Effect of DC Link Capacitor Failure on Free Wheeling Diodes of Inverter Feeding an Induction Motor

Transcription

1 King Saud University From the SelectedWorks of Hadeed Sher 2011 Effect of DC Link Capacitor Failure on Free Wheeling Diodes of Inverter Feeding an Induction Motor Hadeed A Sher, King Saud University Available at:

2 Effect of DC Link Capacitor Failure on Free Wheeling Diodes of Inverter Feeding an Induction Motor Hadeed Ahmed Sher Department of Electrical Engineering King saud University Riyadh,Saudi Arabia Khaled E. Addoweesh Department of Electrical Engineering King Saud University, Riyadh, Saudi Arabia Yasin Khan Department of Electrical Engineering King Saud University, Riyadh, Saudi Arabia Abstract Modern industrial progress is very much dependent on the ruggedly constructed induction motors. Almost every sophisticated process of industry is based on induction motors. Mostly these motors are controlled by means of varying the frequency. Inverter fed induction motors are thus a vital part of such industrial systems. For such reasons it is important to understand the fault behavior of different components of drive system. This research work is a continuation of our fault analysis of an inverter fed induction motor drive. We represent here the effect of capacitor short circuit on free wheeling diodes that are connected in anti parallel direction to the power switches of inverter. This research investigates the after shocks of one of the faults that may occur on DC link of an inverter fed induction motor. DC link capacitors are well designed and even the probability of capacitor failure is high, it is always a rare case if they puncture, however this analysis will add to the reliability of the induction machine under variable operating condition. Index Terms DC Link Capacitor, Free-Wheeling Diodes, Back EMF. I. INTRODUCTION Reliability analysis of induction motor is an important issue in the research community and fault analysis of induction machine is an area that is directly related to the reliability of electric drive system. In industry induction motors are usually fed via an inverter to produce the mechanical characteristics of user own choice. Industrial plants that utilize the inverter fed induction motors need a very accurate fault tolerant system for smooth operation of industrial work. In literature, a lot of work has been done on the fault analysis, monitoring and diagnosis of inverter fed induction motor [1- [2. These faults includes the following Transistor failure Gate drive pulse failure Inverter leg open DC link capacitor faults Some researchers [2, [3 have studied the probability of different faults in inverter fed induction motor drives. According to them, in the past the failure probability for semiconductor components was high due to the non availability of sophisticated and reliable Pulse Width Modulation (PWM) chips. However, this high failure rate is now considerably reduced after the availability of microcontroller and FPGA based control systems. Since, capacitors are used in a lot of power electronics applications including the inverters and Switch Mode Power Supply (SMPS), Fuchs [3 work showed that the probability of capacitor failure is 60 %. Lahyani also studied the capacitor failure symmetrically in fault analysis and concluded that more than half of the faults in SMPS are related to capacitor failure [2. It is pertinent to mention that the Equivalent Series Resistance (ESR) of the capacitor increases with the passage of time. In ordinary circuits it may not be a problem but when the capacitor is used in a switching circuit the ESR can combine with the switching frequency and causes self heating and indirectly leads towards the failure of capacitor. Many researchers have mentioned a DC link short circuit while formulating the possible faults in an inverter operation [4 [8. However, they did not investigated the said problem very deeply. Some researchers declared that the said problem may not be a serious issue since almost every commercially available electric drive is well equipped with the protection schemes [4, [8. A. Ebrahim,et.al. [5 discussed the voltage drop in DC link but did not considered a very low voltage for sustained period. B.Biswas et.al [6 presented a fault analysis and deduced results in term of the current harmonics. But it aims to focus on semiconductor based faults and did not cater for the effect of DC link voltage drop on current harmonics. Peuget et.al. [7, [9 classified the DC link capacitor failure under the umbrella of DC bus faults but they stated that they are interested in semiconductor components failure. As a conclusion most of the work done in this regard is related to fault analysis at semiconductor based components. Keeping in view the need of analysis for the said problem this paper analyzes the effect of capacitor short circuit on DC-AC inverter. In this paper, an inverter feeding a three phase induction motor is considered. The effect of DC link capacitor short circuit is studied thoroughly and its effect on free wheeling diode in particular is considered. The results can be incorporated in the designing of a fault tolerant system and the optimal protection system design.

3 II. PROBLEM DESCRIPTION For a three phase inverter feeding an inductive load it is an essential practice to counter the stress on power transistor. Free wheeling diodes are connected in antiparallel direction that provides a path for current to flow in the same direction during the time motor inductance changes its polarity. In normal condition the current drawn by the motor is controlled by the state of switches and the generation of back e.m.f. In any case if the back e.m.f exceeds the applied voltage then the motor starts behaving like a generator. If the terminal voltages becomes zero while the motor is running then the back e.m.f becomes greater than the applied voltage. There may be serval reasons for terminal voltages to be zero e.g Inverter output terminal open circuit Shut down of input supply DC Link capacitor short circuit The problem for inverter output terminal open circuit may occur due to loose connection but this will only hamper the normal operation of motor and will shut down the motor making its speed zero. The shut down of input supply will off course make the output voltage zero and for a small time the free wheeling diode will conduct and motor will stop depending upon the inertia of load. The last case is very interesting that is a short circuit across a DC link capacitor. In this case the output of inverter will go to zero thus making the back e.m.f greater than the applied voltage. So the induction motor start behaving like an induction generator. The point of interest is the short circuit that will appear at the terminals of the motor via faulty DC link capacitor. This short circuit will draw currents from the motor. The short circuit current will pass through the switch or freewheeling diode depending upon the direction of flow of current. However, in this paper the analysis of capacitor short circuit on the free wheeling diodes is presented. III. MATHEMATICAL MODELING OF INDUCTION MOTOR Induction motor is represented by the equivalent models as shown in fig. 1 [10, [11. The conventional per phase equivalent circuit is sustainable only in the analysis of induction motor in steady state. The mathematical model of induction motor is presented here since the motor used in our simulation setup is tested for a d-q modeling scenario. For analysis in transient state the three phase induction motor stationary reference frame (A s -B s and C s ) is converted into two phase stationary reference frame (d s -q s ) which is then transformed into the rotating two phase frame of reference (d e -q e ). Here the superscripts s refers to stationary reference frame and e for rotating reference frame. Axis transformation from the three phase stationary axis A s -B s -C s to q s -d s is given below [11: [ Vqs s V ds s V os s = 2 3 [ cosθ cos(θ 120 ) cos(θ ) sinθ sin(θ 120 ) sin(θ ) [ Vas V bs Vcs where, V os s is a zero sequence component that may be present. Solving eq.(1) gives (1) Fig. 1. DQ equivalent model of Induction Motor. [10 V qs s = 2 3 V as 1 3 V bs 1 3 V cs (2) V ds s = 1 3 V bs V cs (3) This stationary frame of reference is then converted to rotating two phase frame of reference. This rotating reference frame rotates at a synchronous speed ω e w.r.t. d s -q s and the angle Θ e = ω e t. The realization of this synchronously rotating reference frame is given below [11 V qs = V qs scosθ e V ds ssinθ e (4) V ds = V qs ssinθ e + V ds scosθ e (5) The flux linkage equations on the basis of DQ transformations are given below as described in Krause model [10. Based on fig. 1, the transient model of electric machine in terms of voltage and current can be written as below [10, [11. [ [ Vqs V ds V qr V dr = (6) R s + SL s ω el s SL m ω el m) ω el s R s + SL s ω el m SL m (ω e ω r)l m SL m (ω e ω r)l r R r + SL r [ i qs i ds i qr i dr where, S is a Laplace operator and as in our case for a squirrel cage induction motor the two values i.e. V qr and V dr will be zero [10, [11. From eq. 6 above, the speed ωr is related to the torque and cannot be considered as a fixed entity. The torque is given as [11 T e = T L + J dω m dt = T L + 2 p J dω r dt (7)

4 Fig. 2. AC-DC-AC Inverter. where, T L represents the load torque ω m the mechanical speed and J is the rotor inertia After keeping in mind the interaction of air flux gap and the rotor m.m.f. relating the d-q components of variables the value of torque derived can be expressed as following [11. T e = 3 ( p ψ m I r (8) 2 2) In terms of d e -q e components, the eq.(8) can be expressed as T e = 3 ( p (ψ dm I qr ψ qm I dr ) (9) 2 2) The developed torque in terms of inductance and the current values with d-q phases can be rewritten as T e = 3 ( p L m (I qs I dr I ds I dr ) (10) 2 2) Eqs. (6), (7) and (10) above represents as a whole the complete dynamic behavior of induction motor which is obviously a non linear model [11. IV. MATHEMATICAL ANALYSIS Figure 2 shows the diagram of the AC-DC-AC inverter feeding a three phase induction motor. The input three phase voltages are converted into DC by using an uncontrolled three phase bridge rectifier. The DC obtained is supplied to the DC bus and is smoothed using DC link capacitor C as shown in fig. 2 and its equivalent circuit in fig. 3. The inverters are switched in regular intervals using PWM technique. The voltage at the output of inverter is a stepped waveform and the current drawn by the motor is usually sinusoidal in nature. Here the back e.m.f. is represented as a source of sinusoidally varying voltage and the windings parameters are represented as series resistance R and inductance L per phase. The transistors work as a switch so they either make or break the connection of DC link with the motor. In fig. 3 the MOSFETS Q1, Q2 and Q6 are responsible for conduction at that time and are represented as a short circuit and Q3, Q4 and Q5 are not conducting at that instant so are represented as an open circuit. All diodes are represented as it is, without any change. For an instance lets consider a scenario where the current is such that phase Fig. 3. Equivalent circuit of a motor as a load A contains the current equal to the sum of phase B and phase C as given in eq.(11). I a = I b + I c (11) Since the current is going from phase A that is connected to the positive side of DC bus therefore according to KVL the equation for phase A is given in eq.(12) where V ao = V z + E ba (12) V ao = Phase A voltage with respect to ground V z = Voltage drop across the line impedance E ba = Back E.M.F of motor for phase A Therefore eq.( 12) becomes V ao = ir + L di dt + E ba (13) After the short circuit at DC bus the voltage at the inverter output will become zero as shown in [12. Therefore the eq.(12)becomes E ba = ir + L di (14) dt here the (-) sign shows that the current flow convention is from negative polarity to the positive polarity. The simplified circuit of the said problem is depicted in fig. 3. Before the time t= (0-) the expression for current is given by i r (0 ) = i l (0 ) = E ba V dc (15) R At time t=0 the switch S1 is turned ON, thus creating a short circuit along V dc. Therefore V dc becomes zero. So eq.(15) becomes i r (0+) = i l (0+) = E ba (16) R Since there is inductance L in the loop so it will oppose sudden change in current, therefore the current can not change instantaneously. However,the current will follow the particular solution as given below in eq. (17) [13 i = E ba t (1 e τ ) (17) R where τ = ( L R ) of the equivalent circuit

5 Fig. 4. Simulation setup V. SIMULATION SETUP The simulation setup for the said problem is shown in fig. 4. The simulation is performed in MATLAB/ SIMULINK. Switch S1 is used to simulate the failure of capacitor. Since the failure of capacitor is mostly due to main insulation breakdown, it is actually an internal short circuit [14. To carry out the failure of capacitor, switch S1 is turned ON at time t=1.5sec. The motor model used has the following rated parameters Power = 5hp Rated voltage = 460 V Rated frequency = 60 Hz Rated speed =1750 RPM The inverter consists of MOSFET switches. They are controlled using the most widely used sinusoidal PWM. The carrier frequency used for PWM is 2kHz and the frequency of sine wave is 50 Hz. System is simulated with a load of 5 N-m torque. Fig. 5. Motor electrical parameters before fault VI. RESULTS The induction motor is connected with three phase where the input line voltage is 381 V p (for 220 V) with 50Hz frequency. The generated ripple frequency will be 300Hz which is 6 times the input frequency. For a three phase rectifier the average output voltage is given as V dc = V m (18) For the circuit diagram (fig. 4) two comprehensive analysis studies of induction motor behavior are carried out Without creating a fault By creating a fault at DC link capacitor The motor runs very smooth in the steady state condition before the fault is created as observed by the waveforms shown in fig. 5 and fig. 6. The motor attains its speed as well, in a very short time (0.4sec) as depicted in fig. 6. After testing Fig. 6. RPM and Torque before fault the system in healthy condition, the system is also tested for a fault in DC link capacitor C. The possibilities of capacitor failure is very high in case of inverters and SMPS. The time instant t=1.5 sec is selected so that motor comes to steady state condition and to prevent the fault in transient condition when motor draws heavy current. When the switch S1 is turned on at t=1.5 sec the dc link gets short circuited and the terminal voltages of the motor goes to zero, since the motor is running at a speed of 1500 RPM it can not stop immediately rather it

6 Fig. 7. Equivalent circuit after the fault Fig. 10. Current and voltage of DC link Fig. 8. Stator currents after the fault a serious issue if motor is running at high speed and inertia. Larger the inertia longer will be the generation of back emf. So in case of the said problem the free wheeling diodes will bear the current for longer time. The reverse current depends on the speed of motor therefore it can damage the free wheeling diode in the inverter. While troubleshooting if only a capacitor is replaced without verifying the inverter then the high reverse current can cause damage to the switch as well. The nature of change in various parameters of drive system provides basis for the diagnosis and designing of fault tolerant system. Proper monitoring of inverter semiconductors can make the basis of fault tolerant system. ACKNOWLEDGMENT Authors like to thank Dr.Ali M Eltamaly and Deanship of Scientific Research, King Saud University, for support in carrying out this research work. Fig. 9. Electrical parameters observed at free wheeling diode will work like a generator with the output short circuited as shown in fig. 7. Simulation results shown in fig. 8 depicts the unusual hike of current right after the fault. This fault current is approximately 8 times more than the steady state current that is almost equal to the max current drawn by motor in transient condition. Figure 9 shows the current through the freewheeling diode. At the time of fault this current goes high enough and rises up to 40A and then decays very swiftly as the motor tends to decelerate with the passage of time. Figure 10 shows the current through the DC link and voltage across the DC link capacitor at the time of fault. This negative current spike through the DC link is due to the reverse flow of current from the motor to DC link capacitor during fault. VII. CONCLUSION This paper mainly highlights the effect of capacitor short circuit on free wheeling diode as well as the after shocks of one of the fault that may occur on DC link of inverter fed induction motor. The failure of DC link capacitor could be REFERENCES [1 A. Mendes and A. Cardoso, Performance analysis of three-phase induction motor drives under inverter fault conditions, in Diagnostics for Electric Machines, Power Electronics and Drives, SDEMPED th IEEE International Symposium on. IEEE, 2003, pp [2 A. Lahyani, P. Venet, G. Grellet, and P. Viverge, Failure prediction of electrolytic capacitors during operation of a switchmode power supply, Power Electronics, IEEE Transactions on, vol. 13, no. 6, pp , [3 F. Fuchs, Some diagnosis methods for voltage source inverters in variable speed drives with induction machines-a survey, in Industrial Electronics Society, IECON 03. The 29th Annual Conference of the IEEE, vol. 2. IEEE, 2004, pp [4 D. Kastha and B. Bose, Investigation of fault modes of voltage-fed inverter system for induction motor drive, Industry Applications, IEEE Transactions on, vol. 30, no. 4, pp , [5 E. Ebrahim and N. Hammad, Fault analysis of current-controlled PWMinverter fed induction-motor drives, in Properties and Applications of Dielectric Materials, Proceedings of the 7th International Conference on, vol. 3. IEEE, 2003, pp [6 B. Biswas, S. Das, and P. Purkait, Current Harmonics Analysis of Inverter-Fed Induction Motor Drive System under Fault Conditions. [7 R. Peuget, S. Courtine, and J. Rognon, Fault detection and isolation on a PWM inverter by knowledge-based model, in Industry Applications Conference, Thirty-Second IAS Annual Meeting, IAS 97., Conference Record of the 1997 IEEE, vol. 2. IEEE, 2002, pp [8 R. Ribeiro, C. Jacobina, E. da Silva, and A. Lima, Fault detection in voltage-fed PWM motor drive systems, in Power Electronics Specialists Conference, PESC IEEE 31st Annual, vol. 1. IEEE, 2002, pp [9 R. Peuget, S. Courtine, and J.-P. Rognon, Fault detection and isolation on a pwm inverter by knowledge-based model, Industry Applications, IEEE Transactions on, vol. 34, no. 6, pp , 1998.

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