The design of reliable power filters that mitigate current and voltage harmonics to meet the power quality requirements of the utility grid is a major requirement of present-day power systems. In this paper, a detailed systematic approach to design a shunt active power filter (SAPF) for power quality enhancement is discussed. A proportional–integral (PI) controller is adopted to regulate the DC-link voltage. The instantaneous reactive power theory is employed for the reference current’s extraction. Hysteresis current control is used to obtain the gate pulses that control the voltage source inverter (VSI) switches. The detailed SAPF is developed and simulated for balanced nonlinear loads and unbalanced nonlinear loads using MATLAB/Simulink. The simulation results indicate that the proposed filter can minimize the harmonic distortion to a level below that deployed by the Institute of Electrical and Electronics Engineers (IEEE) standards.

Modeling and Simulation of a PI Controlled Shunt Active Power Filter for Power Quality Enhancement Based on P-Q Theory

Distribution system possesses high resistance to reactance ratio and unbalanced load profile. Introduction of power electronic devices such as solar photovoltaic (PV) inverter in the distribution system leads to power imbalance and unregulated voltage profile at the point of common coupling (PCC) because these devices having low-voltage ride through feature remain connected to grid during fault and inject balanced current to unbalanced fault. Also, high PV integration may increase the voltage at PCC beyond its desired limit. To overcome such unbalanced conditions and to maintain voltage at PCC, a positive, negative and zero sequence-based current controller with reactive power compensation is proposed in this work. The sequence controller controls the sequence currents to their reference command. DC-link voltage regulator, reactive power compensator and PCC voltage regulator decide the references for sequence currents. The proposed reactive power compensator uses both positive sequence voltage at PCC and reactive power supplied by the filter capacitor to generate the reactive power reference value. Feedforward and feedback actions are involved in the controller to produce switching pulses for inverter. The proposed method is tested in OPAL-RT for real-time validation using IEEE 13 bus distribution system model and found to be accurate.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rpg.2018.6219

Voltage control of PV inverter connected to unbalanced distribution system

Active damping is a common way to stabilize the current control of LCL -filtered converters. In this paper, the stable region of −180° phase crossing is first identified within a predefined range of grid impedance and LCL parameter variations. Once the phase of the current control loop is in the identified region, a stabilization control can be attained. Subsequently, digital filters can be adopted to achieve active damping by reshaping the open-loop phase. Various digital filters are selected and benchmarked in this paper. It is confirmed that the all-pass filter has a unity gain and adjustable lagging phase before the Nyquist frequency, thereby being a promising solution to the phase reshaping. Therefore, the all-pass filter is employed to move the phase of the open-loop control (i.e., −180° phase crossing) into the targeted region for active damping. Notably, the current controller and the all-pass filter-based active damping can be separately designed, indicating the easy implementation of the active damping. Experimental tests demonstrate that the proposed method can ensure the system stability over a wide range of parameter variations (e.g., grid impedance changes and LCL -filter parameter drifts) while maintaining fast dynamics with the grid-side current control.

Phase Reshaping via All-Pass Filters for Robust LCL -Filter Active Damping

An effective compensation control strategy for power quality enhancement of unified power quality conditioner

The single-phase unified power quality conditioner (UPQC) is widely used to improve power quality for sensitive end-users. However, the inherent instantaneous power difference between parallel and series converter will generate significant low-frequency dc-link voltage ripple and degrade the compensation performance of UPQC. The mechanism of dc-link voltage ripple generation and its influences on compensation voltage and current are analyzed in this article. To suppress the influence, a control strategy is proposed for the single-phase UPQC. For parallel converter, a notch filter is introduced in the outer voltage loop to prevent the voltage ripple entering the control loop, the specific order harmonics compensation is proposed in the inner-current loop to reduce the grid current harmonics. For the series converter, the dc-link voltage feedback is adopted to suppress the influence of voltage ripple on the compensation performance. Simulation and experimental results show that the grid current total harmonic distortion (THD) can be reduced effectively with the fixed dc-link capacitance compared with the traditional control strategy. More than half of the load voltage THD are also decreased by the dc-link voltage feedback control. Moreover, with the smaller dc-link capacitance, single-phase UPQC can maintain the better performance under the proposed control strategy.

Control Strategy of Single-Phase UPQC for Suppressing the Influences of Low-Frequency DC-Link Voltage Ripple

In a distribution system, to compensate for the effect of voltage sag/swell and mitigate harmonic currents, a unified power quality conditioner (UPQC) is used. Conventionally, the power rating of UPQC is determined based on the expected sag/swell, linear and nonlinear loads. However, with the shift towards usage of electronic loads and electrical drives, the composition of nonlinear loads is increasing. Hence, the UPQC deployed earlier may not be capable of handling increased harmonic currents due to its limited current rating. While trying to compensate for both harmonic currents and voltage sag/swell, the shunt converter (of UPQC) may reach its current limit and would result in undesirable behaviour. To address this issue, a technique to prioritise sag/swell compensation over active filtering is proposed in this study. The remaining power capacity of UPQC (after sag/swell compensation) is utilised for harmonic filtering. Adaptive determination of harmonic compensation current also allows prioritising the various harmonics to be compensated. This helps in case passive tuned filters are installed for some harmonic frequencies for which UPQC need not respond. Detailed simulation studies are performed on MATLAB/SIMULINK platform and results are included in this study. A laboratory prototype of single phase UPQC is developed to validate the performance of the proposed scheme.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-pel.2018.5458

Adaptive shunt filtering control of UPQC for increased nonlinear loads

Dynamic voltage restorer (DVR) is a custom power device used in electrical distribution system for power quality improvement. It ensures regulated voltage supply to the sensitive loads, even in case of voltage sag and swell disturbances in the distribution network. It is a series connected device and compensates voltage sag and swell by injecting a voltage with the help of a series transformer. 
The injection of an appropriate voltage component in the event of a voltage disturbance requires a certain amount of real and reactive power. Conventionally, DVR consists of an energy storage device, which supplies the required power over the limited duration of the sags. Large magnitude and long duration of sags lead to heavy financial investment in energy storage unit. To overcome this limitation, a single-phase back-to-back converter-based DVR is implemented in this work, which eliminates energy storage requirement. The integration of series and shunt converter makes the DVR capable of bidirectional flow of energy. Therefore, the key advantage of this topology is its capability to compensate for long-term voltage sag and swell. Modelling of the DVR and its controller design is included in this paper. The effectiveness of control schemes, protection schemes and starting sequence of operation of DVR is verified through detailed simulation studies. A scaled down laboratory prototype of DVR is developed. The viability of these schemes is confirmed by the experimental results generated from the laboratory prototype. Various challenges faced during the prototype development and corresponding solutions are also discussed in this paper.

/pdf/design-and-control-of-single-phase-dynamic-voltage-restorer-21d4qscwta.pdf

Yogesh Sonawane

Papers

Adaptive shunt filtering control of UPQC for increased nonlinear loads

Design and control of single-phase dynamic voltage restorer

Optimization and Modelling of EGR rate and MIS for POME fuelled CRDI diesel engine