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.
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