Grid Voltage Harmonics Research Articles

Full Feedforward of Grid Voltage for Discrete State Feedback Controlled Grid-Connected Inverter With LCL Filter

Due to multifeedback of state variables, a discrete state-space controller offers outstanding control bandwidth as well as control stability for popular LCL-type grid-connected inverters, while the grid current is still vulnerable to the grid-voltage harmonics. The feedforward of grid voltage, usually employed in continuous controllers to solve such a problem, is too complicated to be applied for the discrete state-space controller. By means of continuous transformation, a full grid-voltage feedforward (GVFF) decoupling strategy is successfully proposed in this paper to make the feedforward possible for the discrete state-space controller. Based on the transfer function analysis and comparison of discretized and continuous system, comprehensive verification is also provided to verify the effectiveness of the derivation of the GVFF path. Subsequently, the robustness analysis of the proposed strategy to the grid impedance is also performed. Moreover, a grid-voltage estimator instead of the measured voltage is employed for the full GVFF, which not only retains the important information of grid voltage but also eliminates the influence of accompanied noises. The distinct features of the proposed feedforward controller plus implementation strategy are the super steady waveform, dynamic response, and robustness to the variation of grid impedance. Besides, the complexity of the algorithm is moderate and the computational burden is not significantly increased. Finally, simulation and experimental results are provided to verify the feasibility and validity of the proposed strategy.

Stator Current Harmonic Control With Resonant Controller for Doubly Fed Induction Generator

Voltage harmonics in the grid can introduce stator current harmonics in a doubly fed induction generator (DFIG) wind turbine system, which may potentially impact the generated power quality. Therefore, wind turbine current controllers need to be designed to eliminate the impact of grid voltage harmonics, especially low-order harmonics. This paper proposes a stator current harmonic suppression method using a sixth-order resonant controller to eliminate negative sequence fifth- and positive sequence seventh-order current harmonics. A stator current harmonic control loop is added to the conventional rotor current control loop for harmonic suppression. The overall control scheme is implemented in dq frame. Based on a mathematical model of the DFIG control system, the effects on system stability using the resonant controller, an analysis of the steady-state error, and the dynamic performance, are discussed in this paper. Taking these effects into account, the parameters of the resonant controller can be designed and effectively damp the influence from the grid voltage harmonics. As a result, the impacts of the negative sequence fifth- and positive sequence seventh-order voltage harmonics on the stator current as well as the electromagnetic torque are effectively removed. Simulations and experiments are presented to validate the theoretical analysis.

Operation of Wind-Turbine-Driven DFIG Systems Under Distorted Grid Voltage Conditions: Analysis and Experimental Validations

This paper presents enhanced control strategies for doubly fed induction generator (DFIG)-based wind power generation systems under distorted grid voltage conditions. The mathematical model of DFIG, in view of the fifth- and seventh-order components of grid voltage harmonics, is proposed and analyzed in detail. Based on the analytical model, further studies are conducted on the distortions of stator/rotor currents and the oscillations in the stator active/reactive powers as well as the electromagnetic torque, where the impact of DFIG's load conditions is considered. Meanwhile, alternative rotor current references are calculated to enhance the uninterruptable operation capability of the wind-turbine-driven DFIG systems under distorted grid conditions. An improved software PLL is designed, which is capable of accurately and rapidly tracking the frequency and phase angle of the fundamental grid voltage under distorted grid conditions. A proportional integral plus resonant (PI-R) current controller in the synchronously rotating (dq) reference frame is employed to simultaneously regulate the fundamental and harmonic components of rotor currents without any sequential component decomposition. Experiment results on a 3-kW DFIG prototype demonstrate the correctness of the analytical results and the effectiveness of the software PLL and PI-R current controller when the grid voltage is distorted.

Dead-Beat Control Algorithm for Single-Phase 50-kW AC Railway Grid Representation

The control of four-quadrant converters (4QC) of traction vehicles fed by an ac railway grid has to meet very restrictive limitations with regard to harmonics and stability. The challenge is increased by continuously changing the position-dependent grid impedance and voltage harmonics already contained in the grid voltage. Testing a 4QC control in a lab, therefore, requires a railway grid representation that both emulates variable grid impedance and grid-voltage harmonics. The paper presents an innovative solution to this problem that employs a 4Q inverter with a suitably designed new control scheme. As the rated power is not high, compared to the traction vehicle fed, the control has to be effective, taking each switching event of the inverter into account. For the design of the control, a suitable simulation is needed, which is also presented in this paper. A comparison of simulation and measurement results is given, proving excellent results.

계통전압 관측기를 이용한 계통연계형 인버터의 예측전류제어

분산전원을 계통에 연계하기 위해서는 전류제어가 필수적이며, 최근에 고성능 DSP(Digital Signal Processors)를 기반으로 빠른 동특성을 만족시키는 예측전류제어에 대한 연구가 활발히 진행되고 있다. 하지만 예측전류제어는 디지털 구현 시 발생하는 시지연, 파라미터 및 입력값의 오차, 노이즈에 의한 간섭으로 인해 제어 성능이 감소할 뿐만 아니라 시스템을 불안정하게 하는 단점을 갖고 있다. 따라서 본 논문은 계통연계형 인버터 응용에 있어서 계통전압 관측기를 이용한 예측전류제어를 제안한다. 전압 관측기 이득 선정을 위해, 계통전압에 존재하는 저차 고조파에 의한 영향을 고려하며, 필터 파라미터 오차에 의한 영향을 분석한다. 제안된 방법은 빠른 전류응답특성 뿐만 아니라, 전압센서를 사용하지 않음으로 인해 노이즈에 강인하며 시스템 구현이 간단하고 계통의 저차 고조파에 강인한 전류제어성능을 갖는다. 제안된 방법의 타당성은 시뮬레이션과 실험을 통하여 검증한다. For a grid-connected inverter in distributed generation systems, the current control is essential, and recently, the predictive current control based on a high performance digital signal processors (DSP) to satisfy a fast dynamic response has been widely investigated. However, the performance of predictive current control is degraded by the time delay due to digital implementation, the parameter and measured value errors and the interference of noise, and also theses make system even unstable. Therefore, this paper proposes the predictive current control using grid voltage observer for grid-connected inverter applications. To determine the relevant voltage observer gain, the low-order harmonics of grid voltage are considered, and the effect of filter parameter errors is analyzed. The proposed method has a fast current response capability, the robustness to noise and simple implementation due to voltage sensorless control and the robust current control performance to low-order grid harmonics. The feasibility of the proposed method is verified by simulation and experimental results.

Bidirectional Front End Converter for DG With Disturbance Insensitivity and Islanding-Detection Capability

This paper introduces and develops a new control strategy for the bidirectional front end converter for grid-connected distributed generation units. In the method, the switching technique directly controls the current of the voltage-source converter. The switching method is based on the evaluation of the inductor-flux error trajectory which allows to predict the next commutation instant. The control is fast and the line-current waveform is insensitive to the grid voltage perturbations or harmonics, and dc bus disturbances. A new frequency detection algorithm, robust against disturbances and with fast response in case of islanding occurrence, is introduced and applied. The proposed control method has been examined under various scenarios, including mains supply faults, distorted and unbalanced grid, and IEEE 1547 anti-islanding test. The results show the feasibility and applicability of the proposed control for the considered electronically-interfaced distributed generation units.

Behavior of PWM Active Front Ends in the Presence of Parallel Thyristor Converters

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper investigates the effects of grid voltage distortions caused by thyristor converters on the steady-state performance of a pulsewidth modulation (PWM) rectifier with dc voltage link for low-voltage applications. The disturbance transfer function of the current control loop is used to analyze the effects of grid distortions. Influences of the current controller, the phase-locked loop parameters, and the filter inductance on the generated current harmonics are presented. For existing grid voltage harmonics, the amplitude of the corresponding current harmonics are determined and compared to measured current harmonics using a 400-V 40-kW test bench consisting of a PWM rectifier with voltage dc link parallel to a 40-kW thyristor converter. </para>

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

Mitigating grid voltage harmonics using a line converter with active filtering feature

A line converter equipped with an LCL-line filter is used as an active filter directly compensating grid voltage harmonics. The control of four individual grid voltage harmonics is implemented using an individual control of corresponding harmonic line currents. The knowledge of the grid impedance is essential to efficient control of the grid voltage harmonics. To overcome the problem of an unknown grid impedance an identification method is used. The method uses the converter control system to measure the grid impedance at selected frequencies. The active voltage harmonic cancellation is tested with a 19 kVA line converter by compensating background voltage distortion and distortion caused by a nonlinear load.

Synchronisation methods for grid-connected voltage source converters

Four different synchronisation methods for a voltage source converter connected to a three-phase grid are investigated. The methods are adapted for use in a digital controller. The performance of the synchronisation methods is studied by response characteristics of phase-shift steps, frequency steps and low-frequency grid voltage harmonics. The low-pass filtering method can be used only if the frequency of the grid is constant and phase jumps do not occur. If phase jumps occur, the novel space vector filtering method is recommended. The extended space vector filtering method is adapted to handle frequency variations and is also preferred if fast frequency variations occur. This method even has a higher performance than the extended Kalman filter method, in spite of the large number of calculations that must be performed.

The Unilateral Dynamic Characteristics of Three-Electrode Vacuum Tubes

Unilateral dynamic characteristics of vacuum tube when plate circuit includes resistance, inductance or capacity.---(1) Theoretical equations. For the case of pure resistance ($R$), the Van der Bijl parabolic relation between plate current and effective grid voltage is expressed as a power series in $e$ sin pt, the impressed simple harmonic grid voltage. The coefficients of the various harmonics involve $R$, the $n$th harmonic reaching a maximum value when $R$ equals $\frac{(n\ensuremath{-}2)}{3}$ times R0 the tube resistance. For the fundamental the maximum energy output for a given plate battery is secured when $R=0.81 {R}_{0}$. The dynamic characteristic was obtained by compounding the harmonics into a single curve; it approaches a straight line as the resistance is increased. For the case of pure inductance, the plate current is expressed as a Fourier series. The dynamic characteristic is a closed loop whose area is proportional to the energy in the inductance. This loop reduces to an ellipse for small values of $e$, in which case the tube functions as an alternator whose internal impedance is a function of the external load. The insertion of a condenser instead of an equivalent inductance gives identical results except that the phase angle of the various harmonics is shifted. (2) Experimental verification. The effects on the plate current of varying the alternating grid voltage $e$, the static grid voltage ${E}_{c}$ and the plate voltage ${E}_{b}$, for a given value of resistance $R$ or inductance $l$, and the effect of varying $R$ or $l$ with constant ${E}_{b}$, ${E}_{c}$ and $e$ (15 or 20 volts), were determined and are shown in curves together with the corresponding theoretical values. A. W. E. 205B tube was used. The results show that the equations predict the harmonic constituents of the plate current as high as the fourth, for values of $e$ up to 15 or 20 volts (depending on ${E}_{b}$), the range for which the fundamental equation holds. For this range the coefficients of the various harmonics in the equation are proportional simply to ${e}^{n}$. The fundamental becomes greater while the other harmonics diminish as we approach the straight portion of the static characteristic and as we increase the plate potential.Circuit for producing pure sine wave electromotive force with frequency of 200,000 cycles.---The oscillating circuit and filters used are shown diagrammatically in Fig. 1.Pure resistance for high frequencies.---A platinized quartz fiber (diameter 0.01 mm.) with a resistance of 100 ohms per inch will carry 0.06 ampere when immersed in acidfree paraffin oil and has a negligible skin effect.