Negative Incremental Impedance Research Articles

Smart Resistor: Stabilization of DC Microgrids Containing Constant Power Loads Using High-Bandwidth Power Converters and Energy Storage

Power electronic converters and electric motor drives have found increased application in dc microgrids, electric automobiles, and ships. Such converters when driving loads in a regulated fashion act as constant power loads (CPLs). These CPLs can cause destabilizing effects on the grid because of their negative incremental impedance. This paper proposes a method to dynamically stabilize CPLs at the point of load by making them behave as adaptive Smart Resistors by using high-bandwidth power converters and energy storage units. These Smart Resistors can work with smart sources to realize ultimate intelligent power networks. The control strategies needed to achieve the concept, including voltage and current trajectory control principles, are analyzed and validated with a hardware test setup.

Interconnection and Damping Assignment Passivity-Based Control Applied to On-Board DC–DC Power Converter System Supplying Constant Power Load

In the more electric aircraft context, dc distribution systems have a time-varying structure due to the flexible distributed loads and complex operation conditions. This feature poses challenges for system stability and increases the difficulty of the stability analysis. Besides, the risk of instability may be increased under constant power load condition due to the negative incremental impedance characteristic. To this end, this article proposes an improved interconnection and damping assignment passivity-based control scheme. Particularly, an adaptive interconnection matrix is developed to establish the internal links in port-controlled Hamiltonian models and to generate the unique control law. The damping assignment technique is addressed to tune the dynamic characteristic. In order to meet the load requirements of different voltage levels, the design procedures were given for determining the control law in both boost converter and buck converter cases. The simulation and experimental results are performed to demonstrate the validity of the proposed control approach.

Voltage-Controlled Quadratic Buck Converter for a DC Power Management System with Constant Current Load

Development of the medium-voltage DC power management systems is an important objective for future submarines and surface combatants. In such a system, DC/DC converters are usually used to supply constant current/power for electrical energy devices. However, constant current loads inherit the negative incremental impedance characteristics, resulting in deteriorated response of the system against load disturbances, and therefore, the system should be suitably controlled. To solve this problem, this research proposes a voltage-controlled quadratic buck converter based on a sliding-mode (SM) control approach which guarantees the system stability and delivers the required energy for constant output current to be supplied. The proposed voltage-mode control is capable of stabilizing the DC power system in different operation modes and over the significant variations in the input voltage and load. The asymptotic stability of the switching surface and existence of SM operation are proved in this paper. The proposed control solution is confirmed by means of simulations through PSIM program, and experimental evidences are provided to validate the expected features for the voltage stability of a quadratic buck converter with constant loads.

An Energy Buffer for Controllable Input Impedance of Constant Power Loads

Power electronic circuits often regulate load power and present a constant power profile to the utility or other electrical source. These constant power loads therefore exhibit a negative incremental input impedance and pose stability challenges when present in either dc or ac systems. This paper presents an energy buffer power converter for a constant power LED lighting load that presents a controllable input impedance to the electrical source. The use of an energy buffer allows the converter to independently control input and output power. The input power is controlled to resemble a resistive load, ensuring that the device exhibits a positive incremental input impedance over short-term input disturbances. Simultaneously, the output power is held constant through high-bandwidth regulation. The control scheme balances the power flow in the long term. Experimental results are presented that demonstrate independence between input and output performance, as well as long-term power balance.

DC bus stabilization using passive damping network in distributed power system with constant power load

In distributed power system (DPS), the stability performance is always associated with the behaviour of constant power load (CPL). Since DPS large complex system build up from many power electronic converters, the DC bus of the system becomes unstable due to the strenuous action from power converters. When these converters tightly regulated, it behaves as constant power load and exhibit negative incremental input impedance which becomes the main reason in stability degradation of DPS. In this paper, four passive damping network topologies was proposed to reduce the DC bus instability. The best damping performance of the topologies was chosen and analysed using MATLAB/Simulink. The DC bus performances was studied in four cases which are damping behavior due to CPL power level, CPL disconnection, effects of filter and damping capacitor, and effects of filter cut-off frequency. Simulation results obtained shows that the DC bus was successfully stabilized and the resonance damped when passive damper installed. An experimental hardware tests was conducted to verify the proposed damping method and the results were compared with the simulated output waveforms. The analysis results in overshoot, settling time and steady state error of bus voltage shows system improvement with the proposed damper network.

A Comprehensive Review on Constant Power Loads Compensation Techniques

Microgrid, because of its advantages over conventional utility grids, is a prudent approach to implement renewable resource-based electricity generation. Despite its advantages, microgrid has to operate with a significant proportion of constant power loads that exhibit negative incremental impedance and thus cause serious instability in the system. In this paper, a comprehensive review is presented on accomplished research work on stabilization of dc and ac microgrid. After reviewing these, microgrid system stabilization techniques are classified with required discussions. As found out in this paper, the stabilization techniques can basically be classified as compensation done: 1) at feeder side; 2) by adding intermediate circuitry; and 3) at load side. Finally, after analyzing the merits and drawbacks of each generalized technique, several infographics are presented to highlight the key findings of this paper.

Voltage Stability Analysis and Sliding-Mode Control Method for Rectifier in DC Systems With Constant Power Loads

Voltage stability draws more and more attentions in dc systems due to the high penetration of constant power loads. This paper gives comprehensive analysis for the voltage stability and proposes a sliding-mode control method to enhance the voltage stability margin of dc systems. First, by the impedance-based stability analysis approach, the voltage stability of the three-phase rectifier serving as the source converter is examined when interacted with tightly regulated load converter. The dynamic input characteristics of the load side converter are investigated to find the relationship between the negative incremental impedance and the output power of the load converter. Second, a new sliding-mode control method is proposed to achieve the large-signal stability. Lyapunov function is used to ensure the stability of the proposed controller. Finally, the effectiveness of the proposed control method is verified by simulation results.

Impedance Decoupling in DC Distributed Systems to Maintain Stability and Dynamic Performance

DC distributed systems are highly reliable and efficient means of delivering DC power or adopting renewable energy resources. However, DC distributed systems are prone to instability and dynamic performance degradation due to the negative incremental input impedance of DC-DC converts. In this paper, we propose a generic method to eliminate the impact of the negative input impedance on DC systems by shaping the source output impedance such that its bode-plot is restricted in the area that is contained below the product of the source’s duty ratio and its characteristic impedance. The performance deterioration originates whenever the output impedance of the source exceeds, in magnitude, the input impedance of the load converter due to deficiency in stability margins. Hence, confining the impedance in the proposed region helps decouple the interaction between the converters and preserve their own dynamic performances. The proposed method was proven by analytical analysis, time-based simulation, and practical experiments. All of their outcomes were in agreement, proving the effectiveness of the proposed method in preserving the dynamic performance of distributed systems.

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.

Active stabilization methods of electric power systems with constant power loads: a review

Modern electric power systems have increased the usage of switching power converters. These tightly regulated switching power converters behave as constant power loads (CPLs). They exhibit a negative incremental impedance in small signal analysis. This negative impedance degrades the stability margin of the interaction between CPLs and their feeders, which is known as the negative impedance instability problem. The feeder can be an LC input filter or an upstream switching converter. Active damping methods are preferred for the stabilization of the system. This is due to their higher power efficiency over passive damping methods. Based on different sources of damping effect, this paper summarizes and classifies existing active damping methods into three categories. The paper further analyzes and compares the advantages and disadvantages of each active damping method.

A Sliding-Mode Duty-Ratio Controller for DC/DC Buck Converters With Constant Power Loads

Incorporating a medium-voltage dc (MVDC) integrated power system is a goal for future surface combatants and submarines. In an MVDC shipboard power system, dc/dc converters are commonly employed to supply constant power to electric loads. These constant power loads have a characteristic of negative incremental impedance, which may cause system instability during disturbances if the system is not properly controlled. This paper proposes a sliding-mode duty-ratio controller (SMDC) for dc/dc buck converters with constant power loads. The proposed SMDC is able to stabilize the dc power systems over the entire operating range in the presence of significant variations in the load power and input voltage. The proposed SMDC is validated by both simulation studies in MATLAB/Simulink and experiments for stabilizing a dc/dc buck converter with constant power loads. Simulation studies for an MVDC shipboard power system with constant power loads for different operating conditions with significant variations in the load power and supply voltage are also provided to further demonstrate the effectiveness of the proposed SMDC.

DC Bus Instability Driving by Photovoltaic Source with Constant Power Load

This study presented the DC bus instability in distributed power system driving by Photovoltaic (PV) source with constant power load (CPL). The aim of this study is to investigate the effect of PV source towards the performances of DC bus voltage stability using constant power load. The system first was tested using linear DC supply as voltage source and CPL was modeled and connected to the voltage source through LC filter network. A passive damping circuit was chosen to stabilize the DC bus instability. The system was repeated using PV source. PV array was modeled based on the parameters obtained from a commercial PV data sheet. A circuitry simulation was performed under the similar design excluding the DC source in order to investigate DC bus instability. Results showed that the stability of DC bus become unstable when using linear DC voltage supply and slightly affected when using PV source. The instability phenomenon due to the negative incremental impedance of a constant power load can be overcome by using damping technique. For linear DC voltage source, DC bus voltage system tends to oscillate. However, with PV source which is actually a current source input seems not much affecting the DC bus voltage stability.

Active damping network in DC distributed power system driven by photovoltaic system

An instability phenomenon may occur in DC electrical systems due to undesirable source–load interaction and the behavior of power electronic loads that act as a constant power load. The present study focuses on DC electrical systems associated with constant power behavior. The phenomenon occurs such that each converter has its own internal control function for its output voltage regulation. The converter tends to draw a constant power which therefore produces negative incremental input impedance within its bandwidth. An active damping technique is proposed to improve the stability of a DC electrical system consisted of photovoltaic arrays, an LC filter and a constant power load. The active damping network was controlled to behave as a small-signal effective damping resistor over a DC network so that any instability in the DC bus will be reduced and eliminated. This technique eliminates the oscillations produced by the constant power load. MATLAB/SIMULINK modeling and simulation are used to confirm the validity of the proposed technique.

Constant-Power Load System Stabilization by Passive Damping

This paper addresses stability problems in power systems with loads that exhibit constant-power behavior. Instability may occur in such systems due to the negative incremental impedance of constant-power loads (CPLs). Existing approaches to stabilizing such systems require modification of the source and/or the load control characteristics, or isolating the CPL from the rest of the system by additional active devices, which are difficult to implement and often conflict with other system requirements such as control bandwidth, size, weight, and cost. In this paper, we investigate passive damping as a general method to stabilize power systems with CPL. Using a representative system model consisting of a voltage source, an LC filter, and an ideal CPL, we demonstrate that a CPL system can be stabilized by a simple passive damping circuit added to one of the filter elements. Three different damping methods are considered and analytical models are developed for each method to define damping parameters required for stabilizing the system. Time- and frequency-domain measurements from an experimental system are presented to validate the methods.

Stability Criteria for Constant Power Loads With Multistage <formula formulatype="inline"> <tex Notation="TeX">$LC$</tex></formula> Filters

In high-voltage dc power systems of advanced aircraft, most of the electrical motor drives and power electronic converters behave as constant power loads (CPLs) when tightly regulated and usually cause negative impedance instability problems. This paper presents stability criteria for an electrical system constituted of a dc power supply, multistage LC filters, and an ideal CPL. The derived criteria are based on the Brayton-Moser mixed potential theory and can be extended to actual CPLs. The dynamic negative incremental impedance of the CPL is used to place design constraints on the filter parameters to prevent instabilities. Simulation and experimental results confirm that the proposed criteria would guarantee system stability under different voltage disturbances.

Suitability of the pulse adjustment technique to control single DC/DC choppers feeding vehicular constant power loads in parallel with conventional loads

The increasing use of power electronic and motor drives in land vehicles, marine vessels and aircraft, for lots of applications such as propulsion, braking, steering, and actuation is creating particular problems when the power distribution system is in the form of multi-converter dc electrical systems. This is because of many power electronic loads and associated controllers which have a 'constant power' characteristic resulting in negative incremental input impedance. The effect of such loads in a dc system is to cause instability. The purpose of this paper is first to identify the most appropriate theoretical techniques for analysis of this nonlinear instability, and then to devise a novel technique for damping the instabilities and ensuring stable operation of system. We call this new control technique, Pulse Adjustment. In spite of its simplicity it offers a very fast and smooth response. Simulation and experimental results are presented to verify the proposed technique.

Modeling of Power Electronic Loads in AC Distribution Systems Using the Generalized State-Space Averaging Method

Most of the loads in ac distribution systems have positive incremental impedance characteristic. However, power electronic loads, when tightly regulated, sink constant power from the system. Therefore, they have negative incremental impedance characteristic. This can cause negative impedance instability. Power electronic loads usually have a controlled or uncontrolled rectifier at the front end. In this paper, these loads are modeled using the generalized state-space averaging method. An assessment of their effects in ac distribution systems is also presented. Experimental results are presented to verify the proposed analysis.