Simulation of 40 kVA, 400 Hz Power Converter System

Simulation of 40 kVA, 400 Hz Power Converter System

A.A.Supekar1, K.P. Rathod2 , Arbind Kumar 3, P.M.Kurulkar4

Research & Development Establishment (Engrs.), DRDO, Alandi Road, Dighi, Pune, India

Abstract This paper discusses the design and simulation of Power converter System (PCS) of capacity 40 kVA for various applications such as defence, avionics, navy etc. The above simulation is based on double conversion and Sine Pulse width modulation (SPWM) technique. Solid state power converter uses high switching frequency devices like Insulated Gate Bipolar Transistor (IGBT). Simulation has been carried out using CASPOC software to determine the performance of the system. The several design aspects and number of iterations have been carried out to achieve optimum results and same have been presented.

Keywords Power Converter; IGBT; double conversion technique; Sine Pulse width modulation; Switching frequency; Axial Flux Permanent Magnet Alternator;

1. INTRODUCTION

   By seeing today's scenario for several applications, it is necessary to process electric power to suit various equipments used in environments where dissimilar voltages and frequencies are mandatory[1]. Solid state power or frequency converters are used for converting either 50 Hz or 60 Hz utility line power to 400 Hz power. They are more efficient than motor-generator sets. In addition, they offer harmonic cancellation, power factor correction, phase conversion, voltage conversion with balanced, smooth, and controlled power output. Many varied applications like aircraft, radar, communication equipments etc in power electronics requires sinusoidal outputs at frequency of 400 Hz. Major issue of using electrical power system operates at 400 Hz instead of conventional 50 Hz / 60 Hz is to make

   From the basic circuit theory, design considerations, simulation of complete Power Converter at rated load, iterations on switching frequency and their results on CASPOC (Computer Aided Software for Power Converter) simulation software have been presented.

2. System Concept

   Main aim of power converter system (PCS) is to provide constant and well-regulated ac voltage and frequency. It is connected between the ac voltage source from the three phase mains supply or very compact and high dense Axial Flux Permanent Magnet Alternator (AFPMA) based DG set. The variable voltage and frequency output of AFPMA can also be given as a input to Power Converter. This Power Converter unit works on sine triangle pulse width modulation (SPWM) IGBT based technique with inbuilt protections and control circuit.

   Fig. 1 shows schematic of the PCS. Here, three phase mains of 415 V, 50 Hz has been taken for simulation purpose, further AFPM based Diesel Generator can be used to carry out simulation studies. Three phase uncontrolled rectifier converts the three phase AC signal from the mains to a constant regulated DC supply [3]. This DC is further given to 6 pulse IGBT based inverter. For battery backup, the DC link bus can be tapped if required.

   Three phase input 415V, 50Hz

   Three phase uncontrolled AC to DC converter					IGBT based DC to AC inverter

   Three phase uncontrolled AC to DC converter					IGBT based DC to AC inverter

   DC Bus Constant AC output

   equipments smaller, lighter and efficient. Volume and weight are main constraints during design of electrical equipment for aerograde applications, use of 400 Hz supply offers many advantages[2].

   To fulfill this requirement, Power/frequency Converter plays a vital role to regulate and control the power quality (voltage and frequency) for proper operation of various loads like radar, communication equipments, missile launchers etc for defence applications to perform its intended function. Hence it is required to do design, analysis and simulation of Power Converter.

   Mains

   AFPM

   based DG set

   PWM based Control strategy

   PWM based Control strategy

   Fig. 1: Schematic of Power Converter

   voltage 200V,400Hz

   Current Feedback

   This paper describes the simulation of a power converter topology and control techniques for realizing sinusoidal output systems. Three-phase power supply, whose line voltage and frequency are 415V and 50 Hz, is converted via a un -controlled rectifier to generate a DC bus and further it is inverted using IGBT based inverter for 200 V, 400 Hz output.

   It converts DC voltage from the capacitor filter to a regulated ac supply. To achieve this, gates of IGBTs are triggered using a SPWM control technique [4]. If required output of the inverter can be further connected to transformer, which gives galvanic isolation between input and output also it will step down the voltage as per requirement. The input for the transformer is three phases and the output is three phase and neutral.

3. FORMULATION OF PWM CONTROL STRATEGY ON CASPOC

   IGBT bridges are required to be effectively switched ON and OFF with PWM control technique for getting required AC output voltage. Using a pulse width modulation switching scheme the switches of the inverter are switched ON and OFF several times per half cycle. The duty cycle can be controlled by varying the on-time per switching period. The average

4. SIMULATION CIRCUITS

   In order to start the simulation of PC, initially the preliminary design calculations have been carried out to finalize the optimum values of components using standard formulas mentioned below: [5][6]

   1] Load Calculations :

   Real Power Reactive Power P Power Factor

   P VIcosø

   output voltage can be maintained at the desired set point by controlling the duty cycle. To achieve this, the PWM control circuit has been designed as shown in Fig. 2 wherein the

   Vph = VL

   3

   Z = V ph

   Iph

   output of three phase signal ( load current) is fed back to comparator which get compared with the reference signal. The output of this comparator is compared with triangular carrier signal to generate three prime phase shifted by 120 degree PWM signals. By using three NOT gates total 6 PWM signals are generated to drive the 6 IGBTs. The frequency of the carrier signal will determine the switching frequency. The pulse widths can be modified by changing the amplitude of the modulation signal. The ratio of the amplitude of the modulation and carrier signals is the modulation index.

   M = Vm / Vc

   R Z cos (

   XL Z sin

   Using above equations the optimum values of R and L are calculated and selected for 40 kVA capacity.

   2] Six Pulse Diode Based Uncontrolled Rectifier :

   For a three phase full bridge rectifier the average value

   of

   dc voltage can be obtained over a radian interval as shown in

   Equation (10)

   The amplitude values of carrier and control

   Vdr = 1 /6 2Vin costdt

   signal for PWM generation are adjusted to achieve modulation index for optimum results. Where M is the modulation index, Vm is the amplitude of the control signal and Vc is the amplitude of the carrier signal. On the designed circuit various switching frequencies from 10 kHz to 20 kHzwere tried and finally the optimum switching frequency is

   /6

   3

   Vdc(avg) = 1.35Vin (8)

   Vdr=1.654Vm

   VL =1.655 Vm (10)

   Vpp Vpositive_p Vneg_p

   3] Six Pulse IGBT Based DC to AC Converter (Inverter) :

   selected out of several iterations.

   Amplitude modulation ratio (ma)= Vm

   Vc

   = 1

   LL

   LL

   V = 3

   2

   VAN

   VLL = 0.612 ma Vd (ma 1-0)

   DC input voltage (Vd) = VLL

   0.612ma

   Peak rating of switch

   VT Vdmax

   IT = 2 Iomax

   DC side current (Drive input current)

   LL

   LL

   Inverter id = 3 V Io

   Vd0.8

   Fig. 2: PWM generation circuit

   using above basic equations the circuit has been designed and its various parameters are given in Appendix.

   1. Complete Circuit Simulation

      After the design calculations, the components were selected and simulated model of power converter was build up in steps. Some of the off the shelf models which were readily available was taken as it is. Fig. 3 shows the simulated model of PCS which is based on double conversion technique [7]. In this circuit primarily the mains circuit block which was available in CASPOC is connected as input to uncontrolled rectifier. The output of rectifier is connected to filter capacitor bank to get ripple free DC output. Based on the amount of ripple contained in the rectified output the number of iterations have been performed to finalize the value of filter capacitors. This DC output is connected to IGBT based 3-phase inverter circuit. In order to obtain the distortion less three phase output of inverter

      Fig. 3: Simulated circuit diagram in CASPOC simulation software

      at desired frequency of 400 Hz, the LC filters are connected between each phase of inverter.

      The values of LC filter was also finalized based on the various iterations and refined results obtained. Three phase inverter is loaded with the calculated values of R and L for getting load of 40 kVA.

   2. PWM Generation Circuit

      In order to trigger IGBT's of inverter circuit, fine PWM control is required. For the same the circuit was made as per Fig. 2. In this circuit load current of three phases was sensed through current transformer and compared with the three reference voltages. The generated six PWM signals were fed to gates of IGBT's which drives the power devices.

   3. Load Connection in STAR connection

   STAR connected loading arrangement is shown in Fig. 4. Load analysis was carried out for STAR connection. Accordingly the values of R and L was calculated and set for 1 kVA to 40 kVA load requirement.

   Fig. 4: Loading arrangement

   The connections are made through CT from where the load current was sensed and fed back to the control circuit.

   After checking all probe connections, the circuit simulation was started. The results are discussed in subsequent sections. [8]-[10]

5. SIMULATION RESULTS

   1. Input signal to PC

      Fig.5: Three phase mains input

      Three phase input of 415V, 50 Hz is shown in Fig. 5. This input is given to Power Conditioning System.

   2. Rectified output

      Fig. 6 shows output voltage of three phase uncontrolled rectifier w. r.t. time along the high value of filter capacitor.This reduces ripple content to some extent from rectified output. Output voltage is fluctuating between 235 V to 240V.

      Fig.7: PWM output waveform

      Figure 7 shows the PWM output wave at 17.6 kHz switching frequency.

6. OPTIMIZATION OF SWITCHING FREQUENCIES

   On simulated model in CASPOC, various switching frequencies (from 10 kHz to 20 kHz) were tried out on rated capacity to optimize the switching frequency which gives symmetrical undistorted load current waveforms[11]. The most optimistic results at different switching frequencies are shown in Fig. 8(A) to 8(D) and the inference of all iterations at various switching frequencies are also given below. At around 17.6 kHz the result is more optimistic.

   1. Output current waveform

      Fig.6: Rectified output signal Fig.8(A):. Output Current @ SF=10kHz

      C. PWM Generation for driving gates of IGBT

      In PWM generation circuit primarily three basic signals are PWM generated and same are inverted for driving six numbers of IGBTs. Fig. 7 shows the generated PWM output waveform. The circuit uses triangular modulation control technique which is explained in part III. For the same, the circuit simulation was carried out for various switching frequencies from 10 kHz to 20 kHz.

      Fig.8(B). Output Current @ SF=15kHz

      Fig.8(C): Output Current @ SF=17.6kHz

   2. OUTPUT Voltage waveform

      Voltage waveform was also captured at rated capacity and is as shown in Fig. 10.

   3. DFT Analysis

   Fig.10: Output voltage waveform

   Fig.8(D): Output Current @ SF=20kHz

   After carrying out the various iterations of SF, it is observed that, at 10 kHz the entire current waveform is distorted and at 20 kHz the waveform peaks are flatten at upper side for more duration whereas at lower side it is sharp in nature.

   After doing the simulation at various SFs, Discrete Fourier Transform (DFT) is also carried out at each SFs to study the harmonic spectrum. It is observed that between 15 kHz to 18 kHz range of SF, harmonic spectrum is optimal for selected filter values. The graph of DFT for voltage and current is as shown in Fig. 11 and 12 respectively.

7. OUTPUT RESULT WAVEFORMS

   1. OUTPUT Current waveform

      The simulated circuit model is loaded partially at various switching frequencies. After confirming the performance of the circuit at full load i.e. 40 kVA @ 17.6 kHz switching frequency, captured load current waveform w.r.t time is presented in Fig. 9.

      Fig.9: Three-phase load current waveforms

      Fig.11: Voltage harmonic spectrum @ 17.6 kHz

      Fig.12: Current harmonic spectrum @ 17.6 kHz

      After finalization of switching frequency, the simulation circuit was loaded in steps from 1 kVA to 40 kVA. The results are tabulated as given in Table 1. It is observed that the calculated and measured electrical load currents were matching approximately. Further isolated three phase step down transformer is required at the output side of the inverter for getting 200 V at the load [12].

      Load (kVA)	Readings at 17.6 kHz SF
      	Calculated	Measured output
      	I (A)	V1(V)	I1(A)	V2(V)	I2(A)	V3(V)	I3(A)
      1	2.88	309.4	5.84	306.0	6.06	310.91	5.92
      5	14.43	290.1	26.63	293.5	25.03	291.77	25.21
      8	23.09	287.2	39.11	288.7	37.74	288.51	38.23
      10	28.86	280.0	46.63	277.6	47.46	278.09	46.57
      20	57.73	253.6	77.52	249.2	72.51	254.32	76.50
      30	86.60	238.7	95.14	236.4	96.79	240.07	99.11
      40	115.47	228.0	114.58	229.7	114	229.14	112.4

      Load (VA)	Readings at 17.6 kHz SF
      	Calculated	Measured output
      	I (A)	V1(V)	I1(A)	V2(V)	I2(A)	V3(V)	I3(A)
      1	2.88	309.4	5.84	306.0	6.06	310.91	5.92
      5	14.43	290.1	26.63	293.5	25.03	291.77	25.21
      8	23.09	287.2	39.11	288.7	37.74	288.51	38.23
      10	28.86	280.0	46.63	277.6	47.46	278.09	46.57
      20	57.73	253.6	77.52	249.2	72.51	254.32	76.50
      30	86.60	238.7	95.14	236.4	96.79	240.07	99.11
      40	115.47	228.0	114.58	229.7	114	229.14	112.4

      TABLE I LOADING ANALYSIS

8. CONCLUSION

   Design and simulation of 40 kVA, 200V, 400 Hz Power Converter using double conversion technique has been successfully completed in Computer Aided Software for Power Converter (CASPOC) simulation software. The exhaustive analysis was carried out by changing electrical parameters like modulation index, switching frequency, loads etc. The system was also simulated and studied with the STAR load configuration [13].

   Theoretical and simulation results are compared with step load at the optimum switching frequency. The overall effect on load current waveform is observed and studied. For pure sine wave outputs of 40 kVA, 200 V, 400 Hz further fine tuning of output filter values are required to be done.

   Based on this simulation study, the hardware realization is under development for several strategic application.

   APPENDIX TABLE II

   The major design parameters

9. ACKNOWLEDGMENT

The authors wish to thank Shri. V. V. Parlikar Director, Research and Development Establishment (Engrs.), Alandi Road, Dighi, Pune-411015 for constant encouragement and valuable guidance to publish this paper in International Journal of Engineering Research and Technology (IJERT). Thanks to the entire design team for their valuable suggestions and support.

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