K. Uhlen - Academia.edu (original) (raw)
Papers by K. Uhlen
IEEE Transactions on Power Systems, 2012
ABSTRACT This paper discusses the impact of dc transmission voltage drops on the distribution of ... more ABSTRACT This paper discusses the impact of dc transmission voltage drops on the distribution of dc grid balancing power when dc voltage droop control is applied. DC line voltage drops in a multiterminal VSC-HVDC (MTDC) system result in nonuniform variations of dc bus voltages when changes in dc grid power flow occur. This in turn affects the distribution of instantaneous balancing power in a MTDC that uses dc voltage droop control. The values of dc voltage droop constants determine the degree of impact that dc voltage drops will have on the sharing of balancing power in the dc grid. In this paper, an analytical expression for estimating the distribution of balancing power which accounts for dc line voltage drops is derived. A five-terminal MTDC was modelled in PSCAD for demonstrating the effects of dc line voltage drops as well as for validating the proposed analytical expression which estimates balancing power distribution.
2017 IEEE Manchester PowerTech, Jun 1, 2017
This paper presents a method for voltage stability monitoring based on the maximum power transfer... more This paper presents a method for voltage stability monitoring based on the maximum power transfer to the load. The only required information is the system topology, the data from PMUs and the operational status of synchronous generators. With this information, the Thevenin impedance seen from a load bus can be estimated and by using the Thevenin theorem for maximum power transfer an impedance based stability index can be established. Since the Thevenin impedance is influenced by the operational conditions of the synchronous generators, it is consequently not a constant value and differs from the short circuit impedance of the bus. It is emphasized that the method requires only the information of the considered subsystem. Therefore, since the computation requirement is insignificant, the algorithm can be used for online monitoring. The validation of the approach is achieved by simulating a simple transmission system.
In Norway distribution networks are evolving rapidly into active systems due to the increasing nu... more In Norway distribution networks are evolving rapidly into active systems due to the increasing number of connections of small scale hydro generation units. As most of the distributed generation is based on synchronous technology their dynamic response on the bulk transmission power system cannot longer be neglected. In this paper a method for power systems reduction and aggregation of generators is applied to obtain dynamic equivalents of distribution power systems with DG units. Criteria to validate this method for distribution power systems are proposed.
SUMMARY This paper reports the initial phase of an Information Technology (IT) initiative to impr... more SUMMARY This paper reports the initial phase of an Information Technology (IT) initiative to improve the utilisation of the Norwegian power grid. We adopt a scalable approach, beginning with a focus on the use of local intelligence ("agent"), and gradually integrating these agents into the existing IT infrastructure. In particular, the paper reports on the implementation and testing of two new applications. These are the Voltage Instability Predictor for on-line tracking of distance to voltage collapse, which is primarily a local device, and a Secondary Voltage Regulation scheme for coordinated control of SVCs and synchronous condensers, which is presently a control centre application.
CIRED 2012 Workshop: Integration of Renewables into the Distribution Grid, 2012
Based on computer simulations of a simplified radial 132, 66 and 22 kV system including a 5 MVA h... more Based on computer simulations of a simplified radial 132, 66 and 22 kV system including a 5 MVA hydro power unit, it is concluded that the assessment of the unit's fault-ridethrough (FRT) capability should be based not only on a voltage magnitude profile, but also on the change in the voltage phase angle. It is assumed that erroneous conclusions might be the result when FRT-capability studies are performed for distributed generation without taking into account the influence of the phase-angle change in this context. Further work will be conducted in order to quantify the effect of phase angle influence on the FRT capability of distributed generation units.
International Journal of Electrical Power & Energy Systems
Power market analysis should be incorporated in reliability assessments of deregulated power syst... more Power market analysis should be incorporated in reliability assessments of deregulated power systems. For the Nordic power system, this is done by using The Multi-area Power-market Simulator (EMPS) for long-term power market analysis, where EMPS finds the optimal socio-economic dispatch on a weekly basis, with respect to, e.g., hydro reservoir levels. The EMPS analysis results in a set of load and generation scenarios, and these scenarios are interpreted as a sample of future power market behaviour, and is used as basis for a reliability assessment. These load and generation scenarios are referred to as power market scenarios. The power market analysis produces a large number of power market scenarios, and to include all these scenarios in a reliability assessment results in excessive computation time. The scenario selection method is presented and discussed. Scenario selection is used to pick out a subset of the generated power market scenarios, to only use this subset of scenarios...
2012 IEEE Power and Energy Society General Meeting, 2012
ABSTRACT DC line voltage drops as well as converter power losses have significant impacts on prec... more ABSTRACT DC line voltage drops as well as converter power losses have significant impacts on precise control of power flow in multiterminal VSC-HVDCs using DC voltage droop. When DC voltage droop controls are used in multiterminal VSC-HVDC, due to unequal DC bus voltages the DC line voltage drops causes large amount of power flow deviations in the DC network. DC line power losses and converter power losses also cause DC grid power flow deviations. To achieve precise control of power flow in the DC network, it is necessary that the power deviations occurring due to each of these factors be eliminated. With simulation of a five terminal VSC-HVDC system in PSCAD/EMTDC it is demonstrated how to achieve precise control of power flow in multiterminal VSC-HVDC by accounting for each of these three factors.
Power Systems …, 2006
In this paper, a Wide Area Monitoring System (WAMS) for the 420kV Norwegian bulk transmission gri... more In this paper, a Wide Area Monitoring System (WAMS) for the 420kV Norwegian bulk transmission grid has been evaluated. The robustness of the PMU measurements has been demonstrated by a direct comparison of phasor information from the PMUs and a state estimator (EMS). The paper highlights benefits of both off-line and on-line WAMS applications: a method for real-time detection of power oscillations, as well as the use of WAMS as input to power system stabilizers (PSS), is presented.
European Transactions on Electrical Power, 2007
The Norwegian system operator plans to change from (N-1) to a more flexible cost-based security c... more The Norwegian system operator plans to change from (N-1) to a more flexible cost-based security criterion. The approach is illustrated using a simple example and then compared with alternative decision paradigms. The comparison shows how different paradigms may change ranking of operating alternatives and that the new cost-based criterion still tends to favor alternatives close to (N-1), provided application of a penalty factor giving additional and significant weight on interruption costs.
In recent years there have been plans to develop large scale offshore wind farms in the North Sea... more In recent years there have been plans to develop large scale offshore wind farms in the North Sea. Dealing with variability of power from such wind farms is an important issue and should be addressed properly in the project developments. An HVDC connection between the offshore wind farm and Norway grid enables the offshore wind farms to benefit from the
Wind Energy, 2005
The Nordic energy market is in need of new power generation capacity. Norway is today in shortage... more The Nordic energy market is in need of new power generation capacity. Norway is today in shortage of electrical energy, and the hydropower-based system must rely on importing electrical power from its neighbours in a normal year of hydro inflow. One solution to this problem is to invest in new generation capacity, e.g. by integrating more wind power into the Norwegian grid. The Norwegian government has announced that 1000 MW of wind power will be installed in Norway by 2010, corresponding to an eventual energy production of approximately 3 TWh year-1 .This article investigates the impact of wind power integration in Norway on the damping of interarea mode oscillations in the Nordic grid. The impact on the interarea mode oscillations is tested for various types of generators. Results from computer analyses of a simplified yet realistic equivalent of the Nordic power system are used to compute the interarea mode oscillations.
Electric Power Systems Research
2016 Power Systems Computation Conference (PSCC), 2016
A novel approach to transient stability analysis in multi-terminal high voltage direct current (M... more A novel approach to transient stability analysis in multi-terminal high voltage direct current (MTDC) grids is presented in this paper. A symmetrical three-phase fault in an ac grid connected to a rectifier terminal of the MTDC grid causes the power injected into the dc grid to decrease, which in turn leads to a lower dc voltage in the MTDC grid. If dc voltage drops below a critical voltage limit before the ac fault is cleared, then the dc grid becomes unstable and its operation is disrupted. An analytical approach is proposed in this paper to calculate the critical clearing time of a fault in an ac grid behind a rectifier terminal beyond which dc voltage collapse occurs. A fiveterminal MTDC grid modeled in EMTDC/PSCAD is used to validate the results obtained with the analytical method.
2013 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC), 2013
ABSTRACT Power market models can generate load and generation scenarios, for a given market regul... more ABSTRACT Power market models can generate load and generation scenarios, for a given market regulation. The generated scenarios can be interpreted as a sample of the future utilisation of the power network, and be used as a basis for a contingency and reliability analysis. However, to use all the generated scenarios as input in a contingency and reliability analysis can lead to quite extensive computational requirements. A data reduction framework, which finds groups of similar scenarios, and only uses the group characteristics as input in a contingency and reliability analysis, is presented and discussed. It is shown that the data reduction framework can reduce the computational requirements by about 90% with little loss of accuracy. However, the success of this approach is highly dependent on which features that are used to quantify similarity between scenarios, and it is shown that choosing a set of nonoptimal features leads to large errors. The feature selection is compared with the choice of clustering algorithm, and shows that the feature selection process has a much large impact on the results than the choice of clustering algorithm.
CIRED 2012 Workshop: Integration of Renewables into the Distribution Grid, 2012
Power systems are passing through profound changes, mainly due to the liberalization of electrici... more Power systems are passing through profound changes, mainly due to the liberalization of electricity markets, the depletion of primary energy resources and the concern about climate change. The aforementioned premises have created a favorable frame for development of distributed energy resources. As a result, the level of integration of distributed generation (DG) technologies, especially in distribution networks has increased. In order to counteract the impact of DG on the stability and reliability of power systems, the transmission and distribution systems operators have started to reconsider and update their national grid codes. This paper provides an overview of the most recent and comprehensive grid codes regarding the DG integration at distribution level.
IEEE Transactions on Power Systems, 2012
ABSTRACT This paper discusses the impact of dc transmission voltage drops on the distribution of ... more ABSTRACT This paper discusses the impact of dc transmission voltage drops on the distribution of dc grid balancing power when dc voltage droop control is applied. DC line voltage drops in a multiterminal VSC-HVDC (MTDC) system result in nonuniform variations of dc bus voltages when changes in dc grid power flow occur. This in turn affects the distribution of instantaneous balancing power in a MTDC that uses dc voltage droop control. The values of dc voltage droop constants determine the degree of impact that dc voltage drops will have on the sharing of balancing power in the dc grid. In this paper, an analytical expression for estimating the distribution of balancing power which accounts for dc line voltage drops is derived. A five-terminal MTDC was modelled in PSCAD for demonstrating the effects of dc line voltage drops as well as for validating the proposed analytical expression which estimates balancing power distribution.
2017 IEEE Manchester PowerTech, Jun 1, 2017
This paper presents a method for voltage stability monitoring based on the maximum power transfer... more This paper presents a method for voltage stability monitoring based on the maximum power transfer to the load. The only required information is the system topology, the data from PMUs and the operational status of synchronous generators. With this information, the Thevenin impedance seen from a load bus can be estimated and by using the Thevenin theorem for maximum power transfer an impedance based stability index can be established. Since the Thevenin impedance is influenced by the operational conditions of the synchronous generators, it is consequently not a constant value and differs from the short circuit impedance of the bus. It is emphasized that the method requires only the information of the considered subsystem. Therefore, since the computation requirement is insignificant, the algorithm can be used for online monitoring. The validation of the approach is achieved by simulating a simple transmission system.
In Norway distribution networks are evolving rapidly into active systems due to the increasing nu... more In Norway distribution networks are evolving rapidly into active systems due to the increasing number of connections of small scale hydro generation units. As most of the distributed generation is based on synchronous technology their dynamic response on the bulk transmission power system cannot longer be neglected. In this paper a method for power systems reduction and aggregation of generators is applied to obtain dynamic equivalents of distribution power systems with DG units. Criteria to validate this method for distribution power systems are proposed.
SUMMARY This paper reports the initial phase of an Information Technology (IT) initiative to impr... more SUMMARY This paper reports the initial phase of an Information Technology (IT) initiative to improve the utilisation of the Norwegian power grid. We adopt a scalable approach, beginning with a focus on the use of local intelligence ("agent"), and gradually integrating these agents into the existing IT infrastructure. In particular, the paper reports on the implementation and testing of two new applications. These are the Voltage Instability Predictor for on-line tracking of distance to voltage collapse, which is primarily a local device, and a Secondary Voltage Regulation scheme for coordinated control of SVCs and synchronous condensers, which is presently a control centre application.
CIRED 2012 Workshop: Integration of Renewables into the Distribution Grid, 2012
Based on computer simulations of a simplified radial 132, 66 and 22 kV system including a 5 MVA h... more Based on computer simulations of a simplified radial 132, 66 and 22 kV system including a 5 MVA hydro power unit, it is concluded that the assessment of the unit's fault-ridethrough (FRT) capability should be based not only on a voltage magnitude profile, but also on the change in the voltage phase angle. It is assumed that erroneous conclusions might be the result when FRT-capability studies are performed for distributed generation without taking into account the influence of the phase-angle change in this context. Further work will be conducted in order to quantify the effect of phase angle influence on the FRT capability of distributed generation units.
International Journal of Electrical Power & Energy Systems
Power market analysis should be incorporated in reliability assessments of deregulated power syst... more Power market analysis should be incorporated in reliability assessments of deregulated power systems. For the Nordic power system, this is done by using The Multi-area Power-market Simulator (EMPS) for long-term power market analysis, where EMPS finds the optimal socio-economic dispatch on a weekly basis, with respect to, e.g., hydro reservoir levels. The EMPS analysis results in a set of load and generation scenarios, and these scenarios are interpreted as a sample of future power market behaviour, and is used as basis for a reliability assessment. These load and generation scenarios are referred to as power market scenarios. The power market analysis produces a large number of power market scenarios, and to include all these scenarios in a reliability assessment results in excessive computation time. The scenario selection method is presented and discussed. Scenario selection is used to pick out a subset of the generated power market scenarios, to only use this subset of scenarios...
2012 IEEE Power and Energy Society General Meeting, 2012
ABSTRACT DC line voltage drops as well as converter power losses have significant impacts on prec... more ABSTRACT DC line voltage drops as well as converter power losses have significant impacts on precise control of power flow in multiterminal VSC-HVDCs using DC voltage droop. When DC voltage droop controls are used in multiterminal VSC-HVDC, due to unequal DC bus voltages the DC line voltage drops causes large amount of power flow deviations in the DC network. DC line power losses and converter power losses also cause DC grid power flow deviations. To achieve precise control of power flow in the DC network, it is necessary that the power deviations occurring due to each of these factors be eliminated. With simulation of a five terminal VSC-HVDC system in PSCAD/EMTDC it is demonstrated how to achieve precise control of power flow in multiterminal VSC-HVDC by accounting for each of these three factors.
Power Systems …, 2006
In this paper, a Wide Area Monitoring System (WAMS) for the 420kV Norwegian bulk transmission gri... more In this paper, a Wide Area Monitoring System (WAMS) for the 420kV Norwegian bulk transmission grid has been evaluated. The robustness of the PMU measurements has been demonstrated by a direct comparison of phasor information from the PMUs and a state estimator (EMS). The paper highlights benefits of both off-line and on-line WAMS applications: a method for real-time detection of power oscillations, as well as the use of WAMS as input to power system stabilizers (PSS), is presented.
European Transactions on Electrical Power, 2007
The Norwegian system operator plans to change from (N-1) to a more flexible cost-based security c... more The Norwegian system operator plans to change from (N-1) to a more flexible cost-based security criterion. The approach is illustrated using a simple example and then compared with alternative decision paradigms. The comparison shows how different paradigms may change ranking of operating alternatives and that the new cost-based criterion still tends to favor alternatives close to (N-1), provided application of a penalty factor giving additional and significant weight on interruption costs.
In recent years there have been plans to develop large scale offshore wind farms in the North Sea... more In recent years there have been plans to develop large scale offshore wind farms in the North Sea. Dealing with variability of power from such wind farms is an important issue and should be addressed properly in the project developments. An HVDC connection between the offshore wind farm and Norway grid enables the offshore wind farms to benefit from the
Wind Energy, 2005
The Nordic energy market is in need of new power generation capacity. Norway is today in shortage... more The Nordic energy market is in need of new power generation capacity. Norway is today in shortage of electrical energy, and the hydropower-based system must rely on importing electrical power from its neighbours in a normal year of hydro inflow. One solution to this problem is to invest in new generation capacity, e.g. by integrating more wind power into the Norwegian grid. The Norwegian government has announced that 1000 MW of wind power will be installed in Norway by 2010, corresponding to an eventual energy production of approximately 3 TWh year-1 .This article investigates the impact of wind power integration in Norway on the damping of interarea mode oscillations in the Nordic grid. The impact on the interarea mode oscillations is tested for various types of generators. Results from computer analyses of a simplified yet realistic equivalent of the Nordic power system are used to compute the interarea mode oscillations.
Electric Power Systems Research
2016 Power Systems Computation Conference (PSCC), 2016
A novel approach to transient stability analysis in multi-terminal high voltage direct current (M... more A novel approach to transient stability analysis in multi-terminal high voltage direct current (MTDC) grids is presented in this paper. A symmetrical three-phase fault in an ac grid connected to a rectifier terminal of the MTDC grid causes the power injected into the dc grid to decrease, which in turn leads to a lower dc voltage in the MTDC grid. If dc voltage drops below a critical voltage limit before the ac fault is cleared, then the dc grid becomes unstable and its operation is disrupted. An analytical approach is proposed in this paper to calculate the critical clearing time of a fault in an ac grid behind a rectifier terminal beyond which dc voltage collapse occurs. A fiveterminal MTDC grid modeled in EMTDC/PSCAD is used to validate the results obtained with the analytical method.
2013 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC), 2013
ABSTRACT Power market models can generate load and generation scenarios, for a given market regul... more ABSTRACT Power market models can generate load and generation scenarios, for a given market regulation. The generated scenarios can be interpreted as a sample of the future utilisation of the power network, and be used as a basis for a contingency and reliability analysis. However, to use all the generated scenarios as input in a contingency and reliability analysis can lead to quite extensive computational requirements. A data reduction framework, which finds groups of similar scenarios, and only uses the group characteristics as input in a contingency and reliability analysis, is presented and discussed. It is shown that the data reduction framework can reduce the computational requirements by about 90% with little loss of accuracy. However, the success of this approach is highly dependent on which features that are used to quantify similarity between scenarios, and it is shown that choosing a set of nonoptimal features leads to large errors. The feature selection is compared with the choice of clustering algorithm, and shows that the feature selection process has a much large impact on the results than the choice of clustering algorithm.
CIRED 2012 Workshop: Integration of Renewables into the Distribution Grid, 2012
Power systems are passing through profound changes, mainly due to the liberalization of electrici... more Power systems are passing through profound changes, mainly due to the liberalization of electricity markets, the depletion of primary energy resources and the concern about climate change. The aforementioned premises have created a favorable frame for development of distributed energy resources. As a result, the level of integration of distributed generation (DG) technologies, especially in distribution networks has increased. In order to counteract the impact of DG on the stability and reliability of power systems, the transmission and distribution systems operators have started to reconsider and update their national grid codes. This paper provides an overview of the most recent and comprehensive grid codes regarding the DG integration at distribution level.