Optimal control strategy of voltage source converter‐based high‐voltage direct current under unbalanced grid voltage conditions

This paper presents an independent operation of the rotor-side converter (RSC) and grid-side converter (GSC) for a doubly fed induction generator (DFIG)-based wind energy conversion system under unbalanced grid voltage conditions. In this paper, the RSC is controlled to achieve four different control targets, including balanced stator current, sinusoidal rotor current, smooth stator active and reactive powers, and constant DFIG electromagnetic torque. The GSC is commanded to keep the dc voltage at a constant value. Additional feedback compensators using resonant regulators for the RSC are employed, and the decompositions of the positive and negative sequence components and calculations of the rotor negative current references can be avoided. Another similar compensator is used in the GSC to suppress the dc voltage fluctuates and remove the GSC reactive power oscillations without the stator or rotor power information. The proposed method can make the RSC and GSC available to an independent operation with a simple implementation for higher reliability. The experimental results demonstrate the effectiveness of the proposed control strategy for both the RSC and GSC under unbalanced grid voltage conditions.

This paper presents a new configuration of doubly fed induction generator (DFIG)-based wind farm with multilevel cascaded H-bridge (CHB) converter and its control system, which is suitable not only for unbalanced grid voltage conditions, but also for small voltage sags. The proposed configuration of DFIG can balance the point of common coupling (PCC) voltage without needing additional DC link capacitor or energy storage unlike other configurations and methods. Consequently, the stator voltage and the other loads voltage connected to the PCC are kept constant at the nominal value. The new configuration improves all the DFIG signals under unbalanced grid voltage and small voltage sag conditions unlike other methods. Some of the DFIG units in wind farm require to connect the CHB converter, therefore, it is cost-effective. Matlab/Simulink is used for simulation of a 13.5-MW wind farm consisting of nine 1.5-MW DFIG units and the results demonstrate the good efficiency of the proposed scheme.

In this paper, a dual-converter three-phase pulse width modulation (PWM) rectifier based on unbalanced one-cycle control (OCC) strategy is proposed. The proposed rectifier is used to eliminate the second harmonic waves of DC voltage and distortion of line currents under unbalanced input grid voltage conditions. The dual-converter PWM rectifier employs two converters, which are called positive-sequence converter and negative-sequence converter. The unbalanced OCC system compensates feedback currents of positive-sequence converter via grid negative-sequence voltages, as well as compensates feedback currents of negative-sequence converter via grid positive-sequence voltages. The AC currents of positive- and negative-sequence converter are controlled to be symmetrical. Thus, the workload of every switching device of converter is balanced. Only one conventional PI controller is adopted to achieve invariant power control. Then, the parameter tuning is simplified, and the extraction for positive- and negative-sequence currents is not needed anymore. The effectiveness and the viability of the control strategy are demonstrated through detailed experimental verification.

Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

This paper presents a mathematical model for Brushless Doubly-Fed Machine (BDFM) based on Stator Flux Orientation (SFO) in positive and negative reference frames under unbalanced grid voltage conditions. A negative controller is designed to eliminate the effects of unbalanced conditions on electrical torque. The model and controller performance during unbalanced conditions is validated by time domain simulation in MATLAB/Simulink.

Voltage source converters (VSCs) have been widely used due to their flexibility to control voltages and power independently and bi-directionally. Typically, the control system of VSCs mainly consists of two parts; outer voltage and power controllers and inner current controllers. The vector current control based dq decoupling technique enables to control the active power, reactive power, DC voltage and AC voltage. However, the d- and q-axis of grid voltages and currents comprise AC and DC components under unbalanced grid conditions. The AC components of the d- and q-axis currents make the grid currents unbalanced. In this paper, a novel current control is presented and investigated under unbalanced grid condition for a Back-to-Back Modular Multilevel Converter (B2B-MMC) based HVDC system and validated using the Real Time Digital Simulator (RTDS). Further, the active power oscillation under fault is eliminated by controlling the AC component of grid currents in the dq frame. The RTDS results demonstrate the feasibility of the proposed controllers under unbalanced grid voltage conditions.

Optimized hierarchical power oscillations control for distributed generation under unbalanced conditions

In order to ride through the unbalanced grid fault, the output power of photovoltaic inverter will fluctuate and its output current will rise and distort under unbalanced grid voltage. This paper proposes an improved formula of reference current and its optimal coefficients method for the photovoltaic inverter under unbalanced grid voltage sag, taking into account the requirements of active and reactive power control. Then the analytic formula of its current harmonic distortion, three-phase current peak, active and reactive power fluctuation, DC voltage fluctuation and the control factor of its output current reference were derived. Considering the constraints of inverter phase current and the dc voltage fluctuation of capacitance in dc side, the optimal model of inverter current reference was established with minimum integrated amplitude of the active and reactive power function as a goal. The correctness of the proposed method has been verified by PSCAD/EMTDC simulation software.

This paper presents a PIR regulator using direct torque control strategy of grid connected wind turbine driven doubly fed induction generators (DFIGs) when the grid voltage is unbalanced. Under the unbalanced grid voltage condition, the stator voltage and current quality is strongly affected due to the negative and distorted components. It will be reducing the performance of other normal loads connected to the DFIG. That control scheme consisting of indirect matrix converter using DTC. To verify the value of the proposed control strategy, simulation results with 500 MVA DFIG topology are presented and discussed in the paper. Finally,the simulation studies are carried out on a 500 MVA wind-turbine driven DFIG system under unbalanced grid voltage conditions. All the results are validated by using PSCAD simulation.

A photo-voltaic (PV) system based grid-connected current source inverter has been investigated in this paper. In this paper, a photovoltaic (PV) system based grid-connected current source inverter has been investigated. For obtaining the maximum power from the PV system, we have developed a Lyapunov energy function-based control scheme. Operation of the system under unbalanced grid voltage condition has been reported and a methodology to extract maximum power from the PV system is articulated. In addition, to achieve optimal performance of the Lyapunov energy function based power controller, an existing parameter update scheme is employed and an optimal performance of the overall system is achieved. Detailed analysis of the proposed power controller with the feature to operate successfully under unbalanced grid voltage condition has been substantiated and discussed along with the effect of parameter update on the overall control architecture. The proposed system is verified via software simulations in MATLAB/Simulink and PLECS domain and various case study results are presented which corroborate the efficacy of the proposed framework.

Unbalanced grid is a normal fault in power system, and in order to ensure the rectifier current tracking precision under this condition, the design and implementation of rectifier control strategies become more complex. In view of this, a simplified grid current tracking strategy is proposed, which is suitable for LCL-based rectifier operating in both balanced and unbalanced conditions. The grid current sinusoidal is achieved by tracking converter-side current of LCL filter. The current reference is only calculated by grid voltage and its 1/4 period lagging signal at stationary coordinates. Compared with conventional strategies, sequence separation and Phase-Locked Loop (PLL) are avoided. Besides, an improved active damping method is applied, which can accurately extract harmonic voltage and enhance the resonance suppressing effect. Simulation and experimental results verify the effectiveness of the proposed strategy.

The grid-connected inverter works as a controlled current source in grid-connected mode, while operates as a controlled voltage source in island mode. The grid-connected inverter should continue to operate normally under unbalanced grid voltage. In case of utility serious faults or intentional islanding, the inverter should be able to seamlessly transfer from grid-connected mode to island mode. In order to achieve these aims, this paper proposed a new feedback multi-loop control strategy with capacitor current, capacitor voltage and grid-side current in αβ reference frame, in which a proportional resonant (PR) regulator is adopted. Compared with the dual-PI control scheme in dq reference frame, the PR controller does not require decomposition of positive-and negative-sequence grid side currents, the transient performance will be improved and low voltage ride through (LVRT) capability of the grid-connected inverters during unbalanced grid voltage condition. The proposed scheme can realize the adjustable power factor, and produce symmetrical and balanced currents in case of unbalanced voltage dips. With this control method, seamless transfer can be achieved between island and grid-connected mode, and load voltage quality during the transition is improved with the proposed control method. Simulations are carried out to verify the proposed controllers and algorithms.

An augmented state observer-based sensorless control of grid-connected inverters under grid faults

This paper analyzes the effects of unbalanced voltage on doubly fed induction generators. It also presents a novel control strategy based on direct power control (DPC+) applied to this type of generators, predominant in wind energy applications, that enables them to work under perturbed conditions and achieve optimum results. Although the technique can be implemented to control both rotor converters and grid converters, we will hereby exemplify the former which regulates stator active and reactive power. The results obtained with DPC+ are then compared through experimental tests to indicate that the technique is suitable and achieves good dynamic responses while controlling current distortion, power or torque oscillations. The validation of results has been performed through experimental tests on a 20-kW generator.

Under unbalanced grid voltage conditions, negative-sequence will generate the double fundamental frequency in the dq synchronous rotating reference frame. In order to detect the positive-sequence component of the grid voltages fast, this paper proposes a new phase-locked loop method, called double fundamental frequency phase-locked loop (DFF-PLL). The operation principle is analyzed, giving the extract formulas of the positive- and negative-sequence with double fundamental frequency, the process of DFF-PLL and the detection of positive-sequence component of grid voltages. The paper improves the second order generalized integrator quadrature-signals generation (SOGI-QSG), in order to remove the DC bias voltage and guarantee the orthogonality of SOGI-QSG. The performance of DFF-PLL is verified by using simulation and experimental results. Compared with DSOGI-PLL, this method increases the detection speed and accuracy under unbalanced grid voltage conditions.