Steady state characteristics
For simplicity, assume a network consisting of only one source and one load, operating in steady state. The latter assumption implies that the network can be regarded as purely resistive and that only the steady state droop characteristic has to be considered, see Figure 3.7. Figure 3.7 The simplified Thevenin equivalent for steady state analysis. The current supplied to the load, with reference to Figure 3.7, is written
Smallsignal stability based on impedance specification
A system with an arbitrary number of DC-DC source converters paralleled using master slave control, is investigated in 61 . Small-signal stability and dynamic performance are investigated. Also, a current sharing compensator is developed. The compensator is designed in the frequency-domain and verified through time-domain simulations. A small-signal model for a parallel rectifier system is derived in 12 . Dynamic properties in the case of long inductive and resistive cables are investigated. It...
References
1 M. Baran and N.R. Mahajan, DC Distribution for Industrial Systems Opportunities and Challenges, IEEE Industrial and Commerical Power Systems Technical Conferenc, I amp CPS 2002 Conf. Rec., Savannah, GA, USA, May 5-8. 2002, pp. 38-41. 2 M. Belkhayat, R. Cooley and A. Witulski, Large Signal Stability Criteria for Distributed Systems with Constant Power Loads, IEEE Power Electronics Specialists Conference, PESC '95 Conf. Rec., Atlanta, GA, USA, Jun. 18-22, 1995, vol. 2, pp. 1333-1338. 3 A....
Acknowledgements
First, I would like to thank my colleague and friend Jorgen Svensson who I have worked with the last three years. I would also like to express my sincere gratitude to my friend and former colleague Martin Bojrup, who I worked with during 1996-1999. I am very grateful to my two mentors, Professor Lars Gertmar and Professor Sture Lindahl. Both Lars and Sture have time to discuss all kinds of topics, not only technical. I would also like to thank my supervisors, Professor Mats Alakula and...
kn
for k 1,5,7,11,13, etc. This is an important feature of three-phase diode and thyristor rectifiers with inductive DC links. Classical HVDC utilises an inductive DC bus, as shown in Figure 2.1. Since the overhead lines are also inductive, the commutations take a finite time. This time is termed commutation overlap. Note that the commutation overlap affects the spectrum of a thyristor based current source converter CSC , since the edges are slightly smoother due to the finite current derivatives....
Voltage control and load sharing
There are basically two different methods to achieve load sharing. One of the methods is referred to as the droop concept and the other as the master slave concept. For the droop concept, a finite loop gain for the DC voltage controller is adopted. A droop characteristic is obtained from the transfer function slope in the P-V plane at the desired DC bus voltage. Since this droop characteristic appears as a negative slope in the P- V plane, load sharing is obtained. The quality of load sharing...
Masterslave DC bus voltage control
In the master slave DC bus voltage control scheme investigated here, only the master is responsible for voltage control. The voltage controller for the master converter is shown in Figure 3.6. Information on actual power for all the other converters is fed forward to the master, where appropriate power references for the slaves, i.e. the other converters operated as sources, are calculated. Even if not used here, the voltage controller output could be included in the slave power references....
Info Mab
which implies that the 17 higher rated power possible at rectifier operation for the same amount of losses as for rated inverter operation is not sufficient. On the other hand, a DC network design without margins to avoid overmodulation, is not likely. Also, cable losses approximately equal to 0.2 p.u. seems high. The p.u. droop characteristic for a three-phase converter based on Semikron SKM300GB123D modules, including over-modulation and loss limits, is shown as a function of current in...
Q.s. Fuelless Generators Inc.
This is rewritten to form a second order algebraic equation V - V-crefVr droop Kable r 0 3.34 If this is expressed in one common p.u. system, e.g. the one of the source, it is found that Sr 2 _ i_ r . 8n rcabie 0 3.35 where 1- and pr are the p.u. receiving end voltage and power, respectively. The p.u. cable resistance is denoted rahU. The p.u. base is given in Appendix D. The stable solution to this equation is written
Simulation of DAB for HVDC transmission
Figure 6.10 shows the simplified schematic of the simulated HVDC DAB system, which in this schematic form is equal to the back-to-back test system. Figure 6.10 DAB transmission system investigated in simulations. The simulation result for the DAB transmission system of Figure 6.10 is shown in this section. All power references are positive from left to right, see Figure 6.10. The data for the DAB converters are given in Table 6.1. The transformer short circuit resistance Rsc and self-inductance...
Load flow and short circuit calculations
DC power system analysis in terms of load flow and short circuit calculations is investigated in 26 . Numerous source models i.e. battery charger, rectifier and generator models are given. Also connector models for load flow and short circuit calculations are investigated. Loads and branches are modeled. According to 26 , the present standards in the year 1996 are not complete and need further work. In 3 , the methods of the IEC draft standard Calculation of short circuit-currents in dc...
VSC based HVDC
HVDC Light trademark of ABB 19 and HVDC Plus trademark of Siemens are basically extensions of the IGBT back-to-back converter, operating at higher power and voltage levels and with an intermediate cable between the converters. Even though this might seem as small step from a development point of view it has resulted in numerous new patents. The main reason is that the development of VSC based HVDC has forced the limits of power electronic converter technology. For example, IGBT valves...
DC Distributed Power Systems
Analysis, Design and Control for a Renewable Energy System Doctoral Dissertation in Industrial Electrical Engineering Department of Industrial Electrical Engineering and Automation Department of Industrial Electrical Engineering and Automation Lund University Box 118 2002 Per Karlsson Printed in Sweden by Media-Tryck, Lund University Lund 2002
Robustness against parameter uncertainty
In 47 , DC power system stability is investigated with focus on robustness against parameter uncertainty. Robust control theories are applied to DC power systems. According to 47 , this is done to cope with the sensitivity to parameter and load variation resulting in DC power systems prone to instability. The stability robustness is investigated by computing the multivariable stability margin, km, and the structured singular value, 1. Calculating these quantities exactly is cumbersome....
Publications
The results presented in this thesis regard DC bus voltage control and fault detection and clearance, found in Chapters 3, 4, 5 and 7. The main results of these chapters are also presented in P. Karlsson and J. Svensson, DC Bus Voltage Control for Renewable Energy Distributed Power Systems, IASTED Power and Energy Systems Conference, PES 2002 Conf. Proc, Marina del Rey, CA, USA, May 13-15, 2002, pp. 333-338. P. Karlsson and J. Svensson, Fault Detection and Clearance in DC Distributed Power...
K Cdc
Note that the gain is the same as for the P-controller used for regular droop control. Thus, the same controller can be used in both cases and the only difference is whether the integral part should be added or not. For PI-control, the source converter output voltage is written s3 ps 2 K Cdc 0 lps K ip CdcTi
Dynamic properties
An impedance specification for individual loads of DC distribution systems, based on the small-signal stability criterion, is presented in 21 . Here, stability is studied in terms of location of the closed loop poles and the root-locus 51 as a function of the cable parameters. Therefore, the cables are from now on modeled as resistive and inductive, with the cable capacitance neglected or lumped with the converter DC side capacitance, i.e. with Zcable sLcable Rcable Rcable ' sT cable 3.55 The...
DC bus voltage droop control
In common voltage droop control, the bus voltage decreases linearly as the output current, or in some cases power, for the converter increases, in order to give stable operation Figure 3.4 . This, of course, results in a stationary error in the voltage level. Figure 3.4 Steady state droop characteristic. The controller structure for DC bus voltage control is shown in Figure 3.5. A proportional P controller is used for the droop scheme, similar to the PI-controller derived in 6 for back-to-back...
DC bus voltage droop control with error restoration
Figure 3.17 and Figure 3.18 show the simulation results for load sharing in the case of DC bus voltage droop control with a Pi-controller, i.e. including voltage error restoration. The simulation result of Figure 3.17 is for a controller equipped with a constant integration time constant and the one of Figure 3.18 with a load dependent integration time constant. The integration is inhibited when the DC bus voltage at each source is inside a dead-band of 5 V located on both sides of the DC bus...
DC bus voltage control
Two different DC bus voltage control schemes are investigated. One of the methods, referred to as the communication or master slave method, strongly relies on fast communication between the source and load converters. The output power, with sign, is calculated for each of the converters. Information on total output power is fed forward to the converters. One of the converters, the master, is responsible for controlling the DC bus voltage. Feed forward of the total output power allows for high...