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- Farhang, P., et al.
(författare)
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Combined design of VSC-HVDC and PSS controllers for LFO damping enhancement
- 2014
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Ingår i: Transactions of the Institute of Measurement and Control. - : SAGE Publications. - 1477-0369 .- 0142-3312. ; 36:4, s. 529-540
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this paper is to present a novel combined design of the voltage source converter–high voltage direct current (VSC-HVDC) and the power system stabilizer (PSS) controllers to obtain a better dynamical response. The proposed technique is applied to enhance the damping of the power system low-frequency oscillations (LFOs) and results are compared with traditional design. A fuzzy logic controller is designed for PSS (FPSS). Then, a chaotic optimization algorithm, which has a strong ability for finding the most optimistic results, is employed, in presence of FPSS, to search for optimal VSC-HVDC output feedback controller parameters. Moreover, a singular value decomposition method is utilized to select the most effective damping control signal of the VSC-HVDC output feedback controllers. The novel proposed controllers are evaluated on an AC/DC power system. The simulation results demonstrate that the combined controllers have an excellent capability for damping power system LFOs and greatly enhance the dynamic stability of the power system. Also, the system performance analysis under different operating conditions and some performance indices show the effectiveness of the proposed controllers. The benefit of the suggested procedure is greatly improving the dynamic response of the system. In addition, the overshoots, undershoots and the settling times are dramatically reduced by applying the proposed method.
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| 4. |
- Safari Tirtashi, Mohammad Reza
(författare)
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Control of Voltage and Damping in Bulk Power Systems
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern power system is a complex dynamical system and one of the largest man-made systems. With recent driving forces like environmental concerns over air emissions, the modern power system is evolving towards an even more complex system. So it is necessary to handle the current challenges in power systems with simple approaches and avoid adding further complexity as much as possible. Also the implementation issues should be taken into account to meet the Transmission System Operators’ (TSOs’) interests. The considered problems in this thesis are related to voltage control and damping control which are two important issues challenging secure power system operation. The first voltage control problem addressed in the thesis occurred during the restoration of the Swedish power system after the blackout in 2003 and is called reactor hunting. Large scale voltage fluctuations are the consequence of the reactor hunting. The common practice used by the Swedish TSO to handle the reactor hunting is to turn off voltage control automatics during the restoration period. That leaves the shunt reactors in manual operation which leads to a longer restoration process. To prevent reactor hunting, an adaptive tolerance band strategy is proposed in the thesis together with two ways to implement it. One is model based and uses short circuit capacity of buses which are going to be energized during the restoration. The short circuit capacity associated to each bus is normally available in the Energy Management System (EMS) in the TSO control center. The second implementation can be completely local and independent of a model. By implementing this strategy, the automatic operation of the reactive shunts will continue during the restoration time, and reactor hunting is eliminated. This should shorten the restoration process. The second voltage control issue addressed in the thesis is related to control of shunt capacitors. Shunt capacitors are commonly controlled using a local scheme, which switches in the capacitor when the voltage at the locally monitored bus is outside a tolerance band. In some cases a shunt capacitor remains unused in a region lacking reactive power just because the local voltage is within the tolerance band. An alternative control strategy proposed in the thesis is called the neighboring scheme. It uses both the local voltage and the voltage at neighboring buses. The neighboring bus voltage is estimated from measurements at the local bus, so this strategy can be implemented locally and communication free which is important for TSOs. In a situation near voltage collapse, this strategy has better performance in the sense of improving the voltage control by connecting more shunt capacitors or connecting them earlier compared to the local scheme. For some scenarios, the voltage collapse that occurs using the local scheme is avoided when using the neighboring scheme. The second actuator used in the thesis for voltage control improvement is VSC-HVDC converters which have the capability to control active and reactive power independently. For emergency voltage control this thesis suggests adjusting active and reactive power set-points to change the AC system power flow. Based on the considered strategy, the active and reactive power set-points are adjusted depending on the disturbance. This control strategy improves the AC system long-term voltage stability and could prevent voltage collapse in some severe scenarios. When designing voltage control systems, the lack of a simple text book size version of NORDIC32 test system for long-term voltage stability study is another issue addressed in the thesis. The NORDIC32 test system is a reduced order model of the Swedish power system but in some cases still a complex test system. In this thesis, we propose the N3area test system which is a text book size version of NORDIC32 with minimum model complexity for our purposes. Applying complex control algorithms to the N3area system and analyzing them is much easier than to the NORDIC32 system. Still it retains a dynamic behaviour quite close to NORDIC32 and reality. The last problem addressed in the thesis is related to inter-area oscillations damping in power systems. These oscillations are becoming a big concern for TSOs since the power systems are getting more and more interconnected. Inter-area oscillations are often limiting the transfer capacity of transmission lines and may even lead to system break up as in the 1996 western North America blackout. Active power modulation is an effective solution to damp out such oscillations. This can be implemented by active power modulation at two points in the network, using for example VSC-HVDC links. Also single-point active power modulation using actuators like Energy Storage (ES) works well. Single-point reactive power modulation using actuators like SVC indirectly controls the active power and is also efficient. Proportional control of active power with local frequency as input is used in reality today for HVDC links. This type of damping controller can be applied for the ES and can also be translated for SVC damping controller. Implementing such proportional damping controllers is simple as they use local feedback signals. However, the damping of the inter-area mode is limited due to nearby zeros, which is evident in the associated root locus plot. It is therefore important to use the optimum gain to achieve the maximum possible damping. Gain selection is normally done using visual inspection of the root locus or through optimization. In this thesis, we propose the impedance matching based gain selection for the VSC-HVDC, ES and SVC damping controllers. It gives a physically based criterion for the optimum gain selection to reach the maximum possible damping of the mode with the greatest mode observability and controllability which depends on the actuator location while not affecting negatively the other modes in the system. The proposed approach may be used as basis for a controller that is self-tuning which is an important feature since the power system operating points are changing a lot. Also it is simpler for implementation in reality compared to the root locus inspection or application of advanced optimization methods for gain selection.
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| 5. |
- 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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| 6. |
- Safari Tirtashi, Mohammad Reza, et al.
(författare)
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Impedance Matching for VSC-HVDC and Energy Storage Damping Controllers
- 2018
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Ingår i: IEEE Transactions on Power Delivery. - 0885-8977. ; 33:2, s. 1016-1017
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Tidskriftsartikel (refereegranskat)abstract
- The small-disturbance electro-mechanical dynamics of a power system can be translated to an equivalent circuit model with inductances and capacitances. Damping control of active power in proportion to local frequency as with a VSC-HVDC link or an energy storage then corresponds to introducing a resistor in the circuit model. This letter shows that impedance matching can be used to select resistor value, and equivalently damping controller gain, that gives maximum damping ratio. It is also shown how the concept is employed without a circuit model.
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| 7. |
- Safari Tirtashi, Mohammad Reza, et al.
(författare)
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Long-Term Voltage Collapse Analysis on a Reduced Order Nordic System Model
- 2014
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Ingår i: 2014 49th International Universities Power Engineering Conference (UPEC). - 9781479965564 ; , s. 1-6
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Konferensbidrag (refereegranskat)abstract
- This paper analyzes and explains the mechanism behind long-term voltage collapse in the NORDIC32 test system. For this purpose a simplified test system called N5area, reflecting the key voltage collapse characteristics of NORDIC32 is proposed. Applying control algorithms is much easier in N5area than in NORDIC32. Load recovery and generator excitation current limiter actions which are two important factors contributing to long-term voltage collapse are considered. Dynamic simulation results for a specified long-term voltage instability scenario are explained and discussed. The effect of generator current limiters is analyzed using PV curves. Furthermore, two different control strategies for controlling the shunt capacitors are applied as countermeasures to save the system. The two strategies are explained and compared and it is shown that control using the voltage at neighboring buses gives better performance.
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