A Dynamic Voltage Restorer for Voltage Sag Mitigation in a Refinery with Induction Motors Loads

Transcription

1 merian J. of Engineering and pplied Sienes 3 (): 44-5, 00 ISSN Siene Publiations Dynami Voltage Restorer for Voltage Sag Mitigation in a Refinery with Indution Motors Loads Tarek I. El-Shennawy, bdel-mon em Moussa, Mahmoud. El-Gammal and mr Y. bou-ghazala Department of Eletrial Engineering, lexandria National Refining and Petrohemial o. (NRP), El-Sad El-aly St., Zip ode: 3, Wady El-Kamar, El-Max, lexandria, Egypt Department of Eletrial Power Engineering, Faulty of Engineering, lexandria University, lexandria, Egypt bstrat: Problem statement: NRP is a refinery based in lexandria, Egypt. The plant was subjeted to several shutdowns due to tripping of large indution motors, either by under voltage or by over urrent relays, sometimes by the mehanial protetion. The main ause for suh unplanned shutdowns was voltage sags. The Dynami Voltage Restorer (DVR) has reently been introdued to protet the industrial failities from voltage sags and other voltage disturbanes. Existing onfigurations and ontrol tehniques for the DVR aim at proteting industries of high-teh, loads with adjustable speed drives and other power-eletroni based loads. Industries with indution motors loads require a omplete different approah for the design and ontrol of a suitable DVR. Owing to the inherit inertia of the indution motors and their apability to withstand short-duration, shallow sags, in addition to its tolerane to phase jumps, a DVR with low ost, fast response and simple ontroller ould be onfigured to fulfill the voltage restoration requirements. pproah: In this study, a simple DVR was proposed, whih utilized the lassial Fourier Transform (FT) for sag detetion and quantifiation, a ontroller based on feed-foreword tehnique whih utilized the error signal (differene between the referene voltage and atual measured voltage) to trigger the swithes of an inverter using a Pulse Width Modulation (PWM) sheme. The proposed DVR utilized energy from other available feeder or from energy storage unit through a retifier. Modeling and simulation of the proposed DVR was implemented in the Matlab/Simulink workspae. Results: Simulation results showed that the proposed DVR was effiient in mitigating balaned, unbalaned, multistage and onseutive sags, as well as swells (over-voltages). The main shortoming of the DVR, being a series devie, is its inability to mitigate omplete interruptions. onlusion: The DVR should be onfigured with respet to the load requirements. With respet to indution motors loads with inherit inertia and insensitivity to phase jumps, the proposed DVR would be of lower ost, simpler ontroller and faster response. Key words: Power quality, voltage sag, Dynami Voltage Restorer (DVR), ustom power INTRODUTION Problem statement: lexandria National Refining and Petrohemials o. (NRP) is a refinery based in lexandria, Egypt, with the purpose of produing lead-free, high-otane gasoline. With an average load of 0 MW, the plant suffers from several shutdowns due to voltage sags. lthough lasting for durations in the range of a quarter seond to slightly more than one seond, these voltage sags ause large Indution Motors (IMs) onneted diretly to the supply bus to trip, either by undervoltage or by overurrent relays, sometimes by the mehanial protetion. s the motor torque is diretly proportional to the square of the supply voltage, a derease (sag) to 70% of the rated voltage will ause the motor torque to derease to 49%, whih may not be suffiient for driving the load. To avoid the risk of damage of the motors shafts during voltage sags, strit protetion settings are applied, leading to numerous (sometimes unneessary) shutdowns. previous study on the effets of voltage sags on IMs showed that IMs are insensitive to very short duration sags (and interruptions). IMs were also unaffeted by phase angle jumps assoiated with most of the voltage sags (ElShennawy et al., 009). orresponding uthor: Tarek I. El-Shennawy, Department of Eletrial Engineering, lexandria National Refining and Petrohemial o. (NRP), El-Sad El-aly St., Zip ode: 3, Wady El-Kamar, El-Max, lexandria 44

2 m. J. Engg. & pplied Si., 3 (): 44-5, 00 It follows that a DVR with different approah has to be designed and ontrolled to ountermeasure suh voltage disturbanes. Voltage sags: The IEEE defines voltage sag as: derease to between 0. and 0.9 p.u. in rms voltage or urrent at the power frequeny for durations of 0.5 yle to min. The amplitude of voltage sag is the value of the remaining voltage during the sag (IEEE Std ). The IE terminology for voltage sag is dip. The IE defines voltage dip as: sudden redution of the voltage at a point in the eletrial system, followed by voltage reovery after a short period, from half a yle to a few seonds. The amplitude of a voltage dip is defined as the differene between the voltage during the voltage dip and the nominal voltage of the system expressed as a perentage of the nominal voltage (International Eletrotehnial ommission, 990). Figure shows an rms representation of voltage sag, the sag starts when the voltage dereases to lower than the threshold voltage V thr (0.9 p.u.) at time T. The sag ontinues till T at whih the voltage reovers to a value over the threshold value. The duration of the voltage sag is (T -T ) and the magnitude of the voltage sag is a sag to V sag (Won et al., 005). The prinipal ause of all voltage sags is a short duration inrease in urrent, mainly due to faults as shown in Fig.. Large motor starting and transformer energizing an also result in shallow sags (Bollen, 999). To alulate the sag magnitude at the Point of ommon oupling (P) in radial systems (whih is the ase in industrial distribution networks), it is familiar to use the voltage divider model, shown in Fig., where the voltage magnitude at the P is given by: U sag Zf = E Z + Z s s () Where: Z s = The soure impedane inluding the transformer impedane Z f = The impedane between the P and the fault inluding fault and line impedanes (Bollen, 996) Sag is also assoiated by a hange in voltage phase angle. This hange is alled phase angle jump (i.e., the phase angle between during sag and pre-sag voltages) and is alulated as the argument of the omplex voltage U sag (Djoki and Milanovi, 006). The Dynami Voltage Restorer (DVR): The Dynami Voltage Restorer (DVR), is a devie that utilizes solid state (or stati) power eletroni omponents and is onneted in series to the utility primary distribution iruit. The DVR provides a three phase independently-ontrolled voltage soure, whose voltage vetor (magnitude and angle) adds to the soure voltage to restore the load voltage to pre-sag onditions (Woodley et al., 999). Fig. : Voltage sag Fig. : Faults on parallel feeder ausing voltage sag 45 Fig. 3: Dynami Voltage Restorer (DVR)

3 m. J. Engg. & pplied Si., 3 (): 44-5, 00 Figure 3 is a simplified iruit for the role and loation of the DVR in the distribution system. When a fault ours on the line feeding Load, its voltage ollapses to zero. Load experienes a sag equals to the voltage at the P and the voltage of sensitive load proteted by the DVR is restored to its pre-fault value (Nielsen et al., 004). With referene to Fig. 3, the main omponents of DVR are (Benahaiba and Ferdi, 009): Energy storage unit: The required energy for ompensation of load voltage during sag an be taken either from an external energy storage unit (batteries) or from the supply line feeder through a retifier and a apaitor. Inverter iruit: Sine the vast majority of voltage sags seen on utility systems are unbalaned, mostly due to single-phase events, the VS will often be required to operate with unbalaned swithing funtions for the three phases and must therefore treat eah phase independently. Moreover, a sag on one phase may result in a swell on another phase, so the VS must be apable of handling both sags and swells simultaneously. The variable output voltage of the inverter is ahieved using PWM sheme. Filter unit: The nonlinear harateristis of semiondutor devies ause distorted waveforms assoiated with high frequeny harmonis at the inverter output. To overome this problem and provide high quality energy supply, a harmoni filtering unit is used. This an ause voltage drop and phase shift in the fundamental omponent of the inverter output and has to be aounted for in the ompensation voltage. Series injetion transformers: Three single-phase injetion transformers are used to injet the missing voltage to the system at the load bus. To integrate the injetion transformer orretly into the DVR, the MV rating, the primary winding voltage and urrent ratings, the turn-ratio and the short-iruit impedane values of transformers are required. The existene of the transformers allow for the design of the DVR in a lower voltage level, depending upon the stepping up ratio. In suh ase, the limiting fator will be the ability of the inverter swithes to withstand higher urrents. ompensation (voltage injetion) strategy: The most popular voltage injetion strategies are pre-sag and inphase ompensation methods (Etxeberria-Otadui et al., 00). Pre-sag ompensation method: The DVR injets the differene (missing) voltage between during-sag and pre-sag voltages to the system, the DVR must ompensate for both magnitude and angle, as shown in Fig. 4a. It is the best solution to obtain the same load voltage as the pre-fault voltage and is best suited for loads sensitive to phase angle jumps like SDs and angle-triggered thyristors-ontrolled loads. In-phase voltage injetion method: The injeted voltage is in phase with supply voltage, as shown in Fig. 4b. The phase angles of the pre-sag and load voltage are different but the magnitude of load voltage is the same as the pre-fault voltage. loser look at Fig. 4a shows the following: In normal onditions (pre-sag), the system or supply voltage is equal to the load voltage V L, both are equal to p.u. with zero angle. During sag, the system voltage dereases to a value of V s less than p.u., this redution in voltage is assoiated with a phase angle jump δ. The DVR reats to the sag event and injets a ompensating voltage V dvr to restore the voltage at the load to pre-sag onditions of both magnitude and angle. The method gives nearly undisturbed load voltage. The magnitude of the required voltage by the DVR an be alulated as follows: Vdvr VL Vs VL Vs osδ = + () Whereas the required phase angle θ to be ompensated an be alulated as follows: θ = dvr V sin δ s tan V s os δ V L (3) ontroller and auxiliary iruits: By-pass swithes, breakers, measuring and protetion relays are some auxiliaries to the DVR blok, in addition to the ontroller of the DVR. 46 Fig. 4a: ompensation to pre-sag onditions (magnitude and phase)

4 m. J. Engg. & pplied Si., 3 (): 44-5, 00 In the other strategy, presented in Fig. 4b, the DVR is designed to ompensate the voltage magnitude only. gain, the pre-sag voltage is p.u. with zero angle and during the sag, the system voltage dereases to V L with a phase angle δ. The DVR injets a ompensating voltage V dvr in phase with the system voltage V L, to boost the voltage up to the pre-sag voltage magnitude V s, with no attention to the angle δ. This method is suitable for loads that an withstand phase angle jumps, whih is a typial ase for indution motor loads whih omprise a large portion of the industrial power system, with no sensitive equipment suh as SDs or any equipment depending in its operation on phasetriggered swithes. This method is very simple in implementation, very fast espeially in alulating the DVR ompensation voltage, whih is obviously alulated as: V dvr = V L - V s (4) nd there is no intention for traking or ompensating the phase angle. ontrol shemes: The DVR is operated in: Standby mode, where the inverter is not ative in the iruit to keep the losses to a minimum tive mode, where the DVR senses the sag and reats as fast as possible to injet the required three phase ompensation voltages Bypass mode, where the DVR is disonneted and bypassed in ase of short iruit ourring inside the faility to protet its sensitive omponents from exessive short iruit urrents (Benahaiba and Ferdi, 008) ontrol strategies fall mainly in one of the following two ategories: Linear ontrol methods omprising the feedbak, the feed-foreword and the ombined feed ontrollers Non-Linear ontrol methods omprising the rtifiial Neural Networks (NN), the Fuzzy Logi (FL) and the Spae Vetor (SV) ontrollers lthough feedbak ontrollers are popular, they require load and soure traking, whereas feedforeword ontrollers are muh simpler yet open-looped, there is no feedbak from the load voltage or urrent (Jing et al., 008). MTERILS ND METHODS In this study DVR ontrol strategy based on Inphase ompensation strategy was used, as it had proven to be muh simpler and hene, the ontroller and onsequently the response time would be faster. simple feed-foreword ontroller aquires its voltage values from the soure, with no feedbak from the load was used, aiming at simple and fast response. Fig. 5 shows the blok diagram of the proposed ontroller for the DVR. The ontroller of the proposed DVR onsists of the following bloks: Sag detetor: This inludes determination of the sag start instant, the depth of the sag, the phase jump angle and the sag end instant. In this study, the FT tehnique is used. It requires at least one operating yle to detet the sag start/end events. Other methods in literature inlude the use of rms values, the use of peak values, the transformation of the three phase voltages to a two dimensional frame (dq frame) and therefore to one phasor and the Wavelet Transform (WT) as a multiresolution analysis tehnique. Fig. 4b: In-phase ompensation (magnitude ompensation only) 47 Fig. 5: The blok diagram of the proposed ontroller for the DVR

5 Fig. 6: Pulse Width Modulation (PWM) sheme (a) (b) The referene voltage is the supply kv with ertain tolerane. The DVR will not operate on small voltage variation events to keep the operational losses to a minimum. In this study, a tolerane of 0 V is onsidered. However, this value is very small in pratie and it an be inreased to 550 V (5% of rated voltage). It is worth mentioning that, the proposed DVR, although does not ompensate for the phase angles, yet it traks them. The sag detetor blok uses the traditional Fourier Transform (FT) to alulate both the magnitude and angle of the fundamental omponent of voltage, to make sure that the injeted sine wave will be in-phase with the remaining sine wave during the sag event, to have a onstrutive vetor addition of the DVR and the supply voltages. omputation of the ompensating voltage is done using a omparator with one input as the variable system voltage and the other being the fixed referene voltage. The omparison (subtration) is done for magnitude only, sine the ompensation strategy used is the In-phase method. The output of the omparator determines the voltage required to be injeted by the DVR and is alled the error signal. Note that this is done for eah phase independently. ompensating voltage generation: The inverter is the ore omponent of the DVR and its ontrol will diretly affet the performane of the DVR. s mentioned earlier, a sinusoidal PWM sheme will be used. The m. J. Engg. & pplied Si., 3 (): 44-5, inverter used in this study is a six-pulse inverter, the arrier waveform is a triangular wave with higher frequeny. The modulating index will vary aording to the input error signal from the omparator. The basi idea of PWM is to ompare a sinusoidal ontrol signal of normal 50 Hz frequeny with a modulating (or arrier) triangular pulses of higher frequeny, as shown in Fig. 6. When the ontrol signal is greater than the arrier signal, three swithes of the six are turned on and their ounter swithes are turned off. s the ontrol signal is the error signal, therefore, the output of the inverter will represent the required ompensation voltage (Mohan et al., 00). In this study, the frequeny of the arrier waveform in the PWM was hosen to be 000 Hz. The thyristors in the inverter iruit are hosen to be of type Integrated Gate Bipolar Transistors (IGBT) for their fast response and robust operation. The d voltage might be utilized from alternative supply soure if available. Otherwise, the line voltage is retified and the d energy is stored in large apaitor banks. Injetion of the ompensating voltage: One the error signal magnitude exeeds the tolerane for dynami voltage variation, the iruit breakers lose to onnet the DVR into the iruit via the injeting series transformers. ompensation of any drop in the series voltage injetion was mainly done to ount for the voltage drop and phase shift introdued by the filter and injeting transformers. In this study, the NRP load is in the range of 600, whih will introdue a large voltage drop aross the transformer windings and the filter indutane. 0% over-ompensation was introdued by the ontroller to ounterat any drops. Modeling and simulation: The performane of the DVR was evaluated by using the Matlab/Simulink program as a simulation tool (Tumay et al., 005; Boonhiam and Mithulananthan, 006; Omar et al., 009). The DVR is onneted in series between a three phase programmable (ontrollable) voltage soure with kv line to line rms voltage, 50 Hz and a refinery load of ative power p = 0 MW and reative power Q = MVR (with installation of power fator orretion apaitors). The Simulink model of the proposed DVR is shown in Fig. 7. RESULTS ND DISUSSION In all ases, the results will be arranged as follows: (a) the supply voltage, (b) the DVR voltage and () the load voltage. The simulation time will be se. Note that the minimum operation time of the DVR is yle or 0 m se.

6 m. J. Engg. & pplied Si., 3 (): 44-5, 00 Disrete, s = e-005 s powergui Vab Vab N B Programmable Voltage Soure Vab a B b V-I Measurement Input voltage measurement Load voltage measurement Vab a B B B b Series RL Branh V-I Measurement -0 Minimum allowed swell lok OR 0 Swith Minimum allowed sag 0 Vab DVR voltage measurement magnitude signal -K- angle Fourier magnitude signal angle Fourier magnitude signal -Kdeg-rad -Kdeg-rad Line rms - Phase peak -K- Filter & Trans. ompensation -K- Filter & Trans. ompensation -K- Filter & Trans. ompensation OR OR Swith angle Fourier -Kdeg-rad Swith dva 000 Refrene dvb dv <a gates <b < Vd 3phase SPWM gates a + b - 3 Phase Inverter Ba Bb B B B L Filter Vab a B b V-I Measurement B v Vd Universal Bridge Fig. 7: Simulink model for the proposed DVR using Matlab/Simulink Fig. 8: Three phase balaned voltage sag Three phase balaned sag: The voltage will be dereased to 70% of its normal value, for a duration of 0.4 se from t = 0.3 till t = 0.7, as shown in Fig. 8, where the line voltage between two phases is shown. Fig. 9: onseutive voltage sags Multi-stage sag: The voltage sags on the three phases to 50% for 0. se from t = 0.3 till t = 0.5, after that, the sag prolonged on another stage to 75% for 0.35 se from t = 0.5 till t = 0.85 as shown in Fig. 0. One more, line voltage between two phases is shown. Two onseutive sags: The voltage sags on the three Three phase unbalaned sag: To this ategory belong phases to 65% for 0.5 se from t = 0. to t = 0.35, the most relevant Single-Line-To-Ground (SLG) faults. followed by voltage reovery to 00% and then the next Phase will be sagged to 60% for 0.5 se from t = 0.5 sag ours after 0.3 se with magnitude of 80% and till t = Note that the line voltages Vab and Va lasts for 0. se from t = 0.65 till t = 0.75, as shown in will be affeted, whereas Vb will not. Figure shows Fig. 9. gain line voltage between two phases is shown. the rms representation of Vab. 49

7 m. J. Engg. & pplied Si., 3 (): 44-5, 00 Fig. 0: Multi-stage voltage sags Fig. : Voltage swell Fig. : Single phase voltage sag Fig. 3: Three phase interruption Swell: The sudden removal of large loads or appliation of large apaitor banks may lead to transient voltage rise. This inrease in voltage (swell), although not as destrutive as sags, may lead to insulation failure of the equipment upon times. The DVR must respond to this disturbane as well. In the simulation, voltage swells to 50% on the three phases for 0.3 se from t = 0.3 till t = 0.6, as shown in Fig.. Only the line voltage between two phases is shown. Interruption: s a series ompensator, the DVR must reognize interruptions (omplete loss of power on at least one of the three phases, or voltage drop to less than 0%) and in this ase, the DVR will be bypassed. The interruption lasts for 0.4 se from t = 0.3 till t = 0.7, as shown in Fig ONLUSION In this study, a simple, fast and ost effetive Dynami Voltage Restorer (DVR) was proposed for mitigating the problem of voltage sags in industrial distribution systems, with a large portion of its load onsisting of indution motors. The modeling and simulation of the proposed DVR using Matlab/Simulink had been presented. Detetion and quantization of sags was done using the lassial Fourier Transform (FT) tehnique. alulation of the ompensating voltage was done with referene to voltage only, sine indution motors are not sensitive to hanges in phase angle. ontroller based on feed-foreword tehnique is used whih utilizes the error signal (differene between the referene voltage and atual measured voltage) to

8 m. J. Engg. & pplied Si., 3 (): 44-5, 00 trigger the swithes of an inverter using a Pulse Width Modulation (PWM) sheme. The proposed DVR utilizes energy drawn from the supply line soure during normal operation and stored in apaitors and whih is onverted to an adjustable three phase a voltage suitable for mitigation of voltage sags. The simulation shows that the DVR performane is satisfatory in mitigating voltage sags/swells. The DVR handles both balaned and unbalaned situations without any diffiulties and injets the appropriate voltage omponent to orret rapidly any deviation in the supply voltage to keep the load voltage onstant at the nominal value. The main advantages of the proposed DVR are simple ontrol, fast response and low ost. Future work will inlude a omparison with a laboratory experiments on a low voltage DVR in order to ompare simulation and experimental results. Further attention to the filter onstrution and its parameters seletion would be paid in future work. The passive filter is designed to ut off high order harmonis after the inverter iruit and introdue a negligible voltage drop. Other omponent to be looked for is the injeting transformer. The seletion of the transformer parameters and saturation issues require further investigation. REFERENES Benahaiba,. and B. Ferdi, 008. Voltage quality improvement using dynami voltage restorer. Elet. Power Quality Utiliz. J., 4: Benahaiba,. and B. Ferdi, 009. Power quality improvement using dynami voltage restorer. m. J. pplied Si., 6: Bollen, M., 996. Fast assessment methods for voltage sags in distribution systems. IEEE Trans. Ind. ppli., 3: Bollen, M., 999. Understanding Power Quality Problems: Voltage Sags and Interruptions. st Edn., IEEE Press, Pisataway, NJ., ISBN: 3: , pp: 67. Boonhiam, P. and N. Mithulananthan, 006. Understanding of dynami voltage restorers through Matlab simulation. Thammasat Int. J. Si. Tehnol., : -6. Djoki, S. and J. Milanovi, 006. dvaned voltage sag haraterization. Part I: Phase shift. IEEE Pro. Generat. Transm. Distribut., 53: DOI: 0.049/ip-gtd: ElShennawy, T., M. El-Gammal and. bou-ghazala, 009. Voltage sag effets on the proess ontinuity of a refinery with indution motors loads. m. J. pplied Si., 6: Etxeberria-Otadui, I., U. Visarret, S. Baha, M. aballero and R. Reyero, 00. Evaluation of different strategies for series voltage sag ompensation. Proeeding of the IEEE 33rd nnual Power Eletronis Speialists onferene, June 3-7, airns, Queensland, ustralia, pp: IEEE Std , IEEE reommended pratie for monitoring eletri power quality. pp: 9. International Eletrotehnial ommission, 990. IE TR Eletromagneti ompatibility (EM)-Part : Environment-setion : Desription of the environment. pp: 8. Jing, W., X. iqin and S. Yueyue, 008. survey on ontrol strategies of dynami voltage restorer. Proeeding of the IEEE 3th International onferene Harmonis and Quality of Power (IHQP), Sept. 8-Ot. I, Wollongong, NSW., pp: -5. DOI: 0.09/IHQP Mohan, N., T.M. Undeland and W.P. Robbins, 00. Power Eletronis: onverters. ppliations and Design. nd Edn., John Wiley and Sons, ISBN: , pp: 84. Nielsen, J.G., M. Newman, H. Nielsen and F. Blaabjerg, 004. ontrol and testing of a dynami voltage restorer (DVR) at medium voltage level. IEEE Trans. Power Elet., 9: DOI: 0.09/TPEL Omar, R., N.. Rahim and M. Sulaiman, 009. Modeling and simulation for voltage sags/swells mitigation using dynami voltage restorer. J. Theor. pplied Inform. Tehnol., 5: Tumay, M.,. Teke, K. agatay, M. Bayındır and U. uma, 005. Simulation and modeling of a dynami voltage restorer. Proeeding of the 4th International onferene Eletrial and Eletroni Engineering, De. 7-, Turkey, pp: -5. Won, D., S. hn and S. Moon, 005. modified sag haraterization using voltage tolerane urve for power quality diagnosis. IEEE Trans. Power Delivery, 0: DOI: 0.09/TPWRD Woodley, N., R. Morgan and. Sundaram, 999. Experiene with an inverter-based dynami voltage restorer. IEEE Trans. Power Delivery, 4: