load) [17], [18]. Fig. 2 illustrates a typical kilovar demand over a 24 h period [5]. This curve can be determined by a recording kilovar meter or calculated using kilowatt and power factor measurements. The fixed banks satisfy the base load requirements, and the switched banks compensate for the inductive kilovar peak during the heavier load periods [5], [17]. Fig. 2. Application of switched and fixed capacitors for a time varying kvar demand condition. Fig. 3. Conventional strategy for power factor correction through switched capacitor banks. In order calculate the capacitive kilovars necessary to correct to a new, higher power factor, one must subtract the inductive kvar of the corrected power factor from the existing power factor. The difference is the amount of capacitive kvar to be added to the system. The following formula is a convenient way of doing this [5]: where: kw kvar is the system kilowatt load; is the amount of capacitive kilovar to be added. The capacitor switching control scheme illustrated in Fig. 1 is based on a local automatic controller. The control senses voltage and current, and uses either these parameters directly or a derived parameter like power factor to compare against a threshold. Fig. 3 shows a flowchart that illustrates the conventional strategy of the automatic controller based on the measured power factor. Accordingtotheflowchart above, the following steps are performed for PF correction [4], [19]: (1)

3 1564 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 3, SEPTEMBER 2012 Fig. 4. Parallel resonance at a point of common coupling (PCC). 1) The power factor control is performed by controlling the opening and closing of the capacitor switches based on the measured power factor. 2) The control unit measures voltage (VT) and current (CT) on the feeder side (as shown in Fig. 1) and the results of the computed power factor is compared to the predetermined target power factor setting. 3) Using measured and target power factors and the capacitor information, the control unit determines if one or more capacitor banks need to be switched on or off to bring the actual power factor as close as possible to the targeted power factor setting. Based on the above considerations, automatic capacitor controllers have been developed and marketed for commercial power system designers and operators. While such controllers work well for traditional passive loads such as motors, more and more facility operators have reported frequent capacitor failures or trips. As a result, reactive power compensation cannot be achieved. This problem is caused by the parallel resonance between the capacitor and the upstream impedance. This resonance is excited by the harmonic currents produced by modern facility loads such as office electronics and variable frequency drives. Industry, therefore, has a strong need for capacitor controllers that can perform power factor correction on one hand and can avoid harmonic resonance on the other. In addition, the controller shall not require additional sensors or inputs and can retrofit current controllers with zero alteration to the existing facility. In the next section, the resonance problem is explained in detail. III. HARMONIC RESONANCE PROBLEM With any application of capacitor banks, there is always the risk of resonance. This is due to the interaction of the bank s capacitance with the inductive reactance characteristics of the supply system. Harmonic currents at or near the resonant frequency can create high harmonic voltages across the high parallel impedance and the capacitor may not be able to withstand the resonance voltage, leading to fuse blowing or capacitor damage [5], [18]. In order to facilitate the description of the resonance problem, Fig. 4 is used to represent a harmonic-producing commercial facility with a shunt PF correction capacitor connected at the PCC with the distribution system. In this figure, the impedance and current source represent the linear and nonlinear loads of the facilities, respectively [20]. Assume that the supply system can be represented by a Thévenin impedance of,whereh Fig. 5. Frequency response of the combined system and capacitor impedances. is the harmonic order (or per-unit frequency normalized to the fundamental frequency). The total impedance seen by the harmonic current source can be determined as The inductive reactance of the supply system impedance increases and the capacitive reactance decreases as the frequency increases, or as the harmonic order increases. At a given harmonic frequency in any system where a capacitor exists, there will be a crossover point where the inductive and capacitive reactances are equal. Consequently, the total impedance approaches infinity and a very high voltage harmonic may result if the commercial facility harmonic current has a frequency close to where is the system short-circuit level and is the capacitor size. The above frequencyiscalledtheresonancefrequency of the system. In this case, the resonant components and are in parallel. The resulting resonance is called parallel resonance. The parallel resonance phenomenon can also be visualized from a frequency scan plot, as shown in Fig. 5. This figure illustrates how both system and capacitor reactances change with the frequency. At the resonance frequency both reactances are equal and total impedance seen from the capacitor location will tend to a very large value. It is extremely unlikely that these two impedances are exactly identical, but near resonance can be very damaging as well. For example, consider a system fault level of 250 MVA and a capacitor bank rating of 10.8 Mvar. Substituting these numbers on (3) yields the following: The parallel resonance order of 4.83 is too close to the 5th harmonic order and if any magnitude of 5th harmonic current flows from the harmonic-producing loads into the power system at the capacitor bus, the capacitor may not be able to withstand the resonance voltage, leading to fuse blowing or capacitor damage. (2) (3)

4 LIN et al.: AN INTELLIGENT CONTROL STRATEGY FOR POWER FACTOR COMPENSATION 1565 TABLE I CAPACITOR LOADING LIMITS ESTABLISHED BY THE IEEE STANDARD A practical (rule of thumb) way to find out whether parallel resonance should be a concern is to use (4), which shows how further away the resonance frequency should be from any dominant harmonic frequency. However, the condition given by (4) is not sufficient because resonance frequency shift can occur due to capacitance deviation [21], for example. Therefore, the final condition to decide if a certain combination of capacitors should be switched is to verify if the stress levels on the capacitors bank meet the limits defined in Table I. When large levels of voltage and current harmonics are present, the ratings are quite often exceeded, resulting in failures. Therefore, the consideration of power capacitor installation should include harmonic resonance analysis at the design stage. Several solutions can be employed for harmonic resonance damping as follows [5]: 1) ungrounding grounded-wye capacitors; 2) changing capacitor bank sizes and/or locations; 3) adding a reactor to an existing capacitor bank; 4) adding a filter capacitor; 5) controlling the capacitor switching scheme to avoid resonance. The installation of filters can bring unacceptable additional operational and capital costs to the PF correction scheme and, furthermore, a detailed harmonic study must be conducted to ensure that the application of the filters will not cause other side effects on both the facility and the distribution power system, such as parallel resonance at harmonic frequencies other than the one targeted by the filter. We think that focusing on a more intelligent algorithm to control the capacitor switching scheme to achieve both power factor correction and resonance avoidance requirements is more of interest for consumers and utilities. Employ an adaptive control to monitor the harmonic distortion and switch the capacitors to avoid resonance might be appropriate for commercial loads where there are numerous switched capacitors coming on and off line randomly [18]. Basically, the idea is to develop a controller that relies on common low cost sensing devices and does not require additional measurements. As a result, the controller can be constructed as a retrofitting device to replace existing power factor correction controllers with little effort. Therefore, a new strategy for the PF correction capacitors bank controller is proposed in this paper and it is discussed in detail in the next section. It is important that the controller is able to achieve both power factor and resonance avoidance requirements under varying demand conditions. (4) IV. PROPOSED CONTROL ALGORITHM Based on the previous discussion, the problem to be solved is to determine the number of capacitor units to be switched that can yield the highest power factor for the facility without causing excessive harmonic stress on the capacitors. Since there are limited numbers of capacitor combinations, the simplest algorithm is to scan through these combinations and pick the best candidate. This approach is doable but is not efficient. Another extreme is to formulate the problem as an optimization problem. Such an approach complicates the problem, it is not guaranteed to converge and it might be time-consuming. More importantly, they cannot be easily implemented into a microcontroller. In this paper, a practical, efficient, and robust algorithm is proposed. Easy implementation is one of the main considerations of the algorithm. It is important to note that the switching control algorithm is only one of the components of the controller. The algorithm needs the system impedance information as input. There is also a need to detect if a capacitor is being overstressed due to changing harmonic conditions. Therefore, the proposed controller actually has at least the following three major functions: A. measurement of the system impedance; B. detection of resonance condition; C. determination of capacitor units. The following subsections provide description of each one of the above functions. A. Measurement of the System Impedance In the previous section, it was discussed that in order to detect a resonance condition, it is necessary to determine the system impedance. One important issue is that the system impedance is not constant, but varies due to loading and topological changes on the system. Therefore, the following issue must be solved, how the harmonic resonance condition can be determined for a time varying load demand and topology? A number of impedance measurement methods have been developed, which can be classified into two types: the transients-based methods and the steady-state-based methods. The transients-based methods inject transient disturbances into the system. The frequency-dependent network impedances are extracted from voltage and current transients [14]. The main problems associated with these methods are the need for a high-speed data acquisition system and for the source of disturbances. The steady-state-based methods use pre- and postdisturbance steady-state waveforms [14] [16]. Typical disturbances are harmonic current injections produced by an external source or switching of a network component. Since there are no transients involved, the methods can only determine network impedances at harmonic frequencies. Since there is no need for a high-speed data acquisition system, the steady-state method can be implemented with many common, low-cost power quality monitors and it relies on the common voltage and current transformer sensors illustrated on Fig. 1. The simplest form of the steady-state measurement method involves the switching of a network component at the location where the network impedance is to be measured. Assuming that there is a shunt capacitor available for switching, the basic idea of this method can be summarized as follows [14], [15]:

5 1566 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 3, SEPTEMBER 2012 Fig. 6. Relationship of the resonance frequency and the number of switched shunt capacitor units. 1) Record the steady-state waveforms of the capacitor voltages and currents. If the capacitor is not connected, its currents are treated as zero. 2) Changes are then made to the status of the capacitor. For example, a capacitor unit can be switched on or off to meet the power factor requirement. 3) The postdisturbance steady-state voltage and current waveforms are recorded. 4) Discrete Fourier transform (DFT) is applied to the pre- and postdisturbance waveforms. For each harmonic, the following system equations can be developed: (5) (6) where and are the predisturbance hth harmonic current and voltage, and and are the post-disturbance hth harmonic current and voltage. and are the internal system voltage and system impedance, respectively. 5) The system harmonic impedances can be determined from the above two equations as follows: The impedance does not include the switched capacitor impedance. Practical implementation and field experiences regarding this method to calculate can be found on [16]. B. Detection of Resonance Condition As mentioned before, from the system impedance and the existing capacitor impedance, the resonance frequency can be calculated through (3). For a certain system impedance (or system fault level, ), the number of capacitor units that lead to a harmonic resonance frequency equal or close to the dominant harmonic frequencies. Fig. 6 illustrates, for a particular system impedance, how the harmonic resonance frequency changes as more and more capacitor units are switched on. For the figure below, the system is represented by a transformer of 1600 kva with reactance of 6.0% and each capacitor (7) unit has a capacity of 50 kvar. From this figure, it is clear that the resonance frequency can be shifted from a harmonic frequency by changing the number of switched capacitor units. A practical way to verify if is too close to any harmonic frequency is to apply (4). In the example of Fig. 6, if 11 capacitors are switched ON, the resonance frequency is too close to the 7th harmonic order (point A in the figure), therefore, the bank should be increased or decreased. If two more capacitor are switched (13 in total), is around 6.4 (point B in the figure), which is further away from 5th and 7th harmonic orders. However, it is also necessary to evaluate for the current combination of capacitor units if its loading conditions meet the limits specified by Table I. One can also observe from Fig. 6 that more than one combination of capacitor units can be considered to avoid resonance. In the following subsections, it will be shown the criteria to select the most appropriate combination. In this paper, each combination refers to a particular number of capacitor units to be switched on to the circuit. C. Determination of Capacitor Units The final step is to determine the number of capacitor units that can be switched without violating power factor and resonance constraints. From the previous subsection, it is possible to estimate, from the current system impedance, the combinations of capacitor units that can be switched so that the resonance frequency is further away from the harmonic frequencies. This can be done through the following steps: 1) The system impedance calculated from the last capacitor switching is used as input. 2) Substituting (3) in (4) yields (8), from which it is possible to determine the combinations of capacitors that can be switched. Normally, the dominant harmonic frequencies are the odd harmonic orders from 3 to 29 3) Among the combinations found in step 2), it is possible to determine which combinations (kvar) lead to a power factor between utility lower and upper limits. This verification can be done as follows: 4) From the combinations found in step 3), select the combination that lead to minimum switching relative to the current capacitor bank configuration. 5) Calculate the anticipated loading for this combination this using the indices presented in Table I. 6) If loading indices meet the standard limits, switch the combination, otherwise discard this option from the combinations obtained in step 3) and go back to step 4) to select a suboptimal solution. The next subsection combines the previously discussed functionalities into a single flowchart. (8) (9)

6 LIN et al.: AN INTELLIGENT CONTROL STRATEGY FOR POWER FACTOR COMPENSATION 1567 TABLE II PARAMETERS OF A TYPICAL DISTRIBUTION POWER TRANSFORMER TABLE III ACTIVE AND REACTIVE POWER DEMAND OF THE CASE STUDY FACILITY Fig. 7. Flowchart of the proposed capacitor bank controller. D. Proposed Controller Flowchart Fig. 7 presents the flowchart of the proposed capacitor bank controller combining the previously discussed functionalities to achieve acceptable utility power factor level as well as harmonic resonance avoidance. The flowchart is composed of the following steps: 1) Read voltage and current from VT and CT sensors as illustrated in Fig. 1 and switch on a capacitor unit. 2) Read postswitching voltage and current and calculate harmonic system impedance from (7). 3) Determine the combinations of capacitor units that meet the resonance constraint given by (8). 4) Read voltage/current and calculate power factor (PF). 5) If PF is within utility lower and upper limits go to step 6), otherwise go to step 7). Normally, is around 0.92 (depending on each utility company) and is equal to 1. 6) If the capacitor bank loading indices (definedintablei) are below standard limits go back to step 4), otherwise go to step 7). 7) Using (9), determine the combinations of capacitor units from those determined in Step 3) that leads to PF between and. 8) Select the combination that leads to minimum switching relative to the current capacitor bank configuration. 9) If the bank loading indices (defined in Table I) are below the standard limits switch the selected combination, otherwise go to step 11). 10) After the switching, if the capacitor bank loading indices are below standard limits go back to step 2), otherwise go to step 11). 11) Remove the combination selected in step 8) from those determined in step 7). Go back to step 8). One can observe from the flowchart of Fig. 7 that the proposed controller is simple and practical and has other advantage as follows: It does not require additional measurements relying on common voltage and current transformer sensors. The controller does not require the installation of reactors and filters. Furthermore, the controller not only checks if the harmonic resonance frequency is further away from any harmonic frequency, but the capacitor stress levels are also verified to ensure the selection of the most appropriate combination of capacitor units. The controller takes into account the time-varying system conditions to determine both the power factor and the resonance condition. The proposed controller also preserves the objective of conventional controllers, which is to achieve the highest facility power factor. In the following section, some case studies are conducted to evaluate how the proposed controller performs. V. CASE STUDIES A case study is analyzed in this section to illustrate the proposed control method. Suppose the parameters of a typical power transformer feeding a commercial building as shown in Table II.

7 1568 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 3, SEPTEMBER 2012 TABLE IV HARMONIC CURRENT LIMITS OF THE INVESTIGATED SYSTEM ACCORDING TO IEEE [20] Fig. 9. Harmonic resonance frequencies obtained for different combinations of switched on capacitors (n). Fig. 8. Equivalent circuit of the transformer and switched on shunt capacitors. TABLE V COMBINATIONS OF SWITCHED ON CAPACITORS LEADING TO HARMONIC RESONANCE FREQUENCY CLOSE TO TYPICAL HARMONIC ORDERS From the parameters of the power transformer, the secondaryside short-circuit current at the PCC is The facility is equipped with a PF correction scheme composed of units of shunt capacitor of 20 kvar each one. The demand of active and reactive power from the facility is provided in Table III. In this table, the maximum demand load current (fundamental frequency) is Therefore, the ratio of to is approximately equal to 24. According to the standard IEEE [20], the harmonic current limits corresponding to each harmonic order could be calculated, shown in Table IV. It is known that the system impedance mostly depends on the impedance of the power transformer for LV distribution system. According to the parameter of the power transformer and shunt capacitors, the equivalent circuit of the transformer and shunt capacitor could be drawn in Fig. 8, in which, the values are in p.u. units and base value is 1000 kva. and are the resistance and inductance of the power transformer, is the harmonic order, is the number of the switched on capacitors and are the resistance and capacitance of the combination of switched on capacitors. From Fig. 8, the resonance frequency corresponding to different combinations of switched on capacitors can be determined, as shown in Fig. 9. Fig. 9 shows that when the number of switched on capacitors is 6, 8, 12, 19, or 20, the resonance frequency does not meet the constraint defined by (8), which means that the resonance frequency is so close to the characteristic harmonics that the Fig. 10. Total impedance obtained for different harmonic orders and combinations of switched on capacitors. capacitors could be damaged. Table V shows the values of the resonance frequency for these combinations. Taking the base impedance as 0.23 ohms, the total impedance from Fig. 8 can be calculated for different and, as shown in Fig. 10. Table IV gives the current limits corresponding to each harmonic order in LV distribution system. If multiplying the current limits and the total impedance, the voltage limit in each harmonic order can be determined. Therefore, the total RMS working voltage of the capacitors can be expressed as (10) where is the fundamental voltage, istheharmonicvoltage and is the maximum harmonic order. The maximum working voltage of the capacitors for different combinations of switched on capacitors considering the case study is shown in Fig. 11.

8 LIN et al.: AN INTELLIGENT CONTROL STRATEGY FOR POWER FACTOR COMPENSATION 1569 Fig. 13. Number of the switched on capacitors and corresponding power factors before and after compensation using proposed control strategy. Fig. 11. Total working voltage of capacitor bank for different combinations of switched on capacitors (n). B. PF Correction With the Proposed Control Strategy Supposing the system harmonic impedance could be precisely measured by using the capacitor switching, for simplicity, the system harmonic impedance is assumed to be Fig. 12. Number of the switched on capacitors and corresponding power factor before and after compensation using conventional control strategy. For the first sample point given in Table III, the number of switched on capacitors can be firstly determined as 6 to meet. From the system impedance,the resonance frequency is calculated as, which does not respect the requirement definedby(8)andmeansthat cannot be selected as 6. Instead, if, the new PF can be estimated as, which is acceptable. Similarly for other cases, the corrected PF can be calculated for all the sample points given in Table III, as shown in Fig. 13. One can observe from both Figs. 13 and 11 that the new control strategy avoids the harmonic resonance problem, which, in turn, it can prevent damage to the capacitor bank of the facility. It is observed that the working voltage of the capacitors exceed the 110% when the number of the switch-on capacitors is equal to 6, 8, 12, or 20. A. PF Correction With Traditional Control Strategy Assume the required threshold PF range for the facility is. According to the flowchart of the traditional capacitor controller shown in Fig. 3, the required bank size necessary to meet this PF range can be calculated from (1). Fig. 12 shows the existing and corrected power factor with the traditional strategy and the number of switched on capacitor units considering the demand profile provided in Table III. It is obvious that, the number of the switched on capacitors is equal to 6 for sample points 1 and 2 and 8 for sample points 3, 4, and 5. From Fig. 11, one can observe that these combinations cause the bank working voltage to exceed the 110% limit defined in [5]. VI. CONCLUSION This paper presents a new control strategy for power factor compensation on distorted low voltage power systems. The proposed strategy can perform power factor correction without exciting harmonic resonance under varying demand conditions. Practical and robust control algorithms are proposed for the purpose of easy implementation in a microcontroller. In addition, the controller relies on common low cost sensing devices and does not require additional hardware circuits. As a result, the proposed controller can be constructed as a retrofitting device to replace existing power factor correction controllers with little effort and low cost. Analysis of representative case studies validates the proposed strategy and illustrates how the proposed controller performs. REFERENCES [1] IEEE Recommended Practice for Electric Power Systems in Commercial Buildings, ANSI/IEEE Std , Gray Book. [2] T. M. Blooming and D. J. Carnovale, Capacitor application issues, IEEE Trans. Ind. Appl., vol. 44, no. 4, pp , Jul

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