VOLTAGE SAG INDEPENDENT OPERATION OF INDUCTION MOTOR BASED ON Z-SOURCE INVERTER Uroš Flisar, Danjel Vončina, Peter Zajec University of Ljubljana, Faculty of Electrical Engineering, Ljubljana, Slovenia Key words: Z-Source inverter, space vector modulation, field oriented control, field weakening Abstract: This paper describes an adjustable speed drive system for driving an induction motor beyond its nominal speed, even in the presence of input voltage sags. The system is based on the Z-Source inverter, which offers several advantages over traditional current or voltage source inverters as it can operate in both buck or boost mode. The boost operation is achieved with controlled short-circuiting of the inverter phase legs that is otherwise forbidden in traditional inverters. This shoot through states are accomplished with the modification of the space vector modulation which is thoroughly explained. In order to assure the required output voltage and ride-through ability during voltage sags, the method for selecting the proper inverter voltage is introduced. The control of the induction motor is carried out with the field oriented control coupled with the field weakening regime of the induction motor. The experimental setup is based on a prototype with a DSP control system to verify the operation of the proposed system. Nemoteno obratovanje asinhronskega motorja na osnovi pretvornika z impedančnim prilagodilnim vezjem Kjučne besede: pretvornik z impedančno prilagodilnim vezjem, modulacija s prostorskim vektorjem, vektorska regulacija polja, slabljenje polja Izvleček: Članek opisuje izvedbo nemotenega obratovanja asinhronskega motorja, ki temelji na trifaznem razsmerniku z impedančnim prilagodilnim vezjem. Razsmernik s prilagodilnim vezjem omogoča vrsto prednosti v primerjavi s klasičnimi napetostno ali tokovno napajanimi razsmerniki, saj poleg pretvorbe napetosti navzdol, hkrati omogoča tudi pretvorbo napetosti navzgor. Dvig vhodne napetosti dosežemo z reguliranim proženjem kratkih stikov v vejah razsmernika, kar je sicer prepovedano v omenjenih klasičnih razsmernikih. Tak način proženja kratkih stikov smo dosegli s prilagoditvijo pulzno-širinske modulacije napetosti na podlagi prostorskih vektorjev. Prav tako je prikazan izračun napetosti razsmerniškega mostiča, s katero zagotovimo zahtevano izhodno napetost za nemoteno obratovanje pri upadih vhodne napetosti. Regulacija vrtenja asinhronskega motorja je izvedena s pomočjo vektorske regulacije na osnovi polja, ki omogoča preprosto nadgradnjo za vključitev slabljenja magnetnega polja v rotorju, s katerim ga zavrtimo preko nazivne hitrosti. Eksperimentalni sistem, ki ga sestavljajo napetostni vir, impedančno prilagodilno vezje, trifazni razsmernik s priključenim asinhronskim motorjem in DSP nadzornim vezjem, je uporabljen za verifikacijo predlaganih rešitev. 1 Introduction The traditional power converters used for the control of motor drives are voltage source inverter (VSI) and current source inverter (CSI). However, both have limitation in their operation. Because the ac output voltage of the VSI is limited below the dc bus, the VSI usually requires an additional boost converter. Similarly the buck converter is often added to the CSI. This additional converter stage increases cost and complexity and lowers the overall efficiency of the power conversion system. The Z-Source inverter (ZSI) overcomes the restrictions of the previously mentioned topologies /1/. Its structure is comprised of two capacitors and inductors connected in a unique impedance network that is usually coupled with a voltage source and Inverter Bridge. The modus operandi of the ZSI includes a controlled short circuiting (also called shoot through) of the inverter phase legs. This enables the boost operation, whereas without it, the Z-Source inverter acts as a traditional voltage source inverter. The Z-Source inverter was originally intended for power systems with fuel cells /2-4/, due to their distinctive operating curve. As their output voltage dramatically decreases with the increased current demand, the need for the boost operation becomes essential. A similar conclusion can be drawn in conjunction with solar cells, since their output voltage changes according to the change in temperature and sun radiation. Another demanding power conversion process takes place in wind turbines, where the wind energy is transformed into the electrical energy. Because the output of the wind turbine is directly proportional to the change in wind speed, the uninterrupted power delivery to the electrical grid is essential. Recently, the use of the ZSI is advancing into the motor drive applications /5/, because it offers several advantages over traditional solutions. The nature of ZSI's operation makes it less sensitive to EMI which could short-circuit inverter phase legs that would normally destroy the switching devices. Because this kind of shoot through is allowed, the insertion of dead time is not necessary anymore. Consequently, this reduces the current and voltage harmonics. Controlling the short-circuiting of inverter phase legs can theoretically step up the input voltage to any value up to the infinity. The practical values are however limited with the device voltage ratings. Nevertheless, with controlled boost of the input voltage an important benefit of the ZSI is gained - the ability to provide ride-through during voltage sags. The ac output of the VSI is usually controlled with sinusoidal pulse-width modulation (SPWM) or with a computational more intensive space vector modulation (SVM). Regard- ing the utilization of the inverter voltage, the SVM is preferable as it is capable to utilize the inverter voltage of about 15 percent more than the SPWM. Both strategies have to be modified in order to include the shoot through states, needed for the ZSI's boost operation /6-8/. Another factor that favors the use of the SVM is the compatibility with Field Oriented Control (FOC) of the induction motor (IM). This type of control is frequently used with the operation in the extended speed range, where the field weakening enables the motor operation with the constant power above base speed. 2 Configuration and control of the zsi for motor drives The voltage source PWM inverter is the common choice for powering the induction motor drives. The preferred PWM method for voltage source inverter is SVM, where the concept of space vectors (representing the states of the inverter switches) is used to control the ac output of the inverter bridge. Figure 1 depicts how the output voltage vector V can be expressed as a linear combination of adjacent space vectors V1 and V2 V = V,+V,= 5- + V,,, ^ + (V^ + V,,,)^ (1) T0 is time duration of zero state V000 and V111, T1 and T2 are time durations for any of the neighboring active states and Ts is the switching period. In the linear or undermodu-lation region the output voltage vector V always remains within the inscribed circle in the hexagon formed by the six space vectors. The pulse pattern of SVM for three phase inverter is illustrated in Fig. 2. The state sequence begins with the zero state where the V000 is impressed (all upper transistors are open). This is followed by the two active states and ends with another zero state V111, where all up- P Voio Viio \ \ a ^ Voii\ 1 . 1 ' ^ / 2 \ / \ /V100 Vooi Vioi Fig. 1: Space vectors of three phase bridge inverter showing voltage trajectory and voltage vector limit per transistors are closed. After half of the switching period Ts, this sequence repeats in reverse order. The SVM with the symmetrical pulse pattern has the two zero states distributed equally on both ends of the active states as illustrated in Figure 2. S1 1 1 1 1 1 1 1 1 1 1 1 S2 1 1 1 1 S3 1 1 1 S4 1 1 1 1 1 1 1 S5 1 1 1 1 Se 1 1 1 1 To 1 ri 1 72 7-0 1 To T2 T, To Ts Fig. 2: Symmetrical pulse pattern of SVM for three phase Inverter Bridge The maximum line-to-line rms voltage (Uab) that can be utilized with the SVM from the inverter voltage u, of the VSI is U (2) When using an induction motor with the nominal line-to-line rms voltage of 177 V as an example, it requires a minimum inverter voltage of 250 V. This value needs to be further increased if the compensation of voltage sags is required. Although the VSI usually includes input capacitor for such occurrences, it is a low energy storage element and is inefficient in case of severe voltage sags. Furthermore, increasing the input voltage is not always a viable solution. Often an additional converter stage is inevitable, which can be an additional boost converter or an addition of the Z Source impedance network. 2.1 Z-Source inverter operation If the impedance network is added to existing VSI, the later is transformed into a Z-Source inverter shown in Figure 3. It can be separated into four major parts: the voltage source Udc and diode D1 to block the input voltage during shoot through; the symmetrical Z-Source impedance network, with capacitors C1 = C2 and inductors L1 = L2; a three phase inverter bridge and the induction motor as the load. The basic operating principle and control of the ZSI have been detailed in /1/. The summary of the ZSI main operating modes are: ll. Fig. 3: Z-Source Inverter with induction motor Mode 1): The inverter bridge is operating in one of the six active states and diode D1 is conducting. From the load point of view, the inverter bridge behaves as a current source as depicted in Figure 4. The voltage and current relationships are "L = ^Dc ~ '*c' "i = - Z/^c (3) he = h + ic>h= h -^c ioc = - if (4) Fig. 4: ZSI operating in active mode Mode 2): The input diode is still conducting and the inverter bridge is operating in the zero state. The upper or the lower transistors are closed and the inverter bridge acts as an open circuit viewed from the Z-Source network. The voltage relationships are the same as in (3) while the currents are (5) Mode 3): Figure 5 illustrates the shoot through mode, where the input diode is reverse biased because the sum of the capacitor voltage is higher than the input voltage. The shoot through can be done in one phase leg, two or in all three phase legs. The voltage and current relationships in the shoot through mode are «L=Wc.«i=0 (6) ioc = 0; «Li= -«ci; hi = -hi- (7) The duration of the shoot through (Tsh) depends on the required boost ratio (B) and can be calculated with T -^ T 2.B where B is /L1 Udc Ci C2 2 ' - /L2 Fig. 5: ZSI operating in shoot through state B = Uoc (9) The maximum duration of the shoot through is limited to half of the switching period, where the resulting inverter voltage is theoretically boosted to infinity. However in practical application the maximal boost or the maximal inverter voltage is limited with the device voltage ratings. To summarize, the ZSI behaves in a similar way as a traditional VSI, however it also introduces a new operating state, called shoot through state that boosts the inverter voltage. This enables the ZSI to produce any output voltage, provided the inverter voltage stays within device voltage ratings. 2.2 Modified SVM In order to include the shoot through states, we must modify the SVM (MSVM). We do this by positioning the shoot Si S2 S3 S4 S5 Se T I T I I I I TT I I I I I I T'o Ti T2 T'qT'O T2 _Isä_üii_Isb_ 7-1 n JLh_Isb, Fig. 6: Modified SVM switching pattern showing the uniform distribution of Tsh through states at the transients of the switching states so that upper and lower transistors on-time is overlapping as illustrated in Figure 6. The shoot through states are distributed equally among all three phase legs and they utilize the zero states symmetrically. This kind of placement allows them to leave the existing active states (T1 and T2) uncompromised and more importantly, the number of switchings remains unchanged, however, at the cost of reduced duration of zero state (T'q). It can be evident from (9) and Fig. 6 that the boost ratio depends on the available duration of the zero state. If the required Tsh is greater than the available Tq, the required voltage boost can not be achieved. Because the voltage boost has a higher priority, the duration of the active states has to be reduced. Figure 7 illustrates the clamped maximum voltage vector (V'max), which is reduced according to the required Tsh in order to assure the required voltage boost of inverter voltage (10) Fig. 7: Modified trajectory of the damped voltage vector V'max The total voltage gain Gu is the combination of output voltage vector and the boost ratio. The relationship between Gu and Tsh is illustrated in Fig. 8 and can be calculated with (11) The choice of the optimal Gu depends on the requirements for the intended application. However in general, we should maximize V and minimize B to reduce the voltage stress of the switching devices. The line-to-line rms voltage of the ZSI with MSVM is (12) tion during voltage sags. Usually the inverter voltage is kept constant and the boost ratio is adapting according to the change in input voltage. The inverter voltage that will enable uninterrupted motor operation during voltage sag can be calculated by solving the (11) and (12) for ui (13) The resulting ui is valid for SVM based Z-Source inverter. If a different PWM technique is used, such as the SPWM, the (12) should be modified accordingly. It should also be noted, that the (13) considers the voltage vector is maximal and boost ratio is minimal. If a different voltage gain strategy is preferred, the (10) should be taken into consideration when choosing the desired inverter voltage. Operating an induction motor with a ZSI requires careful control of the voltage gain to guarantee the proper opera- 0.2 0.3 Fig. 8: Maximum voltage gain of ZSI with MSVM 3 Experimental results The experimental verification has been carried out on the proposed system with the following parameters: - Input line voltage: 100 - 300 V DC - Z-Source network: L1 = L2 = 165 ^H; C1 = C2 = 1000 ^F - Switching frequency: fS = 10 kHz - Load: Induction motor (Un = 177 V, In = 14,8 A, nn = 1456 rpm, Mn = 20 Nm, fn = 50 Hz, cosf^j = 0,785) - Position sensor: 1024-lines incremental encoder - Control system: DSP TMS320F2808 The IM was controlled with the field oriented control which is often used for adjustable speed drive applications, where the field weakening is applied to extend the speed range. The principle of field weakening was achieved by decreasing the flux producing current id according to the "1/rar" method, while increasing the torque producing current iq. A typical functional diagram of the indirect FOC for the induction motor can be seen on Figure 9. The implementation of the existing FOC for VSI based induction motor requires minor modifications. These are mostly related to the chosen PWM method and to the control of the voltage boost. The measuring of the inverter voltage Ui is somewhat cumbersome because the Ui is zero at the time of the shoot through. A more elegant solution is measuring the capacitor voltage uc and calculating the inverter voltage Ui from The same measurements were repeated with a ZSource inverter and the results are shown in Fig. 11. The input voltage was decreased to 180 V to demonstrate the capabilities of the ZSI. The waveforms verify the proper operation of the ZSI where the rotor speed is again increased to 2400 rpm at the 50 % of nominal torque. T. (14) From here forth, the control of the Ui is performed with a PI regulator, which outputs the required B. The Tsh is calculated with (8). Fig. 9: Functional diagram for FOC with the MSVM and control of the inverter voltage The operation of the induction motor was first verified with the VSI without the Z-Source network. For the VSI, the minimal inverter voltage according to (2) is 250 V. Including a safe margin of 10 %, the ui was set to 280 V. Figure 10 shows the ramp up of the rotor speed from nominal value up to 2400 rpm, together with the waveforms of id current. The motor was loaded with the 50 % of the nominal torque, because when the motor is operating at high speed, the required torque is normally low. 2 ^1.5 3 3 1 0.5 ............- i 10 15 20 d A 0.5 1 10 time (s) 15 20 Fig. 10: Ramp up of the speed of IM with VSI (Udc = 280 V) d ft 0.5 ■J" 10 time (s) 15 20 Fig. 11: Ramp up of the speed of IM with ZSI (Udc = 180 V) When operating the motor in the field weakening regime, voltage sags at the DC input present an even bigger problem, which can lead to rapid reduction of the rotor speed. Figure 12 illustrates the effect of simulated voltage sag of about 25 % of the input voltage for the VSI. Because the voltage sag is too severe, the rotor speed decreases below its nominal value. On the contrary, the flux producing current id greatly increases, which indicates the disrupted operation of the IM in the field weakening regime. 2 \ \ \ 1 - X ...... ................L.................................. nil "o 1 5 10 15 20 1 5 ..... .....................:............ "o ini- 5 10 15 20 .................--- 10 lU : : 0.-;-±-±- 15 20 time (s) Fig. 12: Motor operation with VSI when exposed to voltage sag For the ZSI to be able to endure the input voltage drop of 25%, the minimal inverter voltage has to be set according to (13). The resulting u is 365 V; however the inverter volt- age was set 10 % higher, at 400 V. Figure 13 shows the IM operating at 2400 rpm and at 50 % of the nominal torque. The ZSI successfully adapts its boost ratio during the voltage sag and the operation of the IM remains uninterrupted. The waveforms clearly demonstrate that the inverter voltage is boosted without any negative influence on the flux current id, which is maintained at a desired level without any interruptions. 10 time (s) Fig. 13: Motor operation with ZSI when exposed to voltage sag References /1 / Peng, F.Z. , "Z-source inverter', IEEE Trans. Ind.Applications.,Vol 39, No 2,pp 504-510, March/April 2003. /2/ Shen, M.; Joseph, A.; Wang, J.; Peng, F.Z.; Adams, D.J. "Comparison of traditional inverters and Z-source inverter for fuel cell vehicles", Power Electronics in Transportation, IEEE, 125 - 132, February 2005. /3/ Peng, F.Z, Shen, M., Holland, K., "Application of Z-Source Inverter for Traction Drive of Fuel Cell-Battery Hybrid Electric Vehicles", IEEE Trans. Power.Electronics., Vol. 22, No. 3, pp. 1054 - 1061, May 2007. /4/ Holland, K. ; Shen, M. ; Peng, F.Z., "Z-source inverter control for traction drive of fuel cell - battery hybrid vehicles", Industry Applications Conference, Vol. 3, pp. 1651 - 1656, October 2005 /5/ Peng, F.Z., "Z-source inverter for motor drives", Power Electronics Specialists Conference, 2004. PESC 04. 2004 IEEE 35th Annual, Vol. 1, pp. 249 - 254, November 2004. /6/ Poh Chiang Loh; Vilathgamuwa, D.M.; Lai, Y.S.; Geok Tin Chua; Li, Y., "Pulse-width modulation of Z-source inverters", IEEE Transactions on Power Electronics, Vol. 20, No. 6, pp. 1346 - 1355, November 2005 /7/ Peng, F.Z.; Shen, M.; Qian, Z., "Maximum boost control of the Z-source inverter', IEEE Transactions on Power Electronics, Vol. 20, No. 4, pp. 833 - 838, July 2005 /8/ Miaosen Shen; Jin Wang; Joseph, A.; Fang Zheng Peng; Tol-bert, L.M.; Adams, D.J., "Constant boost control of the Z-source inverter to minimize current ripple and voltage stress", IEEE Trans. Ind.Applications., Vol. 42, No. 3, pp. 770 - 778, June 2006 4 Conclusions This paper has presented the motor drive system based on the Z-Source inverter and the induction motor. The field oriented control which is extensively used in high-performance drive applications was successfully adapted to the Z-Source inverter, where the modification of the SVM and control of the inverter voltage was explained. The operation of the system was verified with the practical use with the DSP control system together with the operation in the extended speed range. The use of the Z-Source based inverter is beneficial if the input voltage is limited, or if the voltage source is subjected to frequent voltage sags. The ZSI can easily adapt to such disturbances and keep the operation of the motor system reliable. The findings of the performed experiments could also help when designing power converters with renewable energy sources in mind. In such cases, the changes in input voltage are seldom that sudden, nevertheless an insight into the behavior of the ZSI under comparable circumstances has been presented. U. Flisar University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia uros.flisar@fe.uni-lj.si D. Vončina University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia voncina@fe.uni-lj.si P. Zajec University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia peter.zajec@fe.uni-lj.si Prispelo (Arrived): 10.05.2010 Sprejeto (Accepted): 09.09.2010