Parag Kanjiya

“SRF Theory Revisited” to Control Self-Supported Dynamic Voltage Restorer (DVR) for Unbalanced and Nonlinear Loads

The protection of the sensitive unbalanced nonlinear loads from sag/swell, distortion, and unbalance in supply voltage is achieved economically using the dynamic voltage restorer (DVR). A simple generalized algorithm based on basic synchronous-reference-frame theory has been developed for the generation of instantaneous reference compensating voltages for controlling a DVR. This novel algorithm makes use of the fundamental positive-sequence phase voltages extracted by sensing only two unbalanced and/or distorted line voltages. The algorithm is general enough to handle linear as well as nonlinear loads. The compensating voltages when injected in series with a distribution feeder by three single-phase H-bridge voltage-source converters with a constant switching frequency hysteresis band voltage controller tightly regulate the voltage at the load terminals against any power quality problems on the source side. A capacitor-supported DVR does not need any active power during steady-state operation because the injected voltage is in quadrature with the feeder current. The proposed control strategy is validated through extensive simulation and real-time experimental studies.

Artificial-Neural-Network-Based Phase-Locking Scheme for Active Power Filters

This paper presents a phase-locking control scheme based on artificial neural networks (ANNs) for active power filters (APFs). The proposed phase locking is achieved by estimating the fundamental supply frequency and by generating a phase-locking signal. The nonlinear-least-squares-based approach is modified to estimate the supply frequency. To improve the accuracy of frequency estimation, when the supply voltage contains harmonics that are not known, a prefiltering stage is introduced. In shunt APF applications, not only the information of frequency is sufficient but also the phase information of the supply voltage is required to generate a unit template that is phase-locked to the supply voltage. Therefore, in this paper, an adaptive-linear-neuron-based scheme is proposed to extract the phase information of the supply voltage. The estimated system frequency and phase information are then utilized to generate a phase-locking signal that assures a perfect synchronization with the fundamental supply voltage. To demonstrate the effectiveness of the proposed approach, the synchronous reference frame ( d-q theory) shunt APF control method with the proposed ANN-based phase-locking scheme is adopted. The performance of the proposed ANN-based approach is verified experimentally with different supply systems and load conditions.

A Noniterative Optimized Algorithm for Shunt Active Power Filter Under Distorted and Unbalanced Supply Voltages

In this paper, a single-step noniterative optimized algorithm for a three-phase four-wire shunt active power filter under distorted and unbalanced supply conditions is proposed. The main objective of the proposed algorithm is to optimally determine the conductance factors to maximize the supply-side power factor subject to predefined source current total harmonic distortion (THD) limits and average power balance constraint. Unlike previous methods, the proposed algorithm is simple and fast as it does not incorporate complex iterative optimization techniques (such as Newton-Raphson and sequential quadratic programming), hence making it more effective under dynamic load conditions. Moreover, separate limits on odd and even THDs have been considered. A mathematical expression for determining the optimal conductance factors is derived using the Lagrangian formulation. The effectiveness of the proposed single-step noniterative optimized algorithm is evaluated through comparison with an iterative optimization-based control algorithm and then validated using a real-time hardware-in-the-loop experimental system. The real-time experimental results demonstrate that the proposed method is capable of providing load compensation under steady-state and dynamic load conditions, thus making it more effective for practical applications.

Optimal Control of Shunt Active Power Filter to Meet IEEE Std. 519 Current Harmonic Constraints Under Nonideal Supply Condition

A shunt active power filter (APF) is a well-mature technology for the compensation of nonlinear and/or reactive loads. Normally, the shunt APF is controlled such that it eliminates the load current harmonics and supplies load reactive power to achieve harmonic-free source currents at unity power factor. However, these control objectives cannot be achieved simultaneously when the supply voltages are distorted and unbalanced (nonideal). Hence, under such situation, the shunt APF should be controlled optimally to achieve a maximum possible power factor without violating the current harmonic constraints recommended by IEEE Std. 519. This paper presents an optimal algorithm to control a three-phase four-wire shunt APF under nonideal supply conditions. The optimization problem aiming at maximizing the power factor subject to current harmonic constraints as per IEEE Std. 519 has been formulated and solved mathematically using the Lagrangian formulation. The proposed algorithm avoids the use of complex iterative optimization techniques and thus is simple to implement and has fast dynamic response. The effectiveness of the proposed method is evaluated through a detailed experimental investigation using a digital signal processor controlled shunt APF prototype developed in the laboratory.

A Novel Fault-Tolerant DFIG-Based Wind Energy Conversion System for Seamless Operation During Grid Faults

A novel fault-tolerant configuration of doubly fed induction generator (DFIG) for wind energy conversion systems (WECSs) is proposed in this paper for the seamless operation during all kinds of grid faults. The proposed configuration is developed by replacing the traditional six-switch grid-side converter (GSC) of DFIG with a nine-switch converter. With the additional three switches, the nine-switch converter can provide six independent output terminals. One set of three output terminals are connected to the grid through interfacing inductors to realize normal GSC operation while, the other set of three output terminals are connected to neutral side of the stator windings to provide fault ride-through (FRT) capability to the DFIG. An appropriate control algorithm is developed for the proposed configuration that: 1) achieves seamless fault ride-through during any kind of grid faults and 2) strictly satisfies new grid codes requirements. The effectiveness of the proposed configuration in riding through different kind of faults is evaluated through detailed simulation studies on a 1.5-MW WECS.

“SRF theory revisited” to control self supported dynamic voltage restorer (DVR) for unbalanced and nonlinear loads

The protection of the sensitive unbalanced nonlinear loads from sag/swell, distortion, and unbalance in supply voltage is achieved economically using the dynamic voltage restorer (DVR). A simple generalized algorithm based on basic synchronous-reference-frame theory has been developed for the generation of instantaneous reference compensating voltages for controlling a DVR. This novel algorithm makes use of the fundamental positive-sequence phase voltages extracted by sensing only two unbalanced and/or distorted line voltages. The algorithm is general enough to handle linear as well as nonlinear loads. The compensating voltages when injected in series with a distribution feeder by three single-phase H-bridge voltage-source converters with a constant switching frequency hysteresis band voltage controller tightly regulate the voltage at the load terminals against any power quality problems on the source side. A capacitor-supported DVR does not need any active power during steady-state operation because the injected voltage is in quadrature with the feeder current. The proposed control strategy is validated through extensive simulation and real-time experimental studies.

Optimal Current Harmonic Extractor Based on Unified ADALINEs for Shunt Active Power Filters

Adaptive linear neuron (ADALINE) is widely used in parameter estimation due to its algorithmic simplicity and parallel computing nature. One of the most popular training schemes for ADALINE is the least-mean-squared (LMS) rule, which can be implemented online to reduce the computation and storage requirements greatly. In this paper, an optimal current harmonic extractor based on unified ADALINEs for the shunt active power filter (APF) is proposed to achieve a better dynamic performance and reduced computation burden. The proposed control algorithm consists of three ADALINEs. Two ADALINEs are used for frequency estimation and supply voltage synchronization, while the third ADALINE is used to extract the fundamental active component of the load current. The main factor that affects the estimation speed and accuracy is the learning rate involved in LMS weight-update rule. Generally, this learning rate is selected by trial and error. In this paper, the learning rate of each ADALINE is tuned using particle swarm optimization to achieve the best dynamic performance. Furthermore, an adaptive learning rate for the frequency-ADALINE is proposed to enhance the estimation speed. The proposed ADALINE-based control structure is validated with a detailed experimental study.

Enhancing power quality and stability of future smart grid with intermittent renewable energy sources using electric springs

The concept of “electric spring” is recently proposed to enhance the stability of smart grid with intermittent renewable energy sources. Electric springs can be integrated into the non-critical electrical loads to form smart loads. The smart load has the characteristics of (i) following intermittent power generation by changing its load demand dynamically and (ii) regulating the voltage at the point in distribution system where it is connected. Therefore, it is projected as futuristic device to mitigate voltage fluctuation problems in future smart grid where large number of small scale intermittent renewable energy sources are connected. However, further research has shown that the full capability of smart loads has not been explored. Due to the presence of power electronic converter in smart load, it also has the potential of improving power quality of the supply voltage by reducing voltage harmonics. In this paper, an enhanced control algorithm for smart load is proposed to add the feature of improving power quality of the supply voltage over and above aforementioned characteristics.

A Novel Type-1 Frequency-Locked Loop for Fast Detection of Frequency and Phase With Improved Stability Margins

The synchronous reference frame phase-locked loop (SRF-PLL) is a widely used synchronization technique in power electronics and power systems applications due to its ease of implementation and robust performance. The conventional SRF-PLL is a type-2 control system due to the use of proportional-integral controller as loop filter. With higher bandwidth design, it can achieve fast detection of frequency and phase under ideal grid conditions. However, its bandwidth should be sufficiently lowered to obtain proper disturbance rejection under unbalanced and distorted grid conditions. This results in a slower detection speed. Recently, several advanced PLLs with pre/in-loop filtering stage have been proposed to improve the detection speed. A major challenge with the PLLs is how to further improve their dynamic performance without compromising the disturbance rejection capability and stability. To resolve this issue, in this paper, a novel type-1 frequency-locked loop (FLL) is proposed. The disturbance rejection capability of the proposed FLL is improved using a modified structure low-pass filter with selective harmonics filtering ability. As the proposed FLL is type-1 control system, it achieves better dynamic performance with higher stability margins. The effectiveness of the proposed FLL is confirmed through experimental results and comparison with advanced type-2 PLLs.

A Low Component Count Series Voltage Compensation Scheme for DFIG WTs to Enhance Fault Ride-Through Capability

Fault ride-through (FRT) operation has been a challenge for the doubly fed induction generator (DFIG) based wind turbines (WTs) as the stator winding is directly connected to the grid. Additionally, several grid codes have been established for the grid interconnection of WTs, which demand the WT to stay connected and provide the predefined reactive current support to the grid during FRT operation. The series voltage compensation (SeVC) based FRT schemes for DFIG WTs outperforms all others in terms of smooth transient performance. However, such FRT schemes require an additional voltage-source converter (VSC) and a bulkier series transformer to provide the SeVC. This paper proposes a low component count SeVC scheme that is applicable to both individual WTs and wind parks to cope up with the recent grid codes requirements. The proposed configuration eliminates the need of a series transformer and an additional VSC for the SeVC operation with the use of three additional IGBTs/switches. Furthermore, a coordinated control strategy is proposed to control the WT that enhance its FRT capabilities. The proposed low component count FRT scheme and coordinated control strategy are validated using the detailed mathematical modeling and simulation of 1.5 MW DFIG WT.