This paper suggests a combined novel control strategy for DFIG based wind power systems (WPS) under both nonlinear and unbalanced load conditions. The combined control approach is designed by coordinating the machine side converter (MSC) and the load side converter (LSC) control approaches. The proposed MSC control approach is designed by using a model predictive control (MPC) approach to generate appropriate real and reactive power. The MSC controller selects an appropriate rotor voltage vector by using a minimized optimization cost function for the converter operation. It shows its superiority by eliminating the requirement of transformation, switching table, and the PWM techniques. The proposed MSC reduces the cost, complexity, and computational burden of the WPS. On the other hand, the LSC control approach is designed by using a mathematical morphological technique (MMT) for appropriate DC component extraction. Due to the appropriate DC-component extraction, the WPS can compensate the harmonics during both steady and dynamic states. Further, the LSC controller also provides active power filter operation even under the shutdown of WPS condition. To verify the applicability of coordinated control operation, the WPS-based microgrid system is tested under various test conditions. The proposed WPS is designed by using a MATLAB/Simulink software.
Due to the significant growth of grid integrated renewable energy-based distributed generators (DGs), the power quality (PQ) and reliability (PR) aspects have become a major challenge [
To overcome the PR issues, many researchers suggest various improved control methods for the operation of grid-connected double fed induction generator (DFIG) based WPS. From the literature, few important techniques such as the field-oriented control (FOC) or vector control (VC) approaches are widely applied for similar applications [
To overcome the PQ issues, many researchers suggest various improved control approaches for the operation of grid-connected DFIG based WPS. In [
The major aspects of the study are presented as follows.
Detailed mathematical modeling of grid integrated DFIG based WPS is proposed for non-linear/unbalanced load applications.
To reduce the complexity in design and decrease the computational burden of the system, two novel coordinated control model is proposed as the machine side converter (MSC) and the load side converter (LSC) control approach.
To provide a continuous power supply and appropriate converter operation, an improved MPC controller is selected to develop the MSC controller for appropriate voltage vector generation and offer better power reliability operation.
To mitigate the harmonics obtained by the non-linear load, a novel current decomposition and MMT-based LPF is selected to develop the LSC controller for better PQ operation.
Due to the above-coordinated control approach, the LSC converter can perform as a SAF even during the shutdown condition of WPS.
The complete structure of the proposed
Before discussing about the proposed control strategy, it is essential to develop the detailed mathematical modeling of the undertaken
where
Substitute
The electrical torque ‘
By using the complex conjugate of stator flux (
The complex power (
where P is the real power and Q is the reactive power of the system.
To control the proposed DFIG based WPS, two control structures are required as MSC and LSC control approaches. The detailed explanations related to the proposed control approaches are presented in the following sections.
In this proposed approach, the MPC technique is selected to accurately forecast the requirement of real and reactive power demand of load and grid. After the accurate forecasting, the desired voltage vector is chosen by using the optimization cost function with properly tracking the forecasting value and the reference value. By neglecting the
By using
where
By analyzing
The time derivate of
By canceling the rotor and stator resistance of
It can be analyzed from
where
The complete MSC control approach is demonstrated in
The LSC control approach is separated into two sections. The first section is proposed for a simpler control design by reducing the design complexity and computational burden, and the second section is proposed for an appropriate dc component extraction concept through a novel mathematic morphological technique (MMT) by reducing the harmonic and non-linearity current.
The traditional
In the
As per the PQ theory discussed in [
By accumulating
where
The dc-components of
As illustrated in
From
where
By considering the fundamental and phase sequence voltage and current component, the proposed A-
In
where
As shown in
As shown in
The constraints of the PI regulator are computed by sensing the actual and reference voltage component. To confirm the stability, the frequency response of
The stability of the proposed approach is analyzed through different responses like frequency responses, impulse responses, and step responses respectively as indicated in
After generating
MMT method is suggested for the appropriate real and reactive DC extraction. The design MMT model is illustrated in
To design the MMT approach, two sets like input current component
In the proposed MMT technique,
The opening of ‘
Similarly, the closing of
The important aim for selecting the opening (
Let
At n = 1, F0 = F is the input current signal. Till the error between the output signal (
where
To justify the proposed MSC and LSC control approach need for the DFIG based WPS during nonlinear/unbalanced load conditions, different test conditions are presented in this section. Further, to give a clear idea about the improvement percentage (IP) of the proposed method, the obtained results are compared with the traditional STDPC [
A Similar 20 kW rating of WPS performance is tested by using the traditional STDPC [
Here only magnified figures are illustrated and the detailed explanation related to the undertaken conditions is provided in the following sections. At last, during an unbalanced load condition, the utility current is slightly increased to balance the non-linear load current. The respective stator and grid current outputs are demonstrated in
Moreover, to get more clarity about the performance of the traditional STDPC approach, in
To test the performance of grid integrated WPS, it is undergone through different case studies like constant wind speed, without wind speed, gradually fall in wind speed, and unbalanced load condition. To show the effectiveness of the proposed MSC control approach, the power reliability conditions are illustrated by properly adjusting the real and reactive power flow. In addition to that, to show the effectiveness of the proposed LSC control approach, the power quality conditions are illustrated by compensating the generated harmonics. The corresponding test cases are presented in the following section.
As per the proposed approach, the LSC generates appropriate harmonic current quadrature to the generated non-linear current as demonstrated in
As per the load demand,
Performance analysis Table | |||||||
---|---|---|---|---|---|---|---|
Conditions | THD % of Load | THD% of Grid Current | IP of Grid | THD% of Stator Current | IP of Stator | ||
TraditionalSTDPC [ |
ProposedMSC+LSC | TraditionalSTDPC [ |
ProposedMSC+LSC | ||||
At 10.62 m/s Wind speed | I |
74.89% | 82% | ||||
75.51% | 89% | ||||||
75.09% | 76% | ||||||
At 0 m/sWind speed | I |
76.56% | 0% | ||||
87.33% | 0% | ||||||
78.78% | 0% | ||||||
Decreased wind speed | I |
60.05% | 60% | ||||
60.57% | 42.33% | ||||||
66.42% | 53.98% | ||||||
Unbalanced load condition | I |
70.57% | 74.83% | ||||
72.96% | 69.82% | ||||||
56.34% | 74.80% | ||||||
Overall Improvement Percentage (OIP) | 71.25% | 69.19% |
In this proposed approach, a simplified and coordinated MSC and LSC controller approach is proposed for DFIG based WPS under sensitive load conditions. The important contribution of the paper is to use the MPC technique for appropriate switching pulse generation to operate MSC. Due to the proposed approach, the appropriate rotor voltage is selected according to the minimum cost function in every sampling instant. In addition to that, the design of the MSC controller avoids any switching tables, transformation techniques, and PWM techniques. Therefore, the proposed MSC approach improves the PR operation during both steady and dynamic state operations. On the other hand, the proposed LSC control approach is proposed for offering better PQ of the DFIG based microgrid system. MMT based LSC controller is used to extract the fundamental load current component. By using the LSC approach, the system able to improve PQ by compensating the harmonic component generated by the sensitive load. Further, in the shutdown condition of the WPS, the LSC controller also facilitate the active power filter operation by taking power from the grid. Therefore, the overall performance of the WPS is improved by using the coordinated MSC and LSC approach. The above obtained and analyzed simulated outcomes used as a basis of both MSC and LSC approach for the WPS on a real-time system application.
Assistance provided by Council of scientific and industrial research (CSIR), Government of India, under the acknowledgment number 143460/2K19/1 (File: 09/969(0013)/2K20-EMR-I) and Siksha O Anusandhan (Deemed to be University).
WPS: Maximum Power (