AN IMPROVED THREE STAGES CASCADING PASSIVITY-BASED CONTROL OF GRID-CONNECTED LCL CONVERTER IN UNBALANCED WEAK GRID CONDITION
ABSTARCT :
This paper proposes an improved three-stages cascading passivity-based control (PBC) for a grid -connected LCL converter in unbalanced weak grid condition. In general, the traditional double-loop control based on positive and negative sequence transformations is used in grid-connected converter control in unbalanced weak grid condition. However, it is time consuming for second harmonic filtering, and positive and negative sequence currents need to be controlled separately. The PBC has strong robustness to interference, and the line voltage based PBC can deal with the voltage unbalance effectively and easily without negative sequence transformation. But the traditional PBC needs six variables and three damping coefficients for the grid-connected LCL converter in unbalanced grid condition, and it has the disadvantage of difficult implementation. The improved PBC can realize the same control effect with three-stages cascading PBCs of two variables and one damping coefficient, and it has the advantages of easy implementation, good performance and high stability. First, the modeling and controller design are detailed described. Then the SIMULINK simulation results demonstrate the benefits of the improved control strategy. Finally, a grid-connected LCL converter prototype of 5kW is built and the experimental results verify the correctness and effectivity of the improved three-stages PBC strategy.
EXISTING SYSTEM :
An unbalanced grid does exist because of the asymmetry of power grid parameters, the access of nonlinear loads and power grid faults.
In order to overcome the drawback of existing control strategies for the three-phase cascaded bridgeless rectifier, taking one bridgeless model in each phase of three-phase cascaded bridgeless multilevel converter.
The main advantage of the proposed control scheme with respect to the similar existing one is the simultaneous control of the negative-sequence perturbation and positive-sequence current by means of a single unified controller.
DISADVANTAGE :
The unidirectional rectifier has the problem of input current zero-crossing distortion in essence.
It can fundamentally solve the problem of large-scale asymptotic stability of nonlinear system.
In, this problem is addressed for an inverter with an LCL filter where the controller and also a state observer are optimally designed.
One more important concern with this approach is the impact of those controller dynamics in aggravating the system oscillations under weak grid conditions.
PROPOSED SYSTEM :
In this proposed control strategy, using line voltages and line currents as state variables, the mathematical model of rectifier has been established without considering the positive and negative sequence components of the above variables.
The proposed method does not require a phase-locked loop (PLL), it employs an observer to estimate positive- and negative-sequence components of the grid voltage.
The proposed controller first determines the double-frequency current references and then uses a sixth-order two-input two-output proportional-integral-resonance (PIR) structure, which is optimally designed.
ADVANTAGE :
A low-pass passive filter is normally used to eliminate harmonics and improve power quality of the grid-connected converter.
A valid nonlinear control strategy, which has fewer parameters, simple design, easy application and good performance, should be conducted to improve the grid connected LCL converter’s performance in unbalanced weak grid condition.
The PD is easily applied with a paralleled or a series connected damping resistor to damp the resonant frequency, but it increases power loss and decreases power efficiency.
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