Study of Synchronous Reference Frame based Current Controller Introduction A suitable current controller needs to be selected to track the instantaneous converter currents with the calculated reference currents. Several current control schemes for voltage source converter are discussed in the literature. The selection of the current controller essentially depends on the type of current references, the modulation scheme that follows the current controller, the power converter topology (three-leg or four-leg) and the type of application. The inputs to current controller block are: a set of sensed currents and the reference currents generated for the requisite compensation. The output is a set of six or eight PWM pulses to drive the inverter switches. In a closed loop, the sensed currents try to track their respective reference currents. In literature, three type of current controllers are discussed for active filtering. They are as follows: 1. Synchronous Reference Frame based linear current controller 2. Hysteresis current controller 3. Dead beat current controller A comparative evaluation of the above current control techniques suitable for active power filters is given in [1],[2]. The first scheme is very popular in DSP based motor drive applications. However, the present example deals with a grid connected PWM converter. Simulation Example The simulation example consists of a three phase boost PWM voltage source converter connected to a three phase grid. The objective is to draw three phase currents similar to the three phase current references. The modulator section is a simple sine-triangle modulator. The current controller is based on synchronous reference frame based current controller as shown in Fig. 1. A three phase PLL is used to synchronise the grid freuqency. 1 sin ia_ref Reference Currents abc to αβ αβ to dq abc to αβ αβ to dq Currents vd_ref1 sin PI diff scale pole_1 diff i_d ia_act Sensed cos vq_ref1 PI i_d_1f pole_1 i_q_1f va_pll sin Three Phase PLL cos cos omega OSC PWM Pulses for Three Phase Inverter Sine−Triangle Modulator αβ to abc dq to αβ Figure 1: The current controller scheme Sample Plots The sample simulation plots are shown in Fig. 2. Here, the first upper plot shows the actual and reference currents in q-d reference frame plotted over the full time scale. The lower plot shows the steady state waveforms of the R-phase utility voltage, the inverter currents and the reference currents. The desired current tracking is apparent from the plots. Few sample exercises are given here to get the complete understanding of the topic. Exercises 1. Change the amplitude of the current references and run the simulation. Observe the inverter currents and reference currents on the same plot 2. Change the phase angles of the current references and observe the output for new conditions. 3. Change the gains of the proportional-integral controller and see the effect on the output response. 4. Add the harmonic content in the reference currents and observe the performance at the sharp edges corresponding to the high frequency components. 5. Change the dc bus voltage according to the grid voltages and re-run the simulation. References [1] Simone Buso, Luigi Malesani, and Paolo Mattavelli, “Comparison of Current Control Techniques for Active Filter Applications,” IEEE Transactions onn Industrial Electronics, vol. 45, no. 5, Oct. 1998, pp. 722-729 2 PWM Rectifier with SRF (linear) Current Controller 100 vsa 50 0 i_q_1f i_d_1f i_q_m_1f i_d_m_1f -50 0 0.2 0.4 0.6 0.8 1 300 200 100 0 -100 -200 -300 0.92 0.94 0.96 0.98 Time Figure 2: Simulation Plots [2] Marian P. Kazmierkowski, Luigi Malesani, “ Current Control Techniques for three phase voltage source PWM converters: A survey,” IEEE Transactions on Industrial Electronics, vol. 45, no. 5, Oct. 1998, pp. 691-703 [3] Joseph Mossoba, Peter W. Lehn, “A Controller Architecture for High Bandwidth Active Power Filters,” IEEE Trans. on Power Electronics, vol. 18, no. 1, Jan. 2003, pp. 317-325 [4] M. Sedighy, S. Dewan, and F. Dawson, “A robust digital current control method for active power filters,” in Proc. IEEE APEC99 Conf., 1999, pp. 635641. [5] L. Malesani, P. Mattavelli, and S. Buso, “Robust dead-beat current con trol for PWM rectiers and active lters,” IEEE Trans. Ind. Appl., vol 35, no. 3, pp. 613620, May/Jun. 1999. [6] Cristian Lascu, Lucian Asiminoaei, Ion Boldea, and Frede Blaabjerg, High performance current controller for selective harmonic compensation in Active Power Filters, IEEE Trans. Ind. Appl., vol 22, no. 5, Sept. 2007 pp. 1826-1835, 2007. [7] Hossein Madadi Kojabadi, Bin Yu, Idris A. Gadoura, L. Chang and Mohsen Ghribi, “A Novel DSP-Based Current-Controlled PWM Strategy for Single Phase Grid Connected Inverters,”, IEEE Trans. on Power Electronics, Vol. 21, No. 4, pp. 985-993 JULY 2006. 3