Harmonic Resonant Controller Research Articles

Passivity-based analysis and design for selective harmonic resonant control

This paper analyses the impedance passivity of selective harmonic resonant control for voltage-source converter. It is found that the phase-lead compensation of harmonic resonant controller may fail to eliminate the negative damping above certain harmonic frequency, depending on the size of time delay. Further, a trade-off between the internal stability margin of harmonic control and the impedance-passivity shaping is identified. To overcome these drawbacks, a feedforward control scheme is proposed in this work, which guarantees both impedance passivity and sufficient internal stability margin for the selective harmonic resonant control. Electromagnetic transient simulations verify the theoretical analyses and the effectiveness of the control method.

Parameter co-design of active damping loop and grid current controller for a 3-phase LCL-filtered grid-connected inverter

Under non-ideal grid conditions, especially the harmonic grid condition, the proportional resonant controller with multiple harmonic resonant controllers (HR-PR controller) is recommended to provide high quality grid currents for the 3-phase LCL-filtered grid-connected inverter. Besides, in order to solve the resonance problem, the active damping loop with the grid-side currents feedback only is preferred. However, parameters of the HR-PR controller and the active damping loop both affect the performance of LCL filter resonance suppression and stability of the inverter, which is analyzed in this paper. Moreover, the stability constraint and the resonance suppression constraint is derived to maintain the good ability of LCL filter resonance suppression and satisfy the stability requirements of the inverter. Considering the wide control bandwidth for the HR-PR controller, a co-design method of the active damping loop and HR-PR controller is proposed in this paper. By introducing the Lagrange multiplier method, the proposed co-design method obtains the optimized parameters for the active damping loop and grid currents controller in z-domain appropriately. A prototype of 3-phase LCL-filtered three-level neutral point clamped grid-connected inverter is established to validate the proposed method.

Stability Assessment of Current Controller with Harmonic Compensator for LCL-Filtered Grid-Connected Inverter under Distorted Weak Grid

An assessment of the stability and performance of current controllers with harmonic compensators is presented for an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter under distorted weak grid conditions. By using two typical current control schemes which are the direct current controller with the capacitor current-based active damping and integral-resonant state feedback current controller, the closed-loop system stability and current control performance are investigated in the presence of both uncertain grid impedance and distorted grid. Even though the controller stability has been investigated under weak grid in several studies, the stability assessment of the entire current control scheme, including the harmonic resonant controllers, still needs a further comprehensive investigation. The system stability is analyzed by obtaining the movement of the closed-loop poles in the discrete-time domain when the grid impedance varies. To fully study the impact of distorted weak grid condition on the LCL filters, three LCL filter parameter sets giving the resonance frequency in different frequency bands are chosen for the purpose of evaluating the system robustness and grid-injected current quality. In order to support the presented theoretical analyses, comprehensive simulation and experimental results based on 32-bit DSP TMS320F28335 to control 2 kVA grid-connected inverter are presented in terms of grid current quality and control stability in the environment of both uncertain grid impedance and distorted grid.

A novel harmonic impedance compensator based on POHMR-type RC under gird voltage distortion

The LCL-type grid-connected inverter can suppress high-frequency current harmonics but the inject grid current of the inverter will be worse under grid voltage distortion, therefore harmonic impedance compensation technology is required for this condition. The proportional Multi Resonant (PMR) controller is a feasible method for this purpose, but this scheme needs a large calculation burden and complex design. To overcome these drawbacks, a novel Proportional Odd harmonic Multi Resonant-type Repetitive Controller (POHMR-type RC) is proposed to compensate for the harmonic impedance, which means the grid voltage distortion hardly influence injected grid current. Compared to the PMR controller, the proposed scheme omits the even harmonic resonant controllers and has much more odd harmonic resonant controllers, which also reduces the memory space consumption, calculation burden and controller complexity. The proposed scheme also has better dynamic performance than conventional RC. When the proposed scheme is proposed, the inject grid current is still sinusoidal under strong grid voltage distortion and the transient response is desired. Experimental results verify the feasibility of the POHMR-type RC.

Harmonics and Stability Analysis of Single-Phase Grid-Connected Inverters in Distributed Power Generation Systems Considering Phase-Locked Loop Impact

In the distributed power generation systems (DPGSs) based on the renewable energies, the stability and harmonics of the grid-connected inverter are seriously affected by the uncertainties of the grid at the point of common coupling (PCC). More importantly, as several inverters are usually tied at the PCC together, the interactions among the inverters and the grid impedance can cause more serious harmonics and instabilities at the low frequencies. Hence, the phase-locked loop (PLL) affecting the low-frequency behaviors must be considered. This study aims to provide a comprehensive study of the stability and harmonics of single-phase inverters in the weak grid. Modelings of single-inverter and multiple-inverter systems considering the PLL are established. The impacts of different PLLs on the low-order current harmonics and stability are then studied. It has been proved that the maximum ability of inverters in suppressing the low-order grid current harmonics closely relates to the PLL, even if a harmonic resonant controller and/or PCC voltage feedforward is used. Thus, using different PLLs yields quite different performances. Simulation and experimental results will be given to demonstrate the analysis, which is helpful for the readers to design a proper PLL for DPGSs.

Optimal and Systematic Design of Current Controller for Grid-Connected Inverters

A design approach for grid-connected inverter controllers for distributed and renewable energy system applications is proposed, where a high number of controller gains can be designed optimally in a fairly systematic way. The linear quadratic regulator problem is first modified to accommodate sinusoidal signal tracking through a meaningful state-space transformation. The cost function is modified to explicitly include the tracking error, so that its weights are designed in a transparent and systematic way. The proposed technique is applied to the well-known control structure comprising the fundamental and harmonic resonant controllers. Second, the control structure is rearranged to reject the distortions from both the grid voltage and the reference signal, and the proposed design technique is applied to this new structure. It is shown that the desired features of active damping for $LCL$ filters and robust performance against system uncertainties, harmonics, and disturbances are achieved. Third, the controller and the systematic design procedure are extended for inverters with $LLCL$ filters. Proposed control structures are designed and simulated and then implemented on a digital signal processor. Results confirm features of the method and its ability to address control system challenges in high power density and efficient inverter applications.

Harmonic elimination of quasi-sine rotor injected DFIG-based wind power generation systems connected to electric power networks

Abstract In this study, a novel control scheme including a proportional–integral–harmonic resonant (PI–R) controller is presented to eliminate the produced harmonics resulting from the quasi-sine rotor voltage injection in a doubly fed induction generator (DFIG) connected to power electric networks. The control scheme is implemented in the rotor reference frame ( α r β r ) under quasi-sine rotor injection by performing the well-known six-step switching technique. The contribution of this study is to propose a control scheme that not only keeps a grid-connected DFIG in an acceptable operating margin but also effectively eliminates the harmonic and pulsation components both in the stator and the rotor circuits caused by quasi-sine rotor injection. To validate theoretical results, a DFIG practically used to implement the available wind power plants is simulated using MATLAB/Simulink. The simulated results not only validate the theoretical results but also explicitly verify the excellent performance of the proposed control scheme.

Control of Parallel-Connected Modular Multilevel Converters

The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for high-voltage and high-power applications. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs), which assumes that the multiple MMCs are directly connected at both ac and dc sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions: voltage balancing of submodules and mitigation of circulating currents, where the novel transformed third-order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second-order and fourth-order circulating currents and a sixth-order harmonic resonant controller is used to attenuate the zero-sequence sixth-order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a submodule, respectively. Carefully controlling the healthy submodules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for the short-circuit switch fault operation which was seldom discussed in the published literature works with respect to the MMC fault-tolerant operation schemes. MATLAB simulations and the constructed experimental prototype have verified the performance of the proposed control strategy.

Research on low‐order current harmonics rejections for grid‐connected LCL‐filtered inverters

The current quality is an important indicator to evaluate the performance of a grid‐connected inverter. In practice, many factors including the grid voltage distortion, dead‐time effect, non‐ideal switches and dc‐link disturbances would result in many low‐order harmonics in the injected grid current. The inverter with either the typical grid voltage feedforward or the harmonic resonant (HR) control had some difficulties in rejecting the impact of multi‐harmonic sources. In this study, the combination of the grid voltage feedforward and the multi‐HR control is proposed to suppress low‐order harmonics. It is found that all those harmonic sources are classified into two kinds, and a general way is provided to analyse the harmonic rejection of the inverter with the typical or the proposed strategy. The low‐order current harmonics of the inverter with different strategies are clearly explained. The comparative analysis, simulations and experiments all indicate that the proposed strategy greatly improves the ability of the inverter to reject the current harmonics induced by multi‐harmonic sources as long as the grid feedforward and the resonant control are complementary.

Evaluations of current control in weak grid case for grid‐connected LCL‐filtered inverter

For grid-connected inverters, switching harmonics can be effectively attenuated through an LCL-type filter. In order to suppress resonance and guarantee good performance, many strategies (e.g. active damping (AD), harmonic resonant control, repetitive control and grid feedforward) have been proposed. However, the wide variation of grid impedance value challenges system stability in practical applications. The aforementioned methods need to be investigated. This study evaluates the applicability of each part of the overall control in a weak grid case with the use of a stability criterion. It has been demonstrated that the feedback-based AD control can work well in a wide range of grid conditions. However, the resonant and repetitive control methods meet constraints. The grid feedforward method brings in an extra positive feedback path, and consequently results in high harmonics or even instability. Finally, a recommendation for system design has been presented. Simulations and experiments have been provided to verify the analysis.

Grid-Tie Control of Cascade Dual-Buck Inverter With Wide-Range Power Flow Capability for Renewable Energy Applications

This paper presents a grid-tie control system for cascade dual-buck inverter with both active and reactive power flow capability in a wide range under two types of renewable energy sources. A fuel-cell power-conditioning system (PCS) is a Type I system with active power command generated by balance of plant of each unit; and photovoltaic or wind PCS is a Type II system with active power harvested by each front-end unit through maximum power point tracking. Reactive power command is generated by distributed generation control site for both systems. Selective harmonic proportional resonant controller and admittance compensation controller are first introduced to cascade inverter grid-tie control to achieve better steady-state and dynamic performance. Detailed analysis and derivation of both control systems were conducted. A 1-kV·A cascade dual-buck inverter system was designed and interfaced to 120-V, 60-Hz grid to verify the control strategy. Pure active power, pure reactive power, and mixed power flow with leading and lagging angles were fully tested under steady-state and command-dynamic conditions for both systems. The experimental results proved the effectiveness of the designed grid-tie control for both systems.

Research on an Improved Control Strategy for Harmonics Restraint of DFIG System

An improved control strategy for harmonics restraint of doubly fed wind power system is studied in this paper. Considered low order current harmonics (5th, 7th and so on) caused by the distortion or unbalanced grid voltage conditions and the variations of generator itself parameters, an improved control strategy is proposed. In this control strategy, a proportional integral (PI) regulator paralleled with a harmonic resonant (R) controller in the synchronous reference frame is proposed for restraining the stator current harmonics. The control strategy presented in this paper has not effect on the control of fundamental current, and need not the harmonic separation filter and the sequence decomposition in multi reference frames. To verify the effectiveness of this control strategy, simulation model of DFIG system is developed in Matlab/Simulink, and experiments are implemented on the doubly fed experiment platform. The results of both simulations and experiments show that the control strategy proposed in this paper could restrain harmonic currents effectively and it is easy to implement digital control.

Adaptive rotor current control for wind-turbine driven DFIG using resonant controllers in a rotor rotating reference frame

This paper proposes an adaptive rotor current controller for doubly-fed induction generator (DFIG), which consists of a proportional (P) controller and two harmonic resonant (R) controllers implemented in the rotor rotating reference frame. The two resonant controllers are tuned at slip frequencies ωslip+ and ωslip−, respectively. As a result, the positive-and negative-sequence components of the rotor current are fully regulated by the PR controller without involving the positive-and negative-sequence decomposition, which in effect improves the fault ride-through (FRT) capability of the DFIG-based wind power generation system during the period of large transient grid voltage unbalance. Correctness of the theoretical analysis and feasibility of the proposed unbalanced control scheme are validated by simulation on a 1.5-MW DFIG wind power generation system.

Multiple harmonics control for three-phase grid converter systems with the use of PI-RES current controller in a rotating frame

Voltage source inverters connected to the grid in applications such as active rectifiers, active filters, uninterruptible power supplies, and distributed generation systems need an optimal ac current control. To obtain zero steady-state error at the fundamental frequency (i.e., unity power factor), the use of a standard integrator in a rotating frame is as effective as the use of a resonant controller in a stationary frame. However, the grid voltage harmonics influence the current controller and generate current harmonics unless several integrators in multiple rotating frames or resonant compensators in a stationary frame are adopted. In this letter, a hybrid system consisting of a proportional integral (PI) controller plus a generic hth harmonic resonant controller implemented in a frame rotating at the n th harmonic frequency is discussed in detail. The hth harmonic controller is able to decrease both the (h - n)th and (h + n)th harmonics, while the PI controller is able to decrease other harmonics if the synchronization phase signal adopted for the frame transformation is unfiltered. It is demonstrated that the use of a PI and sixth harmonic resonant compensator is effective for both positive and negative sequence fifth and seventh harmonics; hence, four harmonics are compensated with the proportional integral-resonant (PI-RES) controller implemented in a synchronous frame. Simulation and experimental tests validate the proposed analysis