| 1. |
- de Toledo, Paulo Fischer, et al.
(författare)
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Frequency domain model of an HVDC link with a line-commutated current-source converter. Part I : fixed overlap
- 2009
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Ingår i: IET Generation, Transmission & Distribution. - : Institution of Engineering and Technology (IET). - 1751-8687 .- 1751-8695. ; 3:8, s. 757-770
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a frequency-domain model of an high voltage direct current (HVDC) transmission link with a line-commutated current-source converter. Using space-vector transfer functions between superimposed oscillations in the control signal and the AC and DC sides, expressions for voltages and currents have been derived. The dynamic properties of the HVDC link, taking the characteristics of the networks on both the AC and the DC sides into consideration, can be studied by applying classical Bode/Nyquist/Nichols control methods. The resulting model was validated by time-domain studies in PSCAD/EMTDC. The model is described in two papers. In this paper (Part I), it has been assumed that the overlap angle during commutation remains constant. It is shown in the validation that this assumption introduces resonances that cause severe errors at certain network conditions. In the second paper (Part II), the model is extended so as to cope with the varying overlap angle in order to bring the frequency-domain model into agreement with the results obtained from time-domain simulations.
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| 2. |
- Fischer de Toledo, Paulo, et al.
(författare)
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Frequency-domain modelling of sub-synchronous torsional interaction of synchronous machines and a high voltage direct current transmission link with line-commutated converters
- 2010
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Ingår i: IET GENERATION TRANSMISSION & DISTRIBUTION. - : Institution of Engineering and Technology (IET). - 1751-8687. ; 4:3, s. 418-431
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Tidskriftsartikel (refereegranskat)abstract
- The authors describe a model of a system that includes a high voltage direct current (HVDC) transmission link with line-commutated current source converters (LCC) closely connected to a synchronous generator used to perform analysis of sub-synchronous torsional interaction (SSTI) in the frequency domain. The model of the HVDC transmission link adequately represents the converters in the frequency domain and includes all essential controls for the operation of the converters, including a new sub-synchronous damping control used to mitigate interaction between the recti. er LCC and the shaft of the synchronous machine. The frequency domain model has been validated against time domain simulations showing good agreement.
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| 3. |
- Fischer de Toledo, Paulo, 1954-
(författare)
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Modelling and control of a line-commutated HVDC transmission system interacting with a VSC STATCOM
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interaction of an HVDC converter with the connected power system is of complex nature. An accurate model of the converter is required to study these interactions. The use of analytical small-signal converter models provides useful insight and understanding of the interaction of the HVDC system and the connected system components. In this thesis analytical models of the HVDC converters are developed in the frequency-domain by calculating different transfer functions for small superimposed oscillations of voltage, current, and control signals. The objective is to study the dynamic proprieties of the combined AC-DC interaction and the interaction between different HVDC converters with small signal analysis. It is well known that the classical Bode/Nyquist/Nichols control theory provides a good tool for this purpose if transfer functions that thoroughly describe the 'plant' or the 'process' are available. Thus, there is a need for such a frequency-domain model. Experience and theoretical calculation have shown that voltage/power stability is a very important issue for an HVDC transmission link based on conventional line-commutated thyristor-controlled converters connected to an AC system with low short circuit capacity. The lower the short circuit capacity of the connected AC system as compared with the power rating of the HVDC converter, the more problems related to voltage/power stability are expected. Low-order harmonic resonance is another issue of concern when line-commutated HVDC converters are connected to a weak AC system. This resonance appears due to the presence of filters and shunt capacitors together with the AC network impedance. With a weak AC system connected to the HVDC converter, the system impedances interact through the converter and create resonances on both the AC- and DC-sides of the converter. In general, these resonance conditions may impose limitations on the design of the HVDC controllers. In order to improve the performance of the HVDC transmission system when it is connected to a weak AC system network, a reactive compensator with a voltage source converter has been closely connected to the inverter bus. In this thesis it is shown that the voltage source converter, with an appropriate control strategy, will behave like a rotating synchronous condenser and can be used in a similar way for the dynamic compensation of power transmission systems, providing voltage support and increasing the transient stability of the converter.
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| 4. |
- Fischer, P., et al.
(författare)
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A new control scheme for an HVDC transmission link with capacitor-commutated converters having the inverter operating with constant alternating voltage
- 2012
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Ingår i: 44th International Conference on Large High Voltage Electric Systems 2012. ; , s. 1-11
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Konferensbidrag (refereegranskat)abstract
- Experience and theoretical analysis show that the voltage and power stability becomes an important issue when high-voltage direct current (HVDC) systems using line-commutated converters (LCC) are connected to AC systems with low short-circuit capacity and that these problems become more pronounced the lower the short-circuit capacity of the connected AC system is, as compared with the rating of the HVDC converter station. Normally some additional equipment, like static or synchronous compensators, will be installed in order to improve the performance of the HVDC transmission system, when it is connected to a weak AC system. The cost of the substation thereby will be increased. The Capacitor-Commutated Converter (CCC), which has been developed by ABB, offers an alternative to the LCC in this kind of application. It reduces the reactive power interaction between the converter and the connected AC system. The CCC is, in principle, a classical converter provided with a series capacitor placed between the converter valves and the converter transformer. In this paper a new control strategy that can be used together with such a CCC is presented. In the new control system the inverter emulates the operation of a Voltage Source Converter, in the sense that, beside the control of the active power flow through the converter, the reactive power exchange with the AC network can also be managed in order to adjust the alternating voltage of the converter bus. Studies have shown that the interaction between the inverter and the connected AC system is significantly reduced, when the CCC is used together with this new control strategy. Keeping the alternating voltage constant makes it possible to control the DC-side voltage of the HVDC transmission system, allowing the rectifier converter to control the active power by means of controlling the direct current. This combination allows stable operation of the HVDC transmission system even under severe network conditions associated with low short-circuit power in the connected AC network at the inverter side. In the paper it is shown that, with the new control scheme, it is possible to operate the inverter into an almost passive AC network (a network with very few rotating generators, resulting in almost no short circuit power). Simulation results have indicated satisfactory operation of the HVDC transmission with the inverter operating into a network having a Short-Circuit Ratio as low as 0.2 (SCR ≈ 0.2). To the knowledge of the authors, operation of a line-commutated converter at such low SCR values has never been reported previously. A description of the new control scheme will be presented. The calculated performance in a practically implemented installation, the Rio Madeira Back-to-Back system, will be illustrated.
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