Academic literature on the topic 'Line commutated converter (LCC)'

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.<br>QC 20100708

Power electronic converters are used in a wide range of applications as well as being the enabling technology for interfacing the alternative energy resources and many loads in modern power systems. The methodology of developing the so-called dynamic average-value models (AVMs) for such converters is based on averaging the variables (currents and voltages) within a switching interval resulting in numerically efficient models that are much more suitable than the detailed switching models for system-level studies as well as numerical linearization and the respective small-signal analysis. However, the AVMs available in the literature for line-commutated converters have several limitations such as neglecting the effects of losses, being only valid in certain operational modes and under balanced excitation, as well as employing a simplified representation of the multi-phase transformer in high-pulse-count converters. Moreover, a unified AVM methodology for high-pulse-count converters has not yet been established. In this thesis, a generalized AVM methodology is developed for voltage-source- and rotating-machine-fed multi-pulse line-commutated converters for both classes of transient simulation software packages, i.e., state-variable-based and nodal-analysis-based electromagnetic transient program (EMTP) type. The previously-developed AVM approaches, i.e., analytical and parametric, are extended to the EMTP-type programs, and the indirect and direct methods of interfacing the models with external circuit-network are introduced and compared. For the machine-converter systems, the effects of machine and bridge losses are taken into account in the new AVM. Finally, a generalized dynamic AVM methodology is developed for high-pulse-count converters based on the parametric approach. An effective multi-phase transformer model is developed in transformed (qd0) and phase (abc) variables. An efficient transformer model is also developed, which accurately represents the multi-phase transformer using an equivalent three-phase formulation. The proposed generalized AVM remains valid for all operational modes under balanced and unbalanced excitation. This model is employed for AVM implementation in state-variable-based and EMTP-type programs. Extensive simulation and experimental studies are carried out on several example systems in order to compare the developed AVMs against the detailed and previously-developed average models in time- and frequency-domains. The results demonstrate the great accuracy of the proposed AVMs and a significant improvement compared to the previously-developed models.

This thesis describes a new concept, applicable to high-power current-sourced conversion (CSC), where a controllable firing-angle shift is introduced between series and parallel converters to enable independent active and reactive power control. The firing-shift concept solves a difficult problem, by giving thyristor based CSCs the control flexibility of pulse-width modulated (PWM) converters, but without a loss in efficiency or rating. Several configurations are developed, based on the firing-shift concept, and provide flexible, efficient solutions for both very high power HVDC transmission, and very high current industrial processes. HVDC transmission configurations are first developed for 4-quadrant high-pulse operation, based on the series connected multi-level current reinjection (MLCR) topology. Independent reactive power control between two ends of an HVDC link are proven under firing-shift control, with high-pulse operation, and without on-load tap changing (OLTC) transformers. This is followed by application of firing-shift control to a bi-directional back-to-back HVDC link connecting two weak systems to highlight the added dc voltage control flexibility of the concept. The fault recovery capability of an MLCR based ultra-HVDC (UHVDC) long distance transmis-sion scheme is also proven under firing-shift control. The scheme responds favourably to both ac disturbances and hard dc faults, without the risk of commutation failures and instability experienced during fault recovery of line-commutated conversion. The two-quadrant capability of very high current rectification is also proven with configurations based on phase-shifted 12-pulse and MLCR parallel CSCs. The elimination of the electro-mechanical OLTC/satruable reactor voltage control, the high-current CSC’s biggest shortcoming, greatly improves controllability and with firing-shift control, ensures high power-factor for all load conditions. This reduces the reactive power demands on the transmission system, which results in more efficient power delivery

Traditionally energy has been generated with large synchronous generators. These large plants have characteristics that are well understood and are the basis for the operation of the electricity grid. Most grid codes are based on the assumption that new plant will be composed of synchronous generators. Most of these plants are powered by non-renewable fuels that come with significant carbon emissions. The realisation that there is not an infinite supply of these fuels and their emissions are harming the world’s environment has resulted in policies being implemented aiming at reducing these sources of emissions. This energy is to be replaced with energy from renewable sources. There are many renewable generator types available but wind generation has the highest focus in most countries. As of 2013 there is approximately 318 GW of wind energy installed worldwide. Integrating all of this wind generation into the synchronous power system presents many challenges to grid companies. Wind generation usually does not have the same characteristics as synchronous plant as it is asynchronous. Many of the services that are assumed to be provided by synchronous plant such as inertia or fault contribution are unavailable or come with additional cost. Compounding this wind generation will displace synchronous plant, reducing the system strength further. It is important for grid companies to gain an understanding of the impact of wind generation on the electrical system before the wind integration becomes an issue. Usually when issues begin to arise it is too late to alter existing plant. This means any mitigation of system issues will be expensive or result in an inefficient market. This means that new generators would be required to meet much higher connection standards as there is little system strength left to allocate to the new generators. Ireland has tacked this integration issue by adopting a simple wind integration metric System Non Synchronous Penetration (SNSP) to flag when the system is approaching critical non-synchronous generation levels. This thesis aims to investigate wind generation integration issues in small power systems, in particular ones that are not connected or only weakly connected to other larger grids. It will: • Develop a wind integration metric similar to that used in Ireland or determine application guidelines for the Irish SNSP; • Determine what regulatory approach may reduce the impact of new wind generation minimising the requirement for the integration metric; and • Determine what effect wind generation may have on other plant, particularly those that will not be mitigated by the first two points. For this study the power system of Tasmania is used as the case study. Tasmania is a relatively small (~1700 MW peak load, ~900 MW minimum load) power system connected weakly to the much larger mainland Australia power system via a single HVDC interconnector. This interconnector has a transfer capability of 500 MW into Tasmania and 630 MW out of Tasmania. Additionally this connector is monopolar and can lose all transfer capability in a single fault. This means that during low load approximately half of Tasmania’s generation needs to be able to be tripped at any moment. This is before any response from wind farms is taken into account. Tasmanian generation is predominantly hydro. This type of plant is very flexible. It can be started and shut down very quickly and has no real minimum operating level. This means that when wind generation is high it will tend to shut down rather than operate at a minimum level. This thesis is presented in five sections: Chapter 1. Introduction: This chapter introduces this thesis and its objectives. It also summarises the experiences of other jurisdictions and how they may be similar to the study case. Chapter 2. Mathematical description of a wind plant: This chapter describes a wind plant in mathematical terms, and then it shows how a wind plant responds differently to grid disturbances. Chapter 3. Impact of wind generation on a small power system: This chapter studies the impact of wind generation on the case study power system and investigates how this impact may be mitigated. Chapter 4. Conclusion: This chapter summarises this thesis and explains its outcomes.