| 1. |
- Karatsivos, Evripidis, et al.
(författare)
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A general control system structure for multi-terminal VSC-HVDC systems
- 2014
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Ingår i: IEEE PES Innovative Smart Grid Technologies, Europe.
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Konferensbidrag (refereegranskat)abstract
- Renewable technology expands geographically and in capacity. The long transmission distance and bulk power transfer involved make multi-terminal VSC-HVDC systems a key technology for future power systems. This expansion could be hampered by the pace of development of multi-terminal systems especially when standardization is not reached in hardware or the control system. A general control system structure is proposed in this paper, dividing the control system in control levels for primary and secondary control implementation and focusing on the interface among them for modularity and expandability. Uniform interfaces standardize data exchange and decouple the overall system control from the local substation control. A version of the control system is tested on a simplified model for different scenarios and number of terminals to demonstrate its functionality and ability to integrate additional terminals. The control system structure provides a platform where each part of the control system can be programmed independently.
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| 2. |
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| 3. |
- Karatsivos, Evripidis, et al.
(författare)
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Disturbance Management in a DC Grid using a Hierarchical Control Structure
- 2015
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Ingår i: Lund Symposium 27/28 May, 2015.
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Konferensbidrag (refereegranskat)abstract
- DC grids will play a key role in the future, interconnecting generation and load centers as well as energy markets. Large multi-terminal VSC-HVDC systems of high capacity will require automated coordination for several operation scenarios especially when operating under disturbances. With this in mind, and considering the dynamics of a DC system, communication loss events are considered severe disturbances in the operation of the multi-terminal system along with major electrical faults such as 3-phase faults on the AC sides of the converters that require considerable coordination efforts. This work aims at enabling the multi-terminal system with communication loss ride-through and automatic post-fault rescheduling capabilities. This is achieved within a hierarchical control structure whose features are briefly described. A 3-terminal VSC-HVDC system is modelled for the needs of demonstration under different operation scenarios. Communication loss ride-through capability is demonstrated by comparing the behavior of the 3-terminal system during a normal operation scenario and a worst-case scenario where communications with all-three terminals fail. Its behavior does not change but since communications fail, the operation is uncoordinated greatly reducing the security of the system regarding upcoming events. During 3-phase faults DC droop performs the primary control stabilizing the system. Secondary control is performed by the overall control after the system state is identified. Automatic, post-fault rescheduling takes place in an effort to follow the power flow schedule where possible without exceeding substation limits.
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| 4. |
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| 5. |
- Leisse, Ingmar, et al.
(författare)
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Coordinated voltage control in medium and low voltage distribution networks with wind power and photovoltaics
- 2013
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Ingår i: PowerTech (POWERTECH), 2013 IEEE Grenoble.
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Konferensbidrag (refereegranskat)abstract
- Distributed Generation (DG) installations have been increasing during the last years. Wind power and photovoltaics are two of the most common renewable energy sources for DG typically connected to the distribution network (DN) originally planned and built to supply loads. DG units connected to the DN impact the voltage where customers are connected. Network voltage is an important quality criterion in DN. Voltage rise caused by DG units may become one of the limiting factors for the hosting capacity of wind power and photovoltaics in DNs. Increasing the hosting capacity by network rebuilding is possible but it is expensive and time consuming. Coordinated voltage control has been proposed to increase network capacity without the need of reinforcement. Simulations based on an existing medium and low voltage DN with wind power and photovoltaics are presented. It is shown that coordinated voltage control can increase the hosting capacity and avoid network reinforcement.
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| 6. |
- Leisse, Ingmar, et al.
(författare)
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Electricity Meters for Coordinated Voltage Control in Medium Voltage Networks with Wind Power
- 2010
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Ingår i: Innovative Smart Grid Technologies Conference Europe (ISGT Europe), 2010 IEEE PES. - 9781424485086 - 9781424485093 ; , s. 1-7
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Konferensbidrag (refereegranskat)abstract
- During the last years the amount of electricity generated by Distributed Energy Resources (DER), especially wind turbines, has been increasing a lot. These Distributed Generation (DG) units are often connected to rural distribution networks, where they have a large impact on the voltage and the network losses. The network voltage at the customers point of connection is an important quality criteria and has to follow different standards as e.g. EN 50160. Therefore the voltage change caused by the integration of production units in the distribution network is an important aspect when integrating more DG in distribution networks and often a limiting factor for the maximum DG capacity which is possible to integrate into an existing network without reinforcement. Using the available voltage band more efficient by applying coordinated voltage control is a possibility to increase the hosted DG capacity in an existing distribution network without reinforcement of the network. To get the actual network status the new generation of electricity meters, which have the feasibility to communicate real time voltage measurements from the customers side to a network controller, give some benefits to a more flexible and coordinated voltage control in the network. The voltage range in the network will be used adapted to the actual load and generation situation instead of using worst case assumptions as it is good practice until now. A main part of the voltage control in medium voltage distribution networks is done by the on-load tap changer (OLTC) which takes the voltage at the consumers point of connection into account. A generic 10 kV distribution network with three typical types of feeders, as pure load, pure generation and mixed load and generation feeder, has been outlined. Coordinated voltage control is implemented by a central voltage controller. Simulations on the voltage and the network losses have been done and will be presented in this paper. The maximum DG capacity in the test system increases most when introducing coordinated control of the OLTC but also the use of reactive power adds some benefit. Further increase of the DG capacity by more extensive use of curtailment is always possible but due to economical aspects not favoured.
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| 7. |
- Leisse, Ingmar, et al.
(författare)
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Increasing DG Capacity of Existing Networks through Reactive Power Control and Curtailment
- 2010
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Konferensbidrag (refereegranskat)abstract
- Renewable energy sources (RES), especially wind turbines, have become more important during the last years. An increasing number of distributed generation (DG) units are connected to weak medium voltage distribution networks in rural areas where they have a large influence on the voltage and the line losses. Voltage rise is in this case often a limiting factor for the maximum amount of DG capacity. Already current wind turbines with a capacity of 2 MW can often not easily be connected to existing 10 kV feeders. To increase the DG capacity of existing networks without reinforcement DG units can be controlled. This paper proposes abandoning unity power factor used today and letting the converters used as network interface of many new wind turbine generators absorb reactive power to reduce the voltage level. Since reactive power has great influence on losses in the network the use of reactive power is limited. Line losses due to the transfer of reactive power are taken into account in this study. Furthermore the use of curtailment is analysed. Simulations of voltage change and line losses when using reactive power control by the connected wind turbines and curtailment in a simple test system are presented. Without reinforcement of the network it was possible to increase the DG capacity from 2;7MW to more than 4MW in the test network without violating voltage limits. Line losses increase but to a reasonable extent and lost energy due to curtailment is insignificant.
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| 8. |
- Maqbool, Imran, et al.
(författare)
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Transformer Energization Using Grid Forming Converter
- 2022. - 25
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Ingår i: IET Conference Proceedings. - 2732-4494. ; 2022:25, s. 231-236
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Konferensbidrag (refereegranskat)abstract
- Increasing renewable integration is posing new challenges to reliability of power networks. One of these challenges is to restart the power network and energize power transformers in the event of a blackout. Grid forming converters can provide a potential solution to this issue because they can be controlled as independent, self starting, voltage sources. This study analyzes and compares four grid forming controllers to assess their capabilities to energizing the transformer. Simulations are performed with common parameters with different line lengths and transformer sizes. The results demonstrate that the inrush current and voltage show similar trends for these four controllers but with different magnitudes. The highest magnitude is observed for the matching control while other control schemes demonstrated similar performance. Moreover, each controller show different harmonic response and it is important to consider resonant frequencies of the network.
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| 9. |
- Safari, Reza, et al.
(författare)
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Impedance Matching for VSC-HVDC Damping Controller Gain Selection
- 2018
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Ingår i: IEEE Transactions on Power Systems. - 0885-8950.
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Tidskriftsartikel (refereegranskat)abstract
- The impedance matching concept is employed in this paper to select the optimum gain of a VSC-HVDC damping controller. The damping controller of the DC link is based on control of active power in proportion to the difference in local frequency at the VSC-HVDC converter stations. To explain impedance matching, the small-disturbance electro-mechanical dynamics of a power system is transformed to an equivalent LC-circuit. The VSC-HVDC damping controller corresponds to introducing a resistor in the circuit model. Using impedance matching to select resistor value in the circuit model is equivalent to selecting damping controller gain that gives maximum damping ratio. An analytical derivation is conducted for a linearized model of a generic two-area power system, including an expression for the optimum gain. It is also shown how the concept may be employed without a circuit model and that the optimum value is hardly affected by changes in power flow. Then the performance of the proposed approach in multi-mode test systems and with nonlinearities is evaluated using dynamic simulations in DIgSILENT PowerFactory. The results show that impedance matching maximizes the targeted mode damping ratio while non-targeted modes are not negatively impacted.
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| 10. |
- Safari Tirtashi, Mohammad Reza, et al.
(författare)
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Control strategies for reactive shunts to improve long-term voltage stability
- 2013
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Ingår i: Power Engineering Conference (UPEC), 2013 48th International Universities.
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Konferensbidrag (refereegranskat)abstract
- Voltage collapse was one of the main causes for many recent blackouts. The direct link between voltage stability and reactive power balance in the system leads to more attention toward reactive power resources in the power systems. Shunt reactors and capacitors are used to balance reactive power in the power systems. The strategy to control them in both normal and emergency conditions is an important issue. This paper deals with two different strategies for automatic switching of shunt reactors and capacitors in the power systems. The first control strategy, called the local scheme, switches the shunt when the voltage at the local bus is outside the tolerance band. In the second control strategy, called neighboring scheme, local voltage as well as voltage at neighboring buses are used. Dynamic simulations of the NORDIC 32 test system show that the neighboring scheme improves voltage compared to the local one. In the simulated scenario a blackout is avoided by using the neighboring scheme. This is explained using PV curves for a new test system reflecting the key behavior of NORDIC 32.
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