Stability Analysis of DC Distribution System with Constant Multiple Power Loads

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Abstract
In recent times dc distribution system is become a very complex which consist different types of multiple power converters. But system is suffered from stability related problem which arise due to negative incremental impedance of constant power loads. There are several methods for stability analysis of dc distribution system such as Middelbrook criterion, phase and gain margin criterion, energy source consortium criterion and the Passivity- Based stability criterion (PBSC). Furthermore, one another technique which name is Positive Feed-Forward control which is used with PBSC to improve the stability and to solve the system interaction problem. The main aim of the project is to run whole system into simulation mode in MATLAB and try to make the system stable.

Introduction

Now day dc distribution systems are mostly based on the power electronics Which used power converter and semiconductor devices. As a result stability and dynamic performance developed due to converter interconnection system. As we mentioned above there are most of criterion can be used only for single bus system. However, the power electronics based system consist multiple converter and multibus system so for this complex scenario the most reliable and accurate technique is the Passive Based Stability Critrion (PBSC). In this technique, stability of the any system may be derived by evaluating the system bus impedance. (Siegers, Arrua and Santi, 2017)
Furthermore, in PBSC technique the system may be stable if the bus impedance of the system is analysed as a passive therefore this system also need to couple with positive feed-forward technique which is used to design stabilizing controllers that force the system bus passivity by damping impedances. The main concept of dynamic performance is based on impedance region of the system so suitable damping impedance can be developed or calculated in the system using the PFF control.

Fig1.0 multi-bus system with power converters

Motivation

In recent time, to work with dc distribution system becomes very easier because of developed semiconductor technology and power electronics converters. In power system network generally stability of any system is very import if it is dc or ac distribution. Power quality is generally related to voltage quality of the system. At this stage, the main tendency is to change ac distribution system with high level dc distribution system. There are several technique are available for stability analysis of dc distribution system but the impedance based stability criterion such as PBSC is widely used. When any system is connected with the constant power load then it may be suffer from instability because it causes to increase the current. As a result it will definitely decrease the voltage. (Hodge & Flower, 2009).
In addition, PBSC is recently developed stability analysis technique which is exhibit better stability margins and establishes certain performance. Furthermore, this technique is power electronics based so it may consist of multiple power converters. There is one basic architecture model is given in fig 1.0. This technique is now applied to several networks such as automotive power system, telecommunication system, electric- ship and electric- aircraft, as well as electric and hybrid- electric vehicles. (Siegers, 2017).

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Moreover, firstly PBSC technique could be apply only to single bus system which consist of source and load converter. But after dynamic closed loop response of the converters is establish using standard resistively intermediate converter which is used to limit the analysis of single bus system. In general, the multi-bus power converter system has n numbers of buses and also has large number of switching converters, sources and loads. Multi-bus system is evaluated to an equivalent network (n-Port) to each bus. Mostly, passive based stability criterion is developed in frequency domain.

Objectives

The main objective of this project is how DC distribution system should be become stable during constant power load using PBSC technique. PBSC is one of the different techniques which can be used for both single and multiple bus system so it will be helpful to understand the switching system of converter.

Need to analysis of criteria for the stability of dc distribution system.
To create the circuit of dc distribution using switching converter for different block system such as open loop, feed forward input control.
Create the matrix diagram and its calculation of transfer function.
To run whole DC distribution model in simulation mode in MATLAB.
To compare and analysis of the actual calculation and simulation result for stability.

Significance

The main significance of this method is that system may be stable if the network is passive. PSBC is mostly used for multi-bus system so it has n number of load converter and m number of source converter. Therefore, mainly two criteria for system stable which related to total equivalent impedance.

Z(bus) has no poles on right half plane(RHP)
Re{Z(bus(jw) >=0} or Z(jw) has a contour of Nyquist which is lies totally in the RHP.

The main goal of the technique is to make system stable. So, positive feed- forward control (PFF) is using damping impedance in parallel with the existing impedance. The main reason of using damping impedance is to stabilize the DC bus voltage by changing the bus impedance in the frequency domain. Furthermore, there are mainly three types of parallel damping cases such as Capacitor parallel damping, R-C parallel damping and L-R-C parallel damping.

Proposed Approach

The project work can be divided into a number of tasks that lead to complete work sequentially and successfully for achieving the main objective. There are mainly four task that need to be done such as research or understand the DC system, Principle of PBSC technique and how it is different from the other stability technique, matrix analysis and mathematical approach and finally to establish dc distribution model in simulation mode in MATLAB.
In the first task, it is necessary to understand the basic principle of dc system such as how dc system works and why dc system becomes unstable in certain condition. Furthermore, PBSC is the main part of this project so it is necessary to understand other stable system first then how PBSC can be different technique than other for example; these only one system which can be used for multi-bus system. In addition, the main role is that it works with switching converter because dc system can work only in resistive part but due to switching approach in time domain analysis it can work with inductive and capacitive part.
The third part is to establish mathematical model or calculation of any system by using matrix formation. It will give the actual value of impedance for stability of system because whole method is depends on impedance of the system. Finally, the last part of this project is very important. It is necessary to understand the MATLAB software and then apply simulation mode for DC grid system.

Timeline

One grant chart is given in the appendix which shows the whole timeline of the project. In the first semester, project is divided into 13 weeks excluding holidays and exams. The description and time is also given in the grant chart. Furthermore, for semester 2 dates and description is not fixed but given approximate nearly. During the project, it may come some error and difficulties then some changes will occur. Each task is given sequentially and it may help to complete the whole task in given period.

Risk Assessment

There is another attachment is given in appendix which shows the risk of the project. In general, the risk of the project is very low because mostly work in simulation mode not in the real world. There are several factors which can be affecting on the project such as supervisor, health, personal, software, equipment and computer. All factors are defined by code which is given below.

SUP- Supervisor
Per- Personal
HLTH- health
EQU- equipment
CMP- computer
SFT- software

As mentioned above the overall risk of this project is low. Some risk factors are near to zero such as supervisor, personal. The health risk also low but sometimes it is dangerous for eyes due to sitting in front of computer but it can be overcome wearing the glasses. Instrument risk sometimes high because of awareness of using but it can solve by taking care properly. Computer data risk is very low and it can be overcome by back up data in USB. Software risk can be moderate.

Progress to Date

The current level of the project work is at initial stage. Firstly I try to understand the how DC system is different than AC system. Furthermore, Try to find research paper related to the project work. Research is started on PBSC (Passive based stability criterion) technique and its main principal of this technique. Try to understand that why PBSC is used for stability analysis rather than another method. I am trying to understand matrix equation of stability criterion. In addition, in the last session I understand how DC system works with capacitor and inductor. Also I get broad knowledge about using capacitor in parallel with any circuit. In further session we will learn whole system and after we will learn the MATLAB software for future simulation.

Conclusion

After completing all task of this report, the stability of dc distribution system is quit complex but it is very useful for high voltage distribution system. It is very reliable and easier than AC system. The PBSC technique is also better than other technique because multi- bus system stability developed. Also PBSC is also analysis the passivity for individual bus system within MVDC system present. This technique is also validated or applies for both simulation and experimental model of four converter system. Also PBSC can reduce design and sensitivity to component. There are some benefits of this system such as reduce power dissipation, large currents, weight and cost.

References

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