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Voltage sag and swell mitigation based on modulated carrier PWM
Electrical Power and Energy Systems 66 (2015) 78–85
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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Voltage sag and swell mitigation based on modulated carrier PWM S. Abdul Rahman a, P.A. Janakiraman a,⇑, P. Somasundaram b a b
Mohamed Sathak A.J. College of Engineering, Chennai 603103, India Anna University, Chennai 600025, India
a r t i c l e
i n f o
Article history: Received 17 December 2013 Received in revised form 23 August 2014 Accepted 12 September 2014
Keywords: Direct Voltage Restorer (DVR) Voltage sag Voltage swell Modulated Carrier Pulse Width Modulation (MCPWM)
a b s t r a c t The voltage sags or swells which are normally encountered in distribution systems can be compensated by three-phase Direct Voltage Regulators (DVR). In general, when the sag or swell in any phase is sought to be compensated by using the power available in the other phases, extensive signal processing is required, since the magnitudes and phase-angles differ very much. A new control procedure for generating the pwm signals is presented, which is mostly analog and not at all computation-intensive. Illustrations are provided in which the swell or sag can be brought down by using the power either from the other two phases or power from the same phase. The emphasis is not to establish the superiority or otherwise of these arrangements of diverting power from any one phase to the other phase for mitigating the sag or swell. The core of the work deals with the simplicity of the new pwm generation procedure employed for such diversion of power. The feed-forward nature of the control leads to stable operation. Its effectiveness has been demonstrated by simulations. Ó 2014 Elsevier Ltd. All rights reserved.
Introduction Most of the electrical equipment are sensitive to the voltage swells and sags. Dynamic Voltage Restorers (DVR’s) have been installed in the three-phase distribution networks as well as the transmission lines to maintain the desired voltage-magnitude, symmetry and the relative phase angles. The necessity for providing voltage restorers for performance enhancement, as well as the mitigation of sags and swells in voltage are discussed in [1–3]. The financial risks, losses and the associated failure of industrial processes is available in [4]. The classification of power quality disturbances and their cost effective treatment are considered in [5–7]. The voltage sag profile can be estimated by modeling the process cycle, plant-load profile, and voltage sag-records. The application of DVR’s for sag or swell mitigation, after the islanding of a portion of the grid has been discussed in [8]. Shunt and series compensators to eliminate sag, swell and harmonics have been considered in [9]. Of the various power disturbances, the voltage sags due to the starting of heavy induction motors and short-circuit faults appear to be the major issues [10–13]. Switching large capacitors or the removal of large loads and single phase to ground faults may be the reason for the occurrence of a voltage swell [14]. To prevent the excessive load being taken by a particular PV cell array or a wind turbine in micro-grids, during the voltage sags, the ⇑ Corresponding author. Tel.: +91 98841 33821. E-mail addresses: [email protected] (S. Abdul Rahman), pajraman@ yahoo.com (P.A. Janakiraman), [email protected] (P. Somasundaram). http://dx.doi.org/10.1016/j.ijepes.2014.09.017 0142-0615/Ó 2014 Elsevier Ltd. All rights reserved.
DVR’s have been found to be useful [15,16]. Asymmetrical voltage sags have been found to have a serious effect on the operation of wind generators. The DVR’s are becoming popular in the feeders of residential distribution systems [17]. Different schemes exist for the quick estimation of the sag, so that control action can be carried out as early as possible [18–23]. Two types of voltage restorers or compensators are seen in the technical literature. Of these, the d.c. link type uses a rectifier with large capacitor storage banks, d.c. batteries, flywheels, photo-voltaic arrays or even super capacitors. The d.c. sources in conjunction with an inverter generate the required single-phase or three-phase compensating voltages. Many interesting references are available on this topic [24–28]. As against the d.c. link type of a.c. compensators, another type, known as the direct a.c to a.c converters are available, which are interesting in the sense that they do not require much storage elements like the batteries or condensers. Dynamic Voltage Restorers (DVRs) have been developed for regulating the power supply to critical loads which inject a voltage of required magnitude, phase angle, and frequency in series with the line and the load to mitigate the sag or swell [29,30]. The series voltage compensation can be achieved by using the AC to AC direct converters. These do not suffer from the disadvantages of bulky d.c. links [31–34]. Power can be drawn from the same phase or the other phases of a three phase system to mitigate the sag or swell in any or all the phases. Frequently, three single-phase direct a.c. to a.c. converters are designed and put into use for individual control of the three phases [35,36]. The control procedure itself is varied. Different
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schemes have been proposed for the compensation of unbalance in three-phase systems. Symmetrical components procedure has been attractive since it eliminates explicitly the negative sequence and zero sequence components [37–39]. The direct and quadrature split up of the voltages helps in achieving unity power factor after compensation [1]. Cross phase compensation of three phase systems has been described in [40]. A new vector control strategy has been proposed in [41]. Different control schemes for the control of DVR’s have been extensively reported in [42–45]. In [46] a direct converter based DVR with series boosting transformer (of ratio 1:1) has been presented with five bidirectional switches per phase (including the bypass switch). The duty ratios for switches were computed throughout the compensation process. The compensation range of voltage sag is 33% and 100% for the swell. A matrix converter based DVR with four bi-directional switches reported in [47], enabled the compensation of maximum sag of 25% and a swell of 50% with a series boosting transformer of ratio 3:1. While the comparison of the performance of different strategies is interesting, one should not lose sight of the enormous computational burden and the complexity of the existing procedures. In order to avoid intensive duty ratio computations, a modified PWM procedure is proposed in which, the switches are controlled by a modulated carrier, making the control very simple [48]. DVR topology The DVR schematic is shown in Fig. 1, which has been synthesized using a minimum number (=4) of bidirectional switches (Fig. 2) per phase. It consists essentially of three independent single-phase direct converters which can be connected to the series (1:1) injection transformers. The dither in the p.w.m. is filtered out by RLC filters on the load side. The dither induced high frequency currents are prevented from flowing into the mains by the inductors Za, Zb, Zc and the shunt capacitors. However, the voltage drop across the filter inductors, (due to harmonic currents drawn while compensation), cause load voltage distortion. Hence these are kept as small as possible. The by-pass switches Sga, Sgb, Sgc are used in conjunction with the appropriate converter switches during the DVR operation. When there is no sag or swell, these switches are kept closed fully.
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Fig. 2. Bi-directional switch.
voltage vA(t) is unity under normal conditions (no swell/no sag). The measurement block shown in Fig. 3, determines the peak of the fundamental component of a-phase voltage. A single phase digital adaptive observer [49] has been employed to achieve this. The on-line observer acts as a low pass filter and provides the fundamental component in the mains voltage u1 = Va sin(xt) as well as the quadrature voltage u2 = Va cos(xt). These signals, which are devoid of the harmonics, have been used to estimate the peak qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi as: V a ¼ u21 þ u22 . Now, Va is compared with the unity (Vref = 1) to estimate the magnitude (=lA) of voltage sag or swell. The adaptive observer acts as phase-locked-loop, while simultaneously dividing the period of the signal tracked into 128 parts. At the nominal mains frequency of 50 Hz, it also provides a pulse-train at 25.6 kHz (=4 128 50), which is used for generating the triangular dither at 6.4 kHz [49]. The frequency of the triangular dither signal would naturally vary in tune with the wandering mains frequency, so that the number of divisions of the mains-sine-wave remains constant at 128, enabling the use of commonly available DSP algorithms [50]. This scheme is also programmed to give a unit ^ a ðtÞ, so that the compensation signal lA sin(xt) can be sine wave v generated as shown in Fig. 3.
Control methodology Sag and swell estimation The individual phase voltages are stepped down by potential transformers and scaled such that (Va) the peak of the secondary
Fig. 1. DVR topology.
Fig. 3. Sag and swell estimation for a-phase.
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Compensation using power from the same phase An example to illustrate the new control procedure for generating the pwm signals is now considered. The compensation is done by injecting a voltage derived from the same phase in this illustration. The basic switching pulse generation schematic is shown in Fig. 4. A positive difference between Vref (=1) and the peak value of the normalized load voltage Va, indicates a sag lA in the a-phase. The supply voltage is not a constant d.c., but the voltage-sinusoid of phase-A. The saw tooth carrier (S) is modulated by the scaled down power supply voltage signal |vA(t)|. The error signal |eA(t)| = |lAsin(xt)| is generated using lA and the unit sine-wave v^ aðtÞ. This is compared with the modulated carrier signal |vA(t) S(t)|. Using the comparator and logic gates, the ‘‘on–off’’ driving pulses for the switches Saa and Sga are generated. It may be noted that the switch Saa is in series and Sga is in parallel with the injection transformer of phase-A. Incidentally, this arrangement takes care of the effect of power supply voltage fluctuations [48]. A shortfall in the magnitude of phase-A due to sag, causes the corresponding reduction in the carrier amplitude, thereby increasing the pwm-gain in a synchronous manner. Likewise, for swell compensation, an increase in the magnitude of the phase voltage causes a reduction of the pwm-gain synchronously. The procedure obviates the necessity of point by point computations throughout a cycle. It is not even necessary to insert a voltage feed-back loop to make the compensation quite precise. The generation and the use of modulated carrier |vA(t)| S(t) for sag compensation is shown in Fig. 4. Compensation using power from other phases In order to compensate the voltage fluctuations in any phase, power from the other two phases can be utilized by the converter. For a given phase, (for example, the a-phase), the duration of the converter switches for compensation is illustrated in Fig. 5. The
Table 1 Active switches for swell compensation. Switches used
a-Phase swell
b-Phase swell
c-Phase swell
(+) Sab, Sga () Sac, Sga
(+) Sbc, Sgb () Sba, Sgb
(+) Sca, Sgc () Scb, Sgc
error in phase-a viz., eA(t) is shown to an enlarged scale, which has been used for control purposes instead of the voltage signal vA(t). (in Fig. 5, all the voltages have been normalized by 100 V) Now, a swell in a-phase can be compensated by diverting some power from b-phase during the interval T1a (see Fig. 5) for the positive half cycle of eA(t) and during T3a for the negative half cycle. Similarly, c-phase is used during T2a, for the positive half cycle of eA(t) and T4a, for the negative half cycle. So the bidirectional switch-pairs (Sab, Sga) and (Sac, Sga) will be alternatively fired in the proper sequence. Also the series switch and the parallel switches should not be ‘on’ simultaneously. A dead time is required for the transition between the series-switch to parallel switch (and vice versa). Semi soft commutation technique is contemplated in this work. The switches which are active for swell compensation are listed in Table 1. The amplitude of the normalized grid voltage is compared with unity for estimating the error voltage (swell) to be compensated. Three phase-locked loops have been employed for tracking the individual phase voltages. The PLL associated with the swelling ^ a ðtÞ, which when used in conphase, provides a unit-sine-wave v junction with the magnitude of the swell ‘lA’, results in the generation of the real-time sinusoidal error voltage eA(t) = lA sin(xt). This is compared with the ‘modulated carrier’ signal for generating the required switching pulses. The modulating voltage depends on the interval under consideration. During the interval T1a to T3a, the modulating voltage for the saw tooth carrier is |(vB(t))| and during T2a, T4a, it is |(vC(t))|. The block schematic for swell compensation is shown in Fig. 6. This arrangement is very similar to the scheme suggested in [48] for getting a very pure sinusoidal voltage from an inverter which has to work with a fluctuating power supply. The above arrangement, which uses the power derived from the other two phases, can be used for sag compensation also,
Carrier modulated sinusoidal PWM generation
Fig. 4. Switching pulse generation in a-phase, using power from the same phase.
It could be observed from Fig. 5, that, for the positive half cycle of a-phase swell eA(t), the negative portion of the b-phase during the interval T1a and the negative portion of c-phase during the interval T2a will provide the power supply for the direct converter. Taken together, the power supply voltage of the direct converter is neither sinusoidal nor a fixed voltage but it is a variable voltage
Fig. 5. Switching intervals for compensation in a-phase using power from b-phase and c-phase.
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Fig. 6. Compensation of a-phase using power from b and c phases.
Fig. 7. Generation of modulated carrier (a) |vB(t)|, |vC(t)| (b) Carrier (c) Modulated by |vB(t)|, |vC(t)|.
Fig. 8. Mitigation of sag and swell – control schematic.
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with two peak voltage envelopes for each half cycle of a-phase voltage as shown in Fig. 7a. From this fluctuating non-sinusoidal input voltage, the converter has to generate a sinusoidal phase voltage. In the new control approach presented in this paper, the normal triangular carrier wave is amplitude modulated by the fluctuating input voltage itself, so as to generate the pulses required for synthesising a sinusoidal compensating voltage [48]. The upper half of the SIMULINK model shown in Fig. 8, depicts the sag-control, while the lower half generates the modulated carrier signal to mitigate the swell in a-phase. The required portions of the stepped down b-phase and c-phase voltages are gated using pulses like (T1a, T3a) or (T2a, T3a) and concatenated to yield the double humped waveform already seen in Fig. 7a. The plain triangular carrier signal shown in Fig. 7b is modulated by (multiplied with) this double humped waveform and the result is seen Fig. 7c. This procedure obviates the necessity for keeping track of the fluctuations in vB(t) and vC(t), their phase deviations, and other computations as stated earlier. The compensation for swell in real time for the a-phase can be summarized follows: (a) The swell magnitude is determined by comparing the amplitude of the a-phase voltage with a fixed reference. ^ a ðtÞ is derived using a PLL which (b) A real-time unit sine-wave v tracks the a-phase voltage.
(c) The unit-sine wave is multiplied with the swell-error-magnitude to get the real time swell error eA(t). (d) The usual triangular dither (carrier) is modulated (multiplied) with the double humped waveform generated from the other two (viz., b and c) phases. (e) The gating pulses for Sba, Sca and Sga are derived from this modulated carrier and the real time swell error signal eA(t). Simulation results In the simulations, the input voltage was assumed to be 100 V r.m.s (1 p.u.) at 50-Hz to signify 100%. The load comprised of three 20 X resistors put in the star formation. The saw-tooth carrier signal frequency was set at 6.4 kHz. Three passive RLC filters (0.1 mH, 0.5 X || 85 lF) were used to remove the dither signals in the output. The injection transformers have a turns-ratio of 1:1. The capacity of the DVR is approximately 500VA. The DVR is started from the initially relaxed condition. This is the reason for the transients near the starting point (t = 0). However the output (load) voltage quickly reaches the 100% within a cycle. Next, a balanced sag of 50% was induced in all the phases, which starts at t = 20 ms and lasts continuously. For a balanced sag of 50%, the voltage signals before compensation, the compensating-voltages, the load voltage after sag compensation are illustrated in
Fig. 9. Balanced sag compensation. (a) Sag on all the 3 lines: 50%. (b) Compensation voltages. (c) 3-Phase load voltage.
Fig. 10. Unbalanced sag compensation. (a) Sag in A 50%, sag in B 25% sag in C 10%. (b) Compensation voltages. (c) 3-Phase load voltage.
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Fig. 11. Balanced swell compensation. (a) Balanced swell of 100%. (b) Compensation voltages. (c) 3-Phase load voltage.
Fig. 12. Mitigation of unbalanced voltage swell. (a) Swell-A 20%, B 40%, C 60%. (b) Compensation voltage. (c) 3-Phase load voltage.
Fig. 9. The ability of the DVR, to mitigate unbalanced voltage-sag of 50% in a-phase, 25% in b-phase, and 10% in c-phase is shown in Fig. 10. During a balanced three-phase swell, voltage of all the three lines may reach a level of 200%, which can be mitigated by the voltages picked up from the alternate phases. Fig. 11 shows the ability of the DVR to mitigate balanced voltage swell of 100%. It may however be noted that only 50% of swell happening in just one phase can be successfully compensated due to the lack of the required voltage in the other two phases. However, in Fig. 12, the compensation of unbalanced swell of 60% in C is shown. The success is due to the excess voltage available in the other two phases, viz., 20% swell in A and 40% swell in B.
Discussion For a small sag in voltage, compensation can be done by extracting power from the same phase, provided, the input line current does not become excessive. The advantage of taking power from the same phase is that, there will be no harmonic currents pushed into the line by the compensation process. For large sags of course, the power from the other two lines can be taken, assuming that these lines have not experienced voltage sag. In such case, there may not be any harmonics in the load voltage or the load current. However, on the mains side, there would be harmonic currents. It
may be noted that the dual of the scheme, namely, the correction of the sag in any phase using the other two phases, and the swell using the same phase would also work well. Only small modifications are required to the layout shown in Fig. 8. The top block now ascertains the swell (peak error should be now < 0). The bottom two blocks are used after ascertaining sag (peak error > 0). After these modifications, the polarity of the three injection transformers must also be reversed. Under such a condition, the circuit was tested for a swell of 20% in phase-A with simultaneous sag of 20% in phase-B and phase-C. The compensating voltages are shown in Fig. 13. The filtered 3-phase mains currents show the presence of harmonics in Fig. 14. This is caused by the double humped voltages used for deriving the power from the other two phases. The THD of the compensating voltages are less than 1.5%. However, the THD of the compensated voltage would be very small (<0.8%).As for as the current is concerned, to prevent the high frequency currents from flowing into the mains, LC filters can be used. However, the larger the inductance, the greater will be distortion in the load voltage due to the current-harmonics. A smaller inductance would necessarily mean a higher carrier frequency current flowing into the mains. In the simulations, the LC filter at the input (about 25– 100 lH with a shunt 85 lF capacitor) was used. While the compensation is reasonably perfect, a small error in the output voltage was observed. This error increases with the load current. The error could be modeled to be due to a ‘series resistance-drop’ of the regulator. The main reason for the dip in voltage (about 4 V) under a full load
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precision of compensation by the feed forward strategy is quite high. The total harmonic distortion (THD) in the correction voltages depends on the filter inductor on the mains side, which can be reduced by using a higher carrier frequency and a smaller filter inductor. It is less than 1.5% on full load. However the THD in the output voltage after compensation would be far less (0.8%). After modifying the error signal to include the current induced voltage drop, the persistent amplitude error was found to be about 0.5%. Fig. 13. Compensating voltages for swell-A 20%; sag-B 20%; sag-C 20%.
Fig. 14. Filtered mains current of the DVR under full load (6 A).
current (of 6 A) is that, the voltage pulses developed across the injection transformers do not reach their full level within the pulse-width-time available. The pulses are not of the perfectly rectangular shape due to the non-completion of their transient development. This can be overcome by adding to the voltage error signal, a proportional amount of filtered current signal. The filter time constant was about 1 ms. With this arrangement, the output voltage reached 99.6% even on full load. Interestingly, the sag and swell compensation as well as the compensation due to the load current are of the feed-forward type. Hence, no stability problems would arise. Extra feed-back compensation also need not be provided because, the open-loop compensation itself takes care of the voltage fluctuations as well as the voltage drop due to the load current. The control procedure can be made more general. Even while attempting to correct the sag or swell using the power supply derived from the same phase, a small phase shift, for eg., between vA(t) and eA(t), may necessitate the logical use of power supply available in the other two phases. For such algorithms also, the modulated carrier scheme would be very helpful, obviating the necessity of extensive computations. Conclusion The DVR control schemes presented do not require the dc link as in conventional schemes. The compensation for swell/sag in any phase can be carried out using the voltages taken from the other two phases in succession. This can be modeled as if a sinewave inverter is constrained to work under a fluctuating power supply. Under such conditions, modulating the carrier by the fluctuating power supply enables a very pure sinusoidal output voltage to be generated. Since the amplitude of the triangular dither is altered in synchronism with these fluctuations, the p.w.m. gain gets suitably modified so as to make the compensation perfect, in spite of the sag, swell or fluctuations in the power supply. Many other topologies can also be considered for which the gating pulses could now be generated using a modulated carrier wave, resulting in an enormous reduction of the computation-burden. The
References [1] Omar R, Rahim NA, Ahmad A. Voltage swells improvement in low voltage network using dynamic voltage restorer. Am J Appl Sci 2011;8(1):55–62. [2] Bhumkittipich Krischonme, Mithulananthan Nadarajah. Performance enhancement of DVR for mitigating voltage sag/swell using vector control strategy. Energy-Procedia 2011;9:366–79. [3] Elphick S, Smith V, Gosbell V, Drury G, Perera S. Voltage sag susceptibility of 230 V equipment. Gener Trans Distrib IET 2013;7(6):576–83. [4] Chan Jhan Yhee, Milanovic JV, Delahunty A. Risk-based assessment of financial losses due to voltage sag. IEEE Trans Power Delivery 2011;26(2):492–500. [5] Bulent V, Ali K, Mehmet U. A power quality monitoring system based on MATLAB server pages. Turk J Elec Eng Comp Sci 2010;18:313–26. [6] Brumsickle WE, Schneider RS, Luckjiff GA, Divan DM, Mcgranaghan MF. Dynamic sag correctors: cost-effective industrial power line conditioning. IEEE Trans Ind Appl 2001;37:212–7. [7] Brahmadesam Viswanathan K, Baskaran K. Image pattern recognition technique for the classification of multiple power quality disturbances. Turk J Elec Eng Comp Sci 2013;21:656–78. [8] Trindade FCL, Do Nascimento KV, Vieira JCM. Investigation on voltage sags caused by DG anti-islanding protection. IEEE Trans Power Delivery 2013;28(2):972–80. [9] Jowder FAL. Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation. Gener Trans Distrib IET 2009;3(6):547–60. [10] Chandrasekar Perumal, Kamaraj Vijayarajan. Detection and classification of power quality disturbance waveform using MRA based modified wavelet transform and neural networks. J Elect Eng 2010;61:235–40. [11] Klaic´ Zvonimir, Sljivac Damir, Baus Zoran. Probability density functions of voltage sags measured indices. J Elect Eng 2011;62:335–41. [12] Woodley NH, Morgan L, Sundaram A. Experience with an inverter-based dynamic voltage restorer. IEEE Trans Power Delivery 1999;14:1181–6. [13] Sánchez PR, Acha E, Calderon Jeo, Feliu V, Cerrada AG. A versatile control scheme for a dynamic voltage restorer for power-quality improvement. IEEE Trans Power Delivery 2009;24:277–84. [14] Boonchiam P, Mithulananthan N. Dynamic control strategy in medium voltage DVR for mitigating voltage sags/swells. PowerCon 2006. In: International conference on power system technology, October 22–26, 2006. p. 1–5. [15] Ramasamy M, Thangavel S. Experimental verification of PV based dynamic voltage restorer with significant energy conservation. Int J Electr Power Energy Syst 2013;49(July):296–307. [16] Eskander Mona Naguib, Amer Sanaa Ibrahim. Mitigation of voltage dips and swells in grid-connected wind energy conversion systems. IETE J Res 2011;57(6):515–24. [17] Shahni Farhad, Ghosh Arindam, Ledwich Gerard, Zare Firuz. Voltage unbalance improvement in low voltage residential feeders with rooftop PVs using custom power devices. Int J Elect Power Energy Syst 2014;55(February):362–77. [18] Najafi E, Yatim AHM. A novel current mode controller for a static compensator utilizing Goertzel algorithm to mitigate voltage sags. Energy Convers Manage 2011;52(4):1999–2008. [19] Gencer Özgür, Öztürk Semra, Erfidan Tarik. A new approach to voltage sag detection based on wavelet transform. Int J Electr Power Energy Syst 2010;32(2):133–40. [20] Bae B, Lee J, Jeong J, Han B. Line-interactive single-phase dynamic voltage restorer with novel sag detection algorithm. IEEE Trans Power Delivery 2010;25(4):2702–9. [21] Goswami AK, Gupta CP, Singh GK. Voltage sag assessment in a large chemical industry. IEEE Trans Ind Appl 2012;48(5):1739–46. [22] Bae B, Jeong J, Lee J, Han B. Novel sag detection method for line-interactive dynamic voltage restorer. IEEE Trans Power Delivery 2010;25(2):1210–1. [23] Badrkhani Ajaei F, Afsharnia S, Kahrobaeian A, Farhangi S. A fast and effective control scheme for the dynamic voltage restorer. IEEE Trans Power Delivery 2011;26(4):2398–406. [24] Omar R, Rahim NA. Voltage unbalanced compensation using dynamic voltage restorer based on super capacitor. Int J Electr Power Energy Syst 2012;43(1):573–81. [25] Tiwari HP, Gupta Sunil Kumar. DC energy storage schemes for DVR voltage sag mitigation system. Int J Computer Theory Eng 2010;2(3):1793–8201. [26] Zaveri Tejas, Bhalja BR, Zaveri Naimish. Load compensation using DSTATCOM in three-phase, three-wire distribution system under various source voltage and delta connected load conditions. Int J Electr Power Energy Syst 2012;41(1):34–43. [27] Singh Bhim, Jayaprakash P, Kothari DP. New control approach for capacitor supported DSTATCOM in three-phase four wire distribution system under
S. Abdul Rahman et al. / Electrical Power and Energy Systems 66 (2015) 78–85
[28]
[29]
[30] [31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
non-ideal supply voltage conditions based on synchronous reference frame theory. Int J Electr Power Energy Syst 2011;33(5):1109–17. Wang Bingsen, Venkataramanan G. Dynamic voltage restorer utilizing a matrix converter and flywheel energy storage. IEEE Trans Ind Appl 2009;45(1):222–31. Omar Rosli, Rahim NA. New configuration of a three phase dynamic voltage restorer (DVR) for voltage disturbances mitigation in electrical distribution system. Arab J Sci Eng 2012;37:2205–20. Muhamad M, Nasrudin A. Phase angle analysis for three-phase PWM switched autotransformer voltage-sag compensator. Arab J Sci Eng 2012;37:1987–2001. Al-hadidi HK, Gole AM, Jacobson DA. A novel configuration for a cascade inverter-based dynamic voltage restorer with reduced energy storage requirements. IEEE Trans Power Delivery 2008;23:881–8. Li YW, Vilathgamuwa DM, Blaabjerg F, Loh PC. Investigation and improvement of transient response of DVR at medium voltage level. IEEE Trans Ind Appl 2007;43:1309–19. Meyer C, De Doncker RW, Li YW, Blaabjerg F. Optimized control strategy for a medium-voltage DVR theoretical investigations and experimental results. IEEE Trans Power Electron 2008;23:2746–54. Vilathgamuwa DM, Perera ADR, Choi SS. Voltage sag compensation with energy optimized dynamic voltage restorer. IEEE Trans Power Delivery 2003;18:928–36. Won Dong-Jun, Park Sang-Young, Moon Seung-Il, Park Jong-Keun, Chung IlYop. The DC link energy control method in dynamic voltage restorer system. Int J Electr Power Energy Syst 2003;25(7):525–31. Babaei Ebrahim, Kangarlu Mohammad Farhadi. Operation and control of dynamic voltage restorer using single-phase direct converter. Energy Convers Manage 2011;52(8–9):2965–72. Ding Hongfa, Shuangyan Shu, Xian zhong Duan, Jun Gao. A novel dynamic voltage restorer and its unbalanced control strategy based on space vector PWM. Int J Electr Power Energy Syst 2002;24(9):693–9. Abdollahzadeh Hamed, Jazaeri Mostafa, Tavighi Arash. A new fast-converged estimation approach for Dynamic Voltage Restorer (DVR) to compensate voltage sags in waveform distortion conditions. Int J Electr Power Energy Syst 2014;54:598–609.
85
[39] Li Yun Wei, Mahinda Vilathgamuwa D, Blaabjerg F, Loh Poh Chiang. A robust control scheme for medium-voltage-level DVR implementation. IEEE Trans Ind Electron 2007;54(4):2249–61. [40] Babaei Ebrahim, Kangarlu Mohammad Farhadi. Cross-phase voltage sag compensator for three-phase distribution systems. Int J Electr Power Energy Syst 2013;51:119–26. [41] Bhumkittipich Krischonme, Mithulananthan Nadarajah. Performance enhancement of DVR for mitigating voltage sag/swell using vector control strategy. Energy Procedia 2011;9:366–79. [42] Massoud AM, Ahmed S, Enjeti PN, Williams BW. Evaluation of a multilevel cascaded-type dynamic voltage restorer employing discontinuous space vector modulation. IEEE Trans Ind Electron 2010;57(7):2398–410. [43] Babaei E, Kangarlu MF, Sabahi M. Mitigation of voltage disturbances using dynamic voltage restorer based on direct converters. IEEE Trans Power Delivery 2010;25(4):2676–83. [44] Roncero-Sanchez P, Acha E, Ortega-Calderon JE, Feliu V, Garcia-Cerrada A. A versatile control scheme for a dynamic voltage restorer for power-quality improvement. IEEE Trans Power Delivery 2009;24(1):277–84. [45] Soeiro TB, Petry CA, dos S Fagundes JC, Barbi I. Direct AC–AC converters using commercial power modules applied to voltage restorers. IEEE Trans Industr Electron 2011;58(1):278–88. [46] Cheng Po-Tai, Chen Jhao-Ming, Ni Chia-Lung. Design of a state-feedback controller for series voltage-sag compensators. IEEE Trans Ind Appl 2009;45(1):260–7. [47] Pérez J, Cárdenas V, Morán L, Núñez C. Single-phase ac-ac converter operating as a dynamic voltage restorer (DVR). IEEE Ind Electron IECON 2006;2006:1938–43. [48] Janakiraman PA, Abdul Rahman S. Linear pulse width modulation technique under fluctuating power supply. IEEE Trans Industr Electron 2013;61:1769–73. [49] Sumathi P, Janakiraman PA. Integrated phase locking scheme for SDFT based harmonic analysis of periodic signals. IEEE Trans Circuits Syst II Express Briefs Jan 2008;55(1):51–5. [50] Sumathi P, Janakiraman PA. Phase locking scheme based on LUT assisted SDFT for low power acoustic signals. IET Circ Dev Syst 2011;5(6):491–504.
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Voltage sag and swell mitigation based on modulated carrier PWM S. Abdul Rahman a, P.A. Janakiraman a,⇑, P. Somasundaram b a b
Mohamed Sathak A.J. College of Engineering, Chennai 603103, India Anna University, Chennai 600025, India
a r t i c l e
i n f o
Article history: Received 17 December 2013 Received in revised form 23 August 2014 Accepted 12 September 2014
Keywords: Direct Voltage Restorer (DVR) Voltage sag Voltage swell Modulated Carrier Pulse Width Modulation (MCPWM)
a b s t r a c t The voltage sags or swells which are normally encountered in distribution systems can be compensated by three-phase Direct Voltage Regulators (DVR). In general, when the sag or swell in any phase is sought to be compensated by using the power available in the other phases, extensive signal processing is required, since the magnitudes and phase-angles differ very much. A new control procedure for generating the pwm signals is presented, which is mostly analog and not at all computation-intensive. Illustrations are provided in which the swell or sag can be brought down by using the power either from the other two phases or power from the same phase. The emphasis is not to establish the superiority or otherwise of these arrangements of diverting power from any one phase to the other phase for mitigating the sag or swell. The core of the work deals with the simplicity of the new pwm generation procedure employed for such diversion of power. The feed-forward nature of the control leads to stable operation. Its effectiveness has been demonstrated by simulations. Ó 2014 Elsevier Ltd. All rights reserved.
Introduction Most of the electrical equipment are sensitive to the voltage swells and sags. Dynamic Voltage Restorers (DVR’s) have been installed in the three-phase distribution networks as well as the transmission lines to maintain the desired voltage-magnitude, symmetry and the relative phase angles. The necessity for providing voltage restorers for performance enhancement, as well as the mitigation of sags and swells in voltage are discussed in [1–3]. The financial risks, losses and the associated failure of industrial processes is available in [4]. The classification of power quality disturbances and their cost effective treatment are considered in [5–7]. The voltage sag profile can be estimated by modeling the process cycle, plant-load profile, and voltage sag-records. The application of DVR’s for sag or swell mitigation, after the islanding of a portion of the grid has been discussed in [8]. Shunt and series compensators to eliminate sag, swell and harmonics have been considered in [9]. Of the various power disturbances, the voltage sags due to the starting of heavy induction motors and short-circuit faults appear to be the major issues [10–13]. Switching large capacitors or the removal of large loads and single phase to ground faults may be the reason for the occurrence of a voltage swell [14]. To prevent the excessive load being taken by a particular PV cell array or a wind turbine in micro-grids, during the voltage sags, the ⇑ Corresponding author. Tel.: +91 98841 33821. E-mail addresses: [email protected] (S. Abdul Rahman), pajraman@ yahoo.com (P.A. Janakiraman), [email protected] (P. Somasundaram). http://dx.doi.org/10.1016/j.ijepes.2014.09.017 0142-0615/Ó 2014 Elsevier Ltd. All rights reserved.
DVR’s have been found to be useful [15,16]. Asymmetrical voltage sags have been found to have a serious effect on the operation of wind generators. The DVR’s are becoming popular in the feeders of residential distribution systems [17]. Different schemes exist for the quick estimation of the sag, so that control action can be carried out as early as possible [18–23]. Two types of voltage restorers or compensators are seen in the technical literature. Of these, the d.c. link type uses a rectifier with large capacitor storage banks, d.c. batteries, flywheels, photo-voltaic arrays or even super capacitors. The d.c. sources in conjunction with an inverter generate the required single-phase or three-phase compensating voltages. Many interesting references are available on this topic [24–28]. As against the d.c. link type of a.c. compensators, another type, known as the direct a.c to a.c converters are available, which are interesting in the sense that they do not require much storage elements like the batteries or condensers. Dynamic Voltage Restorers (DVRs) have been developed for regulating the power supply to critical loads which inject a voltage of required magnitude, phase angle, and frequency in series with the line and the load to mitigate the sag or swell [29,30]. The series voltage compensation can be achieved by using the AC to AC direct converters. These do not suffer from the disadvantages of bulky d.c. links [31–34]. Power can be drawn from the same phase or the other phases of a three phase system to mitigate the sag or swell in any or all the phases. Frequently, three single-phase direct a.c. to a.c. converters are designed and put into use for individual control of the three phases [35,36]. The control procedure itself is varied. Different
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schemes have been proposed for the compensation of unbalance in three-phase systems. Symmetrical components procedure has been attractive since it eliminates explicitly the negative sequence and zero sequence components [37–39]. The direct and quadrature split up of the voltages helps in achieving unity power factor after compensation [1]. Cross phase compensation of three phase systems has been described in [40]. A new vector control strategy has been proposed in [41]. Different control schemes for the control of DVR’s have been extensively reported in [42–45]. In [46] a direct converter based DVR with series boosting transformer (of ratio 1:1) has been presented with five bidirectional switches per phase (including the bypass switch). The duty ratios for switches were computed throughout the compensation process. The compensation range of voltage sag is 33% and 100% for the swell. A matrix converter based DVR with four bi-directional switches reported in [47], enabled the compensation of maximum sag of 25% and a swell of 50% with a series boosting transformer of ratio 3:1. While the comparison of the performance of different strategies is interesting, one should not lose sight of the enormous computational burden and the complexity of the existing procedures. In order to avoid intensive duty ratio computations, a modified PWM procedure is proposed in which, the switches are controlled by a modulated carrier, making the control very simple [48]. DVR topology The DVR schematic is shown in Fig. 1, which has been synthesized using a minimum number (=4) of bidirectional switches (Fig. 2) per phase. It consists essentially of three independent single-phase direct converters which can be connected to the series (1:1) injection transformers. The dither in the p.w.m. is filtered out by RLC filters on the load side. The dither induced high frequency currents are prevented from flowing into the mains by the inductors Za, Zb, Zc and the shunt capacitors. However, the voltage drop across the filter inductors, (due to harmonic currents drawn while compensation), cause load voltage distortion. Hence these are kept as small as possible. The by-pass switches Sga, Sgb, Sgc are used in conjunction with the appropriate converter switches during the DVR operation. When there is no sag or swell, these switches are kept closed fully.
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Fig. 2. Bi-directional switch.
voltage vA(t) is unity under normal conditions (no swell/no sag). The measurement block shown in Fig. 3, determines the peak of the fundamental component of a-phase voltage. A single phase digital adaptive observer [49] has been employed to achieve this. The on-line observer acts as a low pass filter and provides the fundamental component in the mains voltage u1 = Va sin(xt) as well as the quadrature voltage u2 = Va cos(xt). These signals, which are devoid of the harmonics, have been used to estimate the peak qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi as: V a ¼ u21 þ u22 . Now, Va is compared with the unity (Vref = 1) to estimate the magnitude (=lA) of voltage sag or swell. The adaptive observer acts as phase-locked-loop, while simultaneously dividing the period of the signal tracked into 128 parts. At the nominal mains frequency of 50 Hz, it also provides a pulse-train at 25.6 kHz (=4 128 50), which is used for generating the triangular dither at 6.4 kHz [49]. The frequency of the triangular dither signal would naturally vary in tune with the wandering mains frequency, so that the number of divisions of the mains-sine-wave remains constant at 128, enabling the use of commonly available DSP algorithms [50]. This scheme is also programmed to give a unit ^ a ðtÞ, so that the compensation signal lA sin(xt) can be sine wave v generated as shown in Fig. 3.
Control methodology Sag and swell estimation The individual phase voltages are stepped down by potential transformers and scaled such that (Va) the peak of the secondary
Fig. 1. DVR topology.
Fig. 3. Sag and swell estimation for a-phase.
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Compensation using power from the same phase An example to illustrate the new control procedure for generating the pwm signals is now considered. The compensation is done by injecting a voltage derived from the same phase in this illustration. The basic switching pulse generation schematic is shown in Fig. 4. A positive difference between Vref (=1) and the peak value of the normalized load voltage Va, indicates a sag lA in the a-phase. The supply voltage is not a constant d.c., but the voltage-sinusoid of phase-A. The saw tooth carrier (S) is modulated by the scaled down power supply voltage signal |vA(t)|. The error signal |eA(t)| = |lAsin(xt)| is generated using lA and the unit sine-wave v^ aðtÞ. This is compared with the modulated carrier signal |vA(t) S(t)|. Using the comparator and logic gates, the ‘‘on–off’’ driving pulses for the switches Saa and Sga are generated. It may be noted that the switch Saa is in series and Sga is in parallel with the injection transformer of phase-A. Incidentally, this arrangement takes care of the effect of power supply voltage fluctuations [48]. A shortfall in the magnitude of phase-A due to sag, causes the corresponding reduction in the carrier amplitude, thereby increasing the pwm-gain in a synchronous manner. Likewise, for swell compensation, an increase in the magnitude of the phase voltage causes a reduction of the pwm-gain synchronously. The procedure obviates the necessity of point by point computations throughout a cycle. It is not even necessary to insert a voltage feed-back loop to make the compensation quite precise. The generation and the use of modulated carrier |vA(t)| S(t) for sag compensation is shown in Fig. 4. Compensation using power from other phases In order to compensate the voltage fluctuations in any phase, power from the other two phases can be utilized by the converter. For a given phase, (for example, the a-phase), the duration of the converter switches for compensation is illustrated in Fig. 5. The
Table 1 Active switches for swell compensation. Switches used
a-Phase swell
b-Phase swell
c-Phase swell
(+) Sab, Sga () Sac, Sga
(+) Sbc, Sgb () Sba, Sgb
(+) Sca, Sgc () Scb, Sgc
error in phase-a viz., eA(t) is shown to an enlarged scale, which has been used for control purposes instead of the voltage signal vA(t). (in Fig. 5, all the voltages have been normalized by 100 V) Now, a swell in a-phase can be compensated by diverting some power from b-phase during the interval T1a (see Fig. 5) for the positive half cycle of eA(t) and during T3a for the negative half cycle. Similarly, c-phase is used during T2a, for the positive half cycle of eA(t) and T4a, for the negative half cycle. So the bidirectional switch-pairs (Sab, Sga) and (Sac, Sga) will be alternatively fired in the proper sequence. Also the series switch and the parallel switches should not be ‘on’ simultaneously. A dead time is required for the transition between the series-switch to parallel switch (and vice versa). Semi soft commutation technique is contemplated in this work. The switches which are active for swell compensation are listed in Table 1. The amplitude of the normalized grid voltage is compared with unity for estimating the error voltage (swell) to be compensated. Three phase-locked loops have been employed for tracking the individual phase voltages. The PLL associated with the swelling ^ a ðtÞ, which when used in conphase, provides a unit-sine-wave v junction with the magnitude of the swell ‘lA’, results in the generation of the real-time sinusoidal error voltage eA(t) = lA sin(xt). This is compared with the ‘modulated carrier’ signal for generating the required switching pulses. The modulating voltage depends on the interval under consideration. During the interval T1a to T3a, the modulating voltage for the saw tooth carrier is |(vB(t))| and during T2a, T4a, it is |(vC(t))|. The block schematic for swell compensation is shown in Fig. 6. This arrangement is very similar to the scheme suggested in [48] for getting a very pure sinusoidal voltage from an inverter which has to work with a fluctuating power supply. The above arrangement, which uses the power derived from the other two phases, can be used for sag compensation also,
Carrier modulated sinusoidal PWM generation
Fig. 4. Switching pulse generation in a-phase, using power from the same phase.
It could be observed from Fig. 5, that, for the positive half cycle of a-phase swell eA(t), the negative portion of the b-phase during the interval T1a and the negative portion of c-phase during the interval T2a will provide the power supply for the direct converter. Taken together, the power supply voltage of the direct converter is neither sinusoidal nor a fixed voltage but it is a variable voltage
Fig. 5. Switching intervals for compensation in a-phase using power from b-phase and c-phase.
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Fig. 6. Compensation of a-phase using power from b and c phases.
Fig. 7. Generation of modulated carrier (a) |vB(t)|, |vC(t)| (b) Carrier (c) Modulated by |vB(t)|, |vC(t)|.
Fig. 8. Mitigation of sag and swell – control schematic.
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with two peak voltage envelopes for each half cycle of a-phase voltage as shown in Fig. 7a. From this fluctuating non-sinusoidal input voltage, the converter has to generate a sinusoidal phase voltage. In the new control approach presented in this paper, the normal triangular carrier wave is amplitude modulated by the fluctuating input voltage itself, so as to generate the pulses required for synthesising a sinusoidal compensating voltage [48]. The upper half of the SIMULINK model shown in Fig. 8, depicts the sag-control, while the lower half generates the modulated carrier signal to mitigate the swell in a-phase. The required portions of the stepped down b-phase and c-phase voltages are gated using pulses like (T1a, T3a) or (T2a, T3a) and concatenated to yield the double humped waveform already seen in Fig. 7a. The plain triangular carrier signal shown in Fig. 7b is modulated by (multiplied with) this double humped waveform and the result is seen Fig. 7c. This procedure obviates the necessity for keeping track of the fluctuations in vB(t) and vC(t), their phase deviations, and other computations as stated earlier. The compensation for swell in real time for the a-phase can be summarized follows: (a) The swell magnitude is determined by comparing the amplitude of the a-phase voltage with a fixed reference. ^ a ðtÞ is derived using a PLL which (b) A real-time unit sine-wave v tracks the a-phase voltage.
(c) The unit-sine wave is multiplied with the swell-error-magnitude to get the real time swell error eA(t). (d) The usual triangular dither (carrier) is modulated (multiplied) with the double humped waveform generated from the other two (viz., b and c) phases. (e) The gating pulses for Sba, Sca and Sga are derived from this modulated carrier and the real time swell error signal eA(t). Simulation results In the simulations, the input voltage was assumed to be 100 V r.m.s (1 p.u.) at 50-Hz to signify 100%. The load comprised of three 20 X resistors put in the star formation. The saw-tooth carrier signal frequency was set at 6.4 kHz. Three passive RLC filters (0.1 mH, 0.5 X || 85 lF) were used to remove the dither signals in the output. The injection transformers have a turns-ratio of 1:1. The capacity of the DVR is approximately 500VA. The DVR is started from the initially relaxed condition. This is the reason for the transients near the starting point (t = 0). However the output (load) voltage quickly reaches the 100% within a cycle. Next, a balanced sag of 50% was induced in all the phases, which starts at t = 20 ms and lasts continuously. For a balanced sag of 50%, the voltage signals before compensation, the compensating-voltages, the load voltage after sag compensation are illustrated in
Fig. 9. Balanced sag compensation. (a) Sag on all the 3 lines: 50%. (b) Compensation voltages. (c) 3-Phase load voltage.
Fig. 10. Unbalanced sag compensation. (a) Sag in A 50%, sag in B 25% sag in C 10%. (b) Compensation voltages. (c) 3-Phase load voltage.
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Fig. 11. Balanced swell compensation. (a) Balanced swell of 100%. (b) Compensation voltages. (c) 3-Phase load voltage.
Fig. 12. Mitigation of unbalanced voltage swell. (a) Swell-A 20%, B 40%, C 60%. (b) Compensation voltage. (c) 3-Phase load voltage.
Fig. 9. The ability of the DVR, to mitigate unbalanced voltage-sag of 50% in a-phase, 25% in b-phase, and 10% in c-phase is shown in Fig. 10. During a balanced three-phase swell, voltage of all the three lines may reach a level of 200%, which can be mitigated by the voltages picked up from the alternate phases. Fig. 11 shows the ability of the DVR to mitigate balanced voltage swell of 100%. It may however be noted that only 50% of swell happening in just one phase can be successfully compensated due to the lack of the required voltage in the other two phases. However, in Fig. 12, the compensation of unbalanced swell of 60% in C is shown. The success is due to the excess voltage available in the other two phases, viz., 20% swell in A and 40% swell in B.
Discussion For a small sag in voltage, compensation can be done by extracting power from the same phase, provided, the input line current does not become excessive. The advantage of taking power from the same phase is that, there will be no harmonic currents pushed into the line by the compensation process. For large sags of course, the power from the other two lines can be taken, assuming that these lines have not experienced voltage sag. In such case, there may not be any harmonics in the load voltage or the load current. However, on the mains side, there would be harmonic currents. It
may be noted that the dual of the scheme, namely, the correction of the sag in any phase using the other two phases, and the swell using the same phase would also work well. Only small modifications are required to the layout shown in Fig. 8. The top block now ascertains the swell (peak error should be now < 0). The bottom two blocks are used after ascertaining sag (peak error > 0). After these modifications, the polarity of the three injection transformers must also be reversed. Under such a condition, the circuit was tested for a swell of 20% in phase-A with simultaneous sag of 20% in phase-B and phase-C. The compensating voltages are shown in Fig. 13. The filtered 3-phase mains currents show the presence of harmonics in Fig. 14. This is caused by the double humped voltages used for deriving the power from the other two phases. The THD of the compensating voltages are less than 1.5%. However, the THD of the compensated voltage would be very small (<0.8%).As for as the current is concerned, to prevent the high frequency currents from flowing into the mains, LC filters can be used. However, the larger the inductance, the greater will be distortion in the load voltage due to the current-harmonics. A smaller inductance would necessarily mean a higher carrier frequency current flowing into the mains. In the simulations, the LC filter at the input (about 25– 100 lH with a shunt 85 lF capacitor) was used. While the compensation is reasonably perfect, a small error in the output voltage was observed. This error increases with the load current. The error could be modeled to be due to a ‘series resistance-drop’ of the regulator. The main reason for the dip in voltage (about 4 V) under a full load
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precision of compensation by the feed forward strategy is quite high. The total harmonic distortion (THD) in the correction voltages depends on the filter inductor on the mains side, which can be reduced by using a higher carrier frequency and a smaller filter inductor. It is less than 1.5% on full load. However the THD in the output voltage after compensation would be far less (0.8%). After modifying the error signal to include the current induced voltage drop, the persistent amplitude error was found to be about 0.5%. Fig. 13. Compensating voltages for swell-A 20%; sag-B 20%; sag-C 20%.
Fig. 14. Filtered mains current of the DVR under full load (6 A).
current (of 6 A) is that, the voltage pulses developed across the injection transformers do not reach their full level within the pulse-width-time available. The pulses are not of the perfectly rectangular shape due to the non-completion of their transient development. This can be overcome by adding to the voltage error signal, a proportional amount of filtered current signal. The filter time constant was about 1 ms. With this arrangement, the output voltage reached 99.6% even on full load. Interestingly, the sag and swell compensation as well as the compensation due to the load current are of the feed-forward type. Hence, no stability problems would arise. Extra feed-back compensation also need not be provided because, the open-loop compensation itself takes care of the voltage fluctuations as well as the voltage drop due to the load current. The control procedure can be made more general. Even while attempting to correct the sag or swell using the power supply derived from the same phase, a small phase shift, for eg., between vA(t) and eA(t), may necessitate the logical use of power supply available in the other two phases. For such algorithms also, the modulated carrier scheme would be very helpful, obviating the necessity of extensive computations. Conclusion The DVR control schemes presented do not require the dc link as in conventional schemes. The compensation for swell/sag in any phase can be carried out using the voltages taken from the other two phases in succession. This can be modeled as if a sinewave inverter is constrained to work under a fluctuating power supply. Under such conditions, modulating the carrier by the fluctuating power supply enables a very pure sinusoidal output voltage to be generated. Since the amplitude of the triangular dither is altered in synchronism with these fluctuations, the p.w.m. gain gets suitably modified so as to make the compensation perfect, in spite of the sag, swell or fluctuations in the power supply. Many other topologies can also be considered for which the gating pulses could now be generated using a modulated carrier wave, resulting in an enormous reduction of the computation-burden. The
References [1] Omar R, Rahim NA, Ahmad A. Voltage swells improvement in low voltage network using dynamic voltage restorer. Am J Appl Sci 2011;8(1):55–62. [2] Bhumkittipich Krischonme, Mithulananthan Nadarajah. Performance enhancement of DVR for mitigating voltage sag/swell using vector control strategy. Energy-Procedia 2011;9:366–79. [3] Elphick S, Smith V, Gosbell V, Drury G, Perera S. Voltage sag susceptibility of 230 V equipment. Gener Trans Distrib IET 2013;7(6):576–83. [4] Chan Jhan Yhee, Milanovic JV, Delahunty A. Risk-based assessment of financial losses due to voltage sag. IEEE Trans Power Delivery 2011;26(2):492–500. [5] Bulent V, Ali K, Mehmet U. A power quality monitoring system based on MATLAB server pages. Turk J Elec Eng Comp Sci 2010;18:313–26. [6] Brumsickle WE, Schneider RS, Luckjiff GA, Divan DM, Mcgranaghan MF. Dynamic sag correctors: cost-effective industrial power line conditioning. IEEE Trans Ind Appl 2001;37:212–7. [7] Brahmadesam Viswanathan K, Baskaran K. Image pattern recognition technique for the classification of multiple power quality disturbances. Turk J Elec Eng Comp Sci 2013;21:656–78. [8] Trindade FCL, Do Nascimento KV, Vieira JCM. Investigation on voltage sags caused by DG anti-islanding protection. IEEE Trans Power Delivery 2013;28(2):972–80. [9] Jowder FAL. Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation. Gener Trans Distrib IET 2009;3(6):547–60. [10] Chandrasekar Perumal, Kamaraj Vijayarajan. Detection and classification of power quality disturbance waveform using MRA based modified wavelet transform and neural networks. J Elect Eng 2010;61:235–40. [11] Klaic´ Zvonimir, Sljivac Damir, Baus Zoran. Probability density functions of voltage sags measured indices. J Elect Eng 2011;62:335–41. [12] Woodley NH, Morgan L, Sundaram A. Experience with an inverter-based dynamic voltage restorer. IEEE Trans Power Delivery 1999;14:1181–6. [13] Sánchez PR, Acha E, Calderon Jeo, Feliu V, Cerrada AG. A versatile control scheme for a dynamic voltage restorer for power-quality improvement. IEEE Trans Power Delivery 2009;24:277–84. [14] Boonchiam P, Mithulananthan N. Dynamic control strategy in medium voltage DVR for mitigating voltage sags/swells. PowerCon 2006. In: International conference on power system technology, October 22–26, 2006. p. 1–5. [15] Ramasamy M, Thangavel S. Experimental verification of PV based dynamic voltage restorer with significant energy conservation. Int J Electr Power Energy Syst 2013;49(July):296–307. [16] Eskander Mona Naguib, Amer Sanaa Ibrahim. Mitigation of voltage dips and swells in grid-connected wind energy conversion systems. IETE J Res 2011;57(6):515–24. [17] Shahni Farhad, Ghosh Arindam, Ledwich Gerard, Zare Firuz. Voltage unbalance improvement in low voltage residential feeders with rooftop PVs using custom power devices. Int J Elect Power Energy Syst 2014;55(February):362–77. [18] Najafi E, Yatim AHM. A novel current mode controller for a static compensator utilizing Goertzel algorithm to mitigate voltage sags. Energy Convers Manage 2011;52(4):1999–2008. [19] Gencer Özgür, Öztürk Semra, Erfidan Tarik. A new approach to voltage sag detection based on wavelet transform. Int J Electr Power Energy Syst 2010;32(2):133–40. [20] Bae B, Lee J, Jeong J, Han B. Line-interactive single-phase dynamic voltage restorer with novel sag detection algorithm. IEEE Trans Power Delivery 2010;25(4):2702–9. [21] Goswami AK, Gupta CP, Singh GK. Voltage sag assessment in a large chemical industry. IEEE Trans Ind Appl 2012;48(5):1739–46. [22] Bae B, Jeong J, Lee J, Han B. Novel sag detection method for line-interactive dynamic voltage restorer. IEEE Trans Power Delivery 2010;25(2):1210–1. [23] Badrkhani Ajaei F, Afsharnia S, Kahrobaeian A, Farhangi S. A fast and effective control scheme for the dynamic voltage restorer. IEEE Trans Power Delivery 2011;26(4):2398–406. [24] Omar R, Rahim NA. Voltage unbalanced compensation using dynamic voltage restorer based on super capacitor. Int J Electr Power Energy Syst 2012;43(1):573–81. [25] Tiwari HP, Gupta Sunil Kumar. DC energy storage schemes for DVR voltage sag mitigation system. Int J Computer Theory Eng 2010;2(3):1793–8201. [26] Zaveri Tejas, Bhalja BR, Zaveri Naimish. Load compensation using DSTATCOM in three-phase, three-wire distribution system under various source voltage and delta connected load conditions. Int J Electr Power Energy Syst 2012;41(1):34–43. [27] Singh Bhim, Jayaprakash P, Kothari DP. New control approach for capacitor supported DSTATCOM in three-phase four wire distribution system under
S. Abdul Rahman et al. / Electrical Power and Energy Systems 66 (2015) 78–85
[28]
[29]
[30] [31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
non-ideal supply voltage conditions based on synchronous reference frame theory. Int J Electr Power Energy Syst 2011;33(5):1109–17. Wang Bingsen, Venkataramanan G. Dynamic voltage restorer utilizing a matrix converter and flywheel energy storage. IEEE Trans Ind Appl 2009;45(1):222–31. Omar Rosli, Rahim NA. New configuration of a three phase dynamic voltage restorer (DVR) for voltage disturbances mitigation in electrical distribution system. Arab J Sci Eng 2012;37:2205–20. Muhamad M, Nasrudin A. Phase angle analysis for three-phase PWM switched autotransformer voltage-sag compensator. Arab J Sci Eng 2012;37:1987–2001. Al-hadidi HK, Gole AM, Jacobson DA. A novel configuration for a cascade inverter-based dynamic voltage restorer with reduced energy storage requirements. IEEE Trans Power Delivery 2008;23:881–8. Li YW, Vilathgamuwa DM, Blaabjerg F, Loh PC. Investigation and improvement of transient response of DVR at medium voltage level. IEEE Trans Ind Appl 2007;43:1309–19. Meyer C, De Doncker RW, Li YW, Blaabjerg F. Optimized control strategy for a medium-voltage DVR theoretical investigations and experimental results. IEEE Trans Power Electron 2008;23:2746–54. Vilathgamuwa DM, Perera ADR, Choi SS. Voltage sag compensation with energy optimized dynamic voltage restorer. IEEE Trans Power Delivery 2003;18:928–36. Won Dong-Jun, Park Sang-Young, Moon Seung-Il, Park Jong-Keun, Chung IlYop. The DC link energy control method in dynamic voltage restorer system. Int J Electr Power Energy Syst 2003;25(7):525–31. Babaei Ebrahim, Kangarlu Mohammad Farhadi. Operation and control of dynamic voltage restorer using single-phase direct converter. Energy Convers Manage 2011;52(8–9):2965–72. Ding Hongfa, Shuangyan Shu, Xian zhong Duan, Jun Gao. A novel dynamic voltage restorer and its unbalanced control strategy based on space vector PWM. Int J Electr Power Energy Syst 2002;24(9):693–9. Abdollahzadeh Hamed, Jazaeri Mostafa, Tavighi Arash. A new fast-converged estimation approach for Dynamic Voltage Restorer (DVR) to compensate voltage sags in waveform distortion conditions. Int J Electr Power Energy Syst 2014;54:598–609.
85
[39] Li Yun Wei, Mahinda Vilathgamuwa D, Blaabjerg F, Loh Poh Chiang. A robust control scheme for medium-voltage-level DVR implementation. IEEE Trans Ind Electron 2007;54(4):2249–61. [40] Babaei Ebrahim, Kangarlu Mohammad Farhadi. Cross-phase voltage sag compensator for three-phase distribution systems. Int J Electr Power Energy Syst 2013;51:119–26. [41] Bhumkittipich Krischonme, Mithulananthan Nadarajah. Performance enhancement of DVR for mitigating voltage sag/swell using vector control strategy. Energy Procedia 2011;9:366–79. [42] Massoud AM, Ahmed S, Enjeti PN, Williams BW. Evaluation of a multilevel cascaded-type dynamic voltage restorer employing discontinuous space vector modulation. IEEE Trans Ind Electron 2010;57(7):2398–410. [43] Babaei E, Kangarlu MF, Sabahi M. Mitigation of voltage disturbances using dynamic voltage restorer based on direct converters. IEEE Trans Power Delivery 2010;25(4):2676–83. [44] Roncero-Sanchez P, Acha E, Ortega-Calderon JE, Feliu V, Garcia-Cerrada A. A versatile control scheme for a dynamic voltage restorer for power-quality improvement. IEEE Trans Power Delivery 2009;24(1):277–84. [45] Soeiro TB, Petry CA, dos S Fagundes JC, Barbi I. Direct AC–AC converters using commercial power modules applied to voltage restorers. IEEE Trans Industr Electron 2011;58(1):278–88. [46] Cheng Po-Tai, Chen Jhao-Ming, Ni Chia-Lung. Design of a state-feedback controller for series voltage-sag compensators. IEEE Trans Ind Appl 2009;45(1):260–7. [47] Pérez J, Cárdenas V, Morán L, Núñez C. Single-phase ac-ac converter operating as a dynamic voltage restorer (DVR). IEEE Ind Electron IECON 2006;2006:1938–43. [48] Janakiraman PA, Abdul Rahman S. Linear pulse width modulation technique under fluctuating power supply. IEEE Trans Industr Electron 2013;61:1769–73. [49] Sumathi P, Janakiraman PA. Integrated phase locking scheme for SDFT based harmonic analysis of periodic signals. IEEE Trans Circuits Syst II Express Briefs Jan 2008;55(1):51–5. [50] Sumathi P, Janakiraman PA. Phase locking scheme based on LUT assisted SDFT for low power acoustic signals. IET Circ Dev Syst 2011;5(6):491–504.