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Sensor Fault Detection and Isolation of a Single Phase PWM Rectifier for Electric Railway Traction Using Pole Placement Technique K.Kavinelavu PG Student Department of Electrical and Electronics Engineering Sri Manakula Vinayagar Engineering College, Puducherry ,India. e-mail:[email protected] Abstract-Modern railway traction power systems are fed by 2×25 kv/50 Hz single phase ac current sources or by 1500 vdc voltage from electric railway substations. This paper presents an easy and a robust sensor fault detection and isolation (FDI) of a single phase PWM rectifier for electric railway traction application. PR controller is used to regulate ac currents in single phase system, in order to obtain both zero magnitude and phase errors. Catenary current sensor and dc link voltage sensor failures of the single phase PWM rectifier are considered. Fault detection and isolation method is mainly depend on pole placement based observed values and threshold values. This method allows a good detection and isolation of the sensor fault. Simulation results are presented to illustrate the effective performance of the fault detection and isolation. Fig. 1. Electrical railway traction system. in electric railway traction system and explain the use of this converter in modern locomotives [4],[15]. In mostly, the interest on the single phase PWM rectifier for electric railway power systems has greatly increased. For the line side converter, control strategies, dc voltage ripple reduction, power quality, and harmonic analysis are the most treated problems in the literature [6]-[10]. In electric railway traction system, the motor side converter and induction motor performances are mostly depend on the availability of the grid side converter. The GsC utilizes a closed loop control to regulate the dc link voltage as well as the input current. The control loop requires one current sensor, a dc-link voltage sensor, and a grid voltage sensor. Therefore, invalid measurement, due to sensor failures, perturbs seriously the performance of the GsC and may cause system shutdown. In this circumstance, sensor fault detection and isolation (FDI) should be achieved to improve the rectifier availability. Fault detection and isolation, and fault tolerant control for the three phase drive systems have been Index terms-Electric traction, fault detection and isolation (FDI), pole placement, sensor fault, single phase pwm rectifier. I. INTRODUCTION N owadays electric railway traction power systems are fed by 2×25 kv/50 Hz single phase ac current sources or by 1500 Vdc voltage from electric railway substations [1]-[3]. This traction system consists of a typical ac/dc/ac power system is shown in Fig. 1. The grid side converter (GsC) is a single phase PWM boost rectifier. Commonly, the use of a single phase PWM rectifier allows a sinusoidal absorption of the inverter input current, power factor increase, and the dc link voltage control [4]-[5]. These advantages are required 1 largely reported in the literature; however, very few works have dealt with a single phase PWM rectifier especially for electric traction. In [11], the authors have focused on single phase rectifier static switch failures in railway traction and in [12], the authors have investigated speed sensor FDI for induction motor drives used in electric traction. FDI model-based methods like parity space method or observer based methods have been attractive during recent years. Parity space method has been used for FDI in WECS [13]-[15] and electrical drives, but cannot lead to reconfiguration control. Observer β based methods are preferred when reconfiguration is aimed. The proposed method does not require the single phase rectifier model but only the switch control signal value. Therefore, this model overcomes the problem related to power converterβs model described in [16]. The structure of this paper is as follows: In section II, the system description and its control strategy are presented. Section III deals with the PWM rectifier state space model to estimate the catenary current and the dc bus link voltage using observer. Section IV describes the FDI method. Section V concerned with simulation results. Section VI cocerned with conclusion. external vdc control loop using a proportional integrator (PI) controller maintains the dc link voltage equal to its reference vdc*, when current load or voltage grid varies. An inner loop controls the input current rectifier using a proportional-resonant (PR) controller in order to ensure unity power factor operation and sinusoidal current absorption with a zero phase and zero steady-state errors. The control principle is illustrated in Fig. 2. Fig. 2. Control strategy of single phase grid side converter. Grid synchronization is performed using a second order generalized integrator (SOGI)-frequency look up loop (FLL) [17]-[19]. In order to increase robustness against grid faults, voltage grid is shared in positive and negative sequences using a quadrature-signal generation (QSG). II. SYSTEM DESCRIPTION AND CONTROL STRATEGY The system under study is modeled using the following state equations, where, S is the binary switching control signal, and where the dead time is neglected π£π = βπππ β πΏ πππ = ππ πππ ππ‘ A. PR Current Control The PR controller is generally used to regulate ac currents in three phase system [20]-[24], as well as single phase system, in order to obtain both zero magnitude and phase errors. The transfer function of single and three phase PR controllers and filters can be derived using internal model control, modified state transformation or frequency domain approach. The grid current controller is implemented using the proportional resonant PR controller which is defined as + π£ππ ππ£ππ ππ‘ πππ = πππ + πππ (1) where, as reported in Fig. 2, ic is the catenary inverse current, πππ is the current in the capacitor, and πππ is the load current. π£π is the grid voltage and vdc is the dc-link voltage. The output current converter current πππ and the input rectifier voltage Vin are expressed as π£ππ = (2π β 1)π£ππ πππ = β(2π β 1)ππ . (2) πΊπ (π ) = πΎπ + πΎπ The goal of the system is to ensure unity power factor operation and dc link voltage regulation. An π π 2 +ππ2 (3) where the coefficients πΎπ and πΎπ are the resonant control and the proportional gain, respectively. 2 The system desired dynamic has been tuned through a proportional gain in terms of phase, gain margins, and bandwidth. The PR controller transfer function is expressed by (3), where the second term of the right side represents the generalized integrator block (GI). This is characterized by an infinite gain at all resonance frequency and a null gain at all other frequencies. Therefore, the PR is able to track a sinusoidal current reference without a steady state error. Fig. 3 illustrates the structure of SOGI-QSG-FLL, where πππ = 314 rad/sec and k are the fixed frequency and gain. The πΎ is a negative gain used to eliminate the error in the dc component by varying π of the SOGI until to the input frequency [17]. III. STATE SPACE MODEL FOR A PWM RECTIFIER By using equations (1) and (2), the state space model of the rectifier is given by π₯Μ = π΄π₯ + π΅π’ + π·π£ π¦ = πΆπ₯ (7) where A is the system matrix B. Reference Current for PR Current Control Measuring the current references in terms of phase, frequency, and amplitude is very important when the power flow between the grid and loads directly depend on it. Therefore, the reference current ic* is giyen by ππ β = πΌπ β2sin(ππ ) β A π 2πβ1 πΏ =[β2π+1 πΏ 0 ππ AS= 0 =[ π β1 πΏ πΏ 1 ππ Μπ¦= Cπ₯Μ To obtain unity power factor, the fundamental phase angle π of ππ should be equal to the fundamental phase angle of ππ of π£π . The SOGIQSG is used to generate π£π+ , π£πβ of the supply voltage π£π . Their transfer functions are expressed as + Q(s) = π£π β π£π π£π ππ€β²π (s) = π 2+ππ +πβ²2 ππ€β²2 (s) = π 2+ππ +π€β² (9) 0 π 1 πΏ AS= 1 =[ β1 πΏ β ], ππ 0 ]. (11) A. Design of Observer Luenberger state observer [25],[26] is used for the estimation of the dc link voltage π£ππ and catenary current ππ . This observer can be expressed by Μπ₯Μ = Aπ₯Μ + Bu +Dv + πΏππ (π¦ β π¦Μ) Fig. 3. SOGI-FLL structure. π£π (8) Now, the state vector is defined by the catenary current ππ and the dc bus voltage π£ππ π₯ = [ππ π£ππ ] π’ = π£π and π£ = πππ (10) S is the switching control signal. It can take two values 1 or 0. β = 0]. The input matrices B and D are 1 0 β π΅ = πΏ , π· = β1 ππ 0 πΌπ β2 = πππ + πππΌ (4) where πππΌ is the output of the dc bus voltage of PI controller and ππ is the phase angle obtained using the SOGI-QSG-FLL. D(s) ] , πΆ = [1 (12) A state correction factor πΏππ (π¦ β π¦Μ) is defined as the difference between the measurement and observer quantity, where L is matrix gain is to chosen to define the observer dynamic. Pole placement based poles should be proportional to system poles. Therefore, pole placement is a method employed to place the closed loop poles of a plant in pre-determined locations. Placing poles is desirable because the location of the poles corresponds directly to the eigenvalues of the system, (5) (6) 3 which control the characteristics of the response of the system. V. SIMULATION RESULTS IV. FDI METHOD To illustrate the FDI method development, different performance conditions are examined by using simulation in Matlab/Simulink software. System parameters are given in Table II. Simulation model of the single phase PWM rectifier is shown in below Fig. 4. The single phase PWM rectifier control requires accurate dc bus voltage and line current measurements. If any sensor fault occur in the converter, it will degrade the performances of the converter and will cause the system shutdown. Therefore, to ensure the operation continuity and prevent from system shutdown,a fast FDI method should be performed. Faults that can affect current or voltage sensor are presented in Table I, where ππ and ππ are the real and sensor output quantities, respectively. During the system operation, only one sensor fault is considered. In this paper, gain fault is considered during operation. Fig. 4. Simulink model of single phase PWM rectifier with FDI. TABLE I ELECTRIC SENSOR FAULT Fault type Measured signal Gain fault Xm = (1+G) Xr Offset Xm = Xr + Xoffset Noise Xm = Xr + n(t) Simulink model for control strategy of the single phase PWM rectifier is shown in Fig. 5. A. Threshold establishment The FDI method is based on comparison of observed values to a defined threshold value. The threshold value is carefully chosen to minimize the false alarm rate due to system parameter variations, operating points variation [27] and [28]. If the threshold value is low, fault detection increases. If the threshold value is high, fault detection decreases and the fault may not be detected. The threshold values for dc link voltage is ππ£ = 0.1 and the threshold value for catenary current ππ = 0.05. Fig. 5. Model for control strategy of single phase PWM rectifier. For simulations, the dc link voltage sensor fault occurs at 0.6s to 0.8s is shown in Fig. 6. B. FDI FDI is based on the comparison of observed values and threshold values ππ£ and ππ . Therefore, this comparison block is used to generate Boolean error is in the form of square waveforms. Therefore, fault detection is achieved by using Boolean error. The Boolean error is set to 1 or 0. If it is set to 1, the dc link voltage sensor may be faulty. If it is set to 0, the current sensor may be faulty. Fig. 6. DC link voltage sensor Fault detection at 0.6s to 0.8s. 4 VI. CONCLUSION An easy and fast pole placement based sensor fault detection and isolation of a single phase PWM rectifier has been developed and simulated. A proportional-resonant (PR) controller is used to ensure unity power factor operation and sinusoidal current absorption with zero phase and zero steadystate errors. The FDI method is based on observed values and threshold values. Simulation results shows the effectiveness of the FDI method. Fig. 7. DC link voltage sensor fault isolation at 0.6s to 0.8s. REFERENCES [1] M. Brenna and F. Foiadelli, βAnalysis of the filters installed in the interconnection points between different railway supply systems,β IEEE Trans. Smart Grid, vol. 3, no. 1, pp. 551β558, Mar. 2012. [2] M. Brenna, A. Capasso, M. C. Falvo, F. Foiadelli, R. Lamedica, and D. Zaninelli, βInvestigation of resonance phenomena in high speed railway supply systems: Theoretical and experimental analysis,β Elect. Power Syst. Res., vol. 81, no. 10, pp. 1915β 1923, 2011. [3] C. Bae, D. Jang, Y. Kim, S. Chang, and J. Mok, βCalculation of regenerative energy in DC 1500 V electric railway substations,β presented at the 7th Int. Conf. Power Electronics, Daego, Korea, Oct. 2007. [4] O. Stihi and B. T. Ooi, βA single phase controlled current PWM rectifier,β IEEE Trans. Power Electron., vol. 3, no. 4, pp. 453β459, Oct. 1988. [5] D. Dong, T. Thacker, I. Cvetkovic, R. Burgos, D. Boroyevich, F. Wang, and G. 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