Bidirectional Harmonic Current Control of Brushless Doubly Fed Motor Drive System Based on a Fractional Unidirectional Converter Under a Weak Grid


The brushless doubly fed machine (BDFM) drive system based on a fractional unidirectional converter is a promising low-cost variable-speed drive system, which shows great potential in applications of driving fans and pumps. However, the harmonic current generated by a diode rectifier can flow into the machine and the grid under a weak grid, which will cause a 6th-order frequency torque ripple and lead to voltage distortion. A steady equivalent circuit considering the uncontrolled rectifier and the grid impedance is built firstly to study the harmonic distribution characteristics.


To eliminate the influence of harmonic currents, the harmonic equivalent impedance of the machine system should be regulated to change the harmonic distribution characteristics. This paper improves the conventional control method through adding a harmonic control loop to prevent harmonic currents from being injected into the machine or the grid, which is then applied in the fundamental synchronous frame. Two indirect parameters are selected to realize the two control targets. Afterwards, the influence of the control system on the harmonic equivalent impedance of the machine system under the conventional method and the proposed method are compared. Finally, experimental results obtained from a 30 kW BDFM prototype verify the proposed method.


  1. Brushless doubly fed machine
  2. Fractional unidirectional converter
  3. Harmonic current control
  4. Weak grid



Figure 1. The Structure Of The BDFM Drive System With A Unidirectional Converter.


Figure 2. Simulation Results Of The Msogi-Fll With An 11 Kw Diode Rectifier Load Switched In The Weak Grid.

Figure 3. Pw Currents And The Grid Current Sensitivity To The Diode Rectifier Current In Proposed Control Structure: (A) Magnitude Frequency Response Of G1irecpir ; (B) Magnitude Frequency Response Of G2irecpir .

Figure 4. Bode Diagrams Of The Open-Loop Transfer Function In (28) With Different Kr : (A) Target I; (B) Target Ii.

Figure 5. The Maximum Cw Voltages Of The Two Control Targets At Different Speeds.


The BDFM variable speed drive system based on a fractional unidirectional converter has wide application prospects for loads that require only limited range speed regulation. To solve the influence of harmonics on system, this paper studies the harmonic characteristics through a new equivalent circuit and proposes a direct harmonic control strategy. It is shown that:

1) The harmonic current contents in the machine and the grid are determined by the grid harmonic impedance and the harmonic equivalent impedance of the machine system. Calculation results agree with the experiment data;

2) The nature of the bidirectional harmonic currents control between the weak grid and the machine is changing the harmonic distribution characteristics of the BDFM driving sys- tem shown in Fig. 1, while the conventional control method can hardly realize this target;

3) The torque ripples and the harmonic currents injecting into the grid can be controlled indirectly through iph and uph;

4) The proposed control method can regulate the harmonic equivalent impedance of the machine system effectively, which commendably realizes the bidirectional harmonic current control. The equivalent circuit structure and control method pro- posed in this paper are also suitable for distorted grid with a large number of nonlinear loads. In this operation environment, a single BDFM control system is insufficient to improve the grid, which will be studied in the future.


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