Presence Of Constant Power Loads Research Articles

Stability Analysis of MIMO Microgrid Systems in d–q Axis Platform with Constant Power Loads Based on Popov Absolute Stability Criterion

Microgrid has several advantages over the conventional utility grid system, thus naturally being the gateway to implement the renewable energy resources. Now it is considered as the primary solution to solve the next generation power demand. However, in practice, it is very challenging to retain the microgrid operational stability due to the dense presence of constant power loads in the system. In this article, we considered a technique of compensating the load side to mitigate the instability issues introduced by CPL. Adopting this technique, a stability analysis was presented here for MIMO (multiple input, multiple output) AC microgrid system in dq axis platform by Popov Absolute Stability Criterion using the sector conditions and Popov plots. Besides the recent researches regarding the microgrid stability techniques and the previous analysis using Popov Absolute Stability Criterion, the modeling of the energy storage-based load side compensated microgrid system is also provided in this article. All the cases and results are rigorously scrutinized in virtual platform such as MATLAB/Simulink and verified through experimental results as well.

Stability improvement of microgrids in the presence of constant power loads

Renewable energy sources, the most reasonable fuel-shift taken over the naturally limited conventional fuels, necessarily deal with the self-sustainable microgrid system rather than the traditional grid distribution system. In practice, the microgrid system experiences the challenge of instability due to the Constant Power Load (CPL) from many electronic devices. In this paper, AC microgrid stability, besides the stability analysis and pole zero movement, is thoroughly investigated for each and every considerable parameter of the system in order to improve the stability scenario. After demonstrating all cases regarding the instability problem, the storage based virtual impedance power compensation method is introduced to retain the system stability and extend the loading limit of the microgrid system. Here, in this proposed method, both the active and reactive power compensation techniques are suggested for a potential solution. Besides this, a PID controller is implemented to maintain the constant terminal voltage of CPL via current injection method from storage. All the cases and results are rigorously scrutinized in the virtual platform such as MATLAB/Simulink with appreciable aftermaths. Hardware implementation of the proposed method is also conducted to find out its effectiveness.

Investigation on the Development of a Sliding Mode Controller for Constant Power Loads in Microgrids

To implement renewable energy resources, microgrid systems have been adopted and developed into the technology of choice to assure mass electrification in the next decade. Microgrid systems have a number of advantages over conventional utility grid systems, however, they face severe instability issues due to the continually increasing constant power loads. To improve the stability of the entire system, the load side compensation technique is chosen because of its robustness and cost effectiveness. In this particular occasion, a sliding mode controller is developed for a microgrid system in the presence of constant power loads to assure a certain control objective of keeping the output voltage constant at 480 V. After that, a robustness analysis of the sliding mode controller against parametric uncertainties was performed and the sliding mode controller’s robustness against parametric uncertainties, frequency variations, and additive white Gaussian noise (AWGN) are presented. Later, the performance of the proportional integral derivative (PID) and sliding mode controller are compared in the case of nonlinearity, parameter uncertainties, and noise rejection to justify the selection of the sliding mode controller over the PID controller. All the necessary calculations are reckoned mathematically and results are verified in a virtual platform such as MATLAB/Simulink with a positive outcome.

Development of Lyapunov redesign controller for microgrids with constant power loads

In microgrid applications, stability issues have been raised into a matter of concern due to the continually increasing modern electronic loads and inverter-based power electronic loads. In this paper, adopting storage system based load side compensation technique, a Lyapunov redesign controller is proposed to considerably improve the microgrid stability in the presence of constant power loads. Besides the controller design, the robustness analysis of the proposed Lyapunov redesign controller against parameter uncertainties is depicted. After that, the proposed Lyapunov redesign controller robustness against parametric uncertainties, frequency variations, and additive white Gaussian noise (AWGN) is presented. Later, performance of the PID and Lyapunov redesign controller is compared in the case of nonlinearity, parameter uncertainties, and noise rejection to justify the selection of the Lyapunov redesign controller over the PID controller. Besides reckoning the necessary calculations, all of the results are verified in MATLAB/Simulink with the appreciable aftermath.

Mitigation of destabilising effect of CPLs in island DC micro‐grid using non‐linear control

In recent years DC micro-grid has been widely accepted as one of the promising solutions to integrate renewable energy sources and to supply power to critical loads such as data centres, remote villages and communication stations. However, DC micro-grid has a fundamental stability challenge due to constant power load (CPL) characteristics of point-of-load converters, which introduce destabilising effect in the system. This study presents a sliding mode control based non-linear control scheme for a solar photo-voltaic based DC micro-grid in the presence of CPLs. The objective of the proposed control scheme is to tightly regulate the DC bus voltage and mitigate the destabilising effects of CPLs. The stability of the system is analytically established and a limit of CPL is obtained. Furthermore, a charging/discharging algorithm is implemented for battery bank interfacing bidirectional converter which facilitates three modes charging namely constant current, constant voltage, and float mode, to enhance the battery life. The validation of the effectiveness of the proposed scheme is done through simulation and experimental results. It is found that the proposed control scheme ensures desired operation of the DC micro-grid under various operating modes and maintains system stability with CPL under variations in the primary resource and load demand.

Implementation of Hybrid Energy Storage Systems to Compensate Microgrid Instability in the Presence of Constant Power Loads

Microgrid systems have been adopted globally to implement the renewable energy-based electrification, but the CPL has caused instability issues. To improve the stability of the microgrid system, a virtual impedance-based load side compensation technique is used. In this paper, to implement this storage-based compensation technique, the hybrid energy storage system (HESS), with a battery unit as well as ultracapacitor unit, is introduced to reduce the deficiency in the case of using either battery-only or ultracapacitor-only storage system and offer the combined features with higher energy and higher power density. Here, the storage will provide high power density with quick charging/discharging time and the ultracapacitor will compensate the transient demand for a short period of time; therefore compensating the required power by the combined features of its constituents. Besides HESS is operated by a simple implementable algorithm, it improves overall efficiency, cost effectiveness, life span; reduce the energy storage size and stress on the battery. To verify the performance of the proposed system, necessary results performed at Matlab/Simulink platform are presented in this paper.

Adaptive DC Stabilizer with Reduced DC fault Current for Active Distribution Power System Application

This paper takes a systematic view on the control and protection of medium power DC networks in an active distribution power system considering fault current limiting, system control, and converter design. Reduced terminal capacitance and extra DC impedance are used to limit DC fault current and reduce the required converter current rating for medium power DC networks. An adaptive DC power stabilizer is proposed to alleviate possible system instability brought by the fault current limiting settings in the presence of constant power load. The effect of the current limiting method and the proposed stabilizer on DC fault current and stability enhancement are validated by simulation studies using a simple two-converter DC network and a multiterminal DC network in an active distribution power system.

Small-Signal Stability Analysis of Three-Phase AC Systems in the Presence of Constant Power Loads Based on Measured d-q Frame Impedances

Small-signal stability is of great concern for electrical power systems with a large number of regulated power converters. In the case of dc systems, stability can be predicted by examining the locus described by the ratio of the source and load impedances in the complex plane per the Nyquist stability criterion. For balanced three-phase ac systems the same impedance-based method applies, for which this paper uses impedances in the synchronous rotating reference ( d-q ) frame. Small-signal stability can be determined by applying the generalized Nyquist stability criterion (GNC). This approach relies on the actual measurement of these impedances, which up to now has severely hindered its applicability. Addressing this shortcoming, this paper investigates the small-signal stability of a three-phase ac system using measured d-q frame impedances. The results obtained show how the stability at the ac interface can be easily and readily predicted using the measured impedances and the GNC, thus illustrating the practicality of the approach, and validating the use of ac impedances as a valuable dynamic analysis tool for ac system integration, in perfect dualism with the dc case.

Comprehensive Review of Stability Criteria for DC Power Distribution Systems

Power-electronics-based dc power distribution systems, consisting of several interconnected feedback-controlled switching converters, suffer from potential degradation of stability and dynamic performance caused by negative incremental impedances due to the presence of constant power loads. For this reason, the stability analysis of these systems is a significant design consideration. This paper reviews all the major stability criteria for dc distribution systems that have been developed so far: the Middlebrook Criterion, the Gain Margin and Phase Margin Criterion, the Opposing Argument Criterion, the Energy Source Analysis Consortium (ESAC) Criterion, and the Three-Step Impedance Criterion. In particular, the paper discusses, for each criterion, the artificial conservativeness characteristics in the design of dc distribution systems, and the formulation of design specifications that ensure system stability. Moreover, the Passivity-Based Stability Criterion is discussed, which has been recently proposed as an alternative stability criterion. While all prior stability criteria are based on forbidden regions for the polar plot of the so-called minor loop gain, which is an impedance ratio, the proposed criterion is based on imposing passivity of the overall bus impedance. A meaningful simulation example is presented to illustrate the main characteristics of the reviewed stability criteria.