BIDIRECTIONAL SOFT-SWITCHING SERIES AC-LINK INVERTER WITH PI CONTROLLER

Transcription

1 BIDIRECTIONAL SOFT-SWITCHING SERIES AC-LINK INVERTER WITH PI CONTROLLER PUTTA SABARINATH M.Tech (PE&D) K.O.R.M Engineering College, Kadapa Affiliated to JNTUA, Anantapur. ABSTRACT This paper proposes a modified configuration for the dc ac and ac-dc power conversion using PI controller. Here we are using PI controller rather than other controllers. This paper proposes a novel bi-directional inverter, named series ac-link inverter. This three-phase inverter belongs to a new class of partial resonant ac-link converters in which the link is formed by a series ac inductor/capacitor (LC) pair having low reactive ratings. In all of these configurations the ac capacitor is the main energy storage element, and the inductor is merely added to facilitate the zero current turn-off of the switches and their soft turn-on. Due to zero current turn-off of the switches in the proposed converter, the use of SCRs with natural commutation is possible as well. Since the current and voltage of the link are both alternating, no bulky dc electrolytic capacitors are required in this converter. This paper mainly focuses on bidirectional dc to three-phase ac conversion. The performance of the proposed system is verified by simulation using MATLAB/ Simulink software. Index Terms Harmonic resonance, hybrid active filter, industrial power system, PI controller. I. INTRODUCTION Soft-switching ac-link universal power converter also called partial resonant ac-link converter. Being universal, the input and output of this converter may be dc, ac, single phase, or multiphase. Therefore, it can appear as dc dc, dc ac, ac dc, or ac ac configurations. In the ac ac and ac dc configurations were studied in detail. The bidirectional and unidirectional three-phase inverter configurations for battery utility and photovoltaic applications were presented and thoroughly studied.the soft-switching ac-link universal power converter has several ad-vantages over the other types of converters. This converter is an extension of the dc dc buck boost converter. Therefore, unlike matrix converters and three-phase voltage source inverters; it is capable of both stepping up and stepping down the voltage. It can also change the frequency in a wide range. By adding the complementary switches and by modifying the switching scheme, the link inductor, which is the main energy storage element in this converter, can have alternating current MUKKARA REDDY PRASANNA Assistant Professor and H.O.D of EEE K.O.R.M Engineering College, Kadapa Affiliated to JNTUA, Anantapur. instead of the direct current. This approach improves the performance of the converter and significantly increases the utilization of the link inductor. In this converter, the frequency of the link current and voltage is only limited by the characteristics of the switches and the sampling time of the microcontroller. Therefore, the frequency can be very high, which results in compact link and filter components. By placing a small capacitor in parallel with the link inductor, the converter benefits from the soft switching. Therefore, the LC pair has small reactive ratings; and there is low power dissipation in the link. The alternating link current and voltage of the soft-switching ac link universal. In this converter, unlike the resonant converters introduced in the link resonates just for a short time in each link cycle. power converter, eliminates the need for the dc electrolytic capacitors at the link. Electrolytic capacitors are integral parts of the conventional dc-link ac ac converters and the two-stage bidirectional inverters, which are formed by bidirectional dc dc converters and voltage source inverters to pro-vide the possibility of regulating the dc-side current, as shown in Fig. 1. The main problem with the electrolytic capacitor is its highfailure rate, especially at high temperatures. Fig.1. Conventional bidirectional inverter Therefore converters containing electrolytic capacitors are expected to have shorter lifetime. Another advantage of the ac-link converters is the possibility of having galvanic isolation with a single-phase high-frequency transformer added to the link. In a three phase voltage source inverter, galvanic isolation is provided by a bulky three-phase low-frequency transformer. Considering the aforementioned merits,

2 the soft-switching ac-link universal power converter is expected to be more compact and more reliable than conventional converters. II. PROPOSED CONFIGURATION AND PRINCIPLES OF OPERATION The schematic of the dc ac configuration, called ac-link buck boost inverter in this paper, is represented in Fig. 2. Although this inverter is still a new technology, it has a very promising future. Therefore, in case of the dc-ac conversion, the link capacitor will be charged through the dc side and then discharged into the ac side. flows from the dc-side to the ac-side is depicted in Fig. 3. Fig. 4 shows the link current, link voltage and unfiltered line to line voltages in this inverter. Phase pair AB at the ac-side is assumed to have the maximum ac-side line to line voltage. It is obvious that the phase pair carrying the highest instantaneous voltage will continually shift during the operation. It is also assumed that for the time span shown in Figs. 3 and 4 the absolute value of the current of phase A at the ac-side is higher than that of the phase B. Fig.2. Proposed bidirectional ac-link inverter (Series ac-link inverter) However, there are three-phases at the ac side and just one link to be discharged. In order to have lower THDs, the discharging mode will be split into two modes so all three phases are involved. During the second half cycle of the link, the link capacitor will be charged from the input and then discharged into the output phases again; however, this time the link capacitor is charged and discharged in a reverse direction. This is feasible due to the existence of the bidirectional switches and leads to an alternating link voltage, and consequently the elimination of the dc capacitor. In this converter, there is resonating mode between each power transfer mode (charging or discharging modes). The proposed switching scheme is developed such that these resonating modes result in the zero current turn-off of the switches and their soft turn-on. In a three-phase ac to dc conversion, the link capacitor is charged through the ac side, and then discharged into the dc side. Therefore, in this case, the charging mode is split into two modes. Similar to the dc-ac conversion, the link will be charged and discharged in both positive and negative directions and a resonating mode occurs between each power transfer mode. In both dc to three-phase ac and three-phase ac to dc conversion, the link cycle is divided into 12 modes, including 6 power transfer modes and 6 resonating modes. Figs. 3 and 4 represent the principle of the operation of this inverter in more detail. The circuit s behavior during each mode when the power (a) dc-ac power flow-mode1 (b) dc-ac power flow-mode2 (c) dc-ac power flow- Mode3 (d) dc-ac power flow-mode4

3 MODE 3: This is the first discharging mode. The link discharges during mode 3, until the voltage across the output phase pair BC_o meets its reference. (e) dc-ac power flow-mode5 Although the voltage across both BC_o and AB_o are non-zero in this mode, we control the duration of mode 3 with the voltage of BC_o because the other phase pair (AB_o) has the maximum voltage and it will be non-zero during both modes 3 and 5. When the voltage across the phase pair BC_o meets its reference we short the link and this phase pair by turning on switch S23. (f) dc-ac power flow-mode6 (g) dc-ac power flow-mode 7 Fig.3. Behavior of the proposed inverter during different modes of operation (dc to three-phase ac conversion) MODE 1: The dc-side current charges the link capacitor; therefore, the link current is equal to the dc-side current. The ac side switch bridge provides a path for the link current and also paths for each output phase current. MODE 2: This is a resonating mode, starts by shorting the link. For this purpose switches S0, S1, S3, and S4 are turned on. Considering the polarity of the link voltage, these switches are forward biased and they can start to conduct. Therefore, the input voltage becomes zero and the link current decreases. The link current continues to decrease in the reversed polarity until its absolute value reaches the absolute value of the current of phase B at ac side. This happens when the current of switches S15, S16, and S17 are all zero, S22 carries current of phase B because switch S13 that was conducting in the previous cycle is reversed biased and the current of phase B is now carried by switch S22. Fig.4. Link voltage, Link current, and unfiltered line-to-line voltages in the dc-to-ac power conversion MODE 4: This is another resonating mode. Once the link current becomes equal to the current of phase A_o, it cannot decrease further. This happens when the current of S14 becomes zero. During mode 4 the LC link resonates and the conduction of current of phase C_o transfers from S14 to S23. Turning on S23, turns off S14 and allow the link current to further decrease to - IA_o. MODE 5: The link current is equal to IA_o, and the link continues to discharge, until there is sufficient energy remained in the link to swing to a maximum current (- Imax ), the absolute value of which is slightly higher than the maximum input and output currents.

4 MODE 6: To allow the link to resonate during mode 6, again the link and the output phase pairs should be shorted. For this all the output side switches that are forward biased (S21, S22, S23, S18, S19, and S20) are turned on. Three of these switches are conducting during mode 5, and turning on the other three switches shorts all the output line to line voltages and the link. The link will resonate during mode 6, and its current will decrease further to - Imax. Once the link current reaches its maximum, we have to turn off the input-side switches that conducted during mode 1. Switches will be forward biased and to prevent them from conducting, they have to be turned off before mode 7 starts. During mode 7, switches S3 and S1 should conduct because the link voltage and current are both negative. Also, we have to turn on another two switches at the output switch bridge that will be conducting during modes These switches belong to the phase pair having the maximum line-to-line voltage (VAB_o). VAB_o is the maximum line to line voltage and it is negative. Modes 7-12 are similar to modes 1-6 except that the polarity of the link voltage/current is reversed. during which the link is discharged into the dc-side. Fig. 5 shows the waveforms corresponding to the ac-dc conversion. The polarity of the line to line reference voltages along with the polarities of the link current and voltage determine the main switches that will be conducting during modes 3 and 5. The phase that is not involved in forming the maximum line to line voltage (Phase C_o in Fig. 4 and phase A_o in Fig. 5) helps with the transition from mode 3 to mode 5 in dc-ac conversion or from mode 1 to 3 in the ac-dc conversion. Fig.6. proposed inverter with galvanic isolation The series ac-link inverter can provide galvanic isolation by adding a single-phase high frequency transformer to the link. In this case the leakage inductance of the transformer may play the role of the link inductance. The schematic of the inverter with galvanic isolation is depicted in Fig. 6. III. DESIGN AND ANALYSIS To simplify the design procedure, the resonating time which is much shorter than the power transfer time, will be neglected. Moreover, for the dc to three-phase ac conversion, the discharging is assumed to take place in one equivalent mode instead of two modes during each power cycle. For this operation, the link is assumed to be charged through the dc source and then discharged into a virtual load with output equivalent current. Considering the principles of the operation, it can be shown that the output equivalent voltage, which is the voltage across the virtual load, is: V, = 3 3 π V, (1) Fig.5. Link voltage, Link current, and unfiltered line-to-line voltages in the ac-to-dc power conversion The principles of the operation can be easily extended to ac to dc conversion. The only difference is that in this case, modes 1 and 3 are charging modes, during which the ac-side currents charge the link, and mode 5 is a discharging mode The output equivalent current can then be calculated as: I, = π 2 3 I, cos θ (2) As shown in Fig. 3, during the charging mode (mode 1), the dc-side current charges the link capacitor and the voltage across this capacitor increases linearly. During the equivalent deenergizing mode (modes 3 and 5), the charged link

5 capacitor is discharged into the virtual load. Fig. 7 shows one cycle of the link voltage when simplified for the design procedure. The following equations describe the behavior of the circuit during the charging and discharging modes, respectively: V _ = I t C V _ = I, t C (3) (4) In the above equations, Idc, VLink,peak, tcharge and tdischarge represent the average of the dc-side current, peak of the link capacitor voltage, total energizing time during mode 1, and total deenergizing time (during modes 3 and 5), respectively. system and the characteristics of the available switches. Once the link frequency is chosen the following equation determines the link capacitance: P C = V _ f (8) Where P is the rated power. This equation shows that by choosing a higher link frequency, a smaller link capacitor can be used. Link inductance should be chosen so as to minimize the resonating periods at full power. Eq. (8) can be rewritten as: f = P 2V + V, C (9) This equation calculates the link frequency at different power levels. IV. P-I CONTROLLER Fig.7. One cycle of the link voltage in the series aclink inverter (simplified for the design procedure) Equations (3) and (4) determine the relationship between the charge time and discharge time as follows: P-I controller is mainly used to eliminate the steady state error resulting from P controller. However, in terms of the speed of the response and overall stability of the system, it has a negative impact. This controller is mostly used in areas where speed of the system is not an issue. Since P-I controller has no ability to predict the future errors of the system it cannot decrease the rise time and eliminate the oscillations. If applied, any amount of I guarantees set point overshoot. t = I, t I (5) The average of the dc-side voltage can be determined as follows: V = 1 T V _ t (6) Where T is the period of the link voltage, and it is twice the sum of the charging time and discharging time. Using (5) and (6), the link peak voltage can be calculated based on the input and output peak voltages: V _ = 2V π V, (7) In the battery-utility application, since the average of the dc-side voltage and the amplitude of the ac-side voltage are almost constant at any operating points, the link peak voltage will be almost constant over a range of power. Fig.8: PI controller A proportional integral controller (PI controller) is a control loop feedback mechanism(controller) commonly used in industrial control systems. A PI controller continuously calculates an error value as the difference between a measured process variable and a desired set point. The controller attempts to minimize the error over time by adjustment of a control variable, such as the position of a control valve, a damper, or the power supplied to a heating element, to a new value determined by a weighted sum. The frequency of the link at full power (f) can be chosen based on the power rating of the u(t) = MV(t) = K e(t) + K e(τ)dτ (10)

6 have been corrected previously. The accumulated error is then multiplied by the integral gain ( ) and added to the controller output. The integral term is given by: I = K e(τ)dτ (13) Equivalently, the transfer function in the Laplace Domain of the PI controller is L(s) = K + K s (11) The integral term accelerates the movement of the process towards set point and eliminates the residual steady-state error that occurs with a pure proportional controller. However, since the integral term responds to accumulated errors from the past, it can cause the present value to overshoot the set point value (see the section on loop tuning). Where : complex number frequency Proportional Term: The proportional term produces an output value that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant K p, called the proportional gain constant. The proportional term is given by: P = K e(t) (12) Fig.9: Plot of PV vs time, for three values of K p (K i and K d held constant) A high proportional gain results in a large change in the output for a given change in the error. If the proportional gain is too high, the system can become unstable (see the section on loop tuning). In contrast, a small gain results in a small output response to a large input error, and a less responsive or less sensitive controller. If the proportional gain is too low, the control action may be too small when responding to system disturbances. Tuning theory and industrial practice indicate that the proportional term should contribute the bulk of the output change. Fig.10: Plot of PV vs time, for three values of K i (K p and K d held constant) V. SIMULATION RESULTS Simulation studies are carried out using MATLAB/ Simulink. In this part the simulation results corresponding to a dc to three-phase ac power conversion system using the proposed converter will be presented. Simulation is carried out for 200 W systems, to show its consistency with the experimental results, and also for a 5 kw system to extend the operation to higher power levels. Table I summarizes the parameters of the converter. TABLE I: Parameters of the Series Ac-Link Inverter Integral Term: The contribution from the integral term is proportional to both the magnitude of the error and the duration of the error. The integral in a PID controller is the sum of the instantaneous error over time and gives the accumulated offset that should

7 Figure 13 Link current and scaled voltage in the simulated 200 W system Fig.10. Matlab model of proposed system Figure 14 dc-side current in the simulated 5 kw system Figure 11 dc-side current in the simulated 200 W system Figure 15ac-side currents in the simulated 5 kw system (with LC filter) Figure 12 ac-side currents in the simulated 200 W system (with LC filter) Fig.16 Link voltage and scaled current in the simulated 5 kw system VI. CONCLUSION This paper introduced a bidirectional softswitching ac link inverter using PI controller. This inverter belongs to a new class of power converters called series partial resonant converters (SEPARC) or Series ac-link universal power converters. Here we are using PI controller rather than other controllers. The proposed inverter has all the advantages of the ac-link buck boost inverter, including zero voltage turn on and soft turn-off of the switches, alternating link current, and the possibility of having galvanic isolation with the

8 addition of a single-phase high-frequency transformer. In this paper the bidirectional dc to three-phase ac conversion was studied. In the proposed inverter the link current and voltage are both alternating. Therefore, no dc electrolytic capacitor or low frequency transformers are used in this inverter. Galvanic isolation can be provided by adding a single-phase high frequency transformer to the link. In this converter all the switches have a soft turn-on and they are turned off at zero current. Therefore, it is expected that SCRs with natural commutation be usable in this converter. This paper studied the principles of the operation in both dc to three-phase ac and three-phase ac to dc conversion modes. Design and analysis of this inverter were also studied in this paper and the converter was evaluated through simulation results. REFERENCES [1] C. D. Parker, Lead-acid battery energy storage systems for electricity supply networks, J. Power Sources, vol. 100, pp.18-28, [2] R. Margolis, A review of pv inverter technology cost and performance projections, National Renewable Energy Laboratory, NREL/SR , [3] M. Amirabadi, H. A. Toliyat and W. Alexander, Battery-Utility Interface using Soft Switched AC Link supporting Low Voltage Ride Through, in Proc. IEEE Energy Conversion Congress and Exposition Conference (ECCE), pp , [4] W. C. Alexander, Universal Power Converter, US patent 2008/ A1, Jan.17, [5] M. Amirabadi, A. Balakrishnan, H. Toliyat, and W. Alexander, High Frequency AC-Link PV Inverter, IEEE Transactions on Industrial Electronics, vol. 61, pp , [6] M. Amirabadi, H. A. Toliyat, and W. C. Alexander, A Multi-Port AC Link PV Inverter with Reduced Size and Weight for Stand-Alone Applications, IEEE Transactions on Industry Applications, vol.49, pp , [7] M. Amirabadi, H. A. Toliyat, and W. C. Alexander, Partial resonant AC link converter: A highly reliable variable frequency drive, in Proc. Annual Conference on IEEE Industrial Electronics Society (IECON), pp , [8] M. Amirabadi, H.A. Toliyat, A New Class of PV Inverters: Series Partial Resonant Converters, in Proc. IEEE Energy Conversion Congress and Exposition Conference (ECCE), pp , PUTTA SABARINATH Completed B.Tech in Electrical & Electronics Engineering in 2014 from K.O.R.M Engineering college, Kadapa Affiliated to JNTUA, Anantapur and M.Tech in POWER ELECTRONICS AND DRIVES in 2016 from K.O.R.M Engineering College, Kadapa Affiliated to JNTUA, Anantapur.. Area of interest includes Electrical Power Systems. id: shabarish229@gmail.com MUKKARA REDDY PRASANNA Completed B.E. in Electrical & Electronics Engineering in 2005 from S.K.I.T Engineering College, Srikalahasti Affiliated to JNTUH, Hyderabad, and M.Tech in ELECTRICAL POWER SYSTEMS in 2012 from K.S.R.M Engineering College Affiliated to JNTUA, Anantapur. Working as Assistant Professor and H.O.D of EEE Department at K.O.R.M Engineering College, Krishnapuram, Thadigotla Chintakommadinne(M), Kadapa Rd, Andhra Pradesh, India. Area of interest includes Power Electronics. id: prasanna2029.eee@gmail.com