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On the Efficiency and Accuracy of Simulation Methods for Optimal Power System Operation: Convex Optimization Models for Power System Analysis, Optimal Utilization of VSC-type DC Wind Farm Grids and FACTS Devices
KTH, School of Electrical Engineering (EES), Electric Power Systems. (EMREG)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Recently, significant changes in electric power systems such as rapid developmentof smart grid and electricity market and integration of non-dispatchablesources have added more complexity to the Power Flow Scheduling (PFS) andPower Balancing (PB) models. For instance, non-dispatchable sources introducean increasing level of uncertainty in the electricity market and power system operation.One of the solutions for handling these uncertainties in the power systemoperation is the improvement of system flexibility through a more efficient operationof power systems. On the other hand, efficient operation can be achieved bywell capturing variable behavior of uncertain sources such as wind power sourceswhich in turn demands efficient and robust PFS/PB models. This way, a moreflexible system, capable of efficiently accommodating higher levels of wind powerchanges, can be achieved. All these factors increase a need for PFS/PB models suchas Power Flow (PF) and Optimal Power Flow (OPF) models which can addressthese new challenges in an efficient, reliable, and economic way while supportingthe power system operation and control. In this regard, various solution methodshave been developed for solving different forms of PF/OPF formulation. The difficultyof solving OPF problems increases significantly with increasing network sizeand complexity. One of these complexities is how to model advanced controllable devices such as HVDC grids and Flexible AC Transmission Systems (FACTS) devices.Accurate handling of these complexities has limited the use of OPF in manyreal-world applications mainly because of its associated computational challenges.The main reasons behind computational challenges are nonlinearity and especiallynon-convexity of constraints representing power system and its components. Inthis regard, OPF problems are classified into two main groups. In the first group,researchers adopt Nonlinear programming (NLP) approach to fully represent thenonlinearity of the power system for the sake of accuracy but with the cost of complexityin the model. Computational and theoretical challenges associated withNLP approaches are then used as a motivation towards developing a more simplifiedOPF model, leading to the second group of OPF models known as LinearProgramming (LP) based OPF models. LP approaches are fast, reliable, and especiallyconvex, and therefore guarantee a global optimum to the simplified OPFproblem. The problem of LP approach to OPF is that the LP solution of OPF may not even be a feasible solution of original nonlinear OPF at all. Another issueassociated with LP models is that complex power system devices such as HVDClinks are difficult to be incorporated. These limitations have restricted the applicationof LP approaches for many OPF problems. According to the mentionedadvantages and disadvantages of NLP and LP based OPF models, what we seeks isan OPF model which can have main advantages of both LP OPF models (Efficientnumerical solvers) and full AC OPF models (Results accuracy). In this thesis, wedevelop convex optimization problems which can be adopted as both PF and OPFmodels which are capable of catching the nonlinear nature of power systems asmuch as possible while can be solved by efficient solution methods such as InteriorPoint Methods (IPMs). These OPF models can incorporate HVDC links, windfarm Multi Terminal HVDC (MTDC) grids, and shunt FACTS devices.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xiv, 97 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:022
National Category
Engineering and Technology
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-166383ISBN: 978-91-7595-573-5 (print)OAI: oai:DiVA.org:kth-166383DiVA: diva2:810658
Public defence
2015-06-09, H1, Teknikringen 33, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150521

Available from: 2015-05-21 Created: 2015-05-07 Last updated: 2015-05-21Bibliographically approved
List of papers
1. AC Power Flow Representation in Conic Format
Open this publication in new window or tab >>AC Power Flow Representation in Conic Format
2015 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 30, no 1, 546-547 p.Article in journal (Refereed) Published
Abstract [en]

This letter presents a second-order cone formulation for AC power flow problem. The power flow equations are first derived as functions of more practical variables of power systems and then placed in a second-order cone programming (SOCP) problem. The proposed conic power flow (CPF) model can be solved efficiently through IPMs, and at the same time, it has a very good accuracy as compared to the full AC power flow model. Also, the proposed CPF can efficiently handle the ill-conditioned networks. The numerical efficiency and good accuracy of the model are shown by simulating various case studies.

Keyword
AC power flow, second-order cone programming
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-159359 (URN)10.1109/TPWRS.2014.2326980 (DOI)000346734000056 ()2-s2.0-84919915854 (Scopus ID)
Note

QC 20150130

Available from: 2015-01-30 Created: 2015-01-29 Last updated: 2017-12-05Bibliographically approved
2. A Multi-Option Unified Power Flow Approach for Hybrid AC/DC Grids Incorporating Multi-Terminal VSC-HVDC
Open this publication in new window or tab >>A Multi-Option Unified Power Flow Approach for Hybrid AC/DC Grids Incorporating Multi-Terminal VSC-HVDC
2013 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 28, no 3, 2376-2383 p.Article in journal (Refereed) Published
Abstract [en]

This paper proposes a multi-option power flow approach for hybrid AC/DC grids. A unified AC-DC unit is introduced which can be used in two different cases: case a) AC grids with embedded VSC-based MTDC grids, and case b) asynchronous AC grids connected via a common VSC-based MTDC grid. In the proposed method for each MTDC grid (regardless of the number of converters), a new state variable is introduced to handle any kind of converter loss models. For the case (b), the AC-DC unit can be used in two different analyses, namely, a1) the separated analysis and a2) the integrated analysis. Both a1) and a2) can be used in the practical analysis of the real-size power systems. However, it is shown that the separated analysis not only offers a shorter computational time but it is also very suitable for the future connection between large existing AC systems and other AC systems or remote renewable energy sources through the DC grids. The proposed methodology is implemented in MATLAB software and verified using commercial software SIMPOW.

Place, publisher, year, edition, pages
IEEE Power system transaction on: , 2013
Keyword
DC grid, multi-terminal HVDC, power flow calculation, unified AC-DC unit, voltage source converter
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-116687 (URN)10.1109/TPWRS.2012.2236366 (DOI)000322989900033 ()2-s2.0-84880923653 (Scopus ID)
Note

QC 20130828

Available from: 2013-01-23 Created: 2013-01-23 Last updated: 2017-12-06Bibliographically approved
3. Second-order cone programming for optimal power flow in VSC-type AC-DC grids
Open this publication in new window or tab >>Second-order cone programming for optimal power flow in VSC-type AC-DC grids
2013 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 28, no 4, 4282-4291 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a second order cone programming (SOCP) formulation of the optimal power flow problem for AC-DC systems with voltage source converter (VSC) technology. Approximation techniques have been used to derive the SOCP formulation of the AC-DC OPF problem. Later, the SOCP formulation can be solved using the interior point method (IPM) by considering the limits on AC-DC grid. The accuracy of SOCP formulation of AC OPF has been proven with numerical examples using IEEE 14-bus, IEEE 30-bus, and IEEE 57-bus example systems. The results of the SOCP formulation are compared with available commercial software. Then a DC system with VSC technology is modeled in the IEEE 30-bus example system. The SOCP formulation of AC-DC OPF is applied to the modified IEEE 30-bus example system and the results are discussed. The limitations of derived SOCP formulation are also discussed.

Keyword
AC-DC optimal power flow, multi-terminal HVDC systems, second-order cone programming
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-128579 (URN)10.1109/TPWRS.2013.2271871 (DOI)000326184100079 ()2-s2.0-84886089694 (Scopus ID)
Note

QC 20131203

Available from: 2013-09-13 Created: 2013-09-13 Last updated: 2017-12-06Bibliographically approved
4. The Modeling Multi-Terminal VSC-HVDC in Power Flow Calculation Using Unified Methodology
Open this publication in new window or tab >>The Modeling Multi-Terminal VSC-HVDC in Power Flow Calculation Using Unified Methodology
2011 (English)In: Innovative Smart Grid Technologies (ISGT) Conference, 2011Conference paper, Published paper (Refereed)
Abstract [en]

In this paper, a new unified method for power flowcalculation in AC grids with embedded multi-terminal HVDCsystems based on voltage source converter is proposed. In thismethod all DC and AC equations are solved simultaneously inthe same iteration while there is no need to rely on resultsobtained from other iterative loops unlike the other methods.The method can be applied for any number of converters,any DC network configuration and any converter loss model.The algorithm is implemented in MATLAB and to validate theresults, they are compared to results obtained from the simulationsoftware SIMPOW.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-51262 (URN)10.1109/ISGTEurope.2011.6162666 (DOI)2-s2.0-84860786140 (Scopus ID)978-145771421-4 (ISBN)
Conference
Innovative Smart Grid Technologies (ISGT) Conference Europe 2011 5th – 7th December 2011, Manchester Central, UK
Projects
The impact of Multi-Terminal HVDC (MTDC) systems on bulk power system stability
Note

QC 20120117

Available from: 2011-12-12 Created: 2011-12-12 Last updated: 2015-05-21Bibliographically approved
5. Power flow calculation of hybrid AC/DC power systems
Open this publication in new window or tab >>Power flow calculation of hybrid AC/DC power systems
2012 (English)In: Power and Energy Society General Meeting, 2012 IEEE, IEEE , 2012, 6343958- p.Conference paper, Published paper (Refereed)
Abstract [en]

Multi-terminal HVDC systems have recently become an attractive option for interconnection of isolated AC systems such as offshore wind farms and oil platforms to asynchronous large AC systems. This paper deals with power flow calculation (PFC) of hybrid AC/DC power systems where several asynchronous AC systems are interconnected via a common multiterminal VSC-HVDC system. This paper proposes a unified AC-DC approach for PFC of a hybrid AC/DC power system. The proposed approach is then employed for two different analyses, namely a) the separated analysis where the entire hybrid AC/DC system is divided into two groups. The first group (named external AC system) comprises all asynchronous AC systems which are not directly connected to the slack convertor of the DC network, and the second group comprises an AC/DC system where the selected AC system is directly connected to the slack convertor. In this method, a PFC is firstly performed for the the first group, and its relevant obtained results will be used for PFC of the second group. b) the integrated analysis where the entire hybrid system is considered as a unit. Both a) and b) can be used in the practical analysis of the real-size power systems. However, due to practical issues and computational costs the separated analysis may be a more acceptable method. The simulations have been performed using MATLAB, and the obtained results have been compared with those obtained in SIMPOW.

Place, publisher, year, edition, pages
IEEE, 2012
Series
IEEE Power & Energy Society General Meeting : [proceedings], ISSN 1944-9925
Keyword
Isolated AC systems, Multi-terminal VSC-HVDC systems, Power flow calculation, Unified AC-DC unit
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-51263 (URN)10.1109/PESGM.2012.6343958 (DOI)000312493700039 ()2-s2.0-84870576751 (Scopus ID)978-146732727-5 (ISBN)
Conference
2012 IEEE Power and Energy Society General Meeting, PES 2012; San Diego, CA; 22 July 2012 through 26 July 2012
Projects
The impact of Multi-Terminal HVDC (MTDC) systems on bulk power system stability
Note

QC 20130115

Available from: 2011-12-12 Created: 2011-12-12 Last updated: 2015-05-21Bibliographically approved
6. Modelling of multi-terminal HVDC systems in optimal power flow formulation
Open this publication in new window or tab >>Modelling of multi-terminal HVDC systems in optimal power flow formulation
2012 (English)In: 2012 IEEE Electrical Power and Energy Conference, EPEC 2012, IEEE , 2012, 170-175 p.Conference paper, Published paper (Refereed)
Abstract [en]

The multi-terminal HVDC systems and their embedded DC networks are considered as smart grids technology which improve economic efficiency of the power system. This technology allows better voltage profile in the power system by better allocation of the generation sources. Also, it can help in improving the economic efficiency of the system by substituting the high-cost generation with low-cost generation. In order to assess the technical benefit of this smart grids technology, this paper presents an optimal power flow formulation for AC grids with embedded DC networks built from multi-terminal HVDC systems. The objective function of this AC-DC OPF formulation is the total active dispatch costs. The constraints consist of (a) AC grid constraints, (b) constraints from multi-terminal HVDC systems, and (c) DC grid constraints. The formulated AC-DC OPF is a mixed-integer nonlinear optimisation problem. The formulation is coded in GAMS platform and tested on IEEE 30 Bus system.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
Economic Efficiency, Multi-terminal HVDC systems, Optimal Power Flow
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-121221 (URN)10.1109/EPEC.2012.6474944 (DOI)000317098400029 ()2-s2.0-84875621012 (Scopus ID)978-146732080-1 (ISBN)
Conference
2012 IEEE Electrical Power and Energy Conference, EPEC 2012; London, ON; Canada; 10 October 2012 through 12 October 2012
Note

QC 20130424

Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2015-05-21Bibliographically approved
7. Ohmic loss minimization in AC transmission systems with embedded DC grids
Open this publication in new window or tab >>Ohmic loss minimization in AC transmission systems with embedded DC grids
2013 (English)In: 39th Annual Conference of the IEEE Industrial Electronics Society, IECON, IEEE conference proceedings, 2013, 2117-2120 p.Conference paper, Published paper (Refereed)
Abstract [en]

The HVDC systems built based on the voltage source converters (VSC) can bring several benefits to the AC power systems. Better voltage profile, increasing power flow controllability, lower ohmic loss, and higher transfer capability are some major benefits of such systems. This paper investigate the impact of VSC-type DC grids installed in the AC power systems on the ohmic network losses. This is done by formulating a convex optimization problem which minimises the ohmic losses (both in AC and DC grids) subject to the technical constraints of both AC and DC system. The formulated optimisation problem is a conic optimisation problem which can be solved using the commercially available optimisation softwares. The conic AC-DC optimal power flow, CAD-OPF, is coded in GAMS platform and solved using the MOSEK solver. The IEEE 30-bus example system is modeled and studied.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2013
Series
IEEE Industrial Electronics Society. Annual Conference. Proceedings, ISSN 1553-572X
Keyword
High Voltage DC Grids (HVDC), Ohmic loss, Voltage Source Converter (VSC)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-143163 (URN)10.1109/IECON.2013.6699458 (DOI)000331149502013 ()2-s2.0-84893555488 (Scopus ID)978-147990224-8 (ISBN)
Conference
39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013; Vienna; Austria; 10 November 2013 through 14 November 2013
Note

QC 20140318

Available from: 2014-03-18 Created: 2014-03-17 Last updated: 2015-05-21Bibliographically approved
8. Calculating Negative LMPs from SOCP-OPF
Open this publication in new window or tab >>Calculating Negative LMPs from SOCP-OPF
2014 (English)In: ENERGYCON 2014 - IEEE International Energy Conference, IEEE Computer Society, 2014, 1461-1466 p.Conference paper, Published paper (Refereed)
Abstract [en]

Recent research shows that non-convex OPF problem can be recast as a convex Semidefinite Programming (SDP) problem or Second Order Cone Programming (SOCP) problem. However, in the most SOCP OPF problems, there are some cases that conic relaxation results in a miscalculation of negative Local Marginal Prices (LMPs). This paper reviews the SOCP formulation of the optimal power flow problem proposed in [1] and then proposes one way of generating negative Locational Marginal Prices, LMPs, using this SOCP formulation. The proposed model is coded in GAMS and its built MOSEK solver and tested on a modified version of IEEE-30 test system.

Place, publisher, year, edition, pages
IEEE Computer Society, 2014
Series
IEEE International Energy Conference, ISSN 2164-4322
Keyword
Second-Order Cone Programming, OPF, Negative LMP
National Category
Energy Systems Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-157239 (URN)10.1109/ENERGYCON.2014.6850615 (DOI)000343646400219 ()2-s2.0-84904962161 (Scopus ID)978-1-4799-2449-3 (ISBN)
Conference
2014 IEEE International Energy Conference, ENERGYCON 2014; Dubrovnik; Croatia; 13 May 2014 through 16 May 2014
Note

QC 20141208

Available from: 2014-12-08 Created: 2014-12-08 Last updated: 2015-05-21Bibliographically approved
9. A Stochastic SOCP Optimal Power Flow With Wind Power Uncertainty
Open this publication in new window or tab >>A Stochastic SOCP Optimal Power Flow With Wind Power Uncertainty
2014 (English)In: 2014 IEEE PES General Meeting | Conference & Exposition, IEEE Computer Society, 2014, 6939790- p.Conference paper, Published paper (Refereed)
Abstract [en]

Recent research shows that non-convex AC OPF problem can be recast as a convex Semidefinite (SD) problem or Second Order Cone Programming (SOCP) problem. This paper presents a stochastic SOCP OPF (SSOCP-OPF) model for power systems connected to the wind farms. This is performed by reformulating the original non-convex OPF problem using more practical parameters of the power system. Finally, we obtain a convex optimization problem through some wellknown approximations and an exact relaxation incorporating the stochastic nature of wind power. One of the advantage of convex SOCP problems, which are a general form of linear problems, is that they can be efficiently solved through Interior PointMethods (IPMs). The proposed OPF model takes advantage of both DC-OPF models (solution efficiency) and full AC-OPF models (solution accuracy). As an application of SSOCP-OPF model, we study the impact of wind power uncertainty on the transmission loss in the power systems. To evaluate the proposed stochastic model modified IEEE 30-bus test system is used. The optimization problem is coded in GAMS platform and solved using its embedded interior point optimizer MOSEK.

Place, publisher, year, edition, pages
IEEE Computer Society, 2014
Series
IEEE Power and Energy Society General Meeting, ISSN 1944-9925
Keyword
Convex Optimization, Stochastic SOCP Programming, Wind Power Uncertainty
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-164498 (URN)10.1109/PESGM.2014.6939790 (DOI)000349551504117 ()2-s2.0-84931003410 (Scopus ID)978-1-4799-6415-4 (ISBN)
Conference
2014 IEEE Power and Energy Society General Meeting, National Harbor, United States, 27 July 2014 through 31 July 2014
Note

QC 20150420

Available from: 2015-04-20 Created: 2015-04-17 Last updated: 2015-07-02Bibliographically approved
10. Multiobjective optimal power flow algorithm to enhance multi-microgrids performance incorporating IPFC
Open this publication in new window or tab >>Multiobjective optimal power flow algorithm to enhance multi-microgrids performance incorporating IPFC
2012 (English)In: Power and Energy Society General Meeting, 2012 IEEE, IEEE , 2012, 6345605- p.Conference paper, Published paper (Refereed)
Abstract [en]

The idea of connecting some adjacent microgrids and making a multi-microgrid (MMG) have recently attracted attention among power system researchers. It helps better operate, control and manage the power system. This paper presents a multiobjective optimal power flow (MOPF) algorithm for improving the performance of MMGs incorporating interline power flow controller (IPFC). The proposed MOPF simultaneously minimizes MMG operating cost and total energy loss, as well as voltage profile deviation of all buses in the system. The proposed multiobjective nonlinear constraint optimization problem is formulated considering the control variables of IPFC. Also, the proposed algorithm guarantees that the final system operating point has a suitable security margin from the voltage instability point. A typical MMG system is used to demonstrate the effectiveness and proficiency of the algorithm.

Place, publisher, year, edition, pages
IEEE, 2012
Series
IEEE Power and Energy Society General Meeting, ISSN 1944-9925
Keyword
interline power flow controller (IPFC), Multi-microgrid, multiobjective nonlinear optimization, operating performance
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-111797 (URN)10.1109/PESGM.2012.6345605 (DOI)000312493706117 ()2-s2.0-84870583736 (Scopus ID)978-146732727-5 (ISBN)
Conference
2012 IEEE Power and Energy Society General Meeting, PES 2012, 22 July 2012 through 26 July 2012, San Diego, CA
Note

QC 20130115

Available from: 2013-01-15 Created: 2013-01-14 Last updated: 2015-05-21Bibliographically approved
11. On-line control of multi-terminal HVDC systems connected to offshore wind farms using the POF-based multi-agent approarch
Open this publication in new window or tab >>On-line control of multi-terminal HVDC systems connected to offshore wind farms using the POF-based multi-agent approarch
2015 (English)Conference paper, Published paper (Other academic)
Abstract [en]

Multi-terminal HVDC systems are an attractive option to connect offshore wind farms to onshore grids. Although scheduling the multi-terminal HVDC system is based on forecasted wind power, the forecasted values may differ from their real time ones. This paper presents a new controller based on multi-agent system which optimally tries to follow the variations of real time wind power outputs. Since a fast optimal power flow algorithm is needed, a convexified AC-OPF model which can be efficiently solved through interior point methods (IPMs) is embedded into the proposed online controller. Simulations are carried out and validated using GAMS platform and MATLAB/Simulink.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2015
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-167129 (URN)10.1109/PTC.2015.7232536 (DOI)000380546800303 ()2-s2.0-84951320180 (Scopus ID)978-147997693-5 (ISBN)
Conference
IEEE Eindhoven PowerTech, PowerTech 2015; Eindhoven; Netherlands; 29 June 2015-2 July 2015
Note

Updated from manuscript to conference paper.

QC 20160202

Available from: 2015-05-21 Created: 2015-05-21 Last updated: 2016-10-06Bibliographically approved

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