Effects Of Constant Power Loads Research Articles

Dynamic Analysis, Stability and Design of Grid Forming Converters With PI-Based Voltage Control in DC and 3-Phase AC Microgrids

This paper analyzes the dynamic behavior of the voltage control loop based on proportional-integral regulators, commonly used for grid-forming converters in 3-phase AC and DC Microgrids and applications that involve a DC-link voltage control. The paper proposes a simple and accurate generalized analysis useful both for the system characterization and design. Two different control schemes, based on linear (Direct Voltage Control, DVC) and quadratic voltage feedback (Quadratic Voltage Control, QVC), are analytically studied, simulated and experimentally tested, demonstrating a superior performance of the QVC under the presence of constant power loads. The operation limits, the system stability and the disturbance rejection capability are analyzed considering the effect of control and plant parameters and the effect of the different types of disturbances and the operating point, taking into account the non-linearities of the system. The analysis is mainly focused on the effect of constant power loads given their negative impact on the system performance. The study provides a generic procedure for the analysis and design of proportional-integral voltage controllers, including the selection of the system capacitance for meeting specific dynamic specifications while considering system characteristics as the load level, the stability margins and the maximum voltage deviation under disturbances.

Smartenance DC-DC On-Board Power Converters

Traditional marine power grids, using diesel motors/generators, pollute the atmosphere with their CO2 emissions and adverse particles. Nowadays, green energy supplies like solar systems and fuel cells play significant roles in the electrification of marine propulsion plants. In general, marine power systems with green energy units are often considered as a kind of mobile DC microgrids under constant power loads (CPLs). However, since the CPLs impose nonlinearity and negative impedance instability on marine power systems, traditional linear controllers no longer stabilize the system under various disturbances. This motivated this brief to develop a non-integer model predictive control for the accurate control of DC/DC converters with CPLs to stabilize the main bus voltage and current in the full-electric ferry ship. In practice, the stability of ship systems is highly threatened due to the negative-independence effect of CPLs. To address the adverse challenges of CPLs, the deep deterministic policy gradient (DDPG) with actor and critic neural networks (NNs) is applied to effectively design the coefficients embedded in the non-integer MPC. In this approach, the actor NN provides the regulatory signals to adjust the non-integer MPC controller while the critic NN is trained to evaluate the quality of actions of actor NN. The experimental results with comparative analysis are carried out to validate the accuracy of the non-integer MPC under the change in the CPL’s power. The results demonstrated that the proposed controller offers a higher level of robustness than the state-of-art-schemes.

Model Predictive Control Based Type-3 Fuzzy Estimator for Voltage Stabilization of DC Power Converters

The stability of power electronic devices is prone to instability when supplying constant power loads (CPL) due to their negative impedance specifications. This necessitates an efficient control methodology to overcome the destabilization issue and regulate the response of the output voltage. In particular, this article proposes a novel interval type-III fuzzy neural network-based model predictive control (IT3FNNMPC) to mitigate the destructive effects of CPL in dc/dc boost converters implemented in the microgrid (MG) and power system distribution. In a typical MGs configuration, the IT3FNNMPC controller is formulated and designed in a constraint control problem to achieve a fast-transient response of system voltage. The feasibility and applicability of the designed IT3FNNMPC controller have been validated by hardware-in-the-loop real-time simulations.

Transient Mitigation Using an Auxiliary Circuit in Cascaded DC–DC Converter Systems With Virtual Impedance Control

Most dc power distribution architectures are organized as a typical cascaded converter system that is composed of source and load converters, and may experience a system oscillation due to the negative impedance effect of constant power loads. The load-side virtual impedance method is a possible solution to such stability problem, which however makes the dynamic response of the load converter slow, and thus increases the requirement of the filter capacitance. In this article, an auxiliary circuit that is connected to the output port of the load converter is proposed to enhance the transient performance of the power converter system, while the system can be stabilized by the virtual impedance method. The control and parameter design of the proposed auxiliary circuit is introduced. The capacitance requirement of the system for an expected transient deviation of the output voltage is significantly reduced by using the proposed auxiliary circuit. An example design is given to verify the feasibility of the proposed method with simulation analysis and experimental results.

Constant Power Load Modeling for a Programmable Impedance Control Strategy

Expanded use of power electronics opens the possibility that input characteristics presented to the conventional utility as well as ac and dc microgrids could be controlled in ways that help support utility operation. Power factor correcting interfaces are one wide-spread example, used around the world to meet regulatory requirements for both power factor and also harmonic content injected to the utility. More support for ensuring healthy utility operation might be demanded from these power electronic interfaces. For example, the potentially destabilizing effects of constant power loads (CPLs) could be remediated on time scales useful for supporting utility operation. This article presents an equivalent circuit model for limited-bandwidth CPLs that quantifies the intrinsic damping properties of regulated converters. A control architecture based on this equivalent circuit model is then analyzed. The control scheme limits the impact of CPL operation by emulating the internal damping of CPLs through programmable input behavior. An intermediate, internal energy buffer is used to support CPL operation while enabling the programmable or selectable input impedance on designed time scales. The effectiveness of this strategy on system stability is shown using a stability analysis and experimental results.

Composite Robust Quasi-Sliding Mode Control of DC–DC Buck Converter With Constant Power Loads

To address the stability issues aroused by the negative-resistance effect of constant power loads (CPLs), this article proposes a composite robust discretized quasi-sliding mode control (DQSMC) scheme for stabilization of buck-converter fed dc microgrids with CPLs. In the outer control loop, a robust DQSMC voltage controller is proposed. First, a discrete integral sliding surface is designed to obtain a fast and robust dynamic response of dc-bus voltage. Then, to tackle the varying load disturbances and model uncertainties, a second-order sliding mode disturbance observer is embedded in the voltage controller for disturbance estimation and compensation. The resulting composite DQSMC scheme features enhanced disturbance rejection, inherent chattering suppression, and guaranteed dynamics. Meanwhile, a PI current controller is retained in the inner control loop to realize the current control and limitation. Robustness and stability analysis of the whole composite controller is proved to assure large-signal stability. Simulation and experimental results confirm the superiority of the presented approach.

Power shaping control of DC–DC converters with constant power loads

This paper deals with the stabilization of DC–DC converters feeding constant power loads (CPLs). It is shown that power/energy dynamics are easily computed via the image of the input/output voltages/currents of the converter. Such image is represented by a high-order differential operator, obtained from first principles, acting on the variables of a stabilizing interconnected controller. This approach is based upon the premise that the destabilizing effect of CPLs, is caused due to a power imbalance in a dissipation equality, which prevents passivity. The proposed stabilizing mechanism shapes the power dynamics via an interconnected controller, inducing a feasible power flow and thus passivity. To do so, we developed a stability test, as well as control design tools in terms of linear matrix inequalities (LMIs), constructed from coefficient matrices of the system dynamics. As a practical advantage, the proposed power shaping approach permits to link stability conditions with the nominal power rate specification of power converters. Finally, we show that the proposed framework can also accommodate other traditional frequency domain, eigenvalue and immittance criteria. The main results are validated via a theoretical analysis and experimental results.

DC/DC Power Converter Control-Based Deep Machine Learning Techniques: Real-Time Implementation

The recent advances in power plants and energy resources have extended the applications of buck-boost converters in the context of dc microgrids (MGs). However, the implementation of such interface systems in the MG applications is seriously threatened with instability issues imposed by the constant power loads (CPLs). The objective is that without the accurate modeling information of a dc MG system, to develop a new adaptive control methodology for voltage stabilization of the dc–dc converters feeding CPLs with low ripples. To achieve this goal, in this letter, the deep reinforcement learning (DRL) technique with the Actor–Critic architecture is incorporated into an ultralocal model (ULM) control scheme to address the destabilization effect of the CPLs under the reference voltage variations. In the suggested control approach, the feedback controller gains of the ULM controller are considered as the adjustable controller coefficients, which will be adaptively designed by the DRL technique through online learning of its neural networks (NNs). It is proved that the suggested scheme will ensure the rigorous stability of the power electronic system, for simultaneous effects of CPL and reference voltage changes, by adaptively adjusting the ULM controller gains. To appraise the merits and usefulness of the suggested adaptive methodology, some dSPACE MicroLabBox outcomes on a real-time testbed of the dc–dc converter feeding a CPL are presented.

A Novel Assorted Nonlinear Stabilizer for DC–DC Multilevel Boost Converter With Constant Power Load in DC Microgrid

The multilevel boost converter (MBC) has been widely adopted in the dc microgrid systems due to its high voltage gain and simple structure. In recent years, the power electronic loads, which usually behave as constant power loads (CPLs), are penetrating in microgrids. The incremental negative impedance of CPLs degrades the stability of microgrid systems. To ensure effective power flow and guarantee system stability, eliminating the undesired effects of CPLs is a necessity. In this article, a novel assorted nonlinear stabilizer, which is integrated with an extended nonlinear disturbance observer (NDO) and an adaptive backstepping controller, is proposed for stabilizing the MBC-fed microgrid system with CPLs. First, the reduced-order MBC model is transformed into the Brunovsky's canonical form using the exact feedback linearization technique. Second, owing to the NDO, fast system dynamic responses are achieved. Using the estimations of NDO, an adaptive backstepping controller is developed to strictly guarantee the microgrid bus voltage stability in the sense of a large signal. Simulation and experiment results are presented to verify the effectiveness and feasibility of the proposed stabilizer.

Analysis and damping of high-frequency oscillations at the presence of distributed constant power loads

Distributed power system can be subjected to a high penetration level of constant power loads (CPLs). In this circumstance, the system stability is a matter of concern. In this paper, the distribution effect of grid-connected CPLs is analyzed in an AC distributed power system. In fact, the interaction dynamics between different arrangements of distributed CPLs and the grid are completely compared. To this aim, the total power of all arrangements of distributed CPLs is considered constant that is called fixed-power arrangements in this work. Accordingly, first, the linearized model of tightly regulated voltage source rectifier (VSR), treating as a CPL, is developed. Then, the obtained model is generalized for multiple VSRs to provide distributed CPLs model. Further, by applying the bode diagram method and eigenvalues-based analysis, the distribution effect of CPLs on their stability is evaluated. The conducted analysis reveals that the distribution of CPLs is accompanied by different responses in their interactions with the grid, which is mainly highlighted at their high-frequency (HF) behavior. Moreover, it is obtained that more distribution of CPLs can deteriorate their HF interaction and lead to a reduction in system stability. According to these achievements, it can be deduced that ideal models of CPLs by negative impedances are not valid anymore, especially at higher frequencies. Afterward, a proper active damper is implemented to reduce the mentioned HF oscillations. The simulation studies of several distributed CPLs cases, as well as a centralized CPL, confirm the validity of the performed analysis.

Seamless Transition of Microgrids Operation From Grid-Connected to Islanded Mode

One of the main features of Microgrids is the ability to operate in both grid-connected mode and islanding mode. In each mode of operation, distributed energy resources (DERs) can be operated under grid-forming or grid-following control strategies. In grid-connected mode, DERs usually work under grid-following control strategy, while at least one of the DERs must operate in grid-forming strategy in islanding mode. A microgrid may experience remarkable fluctuations in voltage and current due to an unintentional islanding event. To achieve a smooth transition to islanding mode and mitigate disturbance effect, this paper proposes a control strategy includes a) a linear voltage controller with capacitor current feedback as an input to the voltage controller and output current feedforward as an input to current controller, and b) modified droop control to emulate the inertia response of a synchronous generator. The proposed controller can suppress voltage, current and frequency fluctuations and also guarantee a smooth transition. A small signal analysis of the proposed control strategy is developed to design its coefficients as well as the destabilizing effect of constant power load (CPL). Experimental results are provided to verify the effectiveness of the proposed control strategy.

Voltage control in a DC islanded microgrid based on nonlinear disturbance observer with CPLs

Abstract In this paper, a nonlinear voltage control for a DC islanded microgrid (MG) is proposed. This control method is a new nonlinear decentralized backstepping based strategy for voltage control of DC MG based on Sepic converter with nonlinear loads such as constant power loads (CPLs). The effect of CPL is to realize a load with negative resistance for DC MG that impacts the power quality of the power system and causes negative damping. The major purpose of the proposed controller is to improve the transient performance of MG with the Plug and Play (PnP) feature in the presence of load change and CPL. To consider the effects of such loads on the proposed control method, a nonlinear disturbance observer (NDO) along with backstepping controller is introduced to estimate the uncertain terms such as CPL and load currents as well as the current transferred between DGUs. The proposed method only measures the local voltage and current quantities without any need for additional information about the grid topology and its parameters. This method successfully regulates the point of common coupling (PCC) voltage. The stability of the method is proved via Lyapunov theory and its features are verified with simulation and some experiment tests.

Model Predictive DC Voltage Control for all-electric ships

With the advent of on-board Direct Current (DC) power and propulsion systems, the transmission and delivery of energy on board of ships can be carried out more efficiently as it is being done using conventional direct-diesel or Alternative Current (AC) power and propulsion systems. However, the stability of DC voltage on-board of all-electric ships with a DC power and propulsion architecture is a critical issue that has drawn attention over the last few years. In this paper, a novel Model Predictive Control (MPC) approach is proposed for the diesel-generator shaft speed control and DC voltage regulation on-board of all-electric ships, focusing on the uncontrolled rectification at the voltage conversion stage. This work considers the prime mover as a Diesel–Generator–Rectifier (DGR) set which feeds propulsive asynchronous motors through a DC-link. First, a state space model dynamic model is developed for the DGR set and the DC-link. Then, the MPC-based approach is presented. The approach is based on Input–Output Feedback Linearization (IOFL) which is used for the linearization of the highly non-linear dynamics of the system. To increase the robustness of the algorithm, a tube-based technique is adopted which is implemented through a linear auxiliary control law. Different analyses are carried out to show that the proposed control strategy is capable of handling sudden changes in load conditions as well as adverse effects of Constant Power Loads (CPL).

Adaptive Stabilization of Uncontrolled Rectifier Based AC–DC Power Systems Feeding Constant Power Loads

It is known that, when tightly regulated, actively controlled power converters behave as constant power loads (CPLs). These loads can significantly degrade the stability of their feeder system. The loop-cancelation technique has been established as an appropriate methodology to mitigate this issue within dc–dc converters that feed CPLs. However, this has not yet been applied to uncontrolled rectifier based ac–dc converters. This paper therefore details a new methodology that allows the loop-cancelation technique to be applied to uncontrolled rectifier based ac–dc converters in order to mitigate instability when supplying CPLs. This technique could be used in both new applications and easily retrofitted into existing applications. Furthermore, the key contribution of this paper is a novel adaptive stabilization technique, which eliminates the destabilizing effect of CPLs for the studied ac–dc power system. An equation, derived from the average system model, is introduced and utilized to calculate the adaptable gain required by the loop-cancelation technique. As a result, the uncontrolled rectifier based ac–dc feeder system is always stable for any level of CPL. The effectiveness of the proposed adaptive mitigation has been verified by small-signal and large-signal stability analysis, simulation, and experimental results.

A distributed control strategy based on droop control and low-bandwidth communication in DC microgrids with increased accuracy of load sharing

A microgrid is obtained from the integration of several distributed energy resources such as fuel cells and storages. Droop control is a method for load sharing between sources or storages in a microgrid. In the conventional droop control method, one can only achieve one of the low voltage drops and accurate load sharing requirements by considering the line resistances. This paper proposes a new non-centralized control approach based on low-bandwidth communication (LBC) and the droop control method for the DC microgrid (DC-MG). The droop gain is calculated with respect to the output currents of the distributed generation (DG) units. Factors like different line resistances, different DG capacities, and the presence of local loads do not affect the controller performance. All calculations and control operations are performed locally and the LBC is used solely for transferring current information—this ensures high reliability. Simulations and experimental results show that the use of the proposed control method decreases voltage deviations and the effects of constant power loads (CPLs), while it increases system efficiency and the accuracy of load sharing.

Constant Power Loads (CPL) with Microgrids: Problem Definition, Stability Analysis and Compensation Techniques

This paper provides a comprehensive review of the major concepts associated with the μgrid, such as constant power load (CPL), incremental negative resistance or impedance (INR/I) and its dynamic behaviours on the μgrid, and power system distribution (PSD). In general, a μgrid is defined as a cluster of different types of electrical loads and renewable energy sources (distributed generations) under a unified controller within a certain local area. It is considered a perfect solution to integrate renewable energy sources with loads as well as with a traditional grid. In addition, it can operate with a conventional grid, for example, by energy sourcing or a controllable load, or it can operate alone as an islanding mode to feed required electric energy to a grid. Hence, one of the important issues regarding the μgrid is the constant power load that results from the tightly designed control when it is applied to power electronic converters. The effect of CPL is incremental negative resistance that impacts the power quality of a power system and makes it at negative damping. Also, in this paper, a comprehensive study on major control and compensation techniques for μgrid has been included to face the instability effects of constant power loads. Finally, the merits and limitations of the compensation techniques are discussed.

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.

Mitigation of Negative Impedance Instabilities in a DC/DC Buck-Boost Converter with Composite Load

A controller to mitigate the destabilizing effect of constant power load (CPL) is proposed for a DC/DC buck-boost converter. The load profile has been considered to be predominantly of CPL type. The negative incremental resistance of the CPL tends to destabilize the feeder system, which may be an input filter or another DC/DC converter. The proposed sliding mode controller aims to ensure system stability under the dominance of CPL. The effectiveness of the controller has been validated through real-time simulation studies and experiments under various operating conditions. The controller has been demonstrated to be robust with respect to variations in supply voltage and load and capable of mitigating instabilities induced by CPL. Furthermore, the controller has been validated using all possible load profiles, which may arise in modern-day DC-distributed power systems.

Mitigation of voltage collapse through active dynamic buffers

Power electronics continues to grow as an element of electrical loads. Most such loads, including motor drives, computer power supplies, and compact fluorescent lighting, serve to 'isolate' the load dynamics from the utility grid. The unfortunate effect is that these loads enforce local constant power behaviour. This tends to destabilise the supply system, resulting in voltage collapse. In this work, the power buffering technique is examined to mitigate the effects of constant power loads. A power buffer is a device that enforces dynamic isolation to bestow beneficial dynamic characteristics upon the grid. A review of constant-power load dynamics and weak power systems is presented. Through simulations, the generalised power buffer concept is shown to stabilise several examples of power systems. Various techniques and topologies for power buffers are then covered and related to the voltage collapse problem, including critical control methods and parameters. Finally, three experiments are presented that demonstrate the stabilising effects of buffers.